Induction of Molecular Organization of Oligomers by Low-Energy Electrons

Communication

634

Induction of Molecular Organization ofOligomers by Low-Energy Electronsa

Yiu-Ting Richard Lau, Lu-Tao Weng, Kai-Mo Ng, Manuel Kempf,Volker Altstadt, Jerold M. Schultz, Chi-Ming Chan*

We reveal that a beam of low-energy electrons (18 eV) can directly trigger long-rangemolecular ordering of an amorphous, semi-flexible oligomer in a few minutes without theprerequisite of pre-orientation. A strong endothermic transition was detected with a micro-thermal analyzer on the areas that had been exposed to the electron irradiation while theareas that were shielded from the irradiation by a protective mask remained amorphous asusual. This result suggests that long-range molecular ordering only developsin the area of the oligomer film underelectron irradiation. This is the first-timeeffort to use electron irradiation to con-trol the long-range ordering of an amor-phous organic thin film above the glasstransition temperature.

C.-M. Chan, Y.T. R. LauDepartment of Chemical and Biomolecular Engineering, HongKong University of Science and Technology, Clear Water Bay,Hong KongE-mail: [email protected]. T. WengMaterials Characterization and Preparation Facility, Hong KongUniversity of Science and Technology, Clear Water Bay, HongKongK. M. NgAdvanced Engineering Materials Facility, Hong Kong University ofScience and Technology, Clear Water Bay, Hong KongM. Kempf, V. AltstadtDepartment of Polymer Engineering, University of Bayreuth,Universitatsstrasse 30, D-95440 Bayreuth, GermanyJ. M. SchultzDepartment of Chemical Engineering, University of Delaware,Newark, DE 19716, USA

a : Supporting information for this article is available at the bottomof the article’s abstract page, which can be accessed from thejournal’s homepage at http://www.mrc-journal.de, or from theauthor.

Macromol. Rapid Commun. 2010, 31, 634–639

� 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Introduction

Many solutions have been developed to align and order

molecules and functional moieties of organic materials at

different length scales. These solutions have used mechan-

ical,[1] thermal,[2] chemical,[3] photochromic,[4] electrical,[5]

and magnetic[6] methods and one has utilized a nanop-

robe.[7] Other indirect methods require an anisotropic

surface as a director for post-alignment ofmolecules on the

top of a substrate. The surface layer can be highly

oriented,[8] groove-patterned[9] or structurally modified

with various orientation techniques.[10]

It isaknownfact thatpositive ionsorelectrons,whichare

destructive, especially at kinetic energyofKeV, candisplace

or even ionize atoms.[11] Under irradiation, the generation

of vacancies and point defects by primary knock-on of the

atoms can induce either crystalline-to-amorphous or

amorphous-to-crystalline phase transformation. The com-

petition between lattice damage and lattice reordering

assisted by defect formation and migration governs

the direction of the phase transformation whereby the

lattice reordering is a thermal process, i.e. it is temperature

DOI: 10.1002/marc.200900730

Induction of Molecular Organization of Oligomers by Low-Energy Electrons

dependent.[11] Normally, organic molecules undergo che-

mical degradation via chain scission, radical formation and

crosslinking under high-energy irradiation, although it is

true that the amorphous phases and the structural defects

of the crystalline phases are more susceptible to degrada-

tion.[12] Amorphization and retarded crystal growth rates

are the natural and irreversible consequences of the

exposure to high doses of irradiation.[12] One research

group[13] utilized a directional low-energy ion beam (70–

300 eV) to fabricateanisotropic carbonaceous surfaces, such

as polyimides, with statistically significant orientation

order of phenyl rings by selective destruction of the rings

with planes normal to the beam direction and not of those

with planes parallel to the beam direction; yet, the virgin

surfaces of the polyimides were eventually converted to

hydrogenated amorphous carbon layers after ion irradia-

tion. These ion-bombarded surfaces, which gave rise to

long-range liquid crystal alignmentwith preferred orienta-

tions, were structurally disordered.

