Comb-shaped diblock copolystyrene for anion exchange membranes

To improve the properties of diblock copolystyrene-based anion exchange membranes (AEMs), a series of AEMs with comb-shaped quaternary ammonium (QA) architecture (QA-PS_m-b-PDVPPA_n-xC where x denotes the number of carbon atoms in different alkyl tail chains and has values of 1, 4, 8, and 10 and C denotes carbon) were designed and synthesized via subsequent quaternization reactions with three different alkyl halogens (methyl iodide and N-alkane bromines (CH₃[CH₂]_x-1Br where x = 4, 8, and 10). Compared with triblock analogues quaternized with methyl iodide in our previous research, QA-PS_m-b-PDVPPA_n-xC (x = 4, 8, and 10) AEMs are more flexible with the introduction of a long alkyl tail chain; this probably impedes crystallization of the rigid polystyrene-based main chain and induces sterically adjustable ionic association. An increase in the pendant alkyl tail chain length generally led to enhanced microphase separation of the obtained AEMs, and this was confirmed using small-angle X-ray scattering and atomic force microscopy. The highest conductivity (25.5 mS cm⁻¹) was observed for QA-PS₁₂₀-b-PDVPPA₈₀-10C (IEC = 1.94 meq g^–1) at 20 °C. Furthermore, the water uptake (<30%) and swelling ratio (<13.1%) of QA-PS_m-b-PDVPPA_n-xC AEMs are less than half of these corresponding values for their triblock counterparts. The QA-PS₁₂₀-b-PDVPPA₈₀-10C membrane retained a maximum stability that was as high as 86.8% of its initial conductivity after a 40-day test (10 M NaOH, 80 °C), and this was probably because of the steric shielding of the cationic domains that were surrounded by the longest alkyl tail chains.