Monomer Partitioning in Emulsion Polymerization of Perfluorinated Ionomers

Polymer electrolyte membrane fuel cells are widely employed for transport applications due to the beneficial aspects of the proton-conducting polymer membrane, which reduces the chance of leakages compared to liquid electrolytes as well as efficiently prevents the flow of electrons. This membrane is characterized by a polymeric framework that is often produced by free radical emulsion copolymerization of perfluorinated monomers, the most abundant of which are typically tetrafluoroethylene and perfluoro-sulfonyl vinyl ether. Therefore, the proper description of monomer partitioning in the multiphase reaction environment is crucial for the development of a predictive kinetic model for polymerization and the optimization of the process. In this study, monomer solubility data have been collected for different combinations of all four phases of the reactive system, namely, gas phase, water phase, droplets of a water-immiscible liquid comonomer, and polymer particles. A thermodynamic partitioning model has been developed to describe these data, exploiting the Sanchez and Lacombe theory for the characterization of the monomer-monomer and monomer-polymer interactions. The comparison with the experimental measurements allowed to estimate important parameters, such as the solubility of the monomers in the different phases and interaction parameters with the polymer, which ultimately allowed evaluation of the monomer mixture composition in the reaction locus. Finally, the developed partitioning model has been validated for the multiphase system and exploited to estimate the reactivity ratios for the two monomers.