AN X-RAY-DIFFRACTION STUDY OF THE DIFFERENT NEMATIC MESOPHASES OF LIQUID-CRYSTALLINE POLY(URETHANE-ESTER)S

An x-ray investigation on powder specimens and stretched oriented fibe rs of poly(urethane-ester)s s TDI-CmCn, derived from various mesogenic alkylene -(omega-hydroxyalkyloxy-4-oxybenzoyl)oxybenzoate]s (CmCn; m = 2, 4, or 6, and n = 4, 6, 8, or 10) and 2,4-toluene-diisocyanate (TD I), is reported. Evidence is provided for the formation of two differe nt nematic mesophases in the polymers, namely a cybotactic nematic and a conventional nematic mesophase. Whereas samples TDI-C2C6, TDI-C6C4, and TDI-C6C10 formed one cybotactic nematic mesophase, samples TDI-C4 C6, TDI-C6C6, and TDI-C6C8 exhibited both cybotactic nematic and conve ntional nematic mesophases in a sequence with increasing temperature, which were connected by a first-order transition. The analysis of the various features of the small-angle x-ray diffraction patterns indicat es that two structural arrangements, namely smectic C-like and convent ional nematic structures, coexist inside the cybotactic nematic mesoph ase of these poly(urethane-ester)s. (C) 1995 John Wiley & Sons, Inc.