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J. Phys. Chem. B 2007, 111, 2174-2180
New Aspects of the Low-Concentrated Aniline Polymerization in the Solution and in SiC Nanocrystals Dispersion A. A. Pud,*,† Yu. V. Noskov,† A. Kassiba,‡ K. Yu. Fatyeyeva,† N. A. Ogurtsov,† M. Makowska-Janusik,§ W. Bednarski,‡ M. Tabellout,‡ and G. S. Shapoval† Institute of Bioorganic Chemistry and Petrochemistry, National Academy of Sciences of Ukraine, 50 KharkiVske shose, 02160, KyiV, Ukraine, Laboratoire de Physique de l’Etat Condense´ , UMR CNRS 6087, Faculte´ des Sciences et techniques - UniVersite´ du Maine, 72085, Le Mans Cedex 9, France, and Institute of Physics, Akademia im. Jana Dlugosza, 13/15 Aleya Armii Krajowej, 42-200 Czestochowa, Poland ReceiVed: August 29, 2006; In Final Form: January 9, 2007
Results of the simultaneous in-situ UV-vis and open-circuit potential (OCP) monitoring of the low-concentrated aniline (An) polymerization in the presence of camphorsulfonic acid (CSA) suggested that during the induction period (IP) step a transition state formed, which probably included anilinium cation and the oxidant anion, antecedent to a propagation step. No aniline oligomers were registered at this stage but they appeared at the beginning of the propagation step under the investigation conditions. The moments of formation of insoluble pernigraniline phase and appearance of emeraldine units in the growing pernigraniline chains were ascertained by the comparison of kinetic and OCP profiles of the polymerization process both in the solution and in SiC dispersion water mediums. It is deduced that pernigraniline reduction by aniline molecules begins earlier than it is generally accepted (i.e., earlier than OCP maximum is reached) and probably in parallel to a continuing appearance of pernigraniline units even in the same chains that undergo the reduction. It was found that an addition of the SiC dispersion phase into the polymerization mixture accelerates differently all stages of the aniline polymerization. Finally, this polymerization process leads to the formation of polyaniline (PANI)CSA shell with thickness in the range from 0.5 nm to a few nm at the SiC nanocrystals surface.
1. Introduction Despite the fact that aniline polymerization is a sufficiently investigated process, a few more papers devoted to refinement of its peculiarities were published recently.1-9 To some extent, such interest appeared because of discovery of additional possibilities of the polymerization process, specifically, of a preparation of nanosized polyaniline (PANI) particles (nanofibers, nanotubes) without the use of any stabilizing additives in the diluted3,4 or nondiluted1,5-7,10,11 polymerization modes. Monitoring of different physical-chemical parameters of the polymerization mixture (pH, temperature, open-circuit potential (OCP)) demonstrated new aspects of this process.2,5,7,10-12 Specifically, a combination of OCP measurements with a determination of molecular weight of the formed PANI allowed discovering the ability of its chains to grow because of attaching aniline or 2-methylaniline even during conversion of pernigraniline to emeraldine salt (ES) after the OCP growth had finished.2 This suggests that the surface of PANI nanofibers, which appear and grow both at pernigraniline and ES stages,5,12 can be grafted with new monomers. On the other hand, in some cases rather complicated shape of pH, temperature, and OCP profiles of the aniline polymerization process5,7,10,11,13 testifies that additional new aspects of this process can be found. In general, investigations of the aniline polymerization without use of any stabilizing additives are a logical develop* To whom correspondence should be addressed. E-mail: echoc@ mail.kar.net. † National Academy of Sciences of Ukraine. ‡ Universite ´ du Maine. § Akademia im. Jana Dlugosza.
