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Effect of a lateral substituent on the
mesomorphic properties of ferroelectric side
chain liquid crystalline polysiloxanes
CHAIN-SHU HSU & CHIA-HSUN TSAI Published online: 06 Aug 2010.
To cite this article: CHAIN-SHU HSU & CHIA-HSUN TSAI (1997) Effect of a lateral substituent on the mesomorphic properties of ferroelectric side chain liquid crystalline polysiloxanes, Liquid Crystals, 22:6, 669-677
To link to this article: http://dx.doi.org/10.1080/026782997208767
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E ect of a lateral substituent on the mesomorphic properties of
ferroelectric side chain liquid crystalline polysiloxanes
by CHAIN-SHU HSU* and CHIA-HSUN TSAI
Department of Applied Chemistry, National Chiao Tung University, Hsinchu, Taiwan 30050, Republic of China
(Received 26 August 1996; in ® nal form 23 January 1997; accepted 31 January 1997 )
The synthesis of side chain liquid crystalline polysiloxanes containing oligooxyethylene spacers and (S)-2-methylbutyl 4-[(4-oxybiphenyl-4¾ -yl)carbonyloxy]-3-¯ uorobenzoate mesogenic side groups is presented. Di erential scanning calorimetry, optical polarizing microscopy and X-ray di raction measurements reveal liquid crystalline properties for all synthesized mon-omers and polymers. All three precursor ole® nic monmon-omers reveal cholesteric and smectic A phases. The ole® nic monomer which contains two oligooxyethylene units in the spacer is the only one which reveals a twist grain boundary A phase and a blue phase, besides the cholesteric and smectic A phases. All three polysiloxanes present enantiotropic smectic A and chiral smectic C phases. The mesomorphic behaviours of the monomers and polymers are compared with those of the corresponding monomers and polymers without the lateral ¯ uoro substituent. The results seem to demonstrate that incorporating a lateral ¯ uoro substituent in the mesogenic cores of the monomers a ects not only the mesophase thermal stability, but also the nature of the mesophases formed. However, incorporating a lateral ¯ uoro substituent in the mesogenic cores of the polymers a ects only the thermal stability of the mesophases formed. The lateral ¯ uoro substituent has a more profound e ect on the mesomorphic behaviour for the monomers than that for the polymers.
1. Introduction ethylene spacer can dramatically increase the thermal The potential application of ferroelectric liquid crys- stability of the chiral smectic C phase.
tals (FLCs) in fast switching, high resolution electro- Investigations of the e ect of lateral substituents on optical devices is well documented [1± 3]. Besides low liquid crystalline behaviour may have several objectives, molar mass FLCs, a number of ferroelectric liquid such as the desire to determine to what extent clearing crystalline side chain polymers have been successfully points and the relative stabilities of di erent mesophases prepared during the past few years [4± 27]. These poly- are a ected by the size and type of lateral substituents mers are interesting because of their ferroelectric proper- or to ® nd out how physical properties, e.g. dielectric ties and this ® eld has been reviewed by LeBarney and anisotropy, elastic constants and viscosity are modi® ed. Dubois [28]. However the detailed structure± property The e ect of molecular structure on the occurrence of relationships of ferroelectric liquid crystalline polymers tilted smectic C phases is very di erent from that on (LCPs) has not been clear until now. It seems necessary nematic or orthogonal smectic phases. The molecules to introduce additional functional groups into the meso- should possess some degree of steric asymmetry, so that genic units, spacers and polymer backbones of side chain they can arrange themselves in a tilted layer when liquid crystalline polymers to create some new property packed together. The in¯ uence of a lateral substituent combinations in order to achieve these goals. on the formation of a chiral smectic C phase for low In our previous reports [29± 31], the e ects of polymer molar mass liquid crystals has been studied by Coates backbone and spacer on the formation and thermal et al. [32]. Their experimental results demonstrated that stability of the chiral smectic C phase for ferroelectric incorporation of a lateral substituent in the mesogenic side chain LCPs have been extensively studied. The core of a molecule reduces the occurrence of higher results demonstrate that the tendency towards a chiral ordered smectic phases, and also decreases their melting smectic C phase increases with increasing spacer length point in most cases. According to some other reports and ¯ exibility of the polymer backbone, and the oligoox- on low molar mass liquid crystals in the literature [33, 34], introducing a suitable lateral substituent, e.g. ¯ uoro, into the mesogenic cores can enhance the formation of a tilted smectic C phase.
