Synthesis and characterization of polyaniline nanoparticles in SDS micellar solutions

Synthetic Metals 122 (2001) 297–304 Synthesis and characterization of polyaniline nanoparticles in SDS micellar solutions Byoung-Jin Kim, Seong-Geun Oh*, Moon-Gyu Han, Seung-Soon Im Division of Chemical Engineering, Hanyang University, 17 Haendang-dong, Seongdong-gu, Seoul 133-791, South Korea Received 1 February 2000; received in revised form 7 February 2000; accepted 21 June 2000 Abstract Polyaniline (PANI) dispersions consisting of 10–20 nm sized nanoparticles were prepared by oxidation with ammonium peroxydisulfate (APS) in sodium dodecylsulfate (SDS) micellar solutions. Coalescence and coagulation were prevented by electrostatic repulsive interaction between anionic SDS micelles. Particle morphology was dependent on the initial shape of surfactant aggregates (micelles) and the molar ratio of SDS to aniline monomer. Spherical particles were obtained at very low monomer concentration and the shape of particles began to be distorted from the spherical shape as monomer concentration increased at constant 0.2 M SDS concentration. The size of spherical particles was the same order of the micellar size or slightly larger. UV–VIS spectroscopy, compositions of PANI powder, morphology, crystalline structure, thermal stability, molecular weight and conductivity of PANI particles synthesized at various monomer and surfactant concentrations were investigated. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Polyaniline; Nanoparticles; SDS micelles; Morphology; Solubilization; Thermal stability 1. Introduction Polyaniline is an electrically conducting polymer with many features that can be exploited in various applications such as microporous electrically conducting materials, anticorrosion protection of metals, molecular sensor, supporting material for catalysts, etc. [1–4]. Among other conducting polymers such as polypyrrole, polythiophene, polyacetylene and polyphenylenevinylene, polyaniline has been most extensively studied because it exhibits a good environmental stability and its electrical properties can be modified by the oxidation state of the main chain and degree of protonation. However, the poor thermal stability and difficult processability of polyaniline should be overcome for the successful application of this electrically conducting polymer. The preparation of polyaniline in colloidal form is one of the attractive alternatives to overcome the poor processability of PANI due to its insolubility in common organic solvents and infusibility. There have been numerous reports on the preparation of PANI dispersions containing 100– 300 nm sized particles. Usually, they are produced with suitable polymeric stabilizers, such as poly(vinyl alcohol), * Corresponding author. E-mail address: seongoh@email.hanyang.ac.kr (S.-G. Oh). poly(N-vinylpyrrolidone), poly(vinyl methyl ether), cellulose ethers or sophisticated tailor-made copolymer architectures [5–8]. Particles are protected from the macroscopic aggregation by the adsorption of steric stabilizers. PANI particles prepared with these methods showed three distinctive morphologies, rice grains, needles and spheres, depending upon the choice of stabilizers, chemical oxidant and reaction conditions [9]. Reactive polymeric stabilizers which have pendant aniline units attached to the main chain and various kinds of inorganic particles were used as promising alternatives resulting in stable polyaniline dispersions [10,11]. Microemulsion, which is defined as a thermodynamically stable and isotropic transparent solution of two immiscible liquids basically consisted of oil, water and surfactant molecules, has been employed as a polymerization medium to obtain spherical latex particles [12,13]. Microemulsion is classified into three types, W/O (oil continuous), O/W (water continuous) and bicontinuous microemulsion. When polymerization is carried out, monomer molecules are incorporated into the submicron oil or water droplets dispersed in the continuous phase. In the case of aniline, W/O microemulsions were employed exclusively as a reaction medium. Gan et al. prepared spherical nanoparticles (10– 35 nm) of PANI in water/petroleum ether/NP-5 (nonylphenol ethoxylate (5 mole)) reverse microemulsion [14]. Also, Tamil et al. obtained highly crystalline nanoparticles of 0379-6779/01/$ – see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 ( 0 0 ) 0 0 3 0 4 - 0 298 B.