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Polyaniline prepared in ethylene glycol or glycerol
Polymer 52 (2011) 1900e1907
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Polyaniline prepared in ethylene glycol or glycerol Elena N. Konyushenko a, *, Stéphanie Reynaud b, Virginie Pellerin b, Miroslava Trchová a, Jaroslav Stejskal a, Irina Sapurina c a
Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, Heyrovsky Sq. 2, 162 06 Prague 6, Czech Republic IPREM-EPCP, CNRS/UPPA UMR 5254, 64053 Pau Cedex 9, France c Institute of Macromolecular Compounds, Russian Academy of Sciences, St. Petersburg 199004, Russia b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 December 2010 Received in revised form 9 February 2011 Accepted 25 February 2011 Available online 4 March 2011
Oxidations of aniline or aniline hydrochloride have been carried out in ethylene glycol or glycerol and in their mixtures with water. Ammonium peroxydisulfate was used as oxidant. The oxidation of aniline is exothermic and changes in temperature were monitored to follow its progress. The effect of the solvents on the course of oxidation, morphology, and properties of final products has been studied by scanning electron microscopy, FTIR spectroscopy, and conductivity measurements. It is proposed that the reduction of dielectric constant of the reaction medium results in the reduced dissociation of ionic species that take a part in oxidative polymerization. Consequently, the addition of an organic solvent has a similar effect as a decrease in the acidity of the reaction mixture. Conductivity and morphology depend on the mole ratio of oxidant and monomer, and the type and volume fractions of organic solvents, viz. ethylene glycol and glycerol, which were used in reactions. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Conducting polymer Polyaniline Nanotubes
1. Introduction Polyaniline (PANI) is one of the most interesting conducting polymers due to the ease of its preparation, its excellent environmental stability and good electron and proton conductivities [1e3]. The electronic conductivity of PANI can be controlled by both the degrees of oxidation and protonation [4,5]. Electrical properties of PANI find applications in electronic devices, sensors, storage of energy, electrorheology, etc. [6e8]. Polyaniline can be prepared by chemical or electrochemical oxidation of aniline in aqueous media [9,10] or in the solid state [11,12]. There are many parameters, which are important in the control of the properties and morphology of the PANI. The pH of the aqueous reaction mixture used for the preparation of the PANI is one of the most important. When aniline was oxidized at low pH, i.e. at high acidity, conducting polymers with molecular weight of 104e105 were produced. The morphology of PANI was represented by granules, nanotubes or nanofibers [13e16]. Self-assembled PANI nanotubes [17e21] can be prepared when the pH varies from the starting value above 5 to a final value below 2.5 [2,22]. This situation is quite
* Corresponding author. Tel.: þ420 296 809 234; fax: þ420 296 809 410. E-mail address: [email protected] (E.N. Konyushenko). 0032-3861/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2011.02.047
often met in the presence of the weak acids. At neutral conditions the different types of morphology were produced, from almost amorphous materials to micro- and nanospheres [16,23] and highly organized hierarchical structures, such as micromats [2,10]. In contrast with this, the non-conducting aniline oligomers were synthesized by the oxidation of aniline in alkaline media [16]. It has been demonstrated in the literature that the addition of miscible organic solvents to aqueous media affects the morphology of the produced PANI. The transition from the globular morphology in favor of nanofibres or nanotubes has been observed when the reaction mixture contained alcohols, such as methanol [24], ethanol [25] or 1,6-hexanediol [26]. The presence of organic solvents thus can be useful in the control of PANI morphology. Chemical oxidative polymerization exclusively in organic solvents, i.e. in the absence of water, has not been performed so far. In the present study, we demonstrate that various factors, including dielectric constant of reaction mixture, can influence the morphology and properties of PANI by controlling the distribution of protonated and deprotonated reaction species in reaction mixture. Moreover, we report the simple preparation of PANI nanostructures, such as granules, nanotubes, or nanospheres in ethylene glycol or glycerol and their mixtures with water. The diol and triol were chosen because they are benign to the environment, they are lowcost chemicals, and polymerization of aniline in them is feasible. Such synthesis may be of benefit in applications of PANI which require the absence of residual water, such as in electrochemistry.
E.N. Konyushenko et al. / Polymer 52 (2011) 1900e1907
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a 4n
NH2
+
+ NH
H N
HSO4
*
5n (NH4)2S2O8
+ NH HSO4
+ N H
3n H2SO4
+
5n (NH4)2SO4
n
b 4n
NH2HCl
+ NH *
Cl
-
+
5n (NH4)2S2O8
H N + NH Cl
+ 5n H2SO4 + 5n (NH4)2SO4 + N H
2n HCl
n
Fig. 1. (a) Aniline and (b) aniline hydrochloride were oxidized with ammonium peroxydisulfate to PANI salt. Sulfuric acid and ammonium sulfate are by-products.
2. Experimental 2.1. Preparation Aniline or aniline hydrochloride (Aldrich) was oxidized by ammonium peroxydisulfate (APS) in mixtures of ethylene glycol or glycerol and water. The aniline concentration was 0.2 M, the mole ratio of oxidant to monomer was 1 or 2. The amount of ethylene glycol or glycerol was varied in the range 0e100 vol.%. This means that the experiments included polymerizations in the absence of water. The reactions were carried out at room temperature, w20 C. The final products of oxidation were collected on a filter, rinsed with 0.1 M sulfuric acid, to remove the residual organic components and inorganic salts, and with acetone. Finally, the product was dried in air overnight and then over silica gel. A part of each sample was converted to the corresponding emeraldine base by 1 M ammonium hydroxide, and dried, prior to characterization by UVevisible and FTIR spectroscopies.
2.2. Characterization Scanning electron micrographs were taken with a JEOL 6400 microscope (Japan). Transmission images have been taken with JEOL JEM 2000FX microscope. Infrared spectra were recorded with a fully computerized Thermo Nicolet NEXUS 870 FTIR Spectrometer with a DTGS TEC detector. The spectra of samples dispersed in potassium bromide were collected in the transmission mode and corrected for the presence of carbon dioxide and humidity in the optical path. UVevisible spectra of the oxidation products dissolved in N-methylpyrrolidone were recorded with a spectrometer Lambda 20 (Perkin Elmer, UK). Conductivity was determined by a four-point van der Pauw method on PANI powders compressed into the pellets 13 mm in diameter and 1 mm thick using the
Keithley 2010 Multimeter equipped with a 2000-SCAN 10 channel scanner card or by a two-point method using a Keithley 6517 electrometer for samples of low conductivity.
3. Results and discussion It is well-known that the acidity of the reaction mixture is the key factor that affects the course of oxidation, properties and morphology of PANI [2,16,21,22,27]. When trying to predict the role of the non-aqueous solvent, which is used as a component of reaction mixture or alone, the consideration of the relative permittivity of the solvent, e, might be pertinent. The permittivity of water is e ¼ 80.1 (20 C). Most solvents have lower permittivity, e.g., ethylene glycol, e ¼ 38.7 (20 C), and glycerol, e ¼ 42.4 (25 C). When added to water, the permittivity of the medium will be reduced. The dissociation of ionic species in such a medium will consequently be suppressed. In the case of acids, which are present or generated in the system, this means that the concentration of protons will be lower by comparison with purely aqueous reaction mixtures. In other words, the effective acidity of the system will be reduced. This concept thus predicts that the addition of a miscible organic solvent will have similar effect to the use of a less acidic aqueous medium. The following transitions may thus be expected when aqueous reaction medium changed to mixed watereorganic or pure organic solvent [2]: 1 The formation of non-conducting aniline oligomers during single-step exothermic oxidation at the low effective acidity when water content is low or absent. 2 Two-step exothermic oxidation process with a long time induction period between the steps and the formation of aniline oligomers in the first step, and conducting polymers in
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the second step, when content of water increases and effective acidity rises. 3 Transformation of two-step oxidation to single-step oxidation with an initial induction period and the synthesis of conducting PANI when the effective acidity will be increased by the presence of acid. As for morphology of the products, the formation of PANI nanotubes may be expected during the two-step oxidation. The formation of one-dimensional structures is a result of the selfassembly of short aniline oligomers, the nucleates, that are produced during induction period between two exothermic oxidation steps. Non-conducting aniline oligomers produced at the first oxidation stage serve as an initial template for the growth of tubular morphology [2,16]. The solubility of the monomer, oxidant, and reaction intermediates is another factor that can influence the course of oxidation, properties and morphology of final oxidation products in the presence of organic solvents. The addition of organic solvents, such as alcohols, can lead to the solubilization of intermediates, and thus to prevent the generation of organized structures. This trend will dominate in the build-up of PANI morphology. Two series of aniline oxidations are demonstrated. In the first, aniline was oxidized by ammonium peroxydisulfate (Fig.1a) in organic solvents, ethylene glycol and glycerol, and their mixtures with water. Reactionwas carried out at a mole ratio [APS]/[aniline] ¼ 1 and at ambient temperature. In the second series, aniline was replaced with aniline hydrochloride (Fig.1b) in order to increase the effective acidity of reaction mixture. Other oxidation conditions were the same. The oxidation is an exothermic reaction and the temperature changes during the oxidation were monitored to follow its progress [16,18,22]. The temperature profiles were measured for all experiments, illustrative examples are shown in Fig. 2.
