Two diamino room temperature ionic liquids, 1,3-di(3-aminopropyl)imidazolium bis[(trifluoromethyl)sulfonyl]imide (monocationic RTIL or mRTIL) and 1,12-di[3-(3- aminopropyl)imidazolium]dodecane bis[(trifluoromethyl)sulfonyl]imide (dicationic RTIL or diRTIL) were synthesized using a Boc protection method. The two RTILs were incorporated within the 6FDA-MDA backbones to tune the solubility properties and improve the separation of CO2 from CH4.
The mRTIL was reacted with 2,2-bis(3,4-carboxylphenyl) hexafluoropropane dianhydride (6FDA) to produce 6FDA-RTIL oligomers. Two oligomers, one with 6.5 repeat units and another with 3.3 repeat units, were further reacted with 6FDA and m-phenylenediamine (MDA) where the compositions of RTIL ranged from 6.5 to 25.8 mol% to form block copolyimides.
The diRTIL was successfully reacted with 6FDA and MDA and formed 6FDA-(MDA/diRTIL) random copolyimides with a concentration of diRTIL up to 30 mol%. An 8 mol% diRTIL based block copolyimide with an oligomer size of 9 repeat units was also synthesized.
The separation performance of all RTIL based copolyimides followed a trade-off relationship and did not exhibit a significant improvement for CO2/CH4 gas pair. The incorporation of RTIL caused the change in the free volumes, free volume distributions of the copolyimides and did not increase the CO2 solubility of the polyimides.
The increase in the RTIL mol% resulted in a decrease in molecular weight, 5% weight loss temperature, glass transition temperatures (Tg) and an increase in density. The long block copolyimides exhibited a higher d-spacing, fractional free volume (FFV) and specific free volume (SFV) than those of the short block copolyimides. The diRTIL based copolyimides exhibited smaller d-spacings, FFVs and SFVs with the increase in mol% of diRTIL. In addition, the 8 mol% diRTIL based block copolyimide exhibited a lower density, higher d-spacing, FFV and SFV than those of the 10 mol% random copolyimide.
The RTIL monomers contain more alkyl groups than that of the MDA, which could increase the chain flexibility of the copolyimides. The incorporation of the RTILs in the 6FDA-MDA backbones caused denser packing that resulted in a lower permeability and higher selectivity compared with the pure 6FDA-MDA. For the mRTIL based block copolyimides, the gas permeability and diffusivity (H2, O2, N2, CH4 and CO2) decreased and the permeability and diffusivity selectivity for CO2/CH4 and O2/N2 gas pair increased with the increase in mol% of the mRTILs. The long block copolyimides have a lower permeability/diffusivity and higher permeability selectivity/ diffusivity selectivity than those of the short block copolyimides. The solubilities of varied gases of the long and short block copolyimides were similar. And the solubilities of O2, N2 and CH4 increased while that of CO2 decreased with the increase in mRTIL content.
For the diRTIL based random copolyimides, the permeability, solubility and diffusivity of varied gases decreased with the increase in mol% of the RTIL. The permeability and diffusivity selectivity of CO2/CH4 and O2/N2 slightly increased while that of N2/CH4 decreased. The solubility selectivity of CO2/CH4 decreased.