Micrometer-sized polystyrene (PS) particles coated with poly(3,4-ethylenedioxythiophene) (PEDOT), an air-stable organic conducting polymer, have been described previously (Langmuir 1999, 15, 3469). X-ray photoelectron spectroscopy (XPS) was used to examine the surface of these coated particles. For the first time, both the conducting polymer overlayer and the steric stabilizer, poly(N-vinylpyrrolidone) (PNVP), could be identified by unique elemental markerssulfur for the PEDOT and nitrogen for the PNVP, respectively. The line shape of the C 1s core-line spectra of a latex coated with a thin PEDOT overlayer (<15 nm) closely resembled that of the uncoated PS latex. However, as the PEDOT overlayer thickness was increased, the line shape came to resemble that of bulk PEDOT. The surface specificity of XPS indicated that the particle surface becomes PEDOT-rich as the conducting polymer loading is increased. Examination of the S 2p core-line spectra revealed that the PEDOT overlayers had relatively low doping levels, which is consistent with the relatively low pressed pellet conductivities. Surprisingly, a N 1s signal could be detected even at the highest PEDOT loadings, where the PNVP stabilizer would be expected to be totally obscured. This suggested that the PEDOT is deposited within the solvated PNVP stabilizer layer (which correlates well with the observed colloidal stability of the coated particles) and that the resulting PEDOT shell might be patchy. Quantification of the surface proportion of PEDOT by XPS confirmed this, with a maximum PEDOT coverage of only 75% being achieved at high loadings. XPS examination of a PEDOTcoated, PNVP-stabilized poly(4-bromostyrene) latex, which contains unique bromine, nitrogen, and sulfur markers for the latex core, steric stabilizer, and conducting polymer components, respectively, confirmed that the PEDOT overlayers do not completely encapsulate the latex core, because bromine is always detected even for PEDOT overlayer thicknesses of 20-25 nm. On the other hand, tetrahydrofuran extraction of the latex core revealed a "broken egg-shell" morphology for the remaining PEDOT overlayer residues, suggesting a reasonable degree of structural integrity.

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.

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