Side-chain engineering of ladder-structured polysilsesquioxane membranes for gas separations

A comprehensive and fundamental gas transport study of ladder-structured polysilsesquioxanes (LPSQs) was systematically performed by investigating the effects of various alkyl substituents, different copolymer ratios, and UV-irradiation induced photo-crosslinking on gas separations. Overall, LPSQ membranes are more suitable for CO₂/N₂ and CO₂/H₂ separations due to a relatively high affinity towards CO₂ as well as rubbery polymer properties. The gas transport in LPSQ membranes was well interpreted by two important parameters, the inter-chain distance and the side chain mobility. A combination of larger inter-chain distance and higher side chain rigidity tends to increase the fractional free volume, resulting in higher gas permeability. Also, it was successfully demonstrated that the separation performance of LPSQ membranes can be predicted by using a logarithmic permeability relationship based on the transport characterization for a series of ladder-structured poly(phenyl-co-methacryloxypropyl)silsesquioxanes. Lastly, the UV-curing process reduced the permeability of LPSQ membranes, increasing the selectivity due to the restricted chain mobility.