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The polymerization of aniline at a solution–gelatin gel interface
European Polymer Journal 45 (2009) 668–673
Contents lists available at ScienceDirect
European Polymer Journal journal homepage: www.elsevier.com/locate/europolj
The polymerization of aniline at a solution–gelatin gel interface Natalia V. Blinova *, Miroslava Trchová, Jaroslav Stejskal Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, Department of Supramolecular Polymer Systems, Heyrovsky Sq. 2, 162 06 Prague 6, Czech Republic
a r t i c l e
i n f o
Article history: Received 17 October 2008 Received in revised form 9 December 2008 Accepted 22 December 2008 Available online 30 December 2008
Keywords: Conducting polymer Polyaniline Gelatin Interfacial polymerization Trans-membrane polymerization
a b s t r a c t Gelatin gel swollen with the solution of aniline hydrochloride was exposed to a solution of ammonium peroxydisulfate. The reactants met at the gel interface, and the redox reaction between them produced a polyaniline (PANI) interlayer, a PANI membrane, at first. The electrons abstracted from the aniline molecules in the gel during the oxidation are transferred through a conducting PANI membrane to oxidant molecules in the external solution. The reaction between aniline and peroxydisulfate thus takes place without the need for the reactant molecules to physically meet. PANI, therefore, grows from the interface into the gelatin gel. When the loci of reactants are reversed, i.e. the oxidant is inside the gelatin gel and aniline hydrochloride in the surrounding solution, PANI grows from the gel interface into the aniline solution but some PANI is produced inside the gelatin gel, too. Composite PANI–gelatin gels were separated and gelatin was removed from them by acid hydrolysis. The resulting PANI had a granular morphology and a conductivity of the order of units S cm 1, slightly lower compared with PANI prepared in a common way by mixing the solutions of reactants. The differences in the details of molecular structure are discussed on the basis of FTIR spectra. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Composite systems comprising a conducting polymer, such as polyaniline (PANI), distributed within a polymer gel represent novel materials that have recently been prepared and investigated. One strategy consisted in the preparation of polyaniline colloids stabilized with poly (vinyl alcohol) followed by c-irradiation, which produced the gel [1]. Other approaches were based on the electrospinning of PANI–gelatin blends [2], entrapment of sulfonated PANI or electropolymerization of aniline in a polyacrylamide gel [3]. Microgels have also served as templates for the polymerization of aniline [4]. Such composite systems are likely to find uses in the design of biocompatible materials [1], tissue engineering [2,5,6], electrically-controlled drug release [7], and related biomedical applications. The preparation of composite gels containing a * Corresponding author. Tel.: +420 296 809 234; fax: +420 296 809 410. E-mail address: [email protected] (N.V. Blinova). 0014-3057/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2008.12.034
conducting polymer is not trivial and the control of the spatial distribution of conducting component within the gel is of challenge. A polymerization technique, in which gelatin gel swollen with aniline solutions was exposed to a solution of an oxidant, ammonium peroxydisulfate, is discussed in the present communication. It has recently been reported that the redox processes between the oxidant and reductant separated by a conducting polymer membrane can proceed by the transfer of electrons from reductant to oxidant through the membrane [8]. Such a process has been illustrated by the reduction of iron (III) ions with ascorbic acid separated by PANI membrane [8,9] and applied to the design of a potentiometric sensor [10]. The electroneutrality of the system is maintained by the simultaneous transfer of protons. In contrast to metals, which conduct electrons but not protons and anions, PANI in the presence of water is both an electron and a proton conductor [11] Anion transport may also play an important role [10,11]. The molecules of oxidant and reductant thus may react without the need
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to be in direct contact by passing electrons through conducting PANI. This concept has recently been extended to the oxidation of aniline with ammonium peroxydisulfate [12], i.e. to the reaction, which is currently used for the preparation of PANI [13]. When solutions of aniline hydrochloride and ammonium peroxydisulfate were separated by a PANI membrane, produced in situ on semi-permeable cellulose template membrane or in advance, PANI was produced entirely on the aniline side of the membrane (Fig. 1). It was confirmed that, also in this case, the reductant (aniline) provides and the oxidant (peroxydisulfate) accepts electrons transferred by the conducting PANI. Polyaniline was produced by this trans-membrane reaction in a high yield, its conductivity being about the same as the conductivity of PANI prepared in the classical way, by mixing the reactant solutions [13]. In the present study, aniline hydrochloride was dissolved in the aqueous solution of gelatin, which was left to produce a gel by lowering the temperature. The solution of an oxidant was then poured over the gelatin gel. The reactants thus were separated, and met at the gel interface with an aqueous solution and produced a PANI membrane there. The subsequent polymerization of aniline is investigated in the present report. The goal of this communication is to provide understanding of the processes involved in the preparation of composite gels comprising a conducting polymer as a component. 2. Experimental 2.1. Preparation of PANI Gelatin (2.5 g; Fluka, Switzerland) was dissolved in 50 mL of an aqueous solution containing various amounts (0.4, 0.8 and 1.2 M) of aniline hydrochloride (Fluka, Switzerland) at 40 °C. The concentration of gelatin in the solution was 5 wt.%. The solutions in beakers were placed in a refrigerator at 4 °C and left overnight to produce a gel. A 50 mL solution of ammonium peroxydisulfate (APS; LachNer, Czech Republic) was poured over the gelatin gel and
