METHYL-GROUP DYNAMICS IN POLY(VINYL METHYL-ETHER) - A ROTATION RATE DISTRIBUTION MODEL

The dynamics of the methyl side group in poly(vinyl methyl ether) (PVM E) is investigated by means of time-of-flight quasi-elastic neutron sc attering in the time range between 10(-11) and 10(-13) s. The quasi-el astic region of the experimental data is analyzed by using two alterna tive different models. One of them assumes that there exist two differ ent families of CH3 in PVME moving at different rates and with a popul ation which depends on temperature. The other model assumes a single t ype of CH3 groups but considers the existence of a random distribution of jumping rates for the methyl group rotation. Both models allow a g ood description of the experimental behavior. However, whereas the for mer results in a peculiar behavior of the model parameters, the latter leads to a simple and physical picture of the methyl group dynamics. In particular, a Gaussian distribution of the energy barriers for the CH3 rotational jumps which is centered at 8.4 kJ mol-1 and has a distr ibution width of +/-0.8 kJ mol-1 is found to be the origin of the dist ribution of jumping rates. Within the experimental uncertainties, the distribution of energy barriers does not change with temperature aroun d the glass transition.