Effect of structure on stress relaxation of polymer blends in glassy state

Short-term stress-relaxation measurements were performed on blends of poly(methyl methacrylate) with poly(styrene-co-acrylonitrile), of polystyrene with poly(2,6-dimethyl-1,4-phenylene oxide) and poly(methyl methacrylate) with poly(vinylidene fluoride), at different temperatures below the glass transition region and a constant strain of 0.5%. Stress-relaxation moduli were analysed using the Williams-Watts equation. For amorphous blends, a universal value for the distribution parameter β of 0.41 was found, and the relaxation time τ showed a power-law correlation with ageing temperature and ageing time. The time and temperature dependences of τ changed with blend composition in systems containing polymers with dissimilar chemical structures, while little variation was observed in systems consisting of molecules of similar structures. Stress relaxation proceeded much faster in semicrystalline systems than in amorphous systems.