ment of works devoted to the formation of PANI nanoparticles in the presence of surface active acids being also dopants of the formed PANI.13-17 Obviously, these new synthetic approaches are favorable because of their less chemical pressure on the environment.3 At the same time, these approaches also give hope for a formation of nanosized PANI layers (core) at the surface of nanoparticles of a different nature. This allows preparing hybrid conducting nanocomposites in the absence of stabilizing additives that can be of a crucial importance for final properties of these materials. Hybrid nanocomposites based on conducting polymers and inorganic nanocrystalline semiconductors are of a great basic and practical interest because of both a specificity of physical processes at their interface and great promises in their diverse applications for integrated nanosized devices.18-20 The success of such materials stems from a synergetic combination at a nanoscale level of properties of all the components and is enhanced with specific interactions along the interface.21 Different approaches have been applied to form these materials, but the subgroup of the approaches on the basis of an incorporation of inorganic building blocks in organic polymers is considered to be the most prominent one.21 Specifically, the formation of inorganic core-conducting polymer shell nanocomposites is a very suitable way to realize intimate interaction between both phases along their interface. This approach allows a quite simple encapsulation of different inorganic micro- or nanosized particles into a conducting polymer shell through a chemical oxidative polymerization of corresponding monomers.20,22 PANI is frequently used as the shell because of its unique combination of conductivity, stability, sensor, electrooptical, catalytic, and other properties with low cost.15 These
10.1021/jp0656025 CCC: $37.00 © 2007 American Chemical Society Published on Web 02/09/2007
Low-Concentrated Aniline Polymerization properties can be easily varied by changing PANI doped state in acid and basic media or by changing dopant.23,24 As a consequence, this capability allows tuning PANI band gap and work function25 that opens additional possibilities in creation of new hybrid nanocomposites of PANI with inorganic semiconductors possessing properties different from those of parent materials.20 Some of these possibilities are given obviously by aniline polymerization conditions which can modify the surface of the inorganic particle (core) through using a quite aggressive oxidant and acid. Specifically, some metal oxide particles can be dissolved in such aggressive media. This phenomenon was observed, for example, by Tang et al.26 for maghemite (γ-Fe2O3) in HCl solution. Obviously, the surface of the materials with inherent ability to oxidation (e.g., II-VI binary compounds, carbon nanoparticles, etc.) also can be transformed by an oxidant, which is used for aniline polymerization. On the other hand, this transformation can probably result in some activation of the surface of the inorganic particles and grafting the formed PANI. As a consequence, this can affect the interface between inorganic core and PANI shell and improve their interactions. As a matter of fact, these changes are of great importance for inert semiconductor nanocrystals, such as silicon carbide (SiC), whose interaction with polymer matrix is necessary to form hybrid nanocomposites with original optical, electrooptical, and other functional characteristics.27,28 Moreover, the charged PANI shell is able to prevent aggregation of the nanoparticles in the matrix medium because of electrostatic repulsion. To shed more light on the above issues, we aimed this work to clear different aspects of aniline polymerization both in “free” conditions (in a reaction mixture without SiC dispersion phase) and in the presence of SiC nanoparticles as well as to evaluate the PANI condition in the formed nanocomposites and their peculiar structural features. 2. Experimental Section 2.1. Materials. SiC nanopowder with nanoparticles of average diameter of 20 nm and Si/C ratio ) 1.04 was synthesized by CO2 laser pyrolysis from gaseous mixture of SiH4 and C2H2 in accord with the technique described elsewhere.29 Annealing at 1300 °C under argon was performed for better homogeneity and crystallization of SiC nanoparticles. Aniline (Merck) was distilled under vacuum and was stored under argon in a refrigerator at 3 °C. Ammonium persulfate (APS) (Ukraine), CSA (Aldrich), and dodecylbenzenesulfonic acid (DBSA) (Acros) were of reagent grade and were used as received. All polymerization experiments were performed in water media prepared with distilled water. 2.2. Aniline Polymerization in Solution and in SiC Water Dispersion. Chemical polymerization of aniline was performed in the presence of CSA in water solution (free conditions) or in water dispersions of the SiC nanocrystals in the range of concentrations 0.062-1.23 wt % at 22 °C without stirring or by constant slow stirring with a magnetic stirrer for 16-24 h. To calculate an aniline quantity, which was necessary to form the PANI shell, at all SiC nanoparticles we presumed that the synthesized PANI covered as a homogeneous layer the surface of each SiC nanoparticle and did not form a separate phase. This presumption is supported by the well-known PANI ability to precipitate at surfaces immersed in an aniline polymerization mixture.30 As parameters for such rough calculations, there were specified the thickness of the PANI layer in the range of 2-5 nm, specific gravity of PANI 1.2 g/cm3, and 100% PANI yield of the polymerization. The necessary quantities and concentra-
J. Phys. Chem. B, Vol. 111, No. 9, 2007 2175 tions of aniline (0.009-0.023 M) and corresponding quantities of APS and CSA were determined from these calculations. There were used the molar ratio aniline:APS ) 1:1.25, which is known to allow a high PANI yield,31 and concentrations of CSA with molar ratios aniline:CSA not less than 1:1.5. Evolution of the aniline polymerization process was watched in situ by both a consecutive survey of UV-vis spectra of the reaction mixtures placed in quartz cuvettes (l ) 1 mm) with the spectrophotometer M-40 (DDR) and simultaneous monitoring of the same mixtures in a separate single-compartment cell by the continued recording of OCP of the redox-electrode GE 105 immersed in the mixture. This compact redox-electrode is the set of the working Pt plate electrode and Ag/AgCl (3 M KCl) reference electrode assembled in one unit in a manner allowing their good contact with the solution under investigation. OCP was recorded with the precision pH/redox/temperature measuring instrument GMH 3530 (Greisinger Electronics) connected to the personal computer. After the polymerization had completed, the formed SiC/ PANI-CSA nanocomposite dispersions were dialyzed against distilled water through cellophane membrane to remove byproducts and impurities. The purified dispersions were placed in Petri dishes and were heated at 70 °C to evaporate water. The separated SiC/PANI-CSA nanocomposites were then dried in vacuum at 70 °C to get the final products for further characterizations. 