PANI membrane Aniline
the system was left in a refrigerator for 7 days. The oxidant-to-aniline molar ratio was 1.25 in all cases. Similar experiments with ammonium peroxydisulfate located inside the gelatin gel and aniline hydrochloride in the external solution have also been carried out. Reference polymerizations, in which the solutions of aniline hydrochloride and APS were simply mixed in the absence of gelatin, have been made at 4 °C. After the oxidative polymerization of aniline, the supernatant liquid was separated from the gel, and any solids were separated by filtration, rinsed with 0.2 M HCl, and acetone, and dried. The gel was placed into excess of 1 M HCl solution and heated to 80 °C for 1 h. Under these conditions, the gelatin was completely hydrolyzed to aminoacids and removed from the PANI [14]. Polyaniline is not damaged by exposure to a strongly acidic medium at elevated temperature [15,16]. The reference PANI samples were prepared in the absence of gelatin and treated in the same way. The solids after the hydrolysis were collected on a filter, rinsed with 1 M HCl, and with acetone, and dried in air. 2.2. Characterization Scanning electron micrographs were taken with a JEOL 6400 microscope (Japan). Infrared spectra were recorded with a fully computerized Thermo Nicolet NEXUS 870 FTIR Spectrometer with a DTGS TEC detector. The spectra of the samples dispersed in potassium bromide were recorded in the transmission mode and corrected for the presence of carbon dioxide and moisture in the optical path. The conductivity was measured with a four-point van der Pauw method on pellets compressed at 700 MPa with a manual hydraulic press using as a SMU Keithley 237 current source and a Multimeter Keithley 2010 with a 2000 SCAN 10channel scanner card. 3. Results and discussion The oxidation of aniline with APS in acidic aqueous medium is a redox reaction yielding PANI [13] (Fig. 2). Sulfuric acid and ammonium sulfate are by-products, at equimolar proportions they produce ammonium hydrogen sulfate [12,17].
Peroxydisulfate – ne
ne
4n
+ ne
PANI
NH2.HA
+
5 n (NH4)2S 2O8
Hydrogen sulfate nH – nH
(nH )
(Anions) Fig. 1. Aniline molecules are oxidized to PANI (left) and peroxydisulfate molecules reduced to hydrogen sulfate (right) by the transfer of electrons through the conducting PANI membrane separating both reactants. Protons accompany electrons in order to maintain the electroneutrality of the system. The possible transport of anions has the same role (adapted from Ref. [12]).
NH A
NH A + 2 n HA
NH
NH n
+ 5 n H2SO4 + 5 n (NH4)2SO4
Fig. 2. Aniline salt is oxidized with ammonium peroxydisulfate to PANI. Sulfuric acid and ammonium sulfate (i.e. ammonium hydrogen sulfate when in equimolar proportion) are by-products. HA is an arbitrary acid, here hydrochloric acid.
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3.1. Polymerization at a solution–gel interface: aniline in the gel In the present paper, one of the reactants is incorporated in the hydrogel, which faces the solution of the second reactant (Fig. 3a). In this case, aniline and oxidant are separated at the beginning of the reaction and are in contact only at the gel interface where both reactants meet. The PANI interlayer is produced at first at this interface, and acts similarly as the PANI membrane in the transmembrane polymerization [8,9]. By analogy with the membrane experiment [12], the electrons abstracted from the aniline molecules are transferred through the conducting PANI to APS, which converts to ammonium sulfate (Fig. 1). Polyaniline is thus expected to be produced at the aniline-containing phase (Fig. 1), i.e. here within the gelatin gel. This is indeed the case (Fig. 3a), and PANI grows from the gel interface into its interior. Over 93 wt.% of PANI
a
(Table 1) from the total amount of PANI produced in the system, was formed in the gel, and the supernatant APS solution contained only traces of the polymer. These traces result from aniline hydrochloride that had diffused into the APS solution before the interfacial PANI layer was produced. The yield of reaction in the gel phase increased from 0.45 to 0.69 g of PANI per gram of aniline hydrochloride with increasing concentration of reactants (Table 1), and was lower than the yield in the reaction carried out in the absence of gelatin by mixing the solutions of aniline hydrochloride and APS (Table 2). Given the stoichiometry shown in Fig. 2 and 1 g of aniline hydrochloride should produce 0.84 g of PANI hydrochloride [13]. The higher yields reported in Table 2 are due to the incorporation of more bulky counter-ions, such as hydrogen sulfate anions [18]. The conductivity of the PANI prepared in the gelatin gel (after the hydrolytic removal of gelatin) is of the order
b Peroxydisulfate solution S2O82–
Aniline hydrochloride solution
2 SO42– PANI
PANI
S2O82– Aniline hydrochloride in gelatin gel
2 SO42–
Peroxydisulfate in gelatin gel
Fig. 3. Aniline molecules are oxidized to PANI at the polymerization site (circle) and abstracted electrons are transferred (arrows) through the conducting PANI to the gel–solution interface where they are accepted by the oxidant molecules. (a) When aniline is located in the gelatin, PANI grows into the gel phase. (b) When APS is in the gel phase, PANI grows on the surface of the gel and partly also penetrates inside the gelatin gel.
