DEUTERIUM NMR-STUDY OF PHENYL GROUP MOTION IN GLASSY POLYSTYRENE AND A BLEND OF POLYSTYRENE WITH POLYPHENYLENE OXIDE

Deuterium solid echo line shapes were measured as a function of temperature and echo delay time on glassy polystyrene and a blend of polystyrene with poly(2,6-dimethyl-1,4-phenylene oxide). The spectra were strongly temperature dependent from room temperature to the glass transition, indicating motion on the time scale of tens to hundreds of microseconds. The line shape signature of pi flips is present in these spectra. The time scale and temperature range overlap the beta process seen in dynamic mechanical spectroscopy on both pure polystyrene and in the blend. A quantitative interpretation of the phenyl dynamics was based on a simultaneous description of phenyl group rotation which allows for small angle jumps and pi flips. A fairly broad distribution of correlation times was introduced through the use of the stretched exponential correlation function. The quantitative interpretation reinforces the time/temperature overlap of the phenyl group motion with the beta process though the apparent activation energies from different approaches are difficult to compare due to the limited temperature range and the overlap with the alpha process.