2H NMR Studies of Liquid Crystal Main Chain Polymers

Abstract

AbstractSemiflexible thermotropic polyesters, specifically deuterated at different positions of the polymer chain, are studied by multipulse dynamic NMR techniques. Analysis of the various 2H NMR experiments is achieved, employing a comprehensive theory, based on the stochastic Liouville equation. Computer simulations provide the orientational distributions and conformations of the polymer chains and the correlation times of the various motions. In the anisotropic melt the correlation times for chain rotation and chain fluctuation are of the order of 10−8s, while trans‐gauche isomerization occurs even faster. Below the melting point two components are observed in the NMR experiments. The dynamics of the liquid crystalline component (40%) continues into the biphasic region. In contrast, the crystalline component (60%) shows a drastic motional decrease at the melting point. The chain order parameters Szz increase with decreasing temperature to limiting values, which depend upon the number n of flexible spacer segments. Polymers with n = even exhibit limiting order parameters of Szz = 0.9, a value considerably higher than those observed in low molecular weight nematogens. In addition, the chains adopt highly extended conformations, evidenced by trans populations of nt= 0.76, exhibited throughout the spacer. The liquid crystalline order is quantitatively retained, when the polymers are cooled below the melting point and glass transition, respectively.