P.E. Vickers et al. Carbon 38(2000)675-689 ve interaction line) greater than I kJ mol. ABA can be 3.3. Wettability studies by dCaa considered within the accuracy of the determination of the deviation from the dispersive interaction line. Therefore, DCAA exploits the Wilhelmy plate technique to de- the interaction of TMP is consistent with its boiling point termine the contact angle made by the appropriate liquid and reveals no unexpected anomalies. probe with the surface of the material under investigation The technique has been described by Chang et al. [40] and 3. 2. 2. Properties of the carbon fibres measures the mass deflection incurred when the probe Table 6 is useful to establish the liquid is raised in order to come into contact with fibre carbon fibre physicochemical properties, that is y -, the urface using the relationship described below dispersive contribution to the surface free energy and cOs 6= acid-base constants derived from the Saint Flour-Papirer approach [30] The value of y. is proportional to the square of AG CH2 where, P is the perimeter of the fibre, g is the gravitational and determined from the slopes shown in Fig. 4 [27] constant, m is the mass(experimental reading from mi- crobalance) y4=(1/4xm)△G/Nacm2) Initially, 0, 25 and 100% dFT fibres were using hexadecane(see Table 2)as the probe where N is the Avogadro number, acH2 is the cross- order to gain an estimate of the fibre perimeter sectional area of an adsorbed CH, group (6 A), y is cane wets the fibre surface efficiently(0<30%)because it the surface free energy of a solid containing only methyl possesses a very low surface free energy, and, being a CI)ps such as polyethylene ()H2=368-0.058T large molecule, is unlikely to fill micropores on the fibre surface, a likely source for contact angle hysteresis. All The relative acid-base properties of the fibres can be three types of fibres gave force curves which showed very investigated by using a combination of the adsorption data little hysteresis, and the fibre diameter of all the fibres was for CHCI, and THF taken as reference Lewis acidic and found to be 6.9 um, in good agreement with Mahy et al basic probes, respectively. Lara and Schreiber[39] defined [31, who reported a fibre diameter of 7. 1 um for the same the following parameters type of fibres 0, 25 and 100% DFT fibres were then analysed using aTHe =-AGa (THF)in kJ mol- ater,glycerol and 1-bromonapthalene as the probe media (7) These were chosen because of they provide a wide range of dispersive and polar contributions to the total surface free energy. as listed in Table 2. The contact angles of BcHCE=-AGA (CHCI, )in kJ mol -I ( 8) fibres with the contact media are reported in Table 8, each of which is an average of five readings. It is apparent that where aThe and BcHcu are the acidity and basicity all receding angles are lower than advancing angles constants, respectively, derived for the solids under test. recorded during the same experiment. This is a widely The constants aTHE and BcHcu together with y, values reported effect [41] and is attributed to a combination of re reported in Table 7. They suggest that carbon fibres chemical heterogeneity and microroughness of the fibre have a high dispersive component to the surface energy, surface. Contact angle hysteresis occurs because the ad y., which however decreases as a result of surface vancing liquid front more easily wets high-energy patches treatment. The value of y. for the untreated fibre is similar present on the surface and is held up by the low-energy to that (129 mJ m at 45C)of HOPG [14] e, resulting in a contact angle more characteristic of treatment,y. value decreases by a value as large as 26.5 the low-energy patches. During emersion, the converse is mJ m- for the 200% DFT fibres. In contrast, the acidity true, with the wetting liquid remaining on the high-energy considerably increases as well as the basicity, although to a phase and holding up the retreat of the liquid from the lesser extent solid surface, thus giving a contact angle characteristic of Dispersive and acid-base constants for carbon fibres with a range of oxidation treatments y(mJ m CTHE 2(aTHE B Untreated 1045 500 l1.8 l1.8 10.86684 P.E. Vickers et al. / Carbon 38 (2000) 675 –689 21 sive interaction line) greater than 1 kJ mol . DBA can be 3.3. Wettability studies by DCAA considered within the accuracy