A Fundamental Study On The Preparation Of Microporous Membranes By Supercritical Fluid Induced Phase Separation

Supercritical CO₂(ScCO₂) induced phase separation is a new process developed recently for membrane preparation,which has following advantages:the phase separation and drying processes are coupled into one step;the dry membrane without structure collapse can be obtained during the drying process;the solvent can be easily removed and recovered via a simple decompression separation process and the whole process is operated without environment pollution.However,the present research work is still in the early stage of the process feasibility.There are little reports on the systemically experimental and theoretical research of the process.This dissertation is aimed to study the technological foundation and operation characteristics of ScCO₂ induced phase separation process.By studying the membrane preparation process for the typical membrane materials of polystyrene(PSt) and cellulose acetate(CA) as well as a new type membrane material of polyvinyl butyral(PVB),the effects of operating parameters on the membranes structure are determined.By the study on the thermodynamics involved in the ternary systerms of ScCO₂/solvent/polymer and the calculation of theoretical ternary phase diagrams,the phase demixing behavior during the membranes formation by ScCO₂ induced phase separation process are described.By studying thermodynamics involved in the quaternary systerms of ScCO₂/solvent/additive/polymer,the effect of a low molecular weight additive on the quaternary phase diagram and dimixing speed are revealed.The main achievements of this study are as following.(1) The experimental apparatus and the related experimental technique of ScCO₂ induced phase separation process are established.The membrane preparation process for typical materials PSt and CA as well as a new type material PVB are investigated experimentally.The results show that the porosity of the membrane prepared by ScCO₂ induced phase separation is higher than that prepared by conventional method;the membrane structures are significantly influenced by the process parameters;compared with the conventional immersion precipitation phase inversion,ScCO₂ induced phase separation process can effectively avoid the formation of macrovoids at suitable operating conditions;during depressurization process,the great differences in the effect of depressurization time on membranes structure for various polymer materials result from that the speed of CO₂ gaseous nuclei escaping from membrane substrate can not keep the pace with the outer depressurization rate. (2) The tendency that the interaction parameters affect the binodal curve position is illustrated by changing the interaction parameters in reasonable ranges.It is found that with the increase of solvent/polymer interaction parameter(x₂₃) or nonsolvent/polymer interaction parameter(x₁₃) the miscibility gap decreaseses.The effect of nonsolvent/solvent interaction parameter(x₁₂) on the binodal curve is relatively complicated.It seems that x₁₂=1 is the transition point of the change tendency of binodal curve position at the certain x₁₃ and x₂₃. When changing x₁₂ in the range of less than 1,miscibility gap increases with the increase of x₁₂. However,when increasing x₁₂ in the range of larger than 1,the change tendency of the binodal curve position is also affected by the values of x₁₃ and x₂₃.(3) The ternary phase diagrams of ScCO₂/acetone/CA,ScCO₂/toluene/PSt and ScCO₂/solvent/PVB systems are calculated using the thermodynamic model.By means of the constructed ternary phase diagrams of these systems,it is found that the binodal curve of ScCO₂/acetone/CA system is quite close to the polymer/solvent axis.Moreover,the metastable miscibility gap consisting between the binodal and spinodal curves is very narrow at the initial CA concentration in casting solution of lower than 25 wt.%.This results in a higher possibility for the components to enter the unstable demixing gap after the rapid phase separation of the system has taken place.Therefore,a bicontinuous structure is easily formed for CA membrane. For the systems of ScCO₂/toluene/PSt and ScCO₂/solvent/PVB,the increase of temperature or decrease of pressure leads to the binodal curves shifting toward the polymer/solvent axes,and thus results in the decrease of miscibility gap.The membrane toplayers of ScCO₂/solvent/PVB systems are dominated by the equilibrium thermodynamics,while the formation of membranes sublayers is the result of the combination of the equilibrium thermodynamics and membrane formation kinetics.(4) The quaternary phase diagrams of ScCO₂/N,N-dimethylacetamide/low molecular weight additive/PVB systems with various solvent/additive ratio are also calculated using the extended quaternary thermodynamic model.The results indicate that the addition of acetone (AC) in the casting solution leads that the outdiffusion of additive and solvent somehow takes on two stages during the membrane formation.The ratio of additive to solvent in polymer-lean phase slightly decreases first and then increases.On the whole of membrane formation process, the solvent is replaced by ScCO₂ prior to DMAC.For the ScCO₂/DMAC/AC/PVB system,the membrane toplayers are controlled by the equilibrium thermodynamics,and the membrane sublayers are the result of the delayed demixing.