Morphological behaviour and instrumented dart impact properties of β-crystalline-phase polypropylene

Specimens consisting of high-purity β-phase polypropylene were prepared by adding a bicomponent β-nucleator consisting of equal amounts of pimelic acid and calcium stearate. Scanning electron microscopy (SEM), differential scanning calorimetry (d.s.c.), dynamic mechanical analysis and instrumented drop weight impact tests were used to characterize the morphology, thermal behaviour and the impact properties of the β-phase polypropylene. SEM examinations show that the β-spherulites exhibit a sheaf-like structure with no clear boundaries between them. This distinct spherulitic morphology results in a substantial improvement in the falling weight impact resistance. Fractographic analysis reveals that microfibrils and voids were formed in the fracture induction area of the β-form specimen. The greater impact strength observed in the β-form material is due to the larger energy dissipation which is associated with the formation of microfibrils. Furthermore, d.s.c. analysis showed that there is no β → α phase conversion during the impact test. Copyright © 1996 Elsevier Science Ltd.