3.1. The course of aniline oxidation and the properties of the products In accordance with our predictions, aniline oxidations are the monotonous single-step reactions (Fig. 2a) in ethylene glycol and glycerol, and also in mixtures of these organic solvents with low water content. The yields of reactions were low compared to the oxidation of aniline in water, the conductivity of the products was also low and sometimes it is on insulating level. The conductivities of products synthesized in ethylene glycol and glycerol are 3.2 109 and 1.7 105 S cm1, respectively (Table 1). The FTIR spectra of the products prepared in ethylene glycol (spectrum A-EG in Fig. 3a) correspond to aniline oligomers, described earlier [16], and are overlapped by strong peaks of ethylene glycol observed at 1083 and 1036 cm1 (shown by asterisks in Fig. 3a). In glycerol, the spectrum of the sample (spectrum AGl in Fig. 3a) is close to the spectrum of PANI prepared in water (spectrum A-W in Fig. 3b) in which, in addition to the most important bands at 1573 and 1498 cm1 (quinonoid and benzenoid ring-stretching vibrations, respectively), at 1245 cm1 (CeNþ 1 stretching vibration in the polaron structure), at 1149 cm (vibration mode of the eNHþ¼ structure) and at 806 cm1 (CeH out-of-plane bending vibrations) [28], the peak of residual glycerol is observed (shown by asterix in Fig. 3a). We conclude that aniline oxidation in pristine organic solvents and also in organic solvent with low water content leads mostly to non-conjugated and non-conducting aniline oligomers. The reason consists in the low effective acidity of reaction mixture and the oxidation of non-protonated species. In this case, the redox interaction proceeds between non-protonated monomer and non-protonated oligomer structure. As a result, the reaction of electrophilic substitution leads to the growth of short chains with mixed ortho- and para-linked constitutional units. The concentrations of protons which are released as sulfuric acid (Fig. 1a) were not sufficient to reach high effective acidity and start the propagation of PANI chains. For that reason, the non-conjugated oligomers with irregular structure are the only products [2]. When oxidation proceeds in the presence of 20 vol.% and 50 vol.% of organic solvent, the effective acidity of reaction mixture raised and one can see the transition of the oxidation course from single-step to two-step exothermic process (Fig. 2a). The presence of the second exothermic phase correlates with the higher conductivity of the products (Table 1) because the conducting
Table 1 The results of oxidations of aniline and aniline hydrochloride in ethylene glycol or glycerol and their mixtures with water.a Solvent
Solvent, vol.%
The oxidation of aniline Water 100 Ethylene glycol 20 50 100a Glycerol 20 50 100a
Fig. 2. The temperature profiles in the oxidation of (a) aniline or (b) aniline hydrochloride in ethylene glycol (EG) or glycerol (Gl) and in their mixtures with water (50 vol.%). The profile obtained in water is included as a reference.
Yield, g (%)
Conductivity S cm1
Conductivity of bases, S cm1
1.80 0.97 0.84 0.67 0.84 1.40 1.70
0.2 5.1 3 3.0 3 3.2 3 0.14 5.0 3 1.7 3
2.5 1010 1.2 108 6.0 108 e 0.45 109 7.5 109 5.3 106
(64) (34) (30) (24) (30) (50) (60)
The oxidation of aniline hydrochloride Water 100 1.90 (89) Ethylene glycol 20 2.00 (93) 50 2.80 (130) 100 0.86 (41) Glycerol 20 1.80 (84) 50 1.77 (85) 100 1.92 (90) a
5.0 1.4 2.0 8.3 4.0 4.5 2.5
102 103 109 103 105
2.0 2.9 3.1 4.5 1.3 6.8 6.6
Only brown non-conducting aniline oligomers were produced.
108 106 107 106 108 108 106
E.N. Konyushenko et al. / Polymer 52 (2011) 1900e1907
Fig. 3. FTIR spectra of samples prepared in water or in non-aqueous media, ethylene glycol or glycerol during the oxidation of: (a) aniline (A) or aniline hydrochloride (AH) in ethylene glycol (EG) and glycerol (Gl) (the spectrum of the sample prepared in water (W) is included for comparison), (b) aniline in water and glycerol and their mixtures, as prepared and in deprotonated forms (B), (c) aniline hydrochloride in water and ethylene glycol and their mixtures, as prepared and in deprotonated forms.
polymer is produced exclusively in the second reaction step. The conductivity of the products increased up to 101 S cm1. We have compared the infrared spectra of the products of oxidation of neutral aniline at two different concentrations of glycerol, 20 and 50 vol.% (Fig. 3b) with the spectrum of the product prepared in water (spectrum A-W in Fig. 3b). The samples were also analyzed in their deprotonated state, i.e. as emeraldine bases (Fig. 3b). In the case of 20 vol.% of glycerol medium (spectrum A-20 Gl) and water [8], the spectra are practically identical. They correspond to the protonated emeraldine due to sulfuric acid
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produced during the oxidation with the most important bands situated at 1573 and 1498 cm1 (quinonoid and benzenoid ring stretching vibrations, respectively), 1245 cm1 (CeNþ stretching vibration in the polaron structure), 1149 cm1 (vibration mode of the eNHþ¼ structure), and at 806 cm1 (CeH out-of-plane bending vibrations) [28]. After deprotonation the spectrum of the product obtained in 20 vol.% of glycerol (spectrum A-20 Gl B) corresponds to completely deprotonated sample, contrary to the case of water (spectrum A-W B). The resistance toward deprotonation of the last sample was noticed earlier [10] and it has been associated with the sulfonation of benzene rings. Two small peaks at 1445 and 1414 cm1 present in the spectrum of deprotonated sample prepared in 20 vol.% of glycerol (spectrum A-20 Gl B) and in water (spectrum A-W B) and marked by arrows in Fig. 3b belong to the aniline oligomers produced at the early stages of aniline oxidation at low acidity of medium [16]. A sample prepared in 50 vol. % of glycerol (spectrum A-50 Gl in Fig. 3b) contains this solvent and its deprotonation was not complete (spectrum A-50 Gl B in Fig. 3b). The formation of conducing polymer in the second oxidation phase follows the synthesis of aniline oligomers in the first oxidation phase [2,16]. The products are the mixture of nonconducting oligomer and conducting polymer. The level of conductivity depends from the proportions of each oxidation product. At the two-step process the first oxidation stage proceeds as before, the oxidation takes place at low effective acidity. The redox interaction between non-protonated monomer and nonprotonated chain resulting in the formation of non-conducting oligomers takes place. The protons are a by-product of the electrophilic substitution reaction, and the effective acidity of reaction mixture increases (Fig. 1a). This leads to the protonation of monomer and intermediates. Aniline monomer becomes protonated the first, and this manifested itself by the start of athermal induction period. The rate of the oxidation reaction decreases because the redox process taking place between protonated monomer and non-protonated chain is reduced. Short aniline oligomers, the nucleates, are the only product at this stage. The second oxidation phase can be achieved when the acidity of reaction mixture increases to the level needed for the protonation of imine constitutional units. Protonated nucleates start to grow to regular PANI chains with para-linked constitutional units. That is why the conducting oxidation product is obtained at the second oxidation stage. When the reaction mixture contained glycerol, the reaction was faster when compared with the polymerization conducted in pure water (Fig. 2a). This fact can be explained by better solubility of aniline in organic solvents compared with water. In contrast to mixed solvents, the oxidation in water starts as heterogeneous process when a large part of monomer exists as droplets due to a limited solubility of aniline in water.
