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

The fracture and failure mode of alpha- and beta-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 alpha- and beta-modifications was assessed by wide-angle X-ray scattering and differential scanning calorimetry. The linear elastic fracture mechanical parameters, viz. fracture toughness (K-c) and fracture energy (G(c)), were determined at room temperature and T = -40 degreesC on notched Charpy specimens. beta-Phase PP-H and PP-B showed superior toughness to the alpha-versions. On the other hand, K-c and G(c) were similar for PP-R in the temperature range studied for both alpha- and beta-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 beta-crystallinity by considering all proved experimental findings. (C) 2002 Published by Elsevier Science Ltd.