Synthesis and Thermal Properties of Ferroelectric Side-Chain Liquid

The polymer PS12NC derivative with one unit of oxyethylene spacer (n = 1) of PSn2NC series reveals a N* phase, a twist grain boundary A (TGBA) phase, ...
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Chem. Mater. 1997, 9, 51-60

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Synthesis and Thermal Properties of Ferroelectric Side-Chain Liquid-Crystalline Polysiloxanes Based on Naphthyl Biphenylcarboxylate Mesogenic Groups and Oligooxyethylene Spacers Jr-Hong Chen, Ging-Ho Hsiue,* and Cheng-Pei Hwang Department of Chemical Engineering, National Tsing Hua University, Hsinchu, Taiwan 300, ROC Received December 12, 1996. Revised Manuscript Received April 24, 1996X

In this work, three series of ferroelectric side-chain polysiloxanes containing oligooxyethylene spacers and (S)-2-methyl-1-butyl (2S)-2-[6-(4-hydroxybiphenyl-4′-carbonyloxy)-2′naphthyl]propionates, (R)-1-methylheptyl (2S)-2-[6-(4-hydroxybiphenyl-4′-carbonyloxy)-2′naphthyl]propionates, and (2S,3S)-2-chloro-3-methylpentyl (2S)-2-[6-(4-hydroxybiphenyl-4′carbonyloxy)-2′-naphthyl]propionates mesogenic groups were synthesized. The mesomorphic behaviors of three series of ferroelectric side-chain liquid-crystalline polysiloxanes were also studied using differential scanning calorimetry, optical polarizing microscopy, and highresolution X-ray diffraction measurements. Some of these side-chain polysiloxanes containing four phenyl rings of ester cores (i.e., -Ph-Ph-COO-naphthyl-) and chiral heptyl tail present a smectic A (SA), and a chiral smectic C (Sc*) phase (around 100 °C). Another two series containing four phenyl rings of ester cores and chiral butyl and pentyl chain tail reveal a cholesteric (N*), an SA phase and an Sc* phase. The polymer PS12NC derivative with one unit of oxyethylene spacer (n ) 1) of PSn2NC series reveals a N* phase, a twist grain boundary A (TGBA) phase, and a Sc* phase. X-ray investigations reveal that the packing of mesogenic groups for PS02NA, PS02NB, PS02NC with the shortest spacers (n ) 0) exhibit a bilayer (two-layer) packing structures. Moreover, polymer PS32NA, PS32NB, PS32NC with the longest spacer chain (n ) 3) reveal a monolayer packing structure. However, these two types of packing structure appear simultaneously in PS12NA (n ) 1), PS12NC (n ) 1), PS12NB (n ) 1), and PS22NB (n ) 2), depending on the chiral tail length. Results obtained in this study again demonstrate that the tendency toward chiral smectic C mesomorphism increases as the rigidity of phenyl ester mesogens increases via the flexible oligooxyethylene spacers system. Furthermore, the thermal stability of the chiral smectic C mesophase is determined by the flexibility of the chiral tail.

Introduction The properties of ferroelectric liquid crystal (FLC) cell (e.g., bistability, spontaneous polarization (Ps), response time (t), and tilt angle (q)) have been extensively investigated.1-6 Interest in this topic has increased since Meyer et al.7 proved that the Sc* mesophase is ferroelectric and Clark and Lagerwall’s8 pioneering work involving the development of surface-stabilized ferroelectric liquid-crystal (SSFLC) display technology. For FLC display devices, it is desirable that FLC material shows a chiral smectic C phase over a wide temperature range including room temperature. More* To whom all correspondence should be addressed. X Abstract published in Advance ACS Abstracts, December 15, 1996. (1) Leslie, T. M. Ferroelectrics 1984, 58, 9. (2) Furukawa, K.; Terashima, K.; Ichihashi, M.; Inoue, H.; Saito, S.; Inukai, T. 6th Liq. Cryst. Conf. Soc. Count. Halle (GDR), Abstract, A37, 1985. (3) Furukawa, K.; Terashima, K. Eur. Pat. Appl. EP 178,647, 1986. (4) Keller, P. Mol. Cryst. Liq. Cryst. 1984, 102, 295. (5) Koden, M.; Katsuse, H.; Itoh, N.; Kaneko, T.; Tamai, K.; Takeda, H.; Shiomi, M.; Numao, N.; Kido, M.; Matsuki, M.; Miyoshi, S.; Wada, T. Abstracts of Fourth International Conference on Ferroelectric Liquid Crystals, 1993, p 146, 369. (6) Tajima, E.; Kondoh, S.; Suzuki, Y. Abstracts of Fourth International Conference on Ferroelectric Liquid Crystals, 1993, pp 147, 371. (7) Meyer, R. B.; Liebert, L.; Strzelecki, L.; Keller, P. J. Phys. Lett. 1975, 36, 69. (8) Clark, N. A.; Lagerwall, S. T. Appl. Phys. Lett. 1980, 36, 898.

