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

The enthalpy relaxation of styrene-maleic anhydride (SMA) copolymers, at various ageing temperatures (T_a) below T_g and for different ageing times (t_a), is described. Values of enthalpy lost (?H(t_a,T_a)) were calculated from experimental data, plotted against log₁₀(t_a) and modelled using the Cowie-Ferguson (CF) semi-empirical approach to give a set of values for three adjustable parameters, ?H₈(T_a), log₁₀(t_c) and ß. These define the relaxation process, which was found to be sensitive to copolymer composition in a non-linear fashion. Increasing MA content initially causes a faster relaxation with a smaller overall enthalpy loss; further increases in MA levels lead to slower changes to a final state of lower equilibrium enthalpy. All experiments indicate that the enthalpy lost by the fully relaxed glass (?H₈(T_a)) is less than the theoretical amount possible on reaching the state defined by the liquid enthalpy line extrapolated into the glassy region (?H_max(T_a)), and that the final enthalpy value is dependent on copolymer composition. The implications of this are discussed.