The effect of excess thermodynamic properties versus structure formation on the physical properties of glassy polymers

The dynamic mechanical properties of glassy polymers, such as atactic polystyrene and poly(ethylene terephthalate), have been followed as a function of the excess enthalpy during isothermal annealing at selected temperatures below their respective glass transition temperatures. For glassy polymers prepared from crystallizable as well as noncrystallizable polymers, the changes in modulus parallel the extent of enthalpy relaxation that occurs during the annealing period as a result of the nonequilibrium nature of the glassy state, i.e., the modulus increases with time and approaches a limiting value that increases with decreasing temperature. These results suggest that the observed changes in the dynamic mechanical moduli are associated with changes in the excess thermodynamic properties of the polymeric glasses, and not with structure formation other than the normal liquidlike packing of the chain segments. Thus the physical properties for corresponding equilibrium glassy states can be determined for crystallizable polymers without interference from structure formation. Such data will provide the basis for a fundamental comparison of the physical properties of glassy polymers and for an assessment of the effect of modes of motion associated with subsidiary relaxation processes on these physical properties.