Geometry of reorientational dynamics in supercooled poly(vinyl acetate) studied by 13C 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 time scale of the motion close to the glass transition. The geometry information is derived from systematic variation of the evolution time in C-13 2D echo NMR measurements. The dynamics can be described as a superposition of angular jumps of approximately 10 degrees and rotational diffusional processes. Both processes are related to the macroscopic alpha-relaxation. On the time scale of one jump process the orientation of a segment changes by about 2 degrees via small step diffusion (< 0.6 degrees). Furthermore the temperature dependence of this reorientatinal scenario is analyzed within the limits 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 from a sequence of many distinct reorientational steps. (C) 1999 American Institute of Physics. [S0021-9606(99)50132-3].