Study on preparation of PVDF-CTFE porous membrane by low temperature thermally induced phase separation
In this paper,PVDF-CTFE porous membranes were prepared by using low temperature thermally induced phase separation(L-TIPS)method.The effects of glycerin content and coagulation bath temperature on the membrane formation mechanism and properties were investigated.The results showed that TIPS and NIPS can be promoted by adding glycerin.On the one hand,glycerin as a non-solvent,with the increase of its content,the temperature difference between cloud point temperature and coagulation bath temperature was increased.Thus the TIPS effect was strengthened,and the membrane structure presented a bicontinuous microporous structure caused by TIPS effect rather than the typical finger-like voids structure caused by NIPS effect.On the other hand,glycerin served as a pore-forming agent during the NIPS process.Therefore double diffusion process was promoted by adding glycerin and porous structure was more likely to generate with the increase of glycerin content,NIPS and TIPS effects competed against each other as the glycerin content increasing and the TIPS effect dominated in the end.Thus,thinner skin layer structure of the membrane and changes of the membrane cross-section structure from dense packing to porous honeycomb was observed.The permeability of the membrane was improve due to aforementioned structural changes.The PVDF-CTFE hollow fiber porous membrane possessed the most comprehensive properties when the content of glycerin was 7.5％(mass fraction),the tensile strength was 2.6 MPa,and the pure water flux reached 860 L/(m2·h·MPa).With the increase of coagulation bath temperature,the difference between cloud point temperature of casting solution and coagulation bath temperature was reduced,TIPS effect was weakened,meanwhile the mass transfer process of NIPS effect was strengthened.The membrane structure changed from honeycomb structure to finger-like voids structure,and the thickness of skin layer increased,which leads to the increase of porosity and pure water flux of membrane and the decrease of mechanical properties.When the coagulation bath temperature reached 80 ℃,the pure water flux of PVDF-CTF E flat porous membrane was 1 510 L/(m2·h·MPa),and the tensile strength was 2.1 MPa.
polyvinylidene fluoride-trifluorochloroethylenelow temperature thermally induced phase sepa-rationmembrane forming mechanismporous membranecoagulation bath temperature
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