Role of edge oxygen atoms on the adhesive interaction between carbon fiber and epoxy resin

A mechanism of the adhesion between carbon fiber and epoxy resin is studied by using density functional theory (DFT) calculations. Surface structures of carbon fiber were modeled by the armchair-edge structure of graphite functionalized with OH and COOH groups. DFT calculations were performed to construct two realistic models of adhesion interface consisting of the functionalized carbon surface and a fragment of epoxy resin. Adhesive properties of the model interfaces were evaluated based on the binding energy (E _b) between the carbon surface and the resin as well as the maximum adhesive force (F_max) acting at the interface. Calculated values of E_b are 13.8 kcal/mol for the OH-functionalized surface and 19.1 kcal/mol for the COOH-functionalized surface. The binding energy per hydrogen bond is calculated to be 6.9 kcal/mol (OH model; two H-bonds) and 6.3 kcal/mol (COOH model; three H-bonds), both of which are virtually similar and reasonable for the bond energy of a typical OH···O hydrogen bond. Analysis of adhesive force-displacement curves derived from energy-displacement plots revealed that F_max is 0.52 nN for the OH model and 0.70 nN for the COOH model. Calculated adhesive properties are in good agreement with those previously reported for the interface between an aluminum oxide surface and an epoxy resin [J. Phys. Chem. C 2011, 115, 11701], strongly suggesting that hydrogen bonds between the oxygen-containing functional groups play a crucial role in the adhesive interaction in the carbon fiber/epoxy resin system.