Structural relaxation in a polymeric glass well below Tg

Glass formers below Tg exhibit slow relaxation toward equilibrium state in a process known as physical aging. The time to reach equilibrium increases drastically as the temperature decreases so that it is impractical to achieve equilibration for temperatures below Tg-15°C. Consequently, it is typically assumed that at temperatures well below Tg physical aging ceases in glasses. We report on linear viscoelastic experiments on an epoxy material, where the linear viscoelastic shear storage and loss isotherms were obtained for a wide temperature range both above Tg and deep in the glassy state. At each temperature a frequency scan from 10^-2 Hz to 10^1.5 Hz was performed first after 2 hr of annealing and then repeated after 6 hr of annealing. Somewhat unexpectedly, even at temperature as low as Tg-280°C there is a noticeable systematic difference between 2 hr and 6 hr isotherms. At room temperature (Tg-160°C) the measurements were performed at longer times of up to 2 months, where systematic shift in the isotherms continued. These results indicate that, although the rate of equilibration slows down progressively with decrease in temperature, not all relaxation processes are frozen even well below Tg.