A two-dimensional (2D) and a novel ‘‘reduced four-dimensional (4D)’’ exchange NMR experiment are applied to investigate slow dynamics in poly(vinyl acetate) 20 K above its ${\mathit{T}}_{\mathit{g}}$. The 2D ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$ NMR spectra indicate a diffusive process with strongly nonexponential loss of correlation. In the ‘‘reduced 4D’’ experiment, the underlying distribution of correlation times is found to be heterogeneous on the time scale of the average correlation time, becoming homogeneous, however, at later times due to fluctuations which are 2 orders of magnitude slower.

• Received 1 April 1991

DOI:https://doi.org/10.1103/PhysRevLett.66.3020