Enthalpy recovery in low molecular weight PMMA

The phenomenological model of Tool-Narayanaswamy-Moynihan (TNM), has been widely used in order to describe enthalpy and volume relaxation experiments in different glass forming systems. However some difficulties were evidenced, especially in polymeric samples. In this work we study the enthalpy relaxation mechanism of a low molecular weight poly(methyl methacrylate) (PMMA) sample via differential scanning calorimetry experiments. We show that the TNM model, with the Scherer-Hodge expression for relaxation times, describes the aging mechanism fairly well so that it was possible to simultaneously reproduce different experimental scans with a single set of parameters. Furthermore the long term enthalpy lost after annealing was found to agree with the prediction of the model. On the contrary, from different papers in the literature it is found that the TNM model shows severe difficulties in describing the enthalpy recovery mechanism of high molecular weight PMMA samples. These results could support the idea that chain entanglements play a role in the difficulties encountered by TNM model in high polymers as suggested by others in the past. (C) 2003 Elsevier B.V. All rights reserved.