Anomalous properties of the local dynamics in polymer glasses

We show how changes in a local, high frequency dynamic process are correlated with the macroscopic behavior of glassy polymers. Polyvinylethylenes vitrified by different chemical and thermodynamic pathways exhibit different densities in the glassy state. We find that the rate and amplitude of a high frequency relaxation mode (the Johari-Goldstein process involving local motion of segments of the chain backbone) can either correlate or anticorrelate with the density. This implies that neither the unoccupied (free) volume nor the configurational entropy governs the local dynamics in any general sense. Rather it is the magnitude of the fluctuations in local density that underlie these nanometer-scale motions. We show how properties of the dynamics and the density fluctuations can both be interpreted in terms of an asymmetric double well potential. Although the results herein are for polymers, the principles are expected to be generally applicable to glassy materials.