On the Order and Orientation in Liquid Crystalline Polymer Membranes for Gas Separation

Abstract

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.

Liquid-Crystalline Polymers: Molecular Engineering, Hierarchical Structures, and Applications

“…For practical applications, a high selectivity and a maximum permeability are desirable, but they are often in conflict with each other. Recent work on LCP membranes highlights the critical role of molecular order and orientation in tuning the permeability and selectivity in a more viable parameter space (Figure d) . The single-gas permeation data show that the permeability decreases with the increase in molecular order while the ideal gas selectivity of He and CO 2 over N 2 increases tremendously (>20 folds) when going from the disordered state to the ordered smectic phase.…”

“…(d) Schematic illustration of LCP membranes with various molecular orders and orientations for gas separation. Reproduced with permission from ref . Copyright 2021, American Chemical Society.…”

Electrically driven liquid crystal network actuators

“…6–9 Therefore, they have been explored in a variety of potential applications, including soft robots, biomimetic devices, energy generators, intelligent textiles, sensors, optical devices and some fields of modern engineering. 10–22…”

“…[6][7][8][9] Therefore, they have been explored in a variety of potential applications, including soft robots, biomimetic devices, energy generators, intelligent textiles, sensors, optical devices and some fields of modern engineering. [10][11][12][13][14][15][16][17][18][19][20][21][22] The characteristic features of the LCN materials and actuators reside in their anisotropy and their anisotropy-determined deformation. The programming of LCN actuators usually involves first arranging rod-shaped mesogens into a uniaxial, 2D or 3D alignment, through mechanical stretching, surface effects, shear extrusion, or electric or magnetic fields, and then carrying out polymer chain crosslinking to lock the orientation state of the LCNs.…”

Molecular Order Determines Gas Transport through Smectic Liquid Crystalline Polymer Membranes with Different Chemical Compositions

“…Self-assembly of liquid crystalline (LC) polymeric materials provides control over the supramolecular organization and alignment of the building blocks at the molecular level and can be used for membrane applications. ,− A variety of ordered nanostructures can be obtained, which, depending on the positional order of the LC monomers and the fabrication process, can differ in molecular order and orientation. Subsequent cross-linking of the LC monomers fixates the nanostructures and results in robust free-standing LC polymer membranes.…”