Session G19: Transport Phenomena in Polymers and Polymer Membranes I

Transport in Ionic Polymer Membranes for Large-Scale Energy Conversion and Storage

New polymer membranes are needed to advance energy storage and conversion technologies for distributed and grid-scale applications. We have recently demonstrated new ion-conducting polymer membranes that have achieved excellent performance and long-lifetime stability in vanadium redox flow batteries, a leading technology candidate for deployment in renewable power networks and grid-scale energy storage systems with sizes ranging from 10s to 100s of megawatts. By tuning the nanoscopic self-assembly of the ionic domains in the polymers, we are able to increase the cycle life of the device by impeding vanadium ion transport through the membrane while facilitating high conductivity in the electrolyte to maintain the battery current density. For instance, by decreasing the vanadium permeability of the membrane by a factor of two, we have been able to double the lifetime of the device, which provides significant life-cycle cost savings. We have also demonstrated membranes with nearly zero vanadium permeability that show 100 % coulombic efficiency in flow battery charge-discharge cycling tests. Currently, we are working on demonstrating these membranes over 100s of charge-discharge cycles. In this talk, transport of ions, water, and active redox species through ion-containing polymer membranes will be discussed.   

Membranes and Sustainability

In the next decade, separation science promises as an important research topic in addressing complex sustainability-driven challenges like reducing carbon footprint, lowering energy cost, accelerating renewable fuel development, and making the chemical processes simpler. Among several emerging separation technologies, membranes have advantages in achieving superior bulk separation efficiency, reducing operating and capital costs, and a lower footprint. Despite having such benefits, the market penetration for membranes is relatively small compared to established technologies like cryogenic distillation for chemical processes, amine-based absorption for CO2 capture, and pressure swing adsorption technologies. Fundamental structure-property development, membrane fabrication, and prototype testing are three key research areas for accelerating membrane development. These three areas sit at the intersection of engineering, basic science, and science policy and partnerships. There is a need to develop a cross-functional, inter-agency membrane platform initiative to drive such goals. The presentation will focus on key membranes application areas like H2 fuel cells, water purification, waste plastic circularity, and carbon capture, emphasizing recent developments, market and societal needs, and proposed research topics.    

Electron tomography reveals new insights on polymer membranes

Membrane technology has relied on intricate control of properties through systematic perturbations to polymer membrane chemistries and processing, yet many fundamental questions remain on the mechanisms that govern water transport and separations. We have leveraged advances in multi-modal electron microscopy to generate new insights on membrane structure and function.  For example, we have combined the focused ion beam with scanning electron microscopy through serial sectioning to reconstruct a 3D representation of ultrafiltration membranes using for virus removal from biopharmaceutical streams.  In addition, we have combined energy-filtered transmission electron microscopy with electron tomography from scanning transmission electron microscopy images to map the variation in density of polyamide films used in reverse osmosis membranes.  Quantitative analyses of imaging products are key to extract mechanistic details that govern water transport and separations.    

Transport in self-assembled polymer membranes with uniform 1 nm transport limiting features

Membranes that exhibit independently tunable solute selectivity and permeance have the potential to transform a number of selective transport applications, with significant implications for water purification, organic solvent separations, and electrochemical device operation. A key but elusive goal is the development of materials with uniform, well-defined pores at the 1 nm scale, the sizes of which can be tuned in small increments with high fidelity. Here, we investigate the fabrication and transport properties of a nanoprous membranes derived from self-assembled liquid crystal mesophases, with the potential to address the aforementioned issues. We highlight the dependence of selectivity on the orientation of the transport vector relative to the nanostructure in membranes derived from direct hexagonal mesophases, and compare the permeability of these systems against membranes produced from double gyroid mesophases. Appropriately normalized, transport in gyroid membranes is demonstrably slower than in direct hexagonal membranes. Good stability in a wide range of conditions and the precision with which the nanostructure can be tuned suggest that these membranes may provide new opportunities for the systematic design of separations with tailored selectivity and permeability, and for understanding and modeling rejection in nanoscale flows.   

Thermal Diffusion in Polymer Electrolyte Membranes

In most mixtures, a concentration gradient develops in response to a temperature gradient. Steady state is quantified by the Soret coefficient, which is the ratio of the thermal diffusion coefficient to the mutual diffusion coefficient. Polymer electrolytes are an interesting class of materials to study in this regard because they could potentially be used in thermogalvanic cells to convert temperature gradients to electricity. Soret coefficients were measured in thermogalvanic cells with lithium electrodes and a dry polymer electrolyte composed of poly(ethylene oxide) (PEO) and lithium bis‐trifluoromethanesulfonylimide (LiTFSI). Voltage and power were measured in response to temperature gradients. Voltage was used to calculate the concentration gradient (and thereby Soret coefficient). Interestingly, the magnitude of Soret coefficient was similar to that in small‐molecule electrolytes and significantly less than that in neutral polymer blends. Surprisingly, the Soret coefficient of this polymer electrolyte depends on salt concentration. In order to understand the concentration dependence, separate measurements of mutual and thermal diffusion of LiTFSI in PEO-based polymers was conducted with time-resolved infrared spectroscopy.

