The observation of a conductivity threshold on the electrorheological effect of p-phenylenediamine oxidized with p-benzoquinone
p-Phenylenediamine was oxidized with p-benzoquinone in the presence of 0.1–5 M methanesulfonic acid (MSA) solutions. The resulting methanesulfonate salts of 2,5-(di-p-phenylenediamine)-1,4-benzoquinone are semiconducting and the particles were further suspended in silicone oil in a weight ratio of 1 : 9 in order to create novel electrorheological fluids. Conductivity measurements using the two-point method along with dielectric spectroscopy were carried out in order to investigate their electrical and dielectric properties, including their silicone-oil suspensions. The higher the concentration of MSA was present during the synthesis, the higher the conductivity was observed. Nevertheless, a certain threshold of the ER effect has been found and a further significant increase in conductivity causes only a slight ER effect enhancement. At an electric field strength of 1.5 kV mm−1, the observed yield stresses read at the low shear rate values were 11.5 Pa, 20.3 Pa, 24.5 Pa, and 28.2 Pa for particles with conductivities 1.5 × 10−12, 8.5 × 10−11, 1.0 × 10−8, and 1.5 × 10−7 S cm−1, respectively. From dielectric spectra, it was observed that the conductivity of the particles determines the relaxation times of their silicone-oil suspensions.