Thermal studies on proton conductive copolymer thin films based on perfluoroacrylates synthesized by initiated Chemical Vapor Deposition

The thermal properties of ion-conductive materials are crucial for their use in temperature-sensitive applications such as polymer electrolyte fuel cells. In this study, 1H,1H,2H,2H,-perfluorodecyl acrylate and methacrylic acid (MAA) copolymers are synthesized by initiated Chemical Vapor Deposition. This method is a solvent-free thin film deposition technique, which allows for the preparation of polymers in distinct stoichiometric compositions. The chemical stability of the copolymers was investigated upon elevated temperatures along with other thermal properties. For this, experimental techniques such as infrared spectroscopy and ellipsometry were used. The data show that samples are chemically stable up to 150 °C, a point above which anhydride formation occurs, resulting in the loss of the conductive groups. A second order thermal transition was found at (95 ± 5) °C for polymers containing 20% MAA, which shifts towards higher temperatures as the MAA content increases. In addition, the water stability was tested. While the membranes show considerable water uptake (over 35% at 60% MAA content), mechanical stability is lacking, resulting in rupture formation and partial dissolution. A possible route to overcome this issue is found in crosslinking, with the addition of 15% ethylene glycol dimethacrylate providing sufficient stability to the polymer.