Plasticization mechanisms and effects of thermal annealing of Matrimid hollow fiber membranes for CO2 removal

CO2 induced plasticization presents a great challenge for sustainable performance of polymeric gas separation membranes for natural gas purification application. Comprehensive understanding of this phenomenon would lead to the formulation of suitable strategies to suppress it. In this study, a nodule structure model based on nucleation and growth mechanism was used to link the membrane microscopic structure to CO2 sorption which causes plasticization. This approach considers the dilation in Henryʹs sites is the major contributor to the plasticization compared to the Langmuir sites. In order to verify the proposed model, gas sorption tests were analysed for Matrimid hollow fiber membranes with different selectivities and the Henryʹs and Langmuir sorption isotherms were decoupled and compared. The modelling results suggest that more gas molecules were adsorbed into the Henryʹs sites than Langmuir sites at lower pressure range for hollow fibers with higher selectivity regardless of the types of the gases used. The modelling results also predicted that hollow fibers with lower selectivity should possess better plasticization resistance which was in agreement with the experimental plasticization observation. Competitive sorption for mixed-gas permeation was also investigated. The simulation study demonstrated that membranes with lower selectivity exhibited stronger competitive sorption than hollow fibers with higher selectivity; therefore plasticization phenomenon is more likely to be masked by competitive sorption for hollow fibers with lower selectivity. Furthermore, thermal annealing at both above and below the Matrimidʹs glass transition temperature was applied on hollow fiber membranes to evaluate its effect on plasticization suppression. Permeation test results indicated that the effect of thermal annealing above 250 °C for more than 30 min was sufficient to suppress plasticization. The dual-mode sites de-coupling technique was also performed on the thermally annealed membrane. The results suggest that the re-distribution of the Henryʹs and Langmuir sites due to thermal annealing could contribute to the suppression of plasticization. Other possible plasticization suppression mechanisms (cross-linking, crystallization and the formation of charge transfer complexes) were also explored using techniques including FT-IR, DSC, and dissolution tests.