Poly(diphenylamine) (PDPA) particles were synthesized as a novel semiconducting electro-responsive electrorheological (ER) material by a chemical oxidative polymerization process and their ER characteristics were investigated at various electric fields. Surface morphology of these N-aryl-substituted polyaniline-derived PDPA particles was examined by scanning electron microscopy and their chemical structure and thermal properties were confirmed by Fourier-transform infrared spectroscopy and thermogravimetric analysis, respectively. Their susceptibility to electric fields and the chain formation of dispersed particles were observed directly using optical microscopy. Rheological analysis of the PDPA-based ER fluid was performed using a combination of steady shear and oscillation tests in a Couette-type rotational rheometer with a high-voltage power supply. The measurements were performed in controlled shear stress and shear rate modes under the varied electric field strengths. The Cho-Choi-Jhon model was found to determine the stress behavior more clearly. Dielectric analysis provided additional information about the electrical polarization properties and thus also the ER performance of the PDPA-based ER fluid. (c) 2017 Elsevier Ltd. All rights reserved.