Session M32: Transport and Separation Phenomena in Polymer Membranes and Molecular Materials: Experiments

Impact of ion mobility on the CO2 solubility in ionic polymers

Increasing concentration of CO₂ in the atmosphere poses a significant threat to the environment, contributing to global warming and triggering disastrous weather events. Ionic polymers have become an important component in the CO₂ capture process. Similar to their monomeric counterparts (e.g., ionic liquids) polymers demonstrate unique solubility and selectivity in capturing CO₂. However, the mechanism underlying these processes remains unclear.  In the case of ionic liquid, substantial research has been dedicated to deciphering the chemistry of the ions. It has been demonstrated that imidazolium–based ionic liquids show a remarkable solubility for CO₂ owing to their specific structural and chemical advantages. In case of polymers, the polymer's structural connectivity introduces an unknown variable of ion mobility to the equation, elevating the challenge of solving equation for CO2 transport to the next level.

In our study, we have conducted both experimental investigation and molecular modeling to explore the role of ion mobility in the polymers containing imidazolium-bistriflimide (TFSI) ion pair on the CO₂ solubility and diffusivity. From the experiment, we demonstrated that when TFSI is mobile, the material exhibits improved solubility and selectivity for CO2 compared to when TFSI is immobile. The simulations confirm that that TFSI ions have a greater affinity for CO₂ when it is mobile.   

The influence of polynorbornene backbone structure on ion clustering, water uptake, and ion transport

Polynorbornene (PNB)-based anion exchange membranes have shown great potential in energy conversion and storage due to their exceptional alkaline stability. However, it is not well understood how the PNB polymer backbone structure affects key fundamental properties such as water absorption and ion transport that have an important influence on device performance. In this study, we designed and synthesized polymers based on bromine-containing polynorbornene (PNB) with similar ion exchange capacity (IEC), molecular weight, and an identical spacer length. The distinctive backbone structures were achieved through vinyl addition polymerization (VAP) and ring-opening metathesis polymerization (ROMP) followed by a hydrogenation reaction. By exposing the obtained bromine-containing polynorbornene thin films to trimethyl amine vapor, quaternary-ammonium (QA) groups were added to the polymer backbone via a four-carbon spacer. These QA-containing polynorbornene thin films allowed us to investigate how different backbone structures influence the properties of polyelectrolytes. We examined QA-VAP-PNB, QA-ROMP-PNB, and QA-hydrogenated ROMP-PNB thin films, all with a thickness of around 90nm, across a humidity range of 25%RH to 90%RH at 25°C. Interestingly, QA-hydrogenated ROMP-PNB exhibited the highest ionic conductivity in relation to water concentration and hydration number despite having comparable IEC, molecular weight, spacer length, and QA group. This can be attributed to the ionic clustering morphology within QA-hydrogenated ROMP-PNB, enabling efficient bromide transport with minimal water uptake and swelling ratio, as confirmed by mid-angle X-ray scattering (MAXS) and Grazing incidence mid-angle X-ray scattering (GI-MAXS). Furthermore, ion clustering facilitates efficient bromide transport in various water states, whether freezable or nonfreezable. The outcomes of this study underscore the significance of hydrophobicity and segmental mobility of the backbone structure in shaping ion clustering morphology, a key factor for facilitating effective anion transport. Ultimately, we believe this study offers valuable insights that can inform the design and synthesis of polyelectrolytes with tailored properties.   

Ion transport in weak polyelectrolyte membranes at varying external pH

Charged polymer membranes are of great interest in various applications, ranging from environment, energy, and health. Understanding of ion transport in charged polymer membranes is critical to the advancement of technologies related to polymer electrolytes in batteries, water purification, critical element extraction, environmental remediation, and medical isotope purification. Here, we designed a systematic library of weak polyelectrolyte membranes based on polyacrylic acid (PAA). A series of polyacrylic acid (PAA) based polymer networks were synthesized with varied charged group contents and controlled water swelling. By adjusting external pH, the number of sodium counter cations dissociated and condensed on the polymer backbone can be systematically changed, leading to different ion transport and dielectric properties in the polymers. We evaluated ion and water transport properties (solubility, diffusivity, and permeability) in the resultant polymer membranes. These transport properties are correlated with dielectric properties in the polymers via dielectric relaxation spectroscopy (DRS). This model system enables us to elucidate the mechanism of ion transport in charged polymer membranes.   