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over, the liquid-crystal material must possess large spontaneous polarization. Consequently, a FLC related device can be operated at reduced driving voltage. These properties are influenced by molecular structures of the liquid-crystal compounds. An increasing interest in the synthesis of low molar mass Sc* liquid crystals has subsequently developed. Numerous FLC compounds and room-temperature mixtures have been prepared for rapid electrooptical applications. Besides low molar mass FLCs, several side-chain liquid-crystalline polymers (LCPs) exhibiting a Sc* mesophase and their ferroelectric properties have been reported in recent years.9-23 LeBarny and Dubois thoroughly re-

(9) Shibaev, V. P.; Beresnev, L. A.; Blinov, L. M.; Kozlovsky, M. V.; Plate, N. A. Polym. Bull. 1984, 12, 299. (10) Decobert, G.; Soyer, F.; Dubois, J. C. Polym. Bull. 1985, 14, 179. (11) Dubois, J. C.; Decobert, G.; LeBarny, P.; Esselin, S.; Friedrich, C.; Noel, C. Mol. Cryst. Liq. Cryst. 1986, 137, 349. (12) Zentel, R.; Rekert, G.; Reck, B. Liq. Cryst. 1987, 2, 83. (13) Hahn, B.; Percec, V. Macromolecules 1987, 20, 2961. (14) Uchida, S.; Morita, K.; Miyoshi, K.; Hashimoto, K.; Kawasaki, K. Mol. Cryst. Liq. Cryst. 1988, 155, 93. (15) Kapitza, H.; Zental, R. Makromol. Chem. 1988, 189, 1793. (16) Vallerien, S. U.; Zentel, R.; Kremer, F.; Kapitza, H.; Fischer, E. W. Makromol. Chem., Rapid Commun. 1989, 10, 333.

© 1997 American Chemical Society

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Scheme 1. Synthesis of Monomer Series MDn2NA, MDn2NB, and MDn2NC

viewed the related field.24 However, the exact relationship between structure and property of Sc* LCPs remains unclear owing to the limited experimental data found in previous literature. As a part of the research program dedicated to developing highly efficient FLC materials, novel low mass FLCs and ferroelectric sidechain liquid-crystalline polymers have been designed and synthesized in our group.25-28,30-33 They exhibit a broad temperature range of chiral smectic C phase and satisfactory electrooptical properties.29 In our previous study (part 1),28 phenyl benzoate/biphenyl benzoate mesogens with three different varieties of chiral moieties were synthesized. The chiral smectic C phase has a tendency of being present in the three phenyl rings of ester core and short chiral moieties (butyl and pentyl chain). However, the chiral heptyl chain seemed too flexible to couple with biphenyl benzoate mesogenic (17) Scherowsky, G.; Schliwa, A.; Springer, J.; Kuhnpast, K.; Trapp, W. Liq. Cryst. 1989, 5, 1281. (18) Shibaev, V. P.; Kozlovsky, M. V.; Plate, N. A.; Beresnev, L. A.; Blinov, L. M. Liq. Cryst. 1990, 8, 545. (19) Dumon, M.; Nguyen, H. T.; Mauzac, M.; Destrade, C.; Achard, M. F.; Gasparoux, H. Macromolecules 1990, 23, 355. (20) Brand, H. R.; Pleiner, H. Makromol. Chem., Rapid Commun. 1990, 11, 607. (21) Endo, H.; Hachiya, S.; Uchida, S.; Hashimoto, K.; Kawasaki, K. Liq. Cryst. 1991, 9, 635. (22) Kapitza, H.; Zentel, R. Makromol. Chem. 1991, 192, 1859. (23) Bomelburg, J.; Heppke, G.; Hollidt, J. Makromol. Chem., Rapid Commun. 1991, 12, 483. (24) LeBarny, P.; Dubois, J. C. In Side Chain Liquid Crystal Polymers; McArdle, C. B., Ed.; Blackie: Glasgow, London, 1989; p 130. (25) Hsu, C. S.; Lin, J. H.; Chou, L. R.; Hsiue, G. H. Macromolecules 1992, 25, 7126. (26) Hsu, C. S.; Shih, L. J.; Hsiue, G. H. Macromolecules 1993, 26, 3161. (27) Chen, J. H.; Chang, R. C.; Hsiue, G. H. Ferroelectrics 1993, 147, 241. (28) Hsiue, G. H.; Chen, J. H. Macromolecules 1995, 28, 4366. (29) Wu, S. L.; Hsieh, W. J.; Chen, D. G.; Chen, S. J.; Shy, J. T.; Hsiue, G. H. Mol. Cryst. Liq. Cryst. 1995, 265, 39. (30) Chen, J. H.; Chang, R. C.; Guu, F. W.; Hsiue, G. H.; Wu, S. L. Liquid Cryst. 1995, 18, 291. (31) Hsiue, G. H.; Lee, G. R.; Chen, J. H. Macromol. Chem. Phys. 1995, 196, 2601. (32) Chen, J. H.; Hsiue, G. H.; Hwang, C. P.; Wu, J. L. Liq. Cryst., in press. (33) Hsiue, G. H.; Hwang, C. P.; Chen, J. H. Liq. Cryst., in press.

