JOURNAL OF APPLIED ELECTROCHEMISTRY 22 (1992) 563-570 Synthesis and properties of polyaniline obtained using sulphamic acid S. K. DHAWAN, D. C. TRIVEDI Central Electrochemical Research Institute, Karaikudi 623 006, India Recieved 16 May 1991; revised 29 August 1991 The results of investigations of the chemical and electrochemical polymerization of aniline in sulphamic acid medium and its characterization by electrochemical and spectroscopic techniques are presented. The investigations reveal that sulphamic acid medium affects, not only the polymerization process, but also its electrochemical characteristics. 1. Introduction Even though the understanding of conduction in many organic polymers is limited, very many appli- cations using these materials have been envisaged [1-7]. The polyaniline family of polymers has been known for more than a century [8, 9]. The efforts of MacDiarmid and his coworkers has made polyaniline (PAn), one of the most thoroughly studied conducting polymers [10, 11, 19]. But there still remains scope to exploit the synthesis of polyaniline, which largely depends upon the synthetic ability to manipulate its molecular structure. Other reports tell that PAn can be made soluble in 80% acetic acid [11], concentrated U2SO 4 [12] and by the use of organic proton acids [13-15]. The solubiliz- ation in 80% acetic acid or concentrated H2 SO4 shows that it is possible to induce dipole polarization in the macromolecular system to yield a soluble polymer. If this is the case then it should be possible to achieve solubilization of PAn by synthesizing it in the presence of acids which can form strong H-bonding with the polymer [13]. Sulphamic acid (SMA) is one such acid whose pKa in water is 1.04 [16] and which exists in the following principle equilibrium: + NH2SO 3H~NH 3SO3 ~ " NH2SO 3 + H + (1) Equation 1 depicts only the principle equilibrium whereas various species like NH 2SO;, NH;, NH/SO~NH/; SO~- also exist along with the prin- ciple equilibrium. The existence of free radicals and radical ions make sulphamic acid one of the best catalysts in polymerization processes (for example, in the manufacture of isoprene, polystyrene, polyvinyl alcohol etc) [17]. In DMSO (dimethyl sulphoxide) solution, sulphamic acid has a pKa of 6.5. The lower acid strength has been attributed to partially weak solvation of the sulphamate ion [18]. This acid, which is milder than H2 SO4, may prove to be a better agent for the formation of H-bonds due to the -NH 2 group. With this in mind, the polymeriz- ation of aniline was carried out in a sulphamic acid 0021-891X/92 9 1992 Chapman & Hall medium by both persulphate oxidative and electro- chemical routes. Thus, by both these methods the PAn obtained is soluble in NMP (N-methyl pyrrolidinone), DMSO and DMF (dimethyl formamide). In this paper we present studies on the chemical and electro- chemical synthesis of polyaniline and its characteriz- ation by cyclic voltammetry, chronoapmerometry, u.v. visible, FTIR studies and 1H NMR of the soluble PAn synthesized in sulphamic acid medium. 2. Experimental details The polymerization of aniline in aqueous sulphamic acid solution (SMA) was carried out by two methods: (i) by chemical oxidative polymerization using ammonium persulphate; and (ii) by anodic polymerization at platinum, indium tin oxide (ITO) and stainless steel electrodes. 2.1. Chemical polymerization The chemical oxidative polymerization of a 0.1:1 molar ratio of aniline and sulphamic acid was carried out by adding 0.1 tool of ammonium persulphate sol- ution drop by drop. (For stoichiometry 0.2moI are required). Stirring of the reaction mixture was con- tinued for 2 h to ensure the completion of the reaction which was indicated by the stabilization of the tem- perature of the reaction mixture. The reaction mixture was then filtered and washed repeatedly with distilled water and finally with 1.0 M sulphamic acid. The poly- mer so obtained was dried under vacuum at 40 ~ for 24h. 2.2. Electrochemical preparation The electrochemical polymerization was carried out from 0.1 M monomer in 1.0 M sulphamic acid aqueous solution. Prior to polymerization, the solution was deoxygenated by passing argon gas for 30 rain. The polymerization was carried out at 0.72V against a saturated calomel electrode (SCE) on either a plati- num electrode, an ITO coated glass plate (resistivity: 20-50~cm) or stainless steel electrodes (charged passed, Q: 0.32 C cm-2). The polymer film growth was 563