Langmuir 1999, 15, 8059-8066
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X-ray Photoelectron Spectroscopy Characterization of Submicrometer-Sized Polypyrrole-Polystyrene Composites D. B. Cairns and S. P. Armes* School of Chemistry, Physics and Environmental Science, University of Sussex, Falmer, East Sussex BN1 9QJ, U.K.
M. M. Chehimi, C. Perruchot, and M. Delamar Institut de Topologie et de Dynamique des Syste` mes de l’Universite´ Paris 7, Denis Diderot, associe´ au CNRS (URA34), 1 rue Guy de la Brosse, 75005 Paris, France Received April 12, 1999. In Final Form: July 14, 1999
The surface compositions of a series of five polystyrene-polypyrrole (PS-PPy) composites and three reference materials (the original poly(ethylene glycol) (PEG) stabilizer, the uncoated PEG-stabilized PS latex, and PPy chloride bulk powder) were examined by X-ray photoelectron spectroscopy (XPS). The uncoated PEG-stabilized PS latex particles had a narrow size distribution with a mean diameter of 129 nm. The N1s XPS signal is a unique elemental marker for the PPy component and was therefore used to determine its surface concentration. As the PPy loading on the PS latex particles was increased from 4.2 to 28.1 wt % the relative intensity of the N1s signal increased, as expected. However, surface doping levels calculated from the Cl/N atomic ratios were relatively low, suggesting that some oxidative degradation of the deposited PPy component had occurred. Raman studies also indicated decreased doping levels at low PPy loadings. Close inspection of the C1s envelopes indicated that the composite particles did not have the expected core-shell morphology, since even at the highest PPy loading these XPS spectra were very similar to that of the original PS latex. These observations were confirmed by scanning electron microscopy (SEM) studies, which revealed the presence of discrete PPy nanoparticles of 20-30 nm diameter. Finally, it was found that more uniform PPy overlayers could be prepared by modifying the synthesis conditions. Thus, reducing both the total latex surface area and the pyrrole monomer concentration led to PS-PPy particles with a much improved core-shell morphology, as judged by both XPS and SEM.
Introduction Polypyrrole (PPy) is a relatively air-stable organic conducting polymer which suffers from poor processability.1 Several research groups have reported the synthesis of PPy latexes using various water-soluble polymers as steric stabilizers.2-5 Although this approach improves the processability of the conducting polymer, the cross-linked nature1 of the PPy component prevents film formation. Thus these latexes are not useful for coatings applications. Recently, workers at DSM Research have coated filmforming, sterically stabilized polyurethane latexes of 60100 nm diameter with PPy.6,7 Here the PPy apparently resides inside the solvated, chemically grafted steric stabilizer layer, thus enabling good colloid stability to be retained. The resulting polyurethane-PPy core-shell particles undergo coalescence and film formation at 50 °C, despite the low Tg polyurethane cores being encapsulated by the conducting polymer. Addition of a plasticizing cosolvent such as N-methylpyrrolidone promotes * To whom correspondence should be addressed. (1) Frommer, J. E.; Chance, R. R. Electrically Conducting Polymers. In Encyclopedia of Polymer Science and Technology; Wiley: New York, 1985; Vol. 5, p 462. (2) Bjorklund, R. B.; Liedberg, B. Chem. Commun. 1986, 1293. (3) Armes, S. P.; Vincent, B. Chem. Commun. 1987, 288. (4) Cawdrey, N.; Obey, T. M.; Vincent, B. Chem. Commun. 1988, 1189. (5) Simmons, M. R.; Chaloner, P. A.; Armes, S. P. Langmuir 1995, 11, 4222. (6) Wiersma, A. E.; vd Steeg, L. M. A. Europ. Pat. No. 589,529. (7) (i) Wiersma, A. E.; vd Steeg, L. M. A.; Jongeling, T. J. M. Synth. Met. 1995, 71, 2269.
film formation at room temperature. Such core-shell latexes are currently being developed by DSM as waterborne anticorrosion and antistatic coatings. Lascelles and co-workers recently modified the DSM protocol in order to synthesize PPy-coated PS latexes.8-10 These PS latexes were prepared via dispersion polymerization in alcoholic media using a poly(N-vinylpyrrolidone) (PNVP) stabilizer, had narrow size distributions, and were in the micrometer size range. The relatively large particle size and rigidity of this model substrate enabled the morphology of the PPy overlayers to be easily assessed by scanning electron microscopy (SEM). SEM studies of the PPy residues after solvent extraction of the PS component revealed a “broken egg-shell” morphology, which was interpreted as strong evidence for the core-shell morphology of the original PS-PNVP-PPy composite particles. One of these composites was examined in detail using X-ray photoelectron spectroscopy (XPS) by Perruchot et al.11 Its PPy loading was ca. 8.7%, which corresponds to a PPy overlayer thickness of approximately 20 nm. Since this overlayer thickness is much greater than the XPS sampling depth of 5 nm, the surface composition of this coated latex was found to be essentially identical to that of PPy bulk powder, with little or no evidence for the underlying PS latex. (8) Lascelles, S. F.; Armes, S. P. Adv. Mater. 1995, 7, 864. (9) Lascelles, S. F.; Armes, S. P. J. Mater. Chem. 1997, 7, 1339. (10) Lascelles, S. F.; Armes, S. P.; Zhdan, P.; Greaves, S.; Brown, A.; Watts, J. F.; Leadley, S.; Luk, S. Y. J. Mater. Chem. 1997, 7, 1349. (11) Perruchot, C.; Chehimi, M. M.; Delamar, M.; Lascelles, S. F.; Armes, S. P. Langmuir 1996, 12, 3245.
10.1021/la990443k CCC: $18.00 © 1999 American Chemical Society Published on Web 09/15/1999
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Langmuir, Vol. 15, No. 23, 1999
Cairns et al.
Recently we have synthesized much smaller submicrometer-sized PS latexes via emulsion polymerization.12 Methoxy-capped poly(ethylene glycol) (PEG) was postgrafted onto the latex surface to produce sterically stabilized particles. These high Tg, non-film-forming PS particles have narrow size distributions and were expected to be a “model” latex for the film-forming polyurethanePPy composite particles being developed by DSM Research. In the present work the XPS characterization of a series of these PS-PEG-PPy composites is described. Experimental Section Synthesis of PEG-Stabilized PS Latex. The submicrometer-sized PS-PEG latex was prepared as described in the preceding paper.12 PPy Deposition Experiments. The initial uncoated PEGstabilized PS latex is abbreviated to PS-PEG. The composites prepared by the deposition of PPy on the PS-PEG latex are abbreviated to PS-PEG-PPy(X), where X is the PPy mass loading. For example, the PS-PEG latex onto which 28.1% PPy was deposited is abbreviated to PS-PEG-PPy(28.1). The abbreviation “PS-PEG-PPyLSA” refers to a PS-PEG latex coated with PPy using both a low latex surface area and also a reduced pyrrole concentration. The PPy deposition experiments were carried out as descibed previously12 and the PPy loadings of the composites examined in this paper ranged from 4.2 to 28.1%. Thus, if it is assumed that these submicrometer-sized composite latexes have a “coreshell” morphology, the theoretical PPy overlayer thicknesses varied from 0.7 to 5.0 nm (these values were calculated using eq 2 in the preceding paper12), which is within the XPS sampling depth of 5 nm. Elemental Microanalyses. A small amount (
