Field strength dependence of the high-frequency viscoelastic relaxation process in polyaniline/silicone oil electrorheological suspensions

Polyaniline (PANI) was synthesized via oxidative coupling polymerization in acid conditions and de-doped in ammonia solution. The electrorheological (ER) properties of the PANI/silicone oil suspensions were investigated in the oscillatory mode shear, with particular focus on the high frequency region, where a crossover in G′(ω) and G′′(ω) signals the onset of a dissipative relaxation process, presumed to be associated with motion of PANI particles within the fibrillar structures generated by the electric field. The relationship between the crossover frequency, ωc, and the electric field strength (E) was investigated as a function of matrix viscosity and shear strain. We find that ωc increases with increasing electric field strength, and decreases with increases of matrix viscosity and strain amplitude. These observations are in qualitative agreement with a theoretical model, which relates the relaxation mechanism to the competition between hydrodynamic and electrostatic forces between PANI particles within thick fibrillar structures. At the crossover point, a critical scaling relation is found relating two dimensionless parameters, the Mason number (Mn), and the Peclet number (Pe), viz. Mn~(Pe)0.09.