Cholesteric Liquid Crystals in Additive Manufacturing

Cholesteric Liquid Crystals in Additive Manufacturing

Citation for published version (APA):Sol, J. A. H. P. (2022). Cholesteric Liquid Crystals in Additive Manufacturing. [Phd Thesis 1 (Research TU/e /Graduation TU/e), Chemical Engineering and Chemistry]. Eindhoven University of Technology.https://doi.org/10.6100/95acbb3d-ab3d-411b-a30c-0be458a2a966

DOI:10.6100/95acbb3d-ab3d-411b-a30c-0be458a2a966

Document status and date:Published: 30/03/2022

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Jeroen Sol

Cholesteric Liquid Crystals in Additive Manufacturing

Cholesteric Liquid Crystals inAdditive Manufacturing

proefschrift

ter verkrijging van de graad van doctor aan de Technische Universiteit Eindhoven,op gezag van de Rector Magnificus, prof. dr. ir. F. P. T. Baaijens, voor eencommissie aangewezen door het College voor Promoties, in het openbaar te

verdedigen op woensdag 30 maart 2022, om 13:30 uur

door

Jeroen Andreas Hermanus Petrus Sol

geboren te Oss

Dit proefschrift is goedgekeurd door de promotoren, en de samenstelling van de pro-motiecommissie is als volgt:

voorzitter: prof. dr. F. Gallucci

1e promotor: prof. dr. A. P. H. J. Schenning

co-promotor: dr. M. G. Debije

leden: prof. dr. N. H. Katsonis Rijksuniversiteit Groningen

prof. dr. K. Neyts Universiteit Gent

prof. dr. L. Moroni Universiteit Maastricht

prof. dr. D. J. Broer

adviseurs: dr. C. Sánchez Somolinos CSIC-Universidad de Zaragoza

dr. ir. A. T. ten Cate Nederlandse Organisatie voortoegepast-natuurwetenschappelijk onderzoek(TNO)

Het onderzoek of onderwerp dat in dit proefschrift wordt beschreven is uitgevoerd inovereenstemming met de TU/e GedragscodeWetenschapsbeoefening.

“One day you choke, your urges overflowAnd obsession wears you down

But don’t you waste the suffering you’ve facedIt will serve you in due time”

Get YourWish, off of Porter Robinson’s albumNurture (2021)

The research described in this thesis was part of theDynamicMaterials for AdditiveMan-ufacturing (“DynAM”) project consortium, which was funded by the Dutch ResearchCouncil (NWO), in the framework of the Innovation Fund Chemistry, and from theDutch Ministry of Economic Affairs and Climate Policy in the framework of the PPP al-lowance.

A catalogue record is available from the Eindhoven University of Technology Library

ISBN: 978-90-386-5472-0

Copyright ©2022 by Jeroen Andreas Hermanus Petrus Sol

Cover and book design: Jeroen Andreas Hermanus Petrus Sol

This thesis is typeset using XƎLATEX, in 11 pt EB Garamond and 9 pt Overpass.

Contents

Prologue 1

Acknowledgements 3

Academic summary 8

1 Introduction and Theoretical Background 101.1 Optically responsive structures: what inspires us . . . . . . . . . . . . . 121.2 Additive manufacturing: bottom-up building . . . . . . . . . . . . . . 131.3 Liquid crystals for responsive optics . . . . . . . . . . . . . . . . . . . . 16

1.3.1 Mesogens and mesophases . . . . . . . . . . . . . . . . . . . . 161.3.2 Reactive mesogens . . . . . . . . . . . . . . . . . . . . . . . . 191.3.3 Responsive structural colour based on cholesterics . . . . . . . . 19

1.4 Using liquid crystals in direct ink writing 3D printing . . . . . . . . . . . 211.4.1 Synthesising a liquid crystal elastomer precursor ink . . . . . . . 211.4.2 Direct ink writing of liquid crystal oligomer inks into smart devices 231.4.3 Direct ink written liquid crystal elastomer optics . . . . . . . . . 24

1.5 Outline of this thesis: where do we go from here? . . . . . . . . . . . . . 25

2 Responsive Photonic Liquid Crystalline Flakes Produced by Ultrasonication 322.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 342.2 Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

2.2.1 Photonic film fabrication . . . . . . . . . . . . . . . . . . . . . 352.2.2 Fracturing of ChLCN films into photonic flakes and their pho-

tonic response . . . . . . . . . . . . . . . . . . . . . . . . . . 372.3 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 432.4 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 442.5 Appendix: video files . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

3 Creating Spatially Patterned Slanted Cholesteric Films through Bar Coatingand Photolithography 503.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 523.2 Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

3.2.1 Synthesis of a cholesteric liquid crystal oligomer . . . . . . . . . 53

3.2.2 Fabrication and characterisation of slanted cholesteric coatings . 543.2.3 Photopatterning visible patterns with anisotropic iridescence . . 58

3.3 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 593.4 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 593.5 Appendix: video files . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

4 Direct Ink Writing of a Cholesteric Oligomer Ink for Freeform Soft Reflec-tors 664.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 684.2 Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . 68

4.2.1 Synthesis of the cholesteric ink for direct ink writing . . . . . . . 684.2.2 Direct ink writing of the cholesteric ink . . . . . . . . . . . . . 694.2.3 Characterisationof the circular polarisationdependence in reflec-

tion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 724.2.4 Demonstrator devices printed using the cholesteric DIW ink . . 73

4.3 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 734.4 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 744.5 Appendix: video files . . . . . . . . . . . . . . . . . . . . . . . . . . . 76

5 4D Printed Structurally Coloured Actuators 785.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 805.2 Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . 80

5.2.1 Synthesis and direct ink writing of the water-responsive choleste-ric ink . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80

5.2.2 Using the water-responsive ink in multi-material 3D printing . . 865.2.3 Scallop-inspired, single-ink, 4D printed structurally coloured ac-

tuator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 875.3 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 895.4 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 895.5 Appendix: video files . . . . . . . . . . . . . . . . . . . . . . . . . . . 93

6 The Future of 4D Printed Cholesterics—Where, and How? 966.1 A brief recap of the work in this thesis . . . . . . . . . . . . . . . . . . . 986.2 Increasing the applicability of cholesteric 3D prints . . . . . . . . . . . . 99

6.2.1 Responsive cholesterics for FFF . . . . . . . . . . . . . . . . . . 996.2.2 Direct ink writing of cholesterics for intricate photonic coatings . 1006.2.3 Cholesterics in direct ink written soft actuators . . . . . . . . . 1026.2.4 The use of direct ink written cholesterics in biological settings . . 104

6.3 Securing a sustainable future for DIWwith cholesterics . . . . . . . . . . 1046.4 Recap: what 4D printed cholesterics can add to the future . . . . . . . . 106

1

Prologue

The thesis youhold before youdescribes the development of newmaterials for 3Dprinting,based on liquid crystals. “Liquid crystals”, a name that I considered to be rather confusingas a student, but have grown to love in the years since. Strictly speaking, there are many,many different types of liquid crystals. Ones that make up cell membranes in organisms,ones that serve as basis for flat-panel displays, and now, novel ones that can be 3D printedinto brightly coloured objects. More on that, later in this dissertation.

During a PhD project, the candidate works at the forefront of science—there wherefew other scientist have gone before. This leads to the uncanny situation that you trulybecome one of the few people in the world with a certain expertise. Specialised in the “syn-thesis anddirect inkwriting of thermotropic cholesteric liquid crystal oligomer inks”. Howabout that? Luckily, the freedom provided during this project has allowed me to venturefar beyond “synthesis” and “direct ink writing”. Given that each research group acts as asmall company—a collective of prospectors along the rocky roads of scientific discovery—there aremany other skills of value besides day-in-day-out data gathering. Communicationof results, in writing and speech; imaging of beautiful samples; visualising data in clearways; and perhaps ultimately, sharing these skills with colleagues towards collective pros-perity.

It makes doing a PhD project seem rather profound, but in the end inmy case—it wasmostly doing things I enjoy. Taking care of the computers within the group, helping outwhenever events made use of audiovisual equipment, and doing the photography at groupevents. This breadth of activities has made “doing a PhD” a very unique and rewardingexperience.

3

Acknowledgements

Obviously, while this book lists a single author on the cover, the research done in these past fouryears can hardly be considered a one-man effort. Many people have helped along the way, big andsmall, all in some way contributors to the pages to follow.

First and foremost: Michael. Your guidance over the past four years has been a tremendousfactor in the success of my research project. I like to think our optimist-realist dynamic worked outreally well! Whenever results were incomprehensible or not conclusive, you would always think ofsomething to try so to not let me succumb to desperation. While we have our differences, mostnotably about the correct way to write that language of yours, there is far more that connects us.Crashing your office with a question usually ended in hourlong dialogs about politics, music, lit-erature, and whatever other news ruled the day. I will surely miss this, and hope you will find aworthy successor. Besides this, I hope you can find someone new that can help you at a moment’snotice, whether it’s about computer issues or students that need instruction.

Albert, your leadership style is something to foster. Our meetings were few and far between,which sometimes made me worried that the research I was doing was lacking your judgment andgoing in the wrong direction. Fortunately, the results seem to have ended up just fine. As I’vementioned in past conversations, when you lectured me on liquid crystals some eight years ago, Ihad no idea what you were going on about. Now, I have written a book on them—how the tableshave turned! Like Michael, you would always think of experiments to carry out in times of need, askill I aim to perfect as well. I would like to thank you for the opportunities you’ve givenmewithinyour group; besides science, I was also involved in some teaching, and took care of the computersin the group. These things take time, and I am oh so happy that I was granted this.

To the rest of the doctoral committee: Dick, your creativity and expertise radiate throughoutthe group. I am honoured that you are willing to be a part of my thesis defence. Carlos, you laidsome of the foundations for SFD’s “4D printing” endeavours and I have re-read your paper on itmany times. Lorenzo and Tessa, you provided the DynAM meetings with valuable insight and Ialways loved to reconvene. Kristiaan Neyts and Nathalie Katsonis, while we have not met before, Ihave been inspired by papers coming from your research groups and am grateful you will join mydefence.

The research in this thesis was part of the “Dynamic Materials for Additive Manufacturing”(DynAM) consortium. Francesco, thank you for organising the meetings. Rint, Hans, Huiyi, andSouma, you showedme the wonderful world of dynamic covalent networks. I hope I can soon buyproductsmade using yourmaterials andmake the human-madeworld a bitmore circular. Lorenzo,Matt, Francis, Agustina, Tobias, the contributions you made to the consortium meeting were in-spiring too. Thank you for the cell tests you did and the experiments we could do on your DLPmachine. To Souma and Francis: I wiss you the best of luck in the final stages of your own PhDprojects and assume you will deliver great theses. Tessa, I admire your skill to keep up with threesuch disparate research lines!

Kees, while you were not actively involved in my side of the DynAM project, you provided

4

useful commentary on my work, but your greatest area of knowledge might be life itself. Duringtrips to Geleen and at conferences you gratuitously shared your experiences, which I appreciated.

During my time at SFD, the group anchored on two stable rocks in a world of fleeting four-year projects. Marjolijn, the website lists “secretary” but you deserve a PhD for your organisationalskills. Whether it was about business trips, car rentals, the road-to-PhDdefence, planningmeetings,printing photomasks, and organising events and conferences. You have supportedme from the firstday in the group, and that support has never faded.

Tom, your role in the smooth operation of SFD cannot be overstated. Keeping the disposablesin stock, ordering chemicals, maintaining equipment. If the PhDs were left to their own devices,the group would soon find itself delapidated. Similarly, you were always available for chats in STO0.26 or at the coffee machines, which were enjoyable especially at times of distraction. Sjoukje, youhave joined the group only a short while ago but I’m certain you will be of great value to the group,too! Johan, your knowledge on liquid crystals and their synthesis has been a huge help towards theresults described in this thesis.

SFD has always been called a family by Albert, and I think he’s right. Family members, likecolleagues, are not always chosen, but can become great friends. The best office, STO 0.21, hasbeen occupied by nothing short of stellar folks. Matthew, you introduced me to the PhD life andalsomade sure the officewas never quiet. We always had something to talk about. Nintendo games,music, graphic design, liquid crystals, too. I have maintained your photoalignment procedure onthe office whiteboard—it feels like a cave painting of ages past. For the longest time I could notimagine myself writing a thesis like you were doing at the time. Anping, we shared the office briefly,but I will never forget the number of grapefruits you worked through. Arne, I think we made fora nice and peaceful office until large sports events such as theOlympics, football championships, orTour de France would occur. These times made for nice distractions! Danielle, it was a joy to haveyou in the office, always open for a chat, yet at the same time you seemed very productive with yourmany colourful prototypes! Wilson, in your thesis you referred to the many talks we had aboutculture and language. These are sorely missed! I hold the honour of having been one of your two“paranymphs” together with Stijn and will always remember your defence day dearly. All the bestin Beijing! Stathis, we have known each other for a long time. It was a great surprise that you joinedSFD as researcher-for-hire and we were both even more pleased that we got to share an office. Acouple of years ago, I had never imagined that youwould join and then surviveme at SFD. I am stillawe-struck by your impressive work ethic, and hope we can continue to discuss many films and alot of music in the future.

Luckily, the other officeswere filledwith people just as amazing. Rob, I got to knowyouduringmybachelor’s project at SFD andwas glad youwere still aroundwhen I started as a doctoral student.It was always nice being around you, cracking jokes and talking sports. Likewise, Gilles, I got toknow you as a student and later returned towork in the greenhouse project with you. Youwere niceto have in the sometimes irresponably ambitious academic world. I wear Kees’s confusion over ournames with pride. Marina, youwere always ready to think along, especially during the first—in thisthesis unreported—year that I made “azobenzene films”. Your creativity and student managementskills are inspirational. Li, you were a lovely colleague and great help during the initial oligomerink syntheses! Simon, your wealth of laboratory experiencemade you a valued colleague, and beinginvited for your wedding party was great. I enjoyed the musical choices, from the bass-heavy dancemusic to the closing Starlight. Sean, we connected on many topics and I could discuss with youpretty much whatever was on my mind, work-related or not. Added bonus: your experience withorganic synthesis has been a very useful resource! Dirk Jan, together withHuub you supervisedmy

5

first contactwith SFD.The amazement from looking at the colourful smecticmembranes using thepolarisedmicroscope has never faded. Roel, unfortunately our trip to San Francisco fell through atthe last moment! I would have loved to spend the week with you. Sterre, you took up photographyand was a huge help in capturing the 10+1 event. Thanks a lot! Yari, you might have one of thethankless jobs in our group—taking care of the DSC machine—but at least this meant I couldalways trust on it working. I hope yourDJ decks spin faster than the autosampler. Stijn, weworkedtogether as paranymphs for Wilson, a project I think worked out fantastic. Your past research intocholesterics has served as inspiration, and I hope your current research will work out just as well.Sebastian, you re-learned me how to run a silica column and I will be forever thankful, as it wasinstrumental for Chapter 5. You were also instrumental in lowering my expectations forDeutscheBahn, sending me into a panic right before my conference trip to Milan. Not so thankful for that!Xinglong, your research into thermoplastic polymers was rather different from my work, but I’mhappy to have been of help with the paper. Alessio, you joined as a guest late in my time as a PhD.It was very enjoyable to have you around and I hope the results you achieve here will suit your ownthesis well. Jeroen (van Aart), we got to know each other during RPK-D. It made the long lecturedays something to look forward to. Danqing, we didn’twork together butMichael and I superviseda group in the master’s course of your making and this being the first group I supervised in “covidconditions” made it a great learning experience. Thank you! Patricia, we didn’t work together atthe university, but perhaps on something greater: a one-hour long interactive experience on the lifeof “Dr. Marc del Pozo Puig”. You were amazing to have as a co-paranymph and I wish you all thebest in your future endeavours.

Yuanyuan andAlberto, our “anime dates” and the dinners attached to themwere always some-thing to look forward to. Alberto, your hospitality, and Yuanyuan, clarifying the Japanese charac-ters on screen, will make these unforgettable, and I hope we will meet many more times like this inthe future.

All the other colleagues that I’ve shared time at SFD and in Helix with: Monali, Sarah, Ellen,Wei, Xiaohong, Fabian,Davey, Pei,Niki, Anahita, Duygu,Mert, Eliza, Jacques,Dongyu, Pengrong,Ievgen, Thierry, Shajeth, Koen,Wanshu, Josué, Bruno, Roberta, Annelore, Evelien, Christian, andStefan—thank you all for making this group and the department a nice place to work at, the ideaswe exchanged, and the great time during events in the summers and winters.

Audrey, I did not make very extensive use of your language services, but I appreciate that youwere always ready to answer any language questions I had. Maartje and I enjoyed the game night alot, and hope we can do one again soon!

I would also like to thank the co-authors I’ve collaborated with these past years: Akhil, Ratna,Lanti, and Nadia. Without your experiments and input, these publications would not have beennearly as nice—if they ever got published at all. Of great importance to the results described inChapter 2, was the filament compounder borrowed fromUltimaker. My thanks to TomHeijmansfor allowing us to use this machine.

A special word of appreciation to the students I have had the opportunity to supervise. Lean:you were the first and were around even before the 3D printer was—the absence of which markedyour project. Nevertheless, you performed many experiments and over time, grew accustomed tothemore fundamental nature of the research. I enjoyed your stay, learnt a lot, andwas happy to joinyou in Heerlen for your bachelor’s defence. I wish you all the best with the master’s in ChemicalEngineering and Chemistry that you are now pursuing. Lana: while you also had your doubtsabout the viability of the project, you worked enthusiastically towards great results. It was enjoy-able to supervise you, see your grow as a researcher, and I am sure your creativity will serve you

6

well in the future. Henk: where do I start? It was obvious from the get-go that we shared someimportant traits. A love for “cholesteric colours”, and a certain degree of stubbornness. That youfinally stopped worrying about NMR spectra lead to the generation of beautiful samples that willserve SFD for years to come, and allowed me to start 3D printing cholesterics. Right at the end ofyour project, the printer finally arrived! Now that you’re a PhD student yourself, I once again wishyou the best of luck but are fully confident your research will work out well (once more frettingabout NMR spectra, though). Robin: we met during the master’s group project I supervised withMichael and you later returned for a graduation project. A highly ambitious piece of research for ahighly ambitious person. I know full well that I had you do plenty of things you didn’t enjoy—allthese syntheses—but your mastering of it is testament to your drive. The results you achieved werebeautiful, so I consider all your hard work to have been worth it. Luc: the work you did was crucialfor what is now Chapter 5. We matched very well on a personal level and I was happy to see youfind joy in laboratory work. I wish you the best for the future!

Alongside these students that I supervised in their individual projects, together with Marc delPozo Puig we also supervised a group of students for the “DBL Energy” practical course. Douwe,Janneke, Renske, Olivia, Dimitar, Kamila, Ahmad, Andrei, and Usama, thank you all for youreffort. While some of you had ambitions towards process engineering rather than chemistry, I hopeyou will remember the brightly coloured coatings youmade. Your results helped Robin get started.

Besides these students working towards 3D printing inks, I have also supervised students thatwere working on projects of their own. Erik: your crossover project between plasmonic optics andliquid crystal technology was perhaps the most enigmatic our group has ever seen. Luckily, all Ireally had to dowas teach the basic procedures in our labs, and off youwent. I enjoyed the frequentcoffee breaks, our discussions on politics, and wish you the best in your future career in science!For now, anyway—I still imagine you as a future policymaker. Nigel: your project was also drivenmostly by yourself. I got to know you as an enthusiastic researcher that loved exploring differentmeasurement techniques toquantify your devices. Your andErik’s independencewas a tremendoushelp during the time I supervised you both, Robin, and Luc at once. Finally, in the final monthsof my project I was asked to aid one final student. Mattia: I enjoyed teaching you how to performbasic chemical research, from weighing chemicals to oligomer ink synthesis and 3D printing. Youwere eager to learn, and I am pretty sure this will pay off in the outcome of your project.

Of course, there are two colleagues that deserve some extra praise for being great paranymphsand even better friends. Marc, we were bound by fate in the same research project: procuring a 3Dprinter and printing liquid crystals with it. I cannot imagine how it would have gonewith someonedifferent. From our days in the ICMS lab, to working in the lab at Maastricht University, and thecountless DynAM meetings. The initial worry about potential overlap quickly faded as we eachfound our own aspects of liquid crystals to focus on, while at the same time, both our research lineswere close enough to discuss problems and new findings with each other. Our only actual part ofcrossover, the review, worked out very well. While it was a product of many days (the whole thingtook a year, I believe), these days were always enjoyable and a nice distraction in the working-from-home culture during the pandemic. At work, I was inspired by your creativity and drive—youmeticulously assembed a liquid crystal mix with nine mesogens for the 2PP-DLWmachine. Out-side work, your way of life, striving for uncompromised comfort and relentless joie-de-vivre makeyou amazing to have around. Your wedding and PhD graduation parties were massive—in partthanks to Wart! I hope you never lose your ways. Jeroen, we weren’t actually colleagues for all thatlong—your defence day was my third day as “a PhD”, and one of your final acts as PhD candidatewas to hand over your IT duties to me. I admire your entrepreneurial spirit and the fact that you

7

are trying to bring one of my favourite projects from SFD, the luminescent solar concentrator, tomarket. Throughout my project you have been keeping up with its progress, in spite of your busyschedule. I really appreciated this, and the experience youhad fromalready completing a PhDmadefor a great example. The many dinners and sleep-overs in Hedel were always enjoyable and I wasalways impressed with your great pleasure in serving coffee—first the automatic machine, later onespressos. What I will not forget are the ballroom dancing classes that Maartje and I joined. Youdrove us there and even safely delivered us back home. I think the experience could not have beenmuch more VIP.

Asmentioned in the prologue, I had the opportunity to also take care of IT-adjacentmatters atSFD. This put me in close contact with the folks of TU/e’s InformationManagement and Services(IMS), especially Frank andMart. Frank, we quickly formed a very good working relationship andit was always nice to catch up with you. Even though you perpetually drown in to-do-tasks, youmanaged to free up time whenever I needed help or advice. While I am not a certified IT profes-sional, I felt taken seriously when we were looking for solutions to computer problems together.Mart, I appreciate the effort you put in whenever I was looking for an IT solution to an SFD issue.

De omstandigheden voor goed onderzoek liggen deels ook buiten de universiteit. Dennis, Bas,en Kevin, ik ken jullie nu bijna mijn halve leven. Jullie vragen over mijn voortgang als promoven-dus en perspectief op mijn werk als niet-materiaalkundigen was altijd lekker relativerend. Elk potjeSuper Smash Bros., en de vele gesprekken en discussies—geopolitiek, taalkunde, memes, muziek, ende staat van de mensheid—waren een nodige afleiding. Ik hoop dat we dit nog vele jaren kunnenblijven doen. Adam, jij bent en blijft voor altijd mijn voetbalorakel. Kaj, wij kunnen altijd heerlijknostalgisch terugkijken op onze tijd in Zweden en als afstudeerstudenten. Gelukkig ben ik niet deenige op wie het veel indruk wekte.

Aan mijn ouders, Pedro en Mariëlle, broertje Adriaan, zusje Annika (en vriend Pim): ik hadnooit verwacht dat we allemaal in de buurt van scheikunde zouden eindigen. De academischeaard van mijn werk was soms lastig voor jullie maar ik desondanks heb ik genoten van jullie pogin-gen om mijn ontdekkingen te begrijpen. De trots waarmee jullie artikelen en posters ontvingenwas enorm motiverend, en liet me realiseren wat voor een leuke werkplek de universiteit is. Langgeleden hebben we ooit een TU/e open dag bezocht—het enige dat ik me nog kan herinneren isfluorescerend kunststof. Pas later zou ik leren dat dit Michael’s werk was. Misschien is hier hetzaadje voor mijn promotie bij SFD geplant! Tevens zei ik vroeger wel eens dat ik scheikundeler-aar wilde worden, waarop als antwoord kwam: “hoogleraar, dat moet je worden”. Geen last vanvervelende studenten, bijvoorbeeld. Dan heb ik nog wel even te gaan! Waardering gaat ook naarmijn “schoonfamilie”: Rinie, Tanja, en Jeroen. Jullie vroegen ook regelmatig naar de voortgangvan mijn werk en probeerden te bedenken hoe ik mijn onderzoek kon verbeteren en de materialenkon toepassen. Helaas heb ik niet de tijd gehad om alles te proberen!

De laatste woorden zijn voor Maartje. Ik ben blij dat mijn onderzoek redelijk soepel verliep;de meeste zorgen kwamen door de treinreis naar Milaan, welke vervolgens ook niet goed liep. Hieren daar heb ik buiten werktijd wat geschreven of vakliteratuur gelezen. Dit beviel je niet altijd,maar je begreep mijn passie voor het werk dus liet me meestal wel mijn gang gaan. Dit waardeer ikzeer. Tevens heb ik je vier jaar lang voorbereid op een “proefschriftcrunch” aan het eind, maar ditis gelukkig niet nodig geweest! We zijn al meer dan negen jaar samen nu, en ik hoop dat we hier nogvele jaren aan toe kunnen voegen.

JeroenMarch, 2022

8

Academic summary

Cholesterics Liquid Crystals in Additive Manufacturing

Since time immemorial, colour and colour changes have been an indispensable tool onearth. In the natural world, it serves as messenger, signaller, or repellent. Later, as art andother forms of visual expression developed, aesthetic aspects gained prominence.

Clearly, colourmatters. Of similar significance, is howmatter becomes coloured. Con-ventionally, absorptive pigments anddyes are used to tintmaterials. In contrast, the naturalworld presents many examples in which it is the structure of matter at the nanoscale thatleads to a strongly coloured appearance, rather than light absorption. “Structural colour”,as this is known, is best represented as the bright, lustrous sheen in scallops, butterflywings,or peacock feathers. Adaptive colouration through nanostructured motifs is particularlyinspiring for materials scientists. It allows for a relatively accessible path to coupling vi-sual appearance to a stimulus-response in thematerial—whether thermal, chemical, photo-induced, or humidity-driven. In recent years,manyof suchphotonic, polymeric responsivematerials have been described and demonstrated.

At the same time, the wider world of polymer manufacturing is undergoing a signifi-cant change. It is expected that in the future, additive manufacturing techniques will serveas production techniques in their own right, for instance for small product runs, for repro-ducing replacement parts no longer available, and for personalised devices used in health-care and surgery. In a future where devices are fully 3Dprinted tomeet the user’s demands,wishes, and needs, it is imperative to have a large library of materials servingmany differentfunctions. Still in the infant stage are printable, responsive structurally coloured polymers.These materials are striking aesthetically, but can also function autonomously and powersource-free as sensors, and as such could find use in many “smart” devices.

Liquid crystals (LCs) are uniquely suited to develop responsive materials. Additionof a chiral compound can organise the formation of a cholesteric mesophase with helicalmolecular ordering. When the characteristic length scale of this ordering is close to visi-ble light wavelengths—hundreds of nanometres—the LC material reflects coloured lightof a single circular polarisation. A bonus is the adaptable chemistry of LCs, which easesincorporation of a stimulus-response. Modifying the LC molecules with reactive groupsthat form a chemically crosslinked network unlocks generation of LC networks (LCNs)or at lower crosslink density, LC elastomers (LCEs). Bringing cholesteric LCs to additivemanufacturing requires appropriating what has made LCNs and LCEs work in “smart”

9

materials, and then translating, adapting, and remixing to meet the requirements of newmanufacturing techniques.

Chapter 2 explores how currentmethods producing responsive cholesterics could finduse in conventional 3D printing using a thermoplastic poly(lactic acid) filament as hostpolymer. It immediately highlights that simply applying known LC formulations to addi-tive manufacturing techniques is problematic. Using sonication, the cholesteric polymernetwork is fractured into submillimetre flakes. After assuming this new shape, response ismaintained, and successful compounding into a 3D printable filament is shown. Unfor-tunately, since the optical response of the LCN is dependent on water migrating into thephotonic polymer, its function is impaired by the impermeable host material.

A method to overcome this impairment is shown in the subsequent Chapters 3 and 4,where a novel LC ink is synthesised for use in direct ink writing (DIW). The functionalityof this ink is first tested using bar coating, which shows that the formulation forms stable,photonic coatings with bright colours. Compared to similar chiral nematic LCNs andLCEs, the helical molecular planes are tilted with respect to the coating substrate, leadingto decreased circular polarisation selectivity from normal incidence. Printing a negativephotomask on an overhead slide demonstrates patterns can be imprinted on the coatingthat retain their unconventional optical properties. It is expected the opening of avenuestowards security features on one hand, and decorative elements on the other will resultfrom this novel ink.

Given the positive bar coating results, the ink was loaded into a syringe for DIW.Some optimisation during the translation from coating to writing was still required—temperatures, printing speeds, layer thicknesses—all of these are shown to be of significantinfluence on the eventual optical characteristics achieved after printing. Programmabilityof the print path with DIW is a significant advantage of this technique, allowing for con-trolling of the direction in which the cholesteric slants, according to computer-designedpaths. Amidst the possibilities demonstrated by this technique with this material, it couldalmost be forgotten that these results also are the first example of DIW 3D printing ofa cholesteric LCE into a soft, structurally coloured material. The optical response of acholesteric LC to water is, especially with future applications in sensing or healthcare inmind, a useful stimulus-response to have in your materials library. The synthesis of an LCink that is both water-responsive and can be processed using DIW is demonstrated in thefinal experimental Chapter 5. The synthetic pathway employed for this ink first requiredsynthesis of a suitable chiral dopant molecule, and with this, turquoise-coloured prints areshown. After activating the printed objects with an acid, water response is seen as a changeof the structural colouration as well as a deformation of the object.

With these results in hand, we believe this thesis sketches a direction for the future ad-ditivemanufacturing of photonic, structurally coloured polymericmaterials with choleste-rics. Chapter 6 discusses the future potential of the additivemanufacturing of cholesterics,highlighting what changes to the ink as well as the printing process can result in.

Chapter 1Introduction and Theoretical Background

This chapter is reproduced from: M. del Pozo+, J. A. H. P. Sol+, A. P. H. J. Schenning, M. G. Debije, Advanced Materials 34, 2104390 (2022).

Recent years have seen major advances in both additive manufacturing and responsive materials. What has so far been “4D printed” using liquid crystals? The 3D printing of responsive, structurally coloured materials can be used to obtain functional optical devices for use in healthcare, energy generation, sensing, and soft robotics. Liquid crystals, a prime contender for the development of responsive optics, only recently have entered the world of additive manufacturing. In this introductory Chapter, a brief primer on additive manufacturing and liquid crystals will be given.

12 | Cholesteric Liquid Crystals in Additive Manufacturing

1.1 Optically responsive structures: what inspires us

Advances in materials science are often inspired by designs found in Nature. Throughoutthe living world, tissues can be encountered that show bright, reflective colour—in somespecies, these vibrant appearances are even able to autonomously change in response toenvironmental changes: the blue wings of theMorpho menelaus change appearance whensoaked in ethanol (see Figure 1.1a) (1, 2), while the relatedMorpho sulkowskyi shows differ-ent colour changes depending on the solvent it is brought into contact with (3). Humidity-responsive effects have been observed in beetles: the longhornbeetle speciesTmesisternus is-abellae has colourful elytra that change in response to humidity (see Figure 1.1b) (4), whiletheHoplia coerulea’s blue colouration shifts to an emerald green with increasing moisturecontent (see Figure 1.1c) (5), and the hercules beetleDynastes hercules actually loses visiblecolour when its shield swells with water (6). In the iridescent feathers of tree swallows(Tachycineta bicolor), a limited change in reflection colour has also been observedwhen theplumage is brought into contact with water.

a)

dry ethanol soaked

b)

dry humid

c)

increasing humidity level in elytra

Figure 1.1: a) Morpho menelaus showing optical response when its wings are exposedto ethanol. Adapted with permission from ref. (2), copyright 2009 American Chemi-cal Society. b) The elytra of Tmesisternus isabellae change colour from an iridescentgreen to dark orange when water infiltrates the elytra. Adapted with permission fromref. (4), copyright 2009 The Optical Society. c) Hoplia coerulea also changes colourupon humidity exposure, from a violet blue to bright green. Adapted with permissionfrom ref. (5), copyright 2009 American Physical Society.

These colour changes share a significant concept inmaterials design: colouration is notonly achieved through dye molecules and pigments, but can also result from the periodicordering of materials (7–12). This teaches us that there is more to materials than just theconstituent molecules—it’s also about how these molecules are spatially positioned in the

Introduction and Theoretical Background | 13

material bulk. When there is a periodic superstructure in the material with characteristiclength scales close to that of visible lightwavelengths, thematerial can be observed to reflectcolour. This optical effect is known as “structural colour”, as it originates in the nanoscalestructuring of the material, either on the surface or in the bulk.

Similar to the natural examples,materials scientists can combine structural colourwitha material’s inherent stimulus response to make environmentally-sensitive devices that re-spondwith colour changes. The ingenuity comes from rationally designing both the chem-istry and photonic structure; to create materials that interact with light and respond tostimuli or analytes, in such a way not seen before in the natural world.

