Polyimides (PI) containing 2,3,5,6-Tetramethyl-1,4-phenylenediamine (TMPD) moiety show a higher gas permeability, but a relatively low gas selectivity because of alkyl substituents disrupting polymer chain packing.
The crosslinking modification tends to increase chain packing and inhibits the intrasegmental and intersegmental mobility, resulting in higher gas selectivity.
Based on this consideration, chemical crosslinking was employed to modify the PI membrane containing TMPD moiety. TMPD based PI was crosslinked by 1,2-Diaminoethane (DAE) and 1,6-Diaminohexane (DAH) to enhance gas transport properties. The crosslinking process was performed by immersing PI film into crosslinking solution for different periods of time.
Fourier transform infrared results show that imide groups were converted into amide groups during crosslinking process. Thermogravimetric analysis study indicated that the degradation temperature of crosslinked PI membranes decreased after crosslinking. This is due to degradation of alkyl group in crosslinking agent. DAE is more efficiency in chemical crosslinking g than DAH because crosslinked PI induced by DAE has higher gel contents than DAH. The d-space of crosslinked PI membranes decreased with increasing crosslinking time, because the crosslinking tightens the network structure.
The permselectivity of CO2/CH4, CO2/N2 and O2/N2 increased during crosslinking except for CO2/N2 permselectivity of crosslinked PI membrane induced by DAE. For the gas pair of CO2/CH4, the maximum increment is about 39.5% after 6 minutes of DAE crosslinking also, that of O2/N2 gas pair is about 20.5% after 6 minutes of DAE crosslinking. According to these result, DAE is more suitable for enhanced permselectivity than DAH. On the contrary, DAE is not useful for CO2/N2 separation due to reduction in CO2/N2 permselectivity after 3 minutes DAE crosslinking. In contrast, the ideal permeability for CH4, N2, O2, and CO2 decreased after crosslinking and the ideal permeability of crosslinked PI membranes induced by DAH is larger than that by DAE.