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

Spectroscopic Characterization of Sulfonate Charge Density in Ion-Containing Polymers

The charge density and hydrogen bonding with water of five different polymer membranes functionalized with various sulfonate side-chain chemistries were investigated using Fourier transform infrared (FTIR) techniques and density functional theory (DFT) calculations. The peak position of the OD stretch of dilute HOD absorbed into the sulfonated poly­(sulfone) membranes was studied using FTIR to compare the charge density of the sulfonate headgroup across the different samples, which can ultimately be related to the acidity of the proton-form sulfonate moieties. The OD peak was deconvoluted to determine the percentage of headgroup-associated, intermediate, and bulk water. DFT modeling was used to calculate the charge density of each headgroup and visualize how the chemistry of the headgroup influenced the conformation of the side-chain tether. FTIR-determined OD peak positions and charge density calculations demonstrated that a perflurosulfonate containing a thioether linkage produced the most acidic sulfonate headgroup. However, the amount of headgroup-associated water calculated for this side chain was low due to the unique cis conformation of the thioether side chain. The biperfluorosulfonate side chain had very low calculated headgroup-associated water due to its bulkiness and water molecules bridging the two sulfonates. These detailed insights on local hydration of sulfonate side chains will point towards new headgroup designs for advanced membranes

Fourier transform infrared spectroscopy investigation of water microenvironments in polyelectrolyte multilayers at varying temperatures

Polyelectrolyte multilayers (PEMs) are thin films formed by the alternating deposition of oppositely charged polyelectrolytes. Water plays an important role in influencing the physical properties of PEMs, as it can act both as a plasticizer and swelling agent. However, the way in which water molecules distribute around and hydrate ion pairs has not been fully quantified with respect to both temperature and ionic strength. Here, we examine the effects of temperature and ionic strength on the hydration microenvironments of fully immersed poly(diallyldimethylammonium)/polystyrene sulfonate (PDADMA/PSS) PEMs. This is accomplished by tracking the OD stretch peak using attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy at 0.25-1.5 M NaCl and 35-70 °C. The OD stretch peak is deconvoluted into three peaks: (1) high frequency water, which represents a tightly bound microenvironment, (2) low frequency water, which represents a loosely bound microenvironment, and (3) bulk water. In general, the majority of water absorbed into the PEM exists in a bound state, with little-to-no bulk water observed. Increasing temperature slightly reduces the amount of absorbed water, while addition of salt increases the amount of absorbed water. Finally, a van't Hoff analysis is applied to estimate the enthalpy (11-22 kJ mol-1) and entropy (48-79 kJ mol-1 K-1) of water exchanging from low to high frequency states.Peer reviewe

Creating cross-linked lamellar block copolymer supporting layers for biomimetic membranes

The long-standing goal in membrane development is creating materials with superior transport properties, including both high flux and high selectivity. These properties are common in biological membranes, and thus mimicking nature is a promising strategy towards improved membrane design. In previous studies, we have shown that artificial water channels can have excellent water transport abilities that are comparable to biological water channel proteins, aquaporins. In this study, we propose a strategy for incorporation of artificial channels that mimic biological channels into stable polymeric membranes. Specifically, we synthesized an amphiphilic triblock copolymer, poly(isoprene)-block-poly(ethylene oxide)-block-poly(isoprene), which is a high molecular weight synthetic analog of naturally occurring lipids in terms of its self-assembled structure. This polymer was used to build stacked membranes composed of self-assembled lamellae. The resulting membranes resemble layers of natural lipid bilayers in living systems, but with superior mechanical properties suitable for real-world applications. The procedures used to synthesize the triblock copolymer resulted in membranes with increased stability due to the crosslinkability of the hydrophobic domains. Furthermore, the introduction of bridging hydrophilic domains leads to the preservation of the stacked membrane structure when the membrane is in contact with water, something that is challenging for diblock lamellae that tend to swell, and delaminate in aqueous solutions. This new method of membrane fabrication offers a practical model for making channel-based biomimetic membranes, which may lead to technological applications in reverse osmosis, nanofiltration, and ultrafiltration membranes. © 2018 The Royal Society of Chemistry.11Nsciescopu

Elastic Long-Chain Multication Cross-Linked Anion Exchange Membranes

Anion exchange membranes (AEMs) are a promising class of materials that enable non-noble metals to be used as catalysts in fuel cells. Compared to their acidic counterparts, typically Nafion and other perfluorosulfonate-based membranes, the low OH^– conductivity in AEMs remains a concern as these materials are developed for practical applications. Cross-linked macromolecular structures are a popular way to optimize the trade-off between the ionic conductivity and the water swelling of AEMs with high ion exchange capacities (IECs). However, common cross-linked AEMs (e.g., <i>x</i>(QH)­QPPO) that have high degrees of cross-linking with low molecular weight between cross-links are usually mechanically brittle. Moreover, the cross-links in AEMs can hinder the transport of OH^–, leading to unsatisfactory conductivities. Here we report a series of elastic and highly conductive poly­(2,6-dimethyl­phenylene oxide) (PPO)-based AEMs (<i>x</i>(QH)₃QPPO) containing flexible, long-chain, multication cross-links. The strength and flexibility of the <i>x</i>(QH)₃QPPO samples are significantly improved as compared to the conventional <i>x</i>(QH)­QPPO membranes and multication un-cross-linked materials reported previously. The high conductivities in these new materials (<i>x</i>(QH)₃QPPO-40, IEC = 3.59 mmol/g, σ_OH^– = 110.2 mS/cm at 80 °C) are attributed to the distinct microphase separation observed in the <i>x</i>(QH)₃QPPO membranes by SAXS and TEM analyses. Furthermore, the <i>x</i>(QH)₃QPPO samples exhibit good dimensional (swelling ratio of <i>x</i>(QH)₃QPPO-40 is 25.0% at 80 °C) and chemical (22% and 25% decrease in IEC and OH^– conductivity in 1 M NaOH at 80 °C for 30 days, respectively) stabilities, making this cross-linking motif suitable for potential membrane applications in fuel cells and other electrochemical devices