Investigation of Phase Separation Behavior for PVDF Membrane Formation

The phase separation mechanisms of thermally induced phase separation (TIPS) and nonsolvent induced phase separation (NIPS) were investigated for poly(vinylidene fluoride) (PVDF)/solvent/diluent system for PVDF membrane fabrication. Di-butyl phthalate (DBP) was used as a diluent and n-methyl-2-pyrrolidone (NMP) was used as a solvent. Image changes via TIPS were successfully captured by using a hot stage and an optical microscope, and NIPS images were observed at the interface between dope solution and nonsolvent. Addition of NMP to PVDF/DBP system lowered the crystallization temperature and retarded the spherulite growth to result in the formation of big spherulites, which was confirmed by the flat membrane fabrication. Addition of DBP to PVDF/NMP system decreased the miscibility of the system for binodal curve to be shifted to the left hand side, which expanded the unstable region. Finger-like structure was disappeared and dense skin layer was formed at the interface due to the rapid extraction of NMR Phase separation mechanism depended on the composition of solvent and diluent, and the corresponding structures were formed.