Molecular dynamics of physical aging in the glassy state

We review first recent results concerning the relation between the relaxation of extensive thermodynamic and of viscoelastic functions. In the underlying theory a central role is assigned to a particular excess‐free volume function h . It was originally introduced in a theory of the equilibrium melt. The time dependence of h can then be derived from volume (or enthalpy) recovery data and serves to predict other quantities, such as mean‐square density fluctuations and viscoelastic‐temperature shift factors during the aging process. Next the physical basis and results of two kinetic theories of volume relaxation are summarized. In both approaches the h ‐function is employed as an expression of the molecular dynamics in the drive to equilibrium. The first describes a gradual elimination of free‐volume gradients through a diffusion process, characterized by a diffusion parameter, varying with the local free volume in accord with a Doolittle relation. The second, a stochastic theory, derives expressions for the matrix of transition rates between different free‐volume states, the resulting spectrum of retardation times, and the size distribution of free volume in the relaxing system. Satisfactory agreement between observed and predicted volume recovery of poly(vinyl acetate) ensues. In the limit of an infinitesimal temperature jump, both theories yield a Williams‐Watts correlation function as an approximate interpolation expression. The exponent β, however, varies in time with a constant value of approximately 0.60 over a limited time interval.