CARBON PERGAMON Carbon38(2000)675-689 The surface chemistry and acid-base properties of a paN-based bon fibre Phil e. vickers John F. Watts,* Christian Perruchot. Mohamed M. Chehimib School of Mechanical and Materials Engineering, University of Surrey, Guildford, Surrey GU2 5XH, UK Institut de Topologie et de dynamique des Systemes(ITODYS) Universite Paris 7, Denis Diderot, associe au CNRS (UPRESA 7086), I rue Guy de la brosse, 75005 Paris, france Received 29 March 1999; accepted 25 June 1999 Abstract Akzo Tenax hta carbon fibres(high strength) with progressive amounts of the standard commercial treatment have been investigated using a number of surface analytical methods. Chemical content and structure have been interrogated by XPs and ToF-SIMS, and compared to thermodynamic and acid-base properties probed by IGC and DCAA. The ys values of the fibres were calculated at ca. 50%C and were found to decrease from 104 mJ m- for untreated fibres to 78 mJ m- for 200% oxidised fibres. The acid-base character at the molecular level and the macroscopic properties of y. are significantly enhanced even at low levels of oxidation, resulting in increased wettability of carbon fibres by polar species. Wettability of the surface by non-polar species appears to be largely unaffected by oxidation. XPS analysis reveals that oxygen introduced progressively at the expense of carbon content, and trace amounts of Na and Cl slowly increase with oxidation oF-SIMS shows that the amount of Ca and Na increases very quickly during the early stages of oxidation, whereas Cl and O increase at a more steady rate. 2000 Elsevier Science Ltd. All rights reserved. Keywords: A. Carbon fibres; C. X-ray photoelectron spectroscopy(XPS): D. Thermodynamic properties, Surface properties 1. Introduction surface area as a result of electrochemical oxidation by BET analysis and other workers have also concluded that The interaction of material(e.g. epoxy resin, surface area of the fibre plays little or no role in the amine curing agent) carbon surfaces is of improvement in the interfacial shear strength(IFSS)[1],or paramount importance to the ultimate mechanical prop. the interlaminar shear strength (ILSS)[4]. Zhdan et al, erties of a carbon fibre reinforced polymer(CFRP)[1, 2]. however, detected an increase in surface roughness of This is because it is the ability of the matrix material to high-strength(HS)carbon fibres by atomic force micro- transfer stress to the reinforcing fibre that results in the scopy(AFM) during in situ electrochemical oxidation [5] high strength weight ratio obtained in high-performanc Previous monitoring of the oxidation of carbon fibre composite materials such as CFRPs. In order to avoid surfaces has involved the use of X-ray photoelectron formation of voids at the fibre-matrix interface, complete spectroscopy(XPS) which has been studied in great detail wetting of the fibre surface by the resin material must be in a series of papers by Sherwood et al. [6-11. These achieved. This is generally attained by oxidising the fibre papers observed the change in C Is and o ls peak shapes surface, thus increasing the surface free energy, and as the oxidation progressed, using a wide variety of therefore wettability, of the fibre surface. The actual reason oxidation methods, such as electrochemical oxidation in for the improvement in performance of the composite is aqueous solutions of sulphuric acid or ammonium bicar unclear, although it is consistently observed. Mahy et al. bonate [6,7]. Scrutiny of the C Is and o Is photoelectron 3 observed that there was no significant increase in peaks indicated that carbonyl groups are introduced to the surface at low oxidation potentials, and the concentration *Corresponding author. Tel. +44-1483-259617, fax: +44 of alcohol-ether groups increases at high oxidation po- 1483-876291 tentials. This observation appears to be consistent with E-mail address: j.watts@surrey. ac uk(F. Watts) cyclic voltammograms recorded during the oxidation pro- 0008-6223/00/S-see front matter 2000 Elsevier Science Ltd. All rights reserved PII:S0008-6223(99)00137-2PERGAMON Carbon 38 (2000) 675–689 The surface