Rheological and mechanical properties of poly(α-methylstyrene-co-acrylonitrile)/poly(methylacrylate-co-methyl methacrylate) blends in miscible and phase-separated regimes of various morphologies -: Part IV.: Influence of the morphology on the mechanical properties (IUPAC technical report)

Influences of the morphology on the thermal and mechanical properties of poly(alpha-methylstyrene-co-acrylonitrile)/poly(methylacrylate-co-methyl methacrylate) (PalphaMSAN/PMMA) blends have been investigated. Differential scanning calorimetry (DSC) measurements confirm that all blends were phase-separated due to the temperature at which they have been extruded and squeeze-molded. Based on the cloudpoints of 17 blends and transmission electron microscopy (TEM) micrographs, the interaction parameters as a function of temperature and composition were calculated for the lower critical solution temperature (LCST) system. Varying the morphology by annealing without changing the composition of the system resulted in a finer morphology for the 85/15 blends, while the 40/60 blend showed an increase in the domain size with annealing time. Tensile strength and fracture toughness indicate that the PalphaMSAN domains in the tougher PMMA matrix cause a deterioration in the mechanical properties of the blends, while the PMMA domains in the PalphaMSAN matrix improve the mechanical properties. No clear conclusions on the influence of morphology on fracture toughness could be drawn because in one case (40/60 blend) the fracture toughness decreases slightly by annealing and in the other case (85/15 blend) fracture toughness values increase slightly with decreasing, phase separation by annealing. In situ strained thin sections in the TEM indicated no effect of annealing on the micromechanical behavior. Shear deformation was observed as the prevailing deformation mechanism in the PaMSAN and fibrillized crazing in the PMMA-rich blends. From fatigue crack growth experiments it was concluded that the fatigue crack propagation threshold is higher for PMMA than for PaMSAN. Tests on the annealed samples of PalphaMSAN/PMMA 85/15 and 40/60 showed that the differences in morphology did not affect the fatigue crack growth resistance significantly. From the features of the fracture surface investigated by scanning electron microscopy (SEM), the conclusion can be drawn that the fatigue crack propagates faster in the more brittle PaMSAN phase, but the overall advance of the crack front is controlled at the interphases, resulting in a crack propagation gradient along the interphase.