2.3. Characterization Methods. Raman spectra of SiC, PANI, and their nanocomposites were performed by Jobin-Yvon Horiba T64000 spectrometer with the specifications reported elsewhere.32,33 In all experiments, the green excitation laser line λ ) 514.5 nm was selected with the laser beam power less than 5 mW to prevent the sample degradation under the laser beam. Transmission electron microscopy (TEM) (JEOL 2010, acceleration voltage 200 kV) was applied to study the morphology of SiC/PANI-CSA nanocomposites. The nanocomposite powder samples were suspended in pure ethanol and were stirred for 10 min by ultrasound to optimize their dispersion state for the measurements. A droplet of the obtained suspension was placed on copper grid and was dried under air. These procedures were followed by covering with amorphous carbon membrane required for charge evacuation. 3. Results and Discussion 3.1. Aniline Polymerization in the Solution. This process at low concentrations of aniline and high concentration (1 M) of CSA or other acids leads to formation of a stable transparent dispersion of PANI-acid nanofibers (∼50 nm) under conditions without stirring.3-5 We have found that a quite stable transparent dispersion of PANI-CSA can be formed also at much lower CSA concentrations. This fact agrees with the recently discovered possibility of formation of nanostructured PANI at mild acid conditions and even in an absence of any additional acid.10 At the early stage of the polymerization process, the UVvis spectra of the reaction solution contain only the bands of anilinium salt and pure CSA without any change of their shape and intensity (Figure 1a). A similar situation was observed for the polymerization of the An-DBSA complex and was explained through the existence of only the anilinium salt in the solution at this stage.16,17 It would seem that this explanation contradicted with the known data on the formation of paminodiphenylamine (PADPA) practically at the very beginning of the aniline polymerization process and on PADPA existence during the induction period (IP).10,13,34,35 However, whereas PADPA is oxidized easier than aniline, its appearance in the
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Figure 2. The OCP and visual color changes during the chemical polymerization of aniline at Caniline ) 0.0091 M, molar ratio An:CSA ) 1:1.5, unbound CCSA ) 0.00455 M.
Figure 1. (a) In-situ UV-vis spectra of the aniline polymerization solution at Caniline ) 0.0091 M, molar ratio An:CSA ) 1:1.5, unbound CCSA ) 0.00455. Interval between spectra, 20 min; except last spectrum (-x-) scanned at the next day at 1860 min. (b) Kinetic profiles of the polymerization solution obtained at wavelength 698 nm for unbound CCSA ) 0.00455 M (1) and 0.0909 M (2). (c) Changes of the position of the exciton band during the aniline polymerization.
solution should result in its fast oxidation followed by a formation of aniline trimer, tetramer, and so forth.13 This means that as soon as PADPA appears in the aniline polymerization solution, it should both trigger off a propagation step of avtocatalytic aniline polymerization and shorten or minimize the IP duration.36 This was not confirmed by our data. Specifically, the unchanged UV-vis spectra registered for ca. 50-55 min (Figure 1a) testified that PADPA did not appear during this noticeable IP, similarly to results of aniline polymerization in the presence of DBSA.16 These observations are in good agreement with the development of the OCP profile of the reaction (Figure 2). Specifically, after addition of the oxidant to the anilinium salt solution, the measured OCP rises to a value corresponding to the initial oxidant concentration. This value is virtually constant for 60 min (Figure 2) that obviously testifies to a temporary absence or a minimal amount of chemical changes in the system and confirms the above UV-vis data on the existence of the significant IP. In turn, these data suggest
formation of a transition state during IP, which is antecedent to an appearance of chemical changes in the reaction solution. It is difficult for the moment to describe precisely the structure of this transition state. This will be done in additional special studies. However, taking into account the charged nature of the reagents, one may expect that the transition state includes anilinium cation and the oxidant anion (e.g., [(PhNH3)+‚‚‚S2O82-] or even [(PhNH3)+‚‚‚S2O82-‚‚‚+(NH3Ph)]). It should be mentioned, however, that for the An-DBSA polymerization system, the formation of the different transition state (the reaction equilibrium intermediate being PADPA radical cation) during IP was supposed.15 After IP has finished, the gradual increase of the OCP value is observed (Figure 2). Visually, at this moment a light blue coloring appears in the solution. These changes are accompanied in the UV-vis spectra by the appearance and very gradual growth of new wide bands centered at ca. 340 and 700 nm (Figure 1a). These bands may be assigned to low molecular aniline oligomers (dimer and higher) containing both benzoid and quinoid units.17,37 Strong oxidation state (pernigraniline form) of these oligoanilines, which do not contain benzoid units bound to the amine nitrogen, causes probably the fact that no polaron band at 400-450 nm is observed at this stage of the aniline polymerization.17 The presence of the quinoid units having oxidant properties37 is confirmed also by the growth of OCP of the polymerization solution (Figure 2). Unlike the IP stage, the propagation step was strongly accelerated with pH decrease (Figure 1b) that confirms the known fact of the acid participation at this step.37 In-situ UV-vis monitoring of the aniline polymerization in the transparent solution showed reproducible spectra up to approximately 174-180 min of the process without any stirring. After this moment, some irreproducibility appeared because of the formation of the insoluble blue PANI phase precipitating at the bottom of the cuvette. This point is easily observed from data scattering at kinetic profiles plotted by optical absorption changes at 698 nm (Figure 1b). The similar scattering point appears in the case of the higher CSA concentration (0.1 M) but earlier (at ca. 93 min) and with much more optical density of the solution, which corresponds to more quantity of the formed blue PANI (Figure 1b, curve 2). The optical transparency of these colloid solutions up to the scattering points at ca. 184 and 93 min, correspondingly, suggests the presence of nanosized PANI (pernigraniline form) in both cases. This suggestion agrees with data of Lee and Kaner12 on the formation of PANI nanofibers in more concentrated (1 M) acid solutions during the pernigraniline formation stage.
