Macromolecules, Vol.53, No.19, 8725-8736, 2020
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-g and 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 (T-g(,matrix)) and interfacial layer T-g (T-g,(inter)) as functions of P2VP molecular weight (MW) and silica content. Relative to neat P2VP T-g (T-g,T-neat), major increases in T-g(,matrix) were observed with increasing silica content in low-MW, 2.1 kg/mol P2VP NCs; T-g(,matrix) - T-g,T-neat = 15 and 35 degrees C at 1.0 and 10 vol % silica, respectively. High-MW, 100 kg/mol P2VP NCs exhibited much smaller increases in T-g(,matrix). To understand better the T-g(,matrix) enhancements, trace levels of dye were labeled to silica to characterize the near- interface (<= 4 nm from the surface) T-g (T-g,(inter)). Profound increases in T-g,(inter) were observed in low-MW P2VP NCs: T-g,(inter) - T-g,T-neat = 25 and 40 degrees C at 1.0 and 10 vol % silica, respectively. In contrast, in high-MW P2VP NCs, T-g,(inter) - T-g ,T-neat = 8 and 16 degrees C at 1.0 and 10 vol % silica, respectively. Because interfacial T-g perturbations propagate tens of nanometers into a matrix, the larger increases in T-g,(inter) in low-MW P2VP and at increasing silica content lead to larger increases in T-g(,matrix). At a given silica content, the greater alteration in T-g,(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 T-g,(inter) which result in greater changes in T-g(,matrix). In contrast, the increase in T-g,(inter) with increasing filler is due to T-g,(inter) perturbations propagating into the matrix. At sufficient silica loading, perturbations to T-g from one interface modify T(g)s at other interfaces. At a constant silica content in the range of 3.0-10 vol %, spatial T-g 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.