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Surface Characterization of Polypyrrole-Coated Polystyrene Latex by X-ray Photoelectron Spectroscopy Christian Perruchot, Mohamed M. Chehimi,* Michel Delamar, Stuart F. Lascelles,† and Steven P. Armes*,† Institut de Topologie et de Dynamique des Syste` mes, Universite´ Paris 7, Denis Diderot, associe´ au CNRS (URA34), 1 rue Guy de la Brosse, 75005 Paris, France, and School of Chemistry and Molecular Sciences, University of Sussex, Falmer, Brighton, BN1 9QJ, U.K. Received January 17, 1996. In Final Form: March 22, 1996X The surface composition of electrically conductive polypyrrole-coated polystyrene latex particles has been examined by X-ray photoelectron spectroscopy (XPS). We have systematically characterized the surface of polystyrene (PS), poly(N-vinylpyrrolidone) (PNVP), chloride-doped polypyrrole (PPyCl) powder, and PNVP-stabilized PS latex in order to determine the relative contributions of PS (core), PNVP (stabilizer), and PPyCl (shell) to the surface composition of the PPyCl-coated PS latex. XPS was found to be very effective in detecting PNVP at the surface of uncoated PS latex using N1s signal as an elemental marker and showed that the former polymer contributes to ca. 50% of the PS latex surface. In contrast, the surface composition of PPyCl-coated PS latex was found to be very PPyCl-rich, since the PPyCl bulk powder and the PPyCl-coated PS latex have very comparable XPS spectra. However, some additional iron chloride species (FeCl2 and/or FeCl3-) were also detected as impurities at the surface of the coated latex.
Introduction Over the last two decades conducting polymers have attracted a great deal of interest owing to their remarkable physical and chemical properties, such as inherent conductivity, redox, and acid-base properties.1,2 Polypyrrole (PPy) is one of the most studied conducting polymers because it offers reasonably high conductivity and has fairly good environmental stability with regard to air and water. In addition, it is easily synthesized in high yield via oxidative polymerization at room temperature in various common solvents, including water. However, polypyrrole is invariably obtained as a black precipitate which is insoluble in all known solvents, probably due to some degree of cross-linking.3 Since this material is not thermoplastic (upon heating, it decomposes below its theoretical softening point), it suffers from very limited processability. Since 1986 various research groups4-8 have reported the preparation of colloidal dispersions of several con† X
University of Sussex. Abstract published in Advance ACS Abstracts, June 1, 1996.
(1) Handbook of Conducting Polymers; Skotheim, T. J., Ed.; Marcel Dekker: New York, 1986; Vols. 1 and 2. (2) Conjugated Polymers: The Novel Science and Technology of Highly Conducting and Nonlinear Optically Active Materials; Bredas, J. L., Sielby, R., Eds.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1991. (3) (a) Bjorklund, R. B.; Liedberg, B. J. Chem. Soc., Chem. Commun. 1986, 1293. Recently, Korean workers have reported the synthesis of non-cross-linked, soluble polypyrrole, see: Lee, J. Y.; Kim, D. Y.; Kim, C. Y. Synth. Met. 1995, 74, 103. (4) (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. (c) Armes, S. P.; Aldissi, M. J. Chem. Soc., Chem. Commun. 1989, 88. (d) Armes, S. P.; Aldissi, M.; Agnew, S. F. Synth. Met. 1989, 28, 837. (e) Armes, S. P.; Aldissi, M. Polymer 1990, 31, 569. (f) Tadros, P.; Armes, S. P.; Luk, S. Y. J. Mater. Chem. 1992, 2, 125. Simmons, M. R.; Chaloner, P. A.; Armes, S. P. Langmuir 1995, 11, 4222. (5) (a) Vincent, B.; Waterson, J. W. J. Chem. Soc., Chem. Commun. 1990, 683. (b) Cawdrey, N.; Obey, T. M.; Vincent, B. J. Chem. Soc., Chem. Commun. 1989, 1189. (c) Markham, G., Obey, T. M.; Vincent, B. Colloids Surf. 1990, 51, 239. (6) (a) Digar, M. L.; Bhattacharyya, S. N.; Mandal, B. M. J. Chem. Soc., Chem. Commun. 1992, 18. (b) Banerjee, P.; Digar, M. L.; Bhattacharyya, S. N.; Mandal, B. M. Eur. Polym. J. 1994, 30, 499. (7) (a) Gospodinova, N.; Mokreva, P.; Terlemezyan, L. J. Chem. Soc., Chem. Commun. 1992, 923. (b) Stejskal, J.; Kratochvil, P.; Gospodinova, N.; Mokreva, P.; Terlemezyan, L. Polym. Int. 1993, 32, 401.
