Enthalpy relaxation of styrene-maleic anhydride (SMA) copolymers Part 1. Single component systems

The enthalpy relaxation of styrene-maleic anhydride (SMA) copolymers, at va rious ageing temperatures (T-a) below T-g and for different ageing times (t (a)), is described. Values of enthalpy lost (Delta H(t(a),T-a)) were calcul ated from experimental data, plotted against log(10)(t(a)) and modelled usi ng the Cowie-Ferguson (CF) semi-empirical approach to give a set of values for three adjustable parameters, Delta H-infinity(T-a), log(10)(t(c)) and b eta. These define the relaxation process, which was found to be sensitive t o copolymer composition in a non-linear fashion. Increasing MA content init ially causes a faster relaxation with a smaller overall enthalpy loss; furt her increases in MA levels lead to slower changes to a final state of lower equilibrium enthalpy. All experiments indicate that the enthalpy lost by t he fully relaxed glass (Delta H-infinity(T-a)) is less than the theoretical amount possible on reaching the state defined by the liquid enthalpy line extrapolated into the glassy region (Delta H-max(T-a)), and that the final enthalpy value is dependent on copolymer composition. The implications of t his are discussed. (C) 2000 Elsevier Science Ltd. All rights reserved.