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

Abstract

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-NH 2 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 H 3 PO 4 (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 H 3 PO 4 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 (∼4fold 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.

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

“…As the grafting of the polymer chains is covalent in nature, the MOF core needs to be digested in order to detach the grafted polymer chains to be analyzed by the GPC technique. This treatment is well-known in the literature to free the surface-grafted polymer chains by the action of HF digesting the framework structure ,,− (see details in the Supporting Information). [Note: Hydrofluoric acid is a highly corrosive inorganic acid that can penetrate the skin easily.]…”

“…Thus, the activated carboxylic acid group (−COO-NHS) when attached covalently with the free primary amine groups (−NH 2 ) exists on the UiO-66-NH 2 surface by means of forming an amide linkage so that the phenyl group stays away from the MOF surface and acts as a Z group to control the reactivity of the C–S functionality of the BSIPA, while the free radical leaving group R • remains attached to the MOF surface during the RAFT propagation process. Also, the methyl group was positioned strategically on the −CH position (which is part of the R group here) adjacent to the trithiocarbonate group [−S-C­(S)-S−] to ensure the desired extra stability to the generated free radical species upon dissociation of the C–S bond present between the trithiocarbonate and the R group . Upon SI-RAFT polymerization, the growing polymer chains (PVPA) will reside between the trithiocarbonate group and the R group.…”

“…Unfortunately, improvement of properties of the resulting MOF hybrid obtained by this method is inadequate because of the limited number of active functionalities available on the modified MOF surface owing to the synthetic challenges to achieve 100% functionalization. ,− Alternatively, amplification of such properties is reported either by impregnation of small molecules in the confined pores or incorporating a polymer with the MOF particles to make a nanocomposite and is based on noncovalent interactions between the nanofiller and the polymer matrix. − However, uneven distribution of the filler (MOF) material in the matrix can encumber the improvement in desired properties. On the other hand, grafting of polymer chains on the MOF surface is found to provide improved homogeneity of the material, resulting in significant improvement in the properties of the MOF hybrid …”