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The issues of helical sense and helical twisting power (HTP) have been addressed using low molar mass analogues of chiral nematic copolymers containing (S)-(+)-2-methoxy-2-phenylethanol, (S)-(-)-1-phenylethanol, and (R)-(-)-methyl mandelate. As reported by other workers, it is found that the empirical rules proposed by Gray et al. for interpreting helical sense are limited in scope. However, the handedness observed in the new copolymeric system and that of the copolymers reported earlier are consistent with the low molar mass counterparts. The HTP's of chiral nematic mixtures based on (S)-(-)-1 -phenylethanol and (/Z)-(-)-methyl mandelate are much lower than those of the corresponding copolymeric structures reported earlier, which is attributed to hindered rotation imposed by polymerization. In contrast, chiral nematic mixtures containing (S)-(+)-2-methoxy-2-phenylethanol show an HTP value only slightly lower than that of the copolymer. In addition, for all three systems, the selective reflection wavelength is observed to increase with decreasing temperature, consistent with Keating's model and with the molecular statistical theory of Kimura et al.

The effect of different ring systems on liquid crystalline (LC) mesomorphism has been addressed in terms of thermotropic side-chain polymers incorporating p-phenylene and trans-cyclohexylene units. It was found that the nematogenic monomer based on a p-n-butylbenzoyl p'-phenyl ester core copolymerized with chiral comonomers containing (S)-(-)-l-phenylethanol and (-)-5-cholesten-3/3-ol (i.e., cholesterol) gives rise to the cholesteric mesophase with selective reflection wavelengths in the visible region. However, partial or complete replacement of the p-phenylene moiety with the trans-cyclohexylene ring results in smectic mesophase formation. The LC mesophases were identified with polarized optical microscopy and wide-angle X-ray diffraction, the latter technique being particularly useful in cases where thermal annealing fails to yield mesophases identifiable with optical microscopy.

The effects of the purification of thermotropic liquid-crystalline siloxane oligomer with supercritical carbon dioxide were investigated. It was found that low-molar-mass contaminants, presumably the two alkene precursors, can be effectively removed at 80 "C and carbon dioxide pressures up to 31,O MPa. In support of this observation is the fact that glass transition temperature goes up as a result of purification. In addition, the selective reflection wavelength was found to increase at increasing purity level, as achieved by extraction at increasing carbon dioxide pressure. 0025-1 16X/89/%03.00