S0743-7463(96)00057-1 CCC: $12.00
ducting polymers, particularly polyaniline (PANI) and PPy. These colloids are prepared via dispersion polymerization, usually employing a polymeric stabilizer which becomes either physically adsorbed or chemically grafted onto the surface of the precipitating conducting polymer particles. Thus further particle growth is prevented, and the well-known steric stabilization mechanism ensures long-term colloid stability.9,10 In principle, these particles represent a more processable, and hence more useful, form of the conducting polymer. However, since the conducting polymer component has a high glass-transition temperature and comprises 60-90% by mass of these colloids, the particles do not normally exhibit film-forming properties at room temperature. Over the last two decades X-ray photoelectron spectroscopy (XPS) has become an established technique for the characterization of polymer surfaces.11-14 More recently, several groups have reported the use of XPS to investigate the surface of conventional polymer latex particles.15-20 For example, the Davies group at Nottingham has published a series of papers on the use of (8) Liu, C.-F.; Moon, D.-K.; Maruyama, T.; Yamamoto, T. Polym. J. 1993, 25, 775. (9) Epron, F.; Henry, F.; Sagnes, O. Makromol. Chem. Macromol. Symp. 1990, 35/36, 527. (10) Napper, D. H. Polymer Stabilization of Colloidal Dispersions; Academic Press: London, 1983. (11) Practical Surface Analysis, Auger and X-Ray Photoelectron Spectroscopy, 2nd edition; Briggs, D., Seah, M. P., Eds.; John Wiley: Chichester, 1990; Vol. 1. (12) (a) Briggs, D. Surf. Interface Anal. 1982, 4, 151. (b) Briggs, D. Surf. Interface Anal. 1983, 5, 113. (13) Briggs, D. Surf. Interface Anal. 1986, 8, 133. (14) High Resolution XPS of Organic Polymers. The Scienta ESCA300 Database; Beamson, G., Briggs, D., Eds.; John Wiley: Chichester, 1992. (15) (a) Lynn, R. A. P.; Davis, S. S.; Short, R. D.; Davies, M. C.; Vickerman, J. C.; Humphrey, P.; Johnson, D.; Hearn, J. Polym. Commun. 1988, 29, 365. (b) Brindley, A.; Davies, M. C.; Lynn, R. A. P.; Davis, S. S.; Heran, J.; Watts, J. F. Polym. Commun. 1992, 33, 1112. (16) Davies, M. C.; Lynn, R. A. P.; Davis, S. S.; Hearn, J.; Watts, J. F.; Vickerman, J. C.; Johnson, D. J. Colloid Interface Sci. 1993, 156, 229. (17) Davies, M. C.; Lynn, R. A. P.; Davis, S. S.; Hearn, J.; Watts, J. F.; Vickerman, J. C.; Paul, A. J. Langmuir 1993, 9, 1637. (18) Zhao, C. L.; Dobler, F.; Pith, T.; Holl, Y.; Lambla, M. J. Colloid Interface Sci. 1989, 128, 437. (19) Dobler, F.; Affrossman, S.; Holl, Y. Colloids Surf., A 1994, 89, 23. (20) Deslandes, Y.; Mitchell, D. F.; Paine, A. J. Langmuir 1993, 9, 1468.
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3246 Langmuir, Vol. 12, No. 13, 1996
Figure 1. Schematic formation of polypyrrole-coated polystyrene latex particles.
