Surface characterization of micrometre-sized, polypyrrole-coated polystyrene latexes: verification of a ‘core–shell’ morphology† Stuart F. Lascelles,a Steven P. Armes,*a Peter A. Zhdan,b Stephen J. Greaves,b Andrew M. Brown,b John F.Watts,b Stuart R. Leadleyc and Shen Y. Lukc aSchool of Chemistry, Physics and Environmental Science, University of Sussex, Falmer, Brighton, UK BN1 9QJ bDept. of Materials Science and Engineering, University of Surrey, Guildford, Surrey, UK GU2 5XH cAnalytical Divison, Courtaulds Research, P.O.Box 111, 101 L ockhurst L ane, Coventry, UK CV 6 5RS Micrometre-sized, polypyrrole-coated polystyrene latexes with various conducting polymer loadings have been extensively characterized using X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectroscopy (TOF-SIMS), Raman and UV–VIS reflectance spectroscopy, scanning force microscopy (SFM) and scanning electron microscopy (SEM). Both XPS and TOF-SIMS studies are consistent with relatively uniform, chloride-doped polypyrrole overlayers.Raman studies also indicated a ‘core–shell’ morphology since only bands attributable to polypyrrole were observed; no evidence was found for the underlying polystyrene component even at the lowest polypyrrole loadings.This is most likely due to remarkably ecient attenuation of the polystyrene bands by the highly absorbing polypyrrole overlayer. UV–VIS reflectance spectroscopy studies confirmed that a coated latex had a much lower reflectance (higher absorbance) than a heterogeneous admixture of polypyrrole and polystyrene with a similar polypyrrole content.High-resolution images of the polypyrrole overlayer nanomorphology were obtained using SFM. At low polypyrrole loadings (1.0 mass%) the overlayer was relatively smooth and uniform, but higher loadings (8.9 mass%) resulted in a rougher, more globular morphology. Finally, the underlying polystyrene latex ‘core’ was quantitatively removed by solvent extraction.SEM studies of the polypyrrole residues revealed a ‘broken egg-shell’ morphology, thus providing irrefutable evidence for the ‘core–shell’ morphology of the original polystyrene/polypyrrole particles. Recently we reported1,2 coating micrometre-sized, sterically stabilized polystyrene latex particles with ultrathin overlayers of polypyrrole (Fig. 1). Polystyrene was selected as a ‘model’ colloidal substrate since it has a relatively high Tg (i.e. the particles are rigid and non-deformable) and latexes can be readily synthesized with narrow size distributions over a wide particle size range (50 nm–10 mm).3,4 Variation of the polystyrene particle concentration (and hence total surface area of latex) at fixed pyrrole polymerization conditions proved to be a very eective method for controlling the conducting polymer loading on the latex particles.Scanning electron microscopy (SEM) studies confirmed that, at polypyrrole mass loadings of <10%, the conducting polymer overlayer had a relatively smooth and featureless morphology. FTIR spectroscopy studies on the dried composite particles suggested that the absorption bands due to the polypyrrole overlayer component were Fig. 1 Schematic representation of an isolated, micrometre-sized, poly- significantly enhanced compared to those of heterogeneous pyrrole-coated polystyrene latex particle mixtures of polypyrrole ‘bulk powder’ and polystyrene latex. Similarly, conductivity measurements on pressed pellets of the thickly coated latex (comprising 8.7 mass% polypyrrole) was dried, coated latexes indicated an anomalously low conduc- predominantly polypyrrole-rich, with surprisingly little evi- tivity percolation threshold.Both spectroscopic and conduc- dence for either the underlying polystyrene component or the tivity data were consistent with a ‘core–shell’ latex morphology. poly(N-vinylpyrrolidone) stabilizer.It was concluded that the Potential applications for these micrometre-sized polypyrrole- polypyrrole overlayer on the polystyrene latex particles was coated polystyrene composite latexes include new stationary relatively uniform, rather than ‘patchy’. phases for electrochromatography5 or possibly novel ‘marker’ In the present work we describe a detailed study of particles for visual agglutination diagnostic assays.6 In the surface composition and nanomorphology of these addition, polypyrrole-coated polystyrene latexes of submicro- micrometre-sized, polypyrrole-coated polystyrene latexes metre dimensions have recently proved7 to be a useful model using six experimental techniques, namely XPS, time-of- system for understanding the behaviour of polypyrrole-coated flight secondary ion mass spectroscopy (TOF-SIMS), film-forming latexes such as those currently being developed8 Raman spectroscopy, UV–VIS reflectance spectroscopy, by