Dopamine-Assisted Polyethyleneimine Modification of Carbon Fiber

Carbon fibers (CFs) used as resin matrix composites need to be treated by surface activation owing to their surface inertness. In this study, the effects and mechanisms of modification of CF by polyethyleneimine (PEI) alone and co-modification by PEI and dopamine (DA) were investigated. PEI alone was not effective in CF modification. However, introduction of DA and the NaIO 4 oxidant allowed PEI molecules to be grafted onto the CF surfaces by the Schiff base reaction. When PEI and DA were simultaneously added for CF surface modification (i.e., one-step modification), different PEI ( m PEI ) to DA ( m DA ) mass ratios showed different copolymerization effects in solution. For m PEI : m DA  = 6:1, copolymers with polydopamine (PDA) cores and PEI branched chains were generated. For m PEI : m DA  = 1:1, copolymers with PEI cores and PDA branched chains were generated. The interfacial shear strengths (IFSSs) of the modified CFs and epoxy resin obtained for the two m PEI : m DA ratios were significantly higher than that of the unmodified CFs. And the IFSS of the modified CFs and polyimide (PI) resin was first increasing and then decreasing. Two-step modification of the CF surfaces with DA before PEI grafting was also performed. PEI was grafted onto the outermost layer of the CFs, and the IFSS of the modified CFs and PI resin reached 73.94 MPa. The IFSS was 31.61% higher than that of the unmodified CFs and 22.67% higher than that of the DA-modified CFs. The advantage of the two-step branched PEI structure on the modified CF surfaces combined with PI was demonstrated.