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X-ray Photoelectron Spectroscopy Studies on Sterically-Stabilized Polypyrrole Particles P. M. Beadle and S. P. Armes* School of Chemistry and Molecular Sciences, University of Sussex, Falmer, Brighton BN1 9QJ, U.K.
S. J. Greaves and J. F. Watts* Department of Materials Science and Engineering, University of Surrey, Guildford, Surrey GU2 5XH, U.K. Received September 11, 1995. In Final Form: January 2, 1996X The surface composition of a sterically-stabilized polypyrrole colloid has been examined by X-ray photoelectron spectroscopy (XPS). The sulfur atom of the polyelectrolyte stabilizer poly(potassium 3-sulfopropyl methacrylate) [PKSPM] and the nitrogen atom of the polypyrrole were used as unique elemental markers for each component. The S/N atomic ratio at the particle surface determined using XPS was found to be significantly higher than the corresponding ‘macroscopic’ S/N atomic ratio calculated from elemental microanalyses. This is the first direct evidence for the presence of a stabilizer-rich layer at the surface of sterically-stabilized conducting polymer particles. Changing to a lower energy X-ray source (Mg KR) did not lead to an increase in the surface S/N ratio; thus, the stabilizer layer is probably ‘patchy’ rather than a continuous coating. XPS also revealed that the surface concentrations of the K+ and Cl- counterions in the colloid were substantially depleted relative to those found in the PKSPM stabilizer and chloridedoped polypyrrole ‘bulk powder’. Moreover, the surface dopant level of the polypyrrole particles calculated on the basis of the remaining chloride ions was unusually low (Cl/N ) 0.07). More realistic surface dopant levels were obtained if it was assumed that some of the sulfonate groups of the polyelectrolyte stabilizer acted as dopant anions for the conducting polymer component. It was concluded that the PKSPM stabilizer is adsorbed onto the surface of the polypyrrole particles as a polymeric dopant anion, with the concomitant expulsion of KCl. Finally, there is some evidence that this stabilizer is present as a polymeric dopant in the interior of the particles as well.
Introduction Over the last two decades X-ray photoelectron spectroscopy (XPS) has become a well-established technique for characterizing polymer surfaces.1,2 In the last ten years or so there has been increasing interest in using XPS to characterize the surface of polymer latex particles. For example, Deslandes et al.3 have examined micron-sized polystyrene latexes stabilized with poly(N-vinylpyrrolidone), whilst Davies and co-workers have published a series of papers on charge-stabilized, surfactant-stabilized, and sterically-stabilized latex particles of submicron dimensions.4-7 It is generally found that the surface composition of the latex particles is consistent with the currently accepted theories of colloid stability.8,9 * Authors to whom correspondence should be addressed. X Abstract published in Advance ACS Abstracts, March 15, 1996. (1) Brundle, C. R., Baker, A. D., Eds. Electron spectroscopy: theory, techniques, and applications; Academic Press: New York, 1977. (2) Briggs, D., Seah, M. P., Eds. Practical Surface Analysis, 2nd ed., Vol. 1, Auger and X-ray Photoelectron Spectroscopy; Wiley: Chichester, 1993. (3) Deslandes, Y.; Mitchell, D. F.; Paine, A. J. Langmuir 1993, 9, 1468. (4) Lynn, R. A. P.; Davies, S. S.; Short, R. D.; Davies, M. C.; Vickerman, J. C.; Humphrey, P.; Johnson, D.; Hearn, J. Polym. Commun. 1988, 29, 365. (5) Davies, M. C.; Lynn, R. A. P.; Davies, S. S.; Hearn, J.; Watts, J. F.; Vickerman, J. C.; Johnson, D. J. Colloid Interface Sci. 1993, 156, 229. (6) Brindley, A.; Davies, M. C.; Lynn, R. A. P.; Davies, S. S.; Hearn, J.; Watts, J. F. Polym. Commun. 1992, 33, 1112. (7) Davies, M. C.; Lynn, R. A. P.; Davies, S. S.; Hearn, J.; Watts, J. F.; Vickerman, J. C.; Paul, A. J. Langmuir 1993, 9, 1637. (8) Napper, D. H. Polymeric Stabilization of Colloidal Dispersions; Academic Press: London, 1983. (9) Hunter, R. J. Foundations of Colloid Science; Clarendon Press: Oxford, 1987; Vol. 1.
