Effect of micromorphologic features on the interfacial strength of iPP/Kevlar fiber microcomposites

Fully transcrystalline, fully spherulitic specimens, and single Kevlar fiber microcomposites were prepared for isotactic polypropylene (iPP) resin with positive, negative, or mixed birefringence and annealed condition. The micromorphologic observation (PLM, PCLM, SEM, AFM) and supermolecular structure study (WAXS, SAXS) show that the optical birefringence change of iPP is related to the contribution of cross-hatch lamellae. Transcrystalline layers along a Kevlar fiber were about 50 μm thick. The interfacial shear strength was determined by means of a modified single fiber pull-out test. The more radial lamellae increase the radial stiffness relative to the tangential one, and produce higher normal stresses on the fiber surface. The results revealed that the optical birefringence in the transcrystalline layer significantly affected the interfacial shear strength, which was related to the mechanical friction, thermal mismatch, and the crystallinity.