* Author for correspondence.
0267± 8292/97 $12´00Ñ 1997 Taylor & Francis Ltd.
and the (S)-2-methylbutyl 4-[(4-oxybiphenyl-4¾ -yl )car-bonyloxy]-3-¯ uorobenzoate mesogenic side group have been synthesized. Their characterization by di erential scanning calorimetry, optical polarizing microscopy, and X-ray di raction is presented. The e ect of a lateral ¯ uoro substituent on the mesomorphic properties of the monomers and polymers obtained is discussed.
2. Results and discussion
The synthetic route used for the preparation of mon-omersIM± IIIMis outlined in scheme 1. The chiral end group was inserted into these mesogenic compounds starting with commercially available (S)-(Õ )-2-methyl-butan-1-ol. This was done by a sequence of reactions which avoided racemization. Three monomers were characterized by di erential scanning calorimetry and
Figure 1. DSC thermograms of monomer IM (10ß C minÕ 1):
optical polarizing microscopy. The DSC thermograms
(a) heating scan; (b) cooling scan.
of monomerIM are presented in ® gure 1. MonomerIM
Scheme 1. Synthesis of monomers IM± IIIM.
(a) (b)
Figure 2. Optical polarizing micrographs displayed by IM: (a) cholesteric texture obtained at 121´7ß C; (b) smectic A texture obtained at 110´0ß C.
(a)
(c)
(b)
(d)
Figure 4. Optical polarizing micrographs displayed by IIM: (a) blue phase obtained at 81´5ß C; (b) cholesteric texture obtained at 80´0ß C; (c) twist grain boundary A texture obtained at 72´8ß C; (d ) smectic A texture obtained at 70´0ß C.
isotropic phase transition at 138´8ß C on the heating scan smectic A and chiral smectic C phases, while monomers IM± IIIM with a lateral ¯ uoro substituent, reveal (curve A). The cooling scan (curve B) looks almost
identical to the heating scan, except that a very small smectic A and cholesteric phases. Monomer IIM is the only one which presents the twist grain boundary A and supercooling ( less than 5ß C) is observed for the
exo-thermic transitions. Figures 2(a) and 2(b) display the blue phases besides the smectic A and cholesteric phases. The results seem to demonstrate that incorporating a typical smectic A and cholesteric textures exhibited
byIM. lateral ¯ uoro substituent in the mesogenic cores
dramat-ically reduces not only the isotropization temperature Monomer IIM reveals a very unusual mesomorphic
behaviour. Five thermal transitions are observed on of a monomer but also the tendency to give a chiral smectic C phase.
both DSC heating and cooling scans (curves A and B
in ® gure 3). Optical microscopic observation veri® ed the The synthesis of polymers IP± IIIP is described in scheme 2. Excess of ole® nic monomers was usually used formation of smectic A, twist grain boundary A,
choles-teric and blue phases for monomerIIM (see ® gures 4(a) to carry the hydrosilylation reaction to completion. The unreacted monomers were removed by several reprecip-to (d)). The twist grain boundary A ( TGBA) phase which
is a new type of liquid crystalline phase, was discovered itations from THF solution into methanol and by pre-parative GPC. Therefore the polymers were isolated by Goodby et al. in 1989 [35, 36]. This mesophase
predicted by de Gennes [37] and Renn and Lubensky with high purity. The polymers were characterized by di erential scanning calorimetry, optical polarizing [38], is made up of smectic A blocks which rotate
around a direction perpendicular to the molecular long microscopy and X-ray di raction. Figure 5 presents rep-resentative DSC thermograms of polymer IP. Polymer axis and in the plane of the smectic layers. The smectic
blocks are separated by a lattice of screw or edge IPshows a glass transition at 0´9ß C followed by a chiral smectic C to smectic A phase transition at 153´4ß C and dislocations. Therefore, the twist grain boundary A phase
is also called the helical smectic A phase SmA*. a smectic A to isotropic phase transition at 168´5ß C. The cooling scan looks almost identical to the heating scan, MonomerIIIM, containing three oxyethylene units in
the spacer, exhibits only smectic A and cholesteric phases except that a very small supercooling ( less than 5ß C) is observed for both exothermic transitions. Both polymers on both DSC heating and cooling scans. Table 1
sum-marizes the thermal transitions and phase transition IIP andIIIP present very similar DSC thermograms to those of polymerIP. On the heating scan, polymerIIP enthalpies of monomers IM± IIIM. The thermal
trans-itions and the thermodynamic parameters of monomers gives a glass transition at Õ 8´0ß C, followed by a chiral smectic C to smectic A phase transition at 157´9ß C and IVM± VIM which have been reported previously [29],
are also reported in table 1 for comparison. As can be a smectic A to isotropic phase transition at 160ß C, while polymer IIIP gives a glass transition at Õ 12´5ß C, fol-lowed by a chiral smectic C to smectic A phase transition at 142´1ß C and a smectic A to isotropic phase transition at 148´0ß C. Parts (a) and (b) of ® gure 6 show representat-ive smectic A and chiral smectic C textures for polymer IP. The phase assignment was also checked by X-ray di raction.
