Session S6: Polymeric Membranes

Ion Specific Effects in Charged Polymer Membranes for Water Purification

Global need for sustainable water and energy sources presents an opportunity for polymer science as many technologies for water purification and some emerging power generation technologies rely on polymeric membranes to control rates of water and ion transport. Ion exchange membranes are used in many of these technologies, and the presence of charged groups on the polymer backbone of such materials influences ion sorption and, subsequently, both rates of ion transport and the selective nature of ion transport. Membranes are often characterized using sodium chloride, which is widely prevalent in nature, but several applications expose membranes to a variety of different ions. For electric field-driven processes, ion transport properties, such as ionic resistance and electrochemical permselectivity, are sensitive to the specific nature of the ions present in the system. This presentation will discuss the influence of ion properties and ion-polymer interactions on the ion transport properties of polymer membrane materials. The observed ion specific effects can be understood in terms of both the thermodynamic and kinetic (mobility) contributors to the overall ion transport process.   

Ion Exchange Polymeric Coatings for Selective Capacitive Deionization

Capacitive deionization (CDI) is an energy-efficient technology for adsorbing and removing scalants and foulants from water by utilizing electric potential between porous carbon electrodes. Currently, industrial application of CDI is limited to low salinity waters due to the limited absorption capacities of carbon electrodes. However, CDI can potentially be used as a low-cost approach to selectively remove divalent ions from high salinity water. Divalent ions such as sulfonates and carbonates cause scaling and thus performance deterioration of membrane-based desalination systems. In this work, we investigated ion-exchange polymer coatings for use in a membrane capacitive deionization (MCDI) process for selective removal of divalent ions. Poly-Vinyl Alcohol (PVA) base polymer was crosslinked and charged using sulfo-succinic acid (SSA) to give a cation exchange layer. 50 um thick standalone polymer films had a permeability of 4.25*10$^{\mathrm{-7}}$~cm$^{\mathrm{2}}$/s for 10mM NaCl feed. Experiments on electrodes with as low as 10~$\upsilon $m thick coating of cation exchange polymer are under progress and will be evaluated on the basis of their selective salt removal efficiency and charge efficiency, and in future we will extend this work to sulfonated block copolymers and anion exchange polymers.   

Dimensional analysis of membrane distillation flux through fibrous membranes

We developed a dimensional-analysis-based empirical modeling method for membrane distillation (MD) flux that is adaptable for novel membrane structures. The method makes fewer simplifying assumptions about membrane pore geometry than existing theoretical (i.e. mechanistic) models, and allows selection of simple, easily-measureable membrane characteristics as structural parameters. Furthermore, the model does not require estimation of membrane surface temperatures; it accounts for convective heat transfer to the membrane surface without iterative fitting of mass and heat transfer equations. The Buckingham-Pi dimensional analysis method is tested for direct contact membrane distillation (DCMD) using non-woven/fibrous structures as the model membrane material. Twelve easily-measured variables to describe DCMD operating conditions, fluid properties, membrane structures, and flux were identified and combined into eight dimensionless parameters. These parameters were regressed using experimentally-collected data for multiple electrospun membrane types and DCMD system conditions, achieving R$^{\mathrm{2}}$ values \textgreater 95{\%}. We found that vapor flux through isotropic fibrous membranes can be estimated using only membrane thickness, solid fraction, and fiber diameter as structural parameters. Buckingham-Pi model DCMD flux predictions compare favorably with previously-developed empirical and theoretical models, and suggest this simple yet theoretically-grounded empirical modeling method can be used practically for predicting MD vapor flux from membrane structural parameters.   

Polymeric water filtration membranes

Nanofiltration (NF) membranes are used for separating salts and small neutral molecules. NF membranes show unique selectivity properties compared to reverse osmosis membranes as it can selectively pass monovalent salts and neutral molecules as a function of charge and molecular weight cut-off which are dependent on membrane characteristics and operating conditions. Dow Water and Process solutions has been a pioneer in the membrane based water purification field and Dow's role was instrumental in developing several NF membranes for different applications. However, the characterization of NF membranes and hence the development of structure-property relationship is challenging due to the nanoscale thin, crosslinked nature of the membrane. Recently significant efforts were employed to develop analytical capabilities to understand polymer structure and composition and it had been possible to achieve a structure-property relationship for NF membranes. This paper will highlight similar relationships and will also focus on the relationships of membrane structure with membrane transport properties and how this relationship influences products for different application areas such as in oil field, sweetener and minimum liquid discharge etc.   

