Influence of polymer backbone rigidity on water and salt transport properties of low water content membrane polymers for desalination

Providing sustainable supplies of purified water and energy is a critical global challenge for the future, and polymer membranes will play a key role in addressing these clear and pressing global needs for water and energy. Polymer membrane-based processes dominate the desalination market because they are more energy efficient than thermal desalination processes, and polymer membranes are crucial components in several rapidly developing power generation and storage applications that rely on membranes to control rates of water and/or ion transport. Much remains unknown about the influence of polymer structure on even basic intrinsic water and ion transport properties, and these relationships must be developed to design next generation polymer membrane materials. For desalination applications, polymers with simultaneously high water permeability and low salt permeability are desirable in order to prepare selective membranes that can efficiently desalinate water, and a tradeoff relationship between water/salt selectivity and water permeability provides a benchmarking tool for evaluating the intrinsic water and salt transport properties of membrane polymers. The tradeoff relationship between water/salt permeability selectivity and water permeability suggests that both sorption and diffusion selectivity properties contribute significantly to water/salt permeability selectivity. The diffusivity tradeoff relationship can be related to free volume and/or polymer water content using established theories. Desalination membrane polymers often contain substantially less water compared to their hydrogel or ion exchange membrane counterparts. For example, cross-linked polyamide desalination membranes, prepared via interfacial polymerization, often absorb less than 10% water (by mass), though this value is sensitive to the specific chemistry and preparation procedure used to prepare the membrane. Transport in highly hydrated ion exchange membranes is often considered to depend more on the mobility of the bulk water sorbed in the polymer than the mobility of the polymer chains. As water content decreases, however, the role of polymer backbone rigidity may become increasingly important as water-polymer interactions become increasingly important relative to bulk sorbed water interactions. To determine the influence of polymer backbone rigidity on water and ion transport in low water content polymers, a series of model polymers were developed where backbone rigidity could be controlled in addition to controlling water content and ion sorption properties. These chemically similar hydrophilic polymers, based on acrylic and methacrylic backbones, have different segmental dynamics, as characterized by the glass transition temperature of the hydrated polymers. Ion and water transport property measurements reveal that, at equivalent water content, polymers with slower segmental dynamics are more diffusion selective than those polymers with more rapid segmental dynamics. Additionally, the more rigid backbone polymers appear to be more size selective than the more flexible backbone polymers at comparable water content. These results suggest that polymer backbone rigidity affects water and salt transport properties of low-water content polymers where water-polymer interactions are likely significant, and they provide fundamental structure-property insight into water and salt transport in hydrated polymers of interest for membrane-based desalination applications

Assembling a Natural Small Molecule into a Supramolecular Network with High Structural Order and Dynamic Functions

Programming the hierarchical self-assembly of small molecules has been a fundamental topic of great significance in biological systems and artificial supramolecular systems. Precise and highly programmed self-assembly can produce supramolecular architectures with distinct structural features. However, it still remains a challenge how to precisely control the self-assembly pathway in a desirable way by introducing abundant structural information into a limited molecular backbone. Here we disclose a strategy that directs the hierarchical self-assembly of sodium thioctate, a small molecule of biological origin, into a highly ordered supramolecular layered network. By combining the unique dynamic covalent ring-opening-polymerization of sodium thioctate and an evaporation-induced interfacial confinement effect, we precisely direct the dynamic supramolecular self-assembly of this simple small molecule in a scheduled hierarchical pathway, resulting in a layered structure with long-range order at both macroscopic and molecular scales, which is revealed by small-angle and wide-angle X-ray scattering technologies. The resulting supramolecular layers are found to be able to bind water molecules as structural water, which works as an interlayer lubricant to modulate the material properties, such as mechanical performance, self-healing capability, and actuating function. Analogous to many reversibly self-assembled biological systems, the highly dynamic polymeric network can be degraded into monomers and reformed by a water-mediated route, exhibiting full recyclability in a facile, mild, and environmentally friendly way. This approach for assembling commercial small molecules into structurally complex materials paves the way for low-cost functional supramolecular materials based on synthetically simple procedures