Low-Concentrated Aniline Polymerization After the formation of the insoluble PANI phase, the solution in the cuvette was stirred before each subsequent UV-vis spectrum scan that allowed getting the spectra with an optical absorption approximately corresponding to a real quantity of the formed PANI. This is confirmed by a typical S-shaped form of the kinetic curves (Figure 1b). Appearance of the insoluble blue phase (precipitate) in the polymerization solution at 174-180 min practically coincides also with the beginning (at ca. 175 min) of the faster growth of OCP (a change of the OCP profile slope) (Figure 2). Specifically, after IP (stage t1) has finished (at ca. 55-60 min), a gradual OCP growth is observed, which develops in two distinguishable stages t2 and t3 (Figure 2) up to the maximum OCP value corresponding to a completion of the formation of PANI in pernigraniline form.38,39 Probably, t2 and t3 stages at OCP profile could be seen for some aniline polymerization systems with slow polymerization rate. However, to our knowledge, these stages were never considered as separate stages corresponding to the pernigraniline formation. For the moment, it is difficult to explain completely from the obtained data the existence of two stages (t2 and t3) instead of one stage. Nevertheless, appearance of the phase of insoluble pernigraniline in the transition point between stages t2 and t3 suggests that the faster growth of the OCP value at stage t3 may be explained through a catalytic acceleration of the process by the formed pernigraniline dispersion phase. Such acceleration in the presence of a dispersion phase is well-known for aniline polymerization.40 This means that changes in the OCP profile as well as the start of the irreproducibility of the UV-vis spectra can be good evidence of a quantitative transformation of the formed pernigraniline from the nanostate (colloid solution) to its insoluble phase (precipitate) in the t2/t3 transition point. The growth of a molecular chain length of pernigraniline and of its quantity is accompanied not only with changes of the OCP profile but also with a blue shift of the exciton band in the UV-vis spectrum up to ca. 590-600 nm (Figure 1c) and with an increase of a total intensity of the spectrum. However, typical of PANI the polaron absorption in the range of 400450 nm does not appear for a long time of the polymerization (Figure 1a) that testifies probably to an invariability of the chemical structure of pernigraniline formed during this period. The polaron absorption becomes apparent only after ca. 230240 min of the polymerization process at low concentration of unbound CSA (0.0045 M) (Figure 1a) or earlier in the case of higher concentration of unbound CSA (under unbound CSA, we understand here a part of the total CSA quantity, which left after interaction with aniline). Obviously, one can say that from this moment the PANI structure starts a qualitative transformation and emeraldine fragments appear in the growing pernigraniline chains. Indeed, from this moment, the exciton band changes the direction of its movement to the blue region of the UV-vis spectrum and shifts back to the red one up to a position at ca. 820 nm, being typical of emeraldine salt, at the end of the process. These band position changes are clearly displayed in their time profile (Figure 1c). As one can see, this profile contains two inflection points. The first one is localized at ca. 174 min of this profile and remarkably concurs both with the scattering point at the kinetic profile of the polymerization process (Figure 1b, curve 1) and with the beginning of the faster growth of OCP (Figure 2, the t2/t3 transition point). Correspondingly, it may be considered as an additional criterion of appearance of insoluble pernigraniline phase (precipitate) during the polymerization. The second point at ca. 224 min, corresponding to the start of the red shift of the exciton band (Figure
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Figure 3. The OCP changes during the chemical polymerization of aniline at Caniline ) 0.0091 M and high unbound CCSA ) 0.0909 M.
