Morphological transformation of extruded isotactic polypropylene film from the Mesophase to a-form crystals

The morphological transformation of extruded polypropylene (PP) films from the mesophase to & alpha;-form crystals has been investigated. Extruded PP films prepared by quenching had the mesophase morphology with a nearly-random orientation of the molecular chains. The mesophase structure of the quenched PP films completely transformed to & alpha;-form crystals by annealing at 100 degrees C accompanied by enhancement of the molecular orientation along the machining direction (MD). Moreover, the annealed films expanded and shrank by approximately 0.5% along the MD and transverse direction (TD), respectively. The unusual expansion behavior during annealing suggests that a small amount of oriented molecular chains formed in the extruded film, and the oriented chains acted as a nucleating agent. Consequently, the molecular orientation along the MD was enhanced owing to the morphological transformation. The oriented chains also acted as nucleating agent during tensile deformation, resulting in a significant improvement of strength in the MD stretching.