3.2. The oxidation of aniline hydrochloride Another series of experiments was carried out using aniline hydrochloride instead of aniline (Fig. 1b). The oxidation thus proceeds at higher acidity compared with the above experiments. The mole ratio [APS]/[aniline] ¼ 1 was also kept in all these experiments, and the volume fractions of diol and triol were varied (Fig. 2b). The oxidations in ethylene glycol and glycerol in the absence of water are much slower. For that reason, the polymerization has not been detected by the increase of temperature, because of the heat dissipation. The polymerization, however, took place, as proved by the high reaction yield and good conductivity of products (Table 1).
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1.5
ethylene glycol
A bsorban ce
1.0
water glycerol
0.5
0.0 400
600
800
Wavelength, nm Fig. 4. UVevisible spectra of the PANI bases prepared by the oxidation of aniline hydrochloride in water, ethylene glycol, or glycerol, and dissolved in N-methylpyrrolidone.
Polyaniline prepared from aniline hydrochloride in water had conductivity of 5.0 S cm1. When the polyaniline is synthesized in a medium containing an increasing amount of ethylene glycol the conductivity slightly increased. Polyaniline prepared in pure ethylene glycol exhibited the maximum conductivity of all the samples, 8.3 S cm1. The fraction of glycerol in the polymerization system had a marginal effect on the conductivity of the PANI produced (Table 1). In this respect, the preparation of PANI by oxidation of aniline hydrochloride in water, ethylene glycol, and glycerol are similar, only the oxidations in an organic solvents have a slower kinetics.
The reasons of this lower rate may be due to poor solubility of peroxydisulfate and aniline salts in these organic solvents. Solubilization of aniline hydrochloride in diol and triol leads to a slight opalescence of the solutions. This means that monomer was agglomerated at these solvents. The FTIR spectrum obtained during polymerization of aniline hydrochloride in ethylene glycol (spectrum AH-EG in Fig. 3a) is close to the spectrum obtained in water (spectrum AH-W in Fig. 3a), only two small peaks at 1083 and 1036 cm1 belonging to ethylene glycol and marked by asterisks in the corresponding spectra signify that this solvent is still present within the sample and is attached most probably due to hydrogen bonding [25]. The situation is different when aniline hydrochloride is used and oxidized in glycerol (spectrum AH-Gl in Fig. 3a). Bands of water molecules present in potassium bromide are detected in the spectrum due to compact consistence of the sample and difficult dispersion in potassium bromide. In the oxidation of aniline hydrochloride in the mixtures of organic solvent and water, the first oxidation step is absent and only the characteristic induction period is found (Fig. 2b). Aniline hydrochloride is easily polymerized with APS in water, and the reaction was complete within 10 min (Fig. 2b). This is a so-called “standard” polymerization [29]. A solvent content up to 50 vol.% did not change the time needed for PANI formation but it slightly decreased the maximum temperature reached during the reaction (Fig. 2b). The polymerization phase is well developed, and this is again in accordance with the good conductivity values of the final products (Table 1). All the PANI obtained from aniline hydrochloride oxidation possess a chain conjugation, as shown by a strong band with maximum of 620 nm observed in UVevisible spectra of deprotonated samples (Fig. 4) [30e32]. The conductivity of the products was as high as 10 S cm1. The infrared spectra of the protonated samples at two different
Fig. 5. Polyaniline prepared by the oxidation of 0.2 M aniline by 0.2 M ammonium peroxydisulfate in (a) 20 vol.% of ethylene glycol, (b) 50 vol.% of ethylene glycol, (c) in 20 vol.% of glycerol, and (d) and 50 vol.% of glycerol.
E.N. Konyushenko et al. / Polymer 52 (2011) 1900e1907
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Fig. 6. The products of the oxidation of 0.2 M aniline hydrochloride by 0.2 M APS in (a) 50 vol.% ethylene glycol, (b) in pure ethylene glycol, (c) in 50 vol.% glycerol, and (d) in pure glycerol.
concentrations of ethylene glycol (20 and 50 vol.%) and spectrum of the product of oxidation of aniline hydrochloride in water are compared in Fig. 3c. In the case of 20 vol.% of ethylene glycol (spectrum AH-20 EG) and water (spectrum AH-W), the spectra of oxidation products are practically identical and correspond to the protonated emeraldine [28]. After deprotonation, the spectrum of product obtained in 20 vol. % of ethylene glycol (spectrum AH-20 EG B) is the same spectrum as that of “standard” emeraldine base (spectrum AH-W B shown in Fig. 3c for comparison) with the blueshifted main bands of quinonoid and benzenoid ring-stretching vibrations (at 1573 and 1498 cm1) and lowered bands character istic of protonation modes of PANI and corresponding to the CeNþ stretching vibrations (1245 cm1) and to the vibration of the eNHþ¼ structure (1149 cm1) [28]. The spectrum of sample prepared in 50 vol.% of ethylene glycol (spectrum AH-50 EG) exhibits lower absorbance, as its compact consistence and difficult dispersion in potassium bromide. After deprotonation (spectrum AH-50 EG B), the sample preparation was much easier and the spectrum was close to the spectrum of emeraldine base. We conclude that the presence of ethylene glycol leads to stronger hydrogen bonding and to more compact structure as compared with the sample prepared in water. In acidic media when aniline hydrochloride was used instead of aniline, monomer exists mostly in its protonated form. Concentration of neutral aniline is very low. Protonated aniline, i.e. the anilinium cation, has higher oxidation potential and probably is not oxidized at the beginning. At the first stage the oxidation of neutral non-protonated aniline proceeds. Short cyclic oligomers, probably trimers of phenazine structure, are produced during this induction
period. In acidic conditions, cyclic oligomers become protonated and initiate the growth of regular para-linked PANI chains. The conjugated chains with a high conductivity level are then produced. Products prepared in acidic conditions mostly contain the polymer, the fraction of oligomers being negligible. 3.3. Morphology Morphology of PANI is determined by the self-association of aniline nucleates produced during the induction period [2]. Phenazine nucleates are the flat molecules that organize into the columns by pep electron interaction. Phenazine stacks formation predetermines the subsequent growth of one-dimensional structures, nanotubes and nanofibers. Non-regular phenazine association into agglomerates of spherical form leads to granular PANI structure. The reaction conditions, especially the acidity profile, during the oxidation of aniline [2], predetermine the morphology of the final PANI product. In the present research, we have varied the nature of the monomer, i.e. aniline or aniline hydrochloride, and the organic solvent, in order to find conditions where PANI structures exhibit morphology of interest. Existence of two-step oxidation process, in which two exothermic steps are separated by athermal induction period, make favorable conditions for nanotubular growth [1]. The collection and regular assembly of phenazine nucleates produced at the induction period, gives a rise to formation of one-dimensional phenazine stacks. At the second oxidation phase, the polymer chains grow from the stacked nucleates and the walls of PANI tubes are produced.
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Fig. 7. Scanning electron micrographs of the products of aniline (0.2 M) oxidation by excess of ammonium peroxydisulfate (0.4 M) in (a) 50 vol.% ethylene glycol and (b) 50 vol. % glycerol. Transmission electron micrograph is shown in inset.