This work was supported by NSF and FSU.   

Engineering Li/Na selectivity in 12-crown-4–functionalized polymer membranes

Isolation of lithium from natural reserves is plagued by time-consuming and costly processes. While polymer membranes could circumvent these challenges by efficiently extracting lithium from aqueous solutions, they exhibit poor ion-specific selectivity. Toward this end, we have incorporated host–guest interactions into a tunable polynorbornene network by copolymerizing: (1) 12-crown-4 ligands to impart ion selectivity, (2) poly(ethylene oxide) side-chains to control water content, and (3) a crosslinker to form robust solids at room temperature. Single salt transport measurements indicate these materials exhibit unprecedented reverse permeability selectivity (~2.3) for LiCl over NaCl—the highest documented to date for a dense, water-swollen polymer. As demonstrated by molecular dynamics simulations, this behavior originates from the ability of 12-crown-4 to bind Na⁺ ions more strongly than Li⁺ in an aqueous environment, which reduces Na⁺ mobility and offsets the increase in Na⁺ solubility due to binding with crown ethers.    

Exploring Macroscopic Manifestations of Microscopic Gas Transport Pathways in Polymer-grafted Nanoparticle Membranes

Polymer nanocomposites have become increasingly relevant materials for use as efficient gas separation membranes in industrial processes. "Matrix-free" polymer-grafted nanoparticles, where all available polymer in the system is chemically tethered to the particle surface, have been explored in this context to overcome difficulties controlling nanoparticle dispersion. Membranes composed of these grafted particles have demonstrated remarkable enhancements in gas transport over that of neat polymer systems, where the degree of enhancement is greater for larger penetrant sizes. Techniques measuring macroscopic properties, such as permeability and activation energy, grant us insight into the microscopic gas transport mechanisms in these systems. These materials have been shown to be spatially heterogeneous, comprised of low-density interstitial regions, and areas of high polymer density close to the particle surface. This work aims to explore the relationship between observable macroscopic properties and proposed mechanisms for penetrant transport in these systems. The effects of preparation methods, penetrant size, and underlying structural formation are key to understanding such enhancements.   

3D Printed Adsorbers for Capturing 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. During intra-arterial chemotherapy infusion to a target organ, typically, more than 50-90% 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, built, and deployed porous adsorbers for capturing chemotherapy drugs before they spread through the body. The porosity was obtained by 3D printing of lattice structures. The surface of porous cylinders was coated with an ion-containing nanostructured block polymer which is responsible for capturing doxorubicin, a widely used chemotherapy drug with significant toxic side effects. Using a swine model, our initial design enables the capture of 69 % of the administered drug without any adverse effects. Additional improvement may be obtained by changing the chemical composition of the selective membrane layer and controlling the lattice structure and size with elastomers.   

Understanding Probe Diffusion in Dense Polymer Networks and the Roles of Mesh Size, Tg, and Segmental Dynamics

The effect of permanent crosslinks on probe diffusion was investigated in dense poly(butyl acrylate), varying the number of repeat units between crosslink junctions from 2-100 repeat units. Fluorescence Recovery After Photobleaching (FRAP) was used to determine the translational diffusion as a function of Tg-normalized temperature (Tg/T) and the size ratio between the dye and average distance between crosslink junctions (d/l). Dielectric spectroscopy was used to determine segmental relaxation times and help understand how probe diffusion was coupled to segmental mobility, as Tg and Tg-breadth significantly increased with decreasing repeat units between crosslink junctions. Probe diffusion exhibited a single exponential decay dependence on both Tg/T and d/l, seeing a two order of magnitude reduction in the diffusion coefficient over the range of crosslink density investigated. Effective diffusion time scales showed a weaker dependence on Tg/T and d/l than predicted from segmental relaxation times, indicating probe diffusion partially decoupled from the segmental mobility and that the increasing breadth may play a critical role in this decoupling.   

