Molecular Order and Motion in Liquid Crystal Polymers Studied by Pulsed Dynamic NMR

Abstract

Main chain liquid crystal polymers have been studied by pulsed deuteron nuclear magnetic resonance 2HNMR) of mascroscopically oriented samples over a wide temperature range, covering several polymer phases. Longitudinal and transverse relaxation times, T 1Z and T2E, have been recorded as a function of temperature, magnetic field strength and macroscopic orientation of the alignment axis with respect to the magnetic field. Analysis of the experiments is achieved in terms of a density operator treatment based on the stochastic Liouville equation. The results show that collective motions contribute to the longitudinal relaxation process in the kilohertz regime, as found for low molar mass liquid crystals, whereas the conventional megahertz range is dominated by local reorientation of individual molecules. The intramolecular motions consist of trans-gauche isomerisation and phenyl ring flips. These motions are the fastest in the hierarchy of time with correlation times of 10−10 sin the nematic melt of the polymers at about 460 K. The intermolecular (whole molecule) motions are interpreted as rotational diffusion in an orienting potential. They have correlation times ranging from 10-9s to 10−7s at this temperature. These motions are coupled to the glass transition and do not contribute to spin relaxation in the glassy state. The slowest motions affecting the 2HNMR observables can be assigned to nematic order director fluctuations, characterised by a broad distribution of thermally activated modes. Analysis of pulse frequence dependent transverse 2H spin relaxation times, T 2E CP , from modified Carr-Purcell-Meiboom-Gill pulse trains provides estimates for the viscoelastic parameters of the liquid crystal polymers. In addition to dynamic information this study also yields values for the orientational and conformation order parameters, which exceed those of the monomeric analogues considerably.