Characterization of a Main-Chain Semiflexible Liquid Crystalline Polymer: Degree of Orientational Order
Selectively deuterated liquid crystalline copolyesters of 4,4′-dihydroxy, α,α′-dimethylbenzalazine were investigated by proton and deuterium n.m.r. N.m.r. spectra were recorded over a wide temperature range: from 100°C (50 to 60°C below the solid to nematic phase transition, Tm to 255°C which is well above the isotropic-nematic phase transition. The main-chain liquid crystalline polymer does not align in the n.m.r. magnetic field below Tm + 20°C, where Tm represents the melting temperature measured by differential scanning calorimetry. However, from the splitting Δν between the peaks in the powder pattern we conclude that the sample is composed of randomly oriented domains. Within these domains the same type of molecular motion occurs as in the monodomain nematic phase at T > Tm + 20°C. Above Tm + 20°C the 2H n.m.r. spectra show the existence of doublets which are characteristic of an aligned monodomain nematic phase. High degrees of orientational order, SZZ, measured by both proton and deuterium n.m.r. are practically independent of sample thermal history. Values of Szz at the nematic to isotropic phase transition and the order parameters of the flexible units are consistent with data reported for other polymers of similar chemical structure.