Protonated benzimidazole segment induced interfacial limited linear grafting in aramid fiber/epoxy resin interface towards strong and tough composites

To induce both high strength and high toughness advantages while ensuring high interfacial grafting density is the intrinsic conflict in fiber reinforced polymer composite structure design. In this article, utilizing the alkalinity of benzimidazole contained aramid fiber (PBIA), we prepared protonated fiber surface via simple Lewis acid solution soaking to adjust polymerization process of epoxy resin matrix, and accomplished linear interfacial polymerization/grafting limited in composite interface as Atomic Force Microscopy (AFM) confirmed, thus strong and tough interface with high grafting density was obtained. As 1 H NMR spectrum confirmed, protonated benzimidazole unit on fiber surface could convert interfacial molecular configuration of composite from crosslinking to linearity, where the linearity configuration dominated interfacial grafting density was also promoted due to their improved surface electron density as Differential Scanning Calorimetry (DSC) and Density Functional Theory (DFT) calculated results revealed. Therefore, ex-situ experiments like Tapered Double-Cantilever Beam (TDCB) tearing test revealed that the protonated benzimidazole segment could highly enhance interfacial tearing toughness (K ⅠC ) by 33.80%, where the practical interfacial shear strength (IFSS) and interfacial shear toughness (G ⅠC ) in the composite also respectively increased by 60.16% and 66.56% at maximum, which leaded to the optimization in mechanical strength and toughness of the composite short beam.