Cyclomatrix-type polyphosphazene coating: Improving interfacial property of carbon fiber/epoxy composites and preserving fiber tensile strength

To improve the interfacial adhesion of carbon fiber reinforced polymer composites (CFRP), carbon fiber usually has to be carried out surface treatment under harsh conditions. However, enhancing the interfacial property is usually accompanied by deteriorating tensile strength of carbon fiber, due to some defects on fiber surfaces after surface modification. In this paper, a layer of amine-capped poly (cyclotriphosphazene-co-4,4′-oxydianiline) coating was introduced onto the carbon fiber surfaces by a facile in-situ polymerization under mild condition. The amines on polyphosphazene coating surface help to improve the surface wettability and activity of carbon fibers, significantly improving the interfacial shear strength of carbon fiber/epoxy composites by 70.5%. Besides the enhanced surface wettability, the chemical bonds also play a key role in such an interfacial enhancement. Moreover, the polyphosphazene sheath can heal the surface defects of fiber effectively, thus increasing the fiber tensile strength by more than 10.0%.