When aniline was oxidized in the presence of 20e50 vol.% of ethylene glycol the two-step oxidation process was observed and PANI nanotubes were indeed obtained (Fig. 5a), the outer diameter of polyaniline nanotubes is 250 nm. This is similar to the case when the oxidation of aniline was made in water [10]. Nanotubes were accompanied by granules and by some spherical particles. PANI granules are produced in more acidic conditions at advanced stages of the polymerization. If pH is low, the induction period is shortened. This means that phenazine nucleates can not self-organized in regular manner [2,16]. The random agglomeration of phenazines gives a raise to PANI granules (Fig. 5b). Others morphologies can be prepared when a triol, glycerol, is used as the solvent in aniline oxidation: mixture of granules and amorphous structures are formed in 20 vol.% glycerol (Fig. 5c). Granules accompanied by rectangular objects are produced in 50 vol.% glycerol (Fig. 5d). The polymerization of aniline hydrochloride at various volume fractions of ethylene glycol or glycerol in the reaction mixture has not lead to any uniform nanostructures. Scanning electron microscopy shows the amorphous or partially granular morphology of products when the polymerization took place in 50 and 100 vol.% ethylene glycol (Fig. 6a,b). We propose that this is connected with a solubility factor. Better solubility of nucleates acts against their organization and manifests itself in a “packed layers’’ morphology in pure glycerol (Fig. 6c,d).
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Absorbance
1583
A-Am
*
A-50 Gl B
* *
A-50 EG B
* *
Glycerol Ethylene glycol
4000
3000
2000
1000
Wavenumbers, cm Fig. 8. FTIR spectra of the samples prepared by the oxidation of 0.2 M aniline with 0.5 M APS, i.e. with an excess of oxidant, in ethylene glycol and glycerol mixtures with water (50 vol.%) after deprotonation to bases, A-50 Gl B and A-50 EG B, respectively. The spectrum of aniline oligomers prepared in 1 M ammonium hydroxide (A-Am) and the spectra of both organic solvents are included for comparison.
3.4. Nanospheres When scanning various reaction conditions, PANI nanospheres have occasionally been observed in the products (Fig. 7). For example, this happened when aniline was oxidized by ammonium peroxydisulfate in 50 vol.% of ethylene glycol and the molar concentration of oxidant with respect to aniline was twice higher, i.e. under the conditions of strong overoxidation. Under such experimental conditions, the pH of the reaction is sufficient for the protonation of reaction intermediates and PANI is formed. The formation of sulfuric acid during the polymerization kept the product at the overoxidized pernigraniline level. Nearly uniform fused PANI nanospheres were obtained and had a mean diameter of 600 nm. Polyaniline prepared in this way had a conductivity of 7 104 S cm1. Such a low value reflects the overoxidation of the final product. The formation of spheres is connected to the increased amount of oxidant in the polymerization mixture. Due to the large concentration of salts, the aniline solubility is reduced and aniline droplets are produced as the result of a salting-out effect. These act as templates [16]. The high concentration of peroxydisulfate anions at the beginning of the oxidation may also lead to the sulfonation of benzene rings not only in the aniline oligomers [13] but also in the aniline molecules. The sulfonated anilines, such as o- and p-aminobenzenesulfonic acids, may be incorporated both into oligomers and polymers. Some authors assume the introduction of sulfate groups to be more probable [33]. Sulfo groups in oligomers produce surfactant-like structures that can stabilize monomer droplets at the submicrometre size level. They gradually convert to PANI nanospheres as the oxidation proceeds. In the case of the polymerization of aniline hydrochloride, the monomer is completely miscible with the reaction medium and, consequently, nanospheres are not produced. The FTIR spectra of the products of oxidation of aniline in 50 vol.% of ethylene glycol (spectrum A-50 EG B in Fig. 8) and glycerol (spectrum A-50 Gl B) with 0.5 M APS in their deprotonated state are compared with the spectrum of the product of oxidation of aniline in 0.2 M ammonium hydroxide solution with 0.25 M APS reported in earlier paper [16] (spectrum A-Am), when oligomeric microspheres were obtained. The spectra are close to each other, in the case of ethylene glycol and glycerol the small peaks at 1445 and 1414 cm1 marked by arrows are also present. It is difficult to distinguish between the peak of sulfonation at about 1040 cm1 in the spectrum of aniline oxidized in ammonium hydroxide (spectrum A-Am) and the peaks of ethylene glycol and glycerol situated close to this peak (marked by asterisk in the spectra). The sharpness of this peak in the spectra points more to the sulfonation, and is
E.N. Konyushenko et al. / Polymer 52 (2011) 1900e1907
higher in the case of glycerol. Instead of well-distinguished and structured peaks present in the spectrum of microspheres obtained in ammonium hydroxide, we have observe the broad bands in the spectra of the products prepared in presence of 50 vol.% of ethylene glycol and glycerol, especially in the region of NH stretching vibrations. We conclude that strong hydrogen bonding occurs in the structure of these products between PANI and diol or triol.
4. Conclusions The feasibility of the preparation of polyaniline by the chemical oxidation of aniline hydrochloride, i.e. of anilinium cations, in nonaqueous media has been demonstrated. Polyaniline was prepared in ethylene glycol and glycerol, and in their mixtures with water. It is proposed that the use of an organic solvent has a similar effect to a reduction in acidity because of reduced dissociation of acid present in the medium. The oxidation of aniline hydrochloride produced PANI of standard emeraldine structure in both organic solvents. The conductivity of the product prepared in ethylene glycol, 8.3 S cm1, was even higher than that of the product prepared in water, 5.0 S cm1. The oxidation of neutral aniline yielded to materials of lower conductivity. Aniline oligomers were present in the final products. In ethylene glycol, non-conducting oligomers were the only product due to the low acidity of the medium. Differences in morphology and departures from the common granular appearance have been found. The morphology is assumed to be controlled by the assembly of aniline oligomers based on their hydrophobic character and by strong hydrogen bondig between the products and organic solvents. For these reasons, the morphology depends on the content of the organic component, which affects the interaction between the oligomers and thus their self-assembly. Oligomeric nanospheres were observed when excess of oxidant was used in the synthesis.
Acknowledgments The authors wish to thank the EGIDE (ECO-NET 16256SA), the Grant Agency of the Academy of Sciences of the Czech Republic (IAA 400500905), and the Czech Grant Agency (203/08/0686) for financial support. Thanks are due to J. Hromádková for the electron microscopy and to J. Prokes for the conductivity measurements.
