Multicationic Anion-Exchange Membranes from Pyridine-Bridged Polybenzimidazole and Polymer-Ionic-Liquid-Grafted Graphene Oxide
Anupam Das, Nilanjan Mukherjee, Ritu Saraswat, Tushar Jana
Abstract
In this study, we functionalized the surface of graphene oxide (GO) nanosheets by grafting polymer ionic liquid (PIL) chains of varying structures and then utilized PIL- g -GO materials as nanofillers with poly(butylated pyridinium benzimidazolium)iodide (PyPBI-BuI-OBA) in order to prepare hydroxide-ion-conducting mixed-matrix alkaline anion-exchange membranes with multiple ion-exchange sites. Three different ionic-liquid-based monomers, namely, [2-(methacryloyloxy)ethyl]trimethylammonium chloride (MATMA), 3-butyl-1-vinylimidazolium iodide [VImBu][I], and 1-butyl-2-methyl-1-(4-vinylbenzyl)imidazolium iodide [VBImBu][I], were polymerized on the activated GO surface via a surface-initiated reversible addition–fragmentation chain-transfer process to obtain three different types of PIL-grafted GOs, namely, pMATMA- g -GO (GOPIL1), pVImBu- g -GO (GOPIL2), and pVBImBu- g -GO (GOPIL3). Exfoliation of GO sheets upon grafting of PIL was observed and verified by electron microscopic studies. The grafted PIL chains were found to have a narrow polydispersity index ( Đ ) and precise control over the molecular weight, confirming polymerization occuring through SI-RAFT technique. The amount of polymer grafted on the GO surface was determined by TGA analysis, and a strong dependence on the polymer structure was found. The mixed-matrix membranes (MMMs) were prepared by solution blending of PyPBI-BuI-OBA with varying loadings of GOPIL materials, and then MMMs were immersed in KOH to prepare alkaline anion-exchange membranes (AAEMs) consisting of imidazolium and pyridinium quaternary ammonium ion-exchange sites. Among all AAEMs, those with 2 wt % GOPIL2 and GOPIL3 exhibited superior hydroxide ion conductivities of 137 and 127 mS/cm, respectively, at 80 °C. The structural, chemical, thermal, mechanical, and conductive properties of the membranes remained unaltered after alkaline treatment at elevated temperatures. These AAEMs exhibited ∼90 and ∼80% retention of ion-exchange capacity (IEC) and ∼90 and ∼70% retention of OH – conductivity after alkaline treatment in 1 M KOH, at 60 °C for 500 h and in 5 M KOH, at 60 °C for 375 h, respectively. To the best of our knowledge, this will be the first report on the utilization of surface PIL-grafted GO nanosheets in AAEMs.