Popularmaterials for emulating these natural, photonic effects are based on liquid crys-tals (LCs)—organic molecules, that because of their shape and molecular characteristicsself-assemble into ordered liquid “mesophases”. Addition of a chiral dopant—a chiralspecies miscible with the LC—leads to the formation of a chiral nematic (“cholesteric”)mesophase in which a helical superstructure reflects light when its periodicity is in the hun-dreds of nanometres. Over the past decades, cholesterics have led to the development of alarge variety of stimuli-responsive optical materials.

At the same time, the processing of materials is at a turning point. Additive manufac-turing (AM) techniques are finding use in a growing array of sectors—nutrition (13, 14),healthcare (15), aerospace (16), and construction (17), among others. The appeal of AMlies in the ever-expanding number of materials that can be processed, and the freedom thatthe techniquesprovide in the free-formdesignof objects. Colloquially knownas “3Dprint-ing”, when stimuli-responsivematerials are processed, it canbe called “4Dprinting”, wherethe additional dimension is derived from the stimulus-induced change after an unspecifiedamount of time.

4D printing of structurally colouredmaterials represents the combination of two chal-lenges: additively manufacturing an object that has internal or external structures suitablefor colouration, and having this structure be responsive. This thesis describes the first stepsin this journey towards the manufacturing of visually responsive materials and devices—the use of cholesteric liquid crystals (ChLCs) as feed material for 3D printing techniques.When combined with additive manufacturing, we are making use of the LC’s tendency to-wards self-organisation with the design freedom of 3D printing techniques to make struc-turally coloured objects.

ThisChapter 1describes thebasic concepts surrounding additivemanufacturing, (cho-lesteric) liquid crystals, how these technologies can be combined, and what the combina-tion of these two has already resulted in. This introductoryChapter concludes with a briefoverview of the following experimental Chapters.

1.2 Additive manufacturing: bottom-up building

One canmake the case that “additivemanufacturing”—the umbrella term for 3Dprintingtechniques (18)—is derived from natural principles. Think of spiders building webs from


protein fibres spun from their silk glands (19), species of birds constructing nests fromdifferent objects and excretions (20), or humans building houses by additively combiningsmaller building blocks (21).

AM has a couple of significant advantages over other manufacturing options. Com-pared to milling or chiselling, additive manufacturing techniques build objects by addingmaterial rather than subtracting it, allowing for possibly large material savings. While dieextrusion or injection moulding also conserve material, these processes are only econom-ically viable at larger production volumes where many objects of identical design are re-quired. 3D printing can fill the niche where specialised objects are needed only in smallerquantities, down to single objects (22). It is for this reason AM is often used for “rapidprototyping”—its original moniker—and has found its place in a variety of industries, in-cluding aerospace, automotive, and in specific the dental industry, where AM processesare gaining traction since it eases personalisation of implants.

Two of the most important additive manufacturing strategies for polymeric materialsare light-based vat polymerisation and microextrusion. Both originate from proceduresdeveloped at the end of the 20th century (23, 24). “Stereolithography” (SLA), the origi-nal vat polymerisation technique, works by irradiating and selectively polymerising a thinlayer of liquid resin using a scanning light source. Since the “printed” object is submergedin a bath of its own monomer, SLA resin formulations are typically densely crosslinkablemonomer mixes to ensure good spatial resolution—both diffusion of generated polymerchains away from the reaction site andmonomer swelling of the newly-formednetwork im-pair resolution. “Digital light processing” (DLP) is similar to SLA, but utilises a miniatureliquid crystal display (LCD) or digital micromirror device (DMD) to irradiate entire layersat once. For examples of what can be achieved with vat polymerisation and 4D printing,the reader is referred to literature reviews on these topics (18, 25).

When considering the nature of the material feedstock, microextrusion techniquesmight seem diametrically opposed to vat polymerisation. Rather than the low-viscosityliquid resins used to fill the bath, extrusion relies on viscoelasticity. After deposition, set-ting is required to maintain the programmed shape, yet the material should freely flowthrough a micronozzle under pressure. Such behaviour can be achieved with different ma-terials. First, before we consider how the extrusion-based printing processes work, it is use-ful to take note of the procedure that takes place before the 3D printer can be used. In 3Dcomputer-aided design (CAD) software, the model-to-be-printed is created and exportedas a “stereolithography” file that contains the three-dimensional shape. Specialised “slicing”software is then used to interpret the 3D model and translate it into “g-code” for the 3Dprinter. This requires programming print headmovement according to user-defined printsettings, following the boundaries of the 3D model. Most significant settings to choosefor common 3D printing methods are the extrusion and bed temperatures (depending onprintingmaterial), the infill patterns that dictate inwhat fashion theobject’s volume is filled(parallel lines, perpendicular lines, hexagonal, honeycomb, spirals, or even others), and the


inkheating mantle

printed objectmicronozzle

a)

b)

nozzle diameterlayer thickness

lateral nozzle speed

c) d)

Figure 1.2: a) Overview of the central element in direct ink writing (DIW): the sy-ringe with micronozzle. Indicated are the locations of the ink, the heating mantle (fortemperature-controlled DIW), the micronozzle and the printed object. The panel tothe right shows a schematic close-up view of the nozzle and some important processparameters in DIW: the nozzle diameter, the thickness of the deposited layer, and themovement speed of the nozzle over the substrate. b) DIW machine in the laboratorysurrounded by the control equipment. c,d) Close-up views of a liquid crystalline inkbeing deposited from the micronozzle.

layer height, which determines the correspondence between the physical 3D object and thedigital 3D object (25).

In “fused filament fabrication” (FFF) spools of solid polymer thread are fed into theprinting nozzle, heated beyond the melting temperature, Tm, and extruded through anorifice typically less than 1 mm in diameter (18). Spatial coordinates dictated by the print-ing code determine the geometry of the final object. Having the feedstock in a solid statewhen not being printedmakes this technique relatively hassle-free; however, it does signifi-cantly limit the technique to thermoplasticmaterials. In FFF, conventional feedstocks usedare thermoplastics including poly(lactic acid) (PLA) poly(acrylonitrile styrene butadiene)(ABS), and copolyesters (CPE). These materials are relatively easy to process, but do notnecessarily provide outstanding mechanical or thermal properties. Electrical conductivityhas beendemonstrated after additionof carbonnanotubes (26), although this is not exactlya time-dependent characteristic, and hard to qualify as “4D”. Closer to 4D was the addi-tion of thermochromic pigment capsules to a static polymer matrix allowing for printingcolour-changing materials using FFF (27).

“Direct ink writing” (DIW) is a different microextrusion technique. By printing froma syringe rather than a filament, the printing ink is also in a viscoelastic state during stor-


age (18, 28). Since there are many material classes that can be found in the form of vis-coelastic liquids, DIW is very versatile (13, 29–35). Mechanical or pneumatic force pushesthe material out through a nozzle. Movement of the nozzle, bed, or both, then leads todeposition of the ink freely in x, y, and z, although care should be taken of gravity effectson overhanging structures (see Figure 1.2a for a schematic overview of DIW).

DIW allows for a greater variety in materials deposited compared to FFF; as long asthe material can be loaded into a syringe from which it can be reliably extruded, and thenproperly sets on the print bed, it can be “direct inkwritten” (see Figure 1.2b,c,d for a typicalDIW setup). A wide range of materials (thermoplastics (18), thermoset resins (32), food-stuffs (13, 33)) and precursors (to ceramics (29), silica glass (30, 31), hydrogels (34), andartificial organs (35)) meeting these demands have been demonstrated with this technique.

1.3 Liquid crystals for responsive optics

1.3.1 Mesogens and mesophases

The term “liquid crystal” covers a large range of organicmaterials that at some temperature,and in some cases a specific concentration, display molecularly ordered liquid phases (seeFigure 1.3a,b). Such materials were first described in the 1860s after being extracted fromplant matter (36); the name “liquid crystal” was minted a few decades later (37); Sincethen, many different subclasses of liquid crystals have been observed, described, and de-fined (38). The LCs used in this thesis are of the “thermotropic” variety—these are LCsthat undergo several transitions upon temperature changes: crystalline to liquid crystalline,between different liquid crystalline phases, and from liquid crystalline to isotropic liquid(see Figure 1.3c). A further specification based on molecular shape leaves us with the rod-like “calamitic” molecules. Calamitics share two general design principles; (1) a stiff corethat can undergo π-π interactions, and (2) flexible aliphatic end groups.

Calamitic molecules can form a variety of liquid crystalline “mesophases”, this namederived fromAncientGreekmésos, meaning “in between”. LCmonomers are called “meso-gens”, deriving from the same etymological origin. Resulting from the distinct elongatedmolecular shape of calamitics are anisotropic material properties, such as differing dielec-tric permittivities and refractive indices along (εe, ne) and perpendicular (εo, no) to themolecular long axis (Figure 1.3d). When the mesogens are forced into large scale align-ment, having two refractive indices manifests itself as birefringence. At temperatures toohigh for liquid crystalline behaviour, the material undergoes a transition to the isotropicphase. This is paired with a loss of molecular order, and thus macroscopic expression ofthe anisotropicmolecular properties—adrop in viscosity and the disappearance of birefrin-gence. Typically, this transition is between “nematic” and isotropic phases, and happensat temperature TNI.

In this “nematic” mesophase (abbreviated as “N”), the molecules align along a com-mon director n as visualised in Figure 1.3e but are otherwise free to flow (typical nematic


nematic (N) smectic A (SmA) smectic C (SmC)

moleculardirector n

plane director

θ

n

εo no

εe ne

crystalline liquid crystalline isotropic liquid

TCrN or TCrSm TNI or TSmI

OO

O

OO

O O

O

O Oa) b) c)

e)d) f)

g)O

O

OO

O

O

O

OO

O

O

O

OO

O

O

O

O O

O

O

O

O

O

OO

O

O

O

O

O

OO

O

Reactive mesogen“RM82”

“RM257”

“LC 242”

Figure 1.3: a) A common reactive liquid crystal monomer (“RM82”) overlaid on theschematic representation that will be used in this thesis to signify calamitic LCs. b)Schematic representation of multiple mesogens with orientational order. c) Liquidcrystal phases typically occur between the crystalline solid and isotropic liquid phasesof matter. Transition temperatures are denoted by “T [ₚₕₐₛₑ] [ₚₕₐₛₑ]”. d) Mesogens areanisotropic, also in their dielectric characteristics. This is manifested as ordinary (εₒ,nₒ) and extraordinary (εₑ, nₑ) relative dielectric permittivities and refractive indices. e)Orientation of a single mesogen, offset from average molecular director n, by angle θ.f) A variety of mesophases occurring for thermotropic, calamitic LCs. Indicated are themolecular director n, and plane director in case of smectic phases. g) Three differentreactive liquid crystals commonly encountered in research works in this field. Notabledifferences are the spacer chain length and the carbonate groups present in “LC 242”.


=O

O

O

O

O

O

O

O

O

O

O

O

O

O

O

OO

O

O

O

O

O

O

O

OO

O

H

H

cholesteric (Ch), chiral nematic (N*)

helicaldirector

n

+

c) d)

a) b)uniaxial nematic

chiral dopant“LC 756”

half-

pitc

h p / 2

Figure 1.4: a) Addition of a chiral dopant to a nematic phase organises the formationof a cholesteric phase. Drawn to the right of the cholesteric phase is the moleculardirector n’s helical orientation pattern (shown here is p/2, half the length of a full rota-tion), and the associated “helical director”. b) Photograph of a cholesteric coating onglass. c) Schematic representation of a reactive mesogen. The reactive groups at theends are typically acrylates, methacrylates, vinyls, epoxides, or oxetanes. d) Chemicalstructure of a common reactive chiral dopant, “LC 756”, overlaid on its schematic rep-resentation in this thesis.

reactive mesogens are shown in Figure 1.3g). Similarly, there are “smectic” mesophases(abbreviated as “SmX” with X being a further subclassification), wherein besides molec-ular orientation, there is stratification through layer formation. Mesogens are not fixedin position—they are free to move within and among layers, but temporally and spatiallyaveraged, the molecules organise into distinct layers. In smectic A (“SmA”), n is orientedperpendicular to the layer direction; in smectic C (“SmC”) n is slightly inclined comparedto the layer normal (Figure 1.3f).

Of particular importance for this thesis is the cholesteric (Ch) mesophase, also knownas chiral nematic (N*), named after the cholesterol derivatives that first revealed the exis-tence of such phases (36, 37, 39). The current interest for cholesteric phases derives fromthe molecular organisation found therein: in a cholesteric, the nematic alignment directorstratifies into a 360° rotation. When the periodicity of this “rotation”—known as pitchlength p—is near the wavelength of incident light, reflection of light with the same hand-edness as the cholesteric helices is the result. Since the mesogens self-organise into thisphotonic structure, it allows for relatively easy material preparation and application (seeFigure 1.4a,b) (40, 41). The following equations govern typical cholesteric reflectors:

λmax = p⟨n⟩ cos (λ) (1.1)

Δλ = p (ne − no) (1.2)

⟨n⟩ = (ne + 2no) /3 (1.3)

p = 1/(HTPc) (1.4)


The wavelength reflected most strongly, λmax, is calculated by multiplying the chole-steric pitch length, p, average refractive index of the LC, ⟨n⟩, and cosine of the angle ofincidence, θ. For a “conventional” planar cholesteric, normal incidence (i.e. θ = 0°) revealsthe longest reflectedwavelength. The reflectionband stretches betweenne- andno-extrema,this determining bandwidth, Δλ. Pitch length p, which signifies the length over which nmakes a full 360° in-plane rotation, is determined by a “chiral dopant” and its weight frac-tion c. The helical twisting power, HTP (conventionally given in µm−1), is a value thatcharacterises the “twisting strength” for each specific chiral dopant-LC combination.

1.3.2 Reactive mesogens

A key aspect in the development of functional materials based on LCs is using “reactivemesogens” (RMs); mesogens that are equipped with polymerisable chemical groups (seeFigures 1.3g and 1.4c) (42). Acrylates, epoxides or oxetanes are such chemical moieties,which can be triggered with suitable (photo)initiators to form polymers, and if difunc-tional reactive mesogens are used, liquid crystal networks (the LCNs). In these tightlycrosslinked networks, the mesophase is fixed and temperature-dependent phase behaviouris (mostly) lost. This widens the applicability of the LC’s anisotropic properties—as theseare now largely decoupled from temperature, and the material no longer flows.

Originally developed as sheathes for fibre optics (43), these materials have since beenused formaterial and device concepts in the fields ofwater purification (44), energy harvest-ing (45), and soft robotics (46), just to name a few. And—as Chapter 1.3.3 will highlight,polymers based on LCs have also been used for environmentally-sensitive optics.

1.3.3 Responsive structural colour based on cholesterics

Similar to reactive mesogens, chiral dopants can also be functionalised with polymerisablechemical groups (e.g. “LC 756”, see 1.4d) (40), aiding incorporation into crosslinked LCs.This also means that the cholesteric mesophase can be fixed in the form of a stable, struc-turally coloured polymer material. When using a cholesteric as “smart” photonic material,changing molecular order influences the reflection characteristics (Figure 1.5a).

Responses to organic solvents are inherent to thematerial, butmay lead to a significantappearance changes nonetheless. This was shown for an LC elastomer (LCE)—an LCNwith a lower degree of crosslinking—where the reflection band would swell to infraredwavelengths, rendering it transparent to a human observer (Figure 1.5b) (47). Several sol-vents were applied, highlighting the LCE did not swell in strongly polar solvents such aswater or alcohols.

Polar solvent response has been demonstrated with LCNs that contain a large fractionof reversible, physical crosslinks based on hydrogen bonded carboxylic acid dimers (52).Such materials require an activation step that forms charges in the material’s interior, forinstance deprotonating the aforementioned carboxylic acid groups into carboxylates. The


b)a)

acetone

drying

trigger

c) d)

f)e)

dry state hydrated state dry state hydrated state

biaxial stretching room temperature 197 °C

Figure 1.5: a) Schematic drawing showing the swelling process in a ChLC. Upon us-ing a suitable trigger, the cholesteric pitch length p increases, leading to reflection oflonger wavelengths, visually observed as a red shifted structural colour. NB: the “he-lices” shown are for ease-of-interpretation, and do not represent the physical meso-gen alignment. b) Cholesteric LCE film showing a reversible optical response to ace-tone. Adapted under the terms of the Creative Commons CC BY-NC licence fromref. (47), copyright 2020 Wiley-VCH. c) A ChLCN film swelling with water, revealinga pre-programmed figure. Adapted under the terms of the Creative Commons CC BY-NC-ND licence from ref. (48), copyright 2018 American Chemical Society. d) A similarChLCN hydrogel, this time fabricated at the microscale using 2PP-DLW. Adapted un-der the terms of the Creative Commons CC BY-NC-ND licence from ref. (49), copyright2020 American Chemical Society. e) A soft cholesteric LCE that shows a blue shift re-sulting from pitch compression upon biaxial stretching. Adapted with permission fromref. (50), copyright 2001 Wiley-VCH. f) A ChLCE that undergoes optical and dimen-sional changes upon temperature changes. Adapted under the terms of the CreativeCommons CC BY-NC-ND licence from ref. (51), copyright 2021 Wiley-VCH.

ionic carboxylates readily attract moisture which leads to the LCN swelling with water.This swelling influences the cholesteric pitch length p, visually observed as a colour shift.Because of the carboxylic acid’s specificity for binding calcium(ii) cations, thin films basedon similar LCN formulations have been proposed for biomedical environments (Figure1.5c) (53). Applicability in food safety has been postulated based on the carboxylic acidinteracting with volatile amines from spoilt fish (52). The reversible degree of physicalcrosslinking these LCmixtures allow has been exploited with two-photon polymerisation


direct laser writing (2PP-DLW), a vat polymerisation technique, to fabricate micrometre-sized, humidity-responsive, reflective photonics (Figure 1.5d) (49). Water-responsehas alsobeen achieved, not by generating charges on the LCN itself, but by impregnating it withanother polymer that serves as hygroscopic agent (54, 55).

Dimensional changes in cholesteric films also influence the pitch length—as the ma-terial is strained along directions perpendicular to the helix director, the pitch contractsand light reflection is blue shifted (48). This effect is most pronounced in “elastomeric”ChLCs, which feature low degrees of crosslinking and are amenable to large dimensionalchanges. The large spectral changes upon deformation have been used to design opticalstrain sensor prototypes (Figure 1.5e) (55–57).

Using a “double polymerisation” strategy, reversible stimulus-responsive dimensionaland colour shifts can be achieved by adding a dithiol and tri- or tetrafunctional thiol to theLCN formulation. First, reactive groups from thiol species react with acrylate groups onthe mesogens. Since there is thiol with functionality higher than 2, a network is formed.This first network allows for deforming the material elastically and fixing a deformed stateby crosslinking excess acrylate groups. Changes in temperature then allow for switching be-tween the shape of the first and second networks (Figure 1.5f) (51). Alternatively, dynamiccovalent chemistry can be used to selectively lock in changes in the reflection characteristicsby straining and then reconfiguring the network topology in situ (58).

1.4 Using liquid crystals in direct ink writing 3D printing

1.4.1 Synthesising a liquid crystal elastomer precursor ink

The examples presented so far demonstrate how LCs can be used to design responsive op-tics. However, most of these devices were made as thin films, or using LC formulationsthat are not suitable for centimetre-scale AM techniques based onmicroextrusion. To thisend, the LCmust bemodified, changing its viscoelastic properties into something that canbe extruded from a nozzle, but retains printed patterns after deposition.

The reactive acrylate groups found in many reactive mesogens provide a good entrypoint for the rheological modification about to be performed. Making use of amine-acrylate “aza-Michael” or thiol-acrylate “thiol-Michael” chemistry allows for chemical con-catenation of the RMs using “chain extenders” such as primary aliphatic amines andaliphatic dithiols (see Figure 1.6) (59, 60). This oligomerisation reaction acts as an addi-tion polymerisation, where the average chain length of the newly formed oligomers growsas the reaction proceeds. The final average length of the oligomer mix (xLC) is primarilydependent on the stoichiometry (25):

ndiacrylate

nextender=

xLCxLC − 1

(1.5)

The resulting elastomer precursor ink is a mix of different LC oligomers. A photo-


activated free radical initiator is added either before or after the chain extension reaction,to crosslink the oligomers into a chemically bound network after processing of the ink.By tuning the reactants employed during oligomer synthesis, it is possible to adjust thenematic-isotropic transition temperature TNI from around the boiling point of water tobelow physiological temperatures (37 °C). For more detailed information, the reader is re-ferred to the literature (25, 61).

Asmentioned in Chapter 1.3.1, chiral dopants lead to the formation of chiral nematicphases; the characteristic pitch length of these nematic phases being dependent on theweight fraction of the chiral species. A further observation that must be considered whensynthesising a cholesteric LCE ink, is that the HTP of a chiral dopant decreases with in-creasing oligomer chain length (47). When taking benchmark chiral dopant “LC 756” asexample, it is found that in low-molecular weight LC mixes a few weight percent givesvisible wavelength reflection (40), while in a chain extended material the required concen-tration is increased. There does not seem to be an obvious trend however; results show that

Typical chain extension agents

for thiol-acrylate: 2,2′-(ethylenedioxy)diethanethiol (EDDET)

H2Nfor amine-acrylate: n-butylamine (nBA)

OO SH

HS1.0 1.2 1.4 1.6 1.8 2.0

235

203050

1

10

100

num

ber o

flin

ked

mes

ogen

s

molar ratio diacrylate to chainextender (mol mol-1)

reactive mesogen chain extension agent catalyst (if applicable)

monomer reaction mixture oligomer mixture

a)

b) c)

d)

LC oligomer mixture crosslinked LC elastomer

triggering of free radical

polymerisationinitiator ( )

Figure 1.6: a) Schematic representation of the oligomerisation reaction, showing thefunction of the reactive mesogen, chain extension agent, and catalyst. After the re-action, a mixture of different oligomer lengths is typically achieved. b) Typical chainextension agents are n-butylamine and 2,2′-(ethylenedioxy)diethanethiol. Both oper-ate as difunctional species in terms of their reactivity. c) Plot showing the relationshipbetween stoichiometric ratio between reactive mesogen and chain extension agent,and the expected average chain length after 100 % conversion. d) Schematic drawingof network formation (LCE) from the printed material (LC oligomer ink).


in a thiol-acrylate oligomer based onmesogen “LC242”, 5.66%w/w “LC756” is sufficientfor visible reflection (62), while in an oligomer made with “RM82” and n-butylamine, therequired concentration is about twice this amount (56, 63).

1.4.2 Direct ink writing of liquid crystal oligomer inks into smart devices

The feasibility of using non-cholesteric LC oligomer inks in a DIW procedure has beendemonstrated quite extensively. Most of these applications make use of an inherent resultfromtheDIWprocess: the shear and elongational forces present during inkdeposition leadto mesogen alignment along the print path direction (see Figure 1.7a,b). As increasing thetemperature decreases order, which in turn leads to a contractile strain α along n (64, 65),DIW has been used successfully for many “soft robotics” concepts where the LC ink isprinted into a device that contracts strongly upon temperature increase (66–70). Someexamples of such “artificial muscles” are presented in Figure 1.7c. Through combiningmultiple LCE muscles in a single device, a sustained rolling motion over a heated surfacewas shown (71).

A significant downside to temperature-sensitivity is that temperature as a trigger israther complicated to target precisely and quickly, since it typically requires heating ofthe larger environment surrounding the LCE actuator. A more precisely controllable trig-ger is light; molecular photoswitches such as azobenzene derivatives have been used to in-corporate a light-response in printed, aligned LCEs, leading to directed bending or con-traction under stress when irradiated with ultraviolet radiation (λ = 365 nm, see Figure1.7d) (72, 75). Alternatively, conventional dyes and pigments can also be used as pho-tothermal additives. Carbon black is such an example, which has been used in a prototypeurological implant (61).

Reconfigurability of direct ink written devices was shown by incorporating vinylgroups in the LCE (73). As the chemical reactivity between vinyl and acrylate towardsfree radical polymerisation is different (76), the vinyl groups are limitedly converted dur-ing the first acrylate crosslinking step. After heating the object to its “actuated state”,the remaining reactive groups can be triggered in a second photo-induced crosslinkingstep, fixing the actuated state in place (see Figure 1.8a). Another type of reconfigura-bility was demonstrated by incorporating hydrogen-bonding 2-ureido-4[1H]-pyrimidone(UPy) groups and furan-maleimide groups, for reversibleDiels-Alder chemistry (see Figure1.8b) (74). These concepts open the way to LCE actuators that can be applied in multipleuse cases through re-designing their shape or actuation pathway.

Recently, it was shown that hydrophilicity can be introduced in LCEs by using (ter-tiary) amines as chain extension agent (77), or alternatively by oxidising the crosslinkedelastomer with aqueous hydrogen peroxide (78). When swelling withwater, the alignmentof the liquid crystals together with the diffusion gradient of liquid within the film deter-mine the final deformation (44). Integration of electrical conductivity has been shown bymixing in a eutectic liquid metal into the LC oligomer ink (79), or by printing “core-shell”


light intensity 200 mW cm-2T < TNI T < TNI T > TNI

ON ONOFF OFF

a) b)

c) d)

Figure 1.7: a) Schematic depiction of mesogen alignment during DIW: in the syringeand nozzle, and after deposition. b) Example showing how mesogenic alignment canbe gauged using crossed polarisers. The crossed-arrowmarks indicate the orientationis these polarisers. NB: the “H” is printed in a top-to-bottom pattern, on top of a fulllayer of LCE, printed at 45° relative to this. Adapted with permission from ref. (67),copyright 2018 Wiley-VCH. c) Soft “artificial muscle” lifting a 20 gram weight againstgravity upon crossing the LCE’s nematic-to-isotropic transition. Adapted with permis-sion from ref. (67), copyright 2018 Wiley-VCH. d) A similar device, but through incor-poration of an acrylate-functionalised photo-responsive azobenzene derivative, sen-sitive to ultraviolet light. Reproduced with permission from ref. (72), copyright 2020American Chemical Society.

architectures where a core of liquid metal is encapsulated by a sheath of LCE (80, 81). Insuchmaterials, the electrical conductivity can be used for two purposes: for sensing, as theresistivity can change after deformation, butmore importantly, for driving the strong LCEcontraction through resistive heating.

1.4.3 Direct ink written liquid crystal elastomer optics

To date, there is little mention of DIW-printed LCEs as optical devices for light manip-ulation, with just two examples: a temperature-responsive lens (see Figure 1.8c), and atemperature-responsive variable transmission element (see Figure 1.8d). Both are basedon static optical materials, relying on the LCE to undergo a dimensional change with tem-perature and with that, influence the optical properties of the device.

What is lacking are examples of direct ink written, centimetre-scale printed objects


[i]

[iii]

[ii]

[iv]

a)

c)

b)

d)

Figure 1.8: a) Reconfigurable LCE actuator that actuates with increasing temperature,and can subsequently be fixed in this state with residual reactive groups present inthe LCE. Reproduced with permission from ref. (73), copyright 2020 Wiley-VCH. b)A different reconfiguration concept that makes use of an azobenzene derivative forlight-induced actuation, and dynamic physical (hydrogen bonding) as well as chemical(dynamic covalent chemistry) crosslinks for both thermal and light-induced networkbond shuffling. Adapted with permission from ref. (74), copyright 2021 Wiley-VCH. c)PDMS-LCE composite that acts as lens at higher temperatures; the LCE ring contracts,forcing the PDMS outwards, leading to light focussing. Reproduced under the terms ofthe Creative Commons CC BY-NC-ND licence from ref. (68), copyright 2018Wiley-VCH.d) A rotating element containing a polarisation filter that can be turned by heating theLCE fibres holding it in place. Reproduced under the terms of the Creative CommonsCC BY-NC-ND licence from ref. (68), copyright 2018 Wiley-VCH.

in which the LCE is the structurally coloured element. Looking beyond cholesterics, atcolloidal array hydrogels and block copolymers, reveals some examples of printed struc-turally colouredmaterials (82–84), but thedesired responsiveness is absent, ornot reported.Cholesterics—their function in “smart” materials proven—are a prime candidate for thedevelopment of stimuli-responsive, structurally coloured 3D printing inks.

1.5 Outline of this thesis: where do we go from here?

AM is rapidly being adopted throughout academia and industry, with an ever-increasingmaterials library at the disposal of the user (18, 85). Cholesteric liquid crystal-based ma-terials can be a valuable addition to this library. Additionally the soft, rubbery nature ofLCEs also allow use in applications where hard materials are undesired, such as when in-terfacing with livingmatter. Of great scientific interest at themoment, are 3D printed softrobotics (86). Adding responsive cholesterics to these could not only increase their aes-


thetic appeal, but could also be the basis of (self-)sensing capabilities, or as optical elementin photonic soft robots.

Chapter 2 demonstrates the possibilities, but also the challenges encountered, when tryingto translate known responsive LCN concepts to FFF 3D printing. A cholesteric LCN ismadeaccording to conventional procedures and fractured into micrometre-sized flakes that re-tain visible optical response to humidity. These particles are then compounded with PLAinto a printable filament. The resulting printed objects show brightly coloured flakes, butno sign of water-response.

Chapter 3 is a report describing the road towards making cholesterics printable by them-selves, so to avoid interfering polymer matrices. Combining a cholesteric LC formulationwith one-pot oligomerisation chemistry yields a vibrant, colourful ink. Bar coating is usedto make coatings with this material and reveals that the helical axis of the cholesteric canbe inclined to generate a slanted photonic structure, amolecular alignment that previouslyrequired intricate alignment conditions. The aesthetic prowess of this material is finally ex-hibited after a photolithography step that embeds two-dimensional designs in the coatings.

Chapter 4 chronicles the direct ink writing of the cholesteric ink into highly customisablepatterned structurally coloured objects. Upon closer inspection, these objects printed withdirect ink writing are not only the first DIW-printed cholesteric elastomers, but also theslanted alignment achievedwith bar coating is successfully translated to thisAMtechnique.Increasing the printing speed leads to this inclined helical structure, the direction of whichcan be freely programmed using the DIW machine. Printed objects can be released fromthe printing substrate to reveal rubbery, structurally coloured polymers.

Chapter 5 revisits the goal of the second Chapter—a printable, water-responsive photonicmaterial; by tuning the chemistry of the cholesteric ink, a water-responsive material suitablefor DIW is shown. Using a slightly different oligomerisation reaction compared to the pre-cedingChapters, a photonic LC ink is synthesised that acts as a hydrogel after an activationtreatment with aqueous acids. Swelling with water is directly transduced into a visual andmechanical response. This shows the road for soft, direct ink written, photonic actuators.

Chapter 6 sketches the road for future developments. The research results shown in thisthesis are discussed in light of the evolving field of “4D printing” and their applicability inthe wider world is discussed. The concluding Chapter closes with a look at how to ensurea life for stimuli-responsive LCs in the coming decades.


Bibliography

[1] C. Fenzl, T.Hirsch, O. S.Wolfbeis, Photonic Crystalsfor Chemical Sensing and Biosensing, AngewandteChemie International Edition 53, 3318–3335 (2014).

[2] O. Sato, S. Kubo, Z.-Z. Gu, Structural Color Filmswith Lotus Effects, Superhydrophilicity, and TunableStop-Bands, Accounts of Chemical Research 42, 1–10(2009).

[3] R. A. Potyrailo, H. Ghiradella, A. Vertiatchikh,K. Dovidenko, J. R. Cournoyer, E. Olson, Mor-pho butterfly wing scales demonstrate highly selec-tive vapour response, Nature Photonics 1, 123–128(2007).

[4] F. Liu, B. Q. Dong, X. H. Liu, Y. M. Zheng, J. Zi,Structural color change in longhorn beetles Tmesister-nus isabellae,Optics Express 17, 16183 (2009).

[5] M. Rassart, P. Simonis, A. Bay, O. Deparis, J. P. Vi-gneron, Scale coloration change following water ab-sorption in the beetle Hoplia coerulea (Coleoptera),Physical Review E 80, 031910 (2009).

[6] M. Rassart, J.-F. Colomer, T. Tabarrant, J. P. Vi-gneron, Diffractive hygrochromic effect in the cuticleof the hercules beetle Dynastes hercules,New Journalof Physics 10, 033014 (2008).

[7] P. Vukusic, J. R. Sambles, Photonic structures in biol-ogy,Nature 424, 852–855 (2003).

[8] M. A. Giraldo, S. Yoshioka, C. Liu, D. G. Stavenga,Coloration mechanisms and phylogeny of Morphobutterflies, The Journal of Experimental Biology 219,3936–3944 (2016).

[9] D. G. Stavenga, H. L. Leertouwer, B. D. Wilts, Mag-nificent magpie colours by feathers with layers of hol-low melanosomes, The Journal of Experimental Biol-ogy 221, jeb174656 (2018).

[10] N. J. Masters, M. Lopez-Garcia, R. Oulton, H. M.Whitney, Characterization of chloroplast iridescencein Selaginella erythropus, Journal of The Royal SocietyInterface 15, 20180559 (2018).

[11] B. J. Glover, H.M.Whitney, Structural colour and iri-descence in plants: the poorly studied relations of pig-ment colour, Annals of Botany 105, 505–511 (2010).

[12] H. K. Snyder, R. Maia, L. D’Alba, A. J. Shultz,K. M. C. Rowe, K. C. Rowe, M. D. Shawkey, Irides-cent colour production in hairs of blind golden moles(Chrysochloridae), Biology Letters 8, 393–396 (2012).

