Nematic liquid crystalline polymer films for gas separation

The gas separation performances of free-standing planar aligned nematic LC polymer films were investigated for gas separations of He, CO₂, CH₄ and Xe. The films consist of derivatives of 1,4-phenylene bis(4-((6-(acryloyloxy)hexyl)oxy)benzoate)s with respective cyano, chloro, methyl and phenyl substituents on the central aromatic cores. Two new LC derivatives of 1,4-phenylene bis(4-((6-(acryloyloxy)hexyl)oxy)benzoate)s were successfully synthesised and fully characterised. Single gas permeation and sorption data show increasing gas permeabilities with increasing steric size of the substituents while the ideal gas selectivity of He over CH₄ and He over CO₂ decreases. The sorption coefficient of all films is independent of the LC substituents, while the subsequently extracted diffusion coefficient for the films with a phenyl substituent is three times higher compared to the films with a cyano substituent, demonstrating that the steric size of the LC substituents mainly affects the diffusion of gasses rather than the solubility of the gases. Irrespective of a methyl or a phenyl substituent, a larger kinetic diameter of Xe gives a 20 times lower diffusion coefficient compared to the smaller species (CO₂).