-J. Kim et al. / Synthetic Metals 122 (2001) 297–304 used as an aqueous phase in micellar solution. Compositions and characteristics for each reaction medium are shown in Table 1. In the first set of experiments, aniline concentration was changed from 0.03 to 0.1 M at fixed SDS and HCl concentration, which were kept as 0.2 and 0.1 M concentrations, respectively. In the second set of experiments, SDS concentration was varied at constant aniline concentration. Before APS was added, total 100 ml of reaction mixtures of different compositions were prepared and purged with nitrogen for 10–15 min to remove the dissolved oxygen. The 10 ml of 0.1 M HCl solutions including APS were added dropwise into 100 ml of reaction mixtures for approximately 20–30 min. After the induction period of about 20–40 min (including time of APS addition), the homogeneous transparent reaction mixtures were turned into bluish tint and the coloration was pronounced as polymerization proceeded. Finally dark green colored PANI dispersions were obtained without any precipitation. In our experiments, APS was dissolved in a relatively small amount of 0.1M HCl solution not to disturb the initial micellar structure. The molar ratio of APS to aniline was kept as 0.5 through the experiments to avoid overoxidation. Since the Krafft point of SDS is around 168C, the polymerization was performed at 20
0:1 C in a two necked round bottom flask mounted in a thermostat for 12 h [21]. Reaction mixtures and APS solutions were equilibrated at experimental temperature before reaction initiated. PANI using cyclohexane as continuous phase, the mixture of AOT (bis-2-ethylhexyl sulfosuccinate sodium salt) and SDS as surfactants [15]. Surfactant aggregates, micelles, have been used for organic synthesis over the past a few decades. Aliphatic and aromatic nucleophilic substitution, hydrolysis of long-chain alkyl sulfates in aqueous solution and reactions involving free radicals have been studied mainly focused on the micellar catalysis [16]. Siswanto and Rathman reported that the reaction rate and the selectivity for N-butyl aniline were improved when Nalkylation of aniline was performed in single-phase aqueous surfactant solution [17]. But in the preparation of PANI dispersions, surfactants have been rarely employed as stabilizer and micelles as reaction medium. Recently, Kuramoto et al. successfully prepared stable green colored transparent PANI dispersions using SDS and dodecylbenzene sulfonic acid (DBSA) [18,19]. They measured UV–VIS spectra of PANI dispersions at various pHs to find out insulator–metal transition point. And they also investigated effects on the product yield, conductivity of oxidant concentration, reaction temperature and kinds of dopant anions. Gospodinova et.al. have reported that they prepared PANI dispersions composed of spherical nanoparticles (5–30 nm) with DBSA and PANI–poly(vinyl alcohol) composites with high electrical conductivity and transparency at film [20]. In this study, we prepared PANI nanoparticles in anionic SDS micelles with ammonium peroxydisulfate (APS) as an oxidant, and correlated the PANI morphology formed in micelles with the size and shape of micelles. This study was focused on the contribution of SDS micelles to the formation of stable dispersions, morphology and electrical properties of PANI particles. Preparation of PANI nanoparticles with DBSA as a stabilizer and doping agent may be interpreted with the similar mechanism. Electrical conductivity, molecular weight, crystalline structure and thermal stability of PANI powders synthesized at different compositions were characterized. 2.2. Precipitation, washing and drying of PANI nanoparticles 2. Experimental Excess amount of methanol was added into the HCldoped PANI dispersion to precipitate PANI powder by breaking the hydrophilic–lyphophilic balance of the system and to stop the reaction. The precipitates were collected with glass filter and washed two times each with methanol, ethanol, and pure water to remove unreacted chemicals, aniline oligomers and SDS. The obtained PANI cakes were dried in a vacuum oven at 408C for 36 h. 2.1. Synthesis of PANI nanoparticles 2.3. Characterization The 0.1 M HCl solution was prepared by adding 37 wt.