Ultraviolet-water: constructing angstrom-sized channels in polymer membranes

Membranes capable of precisely separating ions and small molecules offer great prospects for industry applications, including clean energy production and water desalination. Membrane-based separation techniques have been applied to extract mineral resources such as uranium and lithium from saltwater resources, a process with broad applications in the fields of the nuclear fission and rechargeable lithium-ion batteries. To achieve highly efficient separation, the membrane channels must be formed with angstrom-scale precision. While progress has been achieved in polymer membranes, which serve as the dominant commercial membranes, significant technical challenges of creating angstrom-sized channels remain in these membranes.

Water molecules illuminated by UV could serve as a controllable medium for creating channels in polymer membrane. When water molecules absorb ultraviolet light, they undergo homolysis and photochemical ionization, leading to the production of reactive oxygen species (ROS), particularly oxygen-containing free radicals with unpaired electrons, which are extremely chemically reactive. The highly oxidizing hydroxyl radicals (·OH) are a major type of reactive oxygen species produced under ultraviolet irradiation, which can scissor polymer chains. The study confirmed the generation of hydroxyl radicals by electron paramagnetic resonance (EPR) spectrum. The authors call the water illuminated by UV light ‘ultraviolet-water’ (shortened to UV-W). Simultaneously, authors envisaged that, by introducing UV-W into the interior of the polymer such that water molecules diffuse into this interior, angstrom-sized channels eventually could be formed with the help of hydroxyl radicals within the membrane. The passage of water molecules into the membrane can be achieved through ion irradiation or swelling of the membrane. Inspired by these mechanisms, the study group therefore proposed a UV-W in situ scissoring approach to form angstrom-sized channels in a polymer membrane for ion separation.

To demonstrate the strategy, the study group chooses a polyetherimide ion-irradiated membrane (PEI-IIM) as a precursor, which was obtained by subjecting a polyetherimide membrane (PEIM) to ion irradiation. When the membrane is immersed in water, the water molecules diffuse into the interior of the PEI-IIM. Upon UV light exposure, the water molecules photolyze to produce hydroxyl radicals, which actively scissor the surrounding polymer chains and lead to scission of chains and the formation of new functional groups, ultimately forming angstrom-sized channels inside the PEI ion channel membrane (PEI-ICM). By controlling the UV exposure time, the channel sizes can be tuned at the angstrom scale. The adjustable sizes and functional groups of the angstrom-sized channels formed in the polymer membrane are capable of sieving monovalent ions (e.g., Li⁺, K⁺, Na⁺) from multivalent ions (e.g., Mg²⁺, Ca²⁺, La³⁺).

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