Carbon-13 Two Dimensional Chemical Shift Anisotropy Exchange Patterns in Blends of Polyphenylene Oxide and Polystyrene Near the Glass Transition

Abstract

AbstractTwo dimensional solid state NMR exchange spectroscopy can be used to monitor slow motions on time scales comparable to dynamical mechanical spectroscopy. Polyphenylene oxide, carbon-13 labeled in a methyl position, was blended with polystyrene. The resulting compatible blends have single thermal glass transitions (Tg) at temperatures between those of the homopolymers. The chemical shift anisotropy line shape of the labeled methyl shows reorientational exchange at temperatures near the thermal glass transition. The 2D exchange pattern contains information on the rate and amplitude of the motion. The data indicate Brownian rotational angular diffusion of the phenylene oxide units with rates in the millisecond to second range. The motional description associated with the glass transition is contrasted with those appropriate to single component polymeric glasses. It is concluded that in the blend system the onset of motion occurs at a lower temperature relative to the DSC Tg than in the homopolymer and there is evidence for considerably more dynamic heterogeneity.