Bottom-Up Fabrication of Nanostructured Bicontinuous and Hexagonal Ion-Conducting Polymer Membranes

We report a simple photo-cross-linking process to chemically arrest the different liquid-crystalline structures formed by self-assembly of wedge-shaped amphiphilic mesogens. Using this route, we obtained free-standing polymer membranes with columnar or bicontinuous cubic structures, depending primarily on the relative humidity conditions during UV-induced cross-linking. These cross-linked mesostructures show much higher thermal stability in comparison with that of the liquid-crystalline structures of the initial monomers. The ionic conductivity of the membranes strongly depends on the water uptake preceding the polymerization reaction. According to NMR diffusometry, which can quantify water transport in one or two environments in these materials, the water diffusion rate in the membrane with bicontinuous cubic structures can approach values of commercial ion conducting membranes. These studies show promise for use of this fabrication route in practical applications for selective ion and water transport

Humidity-Modulated Phase Control and Nanoscopic Transport in Supramolecular Assemblies

Supramolecular assembly allows for enhanced control of bulk material properties through the fine modulation of intermolecular interactions. We present a comprehensive study of a cross-linkable amphiphilic wedge molecule based on a sulfonated trialkoxybenzene with a sodium counterion that forms liquid crystalline (LC) phases with ionic nanochannel structures. This compound exhibits drastic structural changes as a function of relative humidity (RH). Our combined structural, dynamical, and transport studies reveal deep and novel information on the coupling of water and wedge molecule transport to structural motifs, including the significant influence of domain boundaries within the material. Over a range of RH values, we employ ²³Na solid-state NMR on the counterions to complement detailed structural studies by grazing-incidence small-angle X-ray scattering. RH-dependent pulsed-field-gradient (PFG) NMR diffusion studies on both water and the wedge amphiphiles show multiple components, corresponding to species diffusing within LC domains as well as in the domain boundaries that compose 10% of the material. The rich transport and dynamical behaviors described here represent an important window into the world of supramolecular soft materials, carrying implications for optimization of these materials in many venues. Cubic phases present at high RH show fast transport of water (2 × 10^–10 m²/s), competitive with that observed in benchmark polymeric ion conductors. Understanding the self-assembly of these supramolecular building blocks shows promise for generating cross-linked membranes with fast ion conduction for applications such as next-generation batteries

Sulfonated Poly(arylene sulfide sulfone nitrile) Multiblock Copolymers with Ordered Morphology for Proton Exchange Membranes

Ordered morphologies in disulfonated poly­(arylene sulfide sulfone nitrile) (SPSN) copolymers were generated via thermal annealing followed by multiblock copolymer synthesis. While SPSN random copolymers (R-SPSN) showed featureless morphologies, the SPSN multiblock copolymers (B-SPSN) exhibited cocontinuous lamellar morphologies with a center-to-center interdomain size of up to 40 nm. In spite of the well-ordered, interconnected hydrophilic domains, the water self-diffusion coefficient (e.g., <i>D</i> = (0.7–2.0) × 10^–10 m² s^–1) and proton conductivity (e.g., σ = 0.16–0.20 S cm^–1 in deionized water at 30 °C) through B-SPSN were lower than those of the corresponding R-SPSN (e.g., <i>D</i> = (3.5–3.9) × 10^–10 m² s^–1 and σ = 0.21 S cm^–1) due to the relatively lower water uptake of the B-SPSN after thermal annealing. The reduced water uptake of B-SPSN was beneficial to reduction of peroxide degradation rate. Thermal annealing produced significant gains in morphological ordering and finer control over desired membrane properties for proton conduction applications