The Physicochemical Structure of Inorganics in PIM-1/Alox Hybrid Membranes Synthesized Via Vapor Phase Infiltration (VPI)

Abstract

Polymers of intrinsic microporosity (PIMs) represent a unique class of polymer membranes defined by their inherent microporosity, leading to excellent separations ability. However, the poor solvent stability of these organic membranes limits their use for organic solvent separations. Vapor phase infiltration (VPI) is an emerging technique adapted from atomic layer deposition (ALD) processes that infiltrates inorganic clusters into polymeric materials, generating hybrid organic-inorganic materials with improved chemical stability. VPI treated polymer of intrinsic microporosity 1 (PIM-1) has shown dramatically improved resistance to solvent dissolution while maintaining its microporosity, making it a much more robust membrane. However, the lack of clarity around the structure of the inorganic clusters within VPI synthesized hybrid materials limits the ability to design these hybrid membranes to optimize potential trade-offs between chemical stability, permeance, and separation performance.Currently, most VPI literature refers to the inorganic components within the hybrid as “metal oxides” or “MOx”. This oversimplification does not fully describe the possible cluster structure or inorganic-to-organic bonding characteristics likely present in these hybrid structures. Moreover, the atomic-scale and long-range disorder of these inorganic clusters make them difficult to describe with traditional methods used for inorganic crystals or polymeric solids. Thus, a more sophisticated understanding of the inorganic structure is needed to inform both process and function design of these hybrid materials. This talk will discuss combined experimental and computational efforts to clarify the structure of inorganic clusters in VPI treated PIM-1/AlOx membranes. XPS and FTIR in conjunction with DFT calculations indicate that the final inorganic species is an oxyhydroxide species. Further, solid state NMR suggests the final inorganic cluster is a 6-coordinate species that is at least dimerized, with water molecules coordinating to the structure to fill out the coordination sphere. DFT calculations validate this claim as the lowest energy structure compared to the monomer form as well as other aluminum oxyhydroxide species. This interdisciplinary work explores the structural characteristics of PIM-1/AlOx hybrid membranes and its impact on the VPI treated material properties such as solvent stability.