Volume and enthalpy recovery of glass-forming polymers and the predictions controlled by internal energy

The volumetric and enthalpic response of poly(methyl metacrylate), a-PhIMA, after a sudden temperature decrease or increase at T g is described. The approach is based on a recently developed thermodynamically consistent non-linear viscoelastic theory of thermal and mechanical behavior of glassy polymers in the glass transition range. The original model is modified by replacing the stretched exponential relaxation functions by the relaxation functions based on a cooperative model where the elementary events tend to occur in clusters due to mutual induction reminiscent of the mechanism underlying Bose-Einstein statistics. The evolution of the specific volume and enthalpy is measured using mercury-in-glass dilatometry and differential scanning calorimetry, respectively. The used thermodynamic description is found to produce good agreement with the observed processes both with regard to its intensity and asymmetry when the evolution to the equilibrium state proceeds from higher and lower temperature.