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Nanostructures produced by conducting polymer, polyaniline

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Název:
Nanostructures produced by conducting polymer, polyaniline
Název v češtině:
Nanostruktury vytvářené vodivým polymerem, polyanilinem
Typ:
Disertační práce
Autor:
Elena Konyushenko, Ph.D.
Školitel:
RNDr. Jaroslav Stejskal, CSc.
Oponenti:
prof. RNDr. Jiří Vohlídal, CSc.
prof. Majda Žigon
Id práce:
112523
Fakulta:
Přírodovědecká fakulta (PřF)
Pracoviště:
Katedra fyzikální a makromol. chemie (31-260)
Program studia:
Makromolekulární chemie (P1405)
Obor studia:
-
Přidělovaný titul:
Ph.D.
Datum obhajoby:
6. 11. 2008
Výsledek obhajoby:
Prospěl/a
Jazyk práce:
Čeština
Abstrakt:
Charles University in Prague Faculty of Science Department of Physical and Macromolecular Chemistry Institute of Macromolecular Chemistry Academy of Sciences of the Czech Republic Nanostructures produced by conducting polymer, polyaniline Abstract of the Doctoral Thesis Elena Konyushenko Supervisor: Jaroslav Stejskal, PhD. Prague 2008 Univerzita Karlova v Praze Přírodovědecká fakulta Katedra fyzikální a makromolekulární chemie Ústav makromolekulární chemie Akademie věd České republiky, v.v.i. Nanostruktury vytvářené vodivým polymerem, polyanilinem Autoreferát disertační práce Ing. Elena Konyushenko Školitel: RNDr. Jaroslav Stejskal, CSc. Praha 2008 Contents List of papers 1. Konyushenko EN, Stejskal J, Šeděnková I, Trchová M, Sapurina I, Cieslar M, Proke J, 1. The aims of the Thesis 5 Polyaniline nanotubes: conditions of formation, POLYMER INTERNATIONAL 55: 31–39 (2006). 2. Preparation of polyaniline 6 2. Trchová M, Šeděnková I, Konyushenko EN, Stejskal J, Holler P, 3. Polymerization of aniline in the solutions of weak acids 7 Ćirić-Marjanović G, Evolution of polyaniline nanotubes: The oxidation of aniline in water, 4. Oxidation of aniline in alkaline media 8 JOURNAL OF PHYSICAL CHEMISTRY B 110: 9461–9468 (2006) 5. Supramolecular morphology of polyaniline 9 3. Konyushenko EN, Stejskal J, Trchová M, Hradil J, Kovářová J, Prokeš J, Cieslar M, Hwang J-Y, Chen K-H, Sapurina I, 6. Preparation of composites of carbon nanotubes and polyaniline 15 Multi-wall carbon nanotubes coated with polyaniline , POLYMER 47: 5715–5723 (2006). 6.1. Carbon nanotubes coated by polyaniline 15 4. Stejskal J, Sapurina I, Trchová M, Konyushenko E, Holler P, 6.2. Properties of the composites 15 The genesis of polyaniline nanotubes, POLYMER 47: 8253–8262 (2006). 7. Carbon nanotubes with nickel catalyst nanoparticles coated by polyaniline 16 5. Konyushenko EN, Kazantseva NE, Stejskal J, Trchová M, Kovářová J, Sapurina I, Abstract (in Czech) 18 Tomishko MM, Demicheva OV, Prokeš J, Ferromagnetic behaviour of polyaniline-coated multi-wall carbon nanotubes containing References 19 nickel nanoparticles, JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS 320: 231–240 (2008). List of papers 21 6. Stejskal J, Sapurina I, Trchová M, Konyushenko EN, Oxidation of aniline: Polyaniline granules, nanotubes, and oligoaniline microspheres, MACROMOLECULES, proofs. 7. Konyushenko EN, Stejskal J, Trchová M, Blinova NV, Holler P, Polymerization of aniline in ice, SYNTHETIC METALS, after revision. 4 21 (Stejskal et al. 2006) Stejskal J, Sapurina I, Trchová M, Konyushenko EN, Holler P, Polymer 1. The aims of the Thesis 47:8253 (2006). The present Thesis consists of 6 papers published in international journals with an (Stejskal et al. 2008)Stejskal J, Sapurina I, Trchová M, Konyushenko EN, Macromolecules extended introduction and a detailed discussion of the content of these papers. This work is in press. divided into two main topics. (Tagowska et al. 2004) Tagowska M, Palys B, Jackowska K, Synth Met 142:223 (2004). (1) The preparation of various polyaniline morphologies, especially nanotubes, at (Trchová et al. 2006) Trchová M, Konyushenko EN, Stejskal J, Šeděnková I, Holler P and various polymerization conditions. Aniline can be oxidized by ammonium peroxydisulfate in Ćirić-Marjanović G, J Phys Chem B 110:9461 (2006). the solutions of strong inorganic or weak organic acids. The acidity of the reaction plays one (Venancio et al. 2006) Venancio EC, Wang P-C, MacDiarmid AG, Synth Met 156:357 of the crucial roles in the formation of supramolecular structures of polyaniline. There are (2006). many others parameters that are regarded to be important in the control the properties and (Vonsovskii et al. 1998) Vonsovskii SV, Magnetism, Nauka, Moscow 1971, Chapter 23, pp. polymer morphology. These include the chemical nature of the oxidant, the acid protonating 800–805. aniline and reaction intermediates during the oxidation, concentration of reactants (especially (Wang et al. 2005) Wang X, Liu N, Yan X, Zhang W, Wei Y, Chem Lett 34:42 (2005). that of aniline and oxidant) and their molar proportions, temperature, solvent components, the presence of additives, etc. We have shown that each of these parameters has influence on the morphology of polyaniline. Many analyses have been done to prove the theoretical prediction of aniline oxidation. These include IR and Raman spectroscopies, UV–visible spectra, gel permeation chromatography, scanning and transmission electron microscopies, etc. (2) The preparation of composites of multi-wall carbon nanotubes (CNT) and polyaniline. Since the discovery of CNT, attention has been given to its surface modification in order to get separated and uniformly dispersed carbon nanotubes and to produce useful composite materials. Multi-wall carbon nanotubes have been coated by conducting polymer polyaniline, as the method to modify the surface of CNT. In such composite materials, PANI acts as an adhesive of the individual carbon nanotube and converts the original hydrophobic surface of CNT to hydrophilic one. Carbon nanotubes prepared by chemical-vapour deposition method often include metal nanoparticles, which serve as catalytic centers for the nanotubular growth. CNT filled with nickel particles were coated by polyaniline film in different conditions. Such composites materials have been studied. CNT/PANI composites materials with nickel particles have the coercivity an order of magnitude higher than that of bulk nickel (Bao et al. 2002, Cao et al. 2001). 20 5 2. Preparation of polyaniline References Aniline is oxidized by ammonium peroxydisulfate in aqueous media (Figure 1). (Bao et al. 2002) Bao JC, Zhou QF, Hong JM, Xu Z, Appl Phys Lett 81:4592 (2002). During the polymerization, the sulfuric acid is produced a by-product. (Bukhaev et al. 1998) Bukhaev AA, Ovchinnikov DV, Nurgazizov NI, Kukovitskii EF, Klaiber M, Weisendanger R, J Phys Solid State 40:1163 (1998). 