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 NMP. Phase separation mechanism depended on the composition of solvent and diluent, and the corresponding structures were formed.

Poly(vinylidene fluoride)(PVDF) 분리막 제조를 위하여 PVDF/희석제/용매 시스템에서의 열유도 상분리와 비용매유도 상분리 메커니즘을 관찰하였다. 희석제로 di-butyl phthalate(DBP)를, 용매로 n-methyl-2-pyrrolidone(NMP) 를 사용하였다. 열유도 상분리 과정의 이미지변화는 hot stage와 광학현미경을 이용하여 관찰하였고, 비용매 유도 상 분리 과정의 이미지변화를 고분자용액과 비용매의 접촉 계면에서 관찰하였다. PVDF/DBP 시스템에 NMP를 첨가할 경우 결정화 온도가 낮아지며 구정 성장이 지연되었고 불규칙하고 큰 구정들이 형성되었음을 평막을 제조하여 확인하였다. PVDF/NMP 시스템에 DBP를 첨가할 경우 시스템의 miscibility가 감소하여 바이노달 곡선이 왼쪽으로 이동하며 불안정 영역이 확장되었다. DBP를 첨가할 경우 finger-like 구조가 사라지며 NMP의 급격한 추출에 의해 계면 부분에서 dense top skin layer가 형성됨을 평막을 제조하여 확인하였다. 용매와 희석제의 혼합공정에서 용매와 희석제의 함량에 따라 상분리 메커니즘은 변화되며 이에 따라 고유한 분리막 구조가 형성되었다.

Keywords: thermally induced phase separation; nonsolvent induced phase separation; PVDF membrane; phase separation mechanism