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Phase TransitionsA Multinational JournalPublication details, including instructions for authors and subscription information:http://www.informaworld.com/smpp/title~content=t713647403

Comparative study of liquid-crystalline ordering in amonomer, linear polymer, and graft copolymer by thephoton transmission techniqueHaluk Özbek a; Sevtap Yldz a; Önder Pekcan a; Yeşim Hepuzer b; Yusuf Yağc b;A. Nihat Berker ac; Giancarlo Galli d; Emo Chiellini da Department of Physics, Istanbul Technical University, 80626 Maslak, Istanbul,Turkeyb Department of Chemistry, Istanbul Technical University, 80626 Maslak, Istanbul,Turkeyc Department of Physics and Center for Materials Science and Engineering,Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA

d Department of Chemistry and Industrial Chemistry, University of Pisa, I-56126 Pisa, Italy

Online Publication Date: 01 December 2003To cite this Article: Özbek, Haluk, Yldz, Sevtap, Pekcan, Önder, Hepuzer, Yeşim, Yağc, Yusuf, Berker, A. Nihat, Galli,Giancarlo and Chiellini, Emo (2003) 'Comparative study of liquid-crystalline ordering in a monomer, linear polymer, andgraft copolymer by the photon transmission technique', Phase Transitions, 76:12, 991 - 998To link to this article: DOI: 10.1080/0141159031000156454URL: http://dx.doi.org/10.1080/0141159031000156454

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Phase TransitionsVol. 76, No. 12, December 2003, pp. 991–998

COMPARATIVE STUDY OF LIQUID-

CRYSTALLINE ORDERING IN A MONOMER,

LINEAR POLYMER, AND GRAFT COPOLYMER

BY THE PHOTON TRANSMISSION TECHNIQUE

HALUK OZBEKa, SEVTAP Y_IILD_IIZa, ONDER PEKCANa,*,YES� IM HEPUZERb, YUSUF YAGC_IIb, A. NIHAT BERKERa,c,

GIANCARLO GALLId and EMO CHIELLINId

aDepartment of Physics, bDepartment of Chemistry, Istanbul Technical University, 80626 Maslak,Istanbul, Turkey;

cDepartment of Physics and Center for Materials Science and Engineering, Massachusetts Instituteof Technology, Cambridge, Massachusetts 02139, USA;

dDepartment of Chemistry and Industrial Chemistry, University of Pisa, I-56126 Pisa, Italy

(Received 26 September 2001; In final form 7 November 2001)

The photon transmission technique was used to study the phase transitions of a liquid crystalline acrylatemonomer, 6-(4-cyanobiphenyl-40oxy)hexyl acrylate (LC6), its homopolymer (PLC6) and its graft copolymer(GLC6) with polytetrahydrofuran grafts. The phase transitions were also confirmed by DSC and polarizingmicroscopy. We observed the phase transition sequence isotropic–nematic–smectic A–smectic C in the LC6monomer. In PLC6 and GLC6 polymers, the nematic and smectic A phases appear dominant. The apparentnematic–smectic A transition is of first order in PLC6 and of second order in GLC6, with the transitiontemperature remaining the same. The effects of quenched random constraints introduced in GLC6 areconsistent with the theory of quenched random interactions. The critical exponents were also evaluated.

Keywords: Photon transmission technique; Liquid crystal homopolymer; Graft copolymer; Nematic order;Quenched random constraints; Critical exponent

1. INTRODUCTION

Polymer liquid crystals are a class of materials that combine the properties of polymerswith those of liquid crystals. These ‘‘hybrids’’ show the same mesophase characteristicsof ordinary liquid crystals, yet retain many of the useful and versatile properties ofpolymers. In order for normally flexible polymers to display liquid crystal characteris-tics, rod-like or disk-like elements (mesogens) must be incorporated into their chains.The placement of the mesogens plays a large role in determining the type of polymerliquid crystals that is formed. Side-chain liquid crystal polymers are formed when

*Corresponding author. E-mail: pekcan@itu.edu.tr

ISSN 0141-1594 print: ISSN 1029-0338 online � 2003 Taylor & Francis Ltd

DOI: 10.1080/0141159031000156454

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the mesogens are connected as side chains to the polymer by a flexible bridge.A polymer-specific property of importance in side-chain polymer liquid crystals is theformation of a glass on cooling because of partial decoupling of the flexible backbonefrom the side chains. Interest in liquid-crystalline side-chain polymers has growntremendously because of their theoretical aspects and their potential applications.A considerable research effort has been made in electro-optic displays using nematicor smectic mesogenic side-chain polymers (Demus et al., 1998).

