CARBON PERGAMON Carbon37(1999)1785-1796 Surface characterization of electrochemically oxidized carbon fibers Z.R. Yue, W. Jiang, L. Wang, S.D. Gardner, C U. Pittman Jr . k Department of Chemistry, Mississippi State University, Mississippi State, MS 39762, USA Department of Chemical Engineering, Mississippi State University, Mississippi State, MS 39762, US Received 7 October 1998; accepted 5 February 1999 Abstract High strength PAN-based carbon fibers were continuously electrochemically oxidized by applying current to the fibers serving as an anode in 1% wt aqueous KNO,. Progressive fiber loss occurred with increasing extents of electrochemical oxidation. XPS studies(C Is and o Is) 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 Is spectra demonstrated that the rise in surface oxygenated unctional groups was mainly due to an increase in carboxyl(COoH)or ester(COOR) groups. An increase in the intensity of the O Is 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 ultramicropore, and by introducing polar oxygen-containing groups over this extended porous surface. FT-IR spectra showed a broad peak at about 1727 cm 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 flowing nitrogen at 550C 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 550C. beyond 550C the progressive decrease in Ag adsorption and Naoh uptake continued at a slower rate and approached 0 umol/g fter heating to 850C. Conversely, after heat treatment I, 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 I2 as CO, and CO evolve. Very narrow pores(<10 a 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%C and was lowest after the 850 treatment. c 1999 Elsevier Science Ltd. All rights reserved Keywords: A. Carbon fibers; B. Electrochemical treatment; Heat treatment; C. X-ray photoelectron spectroscopy (XPS);D. Surface 1. Introduction of carbon fiber-reinforced resin composites depend on the The mechanical properties and environmental stability fiber and the matrix [1-4]. Previous studies have attempted to generate strong adhesion between the fiber surface and sponding author. Tel. +1-601-325-7616: fax: + 1-601 matrix 5-8 to improve stress transfer from the relatively weak and compliant matrix to the strong and stiff reinforc- ddress: pittman @ra. msstate edu(C U. Pittman Jr) ing fibers, Therefore surface treatment of carbon fibers is -6223/99/S-see front matter 1999 Elsevier Science Ltd. All rights reservedPERGAMON Carbon 37 (1999) 1785–1796 Surface characterization of electrochemically oxidized carbon fibers b a b b a, Z.R. Yue , W. Jiang , L. Wang , S.D. Gardner , C.U. Pittman Jr. * a Department of Chemistry, Mississippi State University, Mississippi State, MS 39762, USA b Department of Chemical Engineering, Mississippi State University, Mississippi State, MS 39762, USA Received 7 October 1998; accepted 5 February 1999 Abstract High strength PAN-based carbon fibers were continuously electrochemically oxidized by applying current to the fibers serving as an anode in 1% wt aqueous KNO . Progressive fiber weight loss occurred with increasing extents of 3 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 21 showed a broad peak at about 1727 cm from C5O 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 flowing nitrogen at 5508C 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. 1 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 5508C. Beyond 1 5508C the progressive decrease in Ag adsorption and NaOH uptake continued at a slower rate and approached 0 mmol/g after heating to 8508C. Conversely, after heat treatment I adsorption showed a marked increase as the treatment temperature 2 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 I2 ˚ as CO and CO evolve. Very narrow pores (,10 A diameter) blocked by hydrogen bonding and oxygenated functions 2 become more open. XPS analyses illustrated that the surface oxygen content decreased significantly after heat-treating to 550 or 8508C and was lowest after the 8508C treatment.  1999 Elsevier Science Ltd. All rights reserved. Keywords: A. Carbon fibers; B. Electrochemical treatment; Heat treatment; C. X-ray photoelectron spectroscopy (XPS); D. Surface properties 1. Introduction of carbon fiber-reinforced resin composites depend on the effectiveness of the interfacial bond between the carbon The mechanical properties and environmental stability fiber and the matrix [1–4]. Previous studies have attempted to generate strong adhesion between the fiber surface and *Coresponding author. Tel.: 11-601-325-7616; fax: 11-601- matrix [5–8] to improve stress transfer from the relatively 325-7611. weak and compliant matrix to the strong and stiff reinforc￾E-mail address: cpittman@ra.msstate.edu (C.U. Pittman Jr.) ing fibers. Therefore surface treatment of carbon fibers is 0008-6223/99/$ – see front matter  1999 Elsevier Science Ltd. All rights reserved. PII: S0008-6223(99)00047-0