Synthesis Of Polyimide And Its Mixed Matrix Membranes With CNT For Gas Separation

Polyimide, which is one of the outstanding engineering plastic, possesses high gas separation selectivity, thermal stability and mechanical property and polyimide menbrane has been used in gas separation application. However, the permeability can be restricted by Robeson limitation with high selectivity, and this trade-off relationship between selectivity and permeability of polymer membrane can impede its effectiveness and efficiency in application. To achieve high performances both in selectivity and permeability of polymer membrane, mixing inorganic porous material in polymer membrane to form mixed matrix membrane(MMM) has been proved to be an effective way. As one of the most appealing porous inorganic materials, carbon nanotubes(CNTs) with high aspect raio, smooth inner wall and nano-sized pores attracted tremendous intensions, and these properties are supposed to improve the performance of polyimide membrane in both permeability and selectivity.With proper structural design, we successfully synthesized polyimide via a two-step method with DPDA and 6FDA monomers and the target produce was confirmed by FT-IR, NMR and DSC measurement. DSC and TGA measurement revealed that polyimide membrane had a high T g which indicated that polyimide membrane had excellent mechanical property and thermal stability. In addition, distinguished high compatibility between 6FDA and DPDA had been achieved because of the existence of trifluoromethyl group.Due to the good solubility of polyimide membrane, multi-walled carbon nanotubes(MWCNTs) were modified with carboxyl group via mixed acid method and blended into polyimide membrane in fractions of 1wt%, 2wt% and 3wt% to fabricate MMM.DSC measurement showed that T g of MMMs increased from 256.8 o C to 262.9, 263.4 and 365.0 o C in comparison with pure polyimide membrane when the doping fraction of functional MWCNTs(f-MWCNTs) increased from 0wt% to 1wt%, 2wt% and 3wt%, respectively. This was demonstrated that the addition of f-MWCNTs increased the T g of MMMs. In TGA measurement, MMMs showed better thermal stability and antioxidant capability compared with polyimide membrane. Mechanical evaluation indicated that, with the combination of f-MWCNTs, mechanical strength and toughness of MMM had been well improved, this improvement was attributed to the good compatibility and interaction of f-MWCNTs and polyimide membrane and the MWCNTs suspended in polyimide membrane could transfer stress effectively. DMA results showed that the introduction of f-MWCNTs in MMM could increase the storage modulus and mechanical loss of MMMs, thus increase T g of MMMs.In gas separation test, we chose He, CO2, O2 and N2 as test gases to evaluate the selectivity and permeability of MMMs. In comparison of polyimide membrane, the permeabilities of 1%CNT/PI and 3%CNT/PI MMMs for CO2 improved 102% and 183%. Because f-MWCNTs could suspended in polyimide membrane uniformly on account of the good compatibility between f-MWCNTs and polyimide membrane, the pores of f-MWCNTs could ascend the gas flux through MMM and improve the permeabilities of MMMs. As for the selectivity measurements, selectivities of MMMs for He/N2、He/O2、CO2/N2 and O2/N2 all improved, the selectivities increased as the doping fraction increased. This was an evident that the introduction of f-MWCNTs could improve both permeability and selectivity of MMM and showed potential usage in gas separation application.