Theory of Segmental Relaxation and Physical Aging in Polymer Glasses

A predictive statistical mechanical theory of collective dynamic barriers and segmental relaxation of deeply supercooled polymer melts has been recently developed and widely applied [1]. The theory is based on a dynamic density functional perspective and the concept of a confining nonequilibrium free energy due to interchain forces. Dynamical constraints are primarily quantified by the temperature, pressure and material dependent dimensionless amplitude of long wavelength thermal collective density fluctuations, S0. This theory has now been generalized to the nonequilibrium glass based on the idea of a freezing in of the structural component of density fluctuations. Below Tg an apparent crossover of the segmental relaxation time to an Arrenhius form is predicted. Physical aging is addressed based on a simple first order kinetic equation for the time evolution of S0. At intermediate time scales after a quench the relaxation time generally grows with aging time as a power law with a temperature dependent exponent. The theoretical approach can be generalized to treat nonlinear mechanical properties including stress-strain response, yielding, modulus softening, strain hardening, and stress acceleration of relaxation and aging. [1] K.S.Schweizer and E.J.Saltzman, J.Chem.Phys. 121, 1984 (2004).