SEMIFLEXIBLE LIQUID-CRYSTALLINE POLYESTERS BASED ON TWIN BIS(P-OXYBENZOYL)UNITS .2. EFFECT OF MOLAR-MASS ON MESOMORPHIC BEHAVIOR

The effect of the molar mass (M(n)) and molar mass distribution (M(w)/ M(n)) on liquid-crystalline phase-transition behaviour was investigate d for four polymers of the general class C.C., which were based on two mesogenic bis(p-oxybenzoyl) units and two linear alkylene spacer segm ents containing m (m = 5 or 7) and n (n = 5, 7 or 10) methylene groups , respectively. Each polymer exhibited smectic and nematic mesophases. The polymers were fractionated into various samples having different M(n) and M(w)/M(n) values. The liquid-crystalline phase-transition par ameters of samples C7C7 and C7C10 increased with M(n) until a limiting value of M(n) almost-equal-to 6000-8000 g mol-1, and then remained co nstant. In complete contrast, for samples C5C5 and C5C7 the stability of the smectic mesophase increased, whereas that of the nematic mesoph ase decreased with increasing M(n). In particular, for the former samp les the nematic mesophase turned from enantiotropic to monotropic with increasing M(n), and then disappeared in correspondence to a critical value of M(n) almost-equal-to 7000 g mol-1. In addition, wide M(w)/M( n) values were found to increase the stability of the nematic mesophas e.