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Controlling interfacial microstructure and interactions between carbon fiber (CF) and matrix is of crucial importance for the fabrication of advanced polymer composites. In this paper, a hierarchical reinforcement (CF-g-SiO2) was prepared through directly grafting 3-aminopropyltriethoxysilane (APS) functionalized silica nanoparticles (SiO2-APS) onto CF surface by the covalent linkage for the first time. SiO2-APS nanoparticles distributed onto the fiber surface uniformly, which could increase surface polarity and roughness obviously. CF-g-SiO2 exhibited a low contact angle and high surface free energy, and thus enhanced the wettability between CF and matrix greatly. Simultaneous increases of interlaminar shear strength (ILSS) and interfacial shear strength (IFSS) of CF-g-SiO2 composites were achieved, increasing 53.27% in ILSS and 40.92% in IFSS compared with those of untreated composites. These enhancements can be attributed to the existent of SiO2-APS interface with providing sufficient chemical bonding and strong mechanical interlocking between the fiber and matrix. Moreover, impact resistance of CF-g-SiO2 composites was enhanced with increasing the amplitude of 34.95%. In addition, the introduction of Si-O-Si bonds at the interface by SiO2-APS grafting leads to the remarkable enhancement of the hydrothermal aging resistance.

The gradient modulus intermediate layer between CF and epoxy by designing double “rigid-flexible” structure using CNTs and polyamide (PA) on the CF surface was firstly established, and the effects of modulus intermediate layer on interfacial and mechanical properties of CF composites were investigated. The constructing of double “rigid-flexible” structure on the CF surface can greatly increase the polar functional groups, roughness and wettability of the CF surface as well as the thickness of intermediate layer, which could balance the modulus of fiber and resin to achieve the best matching. The interfacial shear strength (IFSS), interlaminar shear strength (ILSS), flexural strength, and impact strength of CF composites increased by 75.6%, 44.1%, 41.3%, and 34.6%, respectively. In addition, the interfacial failure behaviors and strengthening mechanisms of composites have been studied. This novel design and method built a multistage gradient modulus intermediate layer of composite with strong physicochemical interaction and good wettability, which has greatly potential for high-performance composite.

A novel supercritical method for deposition of polyethyleneimine (PEI) onto carbon fibers (CFs) surface was reported. The surface functional groups, morphology, wettability and interphase properties of CFs grafted with PEI (CF-g-PEI) were studied, and compared to physically-adsorbed and chemically-adsorbed ones (CF-c-PEI and CF-ad-PEI). The CF-g-PEI exhibited the highest deposition amount and interfacial adhesion strength. Interface enhancement mechanism and failure mode have also been explored in details. However, impact resistance and single fiber tensile strength of CF-g-PEI were not optimal. Hence, based on different applications, a property balance of the resulting composite should be considered.

We report on a new application of branched polyethyleneimine as a coupling agent to functionalize carbon fibers in conventional and supercritical methods. The microstructure and mechanical properties of carbon fibers before and after modification were investigated. Polyethyleneimine could increase the polarity, wettability and roughness of the carbon fiber surface, and supercritical methanol was also able to improve the reaction efficiency. The interfacial shear strength and impact toughness improved significantly when using the two methods, especially in supercritical methanol. Reinforcing and toughening mechanisms have also been discussed. Functionalization does not decrease fiber tensile strength.

Atomic oxygen (AO) is a dominant component of the low earth orbit and can erode most spacecraft materials. In this work, both silane and graphene oxide (GO) were introduced onto poly(p-phenylene benzobisoxazole) (PBO) fibers to prevent AO from penetrating into the interface of PBO fiber/epoxy composites. The microstructure, mechanical properties and AO erosion resistance of PBO fibers before and after modification were investigated. Experimental results revealed that the GO was successfully grafted onto PBO fibers using 3-aminopropyltrimethoxysilane (APTMS) as the bridging agent. The surface roughness (Ra) and wettability of the obtained hybrid fibers (PBO–APTMS–GO) were obviously increased in comparison with those of an untreated one. In addition, PBO–APTMS–GO showed simultaneously remarkable enhancement in interfacial shear strength (IFSS) and AO erosion resistance. Meanwhile, single filament tensile strength (TS) had no obvious decrease after the grafting processes. We believe the facile and effective method may provide a novel interface design strategy for developing multifunctional fibers.

A facile and rapid come close to the manufacture of advanced epoxy nanocomposites by grafting of nano-silicon dioxide (nano-SiO2) onto the graphene oxide (GO) through thiol-ene click reaction was reported. The correlations between surface modification, morphology, dispersion/exfoliation, interfacial interaction of sheets and the corresponding mechanical properties of the composites were methodically explored. It appeared that nano-SiO2 have been grafted onto GO via covalent bonding successfully, which effectively improved the interface compatibility and dispersion in epoxy matrix. The reinforcing mechanisms has been also elaborated. It is demonstrated that the tensile strength and elastic modulus for filler with 0.1 wt% GO-SiO2/epoxy composites increased by 32.18% and 22.86% compared to neat epoxy, respectively. It is believed that the simplistic and effectual method may provide a novel and promising strengthen design strategy for next‐generation advanced nanocomposite structures.