Many groups have reported the use of XPS for the characterization of organic conducting polymers such as polypyrrole of polyaniline.10-19 Such studies have focused almost exclusively on chemically-synthesized bulk powders or electrochemically-synthesized thin films. Since XPS is highly surface-specific, with typical sampling depths of 20-50 Å, this technique has proved particularly useful in assessing the surface oxidation and degradation chemistry of conducting polymers.17-19 In 1986 Bjorklund and Liedberg reported the first synthesis of colloid polypyrrole particles using methylcellulose in aqueous media.20 Subsequently, various research groups have shown that a wide range of both commercial21-28 and tailor-made29,30 water-soluble poly(10) Pfluger, P.; Street, G. B. J. Chem. Phys. 1984, 80, 544. (11) Eaves, J. G.; Munro, H. S.; Parker, D. Polym. Commun. 1987, 28, 38. (12) Kang, E. T.; Neoh, K. G.; Tan, K. L. Adv. Polym. Sci. 1993, 106, 135. (13) Neoh, K. G.; Kang, E. T.; Tan, K. L. J. Polym. Sci., Polym. Chem. 1991, 29, 759. (14) Kang, E. T.; Tan, K. L.; Neoh, K. G.; Chan, H. S. O.; Tan, B. T. G. Polym. Bull. 1989, 21, 53. (15) Kang, E. T.; Neoh, K. G.; Ong, Y. K.; Tan, K. L.; Tan, B. T. G. Macromolecules 1991, 24, 2822. (16) Chan, H. S. O.; Hor, T. S. A.; Ho, P. K. H. J. Macromol. Sci., Chem. 1990, A27, 1081. (17) Erlandsson, R.; Inganas, O.; Lundstrom, I.; Salaneck, W. R. Synth. Met. 1985, 10, 303. (18) Inganas, O.; Erlandsson, R.; Nylander, C.; Lundstrom, I. J. Phys. Chem. Solids 1984, 45, 427. (19) Moss, B. K.; Burford, R. P. Polymer 1992, 33, 1902. (20) Bjorklund, R. B.; Liedberg, B. J. Chem. Soc., Chem. Commun. 1986, 1293. (21) (a) Armes, S. P.; Vincent, B. J. Chem. Soc., Chem. Commun. 1987, 288. (b) Armes, S. P.; Miller, J. F.; Vincent, B. J. Colloid Interface Sci. 1987, 118, 410. (22) Armes, S. P.; Aldissi, M.; Idzorek, G. C.; Keaton, P. W.; Rowton, L. J.; Stradling, G. L.; Collopy, M. T.; McColl, D. B. J. Colloid Interface Sci. 1991, 141, 119.
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Figure 2. Chemical structure of the poly(potassium sulfopropyl methacrylate) [PKSPM] stabilizer used to prepare the polypyrrole colloids in this study.
Figure 1. Schematic representation of an isolated, stericallystabilized polypyrrole particle.
