Thermal and dynamic-mechanical characterization of uni- and biaxially oriented polypropylene films

The effect of uni- and biaxial orientation in the morphology of polypropylene films has been investigated by thermal, dynamic-mechanical, X-ray pole figure and diffraction patterns. In uniaxial oriented films the level of damping is roughly three times higher in the MD direction than it is in the TD direction. The stretching always produces crystals of the a form independently of the starting type. Fast DSC scans show two melting peaks indicative of two crystalline species. The Fujiyama et al. model for the crystalline structure can be also applied to the uniaxially stretched films. Upon biaxially orienting, the folded lamellae crystals (kebabs) are the ones to support all the force applied, and when their maximum level of stress slippage is reached they deform following the Peterlin's model, forming a new shish structure. These new shishes are aligned to the TD direction and by linking the original shishes in the MD direction produce a planar orthogonal net of linked shish structures. The space among the shishes is filled with small and imperfect folded lamellae with c-axis in the film plane and preferentially oriented in the MD and TD directions, keeping constant crystallinity density throughout.