Herewe describe an entirely different outcome inwhich

low-energy electron irradiation can directly trigger long-

range molecular ordering of an amorphous, semi-flexible

oligomer in a fewminutes without the prerequisite of pre-

orientation above its glass transition temperature. A strong

endothermic transitionwas detectedwith amicro-thermal

analyzer on the areas that had been exposed to the electron

irradiation while the other areas that were shielded from

electrons by a protective mask remained amorphous as

usual. This result suggests that long-range molecular

ordering only develops in the area of the oligomer film

under electron irradiation. This is afirst study,which shows

that electron irradiation can initiate long-range ordering of

amorphous oligomeric thin films.

Experimental Part

Structural Characterization of the Oligomer

The molecular structure of (HBA-C8)2-FBA-(HBA-C8)2 was con-

firmed with a nuclear magnetic resonance spectrometer (Jeol EX-

400 NMR). 1H NMR (400MHz, CDCl3) d¼7.25 (d, 4H), 7.12 (d, 16H),

6.86 (d,4H), 6.78 (d,16H),3.96 (t, 4H),3.92 (t, 16H),3.40 (t, 4H),1.85 (q,

4H), 1.76 (q, 20H), 1.63 (s, 24H), 1.45 (m, 24H), 1.38 (m, 24H). 13CNMR

(100MHz, CDCl3) d¼ 159.2, 156.8, 142.9, 131.3, 127.6, 125.2, 113.7,

67.9, 63.0, 41.6, 34.0, 32.8, 31.1, 29.3, 28.7, 28.1, 26.1.

Time-of-Flight Secondary Ion Mass Spectrometry

Static spectra and chemical images were obtained on an Ion-ToF V

spectrometer. 25-keV Biþ3 primary ions with an average pulsed

current of 0.1 pAandwith abeamsize of about 0.3mmwereused to

bombard the film surfaces. The total ion flux dosage for each

spectrum and the ion image was less than 2 � 1012 ions cm�2. The

pixel density of each ion image was 128�128.



Local Thermal Analysis (LTA)

LTA of the electron-irradiated and non-irradiated areas of a film

specimenwas performed on a TA Instruments 2990micro-thermal

analyzer. The thermal analyzer was equipped with a Thermo-

MicroscopesWollastonprobeutilizinga 5-mm-diameter platinum/

10% rhodiumwire as the sensing element. The probe temperature

was calibrated by detecting the melting of three standard

polymers: poly(oxymethylene), polyamides-6 and polyamides-66

with respect to the values taken from the conventional bulk

thermal examination using the differential scanning calorimeter

(DSC). The heating rate of the localized thermograms was

10 8C �min�1.

Results and Discussion

In the first part, we report the new finding, whereby the

low-energy electron irradiation directly enables the long-

range molecular ordering of (HBA-C8)n-FBA-(HBA-C8)n,

which is composed of three major units: aliphatic octane

(C8), bisphenol-A (HBA) and hexafluoroisopropyldiphenol

(FBA) units, with bromine as the end groups. The thermal

glass transition temperature of the (HBA-C8)2-FBA-(HBA-

C8)2 oligomerwasmeasured tobe�2 8C (at aheating rateof10 8C �min�1) using a DSC. It is inherently an amorphous

liquid that cannot crystallizequiescently in thebulkor from

the solution in the presence of a non-solvent, such as

methanol. Intriguingly, from the conformational point of

view, these (HBA-C8)n-FBA-(HBA-C8)n molecules are fairly

flexible and do not exhibit the intrinsic features of a liquid

crystal with any planar or axial mesogens acting as

precursors to rod-like alignment under an external field.