1c), designates quite precisely the beginning of the pernigraniline-to-emeraldine transformation. This qualitative pernigraniline-to-emeraldine transformation is not seen as a distinguishable transition point in the OCP profile (Figure 2) under conditions of the investigation. Despite this, the time of appearance of the polaron absorption in the UV-vis data suggests that this transformation can probably be located below the OCP maximum in the OCP profile at the low aniline and acid concentrations (Figure 2). Consequently, at least at low aniline and CSA concentrations under conditions of the investigation, the stage of formation of ES from pernigraniline begins earlier than the OCP maximum is reached even before the moment when all the oxidant (APS) has been consumed. In other words, the pernigraniline reduction by aniline begins earlier than it is generally accepted2,38,39 and probably in parallel to a continuing appearance of pernigraniline units even in the same chains that undergo the reduction. The above data testify to two opposite tendencies in the development of the OCP maximum of the polymerization mixture: (1) the OCP growth resulted from the increase of both the chain length and a total quantity of pernigraniline and (2) the OCP decrease is caused not only by the consumption of the oxidant but also by the appearance of emeraldine units in the growing chains because of reduction of pernigraniline by aniline. These opposite tendencies manifest in the deceleration of the OCP growth followed by the occurrence of the OCP maximum in the OCP profile (Figure 2). After APS has been consumed in the mixture and the accumulation of pernigraniline units in the system has stopped, stage t4 appears in the OCP profile and OCP drops sharply to ca. 610 mV in one more pronounced point of inflection at ca. 340 min (Figure 2). However, unlike the above case of stages t2 and t3, no quantitative or qualitative changes in the system could be observed at this moment visually or in the UV-vis spectra. These facts suggest that during stage t4 this point of inflection appears because of reducing OCP to the value at which the oxidation of aniline decelerates through the decrease of the pernigraniline quantity and aniline concentration. At the same time, it may not be excluded that during stage t4 the formation of emeraldine salt from pernigraniline and the completion of the polymerization process run in distinguishable steps with different kinetics. The last supposition needs to be checked in an additional study. The transition points found in the OCP profile and UV-vis spectra became apparent owing to the low rate of the diluted aniline polymerization. At a high rate of the process, it is difficult to see the time discrepancy between the UV-vis and the OCP data, and the pernigraniline-to-emeraldine transformation point is located at the beginning of the OCP maximum at the higher acid concentration (0.1 M) (Figure 3).
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Figure 5. The OCP changes during the chemical polymerization of aniline in the presence of 0.0625 wt % SiC at Caniline ) 0.009 M, molar ratio An:CSA ) 1:1.5, unbound CCSA ) 0.0045 M.
Figure 4. In-situ UV-vis spectra of the aniline polymerization solution: (a) in the presence of 0.14 wt % of SiC at Caniline ) 0.0137 M, molar ratio An:CSA ) 1:1.5, unbound CCSA ) 0.00685 M. Interval between spectra, 10 min; after curve -0-, 40 min. Insert: kinetic profile of the polymerization process plotted by changes of optical density of the solution at wavelength 698 nm. (b) In the presence of 0.0625 wt % of SiC at Caniline ) 0.009 M, unbound CCSA ) 0.091 M.
3.2 Effect of SiC Dispersion Phase on the Development of Aniline Polymerization. It appeared that the simultaneous reproducible UV-vis and OCP monitoring of the aniline polymerization in the presence of SiC could be realized only under a correct choice of SiC concentration. Specifically, because of the SiC significant bulk specific weight (3.2 g/cm3), it was very difficult to prepare its stable dispersion. It was found earlier that in the absence of surface-active compounds in a solution, the quite stable semitransparent SiC dispersion was possible only at its concentration not higher than 0.05 wt %.27 We have found that this limit can be increased in the investigated An-CSA polymerization system up to 0.14 wt % because of some stabilizing SiC nanoparticles in the solution by adsorption at their surface of the monomer and unbound CSA. Specifically, when the higher concentrations of unbound CSA were used, the more stable and transparent SiC dispersion was. This allowed us to perform not only simultaneous UV-vis and OCP monitoring of the polymerization process but also a preparation of hybrid core-shell SiC/PANI-CSA nanoparticles in quantities being acceptable for materials testing. Moreover, at high stirring rate (150-200 rpm), we could prepare such hybrid core-shell nanoparticles at concentrations of SiC nanoparticles in the dispersion up to 1.23 wt %. Different concentrations of SiC nanocrystals in the An-CSA polymerization mixture resulted not only in the well-known significant acceleration of the PANI-CSA formation process in the presence of a dispersion phase40 but also in changes of the shape of the UV-vis spectra. Specifically, some light scattering in the mixture at 0.14 wt % SiC concentration hampered an observation of distinct bands of growing PANI chains during the polymerization process (Figure 4a). However, an absorption increase in the whole spectrum range could be observed. The kinetic profile of the polymerization process has
