Mechanisms of Primary Relaxation near Orientational-Glass and Structural-Glass Transitions

We discuss the universal features of the relaxation observed in glass formers near the structural-glass transformation temperature Tg. A macroscopic analysis of the relaxation dynamics is given using the correlation-length growth concept and the relevant diffusion exponent zg. Macroscopic relations, which expose temporal and spatial correlations, are proposed in terms of the characteristic temperatures, the exponent zg, and the fragility mg, related to the timescale. A stochastic approach to the problem at a mesoscopic level results in kinetic relations for zg, and the late-time and short-time asymptotic decay-law exponents; the Kohlrausch-Williams-Watts exponent βg and the von Schweidler exponent bg, respectively. A primary relaxation mechanism of anomalous diffusion, established for the amorphous polymers, leads to the kinetic equation zg=3(1/βg−1). Predictions for zg, based on the same mechanism, are given for the family of site-substituted orientational glasses. For the orientational glass in plastic crystals a distinct mechanism yields a new relation, zg=C(2−1/βg) with a constant C, deduced from dielectric loss data.