[13] J. I. Lipton, M. Cutler, F. Nigl, D. Cohen, H. Lipson,Additive manufacturing for the food industry, Trendsin Food Science & Technology 43, 114–123 (2015).

[14] L. Bravi, F.Murmura, AdditiveManufacturing in theFood Sector: A Literature Review, MacromolecularSymposia 395, 2000199 (2021).

[15] L. Moroni, J. A. Burdick, C. Highley, S. J. Lee, Y. Mo-rimoto, S. Takeuchi, J. J. Yoo, Biofabrication strategiesfor 3D in vitromodels and regenerative medicine,Na-ture ReviewsMaterials 3, 21–37 (2018).

[16] H. Klippstein, A. Diaz De Cerio Sanchez, H. Has-sanin, Y. Zweiri, L. Seneviratne, Fused DepositionModeling for Unmanned Aerial Vehicles (UAVs):A Review, Advanced Engineering Materials 20,1700552 (2018).

[17] F. Bos, R. Wolfs, Z. Ahmed, T. Salet, Additive manu-facturing of concrete in construction: potentials andchallenges of 3D concrete printing,Virtual and Phys-ical Prototyping 11, 209–225 (2016).

[18] S. C. Ligon, R. Liska, J. Stampfl, M. Gurr, R. Mül-haupt, Polymers for 3D Printing andCustomized Ad-ditiveManufacturing,Chemical Reviews 117, 10212–10290 (2017).

[19] A. M. T. Harmer, T. A. Blackledge, J. S. Madin, M. E.Herberstein,High-performance spiderwebs: integrat-ing biomechanics, ecology and behaviour, Journal ofThe Royal Society Interface 8, 457–471 (2011).

[20] L. E. Biddle, R. E. Broughton, A.M.Goodman, D. C.Deeming, Composition of Bird Nests is a Species-Specific Characteristic, Avian Biology Research 11,132–153 (2018).

[21] J. Fiala, M. Mikolas, J. Fiala Junior, K. Krejsova, His-tory and Evolution of Full Bricks of Other EuropeanCountries, IOP Conference Series: Materials Scienceand Engineering 603, 032097 (2019).

[22] I. J. Petrick, T. W. Simpson, 3D Printing DisruptsManufacturing: HowEconomies ofOneCreateNewRules of Competition, Research Technology Manage-ment 56, 12–16 (2013).

[23] C. W. Hull, Apparatus for production of three-dimensional objects by stereolithography, US Patent4575330A (1986).

[24] S. S. Crump, Apparatus and method for creat-ing three-dimensional objects, US Patent 5121329(1989).


[25] M. del Pozo, J. A. H. P. Sol, A. P. H. J. Schenning,M.G.Debije, 4D Printing of LiquidCrystals: What’sRight for Me?, Advanced Materials 34, 2104390(2022).

[26] K. Gnanasekaran, T. Heijmans, S. van Bennekom,H. Woldhuis, S. Wijnia, G. de With, H. Friedrich,3D printing of CNT- and graphene-based conductivepolymer nanocomposites by fused deposition model-ing, AppliedMaterials Today 9, 21–28 (2017).

[27] J. Wang, Z. Wang, Z. Song, L. Ren, Q. Liu, L. Ren,Biomimetic Shape–Color Double‐Responsive4D Printing, Advanced Materials Technologies 4,1900293 (2019).

[28] J. Cesarano III, R. Segalman, P. Calvert, Robocastingprovides moldless fabrication from slurry deposition,Ceramic Industry 148, 94–102 (1998).

[29] J. A. Lewis, Direct-write assembly of ceramics fromcolloidal inks,CurrentOpinion in Solid State andMa-terials Science 6, 245–250 (2002).

[30] D. T. Nguyen, C. Meyers, T. D. Yee, N. A.Dudukovic, J. F. Destino, C. Zhu, E. B. Duoss, T. F.Baumann, T. Suratwala, J. E. Smay, R. Dylla-Spears,3D-Printed Transparent Glass, Advanced Materials29, 1701181 (2017).

[31] R. Dylla-Spears, T. D. Yee, K. Sasan, D. T. Nguyen,N. A. Dudukovic, J. M. Ortega, M. A. Johnson, O. D.Herrera, F. J. Ryerson, L. L. Wong, 3D printed gra-dient index glass optics, Science Advances 6, eabc7429(2020).

[32] M. A. Skylar-Scott, J. Mueller, C. W. Visser, J. A.Lewis, Voxelated soft matter via multimaterial multi-nozzle 3D printing,Nature 575, 330–335 (2019).

[33] R. Karyappa, M. Hashimoto, Chocolate-based InkThree-dimensional Printing (Ci3DP), Scientific Re-ports 9, 14178 (2019).

[34] A. Sydney Gladman, E. A. Matsumoto, R. G. Nuzzo,L. Mahadevan, J. A. Lewis, Biomimetic 4D printing,NatureMaterials 15, 413–418 (2016).

[35] D. J. Wu, C. V. C. Bouten, P. Y. W. Dankers, Frommolecular design to 3D printed life-like materialswith unprecedented properties, Current Opinion inBiomedical Engineering 2, 43–48 (2017).

[36] J. Planer, Notiz über das Cholestearin, Justus LiebigsAnnalen der Chemie 118, 25–27 (1861).

[37] O. Lehmann, Über fliessende Krystalle,Zeitschrift fürPhysikalische Chemie 4, 462–472 (1889).

[38] G. Friedel, Les états mésomorphes de la matière, An-nales de Physique 9, 273–474 (1922).

[39] F. Reinitzer, Beiträge zur Kenntniss des Cholesterins,Monatshefte für Chemie 9, 421–441 (1888).

[40] C.Mock-Knoblauch, O. S. Enger, U. D. Schalkowsky,L.7: Novel Polymerisable Liquid Crystalline Acry-lates for the Manufacturing of Ultrathin OpticalFilms, SID Symposium Digest of Technical Papers 37,1673 (2006).

[41] D. J. Mulder, A. P. H. J. Schenning, C. W. M. Bas-tiaansen, Chiral-nematic liquid crystals as one dimen-sional photonicmaterials in optical sensors, Journal ofMaterials Chemistry C 2, 6695–6705 (2014).

[42] D. J. Broer, H. Finkelmann, K. Kondo, In‐situphotopolymerization of an oriented liquid‐crystallineacrylate,DieMakromolekulare Chemie 189, 185–194(1988).

[43] D. J. Broer, On theHistory of ReactiveMesogens: In-terview with Dirk J. Broer, Advanced Materials 32,1905144 (2020).

[44] S. J. D. Lugger, S. J. A. Houben, Y. Foelen,M. G. Debije, A. P. H. J. Schenning, D. J. Mulder,Hydrogen-Bonded Supramolecular Liquid CrystalPolymers: Smart Materials with Stimuli-Responsive,Self-Healing, andRecyclable Properties,Chemical Re-views p. acs.chemrev.1c00330 (2021).

[45] M. G. Debije, P. P. C. Verbunt, Thirty Years of Lu-minescent SolarConcentratorResearch: Solar Energyfor the Built Environment, Advanced Energy Materi-als 2, 12–35 (2012).

[46] M.Pilz daCunha,M.G.Debije, A. P.H. J. Schenning,Bioinspired light-driven soft robots based on liquidcrystal polymers, Chemical Society Reviews 49, 6568–6578 (2020).

[47] E. P. A. van Heeswijk, L. Yang, N. Grossiord, A. P.H. J. Schenning, Tunable Photonic Materials viaMonitoring Step‐Growth Polymerization Kinetics byStructuralColors,Advanced FunctionalMaterials 30,1906833 (2020).

[48] M. Moirangthem, A. P. H. J. Schenning, Full ColorCamouflage in a Printable Photonic Blue-ColoredPolymer, ACS Applied Materials & Interfaces 10,4168–4172 (2018).

[49] M. del Pozo, C. Delaney, C.W.M. Bastiaansen, D.Di-amond, A. P. H. J. Schenning, L. Florea, Direct LaserWriting of Four-Dimensional Structural Color Mi-croactuatorsUsing aPhotonicPhotoresist,ACSNano14, 9832–9839 (2020).


[50] H. Finkelmann, E. Nishikawa, G. G. Pereira,M.Warner, ANewOpto-Mechanical Effect in Solids,Physical Review Letters 87, 015501 (2001).

[51] P. Zhang, G. Zhou, L. T. de Haan, A. P. H. J. Schen-ning, 4D Chiral Photonic Actuators with SwitchableHyper‐Reflectivity, Advanced Functional Materials31, 2007887 (2021).

[52] J. E. Stumpel, C. Wouters, N. Herzer, J. Ziegler, D. J.Broer, C. W. M. Bastiaansen, A. P. H. J. Schenning,An Optical Sensor for Volatile Amines Based on anInkjet-Printed, Hydrogen-Bonded, Cholesteric Liq-uid Crystalline Film, Advanced Optical Materials 2,459–464 (2014).

[53] M. Moirangthem, R. Arts, M. Merkx, A. P. H. J.Schenning, An Optical Sensor Based on a PhotonicPolymer Film to Detect Calcium in Serum, AdvancedFunctionalMaterials 26, 1154–1160 (2016).

[54] J. E. Stumpel, E. R. Gil, A. B. Spoelstra, C.W.M. Bas-tiaansen, D. J. Broer, A. P. H. J. Schenning, Stimuli-Responsive Materials Based on Interpenetrating Poly-mer Liquid Crystal Hydrogels, Advanced FunctionalMaterials 25, 3314–3320 (2015).

[55] X. Shi, Z. Deng, P. Zhang, Y. Wang, G. Zhou,L. T. de Haan, Wearable Optical Sensing of Strainand Humidity: A Patterned Dual‐ResponsiveSemi‐Interpenetrating Network of a CholestericMain‐Chain Polymer and a Poly(ampholyte),Advanced FunctionalMaterials 31, 2104641 (2021).

[56] P. Zhang, X. Shi, A. P. H. J. Schenning, G. Zhou,L. T. de Haan, A Patterned Mechanochromic Pho-tonic Polymer for Reversible Image Reveal,AdvancedMaterials Interfaces 7, 1901878 (2020).

[57] R. Kizhakidathazhath, Y. Geng, V. S. R. Jam-pani, C. Charni, A. Sharma, J. P. F. Lagerwall,Facile Anisotropic Deswelling Method for Realiz-ing Large‐Area Cholesteric LiquidCrystal Elastomerswith Uniform Structural Color and Broad‐RangeMechanochromic Response, Advanced FunctionalMaterials 30, 1909537 (2020).

[58] A. M. Martinez, M. K. McBride, T. J. White,C. N. Bowman, Reconfigurable and Spatially Pro-grammable Chameleon Skin‐Like Material UtilizingLight Responsive Covalent Adaptable CholestericLiquid Crystal Elastomers, Advanced FunctionalMa-terials 30, 2003150 (2020).

[59] T. H. Ware, T. J. White, Programmed liquid crys-tal elastomers with tunable actuation strain, PolymerChemistry 6, 4835–4844 (2015).

[60] C. M. Yakacki, M. Saed, D. P. Nair, T. Gong, S. M.Reed, C. N. Bowman, Tailorable and programmableliquid-crystalline elastomers using a two-stage thiol–acrylate reaction, RSC Advances 5, 18997–19001(2015).

[61] C. P. Ambulo, S. Tasmim, S. Wang, M. K. Abdel-rahman, P. E. Zimmern, T. H. Ware, Processing ad-vances in liquid crystal elastomers provide a path tobiomedical applications, Journal of Applied Physics128, 140901 (2020).

[62] J. A. H. P. Sol, H. Sentjens, L. Yang, N. Grossiord,A. P. H. J. Schenning, M. G. Debije, AnisotropicIridescence and Polarization Patterns in a Direct InkWritten Chiral Photonic Polymer, Advanced Materi-als 33, 2103309 (2021).

[63] P. Zhang, A. J. J. Kragt, A. P. H. J. Schenning, L. T.de Haan, G. Zhou, An easily coatable temperature re-sponsive cholesteric liquid crystal oligomer for mak-ing structural colour patterns, Journal of MaterialsChemistry C 6, 7184–7187 (2018).

[64] D. J. Broer, G. N. Mol, Anisotropic thermal expan-sion of densely cross-linked oriented polymer net-works, Polymer Engineering and Science 31, 625–631(1991).

[65] D. J. Broer, G. N. Mol, G. Challa, In‐situ photopoly-merization of oriented liquid‐crystalline acrylates, 5..Influence of the alkylene spacer on the propertiesof the mesogenic monomers and the formation andproperties of oriented polymer networks,DieMakro-molekulare Chemie 192, 59–74 (1991).

[66] C. P.Ambulo, J. J. Burroughs, J.M. Boothby,H.Kim,M. R. Shankar, T. H. Ware, Four-dimensional Print-ing of Liquid Crystal Elastomers, ACS AppliedMate-rials & Interfaces 9, 37332–37339 (2017).

[67] A. Kotikian, R. L. Truby, J.W. Boley, T. J.White, J. A.Lewis, 3D Printing of Liquid Crystal Elastomeric Ac-tuators with Spatially ProgramedNematic Order,Ad-vancedMaterials 30, 1706164 (2018).

[68] M. López-Valdeolivas, D. Liu, D. J. Broer, C. Sánchez-Somolinos, 4D Printed Actuators with Soft-RoboticFunctions, Macromolecular Rapid Communications39, 1700710 (2018).

[69] D. J. Roach, X. Kuang, C. Yuan, K. Chen, H. J. Qi,Novel ink for ambient condition printing of liquidcrystal elastomers for 4D printing, Smart Materialsand Structures 27, 125011 (2018).

[70] D. J. Roach, C. Yuan, X. Kuang, V. C.-F. Li, P. Blake,M. L. Romero, I. Hammel, K. Yu, H. J. Qi, LongLiquidCrystal ElastomerFiberswithLargeReversible


Actuation Strains for Smart Textiles and ArtificialMuscles, ACS Applied Materials & Interfaces 11,19514–19521 (2019).

[71] A. Kotikian, C. McMahan, E. C. Davidson, J. M.Muhammad, R. D. Weeks, C. Daraio, J. A. Lewis,Untethered soft robotic matter with passive controlof shape morphing and propulsion, Science Robotics 4(2019).

[72] L. Ceamanos, Z. Kahveci, M. López-Valdeolivas,D. Liu, D. J. Broer, C. Sánchez-Somolinos, Four-Dimensional Printed Liquid Crystalline ElastomerActuators with Fast Photoinduced Mechanical Re-sponse toward Light-DrivenRobotic Functions,ACSApplied Materials & Interfaces 12, 44195–44204(2020).

[73] E. C. Davidson, A. Kotikian, S. Li, J. Aizenberg, J. A.Lewis, 3D Printable and Reconfigurable Liquid Crys-tal Elastomers with Light‐Induced ShapeMemory viaDynamic Bond Exchange, Advanced Materials 32,1905682 (2020).

[74] X. Lu, C. P. Ambulo, S.Wang, L. K. Rivera‐Tarazona,H. Kim, K. Searles, T. H. Ware, 4D‐Printing of Pho-toswitchable Actuators,Angewandte Chemie Interna-tional Edition 60, 5536–5543 (2021).

[75] M. del Pozo, L. Liu, M. Pilz da Cunha, D. J.Broer, A. P. H. J. Schenning, Direct Ink Writ-ing of a Light‐Responsive Underwater Liquid Crys-tal Actuator with Atypical Temperature‐DependentShape Changes, Advanced Functional Materials 30,2005560 (2020).

[76] T. A. Garrett, G. S. Park, Reactivity ratios for thecopolymerizationof vinyl acetatewithmethyl acrylate,Journal of Polymer Science Part A-1: Polymer Chem-istry 4, 2714–2717 (1966).

[77] K. Kim, Y. Guo, J. Bae, S. Choi, H. Y. Song, S. Park,K.Hyun, S.-K. Ahn, 4DPrinting ofHygroscopic Liq-uid Crystal Elastomer Actuators, Small 17, 2100910(2021).

[78] M. Javed, S. Tasmim, M. K. Abdelrahman, C. P. Am-bulo, T. H. Ware, Degradation-Induced Actuation

in Oxidation-Responsive Liquid Crystal Elastomers,Crystals 10, 420 (2020).

[79] C. P. Ambulo, M. J. Ford, K. Searles, C. Majidi,T. H. Ware, 4D-Printable Liquid Metal–Liquid Crys-tal Elastomer Composites, ACS AppliedMaterials &Interfaces 13, 12805–12813 (2021).

[80] A. Kotikian, J. M. Morales, A. Lu, J. Mueller,Z. S. Davidson, J. W. Boley, J. A. Lewis, Inner-vated, Self‐Sensing Liquid Crystal Elastomer Actua-tors with Closed Loop Control, Advanced Materials33, 2101814 (2021).

[81] W. Liao, Z. Yang, The Integration of Sensing and Ac-tuating based on a Simple Design Fiber Actuator to-wards Intelligent Soft Robots, Advanced MaterialsTechnologies 2101260, 2101260 (2021).

[82] L. Chen, Y. Dong, C.-Y. Tang, L. Zhong, W.-C. Law,G. C. P. Tsui, Y. Yang, X. Xie, Development ofDirect-Laser-Printable Light-Powered Nanocompos-ites, ACS Applied Materials & Interfaces 11, 19541–19553 (2019).

[83] B. B. Patel, D. J. Walsh, D. H. Kim, J. Kwok,B. Lee, D. Guironnet, Y. Diao, Tunable structuralcolor of bottlebrush block copolymers through direct-write 3D printing from solution, Science Advances 6,eaaz7202 (2020).

[84] B. M. Boyle, T. A. French, R. M. Pearson, B. G. Mc-Carthy, G. M. Miyake, Structural Color for Addi-tive Manufacturing: 3D-Printed Photonic Crystalsfrom Block Copolymers, ACS Nano 11, 3052–3058(2017).

[85] Y. Yang, X. Song, X. Li, Z. Chen, C. Zhou, Q. Zhou,Y. Chen, Recent Progress in Biomimetic AdditiveManufacturing Technology: FromMaterials to Func-tional Structures, Advanced Materials 30, 1706539(2018).

[86] T. J. Wallin, J. Pikul, R. F. Shepherd, 3D printing ofsoft robotic systems,NatureReviewsMaterials 3, 84–100 (2018).

Chapter 2Responsive Photonic Liquid Crystalline Flakes Produced by Ultrasonication

Could a conventional polymer 3D printing filament be made into an optical sensor by incorporating a responsive structurally coloured cholesteric additive? In this Chapter, we demonstrate the use of ultrasonication of responsive cholesteric liquid crystal network films to form structurally coloured flakes that demonstrate colour changes when brought into contact with water, suggesting a scalable technique to form quantities of responsive particles that could conceivably be embedded in permeable hosts to allow the optical detection of humidity or specific chemical species.

This chapter is reproduced from: J. A. H. P. Sol, L. M. Kessels, M. del Pozo, M. G. Debije, Advanced Photonics Research 2, 2000115 (2021).


2.1 Introduction

The perception of colour has played a critical role in human development, helping the earli-est peoples to judge the ripeness of food, vital for their safety (1, 2). For this reason, humanvision has evolved to be most sensitive in the range of 380 to 780 nm (3). Visible coloursresult from light absorbance, luminescence by pigments, or photonic systems that generatevivid colours via selective reflection of the electromagnetic spectrumas a result of nanoscalearrangement of their constituent materials (4). Using structural rather than pigmental orluminophore-generated colour has some key advantages. First is that structural colour isgenerally less susceptible to degradation. Secondly, the structured materials may also bemade dynamic: when the material is swelled or contracted by exposure to environmentalfactors, the result may be dramatic colour changes, which could be harnessed for use assensors (5).

As discussed in Chapter 1, liquid crystals (LCs) are positioned to be the basis of manyresponsive structurally coloured materials. Addition of a chiral dopant to a nematic LCleads to the formationof a light-reflecting chiral nematic phase, inwhich thedepth spannedby a 360° rotation of the directors of the LC planes is labelled the “pitch”, p. The productof p, average refractive index ⟨n⟩, and the angle of observation θ yields the wavelength ofmaximum reflection: λmax = p⟨n⟩ cos (θ).

By polymerizing the reactive ChLCmonomers into a ChLC network (ChLCN), onemakes a step towards materials applicable as photonic plastics. Almost always described inthin-film format, these materials have served as visual security features (6), and indicatorsfor temperature (7, 8), medical conditions (hypo- or hypercalcaemia) (9) or chemical ana-lytes, such as for volatile low molecular weight amines (10). Furthermore, owing to theirvibrant, angle-dependent colour, ChLCmaterials have beenused in the decorative coatingsindustry as effect pigments (11), and recently as organic solvent-responsive coatings (12).An alternative to a fully cholesteric coating is based on spherical cholesteric particles embed-ded in a non-LC binder, although high concentrations of the particles are required (13).

In this Chapter, we present a process that will allow broader application of a wide vari-ety of ChLC-based materials by generating small form factor responsive flakes that canbe used as additives for otherwise static hosts to expand their application towards sens-ing applications. LCN flakes have found use in display technology (14–18) as well as fordecorative elements (19, 20) before, and can be made with a variety of manufacturingmethods, including cryogenic (19) and mechanical (21, 22) fracturing, and “soft lithog-raphy” (14, 18, 23). Ultrasonication, a technique used in cellular biology and chemistryto release cell contents by rupturing cell membranes (24) and as method to de-aggregatenanoparticles (25), has been presented once, but not for responsive LCNs (26). Herein,we present ultrasonication as a viable method of reducing ChLCN film dimensions whilepreserving stimulus-responsive colour change capabilities of the particles, and with thor-ough characterisation of the material along all steps of fabrication and incorporation in abinder material.

Responsive Cholesteric Flakes for 3D Printing | 35

2.2 Results and discussion

2.2.1 Photonic film fabrication

The photonic ChLCN films were made by combining two reactive mesogens, 1 and 2,with chiral dopant 3 to induce a helical twist in the nematic structure of the liquid crystal(see Figure 2.1 for relevant molecular structures). Hydrogen-bonded dimers 4 and 5 wereadded to allow for both reversible reduction of the crosslink density in the film as well astuning its polarity, with the goal of sensitizing the reflected colour to H2O content. Non-reactive mesogen 6 is added as a sacrificial species: before and during polymerisation ithelps maintain liquid crystalline order, while after fashioning of the film, it is removedto leave behind a permeable interior in the ChLCN film. Photo-initiator 7 initiates thefree radical polymerisation that links up the acrylate groups, immobilizing the photonicChLC alignment. The resulting LC transitions to and from the isotropic phase around 68°C before polymerisation (see Figure 2.1).

Depending on the size of cholesteric film desired, up to 100 µL of monomer mixturewas placed on a glass slide heated to 80 °C. Shearing the LC mixture between poly(vinylalcohol)-coated (PVA) glass substrates gave rise to a vibrant photonic reflector. After induc-ing polymerisation with intense ultraviolet light and removing the material from the LCalignment cell, a strongly coloured film is obtained (ChLCN λmax ≈ 605 nm, Figure 2.2a).

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Figure 2.2: a) ChLCN coating after polymerisation (1), after porogen 6 extraction (2),after 0.1 M KOH treatment and drying to air (3), and after swelling in water (4). Thescale bar corresponds to 10 mm. b) FT-IR spectroscopy before (blue line), and afterphoto-induced crosslinking (violet line, N.B. 985, 1410 cm−¹), after extraction of poro-genic mesogen 6 (green line, N.B. 2225 cm−¹), and after treatment in 0.1 M KOH so-lution (yellow line, N.B. 1380, 1550, 1680 cm−¹). c) UV-Vis spectroscopy after eachprocessing step of the ChLCN material: the initial film (violet line), after the THF treat-ment (blue line), after 1.0 M KOH treatment which opens the pores, dried (turquoise),after swelling (green) and after 0.5 M aqueous HNO3 treatment to close the hydrogen-bonds in the ChLCN (yellow). d) Circular polarisation-controlled UV-vis transmittancemeasurements for the ChLCN after removal of 6, and after the base treatment in thedry and swollen (“wet”) states.


Acrylate conversion was confirmed by the disappearance of the Fourier-transform infrared(FT-IR) signature peak for the acrylate (ν̃ ≈ 985, 1410cm−1, Figure 2.2b) (27). Placing thecell filled with the solid, polymerised film in 50 °C water quickly dissolved the PVA sac-rificial layer, releasing the ChLCN. After soaking in tetrahydrofuran (THF), porogenic,non-reactive compound 6 was removed (Figure 2.2b, ν̃ ≈ 2225 cm−1), resulting in partialcollapse of the network and a subsequent blueshift of the reflection (to λmax ≈ 480 nm,Figure 2.2c). A treatment with an alkaline KOH solution (0.1–1 m in H2O) resulted ina strongly hygroscopic network that easily absorbs moisture from the surrounding air—for instance by breathing over the film surface or submerging in water. FT-IR confirmsscission of the hydrogen-bonded carboxylic acid dimers (ν̃ ≈ 1680 cm−1) as well as theformation of a polymer salt featuring negatively charged carboxylate groups ( CO –

2 , atν̃ ≈ 1380, 1550 cm−1) (53). The maximum of the selective reflection band shifted toλmax ≈ 500 nm after treatment.

Subsequent soaking in water of the lye-exposed film reveals a red colour (λmax ≈600 nm). From past work it is known that these colour shifts are directly related to thephysical swelling and de-swelling of the polymer network (6, 28). Additionally, FT-IR re-sults show no significant weakening of the ester bond peaks (see ν̃ ≈ 1730 cm−1 for C O,ν̃ ≈ 1200-1300 cm−1 for C O Cbonds). As described previously for this ChLC compo-sition (6), the polymer salt can be reverted back into a hydrogen-bonded material throughan acid treatment, using for instance aqueous HNO3; we demonstrate partial reversionafter a 90 min exposure (see Figure 2.2c).

Characteristic of cholesterics is their selectivity towards reflecting light of a single cir-cular polarisation, in the case of chiral dopant 3, right-circular polarised. This behaviouris maintained through the base treatment, in both dry and swollen states of the polymersalt (see Figure 2.2d). This is significant for possible future uses wherein circular polarisedreflection is used as information carrier. While the circular polarisation reflection is main-tained, narrowing of the reflection bands is seen, an effect resulting from the loss of bire-fringence during formation of the polymer salt (29).

2.2.2 Fracturing of ChLCN films into photonic flakes and their photonic re-sponse

For breaking up of the ChLCN films into flakes, the films were transferred to a vial filledwith demineralisedwater (ca. 5mL).A sonication probe tipwas placed in the liquid and ul-trasonication initiated. Within the first few seconds, the large centimetre-sized film breaksup into a myriad of small pieces.

The size of the flakes is quantified using digital image analysis. During and after ul-trasonication, samples were taken from the aqueous flake dispersion, dried on a glass slideand imaged using polarised optical microscopy. Between crossed polarisers, the brightlycoloured Grandjean texture provides good contrast with the dark background, whichgreatly eases the image analysis procedure. Micrographs for each sample were analysed to


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Figure 2.3: a) Optical microscopy images taken between crossed polarisers of the dryChLCN flakes observed after 15 min (top) and 100 min (bottom) ultrasonication. Thescale bar corresponds to 500 µm. b) Detail from the image in Figure 2.3a: exampleof how Feret’s diameters are calculated. The scale bar corresponds to 250 µm. c) TheChLCparticle Feret’s diameter versus ultrasonication time for untreated (red) andKOH-treated material (black). d) Photographs taken during the ultrasonication procedureshowing that at longer sonication times, themedium becomes increasingly cloudy. e,f)Scanning electron micrographs of the fractured ChLCN in the polymer salt form at dif-ferent length scales.


give a mean “Feret’s diameter”, as well as the deviation of this value within the sample.The Feret’s diameter that the analysis software ImageJ reports in this case is the diameterof the smallest circle that entirely circumscribes the imaged particle, and this is a measureof the length of a particle along its longest axis. For proposed applications as a dopant infused filament fabrication (FFF), we feel that this is a useful metric to estimate whether theChLCNflakesmight get stuck in the FFF nozzle orifice; for a conventional FFF 3Dprinteras utilised here is the nozzle diameter is 0.4 mm.

Samples collected throughout the ultrasonication procedure show that the medianFeret diameter decreases with sonication time up to a point (see Figure 2.3). The storagemodulus E ′ of hydrogen-bonded LC films drops significantly after base treatment (30)as the crosslinks attained from carboxylic acid dimers are weakened. We assume that thefracturing efficiency during ultrasonication is dependent on the mechanical properties ofthe polymer network, i.e. that with a lower modulus, more rapid fracture is seen. As ex-pected, fracturing proceeds more slowly and yields larger flakes for the untreated ChLCNfilms compared to the KOH treated films. After ca. 55 minutes of ultrasonication of 0.1m KOH treated polymer, a steady state of about 150 ± 75 µm is found. Using a microp-orous glass filter (10-16 µmpore size) the flakeswere removed from the aqueous dispersion,washed with demineralised water, and finally dried to obtain a polymer powder. Scanningelectron microscopy images indicate the periodic alignment pattern is maintained, also inthe swollen polymer salt (see Figure 2.3e,f).

A study of film thickness on ultrasonication efficiency revealed no significant correla-tion: for films made in 6, 10, or 20 µm gap cells, median Feret’s diameters of 189, 198, and176 µmwere found after 40 min of ultrasonication, respectively. Similarly, larger ChLCNfilms fabricated in 69 × 69mm2 cells were sonicated using an identical procedure and re-sulted in similar median Feret’s diameters of 160 µm. Processing multiple individual filmsin a single batch demonstrated at least 4 large films (≈200 mg) can be fractured simultane-ously without significantly affecting the final flake sizes. In terms of upscaling, this meansthat flake production rates can be increased more than an order of magnitude (from 10mg per 30 × 30mm2 film, to 200 mg for 4 films of 69 × 69mm2) without changing theprocedure or its parameters.

Interestingly, in some caseswe observed that flakes formed during ultrasonicationweregreen instead of red, the expected colour for the material in its fully hydrated state. UsingFT-IR spectroscopy, it was confirmed that this is a result of hydrogen bond reformation(ν̃ ≈ 1680 cm−1), a reaction that could be easily reversed by treating these flakes with 0.1m KOH solution. While sonication could also be done in an alkaline solution to preventprotonation of the carboxylic acid groups, chemical resistance of the sonication probe andits housing should be confirmed first.

The flakes, after filtration from the sonicationmedium and a washing step in deminer-alised water, show the same responsive behaviour as seen for the intact films (Figure 2.4a).In the dry polymer salt state, a green reflection is observed which red shifts within seconds


after wetting the flakes. Drying is done under ambient conditions but can be sped up byheating thematerial, in this case with a heating plate at 100 °C, which removed water fromthe cholesteric particles within seconds, reverting the colour to green.

In situ UV-vis spectrophotometry was used to record light reflection spectra of theflakes at different humidity and temperature conditions. To aid the recording of reflectancespectra, the opticalmicroscopewas set to brightfield reflectionmode, a spectrophotometerwas connected to the camera port and the cholesteric sample was placed between crossed

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Figure 2.4: a) ChLC flake powder in the dry state (top), after soaking in demineralisedwater (middle), and after drying on a 110 °C hot plate. The scale bar corresponds to10 mm. b) Left, UV-vis reflectance spectra of a single ChLC flake at a constant tem-perature (T = 19 °C) for increasing levels of relative humidity (from 40 to 80 RH%).Right, UV-vis reflectance spectra for a single flake at constant relative humidity (75RH%) at different temperatures (from 18 to 50 °C). c) Optical polarised microscopyphotographs of the flake analysed at constant relative humidity (75 RH%) for differenttemperatures (see Figure 2.4b) starting at 50 °C (leftmost) and ending at 18 °C (right-most). The scale bar corresponds to 20 µm.


linear polarisers. The reflected light that makes it to the spectrometer is in this way ridof light reflected through Fresnel reflection, and only the circularly polarised cholestericreflection is measured (28).

Figure 2.4b clearly demonstrates that at constant temperature, the wavelength of max-imal reflection is strongly dependent on the surrounding humidity, shifting from ca. 570nm at 40 RH% to 630 nm at 80 RH%. As shown in prior research, below 40 RH%, nosignificant shift in the reflection band occurs with increasing relative humidity (28, 31).The colour shift can also be achieved by maintaining a constant humidity level and lower-ing the temperature (see Figure 2.4c and Video S2). This shift relies on the condensationof water into the flakes. An atmosphere with 75 RH% at an air temperature of 23 °C hasa corresponding dew point of 18 °C (32, 33); this means that when the flakes are cooledfrom 50 °C, the evaporation rate of water from the flake progressively decreases, graduallyfilling the polymer withmoisture. Upon crossing the dew point (i.e. T ≤ 18 °C) the watercondensation rate surpasses the evaporation rate leading to water droplets forming ontothe flakes and causing maximum swelling and red shift as is seen clearly in Figure 2.4c (49).

Although not demonstrated here, the chemistry employed for these responsive flakescould allow for tuneable crosslinkingusing ions, calcium(ii) for example (6). DivalentCa2+

species bonds two carboxylic acid groups: combined with the high degree of order in thisliquid crystalline network, the concentration of Ca2+ significantly influences the degreeof water uptake by the material, and thus should also influence the maximum wavelengthshift between dry and wet states.

