Polymer-Nanoparticle Interfacial Interactions in Polymer Nanocomposites: Confinement Effects on Glass Transition Temperature and Suppression of Physical Aging.

Confinement effects on glass transition temperature (Tg) and physical aging are measured via fluorescence in polystyrene (PS), poly(methyl methacrylate) (PMMA) and poly(2-vinyl pyridine) (P2VP) nanocomposites (NCs) containing 10-15 nm diam. silica nanospheres or 47 nm diam. alumina nanospheres. At 0.1-10 vol{\%} nanofiller, Tgs can be enhanced or depressed relative to neat, bulk Tg or invariant with nanofiller content. For alumina NCs, Tg increases relative to bulk Tg by as much as 16 K in P2VP, decreases by as much as 5 K in PMMA, and is invariant in PS. These results are explained by wetted P2VP-nanofiller interfaces with attractive interactions, non-wetted PMMA-nanofiller interfaces, and wetted PS-nanofiller interfaces lacking attractive interactions, respectively. The presence of wetted or non-wetted interfaces is controlled by choice of solvent; silica/PMMA NCs exhibit Tg enhancements or reductions relative to bulk Tg when films are made from methyl ethyl ketone or acetic acid solutions, respectively. A factor of 10 reduction of physical aging rate relative to that of neat P2VP is shown in a 4 vol{\%} alumina/P2VP NC. Hence, a strategy for achieving non-equilibrium, glassy polymer systems that are nearly stable to physical aging is to incorporate well-dispersed nanoparticles possessing attractive interfacial interactions with the polymer.