5032
Langmuir 1998, 14, 5032-5038
Surface Characterization of Polyaniline-Coated Polystyrene Latexes Christelle Barthet,† Steven P. Armes,*,† Mohamed M. Chehimi,‡ Carole Bilem,‡ and Maria Omastova§ School of Chemistry, Physics and Environmental Science, University of Sussex, Falmer, Brighton BN1 9QJ, U.K., 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 Polymer Institute, Slovak Academy of Sciences, Dubravska cesta 9, 842 36 Bratislava, Slovak Republic Received January 26, 1998. In Final Form: June 1, 1998 The surface compositions of seven polyaniline (PAni)-coated polystyrene (PS) latexes have been investigated by X-ray photoelectron spectroscopy. This technique was used to assess the uniformity of the PAni overlayers deposited onto micrometer-sized, poly(N-vinylpyrrolidone) (PNVP)-stabilized PS latex particles under various synthesis conditions. Peak fitting of the N(1s) core-line spectra provided evidence for the presence of both PAni and the PNVP stabilizer at the surface of the PS latex. Nonuniform PAni coatings were obtained using conventional aniline polymerization conditions (aniline monomer, ammonium persulfate, 1.2 M HCl at 25 °C). In contrast, more homogeneous PAni coatings were obtained when polymerizing aniline hydrochloride at 0 °C in water. The relative proportion of PAni at the surface of the PS latex was estimated by comparing the surface nitrogen contents of the coated and uncoated PS latexes to that of a PAni “bulk powder” prepared in the absence of any latex. It was shown that the relatively rapid polymerization at room temperature resulted in nonuniform PAni coatings and reduced PAni surface composition. The maximum PAni coverage was found to be around 57-59%, which is much lower than the surface composition of 94-100% found for polypyrrole (PPy) deposited onto a similar micrometer-sized PS latex (Perruchot et al. Langmuir 1996, 12, 3245). These results indicate that the PAni coatings are much less uniform than the PPy overlayers. Finally, the improved uniformity of the PAni overlayers prepared using aniline hydrochoride in the absence of HCl is consistent with the higher coalescence temperature found for these PAni-coated PS particles in hot-stage optical microscopy studies.
Introduction In the last 10 years there has been increasing interest in using X-ray photoelectron spectroscopy (XPS) to characterize the surface of polymer latex particles. For example, Deslandes et al.1 have examined micrometersized polystyrene (PS) latexes stabilized with poly(Nvinylpyrrolidone) (PNVP). Similarly, a series of elegant studies on charge-stabilized, surfactant-stabilized, and sterically stabilized latex particles of submicrometer dimensions were reported by Davies and co-workers.2-4 In 1996 Beadle et al.5 reported an XPS study of the surface composition of sterically stabilized polypyrrole (PPy) particles. The stabilizer used was poly(potassium 3-sulfopropyl methacrylate), and it was shown that this anionic polyelectrolyte acted as a polymeric dopant anion for the cationic PPy chains. In addition, there was some evidence that this stabilizer was distributed throughout the interior of the PPy particles, rather than simply being confined to the latex surface as an adsorbed surface layer. * To whom correspondence should be addressed. † University of Sussex. ‡ Universite ´ Paris 7. § Slovak Academy of Sciences. (1) Deslandes, Y.; Mitchell, D. F.; Paine A. J. Langmuir 1993, 9, 1468. (2) 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. (3) 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. (4) 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. (5) Beadle, P. M.; Armes, S. P.; Greaves, S. J.; Watts, J. F. Langmuir 1996, 12, 1784.
Maeda et al.6 used XPS for the surface characterization of a series of conducting polymer-silica colloidal nanocomposites. A N(1s) signal was observed in the survey spectrum of each nanocomposite, which confirmed that the conducting polymer component was located at (or near) the surface. This observation is consistent with the relatively high pressed pellet conductivities found for these materials. However, the surface compositions of these conducting polymer-silica nanocomposites (as judged by their silicon to nitrogen XPS atomic ratios) were always significantly more silica-rich than their corresponding bulk compositions as determined by thermogravimetry. Recently, Lascelles and co-workers reported the synthesis of micrometer-sized PPy-coated PS latexes.7-9 Perruchot et al.10 examined the surface of one of these latexes (containing 8.7 wt % PPy, corresponding to an average overlayer thickness of ca. 20 nm) by XPS. It was found that the particle surface was highly polypyrrolerich, with little evidence for either the underlying PS component or the PNVP stabilizer. This is striking evidence for the highly uniform nature of the PPy overlayer. On the other hand, the XPS surface compositions of a series of submicrometer-sized PPy-coated PS latexes are invariably polystyrene-rich.11 While the possibility of inhomogeneous PPy overlayers cannot be (6) Maeda, S.; Gill, M.; Armes, S. P.; Fletcher, I. W. Langmuir 1995, 11, 1899. (7) Lascelles, S. F.; Armes, S. P. Adv. Mater. 1995, 7, 864. (8) Lascelles, S. F.; Armes, S. P. J. Mater. Chem. 1997, 7, 1339. (9) Lascelles, S. F.; Armes, S. P.; Zhdan, P. A.; Greaves, S. J.; Brown, A. M.; Watts, J. F., Leadley, S. R.; Luk, S. Y. J. Mater. Chem. 1997, 7, 1349. (10) Perruchot, C.; Chehimi, M. M.; Delamar, M.; Lascelles, S. F.; Armes, S. P. Langmuir 1996, 12, 3245.
