Study on tunable crosslinking anion exchange membranes fabrication and degradation mechanism

A novel monomer who combined cross linker and functional groups together called 2,2'-(hexane-1,6-diyl)bis(2-methyl-2,3,3a,4,7,7a-hexahydro-1H-4,7-methanoisoindol-2-ium) iodide (HDHM-iodide) was synthesized and polymerized by Ring-opening Metathesis Polymerization (ROMP) to give a series of anion exchange membranes. The resulting monomers and polymers were characterized by 1H and 13C NMR, TGA. Directly cast from the polymerization solution on flat glass to form the clear, flexible anion exchange membrane. Swelling ratio and water uptake are only 6.2% and 28.04% at 60 °C for membrane with 2.32 mmol·g−1 ion exchange capacity (IEC). The conductivity of Poly (HDHM-Norbornene-hydroxide-12) and Poly(HDHM-Norbornene-chlorine-12) AEMs at room temperature up to 27 mS·cm−1 and 14 mS·cm−1 separately, and increased with temperature quickly. After 16 days immersed in 2 M NaOH solution, conductivity and IEC dropped at 60 °C in alkaline solution, 50% and 35% separately. The energy barrier of degradation pathways calculation reveled that Hofmann Elimination case 2 degradation pathway with the lowest energy barrier 12.181 kcal·mol−1 is most likely occurred when faced with hydroxide. Also, the energy barrier of Hofmann Elimination is lower than Nucleophilic Substitution degradation mechanism.