| The uniaxial stretching is one of the main methods to prepare microporous membranes for lithium ion batteries in the industry.The polymer melt forms a cast film with an oriented lamellar crystal structure during the extrusion casting process,and the lamellar crystal structures are arranged in parallel and oriented in a direction perpendicular to the extrusion direction.Subsequently,the microporous structure is obtained through the steps of annealing,cold stretching,hot stretching and heat-setting.Raw material parameters such as molecular weight,molecular weight distribution,and degree of branching,as well as processing processes such as cold drawing process,hot drawing process and heat-setting process,affect the final pore formation structure of lithium battery microporous membranes.At present,lots of research had been carried out at home and abroad to study the pore-forming structure control and pore-forming mechanism of uniaxially stretched polypropylene microporous separators.However,the research and accumulation of the preparation technology of uniaxially stretched polyethylene microporous separators are still not perfectIn the first part of this paper,the effects of different molecular weight distribution on the structure and properties of high-density polyethylene dry-processed microporous membranes were investigated.The effects of three HDPE resins with different molecular weight distribution on rheological properties,entangled molecular weight and crystalline properties were analyzed,and the mechanical properties and pore-forming properties of the corresponding annealed HDPE films were characterized.In the second part,the HDPE1 in the first part was selected as the research object,and the structural change process of the annealed HDPE1 film at different cold stretching temperatures was tracked in real time by using the small-angle/wide-angle X-ray diffraction(SAXS/WAXS)technology and in-situ infrared spectroscopy technology.Based on the second part,in-situ tracking technique was used to explore the effect of different cold stretching rate on annealed HDPE1 film,and to clarify the effect of different cold stretching process on the pore forming properties of microporous membraneThe experimental results showed that1.Characterization of three kinds of HDPE raw material parameters with different molecular weight distributions and various properties of annealed films and microporous membrane prepared from them.Rotational rheological properties and tensile rheological properties of three HDPE raw materials were tested,and the entangled molecular weight was calculated.At the same time,DSC,SEM and mechanical properties of annealed film made of three HDPE resins were tested and the pore forming properties of the corresponding microporous membranes were characterized.The experimental results show that HDPE1 has a wide molecular weight distribution with many high and low molecular weight fractions.The entanglement and nucleation of the high molecular weight part result in the smallest entangled molecular weight of the HDPE1,4833 g/mol,the thickest lamellar crystal thickness,35.2 nm,the largest entanglement density in the amorphous region,7.97×1027 m-3 and the volume fraction of tie chain is 0.26%.HDPE3 molecular weight distribution is the narrowest,and the influence of high and low molecular weight tail is also the least.The entangled molecular weight of HDPE3 is 12064g/mol,and the entangled density of the amorphous region and tie chain volume fraction are the lowest,3.56×1027 m-3 and 0.17%,respectively.HDPE2 and HDPE3 have similar weight average molecular weights,but the molecular weight distribution of HDPE2 is wider than HDPE3.Therefore,the entangled molecular weight of HDPE2 is smaller than that of HDPE3.At the same time,under the influence of the high molecular weight portion,the entangled density of the tie chain volume fractions is larger than HDPE3.Since the entanglement density in amorphous region and tie chain volume fraction of the annealed HDPE3 film are the smallest,the molecular chains in the amorphous region are easier to stretch in the post-tension process,which is not conducive to pore formation,resulting in the largest Gurley value,the smallest average pore size and the worst pore formation performance,while HDPE1 and HDPE2 have better pore formation,which can be used to prepare microporous membranes for lithium ion batteries2.The SAXS/WAXS and infrared spectroscopy were combined with a micro-stretching device to track the stretching process of annealed HDPE1 film at different cold-stretching temperatures in real time and investigated the effect of different cold-stretching temperatures on the pore-forming properties of annealed HDPE1 film under the same hot-drawing and heat-setting process.Here,the so-called cold stretching process is compared with the later hot stretching stage.And it is found that the molecular chain mobility in the crystalline and amorphous regions increases with the increase of cold stretching temperature.It can cause the lamellar crystal to slip and produce bending deformation and improve the deformation ability of the amorphous region,reduce the degree of separation of the lamellar crystal,and reduce the pore forming performance.Therefore,the average pore size of the microporous membrane prepared by cold drawing at 120℃ is 19.8nm,the Gurley value is 342s/100mL,and the pore forming performance is poor,while the average pore size of microporous membrane prepared by cold stretching at 25℃ is 7.2 larger than that prepared by 120 nm,with better air permeability and pore forming performance3.Based on the third chapter,the effects of cold stretching rates across two orders of magnitude on the stretching process of annealed HDPE1 film are tracked in real time,and the effects of different cold stretching rates on the pore-forming properties of annealed HDPE1 film are explored.Similarly,the so-called cold stretching process is compared with the later hot stretching stage.It is found that with the increase of cold stretching rate,under small strain,the amorphous part between the lamellar crystal is mainly deformed,which makes the platelet separation greater.It is beneficial to improve pore forming performance.Therefore,the average pore diameter of the microporous membrane prepared at a cold stretching rate of 150mm/min is the largest,which is 32.3nm,and the pore forming performance is good,while the average pore diameter of the microporous membrane prepared at a cold stretching rate of 7.5mm/min is small,which is 23.9nm.And the pore forming performance is poor. |