4n NH2 + 5n(NH4)2S2O8 (Cao et al. 2001) Cao H, Tie C, Xu Z, Hong J, Sang H, Appl Phys Lett 78:1592 (2001). (Ćirić-Marjanović et al. 2006) Ćirić-Marjanović G, Trchová M, Stejskal J, Collect Czech HX Chem Commun 71:1407 (2006). (Cram et al. 1964) Cram DJ, Hammond GS, Organic Chemistry, McGraw Hill, New York H H N N 1964, pp 210–211. + 3nH2SO4 + 5n(NH4)2SO4 (Fu et al. 1994) Fu Y, Elsenbaumer RL, Chem Mater 6:671 (1994). +. +. n N N (Han et al. 2006) Han J, Song G, Guo R, Adv Mater 18:3140 (2006). H - H - HSO4 HSO4 (Komura et al. 2000) Komura T, Ishihara M, Yamaguchi T and Takachashi K, J Electroanal Figure 1. The sheme of aniline polymerization. Chem 493:84 (2000). (Konyushenko et al. 2006a) Konyushenko EN, Stejskal J, Šeděnková I, Trchová M, Sapurina We have regarded the course of aniline oxidation, the dependences of pH and I, Cieslar M, Prokeš J, Polym Int 55:31 (2006). temperature on time (Konyushenko et al. 2006a). It was shown that when the polymerization (Konyushenko et al. 2006b) Konyushenko EN, Stejskal J, Trchová M, Hradil J, Kovářová J, starts in the solutions of strong acids, the process has two steps (Figure 2). Prokeš J, Cieslar M, Hwang J-Y, Chen K-H, Sapurina I, Polymer 47:5715 (2006). (Konyushenko et al. 2008a) Konyushenko EN, Kazantseva NE, Stejskal J, Trchová M, Kovářová J, Sapurina I, Tomishko MM, Demicheva OV, Prokeš J, J Magn Magn 3 Mater 320:231 (2008). 0.1 M H2SO4 (Konyushenko et al. 2008b) Konyushenko EN, Stejskal J, Trchová M, Blinova NV, Holler P, 35 Synth Met, submitted. oC 2 30 (Liu et al. 2006) Liu XX, Zhang L, Li Y-B, Bian L-J, Huo YQ, Su Z, Polym Bull 57:825 Temperature, (2006). pH 25 (Saez et al. 1993) Saez EI, Corn RM, Electrochim Acta 38:1619 (1993). 1 Figure 2. The changes in (Stejskal et al. 1995) Stejskal J, Kratochvil P, Jenkins AD, Collect Czech Chem Commun 20 temperature (squares) and acidity 60:1747 (1995). (circles) during the oxidation of 0 (Stejskal et al. 2002) Stejskal J in Dendrimers, Assemblies, Nanocomposites, The MML Ser. 0 200 400 600 800 0.2 M aniline with 0.25 M ammonium peroxydisulfate in Vol. 5, Arshady R and Guyot A, Ads, Chap. 6 and 7, pp. 195 – 281, Citus Books, Time, s 0.1 M sulfuric acid. London 2002. 6 19 Abstrakt Aniline molecules are present dominantly in the form of anilinium cations in the Disertační práce se skládá z šesti prací publikovaných v mezinárodních časopisech a solutions of strong acids; the concentration of neutral aniline molecules is very small, that is jednoho rukopisu v recenzním řízení po revizi, s rozšířeným úvodem a podrobným rozborem why the process of oxidation is slow (we can observed the induction period Figure 2). The obsahu těchto článků. Všechny práce se týkají elektricky vodivého polymeru, polyanilinu. induction period extends for several minutes; there is virtually no heat evolved during this Cílem práce byly: stage, and also the changes in pH are hardly spotted. In such conditions, the oxidation of anilinium cations yields oligomers, dimers and trimers. The blue colour at the beginning of (1) Připrava různých nadmolekulárních struktur polyanilinu za různých podmínek the induction period corresponds to the oxidized forms of semedines, amino-diphenylamines přípravy, zejména syntéza polyanilinových nanotrubek. (Ćirić-Marjanović et al. 2006). Anilin může být polymerován oxidací peroxodisíranem ammoným v roztocích The induction period is followed the fast exothermic process associated with the silných nebo slabých kyselin. Kyselost reakční směsi je jedním z nejdůležitejších parametrů, growth of the polymer chains. The reaction mixture becomes heterogeneous because the které mají vliv na vznik nadmolekulárních struktur polyanilinu. Existují i jiné faktory, které reaction intermediates and products are not soluble in the medium. The blue colour of the mohou ovlivňovat vlastnosti a morfologii polymeru, jako jsou povaha oxidačního činidla, reaction mixture deepens (the absorption maximum is located at 690 nm (Stejskal et al. kyseliny, která protonuje polyanilin i reakční meziprodukty v průběhu oxidace, dále 1995), the thin polymer film is produced at the air/solution interface having a violet metallic koncentrace monomerů a oxidantu, teplota, typ rozpouštědla, přítomnost aditiv, atd. tint. Conducting PANI having a granular morphology is obtained as a precipitate. Polyanilin připravený v různých podmínkách byl nasledně analyzován různými metodami. FTIR a Ramanova spektra byla použita pro analýzu molekulární struktury, gelová 3. Polymerization of aniline in the solutions of weak acids permeační chromatografie pro studium molekulových vah, vodivost byla měřena When the polymerization is conducted in the solutions of weak acids, the aniline and čtyřbodovou metodu, a mikroskopické metody byly použity pro studium nadmolekulárních primary amino groups in oligomers become protonated but the intrinsic secondary amino struktur. Na základě analýzy mechanismu oxidace anilinu v různých podmínkách byla groups are not. The temperature profile during the oxidation of aniline with ammonium objasněna vznikající morfologie oxidačních produktů anilinu – granulární struktura, peroxydisulfate has three phases (Figure 3). nanotrubky a mikrokuličky. 40 Oxidation of aniline 38 (2) Příprava kompozitů uhlíkových nanotrubek a polyanilinu. 8 in 0.4M acetic acid 36 Od objevu uhlíkových nanotrubek se věnuje pozornost na jich povrchové modifikaci 34 6 s důrazem na jejich oddělení a rovnoměrnou dispergaci v kompozitních materiálech. Temperature, C 32 Uhlikové nanotrubky byly koaxiálně pokryty polyanilinem v průběhu oxidace anilinu. 30 pH 4 Vzniká struktura podobná polyanilinovým nanotrubkách uvedeným v předchozím bodu. Byly 28 Figure 3. Time o 26 použity dva typy uhlikových nanotrubek: (a) čisté uhlíkové nanotrubky a (b) uhlíkové dependence of pH and 2 24 nanotrubky obsahující nanočástice niklu, použité jako katalyzátor pro jejich přípravu. temperature during the 22 Vlastnosti kompozitů byly analyzovány různými metodami. polymerization of aniline 0 20 0 5 10 15 20 25 30 35 40 45 in 0.4 M acetic acid. Time, min 18 7 The polymerization in the aqueous solutions of weak acids (acetic, succinic, film grows on the surface of CNT producing a core–shell morphology. Magnetic spectra phosphoric acids, etc.) starts at pH ~ 4.5 and drops as sulfuric acid has been produced by the frequency of complex permeability, µ*(f), of PANI–CNT composites with different ratio of decomposition of peroxydisulfate, and hydrogens are removed from aniline as protons. PANI and CNT are presented in Figure 14. Neutral aniline molecules are easily oxidized to oligomers, and the released protons reduce The composites with 10–40 wt% CNT do not exhibit any ferromagnetic properties; the pH (Figure 3). The equilibrium between the aniline molecules and anilinium cations the real part of permeability, µ′, is around the unity and the magnetic losses, µ″, are close to shifts in favour of latter species. Anilinium cations are much more difficult to be directly zero in the frequency range from 1 MHz to the 10 GHz. The ferromagnetic behaviour of oxidized, because the electron pair on nitrogen, which is delocalized in neutral aniline composites appears when the CNT content increases to 50 wt%, which approximately molecules, becomes localized in anilinium cations (Cram et al. 1964). That is why the corresponds to 10 wt.% of nickel. The character of µ*(f) indicates that single-domain nickel reaction virtually stops (Fu et al. 1994, Stejskal et al. 2006), and the temperature starts to nanoparticles (single-domain-size criterion for nickel particles is 6–7 nm) in CNT are decrease (Figure 3). Yet, there is always equilibrium between the protonated and neutral exchange-coupled (Vonsovskii et al. 1998, Bukhaev et al. 1998). This is revealed by species, both species always coexist, and the oxidation of neutral aniline molecules still magnetic dispersion and two resonance peaks on the µ″(f), the first located in low-frequency proceeds with preference even if their content is very low (Stejskal et al. 2008). This is range, 1–10 MHz, and the second in high-frequency range, 1–3 GHz. By increasing CNT up manifested by a continuing decreasing pH under such conditions (Figure 3). The situation to 80 wt.