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Langmuir 1998, 14, 2032-2041
Synthesis and Characterization of Micrometer-Sized, Polyaniline-Coated Polystyrene Latexes Christelle Barthet,† Steven P. Armes,*,† Stuart F. Lascelles,† Shen Y. Luk,‡ and Heather M. E. Stanley† School of Chemistry, Physics and Environmental Science, University of Sussex, Falmer, Brighton, BN1 9QJ, U.K., and Analytical Division, Courtaulds Research, P.O. Box 111, 101 Lockhurst Lane, Coventry, CV6 5RS, U.K. Received September 25, 1997. In Final Form: January 21, 1998 Poly(N-vinylpyrrolidone)-stabilized polystyrene (PS) latexes have been coated with thin overlayers of polyaniline (PANi) to produce electrically conductive “core-shell” particles. In this work we focused on the morphology of the PANi overlayer, the colloid stability of the coated latexes, and electrical conductivity. PANi-coated particles exhibit a nonuniform morphology as observed by scanning electron microscopy (SEM), in comparison with the relatively smooth polypyrrole overlayers synthesized in a previous study (Lascelles, S. F. et al., J. Mater. Chem. 1997, 7, 1339 and 1349). Disk centrifuge photosedimentometry confirmed that the PANi-coated PS latexes were weakly flocculated. The underlying PS latex “core” was quantitatively removed by solvent extraction, and SEM observations of the PANi residues revealed a “broken egg shell” morphology. Vibrational bands due to the PANi component were more intense in both the FT-IR and Raman spectra of the PANi-coated PS latexes, which is also consistent with the “core-shell” morphology. No reduction in the surface roughness of the PANi overlayer was observed when the rate of the aniline polymerization was decreased or if the latex surface was pretreated with either a thin polypyrrole overlayer or a layer of adsorbed surfactant. Similarly, only rough PANi overlayers were observed when coating a sulfonated PS latex. However, a more uniform deposition of PANi and a corresponding improvement of colloid stability were obtained using aniline hydrochloride monomer in the absence of added acid.
Introduction In principle, the preparation of colloidal dispersions of conducting polymers can alleviate the poor processability characteristics associated with these materials.1 For example, sterically stabilized polypyrrole (PPy) colloids can be easily synthesized in aqueous media by chemically polymerizing pyrrole in the presence of a suitable watersoluble polymer such as methyl cellulose2 or poly(vinyl alcohol).3,4 Similarly, polyaniline (PANi) particles can be synthesized via dispersion polymerization using reactive polymeric stabilizers.5,6 Sterically stabilized PPy particles always exhibit a spherical morphology, but PANi dispersions can exhibit three distinct morphologies, rice-grains, needles, or spheres, depending on the choice of steric stabilizer and chemical oxidant.7 An alternative approach, reported by several research groups, involves coating preformed latexes with thin overlayers of conducting polymers to form “core-shell” particles. If the conducting polymer overlayer is continuous, this can lead to relatively high conductivities even at very low conducting polymer loadings. Moreover, the latex † ‡
University of Sussex. Courtaulds Research.
(1) See, for example: Proc. 1992 Int. Conf. Synthetic Metals (ICSM ′92) Synth. Met. 1993, 55-57. (2) Bjorklund, R. B.; Liedberg, B. J. Chem. Soc., Chem. Commun. 1986, 1293. (3) Armes, S. P.; Vincent, B. J. Chem. Soc., Chem. Commun. 1987, 288. (4) Armes, S. P.; Miller, J. F., Vincent, B. J. Colloid Interface Sci. 1987, 118, 410. (5) Armes S. P. In Handbook of Conducting Polymers; Elsenbaumer, R., Ed.; M. Dekker: New York, 1997; Vol. II, in press. (6) DeArmitt, C.; Armes, S. P. J. Colloid Interface Sci. 1992, 150, 134. (7) Armes S. P.; Aldissi M.; Agnew S.; Gottesfeld S. Langmuir 1990, 6, 1745.
