Herstellung von porösen Polyvinylidenfluorid-Membranen mit maßgeschneiderten Eigenschaften durch schnelle und skalierbare dampfinduzierte Phasentrennung
Hydrophobic and highly porous poly(vinylidene fluoride) (PVDF) membranes with isotropic cross section as well as tunable and narrow barrier pore size distribution in the range from ~0.1 to ~1 μm have been prepared using the non-solvent vapor induced phase separation (VIPS) technique. The process conditions have been tuned to suit an industrial scale up, using a short production time and dimethyl sulfoxide (DMSO) as solvent for PVDF, instead of commonly used hazardous chemicals. Factors like kind of solvent, the relative humidity of air, the exposure time to humid air and the mass fraction of PVDF in the casting solutions have been used to tune membrane characteristics. Interestingly, it was revealed that DMSO as a less common solvent for PVDF shows better qualities regarding upscaling than other more frequently used polar aprotic solvents (e.g. dimethylacetamide) when using VIPS under suited conditions. The phenomenon was explained through investigations of the membrane formation step, in particular by water uptake and cloud point measurements, as well as structure and performance analyses, e.g. by scanning electron microscopy, gas flow/liquid dewetting permporometry, gas and water vapor permeability analyses and liquid water entry pressure measurements. Lab-scale manufactured membranes showed pore characteristics and performance desired for membrane contactor applications. Furthermore, fabrication on a roll-to-roll machine using a nonwoven support and the established VIPS conditions was realized in a short manufacturing time; resulting membranes structure and characteristics were found to be similar to the ones for the lab-scale membranes. By rising the dissolution temperature above a critical value, the membrane structure changes from a sponge like to a spherulitic morphology. The size and the construction of the spherulites can be influenced by the above-mentioned factors. Interestingly, for some factors a linear change resulted in a non-linear change of the resulting polymer structure. These findings were investigated through scanning electron microscopy, infrared spectroscopy and dynamic scanning calorimetry. Although interpretation of the results via ternary phase diagram plots and comparison with recent literature was done, a clear explanation is still missing. This gives rise to further studies in this field. Overall, the combination of PVDF with DMSO gives promising opportunities for a more eco-friendly industrial fabrication of porous membranes with advanced properties via VIPS.
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