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

Abstract

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.

Recycling and 3D-Printing Biodegradable Membranes for Gas Separation─toward a Membrane Circular Economy

“…The physicochemical properties of a polymer, such as mechanical, thermal, viscosity, permeability, and others, are dictated by two elements: (a) chemical composition and (b) supramolecular structure. , By demonstrating that the polymer did not undergo any chemical change (Figure S3), we can conclude that any change in permeability is due to changes in the supramolecular structure. Supramolecular structure is affected by polymer chain lengths, polymer crystallinity, and solvent/additive effects. − As crystallinity remained the same (Figure S1) and no additional additives or solvents were used in the process, the data suggest that the cause for permeability changes is due to changes in molecular weight.…”

Polymer-Coated Covalent Organic Frameworks as Porous Liquids for Gas Storage

“…Additionally, we probe diffusion above and below the glass-transition temperature ( T g ) of the polymer coating and liquid matrix. The T g is a parameter of interest because gas diffusion through a polymer is typically associated with molecular motions that produce free volume and dynamic void spaces in the matrix . Above a polymer’s T g , substantial segmental motion typically results in higher gas diffusion coefficients by an order of magnitude or more.…”

“…The T g is a parameter of interest because gas diffusion through a polymer is typically associated with molecular motions that produce free volume and dynamic void spaces in the matrix. 66 Above a polymer's T g , substantial segmental motion typically results in higher gas diffusion coefficients by an order of magnitude or more. Sub-T g chain end motions, such as those associated with the methyl groups in PDMS, are believed to be responsible for facilitating any gas diffusion that does occur below polymer T g .…”

Tuning the Gas Separation Performances of Smectic Liquid Crystalline Polymer Membranes by Molecular Engineering

“…Moreover, the highly ordered smectic C membranes with lamellar structures parallel to the permeation direction (planar alignment) exhibit higher gas permeations but lower selectivities compared to membranes with lamellar structures perpendicular to the permeation direction (homeotropic alignment). However, so far, the role of the dimensions of the nanostructures in the membranes on the gas separation properties has not been reported yet [28]. Moreover, incorporating halogen atoms such as chlorine or fluorine is known to enhance CO 2 permeability and selectivity by affecting both gas solubility and diffusion [29][30][31][32] and in addition, it also provides a more detailed insight into the gas transport in smectic LC polymer membranes.…”