particles can be made over a much wider size range compared to the rather limited size range achieved for sterically stabilized conducting polymer colloids (30-300 nm).3,4,8 In 1987 Garnier and co-workers9 reported that sulfonated and carboxylated polystyrene latexes of ca. 130 nm in diameter could be coated with PPy overlayers using FeCl3. Similarly, Liu et al.10 used a H2O2/Fe3+ catalytic oxidant11 to coat 200 nm diameter styrene-butadienemethacrylate latex particles with PPy. These latter workers achieved PPy contents of up to 35 wt % with apparently no colloidal destabilization. This observation is rather surprising in view of the high Hamaker constant for PPy reported by Vincent’s group.12 However, no convincing experimental evidence (e.g., light scattering or disk centrifuge photosedimentometry) was cited by Liu et al. to justify their colloid stability claims. Beadle et al.13 prepared a range of PANi-chlorinated latex composites and showed that the PANi loading could be easily controlled by varying the latex concentration in the syntheses. In this latter study all the composites were obtained as macroscopic precipitates, as expected. More recently, Xie et al.14 described the synthesis of colloidal composites of PANi with poly(butadiene-co-styrene-co-2(8) (a) Cawdery, N.; Obey, T. M., Vincent, B. J. Chem. Soc., Chem. Commun. 1988, 1189. (b) Armes, S. P.; Aldissi, M.; Idzorek, G. C.; Keaton, P. W.; Rowton, L. J.; Stradling, G. L.; Collopy, M. T.; McColl, D. B. J. Colloid Interface Sci. 1991, 141, 119. (9) Yasser, A.; Roncali, J.; Garnier, F. Polym. Commun. 1987, 28, 103. (10) Liu, C. F.; Maruyama, T.; Yamamoto, T. Polym. J. 1993, 25, 363. (11) Moon, D. K.; Osakada, K.; Maruyama, T.; Yamamoto, T. Makromol. Chem. 1992, 193, 829. (12) Markham, G.; Obey, T. M.; Vincent, B. Colloids Surf. 1990, 51, 239. (13) Beadle, P.; Armes, S. P.; Gottesfeld, S.; Mombourquette, C.; Houlton, R.; Andrews, W. D.; Agnew, S. F. Macromolecules 1992, 25, 2526.
S0743-7463(97)01064-0 CCC: $15.00 © 1998 American Chemical Society Published on Web 03/20/1998
Polyaniline-Coated Polystyrene Latexes
vinylpyridine) latex. Again, a high degree of dispersion was claimed for the resulting composite particles, but in this case it is likely that the aniline polymerization occurs inside the acid-swollen latex. This possibility is admitted by the authors and is certainly consistent with the “classical” conductivity percolation threshold behavior observed for their latex composites; PANi loadings in excess of 16 wt % were required for high conductivities. Noncolloidal (macroscopic) polymeric substrates have also been coated with PPy. For example, Yoshino and co-workers15 deposited PPy onto 10 µm polyethylene particles and Omastova et al.16 coated 35 µm polypropylene particles with PPy (the latter workers also coated 100 nm poly(methyl methacrylate) latex particles with PPy in the same study and obtained only macroscopic precipitates). The Japanese group15 reported conductivities as high as 0.2 S cm-1 for PPy loadings as low as 1 wt %. Wiersma et al. at DSM Research recently demonstrated17 that sterically stabilized latexes can be coated with either PPy or PANi in aqueous media to form composite particles with good colloid stability. In these syntheses the presence of a nonionic steric stabilizer, such as chemically grafted poly(ethylene oxide) or hydroxymethyl cellulose, was reported to be critical for producing stable colloidal dispersions. It was suggested that the conducting polymer was formed as an ultrathin layer at the surface of the latex particles, without significantly interfering with the steric stabilization mechanism conferred by the solvated layer of polymeric stabilizer. The DSM group utilized several low Tg, film-forming colloidal substrates such as polyurethane latexes or alkyd resins. Particle diameters were in the 50-500 nm range, and the commercial potential of these water-borne antistatic or anticorrosion coatings (tradename ConQuest) is currently being explored.18 Lascelles and co-workers19,20 adapted this DSM protocol in order to coat micrometer-sized poly(N-vinylpyrrolidone) (PVP)-stabilized polystyrene latexes with PPy. Polystyrene (PS) was selected as a model colloidal substrate for these studies since it has a relatively high Tg of ca. 100 °C (i.e., the particles are rigid and nondeformable) and PVP-stabilized PS latexes can be easily synthesized with narrow size distributions via dispersion