% HCl solution into doubly distilled and deionized water and Before the addition of APS into reaction mixtures, relative viscosity was measured with an Ubbelohde viscometer to Table 1 The compositions of the reaction mixtures and characteristics for PANI particles obtained Mix. Mix. Mix. Mix. Mix. Mix. 1 2 3 4 5 6 (1a) (1b) (1c) (1d) (2a) (2b) Aniline concentration (M (g)) SDS concentration (M (g)) pH Relative viscosity Inherent viscosity (dl/g) Conductivity (S/cm) 0.03 0.05 0.08 0.10 0.03 0.03 0.20 0.20 0.20 0.20 0.05 0.10 0.75 0.88 1.44 3.73 1.03 1.71 13.52 >100 0.92 0.71 0.55 0.60 0.61 0.62 6.9 5.2 8.3 4.1 8.9 7.1 (0.279) (0.465) (0.745) (0.931) (0.279) (0.279) (5.768) (5.768) (5.768) (5.768) (1.442) (2.884) 299 B.-J. Kim et al. / Synthetic Metals 122 (2001) 297–304 estimate the concentration at which micellar shape begins to deviate from the spherical one. 0.2 M SDS solution (0.1 M HCl) was taken as a reference solution and the measurement temperature was 268C. To compare the molecular weight indirectly, the inherent viscosity of PANI salt was determined at 258C in 0.1% (w/w) solution in concentrated H2SO4 (97%) using an Ubbelohde viscometer. After 12 h of reaction, 0.5 g of reaction mixtures were taken and diluted with 20 g of 0.1 M HCl solution for UV–VIS absorption spectra (UNICAM 8700 SERIES UV–VIS spectrophotometer). Also dried PANI particles were redispersed in 0.1 M HCl solutions with sonicator (bath type) and UV– VIS spectra for those dispersions were measured. The stability and size of redispersed particles were not measured but sedimentation process began within a few hours. Elemental analysis (Eager 200 model, C, H, N, O) was carried out to determine the composition of the PANI salt and base powder. FT-IR spectroscopy (Nicolet, Magna-IR 760) was used to decide the oxidation state of the PANI base powders. PANI base powders were prepared with 5 wt.% NH4OH solution. X-ray patterns of PANI were taken with Ni-filtered Cu-Ka radiation using RAD-C X-ray diffractometer (Rigaku Denki Co.). The detector moved step by step (D2y ¼ 0:05 ) from 5 to 408 at the speed of 78/min and the X-ray power was 40 KV and 100 mA. The morphology of PANI particles was studied by field emission scanning electron microscopy (FE-SEM, Jeol Model JSM-6340F). Samples for the electron microscope were sputter-coated with gold for 60 s. Thermal gravimetric analysis of the PANI–HCl salt was carried out with Shimadzu TGA-50 at the heating rate of 88C/min up to 8008C in N2 atmosphere. About 0.05 g of dried PANI salt powders were compressed into a disk pellet of 13 mm in diameter with a hydraulic pressure at 3000 psi and conductivity was measured by a four-point probe connected to a Keithley voltmeter-constant current source system. PANI denotes polyaniline in the emeraldine salt form unless otherwise mentioned. 3. Results and discussion 3.1. Elemental analysis The compositions of PANI salt and base were given in Table 2. In all samples, sulfur were detected in the range of 3.4–4.7 wt.%. If all the detected sulfur were from the SDS, doping level by SDS was up to 0.21 at maximum (S/N, molar base). As shown in Table 2, after PANI salt was dedoped, amount of SDS incorporated in the samples was reduced to about one half of that in PANI salt. From that, all SDS molecules were not seemed to exist as counter-ion of dopants. In our experiments, it was found that SDS molecules participated in doping process and could not be removed perfectly. Amount of SDS remaining PANI salt as both dopant counter-ion and impurity did not show any relationship with the used amount of SDS in synthesis. Doping level by SDS was considered just to depend on the washing procedure. Although the S/N ratio was around 0.13 for sample 1d, which meant that major dopant was HCl, XRD patterns and thermal degradation profiles showed typical characteristics of PANI doped with sulfonic and sulfuric acid [22,23]. 