Effect of solvent on crosslinking of a polyimide membrane using the liquid-phase crosslinking process for CO2/CH4 separation

Abstract

The crosslinking within a polyimide membrane was investigated by means of a liquid-phase crosslinking process (L-PCP) in which a polymer and crosslinker are mixed in a solvent to prepare crosslinked membranes in a single step. To prepare crosslinked membranes, the polyimide polymer was dissolved in various solvents including tetrahydrofuran (THF), chloroform, dimethylsulfoxide (DMSO), dimethylformamide (DMF), and N-methylpyrrolidone (NMP) with p-phenylenediamine as a crosslinker, and the crosslinked membranes were then obtained by casting the polymer solution. The crosslinked membranes showed different degrees of crosslinking according to the solvent (NMP > DMF > DMSO > THF > chloroform). The highest degree of crosslinking was obtained with the membrane crosslinked in NMP, which also presented the best chemical stability and plasticization resistance against condensable CO2 gas. On the other hand, the membranes crosslinked in THF and chloroform exhibited poor chemical stability and plasticization resistance. This difference in crosslinking degrees arising from the use of various solvents can be correlated with solubility parameters of the polymer and solvent. A good solvent can swell the polymer chains and provide higher crosslinker diffusivity and more effective crosslinking conditions, and vice versa. For CO2/CH4 separation, the highest CO2 permeability was obtained from the crosslinked membrane in THF and chloroform (~8.2 barrer) with a modest CO2/CH4 selectivity of 35 at high pressure (30 barg) despite plasticization. Moreover, the highest CO2/CH4 selectivity (~39) could be obtained with the membranes crosslinked in DMSO and DMF with CO2 permeability of 5 barrer. The membrane crosslinked in NMP exhibited the lowest CO2 permeability of 2.8 barrer at 30 barg due to the highly inter-crosslinked polymer structure, which hinders gas diffusion.