A comparison of laccase-catalyzed polymerization of aniline in the presence of two different “soft” templates - micelle forming surfactant sodium dodecylbenzenesulfonate (SDBS) and sulfonated natural polyelectrolyte calcium lignosulfonate (LGS) - was investigated to check the influences of different templates on the kinetics of the polymerization and on the characteristics of the complexes obtained. Under respective optimal conditions, pH 4.5 for SDBS and pH 3.5 for LGS, the reaction with LGS was much slower than that in SDBS emulsion. The conditions for laccase-catalyzed synthesis of emeraldine salt of polyaniline (PANI-ES) in both cases were optimized using UV-vis spectroscopy. It was found that PANI-ES obtained with LGS template had a higher degree of oxidation with a higher amount of benzene rings than PANI-ES obtained in SDBS emulsion. Since the protonation of PANI occurred on its imine sites, the doping effect of LGS-doped PANI-ES, confirmed by XPS, was thus better than that of SDBS-doped one.
Keywords: laccase-catalyzed polymerization, emeraldine salt of polyaniline, soft templates, sodium dodecylbenzenesulfonate, calcium lignosulfonate
Polyaniline (PANI) is one of the most popular conducting polymers thanks to low price of the monomer, simple preparation, good stabilities and variable optoelectronic properties.1-5 The general formula of PANI is [(-B-NH-B-NH-)x(-B-N=Q=N-)1-x]n, in which B and Q refer to the benzenoid and quinoid rings respectively. PANI varies in oxidation from fully oxidized state (pernigraniline, x=0), through half oxidized state (emeraldine, x=0.5) to fully reduced state (leucoemeraldine, x=1). Emeraldine salt (PANI-ES) is the only one that can be conducting, because PANI gets its conductivity through the protonation of imine nitrogens (=N-), which is also referred as “doping”.1 The structural changes of PANI according to different redox conditions and protonated/deprotonated states are shown in Scheme 1. Since polymer chains of PANI are very long, detailed structural analyses of PANI, such as mass spectrometry and nuclear magnetic resonance spectrometry, are very difficult.
In case of traditional chemical and electrochemical synthesis of PANI in strongly acidic media, intermolecular hydrogen bonds in the polymer chains result in the poor solubility of PANI and limit its practical application.6,7 In order to improve the solubility and expand practical application of the polymer, “hard” or “soft” templates are usually used in aniline polymerization. “Hard” templates refer to silicon, various carbon and plastic materials, as well as metals and metal oxides. “Soft” templates can be negatively charged assemblies of amphiphiles, like micelles8-13 and vesicles,14-16 or negatively charged polyelectrolytes, including synthetic polymers, like sulfonated polystyrene (SPS),17-22 poly(amic acid)23 and poly(2-acrylamido-2-methyl-1-propanosulfonic acid) (PAMPS),24,25 and natural polymer derivatives, like lignosulfonate (LGS).26-29 Compared to pure PANI, complexes composed of these templates and PANI are more promising for practical application. 1,2 The mechanism of anionic template influence on the polymerization of aniline was quite clear now.8-10,14-17 These templates have three main functions: (1) they bind and align the aniline cation, thus resulting in a preferential head-to-tail coupling and therefore leading to the formation of mainly a linear structure of PANI; (2) the template molecules are attached to the backbone of PANI and play the role of a dopant which leads to an increase in the conductivity of the polymer;10 (3) the template help PANI to disperse in water and the suspension obtained are very stable for many months without precipitation and notable color changes. Such suspension can even be used as ink with an inkjet printer.16
Compared with traditional chemical methods, enzymatic catalysis of PANI, usually using “soft” templates, provides an alternative “green” method toward the formation of soluble and processable conducting PANI because of the milder reaction conditions and the simplified purification process of final products.8-12,14-22 Oxidative polymerization of aniline has been conducted for a century and enzymatic syntheses of conducting PANI have been investigated since the 1990s.30,31 By enzymatic syntheses stable suspension of complex, composed of PANI and templates, was obtained. The complex was so called as “water-soluble polyaniline”. Actually, PANI does not real dissolve in water, but disperse in water with the help of soft templates.8-12,14-22 The soft templates also increase the solubility of PANI in some organic solvents.14,21,32
From our previous work, we realized that soft templates are very important for laccase-catalyzed polymerization of aniline and the complexes obtained differ in many properties, based on the templates used.11,32,33 The aim of our present work was to check the influences of different “soft” templates on the kinetics of the polymerization and the characteristics of the complexes obtained. The templates used were typical negatively charged assemblies of amphiphiles (emulsion of sodium dodecylbenzenesulfonate, SDBS) and sulfonated natural polyelectrolyte (calcium lignosulfonate, LGS), respectively. The reaction was monitored and optimized using UV-Vis spectroscopy. FTTR, Raman spectra and XPS analyses of the complexes demonstrated that LGS-doped PANI-ES had a higher degree of oxidation with a higher amount of benzene rings compared with SDBS-doped PANI-ES. Since the results of cyclic voltammetry and thermal analyses proved that the electroactivity and thermal stability of LGS-doped PANI-ES was better than that of SDBS-doped PANI-ES, we believe that LGS could be a promising dopant in enzymatic syntheses of conducting PANI-ES.
2018; 42(2): 175-184
Published online Jan 25, 2018
Key Laboratory of Science and Technology of Eco-Textile, Ministry of Education, Jiangnan University, Wuxi 214122, Jiangsu, China