chemistry and acid–base properties of a PAN-based carbon fibre a a, b b Phil E. Vickers , John F. Watts , Christian Perruchot , Mohamed M. Chehimi * a School of Mechanical and Materials Engineering, University of Surrey, Guildford, Surrey GU2 5XH, UK b Institut de Topologie et de Dynamique des Systemes `´ ´ (ITODYS), Universite Paris 7, Denis Diderot, associe au CNRS (UPRESA 7086), 1 rue Guy de la Brosse, 75005 Paris, France Received 29 March 1999; accepted 25 June 1999 Abstract Akzo Tenax HTA carbon fibres (high strength) with progressive amounts of the standard commercial treatment have been investigated using a number of surface analytical methods. Chemical content and structure have been interrogated by XPS d and ToF-SIMS, and compared to thermodynamic and acid–base properties probed by IGC and DCAA. The g values of the s 22 22 fibres were calculated at ca. 508C and were found to decrease from 104 mJ m for untreated fibres to 78 mJ m for 200% p oxidised fibres. The acid–base character at the molecular level and the macroscopic properties of g s are significantly enhanced even at low levels of oxidation, resulting in increased wettability of carbon fibres by polar species. Wettability of the surface by non-polar species appears to be largely unaffected by oxidation. XPS analysis reveals that oxygen is introduced progressively at the expense of carbon content, and trace amounts of Na and Cl slowly increase with oxidation. ToF-SIMS shows that the amount of Ca and Na increases very quickly during the early stages of oxidation, whereas Cl and O increase at a more steady rate.  2000 Elsevier Science Ltd. All rights reserved. Keywords: A. Carbon fibres; C. X-ray photoelectron spectroscopy (XPS); D. Thermodynamic properties, Surface properties 1. Introduction surface area as a result of electrochemical oxidation by BET analysis and other workers have also concluded that The interaction of matrix material (e.g. epoxy resin, surface area of the fibre plays little or no role in the amine curing agent) with carbon fibre surfaces is of improvement in the interfacial shear strength (IFSS) [1], or paramount importance to the ultimate mechanical prop- the interlaminar shear strength (ILSS) [4]. Zhdan et al, erties of a carbon fibre reinforced polymer (CFRP) [1,2]. however, detected an increase in surface roughness of This is because it is the ability of the matrix material to high-strength (HS) carbon fibres by atomic force micro￾transfer stress to the reinforcing fibre that results in the scopy (AFM) during in situ electrochemical oxidation [5]. high strength:weight ratio obtained in high-performance Previous monitoring of the oxidation of carbon fibre composite materials such as CFRP’s. In order to avoid surfaces has involved the use of X-ray photoelectron formation of voids at the fibre–matrix interface, complete spectroscopy (XPS) which has been studied in great detail wetting of the fibre surface by the resin material must be in a series of papers by Sherwood et al. [6–11]. These achieved. This is generally attained by oxidising the fibre papers observed the change in C 1s and O 1s peak shapes surface, thus increasing the surface free energy, and as the oxidation progressed, using a wide variety of therefore wettability, of the fibre surface. The actual reason oxidation methods, such as electrochemical oxidation in for the improvement in performance of the composite is aqueous solutions of sulphuric acid or ammonium bicar￾unclear, although it is consistently observed. Mahy et al. bonate [6,7]. Scrutiny of the C 1s and O 1s photoelectron [3] observed that there was no significant increase in peaks indicated that carbonyl groups are introduced to the surface at low oxidation potentials, and the concentration *Corresponding author. Tel.: 144-1483-259617; fax: 144- of alcohol–ether groups increases at high oxidation po- 1483-876291. tentials. This observation appears to be consistent with E-mail address: j.watts@surrey.ac.uk (J.F. Watts). cyclic voltammograms recorded during the oxidation pro- 0008-6223/00/$ – see front matter  2000 Elsevier Science Ltd. All rights reserved. PII: S0008-6223(99)00137-2