Chen et al. Scheme 2. Synthesis of Polymers Series PSn2NA, PSn2NB, and PSn2NC

group, thereby resulting in the vanishing of Sc* phase. In this study, several new series of ferroelectric sidechain liquid-crystalline polysiloxanes containing oligooxyethylene spacers, naphthyl biphenylcarboxylate mesogens, and three different kinds of chiral moieties ((S)-2-methyl-1-butyl, (R)-1-methylheptyl, and (2S,3S)2-chloro-3-methyl pentyl) are synthesized. Their characterizations are also made by differential scanning calorimetry, optical polarizing microscopy, and highresolution X-ray diffraction measurements. Experimental Section Materials. Allyl bromide, 2-chloroethanol, 2-(2-chloroethoxy)ethanol, 2-(2-(2-chloroethoxy)ethoxy)ethanol, and 4-hydroxy-4′-biphenylcarboxylic acid were purchased from Aldrich (USA). (R)-2-Octanol and (S)-2-(6-methoxy-2-naphthyl)propionic acid were obtained from TCI (Japan), (S)-2-Methyl-1butanol was from Fluka; and L-isoleucine, boron tribromide, and other reagents were purchased from Merck (Germany) and used as received. Poly(methylhydrosiloxane) (M h n ) 2270) and divinyltetramethyldisiloxane platinum catalyst were obtained from Hu¨ls Inc. (Germany) and used as received. Toluene was used in the hydrosilation reaction over sodium and then distilled under nitrogen. Techniques. A Seiko DSC 220C/5200H differential scanning calorimeter equipped with a liquid nitrogen cooling accessory was used to determine the thermal transitions and thermodynamic parameters. Heating and cooling rates were 10 and 5 °C min-1. The thermal transitions were collected during the second heating and cooling scans. 1H NMR spectra were recorded on a Bruker AM 300 Hz spectrometer. FT-IR spectra were measured on a Bio-Rad FTS-155 spectrometer. Next, polymer samples were cast as films on a KBr tablet for the measurement. A Nikon Microphot-FX optical polarized microscope equipped with a Mettler FP-80 hot stage and FP-

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Table 1. Transition Temperatures for the Series MDn2NA, MDn2NB, and MDn2NCa phase transition, °C (corresponding enthalpy changes, mJ/mg) name

n

MD02NA

0

heating cooling C 105(-) SE 113.2(45.7) SA 168.3(8.5) I I 166.8(7.3) SA 98.6(11) SE 83.8(31.7) C

MD12NA

1

C 98.6(1.2) SE 120.1(48.5) SA 126.3(3.6) I I 124.8(6.2) SA 81.3(-) Sc* 77.1(46.1)b C

MD22NA

2

C 90.6(-) SA 93.2(77.3)c I I 91.7(4.9) SA 84.7(1.1) Sc* 68.6(62.2) C

MD32NA

3

C74.11(36.8) I I 63.5(4.5) SA 60.0(-) Sc* 53.4(24.7)b C

MD02NB

0

C 26.2(3.8) Sx 76.3(4.8) SE 97.6(37.4) SA 167.3(7.7) I I 165.9(6.1) SA 93.7(11.6) SE 74.5(26.4) Sx 23.2(4.5) C

MD12NB

1

C 80.9(64.1) SA 136.2(-) TGBA 136.3(-) Ch 137.9(2.0)d BP 141.1(0.4) I I 139.6(0.8) BP 136.3(-) Ch 136(-) TGBA 135.9 (2.3)d SA 70.8(11.1) SE 36.1(2.6) C

MD22NB

2

C 65.3(31.4) SA 97.5(-) TGBA 97.7(-) CH 102.2(0.8)d BP 103.4(0.5) I I 102.4(0.4) BP 101(-) Ch 97.4(-) TGBA 97(1.0)d SA 59.2(-)e Sc* 25.1(11.2) C

MD32NB

3

C 20.5(15.2) Sx 39(18.7) Sc* 52(-)e SA 70(-) TGBA 72.6(-) Ch 74.2(0.4)d BP 79.6(0.5) I I 78(0.6) BP 74(-) Ch 72.4(-) TGBA 69.8(0.6)d SA 50(-)e Sc* 6.2(17.2) C

MD02NC

0

C 120.0(54.1) SA 164.2(11.0) I I 163.1(11.1) SA 101.4(52.5) C

MD12NC

1

C106.4(71.8) SA 127.3(7.8) I I 125.8(7.8) SA 83.8(0.1) Sc* 72.7(54.9) C

MD22NC

2

C 92.0(-) SA 94.5(72.8)c I I 90.9(5.0) SA 79.7(0.7) Sc* 66.8(55.5) C

MD32NC

3

C 57.2(22.9) SA 63.3(18.1) I I 58.9(-) SA 57.4(5.8)f Sc* 43.8(38.8) C

a I ) isotropic; S ) smectic E; Sc* ) chiral smectic C; S ) smectic A; TGB ) twist grain boundary A; Ch ) cholesteric; BP ) blue E A A phase; C ) crystalline phase). b ∆H(SA- -Sc*- -K). c ∆H(K- -SA- -I). d ∆H(SA- -TGBA- -Ch). e Enthalpies were too small to be evaluated. f ∆H(I-SA- -Sc*).