Molecular Dynamics of Adsorption: Interaction between Benzoic Acid in Solution and Functionalized Carbon Nanotubes

Adsorption of organic contaminants is among the promising applications of carbon nanotubes that have provoked academic and industrial interest. The current work investigates, through molecular dynamics simulations, filtration of benzoic acid-contaminated water by use of single-walled carbon nanotubes. Such a study inevitably resides at the frontier between molecular and environmental research. Extensive theoretical and experimental studies have shown that adsorption on nanotubes is characterized by the presence of large specific surface areas and the availability of different adsorption sites on the nanotubes. In keeping with this understanding we have carboxylated the carbon nanotubes at different percentages relative to the number of the nanotube carbons. The variability of hydrogen bonding, as a function of radial distance from nanotube, between the different hydrogen containing species in our system is reported. Benchmark values of tube length, tube radius, and concentration of benzoic acid in water were used to examine adsorption properties. The observations were further extended to make recommendations on the scalability of nanotube filtration to input values not explored in the present work.      

Structure and Properties of Superhydrophobic membranes of Poly(vinylidene fluoride) and Poly(methyl methacrylate)-r-1H,1H,2H,2H-perfluorodecyl methacrylate

We are studying superhydrophobic membranes for potential applications in oil-water separations. The membranes are blends comprising poly(vinylidene fluoride), PVDF, and a random co-polymer of poly(methyl methacrylate) and 1H,1H,2H,2H-perfluorodecyl methacrylate. PVDF imparts mechanical strength to the membrane, while the copolymer, with highly fluorinated side groups, forms crystals that enhance membrane roughness and hydrophobicity. Composition was varied by controlling the PVDF content of the blends, including 100, 75, 50, 25, and 0 wt. {\%} PVDF. The properties of bulk materials were studied using X-ray diffraction, thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC). The blend's crystallinity and degradation temperature both decreased as the copolymer content increased, as shown by X-ray and TGA respectively. Using fast scanning chip-based calorimetry, spin-cast thin films were heated and cooled at rates up to 2000 K/s. Homogeneous crystal nucleation of ??-phase PVDF crystallites was observed with cooling rates above 500 K/s. Contact angle measurements confirm that the blends are superhydrophobic. Future work will be reported on the formation and properties of electrospun fiber membranes of these materials.   

Weak polyelectrolytes in Confined Geometries

Crucial to the behavior of recently designed charge-rejection and mosaic membranes are the conformations of polyelectrolyte brushes and oligomeric grafts used to control the membranes' surface charge. The use of pH-tunable weak polyelectrolytes with associative interactions enables fine tuning of material transport properties. Here, we apply constant-pH molecular dynamics along with free energy sampling algorithms to understand the subtle tug-of-war between pH, salt concentrations, and solvation forces in confined systems, and determine how each of these effects alters transport within the system. We further discuss the implications of our findings for the design of electrolyte separation membranes.   

Permeability Enhancement in Polymer-Grafted Nanoparticle Membranes with Controllable Free-Volume

Polymer based membranes could play a key role in several industrially important gas separations, for example, separating CO$_{\mathrm{2}}$ from natural gas, with enormous economic and environmental impact. Here we demonstrate a strategy to rationally create hybrid membranes of nanoparticles grafted with polymer. In a novel PMA-grafted silica (d$=$14nm) system, membrane permeability as well as material ``free volume'' is enhanced relative to neat PMA; this is shown through a combination of permeation experiments and a Quartz Crystal Microbalance setup. These equilibrium constructs offer the ability to tune membrane performance through variations in polymer graft density and chain length, while also avoiding the deleterious effect of polymer aging. In addition, the transport properties of PMMA-grafted silica are explored to demonstrate the broad application of these novel grafted systems to different polymer chains with higher intrinsic ``free volume''. Evidently, these hybrid materials are a new template for designing membranes with readily tunable abilities that may be optimized for vital gas separations.   

Chemical characterization of solid polymer electrolyte membrane surfaces in LiFePO$_{\mathrm{4}}$ half-cells.

High temperature (60 \textdegree C) capacity retention of succinonitrile plasticized solid polymer electrolyte membrane (PEM) in a LiFePO$_{\mathrm{4}}$ half-cell was investigated with or without lithium bis(oxalato)borate (LiBOB) modification. Various symmetric cells and half-cells were studied under different thermal and electrochemical conditions. At room temperature cycling, the unmodified PEM in the half-cell appeared stable up to 50 cycles tested. Upon cycling at 60 \textdegree C, the capacity decays rapidly and concurrently the cell resistance increased. The chemical compositions of the solid PEM surfaces on both cathode and anode sides were analyzed. New IR bands (including those belonged to amide) were discerned on the unmodified PEM surface of the Li electrode side at 60 \textdegree C suggestive of side reaction, but no new bands develop during room temperature cycling. To our astonishment, the side reaction was effectively suppressed upon LiBOB addition (0.4 wt{\%}) into the PEM, contributing to increased high temperature capacity retention at 60\textdegree C. Plausible mechanisms of capacity fading and improved cycling performance due to LiBOB modification are discussed.   

