Solubility and Dispersion Characteristics of Polyaniline

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ABSTRACT We have demonstrated that the solubility characteristics of undoped polyaniline can be understood from the point of view of standard solubility parameters which measure the propensity for the polymer and the solvent to engage in dispersive, polar, and hydrogen bonding interactions. By empirical measures and group additive calculations, emeraldine base has been shown to be characterized by solvent interaction parameters: 8d = 17.4 MPal/2, 8P = 8.1 MPal/2, and 8h =

10.7 MPa1/, which together are equivalent to a total solubility parameter: 6 = 22.2 MPa/2. In the case of doped polyanilines, the solubility parameters can influence or even dominate the interactions of the doped polymer. Without the modifying influence of the dopant anion, the doped polyaniline compositions are more strongly polar and hydrogen bonding than the undoped polymer. INTRODUCTION Processing has long been recognized as a key to the successful application of conducting polymers. Historically, a great deal of work has been directed toward developing methods for processing conjugated polymers by solution techniques. Recent efforts with polyanilines have been directed toward identifying solvents and solvent/dopant combinations which allow the formation of solutions of either the undoped or the doped polymer [1,2]. Other efforts have been directed toward forming dispersions of polyaniline in liquid media [3] or in thermoplastic media [4]. The conductive form of polyaniline is an acid/base complex, which depending on the precursor can be produced by either an oxidative or a protonic doping process. The polymer backbone which is a weak organic base, pKa = 5.4 [5], is generally complexed with a strong protonic acid, which may be either a mineral acid such as hydrochloric acid or an organic acid such as p-toluene sulfonic acid. The base form of polyaniline, poly(p-phenylene amine imine), is generally called emeraldine base (EB) and the conductive complex is correspondingly called emeraldine salt (ES). The formation of the complex results in the protonation of the polymer backbone (EB) at the imine nitrogen sites, thereby, converting these sites to amine linkages. The complexing or doping process by protonic acid dopants, therefore, converts the polymer backbone to poly(p-phenylene amine), and also results in the insertion of anions between polymer chains, which compensate the positive charge produced on the polymer backbone. The positive charge which results from the protonation is delocalized along the backbone and is generally thought to exist as a polaronic defect. The base form of polyaniline (EB) consists of diamine and diimine units as illustrated 157 Mat. Res. Soc. Symp. Proc. Vol. 328. ©1994 Materials Research Society

{ [-(C6H4)-NH-(C6H4)-NH-] I-x[-(C6H4)-N =(C6H4) =N-]x~n


Previous studies have indicated that this polymer is essentially a regular copolymer of such diamine and diimine units [6]. Although partial reduction in the presence of protons will increase the amount of amine units and partial oxidation of the base form by e