Optimization and kinetics of phosphoric acid doping of poly(1-vinylimidazole)-graft-poly(ethylene-co-tetrafluorethylene) proton conducting membrane precursors

Optimization of the reaction parameters affecting phosphoric acid (PA) doping behavior of poly(1-vinylimidazole), P(VIm), grafted poly(ethylene-co-tetrafluoroethylene) (ETFE) proton conducting membrane precursors obtained by radiation induced grafting was studied using the Taguchi method. The reaction parameters such as degree of grafting (G%) in the precursors, PA concentration, temperature and doping time were selected as independent parameters while doping level was the response. The optimum parameters for achieving the maximum doping level (7.45 mmol repeat polymer unit−1) were: G% of 54%, PA concentration of 60%, temperature of 100 °C and reaction time of 5 days. The kinetics of the acid doping reaction was also studied and the doping rate was found to be a function of reaction parameters and followed a first order reaction. The PA doping was verified by Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS) analysis of the membranes. The proton conductivity and thermal stability of the membranes were also evaluated. It can be concluded that the Taguchi method provides an effective tool for prediction of acid doping level and optimization of reaction parameters. The kinetics of acid doping is also suggested to be a diffusion-driven reaction following a multi-layers adsorption model.