Temperature- and light-regulated gas transport in a liquid crystal polymer network

Azobenzene-containing liquid crystal polymer networks (LCNs) are developed for temperature- and light-regulated gas permeation. The order in a chiral-nematic LCN (LCN*) is found to be essential to couple the unique structure of the membrane and its gas permeation responses to external stimuli such as temperature and varying irradiation conditions. An LCN membrane polymerized in the isotropic phase exhibits enhanced N ₂ permeation with increasing temperature, like most traditional polymers, but barely responds to exposure with 455 and 365 nm light. In sharp contrast, a reversible decrease of N ₂ transport is observed for the LCN* membrane of exactly the same chemical composition, but molecularly ordered, when submitted to an elevated temperature. More importantly, alternating in situ illumination with 455 and 365 nm light modulates reversibly N ₂ permeation performance of the LCN* membrane, through the trans–cis isomerization of azo moieties. The authors postulate that, besides the anisotropic deformation of LCN*, the decreased order in LCN* membrane caused by external stimuli (i.e., increasing temperature or UV light illumination) is responsible for an inhibition of gas permeation. These results show potential applications of liquid crystal polymers in the gas transport and separation, and also contribute to the development of “smart” membranes.