Aging bulk modulus obtained from enthalpy and volume relaxations of a-PMMA and its blends with PEO

Enthalpy and volume relaxation data measured on atactic poly(methyl methacrylate) and its blends with poly(ethylene oxide) are used to calculate an aging bulk modulus, K a , tentatively defined as the derivative (∂ h∂ v) T . The K a values are mainly lower than the corresponding bulk modulus data determined directly with a piston-type PVT-device. On the whole, however, the K a values can be considered as lying close to the moduli measured in the liquid state. This notion is supported by K a data measured on amorphous selenium, which has a significantly lower compressibility in the liquid state than common amorphous polymers. The results are discussed in terms of thermodynamics of irreversible processes using a single internal variable, yet without arriving at suitable expression for the enthalpy/volume derivative.