Lithium is widely used in contemporary energy applications, but its isolation from natural reserves is plagued by time-consuming and costly processes. While polymer membranes could, in principle, circumvent these challenges by efficiently extracting lithium from aqueous solutions, they usually exhibit poor ion-specific selectivity. Toward this end, we have incorporated host–guest interactions into a tunable polynorbornene network by copolymerizing 1) 12-crown-4 ligands to impart ion selectivity, 2) poly(ethylene oxide) side chains to control water content, and 3) a crosslinker to form robust solids at room temperature. Single salt transport measurements indicate these materials exhibit unprecedented reverse permeability selectivity (∼2.3) for LiCl over NaCl—the highest documented to date for a dense, water-swollen polymer. As demonstrated by molecular dynamics simulations, this behavior originates from the ability of 12-crown-4 to bind Na+ ions more strongly than Li+ in an aqueous environment, which reduces Na+ mobility (relative to Li+) and offsets the increase in Na+ solubility due to binding with crown ethers. Under mixed salt conditions, 12-crown-4 functionalized membranes showed identical solubility selectivity relative to single salt conditions; however, the permeability and diffusivity selectivity of LiCl over NaCl decreased, presumably due to flux coupling. These results reveal insights for designing advanced membranes with solute-specific selectivity by utilizing host–guest interactions.

Polymerization-induced structural transitions have gained attention recently due to the ease of creating and modifying nanostructured materials with controlled morphologies and length scales. Here, we show that order–order and disorder–order nanostructural transitions are possible using in situ polymer grafting from the diblock polymer, poly­(styrene)-block-poly­(butadiene). In our approach, we are able to control the resulting nanostructure (lamellar, hexagonally packed cylinders, and disordered spheres) by changing the initial block polymer/monomer ratio. The nanostructural transition occurs by a grafting from mechanism in which poly­(styrene) chains are initiated from the poly­(butadiene) block via the creation of an allylic radical, which increases the overall molecular weight and the poly­(styrene) volume fraction. The work presented here highlights how the chemical process of converting standard linear diblock copolymers to grafted block polymers drives interesting and controllable polymerization-induced morphology transitions.

Direct lithium extraction via membrane separations has been fundamentally limited by lack of monovalent ion selectivity exhibited by conventional polymeric membranes, particularly between sodium and lithium ions. Recently, a 12-Crown-4-functionalized polynorbornene membrane was shown to have the largest lithium/sodium permeability selectivity observed in a fully aqueous system to date. Using atomistic molecular dynamics simulations, we reveal that this selectivity is due to strong interactions between sodium ions and 12-Crown-4 moieties, which reduce sodium ion diffusivity while leaving lithium ion mobility relatively unaffected. Moreover, the ion diffusivities in the membrane, when scaled by their respective solution diffusivities and free ion fractions, can be collapsed to an almost universal relationship depending on solvent volume fraction.

The influence of dynamical ion–ion correlations and ion pairing on salt transport in ion exchange membranes remain poorly understood. In this study, we use the framework of Onsager transport coefficients within atomistic molecular dynamics simulations to study the impact of ion–ion correlated motion on salt transport in hydrated polystyrene sulfonate membranes and compare with the results from aqueous salt solutions. At sufficiently high salt concentrations, cation–anion dynamical correlations exert a significant influence on both salt diffusivities and conductivities. Anion–anion distinct correlations, arising from the imbalance between the concentration of free (mobile) cations and anions, and the retarding effect of the fixed charge groups on cations, proves to be an additional important feature for polymer membranes. Our results demonstrate that dynamical correlations should become an important consideration in experimental measurements of salt diffusivities and conductivities for non-dilute salt solutions in polymer membranes.

