Stress relaxation and physical ageing in a blend of poly(styrene-co-acrylonitrile) and poly(methyl methacrylate)

The structural recovery during physical ageing of poly(styrene-co-acrylonitrile), poly(methyl methacrylate) and of a 50/50 blend of these polymers has been examined by determining the stress relaxation modulus, G(t), over a range of temperatures below their respective glass transitions. Time-ageing time superposition of the modulus curves is possible and this gives sets of shift factors which reflect the changing mechanical response of the polymers and their blend during isothermal ageing. The ageing rate of poly(styrene-co-acrylonitrile) decreases quite markedly in the region of the glass transition. The rates for poly(methyl methacrylate) and for the blend also decrease on approaching the glass transition, but with a lesser temperature dependence. By scaling this behaviour to the glass transition the rate of ageing of the blend is found not to be intermediate between that of the components but to be similar to that of poly(methyl methacrylate). This may be explained by proposing that the two blend components respond differently to the applied stress field.