Crystalline morphology of polypropylene and rubber-modified polypropylene

The crystalline morphology of injection-molded polypropylene (PP), its relationship with crazing, and the effects of various impact modifiers on the morphology, crystallization, and fusion of PP have been studied. The highly oriented skin layer of an injection-molded tensile bar after deformation was found to be free from crazing in contrast to the heavy craze density in the randomly oriented spherulitic core zone. Reasons for the difficulty in craze nucleation in a preoriented zone are given in light of Argon's theory of craze initiation. Addition of a rubbery phase results in an irregular texture of spherulite, smaller spherulitic diameter, and decrease in the degree of undercooling, but no appreciable change in heats of fusion and crystallization other than a trivial volume effect. The rubbery phase is not pushed by the melt–solid interface to relocate to the interspherulitic boundaries. Rather, it is engulfed by the growing melt–solid interface, leaving behind a random spatial distribution of rubber particles in the PP matrix.