Table 1 The yield and conductivity of PANI produced by the oxidation of aniline with ammonium peroxydisulfate, one reactant being in a gelatin gel, the other in the external aqueous solution in the contact with the gel. Aniline concentration,a mol L
1
Yield,b g g
1
Conductivity,c S cm
Gel
Solution
Aniline hydrochloride in gelatin gel 0.2 0.4 0.6
0.448 (92.9%) 0.610 (98.0%) 0.693 (99.4%)
0.034 (7.1%) 0.012 (2.0%) 0.004 (0.6%)
0.23 1.7 4.4
APS in gelatin geld 0.2 0.4
0.417e (49.2%) 0.284e (28.9%)
0.022 (2.6%) 0.035 (3.6%)
7.2e/1.9f 5.1e/4.4f
a
0.409f (48.2%) 0.663f (67.5%)
1
Oxidant-to-aniline mole ratio was [13] 1.25. Mass of protonated PANI produced per 1 g of aniline hydrochloride. The fractions produced in/at the gelatin and in the solution are given in the parentheses. c Conductivity of PANI after the hydrolytic removal of gelatin. d Gelatin solution did not form a gel after cooling in 0.6 M APS. e Polyaniline fraction grown at the surface of the gel. f Polyaniline fraction produced inside the gel. b
N.V. Blinova et al. / European Polymer Journal 45 (2009) 668–673 Table 2 The properties of a reference PANI prepared in the absence of gelatin. Aniline concentration,a mol L 0.2 0.4 0.6 a b
1
Yield,b g g 0.908 1.013 1.163
1
Conductivity, S cm
1
8.8 18.1 11.7
Oxidant-to-aniline mole ratio was [13] 1.25. Mass of protonated PANI produced per 1 g of aniline hydrochloride.
of units S cm 1 (Table 1), only slightly lower than the conductivity of the reference PANI prepared in the absence of gelatin (Table 2). The morphology of the PANI is represented by fused granules in all the samples (Fig. 4). More uniform PANI granules having the size of 200–400 nm are produced in the gelatin gel. The morphology of the PANI prepared in the absence of gelatin is similar (Fig. 5) as well as the morphology of PANI produced with the membrane-separated reactants and reported elsewhere [12]. Preparations of PANI-modified hydrogels have been reported in the literature in several cases [19–23]. Hydrogels swollen with aniline solution were immersed in an oxidant solution, as in the present case. It has to be noted that the distribution of PANI in such composite hydrogels will not be uniform. PANI-rich regions are likely to be close
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to the gel surface and PANI-poor domains in its interior (Fig. 3a). The analogy between the oxidation of aniline at a solution–gel interface and at PANI membrane extends also to other properties of PANI, such as conductivity. The most important, however, are the common features of the molecular structure, as observed by FTIR spectroscopy (Fig. 6). The FTIR spectra of PANI prepared in the gelatin gel in the present case and the spectrum of PANI prepared on a membrane, as reported in the literature [12], are practically identical (Fig. 6). They both include the absorption bands at 1639, 1444 and 692 cm 1 observed in the spectra of the first oligomeric products of oxidation of aniline [24]. They correspond to the presence of ortho-linked aniline constitutional units and phenazine-like or cross-linked segments in the product of oxidation. The bands located at 3435 and 3226 cm 1 are most probably associated with hydrogen bonding between the chains. The peaks described above are missing in the PANI prepared in the standard way (Fig. 6). The relative sharpness of the peaks in the spectra of PANI prepared in a gelatin gel, in comparison with the spectrum of PANI prepared in the standard way, indicates a lower molar mass of the product. The FTIR spectra of the majority of the PANI product in the gelatin gel and outside, in the surrounding APS solution, differ marginally.
Fig. 4. Micrographs of PANI prepared with aniline hydrochloride in the gelatin phase and APS in the solution (left, Fig. 3a), and with APS in the gelatin phase and aniline hydrochloride in the solution (right, Fig. 3b). The products from the solution phases are at the top, those from the gelatin gel in the bottom. 50 mL of 1 M APS was poured over 50 mL of 0.8 M aniline in 5 wt.% gelatin at 4 °C and vice versa.
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PANI in gelatin gel
b
PANI on gelatin gel
2924 2856
3435
Absorbance
a
PANI in solution
c
4000
3000
2000
Wavenumbers, cm Fig. 5. Polyaniline prepared by polymerization started in solution without gelatin. 50 mL of 0.8 M aniline was mixed with 50 mL of 1 M APS at 4 °C.
Absorbance
Fig. 7. FTIR spectra of PANI prepared by the oxidation of 0.8 M aniline solution in contact with 1 M APS solution in a gelatin gel: (a) in the gel, (b) on the gel, (c) outside the gel in the surrounding solution.
1444
hydrolysis of the PANI–gelatin composites, i.e. the destruction and removal of gelatin was complete. The conductivity of reference PANI sample kept in 1 M HCl at 80 °C for 1 h was reduced from 8.8 to 2.1 S cm 1. Such change can be regarded as small.
