Immobilization of amines at carbon fiber surfaces

The reaction between amines acting as nucleophiles and the C=C bonds on the carbon fiber surface acting as electrophilic vinyl groups has not yet been explored. In this contribution it is demonstrated that both thermal reactions and electrochemical oxidation of amines at carbon fibers allow the covalent bonding of these molecules directly to the carbon fiber surface, presumably via nucleophilic attack of the amine at electrophilic C=C sites at the surface and subsequent formation of C-N bonds between the surface and the amine. A novel strategy for a quantitative assay of the number of amines attached to the surface is developed in which Fe(CN)₆^3- is electrostatically bound to the protonated, cationic amine sites, followed by electrochemical determination of the amount of bound Fe(CN)₆^3- as a function of its concentration in solution. Analysis of the isotherm for this electrostatic binding process then provides a measure of the number of interfacially immobilized amines. The composition of the amine layer is also probed using X-ray photoelectron spectroscopy (XPS). Mechanisms are discussed by which attachment of amines at the electrophilic vinyl groups of the carbon fibers can occur. The likely influence that this type of reaction has on the interfacial shear strength in carbon fiber/epoxy composite materials is also discussed.