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References [1] Gospodinova N, Terlemezyan L. Progress in Polymer Science 1998;23(8): 1443e84. [2] Sapurina I, Stejskal J. Polymer International 2008;57(12):1295e325. [3] Stejskal J, Sapurina I, Trchová M. Progress in Polymer Science 2010;35(12): 1420e81. [4] Blinova NV, Stejskal J, Trchová M, Prokes J. Polymer International 2008;57(1): 66e9. e nková I, Trchová M, Stejskal J, Prokes J. ACS Applied Materials & Interfaces [5] Sed 2009;1(9):1906e12. [6] Choi HJ, Jhon MS. Soft Matter 2009;5(8):1562e7. [7] Kim SG, Lim JY, Sung JH, Choi HJ, Seo Y. Polymer 2007;48(22):6622e31. [8] Quadrat O, Stejskal J. Journal of Industrial and Engineering Chemistry 2006; 12(3):352e61. [9] Ciri c-Marjanovi c G, Trchová M, Stejskal J. Journal of Raman Spectroscopy 2008;39(10):1375e87. e nková I, Konyushenko EN, Stejskal J, Holler P, Ciri [10] Trchová M, Sed cMarjanovi c G. J Phys Chem B 2006;110(19):9461e8. [11] Abdiryim T, Jamal R, Nurulla I. Journal of. Applied Polymer Science 2007;105 (2):576e84. [12] Posudievsky OY, Goncharuk OA, Pokhodenko VD. Synthetic Metals 2010;160 (1e2):47e51. [13] Huang JX. Pure and Applied Chemistry 2006;78(1):15e27. [14] Huang K, Wan M, Long Y, Chen Z, Wei Y. Synthetic Metals 2005;155(3): 495e500. [15] Chiou NR, Epstein A. Advanced Materials 2005;17(13):1679e83. [16] Stejskal J, Sapurina I, Trchová M, Konyushenko EN. Macromolecules 2008;41 (10):3530e6. [17] Huang J, Wan MX. Journal of Polymer Science Part A-Polymer Chemistry 1999;37(9):1277e84. e nková I, Trchová M, Sapurina I, Cieslar M, et al. [18] Konyushenko EN, Stejskal J, S Polymer International 2006;55(1):31e9. [19] Long YZ, Zhang LJ, Ma XJ, Chen ZJ, Wang NL, Zhang Z, et al. Macromolecular Rapid Communications 2003;24(16):938e42. [20] Yang YS, Wan MX. Journal of Materials Chemistry 2002;12(4):897e901. [21] Zujovic ZD, Laslau C, Bowmaker GA, Kilmartin PA, Webber AL, Brown SP, et al. Macromolecules 2010;43(2):662e70. [22] Konyushenko EN, Trchová M, Stejskal J, Sapurina I. Chemical Papers 2010;64 (1):56e64. [23] Jin E, Wang X, Liu N, Zhang WJ. Materials Letters 2007;61(27):4959e62. [24] Huang YF, Lin CW. Polymer 2009;50(3):775e82. [25] Kan JQ, Zhang SL, Jing GL. Journal of Applied Polymer Science 2006;99 (4):1848e53. [26] Huang YF, Lin CW. Synthetic Metals 2010;160(5e6):384e9. [27] Laslau C, Zujovic ZD, Zhang LJ, Bowmaker GA, Travas-Sejdic J. Chemistry of Materials 2009;21(5):954e62. [28] Blinova NV, Stejskal J, Trchová M, Prokes J, Omastová M. European Polymer Journal 2007;43(6):2331e41. [29] Stejskal J, Gilbert RG. Pure and Applied Chemistry 2002;74(5):857e67. [30] Surwade SP, Manohar N, Manohar SK. Macromolecules 2009;42(6):1792e5. [31] Stejskal J, Kratochvíl P, Jenkins AD. Collection of Czechoslovak Chemical Communications 1995;60(10):1747e55. [32] Stejskal J, Kratochvíl P, Radhakrishnan N. Synthetic Metals 1993;61(3): 225e31. [33] Surwade SR, Dua V, Manohar N, Manohar SK, Beck E, Ferraris JP. Synthetic Metals 2009;159(5e6):445e55.
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Polymer journal homepage: www.elsevier.com/locate/polymer
Polyaniline prepared in ethylene glycol or glycerol Elena N. Konyushenko a, *, Stéphanie Reynaud b, Virginie Pellerin b, Miroslava Trchová a, Jaroslav Stejskal a, Irina Sapurina c a
Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, Heyrovsky Sq. 2, 162 06 Prague 6, Czech Republic IPREM-EPCP, CNRS/UPPA UMR 5254, 64053 Pau Cedex 9, France c Institute of Macromolecular Compounds, Russian Academy of Sciences, St. Petersburg 199004, Russia b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 December 2010 Received in revised form 9 February 2011 Accepted 25 February 2011 Available online 4 March 2011
Oxidations of aniline or aniline hydrochloride have been carried out in ethylene glycol or glycerol and in their mixtures with water. Ammonium peroxydisulfate was used as oxidant. The oxidation of aniline is exothermic and changes in temperature were monitored to follow its progress. The effect of the solvents on the course of oxidation, morphology, and properties of final products has been studied by scanning electron microscopy, FTIR spectroscopy, and conductivity measurements. It is proposed that the reduction of dielectric constant of the reaction medium results in the reduced dissociation of ionic species that take a part in oxidative polymerization. Consequently, the addition of an organic solvent has a similar effect as a decrease in the acidity of the reaction mixture. Conductivity and morphology depend on the mole ratio of oxidant and monomer, and the type and volume fractions of organic solvents, viz. ethylene glycol and glycerol, which were used in reactions. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Conducting polymer Polyaniline Nanotubes
1. Introduction Polyaniline (PANI) is one of the most interesting conducting polymers due to the ease of its preparation, its excellent environmental stability and good electron and proton conductivities [1e3]. The electronic conductivity of PANI can be controlled by both the degrees of oxidation and protonation [4,5]. Electrical properties of PANI find applications in electronic devices, sensors, storage of energy, electrorheology, etc. [6e8]. Polyaniline can be prepared by chemical or electrochemical oxidation of aniline in aqueous media [9,10] or in the solid state [11,12]. There are many parameters, which are important in the control of the properties and morphology of the PANI. The pH of the aqueous reaction mixture used for the preparation of the PANI is one of the most important. When aniline was oxidized at low pH, i.e. at high acidity, conducting polymers with molecular weight of 104e105 were produced. The morphology of PANI was represented by granules, nanotubes or nanofibers [13e16]. Self-assembled PANI nanotubes [17e21] can be prepared when the pH varies from the starting value above 5 to a final value below 2.5 [2,22]. This situation is quite
* Corresponding author. Tel.: þ420 296 809 234; fax: þ420 296 809 410. E-mail address: [email protected] (E.N. Konyushenko). 0032-3861/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2011.02.047
often met in the presence of the weak acids. At neutral conditions the different types of morphology were produced, from almost amorphous materials to micro- and nanospheres [16,23] and highly organized hierarchical structures, such as micromats [2,10]. In contrast with this, the non-conducting aniline oligomers were synthesized by the oxidation of aniline in alkaline media [16]. It has been demonstrated in the literature that the addition of miscible organic solvents to aqueous media affects the morphology of the produced PANI. The transition from the globular morphology in favor of nanofibres or nanotubes has been observed when the reaction mixture contained alcohols, such as methanol [24], ethanol [25] or 1,6-hexanediol [26]. The presence of organic solvents thus can be useful in the control of PANI morphology. Chemical oxidative polymerization exclusively in organic solvents, i.e. in the absence of water, has not been performed so far. In the present study, we demonstrate that various factors, including dielectric constant of reaction mixture, can influence the morphology and properties of PANI by controlling the distribution of protonated and deprotonated reaction species in reaction mixture. Moreover, we report the simple preparation of PANI nanostructures, such as granules, nanotubes, or nanospheres in ethylene glycol or glycerol and their mixtures with water. The diol and triol were chosen because they are benign to the environment, they are lowcost chemicals, and polymerization of aniline in them is feasible. Such synthesis may be of benefit in applications of PANI which require the absence of residual water, such as in electrochemistry.
E.N. Konyushenko et al. / Polymer 52 (2011) 1900e1907
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a 4n
NH2
+
+ NH
H N
HSO4
*
5n (NH4)2S2O8
+ NH HSO4
+ N H
3n H2SO4
+
5n (NH4)2SO4
n
b 4n
NH2HCl
+ NH *
Cl
-
+
5n (NH4)2S2O8
H N + NH Cl
+ 5n H2SO4 + 5n (NH4)2SO4 + N H
2n HCl
n
Fig. 1. (a) Aniline and (b) aniline hydrochloride were oxidized with ammonium peroxydisulfate to PANI salt. Sulfuric acid and ammonium sulfate are by-products.
2. Experimental 2.1. Preparation Aniline or aniline hydrochloride (Aldrich) was oxidized by ammonium peroxydisulfate (APS) in mixtures of ethylene glycol or glycerol and water. The aniline concentration was 0.2 M, the mole ratio of oxidant to monomer was 1 or 2. The amount of ethylene glycol or glycerol was varied in the range 0e100 vol.%. This means that the experiments included polymerizations in the absence of water. The reactions were carried out at room temperature, w20 C. The final products of oxidation were collected on a filter, rinsed with 0.1 M sulfuric acid, to remove the residual organic components and inorganic salts, and with acetone. Finally, the product was dried in air overnight and then over silica gel. A part of each sample was converted to the corresponding emeraldine base by 1 M ammonium hydroxide, and dried, prior to characterization by UVevisible and FTIR spectroscopies.
2.2. Characterization Scanning electron micrographs were taken with a JEOL 6400 microscope (Japan). Transmission images have been taken with JEOL JEM 2000FX microscope. Infrared spectra were recorded with a fully computerized Thermo Nicolet NEXUS 870 FTIR Spectrometer with a DTGS TEC detector. The spectra of samples dispersed in potassium bromide were collected in the transmission mode and corrected for the presence of carbon dioxide and humidity in the optical path. UVevisible spectra of the oxidation products dissolved in N-methylpyrrolidone were recorded with a spectrometer Lambda 20 (Perkin Elmer, UK). Conductivity was determined by a four-point van der Pauw method on PANI powders compressed into the pellets 13 mm in diameter and 1 mm thick using the
Keithley 2010 Multimeter equipped with a 2000-SCAN 10 channel scanner card or by a two-point method using a Keithley 6517 electrometer for samples of low conductivity.
