H-2 NMR-STUDY OF HIGH-PRESSURE EFFECTS ON THE MOLECULAR-DYNAMICS IN POLYSTYRENE .2. PHENYL GROUP MOTION

The effect of high pressure on the phenyl group motion in amorphous po lystyrene (PS-d5) has been studied using H-2 n.m.r. quadrupole echo sp ectroscopy. The spectra are superpositions of two contributions from p henyl groups performing relatively slow and fast 180-degrees flipping motion, respectively. The motion of the phenyl groups is characterized by a distribution of correlation times. The mean correlation times of the slowly flipping groups vary from milliseconds to microseconds and are in good agreement with data from other relaxation techniques. The effect of pressure on the molecular motion is strong in the glassy st ate. The reorientational activation volume is of the same order as the volume required for the rotation of a phenyl group. Above the glass t ransition temperature the effect of pressure is much weaker. We have n o evidence that the phenyl group motion is dependent on the main chain motion.