XPS (in tandem with static SIMS) to examine the surface composition of both sterically-stabilized and chargestabilized polystyrene and poly(n-butylmethacrylate) latexes.15-17 Similarly, Holl and co-workers have reported a combined XPS/SIMS study of surfactant-stabilized acrylic latexes.18,19 Perhaps of particular relevance to the present work, Deslandes et al. have reported XPS and SIMS studies of the surface composition of poly(Nvinylpyrrolidone)-stabilized polystyrene latexes.20 In general, it is found that the surface compositions of the latex systems are consistent with the currently accepted theories of colloid stabilization.10 XPS has also been widely used to characterize organic conducting polymers such as polyaniline and polypyrrole.21-25 Usually such studies have focused on chemically-synthesized bulk powders or electrochemicallysynthesized thin films.21-23 The typical sampling depth for XPS is only 2-5 nm, and thus it has proved to be particularly useful for assessing the surface oxidation and degradation chemistry of conducting polymers.24 In addition, Abel and Chehimi have shown that XPS can be useful in monitoring the adsorption from solution of nonconducting polymers such as poly(methyl methacrylate) onto polypyrrole bulk powders.25 Recently Armes and co-workers have published several papers describing the use of XPS to characterize the surface composition of silica-stabilized,26 surfactant-stabilized,27 and polyelectrolyte-stabilized28 polypyrrole colloids. In each case it was confirmed that the surface of the polypyrrole particles was coated with the dispersant (i.e., silica, surfactant, or polymer). Recently, a research group at DSM (The Netherlands) has shown that conventional submicronic stericallystabilized latexes can be coated with polypyrrole or polyaniline under appropriate conditions.29 The resulting coated particles have reasonable colloid stability, apparently because the thickness of the deposited polypyrrole overlayer is less than that of the solvated steric stabilizer layer (see Figure 1). If the original latex particles (e.g., (21) (a) Pfluger, P.; Street, G. B. J. Chem. Phys. 1984, 80, 544. (b) Inganas, O.; Erlandsson, R.; Nylander, C.; Lundstrom, I. J. Phys. Chem. Solids 1984, 45, 427. (22) (a) Kang, E. T.; Neoh, K. G.; Ong, Y. K.; Tan, K. L.; Tan, B. T. G. Macromolecules 1991, 24, 2822. (b) Kang, E. T.; Neoh, K. G.; Tan, K. L. Adv. Polym. Sci. 1993, 106, 135. (23) Malitesta, C.; Morea, G.; Sabbatini, L.; Zambonin, P. G. In Surface Characterization of Advanced Polymers; Sabbatini, L., Zambonin, P. G., Eds.; VCH: Weinheim, 1993; Chapter 5. (24) Moss, B. K.; Burford, R. P. Polymer 1992, 33, 1902. (25) Abel, M. L.; Chehimi, M. M. Synth. Met. 1994, 66, 225. (26) Maeda, S.; Gill, M.; Armes, S. P.; Fletcher, I. W. Langmuir 1995, 11, 1899. (27) Luk, S. Y.; Lineton, W.; Keane, M.; DeArmitt, C.; Armes, S. P. J. Chem. Soc., Faraday Trans. 1995, 91, 905. (28) (a) Greaves, S.; Watts, J. F.; Beadle, P. M.; Armes, S. P. Langmuir 1996, 12, 1784. (b) Simmons, M.; Armes, S. P.; Chaloner, P. A. Polymer, in press. (29) Wiersma, A. E.; van der Steeg, L. M. A.; Jongeling, T. J. M. Synth. Met. 1995, 71, 2269.
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polyurethanes, alkyd resins) possess a sufficiently low Tg then film-forming conductive particles can be obtained; these remarkable composite materials are expected to find applications as novel anticorrosion or antistatic coatings. The Sussex group has used a modified DSM protocol in order to coate micron-sized polystyrene particles with a thin overlayer of polypyrrole.30 These polypyrrolepolystyrene composite particles have been extensively characterized in terms of their chemical composition, colloid stability, electrical conductivity, (nano)morphology, etc., using a wide range of techniques; this work will be reported in detail elsewhere.31 The aim of the present work is the examination of the surface composition of these conducting polymer-coated latex particles using XPS. Experimental Section Materials. Styrene (Aldrich) was purified by passing it through a column of activated neutral alumina. 