Grafting of Polymer Brushes on MOF Surface to Achieve Proton-Conducting Membranes

In recent years, postsynthetic modification (PSM) of a metal-organic framework (MOF) has gained immense interest for the synthesis of hybrid materials with improved functionality. However, further improvement in the PSM strategies to make them scalable and yield a synthetically tunable MOF surface can create polymer chain-grafted MOF hybrid materials in which polymer chains provide the stability, robustness, and desired functional performances and MOF offers interesting physical properties. Toward these goals, in this work we have developed a simple and effective method to functionalize the UiO-66-NH₂ MOF surface by grafting functional polymer brushes using surface-initiated reversible addition-fragmentation chain-transfer (RAFT) polymerization. At first, the MOF surface was transformed into a polymerizable surface by attaching a trithiocarbonate-based chain-transfer agent (CTA) and then, three different sets of polymer brushes consisting of (i) a neutral but basic nitrogen functionality (PGM-N), (ii) a trimethyl-substituted quaternary ammonium functionality (PGM-C), and (iii) a quaternary ammonium along with sulfonate functionality (PGM-Z) were grown on the activated MOF surface. The uniqueness of this work has additionally been enhanced by utilizing these synthesized PGMs as nanofillers into the oxypolybenzimidazole (OPBI) membrane matrix to obtain homogeneous mixed matrix membranes (OPBI@PGM) that show brilliant thermomechanical and tensile properties and exceptional thermal stability. Interestingly, these membranes exhibit superior proton conductivity when doped with H₃PO₄ (PA) and minimal acid leaching. Grafting of functional polymer brushes on the MOF surface induces strong H-bonding and acid-base interactions with OPBI chains and H₃PO₄ molecules, which are responsible for such high acid uptake and excellent acid retention, thereby displaying a proton conductivity as high as 0.241 S cm^-1 at 160 °C (∼4-fold increase when compared to OPBI), which is among the highest values reported till date for an MOF-based proton exchange membrane with excellent mechanical vigor.