Interplay of backbone rigidity and water content on ion/ion selectivity in hydrated polymer membranes

Increasing lithium demand requires new and improved methods for purification. Membrane-based separation processes, such as selective electrodialysis, hold promise for efficiently separating lithium from other contaminants, such as magnesium. Successful implementation of these technologies requires materials with high selectivity of lithium over other ions while maintaining high lithium conductivity. Two factors that are known to play important roles in governing membrane performance are equilibrium water content and polymer segmental dynamics. We perform coarse-grained molecular dynamics simulations to examine the interplay between these two factors and the impact on ion/ion selectivity. These simulations rationalize our observed experimental selectivity trends for cellulose acetate membranes and provide guidance for future investigation into improved membrane design.   

Electrostatic Funneling in Ionic Transport Through Solid Porous Membranes

Advancements in membrane fabrication techniques allowing precise control over pore size distribution, density, and surface chemistry, together with the development of higher resolution characterization methods, have renewed interest in studying ionic conduction in solid pores.

Recently, it has been shown that ionic concentration polarization (ICP) fundamentally hinders ionic transport through charged pores and impedes the scaling from the conduction efficiency of a single pore to a multiple pore membrane, which renders osmotic power extraction inviable. 

In this work, we revise the classical understanding of ICP in light of a concept termed "Electrostatic Funneling" (EF). EF is an out-of-equilibrium phenomenon originating from a mismatch between the ionic concentration inside and outside charged membranes. This effect is driven by the dual imperatives of maintaining near-electroneutrality within the pore while ensuring current continuity. 

Utilizing state-of-the-art all-atom Molecular Dynamics (MD) simulations with explicit water and Poisson-Nernst-Planck (PNP) modeling, we establish that EF is essentially the root cause of ICP. 

Our findings elucidate the conditions under which EF either impedes or enhances ionic transport, thereby offering a pathway for the rational design and optimization of membrane performance in various applications.   

Impact of PEGMA as a blocking group in ion exchnage membranes for CO2 reduction product crossover: Electrochemical Cell Applications.

CO₂ reduction cells are one of the interesting techniques to cope with CO₂ emission issue. A typical CO₂ reduction cell consists of two cells separated by an ion exchange membrane (IEM). The major role of IEM is to prevent the transport of various CO₂ reduction products (formate, acetate, methanol, and ethanol). Therefore, it is necessary to tailor the membranes to block the transport of these products. Previously, to suppress CO₂ reduction product crossover we introduced a series of uncharged comonomers, acrylic acid (AA, n=0, where n is the number of PEG repeat units), hydroxyethyl methacrylate (HEMA, n=1), and poly(ethylene glycol) methacrylate (PEGMA, n=5), where we observed the crossover of carboxylates were significantly suppressed in PEGMA-containing films in co-diffusion. To further understand this, we prepared a series of PEGMA (n=9)- containing films and measured the permeabilities and solubilities of these films to carboxylates (formate and acetate) and alcohols (methanol and ethanol) in one- and two-component mixtures. In one-component permeation, we observed permeabilities to all solutes being increased with increasing PEGMA content (increased water volume fraction). However, emergent behavior was observed for the co-transport of carboxylates with alcohols. For instance, we observed the permeabilities to acetate in co-diffusion are decreased with increasing PEGMA content. This behavior motivates further investigation for rationally designing ion-exchange membranes.   

Ionic drug transport in charged biosponge polymers to capture toxic chemotherapy drugs before they spread through the body

Cancer is becoming the leading cause of death in most developed nations. Despite efforts to develop targeted and personalized cancer therapeutics, dosing of the cancer chemotherapeutics is limited by toxic side effects. Typically, more than 90% of the injected drug is not trapped in the target organ and bypasses the tumor to general circulation, causing toxicities in distant locations. In the context of reducing the toxicity of chemotherapy, we have designed charged biosponge adsorbers for capturing ionic chemotherapy drugs before they spread through the body. Doxorubicin, a widely used chemotherapy drug with significant toxic side effects, is a model drug. The charge concentration and water uptake in the biosponge adsorbers are systematically changed to achieve high binding capacity and fast kinetics. We aim to understand the mechanism of ionic drug transport in these biosponge adsorbers and establish the design principles for multifunctional biosponge polymers. Using a swine model, our initial design enables the capture of 69 % of the administered drug without any adverse effects. The proposed approach may help patients fight cancer.   

