Alternative oxidizers in polyaniline synthesis
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The article describes a modified method of polyaniline synthesis. The classical oxidizer—aqueous solution of hydrogen peroxide was replaced by different solid state hydrogen peroxide complexes, what helped keep constant temperature of the reaction mixture. The highest yield of the polymerization was obtained using urea–hydrogen peroxide.
KeywordsPolyaniline Hydrogen peroxide complex Oxidizing polymerization
Polyaniline (PANI) has been known since mid-XIX century under the name of aniline black, used for colorfast fabrics dyeing. Usually it has been synthesised in a one of three idealized oxidation states—leukoemeraldine, emeraldine, pernigraniline , containing different numbers of benzenoid and quinoid rings. A renaissance of interest in this polymer began in late 1980’s owing to works published by MacDiarmid et al. , who discovered that protonation of emeraldine base made this material electrically conductive. Since that time a lot of effort has been put in improving different polyaniline properties. For example the reported electrical conductivity, has reached values in excess of 1,000 S/cm . Regardless of some processing problems, this polymer has already found numerous applications in different fields. Among the others, it should be mentioned such technologies like electromagnetic shielding , conductive coatings , corrosion protection , artificial muscles , light-emitting diodes , field-effect transistors , photovoltaic cells  and sensors . Polyaniline also can be used as an efficient catalyst in chemical technology, or as a catalyst substrate .
The most frequently implemented methods of aniline polymerization are based on chemical , electrochemical  or less often, biochemical  processes. There exist lots of their variants like polymerization carried on in emulsion  in order to enhance contact surface between organic monomer and inorganic oxidant, polymerization in non-aqueous environment [17, 18] and in low temperature plasma . The polymerization method is determining factor for properties of the final product like chemical composition, oxidation state or average molecular weight. The synthesis able to provide bulk quantities of polyaniline is chemical synthesis. This is a simple process and hence intensively developed and improved. The chemical synthesis is performed in acidic environment and requires a strong oxidant to create aniline radical cations. Next, these cations undergo rearomatization and are coupled each to form growing pernigraniline chain. At the time when the oxidant is consumed, the remained excess of aniline reduces pernigraniline to emeraldine salt. The crucial role of the oxidant was emphasised by many researches. There are some oxidants more often reported in the bibliography than the others. This group constitute peroxydisulfate (VI) sodium or ammonium (MacDiarmid method) [20, 21], ammonium peroxydisulfate/ammonium cerium (IV) nitrate mixture , chromate (VI) potassium  and hydrogen peroxide .
Despite of the progress already achieved, from the industrial point of view the polyaniline is far from being a mature polymer, therefore there still exists need for a further amelioration of polyaniline properties and its synthesis.
The quality of chemically synthesised polyaniline as well as reproducibility of its properties depends a lot on the reaction temperature stability. Hence unpredictable temperature variations represent a significant drawback. A modified method of polyaniline synthesis, removing this effect is herein proposed. The method employs hydrogen peroxide in form of solid state complexes instead of traditional aqueous solution. These complexes are crystalline powders and can be introduced into the reaction environment without causing a sudden and hard to control increase of the reaction medium temperature, encountered if the hydrogen peroxide is applied as aqueous solution.
Starting chemicals used in this work were aniline, FeCl2, hydrochloric acid bought from POCH S.A., complex of urea–hydrogen peroxide (UHP) acquired from Sigma-Aldrich. Complexes of melamine–hydrogen peroxide (MHP) and of carbonate of sodium with hydrogen peroxide (SCHP) were kindly provided by Industrial Chemistry Research Institute in Warsaw, Poland. Characteristics of these complexes need not to be commented because they are widely used in the industry, owing to good oxidative, bleaching and disinfecting qualities .
Prior to the synthesis, aniline was distilled to remove products of aniline oxidation, which might have appeared during the storage. At the beginning 1 mL of aniline (11 mmol) was poured into 30 mL of the aqueous 1 M hydrochloric, and subsequently was added 2 mL of the aqueous solution including 0.02 mmol catalyst (FeCl2).