2,2′-Azobisisobutyronitrile (AIBN), and poly(N-vinylpyrrolidone) (PNVP; manufacter’s nominal molecular weight 44 000) were obtained from BDH and used without further purification. Aliquat 336 (methyltrioctylammonium chloride from Aldrich) was also used as supplied. Pyrrole (BASF) was purified by passing it through a column of activated basic alumina prior to use. The FeCl3‚ 6H2O oxidant was obtained from Aldrich and used without further purification. Synthesis of Polystyrene Latex. The PNVP stabilizer (7.0 g) and Aliquat 336 costabilizer (2.0 g) were dissolved in isopropyl alcohol (400 mL) with magnetic stirring into a 1 L round bottom flask fitted with a condenser. After being heated to 70 °C under a nitrogen blanket, the reaction solution was purged with nitrogen at 70 °C for 1 hour. Following this, a solution of AIBN initiator (50.0 mg) predissolved in styrene monomer (50 g) was added to the stirred reaction solution. The polymerization was then allowed to proceed for 24 h before the milky-white reaction solution was cooled to room temperature. The resulting latex particles were then purified by repeated centrifugation/redispersion cycles using deionized water. Disc centrifuge photosedimentometry (DCP; Brookhaven Instruments) measurements indicated a weight-average diameter of 1.80 µm and a relatively narrow size distribution, which was confirmed by scanning electron microscopy (SEM; Leica Stereoscan 420). C,H,N elemental microanalyses (C 92.1%, H 7.7%, N 0.2%) of the dried polystyrene latex were determined by Medac Ltd. (Brunel University). Synthesis of Polypyrrole-Coated Polystyrene Latex. The FeCl3‚6H2O oxidant (0.91 g) was dissolved in a stirred aqueous dispersion of the latex (35 mL; 1.12 g of dry weight of latex) in a screw-cap bottle. Pyrrole monomer (0.10 mL) was then added via syringe, and the polymerization was allowed to proceed for 24 h. The resulting black dispersion was purified by repeated centrifugation/redispersion cycles using deionized water in order to remove unwanted inorganic byproducts (FeCl2 and HCl) produced during the pyrrole polymerization. A small sample of the purified latex was oven-dried at 60 °C overnight prior to C,H,N elemental microanalyses (Medac Ltd.). The polypyrrole loading of the coated polystyrene particles was determined to be 8.7 wt % by comparing its carbon content (89.0%) to the carbon cotent of the uncoated polystyrene latex (92.1%) and PPyCl bulk powder (56.6%), respectively. Film Preparation. Polystryene (Acros, MW 250 000) and poly(N-vinylpyrrolidone) (Roth, MW 24 000) films were cast from toluene (Prolabo, Normapur grade) and distilled water, respectively. The films were dried under a stream of argon overnight to ensure “clean” polymer surfaces, with a minimum of contamination. XPS. XPS signals were recorded using a VG Scientific ESCALAB MKI system operated in the constant analyzer energy mode. An Al KR X-ray source was used at a power of 200 W (20 mA × 10 kV), and the pass energy was set at 20 eV. The pressure in the analysis chamber was ca. 5 × 10-8 mbar. The PS and (30) Lascelles, S. F.; Armes, S. P. Adv. Mater. 1995, 7(10), 864. (31) (a) Lascelles, S. F.; Armes, S. P., to be submitted. (b) Lascelles, S. F.; Armes, S. P.; Watts, J. F.; Greaves, S.; Brown, A.; Zhdan, P.; Luk, S. Y. manuscript in preparation.
XPS Characterization of Polypyrrole-Coated Latex
Figure 2. XPS survey spectra of (a) poly(N-vinylpyrrolidone)stabilized polystyrene latex and (b) chloride-doped, polypyrrolecoated polystyrene latex particles. PNVP reference samples were analyzed at 90° relative to the surface. Digital acquisition was achieved with a Cybernetix system, and the data collected with a personal computer. Homemade data processing software allowed smoothing, linear, or Shirley background removal, static charge referencing, peak fitting, and quantification. Charge referencing was determined by setting the main C(1s)C-C/C-H component at 285.0 eV. The surface composition (in atomic %) of the various samples was determined by considering the integrated peak areas of C(1s), N(1s), O(1s), Cl(2p), and Fe(2p)3/2 and their respective experimental sensitivity factors. These were determined using a large set of organic and inorganic compounds of well-defined stoichiometries. The fractional concentration of a particular element A, %A, is computed using
/∑(I /s ) × 100
%A ) (IA/sA)
n
n
where In and sn are the integrated peak areas and the sensitivity factors, respectively.
