Structural relaxation of an epoxy resin at temperatures well below T-g

The effects of annealing a glassy polymer at temperatures well below T-g on the dynamic linear viscoelastic properties were investigated. The polymer was an epoxy thermoset with a T-g of 182 degrees C. Specimens were annealed from 2 h to up to 600 h at temperatures from -100 degrees C (T-g -282 degrees C) up to T-g. At annealing temperatures just below T-g, there was no effect of annealing, and annealing at -100 degrees C (T-g -282 degrees C) showed no difference between 2 and 6 h annealing, although annealing effects might be seen at longer times. At temperatures between -50 degrees C (T-g -232 degrees C) and 170 degrees C (T-g -12 degrees C) the storage isotherms increased with annealing while the loss isotherms decreased. In addition, the heat capacity as measured via DSC after annealing at 50 degrees C (T-g -132 degrees C) and 100 degrees C (T-g -82 degrees C) exhibited endothermic peaks approximately 40 degrees C above the annealing temperature. This study clearly shows that a material deep in the glassy state is not frozen, but undergoes continuing evolution. The nature of deep glass aging is different than physical aging in the T-g region, where deep in the glass the relaxation spectrum changes magnitude and shape but does not shift along the log frequency axis in contrast to physical aging in the T-g region.