Session B04: Membrane-Based Separation Processes

Live

Biomimetic pore embedded membranes can mimic highly efficient water filtration processes occurring in nature. In this work, we simulate peptide appended pillar[5]arene (PAP[5]) pore molecules embedded in polybutadiene-polyethylene oxide (PB-PEO) membranes, which imitate aquaporin water channels in cell membranes. These systems offer a possible high-permeability alternative to conventional reverse osmosis membranes. Because of their rigid pore dimension and surface chemistry, PAP[5] molecules have excellent water permeability and high selectivity. In this study, we systematically varied the PB and the PEO block lengths to discover the effect of membrane architecture on water mobility. We measured short-time water diffusivity in these designs and obtained a diffusivity comparable to that inferred from experiments. Further, a long-time water diffusivity for the best design was computed from the random entrances and exits of water molecules from the PAP[5], which was in good agreement with the short-time diffusivity. Finally, to quantify the ion rejection characteristics of the PAP[5] molecule, we measured the work to drag Na⁺ and Cl^- ions into and out of the pore. Our simulation techniques can be readily applied to other candidate structures, to aid in pore design.   

Live

Nanoporous metal–organic frameworks are promising for gas capture, separation, and storage due to their high surface area and tunable chemistry and are amenable to incorporation into membranes. The framework Cu^I-MFU-4l, which contains coordinatively unsaturated Cu^I centers, can engage in backbonding interactions with small molecule guests. We use in situ near edge X-ray absorption fine structure spectroscopy coupled with theory to examine several gases expected to bind to the open Cu^I sites via different electronic interactions, including σ-donation and π-backbonding. We show that in situ Cu L-edge X-ray absorption spectroscopy of Cu^I-MFU-4l in the presence of a gas can elucidate the π-backbonding contributions by directly probing excitations to unoccupied backbonding orbitals with Cu d-character, even for gases where other interactions, such as ligand-to-metal σ-donation, dominate. First-principles calculations based on time-dependent density functional theory reveal the backbonding molecular orbitals associated with these spectroscopic transitions. The energies of the transitions correlate with the energy levels of the isolated small molecule adsorbate, and the transition intensities are proportional to the binding energies of the guest molecules with Cu^I-MFU-4l.   

Live

Lignin-containing hydrogels have garnered attention for use in a variety of aqueous-based separations as lignin is an abundant biopolymer with a high concentration of hydroxyl groups which can be utilized as crosslinking sites during hydrogel fabrication. However, to date, little is understood regarding how the addition of lignins alters the network structure of these composite hydrogels. Herein, a novel series of lignin–poly(vinyl alcohol) (PVA) composite hydrogels were synthesized utilizing ultraclean lignins (UCLs) of well-defined molecular weights (MWs) and low dispersity. The UCLs were obtained via fractionation of crude bulk lignins. The network structures of the hydrated composites were characterized via small angle neutron scattering where a Modified Lorentzian Power Law model was used to obtain correlation lengths for the composite hydrogels. The permeability of various pollutants (e.g., methylene blue, bovine serum albumin) through the hydrated composites was measured via ultraviolet-visible spectroscopy, where penetrant permeability was dependent on the MW of both the lignins and PVA, and the concentration of crosslinking agents utilized during membrane fabrication. In addition, mechanical indentation was used to characterize Young’s modulus of the hydrogel composites.   

Live

Polymer nanocomposites have become increasingly useful materials for a diverse set of applications, including as industrial gas transport membranes for separations processes. Difficulty controlling nanoparticle dispersion in these nanocomposites has led to the use of polymer-grafted nanoparticles in a “matrix-free” configuration, where all polymer present in the system is chemically tethered to the surface of the nanoparticles. Materials composed from these grafted particles have been shown to display a remarkable enhancement in the gas transport properties of these systems. It has been proposed that the activation energy for penetrant motion through a polymeric membrane scales as the square of penetrant kinetic diameter, and can be related to a material-dependent critical size parameter. This work investigates macroscopic manifestations of this microscopic gas transport mechanism in grafted nanoparticle systems.   