1639
3226
PANI in gelatin
a
1000 −1
PANI on membrane
3.3. Polymerization at a solution–gel interface: oxidant in the gel
PANI reference
One would expect that, if the loci of the reactants were reversed, APS now being in the gelatin gel and aniline hydrochloride in the outside solution, the PANI should be produced in the latter solution. It may, therefore, be surprising to observe that nearly all the PANI, over 92 wt.%, is again a part of the gelatin gel (Table 1). Closer inspection, however, shows a marked difference. Polyaniline indeed grows in the aniline phase, as expected by the scheme in Fig. 1, but it is firmly attached to the gel surface (Fig. 3b). By cutting the gel it is easily visible that PANI also penetrates into the gelatin gel in this case, the fractions of PANI produced inside the gel and on its surface being comparable (Table 1). This observation is logical: the growth of PANI in a connective structure is the prerequisite for the efficient transport of electrons between the oxidant in the gel and the aniline monomer close to the PANI produced in the solution (Fig. 3b). That is why PANI grows from the surface of the gel into the surrounding aniline solution. On the other hand, some aniline molecules seem to diffuse into the interior of the gelatin gel and react there with APS in the ordinary way. This is collaborated by a comparison of the FTIR spectra of the product inside the gel (Fig. 7, spectrum a) with that of PANI prepared in the standard way (Fig. 6, spectrum c). Polyaniline grown on the surface of a gel has a spectrum (Fig. 7, spectrum b) typical of the polymerization on a PANI membrane (Fig. 6, spectrum b), including the protonation features. The PANI-assisted and ordinary mechanisms are obviously combined in this way. There can be two reasons for the diffusion of aniline into the gelatin gel: (1) a slower PANI-assisted polymerization and thus a longer time allowed for the diffusion of the
b c d
4000
3000
2000
Wavenumbers, cm
1000 −1
Fig. 6. FTIR spectra of polyaniline prepared (a) in a gelatin gel (aniline hydrochloride inside the gel, APS outside the gel), (b) in the aniline compartment when the solutions of aniline and APS were separated by a cellulose membrane (taken from Ref. [12]), and (c) in the standard way when aniline solution and oxidant solution are simply mixed. (Samples a and b were hydrolyzed in order to remove gelatin. The spectra c and d of as-prepared standard PANI and PANI run through the hydrolysis procedure, respectively, illustrate that the hydrolysis of the gelatin does not affect the molecular structure of the PANI.)
3.2. Hydrolysis A short comment should be made on the removal of gelatin by hydrolysis of the PANI–gelatin products in 1 M HCl at 80 °C, and its potential impact on the structure and properties of the PANI. It has been reported [15,16] that the structure of the PANI is not considerably damaged, even after 1000 h exposure to 1 M HCl at 105 °C. In comparison with the last experiments, the conditions of the hydrolysis (1 h at 80 °C) can be regarded as being mild. This is independently confirmed by comparison of the FTIR spectra of the reference PANI (Fig. 6, spectrum c) and the spectrum of the same polymer exposed to the hydrolytic procedure (spectrum d). It should also be mentioned that no absorption bands of gelatin have been found after the
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residual monomer; (2) the morphology of PANI produced at the interface (Fig. 4), which may be more permeable to the diffusing species. 4. Concluding remark The PANI-assisted mechanism of aniline oxidation, in which the already-produced conducting PANI mediates the transfer of electrons from the newly oxidized aniline molecules to the oxidant, APS, will be operative in all cases where aniline and oxidant molecules are separated in space by a body of PANI. In addition to the experiments discussed here, it may become operative in interfacial polymerizations [25–27] or in emulsion polymerization [28], where aniline and oxidants are preferentially located in separate phases afforded by organic liquid and water, respectively, and PANI is produced at their interface. 5. Conclusions If a gelatin gel swollen with aniline hydrochloride faces a solution of an oxidant, ammonium peroxydisulfate, PANI will grow from the gel interface to the gel interior. The fraction of PANI produced inside the gel increases from 93 to 99 wt.%, when the concentrations of reactants increase three times. This is explained by a PANI-assisted redox reaction, in which electrons from the aniline are transferred through conducting PANI produced at the gel–solution interface to an oxidant. Aniline thus reacts with an oxidant without the need of both types of molecules to be in direct contact. The ability of PANI to transport protons is essential for this mechanism and for maintaining local electroneutrality. The PANI produced in this way has a lower yield but similar conductivity compared with the PANI produced in the common way, by mixing the solutions of reactants. FTIR spectra, however, reveal some differences in the molecular structure of the two types of products. When the loci of reactants are reversed, and gelatin gel containing an oxidant is left to react with aniline hydrochloride in the surrounding solution, the situation becomes more complex. PANI grows at the surface of the gel by a PANI-assisted redox mechanism, but additional PANI is produced inside the gel by classical polymerization, afforded by the diffusion of aniline monomer into oxidant-containing gel. Acknowledgments The authors thank the Grant Agency of the Czech Republic (203/08/0686) and the Grant Agency of the Academy of Sciences of the Czech Republic (IAA 400500905) for financial support. Thanks are also due to Dr. J. Prokeš from the Charles University Prague, Czech Republic, for the conductivity measurements. References [1] Dispenza C, Leone M, Presti CL, Librizzi F, Spadaro G, Vetri V. Optical properties of biocompatible polyaniline nano-composites. J NonCrystalline Solids 2006;352:3835–40. [2] Li M, Guo Y, Wei Y, MacDiarmid AG, Lelkes PI. Electrospinning polyaniline-contained gelatin nanofibres for tissue engineering applications. Biomaterials 2006;27:2705–15.
