Improving the Stability of PIM-1 Via Vapor Phase Infiltration - Atomic Layer Deposition (VPI-ALD)

There are many separation challenges that have the potential to be addressed using membranes, but require membranes that exhibit stable and reliable operation in demanding operating environments. Highly stable membranes exist (e.g., inorganic membranes), but are often associated with high costs and difficult scale-up. Developing methods to improve membrane stability while leveraging the intrinsic scalability of polymer membrane fabrication will enable membranes to address aggressive separation challenges. Two of the most prominent methods for improving membrane stability post-fabrication are pyrolytic carbonization and crosslinking. These are both promising approaches to creating highly stable, high performance membranes, but require near-complete transformation of the precursor in the former case, and chemical/thermal treatments that result in partial conversion of the precursor in the latter case. Here, we discuss a new technique for improving the stability of polymers of intrinsic microporosity (PIMs) while retaining essentially the same microstructure as the precursor polymer. Cyclic vapor phase infiltration of atomic layer deposition (VPI-ALD) precursors (e.g., water and metal precursors) into the structure of PIM-1 (a prototypical PIM) results in the growth of monolayers or bilayers of metal oxide on the surfaces of the interconnected PIM-1 micropores. The resulting oxide-PIM-1 hybrid material maintains the microporosity of PIM-1, and the pore size and pore volume are observed to decrease after additional VPI-ALD cycles. The hybrid materials exhibit excellent solvent resistance to strong solvents for PIM-1 (chloroform, dichloromethane and tetrahydrofuran). The rejection of dye molecules (dissolved in ethanol) from hybrid membranes in hollow fiber format increased markedly from those observed for neat PIM-1: the rejection of Oil Red, Solvent Blue 38 and Rose Bengal rose from 93%, 92% and 28% to 98%, 99% and 80%, respectively.