Study On Several Mechanical Problems Of Polymer Membranes

With the rapid development of the science and technology,different disciplines are developing in an integrated way.Nowadays,energy,information and materials construct the modern science and affect the life of the human beings.Among them,the materials science and technology have received much attention.With the national emphasis on environmental protection and the deep utilization of natural resources,polymer materials that synthesized from high-abundance elements have grown rapidly.Due to the controllable performance,rich source of raw materials,good chemical stability,and high specific strength,polymer materials can be employed for engineering structural materials to functional materials,especially in the micro-electronic devices,wearable devices and flexible batteries.The mechanical performance greatly affects the application of the polymer materials.However,the description of the mechanical properties of polymer materials has been hindered due to the lack of experimental methods.The nanoscale mechanical properties,viscoelasticity and interface mechanics of the polymer materials have not been revealed yet.Thus,some problems in the study of mechanical properties of polymer films are selected as objects,and the method of combining theoretical calculation and experimental characterization is used to describe the microscopic mechanical properties and rheological behavior of polymer films.The contents are as follows:(1)In the first chapter,the concept,classification,preparation and application of polymer films are reviewed,and the conventional methods for studying the physical and mechanical properties of polymer films are introduced.(2)An important content of thermal mechanical behavior of polymer film is its heat-induced deformation.The change of ambient temperature will inevitably cause the deformation of polymer film,and then lead to the size change and performance decline of the device.In the second chapter,a two-layer model and a continuous multi-layer model are established to solve the thermal deformation problem of polymer films.(3)The surface viscoelastic behavior of polymer film is closely related to its application in micro/nano devices.In chapter 3,the force curve of atomic force microscope is used to study the surface viscoelastic behavior of polymer film.The viscoelastic solution of the indentation problem is obtained by modeling the spherical rigid indenter and the viscoelastic surface.This method can fit the indentation curve well.(4)The mechanical properties of polymer fiber films are mainly determined by the intrinsic properties,fiber diameter and node.The size effect of polymer fiber films is common in experimental characterization,but lacks in-depth theoretical analysis.Thus,in chapter 4,the formula of fiber size effect and characteristic scale is obtained according to strain gradient theory.The influence of fiber diameter change and inorganic filler doping on elastic modulus was studied by three-point bending experiment based on AFM,and the characteristic scales of several polymer fibers were obtained,which provided guidance for improving the mechanical properties of polymer fiber films.(5)In chapter 5,a variety of polymer fiber films are prepared by electrospinning technology,and the mechanical properties of the fiber films are regulated by means of inorganic filler reinforcement,fiber size control,and fiber orientation and so on.On this basis,the physical parameters of the fiber film,such as porosity,thermal stability and ionic conductivity,were characterized,and the application of the fiber film in the diaphragm of lithium ion battery was investigated.