In all previously reported cases where ChLC flakes are employed as optically activecomponents for flexible reflective displays (14, 16, 18, 23, 34), windows (26), circularpolarisation-dependent security labels (8), or as effect pigments for decorative coatings (11),the cholesteric material is based on a densely crosslinked polymer network with a staticphotonic structure.

The material described in this Chapter resembles the flakes described in the aforemen-tioned reports, but with the added feature of optical responsivity to environmental waterafter breaking the supramolecular physical crosslinks formed through hydrogen bonding.During the colour change from green to red, the material takes up H2O, causing swellingalong the direction of the photonic cholesteric helix. This makes incorporation in conven-tional matrices difficult, as there is need for both permeability to moisture, as well as roomfor the photonic particles to swell.

Nevertheless, to demonstrate the processability of these responsive flakes, we com-pounded them with poly(lactic acid) (PLA) into a fused filament fabrication (FFF) 3Dprinting filament, at a 1 %w/w ratio ChLC flake to polymer matrix. Flakes were producedin 69 × 69 mm2 cells to generate larger amounts of photonic material. These were acti-vated, sonicated, dried, and added to about 50 g PLA granulate. Mixing was performedby hand, after which the granular blend was poured into a single-screw compounder set tooutput a 2.85 mm diameter filament (Figure 2.5a). As can be seen from the photograph,


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Figure 2.5: a) 2.85 mm PLA filament with incorporated photonic cholesteric flakes.b) Polarised optical microscopy photograph of the projected object in reflection modeshowing the incorporation of the flakes in the isotropic polymer host. The scale bar cor-responds to500µm. c) 3Dobject printedusingblackPLAas supportwith a single layerof photonic flake-doped, transparent PLA. d) Reflectivity of two 3D printed squareswith one (black line) and zero (orange line) 0.2 mm thick ChLCN flake-containing toplayers, and of a 3D-printed object with one 0.1 mm thick ChLCN flake-containing toplayer (blue line), showing the reflectivity of the samples.

the flakes are well dispersed in the filament. After using this filament to print 3D objects,the flakes can be observed through polarised optical microscopy as well as visually (Figure2.5b,c). UV-vis reflectance data shows amodest reflection peak around λ = 610 nm, whichis similar to the LCN flakes in the hydrated state (Figure 2.5d).

The processability limits of these flakes do pose some issues, as hinted at by thermo-gravimetric analysis (TGA, see Figure 2.6). The flake material is heated and maintainedat 200 °C for 20 minutes to simulate filament compounding and printing procedures. Atthis point, 11 % of the original mass is lost in the form of evaporated H2O. Otherwise, thematerial is stable, showingnoother ill-effects fromthe treatment, indicating it is suitable forincorporation in lowmelting point polymers, such as PLA. The composite was processedat 190 °C and 3D printed at 200 °C. However, the barrier properties of PLA, with limitedmoisture uptake (35), limit functionality of the responsive photonic dopant. Trials witha more hydrophilic polymer, polyamide-6 (PA6) (36), showed that these stability limitswere exceeded. The processing temperatures for PA6 are significantly higher, and no flakes


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could be visually identified in the resulting filament. This is likely due to the prolongedexposure to higher temperatures in the filament extruder (≈260-270 °C). At these temper-atures, terminal degradation of the cholesteric polymer flakes sets in.

To show responsivity may be maintained after incorporation of the photonic addi-tive in an appropriate polymer host, the flakes were dispersed in an aqueous solution ofpoly(vinyl alcohol) (PVA) at 5 % w/w ChLCN flakes to PVA. A coating was applied ontoa glass substrate via solvent casting, which, after drying, clearly shows the green reflectionfrom the driedChLCNflakes (Figure 2.7). Since the PVA is not immediately susceptible towater penetration at room temperature, response of the PVA-flake composite coating wastested at 80 °C. At this temperature, droppingwater on the coating leads to fast absorptionof the liquid into the ChLC flakes which swell and change colour (steps 1 to 2). Since thematerial is at a high temperature, water evaporates after a fewminutes, reverting the coatingto its initial state (steps 2 to 3).

2.3 Conclusions

Wehave reported amethod for the fabrication of stimulus-responsive photonic flakes fromliquid crystalline polymer networks. Using ultrasonication, a simple and accessible tech-nique, large polymer films are fractured into small pieces. It was shown that performinga base treatment on the flakes before ultrasonication gave smaller flakes on average; addi-tional time-controlled tests show that the size of the flakes plateaus after about an hour at


1 2

34

Rewritable Drying with paper towel

Patterning

using H2O

Drying to air

5 min

Figure 2.7: Rewritable coating demonstrated at 80 °C. Clockwise from top left: a dryPVA-coating is patterned using demineralised water (1 to 2), excess water is removedusing a paper towel, showing thatwater has penetrated into the coating and theChLCNflakes (2 to 3). After a couple of minutes, water evaporates from the sample (3 to 4),which returns it to its initial state (4 to 1). The scale bar corresponds to 20 mm.

around 150 µm, a size at which the flakes can be incorporated in other media while main-taining good optical visibility, as demonstrated for PLA-based 3Dprinting filament. How-ever, the flakes require sufficient space to swell with water, which favours certain polymerhost matrices over others. To demonstrate the function of these flakes, we have demon-strated a rewritable coating based on a poly(vinyl alcohol) binder doped with these pho-tonic particles that shows a reversible optical response to hot water. The optical responsein the cholesteric flakes is a large shift in the central wavelength of the selective reflectionband, from green (λmax = 570 nm) to red (λmax = 630 nm).

2.4 Experimental

Materials Reactive liquid crystal mesogens 1 (“RM82”, 2-methyl-1,4-phenylene bis(4-(3-(acryl-oyloxy)propoxy)benzoate)) and 2 (“RM105”, 4-methoxyphenyl 4-((6-(acryloyloxy)hexyl)oxy)-benzoate) were obtained from Merck KGaA. Reactive chiral dopant 3 (“Paliocolor® LC756”, (3R,3aR,6S,6aR)-hexahydrofuro[3,2-b]furan-3,6-diyl bis(4-((4-(((4-(acryloyloxy)butoxy)-carbonyl)oxy)benzoyl)oxy)benzoate)) was purchased from BASF SE. Hydrogen-bonded reactiveliquid crystal 4 (4-((6-(acryloyloxy)hexyl)oxy)benzoic acid) was purchased from Synthon Chemi-cals GmbH & Co. KG and Ambeed Inc. Hydrogen-bonded reactive liquid crystal 5 (4-((6-(acryl-oyloxy)hexyl)oxy)-2-methylbenzoic acid) was purchased from Synthon Chemicals GmbH & Co.KG.Non-reactive porogenic liquid crystal 6 (4-cyano-4′-pentylbiphenyl) was purchased fromTCIChemicals Europe N.V. Photoinitiator 7 (2,2-dimethoxy-2-phenylacetophenone) was purchased


from BASF SE. See Figure 2.1 for structures of the components. KOH pellets and poly(vinyl alco-hol) granulate (MW 31,000-50,000, 87-89 % hydrolysed) were purchased from Sigma-Aldrich Inc.Tetrahydrofuran (THF) was purchased from Biosolve B.V. Poly(lactic acid) (PLA) was purchasedfromNatureWorks LLC (IngeoTM 4043D). Polyamide-6 (PA6) was cordially gifted byRoyal DSMN.V. (Akulon® F136).

Liquid crystal alignment cells Glass substrates were cleaned with a two-step procedure: first 20minutes of ultrasonication in 1:1 v/v 2-propanol/ethanol, followed by 20minutes in aUV/O3-oven(UVProducts PR-100). Clean glass substrates (borosilicate 30×30mm2 or soda-lime69×69mm2)were coated with either a poly(vinyl alcohol) solution (4 wt% inH2O) through spincoating at 1500rpm for 30 s (Karl Suss RC6), or for analysis, 3-(trimethoxysilyl)propyl methacrylate (1 vol% in 1:1v/v ratio 2-propanol/water) at the same spin settings. UV-curable glue to control the cell gap wasmade by mixing glue (UV Sealant 91, Norland Products Inc.) with resin spacer beads of varyingdiameter (6 µm, 10 µm, 20 µm;Micropearl SP series, Sekisui Chemical Co., Ltd.).

Liquid crystal mixture The liquid crystalline mixture has been described previously (48): it com-bines the base components in tetrahydrofuran in the following amounts: 17.9 wt% diacrylatemeso-gen 1, 22.9 wt% monoacrylate mesogen 2, 4.6 wt% chiral diacrylate 3, 18.0 wt% of both hydrogen-bonded physical crosslinkers 4 and 5, 18.0 wt% porogenic non-reactive mesogen 6, 0.6 wt% of pho-toinitiator 7. Tetrahydrofuran was removed from the mixture by evaporation at 80 °C under an airflow, followed by a 50 °C vacuum.

Liquid crystal network Production of cholesteric polymer films was initiated by heating themonomer mixture and glass substrate to 80 °C. Using a micropipette, 20 µL (30 × 30 mm2) or100 µL (69 × 69 mm2) of the isotropic monomer mix was transferred to the activated glass sub-strate. Depending on the experiment, different spacer glues were used (“no spacer glue”, “gluewith no spacers”, or glue with either 6, 10, or 20 µm spacers), small dots of which were placednear the corners of the bottom glass slide. At this point, the sample was removed from the hotplate, and a second glass substrate was placed on top of the isotropic mix and sheared uniaxiallyto mechanically align the cholesteric mixture. After short equilibration at room temperature, highintensity UV light (35mW cm−2 at sample, LumenDynamics EXFOOmnicure S2000) irradiatedthe sample for 5 minutes. Films on poly(vinyl alcohol)-treated glass were removed by submersionin demineralised H2O for 30 minutes.

Cholesteric film activation The cholesteric polymer films were washed with tetrahydrofuran toremove non-reactive mesogen 6, after which the thin film was dried in air overnight. A subsequentsoaking in 0.1 m KOH inH2O (pH≈ 13) for 60 minutes created the responsive photonic material.

Cholesteric film breakup KOH-activated polymer films made in 69 × 69 mm2 cells were putin a vial filled with demineralised H2O. An ultrasonic probe (Sonics VC-750 driver with Sonics630-0418 tapered microtip) was placed in the vial and activated for a variable amount of time in apulse-wise fashion (2.0 s sonication, 0.5 s rest) with sonicating tip amplitude set to 35 %, to preventit fromoverheating. Using a fritted glass filter funnel (ROBU®VitraPOR®Por. 4), the flakes wereseparated from the aqueous medium they were fractured in.

Cholesteric flake size quantification After ultrasonication, samples of the aqueous flake disper-sion were taken from the vial using a plastic pipette and placed on a clean microscope slide. Waterwas removed by heating the slide to 100 °C on a hot plate. Polarised optical microscopy was used toobtain representative sample images, which were processed using ImageJ (www.imagej.net) (37,38) and an in-house written macro. Themain quantifier for the flake size is “Feret’s diameter” (39).


Materials characterisation Differential scanning calorimetry (DSC) was performed using a TAInstruments DSC Q2000. Thermogravimetric analysis (TGA) was performed using a TA Instru-ments TGAQ500 in aerobic conditions. ATR FT-IR spectroscopy was done using a Varian Excal-ibur 3100 (GeATR crystal). Polarised optical microscopy (POM)was done using a LeicaDM6000M. In situ POM and UV-vis spectrophotometry at different temperatures and humidity condi-tions were performed using a Leica DM 2700 M microscope, fitted with a Leica MC170 HDcamera, a Linkam TMS 600 temperature-controlled sample stage and a Sensirion SHT3x temper-ature/humidity sensor. The sample environment was enclosed using a custom-built transparenthumidity chamber. Surface profiles of the films were measured with a Veeco DektakXT. Opticalcharacteristics were measured with a PerkinElmer Lambda 750, a Shimadzu UV-3012 PC fittedwith linear polariser-λ/4waveplate stack for circular dichroismmeasurements, and anOceanOpticsHR2000+opticalmicroscope-attachedphotospectrometer for in situmeasurements. During thesein situmeasurements, themicroscope is operated in brightfield reflectionmode with the sample be-tween crossedpolarisers as explained in ref. (28). Scanning electronmicroscopy (SEM)photographswere recorded by sputter coatingmaterial with Au and subsequently imaging it using a FEIQuanta3D scanning electron microscope.

3D print filament fabrication Filaments were compounded using a 3Devo NEXT 1.0 extruder.Cholesteric flakes weremixedmanually with the polymer before loading into the extrusion column(1 % w/w flakes in polymer). The following extruder settings were used for PLA: T4 = 170 °C,T3 = 185 °C, T2 = 190 °C, T1 = 180 °C (4–3–2–1 from material inlet to extrusion die), screwspeed 5.0 rpm, fan speed 70 %, filament diameter 2.85 mm; and for PA6: T4 = 270 °C, T3 = 272°C, T2 = 261 °C, T1 = 258 °C, screw speed 3.4 rpm, fan speed 100 %, filament diameter 2.85 mm.

3Dprinting of responsive filament 3Dmodels are prepared inBlender (www.blender.org) andsliced using Ultimaker Cura to get the g-code file. The material is printed using an unmodified Ul-timaker 3 fused filament fabrication 3D printer, with the PLA-flake composite as the feed material.The print bed temperature is set to Tbed = 60 °C, nozzle temperature to Tnozzle = 200 °C. Allother settings are derived from the standard PLA print settings present in Ultimaker Cura. Blackfilament used for multi-material structures was “Ultimaker PLA Black”, purchased from Lay3rs3DPrinting Eindhoven B.V.


2.5 Appendix: video files

Videofiles supporting this experimental chapter are available free of charge atWileyOnlineLibrary: https://doi.org/10.1002/adpr.202000115.

Video S1. The ultrasonication break-up pro-cess at varying times after startingthe procedure.

Video S2. Real-time response of a singleChLCN flake at constant relativehumidity (75 RH%) at decreasingtemperature (from 30 to 15 °C).


Bibliography

[1] J. Nathans, The Evolution and Physiology of HumanColor Vision,Neuron 24, 299–312 (1999).

[2] E. J. Gerl, M. R. Morris, The Causes and Conse-quences of Color Vision, Evolution: Education andOutreach 1, 476–486 (2008).

[3] D. H. Sliney, What is light? The visible spectrum andbeyond, Eye 30, 222–229 (2016).


[5] I. B. Burgess, M. Lončar, J. Aizenberg, Structuralcolour in colourimetric sensors and indicators, Jour-nal ofMaterials Chemistry C 1, 6075–6086 (2013).


[7] F. Chen, J. Guo, Z. Qu, J. Wei, Novel photo-polymerizable chiral hydrogen-bonded self-assembledcomplexes: Preparation, characterization and theutilization as a thermal switching reflective colorfilm, Journal of Materials Chemistry 21, 8574–8582(2011).

[8] Y. Jiang, R. Wilson, A. Hochbaum, J. Carter, Opti-cal Security andCounterfeit Deterrence Techniques IV ,R. L. van Renesse, ed. (2002), vol. 4677, pp. 247–254.


[10] J. E. Stumpel, C. Wouters, N. Herzer, J. Ziegler, D. J.Broer, C. W. M. Bastiaansen, A. P. H. J. Schenning,An Optical Sensor for Volatile Amines Based on anInkjet-Printed, Hydrogen-Bonded, Cholesteric Liq-uid Crystalline Film, Advanced Optical Materials 2,459–464 (2014).

[11] F. J. Maile, G. Pfaff, P. Reynders, Effect pigments—past, present and future, Progress in Organic Coatings54, 150–163 (2005).


[13] A. Belmonte, M. Pilz da Cunha, K. Nickmans, A. P.H. J. Schenning, Brush‐Paintable, Temperature andLight Responsive Triple Shape‐Memory PhotonicCoatings Based onMicrometer‐Sized Cholesteric Liq-uid Crystal Polymer Particles, Advanced Optical Ma-terials 8, 2000054 (2020).

[14] A. Trajkovska-Petkoska, R. Varshneya, T. Z. Kosc,K. L. Marshall, S. D. Jacobs, Enhanced Electro-OpticBehavior for Shaped Polymer Cholesteric Liquid-Crystal Flakes Made Using Soft Lithography, Ad-vanced FunctionalMaterials 15, 217–222 (2005).

[15] A. Trajkovska-Petkoska, T. Z. Kosc, K. L. Marshall,K. Hasman, S. D. Jacobs, Motion of doped-polymer-cholesteric liquid crystal flakes in a direct-current elec-tric field, Journal of Applied Physics 103, 094907(2008).

[16] K. L. Marshall, K. Hasman, M. Leitch, G. Cox,T. Z. Kosc, A. Trajkovska-Petkoska, S. D. Jacobs,62.3: Doped Multilayer Polymer Cholesteric-Liquid-Crystal (PCLC) Flakes: A Novel Electro-OpticalMedium for Highly Reflective Color Flexible Dis-plays, SID Symposium Digest of Technical Papers 38,1741–1744 (2007).

[17] T. Z. Kosc, K. L. Marshall, S. D. Jacobs, P-94: Poly-mer Cholesteric Liquid Crystal Flakes for Particle Dis-plays, SID Symposium Digest of Technical Papers 34,581 (2003).

[18] W. Liu, Y. Zhou, S. Liu, W. Shao, D. J. Broer,G. Zhou, D. Yuan, D. Liu, Cholesteric Flakes in Mo-tion Driven by the Elastic Force from Nematic Liq-uid Crystals, ACS Applied Materials & Interfaces 11,40916–40922 (2019).

[19] E. M. Korenic, S. D. Jacobs, S. M. Fare, L. Li, Cho-lesteric Liquid Crystal Flakes — A New Form of Do-main,Molecular Crystals and Liquid Crystals ScienceandTechnology. SectionA.MolecularCrystals andLiq-uid Crystals 317, 197–219 (1998).

[20] E. M. Korenic, S. D. Jacobs, S. M. Faris, L. Li, Colorgamut of cholesteric liquid-crystal films and flakes bystandard colorimetry, Color Research & Application23, 210–220 (1998).

[21] S. M. Faris, Aligned Cholesteric Liquid Crystal Inks,US Patent 5364557 (1994).

[22] D. Coates, M. Goulding, A. May, Cholesteric Flakes,US Patent 6207770 (2001).


[23] G. Zhou, S. Liu, W. Liu, D. Yuan, G. Zhou, Op-timized Soft Lithography Method for Polymer Cho-lesteric Liquid Crystal Flakes Fabrication, Microma-chines 10, 441 (2019).

[24] R. B. Brown, J. Audet, Current techniques for single-cell lysis, Journal of The Royal Society Interface 5,S131–S138 (2008).

[25] S. Pradhan, J. Hedberg, E. Blomberg, S. Wold,I. Odnevall Wallinder, Effect of sonication on parti-cle dispersion, administered dose and metal releaseof non-functionalized, non-inert metal nanoparticles,Journal of Nanoparticle Research 18, 285 (2016).

[26] G. Zhou, D. Yuan, Y. Liu, X. Lin, G. Zhou,N. Li, Properties of Liquid Crystal Polymer Films forElectricity-responsive IR Reflective Windows, ActaPhotonica Sinica 46, 331002 (2017).

[27] G. Socrates, Infrared and Raman CharacteristicGroup Frequencies: Tables and Charts (John Wiley &Sons, 2004), third edn.

[28] M. del Pozo, C. Delaney, C.W.M. Bastiaansen, D.Di-amond, A. P. H. J. Schenning, L. Florea, Direct LaserWriting of Four-Dimensional Structural Color Mi-croactuatorsUsing aPhotonicPhotoresist,ACSNano14, 9832–9839 (2020).

[29] B. M. Oosterlaken, Y. Xu, M. M. J. Rijt, M. Pilzda Cunha, G. H. Timmermans, M. G. Debije,H. Friedrich, A. P. H. J. Schenning, N. A. J. M. Som-merdijk, Nanohybrid Materials with Tunable Bire-fringence via Cation Exchange in Polymer Films, Ad-vanced FunctionalMaterials 30, 1907456 (2020).

[30] O. M. Wani, R. C. P. Verpaalen, H. Zeng, A. Pri-imagi, A. P. H. J. Schenning, An Artificial Noctur-nal Flower via Humidity-Gated Photoactuation inLiquid Crystal Networks, Advanced Materials 31,1805985 (2019).

[31] E. P. A. van Heeswijk, J. J. H. Kloos, N. Grossiord,A. P. H. J. Schenning, Humidity-gated, temperature-responsive photonic infrared reflective broadband

coatings, Journal of Materials Chemistry A 7, 6113–6119 (2019).

[32] M. G. Lawrence, The Relationship between RelativeHumidity and the Dewpoint Temperature in MoistAir: A Simple Conversion and Applications, Bulletinof the American Meteorological Society 86, 225–234(2005).

[33] O. A. Alduchov, R. E. Eskridge, Improved MagnusForm Approximation of Saturation Vapor Pressure,Journal of AppliedMeteorology 35, 601–609 (1996).

[34] A. Trajkovska-Petkoska, S. D. Jacobs, Effect of Differ-ent Dopants on Polymer Cholesteric Liquid CrystalFlakes, Molecular Crystals and Liquid Crystals 495,334/[686]–347/[699] (2008).

[35] J. V. Ecker, A. Haider, I. Burzic, A. Huber, G. Eder,S. Hild, Mechanical properties and water absorptionbehaviour of PLA and PLA/wood composites pre-pared by 3D printing and injection moulding, RapidPrototyping Journal 25, 672–678 (2019).

[36] R. C. P. Verpaalen, M. G. Debije, C. W. M. Bas-tiaansen, H. Halilović, T. A. P. Engels, A. P. H. J.Schenning, Programmable helical twisting in orientedhumidity-responsive bilayer films generated by spray-coating of a chiral nematic liquid crystal, Journal ofMaterials Chemistry A 6, 17724–17729 (2018).

[37] C. A. Schneider, W. S. Rasband, K. W. Eliceiri, NIHImage to ImageJ: 25 years of image analysis, NatureMethods 9, 671–675 (2012).

[38] J. Schindelin, I. Arganda-Carreras, E. Frise, V. Kaynig,M. Longair, T. Pietzsch, S. Preibisch, C. Rueden,S. Saalfeld, B. Schmid, J.-Y. Tinevez, D. J. White,V. Hartenstein, K. Eliceiri, P. Tomancak, A. Cardona,Fiji: an open-source platform for biological-imageanalysis,NatureMethods 9, 676–682 (2012).

[39] W. H. Walton, Feret’s Statistical Diameter as a Mea-sure of Particle Size,Nature 162, 329–330 (1948).

Chapter 3Creating Spatially Patterned Slanted Cholesteric Films through Bar Coating and Photolithography

This chapter is reproduced from: J. A. H. P. Sol, H. Sentjens, L. Yang, N. Grossiord, A. P. H. J. Schenning, M. G. Debije, Advanced Materials 33, 2103309 (2021).

How is a suitable cholesteric ink for direct ink writing synthesised, and what optical properties can be created with such a viscous, structurally coloured material? “4D printing” implies the presence of a stimulus-response after printing. In this Chapter, the synthesis of a cholesteric ink designed to be 3D printing feedstock itself will be explored—as shown in Chapter 2, it did not suffice to take a known cholesteric LC formulation and incorporate thin films thereof in a conventional 3D printing filament. After bar coating of the novel cholesteric oligomer ink, formation of a slanted photonic axis is observed, which exhibits atypical iridescence and polarisation selectivi-ty after processing with bar coating. After crosslinking, the coating obtained shows deviating photonic properties compared to planar choles-teric thin-film coatings conventionally used for smart materials.


3.1 Introduction

Through nanoscale molecular motifs, the natural world displays splendid iridescentcolours in both animate (1–3) and inanimate matter (4). The circular dichroism in re-flection found in specific insect exoskeletons can be emulated using cholesteric liquid crys-tals (5). Since the early days of cavewall inscriptions (6), humanshave themselves developedpigmented materials to apply colours according to strict aesthetic demands (7). Throughintricate processing it is possible to achieve full control over the polarisation characteristicsof these thin-filmorganic optics (8). However, what has long been unavailable is combinedcontrol over the iridescence, polarisation selectivity and macroscopic dimensions of poly-mer photonics.

Structurally coloured materials are under renewed scrutiny for possible application asbiomimetic optics in optical sensors, holographic displays, anti-counterfeit labels, intelli-gent skins, and wearable robotics (9–12). Material classes employed to date include inor-ganics (13), laminated polymers (14), colloidal crystals (15), and block copolymers (16).These materials often require intricate, multi-step processing. Generating the chiropti-cal properties seen in Nature (3), allowing selective interactions with specific wavelengthbands of specific polarisation states, is far from trivial, especially if wishing to distributethese interactions over the surface of a single object.

A defining feature of cholesterics is their chiroptical behaviour: selectivity towards re-flecting either left- or right-circular polarised light, maintaining transparency to the oppo-site polarisation. This behaviour results from the chiral molecular arrangement and thedichroism formed by it. Planar aligned cholesteric liquid crystals lose their circular polar-isation selectivity at oblique angles of incidence, a feature that has been both predictedtheoretically (17) and confirmed experimentally (18–20).

The drive towards engineering ChLCmaterials for high-end applications relies on pre-cise control of the helical LC organisation on the molecular scale. In previous works, us-ing light-addressable chiral dopants has allowed direct tuning of the reflected wavelengthband (21), while in crosslinkedChLCpolymers, it has been shown that distorting the verti-cal helical stratification can be used to generate circular and linear polarisation dependentpseudo-Bragg reflectors (8). Forcing the cholesteric into a slanted configuration can beused to make chiroptical diffraction elements (22). While highlighting the versatility ofChLCs in attainable optical characteristics, themanufacturing of these devices is still rathercomplicated.

In this Chapter, it shown that through a one-pot oligomer synthesis, a bar coating-processable cholesteric ink can bemadewhich readily aligns into a slanted cholesteric align-ment. A consequence of the slanted alignment is a loss of circular polarised reflection speci-ficity at normal incidence, as has been known to occur in cholesterics. With photolithog-raphy, it is shown that the slanted alignment can be patterned into the coating.

Photopatterning Slanted Cholesteric Films | 53


3.2.1 Synthesis of a cholesteric liquid crystal oligomer

The ink is synthesised with a thiol-acrylate Michael addition reaction between commer-cially available reactive mesogens 1 and 2 with dithiol 3 (see Figure 3.1a). This reactioncan be catalysed using organic base 4 (1,8-diazabicyclo[5.4.0]undec-7-ene, see Experimen-tal) (23). Molecular structures for the reactive mesogens used can be found in Figure 3.1a.The final reaction product contains 2.6mesogenic units per oligomer (xLC) as judged fromproton nuclear magnetic resonance spectroscopy (1H-NMR, see Figure 3.1b). Followingfrom this, a number-average molar massMn of ca. 2140 g mol−1 is calculated. An impor-tant sidenote is that during the cholesteric ink synthesis, a side product forms as a resultof the molecular structure of the reactive mesogens. Both 1 and 2 have carbonate func-tional groups as linker between the mesogenic core and the alkyl spacers (see Figure3.1a).These groups are susceptible to nucleophilic substitution reactions, for instance by thiolsand amines (24). 1H-NMR confirms that thiocarbonate and phenolic side products areindeed formed (see Figure 3.1b). A signal is found at δ ≈ 3.1 ppm which is not presentin conventional dithiol-extendedmesogenic oligomers, and is indicative of unreacted thiolgroups (see Figure 3.1c) (25). Other signals of interest in NMR are those at δ ≈ 6.9 ppmand δ ≈ 8.1 ppm, which can result from the formation of the displayed phenolic species.This signal points at the formation of a compound in which the spacer has been cleavedfrom themesogen, resulting anewcompoundwithmolecularmass 352.46 gmol−1—it also

a)1

2

OO SH

HS

O

O

O

O

O

O

O

O

O

O

O

O

O

O

O

O

H

H

O

O

OO

O

O

O O

O

O

mesogen diacrylates 1 and 2

dithiol chain extender 3acrylate-terminated ChLC oligomer ink

1+2+3catalyst 4,

CH2Cl2

R1 =R1OO

OO

O

OR1

O

O

SR2

S O

OR1

O

On = 0–4

byproducts thiol-carbonate

reaction

8 7 6 5 4 3 2 1Chemical shift (ppm)

b)

OO SH

HS

O

O

OO

O

O

O O

O

O

O

O

O

O

O

O

OO

O

O

OH

OO

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OO

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O SHSc)

+

Figure 3.1: a) Schematic representation of the main components making up the chiralnematic liquid crystal oligomer ink (mesogenic diacrylates 1, 2, and dithiol chain exten-der 3). b) ¹H-NMR spectrum of the resulting cholesteric oligomer ink in chloroform-d.Indicated are signals attributed to side products as proposed in Figure 3.1c. c) Chem-ical reaction between reactive mesogens 1 and 2 with dithiol 3 can result in the sideproducts shown, a phenolic species and a thiocarbonate molecule. Shown here aremesogen 1 and dithiol 3, and a phenolic as well as thiocarbonate side product.


shows up inmass spectrometry (MALDI-ToF-MS).Nevertheless, the side products do notseem to impact liquid crystalline phase behaviour. Cholesteric-isotropic transition temper-ature TNI is determined to be at 70 °C (DSC), and glass transition temperature Tg = 30

°C (DSC). Visually, the material shows photonic reflection and manifests itself as a stickypaste similar to other LC oligomer mixtures (26). We postulate that further applicabilityfor DIW follows from successful application using bar coating, given that in both tech-niques, shear and elongational flow are the main forces during material deposition.

3.2.2 Fabrication and characterisation of slanted cholesteric coatings

Rather than bar coating from solution (as was shown for oligomeric ChLCs) (27, 28), theChLC oligomer ink was instead directly melted on a pre-treated glass substrate (see Figure3.2a and Experimental). It was found that performing this coating procedure at 46 °C inthe cholesteric mesophase, resulted in a thin film of elastomer that formed a large-scalecholesteric mesophase within seconds.

Itwas immediately apparent that the peak reflectedwavelengthλmax is not viewed fromthe surface normal as expected for a chiral nematic coating. Rather, inspecting thematerialfrom an angle of ca. 50° “into” the bar coating direction discloses a bright orange colour(see Figure 3.2), while viewing from the other end of the filmdoes not appear to display anyvisible colour at all. This would indicate that using Equation 1.1 with θ as the angle fromthe surface normal is not valid, and θ should be taken as angle from the helical directorh. This contrasts strongly with the planar chiral nematic structures formed when usingsimilar oligomeric inks out of solvent (28), pointing towards viscosity of the ink being asignificant reason for distortion of the photonic axis. Perpendicular to this apparent “slantdirection”, tilting the sample results in a blue shift.

Optical characterisation of the coating was performed with angle-controlledultraviolet-visible (UV-vis) spectrophotometry, see Figure 3.2c,d. This data reflects thecharacteristics recognised from Video S1 and Figure 3.2b: a well-defined reflection bandaround λmax ≈ 570 nm is found when the sample is tilted beyond 50° from the normalinto the bar coating direction (numerically labelled as polar angle θ > 50°, azimuthal angleϕ > 0°). Tilting the sample towards θ = −70° shows that the cholesteric reflection bandblue shifts and becomes progressively broader as the light beam interacts with the helixalignment side-on. Rotation of the sample by 90° (ϕ = ±90°) and then tilting shows ablue shift with increasing angle of observation θ in both directions, as would be expectedfrom a slanted alignment. These results are also visualised in Figure 3.2f, which shows atwo-dimensional characterisation of λmax for 0° < ϕ < 180° and 0° < θ < 50° (this datawas recorded with a specialised five-axis spectrometer, schematically visualised in Figure3.2e).

To compare the optical characteristics of our bar coated slantedChLCEwith a conven-tional low molecular weight reactive cholesteric LC network reflector, a benchmark wasmade using amix of 1 and 2with photo-initiator 7 for crosslinking and reactive surfactant


I

II

I

II

planar ChLC off-axis ChLC

bar c

oatin

g di

rect

ion

0°back fwd

0°leftright

0°

backward

forward

0°

rightward

leftward

b) c)

TOP VIEW

FWD TILT

BACK TILT

LEFT TILT RIGHT TILT

polar angle θ

azimuthal angle ϕ

bar coating direction

d)

e) f)

Bar coated with 60 µm gapHeating table set to 46 °CSheared at about 1 cm s-1

a)

20406080

100

Tran

smis

sion

(%)

-70° -50° -30° 0° 30° 50° 70°

ϕ = 0°θ =

400 500 600 700 8000

20406080

100

Tran

smis

sion

(%)

Wavelength (nm)

ϕ = 90°θ =

-70° -50° -30° 0° 30° 50° 70°

01530

4560

75

90

105

120135

150165 180

1020304050

01020304050

slanted ChLCEreference ChLC

azimuthal angle ϕ (°)

pola

r ang

le θ

(°)

1530

4560

75

90

105

120135

150165

400500600700

λmax(θ,ϕ) (nm)

Figure 3.2: a) Scheme of the bar coating procedure wherein the elastomer is spreadover the substrate using a well-defined 60 µm gap. b) Bar coated sample on PET foilshowing anisotropic iridescent colouration. Angle-dependent UV-vis spectral trans-mission data quantifying the reflection colour shift with the sample inclined parallel,c), and perpendicular, d), to the bar coating direction. e,f) Two-dimensional opticalcharacterisation of reflected light for a conventional chiral nematic reflector (left) andthe slanted chiral nematic presented here (right). “I” and “II” illustrate the detectorarm orientation with respect to the perspective-dependent reflected wavelength. Fora colourblind-friendly version of this image, see the Supporting Information on WileyOnline Library (https://doi.org/10.1002/adma.202103309, Figure S11).


