Thermotropic Copolyesters Exhibiting a Cholesteric Phase. II

Abstract

Copolyesters of 4,4′-dihydroxybiphenyl with HOOC-(CH₂)_n-COOH and chiral (+)- 3-methyl adipic acid form thermotropic cholesteric phases. The cholesteric-isotropic transition temperature, and enthalpy and entropy changes, exhibit an even-odd effect in terms of the number, n, of methylene units in the aliphatic dibasic acid. The even members have larger values of all these quantities. The twist angle of the cholesteric phase also exhibits an even-odd effect, with the even members showing a smaller twist angle (larger pitch). The temperature dependence of the twist angle, or inverse optical pitch 1/nP (where n is the refractive index of the copolymer), is approximately constant for n ≤ 8, but increases for still larger values of n. A theoretical treatment of the cholesteric twist angle given by Keating is used to deduce the structural features which might give rise to these observations concerning the pitch and its temperature dependence.