Structural engineering on copolyimide membranes for improved gas separation performance

Abstract The structural engineering and design of 6FDA/BPDA-based copolyimide membranes at a molecular level are developed to achieve advanced membrane materials with improved CO2 plasticization resistance as well as high gas separation performance. The commercially available diamine monomers with/without various electronegative groups (-CH3, –CF3 and -Cl) have been employed to prepare 6FDA/BPDA-based copolyimides via the one-step method. All of the copolyimides show excellent solubility in the common solvent, film-forming ability and thus mechanical properties. More interestingly, the intermolecular charge transfer complex (CTC) effects is finely tuned as confirmed by the fluorescence emission spectrum. The introducing of polar chloride groups in the copolyimides could significantly strengthen the CTC effects to prevent the intrusion of condensable CO2 and limit polymeric chain movements. Thus, the membrane based on the 2,2′,5,5′-tetrachlorobenzidine (e.g. 6FDA:BPDA-TSN:TCDB (1:1:1:1)) shows the best carbon dioxide (CO2) plasticization resistance and slowest physical aging without sacrificing gas separation performance versus the membrane having weak polar groups. The CO2 plasticization pressure as high as 350 psi has been observed for the 6FDA:BPDA-TSN:TCDB membrane with polar chloride groups. This value is much higher than that of the copolyimide membranes with –CH3 or –CF3 groups ranging from 100 psi to 200 psi. Moreover, this membrane shows a good gas separation performance, e.g. CO2 permeability of 45.7 barrer and CO2/CH4 ideal selectivity of 35.2 surpassing the commercial cellulose acetate and Matrimid. Such structurally defined polymer with good mechanical strength, plasticization resistance and solubility harnesses the full potential of promising gas separation membrane materials for hollow fiber spinning. This work offers structural engineering guidance for the preparation of high-performance polymers with good tolerance to CO2-induced plasticization.