mers can be used as steric stabilizers for polypyrrole. These dispersions generally exhibit good long-term colloid stability: in our own laboratory we have samples which are more than six years old and exhibit no signs of flocculation. This suggests that the conducting polymer particles must be completely coated with a thick layer of polymeric stabilizer (see Figure 1), which prevents particle aggregation via a steric stabilization mechanism.8,9 There is some indirect evidence for this surface layer of adsorbed steric stabilizer. The flocculation/gelation at elevated temperatures of polypyrrole particles synthesized using methylcellulose or poly(methyl vinyl ether) stabilizers is almost certainly due to the inverse temperature-solubility behavior of these stabilizers in aqueous solution.20,25 Similarly, the reversible pH-induced flocculation of poly(vinylpyridine)-stabilized polypyrrole particles reported by Armes et al. was rationalized in terms of deprotonation (and subsequent collapse) of the adsorbed stabilizer layer.28,29 Recently the Sussex group has reported that both small molecule surfactants31 and ultrafine inorganic oxide sols32,33 can also be used as dispersants for polypyrrole in aqueous media. In both cases XPS was utilized to study the surface composition of these colloids.34,35 However, as far as we are aware, there have been no publications describing the use of XPS (or any other surface analytical technique) for characterizing the surfaces of polymerstabilized conducting polymer particles. In the present work we report the first XPS study of the surface composition of polymer-stabilized polypyrrole (23) Cawdery, N.; Obey, T. M.; Vincent, B. J. Chem. Soc., Chem. Commun. 1989, 1189. (24) Markham, G.; Obey, T. M.; Vincent, B. Colloids Surf. 1990, 51, 239. (25) Digar, M. L.; Bhattacharyya, S. N.; Mandal, B. M. J. Chem. Soc., Chem. Commun. 1992, 18. (26) Odegard, R.; Skotheim, T. A.; Lee, H. S. J. Electrochem. Soc. 1991, 138, 2930. (27) Rawi, Z.; Mykytiuk, J.; Armes, S. P. Colloids Surf. 1992, 68, 215. (28) Armes, S. P.; Aldissi, M.; Agnew, S. F. Synth. Met. 1989, 28, 837. (29) Armes, S. P.; Aldissi, M. Polymer 1990, 31, 569. (30) Beadle, P. M.; Rowan, L.; Mykytiuk, J.; Billingham, N. C.; Armes, S. P. Polymer 1993, 34, 1561. (31) DeArmitt, C.; Armes, S. P. Langmuir 1993, 9, 652. (32) (a) Maeda, S.; Armes, S. P. J. Colloid Interface Sci. 1993, 159, 257. (b) Maeda, S.; Armes, S. P. J. Mater. Chem. 1994, 4, 935. (33) (a) Maeda, S.; Armes, S. P. Chem. Mater. 1995, 7, 171. (b) Butterworth, M. D.; Armes, S. P.; Simpson, A. W. J. Chem. Soc., Chem. Commun. 1994, 2129. (34) Luk, S. Y.; Lineton, W.; Keane, M.; DeArmitt, C.; Armes, S. P. J. Chem. Soc., Faraday Trans. 1995, 91 (5), 905. (35) Maeda, S.; Gill, M.; Armes, S. P.; Fletcher, I. W. Langmuir 1995, 11, 1899.
particles. The polymeric stabilizer used to prevent macroscopic precipitation of the conducting polymer was an anionic polyelectrolyte: poly(potassium 3-sulfopropyl methacrylate) [PKSPM]. The structure of this stabilizer is shown in Figure 2. Our results provide a detailed insight into the nature of the interaction between this adsorbed polyelectrolyte stabilizer and the polypyrrole particle surface. Experimental Section Instrumentation. Elemental microanalyses were assessed by an external analytical service (Medac U.K. Ltd. of Brunel University). Aqueous GPC analysis was performed using a Polymer Labs aqueous GPC instrument (RI detector, 0.5 M NaNO3/0.01 M NaH2PO4 eluent at pH 3 and PEG/PEO standards). Conductivity measurements were made on compressed pellets using the conventional four-point probe technique at room temperature. Scanning electron microscopy studies were carried out using a Leica Stereoscan 420 instrument (beam current 16 pA at 10 kV). Samples were sputter-coated with thin layers of gold to prevent sampling charging effects. Synthesis of Polyelectrolyte Stabilizer. The PKSPM