Fabrication And Ionic Transport Properties Of Polyimide Porous Membranes With Angstrom-scale Slit Channels

The issues of resources scarcity and energy shortage are the major challenges for human society.Therefore,the exploitation of efficient and sustainable resource and energy capture technologies is of significant research and application value.In particular,membrane separation technology is widely used in lithium resource extraction,seawater desalination and energy conversion due to its high efficiency and energy saving features.Polymeric porous membranes have attracted extensive attention of researchers because of its high flux and ease of large-scale preparation.However,conventional polymer porous membranes with non-uniform pore size,discontinuous channels,and low porosity.Therefore,the performance of conventional polymeric porous membranes is limited by the trade-off effect between selectivity and permeability.In addition,conventional polymeric porous membranes have a wide range of pore size distribution which makes it extremely difficult to achieve accurate separation at the molecular level.Therefore,it is particularly valuable to design and architect polymeric porous membranes with uniform pore size distribution to achieving efficient and accurate material separation.In this thesis,the construction of polyimide(PI)porous membranes with angstrom-scale slit channels and its ionic separation were investigated to address the issues of non-uniform pore size,discontinuous channels,low porosity and difficulty achieving accurate separation of conventional polymeric porous membranes.The mechanism of pore formation in PI by heavy ion irradiation combined with mechanical stretching was elucidated,as well as the mechanism of ion transport in the angstrom-scale slit channels was clarified.In this study,heavy ion track PI membranes were used as the base membranes,and the mechanical stretching method was used to construct angstrom channels in PI ion track membranes.During the applied stress,heavy ion tracks as stress concentrators(defects)cause the stress to concentrate in the edges of the track and bulk material regions,which undergoes more significant deformation than the traces,resulting in the formation of angstrom-scale slit channels in the PI film as the strain gradually increased.PI porous membranes with angstrom-scale slit channels can combine the advantages of two-dimensional channels and polymer membranes.It enhances the ion selectivity while ensuring the ion flux,making it possible to achieve precise separation at the molecular level.This thesis has three main sections:(1)Fabrication of PI porous membranes with angstrom-scale slit channels.Swift heavy ion irradiation combined with mechanical stretching was used to fabricate PI porous membranes with angstrom-scale slit channels.The slit-shaped straight channels were constructed in PI films with slit lengths of about 40-120 nm and effective widths is about 0.9 nm.The height of slit channel is the film thickness,i.e.,12.5μm.The electric-driven ion permeation experiments showed that the PI porous membranes with slit channels had outstanding ion separation performance.This facile and controllable strategy for the preparation of porous membranes with?-scale slit channels has promising applications.The polymeric porous membranes constructed by this strategy will have great potential for applications in water purification,extraction of lithium resources and energy conversion.(2)Study of the effect of external factors on the separation performance of PI porous membranes with?-scale silt channels.To investigate the potential of the polyimide porous membranes with?-scale slit channels in practical applications,we investigated the effects of p H and concentration of feed solutions on the ion separation performance of the PI porous membranes,respectively.In addition,we investigated the effect of different driving voltages on their ion separation performance.We further measured the ionic separation performance of PI porous membranes with?-scale slit channels in a binary ion solution environment.The results provide powerful experimental data to realize practical applications of the PI porous membranes with?-scale slit channels.(3)Exploration of ionic transport mechanism.In order to elucidate the transport mechanism of ions in PI porous membranes with?-scale slit channels,we first systematically measured the I-V curves of ions permeate through the membrane and the contact angle of the membrane surface in PI porous membranes prepared under different strain conditions.The experimental results indicate that the transport of ions in PI porous membranes with?-scale slit channels is mainly related to the ions dehydration and the electrostatic interaction between ions and carboxyl groups(COO~-)on the wall of the channels.Then,the temperature dependence of the transmembrane conductance of different cations were measured,and the energy barriers of different cations across the membrane were calculated in combination with the Arrhenius equation to explain the selectivity between different cations.Finally,drift-diffusion experiments were used to systematically measure the reverse potential generated by different electrolyte solutions across the membrane with the concentration gradient,and then the Henderson equation and the computational equation of conductivity were combined to obtain the ion mobility of different cations across the membrane,which further elucidated the transport mechanism of ions in PI porous membranes with?-scale slit channels.