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

Abstract 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, T g . From the time dependence of enthalpy, h , and volume, v , the aging modulus was defined as K a = (∂ h /∂ v ) T , and was found to be ca. 2 GPa. This corresponds roughly to the inverse value of the compressibility, κ l , in the vicinity of T g . The results of the measurements on amorphous selenium supported the notion that K a assumes values close to 1/ κ l . The measured K a values were about 5 GPa, in fair agreement with compressibility data extracted from pvT measurements.