Formation and Distribution of the Mesophase in Ultrasonic Micro-Injection-Molded Isotactic Polypropylene

The influence of mold temperature and filling velocity on the formation and distribution of the mesophase in ultrasonic micro-injection-molded isotactic polypropylene (iPP) was explored using a wide-angle X-ray diffraction (WAXD) technique. A Gaussian fitting method was used to decompose one-dimensional WAXD profiles into quantitative fractions of the crystalline lamellae, mesophase, and amorphous phase. It was found that the mesophase formation can only be detected up to a mold temperature of 60 °C due to its low thermal stability. Moreover, the content of the mesophase decreases distinctly as the filling velocity is increased. The component is mainly located in the core layer after examining the distribution of each crystal form along the thickness direction, irrespective of mold temperature and filling velocity. All these findings can be attributed to the correlation between the degree of molecular orientation and the cooling rate. A high cooling rate occurred during the micro-injection molding process produces the result that there is less time for the polymeric chains with a low level of orientation to crystallize into a well-ordered structure, thus yielding many mesophase crystals with a paracrystalline smectic arrangement especially in the core layer. Finally, a diagram for mesophase formation was constructed in mold temperature–filling velocity space, which offers a detailed road map for regulating mesophase fraction in ultrasonic micro-injection molding operations.