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 fibers of poly(urethaneester)s TDI‐CmCn, derived from various mesogenic alkylene di[4‐(ω‐hydroxyalkyloxy‐4‐oxybenzoyl)oxybenzoate]s (CmCn; m = 2, 4, or 6, and n = 4, 6, 8, or 10) and 2, 4‐toluenediisocyanate (TDI), is reported. Evidence is provided for the formation of two different 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‐C4C6, TDI‐C6C6, and TDI‐C6C8 exhibited both cybotactic nematic and conventional 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 indicates that two structural arrangements, namely smectic C‐like and conventional nematic structures, coexist inside the cybotactic nematic mesophase of these poly(urethane‐ester)s.