Introduction:

Conductive polymers such as polyacetylene, polypyrrole, etc. have been the subjects of numerous investigations in the past two decades. ^[1-4]  These materials have properties that make them suitable for several applications in sensor, biosensor, rechargeable batteries, molecular electronic devices, electrochromic display devices, corrosion inhibitors, electromagnetic shielding materials, etc.  Among electrically conducting polymers, polyaniline and its derivatives have been the focus of attention due to their environmental stability, ease of synthesis, exciting electrochemical, optical and electrical properties. ^[3-5]  Polyaniline can be doped to highly conducting state by protonic acids or by electrochemical oxidation.  They show moderate conductivity upon doping with protonic acids and have excellent stability under ambient conditions.
The chemistry of polyaniline allows for several routes of synthesis and processing.  The synthesis of polyaniline may broadly be classified as electrochemical and chemical oxidative polymerizations ^[6].  The latter process is of importance to produce polyaniline on a large scale.  In recent years, emulsion, microemulsion, inverse emulsion and dispersion polymerization of aniline have come to the forefront ^[7-9].  High molecular weight and high reaction rates can simultaneously be attained by these methods.
Several oxidizing agents, for example K₂Cr₂O₇, FeCl₃, KMnO₄, KBrO₃, KClO₃ etc. have been used by Cao et al. ^[10] for the synthesis of polyaniline.  Tetrabutylammonium persulphate (TBAP) ^[11] and hydrogen peroxide ^[12-15] have also been employed recently for the polymerization of aniline.  The oxidizing agent traditionally employed in the polymerization of aniline has been ammonium persulphate because of high yield of the polymer obtained and its relatively higher reduction potential.  However, it yields an insoluble and infusible polymer.  Since ammonium persulphate is a strong oxidizing agent and aniline polymerization is exothermic, controlling the reaction temperature is rather difficult and consequently polymers with wide distribution of molecular weights result.  In our previous work we have reported better conductivity of the polyaniline salt doped with organic sulphonic acid as dopant and organic peroxide as the oxidant. ^[16]  It has also found that that solubility and morphology of the polymer is determined by the choice of solvent system and oxidizing agent.
One of the key problems related to polyaniline is its poor processability because of its insolubility in most of the solvents.  Several methods have been attempted to overcome this disadvantage.  One such method is blending of polyaniline with thermoplastic polymers possessing good mechanical strength.
Another method is polymerisation of substituted anilines.  Substituted polyanilines are synthesized to increase the processability of the polyaniline through higher solubility. ^[17-20]  It is known that the torsional angle influences the electronic properties of various conducting polymers with aromatic backbones.  For example, studies on polyaniline indicate that the ionization potential and band gaps are affected by the torsional angle between adjacent rings of the polymer chain and the substituents in polyaniline should affect the torsional angle.  Moreover, the incorporation of the substituents in the polymer backbone is a common technique to prepare soluble polymers.  It has been reported an enhanced solubility in substituted conducting polymers for example poly(3-alkylpyrroles) and poly(2-alkyl anilines) soluble in common organic solvents.^{[17, 21]}
Copolymerization is a process in which two or more monomers are incorporated as integral segments of a polymer, is used to produce copolymers with properties that are different from those of homopolymers.^[22]  In general, copolymers possess physical and mechanical properties intermediate between both homopolymers, depending on the concentration of the monomer units incorporated into the copolymer.  A major consideration in the design of the copolymers results in the improvement in mechanical, optical and electrochemical properties relative to those of parent conducting polymeric materials.  Various copolymers such as poly(pyrrole-co-N-methyl pyrrole), poly(aniline-co-ortho-anisidine), poly(aniline-co-ortho-ethoxyaniline), etc. have been prepared either by chemical or electrochemical polymerization techniques. ^[23]
One more method, which can improve the processability of polyaniline, is polymerisation of aniline in colloidal dispersion form.^[24-26]  The key to preparing such conducting polymer dispersions is to carry out the polymerisation in the presence of a steric stabiliser, which adsorbs (or grafts) to the particles as they form, preventing gross aggregation and precipitation of the polymer being produced.  These particles find their application in separation processes (chromatography); by applying a controlled but variable, electrostatic potential to a packed bed of such particles, ionic species may be selectively adsorbed (and subsequently desorbed) from a flowing solution.  In such a application control of particle morphology is very desirable.  Dispersions of polypyrrole and polyaniline received intense interest in recent years ^[24].  Polypyrrole can readily prepared as spherical particles, ^[27] whereas polyaniline has only been prepared with rice-grain or needle shaped particles ^[28-30].  Dispersions of colloidal particles will often tend to aggregate in chain like structure, to reduce hydrodynamic repulsions between approaching particles.  The key therefore to producing individual spherical particles must be more efficient steric stabilisation during the early stages of dispersion polymerisation process.