Evolution of Oriented Crystals of Polypropylene for Capacitor Films Subjected to High‐Temperature Stretching

ABSTRACT The effect of deformation on the oriented crystal structure of polypropylene, specializing in capacitor film during stretching at high temperature, was verified. Two kinds of polypropylene with different isotacticity were stretched to different deformation ratios at the same temperature, and the microstructures of the oriented crystals were analyzed by synchrotron 2d wide‐angle/small‐angle X‐ray scattering technique. Results showed that during partial melting stretching, the thickness of lamellar stacks remained constant at a fixed temperature, and the thicknesses of the lamellae crystals, and the amorphous region were invariable over a wide range of stretching deformation. However, the increased deformation led to a decrease in the lateral dimensions of the lamellar stack, indicating that the oriented crystals slipped and broke up along the stretch direction, resulting in the formation of microfibrillar structures. This increase was accompanied by an increase in crystal orientation induced by external forces. Additionally, higher isotacticity facilitated the acquisition of a high degree of orientation and crystallinity, but it had less effect on the lamellar stack thickness. This paper elucidated the relationship between the oriented crystal structure and the deformation behavior of polypropylene, thereby providing a fundamental theoretical basis for optimizing the properties of films based on the hot‐stretching process.