(164f) Thermal Cross-Linking of Zwitterionic Copolymers in Nanoporous Membranes for Enhanced Selectivity

Membranes with advanced ion selectivity could provide a sustainable technical solution to global resource shortages. For example, membranes capable of selective ion retention could enrich aqueous feedstocks with desired ions, enabling efficient capture of precious metals. A promising approach to achieving ion selectivity involves the use of zwitterionic polymers, which have strong ion interactions due to their charged functional groups. Random zwitterionic amphiphilic copolymers (r-ZACs) are random/statistical copolymers that combine zwitterionic and hydrophobic repeat units, naturally self-assemble into bicontinuous networks of zwitterionic nanochannels within a hydrophobic nanophase. This unique morphology makes r-ZACs promising candidates for ion-selective membranes. To translate these materials into practical separation technologies, thin film composite (TFC) membranes are prepared by coating a thin (∼1 µm) layer of r-ZACs onto porous supports which is a simple and scalable membrane fabrication process. However, r-ZAC membranes possess pores that are too large to achieve ion selectivity. To address this challenge, cross-linking strategies have proven effective in tuning membrane performance. Our lab previously explored photo-initiated cross-linking of zwitterionic amphiphilic copolymers (X-ZACs) for anion separation, yielding promising results. However, the process was complex, involved multiple steps, and had a higher likelihood of defects. To overcome these limitations, we developed a thermal cross-linking approach that simultaneously cross-links and coats a thin X-ZAC layer onto a porous support. This method utilizes a thermal initiator and click chemistry to cross-link the allyl double bond groups on the X-ZACs, forming a stable and interconnected polymer network. The thermal process simply required adding the thermal initiator to the coating solution and applying heat, significantly simplifying the fabrication while reducing the risk of membrane defects. Despite the simplified procedure, the separation performance remained unchanged. These thermal cross-linked membranes can have effective pore sizes that range from ~ 2 nm down to <1 nm, with the smallest pore sizes achieving ion selectivity. The thermally cross-linked membranes were tested for cations separation, showing promising rejection of divalent and trivalent ions, including rare earth elements (REEs) making them suitable for advanced cation separation applications. Since REEs are critical for renewable energy technologies, such as wind turbines and high-performance magnets, these membranes offer a scalable and efficient platform for sustainable material purification. The results highlight the effectiveness of thermal-initiated click chemistry as a reliable method for tailoring membrane properties, with significant advantages in processing simplicity and scalability.