3.2. FT-IR spectroscopy Oxidation state of PANI-base was identified with FT-IR spectroscopy by comparing two peaks around 1590 (ring stretching in quinoid unit) and 1480 cm1 (ring stretching in bezenoid unit) [24]. All samples were in the emeraldine oxidation state and it was also confirmed with the purplishbrown coloration upon dissolving PANI in pure concentrated sulfuric acid solution [25]. 3.3. UV–VIS absorption spectra UV–VIS absorption spectra of PANI dispersions and redispersed samples in HCl solution were shown in Figs. 1 and 2. Doped PANI shows three characteristic absorption bands at 320–360, 400–420 and 740–800 nm wavelength. The first absorption band arises from p–p electron transition within benzenoid segments. The second and third absorption bands are related to doping level and formation of polaron, respectively [26]. The first two bands are often combined into a flat or distorted single peak with a local maximum between 360 and 420 nm. PANI-dispersions synthesized at 0.03, 0.05 M aniline and 0.2 M SDS concentration showed maximum at 760 nm wavelength. Maximum peak of the other two samples (1c, 1d) appeared at 780–800 nm and the amplitude of the peak at 420 nm increased with increase of monomer concentration in the initial reaction mixture. Table 2 Chemical compositions of PANI salt and base powdera Sample Sample Sample Sample a 1c 1d 2a 2b C (wt.%) H (wt.%) N (wt.%) S (wt.%) S/N 61.5 61.6 61.5 62.3 5.7 5.7 5.8 6.3 10.9 10.9 10.6 9.6 3.8 3.4 3.7 4.7 0.15 0.13 0.15 0.21 (69.3) (70.5) (68.3) (67.0) (5.7) (5.6) (5.6) (5.9) (12.3) (12.6) (12.0) (11.1) S/N ratios were given on molar base and values in parenthesis were for the PANI base. (1.6) (1.5) (1.6) (2.2) (0.06) (0.05) (0.06) (0.08) 300 B.-J. Kim et al. / Synthetic Metals 122 (2001) 297–304 Fig. 1. UV–VIS absorption spectra for PANI synthesized at various aniline concentrations in 0.1 M HCl solution; (a) raw samples, (b) redispersed samples, 1a, 1b, 1c, 1d from the top. Fig. 2. UV–VIS absorption spectra for PANI synthesized at various SDS concentrations in 0.1 M HCl solution: (a) raw samples, (b) redispersed samples, 2a, 2b, 1a from the top. Absorption spectra for redispersed samples showed different features from those of the PANI dispersion in SDS micellar solution (taken from the reaction mixtures after a 12 h reaction). Overall spectra were a little shifted to a longer wavelength by 10–20 nm with greatly diminished localized polaron band or free carrier tail instead of localized polaron band around 780 nm, which is observed for the PANI doped with camphorsulfonic acid exposed to a secondary dopant such as m-cresol [27]. Appearance of free carrier tail has been explained by expanded coil structure by secondary dopant. PANI synthesized in SDS micellar solution showed highly delocalized polaron band without additional doping. Peak at 450 nm was separated and relative intensity of this peak to that of 360 nm band was increased. So doping state seemed to be improved by washing. This could be assigned to the removal of undesired components such as excess SDS, HCl, unreacted aniline monomer. Similar phenomena in emulsion polymerization process using dinonylnaphthalenesulfonic acid as organic dopant and emulsifier were reported by Kinlen et al. [28]. From the comparison of these UV–VIS absorption spectra it is obvious that excess SDS molecules which are not involved in doping have negative effects on the electrical and structural properties of PANI particles. In the case of PANI dispersion in SDS micellar solution, each particle is stabilized with the SDS molecules adsorbed on the surface or incorporated in the PANI particles. On the other hand, redispersed samples consist of the aggregates of primary PANI particles resulting from the removal of SDS molecules through washing procedure. Adsorbed and incorporated SDS molecules seemed to affect the intrachain and inter-chain conduction process unfavorably resulting in the decrease of conjugation length and doping level. PANI dispersions prepared at fixed aniline concentration (0.03 M) varying SDS concentrations showed the similar spectra to the other samples. Any noticeable change in absorption spectra depending on the SDS concentration was not observed. The spectra of the redispersed samples could be explained in the same way discussed before. B.