LOCAL AND COOPERATIVE MOTIONS AT THE GLASS-TRANSITION OF POLYSTYRENE - INFORMATION FROM ONE-DIMENSIONAL AND 2-DIMENSIONAL NMR AS COMPARED WITH OTHER TECHNIQUES

H-2 NMR results from 2D exchange, solid echo, and wide-line absorption spectra as well as spin-lattice relaxation times are analyzed in terms of different reorientation models applied to the C-H-2 bond directions of chain-deuterated polystyrene. The dominant mechanism is rotational diffusion by small angular steps where the mean rotational correlation time agrees with relaxation times from dynamic mechanical experiments over a dynamic range of 10 decades. However, the width of the correlation time distribution extracted from the NMR results varies from about 5 decades at T(g) to not more than 1 decade at T approximately-greater-than T(g) + 40 K, whereas other relaxation techniques yield constant correlation time distributions. The results of the different methods employed for the study of chain motion are compared. It is shown that the motions of well-defined C-H bond directions which are probed by NMR are tightly coupled to the cooperative motion of the alpha-process. Moreover, they also monitor the separation of the fast beta-process from the slow alpha-process when approaching T(g) by cooling down from the melt.