Figure 7 shows the temperature-dependent d-spacing of the smectic layers obtained from powder samples of IP. The d-spacings of the smectic layers are 30´49 AÊ at 154ß C, 29´20 AÊ at 140ß C, 28´56 AÊ at 90ß C and 28´12 AÊ at 30ß C. When the measuring temperature falls from 154 to 140ß C, the d-spacing given by the ® rst-order re¯ ection gradually decreases from 30´49 to 29´20 AÊ. This gives strong evidence for the formation of the tilted chiral smectic C phase. This result is also in agreement with the formation of a chiral smectic C phase since the tilt angle of the side chains is known to increase gradually with decreasing temperature for chiral smectic C side chain LCPs. Temperature-dependent X-ray di raction
Figure 3. DSC thermograms of IIM (10ß C minÕ 1): (a) heating
scan; (b) cooling scan. results for polymers IIP and IIIP are also shown in
Table 1. Phase transitions and phase transition enthalpy changes for monomers IM± IVM. Phase transitions/ß C (Corresponding enthalpy changes/kcal molÕ 1)a
heating
Monomer n cooling
IM 1 Cr 52´7(1´35)SmA 121´4(0´33)N* 138´8(0´06)I
I 131´5(0´08)N* 119´1(0´3)SmA 47´7(1´06)Cr
IIM 2 Cr 25´5(3´15)SmA 73´2(0´26)TGBA 77´6(0´2)N* 82´9(0´13)BP 87´2(0´02)I
I 85´5(0´07)BP 80´6(0´13)N* 75´8(0´17)TGBA 71´5(0´2)SmA 10´3(1´36)Cr
IIIM 3 Cr 16´6(3´02)SmA 44´7(0´03)N* 51´1(0´14)I I 49´6(0´08)N* 42´8(0´26)SmAÕ 10´5(0´84)Cr
IVMb 1 Cr 95´4(5´36)SmA 171´5(1´16)I
I 168´8(1´16)SmA 76´8(Ð )SmC* 58´1(1´48)Cr
VMb 2 Cr 20´5(0´99)SmC* 72´5(0´03)SmA 140´2(0´82)I
I 138´5(0´76)SmA 69´1(0´03)SmC* 12´1(0´98)Cr
VIMb 3 Cr 7´6(0´93)SmC* 57´4SmA 106´6(0´73)I
I 103´2(0´75)SmA 54´5(0´01)SmC* 1´4(0´80)Cr
aCr=crystalline, SmA=smectic A, TGBA=twisted grain boundary A, SmC*=chiral smectic C,
N*=cholesteric, BP=blue phase, I=isotropic.
bData obtained from ref. [30].
Scheme 2. Synthesis of polysilox-anes IP± IIIP.
® gure 7. Both polymers also reveal smectic A and chiral genic moieties. All three polymers form enantiotropic smectic A and chiral smectic C phases, although their smectic C phases. The d-spacings of the smectic layers
of polymer IIP are 33´86 AÊ at 150ß C, 32´95 AÊ at 135ß C, corresponding monomers give cholesteric and smectic A phases. It is well-documented in many cases that the 32´24 AÊ at 85ß C and 31´76 AÊ at 30ß C, while those for
polymer IIIP are 36´51 AÊ at 148ß C, 35´30 AÊ at 130ß C, mesophase formed by a side chain LCP is more organ-ized than that exhibited by the corresponding monomer; 34´81 AÊ at 80ß C and 34´63 AÊ at 35ß C. The ferroelectric
properties of polymers IP± IIIP have also been studied this is the so-called `polymer e ect’ for side chain LCPs. In order to study the lateral substituent e ect on the by broad band dielectric relaxation and are reported
elsewhere [39]. mesomorphic behaviour of the new polymers, table 2
also lists the thermal transitions and the corresponding Table 2 summarizes the thermal transitions and
cor-responding enthalpy changes for polymers IP± IIIP enthalpy changes for polymersIVP± VIP which contain no ¯ uoro substituent in the mesogenic side chains [30]. which contain a lateral ¯ uoro substituent in the
Figure 5. DSC thermograms of polymer IP (10ß C minÕ 1):
(a) heating scan; (b) cooling scan.
