The enthalpy relaxation of styrene-maleic anhydride (SMA) copolymers, at various ageing temperatures (Ta) below Tg and for different ageing times (ta), is described. Values of enthalpy lost (?H(ta,Ta)) were calculated from experimental data, plotted against log10(ta) and modelled using the Cowie-Ferguson (CF) semi-empirical approach to give a set of values for three adjustable parameters, ?H8(Ta), log10(tc) 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(Ta)) is less than the theoretical amount possible on reaching the state defined by the liquid enthalpy line extrapolated into the glassy region (?Hmax(Ta)), and that the final enthalpy value is dependent on copolymer composition. The implications of this are discussed.
- Enthalpy relaxation
- Physical ageing
- Styrene-maleic anhydride copolymers