Segmental dynamics in poly(methyl acrylate) on silica: effect of surface treatment

The effect of surface treatment on the dynamics of adsorbed poly(methyl acrylate) (PMA) was studied using deuterium NMR and temperature-modulated differential scanning calorimetry (TMDSC). The solid-state deuterium NMR experiments were performed using PMA-d(3), deuterated on the methyl group. The line shape changes for PMA-d(3) were followed as a function of temperature and compared for the polymer on untreated silica, organically modified (treated) silica (reacted with hexamethyltrisilazane), and in bulk. The dynamics of PMA-d(3) on treated silica was found to be intermediate between that of the polymer adsorbed on untreated silica and that of the bulk polymer, i.e., the treated silica caused a restriction on the dynamics of the polymer as compared to bulk, but not as dramatically as that on untreated silica. Similar to the dynamics on untreated silica, the dynamics on treated silica showed a broad heterogeneity with a superposition of more-mobile and less-mobile components. Two molecular mass samples were also studied (38 and 77 kDa) with the molecular mass dependence on the treated or untreated silica being weaker than that in bulk. The TMDSC thermograms of the samples were consistent with the NMR results, with the glass transition region for the PMA-d(3) on the treated silica being in between that of the bulk and that on the untreated silica.