Abstract:

This study mainly focused on the hot⁃stretching⁃induced oriented crystallization structure of isotactic polypropylene （iPP） granules specialized for capacitance films （i.e. electrical grade iPP）. A series of characterization tools， including differential scanning calorimetry， wide⁃angle X⁃ray diffraction， two⁃dimensional small⁃angle/wide⁃angle X⁃ray scattering， and dynamic mechanical analysis， were employed to determine the long⁃period spacing， orientation degree， crystallinity， melt point and chain relaxation behavior of iPP. The stretching deformation resulted in a highly oriented fibrillar morphology. A strain⁃induced crystallization enhancement was observed， in which both of the orientation degree and crystallinity increased continuously with an increasing in the strain. Two types of lamellae with thicker or thinner thicknesses were newly formed during the hot⁃stretching process. The thicker one can be attributed to the melt⁃recrystallization mode， while the thinner one is ascribed to the strain⁃induced crystallization mechanism. The dependence of strain on their crystallinity and melt point seemed to be weak. Moreover， the electric density difference between the amorphous and crystalline phases decreased with an increase in the strain， and meanwhile the chain relaxation temperatures related to different condensed states became close to each other.

Key words: polypropylene, hot stretching, strain, fibrillar crystals, crystallinity, melt point