stabilizer was synthesized via the free-radical homopolymerization of potassium sulfopropyl methacrylate (10 g) in aqueous solution (100 mL) at 80 °C using a water-soluble initiator (V-50; 25 mg). After 18 h the resulting viscous aqueous solution was added to excess THF. The precipitated PKSPM stabilizer was isolated and dried in a vacuum oven at 40 °C overnight. Elemental microanalyses gave C 31.95%, H 4.70%, S 13.13%. Aqueous GPC confirmed that the molecular weight distribution of this stabilizer was fairly broad (Mw/Mn ) 2.9), as expected for a free-radical polymerization synthesis. Its weight-average molecular weight (vs PEO/PEG standards) was ca. 860 000 g mol-1. Colloid Synthesis. A series of preliminary experiments were carried out to determine the minimum PKSPM stabilizer concentration required for the quantitative formation of stable polypyrrole particles (rather than macroscopic precipitate). These experiments led to the following formulation: PKSPM (2.00 g) and FeCl3 (5.47 g) were dissolved in 100 mL of water at room temperature for 16 h with stirring. Pyrrole (1.00 mL) was then injected into the stirred solution, which turned black within a few minutes due to the formation of polypyrrole. The resulting dispersion was stirred for 16 h at room temperature. The colloid was vacuum-filtered in order to remove any traces of precipitate prior to centrifugation at 10 000 rpm for 2 h. The resulting black sediment was redispersed in water using mechanical agitation and an ultrasonic bath. This centrifugation-redispersion cycle was repeated in order to ensure the removal of any excess stabilizer and inorganic salts from the colloid. Elemental microanalyses gave C 50.13%, H 4.52%, N 10.71%, S 5.01%, Cl 3.10%. The compressed pellet conductivity of this dried colloid was 0.9 S cm-1. The above colloid synthesis was repeated in the absence of the PKSPM stabilizer. This resulted in the macroscopic precipitation of chloride-doped polypyrrole ‘bulk powder’ which was used as a control sample in the XPS study. This precipitate was filtered, washed with copious amounts of water until the washings were clear, and dried in a 60 °C oven overnight. Elemental microanalyses gave C 58.02%, H 3.68%, N 16.74%, Cl 10.46%. The compressed pellet conductivity of this dried powder was 3 S cm-1. XPS Experiments. Samples were mounted on double-sided adhesive tape: excess loose powder was shaken off, leaving
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Figure 3. Scanning electron micrograph of the PKSPMstabilized polypyrrole particles. The number-average particle diameter is approximately 100-150 nm. enough sample to cover the analysis area (ca. 10 mm2) in the center of the specimen stub. XPS measurements were made using a VG Scientific ESCALAB Mk. II spectrometer interfaced to a VGS 5000S data system based on a DEC PDP 11/73 computer. The operating conditions were as follows: the X-ray source [Al KR (1486.6 eV) or Mg KR (1253.6 eV) radiation] was operated at a power of 450 W (i.e. 13 kV potential and 34 mA emission current). The spectrometer was operated in the fixed analyzer transmission mode at a pass energy of 50 eV (survey spectra) or 20 eV (high-resolution spectra). The base pressure in the sample chamber during analysis was approximately 3 × 10-8 mbar. Good quality survey spectra were obtained with a single scan; coreline high-resolution spectra were integrated over 5-10 scans depending on the intensity of the spectral region of interest. Total spectral acquisition times per sample were typically 1020 min. Spectral analysis was carried out using the standard VGS 5000S software for quantification and peak-fitting; quantification was based on peak areas calculated from the highresolution spectra. Minor sample charging effects on the order of 1 eV were observed for some of the samples. The errors incurred by ignoring the low hydrogen contents of the samples were negligible.