% nickel content increases, and thus enhances both permeability components in corresponds to the induction period observed in the classical polymerization of aniline in the specified frequency regions. acidic media. The situation changes when pH<2 (Figure 3). Such conditions correspond to those of oxidation in the solutions of strong acids, and indeed the sulfuric acid is produced in 1.6 1.0 Permeability, imaginary part 80 wt.% the course of oxidation (Figure 1). The exothermic polymerization then follows (Figure 3). Permeability, real part 1.4 70 wt.% The oxidation products contain both oligomeric and polymeric component, the latter often in 0.5 50 wt.% 1.2 80 wt.% 70 wt.% 50 wt.% nanotubular form. 1.0 − 10−30 wt.% 0.0 − 10−30 wt.% 4. Oxidation in alkaline media 0.8 The oxidation of aniline in alkaline media proceeds when pH>4. Under such acidity 0.6 -0.5 107 108 109 1010 107 108 109 1010 conditions all kinds of amino groups (the aniline monomer, the terminal amino groups in Frequency, Hz Frequency, Hz oligomers and the secondary amino groups) are not protonated. The pH and temperature dependences are represented by a single process (Figure 4). Figure 14. The frequency dependence of real (left) and imaginary (right) parts of complex permeability of CNT The oxidation in 0.2 M ammonium hydroxide starts at pH 10 and pH decreases due to coated with protonated conducting PANI. The content of CNT is given at the individual curves (Konyushenko et al. 2008a). the generation of protons. The exothermic oxidation starts immediately after mixing of reactants, without any induction period and proceeds with a high rate. Temperature monotonously increases during the whole oxidation process and cools only after one of the reactant or both become depleted. The slow-down of the reaction is observed at later stages (Liu et al. 2006). Protons produced in the oxidation reduce pH to 4. The reaction terminates 8 17 conducting PANI bases, a two-point method was used. Before such measurements, circular by the depletion of neutral aniline molecules that have reacted or became converted to gold electrodes were deposited on both sides of pellets. The dependence of conductivity on anilinium cations. The pH does not reach the acidity needed for the protonation of imine CNT content for CNT coated by PANI is shown in the Figure 13. At low fractions of CNT, groups in pernigraniline, i.e. for the polymer growth. Only non-conducting aniline oligomers the conductivity is determined by the PANI as the main component (Figure 13). are produced. The increase in the conductivity of CNT coated with non-conducting PANI base with increasing content of CNT is slow, compared with the CNT coated by protonated conducting 48 polyaniline. At low concentrations of CNT, the direct electrical contact between CNT is 10 44 prevented by the surface coating with a non-conducting PANI base and the percolation 40 threshold is practically not distinguishable. Yet, the coating with PANI base seems to reduce 8 oC 36 the contact resistance between CNT. The resulting conductivity of CNT coated with PANI Temperature, pH 6 base thus reaches 19.6 S cm–1 at 70 wt.% CNT. The conductivity is comparable with that of 32 the CNT coated with protonated polyaniline. We are obvious dealing with interfacial 28 Figure 4. The pH and 4 temperature dependences during phenomena between the CNT separated with a thin coating of PANI. A non-conducting 24 the aniline oxidation in 0.2 M PANI base is not a true insulator, and it can probably mediate the charge-carrier transfer over 2 20 ammonium hydroxide. short distances between the neighboring CNT, similarly to the protonated conducting form 0 10 20 30 40 50 Time, min (Konyushenko et al. 2006b). 5. Supramolecular morphology of polyaniline 101 We have studied the supramolecular morphology of PANI that depends on the starting conditions of aniline oxidation. One of the most interesting polyaniline structures is Conductivity, S cm−1 10-2 represented by nanotubes. We have polymerized aniline in the presence of 0.4 M acetic acid and just in water. The genesis of aniline oxidation was studied by several methods, such as 10-5 PANI-coated CNT gel permeation chromatography, UV–visible, Raman, and FTIR spectroscopies (Stejskal et Protonated PANI PANI base Figure 13. The conductivity of carbon al. 2006, Trchová et al. 2006). 10-8 nanotubes coated with protonated PANI (full During the oxidation of aniline beginning in neutral media, we have observed the circles) and PANI base (open circles) evolution of the water-insoluble products of the reaction. The silicon windows were placed 10-11 0 20 40 60 80 (Konyushenko et al. 2006b). Composition, wt. % CNT into the reaction mixture and removed at various stages of aniline oxidation. The aniline oligomers produced in the early stages of the oxidation are insoluble in the reaction medium. The oxidation products, deposited on the windows, have been studied by optical microscopy. 7. Carbon nanotubes with nickel catalyst nanoparticles coated by polyaniline There is a link between the course of the polymerization and the supramolecular morphology, Polyaniline coating was produced again by the oxidation of aniline hydrochloride here nanotubes, produced at various reaction times (Figure 5). dissolved in ethanol with an aqueous solution of ammonium peroxydisulfate. Polyaniline 16 9 The product obtained in the first stage of aniline oxidation, after 2–8 min, are 6. Preparation of composites materials: carbon nanotubes/polyaniline composed mostly of oligomer crystals, which are obviously insoluble in water, having the Multi-wall carbon nanotubes (CNT) have been coated with a conducting polymer, size of several micrometers (Figure 5 A). The crystals grow to trees (dendrimers) with polyaniline, directly during the oxidation of aniline in ethanol (50 vol.%)–water mixture. The branches of tens of micrometers in length. On the window removed at 21 min (Figure 5 B), content of CNTs in the samples was 0–80 wt.