Physical Aging in Poly(Methyl methacrylate)/Poly(styrene-co-acrylonitrile) Blends. 3. Simulation of Enthalpy Relaxation using the Moynihan Model

The Moynihan model was used to simulate enthalpy relaxation of a series of blends of poly-(methyl methacrylate) (PMMA) with poly(styrene-co-acrylonitrile) (SAN). The optimized Moynihan parameters for each blend composition were obtained from the best fits of specific heat data in the glass transition region measured in the rate-heating approach experiments, whereby samples were reheated immediately after being cooled through the glass transition region. Optimization was carried out by using the Marquardt algorithm. The optimized parameters were then used in simulations of experiments following the isothermal approach, in which a period of isothermal relaxation was introduced between cooling and heating steps. Calculated values for the isothermal enthalpy relaxation agreed with previously reported experimental data within the margins of experimental uncertainty. The discrete formulation of the Moynihan model was meticulously constructed to ensure an accurate representation of that model.