Session N32: Transport and Separation Phenomena in Polymer Membranes and Molecular Materials: Computation

Predicting Covalent Organic Framework (COF) Membrane Performance by Mapping Molecular Interactions in Mixed Solvents via Atomistic Modeling

Complex solvent environments continue to limit the widespread adoption of organic solvent nanofiltration (OSN) in many chemical industry applications. Reactive force field (ReaxFF) and nonreactive force field models have been recently developed to molecularly map separation performance of a commercial covalent organic framework (COF), TpPa-1, and a carboxylated COF (C-COF). Specifically, the following factors have been characterized using these atomistic models: layer stacking, effective vs. designed pore size, and solvated solute size in various single organic solvents or solvent pairs. Model predications can be directly compared with experimental filtration results after normalizing model outcomes and filtration data with a common solvent, such as water, to minimize time and length scale mismatch between atomistic modeling and experiments. Model outputs, such as organic solvent permeance and solute rejection rate, matched experimental filtration results well. These findings demonstrate how solvated solute state and effective pore size in mixed solvents cumulatively dictate membrane performance. Additionally, ion effects on selectivity were probed theoretically and experimentally by adding NaOH and HCl to organic solvents, such as DMF and methanol. In sum, force field models can serve as digital twins of COF membranes to simulate separation processes while capturing the effects of COF structure, chemistry, and crystallinity on membrane performance in complex organic solvent environments. This approach will provide insight into future COF design and synthesis for persisting separation challenges.   

Ion-ion Separations in Biomimetic Water Channels

Demand for critical elements has increased exponentially as the implementation of batteries (Li) and solid-state devices (Nd, Tb, & Eu) have become ubiquitous. A key limiting step in the extraction of these critical elements is understanding ion-membrane interactions that result in high permselectivies. Through molecular dynamics simulations, we analyze an energy efficient and environmentally beneficial membranes for ion-ion separations that are embedded with artificial water channels. Such channels display angstrom-scale separations and tunable ion-ligand binding energies that mimic transmembrane proteins. Using the ligand-appended pillarene chemistry, we have implemented a range of techniques to evaluate ion transport and correlated the effects of channel chemistry to ion permselectivity. The combination of these techniques bridge monomer-ion interactions, which can be resolved on a very fast timescales, to ion-channel interactions that must be resolved through non-equilibrium or milestoning methods due to larger timescales.   

Characterization of ion binding guided by ∆∆G and mobility calculations in ethylene oxide-rich environments

In light of ongoing global energy issues, there is a need for new, innovative materials to address critical problems. One such issue surrounds the capture and recovery of lithium from naturally occurring brines and e-waste recycling. Through the utilization of polymeric membranes tailored with distinctive ligands, Li⁺ can be entrapped through host-guest interactions. In this work, ethylene oxide-rich environment systems are proposed for Li⁺ separation. We utilize Molecular Dynamics simulations for the calculation of relevant properties to ion permeation through a membrane, including the relative solvation free energy of the cations at finite dilution and the ionic mobility. We establish limits for the separation of Li⁺ from Na⁺ as a comparable monovalent ion and discuss the implications of these results for developing lithium capture technologies.   

Studies of Li ion conduction mechanism for zwitterionic polymer electrolytes using molecular dynamics simulations

Polymer electrolytes are attracting attention due to their ability to improve the safety and durability of commercial Li ion batteries traditionally consisting of liquid electrolytes; however, they exhibit a lower Li ion conductivity. Recent studies suggested that a class of polyzwitterions with LiTFSI as the added lithium salt improves low temperature conductivity and Li transference numbers compared with generally known polymer electrolytes such as PEO, but the mechanism of the fast Li transport in polyzwitterion systems is not yet understood. To clarify the mechanism, we built model systems of polyzwitterions that capture the important features of experimentally reported polyzwitterions and analyzed the Li ion motion using all-atom molecular dynamics simulations. We discuss the dynamic properties, detailed structural features, and the interactions between Li ions and polyzwitterions/TFSI anions at the atomic level.   