The rigid aromatic segments of the molecules are tetra-

hedrally arranged and are regarded as ‘‘kinks’’ that are

detrimental to liquid crystalline properties.[14] Our earlier

results showed that (FBA-C8)n is anamorphous polymer.[15]

The fact that (HBA-C8)1-FBA-(HBA-C8)1 and (HBA-C8)2-FBA-

(HBA-C8)2 cannot crystallize quiescently in the bulk is due

to the presence of the FBA unit as a crystal defect, although

it helps the development of long-range ordering under

electron beam irradiation.

We follow a very simple procedure to produce a pattern

on an amorphous thin film with both disordered and

orderedphasesusingelectron irradiation.Ourobjective is to

obtain a comparison of the chemical structures and

morphology between the ordered and disordered phases

on the same specimen. First, we dissolved the oligomer of

(HBA-C8)2-FBA-(HBA-C8)2 in dichloromethane at a concen-

tration of 40mg �mL�1. An amorphous filmwas formed by

spin-coating 40mL of the solution at a speed of 4 000 rpmon

a 6� 6mm2 SiO2 substrate. The film thickness was

measured to be about 200nm with an alpha-step profil-

ometer.Wethenplacedametallicmaskwithmicro-poresof

well-defined size and shape onto the spin-coated thin film

www.mrc-journal.de 635

Y.-T. R. Lau et al.

636

on theSiO2 substrate. Then, thefilmspecimenwasplaced in

the analysis chamber of a time-of-flight secondary ion

secondary mass spectrometer (ToF-SIMS), which was

equipped with an electron flood gun. When the pressure

of the chamberwaspumpeddownto10�8 torr, thefilmwas

irradiated by an 18-eV electron beam. The electron gunwas

positioned 578 to the surface normal. It was able to deliver

electrons to a spot with a diameter of about 2.6mm. The

current density of the beam was about 1.0mA �mm�2. The

experiment was repeated with metallic masks that

afforded different micro-pore sizes. A ToF-SIMS, a LTA, a

polarized optical microscope (POM), and an atomic force

microscope (AFM) were used to characterize the irradiated

samples. POM and LTA results, as shown in Figure 1a–c,

confirmed that the low-energy electron beam triggered

molecular ordering. The areas of the films (the strips) that

were protected from the electron beam by the metallic

mask remained isotropic. Therefore, these areas did not

exhibit any birefringence under the POM and show any

detectable thermal transition on the heating scan by the

LTA.On thecontrary, theareasof thefilmthatwereexposed

to the electron beam exhibited a positive birefringence and

a thermal transition with an endothermic peak at 40 8C.Based on the sensor deflection thermogram, shown in

Figure 1c, the penetration depth of the heating probe of the

LTA at the melting transition was estimated to be about

200nm,whichmatched the film thickness. All these results

convincingly suggest that the films exhibited not only

optical anisotropybutalso long-rangemolecular orderafter

Figure 1. (a) A reflected polarized photomicrograph of a 100-mesh pspecimen that was irradiated for 30 min by a low-energy electron beapolarized photomicrograph of a 200-mesh pattern on a film speirradiated for 30 min by electrons. The insets in (a) and (b) illustraprotective masks used to stamp the corresponding pattern on theenergy electron beam. (c) Localized thermograms over the opticallyisotropic regions, respectively, in terms of the first derivative-powedeflection signals. The letters ‘‘O’’ and ‘‘D’’ in (a)–(c) representdisordered regions, respectively.



electron irradiation. Using the samepatterning techniques,

we successfully fabricated various patterns of ordered and

disordered phases by using the low-energy electron beam

on oligomeric films with different protective masks that

offered openings with sizes from mm (for example, the

meshed grids) to evenmmscales (for example, the opening

shaped as a ‘‘3’’), as shown in Figure 2. This suggests the

versatility and the simplicity of the patterning process for

various arrangements and sizes of the ordered and

disordered phases as imprinted by the patterns on the

protective masks.

