Polymer-nanoparticle interfacial interactions in polymer nanocomposites: Confinement effects on glass transition temperature and suppression of physical aging

The effects of confinement on glass transition temperature (T-g) and physical aging are measured in polystyrene (PS), poly(methyl methacrylate) (PMMA), and poly(2-vinyl pyridine) (P2VP) nanocomposites containing 10- to 15-nm-diameter silica nanospheres or 47-nm-diameter alumina nanospheres. Nanocomposites are made by spin coating films from sonicated solutions of polymer, nanofiller, and dye. The T(g)s and physical aging rates are measured by fluorescence of trace levels of dye in the films. At 0.1-10 vol % nanofiller, T-g values can be enhanced or depressed relative to neat, bulk T-g (T-g,T-bulk) or invariant with nanofiller content. For alumina nanocomposites, T-g increases relative to T-g,T-bulk by as much as 16 K in P2VP, decreases by as much as 5 K in PMMA, and is invariant in PS. By analogy with thin polymer films, these results are explained by wetted P2VP-nanofiller interfaces with attractive interactions, nonwetted PMMA-nanofiller interfaces (free space at the interface), and wetted PS-nanofiller interfaces lacking attractive interactions, respectively. The presence of wetted or nonwetted interfaces is controlled by choice of solvent. For example, 0.1-0.6 vol % silica/PMMA nanocomposites exhibit T-g enhancements as large as 5 K or T-g reductions as large as 17 K relative to T-g,T-bulk when films are made from methyl ethyl ketone or acetic acid solutions, respectively. A factor of 17 reduction of physical aging rate relative to that of neat, bulk P2VP is demonstrated in a 4 vol % alumina/P2VP nanocomposite. This suggests that a strategy for achieving nonequilibrium, glassy polymeric systems that are stable or nearly stable to physical aging is to incorporate well-dispersed nanoparticles possessing attractive interfacial interactions with the polymer. (c) 2006 Wiley Periodicals, Inc.