To prevent greenhouse emissions into the atmosphere, separations like CO2/CH4 and CO2/N2 from natural gas, biogas, and flue gasses are crucial. Polymer membranes gained a key role in gas separations over the past decades, but these polymers are often not organized at a molecular level, which results in a trade-off between permeability and selectivity. In this work, the effect of molecular order and orientation in liquid crystals (LCs) polymer membranes for gas permeation is demonstrated. Using the self-assembly of polymerizable LCs to prepare membranes ensures control over the supramolecular organization and alignment of the building blocks at a molecular level. Robust freestanding LC membranes were fabricated that have various, distinct morphologies (isotropic, nematic cybotactic, and smectic C) and alignment (planar and homeotropic), while using the same chemical composition. Single gas permeation data show that the permeability decreases with increasing molecular order while the ideal gas selectivity of He and CO2 over N2 increases tremendously (36-fold for He/N2 and 21-fold for CO2/N2) when going from randomly ordered to the highly ordered smectic C morphology. The calculated diffusion coefficients showed a 10-fold decrease when going from randomly ordered membranes to ordered smectic C membranes. It is proposed that with increasing molecular order, the free volume elements in the membrane become smaller, which hinders gasses with larger kinetic diameters (Ar, N2) more than gasses with smaller kinetic diameters (He, CO2), inducing selectivity. Comparison of gas sorption and permeation performances of planar and homeotropic aligned smectic C membranes shows the effect of molecular orientation by a 3-fold decrease of the diffusion coefficient of homeotropic aligned smectic C membranes resulting in a diminished gas permeation and increased ideal gas selectivities. These results strongly highlight the importance of molecular order and orientation in LC polymer membranes for gas separation.

Amorphous polymers are often used for gas separation but have a trade-off between gas permeability and selectivity. Here, the effect of chemical composition and temperature on gas permeability and solubility in well-ordered LC polymer membranes is investigated. Membranes with various compositions of a monomethacrylate LC (M1) with a crown ether functionality to enhance CO 2 solubility and a smectic diacrylate (M2) cross-linker were fabricated, while all having the same order (smectic C) and alignment (planar). Single gas sorption and permeation data show for the membranes with 30 wt % M1 a higher CO 2 solubility coefficient compared to membranes without M1, which results in a higher CO 2 permeability and selectivity. For membranes that contain more than 30 wt % M1 decreasing layer spacings lead to reduced gas solubilities that result in lower gas permeabilities without additional selectivity gain toward CO 2 . The effect of temperature is demonstrated by comparing single gas sorption and permeation data below and above the T g of the membranes. The diffusion coefficient increases above the T g of the membranes with increasing M1 content leading to higher CO 2 permeabilities and selectivities. These results show that not only the chemical composition but also the layer spacing of the smectic structures determines the gas separation performance of smectic LC polymer membranes.