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-acrylonit rile), 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 r ange 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 polyme rs and their blend during isothermal ageing. The ageing rate of poly(styren e-co-acrylonitrile) : decreases quite markedly in the region of the glass t ransition. The rates for poly(methyl methacrylate) and for the blend also d ecrease on approaching the glass transition, but with a lesser temperature dependence. By scaling this behaviour to the glass transition the rate of a geing of the blend is found not to be intermediate between that of the comp onents 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.