Physical ageing and modelling of the glass-liquid transition of water and aqueous solutions imbibed in poly-(2-hydroxyethyl-methacrylate) and in the bulk state

Abstract

Physical ageing of glassy water and aqueous solutions of 22 mol% ethylene glycol and 16 mol% lithium chloride imbibed in poly(2-hydroxyethyl-methacrylate) (PHEMA) and of the bulk solutions was studied by differential scanning calorimetry (DSC). The samples were isothermally annealed for a constant time at various temperatures below their glass → liquid transition temperature. The resulting DSC scans were analyzed in terms of the four-parameter model of Moynihan et al., which incorporates both the non-exponentiality of the relaxation function and the structure dependence of the retardation times. The non-exponentiality parameter, β , is smaller for the solutions imbibed in PHEMA than in the bulk, implying a broader distribution of retardation times and/or enhanced cooperativity of the relaxation process. The activation enthalpy and the parameter x , describing the structure dependence of relaxation, are on average the same whether the solutions are imbibed in PHEMA or in their bulk states. However, x varies systematically with thermal history, indicating that the phenomenology is not completely accurate. It is probable that the treatment of non-linearity by the model needs to be improved.