As can be seen from the data listed in table 2, all six polymers exhibit enantiotropic smectic A and chiral smectic C phases.
The experimental results indicate that incorporating a lateral ¯ uoro substituent into the mesogenic core of a side chain LCP does not change the nature of the mesophases formed, but decreases the thermal stability of the mesophases including that of the chiral smectic C phase. Nevertheless, polymersIP± IIIPstill produce very wide temperature ranges (about 150 to 160ß C) of chiral
(a)
(b)
smectic C phase. These results again indicate that the Figure 6. Optical polarizing micrographs displayed by poly-nature of the ¯ exible oligooxyethylene spacer plays a mer IP: (a) smectic A texture obtained at 155´0ß C; (b) chiral very important role with respect to the thermal stability smectic C texture obtained at 140´0ß C.
of the chiral smectic C phase.
Spontaneous polarizations of both series of polymers heated under re¯ ux over sodium and then distilled under were also evaluated. A voltage of 6 V was applied to a nitrogen.
5mm thick cell. Figure 8 presents the polarization (Ps)
as a function of temperature for polymersIIP and VP. 3.2. Techniques
1H NMR spectra (300 MHz) were recorded on a
The Ps values of polymers IIP and VP are 21´4 and
Varian VXR-300 spectrometer. FTIR spectra were meas-12´4 nC cm
Õ
2at 60ß C. This experimental resultdemon-ured using a Nicolet 520 FTIR spectrometer. Polymer strates that incorporating a lateral ¯ uoro substituent
samples were cast as ® lms on a KBr tablet for the IR into the mesogenic core of this LCP decreases its Ps
measurements. Thermal transitions and thermodynamic value.
parameters were determined by using a Seiko SSC/5200 3. Ex perimental di erential scanning calorimeter equipped with a liquid 3.1. Materials nitrogen cooling accessory. Heating and cooling rates Poly(methylhydrogensiloxane) (M
Â
nÅ
=2270) and were 10ß C minÕ
1. Thermal transitions reported weredivinyltetramethyldisiloxane platinum catalyst were collected during the second heating and cooling scans. obtained from Petrarch system Inc. and were used as A Carl Ziess Axiophot optical polarizing microscope received. (S)-(Õ )-2-Methylbutan-1-ol, [a]25
D=Õ 6´5ß equipped with a Mettler FP82 hot stage and an FP80
(from Merck Ltd ), 2-allyloxyethanol and 4-hydroxy- central processor was used to observe the thermal trans-biphenyl-4¾ -carboxylic acid (from Tokyo Kaisei Co.) and itions and to analyse the anisotropic textures. all other reagents (from Aldrich) were used as received. Preparative gel permeation chromatography (GPC) was carried out using a Waters 510 LC instrument equipped Toluene used in the hydrosilylation reaction was ® rst
with a 410 di erential refractometer and a preparative GPC column (22´5 mmÖ 60 cm) supplied by American Polymer Standard Co. X-ray di raction measurements were made with nickel-® ltered CuKa -radiation with a Rigaku powder di ractometer. Optical rotations were measured at 25ß C using a Jasco DIP-140 polarimeter with chloroform as solvent for all compounds. Spontaneous polarization was evaluated using a Displaytech (Boulder, Colorado, USA) APTIII automated property tester.
3.3. Synthesis of monomers
The synthesis of the ole® nic monomersIM± IIIMused for hydrosilylation is outlined in scheme 1. 2-(2-Allyloxyethoxy)ethanol and 2-[2-(2-allyloxyethoxy)-ethoxy]ethanol were synthesized according to a literat-ure procedliterat-ure [40].
The detailed synthetic procedures for compounds1± 6 in scheme 1 were reported in a previous publication [30].