DSM Research for antistatic and anticorrosion applications.scanning force microscopy (SFM) and SEM. Recently, in collaboration with Chehimi and co-workers, one of our micrometre-sized polypyrrole-coated polystyrene latexes was extensively examined by X-ray photoelectron spec- Experimental troscopy (XPS).9 This study confirmed that the surface of this Polystyrene latex synthesis and polypyrrole coating protocol The general latex synthesis procedure is outlined in the preced- † British Crown Copyright 1996/DERA.Published with the permission of the controller of Her Brittannic Majesty’s Stationery Oce.ing paper.2 The two latexes were sized by disk centrifuge J. Mater. Chem., 1997, 7(8), 1349–1355 1349photosedimentometry (DCP) using a Brookhaven BI-DCP preparation was achieved in a similar manner to that used for the TOF-SIMS experiments. instrument, operating in the line start mode as described previously.2 DCP studies confirmed that the two latexes had narrow size distributions and were of very similar size: the Raman spectroscopy. Raman spectra were recorded using a Bruker FRA 106 spectrometer.Excitation was provided by an mass-average particle diameters were 1.57±0.12 and 1.80±0.06 mm, respectively. Throughout this paper these lat- Adlas Nd5YAG laser at a wavelength of 1064 nm, operating at 30 mW for the polypyrrole-containing samples and 300 mW exes are referred to as the ‘1.6 mm’ and the ‘1.8 mm’ latexes. These DCP sizes were confirmed by transmission electron for the uncoated polystyrene latex.Data were acquired at a resolution of 4 cm-1 and spectra were averaged over 2000 microscopy (TEM) (Hitachi 7100 instrument), and SEM (Leica Stereoscan 420 instrument) on gold-coated dried latexes. A scans.portion of each sample was oven-dried at 60 °C overnight prior to CHN elemental microanalyses at an independent UV–VIS reflectance spectroscopy. A diuse reflectance accessory (PE 60 mm integrating sphere) was used in conjunc- laboratory (Medac Ltd at Brunel University, UK). The general polypyrrole coating protocol is also outlined in the preceding tion with a Lambda 9 Perkin-Elmer UV–VIS spectrophotometer. The dried powders were mounted onto double-sided paper.2 adhesive tape. Control experiments on uncoated, micrometresized polystyrene latex indicated ca. 100% reflectance for this Solvent extraction experiments system in the range 350–800 nm, and also confirmed that there Excess THF (20 ml) was added to ca. 100 mg of a dried was no significant contribution to the UV–VIS spectrum from polypyrrole-coated polystyrene latex (1.6 mm polystyrene latex; the underlying adhesive tape.Single-scan spectra were recorded 6.5% polypyrrole loading by mass) at room temperature and at a scan speed of 60 nm min-1. Reflectance measurements this solution was left to stand overnight. The resulting black were performed against a barium sulfate standard, which had residues were filtered, washed with THF and dried in an oven been previously referenced to an NPL-calibrated opal overnight at 60°C.The residues yield was consistent with the standard. mass loss expected for the polystyrene component. The chemical composition and morphology of the residues were analyzed Scanning force microscopy. Samples for SFM studies were by FTIR spectroscopy and SEM respectively (see later). prepared by drying a small drop of the aqueous latex dispersion onto a very flat Si(100) substrate.This procedure produced mechanically stable deposits which were analyzed by SFM in Characterization of polypyrrole-coated polystyrene latexes the contact mode. Standard microfabricated Si3N4 cantilevers Chemical composition and conductivity measurements.The (100 mm in length with a spring constant of 0.40 N m-1) with polypyrrole loadings of each of the coated latexes were deter- integrated pyramidal Si3N4 4 mm tips were employed as force mined by comparing their nitrogen contents to that of the sensors for SFM imaging. The apparatus used in this research corresponding uncoated polystyrene latex (the nitrogen con- was a Nanoscope II SFM (Digital Instruments Inc, Santa tents were 0.17 and 0.18% for the 1.6 and 1.8 mm latexes, Barbara, CA, USA), which has been described in detail else- respectively), and conventional polypyrrole ‘bulk powder’ where.11 The samples were scanned at constant force in the (average nitrogen content of 16.5±0.5%) synthesized in the range 10-7–10-8 N.SFM images from uncoated latex particles absence of latex particles.The conductivities (s) of compressed demonstrated good reproducibility due to a relatively strong pellets of the polypyrrole-coated polystyrene latexes were attractive interaction between the deposited particles. On the determined using standard four-point probe techniques at other hand, SFM