Blend membranes based on N1-alkyl-substituted imidazolium functionalized polymers and aromatic polyethers: influence of N1-alkyl substituent on properties and alkaline stability

N1-alkyl (octyl and dodecyl)-substituted imidazolium-based PVBC homopolymers have been synthesized via N-quaternization reaction of N1-alkyl imidazole and PVBC precursor homopolymer bearing reactive benzyl chloride moieties. Due to their poor film forming properties and water solubility, these homopolymers were blended with aromatic polyethers bearing main chain pyridine units at different compositions in order to study the effect of alkyl chain length on morphology, water uptake, swelling ability and chemical stability of the prepared membranes. The B2 blend membrane with the highest N1-dodecyl-substituted imidazolium PVBC content (65 wt%) exhibited the highest water uptake (54%) despite its lower IEC value compared to the corresponding one containing N1-octyl-substituted imidazolium PVBC, low swelling ratio and a phase separated morphology. Evaluation of the chemical stability in 3.6 M KOH solution at 80 °C for 7 days revealed the degradation of imidazolium via ring opening, as evidenced by ATR-FT-IR spectroscopy. Therefore, new blends having as second constituent, the N1-alkyl-substituted imidazolium functionalized poly(PVBC-co-AA 20 ) copolymers containing acrylic acid units were fabricated targeting to the improvement of chemical stability via ionic cross linking. The prepared D1 and D2 blend membranes containing 60 and 65 wt% dodecyl-imidazolium functionalized poly(PVBC-co-AA 20 ) copolymer content, respectively, were flexible, exhibited moderate IECs (1.47–1.60 meq/g) and sufficient water uptakes (up to 30%). D2 blend membrane showed excellent chemical stability after testing in 3.6 M KOH solution at 80 °C for 30 days, as confirmed by ATR-FT-IR spectroscopy and TGA analysis. The excellent chemical stability can probably be attributed to the steric hindrance effect of N1 dodecyl substituent which effectively protects the C2 position of imidazolium from hydroxide attack as well as to the formation of a dense, ionic cross-linked structure that hinders hydroxide penetration.