Results and Discussion Various cationic polyelectrolytes based on poly(vinylpyridine)28,29 and poly(2-dimethylamino)ethyl methacrylate)30 copolymers have been used as steric stabilizers for polypyrrole colloids. However we are not aware of any reports describing the use of anionic polyelectrolytes. We have recently evaluated a series of such stabilizers as polymeric dispersants for polypyrrole, and a full account of this study will be published elsewhere.36 In the present work we wish to focus on the use of one particular anionic polyelectrolyte, PKSPM, as a steric stabilizer for polypyrrole particles. A typical scanning electron micrograph of the PKSPMstabilized polypyrrole particles is depicted in Figure 3. The particles have a spherical morphology, with particle diameters in the range 100-150 nm. This is consistent with our photon correlation spectroscopy measurements, which indicate an hydrodynamic diameter of ca. 200 nm for the diluted polypyrrole colloid.36 This difference between the SEM and PCS particle diameters is probably due in part to the adsorbed outer layer of the PKSPM stabilizer; this stabilizer layer is solvated during the PCS measurements in aqueous media but collapses down onto the surface of the dried particles under the high-vacuum conditions used in the SEM studies.27 However we note that the polypyrrole particles are somewhat polydisperse, (36) Beadle, P. M.; Armes, S. P. Manuscript in preparation.
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which would inevitably result in the intensity-average PCS particle diameter exceeding the number-average SEM diameter even if there were no adsorbed stabilizer layer. Thus we regard this particle size data as being consistent with, rather than confirming, the presence of the adsorbed stabilizer layer. The high Tg of the stiff, conjugated polypyrrole component ensures that no significant particle coalescence occurs during drying. Thus we are confident that the surface composition observed under the UHV conditions of the XPS experiments is characteristic of the original colloidal particles (in contrast, Deslandes et al. have attributed marked differences in the surface composition of polystyrene latex particles to loss of the original particle morphology3). FTIR spectroscopy studies on the sterically-stabilized polypyrrole particles, the PKSPM stabilizer, and chloridedoped polypyrrole ‘bulk powder’ will be reported in full elsewhere.36 In the context of the present study it is sufficient to note that the IR spectrum of the polypyrrole colloid clearly contains an additional band at 1719 cm-1 (relative to the ‘bulk powder’ spectrum) which can be assigned to the carbonyl group of the PKSPM stabilizer. The stabilizer content of this colloid was calculated to be 36 wt % on the basis of its reduced nitrogen content (10.71%) relative to that of a polypyrrole ‘bulk powder’ control sample synthesized under the same conditions but in the absence of any PKSPM stabilizer (16.74%). A ‘macroscopic’ S/N atomic ratio of 0.20 ( 0.02 was calculated from the elemental microanalysis of the dried colloid. The solid-state conductivity value of 0.9 S cm-1 for the PKSPM-stabilized polypyrrole colloid is slightly lower than the value of 3 S cm-1 found for the chloride-doped polypyrrole ‘bulk powder’. However, it is consistent with the conductivities (typically 10-2 to 100 S cm-1) of various polypyrrole colloids reported by many research groups.20-30 This lower conductivity is not believed to be due to any reduction in the intrinsic conductivity of the polypyrrole. On the contrary, the Cl/N ratio of 0.25 calculated from elemental microanalyses and the IR spectrum36 of the colloid both indicate that the conducting polymer component of the particles is in its heavily-doped, highlyconductive state. Instead, the reduced conductivity of the colloid is probably attributable to the presence of the electrically-insulating PKSPM stabilizer and/or the increased interparticle contact resistances arising from the finely-divided colloidal nature of the polypyrrole. The XPS survey spectrum of the chloride-doped polypyrrole ‘bulk powder’ control sample is depicted in Figure 4a. The spectrum contains peaks attributable to both nitrogen (pyrrole monomer residue) and chlorine (due to the Cl- dopant anions). The Cl/N surface atomic ratio in this sample is approximately 0.25, which indicates a surface doping level of 0.25. This is in excellent agreement with the macroscopic doping level calculated from elemental microanalyses (see above) and also with literature values.37 As expected, there is no evidence for any sulfur in this survey spectrum. Conversely, the survey spectrum for the PKSPM stabilizer contained both sulfur and potassium peaks but no peaks attributable to either nitrogen or chlorine (see Figure 4b). The K/S atomic ratio was calculated to be 1.11 for this latter sample, which is reasonably close to the expected value of unity. Thus these two ‘control’ spectra confirmed that the sulfur atoms in the PKSPM stabilizer and the nitrogen atoms in the polypyrrole particles could indeed be used as unique elemental “markers” for these two components. In principle, measurement of the S/N atomic ratio by XPS should allow a quantitative assessment of the surface (37) Armes, S. P. Synth. Met. 1987, 20, 367.
