Physical Aging in Amorphous Poly(ethylene terephthalate). 2. Isothermal Volume Recovery After Two Temperature Jumps.

Abstract

The dependence of physical aging kinetics on glass-forming histories and thermal treatments was examined for amorphous poly (ethylene terephthalate) (PET) glasses. The samples were subjected to two-step thermal treatment : first, temperature drop from equilibrium at 80°C to 60°C and then isothermal annealing ; second, temperature jump to aging temperature Ta. Isothermal volume recovery was measured after the second step to Ta (70°C> Ta>60°C) below the glass transition temperature (T). The volume recovery curves were represented by a plot of normalized fractional free volume (=(V-V∞)/V∞, where V∞ is the equilibrium value of specific volume V) vs. logarithm of aging time. All curves, showing the well known “memory effect” characteristic, could be described nearly by a six-parameter phenomenological model postulating a multiplicity of independent relaxation modes with distinct relaxation times. The model parameters determined to fit the data were found to be greatly dependent on the glass-forming history, but weakly dependent on the thermal treatment conditions. Furthermore, the individual recovery curves could be superimposed by horizontal shifts along the logarithm axis of aging time.