roughly pronounced S-shape (Figure 4a, insert) with illegible IP and shows data scattering practically right after IP (ca 3040 min) because of SiC precipitation. To make these spectra more reproducible, the reaction mixture in the cuvette was stirred before each spectrum scan. However, in the case of higher acid concentrations and less SiC loadings, this mixture was much more stable and allowed obtaining UV-vis spectra typical of PANI formation. These spectra developed similarly to the above case of the free aniline polymerization but with faster kinetics and shorter IP. Specifically, from these spectra obtained at 0.1 M CSA concentration, one can observe the beginning of the formation of emeraldine units in the growing pernigraniline chains at ca. 80 min, that is, earlier than in the free conditions (Figure 4b). Naturally, unlike the above case of the free aniline polymerization, we could register neither visually nor by UV-vis spectroscopy an appearance of insoluble pernigraniline phase in the dispersion system. However, in accord with the above analysis of the OCP profile of the free aniline polymerization case (Figure 2), this stage at low aniline and acid concentrations begins in the clear transition point between t2 and t3 stages in the OCP profile of the polymerization in the SiC dispersion (Figure 5). Taking into account the known property of formed PANI to precipitate at surfaces immersed in the aniline polymerization mixture,30 one may postulate that this transition point corresponds also to the pernigraniline precipitation at the surface of SiC nanocrystals, that is, to the beginning of the formation of the PANI shell at the SiC core. The accelerating effect of the SiC dispersion phase manifests itself differently in all stages of the aniline polymerization. For example, for the process at the low aniline and acid concentrations, the transition t2/t3 point (start of the formation of the insoluble pernigraniline phase) appears in the OCP profile at ca. 140 min, that is, by 35 min earlier than in the absence of SiC (Figure 5). From this moment, the stages of pernigraniline formation and its transformation to ES state develop much faster. Specifically, the OCP maximum and inflection point were located at ∼185 min and ∼200 min at OCP profile, that is, by ca. 120 and 140 min earlier than in the free aniline polymerization conditions (Figures 2 and 5). This fact suggests that the high surface of the formed nanosized PANI (pernigraniline) shells at SiC nanocrystal cores gives its own catalytic input (in addition to one of the SiC nanocrystal phases) into the aniline polymerization rate. 3.3 Properties of SiC/PANI Nanocomposites. In general, the results of the UV-vis and OCP measurements have proved the possibility of aniline polymerization in the presence of SiC
Low-Concentrated Aniline Polymerization
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Figure 6. Typical Raman spectra of individual SiC (1), PANI-CSA (2), and nanocomposite SiC/PANI-CSA (3).
nanocrystals in the wide range of aniline, doping acid (CSA), and SiC concentrations. These data, however, do not give real information about whether PANI is really immobilized at the surface of SiC nanocrystals or is located among these nanocrystals as a separate phase or as a matrix in which SiC nanocrystals are embedded. It is unclear also if the properties of PANI in these systems differ from those of individual PANI. To clear these aspects, we used Raman and TEM techniques. The detailed EPR investigations of these nanocomposites will be published elsewhere.41 3.3.1. Raman Spectroscopy. Individual SiC nanocrystals have distinct Raman bands overlapping PANI bands in the regions 1340-1350 cm-1 and 1590-1600 cm-1 (Figure 6), which can be assigned to carbon with sp3 and sp2 hybridization layer at the nanocrystals’ surface.32 Typical Raman spectra of individual PANI-CSA and nanocomposite SiC/PANI-CSA are almost identical if the bands of SiC are taken into account (Figure 6). Specifically, both spectra contain bands at 1622 cm-1 and at 1187 cm-1, which are assigned to CdC stretching and C-H bending of benzoid units, correspondingly, and reveal the doped state of PANI.42 This is confirmed with the presence of the wide band at 1241 cm-1 being characteristic of C-N stretching mode of polaron units.43 Besides, the left side of the wide band at 1338 cm-1 can be probably attributed to the >C-N+‚ stretching mode of delocalized polaron charge carriers44 despite its overlapping the SiC band in the region of 1340-1350 cm-1. The presence of all these vibrational modes clearly proves that the formed PANI in the SiC/PANI-CSA nanocomopsite is in the conducting state. Actually, the Raman spectroscopy results suggest that there is no noticeable physical-chemical interaction between organic (PANI-CSA) and inorganic (SiC) components of the obtained nanocomposite. 3.3.2. Transmission Electron Microscopy. TEM images of SiC/PANI-CSA (Figure 7) display that SiC nanocrystals (more dense spots) are surrounded with diffusive polymer areas with thickness from 0.5 nm to a few nm. In most cases, SiC nanoparticles are embedded in the PANI-CSA matrix (Figure 7a). However, it is possible to see the separate SiC nanoparticles (cores) coated with a PANI-CSA layer (shell) of an asymmetric shape (Figure 7b). These TEM images and the above results of UV-vis and OCP monitoring of the aniline polymerization suggest that the nanosized layer (shell) of PANI at the surface of SiC nanoparticles is formed in situ during the polymerization process. After formation of these core-shell nanoparticles in the dispersion mixture, their agglomeration is obviously minimal because of a positive charge of their surface, which appears because of the oxidation of growing PANI chains (propagation
Figure 7. TEM images of SiC/PANI-CSA nanocomposite: (a) agglomerated nanoparticles, (b) separated nanoparticle.
