Surface characterization of poly(3,4-ethylenedioxythiophene)-coated latexes by X-ray photoelectron spectroscopy

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 markers - sulfur for the PEDOT and nitrogen for the PNVP, respectively. The line shape of the C Is 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 Is 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 PEDOT-coated, 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 mn. 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.