The thermal effect due to heat generation from the

inelastic collisions of the electrons with the film surface is

another possible cause of themolecular ordering. However,

if the thermal effect were a crucial factor, we would have

observed the same ordering behavior under thermal

annealing. The major issue is therefore if the induced

molecular ordering is a consequence of selective degrada-

tion of the chemical structure of the oligomers by the

electrons. It is statistically not possible that the oligomers

can be fragmented by the electron beam into many

fragments with similar structures that can undergo long-

range ordering. The fragments of the oligomers should be

relatively volatile. Heavymass losseswere expected during

the electron irradiation under an ultra-high vacuum

(below 10�8 torr). As a result, a crater would have been

created in the area that was exposed to the irradiation.

However, we did not find any crater on the surface of the

sampleafterelectron irradiation.Anothercommonresultof

attern on a filmm. (b) A reflectedcimen that waste the patternedfilm by the low-aniosotropic andr and the sensorthe ordered and

electron irradiation of polymers is the

creation of crosslinks. If created, cross-

links are defects that will hinder mole-

cular ordering.

To establish experimental support for

the absence of chemical degradation of

the oligomers after the electron irradia-

tion, surface analyses by ToF-SIMS were

performed.Toshowthat thematrixeffect

was absent, the intensity ratios between

two secondary ion species, which are

representative of the same structural

unit, were measured.[16] Figure 3 shows

the intensity ratios of both the rigid

aromatic and the flexible CH2 units of

(HBA-C8)2-FBA-(HBA-C8)2 asa functionof

irradiation time. If chemical degradation

of the oligomers occurs, the intensities of

the higher-mass ions will diminish

while those of the fragmented, smaller-

mass ions will be increased when the

irradiation dose is increased.[17] Our

results show that the intensity ratios

remain relatively constant with irradia-

tion time, indicating that the low-energy

DOI: 10.1002/marc.200900730


Figure 2. (a) Photomicrographs of the ‘‘3’’-shaped, 50-mesh, 400-mesh ordered (O)/ disordered (D) patterns on (HBA-C8)2-FBA-(HBA-C8)2oligomeric films irradiated by electrons for 10 min. (b) A reflected polarized photomicrograph of the region that was embossed by the dottedsquare in (a) to confirm the existence of molecular ordering inside the ‘‘3’’. (c) An AFM phase image captured on one of the edges of ‘‘3’’separating the amorphous and the electron-induced ordered regions. (d) Chemical images of the film surfaces with various ordered/disordered patterns using the positive ion with a mass-to-charge ratio (m/z) of 135, representing the HBA unit of the oligomer under ToF-SIMS. (e) Chemical images of the film surfaces with various ordered/disordered patterns using the positive ion with m/z of 243, representingthe FBA unit, of the oligomer under ToF-SIMS. (f) Normalized ion images of the film surfaces with various ordered/disordered patterns usingthe ratio between the intensity of the positive ion with m/z of 213 and the ion with m/z of 135, both of which represent the HBA unit of theoligomer under ToF-SIMS. As compared with the ion images in (d) and the images in (e), the images in (f) do not show any intensity contrastbetween the disordered (D) and the ordered (O) regions. This is consistent with the quantitative intensity analyses presented in Figure 3 inwhich the ordered regions were not degraded by the electron beam.

electrons did not impart any degradation on the

oligomers.

Lastly, we studied the effects of the molecular structure

on the ordering effect induced by the electron beam.

Oligomers of (HBA-C8)n-FBA-(HBA-C8)n with n¼ 1, 2 and 5

were prepared. We found that induced long-range mole-

cular ordering was observed on the oligomers with n¼ 1

and 2, but no ordering occurred on the oligomer with n¼ 5

(additional Supporting Information is provided to illustrate

the time sequence of induced ordering of an amorphous

film of the (HBA-C8)1-FBA-(HBA-C8)1 oligomer under

electron irradiation). Even at n¼ 5, the bulk glass transition

temperature of the oligomer, which was measured to be

13 8C, is still lower than the room temperature (20 8C). Thus,hindered molecular mobility is not the reason for the

