Shear-induced interfacial sheath structure in isotactic polypropylene/glass fiber composites

Abstract

Shear-induced interfacial sheath structure was generated by pulling a single glass fiber (GF) in the supercooled isotactic polypropylene (iPP) melt. Its morphological evolution and microstructure were investigated by means of polarized optical microscopy (POM), ex-situ polarized Fourier transform infrared spectroscopy (FTIR), ex-situ two-dimension wide-angle X-ray diffraction (2D-WAXD) and scanning electron microscopy (SEM). The results revealed that higher fiber-pulling speed was crucial to promote the formation of sheath structure. Such sheath structure was proved to be the most thermodynamically stable α-crystal that could effectively induce the generation of β-cylindrite. Meanwhile, the thickness and orientation level of the sheath structure were increased with the increasing fiber-pulling speed. More interestingly, the sheath structure was composed of aligned shish-kebabs, and the lamellar-branched β-iPP crystals directly "rooted" in these kebabs, viz., β-iPP crystals grew at the growth front of oriented fibrillar lamellae. This study on the interfacial sheath structure provides another direct evidence for the formation mechanism of β-iPP crystals upon shear field.