We have investigated the effect of Cl, Br, C6H5, and C6H13 substituents upon the transition temperatures of poly(p-phenylene terephthalate). The substituted polyesters are thermotropic nematogens.A single substituent lowers the crystal-nematic transition temperature, t^, from 600 °C to the range 290-405 °C, while substitution on both rings reduces tKN to 205-235 °C. The polyester bearing a phenyl substituent on each ring is an amorphous glass. Disubstituted polymers crystallize more slowly and require annealing to exhibit a crystalline X-ray pattern. Substitution also reduces the degree of crystallinity, although most substituted polymers show Bragg spacings of 12 A or greater, indicating regularity along the chain direction. The nematic-isotropic transition temperature, fNI, is also lowered by substitution. A recent treatment which considers the temperature dependence of the unperturbed dimensions leads to the conclusion that all polymers which can be represented by the Kuhn chain model are in corresponding states at tNI. This implies that a substituent which lowers tKN also reduces the chain extension of the polymer.

Measurement of the anisotropy of turbidity of aligned nematic liquid crystals in three selected geometries can yield the splay, twist, and bend elastic constants (K11, K22, and K33, respectively) which, along with the viscosity coefficients of the medium, govern director fluctuations that lead to a strong light scattering. We have utilized this procedure to determine the temperature dependence of the elastic constants of a homologous series of n-alkycyanobiphenyls (n-CB, where n=5, 6, 7, 8) in the nematic range. K33≳K11≳K22 at all temperatures for each member of the series. The absolute values of the moduli are consistently higher than those obtained by magneto- and electro-optic distortion methods (Frederik’s transitions). Odd–even effects are displayed as the number of methylene carbons in the alkyl chain increases. Distinctly larger K33 values for 8CB may be attributed to the short range smectic ordering in this liquid crystal, which has a lower temperature smectic-A phase.

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