Micronecking in thin films of isotactic polypropylene

Thin melt crystallized spherulitic films of isotactic polypropylene have been deformed in uniaxial tension at different temperatures and examined by transmission electron microscopy. As the temperature was increased above approximately -80 degrees C, a transition from high aspect ratio craze-like features to highly voided shear deformation zones was observed, these latter initiating both at spherulite boundaries and along the spherulite diameters perpendicular to the tensile axis. In rapidly crystallized films, the draw ratio in the deformation zones was estimated to be close to the natural draw ratio of the melt entanglement network, and was independent of test temperature in the range investigated. However, for crystallization temperatures above approximately 125 degrees C, the mean draw ratio within the deformation zones was found to increase with crystallization temperature. It was also observed that, although the draw ratios in deformation zones in beta spherulites were similar to those in alpha. spherulites in films containing both modifications, the former showed more homogeneous textures. This suggests that the particularly pronounced textures associated with alpha spherulites in polypropylene thin films are linked to their crosshatched lath-like lamellar morphology.