Characterizing the Swelling of a Crosslinked Organosilicon Polymer

A hydrophobic crosslinked organosilicon polymer, known by the trade name Osorb, absorbs many times its own weight in liquid- and vapor-phase organic solvents. Its Si---O---Si linkage is likely flexible, analogously to physical and spectroscopic measurements in various forms of silica. Infrared spectroscopy is commonly used due to the sensitivity of certain vibrational modes to changes in geometry and environment. We hypothesized that the Si---O---Si bond angle within the Osorb matrix changes when the material swells upon exposure to organic solvents. Density functional theory calculations on a small-molecule model system qualitatively agree with the central force model of glassy solids, which relates the IR vibrational frequency of the system to the angle and force constant of this bond. Treatment of various Osorb samples with decane consistently causes a moderate increase in the frequency of the asymmetric stretching band around 1100 cm$^{\mathrm{-1}}$. However, the spectroscopic changes do not correlate with the swell capacity of the material. In fact, some low-swelling samples showed the same changes in frequency as high-swelling samples, thus demonstrating that the swelling of Osorb is not due to changes in the Si---O---Si bond angle.   

Effects of Humidity on the Dynamics of Hydrophilic and Hydrophobic Polymers

Poly(styrene)-block-poly(ethylene oxide) (PS-$b$-PEO) is a semicrystalline block copolymer (BCP) with interesting properties. It is mechanically tough, amphiphilic, and has a polar phase. The mechanical toughness is due to the crystallinity of PEO and the high glass transition temperature of PS, as well as the morphological structure of the BCP. Due to our interest in membranes for energy-efficient water purification, we have examined the effect of water on polymer properties. We report the effect of water activity on thermal and mechanical properties of the BCP, PEO, and PS. Using a variety of experimental techniques we have found that water sorption induces isothermal dissolution of PEO crystals and it decreases the apparent glass transition temperature of PS.   

A Particle-Field Hybrid Simulation Method for Studying Strong Correlations in Polymeric Systems

The presence of strongly correlated interactions in polymeric materials, such as hydrogen bonding, ionomers, and coordinative bonds, can drastically alter materials' mesoscale structural and dynamical properties. Examples include supramolecular polymeric assemblies and micro-phase separation of ion-containing block copolymers, etc. Understanding of such materials demands computational methods that can faithfully capture strongly correlated interactions, and have also access to materials' mesoscopic behaviors. Among existing approaches, particle-based methods tend to be highly computationally demanding, while field-based methods tend to undercount essential correlations and fluctuations. With these limitations in mind, here we propose a new simulation formalism that is based on expanding the single chain in mean field (SCMF) scheme [1]. As a hybrid particle-field method, the new formalism offers the degree of freedom of separating interactions into ones to be treated using mean-field representations, and those to be preserved in particle-based representations. By doing so, the formalism combines the strength of both approaches via efficient calculation of interactions in dense systems, while being able to capture correlation and fluctuation effects due to interactions of particular interests. We illustrate the approach in the context of two examples. The accuracy and computational advantage of the new formalism are assessed by direct comparisons with Monte Carlo simulations. [1] J. Chem. Phys., 125, 184904 (2006).   

The role of Swelling in Diffusion-Controlled Drug Release: a Lattice Monte Carlo Approach

A common approach used to control drug release rate consists in encapsulating the drug molecules inside a hydrogel (a network of hydrophilic cross-linked polymers). Placed in aqueous media or under the action of an external stimuli, the hydrogel can swell. This phenomenon leads to a non-trivial relation between the swelling of the hydrogel matrix, the diffusion properties of the drug molecules and the performance of a drug delivery system. We propose to investigate this problem using a novel Lattice Monte Carlo approach. In our LMC scheme, both the diffusion coefficient of the drug molecules and the lattice step size evolve in time as the water penetrates inside the system. Due to the resulting time- and space-dependent diffusivity, there are different interpretations of the stochastic term in the relevant overdamped Langevin equation (i.e ``Ito-Stratonovich dilemma''). First, we examine how each calculus changes the release properties of the system in both the swelling-controlled and the diffusion-controlled limits. Second, we discuss the validity of the empirical mathematic models often used to fit the release data. \newline