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Contents lists available at ScienceDirect
European Polymer Journal journal homepage: www.elsevier.com/locate/europolj
The polymerization of aniline at a solution–gelatin gel interface Natalia V. Blinova *, Miroslava Trchová, Jaroslav Stejskal Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, Department of Supramolecular Polymer Systems, Heyrovsky Sq. 2, 162 06 Prague 6, Czech Republic
a r t i c l e
i n f o
Article history: Received 17 October 2008 Received in revised form 9 December 2008 Accepted 22 December 2008 Available online 30 December 2008
Keywords: Conducting polymer Polyaniline Gelatin Interfacial polymerization Trans-membrane polymerization
a b s t r a c t Gelatin gel swollen with the solution of aniline hydrochloride was exposed to a solution of ammonium peroxydisulfate. The reactants met at the gel interface, and the redox reaction between them produced a polyaniline (PANI) interlayer, a PANI membrane, at first. The electrons abstracted from the aniline molecules in the gel during the oxidation are transferred through a conducting PANI membrane to oxidant molecules in the external solution. The reaction between aniline and peroxydisulfate thus takes place without the need for the reactant molecules to physically meet. PANI, therefore, grows from the interface into the gelatin gel. When the loci of reactants are reversed, i.e. the oxidant is inside the gelatin gel and aniline hydrochloride in the surrounding solution, PANI grows from the gel interface into the aniline solution but some PANI is produced inside the gelatin gel, too. Composite PANI–gelatin gels were separated and gelatin was removed from them by acid hydrolysis. The resulting PANI had a granular morphology and a conductivity of the order of units S cm 1, slightly lower compared with PANI prepared in a common way by mixing the solutions of reactants. The differences in the details of molecular structure are discussed on the basis of FTIR spectra. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Composite systems comprising a conducting polymer, such as polyaniline (PANI), distributed within a polymer gel represent novel materials that have recently been prepared and investigated. One strategy consisted in the preparation of polyaniline colloids stabilized with poly (vinyl alcohol) followed by c-irradiation, which produced the gel [1]. Other approaches were based on the electrospinning of PANI–gelatin blends [2], entrapment of sulfonated PANI or electropolymerization of aniline in a polyacrylamide gel [3]. Microgels have also served as templates for the polymerization of aniline [4]. Such composite systems are likely to find uses in the design of biocompatible materials [1], tissue engineering [2,5,6], electrically-controlled drug release [7], and related biomedical applications. The preparation of composite gels containing a * Corresponding author. Tel.: +420 296 809 234; fax: +420 296 809 410. E-mail address: [email protected] (N.V. Blinova). 0014-3057/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2008.12.034
conducting polymer is not trivial and the control of the spatial distribution of conducting component within the gel is of challenge. A polymerization technique, in which gelatin gel swollen with aniline solutions was exposed to a solution of an oxidant, ammonium peroxydisulfate, is discussed in the present communication. It has recently been reported that the redox processes between the oxidant and reductant separated by a conducting polymer membrane can proceed by the transfer of electrons from reductant to oxidant through the membrane [8]. Such a process has been illustrated by the reduction of iron (III) ions with ascorbic acid separated by PANI membrane [8,9] and applied to the design of a potentiometric sensor [10]. The electroneutrality of the system is maintained by the simultaneous transfer of protons. In contrast to metals, which conduct electrons but not protons and anions, PANI in the presence of water is both an electron and a proton conductor [11] Anion transport may also play an important role [10,11]. The molecules of oxidant and reductant thus may react without the need
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to be in direct contact by passing electrons through conducting PANI. This concept has recently been extended to the oxidation of aniline with ammonium peroxydisulfate [12], i.e. to the reaction, which is currently used for the preparation of PANI [13]. When solutions of aniline hydrochloride and ammonium peroxydisulfate were separated by a PANI membrane, produced in situ on semi-permeable cellulose template membrane or in advance, PANI was produced entirely on the aniline side of the membrane (Fig. 1). It was confirmed that, also in this case, the reductant (aniline) provides and the oxidant (peroxydisulfate) accepts electrons transferred by the conducting PANI. Polyaniline was produced by this trans-membrane reaction in a high yield, its conductivity being about the same as the conductivity of PANI prepared in the classical way, by mixing the reactant solutions [13]. In the present study, aniline hydrochloride was dissolved in the aqueous solution of gelatin, which was left to produce a gel by lowering the temperature. The solution of an oxidant was then poured over the gelatin gel. The reactants thus were separated, and met at the gel interface with an aqueous solution and produced a PANI membrane there. The subsequent polymerization of aniline is investigated in the present report. The goal of this communication is to provide understanding of the processes involved in the preparation of composite gels comprising a conducting polymer as a component. 2. Experimental 2.1. Preparation of PANI Gelatin (2.5 g; Fluka, Switzerland) was dissolved in 50 mL of an aqueous solution containing various amounts (0.4, 0.8 and 1.2 M) of aniline hydrochloride (Fluka, Switzerland) at 40 °C. The concentration of gelatin in the solution was 5 wt.%. The solutions in beakers were placed in a refrigerator at 4 °C and left overnight to produce a gel. A 50 mL solution of ammonium peroxydisulfate (APS; LachNer, Czech Republic) was poured over the gelatin gel and
