In situ growth and crystallization behavior of PP crystals induced by CF under supercritical N2

Abstract

High-performance carbon fiber reinforced polypropylene (PP/CF) composite foams were successfully prepared using supercritical N2 as a physical blowing agent, significantly reducing raw material usage. However, the incorporation of supercritical N2 complicates the crystallization behavior of PP/CF, subsequently affects their property. Herein, this study proposes to prepare PP/CF films by solution method and observe in situ growth and crystallization behavior of PP crystals induced by CF under supercritical N2 for the first time. Numerous nucleation sites are formed in CF under supercritical N2, facilitating transcrystalline formation at the interface and confirming CF's potent heterogeneous nucleation capability. The surface tension effect of CF was visualized for the first time, noting the transformation of spherulites into dendritic structures. This transformation, becoming more pronounced with successive isothermal crystallizations, is attributed to changes in the rheological behavior of the PP melt. Furthermore, the research elucidates the mechanism behind CF-induced transcrystalline formation and the periodic growth of the crystal front under supercritical N2. A 20%CF addition reduced grain size by 41.2 % and increased grain density by 2.7 times at 130 °C. However, a higher CF content raises the interfacial free energy required for nucleation, leading to a slower grain growth rate. This work not only addresses the gap in understanding fiber-induced crystallization under supercritical gases but also lays a theoretical foundation for the crystallization behavior of other fibers under similar conditions. It provides critical insights for developing fiber-reinforced composite microcellular plastics with superior properties.