Shear-Induced Interfacial Structure of Isotactic Polypropylene (iPP) in iPP/Fiber Composites

The shear-induced interfacial structure of iPP in pulled iPP/fiber composites was studied by optical microscopy. Although the Kevlar fiber shows basically no nucleation ability for the iPP in quiescent melt under chosen conditions, different interfacial morphologies have been observed by pulling the fiber embedded in the matrix. It was found that the polymorphic nature of cylindrites developed on the sheared layer along the fiber strongly depended on the fiber-pulling rate, the duration of pulling, and the temperatures used for fiber pulling and subsequent crystallization. At lower fiber-pulling rates, α-iPP cylindrites were always the observed morphologies no matter how long the fiber was pulled. At moderate pulling rates, the interfacial morphologies varied from pure α-iPP cylindrites, to mixed α- and β-iPP cylindrites, and finally to the β-iPP cylindrites, depending on the pulling time. At higher fiber-pulling rates, β-cylindrite structures of iPP were the observed morphologies. According to the observed morphological features, it is suggested that there exists an orientation window of the iPP molecules in the molten state, which enables the formation of β-iPP crystals.