Plasticization of Adsorbed Poly(vinyl acetate) on Silica by Deuterium Solid-State NMR

Abstract

Deuterium nuclear magnetic resonance spectroscopy and temperature modulated differential scanning calorimetry (TMDSC) were used to probe the segmental dynamics of methyl-labeled poly(vinyl acetate)-d3 (PVAc-d3) adsorbed on Cab-O-Sil silica in the presence and absence of a plasticizer, dipropyleneglycol dibenzoate. Unlike the effect of this plasticizer on the bulk polymer, where the reduction in the glass transition temperature (Tg), was proportional to the amount of plasticizer added, the effectiveness of the plasticizer on the adsorbed polymer depended on the amount of polymer adsorbed. For samples with very small amounts of adsorbed polymer (i.e., 0.81 mg/m2), there was little or no effect of plasticizer on the dynamics of the adsorbed polymer. For samples with more adsorbed polymer (i.e., 1.42 or 1.81 mg/m2), the efficiency of the plasticizer was easily measurable on a fraction of the adsorbed polymer that can be considered loosely bound. The 2H NMR line shape changes obtained from the adsorbed samples were fitted to a small jump model (based on the vertices of a “soccer-ball” shaped polyhedron) to obtain the distributions of jump rates contributing to the spectra. The same samples were also studied using TMDSC, and the results were consistent with the NMR results in terms of the effects of the plasticizer on the amount of polymer adsorbed. On the basis of the TMDSC experiments, the plasticizer tends to have a greater effect on the more loosely-bound polymer and lesser effect on a fraction that is more-tightly bound. Compared to the bulk polymer, where the glass transition range was on the order of 10 K, the range for the adsorbed polymer was more around 60 K.