Geometry of reorientational dynamics in supercooled poly(vinyl acetate) studied by C-13 two-dimensional nuclear magnetic resonance echo experiments

A study of the reorientational segmental dynamics in supercooled poly(vinyl acetate) is presented, yielding detailed information about geometry and ti me scale of the motion close to the glass transition. The geometry informat ion is derived from systematic variation of the evolution time in C-13 2D e cho NMR measurements. The dynamics can be described as a superposition of a ngular jumps of approximately 10 degrees and rotational diffusional process es. Both processes are related to the macroscopic alpha-relaxation. On the time scale of one jump process the orientation of a segment changes by abou t 2 degrees via small step diffusion (< 0.6 degrees). Furthermore the tempe rature dependence of this reorientatinal scenario is analyzed within the li mits imposed by the experiment. All results are compared with previous H-2 2D NMR measurements on low-molecular glass formers. In both cases the loss of correlation, as described by conventional correlation times, results fro m a sequence of many distinct reorientational steps. (C) 1999 American Inst itute of Physics. [S0021-9606(99)50132-3].