Influence of physical aging processes on electrical properties of amorphous polymers

The influence of sub-T_g (the glass transition temperature) physical aging processes on the electrical properties of amorphous polymethylmethacrylate (PMMA) and polydiancarbonate was investigated by means of the thermally stimulated depolarization currents, isothermal transient charging currents, and AC low-frequency dielectric methods. A marked decrease in polarization and loss factor corresponding to the main α relaxation (glass transition) was observed with increasing aging times, especially for aging temperatures ranging from 10 to 30°C below T_g. This behavior is compatible with the free-volume concept and can be explained by a gradual decrease in segmental mobility following a shift of the relaxation time spectrum of molecular motions to longer time scales. In PMMA, the secondary β relaxation is also significantly affected, which is probably an indication of cooperation of the main chain. The experimental evidence suggests that proper consideration should be given to thermal history and aging affects in standard electrical testing of polymer materials