alpha-Relaxation in the glass-transition range of amorphous polymers .2. Influence of physical aging on the dielectric relaxation

The influence of physical aging on the dielectric a-relaxation of three amorphous polymers is investigated by means of time domain dielectric experiments. It is found that the dielectric relaxation behavior in a, ''isostructural'' state out-of-equilibrium can be well described by the Kohlrausch-Williams-Watts (KWW) function, the dielectric relaxation time increasing strongly with aging time but the KWW shape parameter being weakly affected by physical aging. The dependence of the relaxation time on physical aging is interpreted in terms of the Adam and Gibbs theory. The effects of physical aging on the configurational entropy obtained by applying the Adam and Gibbs equation to the dielectric relaxation times are compared with the effects on enthalpy relaxation data obtained calorimetrically. For the three polymers investigated, both quantities equilibrate in the same time scale. A phenomenological model of physical aging with parameters determined from the equilibrium supercooled melt behavior is developed. Assuming that the structural and dielectric relaxation times are proportional to each other, it is shown that the dynamical parameters obtained from equilibrium dielectric experiments can account for the time dependence of both dielectric and enthalpy relaxation in the nonlinear regime, at least at temperatures not too far below the glass transition temperature.