%. The monomer and then the oxidant solutions we have observed, in addition to the granular precipitate, the first nanotubes growing. After were added to various portions of CNT to start the polymerization of aniline. We have been 31 min (Figure 5 C), well visible nanotubes are observed. The nanotubes extend often to a used two kind of carbon nanotubes: (1) pure carbon nanotubes and (2) carbon nanotubes with few micrometers, their external diameter being ca. 200 nm and internal diameter of the cavity incorporated nickel nanoparticles. ca. 20 nm. The granular PANI precipitate is produced close to the end of polymerization (Figure 5 D). As it sediments, it covers the nanotubular structure deposited on the silicon 6.1. Carbon nanotubes coated by polyaniline film windows. This precipitate virtually always accompanies the nanotubes. The aniline oligomers The uniform coating of carbon nanotubes by conducting polymer film can be see on produced in the early stages of the oxidation are insoluble in the reaction medium. Oligomers the SEM and TEM micrographs (Figure 12).The coated CNT become thicker as the amount precipitate, often with frequent needle-like crystallite offsprings (Figure 5 A). These of deposited PANI increases. The uniform deposition of PANI on the CNT is similarly crystallites serve as the templates for the future nucleation of polyaniline nanotubes. demonstrated by transmission electron microscopy, which shows the bilayered structure of coated CNT (Figure 12). As the internal cavity is well discernible, we conclude that the coating with PANI takes place only at the outer surface of the CNT (Konyushenko et al. 2006b). The conducting polymer is not covalently bonded to the carbon nanotubes. The aniline oligomers are adsorbed (Stejskal et al. 2005) at the surface of CNT and start the growth of polyaniline coating there. (A) (B) Figure 12. SEM (left) and TEM (right) images of CNT coated by PANI (Konyushenko et al. 2006b). (C) (D) Figure 5. Optical microscopy of the oxidation products deposited on silicon windows after reaction times t = 6.2. Properties of the composite of carbon nanotubes and polyaniline The conductivity of the composites is a characteristic of prime interest. It was (A) 5, (B) 21, (C) 31, and (D) 38 min (Stejskal et al. 2006). measured by a four-point van der Pauw method on pellets compressed at 700 MPa. For non- 10 15 on the surface of the droplets is similar to the oxidation at the interface water-organic liquid, When the acidity of the reaction mixture becomes sufficiently high, the phenazine where aniline itself takes a role of an inorganic liquid. units may initiate the propagation of polymer chains. Due to their flat structure, such hydrophobic units adsorb on the available surfaces. The adsorption of phenazine units at the surface of needle-like oligomer offsprings is selective, due to the obvious anisotropy of the crystallites. We assume that the phenazine units are adsorbed on the walls of the oligomer crystallites, leaving the front surfaces uncoated. In this way, the nucleus of the nanotubes is produced as a sleeve on an oligomer needle (Stejskal et al. 2006, Trchová et al. 2006). The size of the crystallites inside determines the inner diameter of the future nanotube. The thickness of the nanotube wall corresponds to the thickness of the deposited PANI film and it is proportional to the molecular weight of the PANI chains. Figure 10. SEM (left) and TEM (right) images of microspheres which are formed during the oxidation of aniline The agglomeration of phenazine units in the solutions leads to the formation of in 0.2 M ammonium hydroxide. precipitate. On the other hand, the sorption of oligomers on the surface immersed in the reaction mixture gives rise to thin films during the subsequent polymerization. The type of Polyaniline nanofibers can be prepared when aniline is oxidized under reduced organization of phenazine cycles on the template surface or in the volume of the reaction oC. temperature –24 The polymerization takes place in the solid state; the originally white ice mixture predetermines the future morphology of the supramolecular polymer growth. becomes gradually dark green. The morphology of polyaniline is different from the Depending on the reaction conditions and nature of the template, the sorption or polymerizations made in the liquid media. Polyaniline is constituted by particles of agglomeration of phenazines may be random or organized. The polyaniline chains are submicrometre size, mutually connected by nanofibers of ca 20 nm thickness (Figure 11). growing from them in the preferred direction given by molecular geometry. Assembly of the terminal phenazine units thus binds the polyaniline chain-beginnings together. The self- ordering produced is further stabilized during the chain growth of polyaniline counter-parts by hydrogen bonding and ionic interactions (Figure 6). That is why the nanotube continues to grow without any guide and with an internal cavity determined by nucleus size. This concept can be supported by the observation of nanotubular growth in polycarbonate membranes, which continue to grow beyond the membrane limits, still preserving the nanotubular morphology (Tagowska et al. 2004). Figure 11. Polyaniline prepared in frozen mixture of 0.1 M H2SO4 (left) and 0.4 M acetic acid (right) (Konyushenko et al. 2008b). Figure 6. The phenazine units at the start of PANI chains stack on each other, and PANI chains extend from them. The individual spiral threads are shown as being separated just for the clarity of illustration; we believe, however, that each thread closely copies the previous one and there is no separation between them (Stejskal et al. 2006). 14 11 The growth of one-dimensional structures can also be initiated without any template. We have synthesized polyaniline nanptubes with different surface morphology. It was In this case, we assume that phenazine terminal units associate with each other more loosely. shown that the kind of acid controls not only the acidity of the reaction but also the surface Stacks of one-dimensional fibrils of phenosafranines (including phenazine structures) of 1 morphology of the nanotubes (Figure 8). nm diameter, which corresponds approximately to the size of a phenazine heterocycle (Saez Polyaniline granular morphology is the most typical; it is prepared in a classical way et al. 1993), have been reported in the literature (Komura et al. 2000). Polyaniline chains in the acidic aqueous media (Stejskal et al. 2002, Konyushenko et al. 2006a) (Figure 9). extending from them produce a brush. A nanorod without an internal cavity is produced Polyaniline nanogranules have a size of tens to a few hundreds of nm. The mechanism of instead of a nanotube. Such nanorods are observed to accompany the nanotubes (Figure 7), their formation has not been analyzed in the literature. We assume that the globules are and their mutual proportions are likely to be the pH-dependent. produced as a result of random association of phenazine nucleates. Such association takes place when the concentration of nucleates is high, the solubility of nucleates is reduced and the polymer chain starts to grow before the phenazine structures are organized. Figure 9. The granular morphology of Figure 7. TEM micrograph of PANI nanotubes and related structures (Stejskal et al. 2006). polyaniline prepared in the presence of 0.1 M sulfuric acid (Konyushenko et al. 2006a). The formation of microspheres, which is observed in the oxidation of aniline (Wang et al. 2005, Venancio et al. 2006, Stejskal et al. 2008) or aniline derivatives (Han et al. 2006) at low acidity and at alkaline conditions, has another background. The neutral aniline is not always completely miscible with aqueous medium and constitutes a separate phase. The oxidation then proceeds at the interface between aniline droplets and liquid phase containing the oxidant. The non-conducting aniline oligomers form microspheres having a diameter of a few micrometres (Figure 10). A B The oxidation of aniline by the oligomerization mechanism takes place at the droplet Figure 8. The polyaniline nanotubes prepared in 0.4 M succinic acid (A) and 0.1 M picric acid (B). interface. The droplet surface becomes coated with a non-conducting oligomer. Aniline diffuses through the interface and the thickness of the oligomer layer increases. The synthesis 12 13
