Study On Preparation Of Microifltration Membranes Using Supercritical C02Phase Inversion Method

Supercritical CO2(ScCO2) is referred to as a promising, environmentally-friendly and effective substitute for conventional liquid nonsolvent used in nonsolvent induced phase separation method (NIPS) because ScCO2has liquid-like density, gas-like diffusivity, and nearly zero surface tension, and a small change in pressure or temperature near the critical point results in large changes in density, allowing many properties to be tuned. Therefore, ScCO2phase inversion method with ScCO2as a nonsolvent has been becoming one of the hottest research topics in the field of membrane preparation. ScCO2phase inversion method has been usually used to prepare various polymer membranes and the prepared membranes have been applied in separation fields of high value-added products. However, the membrane formation mechanism and material selection guide are still not clear. For example, the membranes prepared by the method under various conditions exhibited uniform cellular pore structure instead of finger-like macrovoids formed in instantaneous demixing by NIPS method, although ScCO2phase inversion method was considered as a typically instantaneous demixing. Besides, the cellular diameter and porosity of various polymeric membranes exhibited versatility with the increase in polymer concentration. The cellular diameters of amorphous polymer membranes showed contrary trends with the rise of ScCO2pressure. In this thesis, polyetherimide (PEI) was chosen to prepare membranes by the method to investigate the formation mechanism of cellular pores, kinetic mechanisms determining cellular diameter and porosity, and the effect of interaction between CO2and amorphous polymers on cellular diameter. These researches will become the fundamental theoretical basis of material selection, morphology adjustment and application of membranes prepared by ScCO2phase inversion method.PEI membranes were prepared by the ScCO2phase inversion method to investigate the formation mechanism of cellular pore structure. Due to the high diffusivity of ScCO2, the high instantaneous in-diffusion velocity of ScCO2in the PEI casting solution promoted the simultaneous generation of plenty of new nuclei within the whole cross-section, but weakened the growth of new nuclei. Even though the cellular structure prepared by the method was different from the finger-like macrovoids formed in instantaneou demixing of NIPS method, the pore formation process still agreed with the essence of the Nucleation and growth mechanism that local delayed demixing leads to the formation of macrovoids. The only difference was that ScCO2high instantaneous in-diffusion disturbed the condition of local delayed demixing in the casting solution. Once the viscosity of casting solution was too high, or the volatilization time was long, or the nascently formed skin was relatively dense, the instantaneous velocity of ScCO2in-diffusion will be decreased and local delayed demixing will occur, leading to the formation of macrovoids in the membrane. To perfect the Nucleation and growth mechanism, the unique result of ScCO2phase inversion method could be taken as a supplement.To reveal the kinetic mechanism of membrane formation, the difference (Vo-Vi) of the average diffusion velocity (Vo) of solvent out of the casting solution and average diffusion velocity (V1) of ScCO2into the casting solution through the interface between casting solution and the nonsolvent were obtained by experiments and calculation. The effects of the polymer concentration and ScCO2pressure and temperature on the cellular diameter and porosity of PEI membranes were explained by the main criteria of Vo-Vi. Because of the high diffusivity of ScCO2in polymers, the decrease of V1was less than that of Vo and thus Vo-Vi decreased when the skin layer became thicker with the increased polymer concentration, which led to the increase in cellular diameter and porosity. The above phenomenon is absent in NIPS method. The membrane with large cellular pores can be prepared at the high polymer concnentration by ScCO2phase inversion method because the effect of mass transfer kinetics on the cellular diameter and porosity was amplified by the high diffusivity of ScCO2in polymers. Besides, the various trends behaved by cellular diameter and porosity with the increased polymer concentration in literature can also be explained by the criteria of Vo-ViThe cellular pore variation of three amorphous polymer membranes, such as PEI, polysulfone (PSf) and polymethyl methacrylate (PMMA), with a rise of ScCO2pressure was investigated to study the effect of interaction between CO2and polymer on cellular size in the cross-setion, and to give material selection guide of ScCO2phase inversion method. The results indicated that the cellular size of PEI membrane increased with the rise of the pressure, while cellular sizes of PSf and PMMA membranes showed different tendency. This work indicated that the effect of ScCO2pressure on the cellular size of amorphous polymer membrane had a close relationship with the interaction between CO2and the polymer, which could be characterized by CO2-philic functional groups in the polymer, the difference between the solubility parameter of the polymer and21.8MPa1/2, and the solubility of CO2in the polymer. This guide could be used to explain the opposite effects of ScCO2pressure on cellular size in the cross-section of different amorphous polymer membranes in literature. For the polymer with weak interaction with CO2, the mechanical strength of the polymer rich phase would not decrease obviously with the raise of ScCO2pressure. Thus, the polymer rich phase could support the growth of the polymer lean phase, leading to an increase in cellular size. For the polymer that.has strong interaction with CO2, the mechanical strength of the polymer rich phase had an apparent decrease. Therefore, the polymer rich phase could not support the growth of the polymer lean phase, and the size of the polymer lean phase matched the thermodynamic law, leading to a reduction of cellular size.In order to develop the new application of microporous membranes prepared by SCCO2phase inversion method, the performance of the PEI membrane was characterized. The membrane had low solvent residue, a uniform cellular structure, high porosity, narrow pore size distribution and high tensile strength. The optimized membrane prepared at24MPa and45with a large mean cellular diameter of6.0μm, high porosity of73%, narrow pore size distribution was coated with chitosan to improve its hydrophilicity and coupled with Cibacron Blue F3GA (CB) as a special ligand to form an affinity membrane (PEI-coated chitosan-CB membrane). The PEI-coated chitosan-CB membrane showed a high adsorption capacity of33.9mg/g membrane to bovine serum albumin and was higher than those of most affinity membranes. Moreover, the tensile strength of the PEI-coated chitosan-CB membrane was11,58MPa and was25%higher than the highest one of reported affinity membranes. This work demonstrates that ScCO2phase inversion method is a potential method to prepare an affinity matrix.