Effects of stress and vapor exposure before and during aging on enthalpy relaxation of poly(vinyl chloride)

Enthalpy relaxations in glassy poly(vinyl chloride) following varied pre-aging treatments and under varied aging conditions have been compared through observations of sub-T_g endothermal DSC (differential scanning calorimetry) aging peaks. The extent of enthalpy relaxation for a fixed time and temperature of aging is progressively enhanced by the imposition and release of increasing mechanical stress before aging. The same effect is produced by sorption and desorption of increasing amounts of CO₂ or CH₃Cl vapor before aging. In contrast, the continued application of mechanical stress, or the presence of vapor, during the aging period suppresses enthalpy relaxation. The extent of suppression increases with increasing vapor pressure and solubility or increasing stress. These effects are interpreted as consequences of an increase in the enthalpy of the polymer under mechanical or sorptive stress and an enthalpy relaxation following the release of this stress. In addition to these effects on the DSC endotherm, a pronounced exotherm between the aging peak and T_g is observed for samples which have undergone shear yielding or orientation either before or during aging. This exotherm may be the result of release of stored strain energy during the DSC scan.