Abstract v angličtině:
Charles University in Prague Faculty of Science Department of Physical and Macromolecular Chemistry Institute of Macromolecular Chemistry Academy of Sciences of the Czech Republic Nanostructures produced by conducting polymer, polyaniline Abstract of the Doctoral Thesis Elena Konyushenko Supervisor: Jaroslav Stejskal, PhD. Prague 2008 Univerzita Karlova v Praze Přírodovědecká fakulta Katedra fyzikální a makromolekulární chemie Ústav makromolekulární chemie Akademie věd České republiky, v.v.i. Nanostruktury vytvářené vodivým polymerem, polyanilinem Autoreferát disertační práce Ing. Elena Konyushenko Školitel: RNDr. Jaroslav Stejskal, CSc. Praha 2008 Contents List of papers 1. Konyushenko EN, Stejskal J, Šeděnková I, Trchová M, Sapurina I, Cieslar M, Proke J, 1. The aims of the Thesis 5 Polyaniline nanotubes: conditions of formation, POLYMER INTERNATIONAL 55: 31–39 (2006). 2. Preparation of polyaniline 6 2. Trchová M, Šeděnková I, Konyushenko EN, Stejskal J, Holler P, 3. Polymerization of aniline in the solutions of weak acids 7 Ćirić-Marjanović G, Evolution of polyaniline nanotubes: The oxidation of aniline in water, 4. Oxidation of aniline in alkaline media 8 JOURNAL OF PHYSICAL CHEMISTRY B 110: 9461–9468 (2006) 5. Supramolecular morphology of polyaniline 9 3. Konyushenko EN, Stejskal J, Trchová M, Hradil J, Kovářová J, Prokeš J, Cieslar M, Hwang J-Y, Chen K-H, Sapurina I, 6. Preparation of composites of carbon nanotubes and polyaniline 15 Multi-wall carbon nanotubes coated with polyaniline , POLYMER 47: 5715–5723 (2006). 6.1. Carbon nanotubes coated by polyaniline 15 4. Stejskal J, Sapurina I, Trchová M, Konyushenko E, Holler P, 6.2. Properties of the composites 15 The genesis of polyaniline nanotubes, POLYMER 47: 8253–8262 (2006). 7. Carbon nanotubes with nickel catalyst nanoparticles coated by polyaniline 16 5. Konyushenko EN, Kazantseva NE, Stejskal J, Trchová M, Kovářová J, Sapurina I, Abstract (in Czech) 18 Tomishko MM, Demicheva OV, Prokeš J, Ferromagnetic behaviour of polyaniline-coated multi-wall carbon nanotubes containing References 19 nickel nanoparticles, JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS 320: 231–240 (2008). List of papers 21 6. Stejskal J, Sapurina I, Trchová M, Konyushenko EN, Oxidation of aniline: Polyaniline granules, nanotubes, and oligoaniline microspheres, MACROMOLECULES, proofs. 7. Konyushenko EN, Stejskal J, Trchová M, Blinova NV, Holler P, Polymerization of aniline in ice, SYNTHETIC METALS, after revision. 4 21 (Stejskal et al. 2006) Stejskal J, Sapurina I, Trchová M, Konyushenko EN, Holler P, Polymer 1. The aims of the Thesis 47:8253 (2006). The present Thesis consists of 6 papers published in international journals with an (Stejskal et al. 2008)Stejskal J, Sapurina I, Trchová M, Konyushenko EN, Macromolecules extended introduction and a detailed discussion of the content of these papers. This work is in press. divided into two main topics. (Tagowska et al. 2004) Tagowska M, Palys B, Jackowska K, Synth Met 142:223 (2004). (1) The preparation of various polyaniline morphologies, especially nanotubes, at (Trchová et al. 2006) Trchová M, Konyushenko EN, Stejskal J, Šeděnková I, Holler P and various polymerization conditions. Aniline can be oxidized by ammonium peroxydisulfate in Ćirić-Marjanović G, J Phys Chem B 110:9461 (2006). the solutions of strong inorganic or weak organic acids. The acidity of the reaction plays one (Venancio et al. 2006) Venancio EC, Wang P-C, MacDiarmid AG, Synth Met 156:357 of the crucial roles in the formation of supramolecular structures of polyaniline. There are (2006). many others parameters that are regarded to be important in the control the properties and (Vonsovskii et al. 1998) Vonsovskii SV, Magnetism, Nauka, Moscow 1971, Chapter 23, pp. polymer morphology. These include the chemical nature of the oxidant, the acid protonating 800–805. aniline and reaction intermediates during the oxidation, concentration of reactants (especially (Wang et al. 2005) Wang X, Liu N, Yan X, Zhang W, Wei Y, Chem Lett 34:42 (2005). that of aniline and oxidant) and their molar proportions, temperature, solvent components, the presence of additives, etc. We have shown that each of these parameters has influence on the morphology of polyaniline. Many analyses have been done to prove the theoretical prediction of aniline oxidation. These include IR and Raman spectroscopies, UV–visible spectra, gel permeation chromatography, scanning and transmission electron microscopies, etc. (2) The preparation of composites of multi-wall carbon nanotubes (CNT) and polyaniline. Since the discovery of CNT, attention has been given to its surface modification in order to get separated and uniformly dispersed carbon nanotubes and to produce useful composite materials. Multi-wall carbon nanotubes have been coated by conducting polymer polyaniline, as the method to modify the surface of CNT. In such composite materials, PANI acts as an adhesive of the individual carbon nanotube and converts the original hydrophobic surface of CNT to hydrophilic one. Carbon nanotubes prepared by chemical-vapour deposition method often include metal nanoparticles, which serve as catalytic centers for the nanotubular growth. CNT filled with nickel particles were coated by polyaniline film in different conditions. Such composites materials have been studied. CNT/PANI composites materials with nickel particles have the coercivity an order of magnitude higher than that of bulk nickel (Bao et al. 2002, Cao et al. 2001). 20 5 2. Preparation of polyaniline References Aniline is oxidized by ammonium peroxydisulfate in aqueous media (Figure 1). (Bao et al. 2002) Bao JC, Zhou QF, Hong JM, Xu Z, Appl Phys Lett 81:4592 (2002). During the polymerization, the sulfuric acid is produced a by-product. (Bukhaev et al. 1998) Bukhaev AA, Ovchinnikov DV, Nurgazizov NI, Kukovitskii EF, Klaiber M, Weisendanger R, J Phys Solid State 40:1163 (1998). 4n NH2 + 5n(NH4)2S2O8 (Cao et al. 2001) Cao H, Tie C, Xu Z, Hong J, Sang H, Appl Phys Lett 78:1592 (2001). (Ćirić-Marjanović et al. 2006) Ćirić-Marjanović G, Trchová M, Stejskal J, Collect Czech HX Chem Commun 71:1407 (2006). (Cram et al. 1964) Cram DJ, Hammond GS, Organic Chemistry, McGraw Hill, New York H H N N 1964, pp 210–211. + 3nH2SO4 + 5n(NH4)2SO4 (Fu et al. 1994) Fu Y, Elsenbaumer RL, Chem Mater 6:671 (1994). +. +. n N N (Han et al. 2006) Han J, Song G, Guo R, Adv Mater 18:3140 (2006). H - H - HSO4 HSO4 (Komura et al. 2000) Komura T, Ishihara M, Yamaguchi T and Takachashi K, J Electroanal Figure 1. The sheme of aniline polymerization. Chem 493:84 (2000). (Konyushenko et al. 2006a) Konyushenko EN, Stejskal J, Šeděnková I, Trchová M, Sapurina We have regarded the course of aniline oxidation, the dependences of pH and I, Cieslar M, Prokeš J, Polym Int 55:31 (2006). temperature on time (Konyushenko et al. 2006a). It was shown that when the polymerization (Konyushenko et al. 2006b) Konyushenko EN, Stejskal J, Trchová M, Hradil J, Kovářová J, starts in the solutions of strong acids, the process has two steps (Figure 2). Prokeš J, Cieslar M, Hwang J-Y, Chen K-H, Sapurina I, Polymer 47:5715 (2006). (Konyushenko et al. 2008a) Konyushenko EN, Kazantseva NE, Stejskal J, Trchová M, Kovářová J, Sapurina I, Tomishko MM, Demicheva OV, Prokeš J, J Magn Magn 3 Mater 320:231 (2008). 