Study On Inorganic Crystal Growth Induced By Polymer And Preparation Of Polypropylene Separator Film For Lithium Battery

This thesis is composed of two sections: first, study on inorganic crystal growth induced by polymer; second, preparation of polypropylene separator film for lithium battery.Section 1: Study on inorganic crystal growth induced by polymerBaWO4 with a scheelite structure (tetragonal phase) is an important material in the electro-optical industry due to its emission of blue luminescence. Because of its interesting stimulated Raman scattering (SRS) properties, BaWO4 is also a potential material for designing all-solid-state lasers emitting radiation in a specific spectral region. In this study, spindly BaWO4 crystals are synthesized using a high molecular weight N heterocyclic homopolymer, poly 4-vinylpyridine (P4VP), as a template. The size and shape of the spindly BaWO4 crystals can be controlled through adjusting the pH value and the polymer concentration of the solution. The well-defined shape of BaWO4 crystals are characterized with scanning electron microscopy, transmission electron microscopy and X-ray diffraction. The formation mechanism of the spindly BaWO4 crystals is suggested according to the experimental results. As a result with increasing of the polymer concentration, the dimensions of the obtained spindly BaWO4 crystals become more uniform and smaller; spindly BaWO4 crystals can not be obtained in solution with very low and very high concentration of Ba²⁺ and WO4^2-; as the pH value of solution below 4, spindly BaWO4 crystals can not be observed because of the nitrogen atoms of P4VP are protonized by H⁺ which made the P4VP lose the templating ability; when the P4VP are grafted onto silicon substrate, the BaWO4 is found to form nucleus on the substrate, which can become larger in size and assembled to rod like structures.Section 2: Preparation of polypropylene separator film for lithium batteryLithium battery has attracted more and more attentions due to their excellent performance, and the battery performance is greatly affected by the properties of separator film. Many new methods and materials have been utilized to prepare separator to meet the needs in applications. Stretching is an important method used to prepare polymer separator. In this thesis, separator prepared by stretchingβform polypropylene (β-PP) is systematically studied. The effect of technical factors on the micropore formation and development are discussed. The main results are summarized as follows:(1) Study on the morphology and structure of the PP precursor sheetThe morphology and structure ofαandβ-PP and the growth ofβcrystal induced by nucleating agent are studied by several methods such as Atomic Force Microscopy, Differential Scanning Calorimetry, Scanning Electronic Microscopy (SEM) and Polarized Optical Microscopy. The effect of technical factors on the morphology and structure of the PP precursor sheets are discussed. The results show that there are significant differences in morphology between a andβ-PP: much more defects existing in theβ-crystalline lamellae, while less defects existing in the a-crystalline lamellae. The defects inβ-PP precursor sheet may act as the favor to form pores during stretching. Theβ-crystalline lamellae induced byβnucleating agent form parallel edge-on lamellar structure which perpendicular to specific crystal face of nucleating agent. This parallel lamellar structure also favors to form pores during stretching. Highβ-crystalline content PP precursor sheet can be prepared by casting the PP melt on the casting roller at about 120℃. The parallel lamellar structure can be obtained by increasing the running speed of the casting roller. And the temperature of the extruding die has a significant effect on the properties of PP films.(2) Preparation ofpolypropylene separator film by stretching methodThe micro pore morphology and structure formed after stretching the precursor sheets are characterized by SEM, and porosity is measured by gas permeation equipment. The effect of technical factors on the morphology and structure of micropore and porosity are discussed. It is found that, the pore size and porosity increase with increasing of stretching ratio and further increase of stretching ratio may lead to the decrease of the porosity. Separator with high porosity can be obtained by stretching at lower temperature, however, too low temperature may lead to film broken. Uniform film with narrow size distribution can be obtained at low stretching rate. Although higher stretching rate can increase separator yield, large pores will form in the film surface with too high stretching rate. Meanwhile the connectivity of the pores inside the separator is reduced.