3.3.1. 3-Fluoro-4-hydroxybenzoi c acid (7)
3-Fluoro-4-hydroxybenzoic acid was prepared follow-ing a method reported in the literature [41].
3.3.2. (S)-2- Methylbutyl 3-¯ uoro-4-hydroxy benzoate (8) 3-Fluoro-4-hydroxybenzoic acid (3´0 g, 19´2 mmol),
Figure 7. Temperature dependence of the smectic C layer
(S)-2-methylbutan-1-ol (2´61 g, 29´6 mmol), and
concen-spacings for polymers IP (&), IIP (+) and IIIP ($).
trated sulphuric acid (0´2 ml ) were added to 15 ml of dried benzene. The reaction mixture was heated under re¯ ux until the 3-¯ uoro-4-hydroxybenzoic acid was com-pletely dissolved and 0´35 ml of water had collected in a Dean± Stark trap. After cooling to room temperature,
Table 2. Phase transitions and phase transition enthalpy the solution was washed with 2% aqueous NaHCO
3
changes for polymers IP± IVP. and water, and dried over anhydrous MgSO
4. The
solvent was then removed by rotary evaporation and
Phase transitions/ß C (Corresponding
the solid puri® ed by column chromatography (silica gel,
enthalpy changes/kcal mruÕ 1)a
ethyl acetate/n-hexane=151 as eluent) to yield 3´4 g
heating
Polymer n cooling (78 %) of pale yellow crystals. [a]25
D=+5´6 (c= 0´32 g cm
Õ
3 in chloroform). 1H NMR (CDCl 3, IP 1 g 0´9SmC* 153´4(Ð )bSmA 168´5(0´66)I TMS, ppm): d0´90± 0´99 (m, 6H, ± CH 3), 1´18± 1´59 (m, I 164´6(0´57)SmA 150´4(Ð )bSmC* 2H, ± CH2± ), 1´75± 1´89 (m, 1H, ± CH(CH3)± ), 4´11 (ABd, IIP 2 gÕ 8´0SmC* 157´9(Ð )bSmA 160´0(0´87)I2H, ± COOCH2± ), 6´95 (t, 1H, ± O± ArH3(F)± ), 7´79 (d,
I 157´3(0´71)SmA 155´3(Ð )bSmC*
2H, ± ArH3(F)± COO± ). C12H15O3F: calcd: C 63´71, H IIIP 3 gÕ 12´5SmC* 142´1(Ð )bSmA 148´0(0´87)I 6´68; found: C 63´83, H 6´71 per cent.
I 144´8(0´77)SmA 139´8(Ð )bSmC*
IVPc 1 g 9´8SmC* 215´2(0´07)SmA 234´6(0´80)I 3.3.3. (S)-2- Methylbutyl 4-[[4-(2-allyloxyethox
y)-I 229´2(0´73)SmA 211´7(0´03)SmC* biphenyl-4¾ -yl]carbonylox y]-3-¯ uorobenzoat e
VPc 2 gÕ 11´2SmC* 208´0(Ð )bSmA 211´8(0´85)I (IM), (S)-2- Methylbutyl 4-[[4-[2-(2-
allyloxy-I 212´0(0´49)SmA 206(Ð )bSmC*
ethoxy)ethoxy]biphenyl 4¾ yl]carbonyloxy] -VIPc 3 gÕ 25´1SmC* 168´0(Ð )bSmA 190´3(0´94)I 3-¯ uorobenzoat e (IIM), and (S)-2- Methylbutyl
I 184´2(0´98)SmA 161(Ð )bSmC*
4-[[4-[2-[2- (2-allyloxyeth oxy)ethoxy]ethoxy]-biphenyl-4¾ -yl]carboxylo xy]-3-¯ uorobenzoate
amru=mol repeating unit: g=glassy, S=smectic, SmA=
(IIIM)
smectic A, SmC*=chiral smectic C, I=isotropic.