stage) and dissociation of the formed PANI salt (the emeraldine formation stage) in the shell. After the polymerization process has completed, the separation procedures of these nanoparticles from the reaction mixture lead to a conversion of the PANI salt in the shell to an undissociated state and, respectively, to minimizing Coulomb repulsion among the nanoparticles. As a consequence, these nanoparticles agglomerate to form larger particles that can finally cause their embedding into the PANICSA matrix. Conclusion Simultaneous in-situ UV-vis and OCP monitoring of the low concentrated (diluted) An-CSA salt polymerization displayed that during the IP step (t1) the transition state formed, which probably included anilinium cation, the acid anion, and the oxidant, antecedent to the propagation step. No aniline oligomers were registered at the IP step under the investigation conditions but they appeared after it had finished. Judging by the UVvis, OCP data, and visual observations, the propagation step consists of substeps (t2 and t3) corresponding to some quantita-
2180 J. Phys. Chem. B, Vol. 111, No. 9, 2007 tive and qualitative transitions in the formed PANI. Specifically, the first substep (t2) begins with the formation of soluble aniline oligomers in a colloidal nanosized pernigraniline state and ends in the clear transition point (t2/t3 transition point) concurring in the kinetic and OCP profiles of the polymerization process and when the visible insoluble pernigraniline phase appears. Moreover, in the case of the presence of a dispersion phase of any nanoparticles (e.g., SiC nanocrystals) in the polymerization mixture, this transition point may correspond also to the beginning of pernigraniline precipitation at their surface, that is, to the beginning of formation of PANI shell at SiC core. This important fact opens the possibility both to control the PANI shell thickness at any nanoparticles in the polymerization mixture and to affect properties of the final hybrid nanocomposite. The next substage (t3) lasts until the first emeraldine units appear in the growing pernigraniline chains. It can be clearly seen by changes of the UV-vis spectra by both the appearance of the polaron absorption and the change of the exciton band position. At the low aniline and acid concentrations, this qualitative pernigraniline-to-emeraldine transformation begins obviously even earlier than the OCP maximum is reached and all the oxidant is consumed. As a consequence, we may deduce that pernigraniline reduction by aniline begins earlier than it is generally accepted and probably in parallel to a continuing appearance of pernigraniline units even in the same chains that undergo the reduction. The addition of SiC dispersion phase into the polymerization mixture accelerates differently all stages of the aniline polymerization. Specifically, the stage of formation of colloid (nanosized) pernigraniline (t2) is accelerated less than the stages of formation of the pernigraniline precipitate and its transformation to ES state. This may be explained by an additional effect of high surface of the insoluble PANI (pernigraniline) nanosized shells formed during stage t3. Finally, this polymerization process leads to the formation of the PANI-CSA shell with the thickness in the range from 0.5 nm to a few nm at SiC nanocrystals’ surface. However, the separation and drying procedures cause probably the agglomeration of the composite nanoparticles that can be easily observed from TEM images. No noticeable physical-chemical interaction between organic (PANI-CSA) and inorganic (SiC) components was proved by Raman spectroscopy. As a matter of fact, PANI chains are plausibly only adsorbed on the SiC nanoparticles’ surfaces creating a free volume between the components of the hybrid nanocomposite as it was demonstrated by molecular dynamics methods.45 Acknowledgment. The scientific exchange visits in the frames of Ukrainian-French program DNIPRO were useful for a successful performance of these investigations. The authors greatly appreciate A. Bulou and S. Kodjikian from Universite´ du Maine for the help in performing Raman spectroscopy and TEM measurements, respectively. References and Notes (1) Huang, J.; Kaner, R. B. J. Am. Chem. Soc. 2004, 126, 851855.
