Anodic oxidation of carbon fibers in alkaline and acidic electrolyte: Quantification of surface functional groups by gas-phase derivatization

Carbon fibers were anodically oxidized using two different electrolytes, i.e. an alkaline aqueous ammonium bicarbonate solution (pH approximate to 8) and an acidic dilute sulfuric acid solution (pH approximate to 2). Two different treatment modes were utilized: (1) a static, discontinuous anodic oxidation treatment of a spatially fixed fiber bundle and (2) a dynamic, continuous oxidation of a fiber tow, which was dragged through the electrolyte bath with constant speed. The resulting degree of oxidation of the carbon fiber surface was analyzed by x-ray photoelectron spectroscopy. By gas-phase derivatization of the fiber surfaces the concentrations of surface hydroxyl and carboxyl groups were quantified. Good agreement between the oxygen elemental concentration determined from the derivatized surface functionalities and that extracted from x-ray photoelectron spectroscopy elemental analysis demonstrates the reliability of the derivatization results. While both treatment modes result in the same oxidation, significant differences of the oxidation behavior exist for carbon fibers treated by the two different electrolytes. This is explained by a mechanism involving the effect of electrolyte salt ions.