S0743-7463(98)00102-4 CCC: $15.00 © 1998 American Chemical Society Published on Web 08/06/1998
Polyaniline-Coated Polystyrene Latexes
excluded for these smaller particles, this is most likely because the PPy overlayer thicknesses are less than or comparable to the XPS sampling depth of 2-5 nm. In a recent paper, we reported coating micrometer-sized PNVP-stabilized PS latexes with thin overlayers of polyaniline (PAni) to produce conductive “core-shell” particles.12 It was found that, compared to the PPy-coated PS latexes,7-9 the PAni-coated PS latexes exhibited an inhomogeneous, nonuniform morphology, as judged by scanning electron microscopy (SEM). However, more uniform deposition of PAni, and a corresponding improvement in colloid stability (reduced degree of flocculation), were obtained using the aniline hydrochloride monomer at a polymerization temperature of 0 °C in the absence of added acid. The objective of the present work was to use XPS and hot-stage optical microscopy in order to characterize the uniformity of the PAni overlayers deposited onto micrometer-sized PS latex particles under various synthesis conditions. It is shown that close inspection of the XPS spectra provides quantitative information on the homogeneity of the PAni coatings. Hot-stage optical microscopy studies of the coated and uncoated PS latexes confirmed the XPS results since more uniform PAni overlayers led to higher coalescence temperatures for the coated latexes. Experimental Section PS Latex Synthesis. The PS latex was prepared by dispersion polymerization according to Paine et al.13 The styrene monomer (Aldrich) was purified by passing through an activated neutral alumina column. The PNVP stabilizer (Fluka) (molecular weight 360 000, 42 g) was dissolved in 2-propanol (2.4 L) in a three-necked round-bottom flask fitted with a condenser and a magnetic flea. The reaction vessel was then heated to 70 °C under a nitrogen blanket and purged with nitrogen for 12 h at 70 °C to remove all traces of oxygen. A solution of azoisobutyronitrile initiator purchased from Aldrich (3.0 g) predissolved in styrene monomer (300 g) was added to the reaction vessel, with vigorous stirring. The styrene polymerization was allowed to proceed for 24 h before cooling to room temperature. The PS latex particles were then purified by repeated centrifugation and redispersion cycles, replacing successive supernatants with deionized water. Polyaniline Coating Protocol. Method 1. Aniline (Aldrich) was purified by vacuum distillation over zinc (or magnesium) and stored at -15 °C prior to use. The [NH4]2S2O8 oxidant was dissolved in a 1.2 M HCl solution containing the PNVP-stabilized PS latex particles (4 g dry weight) in a screw-cap bottle with magnetic stirring. Aniline was added via syringe, and the polymerization was allowed to proceed for 24 h at room temperature (25 °C). The initial oxidant/monomer molar ratio was held constant at 1.25. The PS latex concentration (and hence total surface area) was kept constant and the aniline monomer concentration was varied from 1.8 to 9.4 g L-1. The resulting green PAni-coated PS latexes were purified by repeated centrifugation/redispersion cycles (replacing successive supernatants with 1.2 M HCl), to remove any aniline oligomers and inorganic byproducts of the polymerization. In the reduced temperature experiments, the general procedure described for room-temperature syntheses was followed, except that the oxidant/latex reaction mixture was cooled to 0 °C in an ice bath for at least 60 min prior to addition of the aniline monomer. This temperature was maintained for the first 5 h of the polymerization, after which the reaction temperature was allowed to rise to room temperature. Method 2. Aniline hydrochloride monomer (Aldrich, 97%) was dissolved in an aqueous solution containing the PS latex (4 (11) Cairns, D. B.; Armes, S. P.; Chehimi, M. M.; Delamar, M. Manuscript in preparation. (12) Barthet, C.; Armes, S. P.; Lascelles, S. F.; Luk, S. Y.; Stanley, H. M. E. Langmuir 1998, 14, 2032. (13) Paine, A. J.; Luymes, W.; McNulty, J. Macromolecules 1990, 23, 3104.