0.1 M H2SO4 (Konyushenko et al. 2008b) Konyushenko EN, Stejskal J, Trchová M, Blinova NV, Holler P, 35 Synth Met, submitted. oC 2 30 (Liu et al. 2006) Liu XX, Zhang L, Li Y-B, Bian L-J, Huo YQ, Su Z, Polym Bull 57:825 Temperature, (2006). pH 25 (Saez et al. 1993) Saez EI, Corn RM, Electrochim Acta 38:1619 (1993). 1 Figure 2. The changes in (Stejskal et al. 1995) Stejskal J, Kratochvil P, Jenkins AD, Collect Czech Chem Commun 20 temperature (squares) and acidity 60:1747 (1995). (circles) during the oxidation of 0 (Stejskal et al. 2002) Stejskal J in Dendrimers, Assemblies, Nanocomposites, The MML Ser. 0 200 400 600 800 0.2 M aniline with 0.25 M ammonium peroxydisulfate in Vol. 5, Arshady R and Guyot A, Ads, Chap. 6 and 7, pp. 195 – 281, Citus Books, Time, s 0.1 M sulfuric acid. London 2002. 6 19 Abstrakt Aniline molecules are present dominantly in the form of anilinium cations in the Disertační práce se skládá z šesti prací publikovaných v mezinárodních časopisech a solutions of strong acids; the concentration of neutral aniline molecules is very small, that is jednoho rukopisu v recenzním řízení po revizi, s rozšířeným úvodem a podrobným rozborem why the process of oxidation is slow (we can observed the induction period Figure 2). The obsahu těchto článků. Všechny práce se týkají elektricky vodivého polymeru, polyanilinu. induction period extends for several minutes; there is virtually no heat evolved during this Cílem práce byly: stage, and also the changes in pH are hardly spotted. In such conditions, the oxidation of anilinium cations yields oligomers, dimers and trimers. The blue colour at the beginning of (1) Připrava různých nadmolekulárních struktur polyanilinu za různých podmínek the induction period corresponds to the oxidized forms of semedines, amino-diphenylamines přípravy, zejména syntéza polyanilinových nanotrubek. (Ćirić-Marjanović et al. 2006). Anilin může být polymerován oxidací peroxodisíranem ammoným v roztocích The induction period is followed the fast exothermic process associated with the silných nebo slabých kyselin. Kyselost reakční směsi je jedním z nejdůležitejších parametrů, growth of the polymer chains. The reaction mixture becomes heterogeneous because the které mají vliv na vznik nadmolekulárních struktur polyanilinu. Existují i jiné faktory, které reaction intermediates and products are not soluble in the medium. The blue colour of the mohou ovlivňovat vlastnosti a morfologii polymeru, jako jsou povaha oxidačního činidla, reaction mixture deepens (the absorption maximum is located at 690 nm (Stejskal et al. kyseliny, která protonuje polyanilin i reakční meziprodukty v průběhu oxidace, dále 1995), the thin polymer film is produced at the air/solution interface having a violet metallic koncentrace monomerů a oxidantu, teplota, typ rozpouštědla, přítomnost aditiv, atd. tint. Conducting PANI having a granular morphology is obtained as a precipitate. Polyanilin připravený v různých podmínkách byl nasledně analyzován různými metodami. FTIR a Ramanova spektra byla použita pro analýzu molekulární struktury, gelová 3. Polymerization of aniline in the solutions of weak acids permeační chromatografie pro studium molekulových vah, vodivost byla měřena When the polymerization is conducted in the solutions of weak acids, the aniline and čtyřbodovou metodu, a mikroskopické metody byly použity pro studium nadmolekulárních primary amino groups in oligomers become protonated but the intrinsic secondary amino struktur. Na základě analýzy mechanismu oxidace anilinu v různých podmínkách byla groups are not. The temperature profile during the oxidation of aniline with ammonium objasněna vznikající morfologie oxidačních produktů anilinu – granulární struktura, peroxydisulfate has three phases (Figure 3). nanotrubky a mikrokuličky. 40 Oxidation of aniline 38 (2) Příprava kompozitů uhlíkových nanotrubek a polyanilinu. 8 in 0.4M acetic acid 36 Od objevu uhlíkových nanotrubek se věnuje pozornost na jich povrchové modifikaci 34 6 s důrazem na jejich oddělení a rovnoměrnou dispergaci v kompozitních materiálech. Temperature, C 32 Uhlikové nanotrubky byly koaxiálně pokryty polyanilinem v průběhu oxidace anilinu. 30 pH 4 Vzniká struktura podobná polyanilinovým nanotrubkách uvedeným v předchozím bodu. Byly 28 Figure 3. Time o 26 použity dva typy uhlikových nanotrubek: (a) čisté uhlíkové nanotrubky a (b) uhlíkové dependence of pH and 2 24 nanotrubky obsahující nanočástice niklu, použité jako katalyzátor pro jejich přípravu. temperature during the 22 Vlastnosti kompozitů byly analyzovány různými metodami. polymerization of aniline 0 20 0 5 10 15 20 25 30 35 40 45 in 0.4 M acetic acid. Time, min 18 7 The polymerization in the aqueous solutions of weak acids (acetic, succinic, film grows on the surface of CNT producing a core–shell morphology. Magnetic spectra phosphoric acids, etc.) starts at pH ~ 4.5 and drops as sulfuric acid has been produced by the frequency of complex permeability, µ*(f), of PANI–CNT composites with different ratio of decomposition of peroxydisulfate, and hydrogens are removed from aniline as protons. PANI and CNT are presented in Figure 14. Neutral aniline molecules are easily oxidized to oligomers, and the released protons reduce The composites with 10–40 wt% CNT do not exhibit any ferromagnetic properties; the pH (Figure 3). The equilibrium between the aniline molecules and anilinium cations the real part of permeability, µ′, is around the unity and the magnetic losses, µ″, are close to shifts in favour of latter species. Anilinium cations are much more difficult to be directly zero in the frequency range from 1 MHz to the 10 GHz. The ferromagnetic behaviour of oxidized, because the electron pair on nitrogen, which is delocalized in neutral aniline composites appears when the CNT content increases to 50 wt%, which approximately molecules, becomes localized in anilinium cations (Cram et al. 1964). That is why the corresponds to 10 wt.% of nickel. The character of µ*(f) indicates that single-domain nickel reaction virtually stops (Fu et al. 1994, Stejskal et al. 2006), and the temperature starts to nanoparticles (single-domain-size criterion for nickel particles is 6–7 nm) in CNT are decrease (Figure 3). Yet, there is always equilibrium between the protonated and neutral exchange-coupled (Vonsovskii et al. 1998, Bukhaev et al. 1998). This is revealed by species, both species always coexist, and the oxidation of neutral aniline molecules still magnetic dispersion and two resonance peaks on the µ″(f), the first located in low-frequency proceeds with preference even if their content is very low (Stejskal et al. 2008). This is range, 1–10 MHz, and the second in high-frequency range, 1–3 GHz. By increasing CNT up manifested by a continuing decreasing pH under such conditions (Figure 3). The situation to 80 wt.