polymerization.21-24 Conductivity measurements on the dried PSPPy latexes indicated unexpectedly low percolation threshold of 1-2 vol % similar to those reported by Yoshino et al.15 Moreover, scanning electron microscopy studies confirmed that, at PPy loadings of less than 10 wt %, the conducting polymer overlayer had a relatively smooth and featureless morphology. Subsequent X-ray photoelectron spectroscopy (XPS) studies by Perruchot et al.25 confirmed (14) Xie, H. Q.; Liu, H.; Liu, Z. H.; Guo, J. S. Angew. Makromol. Chem. 1996, 243, 117. (15) Yoshino, K.; Yin, X. H.; Morita, S.; Nakayashi, Y.; Nakagawa, S.; Watanuki, T.; Isla, I. Jpn. J. Appl. Phys. 1993, 32, 979. (16) Osmatova, M.; Pionteck, J.; Kosina, S. Eur. Polym. J. 1996, 32, 681. (17) Wiersma A. E.; vd Steeg L. M. A. Eur. Pat. No 589, 529. (18) Wiersma, A. E.; Van Der Steeg, L. M. A.; Jongeling, T. J. M. Synth. Met. 1995, 71, 2269. (19) Lascelles, S. F.; Armes, S. P. Adv. Mater. 1995, 7, 864. (20) (a) Lascelles, S. F.; Armes, S. P. J. Mater. Chem. 1997, 7, 1339. (b) 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. (21) Goodall, A. R.; Wilkinson, M. C.; Hearn, J. J. Polym. Sci., Polym. Chem. Ed. 1977, 15, 2193. (22) Bromley, C. W. A. Colloids Surf. 1986, 17, 1. (23) Ober, C. K.; Lok, K. P.; Hair, M. L. J. Polym. Sci., Polym. Lett. 1985, 23, 103. (24) Paine, A. J.; Luymes, W.; McNulty, J. Macromolecules 1990, 23, 3104.
Langmuir, Vol. 14, No. 8, 1998 2033 Table 1. Reaction Conditions for Aniline Polymerizationsa polymerization temperature (°C) pH
monomer aniline (method 1) aniline hydrochloride (method 2)
0 25 0 0
0.7 0.7 0.3 2-4
oxidant
oxidant/ monomer ratio
[NH4]2S2O8 [NH4]2S2O8 NH4VO3 [NH4]2S2O8
1.25 1.25 2 1.25
a The latex solids concentration was 17 wt % and the total solution volume was 20 mL.
that, for a relatively thick PPy overlayer (ca. 18 nm), there was very little evidence for the underlying PS latex. This observation is consistent with a very homogeneous PPy overlayer and also reflects the highly surface-specific nature of the XPS technique, which has a sampling depth of 2-5 nm. This technique has proved particularly useful in assessing the surface oxidation and degradation chemistry of conducting polymers.26 The uniform morphology of the PPy coating was also supported by a Raman study,20b in which only Raman bands due to the PPy component were observed, even at the lowest PPy loadings. This was assumed to be due to remarkably efficient attenuation of the Raman bands due to the PS component by the highly absorbing PPy overlayer. In the present work a range of micrometer-sized, PANicoated PS latexes have been synthesized and extensively characterized. Particular attention is given to the uniformity of the PANi overlayer, the colloidal stability of the coated latex, and solid-state conductivities, using FTIR and Raman spectroscopy, disk centrifuge photosedimentometry, and scanning electron microscopy. In addition, various synthetic strategies were explored in order to try to achieve PANi overlayers of similar uniformity to that observed for PPy coatings. Experimental Section PS Latex Synthesis. The PS latex was prepared by dispersion polymerization according to Paine et al.24 The styrene monomer was purified by passing through an activated neutral aluminum oxide column. The PVP stabilizer (molecular weight 360 000; 42 g) was dissolved in 2-propanol (2.4 L) in a threenecked 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 (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. The conditions used for the polymerization of aniline are summarized in Table 1. Method 1. Aniline (Aldrich) was purified by vacuum distillation over zinc (or magnesium) and stored at -15 °C prior to use. Either [NH4]2S2O8 or NH4VO3 were dissolved in the PVPstabilized PS latex (4 g of dry weight) in a screw-cap bottle with magnetic stirring. The latex was acidified to pH 0.7 and 0.3 for [NH4]2S2O8 and NH4VO3, respectively, and the initial oxidant/ monomer molar ratios were fixed at 1.25 and 2. Aniline was added via syringe, and the polymerization was allowed to proceed for 24 h at room temperature. The PS latex concentration (and hence total surface area) was kept constant, and the aniline monomer was varied from 2 to 60 g L-1. The resulting green (25) Perruchot, C.; Chehimi, M. M.; Delamar, M.; Lascelles, S. F.; Armes, S. P. Langmuir 1996, 12, 3245. (26) Inganas, O.; Erlandsson, R.; Nylander, C.; Lundstrom, I. J. Phys. Chem. Solids 1984, 45, 427.