Mixed binary alkali halide salt transport in PEO systems

Salt permeability coefficients are important properties of membranes for industrial applications such as water purification and energy generation; however, there are few systematic studies probing the differences in permeability as a function of external solution composition. Understanding how ion transport is affected by the presence of other ions can help advance the design of membranes tailored with specific ion affinities. Here, we use a model cross-linked poly(ethylene glycol) diacrylate membrane to examine how permeability coefficients of binary alkali halide salt mixtures vary as a function of external salt mole fraction. We see that, at constant ionic strength, mixed salt permeability is largely governed by mixed salt partitioning. Furthermore, starting from the Nernst-Planck framework, we have derived a model for predicting permeability coefficients based off a thermodynamic partitioning model and the Mackie-Meares model for diffusion. This new model, with no adjustable parameters, shows great agreement in conditions with minimal ion-ether oxygen coordination (i.e., Na⁺, Cl^-, and Br^-), and shows slight overpredictions in cases with strong cation-oxygen binding (i.e., K⁺).   

A Diafiltration Apparatus for High-Throughput Characterization of Transport Through Polymer Membranes

Creating systems and techniques capable of reducing the time and resources needed to characterize the transport properties of membranes can help to increase the rate of material and process development. In this study, a diafiltration apparatus is developed to rapidly characterizing membrane performance over a broad range of feed solution compositions. The apparatus doses a fixed-concentration diafiltrate solution into a stirred cell to achieve a predetermined change in the retentate concentration. Here, it was shown that membrane performance, within a 1 to 100 mM KCl phase space, could be probed ten times more quickly with one diafiltration experiment (4 h) than with a traditional experimental campaign (47 h). The synergy between data analytics and instrumentation led to the incorporation of inline conductivity probes that monitored the real-time permeate and retentate concentrations. This additional information provided key insights to distinguish between the mechanisms that govern membrane separations (e.g., discriminating between adsorption or rejection-based separations) and allowed for the membrane transport coefficients to be determined accurately. Ultimately, the ability of this device to characterize membranes rapidly will help to address knowledge gaps related to the interfacial processes that govern solute–solute selectivity and the performance of membranes in complex multi-component feed streams.   

Evidence for pressure-induced diffusion of solvent in dense polymer membranes

Knowledge of the nature of solvent transport through polymers is crucial for developing future membranes for novel separations. It has been debated whether pressure-driven transport of solvent through dense polymer membranes occurs via convective flow through nano-scale pores or diffusive flow induced by the mechanical conditions imposed on the polymer. A key distinction between these two models is the role of membrane pressure. In the diffusive model, membrane pressure is approximately constant while a concentration gradient of penetrant exists in the film. In the pore explanation, solvent flow is driven by pressure drop. Historical evidence has favored diffusion models through the direct observation of pressure-induced concentration gradients of water and organic solvents in polymer films. Recently, the diffusion-based explanation for membrane transport has been challenged. However, our work presents evidence for pressure-induced diffusion in polymers, including cellulose acetate, xl-poly(ethylene glycol diacrylate), xl-poly(styrene sodium sulfonate), and Nafion 117. By observing the pressure-induced concentration gradient across a stack of polymer films, we evidence to support the constant pressure assumption. Additionally, we verify the proposed relationship between pressure, water activity, and diffusivity in polymers by measuring water flux at pressures up to 250 bar. Finally, we discuss the validity of the constant pressure assumption in the context of contact mechanics.   

Super-resolution imaging reveals resistance to mass transfer in functionalized stationary phases

Chemical separations are costly in terms of energy, time, and money. Separation methods are optimized with inefficient trial-and-error approaches that lack insight into the molecular dynamics that lead to the success or failure of a separation and, hence, ways to improve the process. We perform super-resolution imaging of fluorescent analytes in four different commercial liquid chromatography materials. Surprisingly, we observe that chemical functionalization can block over 50% of the material's porous interior, rendering it inaccessible to small molecule analytes. Only in situ imaging unveils the inaccessibility when compared to the industry-accepted ex situ characterization methods. Selectively removing some of the functionalization with solvent restores pore access without significantly altering the single-molecule kinetics that underlie the separation and agree with bulk chromatography measurements. Our molecular results determine that commercial “fully porous” stationary phases are over-functionalized and provide a new avenue to characterize and direct separation material design from the bottom-up.   