The simultaneous use of nonequilibrium reaction processing and complex macromolecular architecture is an exciting way to achieve nanostructures that are not easily accessible via standard static block polymer self-assembly. Previous work has shown that the polymerization of styrene in the presence of a poly(styrene)-block-poly(butadiene) (PS-PBD) diblock copolymer induces a nanostructural transition from a lamellar (LAM) to a hexagonally packed cylinder (HEX) morphology. The transition was found to be driven by in situ PS grafting from the PBD block, which transforms the PS-PBD coil−coil diblock copolymer to a poly(styrene)-block-[poly(butadiene)-graf t-poly(styrene)] (PS-b-PBD-g-PS) coil−comb block polymer. In situ small-angle X-ray scattering and oscillatory shear dynamic mechanical spectroscopy measurements show that the order−order transition is not a simple epitaxial transition seen in prototypical block polymers, but undergoes a complex phase path in which the starting LAM phase at room temperature before polymerization initially disorders at elevated temperatures, evolves from a disordered phase to what is presumed to be a hexagonally perforated lamellae phase during the polymerization, and then transitions to a HEX phase on cooling to room temperature. The high-temperature phase persists for extended periods of time during the polymerization process, which allows for both the trapping and the characterization of the structure at room temperature. By utilizing nonequilibrium reactive processing to convert linear block copolymers to comb−coil type polymers, the creation of polymers with complex molecular topologies can be synthetically simplified while simultaneously allowing for the development of new processing modalities.

We conduct molecular dynamics simulations to study the effects of cation–ligand interactions on salt permeation in ligand-functionalized polymer membranes. Our results indicate that salt partitioning into the membrane increases while the salt diffusivity decreases with enhancement in the cation–ligand interaction strength. Moreover, the increase in partitioning dominates the decline in salt diffusivity, leading to a net enhancement in salt permeation at higher cation–ligand interaction strengths. Equimolar binary mixed salt systems (having a common anion) generally exhibit much more selective partitioning of salts into the membrane as compared to their single salt counterparts. However, single salt systems exhibit much higher ratio of salt diffusivities than their mixed salt analogues. The permselectivity data closely resemble the trends in selective partitioning for both single and mixed salt systems, indicating that salt diffusivity selectivity plays a weaker role than salt partitioning selectivity in dictating the selective permeation of salts across the membrane.

Polymer nanocomposites containing carbon nanomaterials such as carbon black, carbon nanotubes, and graphene exhibit exceptional mechanical, thermal, electrical, and gas-barrier properties. Although the materials property benefits are well established, controlling the dispersion of carbon nanomaterials in polymer matrixes during processing is still a difficult task using current methods. Here, we report a simple, yet versatile method to simultaneously achieve the reduction of graphene oxide (GO) and polymerization of styrene to create reduced graphene oxide/poly(styrene) (RGO/PS) nanocomposite materials via microwave heating. The RGO/PS mixture is then processed into films of desired thicknesses by first removing unreacted styrene and then pressing the powder at elevated temperatures. X-ray photoelectron spectroscopy proved that microwave processing was able to reduce GO, which resulted in a change in the carbon-to-oxygen ratio from 2.0 for GO to 4.5 for RGO. Furthermore, the addition of GO to the RGO/PS nanocomposites leads to an increase in the static dielectric constant (ε s ) relative to that of pure PS, with a minimal change in tan δ (∼0.06% at room temperature). The simultaneous microwave reduction/polymerization method described here will potentially lead to the production of polymer-based dielectric nanocomposite materials with tunable dielectric constants for energy-storage applications.

Membranes are an attractive alternative to current thermal separations due to their scalability and energy efficiency in desalinating water. Unfortunately, many of the conventional membrane materials available today are unable to differentiate between ionic solutes, especially alkali cations, compromising their use in ion−ion separations. Inspired by the ion-specific interactions exhibited by biological ion channels, recent research efforts have focused on synthesizing and characterizing new polymeric materials that incorporate ligands into polymer networks to bias solubility and/or diffusivity of one cationic species over another. Despite these efforts, little is known about the influence of incorporating ligands into polymer membranes on solubility and diffusivity of the complexing species. In this study, we first build a qualitative model of salt partitioning, diffusivity, and permeability in generic cationcomplexing ligand-functionalized polymer membranes. Next, to validate our model and hypotheses, we perform atomistic molecular dynamics simulations of a 12-crown-4-functionalized membrane in the presence of alkali halide salts at low concentration. Generally, cation complexation enhances cation solubility but decreases diffusivity. Interestingly, the reduction in diffusivity is predicted to be larger than the enhancement in solubility for materials which operate by the mechanisms proposed in our physical picture, ultimately resulting in a reduction in the permeability of the selectively complexing ion.