Gas permeance in polymer membranes: computational approaches near and far from ideal behavior

The use of highly specialized membranes for gas separations processes presents a valuable opportunity for a global shift away from traditional thermally driven, carbon-intensive methods. The vast design space for polymers makes it desirable to explore new materials in silico before synthesizing them to conserve valuable resources. Using a combination of molecular dynamics and Monte Carlo methods, we explore gas permeance behaviors of several materials including ladder polymers, polyimides, and polysulfones using fully atomistic simulations. From the solution-diffusion method we use Grand Canonical Monte Carlo to deduce the solubility of gas molecules both near and away from the dilute Henry’s Law limit and use molecular dynamics to evaluate gas diffusion coefficients as a function of gas loading. The use of non-equilibrium molecular dynamics is also explored. Finally, we note how the use of detailed simulations with atomistic force fields combined with an iterative feedback loop provides an opportunity for the accelerated development of soft materials in important engineering applications.   

Role of hydration on ion transport in the transition from dry to wet salt-doped PEO

Ion transport properties are widely studied in polymer electrolytes under rigorously dry conditions and in highly swollen hydrogel networks. While our knowledge of ion transport in the extreme cases of dry and highly swollen polymer membranes is relatively well developed, the transition between these extremes, i.e., the low hydration regime, is poorly understood. In this study, we apply atomistic molecular dynamics simulations to probe the role of hydration on salt transport in the transition from dry to poorly hydrated to water-percolated salt-doped PEO. Our results suggest that even small quantities of water decouple ion diffusion from polymer segmental dynamics compared to rigorously dry systems with increasing temperature. We attribute this to the rapid solvation of ions by water over PEO decreasing the impact of segmental motions on ion diffusion. Despite this solvation of lithium by water, we observe significantly faster speedup of water relative to lithium with increasing hydration under the low-hydration regime. We attribute this to competing solvation dynamics between lithium, water, and PEO. In water-percolated networks, we observe good agreement between simulated ion diffusion coefficients and the Mackie-Meares model.   

Microscopic theory of the effects of penetrant shape on activated dynamics and selectivity in polymer melts and crosslinked networks

We generalize the force level self-consistent cooperative hopping theory for the activated relaxation and diffusion of dilute spherical penetrants in polymer melts and crosslinked networks to explicitly address the role of non-spherical molecular shape at fixed space filling volume (1d rod-like, 2d planar-like, 3d globular-like). A rich dependence of penetrant dynamics and degree of decoupling of the penetrant hopping rate from the polymer alpha relaxation on temperature and crosslinking is predicted in the deeply supercooled regime for relatively large penetrants due to the importance of emergent collective elasticity which becomes significantly stronger as penetrant shape anisotropy grows. In contrast, for smaller penetrants or at high/medium temperatures, penetrant hopping is a local process and the effect of penetrant shape at fixed space filling volume is relatively weak. An aspect ratio variable is proposed that organizes well our results. Quantitative comparison with recent diffusion experiments on chemically complex aromatic penetrants in crosslinked poly-(n-butyl acrylate) networks reveals good agreement, and testable new predictions are made. Our results provide new insights for achieving selective transport and separations in membrane applications.   

Investigating the effect of hydration ratios on hydroxide conductivity in anion exchange membranes from non-reactive molecular dynamics simulations

Anion exchange membranes (AEMs) used in alkaline fuel cells rely on the selective transport of hydroxide ions through the membrane to produce electricity. The transport of hydroxide ions depends on hydroxide-polymer, hydroxide-water and hydroxide-hydroxide cooperative interactions. In this study, we implement atomistic molecular dynamics (MD) simulations across three different AEM chemistries with ion contents ranging from 20% to 50%, and water/hydroxide ratios ranging from 0 – 40. We compute the microstructure of the polymer using the cavity energetic sizing algorithm. We calculate conductivity using both Nernst-Einstein and Onsager transport coefficients. We find that at high hydration ratios, trends in conductivity agree well with experiments. We are able to decorrelate the contributions of polymer dynamics, microstructure, and water dynamics on the ion conductivity in these materials and provide design rules that can be used to optimize AEMs in the future.   

