Diffusion Property Of Penetrant In Polyimide Pervaporation Membrane

The pervaporation technology is one of the most promising separation methods in the future.The diffusion of penetrant in the membranes is the key process in membrane separation technology,which greatly affects the separation performance of membranes.So the study of diffusion is very important for the development of membrane separation technology.In this thesis,twelve kinds of polyimide(PI)membranes with different structures were prepared via condensation polymerization by three kinds of dianhydrides and four kinds of diamines.The dynamic adsorption curves of steam permeated in different PI membranes at the temperature of 298K were measured.The relationships between the adsorption property and the PI membrane structure and the diffusion forms were discussed by using the calculated anomalous diffusion index.The dynamic adsorption curves of different solvents(methanol,ethanol,and isopropanol)at different temperatures were measured.The influences of these diamine,dianhydride chemical structures,different solvents and different adsorption temperature in the membrane adsorption property were discussed.The experiment results show that the process of diffusion of penetrant in the polyimide membranes was divided to two segments,which wereⅠandⅡ. The rate of diffusion in sectⅠwas lower than that in sectⅡ.The forms of diffusion of penetrant in the membranes were divided as four kinds according to the value of anomalous diffusion index.They are:Exceeded diffusion as 0＜d_w＜2,Fick diffusion as d_w=2,Anomalous diffusion when d_w＞2 and CaseⅡdiffusion when d_w→∞.The influence of these diamine,dianhydride chemical structures on the membrane absorption characteristics was most.The PI membranes with big density,low d-spacing and FFV,low chain packing density and weak chain mobility have low diffusing ratio of penetrant in them.the diffusion of water in PI membrane obey Fick's law,while the diffusion of alcohol in the PI membranes did not,but showed anomalous diffusion phenomena.The order of diffusing rate of different penetrants in PI membranes were water＞methanol＞ethanol＞isopropanol.Temperature was an other factor influencing the diffusing rate.When the temperature was high, the anomalous diffusion index of penetrant diffusing in the membranes was low,what indicating the high diffusing rate.