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a
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c
Figure 4. Wide scan XPS spectra for (a) polypyrrole bulk powder, (b) PKSPM stabilizer, and (c) PKSPM-stabilized polypyrrole particles. Note the absence of sulfur in spectrum a and the absence of nitrogen in spectrum b.
Figure 5. S 2p and N 1s core-line XPS spectra used for determining quantitative peak integrals and hence the surface S/N atomic ratio. Note the good signal-to-noise ratio of these two spectra.
composition of the sterically-stabilized polypyrrole particles (we have recently reported using a similar elemental “marker” approach to characterize the surface composition of conducting polymer-silica nanocomposite particles35). The survey spectrum of the PKSPM-stabilized polypyrrole particles is shown in Figure 4c. As expected, peaks attributable to both sulfur and nitrogen are clearly present. The corresponding N 1s and S 2p core-line spectra are shown in Figure 5. Peak integration and normalization of these two core-line spectra yielded a S/N atomic ratio of 0.39 ( 0.04. The small error bar is an indication of the high quality (good signal-to-noise ratio) of these two spectra. This surface S/N atomic ratio should be compared with the macroscopic S/N ratio of 0.20 ( 0.02 calculated from microanalyses. Since the difference between these surface and bulk S/N ratios is well outside experimental error, we conclude that the surface of the PKSPMstabilized polypyrrole particles is indeed distinctly “stabilizer-rich”. This is consistent with the currently accepted theory of steric stabilization.8,9 One question which arises from the above discussion is the following: is the PKSPM stabilizer layer around the polypyrrole particles fully continuous or merely ‘patchy’ in the solid state? In an attempt to address this question we repeated the survey and core-line spectra of the PKSPM-stabilized polypyrrole colloid using Mg KR radiation. Switching to this lower energy source reduces the XPS sample depth by ca. 1 nm.38 Thus, if the stabilizer coating is continuous, we might expect to observe a higher S/N atomic ratio by XPS. On the other hand, S/N ratio would be less likely to be affected if the stabilizer layer were ‘patchy’. Within experimental error, we observed no change in the S/N atomic ratio with the Mg KR X-ray source; this observation is more consistent with the ‘patchy’ model for the adsorbed stabilizer layer. Deslandes et al. have claimed a similar ‘patchy’ model for poly(N-vi-
nylpyrrolidone)-stabilized polystyrene latexes.3 We note that the observation of a N 1s peak in the survey spectrum of the polypyrrole colloid is direct evidence for the presence of the conducting polymer at (or very near) the particle surface. Again, this is best explained in terms of a ‘patchy’ model. This would allow reasonably efficient charge transport between individual polypyrrole particles in the solid state and is therefore consistent with the relatively high macroscopic conductivity exhibited by the dried colloid. A closer examination of the XPS data offers a useful insight into the nature of the interaction between the adsorbed PKSPM stabilizer and the surface of the polypyrrole particles. The surface Cl/N atomic ratio measured for the PKSPM-stabilized polypyrrole colloid is only 0.07, which is significantly lower than the value of 0.25 found for chloride-doped polypyrrole ‘bulk powder’. However, doping levels of 0.25-0.33 are normally expected for heavily doped, conductive polypyrrole. If the surface concentration of chloride anion is insufficient to account for the expected polypyrrole doping level, this suggests that there is an additional source of dopant anions present at the surface. One obvious possibility is the anionic sulfonate groups on the PKSPM stabilizer. We note that the S/N surface atomic ratio of 0.39 is more than sufficient to account for a realistic polypyrrole doping level. Alternatively, if we include the dopant contribution from the chloride anions, only approximately half of the stabilizer’s sulfonate groups need to participate as dopants in order to obtain an overall doping level of 0.25. However, if the sulfonate groups do act as dopant anions for the cationic polypyrrole chains, then we might expect a significant reduction in the surface concentration of potassium counterions. Inspection of the K 2p core-line spectra confirms this to be the case. The K 2p peak at 292.9 eV in the PKSPM stabilizer spectrum (see Figure 6a) has completely disappeared from the corresponding colloid spectrum (see Figure 6b). We conclude that the XPS data provide strong evidence that the PKSPM
(38) Fakes, D. W.; Newton, J. M.; Watts, J. F.; Edgell, M. J. Surf. Interface Anal. 1987, 10, 416.
