Enthalpy and volume relaxation of PMMA, PC, and a-Se: evaluation of aging bulk moduli

Enthalpy, h, and volume, v, relaxation of poly(methyl methacrylate), PMMA, and polycarbonate, PC, were measured using differential scanning calorimetry and mercury-in-glass dilatometry. For PMMA both temperature down-jumps and up-jumps were studied. Attention was paid to the influence of cooling/heating rates on aging process with the result that it is related to the corresponding shift of glass transition temperature, Tg. From the time dependence of enthalpy, h, and volume, v, the aging modulus was defined as Ka = (∂h/∂v)T, and was found to be ca. 2 GPa. This corresponds roughly to the inverse value of the compressibility, κl, in the vicinity of Tg. The results of the measurements on amorphous selenium supported the notion that Ka assumes values close to 1/κl. The measured Ka values were about 5 GPa, in fair agreement with compressibility data extracted from pvT measurements.