Photoswitchable gas permeation membranes based on azobenzene-doped liquid crystals

We have fabricated switchable gas permeation membranes in which a photoswitchable low-molecular-weight liquid crystalline (LC) material acts as the active element. Two different LC eutectic mixtures based on cyanobiphenyls and phenyl benzoates, respectively, were doped with mesogenic azo dyes and infused into commercially available tracketched porous polycarbonate membranes with regular cylindrical pores (0.40 to 10.0 μm). Photo-induced isothermal phase changes in the imbibed mesogenic material afforded large, reversible changes in the permeability of the photoswitchable membrane to nitrogen. The membrane imbibed with the photoswitchable cyanobiphenyl LC material demonstrated low permeability in the nematic state, while the photogenerated isotropic state demonstrated a 16×-greater sorption coefficient. Both states obey a high linear sorption behavior in accordance with Henry's Law. The membrane imbibed with the photoswitchable phenyl benzoate LC showed the opposite permeability behavior to the biphenylimbibed membrane, along with nonlinear sorption behavior. Permeability switching response times for the membranes on the order of 5 s were demonstrated using alternating UV and >420-nm radiation at an intensity of 2 mW/cm2. The effect of thermomolecular motion on gas sorption and diffusion over the LC-isotropic phase transitions are, for the first time, evaluated under isothermal conditions. These photoswitchable membranes are the first examples of systems that are capable of rapid and reversible gas permeation switching. Such switchable and/or tunable membranes are in high demand for applications in analytics, screening, and membrane reactors.