Session Q06: Transport and Separation in Polymer Membranes

Engineering ion selectivity in polymer membranes

Selective separation of monovalent cationic species from complex mixtures is an industrially relevant procedure necessary for the recovery of many commodity materials, such as lithium from salt brines. Unfortunately, most conventional membranes lack selectivity between monovalent ions, rendering their use in such applications infeasible. One approach to overcoming selectivity limitations is to incorporate ligands into polymer matrices which specifically interact with target cations in an aqueous environment. In this talk, I will discuss a few recent topics from our work on using atomistic and coarse-grained simulations to uncover the physics of solubility and diffusivity selectivities in polymer membranes. Our findings provide critical molecular-level insight into the interplay between membrane chemistry and monovalent ion selectivity, aiding in the rational design of selective membranes for resource recovery.

    

Bridging insights between ion transport in battery electrolytes and membranes

Ion transport is central to both hydrated polymers for water filtration and dry polymers for electrochemical applications such as batteries. While remarkable progress has been made in the development of polymeric battery electrolytes and hydrated membranes for both desalination and energy applications (e.g., fuel cells, electrolyzers, and solar fuel generators), further progress hinges on developing fundamental insights into mechanisms of ion solvation/solubility and diffusion that are the foundations of ion permeability and ionic conductivity. Developing this insight requires bridging understanding among both dry and hydrated membranes. I will discuss both our efforts to develop superionic membranes that operate in a rigorously dry state for battery applications and also the effects of hydration on ion conduction in both superionic and ionic polymer electrolyte systems.   

Harnessing Structure-Dependent Separation Behavior of Thin Film Membranes

In obtaining highly permeable and selective membranes for separation processes, it is important to achieve well-defined structures in membranes through a detailed understanding of the processing-structure-property relationships. The layer-by-layer (LbL) framework allows us to synthesize membranes with controlled structure and chemistry. Using this framework, two different thin film membranes were synthesized: polyamide membranes embedded with graphene oxide nanoplatelets (GONPs) and carbon nanodots (CNDs) membranes. The GONPs-containing membranes were investigated for water desalination, and the incorporation of GONPs in polyamide membranes resulted in an increase in surface hydrophilicity and salt rejection properties but not a significant increase in flux compared to pristine membranes. Further, upon exposure to chlorine, GONPs embedded membranes retained salt rejection performance better than the pristine membranes, indicating increased chlorine resistivity of membranes. The CNDs containing membranes were obtained by reacting amine-functionalized CNDs with trimesoyl chloride and were also investigated for organic solvent nanofiltration applications. The synthesized membranes manifested high selectivity (up to 90%) when tested for dye molecules such as brilliant blue and disperse red in methanol.  Taking advantage of the distinct fluorescence properties of CNDs and the films built with those, a crack in the film can be easily detected. This property can be harnessed for diagnostic purposes, such as tracking mechanical failure and fouling of the membrane.   

Understanding Nanoconfinement and Atom-Specific Correlations to Manipulate Molecular and Ion Transport

Polymer-based transport media form essential and rate-limiting components in a wide range of technologies that include water purifiers, batteries, fuel cells, and electrolyzers.  Our group has uncovered critical aspects of nanoconfined transport in ionic polymer-based electrolytes by combining temperature- and composition-dependent NMR with molecular dynamics (MD) simulations, leading to new paths of thought for electrolyte design.  Electrophoretic NMR (ENMR) along with NMR spectroscopy and diffusometry give unprecedented access to ion-specific transport processes and local ion and surrounding matrix dynamics, from liquids to solids and everything in between.  I will discuss example systems (including solid-polymer Li+ battery electrolytes, H-bonding liquids, and water-bearing membranes) that build bridges between liquid-like and solid-like transport, thus motivating new compositions and new morphologies for polymer-based electrolytes and separations technologies.     

Processing and Separation Performance Principles of Zeolitic Imidazolate Framework Membranes

Metal-organic frameworks (MOFs) hold promise as gas separation membranes due to their tunable permeation properties enabled by the structural diversity stemming from the plethora of available metal/organic combinations. Among MOFs, zeolitic imidazolate frameworks (ZIFs) have been studied extensively for their potential as selective separation membranes because their unique structural properties allow them to achieve high performance for certain important separations, like that of propylene from propane. Although relatively rapid progress for highly selective ZIF membranes has been achieved, reliable and scalable thin film processing remains a challenge.[1] In this talk, I will discuss synthesis and post-synthesis modification methods we have been developing based on vapor and liquid phase processing and use of electron-beam-, X-ray- and plasma-induced modifications.[2-5] These methods establish a new materials processing paradigm for thin film separation membranes that could allow control of structure, and composition to achieve optimal separation performance. Recent results on

membranes based on amorphous ZIFs (aZIF) made by molecular layer deposition and microfluidic deposition will be discussed.

1. Lee D.T., et al. Annual Review of Chemical and Biomolecular Engineering 13, 529-555 (2022)

2. Ma X., et al. Science 361, 1008-1011 (2018)

3. Eum K., et al. Angewandte Chemie Int. Ed. 58, 16390-16394 (2019)

4. Miao Y., et al. Chemical Communications 60, 9316-9320 (2021)

5. Hayashi M., et al. Angewandte Chemie Int. Ed. 60, 9316-9320 (2021)