82 central processor was used in observing thermal transitions and anisotropic textures. Applied Biosystem 200 instrument equipped with a Viscotek differential refractometer and a preparative GPC column (22.5 mm × 60 cm) was supplied by American Polymer Standard Co. (USA). X-ray diffraction measurements were then taken with a Rigaku R-Axis IIC powder diffractometer. Two imaging plate (abbreviated IP) detectors were used so that reflection spot exposure and readout operations could be performed. This feature provides efficient data collection and minimizes the time required for IP residual image erasure. The monochromatized X-ray beam from nickel-filtered Cu KR radiation with a wavelength of 0.154 06 nm was used. The resolution was 0.05° for scanning direction. Finally, a temperature controller was added to the X-ray apparatus for thermal measurements. The precision of the controller was (0.5 °C in the temperature range studied. Synthesis of Monomers. The synthesis of olefin monomers, series MDn2NA, MDn2NB and MDn2NC for the hydrosilation reaction outlined in Scheme 1, has been described elsewhere.33 Synthesis of Polysiloxanes Series PSn2NA, PSn2NB, and PSn2NC. The synthesis of liquid crystalline polysiloxanes is outlined in Scheme 2. A general synthetic procedure is described below. For example, 0.5 g of the olefin derivative MD22NA (10 mol % excess versus the Si-H groups in polysiloxanes) was dissolved in 50 mL of dry, freshly distilled toluene together

with the proper amount of poly(methylhydrosiloxane). The reaction mixture was heated to 110 °C under nitrogen. Next, 100 µg of divinyltetramethyldisiloxane platinum catalyst was injected with a syringe as a solution in toluene (1 mg/mL). The reaction mixture was then refluxed (110 °C) under nitrogen for 24 h. After this reaction time, FT-IR analysis indicated that the hydrosilation reaction was complete. The polymers were separated and purified by several reprecipitations from tetrahydrofuran solution into methanol, further purified by preparative GPC, and then dried under vacuum. 1H NMR (CDCl3, δ ) ppm) 0.79-0.89 (m, 13H, -[Si(CH3)(CH2CH2-) O]-, -CH(CH3)CH2CH3), 1.05-1.40 (m, 2H, (-CH2CH3), 1.50-1.65 (m, 4H, -CH2CH(CH3)-, -CH(CH3)COO-), 3.604.25 (m, 13H, -CH(CH3)COO-, -OCH2CH(CH3)-, -CH2(OCH2CH2)2O-), 7.0-8.30 (m, 14H, aromatic protons).

Results and Discussion Table 1 summarizes the thermal transition and corresponding enthalpy changes of monomers series MDn2NA, MDn2NB, and MDn2NC. The phase assignments of these olefin monomers have been characterized in detail using DSC, POM, and powder X-ray diffraction measurements.33 The synthesis of polymers series PSn2NA, PSn2NB, and PSn2NC is described in Scheme 2. An excess amount of olefin monomers was usually

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(a)

(b)

(c)

Figure 1. Plots of transition temperatures versus n, the number of oxyethylene spacer chain: (a) series PSn12A; (b) series PSn12B; (c) series PSn12C.

used to carry out the hydrosilation reaction. The unreacted monomers were removed by several reprecipitations: from chloroform solution into methanol and by preparative GPC. Therefore, the polymers were isolated with high purity. Series PSn2NA. This series contained (S)-2-methyl1-butyl (2S)-2-[6-(4-hydroxybiphenyl-4′-carbonyloxy)-2′-

Figure 2. DSC thermogram (10 °C/min) of polymer (a) series PS22NA, (b) series PS22NB, and (c) series PS22NC. Curve A: heating scan. Curve B: cooling scan.

naphthyl]propionates mesogenic side group and oxyethylene spacers. All of the polymers reveal liquidcrystalline mesomorphism (Figure 1a). Table 2 summarizes the thermal transitions and thermodynamic parameters of the obtained polymer PSn2NA series. All of the compounds of this series show enantiotropic cholesteric (N*), smectic A (SA), chiral smectic C (Sc*) phases, and an unknown smectic (Sx) phase, respectively. This is except for the shortest spacer

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Table 2. Phase Transitions and Phase Transition Enthalpies for Polymer Series PSn2NA polymer

na

PS02NAc

0

heating phase transitions, °C cooling (corresponding enthalpy changes; kcal/mru)b G 16.5 Sx 100(1.0) SA 165.3(0.97) I I 164.8(0.914) SA 98.2(1.0) Sx 14 G

