Segmental dynamics in bulk and adsorbed poly(methyl acrylate)-d₃ by deuterium NMR

Deuterium solid-state quadrupole-echo NMR techniques were used to probe the dynamics of bulk and silica-adsorbed poly(methyl acrylate)-d3 (PMAd3). The effects of surface coverage, molecular weight and overlayer materials on the dynamics were investigated. Bulk PMA-d3 with high molecular weight showed a collapse in the 2H NMR powder pattern corresponding to a sudden increase in motional rate at about 50 C(nmr Tg), 40 °C above the reported glass transition temperature. Surface samples were prepared by adsorption of PMA-d3 onto silica. The NMR spectra were collected from surface species at the surface-air interface. The spectra consisted of different components. Some of the adsorbed material had an enhanced mobility attributed to segments near the air interface. The adsorbed PMA-d3 was more rigid as the surface coverage decreased. The effect of molecular weight was also studied. Bulk PMA-d3 with lower molecular weight was more mobile. In contrast, the adsorbed PMA-d3 with higher molecular weight was more mobile. Composite samples were prepared by hot pressing a surface sample with hydrogenated polystyrene and PMA of different molecular weights as the overlayer. The composite samples were studied in the interfacial layer of PMA-d3 between the silica and overlayer. No material in any composite sample had enhanced mobility and the lower the mobility of the overlayer, the more restricted the adsorbed PMA-d3 layer was --Abstract, page iv