Molecular interpretation of relaxation in glasses: Free volume and density fluctuations

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Abstract
The volume relaxation and recovery in amorphous polymers near the glass transition are generally attributed to free volume collapse. In this work, a molecular kinetic theory based on equilibrium and nonequilibrium statistical thermodynamics has been developed to describe the phenomena in terms of free volume distribution. The work provides a coherent interpretation of some recent models with the successful multiparameter equations of Kovacs, Aklonis, and their co-workers being derived from the present theory. The structural contributions to relaxation times may be interpreted as the result of nonequilibrium fluctuations of the system and are related to the change of entropy. When both time and space are included in a non-Markovian stochastic process, the volume recovery of glasses has to be determined by solving multidiffusion equations in order to be successful in describing the memory effects. Finally, the present theory has been employed to analyze the isothermal relaxation of thermal density fluctuations in polymer glasses. The theory reveals that the decay of thermal density fluctuations is always slower than that of volume and its rate depends on annealing temperature and structure.