Results and Discussion A. Spectral Examination. 1. Wide Scans. XPS wide scans of PS and PPyCl-coated PS latexes are depicted in Figure 2. For PS latex, Figure 2a exhibits N(1s) and O(1s) signals arising from the surface layer of adsorbed PNVP stabilizer on the PS latex. FTIR experiments carried out on this PS latex revealed a weak absorption band at 1660 cm-1 due to the carbonyl group of the PNVP stabilizer.31a This observation is in agreement with the
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microanalysis values for the PS latex, which indicate a PNVP stabilizer content of approximately 1 wt %. The wide-scan spectrum of the PPyCl-coated PS latex is displayed in Figure 2b. The Cl(2p) signal arises from the polypyrrole chloride dopant anion species. In addition to the four main peak (Cl(2p), C(1s), N(1s), and O(1s)) signals detected for PPyCl bulk powder, Figure 2b shows an additional weak feature near 700 eV that is attributable to Fe(2p). It is noteworthy that there is a significant increase in the N/C and O/C intensity ratios compared to those of PS latex. In contrast, the PPyCl-coated PS latex survey spectrum is rather similar to that of the PPyCl bulk powder, however, with a relatively more intense Cl(2p) signal. 2. C(1s). The high-resolution C(1s) peaks for the reference polymers and the uncoated and coated latexes are displayed in Figure 3. The C(1s) peak (Figure 3a) of the pristine PS is fitted with two components centered at 285.0 and 291.8 eV. The former corresponds to the CsC and CsH bonds and the latter to a shake-up satellite due to the aromatic ring. Note that the 0.4 eV splitting between aliphatic and aromatic carbons was not taken into account since this is much lower than the spectral resolution.14 Figure 3b shows the C1s peak of the PNVP stabilizer fitted with three components centered at 285 (CsC, CsH), 285.9 (CsN) and 287.7 (NsCdO) eV. The experimental relative areas of these components were evaluated to be 3.6, 2.3, and 1, respectively. This compares reasonably well with the theoretically expected 3:2:1 relative peak area ratio. In Figure 3c, the C(1s) signal from PS latex exhibits a sharp CsC/CsH component mainly due to PS and is centered at 285 eV. Two high binding energy (BE) components centered at 285.9 and 287.7 eV can be assigned to CsN and NsCdO, as mentioned above for PNVP film. A low-intensity feature at 291.7 eV is due to the C(1s) shake-up satellite already observed for pristine PS film. Figure 3d shows the C(1s) region for PPyCl fitted with five components. These are centered at 285 (β carbon), 286.4 (R carbon), and 288.1 and 290 eV, assigned to CdN defects, non R-R bonding,21,23,32 terminal COH and CdO groups,32,33 and Fermi level electron excitation.23 The high BE component centered at 292.5 eV is due to a π f π* shake-up transition.23,32 The C(1s) peak for the PPyCl-coated PS latex (Figure 3e) is fairly distinct from that of the PS latex but strikingly similar to that of PPyCl bulk powder. Moreover, it is wellfitted with the same five components as the latter reference compound. It is important to note that both the C(1s) peaks of the PPyCl bulk powder and the PPyCl-coated PS latex are skewed and, in addition, they each display a significant increase in background intensity (due to electron energy loss) that coincides with the trailing edge of the peak. This contrasts with the C(1s) peaks shown in Figures 3a-c for pristine PS, PNVP, and PNVPstabilized PS latex, respectively. 3. N(1s). The N(1s) peaks are shown in Figure 4 for PS and PPyCl-coated PS latexes. For PS latex (Figure 4a), the N(1s) signal has one single component centered at 399.8 eV due to the NsCdO group from the PNVP stabilizer, in good agreement with the literature.14 Figure 4b shows a N(1s) peak for PPyCl-coated PS latex similar to that observed for bulk polypyrrole, in agreement with the literature.22b The peak is fitted with four components centered at ca. 398-399, 400-401, 402-403, and 403(32) Pireaux, J. J.; Riga, J.; Boulanger, P.; Snauwaert, P.; Novis, Y.; Chtaib, M.; Gregoire, C.; Fally, F.; Beelen, E.; Caudano, R.; Verbist, J. J. Electron Spectrosc. Relat. Phenom. 1990, 52, 423. (33) Lei, J.; Cai, Z.; Martin, C. R. Synth. Met. 1992, 46, 53.
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Figure 3. Characteristic C(1s) core line signals of (a) polystyrene film, (b) poly(N-vinylpyrrolidone) film, (c) poly(N-vinylpyrrolidone)stabilized polystyrene latex, (d) chloride-doped polypyrrole bulk powder, and (e) chloride-doped, polypyrrole-coated polystyrene latex particles. Table 1. Peak Fitting of N(1s) Spectra for PPyCl Bulk Powder and PPyCl-Coated PS Latexa NdC
NH
NIox
NIIox
PPyCl bulk 398.9 (6.2) 400.8 (66.7) 402.3 (19.5) 404.0 (7.7) powder PPyCl-coated 398.9 (9.6) 400.8 (63.1) 402.0 (18.7) 403.5 (8.5) PS latex a Binding energies (eV) of the various N(1s) components and their corresponding relative intensities (%) in parentheses.