absence of ordering of the oligomer. It is clear to us that the

driving forceproducedbytheelectronbeamisnotsufficient

to force the ordering of the molecules when their sizes are



larger. We also modified the chemical structures of the

oligomers of (HBA-C8)1-FBA-(HBA-C8)1 by substituting the

�C(CF3)2� groups in the FBA units with less polar

�C(CH3)2� groups, the structure became a (HBA-C8)1-

HBA-(HBA-C8)1 oligomer, which was a semi-crystalline

molecule.[18] On the other hand, when all the HBA units of

(HBA-C8)1-HBA-(HBA-C8)1 were replaced with the FBA

units to yield (FBA-C8)1-FBA-(FBA-C8)1, no electron-induced

ordering was observed on the amorphous films. All the

above studies have clearly concurred that a molecular

structure with a combination of the HBA and FBA

constituents bearing the specific sequence, as demon-

strated in Figure 2, is a necessity for the phenomenon to

occur. Such molecular specificity provides a clue to the

mechanism of the induced ordering phenomenon.

It is knownthat theelectricfield canbeused to induce the

conformational changes and orientation of the molecules

by the repulsion of the electronegative atoms away from


Y.-T. R. Lau et al.

Figure 3. Structural compositional changes at the film surfaces ofthe oligomer, (HBA-C8)2-FBA-(HBA-C8)2, monitored in-situ withstatic ToF-SIMS as a function of the electron irradiation time. Thestructural composition is quantitatively defined by, Rj/k, whereRj/k¼ (Ij/Ik)t/(Ij/Ik)t¼0. Ij and Ik denote the intensity of the selectedcharacteristic secondary ion at a mass-to-charge ratio (m/z) of jand k, respectively. The subscript t is the electron irradiation time.Four positive molecular ions at m/z of 91, 107, 135 and 213, andthree negative ions at m/z of 25, 73 and 211, which are charac-teristic of the HBA units of the oligomer, were selected.[16] Twoother positive ions at m/z of 83 and 85, which are characteristic ofthe aliphatic CH2 (C8) units of the oligomer, were selected.[16]

Figure 4. A proposed model to describe the mechanism of the inducedC8)2-FBA-(HBA-C8)2 oligomers under electron irradiation. The two CF3unit display the strongest dipole moment in the whole molecular st

638Macromol. Rapid Commun. 2010, 31, 634–639


the electric field.[19]Webelieve that the samemechanism is

operating in the ordering of the amorphous oligomers

underanelectronbeam.Whentheelectronbeamisdirected

to the sample, the oligomers align themselves with their

chain axesparallel to thedirectionof the electric field due to

coupling between the electric field and the strong dipoles of

CF3 groups of the FBA units, as shown in Figure 4. We

suspect that the electron beam activated the chain packing

of the crystalline components by the repulsionof the strong

dipoles of the CF3 groups. The electric field can induce order

in (HBA-C8)1-FBA-(HBA-C8)1 and (HBA-C8)2-FBA-(HBA-C8)2but not in (HBA-C8)5-FBA-(HBA-C8)5 because the repulsive

force is not strong enough to induce the orientation of the

oligomer as its size increases.

Conclusion

To conclude, in this contribution, we demonstrate for the

first time that we now can use a low-energy electron beam

to create long-range molecular ordering of oligomers. The

new finding offers a facile method to directly control the

ordering of an amorphous organic substrate at ambient

temperature. Fabrication of a millimeter-scaled ordered/

disordered structural pattern of an organic thin filmwith a

protectivemask takes just a fewminuteswitha low-energy

electron beam.

Acknowledgements: This work was funded by the Hong KongResearch Grants Council with the grant numbers 600405 and600408.

Received: October 8, 2009; Revised: November 25, 2009; Publishedonline: January 22, 2010; DOI: 10.1002/marc.200900730

ordering of (HBA-groups in the FBAructure.

Keywords: alignment; electron irradiation;ordering

[1] J. M. Schultz, ‘‘Polymer Crystallization:The Development of Crystalline Order inThermoplastic Polymers’’, 1st edition,Oxford University Press, New York2000, p. 113.

