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 (Δ H( t a, T a)) were calculated from experimental data, plotted against log 10( t a) and modelled using the Cowie–Ferguson (CF) semi-empirical approach to give a set of values for three adjustable parameters, Δ H ∞( T a), log 10( t c) and β. 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 (Δ H ∞( T a)) is less than the theoretical amount possible on reaching the state defined by the liquid enthalpy line extrapolated into the glassy region (Δ H max( T a)), and that the final enthalpy value is dependent on copolymer composition. The implications of this are discussed.