PS12NAd

1

G 18.3 Sx 90(0.476) Sc* 140(-)e SA 148.2(-) N* 160(0.864)f I I 158.4(0.715)f N* 147(-) SA 138.9(-) Sc* 88(0.447) Sx 20.8 G

PS22NA

2

G 12.7 Sx 91.1(2.01) Sc* 150(-)g SA 153(-)g N* 156(1.26)f I I 155.7(1.24)f N* 152(-)g SA 148.3(-)g Sc* 79.9(1.85) Sx 12.9 G

PS32NA

3

G 3.5 Sx 79.4(1.292) Sc* 123.0(-) SA 128(-) N* 132(1.12)f I I 130(1.02)f N* 127(-) SA 120(-) Sc* 72.4(1.05) Sx -1.7 G

a n according to Scheme 2. b mru ) mole repeating unit, G ) glassy, Sx ) unknown smectic, S ) smectic A, Sc* ) chiral smectic C, A N* ) cholesteric, I ) isotropic. c The LC phases investigated by X-ray reveal a bilayer structure. d The LC phases examined by X-ray e f reveal a coexistence of bilayer and monolayer structure. The transition was difficult to detect. ∆H (Sc*-SA-N*-I). g The transition was decided by POM.

Figure 3. Temperature-dependent X-ray measurements for polymer PS22NA at (A) 155, (B) 150, (C) 147, (D) 112, (E) 80, and (F) 55 °C.

derivative polymer, PS02NA (n ) 0), which shows enantiotropic smectic A and Sx phases. Representative DSC heating and cooling traces of polymer PS22NA (n ) 2) are presented in Figure 2a. In this figure, the heating scan (curve A) reveals a slight glass transition (Tg) at 12.7 °C. The cooling scan (curve B) shows an isotropic to N* transition, a N* to a SA transition, a SA to a Sc* transition and a Sc* to a Sx transition at 155.7, 152, 148.3, and 79.9 °C, respectively. However, the individual transitional entropies (∆H) of I/N*, N*/SA, and SA/Sc* were difficult to detect due to overlapping; in addition, nearly no glass transition was found on the cooling scan. The phase assignment was conducted by optical polarizing microscopic observation and X-ray diffraction measurements. Figure 3 presents the temperature-dependent X-ray diffraction diagrams obtained

from a powder sample of PS22NA at 155, 150, 147, 112, 80, and 55 °C, respectively. A broad reflection at wide angles (associated with the lateral packings) and a sharp reflection at small angles (associated with the smectic layers) are separately shown by all curves. Curve A exhibits a diffuse reflection at around 4.96 Å, which corresponds to the lateral spacing of two mesogenic side groups, and a weak reflection at 31.15 Å which corresponds to a weak smectic layer structure. The optical polarizing micrograph (Figure 4a) reveals a quasi-focal-conic texture for polymer PS22NA at 152.4 °C. These results correlate with those found in a cholesteric phase. Curve B presents a diffuse reflection at about 4.94 Å, and a sharp first-order reflection at 31.02 Å which corresponds to strong smectic layers. At this temperature range, the optical polarizing micro-

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Figure 4. Optical polarizing micrographs displayed by PS22NA: (a) cholesteric phase at 152.4 °C (400×); (b) focal-conic texture of smectic A phase at 150 °C (400×); (c) Sc* phase at 140 °C (400×); (d) smectic X phase at 40 °C (400×). Table 3. Phase Transitions and Phase Transition Enthalpies for Polymer Series PSn2NB phase transitions, °C (corresponding enthalpy changes; kcal/mru)b

polymer

na

heating cooling

PS02NBc

0

G 2.2 SA 202.4(0.282) I I 200(0.312) SA 2.0 G

PS12NBd

1

G 9.3 Sx 95(-)e Sc* 151.1(0.095) SA 171(0.334) I I 167.1(0.334) SA 150.2(0.063) Sc* 90(-)e S 8 G

PS22NBd

2

G 3.3 Sx 70(-)e Sc* 135.7(0.966) I I 132.5(0.847) Sc* 48(-)e S 3.3 G

PS32NB

3

G 2.0 Sx 65(-)e Sc* 130 (1.18) I I 129.3(0.99) Sc* 40(-)e S 2.4 G

a n according to Scheme 2. b mru ) mole repeating unit, G ) glassy, S ) smectic A, Sc* ) chiral smectic C, S ) unknown smectic, I A ) isotropic. c The LC phases examined by X-ray reveal a bilayer structure. d The LC phases examined by X-ray reveal a coexistence of bilayer and monolayer structures. e The transition was difficult to detect.