-J. Kim et al. / Synthetic Metals 122 (2001) 297–304 301 Fig. 3. X-ray diffraction patterns for PANI powders prepared (a) at different aniline concentrations, sample 1a, 1b, 1c, 1d and (b) at different SDS concentrations, sample 2a, 2b, 1a from the top. 3.4. Crystalline structure and molecular weight The X-ray diffraction patterns of PANI powders polymerized at different compositions were represented in Fig. 3. As mentioned in elemental analysis part, characteristic XRD patterns of organic sulfonates doped PANI were obtained. All profiles were similar to each other indicating that all samples were isomorphous, but the relative intensities of the peaks were different. Sample 1a, 1b and 1c showed relatively the well-developed X-ray diffraction patterns and sample 1d, which was synthesized at the highest monomer concentration (0.1 M), showed a broad amorphous halo centered at 2y  17 . The ratio of intensity at 20 to 178, I20/I17, and I25/I17 increased with the increase in the crystallinity. Many research groups have reported the relationships of crystallinity with reaction temperature, acidity of the reaction medium and molecular weight of PANI [25,27,29]. According to the reports, low reaction temperature (below 58C), and low pH (0–2) of the reaction medium were favored to obtain highly crystalline PANI having high molecular weight with less structural defects. As shown in Table 1, pH of the reaction mixtures was in the range between 0.7 and 3.7. Thus, low crystallinity of PANI synthesized at high aniline concentration is interpreted due to the low acidity of reaction medium and this result is well consistent with the literatures [29]. Molecular weight of PANI estimated by inherent viscosity showed the same trend as crystallinity and it could be also understood from the viewpoint of acidity of the reaction medium. Sample 1c polymerized around pH 1.7 showed most well defined crystalline structure because of the acidity of the reaction medium and aniline concentration in micellar pseudophase (discussed later). We also investigated the effect of SDS concentration on the crystallinity at 0.03 M aniline concentration. Three X-ray diffraction patterns for sample 1a, 2a, 2b are represented in Fig. 3(b). I20/I17 and I25/I17 increased as SDS concentration decreased. Because three samples were synthesized at the almost same pH, the difference in the crystallinity is not due to the acidity of the reaction medium.and it is also obvious that the micellar shape is spherical at this concentration range. The only possible reason is the moles of aniline per SDS micelle. Thus, we can suggest that crystallinity of the PANI is proportional to the moles of aniline per micelle (below solubilization limit), if the other reaction parameters such as pH of the reaction mixture, temperature, addition rate of APS and micellar shape are controlled to ensure the same reaction conditions. 3.5. Thermal stability Fig. 4 shows the TGA thermograph of the PANI powders. In TGA profile, major losses of weight were observed over two temperature periods, beginning around 160 and 4508C. The first decrease of mass was mainly due to the removal of dopant molecules, HCl and also the loss of possible impurities such as remaining monomers. The second weight loss 302 B.-J. Kim et al. / Synthetic Metals 122 (2001) 297–304 Fig. 4. TGA thermograph for PANI powders polymerized at different aniline and SDS concentrations. at the higher temperature indicates a structural decomposition of the polymer. Gradual weight loss over the wide temperature range could be attributed to the good thermal stability of the PANI main chain. Higher degradation temperature than HCl-doped PANI indicated that SDS was also used as dopant counter-ion. PANI powders prepared at the low aniline concentration, 0.03 M (1a, 2a, 2b), began to degrade at the 4388C, rather lower than the others and also showed greater weight loss despite its high molecular weight and crystallinity. It might be related to the large surface area per unit mass, which means extremely large thermal contacting area. 3.6. Electrical conductivity of PANI All samples synthesized at different reaction compositions had the similar electrical conductivity, 4–10 S/cm. It has been known that PANI of high crystallinity has a higher conductivity than amorphous one and conductivity is independent of molecular weight. As expected from the X-ray profiles, sample 1c and 2a showed a higher electrical conductivity. 