Surfactant effect on the stability and electrorheological properties of polyaniline particle suspension

Abstract

In this thesis, surfactant effects on the stability and electrorheological properties were investigated experimentally. The electrorheological fluid was composed of semi-conductive polyaniline particles dispersed in a mineral oil. To promote the phase stability, small amount of nonionic surfactant was added. The polyaniline was synthesized by polymerization of aniline monomer, and the surfactants selected were Span20, Span80 and Span85, which are distinctly different in their molecular structures. Dynamic yield stress of polyaniline suspension, which possessed the quadratic dependence on electric field strength at low electric field, showed the linear dependence at high electric field. In addition, the conductivity of polyaniline particles influenced the electrorheological response and there existed the optimal conductivity for the maximum yield stress. In the presence of nonionic surfactants used here, the yield stress of polyaniline exhibited a local maximum at a certain surfactant concentration. The existence of optimal surfactant concentration was confirmed by the surfactant adsorption isotherm. Below the optimal threshold concentration, the electrorheological activity was enhanced by the increased interfacial polarization induced by the adsorbed surfactant molecules. Above the threshold concentration, however, the electrorheological activity was degraded by the conduction through the field-induced surfactant-rich microstructure. This optimal concentration coincided with the saturation concentration in the adsorption isotherm. The surfactants used in this study showed the improved phase stability due mainly to the induced steric hindrance effect. For the colloidal stability, Span80 was the most effective. However, considering that most of ER activity comes from relatively large particles, Span85 was the best choice for a stabilizing additive in the present ER suspension. In addition, our results showed that the presence of the nonionic surfactants consid...