Electrochemical surface modification of polyacrylonitrile-based ultrahigh modulus carbon fibers and its effect on the interfacial properties of UHMCF/EP composites

The surfaces of polyacrylonitrile(PAN)-based ultrahigh modulus carbon fibers (UHMCFs) were electrochemically oxidized in NH4HCO3 electrolyte solution, with increasing current density. The microphysical topography, functionalization composition and chemical structure were characterized in detail. Longitudinal ridges on UHMCF surfaces became much more well-defined and a large number of oxygen-containing functional groups were introduced onto the UHMCF surfaces after electrochemical surface modification. However, a significant decrease followed by a certain extent of increase happened to the tensile strength of UHMCF with increasing current density. The corresponding step-wise oxidation mechanism of UHMCFs in the electrochemical modification was revealed. In the first stage, the chemical etching and oxidation happened to the fiber surfaces which resulted in an obvious increase in the ID/IG ratio and significant decreases in the tensile strength of UHMCFs. As the electrochemical oxidation progressed, the outermost layer of UHMCF surfaces could possibly be broken or peeled off and thus the inside ordered graphite structure exposed, which led to a further decrease in the ID/IG ratio. Meantime, the chemical crosslinks between the unfolded graphite layers of UHMCF surfaces showed up which could give longitudinal or lateral cohesion to the fiber. Therefore, the tensile strength of oxidized UHMCFs increased as the electrochemical oxidation progressed. Effect of electrochemical surface modification of UHMCFs on the interfacial properties of UHMCF reinforced epoxy resin (EP) composites was also researched. Increased wettability and functionality of UHMCFs due to electrochemical surface oxidation resulted in increases in interfacial chemical bonding between UHMCF and EP. Therefore, the interlaminar shear strength (ILSS) of UHMCFs reinforced composites continued rising in electrochemical surface modification with increasing current density.