3. Results and discussion It is well-known that the acidity of the reaction mixture is the key factor that affects the course of oxidation, properties and morphology of PANI [2,16,21,22,27]. When trying to predict the role of the non-aqueous solvent, which is used as a component of reaction mixture or alone, the consideration of the relative permittivity of the solvent, e, might be pertinent. The permittivity of water is e ¼ 80.1 (20 C). Most solvents have lower permittivity, e.g., ethylene glycol, e ¼ 38.7 (20 C), and glycerol, e ¼ 42.4 (25 C). When added to water, the permittivity of the medium will be reduced. The dissociation of ionic species in such a medium will consequently be suppressed. In the case of acids, which are present or generated in the system, this means that the concentration of protons will be lower by comparison with purely aqueous reaction mixtures. In other words, the effective acidity of the system will be reduced. This concept thus predicts that the addition of a miscible organic solvent will have similar effect to the use of a less acidic aqueous medium. The following transitions may thus be expected when aqueous reaction medium changed to mixed watereorganic or pure organic solvent [2]: 1 The formation of non-conducting aniline oligomers during single-step exothermic oxidation at the low effective acidity when water content is low or absent. 2 Two-step exothermic oxidation process with a long time induction period between the steps and the formation of aniline oligomers in the first step, and conducting polymers in
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the second step, when content of water increases and effective acidity rises. 3 Transformation of two-step oxidation to single-step oxidation with an initial induction period and the synthesis of conducting PANI when the effective acidity will be increased by the presence of acid. As for morphology of the products, the formation of PANI nanotubes may be expected during the two-step oxidation. The formation of one-dimensional structures is a result of the selfassembly of short aniline oligomers, the nucleates, that are produced during induction period between two exothermic oxidation steps. Non-conducting aniline oligomers produced at the first oxidation stage serve as an initial template for the growth of tubular morphology [2,16]. The solubility of the monomer, oxidant, and reaction intermediates is another factor that can influence the course of oxidation, properties and morphology of final oxidation products in the presence of organic solvents. The addition of organic solvents, such as alcohols, can lead to the solubilization of intermediates, and thus to prevent the generation of organized structures. This trend will dominate in the build-up of PANI morphology. Two series of aniline oxidations are demonstrated. In the first, aniline was oxidized by ammonium peroxydisulfate (Fig.1a) in organic solvents, ethylene glycol and glycerol, and their mixtures with water. Reactionwas carried out at a mole ratio [APS]/[aniline] ¼ 1 and at ambient temperature. In the second series, aniline was replaced with aniline hydrochloride (Fig.1b) in order to increase the effective acidity of reaction mixture. Other oxidation conditions were the same. The oxidation is an exothermic reaction and the temperature changes during the oxidation were monitored to follow its progress [16,18,22]. The temperature profiles were measured for all experiments, illustrative examples are shown in Fig. 2.
3.1. The course of aniline oxidation and the properties of the products In accordance with our predictions, aniline oxidations are the monotonous single-step reactions (Fig. 2a) in ethylene glycol and glycerol, and also in mixtures of these organic solvents with low water content. The yields of reactions were low compared to the oxidation of aniline in water, the conductivity of the products was also low and sometimes it is on insulating level. The conductivities of products synthesized in ethylene glycol and glycerol are 3.2 109 and 1.7 105 S cm1, respectively (Table 1). The FTIR spectra of the products prepared in ethylene glycol (spectrum A-EG in Fig. 3a) correspond to aniline oligomers, described earlier [16], and are overlapped by strong peaks of ethylene glycol observed at 1083 and 1036 cm1 (shown by asterisks in Fig. 3a). In glycerol, the spectrum of the sample (spectrum AGl in Fig. 3a) is close to the spectrum of PANI prepared in water (spectrum A-W in Fig. 3b) in which, in addition to the most important bands at 1573 and 1498 cm1 (quinonoid and benzenoid ring-stretching vibrations, respectively), at 1245 cm1 (CeNþ 1 stretching vibration in the polaron structure), at 1149 cm (vibration mode of the eNHþ¼ structure) and at 806 cm1 (CeH out-of-plane bending vibrations) [28], the peak of residual glycerol is observed (shown by asterix in Fig. 3a). We conclude that aniline oxidation in pristine organic solvents and also in organic solvent with low water content leads mostly to non-conjugated and non-conducting aniline oligomers. The reason consists in the low effective acidity of reaction mixture and the oxidation of non-protonated species. In this case, the redox interaction proceeds between non-protonated monomer and non-protonated oligomer structure. As a result, the reaction of electrophilic substitution leads to the growth of short chains with mixed ortho- and para-linked constitutional units. The concentrations of protons which are released as sulfuric acid (Fig. 1a) were not sufficient to reach high effective acidity and start the propagation of PANI chains. For that reason, the non-conjugated oligomers with irregular structure are the only products [2]. When oxidation proceeds in the presence of 20 vol.% and 50 vol.% of organic solvent, the effective acidity of reaction mixture raised and one can see the transition of the oxidation course from single-step to two-step exothermic process (Fig. 2a). The presence of the second exothermic phase correlates with the higher conductivity of the products (Table 1) because the conducting
Table 1 The results of oxidations of aniline and aniline hydrochloride in ethylene glycol or glycerol and their mixtures with water.a Solvent
Solvent, vol.%
The oxidation of aniline Water 100 Ethylene glycol 20 50 100a Glycerol 20 50 100a
Fig. 2. The temperature profiles in the oxidation of (a) aniline or (b) aniline hydrochloride in ethylene glycol (EG) or glycerol (Gl) and in their mixtures with water (50 vol.%). The profile obtained in water is included as a reference.
Yield, g (%)
Conductivity S cm1
Conductivity of bases, S cm1
1.80 0.97 0.84 0.67 0.84 1.40 1.70
0.2 5.1 3 3.0 3 3.2 3 0.14 5.0 3 1.7 3
2.5 1010 1.2 108 6.0 108 e 0.45 109 7.5 109 5.3 106
(64) (34) (30) (24) (30) (50) (60)
The oxidation of aniline hydrochloride Water 100 1.90 (89) Ethylene glycol 20 2.00 (93) 50 2.80 (130) 100 0.86 (41) Glycerol 20 1.80 (84) 50 1.77 (85) 100 1.92 (90) a
5.0 1.4 2.0 8.3 4.0 4.5 2.5
102 103 109 103 105
2.0 2.9 3.1 4.5 1.3 6.8 6.6
Only brown non-conducting aniline oligomers were produced.
