Effect of polymer-grafted carbon nanofibers and nanotubes on the interlaminar shear strength and flexural strength of carbon fiber/epoxy multiscale composites

Multiwalled carbon nanotubes (MWNTs) and vapor-grown carbon nanofibers (VGCNFs) were grafted with poly(styrene-co-maleic anhydride) (SMA) and poly(glycidyl methacrylate) (PGMA) by free radical polymerization, and used as the secondary reinforcing agent for multiscale carbon fiber (CF) reinforced epoxy composites. The polymer-grafted VGCNFs and MWNTs were simply suspended in ethanol and spray-coated onto the surface of CF fabrics to fabricate the multiscale composites. The structures of the polymer-grafted VGCNFs/MWNTs were characterized and their effect on the mechanical properties of multiple composites was compared. Interestingly, polymer-grafted VGCNFs with much larger diameters and lower grafting ratios are more effective than polymer-grafted MWNTs for the reinforcement; and SMA-grafted VGCNFs are more effective than PGMA-grafted VGCNFs. Addition of only 0.4 wt% (on the basis of CF fabrics) of SMA-grafted VGCNFs increases interlaminar shear strength (ILSS) by ∼73% and flexural strength by ∼21%. The increase of ILSS exceeds those of previously reported functionalized nanotubes/nanofibers, graphene oxide or organoclay nanoplatelets as the secondary reinforcement. The high straightness and large aspect ratio of SMA-grafted VGCNFs and the strong interaction with the matrix play the key roles in the reinforcement. This new and simple method simplifies the fabrication of high-performance multiscale composites for broad applications in the future.