PANI membrane Aniline
the system was left in a refrigerator for 7 days. The oxidant-to-aniline molar ratio was 1.25 in all cases. Similar experiments with ammonium peroxydisulfate located inside the gelatin gel and aniline hydrochloride in the external solution have also been carried out. Reference polymerizations, in which the solutions of aniline hydrochloride and APS were simply mixed in the absence of gelatin, have been made at 4 °C. After the oxidative polymerization of aniline, the supernatant liquid was separated from the gel, and any solids were separated by filtration, rinsed with 0.2 M HCl, and acetone, and dried. The gel was placed into excess of 1 M HCl solution and heated to 80 °C for 1 h. Under these conditions, the gelatin was completely hydrolyzed to aminoacids and removed from the PANI [14]. Polyaniline is not damaged by exposure to a strongly acidic medium at elevated temperature [15,16]. The reference PANI samples were prepared in the absence of gelatin and treated in the same way. The solids after the hydrolysis were collected on a filter, rinsed with 1 M HCl, and with acetone, and dried in air. 2.2. Characterization Scanning electron micrographs were taken with a JEOL 6400 microscope (Japan). Infrared spectra were recorded with a fully computerized Thermo Nicolet NEXUS 870 FTIR Spectrometer with a DTGS TEC detector. The spectra of the samples dispersed in potassium bromide were recorded in the transmission mode and corrected for the presence of carbon dioxide and moisture in the optical path. The conductivity was measured with a four-point van der Pauw method on pellets compressed at 700 MPa with a manual hydraulic press using as a SMU Keithley 237 current source and a Multimeter Keithley 2010 with a 2000 SCAN 10channel scanner card. 3. Results and discussion The oxidation of aniline with APS in acidic aqueous medium is a redox reaction yielding PANI [13] (Fig. 2). Sulfuric acid and ammonium sulfate are by-products, at equimolar proportions they produce ammonium hydrogen sulfate [12,17].
Peroxydisulfate – ne
ne
4n
+ ne
PANI
NH2.HA
+
5 n (NH4)2S 2O8
Hydrogen sulfate nH – nH
(nH )
(Anions) Fig. 1. Aniline molecules are oxidized to PANI (left) and peroxydisulfate molecules reduced to hydrogen sulfate (right) by the transfer of electrons through the conducting PANI membrane separating both reactants. Protons accompany electrons in order to maintain the electroneutrality of the system. The possible transport of anions has the same role (adapted from Ref. [12]).
NH A
NH A + 2 n HA
NH
NH n
+ 5 n H2SO4 + 5 n (NH4)2SO4
Fig. 2. Aniline salt is oxidized with ammonium peroxydisulfate to PANI. Sulfuric acid and ammonium sulfate (i.e. ammonium hydrogen sulfate when in equimolar proportion) are by-products. HA is an arbitrary acid, here hydrochloric acid.
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3.1. Polymerization at a solution–gel interface: aniline in the gel In the present paper, one of the reactants is incorporated in the hydrogel, which faces the solution of the second reactant (Fig. 3a). In this case, aniline and oxidant are separated at the beginning of the reaction and are in contact only at the gel interface where both reactants meet. The PANI interlayer is produced at first at this interface, and acts similarly as the PANI membrane in the transmembrane polymerization [8,9]. By analogy with the membrane experiment [12], the electrons abstracted from the aniline molecules are transferred through the conducting PANI to APS, which converts to ammonium sulfate (Fig. 1). Polyaniline is thus expected to be produced at the aniline-containing phase (Fig. 1), i.e. here within the gelatin gel. This is indeed the case (Fig. 3a), and PANI grows from the gel interface into its interior. Over 93 wt.% of PANI
a
(Table 1) from the total amount of PANI produced in the system, was formed in the gel, and the supernatant APS solution contained only traces of the polymer. These traces result from aniline hydrochloride that had diffused into the APS solution before the interfacial PANI layer was produced. The yield of reaction in the gel phase increased from 0.45 to 0.69 g of PANI per gram of aniline hydrochloride with increasing concentration of reactants (Table 1), and was lower than the yield in the reaction carried out in the absence of gelatin by mixing the solutions of aniline hydrochloride and APS (Table 2). Given the stoichiometry shown in Fig. 2 and 1 g of aniline hydrochloride should produce 0.84 g of PANI hydrochloride [13]. The higher yields reported in Table 2 are due to the incorporation of more bulky counter-ions, such as hydrogen sulfate anions [18]. The conductivity of the PANI prepared in the gelatin gel (after the hydrolytic removal of gelatin) is of the order
b Peroxydisulfate solution S2O82–
Aniline hydrochloride solution
2 SO42– PANI
PANI
S2O82– Aniline hydrochloride in gelatin gel
2 SO42–
Peroxydisulfate in gelatin gel
Fig. 3. Aniline molecules are oxidized to PANI at the polymerization site (circle) and abstracted electrons are transferred (arrows) through the conducting PANI to the gel–solution interface where they are accepted by the oxidant molecules. (a) When aniline is located in the gelatin, PANI grows into the gel phase. (b) When APS is in the gel phase, PANI grows on the surface of the gel and partly also penetrates inside the gelatin gel.
