Preparation Of Sub-t_g Crosslinked Thermally Rearranged Phenolphthalein-based Polybenzoxazole Gas Separation Membrane

Polyimide is considered as a promising gas separation material because of its high gas selectivity,excellent thermal stability and good mechanical properties.However,traditional polyimide membranes have low gas permeability and are prone to plasticization when separating higher concerntrations of soluble gases(CO2,CH4,etc.).In order to solve these problems,the lactone ring is introduced as the side group of the polyimide,which can reduce the crosslinking temperature to avoid the collapse of the internal cavity.At the same time,the ortho-hydroxy polyimide can undergo a thermal rearrangement(TR)reaction,which will generate many tiny cavities,so that the polymer has good size screening performance.The dual synergistic effect of Sub-Tg cross-linking and TR reaction can improve the gas permeability property and plasticization resistance of the polymer membrane.In this paper,an ortho-hydroxy diamine 3,3’-diaminophenolphthalide(DAP)is synthesized and polymerize with 4,4’-(hexafluoroisopropyl)diphthalic anhydride(6FDA)to synthesize polyimide 6FDA-DAP.Characterization of polyimide shows that 6FDA-DAP has excellent thermal stability and solubility.The CO2 pure gas permeability of 6FDA-DAP is 30.08 barrer,and the ideal CO2/CH4 selectivity is 36.68.Due to the poor plasticization resistance of polyimide,it can be proved by characterization that the lactone ring of 6FDA-DAP decomposes at about 30℃below Tg to produce benzene radicals,which cross-link between the benzene radicals to form the biphenyl structure.Sub-Tg cross-linking was achieved,and no plasticizing effect is observed in the cross-linked membrane at 350℃(PI350℃)under CO2 pressure of up to 30 atm.The TR polymer is obtained by further heat treatment.The gas permeability property of the polybenzoxazole(PBO)membrane in this study is much higher than other TR membranes with similar structure that have not undergone crosslinking.This is because the TR reaction produces many small and mediumsized cavities inside the polymer,leading to improved gas size screening performance,but the most important thing is happened due to the Sub-Tg crosslinking to prevent the internal cavity collapse.The improvement of the plasticization resistance is attributed to the formation of the rigid cross-linked network structure and the remodeling of the pore structure caused by the TR reaction.Compared with 6FDA-DAP polymer membranes,CO2 permeability increases by more than 20 times while CO2/CH4 selectivity remains unchanged,and gas permeability property of CO2/CH4 at PI-400℃ after thermally rearranged exceeds Robeson 2008 upper bound.In addition,PI-400℃still has excellent gas separation performance when the total pressure of the mixed gas(CO2:CH4=1:1)is 20atm,and the CO2/CH4 selectivity is 30.18.Therefore,the sub-Tg cross-linked phenolphthalein thermally rearranged polymer membranes exhibit strong potential for gas separation.