MULTIDIMENSIONAL H-2 NMR-STUDIES OF THE NONEXPONENTIAL CHAIN RELAXATION OF POLYSTYRENE ABOVE THE GLASS-TRANSITION

Multidimensional exchange nuclear magnetic resonance (NMR) of chain-deuterated amorphous polystyrene has been applied to elucidate the molecular dynamics above the glass transition. In addition to the diffusive aspect of the chain motion, 3D exchange NMR spectra show clear evidence of a jump-type motion, probably due to conformational changes. When the jump has taken place, no indication of long term orientational memory, i.e., back-jumps to the initial position, can be found. With a 4D exchange NMR experiment, the relaxation function of a selected sub-ensemble has been measured for motions on the 100 Hz range. This experiment indicates a broad distribution of correlation times due to parallel processes, for which different spatial regions differ in their correlation times. Another relaxation experiment proves that those spatial regions where the alpha-relaxation is slow also contribute with slow components to the nonexponential T-1 relaxation which is sensitive to motions with spectral densities in the range of the H-2 Larmor frequency of 45 MHz (beta(s) process).