Surface-initiated atom transfer radical polymerization (ATRP) of various hydrophilic methacrylate monomers on submicrometer-sized silica particles in aqueous media at 20 °C leads to polymer-grafted silica particles whose colloid stability depends on the nature of the grafted polymer. These new organic−inorganic hybrid particles have been extensively characterized by thermogravimetry, elemental microanalyses, FT-IR spectroscopy, dynamic light scattering, scanning electron microscopy, and X-ray photoelectron spectroscopy. They are expected to be interesting model colloids for evaluating theories of steric stabilization.

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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.

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 surfactant-free synthesis of colloidal dispersions of vinyl polymer-silica nanocomposite particles in aqueous media using a batch emulsion polymerization protocol has been previously described [Percy, M. J.; et al. Langmuir 2000, 16, 6913]. In the present work 2-hydroxypropyl methacrylate [HPMA] was copolymerized with 4-vinylpyridine [4VP] using ammonium persulfate in the presence of an ultrafine silica sol. 4VP is used as an auxiliary in these syntheses; the strong interaction of this basic monomer with the acidic surface of the silica particles is essential for successful nanocomposite particle formation. HPMA monomer was selected since it has appreciable water solubility (up to 13% at 20°C), but HPMA homopolymer is water-insoluble. This unusual solubility behavior ensured that these nanocomposite syntheses were conducted under true dispersion polymerization conditions. In view of the success of these syntheses, we conclude that emulsion monomer droplets and micelles are not a prerequisite for the formation of nanocomposite particles. Both thermogravimetric analysis and elemental microanalyses were used to determine the silica contents of the nanocomposite particles, which ranged from 5% to 42% by mass and depended on the proportion of 4VP in the comonomer feed. Under the conditions investigated the minimum amount of 4VP auxiliary required was around 15%. Depending on the synthesis conditions, the mean particle diameter of the HPMA-4VP/SiO2 particles varied from 205 to 330 nm, as judged by disk centrifuge photosedimentometry. Scanning electron microscopy studies of selected nanocomposites indicated that discrete particles were obtained on freeze-drying, partial particle coalescence occurred on drying at ambient temperature, and complete particle coalescence occurred on annealing at 50°C. The transmittance of the annealed films was greater than 80% over the entire visible wavelength range, indicating a high degree of dispersion for the ultrafine silica sol within the film. Aqueous electrophoresis measurements combined with X-ray photoelectron spectroscopy studies indicated that the surface compositions of these HPMA-4VP/SiO 2 particles are silica-rich.

Contrary to 4,4'-dipyridinium (i.e., archetypal methyl viologen), which is reduced by two single-electron transfers (stepwise reduction), the 4,1'-dipyridinium isomer (so-called "head-to-tail" isomer) undergoes two electron transfers at apparently the same potential (single-step reduction). A combined theoretical and experimental study has been undertaken to establish that the latter electrochemical behavior, also observed for other polyarylpyridinium electrophores, is due to potential compression originating in a large structural rearrangement. Three series of branched expanded pyridiniums (EPs) were prepared: N-aryl-2,4,6-triphenylpyridiniums (Ar-TP), N-aryl-2,3,4,5,6-pentaphenylpyridiniums (Ar-XP), and N-aryl-3,5-dimethyl-2,4,6-triphenylpyridinium (Ar-DMTP). The intramolecular steric strain was tuned via N-pyridinio aryl group (Ar) phenyl (Ph), 4-pyridyl (Py), and 4-pyridylium (qPy) and their bulky 3,5-dimethyl counterparts, xylyl (Xy), lutidyl (Lu), and lutidylium (qLu), respectively. Ferrocenyl subunits as internal redox references were covalently appended to representative electrophores in order to count the electrons involved in EP-centered reduction processes. Depending on the steric constraint around the N-pyridinio site, the two-electron reduction is single-step (Ar = Ph, Py, qPy) or stepwise (Ar = Xy, Lu, qLu). This steric switching of the potential compression is accurately accounted for by ab initio modeling (Density Functional Theory, DFT) that proposes a mechanism for pyramidalization of the N(pyridinio) atom coupled with reduction. When the hybridization change of this atom is hindered (Ar = Xy, Lu, qLu), the first reduction is a one-electron process. Theory also reveals that the single-step two-electron reduction involves couples of redox isomers (electromers) displaying both the axial geometry of native EPs and the pyramidalized geometry of doubly reduced EPs. This picture is confirmed by a combined UV-vis-NIR spectroelectrochemical and time-dependent DFT study: comparison of in situ spectroelectrochemical data with the calculated electronic transitions makes it possible to both evidence the distortion and identify the predicted electromers, which play decisive roles in the electron-transfer mechanism. Last, this mechanism is further supported by in-depth analysis of the electronic structures of electrophores in their various reduction states (including electromeric forms).

This paper reports on the preparation of poly(methyl methacrylate) (PMMA), poly(n-butyl acrylate) (PBA), and polystyrene (PS) brushes at the surface of conducting materials that were modified by the electrochemical reduction of a brominated aryl diazonium salt BF4-, +N2-C6H4-CH(CH3)-Br (D1). The grafted organic species -C6H4-CH(CH3)-Br was found to be very effective in initiating atom transfer radical polymerization (ATRP) of vinyl monomers. This novel approach combining diazonium salts and ATRP allowed PMMA, PBA, and PS brushes to be grown from the surface of iron electrodes. The polymer films were characterized in terms of their chemical structure by infrared reflection absorption spectroscopy and X-ray photoelectron spectroscopy. Atomic force microscopy studies indicated that the polymer brushes are densely packed. Contact angle measurements of water drops on PS and PMMA brushes were 88.1 +/- 2.0 and 70.3 +/- 2.1 degrees, respectively, which is consistent with the published wettability data for the corresponding polymer sheets.