The ole® nic monomers IM± IIIM were prepared by
bOverlapped transition.
cData obtained from ref. [30]. esteri® cation using (S)-2-methylbutyl 3-¯
Figure 8. Spontaneous polariza-tion Ps versus temperature for
polymer IIP (#) and VP (&).
roxybenzoate and the corresponding acids 4± 6. The respect to the Si± H groups present in polysiloxane) was synthesis of monomerIIM is described below. dissolved in 100 ml of dry, freshly distilled toluene 4-[2-(2-Allyloxyethoxy)ethoxy]biphenyl-4¾ -carboxylic together with the proper amount of poly(methylhydrog-acid (2´0 g, 5´8 mmol) was reacted at room temperature ensiloxane). The reaction mixture was heated to 110ß C with an excess of thionyl chloride containing a drop of under nitrogen and 100mg of divinyltetramethyldisilox-dimethylformamide in 7 ml of methylene chloride for ane platinum catalyst was then injected with a syringe 2 h. The solvent was removed under reduced pressure to as a solution in toluene (1 mg ml
Õ
1). The reactionmix-give the crude acid chloride. The product was dissolved ture was heated under re¯ ux (110ß C) under nitrogen for in 10 ml of methylene chloride and added slowly to 24 h. After this reaction time, FTIR analysis showed that a cold solution of (S)-2-methylbutyl 3-¯ uoro- the hydrosilylation reaction was complete. The polymers 4-hydroxybenzoate (1´56 g, 7´0 mmol ) and 4-dime- were separated, puri® ed by several reprecipitations from thylaminopyridine (0´85 g, 7´0 mmol) in 100 ml of methyl- tetrahydrofuran solution into methanol, further puri® ed ene chloride. The solution was allowed to stir for 2 h by preparative GPC, and then dried under vacuum. and then the solvent was removed by rotary evaporation.
The crude product was puri® ed by column chromato-graphy (silica gel, ethyl acetate/n-hexane=153 as eluent)
4. Conclusions to yield 1´32 g (43%) of white crystals. [a]25
D=+5´86 A series of new ferroelectric side chain liquid
crystal-(c=0´35 g cm
Õ
3 in chloroform). 1H NMR(CDCl3, line polysiloxanes containing oligooxyethylene spacers
TMS, ppm) d 0´94± 1´01 (m, 6H, ± CH3), 1´11± 1´59 (m,
and 4(S)-2-methylbutyl [(4-oxybiphenyl-4¾ -yl )car-2H, ± CH(CH3)± CH2± ), 1´81± 1´95 (ABd, 1H, ± CH
bonyloxy]-3-¯ uorobenzoate side groups has been pre-(CH3)± ), 3´59 (d, 2H, ² CH± CH2± ) 4´11± 4´24 (m, 10H, pared. All the polymers exhibit enantiotropic smectic A
± (OCH2CH2)2± and ± O± CH2± ), 5´14 and 5´32 (2d, 2H,
and chiral smectic C phases. The synthesized polymers H2C=d) 5´81± 5´99 (m, 1H, =dCH± ), 6´96 (d, 2H,
IP± IIIPrepresent a system that contains a lateral ¯ uoro biphenyl protons), 7´37 (t, 1H, ± OArH3(F)± ), 7´60 (d,
substituent. If they are compared with the corresponding 2H, biphenyl protons), 7´71 (d, 2H, ± ArH3(F)± COO± ],
polymers IVP± VIP which contain no lateral ¯ uoro 7´91(d, 2H, biphenyl protons), 8´26 (d, 2H, biphenyl
substituent, the former display the same mesomorphic protons). C32H35O7F: calcd: C 69´80, H 6´41; found: C
behaviour as the latter. However, the mesomorphic 69´83, H 6´50 per cent.
temperature ranges exhibited by the former are much narrower than those displayed by the latter. The experi-3.4. Synthesis of polysiloxane sIP± IIIP
mental results indicate that incorporating a lateral ¯ uoro The synthesis of liquid crystalline polysiloxanes1P± 3P
substituent into the mesogenic core of a polymer can is outlined in scheme 2. A general synthetic procedure is
decrease the thermal stabilities of the mesophases, described below.
The ole® nic derivative 1´0 g (10 mol % excess with including the chiral smectic C phase.
[19] Scherowsky, G., Schliwa, A., Springer, J.,
The authors are grateful to the National Science
Kuhnpast, K., and Trapp, W., 1989, L iq. Cryst., 5, 1281.
Council of the Republic of China for ® nancial support [20] Shibaev, V. P., Kozlovsky, M. V., and Plate’, N. A., of this work. (NSC82-0511-E009-01) 1990, L iq. Cryst., 8, 1281.
[21] Dumon, M., Nguyen, H. T., Mauzac, M., Destrade, C., Achard, M. F., and Gasparoux, H., 1990, Macromolecules, 23, 355.
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