| Polyvinylidene fluoride(PVDF), an engineering material, becomes outstanding from the fluorine polymers due to its excellent physical and chemical properties. And it has attracted much interests because it can be used as functional materials. Unique performance of PVDF is closely related to its profuse crystal forms, especially to its polar β form and γ form. However, in most cases the α-form rather than β-form is usually obtained. In addition to electric polarization and mechanical stretching the β-form can be achieved by tailory of crystallization of PVDF with additiveties. Recently, as a new style of green solvent, ionic liquids(IL) possesses a lot of peculiar properties, such as negligible vapour pressure, broad electrochemical window, high thermal stability, and so on. Ionic liquids generally consists of organic cations and anions under room temperature, thus a composite material with good performance can usually be got through simple blending IL with polymer. Especially in the blends containing PVDF, there has a strong coulombic forces between IL ions and PVDF molecular chains. It influences the crystallization behavior of PVDF significantly, and thus the generation of the β form with piezoelectric effects. Therefore, influence of ILs, the crystallization behavior and phase morphology of PVDF and its blends were systematically studied. In this thesis the main results are as follows:(1) Ionic liquid was incorporated into PA6/PVDF blends through melt mixing. The compatibility, mechanical properties and the crystallization behavior of the blends were systematically studied. It is indicated that the presence of the ionic liquid in the blends promotes the transformation of sea-island structure into co-continuous morphology, which to a large extent accounts for the improvement in toughness. In addition, it is believed that the enhanced mobility of the PA6 matrix by the ionic liquid also contributes to the high impact toughness.(2) PVDF membranes with exclusive piezoelectric β-crystal were directly prepared by immersion precipitation from a mixed solvent containing an ionic liquid, and the effect of coagulation temperatures and IL/DMF ratio on the water flux and crystallization habits, was studied. The addition of the IL in DMF favors the crystallization of PVDF in advance of liquid–liquid demixing, which becomes significant at low coagulation temperatures and high IL loadings. As a result, particulate morphology and high water flux are achieved in the membranes. What is more, the presence of the IL promotes the crystallization of PVDF into exclusive β-form without any trace of α- and γ-forms at low coagulation temperatures, irrespective of the IL loadings. It should arise from the generation of TTTT conformation in the mixed solvents due toiondipole interactions between the IL and polar PVDF chains.(3) IL was blended with PVDF via melt mixing. And effect of IL loadings and temperature on the crystallization properties of PVDF was studied. The results indicated that PVDF with 2wt% IL can get high degree of γ form during isothermal crystallization at 155℃. The form can be easily transformed into β form by mechanical stretching at 60℃。... |
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