In situ fibrillation of polymeric nucleating agents in polypropylene and subsequent transcrystallization propelled by high-pressure water penetration during water-assisted injection molding

In situ microfibrillar reinforced composites (MFCs) parts of polypropylene (PP) and a special polymeric nucleating agent (acrylonitrile-styrene copolymer, SAN) with four weight ratios (PP/SAN; 98/2, 96/4, 94/6, and 92/8) were molded via water-assisted injection molding (WAIM). The phase morphology and crystalline structure of the WAIM MFCs parts were studied. It was found that high shear stress and cooling rate caused by the high-pressure water penetration during the WAIM could result in the in situ fibrillation of the SAN droplet, followed by the formation of transcrystals at the interface between the SAN microfiber and PP matrix. Moreover, β-form spherulites can be induced by the SAN. The contents of the transcrystals and β-form spherulites were dependent on the SAN content and melt temperature. Interestingly, a WAIM PP/SAN MFCs part with nearly half of its thickness totally dominated by transcrystals and the other half containing high contents of β-form spherulites can be obtained at proper SAN content and melt temperature. This work is expected to provide a promising way to tailor the crystalline structure of the molded parts.