Dispersion polymerization of aniline using hydroxypropylcellulose as stabilizer: role of rate of polymerization

Hydroxypropylcellulose (HPC) has been used as a steric stabilizer for preparing polyaniline dispersions using the route of oxidative dispersion polymerization of aniline. Using strongly acidic conditions (1 mol l −1 HCl), low temperature of about 2 °C and a concentration of aniline as low as 0.5%, ammonium peroxodisulfate at 1.25% and hydroxypropylcellulose concentrations at 0.5–1 g d l −1 , unstable dispersions were obtained not only in water but also in aqueous alcohols (ethanol and methanol) up to at least 70 vol% alcohol. In contrast, dispersions that remained stable for at least 72 h were obtained when the alcohol concentration of the medium was as high as about 80 vol%. Kinetic studies of the polymerization systems suggested that success in the latter case was due to a lowering of the rate of polymerization. Transmission electron microscopy studies showed that dispersion particles prepared in 80 vol% alcohol media are spherical in shape and their diameter decreases with increasing stabilizer concentration. However, a change of morphology from spherical to aggregated needle‐shaped was observed when the rate was increased by increasing the aniline concentration from 0.5% to 0.75% g d l −1 in the above recipe. The aggregated particles were found to be broken down to spherical nanoparticles when the as‐prepared dispersions were sonicated for about 30 min. The sonicated dispersion on drying showed the presence of fractal clusters of polyaniline particles in the dried film. The fractal dimension was determined to be 1.77 which agreed well with the theoretical value determined by computer simulation based on a diffusion limited cluster–cluster aggregation model in three dimensions.