2H NMR Study of Benzene-d6 in a Liquid-Crystalline Polysiloxane

Abstract

2H NMR line-shape and spin-lattice relaxation measurements for benzene-d6 in poly[methyl-[8-(7-pentylfluoren-2-yloxy)octyl]siloxane] (PMFS) from 16 to 434 K are presented. The two-component system has both a smectic A and a smectic E phase and forms a partially ordered glass at lower temperatures. The spectra in the smectic E and smectic A phases indicate the presence of two distinct average environments; the one partially ordered and the other isotropic. In contrast to this, the relaxation curves are characterized by a single spin-lattice relaxation time, T1. This discrepancy is explained as resulting from the fact that the spectra reflect the reorientation of the symmetry axis whereas T1 is determined largely by rotations about the 6-fold axis. The glass transition upon cooling is characterized by a marked change in the temperature dependence of T1 and the onset of nonexponential relaxation. In the glass phase the spectra and relaxation curves are dominated by a nonaveraged distribution of the local environments. The distribution of activation energies for the rotation of benzene about its 6-fold axis, g(E), is derived from the spectra and compared to results obtained in other polymer/probe systems. No discernible effects due to the ordering of the matrix are observed. The effect of this ordering on the glass-phase spectra is calculated by using mean-field theory and the order parameter for benzene at Tg. It is shown that the anisotropy of the orientational distribution function is small and has little influence on the slow motion spectra in agreement with the experimental results. © 1991, American Chemical Society. All rights reserved.