Study On Evolution And Regulation Of Pore Structure In Polyolefin Microporous Membrane

Due to their excellent comprehensive properties,polyolefin microporous membranes are widely used in several fields,such as seawater desalination,wastewater treatment,lithium battery separators,air purification,etc.In recent years,with the rapid development of new energy technology,the performance requirements of polyolefin microporous membranes which act as important components of lithium battery membrane are increasingly improved.During the preparation process of microporous membranes,the influence of numerous processing parameters on product quality is so complicated that it is difficult to coordinate thickness and porosity with overall quality and ensure good quality stability.In view of the preparation process of microporous membranes,the relationship between process,structure and performance is discussed in depth to realize the regulation of pore structure,which is of great significance to improve the performance of microporous membranes.In this paper,the effects of processing parameters on the crystalline structure and performance of films were systematically studied,and the evolution mechanism of pore structure was analyzed,which provided some reference for the preparation of high-performance microporous membranes.For the key preparation process of precursor films,based on Taguchi method,taking melt draw ratio,air gap,extrusion temperature and casting roll temperature as experimental factors,taking elastic recovery rate as evaluation index,the influence of processing parameters on the hard elasticity of precursor film was studied by signal-to-noise ratio analysis,and the optimal combination of processing parameters was obtained.In order to further explore the crystalline structure changes of polyolefin materials during casting,the crystallization behavior of polyolefin melts was studied by wide-angle X-ray diffraction and small-angle X-ray scattering analysis.The mechanism of how melt draw ratio influence crystallinity,periodicity,orientation degree and tensile properties of precursor casting films was discussed and the relationship between crystalline orientation and pore-forming properties of precursor films was analyzed.Annealing can improve the crystalline structure,crystallinity and hard elasticity of precursor films.In this paper,taking annealing temperature and annealing time as experimental variables and improving the elastic recovery rate of precursor films as optimization objective,the response surface model between the experimental variables and optimization objective was established.By variance and residual analysis,the effect of annealing parameters on hard elasticity of precursor films was discussed,and the optimum annealing conditions were determined.On this basis,the relationship between annealing temperature and crystalline structure and properties of precursor films was further studied.In view of the tensile-induced micropore formation process which directly affects and determines the final morphology and properties of microporous membranes,the deformation mechanical behavior of precursor films at different temperatures was analyzed,the crystalline structure changes at different strain stages were studied,the formation and evolution of micropore were analyzed,the effects of tensile temperature and draw ratio on the pore structure and properties of microporous membranes were explored,and the different tensile properties are explained.