Effect of aging time on the volumetric and enthalpic glass transition of a-PMMA upon heating

In this paper we model the volumetric and enthalpic response of a-PMMA during heating at constant rate through the glass transition region following an aging period of varying length. The modeling is based on a recently developed thermodynamically consistent non-linear viscoelastic theory of thermal and mechanical behaviors of glassy polymers in the glass transition range [Caruthers JM, Adolf DB, Chambers RS, Shrikhande P. Polymer 2004;45:4577-97. Adolf DB, Chambers RS, Caruthers JM. Polymer 2004;45:4599-621]. The original model is slightly modified by replacing the stretched exponential by a relaxation function based on a simplified cooperative model emulating the commonly observed linear variation of the relaxing quantity with logarithmic time. Good agreement is found with the observed temperature dependence of specific volume and enthalpy. Also the peaks in thermal expansivity and heat capacity are well described, both with regard to their intensity and position along the temperature axis.