Structure determination and phase transition identification of the liquid-crystalpolymers are essentially based on thermal analysis (DSC measurements), optical micro-scopy, and X-ray measurements. In this work, we have investigated the phasetransitions of 6-(4-cyanobiphenyl-40-oxy)hexyl acrylate (LC6) liquid crystal, its sidechain polymer (PLC6) and its graft copolymer (GLC6) upon cooling from the isotropicmelt by using a different technique, in-situ ultraviolet/visible (UV/VIS) photontransmission technique (Ozbek et al., 1999, 2001). The transmitted photon intensityItr from the samples being investigated is monitored as a function of time and thenthe data are replotted versus temperature by using the calibration curve between timeand temperature. The data obtained from Itr(T) curves can be used to determine thetransition temperatures and, for the transitions that are of second order, the criticalexponents. At a phase transition, we have detected a drastic reduction in the trans-mitted photon intensity, which reflects the order parameter and yields the expectedcritical exponents as explained below.

2. EXPERIMENTAL

The general formulae of LC6 monomer, PLC6 polymer, and GLC6 graft copolymersare shown in Fig. 1. The syntheses of the LC6 monomer and the corresponding homo-polymer PLC6 (Mn¼ 18,400, Mw/Mn¼ 1.5) have previously been described (Duboiset al., 1986; Le Barny et al., 1986). Graft copolymerization was as follows: 0.2 g(0.57mmol) LC6 monomer was added to a solution of 1ml dimethylformamide(DMF) containing 0.02 g (4.5� 10�3mmol) of the polytetrahydrofuran macromono-mer (Mn¼ 4500, Mw/Mn¼ 1.1) and AIBN (5� 10�3mol/l). The polytetrahydrofuranmacromonomer was prepared by cationic polymerization of tetrahydrofuran initiatedby the methacryloylchloride silver hexafluoroantimonate combination system followingthe procedure described by Vargas et al. (1980). The tube was degassed and sealedunder nitrogen. It was then placed into a thermostatically controlled bath at 70�C.After a given time, the polymerization mixture was poured into methanol. The polymerwas then dissolved in CH2Cl2 and reprecipitated from methanol to remove unreactedmonomer (Mn¼ 13,350, Mw/Mn¼ 2.2). The precipitated polymer was filtered off anddried in vacuo. Average molecular weights were determined by GPC using universalcalibration.

We have prepared the samples of LC6 and PLC6 separately by placing the powderin-between two glass plates, which were then heated in an oven at 110�C. The GLC6samples was prepared like the other samples except that it was granular at roomtemperature.

In-situ transmitted photon intensity measurements were performed in real time at750 nm wavelength using a Jasco V-530 UV/VIS spectrometer in time-course mode.The samples were separately placed within the spectrometer after heating to the

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isotropic liquid state which was then left to cool down during the UV measurements inreal time. Another glass plate was used as a standard for all UV experiments.Temperature was measured with a Ni–Cr thermocouple during cooling process. Thelinear dependence of temperature on time was confirmed by using the time–temperaturecalibration curve.

The structure of the mesophases was identified by optical microscopy using apolarizing microscope Olympus BHSP.

3. RESULTS AND DISCUSSION

The plots of transmitted intensity versus temperature for the cooling of LC6 liquidcrystal monomer, PLC6 and GLC6 polymers are shown in Fig. 2. It is seen that thetransmitted photon intensity decreases drastically at certain onset temperatures whichcorrespond to the phase transitions, as also confirmed by polarization microscopy.

When the isotropic liquid is cooled, the nematic phase appears at the isotropizationpoint in the form of separate small, round droplets. Upon cooling, the droplets growand coalesce to form structures from which stable textures form. The structural hetero-geneities in thermotropic liquid crystals indicate that, if the wavelength of the incidentlight is much smaller than the size of local domains, light must be reflected. In this workthe diameter of the liquid crystal droplets was larger than the � of the incident light

FIGURE 1 The general formulae of (a) LC6 monomer, (b) PLC6 polymer, (c) GLC6 graft copolymer.

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which caused the loss of the transmitted light intensity during cooling of the liquidcrystal. Upon cooling in the formation of smectic phases, the size of the domainsincreases and as a result reflection of light increases. Thus the transmitted intensityItr is proportional to an overall order parameter, �, which is the quantitative measureof the alignment of the molecules in the liquid-crystalline phases, obeying a power lawnear the critical temperature TC of the phase transition,

j�� �Cj ¼ AjT � TCj�, ð1Þ

where �C is � at TC, � is the critical exponent, and A is the critical amplitude.Accordingly, the transmitted intensity Itr also obeys the power law

jItr � ItrC j ¼ A0jT � TCj�, ð2Þ

where ItrC is the value of Itr at TC and A0 is another critical amplitude. As mentionedearlier, the transmitted light intensity decreased drastically at certain onset tem-peratures corresponding to the phase transitions. The transition temperatures were

FIGURE 2 Transmitted intensity Itr versus temperature T for (a) LC6, (b) PLC6, (c) GLC6.

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determined from the extrema of the first derivative of the Itr(T) curves, which arepresented in Fig. 3.

