Distribution of correlation times in glassy polymers from pulsed deuteron NMR

Abstract

Pulsed deuteron NMR offers new possibilities for the determination of the distribution of correlation times for local motions in glassy polymers. The NMR line shape is a superposition of spectra corresponding to the different values of the correlation times. The weighting factors of these single spectra depend on the distribution function, which therefore can be characterized by a line shape analysis. Moreover, by combination with spin-lattice relaxation experiments the motional behaviour can be probed on a much longer time scale then by a line shape analysis alone. In this way homogeneous and heterogeneous distributions can clearly be distinguished. The method is explained in detail and is demonstrated by a simple example involving the methyl group rotation in glassy polycarbonate. in both polycarbonate itself and in mixtures with low molecular mass additives the results can be described by a log-Gausian distribution of correlation times. This distribution is heterogeneous in nature, probably resulting from a distribution of the activation energies at spatially different sites between 15 and 21 kJ/mole.