graph (Figure 4b) reveals a focal-conic fan texture. Both results are consistent with a smectic A structure. Moreover, decreasing the measuring temperature from 155 to 147 °C, decreases the d spacing of the first-order reflection from 31.02 to 30.93 Å (curve C). The temperature dependence of the layer spacing for series PSn2NA (n ) 0, 1, 2, 3) is presented in Figure 5a. As indicated in this figure, the d spacing decreases as the temperature decreases below the transition point of SA phase for the three polymers PS12NA-PS32NA (n ) 1-3), respectively. This gives an evidence for the formation of the tilted chiral smectic C phase. This result also corresponds to the optical microscopic observation in which a texture with striated lines on the

fan domain is found (Figure 4c). Further cooling the measuring temperature to 80 °C causes (a) the d spacing of first-order reflection to change back to 32.31 Å and (b) the wide-angle reflection to become complicated (curve E). These results indicate the formation of an unknown smectic phase. Figure 4d displays the smectic X(Sx) texture exhibited by PS22NA. Series PSn2NB. This series differed structurally from the PSn2NA series. The (S)-2-methyl-1-butyl terminal chiral tail of PSn2NA is replaced by (R)-1methylheptyl chiral moiety. The thermal transition and corresponding enthalpy changes of the polymer PSn2NB series are summarized in Table 3. The polymers PS12NB, PS22NB, and PS32NB (n ) 1, 2, 3) show an

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Chem. Mater., Vol. 9, No. 1, 1997 57

enantiotropic SA/Sc* or a Sc* mesomorphism; meanwhile, the polymer PS02NB reveals an enantiotropic smectic A phase. Figure 2b presents representative DSC heating and cooling traces for polymer PS22NB. In this figure, the heating scan (curve A) shows a glass transition (Tg) at 3.3 °C, followed by a chiral smectic C phase to isotropic at 135.7 °C. The cooling scan (curve B) closely resembles the heating scan. X-ray diffraction measurements and optical polarized microscopy verified the assignment of the mesophase for these polymers. Figure 6 shows a representative temperature-dependent X-ray diffraction diagram for the powder sample of PS22NB at 138, 131, 118, 90, and 40 °C, respectively. The diffraction patterns obtained at different temperatures closed resemble each other. A broad reflection at wide angles (associated with the random ordering of lateral packing) and two reflections (one is sharp, another is weak) at low angles (associated with the smectic layers) are shown, respectively, by all curves. Only the d spacings are different. Figure 5b presents the d spacing plotted as a function of temperature for PSn2NB series. In this figure, the layer spacing decreases with a decrease in the measuring temperature for PS12NB, PS22NB, and PS32NB. The optical micrograph of polymer PS22NB (Figure 7) shows a typical chiral smectic C texture with striated lines on the fan domains. These results correlate with those found in a tilted Sc* phase. On the other hand, X-ray diffraction measurements reveal an increase in layer d spacing below the temperature of Sc* phase; nevertheless, the wide-angle reflections remain unchanged. Moreover, a change in the texture of Sc* phase is not observed in this smectic-like phase. One possible explanation is owing to the quenching of Sc* phase near the glass transition due to the high viscosity of increasing the order orientation of mesogenic groups.

Figure 5. Temperature dependence of layer spacing for (a) series PSn2NA: (n ) 0, ×); n ) 1, 2); (n ) 2, 9); (n ) 3, (); (b) series PSn2NB: (n ) 0, ×); n ) 1, 2); (n ) 2, 9); (n ) 3, (); (c) series PSn2NC: (n ) 0, ×); (n ) 1, 2); (n ) 2, 9); (n ) 3, ().

Series PSn2NC. The last series of polymers, was covalently incorporated with (2S,3S)-2-chloro-3-methylpentyl chiral moiety instead of (S)-2-methyl-1-butyl and (R)-1-methylheptyl of polymer PSn2NA, PSn2NB series. This type of moiety with two chiral centers has been reported to possess large spontaneous polarization values.19 Four compounds of this series are all mesomorphic. A different chiral moiety normally results in a different mesophasic sequence (Figure 1c). The thermal transition and thermodynamic parameters of PSn2NC series are summarizes in Table 4. All four polymers present a glass transition temperature at 8.5, 9.7, 7.0, and 7.9 °C, respectively. Polymer PS22NC (n ) 2) and PS32NC (n ) 3) exhibit enantiotropic cholesteric, smectic A, and chiral smectic C phases; meanwhile, polymer PS12NC reveals a cholesteric, a twist grain boundary A (TGBA) and a chiral smectic C phase. An unknown smectic phase is also observed for polymer PSn2NC series similar to the PSn2NA series. A representative DSC trace for PS22NC is shown in Figure 2c. Figure 8 presents a representative temperaturedependent X-ray diffraction diagram for polymer PS12NC. Curve A and the optical polarizing micrograph of Figure 9a are both consistent with a cholesteric phase. Moreover, the SA diffraction pattern by curves B and C combined with a filament texture displayed in Figure 9b reveals the presence of the twist grain boundary A phase (TGBA) which is a result of the

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Figure 6. Temperature-dependent X-ray measurements for polymer PS22NB with two unit oxyethylene spacers at (A) 138, (B) 131, (C) 118, (D) 90, and (E) 40 °C.

