(447h) Improving the Solvent Stability of Polymer Membranes through Vapor Phase Infiltration

Membrane filtration is a promising tool for reducing carbon emissions in solvent-solvent separations, as its use avoids the need for energy-intensive distillation processes. However, improvements to membrane permeability, selectivity, thermal stability, chemical stability, and fouling resistance will be required before they are cost-effective enough to handle the acidic, alkaline, organic, and otherwise complex feed streams present in many applications. Of the commercial membranes commonly available, polymeric membranes are relatively low cost but often suffer from low thermal and chemical stability, while other membrane materials (e.g. ceramics) can be difficult or expensive to manufacture. Vapor phase infiltration (VPI), also known as sequential infiltration synthesis or atomic layer infiltration, is one potential cost-effective strategy for making higher-performance membranes. In VPI, a polymer is exposed to vapor phase metalorganic reactants which absorb into, diffuse through, and react with the polymer to form a composite material. This approach could be used to upgrade existing membranes through post-synthetic modification to improve their performance.

In this work, we highlight how VPI can be applied toward improving the solvent stability of polymer membranes. First, polyethersulfone (PES) membranes—often used as a support layer—were treated with trimethylaluminum and water to introduce aluminum oxide via VPI. Exposure duration and process cycles were then modulated to control the infiltration depth of reactants and inorganic loading, respectively, before measuring mechanical properties through burst pressure testing and dynamic mechanical analysis. Results showed that membranes were more resistant to pressure and less brittle when deeper infiltration depths and lower inorganic loadings were used. These trends were found to agree with theoretical models, such as the rule of mixtures and Gibson-Ashby formulations, which predict that the distribution of alumina would mitigate the loss of ductility. Additionally, the chemical stability of VPI-treated membranes in organic solvents and their separation performance were assessed as a function of alumina infiltration depth, suggesting a tradeoffs between chemical and mechanical stability in VPI-modified PES membranes. We also show the impact of this modification of membrane permeability and selectivity for stable solvents, as well as a preliminary exploration of the use of organic reactants that could expand the library of infiltration processes. In doing so, this presentation will showcase the potential of VPI to create next-generation membranes for diverse and challenging separation applications, like solvent-solvent separations.