Effect of solvents on morphology and polymorphism of polyvinylidene fluoride membrane via supercritical CO₂ induced phase separation

Abstract

Poly vinylidene fluoride (PVDF) membranes were prepared via supercritical CO 2 induced phase separation. The effects of solvent power on PVDF membrane morphology and polymorphism were investigated using N-N-dimethylformamide (DMF), triethyl phosphate (TEP), and their mixture respectively. The morphology evolution including cross-section and surfaces were thoroughly studied by scanning electron microscope (SEM) and atomic force microscopy (AFM). The differences of solubility parameters between the solvent and PVDF affected the phase separation and the resultant morphology. The various crystalline phases of the membranes were mainly investigated by Fourier transform infrared spectroscopy (FTIR) and X-ray diffractometer (XRD). Solvent with larger dipole moment tended to form polar b phase. Decreasing the difference of solubility parameters favored the formation of a phase. Furthermore, the effects of salt additive on PVDF membrane morphology and crystalline form were studied as well. Results turned out that lithium chloride (LiCl) induced a porous top surface and boosted the formation of b phase. V C 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 41065.

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