Revisiting Ion Transport in Nafion

Nafion is a charged polymer of significant interest as a fuel cell membrane due to rapid and selective transport of protons. Small molecule transport in Nafion has been explored in considerable detail over the past several decades and draws continued interest. However, this literature has evolved somewhat independently from other fields studying charged polymers (e.g., polyelectrolytes and water purification), with different models applied among these fields. Moreover, many Nafion studies focus on proton transport, while most studies of charged polymers in the membrane community focus on ions other than protons, given the importance of such ions to desalination. This presentation details experimental measurements and theoretical modeling of ion transport in Nafion, connecting teachings from several communities interested in transport through polymers. The importance of counter-ion condensation and ion speciation (i.e., ion pairing) on ion transport in charged polymers are highlighted. The development and applicability of a thermodynamic model, accounting for these phenomena, to predict ion sorption in Nafion without adjustable parameters is discussed. Together, these results provide critical fundamental insights for designing highly selective membrane materials.   

Controlling Nanoparticle Adsorption to Weak Polyelectrolyte Layer-by-Layer Films

Designing materials to control adsorption of nano-sized objects is essential in many applications including the separation and purification of proteins, nanoparticle (NP) synthesis, and water filtration. Essential to this task is an understanding of the underlying interactions that influence nanoparticle transport and adsorption. Using a model layer-by-layer (LbL) system of alternating poly(allyl amine hydrochloride) (PAH) and poly(acrylic acid) (PAA), we study the adsorption of 20 nm poly(ethylene glycol)-carboxylic acid functionalized gold NPs (Au-PEG_12-COOH NPs) to 50 nm LbL films on a silicon or glass substrates. By complimenting macroscopic quartz crystal microbalance with dissipation (QCM-D) with microscopic in situ, label-free interferometric scattering microscopy (iSCAT) and scanning electron microscopy (SEM) measurements, we demonstrate the ability to control adsorption by modifying the capping layer chemistry (PAA vs. PAH), adsorption experiment solution pH, and thermal crosslinking temperature of the LbL films prior to particle exposure and quantify effects of these variables on surface coverage, interaction mechanism, and kinetics of adsorption.   

Withdrawn

Establishing clear relationships between the chemical structure, water/ion transport, and water filtration performance in the salt selective polyamide (PA) layer of thin film composite (TFC) reverse osmosis (RO) is important for designing water filtration membranes, yet these relationships are difficult to establish. For instance, previous Ag⁺ probe Rutherford backscattering experiments by Coronell (https://doi.org/10.1021/es8002712) showed commercial polyamide membranes exhibit negligible amine populations, whereas ¹⁵N {¹H} NMR work by Qiu et al (https://doi.org/10.1016/j.memsci.2019.03.037) showed amines comprise ~10% of the nitrogen in a similar membrane.

 

In this talk, I will demonstrate a chemical separation method for purifying the PA from TFC membranes, the purity of which is quantified via ¹³C CPMAS NMR. Using this process, we report on four trimesoyl chloride (TMC)/isophthaloyl chloride (IPC)/metaphenylene diamine (MPD)-based TFC membranes in which the crosslink density was intentionally reduced by replacing trifunctional crosslinking TMC monomers with their linear IPC difunctional analog. The degree of crosslinking and fractions of unreacted carboxylic acid and amine polar groups are followed using ¹³C CPMAS NMR and FTIR. While the NMR shows a 2x decrease in crosslinking that causes a 30 % increase in salt passage, the addition of the difunctional analog leads to increased polar amine groups that reduce water permeance due to tighter binding of water in the membrane. Our results demonstrate that both crosslink density and polarity are important design criteria in RO membranes, and that ¹³C CPMAS is a powerful method for quantitatively monitoring such values. Furthermore, I will show results from ¹H field cycling and diffusometry NMR experiments in water-swollen samples from which we glean polymer segmental dynamics and water transport.   

Neutron spectroscopy of water dynamics and polymer relaxation in polyamide RO desalination membranes

We investigate model polyamide (PA) membranes with a combination of quasi-elastic and inelastic neutron scattering to elucidate the translational dynamics of confined water and the relaxation of the polymeric matrix under conditions relevant to reverse osmosis (RO). Specifically, PA membranes were synthesised by the interfacial polymerization of m-phenylenediamine (MPD) and trimesoyl chloride (TMC), and hydrated with both H₂O and D₂O to decouple the water dynamics from the polymer network relaxation. We hydrate the membranes with water vapor at precise RH conditions, in order to elucidate the translational dynamics of confined water. For comparison, we also measure PA membranes prepared by direct immersion in water and careful drying, in order to examine the relative contributions of confined and ‘bulk’ water. We isolate contributions from localized diffusion, translational jump-diffusion, and long-range diffusion mechanisms. Overall, our results quantify this multimodal diffusive nature of water in PA membranes that can be related to bulk transport via coarse-grained engineering models.