Study On Tensile Behavior Of Hard-elastic Polypropylene Film

The semi-crystalline polymer hard elastic film is mainly prepared by extrusion-casting method.During the melt-stretching process,the polymer melt is stretched in a short range between the extrude die and the hot roll,and the precursor film with high oriented row-nucleated lamellae is obtained.These hard-elastic materials show high yield stress,high elastic modulus and high elastic recovery properties.However,there still have some controversies about the source of high elastic of hard-elastic materials.Hard elastic materials are mainly used in the preparation of lithium-ion battery separators.In the past 30 years,a lot of research work has been done to analyze the structural evolution of hard elastic materials at different tensile stages,and a series of models about microstructureevaluation and pore structure during the tensile process have been put forward.However,most of these models can only deduce the formation process of the pore phenomenally and cannot fundamentally explain how the tensile process control the pore size.Hence,it is important to understand its hard-elastic behavior and tensile behavior and clarify the role of energy on the deformation of structure and build the relation between the energy,structure and properties.This will be conducive to guiding the service process of the hard-elastic material and the optimization of the production process for the lithium-ion battery separator.In this dissertation,the hard-elastic polypropylene film was chosen.The cyclic stretching process of polypropylene film at room temperature was studied.The evolution of microstructure for hard elastic materials was investigated.From the viewpoint of energy,the contribution of the energy elasticity produced by the surface tension of the micropore and the entropy elasticity of the molecular chain in the amorphous region to the elastic recovery of the hard elastic material was discussed.The annealing process at 45? of stretched film was studied and the response structures of mechanical property and energy in different stretching stages were clarified.The influence of the movement of the molecular chain in the amorphous region on the recovery of the properties of the hard elastic material was clarified,and the relationship between the initial structural failure and the interface failure was illustrated.These will provide a theoretical basis for controlling the size and distribution of the initial pore core in the subsequent production process.The intrinsic relationship between the structure evolution and micropore properties during hot drawing process was studied and the influence mechanism of stretching work and activation energy on the formation and evolution of micropores was clarified.The main experimental work and results were listed as follows:1.The deformation mechanism of the microstructure of the polypropylene hard elastic film during cycling load and the source of high elastic recovery at room temperature were clarified.During the stretching process,the recovery intercrystalline crazing and some nanocavities are formed.In the subsequent cyclic stretching process,the microstructure deformation is basically similar,the lamellar structure and micropore are periodically separated and recovery,the amorphous molecular chain is reversibly extended and curled.In the recovery process,when the recovery strain is larger than 60%,the surface energy is the main driver of strain recovery.While the recovery strain is less than 60%,the entropy effect causes the strain recovery.The nature of high elastic recovery for polypropylene hard elastic system at room temperature is a mixed contribution of energy elasticity and entropy elasticity.2.The annealing process at 45? of cyclic stretched film for polypropylene hard elastic system was studied by S AXS,FTIR and mechanical tests.The effects of structural changes in amorphous phase on mechanical properties of hard elastic films were discussed.It is found that the lamella structure remains basically unchanged during the annealing at 45?.The annealing process mainly leads to the recovery of the tie chain and the amorphous entangled chain.The change of the volume fraction of the tie chain leads to the increase of the initial modulus and the yield strength.The recovery of the entangled chain results in the weakening of the strain hardening and the contraction of the stress plateau.Based on the evolution of micro-structure and mechanical behavior during the annealing process at 45?,a double-network model which contains the lamella-tie chain network and the amorphous entangled chain network is proposed.During the initial stretching phase of the hard elastic material at room temperature,the hard network of structure lamellar stack and tie chain is mainly stretched.In the strain plateau region,the interfacial crazing and the molecular chain between entanglement point are stretched.While in large strain region,the entanglement network is stretched and deformed,resulting in obvious strain hardening.This model helps to understand the relationship between the structural evolution in amorphous phase and mechanical properties of hard-elastic film during the stretching process.In production process for the lithium-ion battery separator,the formation of micropore is mainly controlled by the molecular chain movement in the amorphous region.Increasing the tensile temperature suitably will contribute to pore control.3.The influence of temperature on micropore morphology of hard-elastic polypropylene film during uniaxial tensile deformation was systematically studied.The stretching-induced microstructure such as lamellar clusters,fiber crystal and micropores were tracked by in-situ X-ray scattering.The correlation between micropore formation and the activation volume and the relation between stretching work and the new surface area after stretching were considered from mechanical parameters and the SAXS results.It is found that stretching temperature mainly affects the activation volume and the stretching work,whereas the activation volume influences the thickness of lamellar clusters and micropore density,the stretching work determines the new surface area.The higher stretching temperature induces few micropore nuclei and larger new micropore size,meaning that few pores with larger size are formed.This paper clarifies how the temperature affects the formation and enlarging of micropore during the uniaxial tensile deformation.The essence of the effect of stretching temperature on the performance of microporous membranes is that the difference in energy dominates the evolution of microstructure,further determines the properties of the material.The relationship between the tensile work and the specific surface area of the pores represents the conservation of energy.