Mid-block quaternized polystyrene-b-polybutadiene-b-polystyrene triblock copolymers as anion exchange membranes

A novel mid-block, side-chain-type quaternized triblock copolymer was prepared from commercial polystyrene-b-polybutadiene-b-polystyrene (SBS) triblock copolymers by Markovnikov addition reaction with HBr, azidation, and Cu(I)-catalyzed click reaction for high-performance anion exchange membranes (AEM). After the optimization of reaction conditions, the bromination, azidation and subsequently Cu(I)-catalyzed click reactions have been confirmed to be quantitative according to the NMR results. The anion-conductive, mid-block triblock copolymer (SBS-c-QA) membrane was obtained from casting its DMF solution. As a control experiment, the mid-block quaternized SBS (SBS-QA) membrane was also synthesized by direct quaternization of brominated SBS with trimethylamine. The SBS-c-QA membrane with pendent quaternary ammounium groups showed much lower water uptake than that of SBS-QA membrane. However, comparable bicarbonate conductivities of 5.6 mS/cm at 20 °C and 20.5 mS/cm at 80 °C were obtained for SBS-c-QA membrane in spite of its low ion exchange capacity (IEC) (1.10 meq./g) and low water uptake (18 wt%). Thus, the SBS-c-QA membrane displayed high IEC-normalized conductivity which can be attributed to the side-chain architecture of the triblock copolymer. The alkaline stability testing indicated that SBS-c-QA membrane showed a higher alkaline stability than SBS-QA membrane. A high retention of bicarbonate conductivity (70%) was observed after degradation testing in 1 M NaOH at 80 °C for 216 h. These results suggested that incorporating the side-chain-type QA cations into the mid-block of the triblock copolymer was an effective method to fabricate AEMs with both high ionic conductivity and excellent alkaline stability.