Reflection at normal incidenceUnpolarised

RCP

LCP RCPLCP

400 500 600 700 800Wavelength (nm)

10

20

30

40

Tran

smis

sion

(%)

Figure 3.3: Left: Crosslinked photonic film bar coated and polymerised on a flexibleplastic substrate displaying axially asymmetric colour reflection and variable circularpolarisation selectivity based on angle of incidence. Right: Polarisation-controlled UV-vis reflectance spectrum recorded at normal incidence showing near-equal reflectionof left- and right-circular polarised visible light.

8 to ensure monodomain planar alignment after the coating procedure (29). Juxtaposingthe 2D reflectance data of our coating with the cholesteric standard shows the clear differ-ence in angle dependence.

Photo-induced crosslinking results in a soft rubberyChLCE coatingwhich retains theunconventional photonic structure. With a visible-spectrum circular polarisation filter,the selectivity between left- and right-circular polarised light was inspected, as seen in Fig-ure 3.3. At normal incidence—the green reflection—the circular polarisation reflectionselectivity common forChLCs is not observed. This is also seenwith circular polarisedUV-vis reflectance measurements: here, a conventional right-handed cholesteric should reflect100% of right-circular polarised light (RCP), and 0% of left-circular polarised (LCP). Ourbar coatedmaterial reflects about 35–40 % of both at λmax. The combined observations ofnot seeing λmax at normal incidence and the diminished circular polarisation selectivityindicated formation of a slanted chiral nematic helix (18, 19, 30).

To verify our theory that the off-normal position of λmax is formed during bar coatingand is related to the internal material morphology, cross-sections of the ChLCE coatingon poly(ethylene terephthalate) were made and analysed using atomic force microscopy(AFM, see Figure 3.4a). Since the material is too soft to reliably fracture, even in cryogenicconditions, it is cut using a microtome and the viscoelastic phase data from AFM is usedfor microscale analysis. As can be seen, there is a clear slant angle α of about 45° in this


2 µm

phase

3.6°

–7.3°

α ≈ 45°

p/2 ≈ 0.18 µm

a) b)planar slanted

c)RCP

LCP unpolarised

nearly unpolarised

unpolarisednearly unpolarised

substrate substrate

helix director h

helix director hhelix slant α

Figure 3.4: a) AFM image showing the nano-engineered slanted photonic structure(inset: fast Fourier transform of the AFM phase image). b) Scheme of the molecularalignment in a conventional chiral nematic reflector (left) and slanted photonic (right).c) Scheme detailing the optical response of the object in Figure 3.3 with the knowledgegained from Figure 3.4a.

area of the sample, with a cholesteric half-pitch p/2 ≈ 0.18 µm. Entering p = 0.36 µmwith an assumed ⟨n⟩ = 1.55 (29) in Equation 1 gives λmax,AFM = 557 nm, consistentwith UV-vis data presented in Figure 3.2c,d. From the distorted photonic periodicity, onecan derive the orientation of the chiral nematic mesogens (Figure 3.4b) and subsequentlyexplain the optical characteristics demonstrated in Figure 3.3. At steep angles of incidence(top of Figure 3.4c), the helix director h is in line with the viewer’s perspective, displayingλmax. At normal incidence with respect to the bent film (middle of Figure 3.4c), the helixis effectively tilted, with blue shifted λmax and showing little left/right-circular polarisationselectivity (18, 19). Tilting even further, no colour is seen as the helix director lies perpen-dicular to the viewing direction.

Polarisation gratings (PGs) are typically made using LCs in precisely designed align-


ment devices, that coax the mesogens into forming a slanted cholesteric alignment (22, 30,31). These devices are made by patterning the substrate with a photoalignment layer thatis treated with an interference pattern formed in an optical setup. After a sufficient energydose, an alignment pattern is formed in which n rotates by 360° over the substrate every500–900 nm (30, 31). Such a procedure results in fine control over the structure of the PG,but is likely difficult to scale to larger sample sizes. Bar coating, on the other hand, is simi-lar to the blade coating already used for cholesterics (29). We envision that after sufficientoptimisation—procedural but likely also chemical—the bar coating technique presentedcould facilitate larger-scale production of cholesteric PGs.

3.2.3 Photopatterning visible patterns with anisotropic iridescence

Using a two-step photolithography process, it is possible to imprint images into the slantedChLCE coatings (27), as seen in Figure 3.5. Initially, the coating is deposited in the chole-steric mesophase as before, after which the letters “Sfd” are polymerised through use of anegative photomask (see Figure 3.5a). After the reaction time, themask is removed and the

Unpolarised RCPLCP

Unpolarised RCPLCP

FORWARD TILT

TOP VIEW

c)

d)

b)

a)

Figure 3.5: a) Photograph of the printed mask used for photolithography. b) Afterusing successive photo-masked crosslinking steps, visual patterns can be imprintedin the coating that maintain highly perspective-dependent colouration. The white ar-row in Figure 3.3b denotes the bar coating direction, the scale bar represents 10 mm.c) Comparing the circular polarisation of reflected light at oblique incidence. d) Thecoating as seen from directly above, and its circular polarisation characteristics.


coating is then heated to the isotropic phase (70 °C) and photopolymerised through a floodUV exposure; during the heating step, only the non-crosslinked areas enter the isotropicphase.

As expected, the letters imprinted in the coating retain their perspective-dependentcolour (Figure 3.5b and Video S2). Figure 3.5c,d shows the perspective-dependence onthe circular polarisation selectivity as also seen in Figure 3.3. Observing the coating froman oblique angle reveals a bright red reflection that consists nearly entirely of right-circularpolarised light, whereas from the normal direction, green reflection is seen which is neitherfully left- nor right-circularly polarised.

3.3 Conclusion

We have demonstrated the fabrication of a rubbery chiral nematic liquid crystal materialwith anomalous but tuneable chiroptical properties. The distorted helicoidal structure ofthe liquid crystals leads to a photonic material with its longest reflected wavelength offsetfrom the material’s normal direction at an oblique angle of about 50° in the direction ofthe bar coating. Here, light reflected by the material at the normal direction is nearly polar-isation independent. Themethod used here to achieve the slanted cholesteric architecture,bar coating, is likely to be better scalable than the photopatterned alignment devices usedcurrently. We envision that after optimisation, this could be a viable way for mass produc-ing polarisation gratings.

Through a relatively simple photolithography procedure, we have shown that thisalignment can be photopatterned for spectacular visual effects. The optical patterns em-bedded in the cholesteric coating retain the slanted alignment induced during bar coating,while the isotropic parts of the coating are colourless and show no visible angle depen-dence. Characteristics like these make such coatings appealing for anti-counterfeit labels,but likely also as decorative elements in urban and fashion design.

The key finding here is that the results in thisChapter prove that a cholesteric oligomerink can be coated onto substrates solvent-free, and that the method of application influ-ences the photonic structures that are formed. In direct ink writing, the flow regime afterdeposition superficially resembles bar coating—indicating that the ink from this Chaptershould be suitable for this additive manufacturing technique.

3.4 Experimental

Materials Components for producing the chiral nematic liquid crystal elastomer are 2-methyl-1,4-phenylene bis(4-(((4-(acryloyloxy)butoxy)carbonyl)oxy)benzoate) (1, “Paliocolor® LC 242”, pur-chased from BASF SE), (3R,3aR,6S,6aR)-hexahydrofuro[3,2-b]furan-3,6-diyl bis(4-((4-(((4-(acryl-oyloxy)butoxy)carbonyl)oxy)benzoyl)oxy)benzoate) (2, “Paliocolor® LC 756”, purchased fromBASF SE), 2,2′-(ethylenedioxy)diethanethiol (3, EDDET, purchased from Sigma-Aldrich Corp.)and 1,8-diazabicyclo[5.4.0]undec-7-ene (4, DBU, purchased from TCI Chemicals Europe N.V.).


For photoinduced crosslinking, bis(2,4,6-trimethylbenzoyl) phenylphosphineoxide (PI, Irgacure®819, purchased from BASF SE) was added. Reaction solvent dichloromethane was purchasedfromBiosolve B.V. For the reference chiral nematic samples, two additional components were used,photo-initiator 1-hydroxycyclohexyl phenyl ketone (7, Irgacure® 184, purchased from BASF SE),and surfactant 2-(N -ethylperfluorooctanesulfonamido)ethyl acrylate (8, purchased from AcrosChemicals BVBA). Poly(ethylene terephthalate) substrates were cut from a roll of 75 µm thickTenolan OAN0001. For the preparation of sacrificial layers, poly(vinyl alcohol) (MW 31,000-50,000, 87-89 % hydrolysed) or polyvinylpyrrolidone (MW 10,000) were used; both purchasedfrom Sigma-Aldrich Inc.

Synthesis of the ChLC oligomer ink The chiral nematic liquid crystal elastomer ink were syn-thesised by adding diacrylates 1 and 2 and dithiol 3 (3:2 mol ratio diacrylate to dithiol) to a flaskwith CH2Cl2. Weight ratio between the two diacrylates was 94.3 of 1 to 5.7 of 2. After the so-lution clarified, one of two methods were followed. For photopatterned reflective coatings, 4 wasadded pure (0.3 % w/w to total reactants) to catalyse the thiol-acrylate reaction, at room tempera-ture. After 60 min reaction time, the stirring bar was stopped and the mixture was dried using arotary evaporator at 40 °C, 780 mbar. Final drying was done in a vacuum oven at low vacuum. Foraddition of photo-initiator, an amount of ChLC oligomer ink was re-dissolved in CH2Cl2 and 1wt % (to ChLC oligomers) of PIwas added and dissolved, after which the solvent was removed bydrying on a 46 °C hot plate at atmospheric pressure.

Characterisation of the chiral nematic resin Nuclear magnetic resonance (NMR) spectra wererecorded with a Bruker Avance III HD 400MHz in chloroform-d (purchased from Sigma-AldrichInc., 99.8 atom % D, 0.03 % v/v tetramethylsilane). Average chain length was determined by com-paring the ratio of acrylate to mesogenic core signal (25). Gel permeation chromatography (GPC)plotswere recordedwith a ShimadzuLC-2030.3Dwith 254nmPDAand refractive indexdetectors,using tetrahydrofuran as eluent (purchased from Biosolve B.V., stabilised with butylated hydroxy-toluene). Differential scanning calorimetry (DSC) is performed with a TA Instruments Q2000,between 50 and 100 °C at 10 °C min−1. TNI is defined as the peak maximum of the signal corre-sponding to the nematic-isotropic transition, whileTg is defined as the inflection point of the signalindicating the glass-rubber transition.

Preparation of glass substrates for bar coating Glass substrates for bar coating were preparedby ultrasonicating for 20 minutes in 1:1 v/v ethanol:2-propanol (Branson 2510) followed by 20minutes in a UV/O3-oven (UV Products PR-100) and then spin coating either poly(vinyl alcohol)or polyvinylpyrrolidone (both from 4 % w/w solution in deionised H2O) as sacrificial layer onto3 × 3 cm2 borosilicate or 6.9 × 6.9 cm2 soda-lime glass slides at 1500 rpm for 30 s (Karl Suss RC6).

Preparation of slanted chiral nematic coatings through bar coating Bar coating of the ChLColigomer ink was done with a RK Print Coat Instruments K101 Control Coater table and a Sheen1107/80/1 applicator bar with 60 µm gap spacing. The sample table was heated to 46 °C, afterwhich the ChLC oligomer ink was placed on the glass. The bar coater was operated by hand andslid at circa 1 cm s−1. Photo-polymerisation of the material was conducted for 60 min with a lowpower UV source (Philips Original Home Solaria HB 172, 0.9 mW cm−2 UV-A). Generation ofvisual patterns was done through a photomasking procedure. A mask was printed on transparentfoil using opaque black ink; this mask was placed between the sample and the UV light source.Photo-polymerisationwas carriedout as before savewith themaskbetween the source and substrate.For the second step, the sample was heated to the isotropic transition at TNI, which caused the


non-polymerised areas to become isotropic and transparent. In this state, a UV flood exposurecrosslinked the material in the non-chiral nematic state.

Characterisation of the crosslinked materials Ultraviolet-visible (UV-vis) spectrophotometrywas performed with a PerkinElmer Lambda 750 spectrophotometer with integrating sphere detec-tor or OMT Solutions ARTA goniometer sample stage detector where necessary, or a ShimadzuUV-3102 PC spectrophotometer. For more detailed studies on angle-dependent light reflection, aMelchers Autronic DMS 703 LCD characterisation device was used. Polarised optical microscopywas performedwith aLeicaDM6000Mpolarised opticalmicroscopewithDFC420C-mount cam-era. Heating during optical microscopy was done with a Linkam TMS94 controller and LinkamTHMS600 stage. Photographs were recorded with an Olympus OM-D E-M10Mk III and Olym-pusM.Zuiko ED60mm f /2.8macro lens (files saved in .orf raw format), or a Samsung SM-G920(files saved in .dng raw format). Rawphoto fileswere developed and exported to .jpgusingAdobeLightroom Classic CC.

Microscale analysis of slanted chiral nematic coatings Atomic force microscopy (AFM) wasperformed in ambient conditions using a Bruker FastScan atomic force microscope set to tappingmode at a scan rate of 1 Hz, using TESPA-V2 AFM tips (k = 42 N m−1, f = 320 kHz). BrukerNanoscope Analysis 9.4 software was used for controlling the AFM, while Nanoscope Analysis2.0 was used for analysis of the results. Cross-sections of the crosslinked material were preparedby cryogenic microtomy at 120 °C, cut with a Leica EM UC7 ultramicrotome equipped with adiamond blade (Diatome Ltd). Resulting phase images were stitched using Adobe Photoshop CCand saved as .tiff files, which were opened using ImageJ (www.imagej.net) (32). Fast Fouriertransforms (FFT) were done to find the luminosity periodicity in the images, which correspond tohalf the length of the chiral nematic pitch (p/2), as well as the angle from the substrate normal.

Preparation of the conventional chiral nematic reflector reference Reference chiral nematicsamples were made by dissolving an LC mixture (94.1 wt% 1, 4.4 wt% 2, 1 wt% 7, 0.5 wt% 8) inxylene at 55 parts solvent to 45 parts solids (v/w) and spin coating the solution at 1000 rpm for 40s on PVA coated glass slides (Karl Suss SC6) which had been rubbed over a velvet cloth to generatea uniaxial alignment layer for the LC. The spin coated samples were then flash dried for 10 s at 90°C and photocured using a low power UV source (Philips Original Home Solaria HB 172, 0.9 mWcm−2 UV-A), where the sample remained for 10 min.



Videofiles supporting this experimental chapter are available free of charge atWileyOnlineLibrary: https://doi.org/10.1002/adma.202103309.

Video S1. Coating on PET foil, made withthe ChLC oligomer ink. Videorecordedwith sample rotating at 4°s−1; video playback rate set to 500%.

Video S2. Coating on glass, made with theChLC oligomer ink, with the“Sfd” area polymerised at 40 °C,the remainder at 70 °C. Videorecordedwith sample rotating at 4°s−1; video playback rate set to 500%.


Bibliography


[2] J. M. Douglas, T. W. Cronin, T. H. Chiou, N. J.Dominy, Light habitats and the role of polarizediridescence in the sensory ecology of neotropicalnymphalid butterflies (Lepidoptera: Nymphalidae),Journal of Experimental Biology 210, 788–799(2007).

[3] V. Sharma,M.Crne, J.O. Park,M. Srinivasarao, Struc-turalOrigin ofCircularly Polarized Iridescence in Jew-eled Beetles, Science 325, 449–451 (2009).

[4] J. V. Sanders, Colour of Precious Opal, Nature 204,1151–1153 (1964).

[5] Y. Bouligand, Liquid crystals and biological morpho-genesis: Ancient and new questions, Comptes RendusChimie 11, 281–296 (2008).

[6] C. S. Henshilwood, F. D’Errico, K. L. van Niekerk,Y. Coquinot, Z. Jacobs, S.-E. Lauritzen, M. Menu,R. Garcia-Moreno, A 100,000-Year-Old Ochre-ProcessingWorkshop at Blombos Cave, South Africa,Science 334, 219–222 (2011).

[7] J. Barnett, S.Miller, E. Pearce, Colour and art: A briefhistory of pigments, Optics & Laser Technology 38,445–453 (2006).

[8] A. J. J. Kragt, D. C. Hoekstra, S. Stallinga, D. J. Broer,A. P.H. J. Schenning, 3DHelix Engineering inChiralPhotonicMaterials,AdvancedMaterials31, 1903120(2019).

[9] A. G. Dumanli, T. Savin, Recent advances in thebiomimicry of structural colours,Chemical SocietyRe-views 45, 6698–6724 (2016).

[10] C. Xu, G. T. Stiubianu, A. A. Gorodetsky, Adaptiveinfrared-reflecting systems inspired by cephalopods,Science 359, 1495–1500 (2018).

[11] A. Scarangella, V. Soldan, M. Mitov, Biomimetic de-sign of iridescent insect cuticles with tailored, self-organized cholesteric patterns, Nature Communica-tions 11, 4108 (2020).

[12] F. Fu, L. Shang, Z. Chen, Y. Yu, Y. Zhao, Bioinspiredliving structural color hydrogels, Science Robotics 3,eaar8580 (2018).

[13] G. Pfaff, P. Reynders, Angle-Dependent Optical Ef-fects Deriving from Submicron Structures of Filmsand Pigments, Chemical Reviews 99, 1963–1982(1999).

[14] M. F. Weber, C. A. Stover, L. R. Gilbert, T. J.Nevitt, A. J. Ouderkirk, Giant Birefringent Optics inMultilayer Polymer Mirrors, Science 287, 2451–2456(2000).

[15] Y. Zhao, Z. Xie, H. Gu, C. Zhu, Z. Gu, Bio-inspiredvariable structural color materials., Chemical SocietyReviews 41, 3297–317 (2012).

[16] M. Stefik, S. Guldin, S. Vignolini, U. Wiesner,U. Steiner, Block copolymer self-assembly fornanophotonics, Chemical Society Reviews 44, 5076–5091 (2015).

[17] V. A. Belyakov, V. E. Dmitrienko, V. P. Orlov, Opticsof cholesteric liquid crystals, Soviet Physics Uspekhi 22,64–88 (1979).

[18] H. Takezoe, Y. Ouchi, A. Sugita, M.Hara, A. Fukuda,E. Kuze, Experimental Observation of the Total Re-flection by a Monodomain Cholesteric Liquid Crys-tal, Japanese Journal of Applied Physics 21, L390–L392 (1982).

[19] K. M. Lee, M. Rumi, M. S. Mills, V. Reshetnyak,D. R. Evans, T. J. Bunning, M. E. McConney, A Dif-ferent Perspective on Cholesteric Liquid Crystals Re-veals Unique Color and Polarization Changes, ACSApplied Materials & Interfaces 12, 37400–37408(2020).

[20] V. P. Tondiglia, M. Rumi, I. U. Idehenre, K. M. Lee,J. F. Binzer, P. P. Banerjee, D. R. Evans, M. E. Mc-Conney, T. J. Bunning, T. J. White, Electrical Con-trol of Unpolarized Reflectivity in Polymer-StabilizedCholesteric LiquidCrystals at Oblique Incidence,Ad-vanced OpticalMaterials 6, 1800957 (2018).

[21] H. Wang, H. K. Bisoyi, L. Wang, A. M. Urbas,T. J. Bunning, Q. Li, Photochemically and ThermallyDrivenFull-ColorReflection in a Self-OrganizedHeli-cal SuperstructureEnabledby aHalogen-BondedChi-ral Molecular Switch, Angewandte Chemie Interna-tional Edition 57, 1627–1631 (2018).

[22] J. Kobashi, H. Yoshida, M. Ozaki, Planar optics withpatterned chiral liquid crystals, Nature Photonics 10,389–392 (2016).


[23] J. W. Chan, C. E. Hoyle, A. B. Lowe, M. Bow-man,Nucleophile-Initiated Thiol-Michael Reactions:Effect of Organocatalyst, Thiol, and Ene, Macro-molecules 43, 6381–6388 (2010).

[24] H. Tomita, F. Sanda, T. Endo, Model reaction for thesynthesis of polyhydroxyurethanes fromcyclic carbon-ates with amines: Substituent effect on the reactivityand selectivity of ring-opening direction in the reac-tion of five-membered cyclic carbonates with amine,Journal of Polymer Science Part A: Polymer Chemistry39, 3678–3685 (2001).






[30] Y.-H. Lee, K. Yin, S.-T. Wu, Reflective polariza-tion volume gratings for high efficiency waveguide-coupling augmented reality displays,Optics Express25,27008 (2017).

[31] I. Nys, M. Stebryte, Y. Y. Ussembayev, J. Beeckman,K. Neyts, Tilted Chiral Liquid Crystal Gratings forEfficient Large‐Angle Diffraction, Advanced OpticalMaterials 7, 1901364 (2019).

[32] J. Schindelin, I. Arganda-Carreras, E. Frise, V. Kaynig,M. Longair, T. Pietzsch, S. Preibisch, C. Rueden,S. Saalfeld, B. Schmid, J.-Y. Tinevez, D. J. White,V. Hartenstein, K. Eliceiri, P. Tomancak, A. Cardona,Fiji: an open-source platform for biological-imageanalysis,NatureMethods 9, 676–682 (2012).

Chapter 4Direct Ink Writing of a Cholesteric Oligomer Ink for Freeform Soft Reflectors

This chapter is reproduced from: J. A. H. P. Sol, H. Sentjens, L. Yang, N. Grossiord, A. P. H. J. Schenning, M. G. Debije, Advanced Materials 33, 2103309 (2021).

Is it possible to use a cholesteric oligomer ink with direct ink writing to print freeform structurally coloured objects, and retain the capacity of forming slanted cholesteric alignment? Based on the results shown in Chapter 3, that demonstrate a cholesteric oligomer ink which can be coaxed into forming a slanted photonic axis, direct ink writing is used to spatially program the unconventional cholesteric alignment. Tuning the writing direction and speed leads to programmed formation of a slanted photonic axis which exhibits atypical iridescence and polarisation selectivity. After crosslinking, we obtain a freely-programmable, chiroptical photonic polymer material. The strongly perspective-dependent appearance of the material could function as specialised anti-counterfeit markers, as optical elements in decorative iridescent coatings, or as demonstrated here, in optically-based signalling features.


4.1 Introduction

Recent work has demonstrated that by using direct ink writing (DIW) with a polymericink conducive to shear and elongation, the molecular orientation of the print may be de-fined (1, 2), which has been exploited in the past to print liquid crystal elastomer (LCE)actuators (3, 4). Using DIW to print structurally coloured photonics is also gainingtraction—recently successful printing of bottlebrush block copolymers was reported (5),which shows promise forwriteable, non-chiral polymer photonics, allowing for colour tun-ing based on the printing speed, for instance. Fibre spinning of cellulose-based cholesterichas also beenused tomakephotonic hydrogel filaments featuring a slanted cholesteric align-ment (6).

In thisChapter, wepresent a structurally coloured, printable chiropticalmaterial basedon a ChLC oligomer ink synthesised from a reactive cholesteric LC. This ink combinesstraightforward processing with easy-to-exploit freedom in iridescence and circular polari-sation selectivity of reflected light on a “voxel-by-voxel” basis. DIW is used for the genera-tion of intricate, spatially defined optical patterns—this additivemanufacturing techniquebeing highly adaptable. It is shown that the resulting chiroptical properties of the ink areadjustable by altering deposition rates, and so the optical properties of the final print canbe varied spatially, resulting in spectacular visual effects generated from a single ink, duringa single print.


4.2.1 Synthesis of the cholesteric ink for direct ink writing

The ink is synthesised with a thiol-acrylate Michael addition reaction between commer-cially available reactive mesogens 1 and 2 with dithiol 3 (see Figure 4.1a). This reactionis catalysed using 5, dimethylphenylphosphine (7). The final reaction product containsxLC = 2.3 mesogenic units per oligomer as judged from proton nuclear magnetic reso-nance spectroscopy (1H-NMR),with a calculated number-averagemolarmassMn ≈ 1890

g mol−1, and dispersity 2.06 (from gel permeation chromatography, GPC). Similar to thecholesteric oligomer ink described in Chapter 3, the 1H-NMR spectrum for the DIWink shows thiocarbonate and phenolic side products. Matrix-assisted laser desorption-ionisation-time of flight-mass spectrometry (MALDI-ToF-MS) revealed that the mixturecontains oligomer lengths up to pentamers. Cholesteric-isotropic transition temperatureTNI is determined to be at 76 °C, and glass transition temperature Tg = 22 °C.

Visually, the material exhibits photonic reflection and manifests itself as a sticky pastesimilar to other LC oligomer mixtures, but with a clear structurally coloured property asseen in Figure 4.2a (4). Based on the results achieved inChapter 3, we hypothesise that thisChLC oligomer ink is very well suited for generating bright, selectively reflecting printsusing DIW. Furthermore, we postulate that applicability for DIW follows from successful

Direct Ink Writing of a Cholesteric Oligomer Ink | 69

a)1

2

OO SH

HS

O

O

O

O

O

O

O

O

O

O

O

O

O

O

O

O

H

H

O

O

OO

O

O

O O

O

O

mesogen diacrylates 1 and 2

dithiol chain extender 3acrylate-terminated ChLC oligomer ink

1+2+3catalyst 4,

CH2Cl2

R1 =R1OO

OO

O

OR1

O

O

SR2

S O

OR1

O

On = 0–4

b)

8 7 6 5 4 3 2 1Chemical shift (ppm)

Figure 4.1: a) Synthesis procedure for the cholesteric ink. The procedure is largely sim-ilar to that described in Chapter 3. b) 1H-NMR spectrum of the resulting cholesteric inkin chloroform-d. Coloured peak labels indicate chemical groups highlighted in Figure3.1c.

b)Tink = 90 °C

Tbed = 53 °C

I.D. 335 µm

vnozzle = 1–15 mm s-1

a)

Figure 4.2: a) The ChLC oligomer ink in a vial. b) Scheme illustrating the direct inkwriting of the photonic elastomer (top), mesophase transitions it undergoes (top right)and a photograph of the print procedure in progress (bottom right).

application using bar coating, given that in both techniques, shear and elongation flow arethe main forces during material deposition.

4.2.2 Direct ink writing of the cholesteric ink

Based on the results obtained with bar coating, we set out to pattern the chiral nematic’sphotonic properties with a higher degree of spatial control using DIW. This techniquehas been used successfully to print uniaxial nematic LCEs, where the shear present duringextrusion aligned the ink into a uniaxial alignment following the print path (3, 4, 8). Inour case, to get a photonic reflection from our material, we need the material to form a he-licoidal alignment—which contrasts strongly to the uniaxial nematic alignment presentedinprevious reports. To allow for formationof our requiredmolecular orientation, printingis done from a syringe heated to above TNI, and a print substrate heated to just below thistemperature (see Figure 4.2b). As the material passes through the uninsulated nozzle, it al-ready cools down partially (9). Video S1 shows the rapid mesophase transitions after mate-rial deposition. The effect of different substrate temperatures is highlighted in Figure 4.3a:


b)40 47 53 59 °C

UNPOLARISED

CROSSED POLARISERS

a)

400 500 600 700 8000102030405060708090100

Refle

ctan

ce (%

)Wavelength (nm)

40 °C 47 °C 53 °C 59 °C

Tbed =

c)

AP

Figure 4.3: a) Cholesteric elastomer printed with different substrate temperatures(left-to-right: 40, 47, 53, 59 °C). The scale bar represents 3 mm. b) Stitched opticalmicroscopy images showing unpolarised transmission (top) and transmission throughcrossed polarisation filters (bottom), indicating birefringence of the printed materialsin (a). c) UV-vis reflectance spectra (unpolarised) of the printed strips from (a).

colour is only formedwhen the substrate is held at 53 °C or lower; with lower temperaturesgenerating less intense colours. Thedecreased colour saturation seen at temperatures below53 °C is likely due to the formation of amore disorderly, scatteringmolecular alignment inthe prints, as can be seen in both reflectance and transmission photographs (Figure 4.3a,b).Prints done at lowerTbed are also slightly red shifted (see Figure 4.3c). This change in reflec-tion band is similar to the temperature-dependent pitch length change found for ChLColigomers before (10). Polarised opticalmicroscopy confirms that if the temperature of thebed is held higher than 53 °C, an isotropic phase is formed (Figure 4.3b). After deposition,the prints are illuminatedwith intenseUV light for up to 5minutes (λLED = 365nm,≈ 60

mWcm−2) withinminutes after deposition, which crosslinks thematerial sufficiently evenin aerobic conditions (4, 8, 11).

Printing speed is critical to the appearance of our photonic ink after writing. At lowlateral nozzle speeds, a planar chiral nematic alignment is, which becomes increasinglydistorted as the print speed increases. The transition from planar to slanted cholestericalignment can be seen by eye, but angle-dependent UV-vis transmittance measurementsprovided a much clearer picture of the process (see Figure 4.4a). Increasing printing speedvnozzle from 2 mm s−1 to 8 mm s−1 results in films with increasingly angle-asymmetricalreflection characteristics. This happens by broadening of the reflection band at normalincidence and splitting of the reflection band at large positive angles of incidence, θ > 60°.Upon further increase of the printing speed, the slanted cholesteric reflection band atlonger wavelength prevails, leading to the perspective-dependent appearance. From thereflection band splitting seen in Figure 4.4awe assume that the transition happens throughthe formation of slanted ChLC domains in coexistence with planar ChLC domains. In-


20406080

20406080

Tran

smis

sion

(%)

300 400 500 600 7000

20406080

Wavelength (nm)

+60°ϕ = 0°, θ = –60° +30°–30° 0°

2 mm s-1

6⅔ mm s-1

8 mm s-1

b)a)

Figure 4.4: a) Angle-dependentUV-vis transmission spectra formaterial strips printedat 2, 62

3 , and 8 mm s−1 (-60°> θ > 60°, azimuthal angle ϕ = 0°). b) Butterfly structureprinted at 2 mm s−1 (wings’ outer rim) and 10 mm s−1 (inner wing sections) observedfrom different viewing perspectives. Illumination comes from nearly the same direc-tion as the camera.

creasingly more of the ChLC assumes a slanted orientation when vnozzle is increased,rather than the whole slant angle α increasing gradually but uniformly. Since there doesnot seem to be an overall α we can measure from angle-dependent UV-vis—there is likelya range of tilt angles assumed between the substrate and film surface—we cannot directlyconstruct a numerical relation between α and vnozzle for this material.

Comparing films of differing film thicknesses indicates that this process is highly sheardependent—films of lesser thickness show this band broadening and splitting effect atlower lateral nozzle speeds. Further, the amount of material extruded is also critical to theoptical characteristics. The lower print speeds are prone to over-extrusion, which leadingto a strongly scattering appearance. One possible reason is that the high extrusion rate leadsto a chaotic flow pattern between nozzle and print bed, interfering with the uniaxial align-ment originating in the nozzle tip. Then, during relaxation on the print bed, the poorlyoriented mesogens self-assemble into a polydomain cholesteric mesophase. On the otherhand, high printing speeds generally create layers thinner than programmed, but these areof better optical quality as demonstrated by the higher transmission at shorter wavelengths(Figure 4.4a). The influence of print speed on the deposited layer thickness in LCE DIWhas been reported previously (8), and our decreased optical quality when over-extruding isexpected when considering the molecular alignments formed (12).


020406080

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ctan

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TOP VIEW SIDE VIEW

unpo

laris

edRC

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P

c)

unpolarised LCP RCP

Figure 4.5: a) Polarisation-controlled UV-vis reflectance measurements at normal in-cidence of a photonic sheet printed at different lateral nozzle speeds (2, 62

3 , 10 mms−1). b) Photographs of two DIW printed coatings, back-and-forth 2 mm s−1 and “uni-directional” 10 mm s−1, for unpolarised, and left- and right-circular polarised light. c)Photographs of a Morpho-inspired coating DIW printed with different printing speedsin the same object: the rim of the wings is printed at 2 mm s−1, the inside at 10 mm s−1.

4.2.3 Characterisation of the circular polarisation dependence in reflection

The voxel-by-voxel freedom gained is demonstrated by programming the DIW to print aMorpho-inspired object, which incorporates the possibilities when printing using multi-ple lateral nozzle speeds in a single object (Figure 4.4b). At steep angles of observation(θ ≈ 50°), with the illumination coming from nearly the same direction, the printed but-terfly pattern does something not seen in its natural counterpart: as depicted in Figure 4.4band Video S2, different parts of the “wings” light up depending on the exact position ofthe viewer with respect to the printed object. The juxtaposition of polarisation selectiv-ity and non-selectivity in this single object is inspired by actual photonic wing patterns ofsome butterfly species (13), which feature regions of polarised reflection on unpolarisingbackgrounds.

