Dynamics of poly(vinyl acetate) in bulk and on silica

Quadrupole echo deuterium NMR has been used to probe the dynamics of poly(vinyl acetate-d(3)) in bulk and at saturation coverage (from toluene) on silica at the solid-air interface. In bulk and at lower temperatures, the deuterium powder pattern of the labeled methyl group was found to be consistent with fast threefold jumps with a small asymmetry due to the interaction of the methyl deuteron with the carbonyl oxygen. This apparent asymmetry was a result either of a distortion in the C-C-D bond angle or a reduction of the axial symmetry of the electric field gradient along the C-D bond vector. At higher temperatures, the onset of backbone motion of the polymer caused the collapse of the powder pattern to a single broad resonance over a fairly small temperature range between 65 and 73 degrees C. The collapse was well above the glass transition temperature as measured by differential scanning calorimetry, T-g(DSC) = 32 degrees C, and can be considered the T-g(NMR). The differences in temperatures for these two experiments are due to their different time scales. For the PVAc-d(3) adsorbed at monolayer coverage on a silica surface, a small fraction was found to result in a collapsed powder pattern below the T-g(NMR). This fraction with enhanced mobility was believed to be located near the air-polymer interface. The powder pattern for the majority of the surface-bound polymer was found to collapse gradually with temperature, with a rigid component observable well above the temperature where the splittings for all of the bulk material had collapsed. Thus, from the deuterium NMR spectra, it is possible to deduce that on a surface, different segments from a single molecule exhibit a range of mobilities.