Surface characterization of electrochemically oxidized carbon fibers

High strength PAN-based carbon fibers were continuously electrochemically oxidized by applying current to the fibers serving as an anode in 1% wt aqueous KNO3. Progressive fiber weight loss occurred with increasing extents of electrochemical oxidation. XPS studies (C 1s and O 1s) indicated that the oxygen/carbon atomic ratio rose rapidly to 0.24 as the extent of electrochemical oxidation was increased from 0 to 133 C/g and then remained almost constant as the extent of electrochemical oxidation rose to 10 600 C/g Fitting the C 1s spectra demonstrated that the rise in surface oxygenated functional groups was mainly due to an increase in carboxyl (COOH) or ester (COOR) groups. An increase in the intensity of the O 1s peak (534.6-535.4 eV) after electrochemical oxidation corresponded to chemisorbed oxygen and/or adsorbed water. Electrochemical oxidation increased surface activity by generating more surface area via the formation of ultramicropores, and by introducing polar oxygen-containing groups over this extended porous surface. FT-IR spectra showed a broad peak at about 1727 cm(-1) from C=O stretching vibrations of carboxyl and/or ketone groups, the relative intensity of which increased significantly with the extent of electrochemical oxidation. Post-oxidation heat-treatments in Bowing nitrogen at 550 degrees C for 30 min. caused further weight losses due to decarboxylation of carboxyl groups and other reactions in which oxygenated functions decomposed. These weight losses increased with the extent of electrochemical oxidation. This demonstrated that more oxygenated groups formed on the internal pore surfaces as pores increasingly penetrated deeper into the fibers with increased electrochemical treatment. Weight loss depended on the heat treatment temperature since different types of carbon-oxygen surface groups were formed during the electrochemical oxidations. Different functions have different abilities to decarboxylate or decarbonylate. The amount of Ag+ and NaOH uptake by electrochemically oxidized fibers rapidly decreased as the temperature of the post heat treatment increased to 550 degrees C, Beyond 550 degrees C the progressive decrease in Ag+ adsorption and NaOH uptake continued at a slower rate and approached 0 mu mol/g after hearing to 850 degrees C. Conversely, after heat treatment I-2 adsorption showed a marked increase as the treatment temperature was raised. Thermal decomposition of carbon-oxygen complexes within the pore structure leads to a lower hydrophilicity of the pore surface, The extensive micropore surface area generated by electrochemical oxidation becomes more accessible to I-2 as CO2 and CO evolve. Very narrow pores (<10 Angstrom diameter) blocked by hydrogen bonding and oxygenated functions become more open, XPS analyses illustrated that the surface oxygen content decreased significantly after heat-treating to 550 or 850 degrees C and was lowest after the 850 degrees C treatment. (C) 1999 Elsevier Science Ltd. All rights reserved.