同步辐射X射线散射研究聚乙烯微孔隔膜加工物理

发布时间：2018-03-16 06:09

本文选题：高分子物理　切入点：聚乙烯微孔膜　出处：《中国科学技术大学》2017年博士论文　论文类型：学位论文

【摘要】：锂离子电池广泛应用于3C产品,已经成为人类不可或缺的消费品。延长电池使用寿命、增强充放电稳定性,无疑是消费者对3C产品的普遍期待。微孔隔膜作为锂离子电池非常重要的组成材料,其失效会导致锂离子电池正负电极的直接接触并引发短路。由此可见,微孔隔膜对于锂离子电池的安全性、寿命以及充放电稳定性具有重要的作用。而研究和明确锂离子电池微孔隔膜制备过程中的基础物理问题,才能有效地改善其性能。尽管针对锂离子电池微孔隔膜的制备已经有较多的研究,但是多数的研究还是基于干法聚丙烯(PP)和湿法聚乙烯(PE)微孔膜的研究,对干法PE微孔膜的系统研究还比较少,加工过程中存在的很多问题和难点还没完全认识清楚。本论文为了回答干法PE微孔膜制备过程中涉及的流动场诱导结晶、机械能和热效应对微孔成核与生长的作用、高度取向片晶簇无定形区的变形模式、片晶簇在应变-温度空间的变形机理等物理问题,选取高密度聚乙烯(HDPE)为研究对象,结合高时间、空间分辨的同步辐射X射线散射、扫描电镜(SEM)、透气性测试等研究手段,从预制膜的制备到微孔膜后拉伸研究了微孔膜制备过程中的多尺度结构演变,为高性能HDPE微孔膜的研究积累了基础数据和产品开发思路。本论文的主要结果和结论总结如下:1)通过改变熔体挤出-流延过程中口模温度和流延拉伸比等工艺参数来改变熔体结晶过程中的流动场强度制备了含不同结构的HDPE预制膜,预制膜通过一种新型拉伸方法——受限拉伸制备了微孔膜。通过X射线小角散射(SAXS)和X射线广角散射(WAXS)测得了预制膜的结构参数如长周期、片晶侧向尺寸、取向度和纤维晶含量,微孔膜的形貌则通过SEM进行表征。本论文发现纤维晶含量和取向度在预制膜制备及拉伸成孔过程中是矛盾的。平衡这一对矛盾的窗口为纤维晶含量为6.4%左右,此时微孔膜可实现最大孔隙率和最均匀的微孔结构。本论文还提出了旋转片晶、平行排列的取向片晶、纤维晶对于微孔的形成扮演的角色的模型,并构建了 HDPE预制膜纤维晶含量-预制膜微观结构-成孔能力的关系。2)采用原位和离线SAXS跟踪冷拉后的HDPE膜在低温和升高温度下两步应力松弛的结构演变,应力松弛过程中保持拉伸应变恒定。研究发现冷拉诱导的片晶簇密度涨落在应力撤去时能完全回复,因此将冷拉诱导的密度涨落称为微孔胚胎。低温下机械能释放使微孔胚胎演变成微孔核存在一段诱导期,并在机械能继续释放下非均匀生长,最终通过热拉放大为连通性差的非均匀分布微孔结构。而在升高温度下的机械能释放,微孔胚胎能够转化为均匀分布的微孔和纤维晶,可作为热拉过程中连通孔的扩展位点,进而通过热拉放大的微孔膜不仅微孔分布和孔径均匀而且透气性好。本工作暗示低温下机械能释放导致的微孔成核实际上不利于微孔均匀生长,冷拉后通过热效应固定微孔结构是制备窄孔径分布和高透气性的微孔膜的有利条件。3)采用时间分辨原位SAXS跟踪跨越两个数量级的拉伸速率下含高度取向片晶簇结构的HDPE预制膜的结构演变,发现不同速率对应于不同的变形模式,与无定形区分子链的松弛响应有关。低速率下,松弛时间短的结构,如缠结密度小、末端链等区域容易被拉开,拉开之后变形容易应力集中,进一步扩展成非均匀形变,造成二阶峰在整个形变过程中都较弱。高拉伸速率下,各级结构对应的松弛时间和拉伸速率都不匹配,不容易发生松弛,而容易使无定形区发生均匀形变,导致二阶峰增强,非均匀信号(Icav)减弱。本工作研究的区间Ⅰ(0.04 mm/s-0.4 mm/s)在拉伸过程中主要发生非均匀形变,产生空洞化;区间Ⅱ(0.4mm/s-2.4mm/s)是非均匀形变和均匀形变共存,且发展能力相当;区间Ⅲ(2.4 mm/s-4mm/s)主要发生拉伸诱导片晶簇分离引起密度涨落的均匀形变。本论文有望为无定形区松弛时间谱的分析提供新思路。4)选取含高度取向片晶簇的预制膜,通过改变冷拉参数制备了 HDPE微孔膜。制备过程中片晶簇的演化和微孔的形成用原位和离线SAXS、WAXS以及SEM进行跟踪。研究发现在25℃下冷拉,当应变低于60%时,冷拉应变主要影响片晶簇分离程度,更高应变下则主要发生晶体滑移。冷拉温度低于60℃能够促进片晶簇的分离,而在冷拉温度高于60℃时片晶中的分子链容易被拉出形成微纤。此外,本论文揭示了最后微孔膜的透气性和片晶的分离程度呈线性关系。
[Abstract]:Lithium ion batteries are widely used in 3C products, has become an indispensable consumer goods. Prolonging the service life of the battery charge and discharge, enhance the stability, is undoubtedly the common expectation of consumers 3C products. As an important component of lithium ion battery materials, microporous membrane, its failure will lead to direct contact with the negative electrode of lithium ion batteries and cause short circuit thus, safety of microporous membrane for lithium ion batteries, which plays an important role in life and discharge stability. And the research and preparation of microporous membrane for lithium ion batteries prepared clear basic material in the process of the problem and can effectively improve the performance of lithium ion battery. Although the microporous membrane preparation has been researched however, most research is based on the dry and wet polyethylene polypropylene (PP) (PE) of microporous membrane, system of dry PE microporous membrane research is still relatively Little, many problems and difficulties in the process is not fully understood. In this paper, in order to answer the preparation process of dry PE microporous membrane system involved in flow induced crystallization, mechanical and thermal effects on the pore nucleation and growth effect of deformation mode of highly oriented lamellar clusters of amorphous region, lamellae in the cluster temperature space strain deformation mechanism and other physical problems, selection of high density polyethylene (HDPE) as the research object, combined with the high time, synchrotron radiation X ray scattering spatial resolution, scanning electron microscopy (SEM), permeability test and other research methods, from the pre prepared membrane preparation to microporous membrane was studied after tensile microporous membrane system evolution of multiscale structure in the preparation process, to accumulate basic data and the product development research for high performance HDPE microporous membrane. The main results and conclusions are summarized as follows: 1) by changing the melt extrusion die flow extension process The temperature and the casting process parameters to change the ratio of melt crystallization in the process of flow field strength of prepared HDPE films containing different prefabricated structures, prefabricated film with a new drawing method of constraint were prepared by X microporous membrane. Small angle X-ray scattering (SAXS) and wide angle X ray scattering (WAXS) the structure parameters such as long cycle measured pre film, crystal lateral dimensions, orientation and