Morphologies and crystalline forms of polyvinylidene fluoride membranes prepared in different diluents by thermally induced phase separation

We have studied the morphologies and crystalline forms of polyvinylidene fluoride (PVDF) membranes separately prepared in four different diluents bearing >C=O groups, namely 1,2-propylene glycol carbonate (PGC), dimethyl phthalate (DMP), diphenyl ketone (DPK), and dibutyl phthalate (DBP), by the thermally induced phase separation (TIPS) method. The permittivities of the diluents and PVDF were measured to compare the different PVDF-diluent systems. The results showed the permittivity of PGC to be much greater than that of PVDF, and those of DMP and DBP to be lower than that of PVDF. The permittivity difference between DPK and PVDF was not apparent above 120 °C. On cooling mixtures with a PVDF concentration of 10 wt %, PVDF crystallization was observed in the PVDF-DMP, PVDF-DBP, and PVDF-PGC systems, while liquid-liquid phase separation occurred in the PVDF-DPK system. A cross-section of the PVDF-PGC membrane presented smooth PVDF particles in the β-phase crystalline form. Those of the PVDF-DMP and PVDF-DBP membranes presented PVDF particles consisting of a fibrillar network in the α-phase. The PVDF-DPK membrane preferentially adopted an α-phase bicontinuous channel structure. When the concentration of PVDF was 60 wt %, the cross-sections of the above four membranes revealed PVDF polyhedra, among which the PVDF-DMP, PVDF-DBP, and PVDF-DPK membranes retained the α-phase crystalline form, and the diffraction peak of the α-phase became visible in the X-ray diffraction (XRD) spectrum of the PVDF-PGC membrane. ©