From this assay, we have selected two complementary speeds (vnozzle = 2 and 10 mm


s−1) for further processing—2mms−1 nozzle speed results in a consistent planar cholestericmorphology, while 10 mm s−1 reliably leads to the slanted morphology also seen after barcoating. This confirms our initial conjecture that bar coating results could be translatedsuccessfully to direct ink writing. In both cases, the cholesteric alignment forms withinseconds after deposition, as seen in Video S3. As expected, the prints performed at lowerspeeds reflect only a single circular polarisation of light at normal incidence. At normal in-cidence, the prints deposited at 10mms−1 reflect light independent of circular polarisation,similar to the coatings described previously (shown in Figure 4.5a,b). As also described inChapter 3, Figure 4.5b also shows that when a slanted cholesteric alignment is inspectedat an angle, “into” the cholesteric alignment, conventional left/right-circular polarisationselectivity is exhibited.

See Figure 4.5c for the difference in polarisation selectivity between the inner and outerparts of theMorpho demonstrator. The rim around the butterfly is printed at 2 mm s−1,and reflects as a standard planar, chiral nematic reflection grating; recognised by the greencolour, and the observation that right-circular polarised light is more strongly reflected.The inner area, printed at 10 mm s−1, however, appears as teal from normal incidence.

4.2.4 Demonstrator devices printed using the cholesteric DIW ink

Photonic materials are widespread in Nature and used for all manner of communication;likewise, engineeredphotonicmaterials could also serve communicationpurposes (14). Us-ing our procedure, a practical, functional demonstration of what can be created by DIWis seen in Figure 4.6a and Video S3, which depicts an arrow that is only seen as green whenviewed from behind, thus indicating to a viewer that they are looking in the “correct di-rection”, or following a correct route. Printing such a “unidirectional” slanted alignmentrequires multiple airborne movements—after drawing each line, the nozzle is lifted 5 mmfrom the substrate and moved quickly to the starting position of the next line, where it isthen lowered to printing height and draws the next line, as shown in Video S4. The elas-tomeric nature of the photonic material after crosslinking is further highlighted in Figure4.6b. When writing the material on a substrate functionalised with a sacrificial layer, suchas polyvinylpyrrolidone, the ChLCE can be released to obtain a self-supporting, flexiblephotonic rubber.

4.3 Conclusion

We have demonstrated the fabrication of a rubbery chiral nematic liquid crystal materialwith anomalous but tuneable chiroptical properties. The distorted helicoidal structure ofthe liquid crystals leads to a photonic material with its longest reflected wavelength offsetfrom the material’s normal. Here, light reflected by the material at the normal directionis nearly polarisation independent. When using direct ink writing, the cholesteric align-ment can be distorted in a programmable manner, quickly organising after extrusion into


TOP VIEWLEFT RIGHT

FORWARD

BACKWARD

b)a)

Figure 4.6: a) Demonstration of a direct ink written guidance device reflecting brightcolour when observed from the proper perspective. b) After removal from the sub-strate, the crosslinked elastomer is photographed as a free-standing photonic rubber.The scale bar represents 5 mm.

the slanted chiral nematic alignment. This chiroptical photonic ink, combined with thedirect ink writing method used, pave the way for design of specialised polymeric opticalelements with disparate optical effects, all deposited in one pass using the same ink. Forinstance, these could be written into anti-counterfeiting security tags, or printed directlyinto 4D printed soft robotic assemblies to embed optical sensing capabilities, a feature notwidespread yet (15). Finally, the prints could be used for high-end decorative elementsgiven the material’s attractive and uniquely iridescent appearance.

4.4 Experimental

Materials Components for producing the chiral nematic liquid crystal elastomer are 2-methyl-1,4-phenylene bis(4-(((4-(acryloyloxy)butoxy)carbonyl)oxy)benzoate) (1, “Paliocolor® LC 242”, pur-chased from BASF SE), (3R,3aR,6S,6aR)-hexahydrofuro[3,2-b]furan-3,6-diyl bis(4-((4-(((4-(acryl-oyloxy)butoxy)carbonyl)oxy)benzoyl)oxy)benzoate) (2, “Paliocolor® LC 756”, purchased fromBASF SE), 2,2′-(ethylenedioxy)diethanethiol (3, EDDET, purchased from Sigma-Aldrich Corp.)and dimethylphenylphosphine (5, Me2PPh, purchased from TCI Chemicals Europe N.V.). Forphotoinduced crosslinking, bis(2,4,6-trimethylbenzoyl)phenylphosphineoxide (PI, Irgacure® 819,purchased from BASF SE) was added. Reaction solvent dichloromethane was purchased fromBiosolve B.V. For the preparation of sacrificial layers, poly(vinyl alcohol) (MW 31,000-50,000, 87-89 % hydrolysed) or polyvinylpyrrolidone (MW 10,000), were used, both purchased from Sigma-Aldrich Inc.

Synthesis of the ChLC oligomer ink The chiral nematic liquid crystal elastomer ink was synthe-sised by adding diacrylates 1 and 2 and dithiol 3 (3:2 mol ratio diacrylate to dithiol) to a flask withCH2Cl2. Weight ratio between the two diacrylates was 94.3 of 1 to 5.7 of 2. After the solutionclarified, one of two methods were followed. For direct ink writing, catalyst 5 in CH2Cl2 (ca. 50mg mL−1) was added to catalyse the thiol-acrylate reaction at room temperature (0.15 % w/w toreactants). After 60 min reaction time, the stirring bar was stopped and the mixture was dried by


pouring the reaction mixture in a 120 mL PTFE evaporation dish and leaving it at ambient condi-tions overnight. For addition of photo-initiator, an amount ofChLColigomer inkwas re-dissolvedin CH2Cl2. PI was added to the ChLC oligomer ink (≈2 % w/w) and dispersed through magneticstirring, after which the solvent was removed by drying in a PTFE evaporation dish overnight atambient conditions. If there was still dichloromethane present—signified by the 1H-NMR signalat δ 5.3 ppm (16)—a final drying step at 90 °C at vacuum in a vacuum oven would be done.

Characterisation of the cholesteric ink Nuclear magnetic resonance (NMR) spectra wererecorded with a Bruker Avance III HD 400MHz in chloroform-d (purchased from Sigma-AldrichInc., 99.8 atom % D, 0.03 % v/v tetramethylsilane). Average chain length was determined by com-paring the ratio of acrylate to mesogenic core signal (17). Gel permeation chromatography (GPC)plotswere recordedwith a ShimadzuLC-2030.3Dwith 254nmPDAand refractive index detectors,using tetrahydrofuran as eluent (purchased from Biosolve B.V., stabilised with butylated hydroxy-toluene). Differential scanning calorimetry (DSC) is performed with a TA Instruments Q2000,between 50 and 100 °C at 10 °C min−1. TNI is defined as the peak maximum of the signal cor-responding to the nematic-isotropic transition, while Tg is defined as the inflection point of thesignal indicating the glass-rubber transition. Matrix-assisted laser desorption/ionisation-time offlight-mass spectrometry (MALDI-ToF-MS)was done using a BrukerAutoflex SpeedMALDI-MSwith trans-2-(3-(4-tert-butylphenyl)-2-methyl-2-propenylene)malonitrile (DCTB) and α-cyano-4-hydroxycinnamic acid (CHCA) as matrices.

Preparation of glass substrates for bar coating and direct ink writing Glass substrates for barcoating were prepared by ultrasonicating for 20 minutes in 1:1 v/v ethanol:2-propanol (Branson2510) followedby 20minutes in aUV/O3-oven (UVProducts PR-100) and then spin coating eitherpoly(vinyl alcohol) or polyvinylpyrrolidone (both from 4 % w/w solution in deionised H2O) assacrificial layer onto 3 × 3 cm2 borosilicate or 6.9 × 6.9 cm2 soda-lime glass slides at 1500 rpm for30 s (Karl Suss RC6).

Direct ink writing planar and slanted cholesteric reflectors Direct ink writing of the ink wasdone using a Hyrel EHR equipped with TAM-15 high-operating temperature reservoirs. Using aLuer-Lock adapter, 0.335 mm I.D. (27 ga A.W.G.) micronozzles (Fisnar QuantX Micron-S Red)were fitted to the ink reservoir. To print the chiral nematic ink, temperatures were kept aroundTNIas measured through DSC: 90 °C for the ink syringe, 53 °C for the print substrate. After printing,objects were photo-crosslinked with a printer-attached UVATA 405 nm LED (10 mW cm−2) for1-5 minutes depending on the size of the print, followed by exposure to a high intensity UV lightsource (Excelitas EXFO Omnicure S2000, set to ≈10–15 mW cm−2) for 15 min on both sides ofthe print. Layer spacing for the direct inkwritten patternswas set anywhere in between 50–100 µm.Preparation of print path .gcode files was done in twoways: if the print pattern consists mainly ofsimple lines, such as printed strips, these are written manually in g-code. For more complex shapes,such as theMorpho-inspired butterfly, a combination of software tools was used. First, a vector filewas prepared in Adobe Illustrator CC, which was saved as an .svg (Scalable Vector Graphics) file.This file was imported into Blender (www.blender.org), given a thickness and exported as a .stl(stereolithography) file. Finally, this .stl file was opened using Slic3r 1.3.0 (www.slic3r.org),where a .gcode file was made according to chosen direct ink writing settings: speed, layer height,line spacing.

Characterisation of the crosslinked materials Dynamic mechanical thermal analysis (DMTA),was measured with a TA Instruments DMAQ800 at 1 Hz and 1 mN preload force. Sample thick-


ness was measured using a Sensofar S neox confocal optical profilometer. Ultraviolet-visible (UV-vis) spectrophotometry was performed with a PerkinElmer Lambda 750 spectrophotometer withintegrating sphere detector or OMT Solutions ARTA goniometer sample stage detector wherenecessary, or a Shimadzu UV-3102 PC spectrophotometer). Polarised optical microscopy wasperformed with a Leica DM 6000 M polarised optical microscope with DFC420 C-mount cam-era. Heating during optical microscopy was done with a Linkam TMS94 controller and LinkamTHMS600 stage. Photographs were recorded with an Olympus OM-D E-M10Mk III and Olym-pusM.Zuiko ED60mm f /2.8macro lens (files saved in .orf raw format), or a Samsung SM-G920(files saved in .dng raw format). Rawphoto fileswere developed and exported to .jpgusingAdobeLightroomClassic CC.


Videofiles supporting this experimental chapter are available free of charge atWileyOnlineLibrary: https://doi.org/10.1002/adma.202103309.

Video S1. The print process as it happens,showing the phase transitions thechiral nematic ink is undergoingimmediately after deposition onthe glass substrate at 53 °C.

Video S2. The Morpho-inspired print fromFigure 4.4b in themain text shownon a rotating table. Video recordedwith sample rotating at 4° s−1;video playback rate set to 500 %.

Video S3. The arrow-shaped signalling de-vice from Figure 4.6a in the maintext shown on a rotating table.Video recorded with sample rotat-ing at 4° s−1; video playback rate setto 500 %.

Video S4. The print process for the unidirec-tionally aligned arrow, showing re-peated airborne movements neces-sary for the desired alignment di-rection.


Bibliography






[6] Y. Liu, P. Wu, Bioinspired Hierarchical Liq-uid‐Metacrystal Fibers for Chiral Optics andAdvanced Textiles, Advanced Functional Materials30, 2002193 (2020).

[7] J. W. Chan, C. E. Hoyle, A. B. Lowe, M. Bow-man,Nucleophile-Initiated Thiol-Michael Reactions:Effect of Organocatalyst, Thiol, and Ene, Macro-molecules 43, 6381–6388 (2010).


[9] Z. Wang, Z. Wang, Y. Zheng, Q. He, Y. Wang, S. Cai,Three-dimensional printing of functionally gradedliquid crystal elastomer, Science Advances 6, eabc0034(2020).



[12] D. Mistry, N. A. Traugutt, K. Yu, C. M. Yakacki, Pro-cessing and reprocessing liquid crystal elastomer actu-ators, Journal of Applied Physics 129, 130901 (2021).

[13] J. M. Douglas, T. W. Cronin, T. H. Chiou, N. J.Dominy, Light habitats and the role of polarizediridescence in the sensory ecology of neotropicalnymphalid butterflies (Lepidoptera: Nymphalidae),Journal of Experimental Biology 210, 788–799(2007).

[14] Y. Qi, C. Zhou, W. Niu, S. Zhang, S. Wu, W. Ma,B. Tang, Retroreflection and Wettability ControlledSmart Indicator Based on Responsive Bilayer Pho-tonic Crystals for Traffic Warning, Advanced OpticalMaterials 8, 2001367 (2020).

[15] T. J. Wallin, J. Pikul, R. F. Shepherd, 3D printing ofsoft robotic systems,NatureReviewsMaterials 3, 84–100 (2018).

[16] G. R. Fulmer, A. J. Miller, N. H. Sherden, H. E.Gottlieb, A. Nudelman, B. M. Stoltz, J. E. Bercaw,K. I. Goldberg, NMR chemical shifts of trace impuri-ties: Common laboratory solvents, organics, and gasesin deuterated solvents relevant to the organometallicchemist,Organometallics 29, 2176–2179 (2010).


Chapter 54D Printed Structurally Coloured Actuators

This chapter is reproduced from: J. A. H. P. Sol, L. G. Smits, A. P. H. J. Schenning, M. G. Debije, manuscript in submission.

With the knowledge of how to successfully direct ink write a cholesteric oligomer ink, can the ink now be enhanced with a stimulus-response, towards “4D printed” structurally coloured devices? Combining both a change in shape and appearance in a single 3D printable material remains a challenge. In this Chapter, an ink is developed for direct ink writing that does both—a “4D printing”, cholesteric liquid crystal oligomer ink is synthesised that can be deposited onto 3D printed objects or printed into 3D shapes of its own.


5.1 Introduction

The expected potential of stimuli-responsive polymeric materials in a broad array of ap-plications spanning from healthcare to optical sensing, from soft robotics to energy har-vesting, has made these materials a popular research topic. Concurrently, the viability ofadditive manufacturing (AM) in both industrial and academic settings slowly crystallisinghas opened the race for development of responsive, “4D printing” inks suiting these tech-niques. This hunt often starts from a perusal of Nature, where responsive materials haveevolved overmillions of years. As listed inChapter 1, examples of responsivity canbe foundacross many different species, and these have fruitfully served as inspiration for researchers.

A particularly promising material for developing printable stimuli-responsive materi-als are cholesteric liquid crystals (ChLCs). To date, ChLCs have been used for the gen-eration of structurally coloured materials demonstrating a visible response to (chemical)analytes (1–3), water (3–5), and mechanical forces (6–9). What is lacking in the workto date is the fourth dimension referred to in “4D printing”—a stimulus response afterprinting of the computer-designed object. As it currently stands, there are multiple meth-ods to print structurally coloured designs with direct ink writing: with colloidal photoniccrystals (10, 11), block copolymers (12), hydrogels containing photonic arrays (13), and asshown in Chapter 4, cholesteric LC oligomers (14). However, 4D printing of structurallycoloured, responsive inks has so far gone underreported.

In this Chapter, this deficiency of structurally coloured 4D prints is approached bythe direct ink writing of a water-responsive cholesteric liquid crystal elastomer. First, syn-thesis of a responsive ChLC oligomer ink based on an amine-acrylate “aza-Michael” addi-tion reaction is performed using a custom isosorbide-derived chiral dopant. Then, devicesthat demonstrate the possibilities this ink offers are highlighted in two naturally-inspireddesigns. First, a fully 3D printed, beetle-inspired object combining a static support withthe dynamic ChLCE as final printed layer. Secondly, a photonic, scallop-inspired actua-tor with three-dimensional control over the appearance was designed and fully 4D printedusing the responsive, cholesteric oligomer ink.


5.2.1 Synthesis and direct ink writing of the water-responsive cholestericink

The water-sensitive cholesteric ink in this work is synthesised through a straightforwardamine-acrylate “aza-Michael” addition reaction. The diacrylates used are commerciallyavailable reactive mesogens 1 and 2, custom synthesised reactive chiral dopant 3, and di-amine 4, which is similar to a compound used before to include water-responsivity in anLCE (see Experimental and Figure 5.1a for details) (15).

Reactive mesogens 1 and 2 are chosen since their combination leads to oligomers

Direct Ink Writing of 4D Structural Colour | 81

with a nematic mesophase. Contrary to common usage (7–9, 16, 17), the chiral dopant“Paliocolor® LC 756” ((3R,3aR,6S,6aR)-hexahydrofuro[3,2-b]furan-3,6-diyl bis(4-((4-((-(4-(acryloyloxy)butoxy)carbonyl)oxy)benzoyl)oxy)benzoate)), is not used in this chain ex-tension reaction. LC 756 contains carbonate groups as part of the molecule (18), groupsthat have been shown to be labile during the thiol-acrylate Michael addition during inksynthesis, thereby influencing the final ink (14).

To achieve water-response in the final printed objects, diamine 4 (N,N -dimethyl-1,3-propyldiamine), is used as chain extension agent (15). The primary amine group inthis compound connects two acrylate groups during chain extension, while keeping the

Reactive chiral dopant

O

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Diaminechain extender

Reaction mixture ChLC oligomer ink1. THF, 60 °C,

Ar, reflux

2. bulk, 100 °C, + antioxidant 5

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writing, Tbed, Tsyringe,15 minutes wait

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reactive chiral dopant 3

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reactive mesogen 1

9 8 7 6 5 4 3 2 1Chemical shift (ppm)

mesogen phenyl

chiral dopant phenyl

acrylate

PI

Free radicalphoto-initiator

P

O

OO

Figure 5.1: a) Components used for synthesizing the cholesteric liquid crystal (ChLC)oligomer ink—left-to-right: reactive mesogens 1 and 2, reactive chiral dopant 3, di-amine chain extender 4, and free radical photo-initiator PI. b) Schematic drawing ofthe molecular composition of the ChLC mix before the chain extension reaction, afteroligomerization, and after acrylate crosslinking. Also given are the reaction conditionsfor both steps. c) ¹H-NMR spectrum and d) GPC data for the ChLC oligomer ink. In the¹H-NMR spectrum, “mesogen phenyl” and “chiral dopant phenyl” refer to the protonson theorthopositions of the4-hydroxybenzoic acidmoieties present in thediacrylates.


dimethylamine group free for functionalisation after formation of the oligomers and poly-mer network. Reactions between carbonates and primary amines have also been widelyreported (19, 20); using chiral dopant LC 756 with in an amine-acrylate addition reactionprocedure would almost certainly lead to undesired side products.

Technically speaking, LC 756 could be added after oligomerization to keep the car-bonate groups intact; however, this would increase crosslink density and likely impair thestimulus-response in the polymerised cholesteric network. For this reason, the alternativereactive chiral dopant 3, ((3R,3aR,6S,6aR)-hexahydrofuro[3,2-b]furan-3,6-diyl bis(4-((6--(acryloyloxy)hexyl)oxy)benzoate)), is synthesised from precursors 3a and 3b as describedin Experimental. This compound does not contain the labile carbonate groups and shouldthus lead to a better cholesteric ink.

As the chain extension procedure is effectively an addition polymerisation, the aver-age oligomer length achieved is dependent on the stoichiometric ratio chosen betweenthe diacrylate and primary amine reactants (21). Since formation of the cholesteric me-sophase after deposition is eased by having short oligomers, the molar ratio diacrylate-to-chain extender is set at 1.5, which should result in an average “mesogenic oligomer length”xLC = 3 (22). The oligomerization reaction proceeds over two consecutive days: the re-action mixture in tetrahydrofuran is first refluxed overnight in argon atmosphere at 60 °C,after which 0.05 % w/w (to reactants) of antioxidant 5 (2,6-di-tert-butyl-4-methylphenol)is added to the reaction mixture. Subsequently, the temperature of the reaction mixtureis increased to 100 °C and the flask is left open to air to facilitate removal of the reactionsolvent. During the second night, the reaction proceeds at high temperature in bulk (seeFigure 5.1b).

From proton nuclear magnetic resonance (1H-NMR) spectroscopy, the average chainlength of the resulting oligomers can be found by inspecting the ratio of a specific protonon the mesogen core versus protons that are part of acrylate groups (see Figure 5.1c) (23–25). From this calculation follows xLC = 2.7 (Mn ≈ 2025 g mol−1), indicating the for-mation of short oligomers as planned. The signals observed in NMR do not indicate theformation of side products, as seen before inChapters 3 and 4. Gel permeation chromatog-raphy shows that a significant contributor to the oligomer ink are monomeric mesogens,as expected. The cholesteric-isotropic transition temperature of the ChLC oligomer ink,TNI, is gauged with differential scanning calorimetry (DSC), which showed a transitionsignal in cooling at 59 °C.

Afterwards, 2 %w/w bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide (PI) is addedas free radical photo-initiator, which instigates acrylate crosslinking at command. Theink, now ready for direct ink writing, is transferred to a stainless steel DIW syringe. Asshown previously, for successful printing of a ChLC oligomer ink, the process tempera-tures Tbed and Tsyringe, and the lateral nozzle speed vnozzle are important parameters (seeFigure 5.2a,b) (14). Typically, the bed temperature is below TNI, while the temperature inthe ink reservoir is slightly above it.


a) b)

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Figure 5.2: a,b) Schematic renders showing the key direct ink writing process parame-ters Tbₑd, Tₛyriₙgₑ, vₙₒzzₗₑ, and the photo-induced crosslinking occurring on the print-ing bed after allowing the LC ink to self-align. c) UV-vis reflectance spectrum for theturquoise ChLC oligomer ink cured after 15 minutes dwell time at different Tbₑd tem-peratures.

During temperature optimization, it became apparent the ink required time to self-assemble into a proper cholesteric phase after deposition. The printed ink is initiallystrongly scattering, while after 15 minutes at elevated bed temperature (Tbed = 39 °C),the formation of a reflection peak around λmax = 515 nm is observed. Setting the bedto temperatures higher or lower leads to increased scattering of shorter wavelength light(Figure 5.2c). After optimization, this resulted in Tbed = 39 °C and Tsyringe = 75 °C.

After deposition, the inks are crosslinked, first with a λ = 405 nm LED attached tothe direct ink writing machine (≈ 120 seconds), and subsequently using a high-power UVlight source in N2 atmosphere (900 seconds irradiation each from both top and bottom).Gel fraction values of 78%± 6% for the crosslinked elastomerwere determined inCH2Cl2over 48 hours.

To achieve responsivity to humidity by the cholesterics, the pendant amine groups intheChLCEwere “activated” using an aqueous hydrochloric acid solution (Figure 5.3a). Inthis acid-base reaction, the hydrochloric acid protonates the dimethylamino groups, form-ing a hygroscopic ammonium group in close proximity to the polymer backbone (15). At-tenuated total reflection Fourier-transform infrared (ATR FT-IR) spectroscopy confirmsthat during the acid treatment, tertiary amine groups (ν̃ ≈ 2750, 2850 cm−1) are convertedinto tertiary ammonium groups (ν̃ ≈ 2940 cm−1, see Figure 5.3b) (15). The broad bandformed around ν̃ ≈ 3400 cm−1 can be attributed to H2O absorbed from the atmosphere,as the hygroscopic ChLCE readily attracts moisture into its polar interior.

Following the protonation with reflectance UV-vis spectroscopy, it was found thattreating the material from one side leads to a reflection band shift completed in no morethan 3 hours (see Figure 5.3c). Interestingly, during the acid treatment, the photonic re-flection band does not temporarily broaden, but rather an independent second, red shifted


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pswollen

Figure 5.3: a) Schematic drawing of the ChLCE before and after acid treatment, high-lighting the chemical modification that takes place, and the concurrent change in cho-lesteric pitch length. NB: the depiction of the cholesteric pitch structure is schematic.b) ATR FT-IR measurement for the ChLC oligomer ink, the crosslinked ChLCE afterdirect ink writing, and the maximally-protonated, acid-treated ChLCE. c) UV-vis re-flectance spectra at different acid treatment duration intervals. All samples are indeionised water during the measurement. d) Free-standing ChLCE polymer films sep-arated from the glass DIW substrate showing the visible colour change upon proto-nation and subsequent exposure to water. The scale bar represents 1 cm. e) UV-visreflectance scans showing the ChLCE fixed on glass in different states exposed to left-(LCP) and right-circular polarised light (RCP): in the crosslinked state after direct inkwriting (top), and in the dried (middle) and swollen (bottom) states after acid treat-ment.


band is formed (λmax ≈ 680 nm, see Figure 5.3a), the prominence of which increases withtime at the expense of the original reflection band. This suggests that the protonation ofthe ChLCE does not occur homogeneously throughout the material, but rather as an ad-vancing front. This can be explained knowing that the unprotonated material is apolar,and as such must be first protonated before it can swell in water and displace the acidicfront. Assuming the refractive index of the material remains constant, we can estimate themaximum degree of swelling from the change in λmax. At (680 nm/515 nm) − 1 = 32%,the water uptake is comparable to photonic hydrogels based on tightly crosslinked ChLCnetworks (26) and slightly higher than other humidity-sensitiveChLCE formulations (16).

The change in reflection band was monitored for a range of HCl concentrations (1mm–0.1 m). While the material takes up water at all concentrations, the water uptakeof a 100 µm thick film in 0.1 m HCl happens much more rapidly (< 6 hours) than in10 mm HCl, where the reflection band shift proceeds over the duration of roughly twoweeks. The opposite process, deprotonation of a protonated film in deionisedwater, is alsoslow. After 14 days, the peak reflected wavelength when swollen had slightly shortened toλmax ≈ 650 from680nm, and after another 28 days, to λmax ≈ 610nm. Contrasting to theprotonation during activation, this gradual deactivation seemed to occur homogeneouslythroughout the material as indicated by the shift of λmax, rather than the formation of anindependent, second peak during activation seen previously. The gradual loss of protona-tion in deionised water corroborates with what is likely a low pKa of the ChLCE, i.e. ≪7.

While the amine groups pendant from the ChLCE backbone are in theory basic, theneed for a strongly acidic solution to hydrophilise thematerial is not unexpected. The sameeffect is seen in cholesteric liquid crystal networks with carboxylic acid groups that requirestrongly alkaline solutions to initiate swellingwithwater (26). The need of a strongly acidicsolution (pH = 1) in the case of the amine-functionalised crosslinked cholesteric elastomercan be described in terms of the hydrophobic interactions originating from the mesogenicblocks in the polymer network, as well as many positively charged that species are formedin close proximity. This disfavours protonation, thus requiring a lower pH for activation.

Functionalization of the glass substratewith a polymeric sacrificial layer allows for sepa-ration of the crosslinkedChLCE from the glass after direct inkwriting and polymerisation.This is demonstrated in Figure 5.3d, which shows the cholesteric rubber before and afteracid treatment, showing a clearly, visually appraisable colour change. Initially, the filmis a turquoise colour, which changes to red after the acid treatment. Drying the film re-verts the position of the selective reflection band: there is a noticeable discrepancy betweenthe colour immediately after crosslinking and that in the dry state after acid treatment; atλmax,treated ≈ 555 nm, a remaining pitch length increase of ≈8 % is calculated, see Figure5.3e. Another observation from Figure 5.2e is that the ink does not discern strongly be-tween left- and right-circular polarised light. It is likely that this is rooted in the formationof a slanted cholesteric alignment, which is also recognised visually when the samples are


observed at an angle, as is known from Chapter 4.

5.2.2 Using the water-responsive ink in multi-material 3D printing

To fabricate 4D materials that change colour, a static 3D printed static object was coatedby DIW of the cholesteric ink. Through a conventional FFF 3D printing procedure, blackCPE filament was used to print an object roughly resemblant of the humidity-responsiveTmesisternus isabellae beetle (27) (see Figure 5.4a,b). Since the thermoplastic supports areheated during printing of the cholesteric top layer, thermal characteristics of the FFF fila-ment are not trivial—the commonly used poly(lactic acid) (PLA) has itsTg at≈ 55 °C (28).For this reason, a copolyester (CPE) filament with favourable thermal properties (Tg at≈ 100 °C) was chosen. Temperatures set for the print bedwere re-calibrated to account forthe addition of the CPE slab in between the bed and the extrusion nozzle.

To facilitate direct ink writing on top of the CPE beetle, the top and bottom surfacesof the printed beetle were designed to be flat. On the bottom, this ensures maximum heattransfer from the print bed to the beetle, and for DIW on top, this eases generation ofthe g-code needed for printing the cholesteric. After printing of the plastic beetles, theywere functionalised with surface-boundmethacrylate groups to covalently bond to the de-

d) 6 RH%T = 20 °C 96 RH% 13 RH%

a) b) c)

Tbed = 45 °C

dry wet

Figure 5.4: a) Photographs of a Tmesisternus isabellae beetle demonstrating the colourchange in the elytra resulting from exposure to water. Adapted with permission fromref. (27). Copyright 2009 The Optical Society. b) FFF 3D printed beetle shape usedas base for the artificial water-responsive beetle. The scale bar represents 1 cm. c)With DIW 3D printing, a layer of the water-responsive ChLC oligomer ink is printedon top of the plastic beetle. d) Series of photographs showing a colony of 3D printedwater-responsive beetles at increasing, and then decreasing, relative humidity. Thephotographs were taken at a constant temperature of 20 °C.


positedChLCE—forgoing this stepwould result in delamination of the photonicmaterialafter acid treatment. In our experience, subjecting the co-polyester beetle to a UV/O3 pro-cedure does not negatively impact the applicability in further processing steps.

Using a 3D printed “mask”, the beetles were aligned on the print bed of the DIW 3Dprinter. A poly(vinyl alcohol) glue stick was used to fix the plastic objects in place, afterwhich the print bed was heated to Tbed = 45 °C (see Figure 5.4c). Direct ink writingof the cholesteric oligomer ink to form the elytra was done at the same settings (Tsyringe,vnozzle) as listed previously (see Video S1). After extrusion, thematerial was left at increasedtemperature Tbed for 15 minutes, after which it was photopolymerised with the printer’sLED and subsequently with a high-power UV light source in a nitrogen atmosphere.

Activation of the ChLCE was done by submerging the “elytra” in 0.1 m hydrochloricacid overnight to ensure maximum protonation of the dimethylamino moieties. This wasclearly observed as a shift of the reflected colour to intense red (see Figure 5.4d). Dryingthe photonic beetles in ambient air (≈ 30 RH%) reverts the colour to green. The hygro-scopic nature of the ChLCE was clearly seen during experiments where the relative hu-midity in the surrounding air is increased at constant temperature. Upon humidificationof the atmosphere, the ChLCE quickly changes colour in response, in analogy with itsnatural counterpart (27). Replacing the humid atmosphere with dry N2 demonstrates re-versibility of the material’s photonic response to water. Video S2 shows two full cycles ofhumidification and drying.

5.2.3 Scallop-inspired, single-ink, 4Dprinted structurally coloured actuator

To highlight the full capabilities of the ChLC oligomer ink, an actuator was designedaround the shape of a typical scallop (Pecten jacobaeus), chosen for its recognisable shapeas seen in Figure 5.5a. In the print design, two shells are connected by a hinge that willbe made water-responsive after printing, allowing folding of the device in half (see Fig-ure 5.5b) (15, 29). In addition to imparting a colour change in water, the ink also allowsfor fine control over the appearance resulting from the selected printing conditions. Asknown from Figure 5.2b, Tbed strongly influences the scattering present in the ChLCE,which can be exploited as an aesthetic choice. Therefore, the scallop demonstrator wasdesigned as a bilayer, with the first 100 µm thick layer printed at Tbed = 45 °C and thesecond 90 µm thick layer at Tbed = 39 °C (Figure 5.5c). Indeed, this leads to a clear differ-ence in the two sides as seen in Figure 5.5d—a scattering exterior, and a slanted choleste-ric, anisotropic iridescent interior. The slanted cholesteric mesophase leads to a stronglyperspective-dependent appearance on the interior, whichmirrors the direction of the printpath (see Video S3).

After UV-induced crosslinking, the ChLCE object was removed from the glass sub-strate by dissolution of the polyvinylpyrrolidone sacrificial layer. To initialise the objectfor actuation, the exterior side of the hinge was selectively treated with 0.1 m hydrochloricacid for 3 hours to protonate halfway through the ChLCE. In an enclosed environment,


shellhinge

outer layer Tbed = 45 °C

inner layer Tbed = 39 °C

c)

14 RH% 95 RH% 30 RH%T = 22 °C

backside treated with aqueous acid

e)

water

dry

print direction

inner outer

b)

d)

differentialswelling

a)

Figure 5.5: a) Photograph of a Pecten jacobaeus and how it fits into the demonstratordesign. Reproduced under the terms of the Creative Commons CC BY-SA 4.0 licencefrom commons.wikimedia.org, copyright 2002 Andreas Tille. b) Schematic draw-ing showing the intendedmotion upon triggering the actuator. c) Scheme showing thebilayer design of the object. The first layer is printed at Tbₑd = 45 °C, the second layerat Tbₑd = 39 °C. d) Photographs showing the “inner” and “outer” layers of the photonicactuator (illuminated from overhead). e) Demonstration of the actuation at isothermalconditionswith increasing, and thendecreasing, relative humidity (14 to95 to30RH%).The backside of the clam’s hinge is treated with acid to induce the differential swellingneeded for bending.

the photonic actuator shows reversible actuation in response to atmospheric humidity. Ab-sorption ofwater into the protonated region of the hinge leads to expansion, which in turnorchestrates an out-of-plane bending deformation, closing the scallop and obscuring theiridescent interior. Similar to the colour change in the printed Tmesisternus isabellae bee-


tles, the deformation is fully reversible, as upon the introduction of a dry N2 atmosphere,the scallop undergoes the opposite deformation and opens.

