Mechanisms of primary relaxation near orientational-glass and structural-glass transitions

We discuss the universal features of the relaxation observed in glass forme rs near the structural-glass transformation temperature T-g. A macroscopic analysis of the relaxation dynamics is given using the correlation-length g rowth concept and the relevant diffusion exponent z(g). Macroscopic relatio ns, which expose temporal and spatial correlations, are proposed in terms o f the characteristic temperatures, the exponent z(g), and the fragility m(g ), related to the timescale. A stochastic approach to the problem at a meso scopic level results in kinetic relations for z(g), and the late-time and s hort-time asymptotic decay-law exponents; the Kohlrausch-Williams-Watts exp onent beta (g) and the von Schweidler exponent b(g), respectively. A primar y relaxation mechanism of anomalous diffusion, established for the amorphou s polymers, leads to the kinetic equation z(g) 3(1/beta (g) - 1) Prediction s for z(g), based on the same mechanism, are given for the family of site-s ubstituted orientational glasses. For the orientational glass in plastic cr ystals a distinct mechanism yields a new relation, z(g) = C(2 - 1/beta (g)) with a constant C, deduced from dielectric loss data.