Selective ionic transport in zwitterion-functionalized nanopores

We investigate ionic transport in zwitterion-functionalized nanopores to illuminate the physics underlying selective salt transport in random zwitterionic amphiphilic copolymer (r-ZAC) membranes. By conducting molecular dynamics simulations, we probe the transport of various sodium halides in zwitterion-functionalized nanopores as a function of the (i) pore radius, (ii) zwitterionic chemistry, (iii) grafting density of the zwitterions, and (iv) orientation of the zwitterionic dipoles. The variation in density of water molecules within the nanopore with respect to the abovementioned parameters is accounted for in our work by allowing the pores to attain chemical equilibrium with two external reservoirs containing bulk water. Our results reveal the intriguing possibility for a complete reversal of the anionic diffusivity trends within the zwitterion-functionalized nanopores as compared to salt-in-water solutions at equivalent concentrations. By highlighting the key selectivity motifs underlying salt transport in zwitterion-functionalized nanopores, this study will help unravel the principles for designing novel r-ZAC membranes with exceptional ionic separation characteristics.   

The Effect of Crosslinker Concentration on Drug Release Kinetics of Thermo-Responsive, Lignin-Based Soft Composites

The use of lignin in soft composites has gained recent attention due to its various favorable qualities, such as biocompatibility, antimicrobial and antibacterial properties. Lignin, an abundant biopolymer, can be used in conjunction with stimuli-responsive materials, such as poly(N-isopropylacrylamide) (PNIPAm), as these materials are able to undergo a volume change at a temperature close to that of human physiological conditions. The fabrication of interpenetrating networks (IPNs) containing PNIPAm and other crosslinked polymers have been explored. However, traditional methods investigate the role of exclusively petroleum-based materials in the fabrication of these soft composites. In this study, we fabricated soft composites containing: (1) a thermo-responsive polymer, PNIPAm; (2) a hydrophilic polymer, poly(vinyl alcohol) (PVA); and (3) a sustainable biopolymer, lignin. The concentrations of crosslinkers glutaraldehyde and N,N’-methylenebisacrylamide were varied between 5 and 15 mass percent of their respective polymers. After fabrication, the drug-release kinetics of each soft composites were characterized. Specifically, the diffusion of caffeine through the network via ultraviolet-visible spectroscopy and the equilibrium water uptake were analyzed at room temperature and 40 °C. It was observed that the diffusion of caffeine into water was suppressed with the addition of lignin. Furthermore, the crosslinker concentration was also seen to impact the release kinetics.   

Polymer Architecture Induced Trade-off Between Conductivities and Transference Numbers in Salt-doped Polymeric Ionic Liquids

Recent experiments have demonstrated that polymeric ionic liquids which share the same cation and anion but possessing different architectures can exhibit markedly different conductivity and transference numbers characteristics when doped with lithium salt. In this study, we used atomistic molecular simulations on polymer chemistries inspired by the experiments to probe the mechanistic origins underlying the competition between conductivity and transference numbers. Our results indicate that the architecture of the polycationic ionic liquid plays a subtle, but crucial role in modulating the anion-cation interactions, especially their dynamical coordination characteristics. Chemistries leading to longer-lived anion-cation coordinations relative to lithium-anion coordinations lead to lower conductivities and higher transference numbers. Our results suggest that higher conductivities are accompanied by lower transference numbers and vice versa, revealing that alternative approaches may need to be considered to break this trade-off in salt-doped polyILs.   

Effects of electrostatic correlations on charge transport in single-ion conducting polymer electrolytes

Single-ion conducting polymer electrolytes such as the polymerized Ionic Liquids (polyILs) are of great interest with potential applications in lithium-ion batteries, supercapacitors, fuel cells, and other similar products. However, currently available single-ion conducting polymer electrolytes have ionic conductivity significantly lower than required for use in these applications. Ion-ion correlations quantified in terms of inverse Haven ratio have been shown to be responsible for the reduced conductivity of single-ion conducting polymer electrolytes. However, a microscopic understanding of the inverse Haven ratio has been lacking and is needed to design single-ion conducting polymer electrolytes with superior ionic conductivity. In this talk, we present the results of a coarse-grained molecular dynamics study investigating the effects of local polarization on the inverse Haven ratio and ionic conductivity of single ion conducting polymer electrolytes. In particular, we simulate single-ion conducting polymer electrolytes with a spatially uniform dielectric constant and compare it with the simulations containing polarizable ions. The polarizability is introduced using Drude oscillators and encodes non-uniform polarization in these polymer electrolytes. Effects of cation size, dielectric constant, and polarizability of the ions on the inverse Haven ratio and the ionic conductivity of single ion conducting polymer electrolytes will be presented.