Exploring properties of polyaniline-SDS dispersion: a rheological approach

The paper describes steady and dynamic rheological characterization of a system in which polyaniline (PAn) is dispersed in aqueous medium by the effect of a surfactant sodium dodecyl sulphate (SDS). During polymerization of aniline in SDS medium, large and agglomerated micelle-polymer structures are formed (supported by TEM and DLS) resulting in high viscosity of the medium. On application of steady shear micellar entanglements are ruptured and the system exhibits yield properties followed by shear thinning. From the frequency dependence of storage and loss modulii (G' and G'') it seems that the system behaves more like a viscous fluid rather than an elastic liquid. Carrying out the same experiments on another dispersion in which PAn is stabilized by dodecyl benzenesulphonic acid (DBSA), very different viscoelastic response was received. DBSA molecules become counter-ions to PAn chains and this way large and interconnected PAn-DBSA structures are formed by mutual sharing of DBSA anions and PAn chains. This system therefore, exhibits gel like properties and encounters a gel to sol transition at larger deformation. Detailed studies have established that PAn-SDS is a stabilized dispersion that resembles entangled polymeric solutions to some extent while PAn-DBSA is a partially flocculated system. Therefore, rheological response of the system is mainly governed by the mutual orientation of PAn with respect to the micelles rather than the individual properties of the components. None of these systems, however, follow the established Maxwell's model and a single relaxation time is not obtained. Rather, Rouse model of multiple relaxation times is partially applicable to PAn-SDS dispersion.