Figure 7. Optical polarizing micrographs displayed by polymer (n ) 2) PS22NB at 120 °C (400×).

macrohelix in the direction normal to the long axes of molecules, and parallel to the layers.34-36 Curves D displays small-angle reflection at 27.41 Å. Furthermore, the d spacing versus measuring temperature is presented in Figure 5c. This figure indicated that the d spacing decreases as the temperature decreases below the SA-Sc* or TGBA-Sc* point. These results also correlate with the formation of chiral smectic C phase due to the fact that the tilt angle of side chains typically increases with a decrease in the (34) Nguyen, H. T.; Twieg, R. T.; Nabor, M. F.; Isaert, N.; Destradem, C. Ferroelectrics 1991, 121, 187. (35) Goodby, J. W.; Waugh, M. A.; Stein, S. M.; Chin, E.; Pindak, R.; Patel, J. S. Nature (London) 1989, 337, 449. (36) Navailles, L.; Nguyen, H. T.; Barois, P.; Destrade, C.; Isaert, N. Liq. Cryst. 1993, 15, 479.

temperature for chiral smectic C side-chain LCPs. On the other hand, the striated domains of the Sc* phase grow in the regions of the specimen where the filament texture of the TGBA phase was previously present. Observation of the filament texture reveals that te helix axes of the TGBA phase have crossed each other in the presence of pitch bands and dechiralization lines of Sc* phase. This observation demonstrates that the helical axes in the TGBA and Sc* phases are orthogonal to one another (Figure 9b,c). Curves E and F display that small-angle reflections appeared at 28.13 and 28.27 Å; wide-angle reflections also appeared at 4.94 and 3.93 Å. This occurrence indicates the formation of an unknown structure of smectic phase. Figure 5 plot the smectic layer spacings (estimated from the small angle reflection peak) against temperature for the three series, i.e., PSn2NA, PSn2NB, and PSn2NC. The values of the layer spacings of polymers PS02NA, PS02NB, PS02NC with n ) 0, correlate well with double the calculated length of the side chain in its energy-minimized conformation. This fact indicates that the shortest derivatives PS02NA, PS02NB, and PS02NC exhibit a bilayer (two layer) packing structure. However, the layer spacing values for the longest derivatives PS32NA, PS32NB, and PS32NB (n ) 3) in three series are compatible with the calculated lengths of the side chain in their energy-minimized conformation and, subsequently, form the monolayer (single layer) packing structure. On the other hand, X-ray diffraction patterns obtained from powder samples PS12NA (n ) 1), PS12NC (n ) 2), PS12NB (n ) 1), and PS22NB (n ) 2) at various temperature show two pairs

Ferroelectric Liquid-Crystalline Polysiloxanes

Chem. Mater., Vol. 9, No. 1, 1997 59

Figure 8. Temperature-dependent X-ray diffraction measurements for polymer PS12NC with one unit oxyethylene spacer at (A) 147, (B) 132, (C) 122, (D) 100, (E) 60, and (F) 40 °C. Table 4. Phase Transitions and Phase Transition Enthalpies for Polymer Series PSn2NC polymer

na

PS02NCc

0

heating phase transitions, °C cooling (corresponding enthalpy changes; kcal/mru)b G 8.5 Sx 103.3(0.74) SA 149(0.195) I I 147.6(0.195) SA 79.2(0.39) Sx 3.3 G

PS12NCd

1

G 9.7 Sx 70.7(1.364) Sc* 118.3(-)e TGBA 137.3(-)e N* 150(0.95)f I I 148.3(1.04)f N* 137.1(-)e TGBA 115(-)e Sc* 58(1.171) Sx 10.6 G

PS22NC

2

G 7.0 Sx 82.3(1.658) Sc* 123.4(-) SA 132(-) N* 148(0.974)f I I 143(1.025)fN* 130(-) SA 120(-) Sc* 60.9(1.316) Sx 5.2 G

PS32NC

3

G 7.9 Sx 76.3(1.27) Sc* 100.2(-) SA 113.5(-) N* 120(1.18)f I I 120(1.179)f N* 110(-) SA 98(-) Sc* 55.2(1.27) Sx 0.1 G

a n according to Scheme 2. b mru ) mole repeating unit, G ) glassy, Sx ) unknown smectic, S ) smectic A, Sc* ) chiral smectic C, A TGBA ) twist grain boundary A, Sc* ) chiral smectic C, N* ) cholesteric. I ) isotropic. c The LC phases examied by X-ray reveal a d bilayer structure. The LC phases examined by X-ray reveal a coexistence of bilayer and monolayer structures. e The transition was decided by POM. f ∆H (Sc*-SA-N*-I).