imaging of the polypyrrole-coated latex room temperature.It is estimated that the random error particles was much more dicult, probably because of repulsive associated with such measurements is ca. 10%, with a system- interaction between the particles as a result of electric charge atic error of ca. 5–10%. accumulation during scanning. Such charging could arise from direct mechanical interaction between the insulating Si3N4 tip Time-of-flight secondary ion mass spectrometry.Spectra were and the electrically conductive particles mounted on the Si acquired using a VG Scientific Type 23 system. This instrument wafer, covered by a thin insulating surface layer of oxidized was equipped with a single stage reflectron time-of-flight silicon. It should be noted that the image analysis software is analyser and a MIG300PB pulsed liquid-metal (69Ga) ion designed for planar substrates and is not well suited to features source.Static SIMS conditions10 were employed for the analy- with high curvature such as latex particles. In the present ses (i.e. <1013 ions cm-2 per analysis) which was accomplished study relative morphological dierences between samples are using a primary ion beam pulsed at 20 kHz and 25 ns.The emphasized and no attempt has been made to quantify surface beam was rastered over an area of ca. 1 mm2 at a rate of 50 roughness. frames s-1, and both positive and negative spectra were acquired over a mass range of 5–800 u. Specimens were FTIR spectroscopy. FTIR spectra (KBr disk) were recorded prepared by decanting a small amount of the colloidal disper- using a Nicolet Magna 550 Series II research-grade instrument.sion onto 1 cm diameter sample stubs and allowing the aqueous Spectra were typically averaged over 64 scans at 4 cm-1 phase to evaporate; this provided a uniform coverage of the resolution. latex particles on the stub and no signals from the underlying stub were evident in either the SIMS or XPS spectra.Scanning electron microscopy. Morphology studies were carried out using a Leica Stereoscan 420 instrument operating at 30 kV. The samples were mounted on a double-sided X-Ray photoelectron spectroscopy. XPS spectra were collected using a VG Scientific ESCALAB Mk II spectrometer. adhesive carbon disc and sputter-coated with a thin layer of gold to prevent sample charging problems.This was operated in the constant analyzer mode (CAE) using pass energies of 50 eV for survey spectra and 20 eV for the high-resolution spectra. Al-Ka radiation was employed at an Results and Discussion anode power of 420 W. For spectral acquisition and subsequent data processing the spectrometer was controlled by a VGS The latex syntheses via dispersion polymerization using a poly(N-vinylpyrrolidone) stabilizer in alcoholic media proved 5000 data system based on a DEC PDP 11/73 computer. Quantification was achieved using peak areas and Wagner to be reasonably repeatable. Two polystyrene latexes of ca. 1.6 and 1.8 mm diameter were obtained in high yield. For the sensitivity factors and the manufacturer’s software. Specimen 1350 J. Mater. Chem., 1997, 7(8), 1349–1355conducting polymer coating experiments the synthesis conditions (i.e.the concentrations of the pyrrole and FeCl3 reagents) were held constant and the latex particle concentration was systematically varied in order to control the final polypyrrole loading.1,2 The particle size, conducting polymer loadings and electrical conductivities of the four polypyrrolecoated polystyrene latexes examined in this study are summarized in Table 1.The polypyrrole overlayer thicknesses were calculated as described previously.2 The overlayer was assumed to be uniform and the polypyrrole and polystyrene densities were taken to be 1.46 and 1.05 g cm-3, respectively. Most of the characterization work described in the present study was carried out on samples 1 and 4, which are coated with relatively thick and relatively thin polypyrrole overlayers, respectively.The polypyrrole loading in sample 1 is above the ‘knee’ in the conductivity percolation threshold curve for these composite particles,2 and therefore the conductivity of this material is comparable to that of polypyrrole bulk powder (1 S cm-1). In contrast, the polypyrrole loading in sample 4 is below the percolation threshold and this sample has a much lower conductivity (<10-6 S cm-1).X-Ray photoelectron spectroscopy The survey spectrum of an uncoated 1.8 mm diameter poly(Nvinylpyrrolidone)- stabilized polystyrene latex is shown in Fig. 2. An N 1s peak is clearly visible and is most likely due to the poly(N-vinylpyrrolidone) stabilizer at the surface of the latex particles.However, it is impossible to exclude the possibil- Fig. 2 X-Ray photoelectron survey spectra of: (a) an uncoated, poly(Nvinylpyrrolidone)- stabilized polystyrene latex of 1.8 mm diameter; (b) ity of contributions to the N 1s signal from AIBN-initiated and (c) polypyrrole-coated, poly(N-vinylpyrrolidone)-stabilized poly- polystyrene chains and/or the nitrogen-containing cationic styrene latexes of 1.8 mm diameter with polypyrrole mass loadings of surfactant (Aliquat 336) used in the polystyrene latex syntheses. 