RECENT ADVANCE IN LIQUID CRYSTALLINE ELASTOMERS S. B …

RECENT ADVANCE IN LIQUID CRYSTALLINE ELASTOMERS

S. B UALEK* 1 and S. KOHJI YA * 2

Dedicated to Professor Dr. Shinzo Yamashita on the occasion of his retirement from Kyoto Institute of Technology

ABSTRACT

This short review describes the most recent advances in the study on liquid crystalline

elastomers (LCE). Concise description on liquid cryatals and liquid crystalline polymers is also given. LCE exhibits liquid crystalline state that resembles to its precursor in the un-

crosslinked state. Within the mesophase range of temperature, elastic properties of LCE are different from conventional elastomers in the sense that they are energetic in nature. This is due to the partially ordered alignment of the mesogens. It was noted, however, that the optical anisotropy remained even far above clearing temperature (Ta), up to T~ + 30°C, if the specimen was under stress. Volume change of LCE with applied field is also interesting. This

phenomenon leads to a high tendency of using cholestric or smectic C* LCE as a piezoelectric sensor because of the flexibility and the shape stability.

1. INTRODUCTION

An intermediate phase which lies between

crystalline solid and isotropic liquid has long

been observed since 18881). It was named

mesophase or liquid crystal due to its fluidity

and anisotropies in optical, electrical, magnetic

and mechanical properties. The most essential

nature of the compounds which enables them to

exhibit this phenomenon is their rigid rod- or

disc-like molecular structures that possess a

high polarizability2).

Since late 1970's, it has been realized that

polymers can also display liquid crystalline pro-

perties, if the polymeric main chain contains mesogenic groups, which is called main chain

liquid crystalline polymer. If mesogenic groups

*1 Department of Chemistry , Faculty of Science, Mahidol University, Bangkok 10400, Thailand *2 Department of Materials Science, Kyoto In-

stitute of Technology, Matsugasaki, Kyoto 606, Japan

10

are attached as side groups along the main chain, side chain liquid crystalline polymer results. Recently, mesogenic units were in-

troduced both in the main chain and at the side

groups, and the polymer was named combined main chain/side group liquid crystalline

polymer6) . The main objective of this paper is to review the studies on liquid crystalline

elastomers which have been conducted so far. Elastomers in this article mean crosslinked

polymers which show elastomeric properties at certain conditions. Only thermotropic class of liquid crystalline materials will be considered here, because the authors assume they have higher possibility in technological applications.

Uncrosslinked polysiloxanes which are quite often considered as elastomers with some reasons7~ 9) and crosslinked lyotropic liquid

crystalline polymers1o~11) will not be main con-cerns in this review.

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第65巻 第1号(1992) S.BUALEKandS.KOHJIYA

2. LOW MOLAR MASS LIQUID CRYSTALS

Molecular arrangement in liquid crystalline phase, which has been investigated mainly by X-ray diffraction technique, can be classified into two main types, namely smectic and nematic phases12). In smectic phase (S), as

shown in Figure 1(a) , molecules are aligned so that their molecular long axes are in the same direction and the center of gravity of the molecules are in the same layer. There is no short range interaction between layers. Within a smectic layer the packing of molecules is ran-dom or partially ordered or in some cases the long axis of the molecule is tilted at a certain angle to the normal of smectic plane. This leads to several types of smectic phases, named as smectic A, B, C, K. Smectic liquid crystal can thus be considered as a two-dimensional crystal due to their ordering in two dimensions. Nematic phase (N), on the other hand, has ordering only in one dimension, i.e., the molecules are aligned so that their long axes are more or less in the same direction, called prefer-red direction. However, the center of gravity of the molecules are randomly distributed. See Figure 1(b) for nematic phase. The degree of ordering of nematic phase is described by order

parameter, s, defined by the following equationl3) :

s1/2<3 cost 9-1> (1) where 8 is the angle between the molecular long axix and the preferred direction. If a compound that exhibits nematic phase

contains a chiral element, either by chemical bonding or as a mixture, chiral nematic phase

(N*) or cholesteric phase results14) . In this case, the mesogenic molecules arrange so that the nematic like layers turn into a helical fashion as shown in Figure 1(c). This type of ar-rangement of molecules interacts with the light, and gives rise to so called selective reflection. A circular wave with a direction of polarization be-ing the same as the handedness of the helix is totally reflected, if its wave length (LR) is equal to the optical pitch (P) of the helix (i.e. the length along the helical axis for a 2ir turn of the nematic layer). That is, when Equation (2) is satisfied: AR-Pn (2)

where n is the refractive indexl5). Since the pitch length of the helix depends

Fig.

tic

liq

1 Schematic arrangement of molecules in (A) smec-

(B) nematic and (C) cholesteric or chiral nematic uid crystals.