of the determination of the deviation from the dispersive interaction line. Therefore, DCAA exploits the Wilhelmy plate technique to de￾the interaction of TMP is consistent with its boiling point termine the contact angle made by the appropriate liquid and reveals no unexpected anomalies. probe with the surface of the material under investigation. The technique has been described by Chang et al. [40] and measures the mass deflection incurred when the probe 3.2.2. Properties of the carbon fibres liquid is raised in order to come into contact with fibre The data reported in Table 6 is useful to establish the surface using the relationship described below. d carbon fibre physicochemical properties, that is g , the s mg dispersive contribution to the surface free energy and cos u 5 ] (9) acid–base constants derived from the Saint Flour-Papirer Pglv approach [30]. where, P is the perimeter of the fibre, g is the gravitational d CH2 The value of g s is proportional to the square of DG constant, m is the mass (experimental reading from mi- and determined from the slopes shown in Fig. 4 [27]: crobalance). d CH2 2 Initially, 0, 25 and 100% DFT fibres were analysed g 5 (1/4g )(DG /Na ) (6) s CH2 CH2 using hexadecane (see Table 2) as the probe liquid, in order to gain an estimate of the fibre perimeter. Hexade- where N is the Avogadro number, a is the cross- CH2 2 cane wets the fibre surface efficiently (u ,308) because it ˚ sectional area of an adsorbed CH group (6 A ), g is 2 CH2 possesses a very low surface free energy, and, being a the surface free energy of a solid containing only methyl- large molecule, is unlikely to fill micropores on the fibre ene groups such as polyethylene (gCH2536.8–0.058T surface, a likely source for contact angle hysteresis. All (8C)). three types of fibres gave force curves which showed very The relative acid–base properties of the fibres can be little hysteresis, and the fibre diameter of all the fibres was investigated by using a combination of the adsorption data found to be 6.9 mm, in good agreement with Mahy et al. for CHCl and THF taken as reference Lewis acidic and 3 [3], who reported a fibre diameter of 7.1 mm for the same basic probes, respectively. Lara and Schreiber [39] defined type of fibres. the following parameters: 0, 25 and 100% DFT fibres were then analysed using AB 21 water, glycerol and 1-bromonapthalene as the probe media. a 5 2DG (THF) in kJ mol (7) THF a These were chosen because of they provide a wide range of dispersive and polar contributions to the total surface and free energy, as listed in Table 2. The contact angles of AB 21 b 5 2DG (CHCl ) in kJ mol (8) fibres with the contact media are reported in Table 8, each CHCl3 a 3 of which is an average of five readings. It is apparent that where a and b are the acidity and basicity all receding angles are lower than advancing angles THF CHCl3 constants, respectively, derived for the solids under test. recorded during the same experiment. This is a widely d The constants a and b together with g values reported effect [41] and is attributed to a combination of THF CHCl3 s are reported in Table 7. They suggest that carbon fibres chemical heterogeneity and microroughness of the fibre have a high dispersive component to the surface energy, surface. Contact angle hysteresis occurs because the ad- d g , which however decreases as a result of surface vancing liquid front more easily wets high-energy patches s d treatment. The value of g s for the untreated fibre is similar present on the surface and is held up by the low-energy 22 to that (129 mJ m at 458C) of HOPG [14]. Upon phase, resulting in a contact angle more characteristic of d treatment, g value decreases by a value as large as 26.5 the low-energy patches. During emersion, the converse is s 22 mJ m for the 200% DFT fibres. In contrast, the acidity true, with the wetting liquid remaining on the high-energy considerably increases as well as the basicity, although to a phase and holding up the retreat of the liquid from the lesser extent. solid surface, thus giving a contact angle characteristic of Table 7 Dispersive and acid–base constants for carbon fibres with a range of oxidation treatments d 22 1/2 DFT (%) g (mJ m ) a b 2(a b ) s THF CHCl3 THF CHCl3 Untreated 104.5 3.4 20.223 |0 25 89.2 10.7 1.1 6.86 50 84.3 12.5 2.4 10.95 100 85.2 11.8 1.7 8.96 200 78.0 11.8 2.5 10.86