1.0 and 8.9% respectively (see samples 4 and 1 in Table 1) If we assume that these species do not significantly aect the microanalytical nitrogen content of the latex, a stabilizer content of ca. 1.5% can be calculated. This is consistent with the observation of a very weak feature due to the pyrrolidone latexes would be expected to be higher than that of the uncoated latex. This is indeed the case.The other survey carbonyl group at ca. 1660 cm-1 in the IR spectrum of this material.2 Applying appropriate sensitivity factors we estimate spectra depicted in Fig. 2 are for two polypyrrole-coated polystyrene latexes with polypyrrole mass loadings of 1.0 and an N/C ratio of 0.031 from the XPS spectrum of the uncoated latex.This value is lower than that determined by XPS for the 8.9% and N/C ratios of 0.052, and 0.085, respectively (samples 4 and 1 in Table 1). It is noteworthy that the reduced N/C poly(N-vinylpyrrolidone) stabilizer alone, which suggests that the surface coverage of the latex particles by the stabilizer is ratio found for the lower mass loading is consistent with the polypyrrole overlayer thickness being less than the XPS sam- incomplete.This conclusion is in agreement with studies reported by both Deslandes et al.12 and Chehimi and co- pling depth of 2–5 nm for this sample. Close inspection of the XPS spectra of the coated latexes workers.9 According to their respective structural formulae the theor- reveals additional signals due to Cl 2p.This indicates that the cationic polypyrrole chains are doped with chloride anions etical N/C ratio in polypyrrole is ca. 0.25, whereas the N/C ratio of poly(N-vinylpyrrolidone) is somewhat lower at 0.166. (originating from the FeCl3 oxidant used to polymerize the pyrrole). From the XPS spectra the Cl/N atomic ratios were In XPS studies of conducting polymers the surface carbon signal is generally somewhat more intense than expected on estimated to be 0.21 and 0.27 at polypyrrole mass loadings of 1.0 and 8.9%, respectively.Taking into account the likelihood the basis of elemental microanalyses. This is most likely due to the relatively high surface energies of these materials.14 of surface degradation and concomitant loss of dopant species, these values are in reasonable agreement with the normally Thus, precise agreement between N/C ratios calculated from XPS data and structural formulae is not necessarily expected. accepted doping range for polypyrrole (0.25–0.33).There is no evidence for any Fe signals in the spectral region from 708 to Nevertheless, if the polypyrrole were present as an overlayer on the latex surface, the XPS N/C ratios of the two coated 720 eV, which suggests that these two latexes are significantly Table 1 Summary of the particle size, conducting polymer loading and electrical conductivity of the polypyrrole-coated polystyrene (PS) latexes examined in this study nitrogen PS latex content of polypyrrole polypyrrole sample diametera/mm latexb (mass%) loadingb (mass%) layer thicknessc/nm sd/S cm-1 1 1.8 1.71 8.9 21 1 2 1.6 1.25 6.5 13 3 3 1.8 0.94 4.6 10 0.8 4 1.8 0.34 1.0 2 <10-6 aAs measured by DCP (confirmed by electron microscopy).bFrom CHN elemental microanalyses. Polypyrrole loadings were obtained by calculating reduced nitrogen contents relative to that of polypyrrole ‘bulk powder’ (16.5% N) prepared in the absence of latex.cCalculated assuming a uniform polypyrrole overlayer as described in ref. 2. dFour-point probe measurements on compressed pellets at room temperature. J. Mater. Chem., 1997, 7(8), 1349–1355 1351less contaminated with iron salt(s) than the polypyrrole-coated polystyrene latex examined by Chehimi and co-workers.9 High-resolution C 1s XPS spectra of the same three latexes are shown in Fig. 3. A ‘shake-up’ satellite at ca. 291.5–292.0 eV is clearly visible in the spectra of both the uncoated latex and also the coated latex which contains 1.0% polypyrrole (sample 4). This feature has been previously assigned to a p–p* transition for the aromatic rings of the polystyrene component.9 Its presence suggests that the overlayer is either very thin (i.e.less than the XPS sampling depth) and/or is rather ‘patchy’. This observation is also consistent with the reduced N/C ratio for this sample (see above). No ‘shake-up’ satellite is visible in the C 1s spectrum of the latex containing 8.9% polypyrrole (sample 1), which suggests that the overlayer in this latter sample is suciently thick and/or more uniform to obscure the underlying polystyrene latex.Similar observations were reported by Chehimi and co-workers, who examined a polystyrene latex coated with a similarly thick overlayer of polypyrrole.9 Time-of-flight secondary ion mass spectrometry Fig. 4 depicts three ‘negative ion’ TOF-SIMS spectra in the low mass range. The first spectrum is of the uncoated, poly(Nvinylpyrrolidone)- stabilized polystyrene latex of 1.8 mm diameter.This spectrum is identical to that obtained for the pristine poly(N-vinylpyrrolidone) stabilizer, which is consistent with this component being located at the latex surface. There is a mass peak at 26 u which is assigned to CN- fragments from the pyrrolidone unit of the stabilizer. The second spectrum is that of a polypyrrole chloride ‘bulk powder’ synthesized by conventional precipitation polymerization in the absence of any latex.A peak at 26 u is again prominent: this has been previously assigned to the CN- anion and is therefore charac- Fig. 4 Negative-ion TOF-SIMS spectra of: (a) an uncoated, poly(Nvinylpyrrolidone)- stabilized polystyrene latex of 1.8 mm diameter; (b) polypyrrole chloride bulk powder prepared by conventional precipitation polymerization in the absence of any latex; and (c) a polypyrrolecoated, poly(N-vinylpyrrolidone)-stabilized polystyrene latex of 1.8 mm diameter (8.9% polypyrrole mass loading; sample 1 in Table 1) teristic of polypyrrole.14 It is noteworthy that, in this spectrum, the 26/25 peak ratio is greater than unity (the 25 u peak is assigned to the C2H- anion).In addition, there are two peaks at 35 and 37 which are attributable to 35Cl and 37Cl. The third spectrum is of a polypyrrole-coated, poly(N-vinylpyrrolidone)- stabilized polystyrene latex of 1.8 mm diameter (8.9% polypyrrole loading by mass; sample 1). Again, the 26/25 peak ratio is greater than unity and there are two strong peaks due to the isotopic chloride anions.Thus these TOF-SIMS spectra are consistent with the XPS data and further support the hypothesis that chloride-doped polypyrrole is formed as an overlayer on the surface of the polystyrene latex particles. Raman spectroscopy The Raman results are summarized in Fig. 5. The spectrum of an uncoated polystyrene latex is shown in Fig. 5(d). There are several strong signals which are characteristic of polystyrene at approximately 1002 cm-1 (n1 ring-breathing mode), 1603 cm-1 (n9b ring stretch), and 622 cm-1 (n6b ring deformation). This spectrum is in excellent agreement with the Fig. 3 High-resolution XPS spectra of the C 1s region: (a) an uncoated, poly(N-vinylpyrrolidone)-stabilized polystyrene latex of 1.8 mm diam- Raman spectra of polystyrene reported by previous work- eter; (b) and (c) polypyrrole-coated, poly(N-vinylpyrrolidone)-stabilized ers.15,16 A polypyrrole-coated polystyrene latex is shown in polystyrene latexes of 1.8 mm diameter with polypyrrole mass loadings Fig. 5(b). The conducting polymer loading on this latex is only of 1.0% and 8.9% respectively (see samples 4 and 1 in Table 1). Note 1.0% by mass (N.B.essentially identical spectra were also that the p–p* shake-up satellite at ca. 291.5–292.0 eV due to the obtained at polypyrrole loadings of 3.0 and 8.9%), yet this aromatic rings in the underlying polystyrene latex is absent in spectrum Raman spectrum is identical to that of pure polypyrrole.17 (c). This is consistent with a thicker, more uniform polypyrrole overlayer. Surprisingly, no signals attributable to polystyrene are 1352 J.Mater. Chem., 1997, 7(8), 1349–1355to that of the uncoated polystyrene latex [Figure 5(d)]. Thus, in contrast to the coated latex, the two components in the heterogeneous admixture are essentially additive, as expected. We conclude that the ‘core–shell’ particle morphology of these polypyrrole-coated polystyrene latexes is responsible for the complete attenuation of the signal from the polystyrene core, even at a polypyrrole overlayer thickness of only 2 nm.UV–VIS reflectance spectroscopy During the course of the Raman control experiments described above, it was noted that the polypyrrole-coated polystyrene latexes were distinctly more coloured (i.e. appeared a darker shade of grey) than the corresponding heterogeneous admixtures of dried polypyrrole bulk powder and polystyrene latex with the same conducting polymer loadings.An attempt to quantify this observation was made using diuse reflectance UV–VIS spectroscopy. The results are depicted in Fig. 6 for a polypyrrole-coated polystyrene latex with a polypyrrole mass loading of 4.6% (sample 3) and the equivalent heterogeneous admixture (polypyrrole mass loading 5.0%).It is clear that the coated