Hydrophobic networks for advanced proton conducting membrane: Synthesis, transport properties and chemical stability

The design of interpenetrated networks (IPN) membranes for fuel cell applications requires both an electrolyte and a neutral network. The composition and architecture of the latter are of major importance for the final IPN membrane properties. In this work, networks based on a fluorinated diepoxy oligomer (DFODDE) and a non-fluorinated triepoxy monomer (TMPTGE) were synthesized. The network composition was varied from 100% DFODDE to 100% TMPTGE, resulting in an increase of the crosslinking density and concomitantly a decrease of the fluorine content. The curing process was optimized to achieve a total epoxy conversion and the chemical structure of the networks was characterized by Raman and Infrared spectroscopies. The physical, thermal and chemical membrane properties were studied and discussed as a function of the crosslinking density and of the fluorine content. Increasing the crosslinking density led to a decrease of the membrane permeability to oxygen. Water sorption properties depended on both parameters, with a prevailing role of the fluorine content on the water uptake and a major influence of the crosslinking density on the diffusion parameter. The thermal stability increased also with the fluorine content. All materials exhibited a good stability in water at 80 °C but a significant weight loss after immersion in a concentrated H2O2 solution. Altogether, these results indicate that networks containing 20% of TMPTGE exhibit an interesting set of properties (low oxygen permeability, high Tg, good chemical and thermal stability) to behave as a neutral partner in an IPN membrane. Moreover, the low water uptake and diffusion rate measured in these networks make them attractive for water barrier membranes.