| Poly(vinylidene fluoride) (PVDF) is a multifunctional polymer material. It has attracted much attention due to its excellent performances, e.g., heat resistance, chemical stability and workability. In the field of environmental science, PVDF has often to be used as filtration material. However, the PVDF molecules have a good affinity and limits its applications. In addition, the widely used PVDF microfiltration membranes are often prepared through wet-forming process, which can not endow the membranes with good mechanical properties. So, it is necessary to shed light on the structural development methods. Undoubtedly, the study results will bear very important theoretical meaning and practical value to the production of PVDF microfiltration membranes with excellent comprehensive performaces.Firstly, we have prepared PVDF microfiltration membranes through wet phase inversion process. During this process, polyvinyl alcohol (PVA) has been used as a hydrophilic modified agent. The effects of DMF concentration, molding temperature, PVDF and PVA content on the structure and properties of PVDF microfiltration membranes has been investigated. It is revealed that both the cell size and Fig.er-shaped pore decrease with the increment of DMF. The double defusion is reduced in the high and internal pore structure when the bath temperatures are above 40 ℃. The double defusion increases and then the PVDF microfiltration membranes with bigger pore sizes are formed. The tensile strength of the PVDF microfiltration membranes will be enhanced by adding the concentration of PVDF or reducing the DMF concentration in coagulating bath. Furthermore, the contact angle of PVDF films is reduced from 108° to 63°, suggesting that the addition of PVA helps to improve the hydrophilicity of PVDF films.The thermodynamic compatibility in multi-component polymer blend affect greatly its phase separation and the inner pole structure. Next, we have chosen the thermodynamic compatibility misbible PVDF/PBA and immiscible PVDF/PCL blend systems and studied the effects of blend ratios and temperautes on the phase searation and the inner pole structure. It is shown that the miscibility is improved by the addition of PCL or increasing the temperature in PVDF/PCL blend. At higher temperatures the pore size becomes more uniform in the two blend systems. In addition, the tensile strength of PVDF ultrafiltration membranes increases with the increment of PCL or PBA. This is because the structure of the membranes becomes more imcompact in PCL-rich or PBA-rich blend. Comparatively speaking, the porous structure is more uniform in the miscible PVDF/PBA blend than in the immiscible PVDF/PCL blend. However, the tensile strength of PVDF/PCL is higher than PVDF/PBA.Finally, we have studied the interaction of the two components during the crystallization process to clarify the structural development of the phase separation in PVDF/PCL blend. The results show that the miscibility is different in the blend with different blend ratios. The blend with high PVDF conent is miscible, while the blend with high PCL content is immiscible. The thermodynamic results reveal that the crystallization rate of PCL in 3/7 PVDF/PCL blend is higher than that in pure PCL or other blend ratios. Subsequently, we have prepared highly orientated PVDF films and studied the epitaxial crystallization behavior of PCL on PVDF. The electron diffraction results reveal that the PCL melecules are parallel to the PVDF molecules and there exists a perfect lattice matching between them. As a result, in the PVDF/PCL blend, the pre-crystallized PVDF component acts as the nucleation agent and induces the crystallization of the PCL component. |
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