P.E. Vickers et al. Carbon 38(2000)675-689 Table 0.25 Estimate of surface free energy values for carbon fibre re y(mN m y ( (mN m) 2.9±0.2 34.1±2.3 104+3.3 44.5±3.3 7.0±2.8 the grafting of functional groups of relatively lower surface tension onto clean fibres of relatively high surface energy This is illustrated in Fig 8 which shows a plot of y. vs zdoi =3. 96 mm the sum of the nitrogen and oxygen contents for the various fibres. O and n were preferred to the function- alized carbon species as they constitute elemental markers position/mm of the surface treatments The acid-base interactions of the c fibres markedly(see Table 7) between 0% and 25% after which it appears to reach a maximum, or 0.06 receding slightly increase thereafter. When it is considered that this O=43° technique is used at infinite dilution, i.e. the probes are not concentrated enough in the carrier gas to interfere with 0.04 each other. then the most obvious conclusion here is that high-energy relatively strongly acidic sites are introduced onto the surface initially during the oxidation procedure. In view of the XPS results, that indicate a steady increase in the presence of oxygen on the surface with fibre treatment, zdoi = 3.51 mm it appears that a different species is introduced at this ligher level of treatment. This is possibly because the acid sites could be introduced into a specific point of the position/mm surface structure, such as edge sites, that are energetically more favourable than the further introduction of less acidic 0.15 oxygen containing functional groups that are introduced at seemingly less specific and more numerous surface sites. The untreated fibres appear to have very little or zero basicity. Presumably, the untreated fibres do not possess sufficient electron donating character to interact with hloroform (purely acidic reference probe) to an extent which would influence its retention volume. The situation changes upon treatment as IGC was effective in probing some degree of basicity of the treated fibres. This character s due to the grafted basic(C=O, n containing groups)and photeric (OH, COoH) functional groups. To illustrate zdoi 3. 76 mm the change in acid-base characteristics of the various fibres we have plotted in Fig. 9 the geometric mean value of a and B, 2(aB), constants reported in Table 6 vs the position/mm oxygen and nitrogen surface concentration determined by water;(b)1-bromonaphthalene;and Ce for 100% fibres in:(a) of the fibres. Fig. 9 shows a rapid change in the overall Fig. 6. DCaa force vs. distance XPS. 2(aB)is an IGC measure of the overall behaviour acid-base characteristics of the fibres as a result of treatment, then a steady state beyond 50% DFT. ontaining groups. It is well known [46] that the functional groups have a surface tension lower y d croscopic scale, DCAA results prou ea much y: values compared to IGC, and failed to detect which does not exceed 75 mJ m- at room significant changes with increasing oxidation. The results (except for >C(OH)H group of which the surface in Table 8 show that oxidation improves the wettability of tension contribution is 104 mJ m ) Consequently, the the fibre by water, as would be expected with the intre electrochemical treatment of the carbon fibres resulted in duction of oxygen containing functional groups. The higl686 P.E. Vickers et al. / Carbon 38 (2000) 675 –689 Table 9 Estimate of surface free energy values for carbon fibre d 21 p 21 21 Fibre g (mN m ) g (mN m ) g (mN m ) s ss 0% 37.161.4 2.960.2 39.961.2 25% 34.162.3 10.463.3 44.563.3 100% 37.062.8 10.762.3 47.661.3 the grafting of functional groups of relatively lower surface tension onto clean fibres of relatively high surface energy. d This is illustrated in Fig. 8 which shows a plot of g vs. s the sum of the nitrogen and oxygen contents for the various fibres. O and N were preferred to the function￾alized carbon species as they constitute elemental markers of the surface treatments. The acid–base interactions of the C fibres increase markedly (see Table 7) between 0% and 25% treatment, after which it appears to reach a maximum, or possibly slightly increase thereafter. When it is considered that this technique is used at infinite dilution, i.e. the probes are not concentrated enough in the carrier gas to interfere with each other, then the most obvious conclusion here is that high-energy relatively strongly acidic sites are introduced onto the surface initially during the oxidation procedure. In view of the XPS results, that indicate a steady increase in the presence of oxygen on the surface with fibre treatment, it appears that a different species is introduced at this higher level of treatment. This is possibly because the acid sites could be introduced into a specific point of the surface structure, such as edge sites, that are energetically more favourable than the further introduction of less acidic oxygen containing functional groups that are introduced at seemingly less specific and more numerous surface sites. The untreated fibres appear to have very little or zero basicity. Presumably, the untreated fibres do not possess sufficient electron donating character to interact with chloroform (purely acidic reference probe) to an extent which would influence its retention volume. The situation changes upon treatment as IGC was effective in probing some degree of basicity of the treated fibres. This character is due to the grafted basic (C=O, N containing groups) and amphoteric (OH, COOH) functional groups. To illustrate the change in acid–base characteristics of the various fibres we have plotted in Fig. 9 the geometric mean value 1/2 of a and b, 2(ab) , constants reported in Table 6 vs. the oxygen and nitrogen surface concentration determined by 1/2 XPS. 2(ab) is an IGC measure of the overall behaviour Fig. 6. DCAA force vs. distance curves for 100% fibres in: (a) of the fibres. Fig. 9 shows a rapid change in the overall water; (b) 1-bromonaphthalene; and (c) glycerol. acid–base characteristics of the fibres as a result of treatment, then a steady state beyond 50% DFT. containing groups. It is well known [46] that these organic At the macroscopic scale, DCAA results produced much d functional groups have a surface tension contribution lower g values compared to IGC, and failed to detect s 22 which does not exceed 75 mJ m at room temperature significant changes with increasing oxidation. The results (except for .C(OH)–H group of which the surface in Table 8 show that oxidation improves the wettability of 22 tension contribution is 104 mJ m ). Consequently, the the fibre by water, as would be expected with the intro￾electrochemical treatment of the carbon fibres resulted in duction of oxygen containing functional groups. The high