Live

Block copolymers self-assemble a variety of morphologies useful as porous water-treatment membranes. A key challenge is to determine which morphologies maximize the flux of water while selectively rejecting contaminants. Here, we generate equilibrium and nonequilibrium ABC triblock copolymer morphologies using self-consistent field theory (SCFT) and use them in a kinetic Monte Carlo (kMC) model for solute diffusion. Our model excludes transport through the A-block membrane matrix, confining it to the B-block brush that expands when the sacrificial C-block is removed to establish a pore network. These effects are embedded in the SCFT and kMC workflow to examine the roles of mesoscale morphology and internal pore structure in determining solute diffusion rates in porous block copolymer membranes.   

Live

Polymers of intrinsic microporosity (PIMs) have outstanding gas permeabilities and adequate selectivities. These properties result from the rigid and contorted backbone structure of PIMs, which produce significant non-equilibrium free volume as these polymers vitrify during solvent casting. Unfortunately, it is difficult to manipulate and predict the nascent free volume architecture of these polymers. This presentation will describe two alternative strategies to generate high free volume polymers. The first strategy is to construct bottlebrush polymers with pre-designed side chains that can generate free volume. In this bottom-up approach, rigid side chains are tethered to flexible backbones to generate a new class of PIM materials. The second strategy is a top-down approach, where a polymer is pre-functionalized with a labile protecting group. After casting, in situ solid-state deprotection and crosslinking significantly increases free volume and tightens free volume distribution, resulting in enhanced permeability and selectivity for various gas pairs.   

Live

Using hydrodynamic simulations, we examine the behavior of single polymers flowing through model three-dimensional porous media (face-centered cubic close-packed colloidal crystal), where we observed that solvent properties and flow rates play an import role for polymer transport. In good solvent or high flow rates, the transport dynamics are similar to DNA gel electrophoresis, that is, the polymers showed size dependent Ogston sieving velocity for L_c/L < ~1 and size independent biased reptation velocity for L_c/L > ~1 (here L_c is the polymer contour length and L is the diameter of colloids that forms the porous media). Importantly, in bad solvent and low flow rates, the polymers showed an extra window of size dependent velocity for ~1 < L_c/L < ~2, which, to the best of our knowledge, has not been reported. In this regime, the polymer transport is controlled by a globule-stretch transition at pore throats, and the transport velocity is much slower than reptation. These findings are important for understanding the mechanical entrapment when using polymers for subsurface applications such as polymer flooding, and to design new ways for the separation of (bio)macromolecules.   

Live

Mixed electron/ion transport in organic materials is attracting great attention due to the possibilities in applications such as electrochemical transistors, bioelectronics, sensors, and soft robotics. PEDOT:PSS shows promise as a mixed conductor with a morphology consisting of PEDOT-rich gel particles embedded within a PSS matrix. Our previous study reported a fast and a slow ion mobility channels. Here we show that the faster ion mobility occurs in a PSS-rich top layer. We used different ion barrier layers of increasing hydrophobicity to show high correlation with the measured ion mobility. We also removed the top PSS layer through water sonicating at different stages of film crosslinking which again confirms linear relation between the contact angle of the PEDOT channel and mobility. The results reveal the measured ion mobility can be skewed without taking the PSS top layer into effect. Additionally, we will discuss our investigation toward resolving where the ion goes in the nano structure during doping through resonant soft X-ray scattering measurements. Understanding the correlation between structure and ion transport to transduce an electric signal will enable these devices to potentially use in next generation medical advancement.   

Live

Block copolymers (BCPs) are promising materials for many membrane applications such as ultrafiltration (UF) membranes for water treatment due to their ability to self-assemble into highly ordered structures with uniform pore size and high pore density. Typically, the pore’s size and surface interactions are controlled by the BCP chemistry. In this study, we aim to tailor the size and chemistry of BCP based UF membranes by selectively growing metal oxides inside the pores. PS-b-P4VP BCP were used to fabricate UF membranes in a process combining self-assembly with non-solvent induced phase separation (SNIPS). This results in an integral but asymmetric membrane, with ordered pores at the top of the membrane and sponge-like mechanically robust support layer at the bottom of the membrane. Sequential infiltration synthesis (SIS), a technique that enables selective growth of metal oxides inside of BCP, was used to grow AlO_x inside the P4VP domains of the membranes. By growing metal oxides in the pores, the pore size was reduced, enhancing membrane selectivity.   