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adsorbed amount, Γ, of approximately 17 mg m-2. This value is an order of magnitude higher than that typically found for polymers adsorbed onto solid substrates. Furthermore, polyelectrolytes usually have lower Γ values than neutral polymers, since they tend to adopt relatively “flat” conformations at surfaces.39,40 Thus this Γ calculation led us to suspect that the PKSPM stabilizer was actually present within the polypyrrole particle interior as well as at the surface. Accordingly, using the Cl and N elemental microanalyses of the colloid, we calculated a polypyrrole dopant level of only 0.11, assuming chloride was the sole dopant anion. This is significantly lower than that of the chloride-doped polypyrrole ‘bulk powder’. Again, more realistic doping levels (0.25-0.32) could be calculated if it were assumed that some or all of the sulfonate groups of the PKSPM stabilizer participated as a codopant for the polypyrrole chains. Thus we conclude that the PKSPM stabilizer probably acts as a polymeric dopant within the interior of the polypyrrole particles as well as at the surface. Conclusions
Figure 6. C 1s/K 2p core-line spectra for (a) PKSPM stabilizer and (b) PKSPM-stabilized polypyrrole particles. Note the complete disappearance of the K 2p signal in the latter spectrum.
Figure 7. Schematic representation of the surface interaction between the PKSPM stabilizer and the polypyrrole particles. The polyelectrolyte stabilizer acts as a polymeric dopant anion for the cationic polypyrrole chains, leading to the concomitant expulsion of the K+ and Cl- counteranions.
stabilizer acts as a polymeric dopant at the surface of the polypyrrole particles, with the concomitant expulsion of K+ and Cl- counterions. Thus there is a direct electrostatic (Coulombic) interaction between the stabilizer and the particle surface rather than mere physical adsorption (see Figure 7). Given that the polypyrrole colloid contains 36 wt % PKSPM, and assuming that all of this stabilizer is on the outside of the polypyrrole particles, we calculate an
Our XPS data have confirmed, for the first time, that the surface of a sterically-stabilized conducting polymer colloid is distinctly “stabilizer-rich”. This is consistent with the observed long-term colloid stability exhibited by these dispersions. In the solid state the PKSPM stabilizer layer appears to be ‘patchy’ rather than continuous, which is consistent with the reasonably high macroscopic conductivity of the dried colloid. Furthermore, the surface dopant level of the conducting polymer component and the substantial depletion in the surface concentrations of K+ and Cl- counterions (originating from the PKSPM stabilizer and polypyrrole components, respectively) strongly suggest that the polyelectrolyte stabilizer acts as a polymeric dopant anion for the cationic polypyrrole chains, i.e. that there is a direct electrostatic interaction between the adsorbed stabilizer and the polypyrrole particle surface. Finally there is some evidence that the PKSPM stabilizer is distributed throughout the interior of the polypyrrole particles as a polymeric dopant rather than simply being confined to an adsorbed surface layer. Acknowledgment. P.M.B. wishes to acknowledge the EPSRC and Cabot Plastics for a Ph.D. CASE studentship. J.F.W. wishes to thank the EPSRC for continued funding of electron spectroscopy research at the University of Surrey. We thank Dr. E. Meehan and Mr. S. Oakley of Polymer Laboratories for the aqueous GPC data. LA950746O (39) Aksberg, R.; Einarson, M.; Berg, J.; Odberg, L. Langmuir 1991, 7, 43. (40) Cosgrove, T.; Obey, T. M.; Vincent, B. J. Colloid Interface Sci. 1986, 111, 409.
