Memory effects in glass transition

Abstract

Behavior of glass in the glass transition region is reviewed with the emphasis on memory effects. Three examples of memory effects analyzed are the behavior of the fictive temperature in the cross-over, sub- T g relaxation and simple reheating experiments. In all cases, the non-equilibrium system reveals a temporary neglect of the equilibrium state by moving away from it. A multiexponential model of structural relaxation is employed to model this phenomenon and to investigate the behavior of separate relaxation mechanisms contributing to a total effect. The model characterizes the glassy state by a broad distribution of partial fictive temperatures - low, produced by fast relaxation mechanisms, and high, produced by slow mechanisms. During reheating, each mechanism approaches the equilibrium line T f= T in two stages: first from above (because T f i > T) and then from below (when due to the overshoot T f i < T. In the lower part of the glass transition region where slow mechanisms are still in the first stage of a very slow decrease of their T f i 's, the fast mechanisms are finishing their approach to equilibrium by a vigorous increase of their T f i 's, The T f measured in the experiment is the weighted average of all T f i 's and therefore reflects the behavior of fast mechanisms by increasing its value although T f is still greater than the actual temperature T. A complex thermal treatment designed as a combination of quenching and annealing steps will emphasize different groups of mechanisms and produce many intriguing effects. All these effects can be explained as a superposition of quenched and annealed mechanisms approaching the equilibrium state from opposite directions.