latex has a significantly lower reflectance (i.e. higher absorbance), which is again consistent with the ‘core–shell’ Fig. 5 Raman spectra of: (a) a heterogeneous mixture comprising 3% particle morphology proposed for this material. polypyrrole chloride bulk powder and 97% poly(N-vinylpyrrolidone)- stabilized polystyrene latex; (b) a polypyrrole-coated, poly(N-vinylpyr- Scanning force microscopy rolidone)-stabilized polystyrene latex with a polypyrrole mass loading of 1.0% (sample 4 in Table 1); (c) the dierence spectrum obtained by Our earlier studies1,2 of polypyrrole-coated polystyrene par- subtracting spectrum (b) from spectrum (a); (d) an uncoated poly(N- ticles using scanning electron microscopy suggested that the vinylpyrrolidone)-stabilized polystyrene latex.Note the strong poly- polypyrrole overlayer was remarkably smooth and uniform at styrene bands in spectrum (a); these features are not visible in spec- polypyrrole loadings lower than approximately 10% by mass. trum (b). However, these morphological studies were somewhat restricted by the relatively low SEM resolution.It is well known that observed, even though this relatively strong Raman scatterer scanning force microscopy (SFM) has a much higher resolution has its strongest signal (1002 cm-1) occurring in a region of than SEM so it was decided to use this technique to study the near-baseline Raman intensity for polypyrrole. Why are there nanomorphology of both the uncoated and coated polystyrene no Raman features attributable to polystyrene in a composite latexes.Fig. 7(a) shows an image of an isolated, uncoated which contains 99% polystyrene by mass? This observation is 1.8 mm polystyrene latex. These particles have a relatively believed to be directly related to the ‘core–shell’ particle smooth, featureless morphology, with relatively few imperfecmorphology of the coated latexes.The polypyrrole overlayer tions. In contrast, an image of a polypyrrole-coated polystyrene must either completely absorb the Raman excitation light latex particle (1.0% polypyrrole loading by mass) is shown in and/or attenuate the Raman signal arising from the underlying Fig. 7(b). The nanomorphology of the polypyrrole overlayer is polystyrene latex.In this context it is worth emphasising that clearly somewhat rougher than that of the uncoated latex the excitation wavelength of 1064 nm is very near the strong, particles and appears to be composed of individual nanosized broad optical absorption band due to polypyrrole (lmax ca. features of 10–20 nm. It is noteworthy that these features were 950–1000 nm).We note that similar ‘skin-eect’ observations not observed in our SEM studies,2 presumably due to this have been reported by other workers18 in Raman studies of the surface chemistry of graphite or carbon fibres and, more importantly, by Hearn et al. for the Raman spectra of polypyrrole- coated polyester fibres.19 However, it is remarkable that such ecient attenuation is observed in the present work since, at a conducting polymer mass loading of 1.0%, the thickness of the polypyrrole overlayer on the polystyrene latex particles is extremely thin (ca. 2 nm; see sample 4 in Table 1). In contrast, the conducting polymer overlayers on the polypyrrole-coated polyester fibres studied by Hearn et al. were much thicker (>100 nm).19 In order to verify the unusual observations described above, control experiments were carried out using a heterogeneous admixture with a mass composition of 3% polypyrrole chloride ‘bulk powder’ and 97% uncoated polystyrene latex.The Raman spectrum of this admixture is shown in Fig. 5(a). There is no possibility of a ‘core–shell’ morphology for such an admixture and, as expected, the signals associated with the polystyrene latex are readily observed, even though this control Fig. 6 UV–VIS reflectance spectra (obtained using a diuse reflectance sample has a slightly lower polystyrene content than the core– accessory) for (a) a polypyrrole-coated polystyrene latex (polypyrrole shell latex [Figure 5(b)]. Some polypyrrole bands are also loading 4.6% by mass; sample 3 in Table 1) and (b) a heterogeneous apparent in Fig. 5(a) but these are much less intense than those admixture comprising 5% polypyrrole bulk powder and 95% poly- observed in Fig. 5(b). Finally, spectrum (b) was subtracted from styrene latex (1.8 mm diameter). The observed lower reflectance for the spectrum (a) to obtain a dierence spectrum [Fig. 5(c)]. former spectrum indicates increased light absorption by the core– shell particles.Although somewhat noisier, spectrum (c) is essentially identical J. Mater. Chem., 1997, 7(8), 1349–1355 1353Scanning electron microscopy and IR spectroscopy studies At this point in our investigation all the experimental