Table 1 The yield and conductivity of PANI produced by the oxidation of aniline with ammonium peroxydisulfate, one reactant being in a gelatin gel, the other in the external aqueous solution in the contact with the gel. Aniline concentration,a mol L
1
Yield,b g g
1
Conductivity,c S cm
Gel
Solution
Aniline hydrochloride in gelatin gel 0.2 0.4 0.6
0.448 (92.9%) 0.610 (98.0%) 0.693 (99.4%)
0.034 (7.1%) 0.012 (2.0%) 0.004 (0.6%)
0.23 1.7 4.4
APS in gelatin geld 0.2 0.4
0.417e (49.2%) 0.284e (28.9%)
0.022 (2.6%) 0.035 (3.6%)
7.2e/1.9f 5.1e/4.4f
a
0.409f (48.2%) 0.663f (67.5%)
1
Oxidant-to-aniline mole ratio was [13] 1.25. Mass of protonated PANI produced per 1 g of aniline hydrochloride. The fractions produced in/at the gelatin and in the solution are given in the parentheses. c Conductivity of PANI after the hydrolytic removal of gelatin. d Gelatin solution did not form a gel after cooling in 0.6 M APS. e Polyaniline fraction grown at the surface of the gel. f Polyaniline fraction produced inside the gel. b
N.V. Blinova et al. / European Polymer Journal 45 (2009) 668–673 Table 2 The properties of a reference PANI prepared in the absence of gelatin. Aniline concentration,a mol L 0.2 0.4 0.6 a b
1
Yield,b g g 0.908 1.013 1.163
1
Conductivity, S cm
1
8.8 18.1 11.7
Oxidant-to-aniline mole ratio was [13] 1.25. Mass of protonated PANI produced per 1 g of aniline hydrochloride.
of units S cm 1 (Table 1), only slightly lower than the conductivity of the reference PANI prepared in the absence of gelatin (Table 2). The morphology of the PANI is represented by fused granules in all the samples (Fig. 4). More uniform PANI granules having the size of 200–400 nm are produced in the gelatin gel. The morphology of the PANI prepared in the absence of gelatin is similar (Fig. 5) as well as the morphology of PANI produced with the membrane-separated reactants and reported elsewhere [12]. Preparations of PANI-modified hydrogels have been reported in the literature in several cases [19–23]. Hydrogels swollen with aniline solution were immersed in an oxidant solution, as in the present case. It has to be noted that the distribution of PANI in such composite hydrogels will not be uniform. PANI-rich regions are likely to be close
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to the gel surface and PANI-poor domains in its interior (Fig. 3a). The analogy between the oxidation of aniline at a solution–gel interface and at PANI membrane extends also to other properties of PANI, such as conductivity. The most important, however, are the common features of the molecular structure, as observed by FTIR spectroscopy (Fig. 6). The FTIR spectra of PANI prepared in the gelatin gel in the present case and the spectrum of PANI prepared on a membrane, as reported in the literature [12], are practically identical (Fig. 6). They both include the absorption bands at 1639, 1444 and 692 cm 1 observed in the spectra of the first oligomeric products of oxidation of aniline [24]. They correspond to the presence of ortho-linked aniline constitutional units and phenazine-like or cross-linked segments in the product of oxidation. The bands located at 3435 and 3226 cm 1 are most probably associated with hydrogen bonding between the chains. The peaks described above are missing in the PANI prepared in the standard way (Fig. 6). The relative sharpness of the peaks in the spectra of PANI prepared in a gelatin gel, in comparison with the spectrum of PANI prepared in the standard way, indicates a lower molar mass of the product. The FTIR spectra of the majority of the PANI product in the gelatin gel and outside, in the surrounding APS solution, differ marginally.
Fig. 4. Micrographs of PANI prepared with aniline hydrochloride in the gelatin phase and APS in the solution (left, Fig. 3a), and with APS in the gelatin phase and aniline hydrochloride in the solution (right, Fig. 3b). The products from the solution phases are at the top, those from the gelatin gel in the bottom. 50 mL of 1 M APS was poured over 50 mL of 0.8 M aniline in 5 wt.% gelatin at 4 °C and vice versa.
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PANI in gelatin gel
b
PANI on gelatin gel
2924 2856
3435
Absorbance
a
PANI in solution
c
4000
3000
2000
Wavenumbers, cm Fig. 5. Polyaniline prepared by polymerization started in solution without gelatin. 50 mL of 0.8 M aniline was mixed with 50 mL of 1 M APS at 4 °C.
Absorbance
Fig. 7. FTIR spectra of PANI prepared by the oxidation of 0.8 M aniline solution in contact with 1 M APS solution in a gelatin gel: (a) in the gel, (b) on the gel, (c) outside the gel in the surrounding solution.
1444
hydrolysis of the PANI–gelatin composites, i.e. the destruction and removal of gelatin was complete. The conductivity of reference PANI sample kept in 1 M HCl at 80 °C for 1 h was reduced from 8.8 to 2.1 S cm 1. Such change can be regarded as small.