45 ) 11

RECENT ADVANCE IN LIQUID CRYSTALLINE ELASTOMERS E1*~~

on the temperature, the color of reflected light

changes with temperature and this leads to an

application of chiral nematic phase as a surface

temperature indicator. Analogous to cholesteric

phase, smectic C* is now under active investiga-tions: If a compound containing a chiral ele-

ment exhibits smectic C phase, i.e., the

molecular long axis tilts to a certain angle to the

normal of smectic layer, the axis turns in a

helical way at a constant tilt angle around the

normal of the smectic plane, which is

schematically displayed in Figure 2. Symmetry

of such an arrangement of molecules lacks an in-

version center and therefore it is ferroelectricl6).

Several areas of application of liquid

crystals have been evolved not only in

technological use but also in academics for

about two decades. They are used in the

laboratory as anisotropic solvents for example

in nuclear magnetic resonance for the observa-

tion of dipolar and quadrupole interactions

which otherwise disappear in isotropic

solvents17). Liquid cryatals can be used as a sta-

tionary phase in gas chromatography in order

to separate compounds of very close boiling

points but having different molecular

geometry17). Anisotropic solvents are used in in-frared and ultraviolet-visible spectroscopies in

order to study linear dichroism of the guest

molecules17),18). Liquid cryatals were used as solvents to carry out chemical reactions19) . Fur-thermore, nematic liquids were excellent solvents for chiral molecules to determine their

relative configuration, because the enantiomers induced different handedness of helices20).

Since the molecular arrangement of liquid crystals is influenced by external fields such as electric field, the switching on and off of the field induce a change of optical appearance of the sample. This effect has led to industrial manufacturing of liquid cryatal display devices

(LCD) 21) . It is now well known that LCD is in use for many electronic devices, for examples, watches, calculators, computer and word-pro-cessor displays, to name a few. Recently, even flat-panel type television sets are becoming available, which are realized by use of LCD of matrix system.

3. LIQUID CRYSTALLINE POLYMERS

Schematic representations are given in Figure 3 for typical structures of main chain, side chain, and combined main chain/side

group liquid crystalline polymers. The temperature range and the type of mesomor-

phic phases of the liquid crystalline polymers de-pend on the structures of the polymer backbone and mesogenic groups, and on the length of spacer i.e. a flexible linkage between the

polymer backbone and the rod-like or disc-like mesogenic groups. If the polymer backbone is

polysiloxane, the mesophase range is broad. It quite often transforms from glassy state to liquid crystalline state at room temperature or even at a lower temperature. In the case of

polyacrylates and polymalonates, their solid states are usually crystalline solids8). The length of the spacer also plays an im-

portant role on the transition temperature, type of mesophase, and morphology of the solid state of the polymer. In general, it was found that all transition temperatures rose, when the spacer became longer. Additionally, the odd-even effect was observed for melting and clearing

Fig. 2 Molecular arrangement model

phase.

of ch iral smectic C

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第65巻 第1号(1992) S.BUALEKandS.KOHJIYA

temperatures. The longer spacer also afforded the side chain crystallization and the tendency for mesophase to become smectic rather than nematic22~ . Phase behaviors of mesogens with various

ring systems in low molar mass liquid crystals were reported23), but not yet fully elucidated in liquid crystalline polymers. From the results by changing core ring system such as biphenyl, azobenzene, and azoxybenzene in

polymalonates (i.e. an example of combined main chain/side group liquid crystalline

polymer), it was found that the polymer ex-hibited glassy solid state, whereever the azoxy

group was incorporated, i.e., either in the main chain or at the side group. The glassy state transformed into liquid crystalline state at room temperature or even at lower one. However, the other similar polymers which did not con-tain biphenyl nor azobenzene group, were crystalline solids, and the transition temperatures were approximately 100°C24~ . Another systematic study of the effect of core ring system were carried out on polysiloxanes with ester groups of benzoic acid at side chains24). It was found that the efficiency of the ring system to promote liquid crystalline nature was

dicyclohexyl > biphenyl > phenyl cyclohexyl

Transition temperatures also depended on

molecular weights of polymers. With the in-

crease of molecular weight of the polymers, all

the mesophase ranges shifted to higher

temperatures26). However, elevation of the

phase transition temperatures reached a plateau at a certain upper limit of molecular weight.

