Molecular Packing and Dynamics of the Main Chain and Side Chains in Mesomorphic Poly(vinyl ether)s As Revealed by X-ray Scattering, Dielectric Spectroscopy, Solid State 2H, and 13C-MAS NMR Spectroscopy

Abstract

The conformational states and dynamics of a series of side-chain liquid crystalline poly(vinyl ether)s with phenylbenzoate or stilbenzyloxy as mesogen and with 4−11 methylene spacer groups, where the 11-methylene-spacer polymers had two different tacticities, were assessed by dielectric and NMR spectroscopy. Dielectric spectroscopy confirmed the presence of three subglass processes (β, γ, and δ) exhibiting an Arrhenius temperature dependence. The activation energy (35 ± 5 kJ/mol) of the γ process was insensitive to the morphology, and this process is assigned to localized motions within the spacer group. The absence of the β process in the stilbenzyloxy polymer suggests that this process is due to flips of the phenylene units causing a 180° reorientation of the carboxylic group in the phenyl benzoate moiety. The frequency of the flip of the inner phenylene unit at 0 °C as obtained by 13C-MAS NMR was almost the same as the frequency of the dielectric β loss peak at the same temperature. The relaxation strength of the β process passed through a maximum as the spacer length increased. It is suggested that the participating carboxyl groups were unable to reorient completely in a polymer with a short spacer group since reorientation of the mesogen was more complete in a polymer with longer spacer groups, the latter containing a smaller percentage of mesogens. 13C-MAS NMR data indicated that the gauche content of the spacer group was lower than that of a free polymethylene chain at the same temperature and that it increased with increasing temperature. The X-ray scattering patterns of the 11-methylene-spacer polymers with different tacticities were identical, and it was shown by NMR and dielectric spectroscopy that the molecular dynamics was not influenced by the tacticity of the polymers. The decrease in the thickness of the smectic layer at the transition from smectic B to smectic A is accompanied by a pronounced increase in segmental mobility of the backbone as revealed by 2H NMR spectroscopy.