Abstract
Poly(vinylidene fluoride) (PVDF) membranes were prepared by non solvent induced phase separation (NIPS), melt spinning and the solution-cast method. The effect of preparation methods with different membrane formation mechanisms on crystallization behavior and tensile strength of PVDF membranes was investigated. Fourier transform infrared spectroscopy-attenuated total reflectance (FTIR-ATR) and X-ray diffraction (XRD) were employed to examine the crystal form of the surface layers and the overall membranes, respectively. Spherulite morphologies and thermal behavior of the membranes were studied by polarized light optical microscopy (PLO) and differential scanning calorimetry (DSC) separately. It was found that the crystallization behavior of PVDF membranes was closely related to the preparation methods. For membranes prepared by the NIPS method, the skin layers had a mixture of α and β phases, the overall membranes were predominantly α phase, and the total crystallinity was 60.0% with no spherulite. For melt spinning membranes, the surface layers also showed a mixture of α and β phases, the overall membranes were predominantly α phase. The total crystallinity was 48.7% with perfect spherulites. Whereas the crystallization behavior of solution-cast membranes was related to the evaporation temperature and the additive, when the evaporation temperature was 140 °C with a soluble additive in the dope solution, obvious spherulites appeared. The crystalline morphology of PVDF exerted a great influence on the tensile strength of the membranes, which was much higher with perfect spherulites.
Highlights
Poly(vinylidene fluoride) (PVDF) as a semi-crystalline polymer, is widely used as a commercial polymeric membrane material because of its outstanding properties: excellent chemical resistance, thermal stability and high mechanical strength [1,2,3,4,5]
PVDF membranes were prepared through non solvent induced phase separation (NIPS), melt spinning and the solution-cast method
These results demonstrate again that it is the crystallization rate that determines the crystalline phase of PVDF
Summary
Poly(vinylidene fluoride) (PVDF) as a semi-crystalline polymer, is widely used as a commercial polymeric membrane material because of its outstanding properties: excellent chemical resistance, thermal stability and high mechanical strength [1,2,3,4,5]. It is well-known that PVDF has four crystalline forms: the nonpolar α form and the polar β, γ and δ forms [6,7]. Various methods, which can help to control the crystalline structure and improve the tensile strength of existing PVDF membranes
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