Liquid crystalline polymers possess some

properties different from those of low molecular mass liquid crystals. For example, viscosity in

the mesophase range is very high, and hence it

is difficult to obtain a homogeneous orientation

of mesogenic groups by applying external elec-

tric or magnetic field. Their use in display

devices, therefore, is not practical due to the

rise time and decay time being too long. One

feature that has to be emphasized for polymeric

liquid crystals is that mechanical forces can be

used to induce molecular orientation of the

mesogens. They are oriented by drawing the

polymer film in its liquid crystalline phase, and

Fig. 3 Schematic representation of liquid crystalline

polymers: (A) side chain type, (B) main chain type, and (C) combined main chain/side group type.

Fig. 4 Schematic representation of liquid crystalline elastomers (LCE) from (A) side chain type, (B) main chain type, and (C) combined main chain/side group type liquid crystalline polymers. Crosslinking points are

encircled.

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RECENT ADVANCE IN LIQUID CRYSTALLINE ELASTOMERS ~~~~TaAG

then quenched into glassy state. In the case of crosslinked liquid crystalline polymers (i.e. LCEs), the orientation can be accomplished by stretching or compression27). From this view

point, liquid crystlline polymers, especially the side chain type, are interesting both from academic and industrial view points. For exam-

ple, solid polymeric film can be obtained, when liquid crystal phases are frozen-in in the glassy state. This phenomenon is used to fabricate an optical data storage device which can be erased by heat or by laser light8). Copolymers with

photochromic, photoconductive or hyper-

polarizable groups were reported, which led to variety of applications8).

4. LIQUID CRYSTALLINE

ELASTOMERS: SYNTHESIS

Liquid crystalline elastomers (LCE) are

polymeric networks that exhibit mesomor-

phism. Their shematic representaions are given in Fig. 4. LCEs can be prepared via side chain, main chain or combined main chain/side group liquid crystalline polymers27~39) using one step or two steps procedure. One step preparations were carried out by radical copolymerization of

Table 1 Preparation of liquid crystalline elastomers (LCE)

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65 1-(1992) S. BUALEK and S. KOHJIYA

monomers containing mesogenic groups and those with crosslinkable groups in solution28),31- 33).

In the two steps procedure, linear liquid crystalline polymers with reactive groups for crosslinking are synthesized first, and the resulting polymers are secondly subject to gela-

tion using an appropriate crosslinker27^-30),35^-36) , heat, irradiation in li-

quid crystalline phase37) or in solid state38), or y-ray radiation39> . Photocrosslinking of the

polymer films seem to be more interesting than the others, because the polymers can be subject to orientation in their liquid crystalline states by electric or magnetic field, or by mechanical force before the crosslinking. Synthetic methods of LCEs are summarized in Table 1.

5. LIQUID CRYSTALLINE ELASTOMERS: PHYSICAL

PROPERTIES

Thermal, mechanical, electrical and op-tical properties of LCEs have been published so far. These results are summarized in this chapter. It is noted here that even though LCE is crosslinked, the properties of its precursor are more or less maintained in many cases, as it is the case for usual rubber vulcanizates. 5.1 Thermal Properties

It was found that all mesophases of

polymer precursors remained the same even after the crosslinking. In other words, the crosslinked liquid crystalline polymers showed the same mesophase as that of their mother

polymers 35),36),40) . If the degree of crosslinking is only a few percents, no remarkable change of transition temperatures was observed29),33),40) As the degree of crosslinking increases, the clear-ing temperature (Ta) shifts a few degree to lower temperature. On the other hand, the melting points (Tm) from crystalline state or the

glass transition temperatures (Tg) from glassy state to the liquid crystalline phases could be in-creased or decreased24),28^-30),32),33). As an ex-

treme case, there exists a limit of crossllinking

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density above which the mesophase disappears32),33)

In these examples, the crosslinking points may be pictured as the impurities which inter-rupt the intermolecular interactions and hence the ordering of mesogenic groups. This effect results in the decrease of T~, while Tg increased because the crosslinking reduced the mobility of the polymeric chains. 5.2 Mechanical Properties

As has been recognized in ordinary elastomers, the tensile modulus of LCE was found to increase with the progress of crosslinking33) . Above T6 i. e. at the isotropic

phase, LCE behaves like a conventional elastomer, but at mesophases its mechanical

properties are highly temperature and time dependent33) . It was observed that the initial modulus increased rapidly as the temperature was decreased from T. to Tg. The behavior of these elastomers was clearly influenced by li-

quid crystalline phase. When the tensile stress is applied, the extension of polymeric main chains can result in the side chain alignment to orient parallel to the polymeric backbone, in the case of weak coupling between the mesogenic units and their polymeric backbone. However, in the presence of strong coupling, the side chains align at right angle to the extension direc-tion, i.e., a bottle brush type structure results.