2034 Langmuir, Vol. 14, No. 8, 1998 PANi-coated PS latexes were purified by repeated centrifugation/ redispersion cycles (replacing successive supernatants with 1.2 M HCl), to remove the oligomers and the inorganic byproducts of the aniline 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 g of dry weight). No external acid was added to this reaction solution. 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, and 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. Two further experiments were also carried out, using the same protocol, but in the presence of (i) 2 × 10-3 M hydroquinone and (ii) 9.5 × 10-3 M sodium dodecyl sulfate (SDS) (the critical micelle concentration (cmc) of SDS is ca. 6 × 10-3 M). The polymerization retarder, hydroquinone,27 was added to the precooled aqueous solution containing the PS latex and the aniline hydrochloride monomer, prior to the addition of the [NH4]2S2O8. The SDS was equilibrated for 12 h with the aqueous latex to allow adsorption onto the surface of the particles before the addition of the aniline hydrochloride monomer, followed by the oxidant. Polypyrrole Coating.19,20 FeCl3‚6H2O oxidant (FeCl3/pyrrole molar ratio ) 2.33) was dissolved into a stirred aqueous solution containing the PS latex (5.25 g dry weight) in a screw-cap bottle. Pyrrole was added via syringe, and the polymerization was allowed to proceed for 24 h. The resulting PPy-coated PS latex particles were purified by repeated centrifugation/redispersion cycles, replacing successive supernatants with deionized water. Synthesis of the Sulfonated PS Latex. The PVP stabilizer (molecular weight 360 000; 3.80 g) was dissolved in methanol (200 mL) 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 (0.33 g) predissolved in purified styrene (31.25 g) was added to the reaction vessel, with vigorous stirring. Sodium 4-styrenesulfonate (NaSS; 1.56 g) dissolved in 50 mL of methanol was immediately added to the previous mixture. Methanol was used as the synthesis medium instead of 2-propanol because of its higher dielectric constant. This made dissolution of the NaSS easier and might be expected to increase the fraction of sulfonate groups at the surface of the PS particles. The copolymerization of styrene and NaSS was allowed to proceed for 24 h before cooling to room temperature. The latex was then purified by repeated centrifugation and re-dispersion cycles, replacing successive supernatants with deionized water. Its polydispersity (Dw/Dn) was found by disk centrifuge photosedimentometry (DCP) to be 1.09. The modified latex was then coated with PANi according to method 2. 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 0.19-0.22% due to the PVP stabilizer) and to that of PANi “bulk powder” synthesized in the absence of latex particles (average nitrogen content of 10.95%). In the case of the PPy-PANi bilayer, the PANi content was calculated by comparing the nitrogen contents of the PPy-PANi-coated latex, the PPy-coated latex prior to aniline polymerization, and PANi “bulk powder”. Conductivity Measurements. The conductivities of compressed pellets of the PANi-coated PS latexes were determined using standard four-point probe techniques at room temperature. (27) Wei, Y.; Jang, G. W.; Chan, C. C.; Hsueh, K. F.; Hariharan, R.; Patel, S. A.; Whitecar, C. K. J. Phys. Chem. 1990, 94, 7716.