404 eV, which are attributable to the imine NdC group, the NsH bond in the pyrrole repeat unit, and the two high oxidation states of nitrogen (NIox and NIIox) in doped polypyrrole. Table 1 summarizes the results of N1s peak fitting for the PPyCl bulk powder and the PPyCl-coated PS latex. For the PPyCl-coated PS latex, the NsH component has a relative intensity of 63.1% of the total N(1s) peak area. This value is lower than that for PPyCl bulk powder. On the other hand, the imine component of the coated latex has a relative peak area of 9.6%, which is somewhat higher than the 6.2% determined for PPyCl bulk powder. This could be ascribed to the N(1s) signal arising from PNVP, which could not be resolved as a separate component. It is noteworthy that the sum of the relative areas due to NIox and NIIox is 27.2% for both PPyCl
bulk powder and the PPyCl-coated PS latex, a value comparable to that reported in the literature (29%) for PPyCl bulk powder.22b Since the contribution of NIox and NIIox to the N(1s) peak area is known to be comparable to the overall PPy doping level,22b,33 it is clear that the oxidation states of the PPy components in these two sampels are very similar. Clearly, close inspection of both C(1s) and N(1s) signals from PPyCl-coated PS latex (Figures 3e and 4b, respectively) provides strong evidence that the surface of this material is essentially PPyCl, as expected. 4. Cl(2p). Cl(2p) signals for the PPyCl bulk powder and PPyCl-coated PS latex are shown in Figure 5. The former material displays a structure very similar to that reported by Kang and co-workers.22b,34 It can be fitted with three components centered at 197.5, 199.2, and 201.4 eV of which relative intensities are 4.5:3.2:1. The three Cl(2p) signals arise from anionic chlorine, resulting from charge transfer interactions with the polymer chain, and chlorine covalently bound to the polymer backbone.22b Figure 5b shows the Cl(2p) signal of the PPyCl-coated PS latex fitted with four components in the ratio 2.6:5.1:3.6: (34) Kang, E. T.; Neoh, K. G.; Ong, Y. K.; Tan, K. L.; Tan, B. T. G. Synth. Met. 1990, 39, 69.
XPS Characterization of Polypyrrole-Coated Latex
Figure 4. N(1s) core line signals of (a) poly(N-vinylpyrrolidone)stabilized polystyrene latex and (b) chloride-doped, polypyrrolecoated polystyrene latex particles.
1. The additional one is due to a shoulder on the low BE side and is attributed to iron chloride species (see below). 5. Fe(2p). The Fe(2p) signal is detected for the PPyClcoated PS latex but not for the PPyCl bulk powder reference sample. This peak could be due to a possible insertion of FeCl4- and/or FeCl3- as dopant anions for the cationic polypyrrole chains. Alternatively, the Fe species could be complexed with the PNVP stabilizer, there is some literature evidence for such complexation in both aqueous35 and nonaqeuous media.36 It is also possible that FeCl2 is retained by the latex as a physically occluded contaminant. Figure 6 shows that the shape of PPyClcoated PS latex Fe(2p) signal is typical of Fe(II) species.37 This is evidenced by the position of the shake-up satellite on the high BE side of Fe(2p)3/2. For Fe(II) salts, the shakeup satellite is a high Be shoulder of Fe(2p)3/2, whereas it is a low BE side shoulder of Fe(2p)1/2 in the case of Fe(III).37 Therefore, the iron species in the coated latex is present in its Fe(II) oxidation state. The additional component observed in the Cl(2p) spectrum suggests that the Fe(II) species is probably an iron chloride complex. B. Surface Composition. The surface composition of the PS and PPyCl-coated PS latexes in atomic percent(35) Armes, S. P. Ph.D. Thesis, University of Bristol, Bristol, U.K., 1987. (36) Biederman, H.-G.; Graf, W. Z. Naturforsch. 1974, 29, 65. (37) Castle, J. E.; Ke, R.; Watts, J. F. Corros. Sci. 1990, 30, 771.
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Figure 5. Cl(2p) core line signals of (a) chloride-doped polypyrrole bulk powder and (b) chloride-doped polypyrrole latex particles.