If an external force is applied to LCE to

reach a constant extension, the stress would rapidly decrease to a steady level. This is not

just a stress relaxation observed on ordinary elastomers, but it is due to the ordering of the mesogenic phase having reached its possible maximum value41). On the other hand, if the ap-

plied force is kept at a constant value, the length of the specimen increases as the temperature decreases from TC to T941). This observation is explained by the fact that the higher is the order-ing of the molecules within the mesophase range, the lower is the temperature. It is needless to say that because elastic properties of LCE are anisotropic, the values are dependent

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RECENT ADVANCE IN LIQUID CRYSTALLINE ELASTOMERS

on the direction of deformation.

5.3 Optical Properties Stress optical properties of LCE in

isotropic state (at T > T~ + 30°C) are similar to

those of conventional elastomers at room temperature. Investigations of siloxane elastomers revealed that the birefringence in-creased fast as the temperature was lowered to T~42),43). Birefringence (An) is defined as the difference of refractive indices in the direction

parallel (n11) and perpendicular (nl) to the preferred direction, and stress optical coefficient (C) as the birefringence per unit stress:

An=n//-nl (3) C=d n/Q (4)

where a is mechanical stress. Theoretically, the

product of C and temperature should be constant44), that is, C*T=dn/Q

=2n(n~+2)2(al-a2)/45kn (5)

where al and a2 are polarizability components

parallel and perpendicular to the extension direction, respectively, k is Boltzmann cons-tant, and n is the mean refractive index. The

product rapidly increases or decreases as the temperature approaches T~. This behaviour cor-responds to the strain induced orientation of

pretransitional nematic clusters. The sign of the birefringence depends on the direction of the in-cident beam. This results from the change of the preferred direction of the mesogenic groups. Stress-optical behaviour of LCE in the

mesophase range is determined by the optical

properties of liquid crystalline state. Network properties do not play an important role. Ow-ing to the turbidity of LCE in liquid crystalline

phase, their photoelastic investigations are much more difficult than in isotropic state. 5.4 Electric Properties Electric field effects on LCE that were swollen in low molar mass nematic liquid crystal were reported40),45) . The shape change of the sample of LCE type 3 (see Table 1) was observed, when an electric field of 30 V was applied40) . The other paper45) described the con-

16

traction of polymer type 2 (see Table 1) when subjected to a variable electric field up to 110 V. This contraction reached 20 percents of the original volume as the temperature was raised. In general, the elastic modulus of polymer net-works increases with temperature, whereas that of liquid crystals decreases. Therefore, it can be said that the behaviour of liquid crystalline

phase dominated over that of the polymer net-works under these experimental conditions.

Recently, there appeared reports on the measurement of piezoelectricity of LCE which exhibited smectic C* or cholesteic phase46)>47), and its theoretical interpretation was also

published48) . Piezosignals of LCE type 4 (see Table 1) were measured which contained a chiral group with very high polarization ex-hibiting S*. or n* phase46). It is found that the

piezosignal in S*. phase increased as the temperature decreased from T~. The observed value was about 1000 times larger than that ob-tained in the isotropic state. The signal obsrved in n* phase was smaller than that in the S*~. Nematic LCE swollen in low molar mass cholesteric liquid was also reported to exhibit

piezoelectricity, but the signal was again smaller than that in the S*. phase47) .

6. CONCLUSION

Recent advance in the studies on crosslink-ed liquid crystalline polymers (i.e. liquid crystalline elastomers, LCE) were described. LCE is an elastomeric material showing liquid crystalline natures. Many physical properties were in accord with those of their uncrosslinked

precursors, but in some cases the character was enhanced by being an elastomer. Change of anisotropic nature by mechanical (especially tensile) stress is most intersting. At present,

piezoelectricity is the only possibility for in-dustrial applications, but the idea of controlling various functions by mechanical stress or strain is fascinating. Further developments in this area are expected.

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65 1-(1992) S. BUALEK and S. KOHJIYA

ACKNOWLEDGEMENT

The authors express their sincere thanks to

Japan Society for the Promotion of Science (JSPS) for the assistance to our research cooperation. The cooperation is based on the scientific exchange program between JSPS and National Research Council of Thailand

(NRCT).

