Effects of Thermally Saccharide Units On the Gas Separation Performance of Highly Permeable Polyimide Membranes

Thermal annealing polymeric membranes consisting of thermally saccharide labile units have been proven to be a feasible approach to produce highly permeable gas separation membranes. In this work, thermal labile units with different molecular weights and structures, glucose (180 g/mol), sucrose (342 g/mol) and raffinose (504 g/mol), are chosen to be grafted onto the side chains of a polyimide and the membranes are annealed to investigate the effects of thermally labile units on its properties. The gas separation performance of the membranes for various gases such as O₂, N₂, CO₂, CH₄, C₂H₆, C₃H₆ and C₃H₈ are examined. It is observed that when the grafted and annealed membranes are annealed from 200 to 400 °C, a substantial increase in gas permeability is achieved with moderate gas-pair selectivity. It could be attributed to the formation of microvoids upon the degradation of the thermally labile unit. Depending on the thermally labile unit grafted, a four to eight-fold increase in gas permeability was seen. The variation of the gas separation performance with the thermally labile unit is elucidated by the thermal decomposition behavior of the thermally labile units and the interaction with the polymer matrix. The membrane resistance to CO₂ plasticization is also investigated. The annealed membranes show good flexibility with enhanced gas permeability and CO₂ plasticization resistance. The membranes can have CO₂ permeability up to 1389 Barrer with no sign of CO₂-induced plasticization up to 30 atm, exhibiting excellent CO₂/C₂H₆ and C₃H₆/C₃H₈ separation performance. The selectivity for CO₂/C₂H₆ is over 34.  The separation performance for O₂/N₂, CO₂/N₂ and CO₂/CH₄ gas pairs fall slightly below the upper bound.