evidence suggested a ‘core–shell’ morphology for the polypyrrole-coated polystyrene particles. We were aware that the Lehigh group had successfully used solvent extraction to examine the particle morphology of poly(methyl methacrylate)/polystyrene ‘core– shell’ latexes.22 Since there is some literature evidence that polypyrrole is lightly cross-linked,23 extraction of the uncrosslinked polystyrene ‘core’ with a suitable organic solvent was attempted.If successful, the polypyrrole ‘shells’ should remain as insoluble residues.Accordingly, a dried polypyrrole-coated 1.6 mm polystyrene latex (6.5% polypyrrole loading by mass; sample 2 in Table 1) was treated with THF (see Experimental section). After isolation and drying, the mass loss was found to be consistent with quantitative extraction of the polystyrene component. Analysis of the black residues using FTIR spectroscopy confirmed that this material was essentially just polypyrrole (see Fig. 8), with very little evidence for the original polystyrene component (compare the relatively weak intensity of the band at 702 cm-1 with the corresponding very strong band observed in the IR spectrum of the polypyrrole-coated polystyrene latex).Examination of these polypyrrole residues by SEM revealed a ‘broken egg-shell’ morphology (see Fig. 9), with the ‘egg-shell’ diameter corresponding to that of the original coated particles. Thus, this solvent extraction experiment confirms beyond all reasonable doubt that these composite particles do indeed possess a ‘core–shell’ morphology. There are two possible explanations for the success of the extraction experiment: (1) the THF could permeate the continuous polypyrrole overlayer, causing the polystyrene core to swell and eventually leading to rupturing of the polypyrrole overlayer; (2) the polypyrrole overlayer is not completely continuous and the THF diuses into the latex core via imperfections or defects in the polypyrrole overlayer.Finally, we note that the ‘broken egg-shell’ polypyrrole morphology is highly unusual and may be useful in catalysis applications.Thus, smaller polystyrene latex particles could possibly serve as a colloidal ‘template’ for the synthesis of high surface area polypyrroles with unusual morphologies. Conclusions Several micrometre-sized polypyrrole-coated polystyrene latexes have been characterized in terms of their surface Fig. 7 SFM images of (a) an uncoated 1.8 mm diameter poly(Nvinylpyrrolidone)- stabilized polystyrene latex; (b) a polypyrrole-coated polystyrene latex with a polypyrrole mass loading of 1.0% (sample 4) and (c) a polypyrrole-coated polystyrene latex with a polypyrrole mass loading of 8.9% (sample 1) latter technique’s poorer resolution.On the other hand, similar nanomorphologies for polypyrrole overlayers on textile fibres (STM) were reported using scanning tunnelling microscopy (STM) in an earlier study.20 An SFM image of a polypyrrolecoated polystyrene latex at a higher polypyrrole loading (8.9% by mass) is shown in Fig. 7(c). This thicker polypyrrole overlayer has a distinctly ‘globular’ morphology, with features of the order of 50 nm. These globular features were also observed in our earlier SEM studies2 and are very similar to the morphology of relatively thick polypyrrole coatings on quartz fibres.21 At least ten polypyrrole-coated latex particles were Fig. 8 FTIR spectra of (a) a polypyrrole-coated polystyrene latex (mass analyzed by SFM. In each case, the polypyrrole overlayers loading 6.5%; sample 2 in Table 1) and (b) the polypyrrole broken appear to be reasonably continuous, with little or no evidence egg-shells residues remaining after solvent extraction of the polystyrene of bare patches.This is consistent with both the Raman component using THF (a non-solvent for the polypyrrole component). spectroscopy observations discussed above and also with a The polystyrene bands are much weaker in the latter spectrum, thus confirming quantitative extraction of this component.‘core–shell’ particle morphology. 1354 J. Mater. Chem., 1997, 7(8), 1349–1355Defence Research Agency are both thanked for partially funding the purchase of the FTIR spectrometer. Dr. E. Then (Science Museum, UK) is thanked for his assistance in con- firming our original UV–VIS reflectance spectroscopy studies. References 1 S. F. Lascelles and S. P. Armes, Adv.Mater., 1995, 7, 864. 2 S. F. Lascelles and S. P. Armes, J.Mater. Chem., preceding paper. 3 (a) A. R. Goodall, M. C. Wilkinson and J. Hearn, J. Polym. Sci., Polym. Chem. Ed., 1977, 15, 2193; (b) C. W. A. Bromley, Colloids Surf., 1986, 17, 1. 4 (a) A. J. Paine, W. Luymes and J. McNulty, Macromolecules, 1990, 23, 3104; (b); C. K. Ober, K. P. Lok and M. L. Hair, J. Polym. Sci., Polym. L ett., 1985, 23, 103. 