References

1) Reinitzer, F.: Mh. Chem., 9, 421 (1888) 2) Gray, G. W.: "Molecular Structure and the Properties of Liquid Crystals", Academic Press, London (1962) 3) Blumstein, A. (Ed.) : "Liquid Crystalline Order in Polymers", Academic Press, New York

(1978) 4) Blumstein, A. (Ed.) : "Polymeric Liquid Crystals", Plenum Press, New York (1985) 5) Chapoy, L. (Ed.) : "Recent Advances in Li- quid Crystalline Polymers", Elsevier Appl. Sci., London (1985) 6) Reck, B, and Ringsdorf, H.: Makromol. Chem. Rapid Commun., 6, 291 (1985); ibid., 7, 389 (1986)

7) Kohjiya, S.: Nippon Gomu Kyokaishi, 61, 828 (1986)

8) McArdle, C. B. (Ed.) : "Side Chain Liquid Crystalline Polymers", Blackie, Glasgow & Lon- don (1989)

9) Kohjiya, S., Ono, A., Kishimoto, T., Yamashita, S., Yanase, H., and Asada, T.: Mol.

Cryst. Liq. Cryst., 185, 183 (1990) 10) Kishi, R., Sisido, M. and Tazuke, S.:

Macromolecules, 23, 779 (1990) 11) Lofher, R. and Finkelmann, H.: Makromol. Chem. Rapid Commun., 11, 32 (1990) 12) Friededl, G.: Ann. Phys., 18, 273 (1922); C. R. Acad. Sci., 176, 475 (1923) 13) Maier, W. and Saupe, A.: Z. Naturforsch.,

14A, 882 (1959) 14) Goossens, G. W. A.: Mol. Cryst. Liq. Cryst.,

12, 237 (1971) 15) de Vries, H.: Acta Crystallogr., 4, 219 (1951) 16) Meyer, R. B., Liebert, L., Strzelecki, L., and

Keller, P.: J. Phys. (Paris), 36, L-69 (1975) 17) Keller, H. and Hatz, R.: "Handbook of Li- quid Crystals", Verlag Chemie, Weinheim (1980)

(51)

18) Hansen, T. S.: Z. Naturforsch., 24A, 866 (1969)

19) Blumstein, A. (Ed.) : "Mesomorphic Order in Polymers and Polymerization in Liquid Crystalline Media", ACS Symp. Ser., Vol. 74, American Chemical Society, Washington, D. C.

(1978) 20) Bualek, S.: Dissertation for Dr. Sci. to Univ. of Dortmund (1975)

21) Sprokel, G. L.: "The Physics and Chemistry of Liquid Crystal Devices", Plenum, New York (1980)

22) Richard, H., Mauzac, M., Nguyen, H. T., Sigand, G., Achard, M. F., Hardovin, F., and Gasparoux, H.: Mol. Cryst. Liq Cryst., 155, 141

(1988); Keller, P.: ibid., 157,193 (1988); Kruck- je, B., Schlossarek, M. and Zaschke, H.: Acta

Polym., 39, 607 (1988) 23) Toyne, K. J.: in "Thermotropic Liquid Crystals", Critical Review on Applied Chemistry, Vol. 22, ed. by Gray, G. W., John

Wiley, Chicester, England (1987); Gray, G. W.: Mol. Cryst. Liq. Cryst., 63, 3 (1981)

24) Bualek, S. and Zentel, R.: Makromol. Chem., 189, 791 (1988)

25) Gray, G. W., Hill, J. S. and Lacey, D.: Makromol. Chem., 191, 2227 (1990)

26) Blumstein, R. B., Stickles, E. M., Gauthier, M. M., Blumstein, A., and Volino, F.:

Macromolecules, 17, 177 (1984) 27) Zentel, R. and Benalia, M.: Makromol. Chem., 188, 665 (1987)

28) Finkelmann, H. Koch, H. J. and Rehage, G.: Makromol. Chem. Rapid Commun., 2, 317 (1981)

29) Zentel, R. and Reckert, G.: Makromol. Chem., 187, 1915 (1986)

30) Canessa, G., Reck, B., Reckert, G., and Zentel, R.: Makromol. Chem. Macromol. Symp., 4,

91 (1986) 31) Hessel, F., Herr, R. P., and Finkelmann, H.:

