Kinetic Study of the Emulsion Polymerization of a Perfluorosulfonic Acid: Role of Mass Transfer Limitations

Perfluorosulfonic acid ionomers are noteworthy for their excellent proton conductivity and mechanical strength provided by their molecular structure, making them particularly valuable in the manufacturing of polymer electrolyte membranes and electrode binders for fuel cells and electrolyzers. These copolymers are mainly produced by free radical emulsion polymerization involving a supercritical monomer, i.e. tetrafluoroethylene (TFE), and a liquid perfluorosulfonyl vinyl ether. Hence, reliable descriptions of thermodynamic partitioning and kinetic mechanisms are required for the development of a model capable of predicting the final copolymer composition, which greatly affects its properties in application. In emulsion polymerizations involving a gaseous monomer, the reactant must diffuse to the reaction locus, i.e., polymer particles, and the reactions are frequently affected by gas–liquid mass transfer limitations. For this reason, in this work, a revised kinetic model that considers gas–liquid diffusional limitations has been developed by exploiting an equilibrium partitioning model already described in our previous study. Therefore, first the mass transfer coefficient has been evaluated using sorption tests of TFE in the liquid mixture, and then interrupted conversion polymerizations have been exploited for model fitting of the reactivity ratios. Revised values of the reactivity ratios for this copolymerization are proposed and compared with those available in the literature to emphasize the need to account for mass transfer limitations in these multiphase reacting systems. The evaluated parameters have been confirmed by predicting new compositional data for model validation, showing good predictivity. Finally, the possibility of exploiting the model in the design of an optimized polymerization recipe, minimizing the copolymer compositional drift, has been demonstrated.