5.3 Conclusions

The two demonstrators shown here, as ink in a multi-material printing procedure and asink for photonic, humidity-responsive actuators with multi-coloured appearance, high-light the versatility of water-responsive cholesteric LC oligomer inks. A diacrylate chiraldopant was synthesised to be compatible with the “aza-Michael” reaction conditions, anddirect ink write 3D printing was successfully used in conjunction with this ink. After de-position, the ink organises into a visually impactful cholesteric alignment. The crosslinkedelastomer can be activatedwith an aqueous acid, leading to a large bathochromic shift in re-flection upon exposure to water, an omnipresent environmental stimulus. This photonic,stimuli-responsive ink opens up future avenues for the design of 3Dprinted optical sensorsand actuators, which, by carefully programming the print speed and modulating the bedtemperature during printing, can exhibit a variety of perspective-dependent appearancesin a single object, based on a single ink.

5.4 Experimental

Materials Diacrylate mesogens 1 (“RM82”, 1,4-di(4-(6-acryloyloxyhexyloxy)benzoyloxy)-2-methylbenzene) and 2 (“RM257”, 1,4-di(4-(3-acryloyloxypropyloxy)benzoyloxy)-2-methylben-zene) were purchased from Daken Chemical Ltd. (Zhengzhou, P. R. China), chiral diacrylate3 ((3R,3aR,6S,6aR)-hexahydrofuro[3,2-b]furan-3,6-diyl bis(4-((6-(acryloyloxy)hexyl)oxy)ben-zoate)) was synthesised from 4-(6-acryloyloxyhexyloxy)benzoic acid (3a), purchased fromAmbeedInc. (ArlingtonHeights, U. S. A.), and (3R,3aR,6S,6aR)-hexahydrofuro[3,2-b]furan-3,6-diol (3b),purchased from TCI Europe N.V. (Zwijndrecht, Belgium). Amine 4 (N,N -dimethylpropane-1,3-diamine) was obtained from Huntsman. Photoinitiator PI (phenylbis(2,4,6-trimethylben-zoyl)phosphine oxide) was obtained from BASF SE (Ludwigshafen, Germany). N,N ′-dicyclo-hexylmethanediimine (DCC), N,N -dimethylpyridin-4-amine (DMAP), and 2,6-di-tert-butyl-4-methylphenol (5, BHT), 3-(trimethoxysilyl)propyl methacrylate, and polyvinylpyrrolidone(MW 10,000 g mol−1) were purchased from Sigma-Aldrich (Darmstadt, Germany). Ethanoland 2-propanol were purchased from Avantor (Radnor, U. S. A.). Silica used during columnchromatography (60 Å, 40-63 µm) was obtained from Screening Devices B.V. (Amersfoort, TheNetherlands). Ethyl acetate (AR grade), n-heptane (AR grade), and dichloromethane (AR grade)were purchased from Biosolve B.V. (Valkenswaard, The Netherlands). Sand (“extra pure”) used forsample photography was purchased from Fisher Scientific.

Preparation of glass printing substrates Direct inkwritingwas done on glass substrates preparedfor either easy sample release or covalent bonding to the sample. In both cases, the glass was firstcleanedbyultrasonication (1:1 v/v 2-propanol–ethanol, 25minutes; Branson2510), followedby ac-tivation in a UV/O3 oven (30minutes; UV Products PR-100). With a spin coater (Karl Suss RC6),one of two solutions was cast on the glass: for easy sample release, 10 % w/w polyvinylpyrrolidonein ethanol; for covalent bonding to the substrate, 1 v/v % 3-(trimethoxysilyl)propyl methacrylate


in 1:1 v/v 2-propanol–H2O. For both sample types, the spin coater was set to 1500 rpm, 500 rpms−1 acceleration, 45 s spin time. After spin coating, the treated substrates were post-treated at 100°C for 10minutes. Substrate glasses were either 76× 52mm2 (PaulMarienfeld GmbH&Co. KG)or 50 × 20mm2 (Epredia).

Preparation of the thermoplastic beetle support structure Similar to the design of shapes fortheChLColigomer ink direct inkwriting procedure, a shape resembling the Tmesisternus isabellaewas drawn in Adobe Illustrator CC, exported to Blender, and saved as .stl file. Ultimaker Curawas used to prepare the .gcode file for the Ultimaker 3 FFF 3D printer. The filament used was“Ultimaker CPE+ Black”, which was extruded with the CPE+ preset in the slicing software, ata layer thickness of 0.06 mm. Afterwards, the polymer slabs were placed in a UV/O3 oven (20minutes), after which 3-(trimethoxysilyl)propylmethacrylate (1% v/v, in 1:1 v/v 2-propanol–H2O)was drop cast on the material. Subsequently, this was reacted for 20 minutes on a 90 °C hot plate,after which unreacted methacrylation agent was removed by cleaning the surface of the CPE slabswith ethanol.

Synthesis of chiral diacrylate 3 Esterification reactions were carried out under Steglich con-ditions (30). (3R,3aR,6S,6aR)-hexahydrofuro[3,2-b]furan-3,6-diol was first dried in vacuo overNaOH pellets. To a 250 mL flask was added 3.067 g (20.98 mmol) (3R,3aR,6S,6aR)-hexahydro-furo[3,2-b]furan-3,6-diol, 12.283 g (42.02 mmol) 4-(6-acryloyloxyhexyloxy)benzoic acid, 0.521 g(4.26 mmol) N,N -dimethylpyridin-4-amine, and all were dissolved in 125 mL dichloromethane.The flask was lowered into an ice bath, and after cooling to 0 °C, 9.095 g (44.08 mmol)N,N ′-dicy-clohexylmethanediimine was added to the mixture. The reaction was left overnight, after which avacuumfiltration through a silica plugwas used to get rid of solids formedduring the reaction. Finalpurification of 3 was done using column chromatography over silica with n-heptane/ethyl acetate(2:1 v/v) as eluent, where the second solids fraction contained 3, 3.09 g (4.76 mmol), in a yield of21.2 %. 1H-NMR (400 MHz, Chloroform-d): δ = 8.07 − 7.97 (m, 2H), 8.00 − 7.91 (m, 2H),6.96 − 6.84 (m, 4H), 6.40 (dd, J = 17.4, 1.5Hz, 2H), 6.12 (dd, J = 17.3, 10.4Hz, 2H), 5.82 (dd,J = 10.4, 1.5Hz, 2H), 5.46 (d, J = 3.2Hz, 1H), 5.39 (q, J = 5.5Hz, 1H), 5.04 (t, J = 5.0Hz, 1H),4.67 (d, J = 4.6Hz, 1H),4.17 (td, J = 6.6Hz, 1.3), 4.17−4.06 (m, 2H),4.05 (ddd, J = 17.6, 6.7, 3.8Hz, 5H), 4.00 (d, J = 4.1Hz, 1H), 1.83 (qt, J = 6.3, 3.1Hz, 4H), 1.72 (dq, J = 13.7, 6.8, 6.2Hz,4H), 1.56− 1.40 (m, 6H). FT-IR (ATR, bulk; ν̃, cm−1): 2933, 2868, 2855, 1712, 1635, 1606, 1580,1510, 1463, 1415, 1405, 1387, 1350, 1298, 1253, 1203, 1167, 1123, 1084, 1062, 980, 906, 847,810, 764, 695, 655. Melting point (DSC; Tm, °C): in heating 40.5 °C; in cooling 19.7 °C (–1 °Cmin−1), 8.3 °C (–5 °C min−1), 5.2 °C (–10 °C min−1).

Synthesis of humidity responsive cholesteric oligomer inks Autocatalysed “aza-Michael”amine-acrylate reaction between reactive mesogens 1 and 2, chiral diacrylate 3, and amine 4 wereemployed. To ensure an ink that aligns swiftly after direct ink writing, the targeted average meso-genic chain length is 3, which required amolar ratio diacrylate-to-amine of 1.5 (22). For synthesis ofthe ChLC oligomer ink, the following quantities were used: 2.65 g (3.95 mmol) of 1, 2.32 g (3.93mmol) of 2, 0.48 g (0.67 mmol) of 3, 0.58 g (5.72 mmol) of 4. In this formulation, 3 constitutes≈8.5 wt% of the diacrylate fraction. First, all diacrylates (1, 2, 3) were weighed into a three-necked100mL flask, dissolved in≈25mL tetrahydrofuran assisted by a magnetic stirrer bar, and heated to60 °C using an oil bath. After installing a reflux column and replacing the atmosphere with argon,amine 4 pre-mixed in ≈1 mL tetrahydrofuran was added to the mixture. The reaction continuedovernight under continuous agitation. The next day, an analysis sample was taken from the reac-tion mixture, after which ≈3 mg (≈0.05 wt% to reactants) 5 was added from a pre-mix in ≈1 mL


tetrahydrofuran. Temperature of the oil bath was increased to 100 °C, and one stop was removedfrom the flask to facilitate solvent evaporation. At this point the oligomerisation reaction proceedsin bulk conditions and was left to run overnight. The next day, the oil bath was removed and thereaction mix re-dissolved in ≈10 mL tetrahydrofuran. After dissolution, an analysis sample wastaken from the reactionmixture, after which≈120mg (≈2wt% to reactants) of PIwas added froma pre-mix of ≈1 mL tetrahydrofuran. The solution is quantitatively transferred into a PTFE evap-oration dish and first dried overnight at room temperature in a fume cupboard, and finally driedat vacuum, 85 °C, for 2 hours. After the ink is rid of solvent, it is transferred into a stainless steelsyringe for direct ink writing. 1H-NMR (400 MHz, Chloroform-d): δ = 8.15 (ddt, J = 10.8, 9.2,1.7 Hz), 8.02 (d, J = 8.5Hz), 7.95 (d, J = 8.5Hz), 7.71 (d, J = 6.8Hz), 7.17 (d, J = 8.6Hz), 7.12(d, J = 2.6Hz), 7.08 (dd, J = 8.7, 2.7Hz), 7.02 − 6.85 (m), 6.42 (ddd, J = 17.3, 7.9, 1.5Hz), 6.13(ddd, J = 17.3, 10.4, 4.9Hz), 5.84 (td, J = 10.6, 1.5Hz), 5.45 (s), 5.38 (d, J = 5.3Hz), 5.03 (s), 4.66(d, J = 4.7Hz), 4.39 (t, J = 6.2Hz), 4.29 (td, J = 6.3, 2.0Hz), 4.22 − 3.97 (m), 2.82 − 2.73 (m),2.45 (tq, J = 7.2, 4.2Hz), 2.24 (s), 2.20 (t, J = 2.6Hz), 2.17 (q, J = 6.4, 5.4Hz), 1.83 (t, J = 6.9Hz), 1.77− 1.70 (m), 1.70− 1.64 (m), 1.64− 1.54 (m), 1.54− 1.40 (m). FT-IR (ATR, bulk; ν̃, cm−1):2937, 2857, 2815, 2765, 1718, 1604, 1579, 1509, 1492, 1465, 1405, 1249, 1162, 1062, 1006, 983,896, 844, 809, 763, 692. Melting point (DSC;Tm, °C): in heating 63.9 °C (1 °Cmin−1), 65.9 °C (5°C min−1), 67.5 °C (10 °C min−1); in cooling 62.2 °C (–1 °C min−1), 60.2 °C (–5 °C min−1), 59.1°C (–10 °C min−1).

Direct ink writing of the water-responsive cholesteric liquid crystal oligomer inks A HyrelEHR direct ink writer with TAM-15 high-operating temperature syringe extrusion head was usedfor ChLC oligomer ink writing. 0.335 mm I.D. (27 ga A.W.G.) micronozzles (Fisnar QuantXMicron-S Red) were fitted to the stainless steel ink syringe using a Luer-Lock adapter. Tempera-tures during printing were dependent on the ink being printed. Since the temperature reportedfor the printing bed does not match the printing substrate due to various heat losses, the print bedtemperature was calibrated using a separate digital thermocouple. For the ChLC oligomer ink, weusedTbed = 39 °C andTsyringe = 70 °C.Objectswere photo-crosslinked for 2minutes after printingwith a printer-attachedUVATA405 nmLED (set to 10mWcm−2), followed by exposure to a highintensity UV light source (Excelitas EXFO Omnicure S2000, ≈10-15 mW cm−2) for 15 min onboth sides of the print, in aN2 atmosphere. Layer spacing for the direct ink written patterns was setanywhere between 50–100 µm with line spacing 250–335 µm. Preparation of print path .gcodefiles was done in two ways: if the print pattern consisted mainly of simple lines, such as printedstrips, theywerewrittenmanually in g-code. Formore complex shapes, such as thePecten jacobaeus-inspired clam shell and the Tmesisternus isabellae beetle, a combination of software tools was used.First, a vector file was prepared in Adobe Illustrator CC, and saved as an .svg (Scalable VectorGraphics) file. This file was imported into Blender (www.blender.org), given a thickness (“solid-ify” modifier) and exported as a .stl (stereolithography) file. Finally, this .stl file was openedusing Slic3r 1.3.0 (www.slic3r.org), where a .gcode file was made according to chosen directink writing settings (speed, layer height, line spacing, infill settings). When printing on 4mm thickpolyester slabs, the bed temperature Tbed was set to 53 °C for the ChLC oligomer ink. CPE slabswere fixed to the glass printing bed using poly(vinyl alcohol) glue (Staples glue stick, product no.39610). Alignment of theTmesisternus isabellae supports was done using amask that was designedin Blender and printed with the Ultimaker 3 FFF 3D printer; through the openings in the plasticmask, markings were made on the glass underneath using a permanent marker.

Gel fraction measurements of the ChLCE Small strips of material (25 mm × 3 mm × 100 µm,


minitial ≈ 10 mg) were submerged in CH2Cl2 for 48 hours. After this time, the material was re-trieved from the solvent, dried in vacuo, and weighed. The gel fraction was calculated according towgel = mextracted/minitial.

Fourier-transform infrared characterization AVarian 670-IR with Golden Gate ATRmodule(wavenumber resolution set to 4 cm−1) was used to record infrared absorbance data.

Optical characterization of the ChLCE Prior to optical characterization, sample strips of ma-terial (25 × 3 mm2, thickness d variable) were printed at different printing conditions. Transmit-tance and reflectance measurements were performed using a PerkinElmer Lambda 750 with 150mm integrating sphere detector. Angle-dependent UV-vis transmittance measurements were per-formed with the same spectrophotometer, with an OMT Solutions ARTA detector module. Insitu spectrometry during acid activation was performed by placing sample fixed to methacrylate-functionalised glass in a temporary glass cell on the sample table of a Leica DM 6000 M opticalmicroscope, to which anOceanOptics HR2000+ spectrophotometer was attached (exposure time10 ms, spectra averaged over 128 scans). Photographs of the samples were made with an OlympusOM-DE-M10Mark III (withM.Zuiko 60mm f /2.8macro lens) inmanualmode, files recorded in.orf raw file format. Images were developed in Adobe LightroomClassic CC, where slight clarityand exposure adjustments were made. Sample lighting was provided by a halogen light source.



Video files supporting this experimental chapter are available free of charge at4TU.ResearchData: https://doi.org/10.4121/19181744.

Video S1. Printing process of the humidity-responsive beetle elytra. Playbackrate: 50×.

Video S2. Twocycles of dry-humid-dry air ex-posure on the artificial, humidity-responsive beetles. Data also visu-alised in Figure 5 and Figure S12.Playback rate: 375×.

Video S3. Printing process of the 4D scallopactuator. Playback rate: 50×.

Video S4. Actuation cycle of the 4D scallopactuator, showing response to dry-humid-dry air. Data also visualisedin Figure 6. Playback rate: 125×.


Bibliography


[2] E. P. A. van Heeswijk, A. J. J. Kragt, N. Grossiord,A. P. H. J. Schenning, Environmentally responsivephotonic polymers, Chemical Communications 55,2880–2891 (2019).

[3] M.Moirangthem, A. P. H. J. Schenning, Liquid Crys-tal Sensors, A. P. H. J. Schenning, G. P. Crawford,D. J. Broer, eds. (CRC Press, Boca Raton, FL, 2017),chap. 4, pp. 83–102.

[4] S. J. D. Lugger, S. J. A. Houben, Y. Foelen,M. G. Debije, A. P. H. J. Schenning, D. J. Mulder,Hydrogen-Bonded Supramolecular Liquid CrystalPolymers: Smart Materials with Stimuli-Responsive,Self-Healing, andRecyclable Properties,Chemical Re-views p. acs.chemrev.1c00330 (2021).

[5] B. Liu, T. Yang, X. Mu, Z. Mai, H. Li, Y. Wang,G. Zhou, Smart Supramolecular Self-AssembledNanosystem: Stimulus-Responsive Hydrogen-Bonded Liquid Crystals, Nanomaterials 11, 448(2021).

[6] H. Finkelmann, S. T. Kim, A. Muñoz, P. Palffy-Muhoray, B. Taheri, Tunable Mirrorless Lasing inCholesteric Liquid Crystalline Elastomers, AdvancedMaterials 13, 1069–1072 (2001).

[7] P. Zhang, G. Zhou, L. T. de Haan, A. P. H. J. Schen-ning, 4D Chiral Photonic Actuators with SwitchableHyper‐Reflectivity, Advanced Functional Materials31, 2007887 (2021).


[9] R. Kizhakidathazhath, Y. Geng, V. S. R. Jam-pani, C. Charni, A. Sharma, J. P. F. Lagerwall,Facile Anisotropic Deswelling Method for Realiz-ing Large‐Area Cholesteric LiquidCrystal Elastomerswith Uniform Structural Color and Broad‐RangeMechanochromic Response, Advanced FunctionalMaterials 30, 1909537 (2020).

[10] F. Fu, L. Shang, Z. Chen, Y. Yu, Y. Zhao, Bioinspiredliving structural color hydrogels, Science Robotics 3,eaar8580 (2018).

[11] A.T.L.Tan, S.Nagelberg, E.Chang‐Davidson, J.Tan,J. K. W. Yang, M. Kolle, A. J. Hart, In‐Plane Di-rect‐Write Assembly of Iridescent Colloidal Crystals,Small 16, 1905519 (2020).


[13] J. Chen, L. Xu, M. Yang, X. Chen, X. Chen,W. Hong, Highly Stretchable Photonic Crystal Hy-drogels for a Sensitive Mechanochromic Sensor andDirect InkWriting,Chemistry ofMaterials 31, 8918–8926 (2019).


[15] K. Kim, Y. Guo, J. Bae, S. Choi, H. Y. Song, S. Park,K.Hyun, S.-K. Ahn, 4DPrinting ofHygroscopic Liq-uid Crystal Elastomer Actuators, Small 17, 2100910(2021).

[16] X. Shi, Z. Deng, P. Zhang, Y. Wang, G. Zhou,L. T. de Haan, Wearable Optical Sensing of Strainand Humidity: A Patterned Dual‐ResponsiveSemi‐Interpenetrating Network of a CholestericMain‐Chain Polymer and a Poly(ampholyte),Advanced FunctionalMaterials 31, 2104641 (2021).

[17] A. M. Martinez, M. K. McBride, T. J. White,C. N. Bowman, Reconfigurable and Spatially Pro-grammable Chameleon Skin‐Like Material UtilizingLight Responsive Covalent Adaptable CholestericLiquid Crystal Elastomers, Advanced FunctionalMa-terials 30, 2003150 (2020).


[19] H. Tomita, F. Sanda, T. Endo, Model reaction for thesynthesis of polyhydroxyurethanes fromcyclic carbon-ates with amines: Substituent effect on the reactivityand selectivity of ring-opening direction in the reac-tion of five-membered cyclic carbonates with amine,Journal of Polymer Science Part A: Polymer Chemistry39, 3678–3685 (2001).


[20] J. R. Caldwell, W. J. Jackson, Surface treatment ofpolycarbonate films with amines, Journal of PolymerScience Part C: Polymer Symposia 24, 15–23 (1968).

[21] T. H. Ware, Z. P. Perry, C. M. Middleton, S. T. Ia-cono, T. J. White, Programmable Liquid Crystal Elas-tomers Prepared by Thiol–Ene Photopolymerization,ACSMacro Letters 4, 942–946 (2015).



[24] A. H. Gelebart, M. K. McBride, A. P. H. J. Schen-ning, C. N. Bowman, D. J. Broer, PhotoresponsiveFiber Array: Toward Mimicking the Collective Mo-tion of Cilia for Transport Applications, AdvancedFunctionalMaterials 26, 5322–5327 (2016).

[25] L. Liu, M. del Pozo, F. Mohseninejad, M. G. De-bije, D. J. Broer, A. P. H. J. Schenning, Light Track-ing and Light Guiding Fiber Arrays by Adjusting

the Location of Photoresponsive Azobenzene in Liq-uid Crystal Networks, Advanced Optical Materials 8,2000732 (2020).


[27] F. Liu, B. Q. Dong, X. H. Liu, Y. M. Zheng, J. Zi,Structural color change in longhorn beetles Tmesister-nus isabellae,Optics Express 17, 16183 (2009).

[28] D.Garlotta, ALiteratureReviewofPoly(LacticAcid),Journal of Polymers and the Environment 9, 63–84(2001).

[29] L. T. de Haan, J. M. N. Verjans, D. J. Broer, C. W. M.Bastiaansen, A. P. H. J. Schenning, Humidity-Responsive Liquid Crystalline Polymer Actuatorswith an Asymmetry in the Molecular Trigger ThatBend, Fold, and Curl, Journal of the American Chem-ical Society 136, 10585–10588 (2014).

[30] B. Neises, W. Steglich, Simple Method for the Esteri-fication of Carboxylic Acids, Angewandte Chemie In-ternational Edition in English 17, 522–524 (1978).

Chapter 6The Future of 4D Printed Cholesterics—Where, and How?

This chapter is reproduced from: M. del Pozo+, J. A. H. P. Sol+, A. P. H. J. Schenning, M. G. Debije, Advanced Materials 34, 2104390 (2022), and: J. A. H. P. Sol, A. P. H. J. Schenning, M. G. Debije, Proceedings of SPIE 12023, Emerging Liquid Crystal Technologies XVII (2022), and: J. A. H. P. Sol, R. F. Douma, A. P. H. J. Schenning, M. G. Debije, manuscript in preparation.

Now that responsive cholesterics can be printed, what applications will they be used in, and how? This final, concluding Chapter, puts the results described in this thesis into perspective. What is still needed to take these materials and the associated direct ink writing method out of university laboratories and into the wider world? What functionalities are desirable, and how could these be implemented? And finally: a brief look at the state of cholesteric liquid crystal-based “smart” material research now and what is needed for it to prosper in the future.


6.1 A brief recap of the work in this thesis

Reading the experimental Chapters 2 to 5 in their presented order reveals a story of thedevelopment of a humidity-responsive cholesteric liquid crystal (ChLC) material for ad-ditive manufacturing—one of the many possible “stimulus responsitivities” available toliquid crystallinematerials. In Chapter 2, a path was shown to bring responsive cholestericto fused filament fabrication, one of themost common additivemanufacturing techniquesaround. Another common technique, ultrasonication,was used to breakup a larger filmofa responsive cholesteric LCs, into tiny, micrometre-sized flakes. These flakes retained thestimulus-responsiveness of the parent polymer film, showing a large and optically apprecia-ble colour shift upon submersion in water or in the presence of humid air. Unfortunately,the need forwater to physically infiltrate the cholesteric polymer flakes to trigger the colourshift also proved to be the Achilles’s heel—as understandably, most fused filament fabrica-tion (FFF) printing filaments are engineered to be non-permeable and stable towards waterexposure.

To circumvent the problem introduced by the host polymer—impermeability towater—a cholesteric liquid crystal material itself capable of being 3D printed was devel-oped in Chapters 3 and 4. A one-pot synthetic procedure was used to tune the viscoelasticproperties of the liquid crystal, resulting in a structurally coloured ink with the capabilityof being bar coated and direct inkwritten. Thematerial shows a bright reflection colour af-ter processing, and subsequently can be photopolymerised into either coatings or releasedfrom their substrates to form free-standing objects.

An intriguing finding was that cholesteric oligomer inks, such as those used in Chap-ters 3 and 4, can be readily aligned in a slanted cholesteric phase. Compared to “regular”planar cholesteric alignment, the slanted alignments reflect the longest reflectedwavelengthat an offset from the surface normal direction. A loss of circular polarisation selectivity isalso characteristic for slanted cholesterics.

Based on the inks and procedures developed for the DIW of cholesterics, a humidity-responsive structurally coloured ink for additive manufacturing was designed and demon-strated in Chapter 5. As is common to cholesterics, the reflected colour could be easilytuned by changing the ratio of chiral dopant to reactive nematic liquid crystal. A turquoise-reflecting inkwas synthesisedwhich after crosslinking and acid treatment, manifests a largecolour shift when submerged inwater. The inks versatility was shown by fashioning it intoa free-standing, humidity-responsive photonic actuator, as well as a DIW-printed surfacecoating on a FFF 3D printed free-form substrate, opening the door for use of this respon-sive ink inmulti-material printingprocedures, which conventionally use all-static feedstockmaterials.

The Future of 4D Printing Liquid Crystals | 99

6.2 Increasing the applicability of cholesteric 3D prints

6.2.1 Responsive cholesterics for FFF

Compared to direct ink writing, fused filament fabrication boasts a couple of advantages,mainly in the commercialisation of this technique seeming to be slightly ahead, and as aresult, a more comfortable user experience. Secondly, the filament being in the solid stateallows for easier storage compared to the viscoelastic resins used in DIW.

65 °C535125 31 39 43 46 49

a) b)

c)

Figure 6.1: a) Thermo-responsive cholesteric liquid crystal with a polydimethylsilox-ane top coating. Adapted under the terms of the Creative Commons CC BY-NC-NDlicence from ref. (1), copyright 2021 American Chemical Society. b) One-way shapememory and colour change in a FFF 3D printed object consisting of a thermoplasticpolymer doped with thermochromic pigment particles. Reproduced with permissionfrom ref. (2), copyright 2019 Wiley-VCH. c) Particles of a thermochromic cholestericenclosed in an alginate shell. Reproducedwith permission from ref. (3), copyright 2021Wiley-VCH.

So, while the cholesteric flakes presented in Chapter 2 were not able to be used to theirfullest given the loss of responsivity in the poly(lactic acid) (PLA) filament, there are othermethods that might lead to stimulus-responsive filament: shown previously in the litera-ture was a PLA filament compounded with temperature-responsive particles (2), based onthermochromic organic pigments (4) (see Figure 6.1b). An advantage of using choleste-rics over these pigments is their brighter perceived colour and stability. There are severaldifferent thermochromic cholesteric formulations (see Figure 6.1a) (5), which need to beadapted into particles that can be compounded with appropriate polymers to form FFFprinting filaments. As an example, one such thermochromic ChLC encapsulated in algi-nate gel shells was recently reported (see Figure 6.1c) (3)—although for compounding anFFF filament, the particles would need to be more mechanically robust. Alternatively, UVdosimeters based on a cholesteric LC (6) could conceivably be fabricated in a similar fash-ion. Stimulus-responses to chemicals are more difficult to realise, as the polymer matrix,similar to our experience with the water-responsive flakes, might act as barrier and hinderinteraction with the cholesteric sensor.


6.2.2 Direct ink writing of cholesterics for intricate photonic coatings

The roles as originally defined in Chapters 3 and 4 can also be reversed: instead of devel-oping a cholesteric oligomer ink for direct ink writing, the direct ink writing process is en-gineered to achieve specific optical properties from the cholesteric oligomer ink. Chapter4 demonstrates this, as the printing speed determines the (extent of) formation of a slan-ted cholesteric, and the print path direction dictates the direction in which the cholestericalignment is slanted.

A technique often used for application of ChLCs is photolithography, as described inChapter 3. Initial selective crosslinkingof theChLCs throughamask followedby crosslink-ing the remaining material in a non-reflective state has been the basis of many patterned(multi-coloured) reflectors (7–13). Using DIW to deposit ChLC oligomer ink in specificpre-programmed slant directions before photolithography, allows for coatings with highlycustomisable appearance, as demonstrated in Figure 6.2a.

During the first crosslinking step, slanted cholesteric alignment is locally fixed as a poly-mer network (see Figure 6.2b). After sufficientUV exposure through themask, the sampleis heated to above TNI, and the entire unmasked film is subjected to a flood UV exposurecrosslinking the isotropic regions (see Figure 6.2c). By direct writing the cholesteric ink ina back-and-forth pattern, an “inverse” cholesteric appearance can be created in the planeparallel to the print direction. This effect relies on two underlying principles: (1) by print-ing into a slanted alignment, λmax is seen at an offset from the normal direction, and (2)the colour at λmax is conceived more strongly than other reflected colours. Generating g-code with alternative slicing patterns can yield a variety of optical effects, as seen in thedemonstrators in Figure 6.2d,e, which show imprinted patterns with highly perspective-dependent appearances.

Besides the purely decorative potential of these patterned photonic coatings, a pos-sible commercialisation route is as anti-counterfeit markers, an application that is oftensuggested for ChLC polymer films (9, 14–18). The slanted alignment that can be createdwithDIW, and the unique circular polarisation interactions it imparts provide an extra toolin the design of unique, difficult-to-replicate optical films. Before this would be possible,however, the optical clarity of the cholesteric oligomer ink needs to be enhanced.

As exemplified in reports of cholesterics printed using DIW, it is clear the imperfectmesogen alignment results in a milky, scattering appearance (8). When the printed LCEis to be used in optical processes, such as light-triggered actuation (19, 20) or as reflectiveelements (8), light scattering can be detrimental to the LCE’s function, preventing pre-cise focusing of light for actuation, or a reduced quality reflector. This is exacerbated ifthicker layers are printed, or multiple layers are printed on top of each other. Ideally, itshould be possible to print cholesterics with a homogeneous, optically clear appearance,mostly through tuning the deposition procedure. Another option that might improvetransparency is to include a reactive surfactant in the printing ink to assist LC assemblyat the LC–air interfaces, such as the commonly used 2-(N -ethylperfluorooctanesulfon-


Tink = 90 °C

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Figure 6.2: Scheme detailing the procedure for the two-step direct ink writing–photolithography process: a) direct ink writing deposits the cholesteric ink on a sub-strate, b) initial crosslinking is accomplished through a printed photomask at low UVintensity, for short time, c) the bed temperature is increased beyond TNI, and the en-tire coating is irradiated with high-intensity UV to crosslink the isotropic regions. d,e)Photographs of the patterned, perspective-dependent photonic coatings.

amido)ethyl methacrylate (13, 18), although as a result of the oligomer ink’s increased vis-cosity, it might mean that for proper diffusion of the surfactant and coherent cholestericalignment formation, wait times longer than the 15 minutes after printing suggested inChapter 5 are needed. Even then, it is unlikely that complete objects will be manufacturedout a cholesteric ink, since only a thin layer of a fewmicrometres is needed to reachmaximalreflection. Therefore, it is better to focus on the cholesteric inks as something to print astop layers, or for thin devices.


A final point concerning most LC 3D printing reported in the literature to date: thereproducibility of the oligomer ink synthesis. As prepared in this thesis, the inks are com-monly synthesised in a one-pot procedure where all reactants are added at the start (19–26). While the average degree of polymerisation is controlled by the stoichiometry betweendiacrylate and chain extension agent, this still leaves a lot of room for variation in chainlength distribution, and thus, variation in thermal and rheological properties of the result-ing ink. For future upscaling endeavours, such batch-to-batch variationwould surely limitapplicability. In addition, the influence of chain length dispersity on the helical twistingpower should be investigated—as it’s clear that increasing the degree of polymerisation hasa strong depressing effect on the helical twisting power (7). To combat reproducibility is-sues in future research, more attention should be given to extensive characterisation of thesynthesised inks, as well as standardisation of synthetic procedures in terms of duration,temperatures, reactants, solvents, and catalysts.

6.2.3 Cholesterics in direct ink written soft actuators

Through addition of diacrylate azobenzene derivatives to LC oligomer inks duringoligomer synthesis, light-responsive printed actuators have been fabricated, as highlightedin Chapter 1. These actuators can be upgraded with a light-reflecting feature through amulti-material direct ink writing procedure. Printing both a cholesteric LC oligomer inkand an azobenzene-functionalised LColigomer ink on a single, static support foil and thenirradiating the photo-sensitive stripes causes bending of the assembly; photo-switchablelight transmission is the result (see Figure 6.3a) (26).

However, such a versatile device can also be reimagined to be based on a single inkrather than relying on multiple inks for the same effect, see Figure 6.3b for a representa-tive composition. Adding both azobenzene and a chiral dopant to the reactive mesogensfor oligomerisation creates an ink that is both structurally coloured and responsive to UVirradiation. Perhaps contrary to expectation: it was found that triggering the trans–cis iso-merisationdoes not lead to anoticeable shift inλmax, as the formationof a polymernetworkfixes pitch length p sufficiently to be independent of azobenzene photoisomerisation.