3.7. Particle size and morphology At low concentration, sample 1a, aniline was polymerized into the spherical nanoparticles. The size of particles shown in Fig. 5(a) was 8–15 nm which was the same order of the initial micellar size or slightly larger. The shape of particles synthesized at the other concentrations, 1b, 1c, 1d, 2a, 2b, was expected to slightly deviate from the spherical shape as shown Fig. 5(b). In the case of SDS, the CMC (critical micelle concentration) was reported to be 8.0 mM concentration in aqueous solution without any additives at 208C [21]. Micellar size and aggregation number at CMC are Fig. 5. FE-SEM pictures of the PANI particles: (a) sample 1a, (b) sample 1b, (c) sample 1c, (d) sample 1d. B.-J. Kim et al. / Synthetic Metals 122 (2001) 297–304 approximately 6 nm (in diameter) and 62, respectively, at 208C [21]. At this concentration, the shape of micelle is assumed to be spherical. Generally, as surfactant concentration increases, micellar shape changes from sphere to cylinder, hexagonal and lamellar structure successively. Oh and Shah reported that the transition of SDS micelles from spherical to cylindrical shape occurred at 0.2 M surfactant concentration in aqueous solution without any other additives [30]. If solubilizate, salt, or any other additives are added into the solution, the system becomes so complex that the micellar shape, size and morphology could not be predicted without experimental investigation. Especially, ionic surfactant micelles are significantly influenced by the salt concentration. Salt ions adsorbed onto the micellar surface reduce electric repulsion between hydrophilic head groups of the ionic surfactant, thus increase micellar size and aggregation number and also affect intermicellar interaction as depressing electrical double layer. The aggregation number of SDS at 0.18 M concentration with 0.1 M NaCl was reported to be 92 at 258C, and the size of micelle is assumed to be around 15 nm (in diameter) [21]. When hydrochloride is used instead of NaCl, it acts somewhat different way but is expected to show similar influence on micellar behavior. In the first set of experiments, SDS and HCl concentrations were kept at 0.2 and 0.1 M and aniline concentration varied from 0.03 to 0.10 M. At 0.03 M aniline concentration, it is obvious from the viscosity data that micellar shape would be spherical or at least spherelike, and this was consistent with SEM photograph of PANI powders. Above 0.03 M aniline concentration, the micellar shape seemed to be changed into cylindrical or worm like shape, which coincided with the sharp increase of relative viscosity up to 13.52. Reaction mixture 4 (0.1 and 0.2 M, aniline, and SDS, respectively) was so viscous such as gel that relative viscosity could not be measured (above 100). Sharp increase of viscosity of surfactant solutions could be thought as the transition of micellar shape, for example, spherical to cylindrical or, cylindrical to hexagonal shape. In our experiment, all the reaction mixtures were not birefringent, which means that the formation of hexagonal and lamella phases could be excluded, but from the relative viscosity data, it is certain that the micellar size and aggregation number increase with increase of aniline concentration and also micelle would change from spherical micelle to cylindrical or worm like micelle followed by further growth and change of micellar shape to another one. Exact micellar shape and transition point could not be revealed with viscosity measurement. Since the reaction takes place mainly in the micelle–water interface, if the SDS concentration is high enough to solubilize almost all aniline and anilinium molecules, morphology of the obtained PANI particles would be the same as the initial micellar shape. 