Barthet et al. It is estimated that the random error associated with such measurements is approximately 10%, with a 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,28 PVP chains with a molecular weight of 360 000 have an adsorbed layer thickness of approximately 20-30 nm. Thus the increase in particle size due to this steric stabilizer layer is negligible in comparison to the overall diameter of the PS particles (>1.5 µm). The density of the sterically stabilized PS latex was therefore assumed to be that of pure polystyrene, i.e., 1.05 g cm-3. The centrifuge rate was 3000 rpm and PANi-coated latexes were assumed to have the same scattering characteristics as carbon black. Zeta Potential Measurements. Zeta potential vs pH curves were measured using a Malvern Instruments Zetamaster apparatus. Zeta potentials,29 ζ, were calculated from electrophoretic mobilities (u) using the Smoluchowsky relationship, ζ ) ηu/, where it is assumed that κa . 1 (with η and the viscosity and dielectric constant of the medium and κ and a the Debye-Hu¨ckel parameter and the radius of the particles, respectively). The solution pH was adjusted by the addition of HCl. Fourier Transform Infrared (FT-IR) Spectroscopy. FTIR spectra (KBr disk) of PANi “bulk powder” and both coated and uncoated PANi-PS latexes were recorded using a Nicolet Magna 550 Series II instrument. Spectra were typically averaged over 16 scans at 4 cm-1 resolution. 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. Raman Spectroscopy. Raman spectra were recorded using a Bruker FRA 106 spectrometer. Excitation was provided by an Adlas Nd:YAG laser at a wavelength of 1064 nm, operating at 30 mW for the PANi-containing samples and 300 mW for the uncoated PS latex. Data were acquired at a resolution of 4 cm-1, and spectra were averaged over 2000 scans. Solvent Extraction Experiments. Excess THF (10 mL) was added at room temperature to ca. 50 mg of the dried PANicoated PS latexes. This solution was left overnight. The resulting black residues were filtered, washed with THF, and dried in an oven at 60 °C. The morphology of the PANi residues was examined by SEM. FT-IR spectroscopy was used to confirm the loss of PS.
Results and Discussion The scanning electron micrographs in Figure 1 correspond to thin overlayers of PPy and PANi on PS latex at conducting polymer mass loadings of 10 and 8 wt %, respectively. The PANi overlayer was deposited onto a 1.8 µm PS latex according to method 1. The conditions used for the PPy coating were those described by Armes and co-workers.19,20 The morphology of the PANi overlayer is clearly markedly rougher than that of the PPy. Similar effects have been reported by Armes et al. for the deposition of PPy and PANi onto quartz fibers.30 The thickness of the PPy and PANi overlayers can be calculated using the following equation
(x( 3
δ)R
) )
M2F1 +1 -1 M1F2
(1)
where δ is the conducting polymer overlayer thickness, R is the radius of the uncoated latex particles, M1 and F1 are (28) Liu, C. F.; Moon, D. K.; Maruyama, T.; Yamamoto, T. Polym. J. 1993, 25, 775. (29) Hunter, R. J. Foundations of Colloid Science, Vol. I, 557. (30) Armes, S. P.; Gottesfeld, S.; Beery, J. G.; Garzon, F.; Mombourquette, C.; Hawley, M.; Kuhn, H. H. J. Mater. Chem. 1991, 1, 525.
Polyaniline-Coated Polystyrene Latexes
Langmuir, Vol. 14, No. 8, 1998 2035
Figure 2. Variation of conductivity with conducting polymer loading for PPy-coated latexes ([) (data taken from ref 19), PANi-coated latexes synthesized using method 1 at 25 °C (O) and at 0 °C (0) and PANi-coated latexes prepared using method 2 at 0 °C (3).
