Enthalpy and entropy changes during physical ageing of 20% polystyrene–80% poly(α-methylstyrene) blend and the cooling rate effects

• Aging of polystyrene–poly(α-methylstyrene) blend studied by DSC.
• Model-free thermodynamics used to analyse the temperature, time and cooling rate effects.
• Aging kinetics differ from the α-relaxation dynamics.
• Fictive temperature’s change interpreted in terms of aging kinetics.

Certain compositions of polymer blends remain mixed in the glassy state, and demix on heating and may demix or otherwise change on physical aging. To investigate these effects, we studied the loss of enthalpy and entropy of a 20% polystyrene–80% poly(α-methylstyrene) blend: (i) after aging it for varying periods at a fixed temperature, (ii) after aging it for a fixed period at various temperatures and (iii) after vitrifying it at two different cooling rates prior to the physical aging. The results have been analysed by: (a) fitting the TNM model for a non-exponential, non-linear relaxation, and (b) by determining the enthalpy and entropy loss on aging. A single set of TNM model-fit parameters did not fit the data obtained for different cooling rates, and for the physically aged blend, and for some conditions a sub-Tg feature known from previous studies of pure polymers appeared. The enthalpy and entropy on physical aging are found to decrease non-exponentially, but the exponent, βage, and the characteristic time, τa (i.e., reciprocal of the rate constant of aging-kinetics), differ from the βcal and τ0 determined by fitting the TNM model to the data. This indicates that either τa itself is time-dependent during the relaxation, and/or βage varies with the temperature. Since the characteristic time of the α-relaxation process and its non-exponential parameter refer to density and structure fluctuations in a state of fixed volume and energy, these two are neither expected to be the same as the characteristic time and the parameter observed for aging, nor are they found to be the same. We also discuss the change in the fictive temperature for different aging conditions.