Water Permeabilities Of Ion Track-etched Membranes With Nanochannels

Water shortage has become an urgent global problem with the increasing of population,the rapid developing of urbanization and the aggravating of water pollution.Membrane ultrafiltration is an extensively used technique for water purification which has better selectivity and adaptability.As a key parameter,water permeability has been the focus of fundamental researches and technical applications.Especially as the diameter falls into the nanoscale,it is of great significance to understand the water behavior in the nanochannels.Within the group of materials,track-etched membranes are highly valued for their uniform and identical pore geometry among multiple channels,narrow pore size distribution,thereby this type of membranes can be considered a model system for studying confined water flow behavior.However,up to date there is a lack of unambiguous data and systematic research on the water permeabilities of track-etched membranes with pore diameters less than 200 nm,especially for small diameters below 100 nm.In light of the Hagen-Poiseuille equation,the pressure-driven water permeability in a channel scales as the fourth power of the channel diameter.Hence,the precision of channel diameter plays a pivotal role in precise determining water permeability.However,for non-conductive porous materials,an additional conductive layer should be introduced to cover the material’s surface to facilitate Scanning Electron Microscopy(SEM)observations,which brings errors in pore sizes determination inevitably.In this thesis,water permeabilities of pressuredriven ultrafiltration membranes are precisely determined with unprecedented accuracy.This is realized by applying a newly proposed pore diameter-calibration method based on the replication of pore geometry in ion track-etched membranes.We expect this work will shed light on a controversy over an enhanced and depressed water permeabilities of ultrafiltration membranes.The thesis covers the following topics:(1)In order to accurately determine the diameter of nanochannels,we propose a newly adopted method to calibrate the nanochannel diameters with unprecedented accuracy.This calibration method is based on the geometry replication of nanopores by sputter coating.We believe that the fine calibration based on the highly correlated porenanotube SEM images can maximize elimination of errors in measuring pore diameters.Also,we explain the reason why this pore diameter-calibration method is more reliable in terms of the working mechanism of sputter coating.(2)Polycarbonate(PC)ion track-etched membranes with sub-200 nm elliptical pores are prepared for the first time by stretching method,and the membrane surface properties(long axis,short axis and porosity)of the membrane before and after deformation are statistically analyzed.With increasing membrane stretching,circular pores gradually transform into elliptical pores and both the long axis of pores and the porosity of the membrane increase,while the short axis has no significant change(in comparison with initial circular pores).(3)Based on the newly proposed pore diameter-calibration method,water permeabilities of pressure-driven ultrafiltration membranes was precisely determined,showing a remarkably improved precision as comparing with the previously reported data.As the diameter falls into the sub-100 nm regime,the measured permeabilities are lower than theoretical predictions.The decrease in permeability is explained under the framework of the wettability effect which becomes more dominant as the pore diameter decreases.(4)The permeabilities of the sub-200 nm ion track-etched membranes with different stretching ratios are determined.The results show that the elliptical pore geometry can enhance the water permeability of the membrane whereas the selectivity sustains.In summary,this thesis has studied the water permeabilities of ion track-etched membranes with nanoscale channels.The strategy comprises the steps of: precise calibration of the membrane pore diameter with the method of entire replication of pore geometry;the preparation of membrane with elliptical pores by stretching;and the experimental determination of water permeabilities of ion track-etched membranes.We expect the findings of this thesis will shed light on understanding the separation behavior of ultrafiltration membranes and improving the separation performance of membranes,and also guiding the design of ultrafiltration membrane for separation applications.