Study On The Preparation And Performance Of PEI Resistance Composite Membrane

Resistance composite membrane consists of an asymmetric membrane whose function is predominately responsible for the separation and a coating layer whose function is to plug the pores in the dense layer of the asymmetric membrane. Both the asymmetric membrane structure and the coated materials determine the separating performance of the composite membrane. Immersion precipitation phase inversion method is most commonly used to fabricate asymmetric membranes for its advantage of simple operation and easy control. However, its formation mechanism is complex and still needs to be investigated. Silicon rubber is usually used as a sealing material, but the low solvent resistance has limited its practical application. Consequently, it is much more important to find new coating materials and investigate the sealing of defects during the coating processes. In this thesis, the formation mechanism of the asymmetric polyetherimide (PEI) membranes by phase inversion process was studied, and the effects of technical parameters on membrane morphological structure and properties were discussed. Moreover, polydimethylsiloxane (PDMS) and polyfluoropropylmethylsiloxane (PTFPMS) are used as sealing materials to plug the defects of the dense layer, and the effects of process parameters on the membrane performance were investigated, and the method to select optimal sealing materials to match the defects was also proposed.Firstly, asymmetric PEI membranes were prepared by phase inversion process using water as the coagulant. The thermodynamic and kinetic properties of the membrane formation process were characterized by the measurements of cloud point and light transmittance. The results indicated that the increase of polymer concentration in the casting solution resulted in a denser skin layer. The membrane prepared with N-methyl-2-pyrrolidinone (NMP) as a solvent could extend the phase separation time, also leading to a dense skin layer. The membrane prepared by the casting solution with 28.0 wt.% of PEI/NMP at coagulation bath temperature of 20℃exhibited a H2 permeance of 8.5 GPU (1 GPU= 1×10-6cm3/(cm2·s·cmHg)), with a O2/N2 and H2/N2 separation factor of 94.8 and 4.8, respectively.Secondly, the equations of binodal line and spinodal line for Water/NMP/PEI system were calculated based on Flory-Huggins theory, and the effects of the binary interaction parameters on the binodal line were investigated. The calculated results showed that the membrane with a dense skin layer can be prepared by varying the binary interaction parameters, such as decreasing the nonsolvent-polymer interaction parameter, increasing nonsolvent-solvent interaction parameter or decreasing solvent-polymer interaction parameter. A modified Reuvers model for mass transfer during formation of Water/NMP/PEI system was also presented. The model was applied to calculate the composition path during coagulation and analyze the mechanism of the asymmetric structure of PEI membrane under different polymer concentration. The calculated results showed that the slope of the polymer concentration changing with operating time was high in a very short period of time. The concentration of the polymer was very high near the interface of the membrane and coagulation bath, but it maintained their original concentration in the membrane body. Therefore, the asymmetric structure membrane was formed with a dense skin layer and a loosen support.Additive can regulate the thermodynamic properties of casting solution effectively. As a low molecular component, formamide (FM) has a good miscibility with solvent and a high diffusivity in the coagulant, so it was chosen as the additive in this thesis. It was found that the intermolecular distance of PEI molecules was reduced and the motion of the PEI molecular chains was limited with the addition of FM in the casting solution. Therefore, the skin layer of PEI membrane became denser and the separation performance was improved. At the FM concentration of 3.0 wt.%, the PEI membrane exhibited good performance that the H2 permeance was 24.6 GPU, and O2/N2 and H2/N2 separation factors are 7.0 and 153.0, which exceed the non-additive membranes by 31 and 38%, respectively.Lastly, the effects of molecular weight of the sealing material on the composite membrane performance were studied. The composite membranes were prepared by coating thin sealing layer of polyfluoropropylmethylsiloxane (PTFPMS) and PDMS respectively on asymmetric polyetherimide (PEI) substrates. Using PDMS as coating material, the effects of coating conditions including coating methods, coating time, coating solvents, and PDMS concentration on the composite membranes performance were investigated. The experimental results showed that the PDMS/PEI composite membrane prepared with a concentration of 3.0 wt.% PDMS/pentane coating solution by vacuum dip coating method had the best performance. In order to improve the solvent resistance of PEI resistance composite membranes, PTFPMS with high chemical resistance was used as the sealing layer to prepare composite membrane. The Materials Studio simulation results indicated that the higher the PTFPMS molecular weight, the longer the PTFPMS molecules chain and the higher the degree of the twist distortion. The sizes of defects on the dense layer were calculated by the dusty-gas model and compared with the molecular dimension of PTFPMS. The results showed that the molecular dimension of PTFPMS 1100K was larger than the defect diameter. The large polymer coils of PTFPMS 1100K could hardly penetrate into the defects of the PEI membranes regardless the concentration of the coating solution, resulting in a poor separation performance of the PEI composite membrane. However, the molecular dimension of PTFPMS 20K was smaller than the defect diameter of the PEI membrane. When the concentration of PTFPMS 20K was higher than 5 wt.%, PTFPMS 20K could form suitable polymer coils and seal the defects of the asymmetric PEI membranes. The PEI composite membrane prepared with 20 wt.% of PTFPMS 20K coating solution showed the separation factor of 6.9 and 107.3 for O2/N2 and H2/N2, respectively. Moreover, this membrane maintained its performance after soaked by pentane or petroleum ether. Therefore, PTFPMS/PEI resistance composite membrane demonstrates promising applications in gas separation of petrochemical industry.