Molecular Weight Dependence of the Glass Transition Temperature (T_g)-Confinement Effect in Well-Dispersed Poly(2-vinyl pyridine)-Silica Nanocomposites: Comparison of Interfacial Layer T_gand Matrix T_g

Nanocomposites (NCs) of poly(2-vinyl pyridine) (P2VP) and nanosilica were prepared by spin-coating and characterized by fluorescence to assess the alterations in matrix glass transition temperature (Tg,matrix) and interfacial layer Tg (Tg,inter) as functions of P2VP molecular weight (MW) and silica content. Relative to neat P2VP Tg (Tg,neat), major increases in Tg,matrix were observed with increasing silica content in low-MW, 2.1 kg/mol P2VP NCs; Tg,matrix - Tg,neat = 15 and 35 °C at 1.0 and 10 vol % silica, respectively. High-MW, 100 kg/mol P2VP NCs exhibited much smaller increases in Tg,matrix. To understand better the Tg,matrix enhancements, trace levels of dye were labeled to silica to characterize the near-interface (≤4 nm from the surface) Tg (Tg,inter). Profound increases in Tg,inter were observed in low-MW P2VP NCs: Tg,inter - Tg,neat = 25 and 40 °C at 1.0 and 10 vol % silica, respectively. In contrast, in high-MW P2VP NCs, Tg,inter - Tg,neat = 8 and 16 °C at 1.0 and 10 vol % silica, respectively. Because interfacial Tg perturbations propagate tens of nanometers into a matrix, the larger increases in Tg,inter in low-MW P2VP and at increasing silica content lead to larger increases in Tg,matrix. At a given silica content, the greater alteration in Tg,inter in low-MW P2VP NCs is explained by differences in interfacial conformations of low- and high-MW P2VPs and greater interfacial hydrogen bonding in low-MW P2VP. The stronger attractive interfacial interactions in low-MW P2VP NCs lead to greater alterations in Tg,inter, which result in greater changes in Tg,matrix. In contrast, the increase in Tg,inter with increasing filler is due to Tg,inter perturbations propagating into the matrix. At sufficient silica loading, perturbations to Tg from one interface modify Tgs at other interfaces. At a constant silica content in the range of 3.0-10 vol %, spatial Tg gradients in the NCs and near-interface gradients have little MW dependence. At 0.5 and 1.0 vol % silica, our results suggest that near-interface gradients are larger in low-MW P2VP NCs.