% nickel content increases, and thus enhances both permeability components in corresponds to the induction period observed in the classical polymerization of aniline in the specified frequency regions. acidic media. The situation changes when pH<2 (Figure 3). Such conditions correspond to those of oxidation in the solutions of strong acids, and indeed the sulfuric acid is produced in 1.6 1.0 Permeability, imaginary part 80 wt.% the course of oxidation (Figure 1). The exothermic polymerization then follows (Figure 3). Permeability, real part 1.4 70 wt.% The oxidation products contain both oligomeric and polymeric component, the latter often in 0.5 50 wt.% 1.2 80 wt.% 70 wt.% 50 wt.% nanotubular form. 1.0 − 10−30 wt.% 0.0 − 10−30 wt.% 4. Oxidation in alkaline media 0.8 The oxidation of aniline in alkaline media proceeds when pH>4. Under such acidity 0.6 -0.5 107 108 109 1010 107 108 109 1010 conditions all kinds of amino groups (the aniline monomer, the terminal amino groups in Frequency, Hz Frequency, Hz oligomers and the secondary amino groups) are not protonated. The pH and temperature dependences are represented by a single process (Figure 4). Figure 14. The frequency dependence of real (left) and imaginary (right) parts of complex permeability of CNT The oxidation in 0.2 M ammonium hydroxide starts at pH 10 and pH decreases due to coated with protonated conducting PANI. The content of CNT is given at the individual curves (Konyushenko et al. 2008a). the generation of protons. The exothermic oxidation starts immediately after mixing of reactants, without any induction period and proceeds with a high rate. Temperature monotonously increases during the whole oxidation process and cools only after one of the reactant or both become depleted. The slow-down of the reaction is observed at later stages (Liu et al. 2006). Protons produced in the oxidation reduce pH to 4. The reaction terminates 8 17 conducting PANI bases, a two-point method was used. Before such measurements, circular by the depletion of neutral aniline molecules that have reacted or became converted to gold electrodes were deposited on both sides of pellets. The dependence of conductivity on anilinium cations. The pH does not reach the acidity needed for the protonation of imine CNT content for CNT coated by PANI is shown in the Figure 13. At low fractions of CNT, groups in pernigraniline, i.e. for the polymer growth. Only non-conducting aniline oligomers the conductivity is determined by the PANI as the main component (Figure 13). are produced. The increase in the conductivity of CNT coated with non-conducting PANI base with increasing content of CNT is slow, compared with the CNT coated by protonated conducting 48 polyaniline. At low concentrations of CNT, the direct electrical contact between CNT is 10 44 prevented by the surface coating with a non-conducting PANI base and the percolation 40 threshold is practically not distinguishable. Yet, the coating with PANI base seems to reduce 8 oC 36 the contact resistance between CNT. The resulting conductivity of CNT coated with PANI Temperature, pH 6 base thus reaches 19.6 S cm–1 at 70 wt.% CNT. The conductivity is comparable with that of 32 the CNT coated with protonated polyaniline. We are obvious dealing with interfacial 28 Figure 4. The pH and 4 temperature dependences during phenomena between the CNT separated with a thin coating of PANI. A non-conducting 24 the aniline oxidation in 0.2 M PANI base is not a true insulator, and it can probably mediate the charge-carrier transfer over 2 20 ammonium hydroxide. short distances between the neighboring CNT, similarly to the protonated conducting form 0 10 20 30 40 50 Time, min (Konyushenko et al. 2006b). 5. Supramolecular morphology of polyaniline 101 We have studied the supramolecular morphology of PANI that depends on the starting conditions of aniline oxidation. One of the most interesting polyaniline structures is Conductivity, S cm−1 10-2 represented by nanotubes. We have polymerized aniline in the presence of 0.4 M acetic acid and just in water. The genesis of aniline oxidation was studied by several methods, such as 10-5 PANI-coated CNT gel permeation chromatography, UV–visible, Raman, and FTIR spectroscopies (Stejskal et Protonated PANI PANI base Figure 13. The conductivity of carbon al. 2006, Trchová et al. 2006). 10-8 nanotubes coated with protonated PANI (full During the oxidation of aniline beginning in neutral media, we have observed the circles) and PANI base (open circles) evolution of the water-insoluble products of the reaction. The silicon windows were placed 10-11 0 20 40 60 80 (Konyushenko et al. 2006b). Composition, wt. % CNT into the reaction mixture and removed at various stages of aniline oxidation. The aniline oligomers produced in the early stages of the oxidation are insoluble in the reaction medium. The oxidation products, deposited on the windows, have been studied by optical microscopy. 7. Carbon nanotubes with nickel catalyst nanoparticles coated by polyaniline There is a link between the course of the polymerization and the supramolecular morphology, Polyaniline coating was produced again by the oxidation of aniline hydrochloride here nanotubes, produced at various reaction times (Figure 5). dissolved in ethanol with an aqueous solution of ammonium peroxydisulfate. Polyaniline 16 9 The product obtained in the first stage of aniline oxidation, after 2–8 min, are 6. Preparation of composites materials: carbon nanotubes/polyaniline composed mostly of oligomer crystals, which are obviously insoluble in water, having the Multi-wall carbon nanotubes (CNT) have been coated with a conducting polymer, size of several micrometers (Figure 5 A). The crystals grow to trees (dendrimers) with polyaniline, directly during the oxidation of aniline in ethanol (50 vol.%)–water mixture. The branches of tens of micrometers in length. On the window removed at 21 min (Figure 5 B), content of CNTs in the samples was 0–80 wt.%. The monomer and then the oxidant solutions we have observed, in addition to the granular precipitate, the first nanotubes growing. After were added to various portions of CNT to start the polymerization of aniline. We have been 31 min (Figure 5 C), well visible nanotubes are observed. The nanotubes extend often to a used two kind of carbon nanotubes: (1) pure carbon nanotubes and (2) carbon nanotubes with few micrometers, their external diameter being ca. 200 nm and internal diameter of the cavity incorporated nickel nanoparticles. ca. 20 nm. The granular PANI precipitate is produced close to the end of polymerization (Figure 5 D). As it sediments, it covers the nanotubular structure deposited on the silicon 6.1. Carbon nanotubes coated by polyaniline film windows. This precipitate virtually always accompanies the nanotubes. The aniline oligomers The uniform coating of carbon nanotubes by conducting polymer film can be see on produced in the early stages of the oxidation are insoluble in the reaction medium. Oligomers the SEM and TEM micrographs (Figure 12).The coated CNT become thicker as the amount precipitate, often with frequent needle-like crystallite offsprings (Figure 5 A). These of