Live

Despite the technological innovations spurred by the advent of the lithium-ion battery, lithium production still utilizes antiquated solar evaporation processes that are inefficient, environmentally deleterious, and lack the scalability required to meet projected demand. Direct lithium extraction via membrane separations could revolutionize lithium salt production, but progress in this area is limited by the lack of monovalent ion selectivity exhibited by conventional polymeric membranes.

Recently, a 12-crown-4 functionalized polymer membrane was shown to have one of the largest lithium/sodium permselectivities observed to date. Using atomistic molecular dynamics simulations, we reveal that this selectivity is due to strong interactions between sodium ions and 12-crown-4. These interactions decrease the diffusivity of sodium ions while leaving lithium ions relatively unaffected. We show that diffusive selectivity for lithium over sodium in the membrane persists across a range of water volume fractions and identify several parameters to guide the design of future lithium/sodium reverse-selective membranes.   

Live

Markov state models (MSMs) have found widespread use in interpreting conformation changes of proteins and understanding the complex molecular rearrangements necessary for drug delivery. By discretizing structural data from molecular simulations into several stable and metastable states, fundamental processes underlying a larger process may be elucidated. Here, we explore the use of Markov State Models in constructing free energy landscapes defined on important collective variables. In particular, we explore particle transport applications through porous media where the MSM is utilized within an unbiased advanced sampling method to obtain effective free energies and elucidate the fundamental processes limiting transport. The resulting method is efficient, accurate, and enables massively parallel data acquisition and simple error estimation. We will further discuss extensions of the method to driven, nonequilibrium processes such as solute nanofiltration.   

Live

Gas separation based on polymeric materials has been well established due to low cost. However, there is a trade-off between selectivity and permeability which impede its practical application. Among promising materials, poly(ionic liquids) (PolyILs) composed of anion and cation ions in the system shows good mechanical stability, easy processability, and higher CO₂ absorption.
The mobility of either cation or anion in PolyILs can affect its gas transport because the gas transport properties can be controlled by changing chemistry and flexibility of the polymer chains which lead to the control over free volume. We, therefore, prepare two sets of PolyILs through RAFT polymerization to understand the structural effect of PolyILs for gas transport. Both synthesized PolyILs have the same chemical configuration composed of imidazolium and (trifluoromethane)sulfonamide component, but mobile ion is different for each system.
We found that imidazolium-based PolyILs with (trifluoromethane)sulfonamide as mobile ion have higher CO₂ and helium gas permeability compared to the other PolyILs configuration, resulting in higher selectivity of CO₂/N₂ and He/N₂. Mechanical studies were performed to understand gas transport. We also incorporated MOFs particle to improve CO₂ permeability.   

On Demand

All-cellulose membranes, developed entirely from natural biomass resources, have untapped potentials for a wide range of water purification applications, including wastewater treatment. In this study, we prepared water-resistant but super hydrophilic all-cellulose membranes with high porosity (~80%). The demonstrated membrane system consists of a Lyocell microfiber scaffold infused with cellulose nanofibers (CNF) crosslinked by polyamideamine-epichlorohydrin (PAE); where the system showed good mechanical strength (wet stress: 3.5 - 8.0 MPa), pH resistance, and stability in hot water. The optimized all-cellulose membrane exhibited high permeation flux (8.8 ± 1.5 L/m²×h×psi), excellent separation efficiency (> 99.9%), good flux recovery ratio (> 95%), and self-healing property for wastewater filtration, compared with the commercial polymeric membranes such as polyvinylidene difluoride (PVDF) and polyether sulfone (PES) membranes. Moreover, the fouling mechanism was investigated by the resistance-in-series and three combined cake-filtration models. This study illustrated the promising potential of using all-cellulose membranes for high-efficient wastewater treatment and its superior antifouling performance compared to existing commercial ultrafiltration membranes.