Glass transition dynamics in a compatible blend by two-dimensional solid-state NMR

Chain motion at the glass transition of one component in a compatible blend was observed by two-dimensional solid-state NMR spectroscopy. A blend of polystyrene and carbon-13-labeled poly-(2,6-dimethylphenylene oxide) at two concentrations was studied by carbon-13 spectroscopy as a function of temperature and mixing time in the vicinity of the thermal glass transition. Chain motion of the labeled polymer on a ms time scale commenced at temperature of approx 10 deg C below the thermal glass transition, in contrast to single-component polymeric glasses which only show such motion at temperatures above the glass transition. The motion exhibited the characteristics of rotational Brownian diffusion with an associated broad distribution of exponential correlation times. Again in contrast to single-component polymeric glasses, the distribution is considerably broader and can only be simulated by bimodal or trimodal distribution of correlation times. Each of the modes involves a distribution of exponential correlation times, and that given by a stretched exponential correlation function was employed. The relative weighting of the modes corresponded to local concentration fluctuations; calculated from a lattice model. The dynamic heterogeneity associated with these concentration fluctuations exceeds the heterogeneity generally encountered in single-component polymeric glasses.