of the layer reflections independently at small angles. Figures 6 and 8 reveal the representative two pairs of reflection peaks at small angles for polymer PS22NB and PS12NC. The interplanar distances estimated from the two independent peaks for PS12NA, PS12NC, PS12NB, and PS22NB are superimposed in Figure 5. These values of the two interplanar distances imply the coexistence of the two different packing structures.37 Restated, the layer spacing estimated from the smaller angle is consistent with the bilayer structure and that estimated from the larger angle is compatible with the single-layer packing structure. Results obtained here indicate that an increase in the unit of the oxyethylene spacer is unfavorable to the bilayer packing structure. Therefore, an increasing length of the flexible spacer in side-chain polymeric liquid crystals tends to enable the mesogenic groups to orient themselves more easily in the same liquid-crystalline state. On the other hand, the polymer PS22NA (n ) 2) in series PSn2NA and the (37) Yamaguchi, T.; Asada, T. Liq. Cryst. 1990, 8, 345.

polymer PS22NC (n ) 2) in series PSn2NC exhibiting only monolayer packing structure is different from the series PSn2NB, in which polymer PS22NB (n ) 2) reveals the coexistence of bilayer and monolayer packing structures. Therefore, the above results imply that the long length of the chiral tails tends to entangle with each other and, thereby, result in a widening of the range of the packing structure (i.e., coexistence range: n ) 1-2 for PSn2NB series with heptyl chiral tail; n ) 1 for series PSn2NA and PSn2NC with butyl and pentyl chiral tail). From Figure 5, the increase in the layer spacings (bilayer) of PS02NA-D2, PS12NA-D2, PS02NCD2, PS12NC-D2, PS02NB-D2, PS12NB-D2, and PS22NBD2 on increasing the temperature indicates the side chain’s increased mobility. This fact finding implies that the large free volume of the side chains do not allow the partial overlapping of the tails in the bilayer structure at a higher temperature. A detailed study of bilayer and monolayer packing structure, including molecular simulation and X-ray diffraction, is currently underway.

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crystalline thermal properties because the flexible polymer backbone and oxyethylene spacer in the more rigid four aromatic ring ester system (biphenyl naphthyl carboxylate mesogenic group) achieve a suitable balance for the formation of a liquid-crystalline structure. From DSC studies, the crystallization process is more apparent for series PSn2NA and PSn2NC than series PSn2NB due to the different flexibilities of the three kinds of chiral moieties. As a result, the process causes a sidechain crystallization phenomenon and quite ambiguous glass transition for series PSn2NA and PSn2NC with a short chiral chain tail and a rigid four phenyl rings of ester core. Nevertheless, the longer chiral heptyl ((R)1-methylheptyl) is more inclined to form chiral smectic C phase than the chiral butyl and pentyl tails ((S)-2methyl-1-butyl, (2S,3S)-2-chloro-3-methylpentyl) in the flexible polymer backbone/oxyethylene spacers and four aromatic ring ester system. This evidence correlates with the results of previous studies.28 Conclusions

Figure 9. Optical polarizing micrographs: (a) cholesteric phase of PS12NC at 140.5 °C (400×); (b) twist grain boundary A phase of PS12NC at 120 °C (400×); (c) chiral smectic C phase of PS12NC at 100 °C (400×).

A comparison of the thermal transitions of polymer series PSn2NA, PSn2NB, and PSn2NC reveals that a flexible polysiloxanes backbone tends to have a low glass transition and a wide mesomorphic temperature range. Furthermore, the phase behaviors of the polymers PSn2NA, PSn2NB, and PSn2NC series can be compared by superimposing the plots of the dependencies of their thermal transition temperatures as a function of oxyethylene spacer unit (n, Figure 1a-c). Polymers PSn2NA, PSn2NB, and PSn2NC display good liquid-

In conclusion, four series of new side-chain liquidcrystalline polysiloxanes containing oligooxyethylene, biphenyl naphthyl carboxylate mesogenic group and three different kinds of chiral moieties ((S)-2-methyl1-butyl, (R)-1-methylheptyl, and (2S,3S)-2-chloro-3-methylpentyl) have been synthesized and characterized. Three series, PSn2NA, PSn2NB, and PSn2NC, exhibit smectic mesomorphism. Among the three series, two series (PSn2NA and PSn2NC) with chiral butyl and pentyl moieties reveal a narrower temperature range of the chiral smectic C phase than polymer PSn2NB series. The rigidity of aromatic ring ester core plays an influential role in forming the mesophases. On the other hand, X-ray studies indicate that the packing of mesogenic groups for PS02NA, PS02NB, PS02NC with the shortest spacers (n ) 0) exhibit a bilayer (two layer) packing structures. Moreover, polymers PS32NA, PS32NB, and PS32NC with the longest spacer chain (n ) 3) reveal a monolayer packing structure. However, these two types of packing structures appear simultaneously in PS12NA (n ) 1), PS12NC (n ) 1), PS12NB (n ) 1), and PS22NB (n ) 2), depending on the chiral tail length. The chiral tail’s flexibility has an effect not only on the thermal stability of chiral smectic C phase but also on the packing structure of mesophases. A flexible polymer backbone enhances the decoupling of the motions of the side chain and main chain, thereby giving rise to a higher thermal stability of the mesophases, including the chiral smectic C phase. Acknowledgment. The authors would like to thank the National Science Council of the Republic of China for financial support of this article under Contract NSC-84-2216-E007-029. CM9505963