Impact of Morphology on Water Dynamics in Hydrated Copolymers for Proton Exchange Membranes

We study novel fluorine-free copolymers with transport properties rivaling those of perfluorosulfonic acid polymers that are promising as proton exchange membranes. These random copolymers have one monomer with a 5-carbon backbone and a pendant phenylsulfonate group and a second monomer with the same backbone and a pendant phenyl group. Using atomistic molecular dynamics simulations, we explore the effects of varying the relative humidity and the percent of sulfonated phenyl groups. We identify systems that form percolated nanostructures using a combination of visual inspection and cluster-based analyses. Total scattering functions reveal strong nanophase separation between the hydrophilic and hydrophobic domains as evidenced by the presence of an ionomer peak. We obtain the water diffusion coefficient, water rotational time, and nuclear magnetic resonance (NMR) T1 and T2 times for the copolymers that exhibit percolated nanostructures. We relate these dynamic properties to structural descriptors such as channel width distribution and fractal dimension to better understand the impact of nanostructure morphology on water dynamics.   

Finding non-aqueous proton conductors for polymer-based electrolytes using density functional theory

Polymer electrolytes, used in energy storage and conversion devices like fuel cells and lithium-ion batteries, offer benefits such as high electrochemical stability, mechanical flexibility, and interface stability, positioning them above their liquid and ceramic counterparts. However, their reliance on water restricts their working temperature below the boiling point of water, thus limiting polymer electrolyte membrane fuel cell performance. This study employs density functional theory to investigate amphoteric molecules, capable of both accepting and donating protons, as potential non-aqueous proton conductors. Using the proton affinity energy as a proxy for the proton transfer barrier, we examine how changes in electron donating and withdrawing groups can enhance or reduce the hopping of protons between molecules. Extending previous work by Zawodzinski et al., our simulations help to elucidate how these modifications can be used to identify novel molecules with proton affinities mirroring water. Consequently, this research provides the basis for the use of data science methods that can expedite the design and discovery of high-temperature, water-free electrolytes.   

Computational Analysis of Small Angle Scattering Measurements on Polymer Membranes

Polymers are widely used as materials to engineer membranes for chemical separations and ion/charge transport-related applications. In such applications, the morphology within the polymer films dictates how well the material performs. As such, researchers working with such materials need to characterize the membrane structures at various length scales as a function of polymer design and processing conditions. To characterize polymer membrane structure at multiple length scales and provide insights into the structural evolution at various processing and operating conditions, one can conduct small angle scattering (SAS) measurements. Analysis of the results from such SAS measurements, especially for materials with network morphologies often desired for membranes, is not easy with conventional analytical models. To alleviate this challenge with analysis of SAS measurements of network morphologies, we have developed a computational method using random fields. Using this method we can interpret SAS measurements and output the 3-dimensional (3D) structures of the domains present within the network structure and understand connectivity and domain size distributions. We validate our approach by taking as input the SAS profiles from simulated structures and show that our method is capable of outputting 3D structures that match, visually and quantitatively, the simulated structures that provided the SAS profile.   

Polyethylene-block-polyacrylate based block copolymers for high ionic conductivity polymer electrolytes

Block copolymer-based polymer electrolytes are a unique class of electrolytes due to their ability to harness the properties of different polymer blocks. Typically, block copolymer electrolytes combine the mechanical properties of a high glass transition temperature (Tg) block and the ion-conducting properties of a low Tg block. The ionic conductivity of such systems is usually low due to the restricted segmental mobility of the glassy block. Here, we develop and study the polyethylene-block-polyacrylate based block copolymers doped with ionic liquids, and observe a significantly high ionic conductivity of ~1 mS/cm at room temperature. The ionic conductivity remains high over a broad temperature range. This high ionic conductivity is enabled by the low Tg of both blocks. Furthermore, we study the mechanical properties of these polymers and find them to be elastic solids. We study the role of interfacial width and segmental relaxations to understand the high ionic conductivity of these polymer electrolytes. We expect these polymers to serve as high ionic conductivity solid-state electrolytes with high temperature and mechanical stability.