Enlin Han

Enhanced surface free energy of polyimide fibers by alkali treatment and its interfacial adhesion behavior to epoxy resins

Abstract In this article, polyimide (PI) fibers were modified by alkali treatment, and PI fiber-reinforced epoxy composites were fabricated. The effects of different alkali treatment times on the surface properties of the PI fibers and the adhesion behaviors of PI fiber/epoxy composites were studied. The surface morphologies, chemical compositions, mechanical properties, and surface free energy of the PI fibers were characterized by atomic force microscopy, X-ray photoelectron spectroscopy, single-fiber tensile strength analysis, and dynamic contact angle analysis, respectively. The results show that alkali treatment plays an important role in the improvement of the surface free energy and the wettability of PI fibers. We also found that, after the 3 min, 30 °C, 20 wt% NaOH solution treatment, the fibers possessed good mechanical properties and surface activities, and the interlaminar shear strength of the composites increased to 64.52 MPa, indicating good interfacial adhesion properties.

Effect of discontinuous long polyimide fiber on mechanical properties, fracture morphology, and crystallization behaviors of polyamide-6 matrix composites:

This work has conducted an extensive investigation on the effect of discontinuous long polyimide (PI) fiber on the mechanical properties, fracture morphology, and crystallization behaviors of polyamide-6 (PA6) matrix composites. A series of PA6 matrix composites with different contents of PI fiber were prepared through a standard melt-pultrusion process. The resulting composite specimens not only achieved a prominent reinforcement but also obtain a significant improvement in impact toughness. It is highlighted that the composites achieved a remarkable increase in Izod impact strength by a factor of five compared to pure PA6 when 12 wt% of PI fiber was incorporated. Moreover, the tensile strength of the composites reached 143 MPa at a fiber content of 18 wt%. The mechanical properties could be well predicted by the Cox-Krenchel model, but a negative deviation in experimental data was observed at high fiber concentrations due to the decrease of residual fiber length and fiber aggregation. The morphologic observation of fracture surface indicated that fiber pullout was a major mechanism for tensile failure as a result of long PI fiber-reinforcing effectiveness, and it was also the predominant energy absorption mechanism for the impact fracture of composite specimens. The presence of long PI fiber not only enhanced the crystallinity of PA6 matrix but also induced a well-defined transcrystalline layer on the fiber surface due to its high nucleating ability.