Fracture toughness of α- and β-phase polypropylene homopolymers and random- and block-copolymers

The fracture and failure mode of α- and β-phase polypropylene homopolymers (PP-H), block- (PP-B) and random-type (PP-R) copolymers with ethylene were studied in high speed (1.2 m/s) flexural tests and compared. The crystallinity of the α- and β-modifications was assessed by wide-angle X-ray scattering and differential scanning calorimetry. The linear elastic fracture mechanical parameters, viz. fracture toughness (Kc) and fracture energy (Gc), were determined at room temperature and T=−40 °C on notched Charpy specimens. β-Phase PP-H and PP-B showed superior toughness to the α-versions. On the other hand, Kc and Gc were similar for PP-R in the temperature range studied for both α- and β-modifications. Fracture surfaces of the broken specimens were inspected in scanning electron microscopy and the related failure mode concluded. A model was proposed to explain the toughness improvement via β-crystallinity by considering all proved experimental findings.