•A luminescent labeling method was developed to tag the CFRP composite interface.•The composite interface was tagged with green luminescence.•The interfacial adhesion of CF/epoxy composites was improved by LNs.A luminescent labeling method was developed to tag and enhance the carbon fiber (CF)/epoxy interface by doping sizing agent with upconversion luminescent nanoparticles (LNs). Morphology observation indicated that the value of surface roughness (Ra) decreased after sizing process and the sizing layer was about 110 nm thick. With the addition of LNs, the CF/epoxy interface was tagged with green luminescence, which showed that the interface enlarged to 1.2 μm due to interdiffusion between sizing layer and epoxy matrix. The LNs also presented a positive effect on the interfacial adhesion of CF/epoxy composites through chemical bonding among the LNs, sizing layer and matrix. The unique upconversion luminescence and reactive groups of LNs were responsible for the mechanism of luminescent labeling and interfacial enhancement, respectively.Download high-res image (67KB)Download full-size image

Epoxy matrix toughened by polyethersulfone (PES) and polyamide (PA) microparticles was designed and the in-situ interlaminar toughened carbon fiber/epoxy composites were fabricated. Synergistic toughening effect of PES and PA on epoxy matrix was achieved due to semi-IPN structure of PES toughened matrix and uniform dispersion of PA microparticles. Shear-calender orientation of PA microparticles was found during prepreg processing and the microparticles remained on the surface of prepreg due to fiber-bundle filtration. The in-situ formed toughening interlayer of PA microparticles and interfacial bonding between PA and epoxy matrix were detected, which resulted in enhanced fracture toughness, CAI, and transverse flexural strength of the composite based on the PES/PA synergistically toughened matrix. SEM images of fracture morphology of the composite showed evidence of enhanced plastic deformation created by PES and PA, and crack deflection and bridging by PA microparticles.

MWNTs-EP/PSF (polysulfone) hybrid nanofibers with preferred orientation were directly electrospun onto carbon fiber/epoxy prepregs and interlaminar synchronously reinforced and toughened CFRP composites were successfully fabricated. With MWNTs-EP loading increasing, the oriented nanofibers were obtained accompanying with enhanced alignment of inner MWNTs-EP. Flexural properties and interlaminar shear strength of composites were improved with increasing MWNTs-EP loadings, whereas fracture toughness attained maximum at 10 wt% MWNTs-EP loading and then decreased. Based on these results, multiscale schematic modeling and mechanism schematic of hybrid nanofibers reinforced and toughened composites were suggested. Due to the preferred orientation of nanofibers, MWNTs-EP was inclined to align vertically to carbon fiber direction along the in-plane of interface layer. The proposed network structures, containing four correlative phases of MWNTs-EP/PSF sphere/carbon fiber/epoxy matrix, contributed to simultaneous improvement of strength and toughness of composites, which was realized by crack pinning, crack deflection, crack bridging and effective load transfer.

MWNTs-EP were successfully prepared by functionalization of MWNTs with epoxy-based groups, and MWNTs-EP/polysulfone (PSF) hybrid nanofibers were fabricated to obtain ex-situ dispersion and alignment of MWNTs-EP by electrospinning. The prescribed morphology and interface correlation of hybrid nanofibers reinforced and toughened epoxy matrix (RTEP) were investigated. The alignment degree of hybrid nanofibers was enhanced with increasing MWNTs-EP loadings, and MWNTs-EP were found to be well dispersed and aligned along the nanofiber axis. The dispersion and alignment states of MWNTs-EP during inhomogeneous phase separation of RTEP were proposed and verified. MWNTs-EP dispersed and aligned along the orginal nanofiber axis were enveloped, bridged or pinned by PSF spheres arranged in the nanofiber direction. The interface chemical correlation between MWNTs-EP and resin matrix was generated due to the further reaction of epoxide rings on the surface of MWNTs-EP, which resulted in simultaneous improvement of mechanical and thermal properties of RTEP.Highlights► The ex-situ dispersion and alignment of MWNTs-EP were obtained by electrospinning. ► MWNTs-EP/PSF hybrid nanofibers were used to reinforce and toughen epoxy matrix. ► The prescribed morphology of reinforced and toughened epoxy were proposed and verified. ► The interface chemical correlation between MWNTs-EP and epoxy matrix was generated. ► The simultaneous improvement of mechanical and thermal properties were achieved.