108 106 107 106 108 108 106
E.N. Konyushenko et al. / Polymer 52 (2011) 1900e1907
Fig. 3. FTIR spectra of samples prepared in water or in non-aqueous media, ethylene glycol or glycerol during the oxidation of: (a) aniline (A) or aniline hydrochloride (AH) in ethylene glycol (EG) and glycerol (Gl) (the spectrum of the sample prepared in water (W) is included for comparison), (b) aniline in water and glycerol and their mixtures, as prepared and in deprotonated forms (B), (c) aniline hydrochloride in water and ethylene glycol and their mixtures, as prepared and in deprotonated forms.
polymer is produced exclusively in the second reaction step. The conductivity of the products increased up to 101 S cm1. We have compared the infrared spectra of the products of oxidation of neutral aniline at two different concentrations of glycerol, 20 and 50 vol.% (Fig. 3b) with the spectrum of the product prepared in water (spectrum A-W in Fig. 3b). The samples were also analyzed in their deprotonated state, i.e. as emeraldine bases (Fig. 3b). In the case of 20 vol.% of glycerol medium (spectrum A-20 Gl) and water [8], the spectra are practically identical. They correspond to the protonated emeraldine due to sulfuric acid
1903
produced during the oxidation with the most important bands situated at 1573 and 1498 cm1 (quinonoid and benzenoid ring stretching vibrations, respectively), 1245 cm1 (CeNþ stretching vibration in the polaron structure), 1149 cm1 (vibration mode of the eNHþ¼ structure), and at 806 cm1 (CeH out-of-plane bending vibrations) [28]. After deprotonation the spectrum of the product obtained in 20 vol.% of glycerol (spectrum A-20 Gl B) corresponds to completely deprotonated sample, contrary to the case of water (spectrum A-W B). The resistance toward deprotonation of the last sample was noticed earlier [10] and it has been associated with the sulfonation of benzene rings. Two small peaks at 1445 and 1414 cm1 present in the spectrum of deprotonated sample prepared in 20 vol.% of glycerol (spectrum A-20 Gl B) and in water (spectrum A-W B) and marked by arrows in Fig. 3b belong to the aniline oligomers produced at the early stages of aniline oxidation at low acidity of medium [16]. A sample prepared in 50 vol. % of glycerol (spectrum A-50 Gl in Fig. 3b) contains this solvent and its deprotonation was not complete (spectrum A-50 Gl B in Fig. 3b). The formation of conducing polymer in the second oxidation phase follows the synthesis of aniline oligomers in the first oxidation phase [2,16]. The products are the mixture of nonconducting oligomer and conducting polymer. The level of conductivity depends from the proportions of each oxidation product. At the two-step process the first oxidation stage proceeds as before, the oxidation takes place at low effective acidity. The redox interaction between non-protonated monomer and nonprotonated chain resulting in the formation of non-conducting oligomers takes place. The protons are a by-product of the electrophilic substitution reaction, and the effective acidity of reaction mixture increases (Fig. 1a). This leads to the protonation of monomer and intermediates. Aniline monomer becomes protonated the first, and this manifested itself by the start of athermal induction period. The rate of the oxidation reaction decreases because the redox process taking place between protonated monomer and non-protonated chain is reduced. Short aniline oligomers, the nucleates, are the only product at this stage. The second oxidation phase can be achieved when the acidity of reaction mixture increases to the level needed for the protonation of imine constitutional units. Protonated nucleates start to grow to regular PANI chains with para-linked constitutional units. That is why the conducting oxidation product is obtained at the second oxidation stage. When the reaction mixture contained glycerol, the reaction was faster when compared with the polymerization conducted in pure water (Fig. 2a). This fact can be explained by better solubility of aniline in organic solvents compared with water. In contrast to mixed solvents, the oxidation in water starts as heterogeneous process when a large part of monomer exists as droplets due to a limited solubility of aniline in water.
3.2. The oxidation of aniline hydrochloride Another series of experiments was carried out using aniline hydrochloride instead of aniline (Fig. 1b). The oxidation thus proceeds at higher acidity compared with the above experiments. The mole ratio [APS]/[aniline] ¼ 1 was also kept in all these experiments, and the volume fractions of diol and triol were varied (Fig. 2b). The oxidations in ethylene glycol and glycerol in the absence of water are much slower. For that reason, the polymerization has not been detected by the increase of temperature, because of the heat dissipation. The polymerization, however, took place, as proved by the high reaction yield and good conductivity of products (Table 1).
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1.5
ethylene glycol
A bsorban ce
1.0
water glycerol
0.5
0.0 400
600
800
Wavelength, nm Fig. 4. UVevisible spectra of the PANI bases prepared by the oxidation of aniline hydrochloride in water, ethylene glycol, or glycerol, and dissolved in N-methylpyrrolidone.
Polyaniline prepared from aniline hydrochloride in water had conductivity of 5.0 S cm1. When the polyaniline is synthesized in a medium containing an increasing amount of ethylene glycol the conductivity slightly increased. Polyaniline prepared in pure ethylene glycol exhibited the maximum conductivity of all the samples, 8.3 S cm1. The fraction of glycerol in the polymerization system had a marginal effect on the conductivity of the PANI produced (Table 1). In this respect, the preparation of PANI by oxidation of aniline hydrochloride in water, ethylene glycol, and glycerol are similar, only the oxidations in an organic solvents have a slower kinetics.
The reasons of this lower rate may be due to poor solubility of peroxydisulfate and aniline salts in these organic solvents. Solubilization of aniline hydrochloride in diol and triol leads to a slight opalescence of the solutions. This means that monomer was agglomerated at these solvents. The FTIR spectrum obtained during polymerization of aniline hydrochloride in ethylene glycol (spectrum AH-EG in Fig. 3a) is close to the spectrum obtained in water (spectrum AH-W in Fig. 3a), only two small peaks at 1083 and 1036 cm1 belonging to ethylene glycol and marked by asterisks in the corresponding spectra signify that this solvent is still present within the sample and is attached most probably due to hydrogen bonding [25]. The situation is different when aniline hydrochloride is used and oxidized in glycerol (spectrum AH-Gl in Fig. 3a). Bands of water molecules present in potassium bromide are detected in the spectrum due to compact consistence of the sample and difficult dispersion in potassium bromide. In the oxidation of aniline hydrochloride in the mixtures of organic solvent and water, the first oxidation step is absent and only the characteristic induction period is found (Fig. 2b). Aniline hydrochloride is easily polymerized with APS in water, and the reaction was complete within 10 min (Fig. 2b). This is a so-called “standard” polymerization [29]. A solvent content up to 50 vol.% did not change the time needed for PANI formation but it slightly decreased the maximum temperature reached during the reaction (Fig. 2b). The polymerization phase is well developed, and this is again in accordance with the good conductivity values of the final products (Table 1). All the PANI obtained from aniline hydrochloride oxidation possess a chain conjugation, as shown by a strong band with maximum of 620 nm observed in UVevisible spectra of deprotonated samples (Fig. 4) [30e32]. The conductivity of the products was as high as 10 S cm1. The infrared spectra of the protonated samples at two different
Fig. 5. Polyaniline prepared by the oxidation of 0.2 M aniline by 0.2 M ammonium peroxydisulfate in (a) 20 vol.% of ethylene glycol, (b) 50 vol.% of ethylene glycol, (c) in 20 vol.% of glycerol, and (d) and 50 vol.% of glycerol.
E.N. Konyushenko et al. / Polymer 52 (2011) 1900e1907
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Fig. 6. The products of the oxidation of 0.2 M aniline hydrochloride by 0.2 M APS in (a) 50 vol.% ethylene glycol, (b) in pure ethylene glycol, (c) in 50 vol.% glycerol, and (d) in pure glycerol.
concentrations of ethylene glycol (20 and 50 vol.%) and spectrum of the product of oxidation of aniline hydrochloride in water are compared in Fig. 3c. In the case of 20 vol.% of ethylene glycol (spectrum AH-20 EG) and water (spectrum AH-W), the spectra of oxidation products are practically identical and correspond to the protonated emeraldine [28]. After deprotonation, the spectrum of product obtained in 20 vol. % of ethylene glycol (spectrum AH-20 EG B) is the same spectrum as that of “standard” emeraldine base (spectrum AH-W B shown in Fig. 3c for comparison) with the blueshifted main bands of quinonoid and benzenoid ring-stretching vibrations (at 1573 and 1498 cm1) and lowered bands character istic of protonation modes of PANI and corresponding to the CeNþ stretching vibrations (1245 cm1) and to the vibration of the eNHþ¼ structure (1149 cm1) [28]. The spectrum of sample prepared in 50 vol.% of ethylene glycol (spectrum AH-50 EG) exhibits lower absorbance, as its compact consistence and difficult dispersion in potassium bromide. After deprotonation (spectrum AH-50 EG B), the sample preparation was much easier and the spectrum was close to the spectrum of emeraldine base. We conclude that the presence of ethylene glycol leads to stronger hydrogen bonding and to more compact structure as compared with the sample prepared in water. In acidic media when aniline hydrochloride was used instead of aniline, monomer exists mostly in its protonated form. Concentration of neutral aniline is very low. Protonated aniline, i.e. the anilinium cation, has higher oxidation potential and probably is not oxidized at the beginning. At the first stage the oxidation of neutral non-protonated aniline proceeds. Short cyclic oligomers, probably trimers of phenazine structure, are produced during this induction
period. In acidic conditions, cyclic oligomers become protonated and initiate the growth of regular para-linked PANI chains. The conjugated chains with a high conductivity level are then produced. Products prepared in acidic conditions mostly contain the polymer, the fraction of oligomers being negligible. 3.3. Morphology Morphology of PANI is determined by the self-association of aniline nucleates produced during the induction period [2]. Phenazine nucleates are the flat molecules that organize into the columns by pep electron interaction. Phenazine stacks formation predetermines the subsequent growth of one-dimensional structures, nanotubes and nanofibers. Non-regular phenazine association into agglomerates of spherical form leads to granular PANI structure. The reaction conditions, especially the acidity profile, during the oxidation of aniline [2], predetermine the morphology of the final PANI product. In the present research, we have varied the nature of the monomer, i.e. aniline or aniline hydrochloride, and the organic solvent, in order to find conditions where PANI structures exhibit morphology of interest. Existence of two-step oxidation process, in which two exothermic steps are separated by athermal induction period, make favorable conditions for nanotubular growth [1]. The collection and regular assembly of phenazine nucleates produced at the induction period, gives a rise to formation of one-dimensional phenazine stacks. At the second oxidation phase, the polymer chains grow from the stacked nucleates and the walls of PANI tubes are produced.