Rather, the azobenzene-containing cholesteric LCE can be used for photoinducedbending. In Figure 6.3c, this is demonstrated for a cholesteric oligomer ink containingthe diacrylate azobenzene derivative commonly used in light-responsive LCEs (20, 25, 26).The oligomer ink’s increased viscosity compared to low-molecular weight LC mixtures al-lows for greater control over the molecular alignment obtained after direct ink writing;from Chapter 4 it is known altering print speeds make the difference between depositingplanar-aligned or slanted cholesterics. Intriguingly, the change in molecular alignmentalso leads to a significant change in actuation behaviour. Reducing the molecular order(through increasing temperature, or photoswitch isomerisation) of cholesterics generallyleads to contraction perpendicular to the helical director (x, y) and concurrent expansionalong it (z) (27). When on a static, flexible substrate, contraction along a planar direction


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λ = 532 nm laser

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Figure 6.3: a) A bilayer device consisting of a static glassy foil onto which a light-responsive azobenzene-containing LCE is printed, along with a photonic ChLCE.Adapted under the termsof theCreativeCommonsCCBY-NC-ND licence from ref. (26),copyright 2021 American Chemical Society. b) Molecular ingredients for a light-responsive, light-reflective cholesteric LC oligomer ink. c) Comparison of UV-inducedbending of planar and slanted aligned strips of the azobenzene-functionalised chole-steric LCE.

leads to bending of the bilayer, with the active layer “on the inside of the bend” (26).Contrastingly, slanted cholesteric alignments have an offset helical director, and this

is manifested in their photo- and thermomechanical behaviour. Irradiating the slantedChLCE-polyetherimide bilayer leads to bending with the ChLCE on the outside of thebend, indicating a length increase in (one of the) planar directions. Control over bendingdirection had previously been achieved with the aforementioned azobenzene-containingLCE (20, 26), but in that system, the bending direction is dependent on the temperatureduring actuation. In the ChLCE, planar or slanted alignment is created and fixed duringdevice creation and dictates the actuation pathway irrespective of the temperature while inuse.

A further demonstration of the unique versatility of theseChLColigomer inks is given


by polymerising the oligomer ink immediately after extrusion from the nozzle. Immediatepolymerisation fixes the ink in a uniaxial nematicmesophase with alignment (order param-eter S = 0.3) comparable to literature-reported figures (22). This originally cholesteric inkcould function as contracting artificial muscles in devices designed to be independent ofstatic support layers; a single ink could be used for optical as well as actuating elements,depending on the printing and polymerisation conditions.

6.2.4 The use of direct ink written cholesterics in biological settings

One of the promises of soft, responsive materials is their proposed application in biologi-cal environments because they would not pose a mechanical threat to soft, biological tis-sues (28). Liquid crystalline materials are seen as promising candidates for tissue engineer-ing given their anisotropic molecular alignment and stimuli-responsiveness (29).

Cholesterics can be applied in tissue engineering bymaking use of their dynamic prop-erties in cell scaffolds. An example based on an azobenzene-functionalised cholesteric net-work demonstrated that UV exposure through a mask caused localised deformations asthe trans-cis photoisomerisation leads to expansion in the helix direction (30). Cells seededon this coating deformed to accommodate the dynamic three-dimensional surface reliefs.Since azobenzene photoisomerisation induces significantly larger deformations in LCEsthan it does in LCNs (31), designing similar coatingswithChLCEswill likely lead to largersurface features, and possibly a different cellular response. Another advantage of choleste-ric LCEs is that this thesis has proven their processability with direct ink writing; mean-ing this type of cholesteric, along with their proposed capability to form dynamic surfacetopographies, can be 3D printed into other additively manufactured tissue engineeringscaffolds to handle these light-addressable scaffold-cell interactions.

However—before such tissue engineering or implants can be used, research is neededinto the biocompatibility of theLC inks. Reactivemesogens are not native to biological sys-tems, and could thus pose a threat to organisms after implantation. Fromwhat is reported,it is difficult to provide a conclusive statement over the biocompatibility of LC-basedmate-rials. The few results published do not paint a worrying picture, and mention low degreesof cytotoxicity for LCNs (30, 32–35) and LCEs (36, 37). Another question that goes inhand with investigating the toxicity of polymeric LC materials is the longevity of thesematerials in biological environments. Research has shown that thiol-acrylate based LCEsare sensitive to oxidative damage and which impacts their mechanical properties, althoughno release of chemicals as a result of this damage was mentioned (38).

6.3 Securing a sustainable future for DIW with cholesterics

Recyclability becomes increasingly important as the volumes of LC materials increase tomeet the future demands of producing “smart” devices. Actually, it can be argued that


“smart” devices need “smart” materials, and that materials are not truly “smart” until theycome at no (or very little) burden to the environment.

Most current polymeric LCs are thermosets which are difficult to recycle and eventu-ally end up in landfills after use, or are mechanically ground down into low-quality fillermaterials (39). In the development of sustainable LC materials, there are four options, inascending order considering their expected impact: (1) development of self-healing (dy-namic covalent) crosslinked LCmaterials, (2) LCmesogens that can be degraded into well-defined, reusable species. Finally, and most importantly, (3) the use of biomaterials as cho-lesterics, and adjacent to this, (4) design of liquid crystalline materials based on renewable,biobased sources.

One of the foremost solutions (“option 1”) is to develop thermosets with reversiblecrosslinks, which would allow for reconfiguration of the material into new shapes, or com-plete melting for recycling. At the same time, many chemistries that allow for reconfig-urability also inherently bring (self-)healing capabilities, which would extend the usefullifetime of the materials. Dynamic covalent chemistries, which can be employed for thispurpose, have been briefly highlighted in Chapter 1.

Developing thermoplastic LCEs could be the middle road here, where short chains ofLC oligomers crosslink physically. Examples are polyureas or polythiourethanes based onliquid crystallinemonomers (40, 41). Thesematerials can be synthesised by sequential one-pot reactions, and can be processed relatively easily into polymer materials that show largedimensional changes with temperature increase. Since during the polymer synthesis thereactivemesogens are first transformed into LC oligomers, functional groups known fromLCNandLCE researchmaybe incorporated into the chain, while the physical crosslinkingallows for remoulding, reshaping, and eventually, recycling. When the device survives thetask for which it was intended, it can be reprocessed for a new function.

Nevertheless, self-healing materials deteriorate over time. At that point, breakdownof the material into useable molecular fragments is desirable (“option 2”). This requiresthe design of newmesogens with labile chemical groups on strategic locations in the mole-cule. Non-mesogenic formulations have been developed for 2PP-DLW 3D printing thatmake use of labile groups which disintegrate in alkaline conditions (42), or by enzymaticmeans (43).

Securing a sustainable future for the materials presented in this thesis also requires re-viewing methods of synthesising mesogens from renewable sources instead of from thepetrochemicals that many organic compounds are currently derived from (“option 3”).This can be interpreted in multiple ways, one being to consider alternative natural ma-terials that show LC properties. An example of such natural materials are aqueous dis-persions of cellulose nanocrystals which display cholesteric phases at specific concentra-tions (44, 45), and as highlighted in Figure 6.4a,b,c, have been used for variety of coloured,stimuli-responsive materials. Nanocellulose crystals have also been used as dopants in hy-drogels, to induce anisotropic deformation upon hydration (46); so while bound to aque-


ous environments, it is a possible direction for fully biobased actuators if a suitable biobasedhydrogel matrix is chosen.

Another way towards sustainable cholesterics is by considering renewable, bio-basedmonomer sources (50). It should be noted that while a renewable feedstock is better thanone based on oil or natural gas—petrochemicals will run out at some point—renewablemonomers alone are not the ideal solution, as the nurturing and harvesting of the renew-able source still requires energy, and the creation of new material also creates new wastein the end. For this reason, it is interesting to consider existing waste streams as source ofmesogen building blocks.

Lignin, a waste product from wood processing, can be broken down into phenoliccompounds, which could possibly be used to synthesise the aromatic cores commonly seenin reactivemesogens. Example in point: the lignin obtained fromPopulus trichocarpa treescontains pendant 4-hydroxybenzoate groups (51), an essential part of the reactivemesogendisplayed in Figure 6.4d. Developing a synthetic pathway to cleave this moiety from thebiopolymer would be a great step towards a petrochemical-free future for reactive meso-gens. The C6 spacer present in “RM82” and can furthermore be derived from 1,6-hex-anediol, which can be prepared from d-fructose—in its turn attainable from cellulose—through a couple of intermediates (52). The central ring of the mesogen in question,the 2-methyl-1,4-benzenediol moiety, was found (in small quantity) among a plethora ofcompounds during the hydrothermal breakdown of corn biomass (53). Additionally, theblossoming field of coaxing microbes into performing industrial chemical conversions hasshown that glycerol can be used for the synthesis of acrylic acid (54).

In an ideal world, these options should be developed and used in concert. Currentfunctional materials based on reactive LCs cannot be translated into biopolymer LCsovernight, and neither is a process to make the current reactive mesogens circular devel-oped quickly; to bridge the gap, the synthesis of currentmesogens from renewable orwastestream sources should be swiftly investigated. Then, these could be used to design recy-clable and self-healing functionalmaterials, such as those based on polyurethane chemistry,or containing dynamic covalent crosslinks. Finally, in themore distant future, it should bepossible for functional polymers based on (cholesteric) LCs to be processable circularly,or even be biodegradable (on-demand) into harmless chemicals, although this will requiresignificantly more research on how to guarantee similar liquid crystalline properties (di-electric, optical, thermomechanical) from these newmaterials, as we demand from reactivemesogens currently.

6.4 Recap: what 4D printed cholesterics can add to the future

3D printing techniques—those with static feed materials—are rapidly undergoing adop-tion in academia and industry alike (55, 56). An example that demonstrates the presenceofAM in daily life is its use in dentistry—by some already consideredmainstreamfive yearsago (57). Recent developments in this field have shown the possibility for precise deposi-


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Figure 6.4: a) Optical mechanochromic stress sensor based on a glucose-celluloseelastomer. The scale bar represents 1 cm. Reproduced with permission from ref. (47),copyright 2020 Wiley-VCH. b) Microscopy image of a pressure-sensitive foil based onan aqueous cholesteric cellulose suspension. Adapted under the terms of the CreativeCommons CC BY licence from ref. (48), copyright 2018 Springer Nature. c) Humidity-sensitive optical sensor based on a cellulose doped with sodium chloride. Adaptedunder the terms of the Creative Commons CC BY licence from ref. (49), copyright 2019Wiley-VCH. d) An overview of possible bio-based sourcing methods for the synthesisof the popular nematic reactive mesogen “RM82”.


tion of multiple materials in single printed objects (58, 59). The next step is the additionof “smart”, stimulus-responsive materials to the feedstock library of AM techniques. De-signing autonomous, inherent responses to the environment is not simply an academicexercise; for imagining a future where “micro-bots” carry out functions in medicine (60),harvesters remove pollutants from the ocean (61), and larger soft assemblies operate in oron living beings (62), it is imperative to rethink the materials, and the requirements forsuch materials. For instance, currently, robots rely on electrical control circuits, the de-sign of which gets increasingly complicated at smaller sizes, or when the device should bestrongly deformable.

This raises the question: how can structurally coloured materials add to the develop-ment of relevant devices? Throughout “smart” materials research, light is often named asprime means of remote, “tetherless” control. This creates the need for control over lightfor the design of advanced devices. Light might need to be redirected to different areas ofthe machine, or the reflection of light or the lack thereof serve as indicator for the presenceof some predetermined condition—presence of a chemical or a temperature change—thatsignify what environment encloses the robot. Using our imagination, we envision softdevices aiding in surgeries, and can immediately, report optically on the condition of theaffected site.

Of course, this is just conjecture, but as highlighted earlier, cholesterics have a pedigreeas battery-free sensors for temperature (5), electromagnetic radiation (6), and identifyingchemical species (14, 63). Quantifying these phenomena can yield useful biometric orenvironmental data; hinting at imminent hypo- or hyperthermia (64), the onset of sun-burn (65), or indicating chemical imbalances in the body (66).

This thesis shows a path towards the 4D printing of devices that demonstrate both re-versible colour and shape changes, based on a responsive, cholesteric liquid crystal oligomerink. We envision that this material will serve as starting point for many responsive, struc-turally coloured 4D printing inks of the future, to be used in many different applications:sensing labels and light management systems to fully additively manufactured self-sensing,photonic soft robots.


Bibliography

[1] W. Zhang, A. P. H. J. Schenning, A. J. J. Kragt,G. Zhou, L. T. de Haan, Reversible ThermochromicPhotonic Coatings with a Protective Topcoat, ACSApplied Materials & Interfaces 13, 3153–3160(2021).

[2] J. Wang, Z. Wang, Z. Song, L. Ren, Q. Liu, L. Ren,Biomimetic Shape–Color Double‐Responsive4D Printing, Advanced Materials Technologies 4,1900293 (2019).

[3] J. Kim, Y. Oh, S. Lee, S.-h. Kim, ThermochromicMicrocapsulesContainingChiralMesogens Enclosedby Hydrogel Shell for Colorimetric Temperature Re-porters, Advanced Functional Materials 32, 2107275(2022).

[4] A. Seeboth, R. Ruhmann, O. Mühling, Ther-motropic and Thermochromic Polymer BasedMaterials for Adaptive Solar Control, Materials 3,5143–5168 (2010).

[5] W. Zhang, A. A. F. Froyen, A. P. H. J. Schenning,G. Zhou, M. G. Debije, L. T. de Haan, Tempera-ture‐Responsive PhotonicDevices Based onCholeste-ric Liquid Crystals, Advanced Photonics Research 2,2100016 (2021).

[6] P. Cachelin, H. Khandewal, M. G. Debije, T. Peijs,C. W. M. Bastiaansen, Optical UV Dosimeters Basedon Photoracemization of (R)‐(+)‐1,1′‐Bi(2‐Napthol)(BINOL)within a Chiral Nematic Liquid CrystallineMatrix, ChemistrySelect 6, 5266–5270 (2021).



[9] A. J. J. Kragt, D. C. Hoekstra, S. Stallinga, D. J. Broer,A. P.H. J. Schenning, 3DHelix Engineering inChiralPhotonicMaterials,AdvancedMaterials31, 1903120(2019).

[10] W.-L. Hsu, J. Ma, G. Myhre, K. Balakrishnan, S. Pau,Patterned cholesteric liquid crystal polymer film, Jour-nal of the Optical Society of America A 30, 252–258(2013).

[11] D. C. Hoekstra, B. P. A. C. van der Lubbe, T. Bus,L. Yang, N. Grossiord, M. G. Debije, A. P. H. J.Schenning, Wavelength‐Selective Photopolymeriza-tion of Hybrid Acrylate‐Oxetane Liquid Crystals,Angewandte Chemie International Edition 60,10935–10941 (2021).

[12] D. C. Hoekstra, K. Nickmans, J. Lub, M. G. Debije,A. P. H. J. Schenning, Air-Curable, High-ResolutionPatternable Oxetane-Based Liquid Crystalline Pho-tonic Films via Flexographic Printing, ACS AppliedMaterials & Interfaces 11, 7423–7430 (2019).

[13] A. J. J. Kragt, N. C. M. Zuurbier, D. J. Broer,A. P. H. J. Schenning, Temperature-Responsive,Multicolor-Changing Photonic Polymers, ACSApplied Materials & Interfaces 11, 28172–28179(2019).



[16] E. P. A. van Heeswijk, A. J. J. Kragt, N. Grossiord,A. P. H. J. Schenning, Environmentally responsivephotonic polymers, Chemical Communications 55,2880–2891 (2019).

[17] D. C. Hoekstra, A. P. H. J. Schenning, M. G. De-bije, Epoxide and oxetane based liquid crystals for ad-vanced functional materials, Soft Matter 16, 5106–5119 (2020).

[18] C. Sánchez, F. Verbakel, M. J. Escuti, C. W. M. Basti-aansen, D. J. Broer, Printing ofMonolithic PolymericMicrostructures Using Reactive Mesogens, AdvancedMaterials 20, 74–78 (2008).

[19] L. Ceamanos, Z. Kahveci, M. López-Valdeolivas,D. Liu, D. J. Broer, C. Sánchez-Somolinos, Four-Dimensional Printed Liquid Crystalline ElastomerActuators with Fast Photoinduced Mechanical Re-sponse toward Light-DrivenRobotic Functions,ACSApplied Materials & Interfaces 12, 44195–44204(2020).






[24] A. Kotikian, C. McMahan, E. C. Davidson, J. M.Muhammad, R.D.Weeks, C.Daraio, J. A. Lewis, Un-tethered soft robotic matter with passive control ofshape morphing and propulsion, Science Robotics 4,eaax7044 (2019).

[25] L. Liu, M. del Pozo, F. Mohseninejad, M. G. De-bije, D. J. Broer, A. P. H. J. Schenning, Light Track-ing and Light Guiding Fiber Arrays by Adjustingthe Location of Photoresponsive Azobenzene in Liq-uid Crystal Networks, Advanced Optical Materials 8,2000732 (2020).

[26] M. del Pozo, J. A. H. P. Sol, S. H. P. van Uden, A. R.Peeketi, S. J. D. Lugger, R. K. Annabattula, A. P. H. J.Schenning, M. G. Debije, Patterned Actuators via Di-rect InkWriting of Liquid Crystals,ACS AppliedMa-terials & Interfaces 13, 59381–59391 (2021).

[27] D. J. Broer, G. N. Mol, Anisotropic thermal expan-sion of densely cross-linked oriented polymer net-works, Polymer Engineering and Science 31, 625–631(1991).

[28] M. Sitti, Miniature soft robots — road to the clinic,Nature ReviewsMaterials 3, 74–75 (2018).

[29] A. M. Lowe, N. L. Abbott, Liquid Crystalline Mate-rials for Biological Applications,Chemistry ofMateri-als 24, 746–758 (2012).

[30] G.Koçer, J. ter Schiphorst,M.Hendrikx,H.G.Kassa,P. Leclère, A. P. H. J. Schenning, P. Jonkheijm, Light-Responsive Hierarchically Structured Liquid CrystalPolymer Networks for Harnessing Cell Adhesion andMigration, AdvancedMaterials 29, 1606407 (2017).

[31] M.Pilz daCunha,M.G.Debije, A. P.H. J. Schenning,Bioinspired light-driven soft robots based on liquidcrystal polymers, Chemical Society Reviews 49, 6568–6578 (2020).

[32] D.Martella, P. Paoli, J.M. Pioner, L. Sacconi, R. Cop-pini, L. Santini, M. Lulli, E. Cerbai, D. S. Wiersma,C. Poggesi, C. Ferrantini, C. Parmeggiani, LiquidCrystalline Networks toward Regenerative Medicineand Tissue Repair, Small 13, 1702677 (2017).

[33] C. Ferrantini, J. M. Pioner, D. Martella, R. Coppini,N. Piroddi, P. Paoli, M. Calamai, F. S. Pavone, D. S.Wiersma, C. Tesi, E. Cerbai, C. Poggesi, L. Sacconi,C. Parmeggiani, Development of Light-ResponsiveLiquid Crystalline Elastomers to Assist Cardiac Con-traction, Circulation Research 124, e44–e54 (2019).

[34] G. Babakhanova, J. Krieger, B. Li, T. Turiv, M. Kim,O. D. Lavrentovich, Cell alignment by smectic liquidcrystal elastomer coatings with nanogrooves, Journalof Biomedical Materials Research Part A 108, 1223–1230 (2020).

[35] T. Turiv, J. Krieger, G. Babakhanova, H. Yu, S. V.Shiyanovskii, Q.-H. Wei, M.-H. Kim, O. D. Lavren-tovich, Topology control of human fibroblast cellsmonolayer by liquid crystal elastomer, Science Ad-vances 6, 1–11 (2020).

[36] C. M. Yakacki, M. Saed, D. P. Nair, T. Gong, S. M.Reed, C. N. Bowman, Tailorable and programmableliquid-crystalline elastomers using a two-stage thiol–acrylate reaction, RSC Advances 5, 18997–19001(2015).

[37] R. K. Shaha, D. R. Merkel, M. P. Anderson, E. J. De-vereaux, R. R. Patel, A. H. Torbati, N. Willett, C. M.Yakacki, C. P. Frick, Biocompatible liquid-crystal elas-tomers mimic the intervertebral disc, Journal of theMechanical Behavior of Biomedical Materials 107,103757 (2020).

[38] M. Javed, S. Tasmim, M. K. Abdelrahman, C. P. Am-bulo, T. H. Ware, Degradation-Induced Actuationin Oxidation-Responsive Liquid Crystal Elastomers,Crystals 10, 420 (2020).

[39] W. Post, A. Susa, R. Blaauw, K. Molenveld, R. J. I.Knoop, A Review on the Potential and LimitationsofRecyclableThermosets for Structural Applications,Polymer Reviews 60, 359–388 (2020).

[40] J. Lee, J. Bae, J.H.Hwang,M.Choi, Y. S.Kim, S. Park,J. Na, D. Kim, S. Ahn, Robust and Reprocessable Ar-tificial Muscles Based on Liquid Crystal ElastomerswithDynamic Thiourea Bonds,Advanced FunctionalMaterials p. 2110360 (2021).


[41] S. J. D. Lugger, D. J. Mulder, A. P. H. J. Schenning,One‐Pot Synthesis of Melt‐Processable Supramolecu-lar Soft Actuators,Angewandte Chemie InternationalEdition 61, e202115166 (2022).

[42] D. Gräfe, A. Wickberg, M. M. Zieger, M. Wegener,E. Blasco, C. Barner-Kowollik, Adding chemically se-lective subtraction tomulti-material 3D additiveman-ufacturing,Nature Communications 9, 2788 (2018).

[43] D. Gräfe, M. Gernhardt, J. Ren, E. Blasco, M. We-gener, M. A. Woodruff, C. Barner‐Kowollik,Enzyme‐Degradable 3D Multi‐Material Microstruc-tures, Advanced Functional Materials 31, 2006998(2021).

[44] Y. Liu, P. Wu, Bioinspired Hierarchical Liq-uid‐Metacrystal Fibers for Chiral Optics andAdvanced Textiles, Advanced Functional Materials30, 2002193 (2020).

[45] G. Guidetti, H. Sun, A. Ivanova, B. Marelli,B. Frka‐Petesic, Co‐Assembly of Cellulose Nanocrys-tals and Silk Fibroin into Photonic Cholesteric Films,Advanced Sustainable Systems 5, 2000272 (2021).


[47] C. E. Boott, A.Tran,W. Y.Hamad,M. J.MacLachlan,CelluloseNanocrystal ElastomerswithReversibleVis-ibleColor,AngewandteChemie International Edition59, 226–231 (2020).

[48] H.-l. Liang, M. M. Bay, R. Vadrucci, C. H. Barty-King, J. Peng, J. J. Baumberg,M. F. L.DeVolder, S.Vi-gnolini, Roll-to-roll fabrication of touch-responsivecellulose photonic laminates, Nature Communica-tions 9, 4632 (2018).

[49] T. H. Zhao, R. M. Parker, C. A. Williams, K. T. P.Lim, B. Frka‐Petesic, S. Vignolini, Printing of Re-sponsive Photonic Cellulose Nanocrystal MicrofilmArrays, Advanced Functional Materials 29, 1804531(2019).

[50] D. E. Fagnani, J. L. Tami, G. Copley, M. N. Clemons,Y. D. Getzler, A. J. McNeil, 100th Anniversary ofMacromolecular Science Viewpoint: Redefining Sus-tainable Polymers, ACS Macro Letters 10, 41–53(2021).

[51] Y. Zhao, X. Yu, P.-Y. Lam, K. Zhang, Y. Tobimatsu,C.-J. Liu, Monolignol acyltransferase for lignin p-hydroxybenzoylation in Populus, Nature Plants 7,1288–1300 (2021).

[52] T. Buntara, S. Noel, P. H. Phua, I. Melián-Cabrera,J. G. de Vries, H. J. Heeres, Caprolactam fromRenewable Resources: Catalytic Conversion of5-Hydroxymethylfurfural into Caprolactone, Ange-wandte Chemie International Edition 50, 7083–7087(2011).

[53] K. Yao, Q. Wu, R. An, W. Meng, M. Ding, B. Li,Y. Yuan, Hydrothermal pretreatment for deconstruc-tion of plant cell wall: Part I. Effect on lignin-carbohydrate complex, AIChE Journal 64, 1938–1953 (2018).

[54] T. Dishisha, S.-H. Pyo, R. Hatti-Kaul, Bio-based 3-hydroxypropionic- and acrylic acid production frombiodiesel glycerol via integrated microbial and chemi-cal catalysis,Microbial Cell Factories 14, 200 (2015).

[55] S. C. Ligon, R. Liska, J. Stampfl, M. Gurr, R. Mül-haupt, Polymers for 3D Printing andCustomized Ad-ditiveManufacturing,Chemical Reviews 117, 10212–10290 (2017).

[56] R. L. Truby, J. A. Lewis, Printing soft matter in threedimensions,Nature 540, 371–378 (2016).

[57] A. Dawood, B. M. Marti, V. Sauret-Jackson, A. Dar-wood, 3Dprinting in dentistry,BritishDental Journal219, 521–529 (2015).

[58] M. A. Skylar-Scott, J. Mueller, C. W. Visser, J. A.Lewis, Voxelated soft matter via multimaterial multi-nozzle 3D printing,Nature 575, 330–335 (2019).

[59] D. J. Roach, C. M. Hamel, C. K. Dunn, M. V. John-son, X. Kuang, H. J. Qi, The m⁴ 3D printer: A multi-material multi-method additive manufacturing plat-form for future 3D printed structures,AdditiveMan-ufacturing 29, 100819 (2019).

[60] S. Palagi, P. Fischer, Bioinspired microrobots,NatureReviewsMaterials 3, 113–124 (2018).

[61] M. Pilz daCunha,H. S. Kandail, J.M. J. denToonder,A. P. H. J. Schenning, An artificial aquatic polyp thatwirelessly attracts, grasps, and releases objects,Proceed-ings of the National Academy of Sciences 117, 17571–17577 (2020).

[62] M. Li, A. Pal, A. Aghakhani, A. Pena-Francesch,M. Sitti, Soft actuators for real-world applications,Nature ReviewsMaterials (2021).

[63] H. S. Lim, J.-H. Lee, J. J. Walish, E. L. Thomas, Dy-namic Swelling of Tunable Full-Color Block Copoly-mer Photonic Gels via Counterion Exchange, ACSNano 6, 8933–8939 (2012).


[64] E. P. A. van Heeswijk, T. Meerman, J. de Heer,N. Grossiord, A. P. H. J. Schenning, Paintable Encap-sulated Body-Temperature-Responsive Photonic Re-flectors with Arbitrary Shapes, ACS Applied PolymerMaterials 1, 3407–3412 (2019).

[65] L. N. Lisetski, O. V. Vashchenko, V. D. Panikarskaya,O. T. Sidletskiy, I. P. Terenetskaya, Proceedings ofSPIE 5257, Ninth International Conference on Non-

linear Optics of Liquid and Photorefractive Crystals,G. V. Klimusheva, A. G. Iljin, S. A. Kostyukevych,eds., no. December 2003 (2003), pp. 97–101.


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Curriculum vitae

Jeroen was born on March 30, 1995, in Oss, to a biol-ogy teacher and a chemical process engineer. His youthwas largely marked by a predilection towards Pokémon,recreational swimming and gymnastics, and innumer-ous holidays spent in France. Pre-university educationwas done at Mondriaan College Oss, specialising in thenatural sciences after the third year. During these years,and for a couple after, he was part of the school’s ICT&Media team that managed computers, event photogra-phy, and the light and sound setup during school plays,graduation ceremonies, and other events. In 2012 hestarted his undergraduate studies at the Eindhoven Uni-versity of Technology’s Department of Chemical Engi-neering and Chemistry. After figuring out his way through process engineering courses,he discovered liquid crystal technology and Functional Organic Materials and Devices(SFD) in the third year of the Bachelor’s studies and hasn’t digressed since—obtaining hisBachelor’s after research on “Cholesteric liquid crystal based optical sensors” withMonaliMoirangthem and Albert Schenning, and his Master’s after working on “Colour modu-lation in luminescent solar concentrators for agricultural purposes” with Gilles Timmer-mans and Michael Debije. A bit more than four months were spent in Märsta, Sweden,working onwaterborne coatings for the automotive industry atBeckers Industrial Coatingswith Susana Torrón.

In 2018, he started his doctoral studies with Michael Debije and Albert Schenning inthe now-Stimuli-responsive FunctionalMaterials andDevices group, on the future of bothliquid crystals andmanufacturing. This project was performed in close collaboration withMarc del Pozo Puig, and over the course of four years, both their creativities could reignfree as newmaterials were developed and demonstrators were conjured up. His experiencewith photography proved to be a useful skill in capturing their science in two dimensions.During the PhDproject, Jeroen also supervised a handful of students invidually and as partof practical group courses, and sporadically gave lectures on liquid crystals. Additionally,he was SFD’s “head of IT”, bringing in his past experience in managing digital infrastruc-ture, and also served as interlocutor and bridge to the university’s IT service, taking part indepartmental IT committee meetings as well.

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List of publications

Articles related to this thesis, in order of appearance1. M. del Pozo+, J. A. H. P. Sol+, A. P. H. J. Schenning, M. G. Debije, 4D Printing of Liquid

Crystals: What’s Right for Me?, AdvancedMaterials 34, 2104390 (2022)

2. J. A. H. P. Sol, L.M. Kessels, M. del Pozo,M. G. Debije, Responsive Photonic Liquid Crys-talline Flakes Produced byUltrasonication,Advanced Photonics Research 2, 2000115 (2021)

3. J. A. H. P. Sol, H. Sentjens, L. Yang, N. Grossiord, A. P. H. J. Schenning, M. G. Debije,Anisotropic Iridescence and Polarization Patterns in a Direct Ink Written Chiral PhotonicPolymer, AdvancedMaterials 33, 2103309 (2021)

4. J. A. H. P. Sol, L. G. Smits, A. P. H. J. Schenning, M. G. Debije,manuscript in submission

5. J. A.H. P. Sol, R. F.Douma, A. P.H. J. Schenning,M.G.Debije,manuscript in preparation

+ Shared first-authorship

Conference proceedings1. J. A. H. P. Sol, A. P. H. J. Schenning, M. G. Debije, Image encoding with unconventional

appearance through direct ink writing of a cholesteric liquid crystal oligomer ink, part ofProceedings of SPIE 12023, Emerging Liquid Crystal Technologies XVII (2022)

Articles not related to this thesis, in order of publication1. J. A. H. P. Sol, G. H. Timmermans, A. J. van Breugel, A. P. H. J. Schenning, M. G. Debije,

Multistate Luminescent Solar Concentrator “Smart” Windows, Advanced Energy Materi-als 8, 1702922 (2018)

2. J. A. H. P. Sol, V. Dehm, R. Hecht, F. Würthner, A. P. H. J. Schenning, M. G. Debije,Temperature-Responsive Luminescent Solar Concentrators: Tuning Energy Transfer in aLiquid Crystalline Matrix, Angewandte Chemie 130, 1042–1045; Angewandte Chemie In-ternational Edition 57, 1030–1033 (2018)

3. J. A. H. P. Sol, A. R. Peeketi, N. Vyas, A. P. H. J. Schenning, R. K. Annabattula, M. G.Debije, Butterfly proboscis-inspired tight rolling tapered soft actuators, Chemical Commu-nications 55, 1726–1729 (2019)

116

4. K. Mehta, A. R. Peeketi, J. A. H. P. Sol, M. G. Debije, P. R. Onck, N. Vyas, R. K. Annabat-tula, Modeling of surface waves in photo-responsive viscoelastic liquid crystal thin films un-der a moving light source,Mechanics of Materials 147, 103388 (2020)

5. X. Pan, N. Grossiord, J. A. H. P. Sol, M. G. Debije, A. P. H. J. Schenning, 3D AnisotropicPolyethylene as Light‐Responsive Grippers and Surfing Divers,Advanced FunctionalMate-rials 31, 2100465 (2021)

6. M. del Pozo, J. A.H. P. Sol, S. H. P. vanUden, A. R. Peeketi, S. J. D. Lugger, R. K. Annabat-tula, A. P. H. J. Schenning, M. G. Debije, Patterned Actuators via Direct Ink Writing ofLiquid Crystals, ACS AppliedMaterials & Interfaces 13, 59381–59391 (2021)

7. E. A. P. van Heijst, S. E. T. ter Huurne, J. A. H. P. Sol, G. W. Castellanos, M. Ramezani,S. Murai, M. G. Debije, J. Gómez Rivas, Electric tuning and switching of the resonant re-sponse of nanoparticle arrays with liquid crystals, Journal of Applied Physics 131, 083101(2022)

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This thesis chronicles the development of cholesterics for “4D printing” starting at conventional material formula-tions, then the synthesis of a printable cholesteric oligomer material, ending with a water-responsive, colour changing ink for direct ink writing—and each chapter employing demonstrators to highlight the unique capabili-ties of cholesterics. The final chapter discusses how cholesteric liquid crystals can fulfil their bright, lustrous future, and their newfound role in additive manufacturing.

Cholesteric Liquid Crystals in Additive Manufacturing

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Transcript of Cholesteric Liquid Crystals in Additive Manufacturing