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

Abstract

The effect of layer spacing and halogenation on the gas separation performances of free-standing smectic LC polymer membranes is being investigated by molecular engineering. LC membranes with various layer spacings and halogenated LCs were fabricated while having a planar aligned smectic morphology. Single permeation and sorption data show a correlation between gas diffusion and layer spacing, which results in increasing gas permeabilities with increasing layer spacing while the ideal gas selectivity of He over CO2 or He over N2 decreases. The calculated diffusion coefficients show a 6-fold increase when going from membranes with a layer spacing of 31.9 Å to membranes with a layer spacing of 45.2 Å, demonstrating that the layer spacing in smectic LC membranes mainly affects the diffusion of gasses rather than their solubility. A comparison of gas sorption and permeation performances of smectic LC membranes with and without halogenated LCs shows only a limited effect of LC halogenation by a slight increase in both solubility and diffusion coefficients for the membranes with halogenated LCs, resulting in a slightly higher gas permeation and increased ideal gas selectivities towards CO2. These results show that layer spacing plays an important role in the gas separation performances of smectic LC polymer membranes.

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

“…Attenuated total reflection Fourier transform infrared spectroscopy (ATR FT-IR) spectra, polarizing optical microscopy (POM) images, differential scanning calorimetry (DSC) measurements, and medium- and wide-angle X-ray scattering (MAXS/WAXS) measurements have been carried out according the same procedure as described in the literature. , …”

“…Gas sorption of CO 2 was measured with a Rubotherm series IsoSORP sorption instrument at 6 bar and 20, 40, and 70 °C to determine the solubility coefficient of all membranes. The measurement procedure and solubility coefficients of CO 2 were determined according the same procedure as described in our previous publications. , The equilibrium time of each measurement, which was determined by monitoring the sorption over time, was 3 h.…”

“…Single gas permeation experiments using He, CO 2 , Ar, or N 2 were performed in a custom-built permeation setup and have been performed according a procedure described in our previous publications. , Single gas permeabilities were determined from the steady-state pressure increase in time in a calibrated volume at the permeate side of the membrane at temperatures of 20, 40, and 70 °C and a feed pressure of 6 bar. The order in which the gases were measured was kept constant (He, Ar, N 2 , and CO 2 ) since CO 2 could induce swelling of the membrane.…”

“…Subsequent cross-linking of the LC monomers fixates the nanostructures and results in robust free-standing LC polymer membranes. Although LC polymer membranes have already been investigated for water separations, − these materials are hardly investigated for gas separations. ,, In a very recent publication we reported the gas separation performances of smectic LC polymer membranes by molecular engineering . In other work, we studied the role of supramolecular organization and orientation in free-standing thermotropic LC polymer membranes based on crown ether functionalized LCs for gas separation by using LC membranes with various distinct morphologies and alignment while using the same chemical composition (Figure a for chemical structures) .…”

“…9,15,31 In a very recent publication we reported the gas separation perform- ances of smectic LC polymer membranes by molecular engineering. 31 In other work, we studied the role of supramolecular organization and orientation in free-standing thermotropic LC polymer membranes based on crown ether functionalized LCs for gas separation by using LC membranes with various distinct morphologies and alignment while using the same chemical composition (Figure 1a for chemical structures). 15 We found that control over the molecular order and orientation of the LC building blocks are important parameters that influence the gas separation performances to a great extent.…”