Two of the greatest challenges facing the 21st century involve providing sustainable supplies of clean water and energy, two highly interrelated resources, at affordable costs. Membrane technology is expected to continue to dominate the water purification technologies owing to its energy efficiency. However, there is a need for improved membranes that have higher flux, are more selective, are less prone to various types of fouling, and are more resistant to the chemical environment, especially chlorine, of these processes. This article summarizes the nature of the global water problem and reviews the state of the art of membrane technology. Existing deficiencies of current membranes and the opportunities to resolve them with innovative polymer chemistry and physics are identified. Extensive background is provided to help the reader understand the fundamental issues involved. Ph.D. students and two postdoctoral fellows performing fundamental research in gas and liquid separations using polymer membranes and barrier packaging. His research group focuses on include structure/property correlation development for desalination and vapor separation membrane materials, new materials for hydrogen separation and natural gas purification, nanocomposite membranes, reactive barrier packaging materials, and new materials for improving fouling resistance and permeation performance in liquid separation membranes. His research is described in more than 250 publications, and he has coedited four books on these topics. He has won a number of national awards for his research contributions, including the ACS
Salinity gradient energy technologies, such as reverse electrodialysis (RED) and capacitive mixing based on Donnan potential (Capmix CDP), could help address the global need for noncarbon-based energy. Anion exchange membranes (AEMs) are a key component in these systems, and improved AEMs are needed in order to optimize and extend salinity gradient energy technologies. We measured ionic resistance and permselectivity properties of quaternary ammonium-functionalized AEMs based on poly(sulfone) and poly(phenylene oxide) polymer backbones and developed structure−property relationships between the transport properties and the water content and fixed charge concentration of the membranes. Ion transport and ion exclusion properties depend on the volume fraction of water in the polymer membrane, and the chemical nature of the polymer itself can influence fine-tuning of the transport properties to obtain membranes with other useful properties, such as chemical and dimensional stability. The ionic resistance of the AEMs considered in this study decreased by more than 3 orders of magnitude (i.e., from 3900 to 1.6 Ω m) and the permselectivity decreased by 6% (i.e., from 0.91 to 0.85) as the volume fraction of water in the polymer was varied by a factor of 3.8 (i.e., from 0.1 to 0.38). Water content was used to rationalize a tradeoff relationship between the permselectivity and ionic resistance of these AEMs whereby polymers with higher water content tend to have lower ionic resistance and lower permselectivity. The correlation of ion transport properties with water volume fraction and fixed charge concentration is discussed with emphasis on the importance of considering water volume fraction when interpreting ion transport data.
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.
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