5 H. Ge, P. R. Teasdale, G. G. Wallace, J. Chromatogr., 1991, 544, 305. 6 (a) H. Kawaguchi, Microspheres for Diagnosis and Bioseparation in PolymerMaterials for Bioanalysis and Bioseparation, ed. T. Tsuruta et al., CRC Press, London, 1993; (b) P. J. Tarcha, D. Misun, M. Wong and J. J. Donovan, Polymer L atexes: Preparation, Characterisation and Applications, ed. E. S. Daniels, E. D.Sudol and M. S. El-Aassar, ACS. Symp. Ser. no. 492, 1992, 22, 347; (c) M. R. Pope, S. P. Armes and P. J. Tarcha, Bioconjugate Chem. 1996, 7, 436. 7 D. B. Cairns, S. P. Armes, M. M. Chehimi, M. Delamar and S. Y. Luk, Macromolecules, to be submitted. 8 (a) A. E. Wiersma and L. M. A. vd Steeg, Eur. Pat., 589 529; (b) A. E. Wiersma, L. M. A. vd Steeg and T. J. M. Jongeling, Synth. Met., 1995, 71, 2269. 9 C. Perruchot, M. M. Chehimi, M. Delamar, S. F. Lascelles and S. P. Armes, L angmuir, 1996, 12, 3245. 10 J. C. Vickerman, A. Brown and N. M. Reed, Secondary Ion Mass Fig. 9 Scanning electron micrographs of (a) a polypyrrole-coated poly- Spectrometry5Principles and Applications, Clarendon Press, styrene latex (mass loading 6.5%; sample 2) and (b) the resulting Oxford, 1989.polypyrrole residues after solvent extraction of the underlying poly- 11 B. Drake, C. B. Prater, A. L. Weisenhorn, S. A. C. Gould, T. R. styrene core particles using THF (a non-solvent for the polypyrrole Albrecht, C. F. Quate, D. S. Cannell, H. G. Hansma, and P. K. component). The broken egg-shells morphology clearly evident in the Hansma, Science, 1989, 243, 1586. latter micrograph confirms the core–shell particle morphology of the 12 Y.Deslandes, D. F. Mitchell and A. J. Paine, L angmuir, 1993, original polypyrrole-coated latex particles. 9, 1468. 13 (a) M. M. Chehimi, S. F. Lascelles and S. P. Armes, composition and nanomorphology using a wide range of Chromatographia, 1995, 41, 671; (b) M. M. Chehimi, M. L. Abel, Z. Sahraoui, K. Fraoua, S. F.Lascelles and S. P. Armes, Int. J. Adhes. techniques. XPS and TOF-SIMS studies were consistent with Adhes., 1997, 17, 1. a relatively uniform conducting polymer overlayer containing 14 (a) M. L. Abel, S. R. Leadley, A. M. Brown, J. Petitjean, M. M. chloride dopant anions. XPS provided some evidence for the Chehimi and J. F. Watts, Synth. Met., 1994, 66, 85; (b) S. Y. Luk, underlying polystyrene latex at lower polypyrrole loadings. W.Lineton, M. Keane, C. DeArmitt and S. P. Armes, J. Chem. Raman studies were also consistent with a core–shell particle Soc., Faraday T rans., 1995, 91, 905. morphology, since only vibrational bands due to polypyrrole 15 R. S. Venkatachalam, F. J. Boerio, P. G. Roth and W. H. Tsai, J. Polym. Sci., Part B: Polym. Phys. 1988, 26, 2447.were observed. Remarkably, no evidence was found for the 16 (a) C. H. Jones and I. J. Wesley, Spectrochim. Acta, Part A, 1991, underlying polystyrene component, even for polypyrrole over- 47, 1293; (b) R. A. Nyquist, C. L. Putzig, M. A. Leugers, R. D. layer thicknesses as low as 2 nm. This is attributed to unusually McLachlan and B. Thill, Appl. Spectrosc., 1992, 46, 981. ecient attenuation of the polystyrene bands by the polypyr- 17 C. M. Jenden, R. G. Davidson and T. G. Turner, Polymer, 1993, role overlayer. The enhanced absorbance (reduced reflectance) 34, 1649. of the polypyrrole-coated polystyrene latex particles relative 18 (a) F. Tuinstra and J. L. Koenig, J. Chem. Phys., 1970, 53, 1126; (b) M. A. Tadayyoni and N. R. Dando, Appl. Spectrosc., 1991, 45, to their corresponding heterogeneous admixtures was con- 1613. firmed by UV–VIS reflectance spectroscopy. At low polypyr- 19 M. J. Hearn, I. W. Fletcher, S. P. Church and S. P. Armes, Polymer, role loadings (1.0 mass%) the nanomorphology of the 1993, 34, 262. conducting polymer overlayer was relatively smooth, but a 20 S. P. Armes, M. Aldissi, M. Hawley, J. G. Beery and S. Gottesfeld, rougher, more globular nanomorphology was observed using L angmuir, 1991, 7, 1447. SFM at higher loadings (8.9 mass%). Solvent extraction of 21 S. P. Armes, S. Gottesfeld, J. G. Beery, F. Garzon, M. Mombourquette, M. Hawley and H. H. Kuhn, J.Mater. Chem., the underlying polystyrene latex core was quantitative and 1991, 1, 525. revealed a ‘broken egg-shell’ morphology for the polypyrrole 22 S. Shen, M. S. El-Aasser, V. L. Dimonie, J. W. Vanderho and residues, thus providing irrefutable evidence for a ‘core–shell’ E. D. Sudol, J. Polym. Chem., Part A: Polym. Chem., 1991, 29, 857. particle morphology. 23 F. P. Bradner, J. S. Shapiro, H. J. Bowley, D. L. Gerrard and W. F. Maddams, Polymer, 1989, 30, 914. This work has been carried out with the support of the Defence Research Agency, Fort Halstead, UK. DSM Research and the Paper 7/00236J; Received 10th January, 1997 J. Mater. Chem., 1997, 7(8), 1349–1355 1355