Grafting of Poly(vinyl phosphonic acid) to MOF Surfaces to Achieve Proton Conducting Hybrid Materials

In recent times, surface-modified metal organic frameworks (MOFs), obtained using different postsynthetic modifications (PSMs) on MOFs, are being explored frequently to develop hybrid materials for numerous applications. In continuation of our recent work on the grafting of polymer chains on the MOF surface to make hybrid materials (polymer-MOF conjugates) with enhanced physical properties, in this work, we grafted a proton conducting polymer, namely, poly(vinyl phosphonic acid) [PVPA] chains of various length and grafting density, on UiO-66 MOF, using surface-initiated reversible addition-fragmentation chain transfer (SI-RAFT) polymerization in order to develop an advanced class of proton conducting MOF-based hybrid materials with superior performance. Grafting of PVPA chains was confirmed by TGA, GPC, FT-IR, NMR, FESEM, TEM, EDX, and mapping studies. The stability of the freshly synthesized hybrid material under various harsh environments was further analyzed. Pendent phosphonic acid groups of the PVPA chains grafted on the MOF surface create strong H-bonding, acid-base, ionic, and noncovalent interactions with the neighboring PGMs and water molecules, which are responsible for displaying high stability, water sorption ability, and high proton conductivity of 1.26 × 10^-2 S cm^-1 at 80 °C for the PGM-L3 sample under 98% relative humidity and 9.8 × 10^-3 S cm^-1 at 60 °C for the PGM-H1 sample under 75% relative humidity, which are among the highest values reported so far for MOF-based systems.