Pud et al. (2) Kolla, H. S.; Surwade, S. P.; Zhang, X.; MacDiarmid, A. G.; Manohar, S. K. J. Am. Chem. Soc. 2005, 127, 16770-16771. (3) Chiou, N. R.; Epstein, A. J. AdV. Mater. 2005, 17, 1679-1683. (4) Chiou, N. R.; Epstein, A. J. Synth. Met. 2005, 153, 69-72. (5) Li, D.; Kaner, R. B. J. Am. Chem. Soc. 2006, 128, 968-975. (6) Huang, J.; Kaner, R. B. Chem. Commun. 2006, 4, 367-376. (7) Blinova, N. V.; Stejskal, J.; Trchova, M.; Prokes, J. Polymer 2006, 47, 42-48. (8) Malinauskas, A.; Malinauskiene, J. Chemija 2005, 16, 1-7. (9) Yang, Y.; Yang, W. Polym. AdV. Technol. 2005, 16, 24-31. (10) Konyushenko, E. N.; Stejskal, J.; Sedenkova, I.; Trchova, M.; Sapurina, I.; Cieslar, M.; Prokes, J. Polym. Int. 2006, 55, 31-39. (11) Trchova, M.; Sedenkova, I.; Konyushenko, E. N.; Stejskal, J.; Holler, P.; Ciric-Marjanovic, G. J. Phys. Chem. B 2006, 110, 9461 -9468. (12) Li, D.; Kaner, R. B. Private communication. (13) Kinlen, P. J.; Liu, J.; Ding, Y.; Graham, C. R.; Remsen, E. E. Macromolecules 1998, 31, 1735-1744 and citations herein. (14) Pron, A.; Rannou, P. Prog. Polym. Sci. 2002, 27, 135 -190. (15) Madathil, R.; Ponrathnam, S.; Byrne, H. J. Polym. 2004, 45, 54655471. (16) Shreepathi, S.; Holze, R. Langmuir 2006, 22, 5196-5204. (17) Haba, Y.; Segal, E.; Narkis, M.; Titelman, G. I.; Siegmann, A. Synth. Met. 1999, 106, 59-66. (18) Kruis, E.; Fissan, H.; Peled, A. J. Aerosol Sci. 1998, 29, 511535. (19) Nelson, J. Curr. Opin. Solid State Mater. Sci. 2002, 6, 87-95. (20) Gangopadhyay, R.; De, A. Chem. Mater. 2000, 12, 608-623. (21) Kickelbick, G. Prog. Polym. Sci. 2003, 28, 83-114. (22) Armes, S. P.; Gottesfeld, S.; Beery, J. G.; Garzon, F.; Agnew, S. F. Polymer 1991, 32, 2325-2330. (23) Stejskal, J.; Kratochvil, P.; Jenkins, A. D. Polymer 1996, 37, 367369. (24) Cao, Y.; Smith, P.; Heeger, A. J. Synth. Met. 1993, 57, 35143519. (25) Janata, J.; Josowicz, M. Nat. Mater. 2003, 2, 19-24. (26) Tang, B. Z.; Geng, Y.; Lam, J. W. Y.; Li, B. Chem. Mater. 1999, 11, 1581-1589. (27) Boucle´, J.; Herlin-Boime, N.; Kassiba, A. J. Nanopart. Res. 2005, 7, 275-285. (28) Konorov, S. O.; Ivanov, A. A.; Alfimov, M. V.; Fornarini, L.; Carpanese, M.; Avella, M.; Errico, M. E.; Petrov, A. N.; Fantoni, R.; Zheltikov, A. M. J. Opt. A: Pure Appl. Opt. 2004, 6, 253-258. (29) Charpentier, S.; Kassiba, A.; Emery, J.; Cauchertier, M. J. Phys.: Condens. Matter. 1999, 11, 4887-4897. (30) Stejskal, J.; Quadrat, O.; Sapurina, I.; Zemek, J.; Drelinkiewicz, A.; Hasik, M.; Krivka, I.; Prokes, J. Eur. Polym. J. 2002, 38, 631-637. (31) Stejskal, J.; Gilbert, R. G. Pure Appl. Chem. 2002, 74, 857-867. (32) Kassiba, A.; Makowska-Janusik, M.; Boucle, J.; Bardeau, J.-F.; Bulou, A.; Herlin, N.; Mayne, M.; Armand, X. Diamond Relat. Mater. 2002, 11, 1243-1247. (33) Tabellout, M.; Fatyeyeva, K.; Baillif, P.-Y.; Bardeau, J.-F.; Pud, A. A. J. Non-Cryst. Solids 2005, 351, 2835-2841. (34) Bremer, L. G. B.; Verbong, M. W. C. G.; Webers, M. A. M.; van Doorn, M. A. M. M. Synth. Met. 1997, 84, 355-356. (35) Gill, M. T.; Chapman, S. E.; DeArmit, C. L.; Baines, F. L.; Dadswell, C. M.; Stamper, J. G.; Lawless, G. A.; Billingham, N. C.; Armes, S. P. Synth. Met. 1998, 93, 227-233. (36) Duic´, L.; Kraljic´, M.; Grigic´, S. J. Polym. Sci., Part A: Polym. Chem. 2004, 42, 1599-1608 and citations herein. (37) Kang, E. T.; Neoh, K. G.; Tan, K. L. Prog. Polym. Sci. 1998, 23, 211-324. (38) Manohar, S. K.; MacDiarmid, A. G.; Epstein, A. J. Synth. Met. 1991, 41, 711-714. (39) Wei, Y.; Hsueh, K. F.; Jang, G.-W. Polymer 1994, 35, 3572-3575. (40) Tzou, K.; Gregory, R. V. Synth. Met. 1992, 47, 267-277. (41) Bednarski, W.; Kassiba, A.; Pud, A.; Kodjikian, S.; Bulou, A.; Nguyen, T. P.; Hilali, M. A.; Fatyeyeva, K.; Ogurtsov, N. Mater. Sci. Eng. B, sent for publication. (42) Pereira da Silva, J. E.; Temperini, M. L. A.; Cordoba de Torresi, S. I. Electrochim. Acta 1999, 44, 1887-1891. (43) Laska, J.; Girault, R.; Quillard, S.; Louarn, G.; Pron, A.; Lefrant, S. Synth. Met. 1995, 75, 69-74. (44) Furukawa, Y.; Ueda, F.; Hyodo, Y.; Harada, I.; Nakajima, T.; Kawagoe, T. Macromolecules 1988, 21, 1297-1305. (45) Makowska-Janusik, M.; Kassiba, A.; Failleau, G.; Boucle´, J. Mater. Sci. J. (Poland) S, to be published.