Figure 1. Scanning electron micrographs showing (a) a PPycoated (10 wt %) PS latex and (b) a PANi-coated (8 wt %) PS latex. The PS latexes were 1.6 µm diameter for PPy coating and 1.8 µm diameter for PANi coating. The oxidants used were FeCl3 and (NH4)2S2O8, respectively, for pyrrole and aniline polymerizations. Deposition of both conducting polymers was carried out at room temperature. Scale bar ) 2 µm.
the mass fraction and density of the PS component, and M2 and F2 are the mass fraction and density of the conducting polymer component. The densities of the PS latex and PPy and PANi bulk powders were found to be 1.05, 1.46, and 1.40 g cm-3, respectively, by helium pycnometry. According to eq 1, the PPy and PANi overlayers thicknesses for the coated latexes depicted in Figure 1 are close to 21 and 20 nm, respectively. The calculated PPy thickness is considered to be accurate given the uniformity of this overlayer. In contrast, the nonuniform nature of the PANi overlayer suggests that its calculated thickness of 20 nm is likely to be a lower limit value. DCP analyses of the uncoated PS latexes used in this study showed them to be reasonably monodisperse. In contrast, the presence of multiplets in the DCP traces of PANi-coated latexes (see later) indicates that the inhomogeneous PANi overlayer leads to a similar degree
of flocculation as the PPy coating for these sterically stabilized particles. Ultrasonic treatment reduces aggregation, but the multiplets are not completely removed. Effect of Lowering the Polymerization Temperature. Stejskal et al.31 reported that the polymerization temperature affected the morphology of colloidal PANisilica particles. These nanocomposites had an irregular shape when polymerization proceeded at 25 °C, whereas particles prepared at 0 °C were more spherical. This may be due to the reduced rate of aniline polymerization at this temperature. Although we were coating micrometersized PS latex (rather than ultrafine silica sols), it was nevertheless envisaged that a reduced polymerization temperature might also lead to a more controlled polymerization. A 1.66 µm PS latex was selected for our coating experiments at 0 °C and at room temperature. The conductivities of PANi-coated PS latexes prepared at room temperature and 0 °C are compared in Figure 2. The conductivities of the PANi-coated latexes are also compared to those of PPy-coated PS latexes prepared at room temperature.20a The somewhat lower conductivities of the PANi-coated PS latexes prepared at 0 °C compared to those obtained for PPy-coated latexes synthesized at 25 °C suggest a fundamental difference in either the quality (i.e., average conjugation length and number of charge carriers) or morphology (overlayer uniformity) between the PANi and the PPy overlayers. However, since the conductivities of the PPy and PANi “bulk powders” are similar, it is most likely that the decrease in conductivities is due to a different morphology of the PANi overlayer. Moreover, the conductivities of the PANi-coated latexes were not improved by decreasing the polymerization temperature. Subsequent SEM studies of the PANi-coated latexes indicated that lowering the polymerization temperature had essentially no effect on the morphology of the PANi overlayer. Moreover, the DCP traces of PANi-coated latexes prepared at 0 and 25 °C are (31) Stejskal, J.; Kratochvil, P.; Armes, S. P.; Lascelles, S. F.; Riede, A.; Helmstedt, M.; Prokes, J.; Krivka, I. Macromolecules 1996, 29, 6814.
2036 Langmuir, Vol. 14, No. 8, 1998
Figure 3. Polymerization yields obtained for PANi syntheses performed at room temperature ([) and at 0 °C (4). The yields were calculated assuming that the PANi deposited onto the PS has a doping level of 50% (i.e., one chloride ion for every two aniline repeat units).