Figure 6. Fe(2p) core line signal of chloride-doped, polypyrrolecoated polystyrene latex particles.
ages and the relative proportions of each polymer were calculated, assuming that the composition is homogeneous in the volume analyzed by XPS. This is almost certainly not true since the PS latex is coated with PNVP stabilizer and the PPyCl-coated PS latex is expected to have a “core/ shell” morphology. Therefore, the surface compositions reported below should be regarded only as “apparent”
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Table 2. Apparent XPS Surface Composition of Polymers and Latexes (atom %) materials
C
O
N
Cl
Fe
PS PNVP PS latex PPyCl bulk powder PPyCl-coated PS latex
99 75 86 73 70
1 13 7 3 5
12 7 18.6 16
5.2 8.1
traces 1.5
compositions and used for a semiquantitative comparison between the various materials surfaces. Table 2 reports the surface composition (in atom %) of PS, PNVP, PPyCl, and PS and PPyCl-coated PS latexes. For PNVP, the C/N and C/O ratios are ∼6:1 which compare well with the theoretical stoichiometric formula. The PS latex coated with the PNVP stabilizer has higher C/N and C/O ratios than the pristine PNVP film. This is consistent with the detection of the C(1s) shake-up satellite from the underlying PS. The PNVP overlayer is thus thinner than the XPS analysis depth and/or is discontinuous. For the PPyCl bulk powder, the C/N ratio is 3.9:1, which is very close to the theoretical value of 4:1. The PPy doping level (Cl/N ratio) is 0.28. For the PPyCl-coated PS latex, the C/N ratio is ∼4.4:1, comparable to that of PPyCl bulk powder. However, the apparent doping level is 0.51; the surface chlorine concentration is higher due to the iron chloride species retained at the PPyCl-coated PS latex particle surface. We have shown above that the contribution of oxidized nitrogens to N(1s) (27.2%) is the same for both the PPyCl-coated PS latex and PPyCl bulk powder. Therefore, the doping level of the PPyCl shell in the former sample should be approximately 0.28. We have used the approach reported by Abel and Chehimi for the analysis of PMMA-coated PPy powders25 to determine the relative proportion of the reference polymers at the surface of the uncoated PS and PPyClcoated PS latexes. For the uncoated PS latex:
CPSL ≈ CPS + CPNVP where CPSL, CPS, and CPNVP are the atom % of carbon for PS latex, pristine PS, and PNVP films. For PNVP,
CPNVP ) 6.25 × NPNVP where NPNVP is the atom % of N due to the PNVP component in the PS latex. It follows that CPNVP ≈ 44% and
CPS ) CPSL - CPNVP ) 42% PNVP and PS repeat units contain six and eight carbon atoms, respectively. Therefore, the relative proportion of PNVP shell is
%PNVP ) (44/6)/[(44/6) + (42/8)] Thus PNVP occupies ca. 58% of the PS latex surface. Since the only source of nitrogen in the PS latex is the PNVP stabilizer, its relative proportion at the latex surface can also be estimated simply by ratioing the two N(1s) signals observed in the PNVP and the PS latex. Thus %PNVP ) NPSL/NPNVP ) 7/12 ) 58%, which is in excellent agreement with the above value. An alternative to these two approaches consists of using the C(1s)/N(1s) intensity ratio for PNVP (3.5) to determine the contribution of PNVP to C(1s) peak area of the PS latex. By subtraction, one can estimate the contribution of PS to C(1s) intensity for PS. Taking into account the stoichiometric formula of both polymers, the %PNVP is
[C(1s)(PNVP)/6]/[C(1s)(PNVP)/6 + C(1s)(PS)/8] × 100 This approach yields 51%, which is in good agreement
with the other two values. It is noteworthy that the relative proportion of PNVP at the surface of our PS latex particles is approximately twice the value of 30% obtained by Deslandes et al.20 for their synthesis of PS latex. Similarly, we have assesed the relative proportions of PS, PNVP, and PPyCl at the PPyCl-coated PS latex particle surface by estimating the contribution of each component to the overall C(1s) signal. The starting point is the estimation of the chlorine content in the PPyCl overlayer. Unfortunately, this is somewhat problematic because iron is not detected in the PPyCl bulk powder. Thus, the Fe species in the coated latex should be regarded as a contaminant present either in the form of FeCl2 or as the possible codopant FeCl3- as mentioned above and the contribution of iron(II) chloride species to Cl(2p) signal should first be assessed. This can be done by two methods that rely on the