Methyl group dynamics in poly(vinylacetate): A neutron scattering study

The methyl group dynamics in a glassy polymer, poly(vinyl acetate) (PVAc), has been investigated by quasielastic neutron scattering in a temperature range from 50 to 300 K, below its glass transition temperature (T-g congruent to 315 K). We have used three different neutron spectrometers covering a wide dynamic range from 10(-8) to 10(-12) s. The data have been analyzed in the framework of the rotation rate distribution model, which was previously introduced by us to interpret methyl group dynamics in glassy polymers. This framework has proved to be a better approximation than an alternative approach based on the Kohlrausch-Williams-Watts function, when a wide dynamical range is considered. The results obtained for PVAc have been compared with those corresponding to four other polymers: poly(vinyl methyl ether), poly(methyl methacrylate), polyisoprene, and poly(dimethylsiloxane), which have also been analyzed in the same theoretical framework. While the average activation energy barrier for methyl group rotation is determined by the chemical structure of the monomer, the width of the distribution of activation energy barriers seems to be mainly controlled by intermolecular disorder and packing conditions.