deposited PANI increases. The uniform deposition of PANI on the CNT is similarly crystallites serve as the templates for the future nucleation of polyaniline nanotubes. demonstrated by transmission electron microscopy, which shows the bilayered structure of coated CNT (Figure 12). As the internal cavity is well discernible, we conclude that the coating with PANI takes place only at the outer surface of the CNT (Konyushenko et al. 2006b). The conducting polymer is not covalently bonded to the carbon nanotubes. The aniline oligomers are adsorbed (Stejskal et al. 2005) at the surface of CNT and start the growth of polyaniline coating there. (A) (B) Figure 12. SEM (left) and TEM (right) images of CNT coated by PANI (Konyushenko et al. 2006b). (C) (D) Figure 5. Optical microscopy of the oxidation products deposited on silicon windows after reaction times t = 6.2. Properties of the composite of carbon nanotubes and polyaniline The conductivity of the composites is a characteristic of prime interest. It was (A) 5, (B) 21, (C) 31, and (D) 38 min (Stejskal et al. 2006). measured by a four-point van der Pauw method on pellets compressed at 700 MPa. For non- 10 15 on the surface of the droplets is similar to the oxidation at the interface water-organic liquid, When the acidity of the reaction mixture becomes sufficiently high, the phenazine where aniline itself takes a role of an inorganic liquid. units may initiate the propagation of polymer chains. Due to their flat structure, such hydrophobic units adsorb on the available surfaces. The adsorption of phenazine units at the surface of needle-like oligomer offsprings is selective, due to the obvious anisotropy of the crystallites. We assume that the phenazine units are adsorbed on the walls of the oligomer crystallites, leaving the front surfaces uncoated. In this way, the nucleus of the nanotubes is produced as a sleeve on an oligomer needle (Stejskal et al. 2006, Trchová et al. 2006). The size of the crystallites inside determines the inner diameter of the future nanotube. The thickness of the nanotube wall corresponds to the thickness of the deposited PANI film and it is proportional to the molecular weight of the PANI chains. Figure 10. SEM (left) and TEM (right) images of microspheres which are formed during the oxidation of aniline The agglomeration of phenazine units in the solutions leads to the formation of in 0.2 M ammonium hydroxide. precipitate. On the other hand, the sorption of oligomers on the surface immersed in the reaction mixture gives rise to thin films during the subsequent polymerization. The type of Polyaniline nanofibers can be prepared when aniline is oxidized under reduced organization of phenazine cycles on the template surface or in the volume of the reaction oC. temperature –24 The polymerization takes place in the solid state; the originally white ice mixture predetermines the future morphology of the supramolecular polymer growth. becomes gradually dark green. The morphology of polyaniline is different from the Depending on the reaction conditions and nature of the template, the sorption or polymerizations made in the liquid media. Polyaniline is constituted by particles of agglomeration of phenazines may be random or organized. The polyaniline chains are submicrometre size, mutually connected by nanofibers of ca 20 nm thickness (Figure 11). growing from them in the preferred direction given by molecular geometry. Assembly of the terminal phenazine units thus binds the polyaniline chain-beginnings together. The self- ordering produced is further stabilized during the chain growth of polyaniline counter-parts by hydrogen bonding and ionic interactions (Figure 6). That is why the nanotube continues to grow without any guide and with an internal cavity determined by nucleus size. This concept can be supported by the observation of nanotubular growth in polycarbonate membranes, which continue to grow beyond the membrane limits, still preserving the nanotubular morphology (Tagowska et al. 2004). Figure 11. Polyaniline prepared in frozen mixture of 0.1 M H2SO4 (left) and 0.4 M acetic acid (right) (Konyushenko et al. 2008b). Figure 6. The phenazine units at the start of PANI chains stack on each other, and PANI chains extend from them. The individual spiral threads are shown as being separated just for the clarity of illustration; we believe, however, that each thread closely copies the previous one and there is no separation between them (Stejskal et al. 2006). 14 11 The growth of one-dimensional structures can also be initiated without any template. We have synthesized polyaniline nanptubes with different surface morphology. It was In this case, we assume that phenazine terminal units associate with each other more loosely. shown that the kind of acid controls not only the acidity of the reaction but also the surface Stacks of one-dimensional fibrils of phenosafranines (including phenazine structures) of 1 morphology of the nanotubes (Figure 8). nm diameter, which corresponds approximately to the size of a phenazine heterocycle (Saez Polyaniline granular morphology is the most typical; it is prepared in a classical way et al. 1993), have been reported in the literature (Komura et al. 2000). Polyaniline chains in the acidic aqueous media (Stejskal et al. 2002, Konyushenko et al. 2006a) (Figure 9). extending from them produce a brush. A nanorod without an internal cavity is produced Polyaniline nanogranules have a size of tens to a few hundreds of nm. The mechanism of instead of a nanotube. Such nanorods are observed to accompany the nanotubes (Figure 7), their formation has not been analyzed in the literature. We assume that the globules are and their mutual proportions are likely to be the pH-dependent. produced as a result of random association of phenazine nucleates. Such association takes place when the concentration of nucleates is high, the solubility of nucleates is reduced and the polymer chain starts to grow before the phenazine structures are organized. Figure 9. The granular morphology of Figure 7. TEM micrograph of PANI nanotubes and related structures (Stejskal et al. 2006). polyaniline prepared in the presence of 0.1 M sulfuric acid (Konyushenko et al. 2006a). The formation of microspheres, which is observed in the oxidation of aniline (Wang et al. 2005, Venancio et al. 2006, Stejskal et al. 2008) or aniline derivatives (Han et al. 2006) at low acidity and at alkaline conditions, has another background. The neutral aniline is not always completely miscible with aqueous medium and constitutes a separate phase. The oxidation then proceeds at the interface between aniline droplets and liquid phase containing the oxidant. The non-conducting aniline oligomers form microspheres having a diameter of a few micrometres (Figure 10). A B The oxidation of aniline by the oligomerization mechanism takes place at the droplet Figure 8. The polyaniline nanotubes prepared in 0.4 M succinic acid (A) and 0.1 M picric acid (B). interface. The droplet surface becomes coated with a non-conducting oligomer. Aniline diffuses through the interface and the thickness of the oligomer layer increases. The synthesis 12 13
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