Langmuir, Vol. 14, No. 18, 1998 5033 g dry weight). No external acid was added to this reaction solution and the monomer concentration was varied from 1.8 to 19.4 g L-1. The monomer/latex reaction mixture was cooled to 0 °C in an ice bath for at least 60 min prior to addition of the [NH4]2S2O8. The polymerization temperature was maintained at 0 °C for the first 5 h of the polymerization. The cleanup procedure was the same as that described in method 1. Characterization Techniques. Chemical Composition. CHN elemental microanalyses of both the dried coated and uncoated latexes were carried out at Medac Ltd. at Brunel University, U.K. The PAni loadings of the coated latexes were determined by comparing their nitrogen contents to that of the corresponding uncoated PS latex (this reference material had an average nitrogen content of ca. 0.20% due to the PNVP stabilizer) and to that of PAni “bulk powder” synthesized in the absence of latex particles (N ) 10.96% for PAni synthesized at room temperature and N ) 10.89% for polymerization at 0 °C). Conductivity Measurements. The conductivities of compressed pellets of the PAni-coated PS latexes were determined using the standard four-point probe technique at room temperature. It is estimated that the random errors associated with such measurements are approximately 10%, with an additional systematic error of ca. 5 to 10%. Disk Centrifuge Photosedimentometry (DCP). This method was used to obtain the weight-average particle size distribution of the coated and uncoated latexes. According to Yamamoto and co-workers,14 PNVP chains with a molecular weight of 360 000 have an adsorbed layer thickness of approximately 20-30 nm. The increase in particle size due to this thin steric stabilizer layer was considered to be negligible compared to the overall diameter of the PS particles (>1500 nm). Thus the effective particle density of the sterically stabilized PS latex was assumed to be that of pure polystyrene, i.e., 1.06 g cm-3. The centrifuge rate was 3000 rpm, and the PAni-coated latexes were assumed to have the same scattering characteristics as carbon black. Scanning Electron Microscopy (SEM). Morphology studies were carried out using a Leica Stereoscan 420 instrument operating at 20 kV with a probe current of 10 pA. The samples were mounted on a double-sided adhesive carbon disk and sputter-coated with a thin layer of gold to prevent samplecharging problems. X-ray Photoelectron Spectroscopy (XPS). XPS spectra 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. For uncoated PS latex charge referencing was determined by setting the main C(1s)C-C/C-H component at 285.0 eV. In the case of the PAni-coated PS latexes, the spectra were calibrated by setting the main N(1s) of amine nitrogen (-NH) in PAni at 399.5 ( 0.1 eV according to Neoh et al.15 and Kumar et al.16 The surface composition (in atomic %) of the various samples was determined by considering the integrated peak areas of C(1s), N(1s), O(1s), and Cl(2p) and their respective experimental sensitivity factors (determined using a large set of organic and inorganic compounds of well-defined stoichiometries). The following experimental sensitivity factors were used: 1 for C(1s), 1.6 for N(1s), 3.1 for O(1s) and 2.8 for Cl(2p). The fractional concentration of a particular element A, %A, was computed using:
%A ) (IA/sA)/
∑(I /s ) × 100 n
n
(1)
where In and sn are the integrated peak areas and the sensitivity factors, respectively. Digital acquisition was achieved with a Cybernetix system, and the data were collected using a PC. Homemade data processing software allowed smoothing, linear or Shirley background removal, static charge referencing, peak fitting, and quantification. We used a nonlinear least-squares analysis to (14) Liu, C. F.; Moon, D. K.; Maruyama, T.; Yamamoto, T. Polym. J. 1993, 25, 775. (15) Neoh, K. G.; Kang, E. T.; Tan, K. L. J. Phys. Chem. 1991, 95, 10151. (16) Kumar, S. N.; Gaillard, F.; Bouyssoux, G.; Sartre, A. Synth. Met. 1990, 36, 111.
5034 Langmuir, Vol. 14, No. 18, 1998
Barthet et al.
Table 1. Summary of the Particle Size, Conducting Polymer Loading, and Electrical Conductivity of the PAni-Coated PS Latexes Examined in This Study sample
monomer
solvent
reaction temp/°C
PS latex diametera/µm
PAni loadingb/ mass %
PAni layer thicknessc/nm
σd/S cm-1
PAni/0.5RTe PAni/5.4RTe PAni/1.0ZTe PAni/6.1ZTe PAni/0.8AnClf PAni/5.6AnClf PAni/11.2AnClf
aniline aniline aniline aniline aniline hydrochloride aniline hydrochloride aniline hydrochloride
HCl HCl HCl HCl water water water
25 25 0 0 0 0 0
1.6 1.6 1.6 1.6 1.8 1.8 1.8
0.5 5.4 1.0 6.1 0.8 5.6 11.2
0.9 11.3 2.1 13.4 1.9 13.0 27.3
