Crystal Structure and Thermal Properties of Polypropylene Prepared by Variable Speed Pressurization

In the high-pressure molding process of polymers, the pressure, pressurization rate and molecular weight of the polymers will affect the final crystalline structure of the products. Studying the relationship between different high-pressure processing methods and the crystalline structure of isotactic polypropylene (iPP) is helpful to deeply understand the influence of high-pressure molding process on the crystallization behavior of polymers. In this paper, wide-angle X-ray diffraction and differential scanning calorimetry were used to study the crystallization behavior and thermal properties of two kinds of iPP with different molecular weights at different pressurization rates and pressures. The results show that pressure is a key factor in determining the formation of mesophase iPP. The complete mesophase iPP can be prepared under sufficient pressure. Under enough pressure, the higher the pressurization rate, the easier to form mesophase iPP, ortherwise, to form γ-phase iPP. The higher the molecular weight, the higher pressure required for the preparation of mesophase iPP, and the greater the required critical pressurization rate. The thermal property analysis of the products shows that the thermal properties of the mesophase products obtained under different conditions are basically the same, but the crystal perfection of the γ-phase iPP is related to the pressurized condition.