content of Xian Weijing, morphology of microporous membranes were characterized by SEM. It is found in this paper are contradictory Xian Weijing content and degree of orientation in preformed membrane preparation and micro pore forming process. The balance of the contradiction between the window for Xian Weijing the content is about 6.4%, the microporous membrane can achieve maximum porosity and the uniform pore structure. This paper also puts forward rotation of lamellae, lamellae oriented parallel to the formation of microporous, Xian Weijing plays the role of the model, and the Construction of HDPE prefabricated fibrous membrane - membrane micro - structure content of prefabricated hole forming ability.2) HDPE membrane by in situ and off-line SAXS tracking after cold drawing should structure evolution of stress relaxation at low temperature and high temperature under the two step, should maintain a constant tensile strain relaxation process. The study found that cold induced crystal clusters the density fluctuation in the stress response can be completely removed, so the density fluctuation induced by cold called microporous embryos. Low temperature mechanical energy release of the porous embryo evolved into microporous nucleus there is a section of the induction period, and the mechanical energy to continue to release under non uniform growth, through thermal amplification for the poor connectivity of non uniform distribution the microporous structure at elevated temperatures. And the release of mechanical energy into uniform micropore and microporous fibrous embryos can be, can be used as the extension locus in the process of hot hole, then through thermal amplification Microporous membrane not only microporous distribution and uniform pore size and good permeability. This work suggests that low temperature mechanical energy release caused by microvoid nucleation in fact is not conducive to the uniform microporous growth, after the cold drawing of the thermal effect of fixed pore structure is prepared with narrow pore distribution and favorable conditions of high permeability microporous membrane of.3) by the time evolution of structure resolution in situ SAXS tracking across HDPE prefabricated membranes containing highly oriented lamellae cluster structure is two orders of magnitude of strain rate under the different rate corresponding to different modes of deformation, and no relaxation of the amorphous region of molecular chain response. Low rate, structural relaxation time is short, such as entanglement density is small, the terminal chain etc. area easily opened, opened after the easy deformation of stress concentration, and further extended into non homogeneous deformation, caused by the two order peak is weak in the whole deformation process. High tensile rate, the structure of the corresponding levels The relaxation time and stretching rate does not match, not prone to relaxation, and easy to make the amorphous region of uniform deformation, resulting in two order peak enhancement, inhomogeneous signal (Icav) decreased. In this work, the interval I (0.04 mm/s-0.4 mm/s) non homogeneous deformation mainly occurred during the stretching process, produce hollowing; section II (0.4mm/s-2.4mm/s) is a non uniform deformation and uniform deformation coexist, and development ability; section III (2.4 mm/s-4mm/s) occurred mainly stretch induced lamellar cluster separation caused by uniform deformation density fluctuation. This thesis is expected to provide new ideas for the analysis of.4 no area of the relaxation time spectrum) from prefabricated membranes containing highly oriented crystal clusters, by changing the parameters of cold drawn HDPE microporous membrane was prepared. The preparation process and the evolution of micro crystal clusters formed by in situ and offline SAXS, WAXS and SEM on track. Now 25 DEG C when drawn, Strain is less than 60%, the main effect of cold drawn strain lamellae cluster separation degree, higher strain is mainly crystal slip. Cold temperature lower than 60 DEG C can promote the separation of crystal clusters, and in the cold temperature higher than 60 DEG C when the molecular chains in crystalline form is easy to pull out micro fiber. In addition, this paper reveals there was a linear relationship between the final microporous membrane permeability and lamellar separation degree.

【学位授予单位】：中国科学技术大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：TM912;TB383.2

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