[2] T. R. Taylor, S. L. Arora, J. L. Fergason, Phys.Rev. Lett. 1970, 25, 722.

[3] M. Muthukumar, C. K. Ober, E. L. Thomas,Science 1997, 277, 1225.

[4] K. Ichimura, Chem. Rev. 2000, 100, 1847.[5] [5a] J. X. Geng, E. Zhou, G. Li, J. W. Y. Lam,

B. Z. Tang, J. Polym. Sci., Part B: Polym.Phys. 2004, 42, 1333; [5b] I. Lelidis, G.Durand, Phys. Rev. Lett. 1994, 73, 672.

DOI: 10.1002/marc.200900730


[6] T. Kawai, T. Kimura, Polymer. 2000, 41, 155.[7] K. Kimura, K. Kobayashi, H. Yamada, T. Horiuchi, K. Ishida,

K. Matsushige, Appl. Surf. Sci. 2006, 252, 5489.[8] C. Yan, H. H. Li, J. M. Zhang, Y. Ozaki, D. Y. Shen, D. D. Yan, A. C.

Shi, S. K. Yan, Macromolecules 2006, 39, 8041.[9] Q. H. Lu, X. M. Lu, J. Yin, Z. K. Zhu, Z. G.Wang, H. Hiraoka, Jpn. J.

Appl. Phys. 2002, 41, 4635.[10] [10a] B. W. Lee, N. A. Clark, Science. 2001, 291, 2576; [10b] P. J.

Martin, Recent Pat. Mater. Sci 2008, 1, 21.[11] P. M. Ossi, ‘‘Radiation-induced Phase Transitions’’, in: Radi-

ation Effects in Solids, 1st edition, K. E. Sickafus, E. A. Kotomin,B. P. Uberuaga, Eds., Springer, Dordrecht 2007, p. 259.

[12] J. Schultz, ‘‘Polymer Materials Science’’, 1st edition, Prentice-Hall, New Jersey 1974, p. 104.

[13] J. Stohr, M. G. Samant, J. Luning, A. C. Callegari, P. Chaudhari,J. P. Doyle, J. A. Lacey, S. A. Lien, S. Purushothaman, J. L.Speidell, Science 2001, 292, 2299.



[14] A. M. Donald, A. H. Windle, ‘‘Liquid Crystalline Polymers’’,1st edition, Cambridge University Press, Cambridge 1992,p. 50.

[15] Y. G. Lei, Z. L. Cheung, K. M. Ng, L. Li, L. T. Weng, C. M. Chan,Polymer 2003, 44, 3883.

[16] [16a] Y. T. R. Lau, L. T. Weng, K. M. Ng, C. M. Chan, Appl. Surf.Sci. 2008, 255, 1001; [16b] L. Li, C. M. Chan, K. M. Ng, Y. G. Lei,L. T. Weng, Polymer 2001, 42, 6841.

[17] G. L. Fisher, R. E. Lakis, C. C. Davis, C. Szakal, J. G. Swadener, C. J.Wetteland, N. Winograd, Appl. Surf. Sci. 2006, 253, 1330.

[18] Y. T. R. Lau, J. M. Schultz, L. T. Weng, K. M. Ng, C. M. Chan,Langmuir 2009, 25, 8263.

[19] [19a] H. J. Kreuzer, Surf. Interface Anal. 2004, 36, 372; [19b] A.Troisi, M. A. Ratner, J. Am. Chem. Soc. 2002, 124, 14528;[19c] A. Yoshizawa, S. Segawa, F. Ogasawara, Chem. Mater.2005, 17, 6442; [19d] S. Chiba, A. Yoshizawa, Jpn. J. Appl. Phys.2008, 47, 6386.


Induction of Molecular Organization of Oligomers by Low-Energy Electrons

Documents

Transcript of Induction of Molecular Organization of Oligomers by Low-Energy Electrons