reasonably similar for a given PANi content. This suggests a similar degree of flocculation in each case. It is noteworthy from Figure 3 that much higher polymerization yields were obtained at 0 °C. The theoretical PANi contents were calculated assuming a doping level of 50% (one chloride anion for every two aniline residues). The reduced yields obtained for PANi contents lower than 5 wt % may be explained by the low reagent concentrations in the reaction medium (and hence slower polymerization). Reduced polymerization yields and conductivities were obtained at room temperature for PANi loadings higher than 17 vol %. This is probably attributable to the temperature rises caused by the exothermic polymerization (up to 20-30 °C for the highest PANi loadings). Boara et al.32 postulated that, under similar conditions, residual oxidant may cause overoxidation of the PANi chains, forming soluble byproducts. Shen et al.33 reported the use of solvent extraction to examine the particle morphology of poly(methyl methacrylate)/PS “core-shell” latexes. Similarly, Lascelles and co-workers20b treated a dried PPy-coated PS latex (6.5 wt % PPy loading, corresponding to an overlayer thickness of 13 nm) with THF to remove the non-cross-linked underlying PS core. Examination of the PPy residues by SEM revealed a “broken egg shell” morphology which confirmed the “core-shell” morphology of the original PPycoated PS latex. Similar extraction experiments were carried out on the PANi-coated PS latex. For relatively high PANi loadings (11 wt %, 27 nm thickness), SEM studies of the PANi residues after THF extraction also revealed a “broken egg shell” morphology (see Figure 4), with the “egg shell” diameter corresponding to that of the original coated particles. The FT-IR spectra of the residues after extraction showed no evidence for the presence of PS. At lower PANi loadings (6 wt %, 13 nm) rather less distinct, collapsed egg shells were observed. These observations are consistent with the less uniform overlayer morphology observed for PANi in Figure 1. Nevertheless, (32) Boara, G.; Sparpaglione, M. Synth. Met. 1995, 72, 135. (33) Shen, S.; El-Aasser, M. S.; Dimonie, V. L.; Vanderhoff, J. W.; Sudol, E. D. J. Polym. Chem., Part A: Polym. Chem. 1991, 29, 857.
Barthet et al.
these solvent extraction experiments confirm that at higher loadings PANi coats the PS latex in a continuous or near-continuous overlayer. The FT-IR spectrum of a PANi-coated PS latex (PANi loading 8.8 wt %) was compared to those of three heterogeneous admixtures of uncoated PS latex with PANi “bulk powder” containing 5, 10, and 15 wt % PANi, respectively. (We only show in Figure 5 the spectrum corresponding to the 15 wt % PANi.) FT-IR spectra of PANi “bulk powder” and uncoated PS latex reference materials were also recorded to aid assignment the characteristic peaks for the PANi and PS components (Figure 5). For the PANi-coated latex (see spectrum c), the main peaks due to the PS component are at 1492, 1455, 762, and 695 cm-1. Three strong absorption bands attributed to PANi are observed at 1295, 1238, and 1140 cm-1. Even allowing for PANi being a strong IR absorber, these absorption bands seem relatively intense for a PANi loading of only 8.8 wt %. On the other hand, the FT-IR spectra of the admixtures only show very weak PANi bands, even for a PANi content of 15 wt % (see spectrum d in Figure 5). This observation suggests that the “coreshell” morphology of the PANi-coated PS latex leads to a significant enhancement of the IR absorption of the conducting polymer component. Similar results were obtained by Lascelles and co-workers for PPy-coated latexes.20a Lascelles et al.20b characterized various PPy-coated PS latexes by Raman spectroscopy. Similar Raman studies were carried out on the PANi-coated PS latexes. The Raman spectra for a PANi “bulk powder”, a heterogeneous admixture of PANi/PS latex (10 wt % PANi loading), and uncoated and PANi-coated PS latexes are depicted in Figures 6 and 7. A very strong Raman band at 1003 cm-1, due to the ν1 ring-breathing mode of the PS component, is clearly visible in the admixture spectrum (see Figure 6). However, this PS band is absent in the spectrum of the PANi-coated PS latex (PANi thickness, 23 nm), despite this composite containing nearly 90% PS latex by mass. In fact, the spectrum of this “core-shell” latex is essentially the same as that recorded for PANi bulk powder. It seems that, as with the PPy-coated PS particles,20b the Raman signals due to the PS core are completely attenuated by the PANi overlayer at this loading. Raman spectra of PANi-coated PS latexes were also recorded at lower PANi loadings (see Figure 7). The 1003 cm-1 band is present as a very weak feature in the spectra of the PANi-coated PS latexes containing between 0.5 and 4.5 wt % PANi (1-9 nm PANi overlayer). These observations on PANicoated latexes are qualitatively different from those reported for PPy-coated PS latexes, for which uniform PPy loadings as low as 1 wt % (ca. 2 nm) were sufficient to completely attenuate the Raman signal arising from the PS core of the particles.20b We interpret this as evidence for the inhomogeneous nature of the PANi overlayer. In summary, the PANi overlayers have considerable surface roughness and appear to be discontinuous at low loadings (
