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The morphology and fracture behavior of isotactic polypropylene toughened by styrene‐ethylene‐propylene (PP/SEP) were investigated. The SEP rubber, having an average particle size of 0.2 µm, is found to be well dispersed in the PP matrix. The fracture toughness of SEP‐modified PP is greatly improved. The toughening mechanism investigation shows that a widespread crazing zone is generated in the crack tip damage zone. An intense narrow damage band in the center of crazed zone is formed. Crazing and shear yielding are found to be the dominant toughening mechanisms in PP/SEP. The crazes are initiated only by large SEP particles in the blend. The small SEP particles (< 0.3 µm) can neither cavitate nor trigger crazing. As a result, large scale shear deformation is suppressed in this blend. These findings are consistent with the notion that the crack tip plane strain constraint has to be relieved in magnitude in order for the deviatoric stress to reach a critical value for widespread shear banding.

ABSTRACT:Fracture toughness and failure mechanisms in preformed poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) particle-modified bismaleimide (BMI) systems are investigated. The fracture toughness of BMI can be significantly improved by incorporating preformed PPO particles without causing significant deterioration in other mechanical and thermal properties. The fracture mechanisms in BMI/PPO appear to be dominated by craze-like damage. Further investigation of the craze-like damage zone using transmission electron microscopy reveals that crazes are formed inside the PPO particle phase and dilatation bands, which appear to be triggered by the crazes inside the PPO particle, are formed in the BMI matrix. Particle bridging is also found to contribute to the toughening of BMI/PPO. The benefits of using preformed PPO particles to toughen BMI and other brittle thermosets for composite and adhesive applications are discussed.