Cl(2p) spectra shown in Figure 5. (i) The high-resolution scan of the Cl(2p) region indicates an additional low BE component due to the uptake of iron(II) chloride species. Using the peak areas of the low BE component in the Cl(2p) spectrum and that of Fe(2p)3/2 and considering the sensitivity factors, we obtain a stoichiometric formula of FeCl1.2. Since the Cl/N atomic ratio is 0.28:1 for PPyCl bulk powder (Table 2), it follows that the %N in the PPyCl-coated PS latex is ∼21%, which is much greater than the value of 16% reported in Table 2. Therefore, the surface of PPyCl-coated PS latex is essentially composed of PPyCl (with some iron(II) chloride present as a contaminant). (ii) A second approach lies in Cl(2p) peak fitting. For PPyCl bulk powder, the signal can be fitted with three Cl(2p) components, the intensity ratios of which are 4.5:3.2:1 (Figure 5a). In the case of PPyCl-coated PS latex, the Cl(2p) signal is best fitted with four components, for which the intensity ratios are 2.6: 5.1:3.6:1. It follows that the additional iron(II) chloride species make a significant contribution to the Cl(2p) signal. Assuming that the intensity ratios of the Cl(2p) components for PPyCl bulk powder hold for the latex, we can determine the contribution of the additional iron(II) chloride species to the total Cl(2p) peak area. Combining this fraction and the peak area of Fe(2p)3/2 from PPyClcoated PS latex, we obtain a Cl/Fe atomic ratio of ∼2.5:1. The remaining %Cl for the PPyCl shell is thus
%Cl(PPyCl-coated PS latex) [(Cl/Fe) × (atom %Fe)] ) 8.1 - (2.5 × 1.5) ) 4.4 where Cl/Fe is the atomic ratio found for the additional iron(II) chloride species retained in the PPyCl shell. By taking into account the doping level of PPyCl (∼28%, or N/Cl ) 3.6), the contribution of the PPyCl shell to the total %N of the PPyCl-coated PS latex particle is 15.7%. Thus, by difference, the contribution of the PNVP stabilizer to the overall %N is only 0.3%. Using the theoretical C/N atomic ratios of 4 and 6 for PPyCl and PNVP, respectively, it follows that the contribution of these polymers to the total carbon content in the PPyCl-coated PS latex is (15.7 × 4) + (0.3 × 6) ≈ 65%. Therefore, the contribution of PS to the overall %C is 5%. Given this, one can calculate the apparent relative proportions of the three polymers with the following:
%X )
(%CX/nC) (%CPS/8) + (%CPNVP/6) + (%CPPyCl/4)
× 100
where %X is the relative proportion of one of the three components constituting the latex material, %CX is the corresponding %C, and nC is the number of carbon atoms per repeat unit of the polymer for which the relative proportion is calculated.
XPS Characterization of Polypyrrole-Coated Latex Table 3. Relative Proportions of PS, PNVP, and PPyCl at the Surface of the PS and PPyCl-Coated PS Latexes by XPS latex/polymer
PS
PNVP
PPyCl
PS latex PPyCl-coated PS latex
49 0-4
51 0-2
94-100
As an example, the relative proportion of PPyCl at the PPyCl-coated PS latex particle surface is
%PPyCl )
(15.7) × 100 ) 94% (0.63) + (0.3) + (15.7)
Table 3 summarizes the approximate relative proportion of PS, PNVP, and PPyCl (in % of repeat units) at the PS and PPyCl-coated PS latex surfaces. Thus, Table 3 verifies that the PPyCl-coated PS latex surface composition is dominated by the PPyCl component, at least within the XPS analysis depth of 2-5 nm. This contrasts very much with the surface composition of silica-polypyrrole nanocomposites,26 which have a relatively low surface composition (23%) with respect to PPyCl. Thus, the XPS data are consistent with the PS latex being coated with a thin, uniform overlayer of conducting polymer. Conclusions The surface composition of PS/PPyCl core/shell latexes have been studied using XPS. Comparisons have been
Langmuir, Vol. 12, No. 13, 1996 3251
made with PS, PNVP, PPyCl bulk powder, and PS latex reference materials. We have reached the following results: (1) XPS is very effective in detecting nitrogen, oxygen, and carbon features due to the PNVP stabilizer on the original uncoated PS latex particles. (2) The surface composition of the uncoated PS latex was found to contain approximately 58% PNVP stabilizer. (3) The XPS survey and high-resolution scans of both the PPyCl bulk powder and PPyCl-coated PS latex are very similar, suggesting that the surface of the latter material is rich in PPyCl. (4) Contrary to PPyCl bulk powder, the PPyCl-coated PS latex contains a small amount (