The Role of Experimental Factors in Membrane Permselectivity Measurements

The apparent permselectivity of a membrane is a critical ion transport property that influences the efficiency of electric field-driven membrane technologies and often is measured using a pseudo-steady-state measurement technique. In some cases, nonphysical apparent permselectivity values (greater than unity) are attributed to experimental uncertainty factors. The accuracy of the apparent permselectivity measurement can be influenced by variations in temperature, inaccurate solution concentrations, and fluctuations in membrane potential measurements, and these sources of uncertainty and their magnitudes were investigated both experimentally and using error analysis. Temperature had a small influence on the apparent permselectivity properties of two commercially available cation exchange membranes, as the value of the apparent permselectivity decreased by approximately 2% as the temperature increased from 14 to 31 °C. Membrane potential measurement fluctuations contributed approximately 0.2–0.5% uncertainty to the apparent permselectivity measurement. Deviations from target sodium chloride solution concentrations of 10 ppm introduced approximately 0.015–0.1% error, respectively, in apparent permselectivity. The magnitudes of these uncertainties typically are comparable to the magnitude of the measurement variability associated with disassembling and reassembling the measurement cell between replicate measurements made on the same sample, so the overall influence of the experimental factors considered in this study on apparent permselectivity is expected to be generally small

Counterion Mobility in Ion-Exchange Membranes: Spatial Effect and Valency-Dependent Electrostatic Interaction

Enhancing the selectivity of ion-exchange membranes (IEMs) is an important need for environmental separations but is hindered by insufficient understanding of the fundamental transport phenomena. Specifically, existing models do not adequately explain the order of magnitude disparity in diffusivities of mono-, di-, and trivalent ions within the membranes. In this study, a transport framework is presented to describe counterion migration mobility using an analytical expression based on first-principles. The two governing mechanisms are spatial effect of available fractional volume for ion transport and electrostatic interaction between mobile ions and fixed charges. Mobilities of counterions with different valencies were experimentally characterized and shown to have high R2s in regression analyses with the proposed transport model. The influence of membrane swelling caused by different counterions was further accounted for to better model the spatial effect. The frictional effect of electrostatic interaction was quantitatively linked to the membrane structural and electrical properties of fixed charged density and dielectric constant. Additionally, the anion-exchange membrane exhibited a weaker electrostatic effect compared to cation-exchange membranes, which was attributed to steric hindrance caused by hydrocarbon chains of the quaternary amine functional groups. The insights offered in this study can inform the rational development of IEMs and membrane processes for ion-specific separations

Increased Hydrogel Swelling Induced by Absorption of Small Molecules

The water and small molecule uptake behavior of amphiphilic diacrylate terminated poly­(dimethylsiloxane) (PDMSDA)/poly­(ethylene glycol diacrylate) (PEGDA) cross-linked hydrogels were studied using attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy. These hydrogel networks absorbed more water as the PEGDA content of the network increased. In contrast to typical osmotic deswelling behavior that occurs when liquid water equilibrated hydrogels are immersed in small molecule solutions with water activities less than unity, water-swollen gels immersed in 2-acrylamido-2-methylpropanesulfonic acid (AMPS-H) solutions rapidly regained their water content within 4 min following an initial deswelling response. In situ ATR-FTIR analysis of the hydrogel film during the dynamic swelling experiment indicated that small molecule absorption into the gel played an important role in inducing gel reswelling in low water activity solutions. This aspect of polymer gel water uptake and interaction with small molecules is important for optimizing hydrogel coatings and hydrophilic polymer applications where there is an interaction between the internal chemical structure of the gel and electrolytes or other molecules in solution

Water and Salt Transport Properties of Triptycene-Containing Sulfonated Polysulfone Materials for Desalination Membrane Applications

A series of triptycene-containing sulfonated polysulfone (TRP-BP) materials was prepared via condensation polymerization, and the desalination membrane-relevant fundamental water and salt transport properties (i.e., sorption, diffusion, and permeability coefficients) of the polymers were characterized. Incorporating triptycene into sulfonated polysulfone increased the water content of the material compared to sulfonated polysulfone materials that do not contain triptycene. No significant difference in salt sorption was observed between TRP-BP membranes and other sulfonated polysulfone membranes, suggesting that the presence of triptycene in the polymer did not dramatically affect thermodynamic interactions between salt and the polymer. Both water and salt diffusion coefficients in the TRP-BP membranes were suppressed relative to other sulfonated polysulfone materials with comparable water content, and these phenomena may result from the influence of triptycene on polymer chain packing and/or free-volume distribution, which could increase the tortuosity of the transport pathways in the polymers. Enhanced water/salt diffusivity selectivity was observed for some of the TRP-BP membranes relative to those materials that did not contain triptycene, and correspondingly, incorporation of triptycene into sulfonated polysulfone resulted in an increase, particularly for acid counterion form TRP-BP materials, in water/salt permeability selectivity, which is favorable for desalination membrane applications