Photocrosslinked films composed of polydimethylsiloxane bearing alicyclic epoxy units and their CO₂-selective permeation properties

Various crosslinked PDMS films incorporating cyclic epoxy groups were prepared by UV-induced acid generation and thermal curation and evaluated as CO₂-selective permeable membranes. These free-standing, ultrathin PDMS films (~100 nm thick) were formed by crosslinking side-epoxy-PDMS, which contains multiple epoxy groups, and end-epoxy-PDMS, which has epoxy groups at the polymer ends only. Gas permeation tests revealed that the films crosslinked with end-epoxy-PDMS exhibited high CO₂ permeance. Specifically, the membrane composed of UV-crosslinked end-epoxy-PDMS (Mn = 20,000, thickness ~ 200 nm) achieved a CO₂ permeance of 5200 GPU and a CO₂/N₂ selectivity of 11.0. Reducing the membrane thickness increased the permeance without affecting selectivity. However, shortening the siloxane chain, using side-epoxy-PDMS, or reducing the linker length led to decreases in both permeance and selectivity. For example, side-epoxy-PDMS (Mn = 30,000, Si-H/O-Si-O ratio = 37%, thickness ~ 200 nm) had a CO₂ permeance of 400 GPU and a CO₂/N₂ selectivity of 1.16. These results indicate that a lower crosslinking density and longer end-epoxy-PDMS siloxane chains are advantageous for CO₂ dissolution and diffusion, resulting in superior CO₂ permeance and selectivity compared with composed of side-epoxy-PDMS. UV-crosslinked PDMS membranes with site-selective alicyclic epoxy units exhibited outstanding CO₂ permeance and selectivity. By comparing end- and side-functionalized PDMS structures, we reveal that low crosslinking density and long siloxane chains favor CO₂ diffusion. Ultrathin (~100 nm), freestanding films fabricated via photoacid-induced curing achieved a CO₂ permeance of up to 5200 GPU with a CO₂/N₂ selectivity of 11.0. This study demonstrates the potential of molecularly engineered siloxane networks for high-performance gas separation, especially in membrane-based direct air capture (m-DAC) applications.