In Fig. 2a, it is seen that the intensity Itr from LC6 monomer presented a discontin-uous decrease at 106.27�C, indicating a first-order phase transition. This transitionbetween the isotropic and nematic phases of LC6 is consistent with the Landautheory of de Gennes (1969, 1971). The mean-field theories of Kobayashi (1970a,1970b) and Mc Millan (1971) have predicted that the nematic–smectic A transitioncan be first or second order, depending on whether the ratio of the nematic–smecticA transition temperature to the isotropic–nematic transition temperature, TNA/TIN,is greater or less than 0.87. In Fig. 2a the nematic–smectic A (N–SmA) phase transitionoccurs at 89.09�C, as second-order. The observed transition temperature ratio isTNA/TIN¼ 0.84<0.87, which, together with the observed second-order nature, agreeswith the Kobayashi–McMillan theory. Over the reduced temperature range of 1.33�10�3<j1�T/TCj<4.35� 10�3, the critical exponent is found to be �¼ 0.219� 0.004,which is less than the helium-like value (�¼ 0.346) (Fig. 4a). In Fig. 2a, the thirdand final phase transition occurs at 74.43�C between the smectic A and C phases ofLC6, as second-order. This transition was not observed on cooling by DSC measure-ments (Dubois et al., 1986). Over the reduced temperature range of 4.58�10�3<j1�T/TCj<5.72� 10�3, the critical exponent is found to be �¼ 0.533� 0.001

FIGURE 3 The first derivative of Itr versus temperature T for (a) LC6, (b) PLC6, (c) GLC6.

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which is consistent with mean-field theory (Fig. 5). According to the de Gennes’s sug-gestion (1973) the smectic A–smectic C phase transition may be continuous and canexhibit helium-like critical behaviour, which has been of considerable interest in thefield of critical phenomena (Schaetzing and Litster, 1979). It has also been reported

FIGURE 4 Log–log plot of intensity Itr versus temperature T at the nematic–smectic A phase transition for(a) LC6, (b) GLC6.

FIGURE 5 Log–log plot of intensity Itr versus temperature T at the smectic A–C phase transition for LC6.

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that critical exponents vary from mean-field (Delaye and Keller, 1976; Meiboom andHewitt, 1977; Safinya et al., 1980) to helium-like behaviour (Delaye, 1979; Flanders,1976; Galerne, 1981) or may have values in between (Wise et al., 1973; Garoff andMeyer, 1977; Galerne, 1978; Lim and Ho, 1978; Lim et al., 1980). In Fig. 2b, it isseen that the transmitted intensity Itr from PLC6 polymer, upon cooling, presents a dis-continuous decrease at 106.07�C. This first-order transition appears to be betweennematic and smectic A phases of PLC6 polymer. The phases on each side of thephase transition was identified by observation of textures under polarization micro-scopy. The transition temperature obtained by DSC measurement (Dubois et al.,1986), 124�C for PLC6 was higher than the temperature was measured. In Fig. 2c,the transmitted intensity Itr from the graft copolymer GLC6, on cooling, decreasesat 106.5�C showing continuous behaviour. This second-order phase transitionalso appears to be between the nematic and smectic A phases of GLC6, as also identi-fied by polarization microscopy. Over the reduced temperature range of 8.85�10�4<j1�T/TCj<3.28� 10�2, the critical exponent is found to be �¼ 0.532� 0.005(Fig. 4b). For these polymers, the nematic–isotropic transition thus appears to occurat temperatures higher (>120�C) than our experimental temperature range.

In summary, we have reported phase transitions of liquid crystalline monomer LC6,its side-chain polymer PLC6, and graft copolymer GLC6. It was observed that the mul-tiple phase transition characteristic of LC6 monomer is reduced in either polymerand the nematic and smectic A phases appear dominant. The N–SmA transition isfirst-order of the PLC6 polymer. For the graft copolymer GLC6, the N–SmA transitiontemperature is the same as for PLC6, but the N–SmA transition is of second order.Transition temperatures measured by polarizing microscope are consistent with thetemperatures obtained by the photon transmission technique. It has been argued, ongeneral theoretical grounds, that first-order phase transitions are converted tosecond-order phase transitions, by the introduction of frozen disorder into a system(Hui and Berker, 1989; Berker, 1991, 1993). In the current cases, in going fromPLC6 to GLC6, a frozen randomness, in the form of random connectivities, is intro-duced. Thus, the conversion from first-order to second-order is consistent with thetheoretical arguments. Furthermore, theoretical calculations (Falicov and Berker,1996) on model systems show that the order of the transition changes, but the transitiontemperature remains essentially unchanged, which is also consistent with our currentfindings.

Acknowledgements

This work was supported by The Scientific and Technological Research Council ofTurkey (TUBITAK) under Grant No: TBAG-1951 (100T074), and the Research Fundof _IIstanbul Technical University, Grant No. 1426 and 1538. This work was performedwith partial financial support from the Turkish TUBITAK and the Italian CNR.

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