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Fig. 7. Scanning electron micrographs of the products of aniline (0.2 M) oxidation by excess of ammonium peroxydisulfate (0.4 M) in (a) 50 vol.% ethylene glycol and (b) 50 vol. % glycerol. Transmission electron micrograph is shown in inset.
When aniline was oxidized in the presence of 20e50 vol.% of ethylene glycol the two-step oxidation process was observed and PANI nanotubes were indeed obtained (Fig. 5a), the outer diameter of polyaniline nanotubes is 250 nm. This is similar to the case when the oxidation of aniline was made in water [10]. Nanotubes were accompanied by granules and by some spherical particles. PANI granules are produced in more acidic conditions at advanced stages of the polymerization. If pH is low, the induction period is shortened. This means that phenazine nucleates can not self-organized in regular manner [2,16]. The random agglomeration of phenazines gives a raise to PANI granules (Fig. 5b). Others morphologies can be prepared when a triol, glycerol, is used as the solvent in aniline oxidation: mixture of granules and amorphous structures are formed in 20 vol.% glycerol (Fig. 5c). Granules accompanied by rectangular objects are produced in 50 vol.% glycerol (Fig. 5d). The polymerization of aniline hydrochloride at various volume fractions of ethylene glycol or glycerol in the reaction mixture has not lead to any uniform nanostructures. Scanning electron microscopy shows the amorphous or partially granular morphology of products when the polymerization took place in 50 and 100 vol.% ethylene glycol (Fig. 6a,b). We propose that this is connected with a solubility factor. Better solubility of nucleates acts against their organization and manifests itself in a “packed layers’’ morphology in pure glycerol (Fig. 6c,d).
1504
Absorbance
1583
A-Am
*
A-50 Gl B
* *
A-50 EG B
* *
Glycerol Ethylene glycol
4000
3000
2000
1000
Wavenumbers, cm Fig. 8. FTIR spectra of the samples prepared by the oxidation of 0.2 M aniline with 0.5 M APS, i.e. with an excess of oxidant, in ethylene glycol and glycerol mixtures with water (50 vol.%) after deprotonation to bases, A-50 Gl B and A-50 EG B, respectively. The spectrum of aniline oligomers prepared in 1 M ammonium hydroxide (A-Am) and the spectra of both organic solvents are included for comparison.
3.4. Nanospheres When scanning various reaction conditions, PANI nanospheres have occasionally been observed in the products (Fig. 7). For example, this happened when aniline was oxidized by ammonium peroxydisulfate in 50 vol.% of ethylene glycol and the molar concentration of oxidant with respect to aniline was twice higher, i.e. under the conditions of strong overoxidation. Under such experimental conditions, the pH of the reaction is sufficient for the protonation of reaction intermediates and PANI is formed. The formation of sulfuric acid during the polymerization kept the product at the overoxidized pernigraniline level. Nearly uniform fused PANI nanospheres were obtained and had a mean diameter of 600 nm. Polyaniline prepared in this way had a conductivity of 7 104 S cm1. Such a low value reflects the overoxidation of the final product. The formation of spheres is connected to the increased amount of oxidant in the polymerization mixture. Due to the large concentration of salts, the aniline solubility is reduced and aniline droplets are produced as the result of a salting-out effect. These act as templates [16]. The high concentration of peroxydisulfate anions at the beginning of the oxidation may also lead to the sulfonation of benzene rings not only in the aniline oligomers [13] but also in the aniline molecules. The sulfonated anilines, such as o- and p-aminobenzenesulfonic acids, may be incorporated both into oligomers and polymers. Some authors assume the introduction of sulfate groups to be more probable [33]. Sulfo groups in oligomers produce surfactant-like structures that can stabilize monomer droplets at the submicrometre size level. They gradually convert to PANI nanospheres as the oxidation proceeds. In the case of the polymerization of aniline hydrochloride, the monomer is completely miscible with the reaction medium and, consequently, nanospheres are not produced. The FTIR spectra of the products of oxidation of aniline in 50 vol.% of ethylene glycol (spectrum A-50 EG B in Fig. 8) and glycerol (spectrum A-50 Gl B) with 0.5 M APS in their deprotonated state are compared with the spectrum of the product of oxidation of aniline in 0.2 M ammonium hydroxide solution with 0.25 M APS reported in earlier paper [16] (spectrum A-Am), when oligomeric microspheres were obtained. The spectra are close to each other, in the case of ethylene glycol and glycerol the small peaks at 1445 and 1414 cm1 marked by arrows are also present. It is difficult to distinguish between the peak of sulfonation at about 1040 cm1 in the spectrum of aniline oxidized in ammonium hydroxide (spectrum A-Am) and the peaks of ethylene glycol and glycerol situated close to this peak (marked by asterisk in the spectra). The sharpness of this peak in the spectra points more to the sulfonation, and is
E.N. Konyushenko et al. / Polymer 52 (2011) 1900e1907
higher in the case of glycerol. Instead of well-distinguished and structured peaks present in the spectrum of microspheres obtained in ammonium hydroxide, we have observe the broad bands in the spectra of the products prepared in presence of 50 vol.% of ethylene glycol and glycerol, especially in the region of NH stretching vibrations. We conclude that strong hydrogen bonding occurs in the structure of these products between PANI and diol or triol.
4. Conclusions The feasibility of the preparation of polyaniline by the chemical oxidation of aniline hydrochloride, i.e. of anilinium cations, in nonaqueous media has been demonstrated. Polyaniline was prepared in ethylene glycol and glycerol, and in their mixtures with water. It is proposed that the use of an organic solvent has a similar effect to a reduction in acidity because of reduced dissociation of acid present in the medium. The oxidation of aniline hydrochloride produced PANI of standard emeraldine structure in both organic solvents. The conductivity of the product prepared in ethylene glycol, 8.3 S cm1, was even higher than that of the product prepared in water, 5.0 S cm1. The oxidation of neutral aniline yielded to materials of lower conductivity. Aniline oligomers were present in the final products. In ethylene glycol, non-conducting oligomers were the only product due to the low acidity of the medium. Differences in morphology and departures from the common granular appearance have been found. The morphology is assumed to be controlled by the assembly of aniline oligomers based on their hydrophobic character and by strong hydrogen bondig between the products and organic solvents. For these reasons, the morphology depends on the content of the organic component, which affects the interaction between the oligomers and thus their self-assembly. Oligomeric nanospheres were observed when excess of oxidant was used in the synthesis.
Acknowledgments The authors wish to thank the EGIDE (ECO-NET 16256SA), the Grant Agency of the Academy of Sciences of the Czech Republic (IAA 400500905), and the Czech Grant Agency (203/08/0686) for financial support. Thanks are due to J. Hromádková for the electron microscopy and to J. Prokes for the conductivity measurements.
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