Synthesis and Characterization of Partially Renewable Oleic Acid-Based Ionomers for Proton Exchange Membranes

The future availability of synthetic polymers is compromised due to the continuous depletion of fossil reserves; thus, the quest for sustainable and eco-friendly specialty polymers is of the utmost importance to ensure our lifestyle. In this regard, this study reports on the use of oleic acid as a renewable source to develop new ionomers intended for proton exchange membranes. Firstly, the cross-metathesis of oleic acid was conducted to yield a renewable and unsaturated long-chain aliphatic dicarboxylic acid, which was further subjected to polycondensation reactions with two aromatic diamines, 4,4′-(hexafluoroisopropylidene)bis(p-phenyleneoxy)dianiline and 4,4′-diamino-2,2′-stilbenedisulfonic acid, as comonomers for the synthesis of a series of partially renewable aromatic-aliphatic polyamides with an increasing degree of sulfonation (DS). The polymer chemical structures were confirmed by Fourier transform infrared (FTIR) and nuclear magnetic resonance (1H, 13C, and 19F NMR) spectroscopy, which revealed that the DS was effectively tailored by adjusting the feed molar ratio of the diamines. Next, we performed a study involving the ion exchange capacity, the water uptake, and the proton conductivity in membranes prepared from these partially renewable long-chain polyamides, along with a thorough characterization of the thermomechanical and physical properties. The highest value of the proton conductivity determined by electrochemical impedance spectroscopy (EIS) was found to be 1.55 mS cm−1 at 30 °C after activation of the polymer membrane.