1639
3226
PANI in gelatin
a
1000 −1
PANI on membrane
3.3. Polymerization at a solution–gel interface: oxidant in the gel
PANI reference
One would expect that, if the loci of the reactants were reversed, APS now being in the gelatin gel and aniline hydrochloride in the outside solution, the PANI should be produced in the latter solution. It may, therefore, be surprising to observe that nearly all the PANI, over 92 wt.%, is again a part of the gelatin gel (Table 1). Closer inspection, however, shows a marked difference. Polyaniline indeed grows in the aniline phase, as expected by the scheme in Fig. 1, but it is firmly attached to the gel surface (Fig. 3b). By cutting the gel it is easily visible that PANI also penetrates into the gelatin gel in this case, the fractions of PANI produced inside the gel and on its surface being comparable (Table 1). This observation is logical: the growth of PANI in a connective structure is the prerequisite for the efficient transport of electrons between the oxidant in the gel and the aniline monomer close to the PANI produced in the solution (Fig. 3b). That is why PANI grows from the surface of the gel into the surrounding aniline solution. On the other hand, some aniline molecules seem to diffuse into the interior of the gelatin gel and react there with APS in the ordinary way. This is collaborated by a comparison of the FTIR spectra of the product inside the gel (Fig. 7, spectrum a) with that of PANI prepared in the standard way (Fig. 6, spectrum c). Polyaniline grown on the surface of a gel has a spectrum (Fig. 7, spectrum b) typical of the polymerization on a PANI membrane (Fig. 6, spectrum b), including the protonation features. The PANI-assisted and ordinary mechanisms are obviously combined in this way. There can be two reasons for the diffusion of aniline into the gelatin gel: (1) a slower PANI-assisted polymerization and thus a longer time allowed for the diffusion of the
b c d
4000
3000
2000
Wavenumbers, cm
1000 −1
Fig. 6. FTIR spectra of polyaniline prepared (a) in a gelatin gel (aniline hydrochloride inside the gel, APS outside the gel), (b) in the aniline compartment when the solutions of aniline and APS were separated by a cellulose membrane (taken from Ref. [12]), and (c) in the standard way when aniline solution and oxidant solution are simply mixed. (Samples a and b were hydrolyzed in order to remove gelatin. The spectra c and d of as-prepared standard PANI and PANI run through the hydrolysis procedure, respectively, illustrate that the hydrolysis of the gelatin does not affect the molecular structure of the PANI.)
3.2. Hydrolysis A short comment should be made on the removal of gelatin by hydrolysis of the PANI–gelatin products in 1 M HCl at 80 °C, and its potential impact on the structure and properties of the PANI. It has been reported [15,16] that the structure of the PANI is not considerably damaged, even after 1000 h exposure to 1 M HCl at 105 °C. In comparison with the last experiments, the conditions of the hydrolysis (1 h at 80 °C) can be regarded as being mild. This is independently confirmed by comparison of the FTIR spectra of the reference PANI (Fig. 6, spectrum c) and the spectrum of the same polymer exposed to the hydrolytic procedure (spectrum d). It should also be mentioned that no absorption bands of gelatin have been found after the
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residual monomer; (2) the morphology of PANI produced at the interface (Fig. 4), which may be more permeable to the diffusing species. 4. Concluding remark The PANI-assisted mechanism of aniline oxidation, in which the already-produced conducting PANI mediates the transfer of electrons from the newly oxidized aniline molecules to the oxidant, APS, will be operative in all cases where aniline and oxidant molecules are separated in space by a body of PANI. In addition to the experiments discussed here, it may become operative in interfacial polymerizations [25–27] or in emulsion polymerization [28], where aniline and oxidants are preferentially located in separate phases afforded by organic liquid and water, respectively, and PANI is produced at their interface. 5. Conclusions If a gelatin gel swollen with aniline hydrochloride faces a solution of an oxidant, ammonium peroxydisulfate, PANI will grow from the gel interface to the gel interior. The fraction of PANI produced inside the gel increases from 93 to 99 wt.%, when the concentrations of reactants increase three times. This is explained by a PANI-assisted redox reaction, in which electrons from the aniline are transferred through conducting PANI produced at the gel–solution interface to an oxidant. Aniline thus reacts with an oxidant without the need of both types of molecules to be in direct contact. The ability of PANI to transport protons is essential for this mechanism and for maintaining local electroneutrality. The PANI produced in this way has a lower yield but similar conductivity compared with the PANI produced in the common way, by mixing the solutions of reactants. FTIR spectra, however, reveal some differences in the molecular structure of the two types of products. When the loci of reactants are reversed, and gelatin gel containing an oxidant is left to react with aniline hydrochloride in the surrounding solution, the situation becomes more complex. PANI grows at the surface of the gel by a PANI-assisted redox mechanism, but additional PANI is produced inside the gel by classical polymerization, afforded by the diffusion of aniline monomer into oxidant-containing gel. Acknowledgments The authors thank the Grant Agency of the Czech Republic (203/08/0686) and the Grant Agency of the Academy of Sciences of the Czech Republic (IAA 400500905) for financial support. Thanks are also due to Dr. J. Prokeš from the Charles University Prague, Czech Republic, for the conductivity measurements. References [1] Dispenza C, Leone M, Presti CL, Librizzi F, Spadaro G, Vetri V. Optical properties of biocompatible polyaniline nano-composites. J NonCrystalline Solids 2006;352:3835–40. [2] Li M, Guo Y, Wei Y, MacDiarmid AG, Lelkes PI. Electrospinning polyaniline-contained gelatin nanofibres for tissue engineering applications. Biomaterials 2006;27:2705–15.
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