303 reactants in the micelles, is one of the most important factors, which significantly affects the reaction kinetics, selectivity and yield. The site of incorporation of the solubilizate is closely related to its chemical nature. It has been well accepted that non-polar, highly hydrophobic solubilizates are located in the hydrocarbon core of the micelles. Polar or surface active molecules are solubilized in the micelle–water interface. In the case of aniline existing in the form of anilinium cation in acidic aqueous solution, Kuramoto et al. suggested that anilinium cations are adsorbed on the micellar surface by electrostatic interaction with anionic SDS molecules being fully exposed to the aqueous phase [19]. But, some portion of Naþ counter-ions would be bound with the micelles and electrical double layer might be depressed in acidic aqueous solution leading to the reduction of the charge density of micelles. Besides, exposure of the aromatic moieties to the aqueous phase is not acceptable if the transfer free energy change of the aniline and anilinium molecules from aqueous phase to micellar pseudophase is considered [31,32]. Thus, we assumed, referring to the literatures, that most aniline monomers were solubilized in the micelle-water interface as shown in Fig. 6. Obviously, some of them adsorb on the micellar surface and maybe also exist in the aqueous phase. Kandori et al. reported that phenol molecules in dodecyltrimethylammonium bromide micellar solution first were incorporated in the palisade layer and began to adsorb on the micellar surface when palisade layer had been saturated with solubilized phenol molecules [33]. We assumed that solubilization locus of aniline-HCl salt was not so different from that of the phenol, although investigated compositions were not in the same range of our reaction compositions. Solubilized aniline or anilinium molecules were polymerized oxidatively by APS existing in the aqueous phase. The reaction took place mainly in the micelle-water interface adjacent to the surfactant head groups, because hydrated APS molecules could not penetrate into the micellar surface. Here, one must note that the micelles are not static and rigid entities. Micelles are in a dynamic equilibrium state with surfactant monomers in 3.8. Formation of PANI nanoparticles When organic synthesis or polymerization is carried out within the micelles, solubilization locus, the location of the Fig. 6. Schematic diagram for the solubilization locus of aniline–HCl salt in SDS micelle. 304 B.-J. Kim et al. / Synthetic Metals 122 (2001) 297–304 solution. Namely, micelles are dissociated into monomers and monomers are associated into micelles continuously. Thus all segments of the surfactant and solubilizates could be exposed to the water. The explanation for polymerization mechanism of aniline in micelles is totally based on the time-averaged observations. p-Aminodiphenylamine which is known as an intermediate produced in polymerization of aniline has very low solubility in pure water, <0.1 g/100 ml. So possible aniline molecules attacked in the aqueous phase would be incorporated readily into the micelles as they grow into dimer, trimer or tetramer due to their increased hydrophobicity. After the reaction is finished, produced PANI nanoparticles are stabilized by adsorbed and incorporated SDS molecules by electrostatic repulsive interactions. References 4. Conclusions [13] [14] [15] In this study, SDS micelles were used as polymerization medium to produce PANI nanoparticles. All PANI dispersions prepared at various compositions were highly transparent spherical PANI nanoparticles were obtained when SDS concentration was high enough to solubilize almost all monomer molecules and the micellar shape remained spherical. Small amount of SDS could not be removed after washing and also participated in the doping process. XRD patterns and thermal degradation profiles showed the characteristics of PANI doped with organic sulfonic or sulfuric acid. More structured PANI particles showed a higher conductivity. Electrical properties were affected by the presence of the SDS molecules and did not depend on the molecular weight and morphology as reported in the literatures. 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