Toughness profile in injection-molded polypropylene:: The effect of the β-modification

Fracture toughness characterized by the J-integral method has been evaluated along injection-molded bars of isotactic polypropylene. A significant increase in toughness with decreasing distance from the specimen gate has been observed with maximum steepness of the dependence in the central part of the specimen length. Parallel analysis of wide-angle X-ray diffraction patterns has shown a similar dependence of the crystalline beta-phase concentration. It is suggested that polypropylene with beta-crystallites has better structural continuity in the amorphous phase and, consequently, higher inherent ductility and superior macroscopic toughness than the related material containing only a-crystallites. Consequences of beta-phase gradients for practical toughness testing are also discussed. (C) 1998 John Wiley & Sons, Inc.