The mixture was intensively stirred and thermostated between −5 and 0 °C to minimize aniline side reactions. At the moment when temperature stabilized, 0.434 g (4.6 mmol) of the adduct UHP (oxidizing agent) was added in one shot but breaking of temperature stabilization was not observed. Afterwards the synthesis continued for either 6 or 24 h. The resulting deposit was filtered off from reaction mixture, washed with abundance of distilled water until pH of 6–7 was reached and converted to its base form by treatment in ammonia solution for about 6 h. At the end, the final product was extracted with methanol and tetrahydrofurane using Soxlet apparatus in order to remove oligomers residues.
The reaction was repeated replacing UHP oxidizer by melamine–hydrogen peroxide complex (MHP) in quantity 0.71 g (4.4 mmol) and carbonate of sodium hydrogen peroxide complex (SCHP) in quantity 0.7 g (5.0 mmol).
Oxidation polymerization reaction yields using three different complexes of hydrogen peroxide as an oxidizing agent and FeCl2 as a catalyst
Process duration (h)
In polyaniline UV–vis absorption spectrum it can be distinguished four characteristic bands at 305–330 nm, 420–450 nm, 620–650 nm and 760–800 nm, related to following transitions—π → π* in the benzenoid ring, polaron-π*, π → π* in the quinoid ring and π-polaron . Relative intensities of these bands depend on the sample electrical conductivity. As a fingerprint of electrical conductivity is considered, part of the spectrum continuously rising towards NIR which is often referenced as free carrier tail. In the case of poor electrical conductivity transitions 620–650 nm and 760–800 nm should not be present.
Electrical conductivity found for HCl-doped polyanilines
2.8 ± 0.5 × 10−2
6.5 ± 0.3 × 100
5.5 ± 0.8 × 101
The first exhaustive and systematic work  published at the beginning of 1990 systematizing and explaining crystalline structures of polyaniline, still remains main reference point for structural studies of different form of this polymer. In  two substantially different systems were proposed. They consisted of EB-I and EB-II structures (for emeraldine base form) and related ES-I and ES-II (emeraldine). An EB can be converted in respective ES through protonation. Deprotonation does vice versa. ES-I structure is naturally adopted by the product directly resulting from chemical polymerization. The EB-I constitutes an amorphous structure what expresses in diffraction pattern consisting of only one very large maximum centred around 2θ ≈ 19° (related to the CuKα line). The EB-II form is observed in samples processed from some organic solvents like THF, NMP, DMSO. Beside the very distinct peak at 2θ ≈ 19° EB-II diffraction pattern exhibits also two less intense peaks at 2θ ≈ 24° and 2θ ≈ 31° what is explain as a proof of partial crystallinity. Shape of diffraction patterns of ES-I and ES-II is considerably different. In order to interpret patterns not fitting in these systems, it has been developed modifications taking into account a range of factors like water content, doping agent nature, preparation conditions or presence of plastifier . Certain authors claimed also partial crystallinity in the case of EB-I , however, represented by a different diffraction pattern than characteristic of EB-II.
A modified method of aniline polymerization in the presence of solid state oxidizers (hydrogen peroxide complexes UHP, MHP, SCHP) has been developed. It is a practical, cost effective technology.
The polymerization yields were similar (or higher) than observed in classical polymerization using aqueous solution of hydrogen peroxide. The highest reaction output, approaching 60–65%, was obtained when urea–hydrogen peroxide was employed as the oxidizing agent and aqueous FeCl2 solution as a catalyst. Other catalysts (CoCl2, Ni(NO3)2 and CuCl2) gave maximum yields of 40, 20 and 10%, respectively (details will be published elsewhere). It is necessary to stress that 65% yield was reached just after 6 h of reaction. Despite the fact that the reaction was not optimized, this yield was close to reported by researchers from DuPont . Physical experiments evidenced that the synthesized product was of qualities comparable to other polyanilines described in available bibliography.
It can be concluded that urea–hydrogen peroxide (UHP) may be successfully applied as a convenient oxidant in the polymerization of aniline, although this complex have been known so far as a component of pharmacological, cosmetics or household chemistry products with regard to its antiseptic, bleaching as well as disinfecting properties.
Jacek Niziol and Maciek Sniechowski acknowledge active support from their home institution indicated in the affiliations.
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