Makromol. Chem., 188, 1597 (1987) 32) Davis, F. J., Gilbert, A., Mann, J., and Mit- chell, G. R.: J. Chem. Soc., Chem. Commun., 1333

(1986) 33) Mitchell, G. R., Davis, F. J. and Ashman, A.: Polymer, 28, 639 (1987) 34) Schaetzle, J., Kaufhold, W, and Finkelmann, H.: Makromol. Chem., 190, 3269 (1989) 35) Bualek, S., Kapitza, H., Meyer, J., Schmidt , G. F., and Zentel, R.: Mol. Cryst. Liq. Cryst., 155, 47 (1988)

36) Zentel, R., Reckert, G., Bualek, S., and Kapit- za, H.: Makromol. Chem., 190, 2869 (1989)

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RECENT ADVANCE IN LIQUID CRYSTALLINE ELASTOMERS ~ ~ 17D1 T-. piL

37) Zentel, R., Kapitza, H., Kremer, F., and Vallerian, S. U.: in "Functionalized Liquid

Crystalline Polymers", Am. Chem. Soc., Washington, D. C. (1990)

38) Keller, P.: Chem. Materials, 2, 3 (1990) 39) Talroze, R. V., Gubana, T. I., Shibaev, V. P., and Plate, N. A.: Makromol. Chem. Rapid Com- mun., 11, 67 (1990) 40) Barnes, N. R., Davis, F. J. and Mitchell, G. R.: Mol. Cryst. Liq. Cryst., 168, 13 (1989) 41) Davis, F. G. and Mitchell, G. R.: Polym. Com- mun., 28, 8 (1987) 42) Gleim, W. and Finkelmann, H.: Makromol.

Chem., 188, 1489 (1987) 43) Hammerschmidt, K. and Finkelmann, H.:

Makromol. Chem., 190, 1089 (1989) 44) Kuhn, W. and Gruen, F.: Kolloid Z., 101, 248

(1942) 45) Zentel, R.: Liq. Cryst., 1, 5889 (1986) 46) Vallerien, S. U., Kremer, F., Fisher, E. W., Kapitza, H., Zentel, R., and Poths, H.:

Makromol. Chem. Rapid Commun., 11, 593 (1990) 47) Meier, W. and Finkelmann, H.: Makromol. Chem. Rapid Commun., 11, 599 (1990) 48) Brand, H. R. and Pleiner, H.: Makromol. Chem. Rapid Commun., 11, 607 (1990)

和文要旨

液 晶 性 エ ラ ス トマ ー に お け る最 近 の 進 歩

サバロープ ・プアレック*1,鞠 谷信三*2

異 方 性 の 液 体 で あ る液 晶 へ の興 味 とそ の有 用 性 か ら,液 晶性 高 分 子 の 研 究 が活 発 に行 わ れ て い る.

液 晶分 子(メ ソ ー ゲ ン)の 応 力場 を含 め た 外 部 場 に よる配 向 を 利 用 す る うえ で,液 晶 性 エ ラス トマ ー

(LCE)は 極 め て興 味 深 い材 料 で あ ろ う.

こ の短 い総 説 で は,ま ず 初 め に 液 晶 と液 晶 性 高 分 子 に つ い て 簡 単 な説 明 の後,ゴ ム弾 性 を示 す(そ

の た め に こ こ で は 架橋 され た 系 を対 象 と して い る)LCEに つ い て 最 近 の進 歩 を紹 介 した.LCEが 示

す 液 晶 性 は,一 般 に 前 躯 体 で あ る架 橋 前 の液 晶 性 高 分 子 の それ に近 い.し か し,液 晶温 度 領 域 に お い

て:LCEが 示 す 弾 性 は エ ネ ル ギ ー的 で,通 常 の エ ラ ス トマ ーが エ ン トロ ピー 的 で あ る の と異 って い

る.非 常 に 興 味 深 い こ とに,応 力場 に お い ては 光 学 的 異 方性 が澄 明点(乃)を 越 え て,鴇+30。C位 ま で

維 持 され る.応 力 印 加 に よ るLCEの 体 積 変 化 も注 目す べ き点 で あ ろ う.焦 電 性 セ ンサ ー材 料 と し

て,コ レス テ リ ッ クあ るい は ス メ クチ ッ クC*相 を 有 す るLCEが 試 み られ て い る の は,そ の フ レキ

シ ビ リテ ィ と(架 橋 され て い る こ とに よ る)形 状 安 定 性 を利 用 した も の であ る.

(*1マヒドン大学理学部化学科,*2京 都工芸繊維大学工芸学部物質工学科)

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