Effect of polar rotation on the formation of porous poly(vinylidene fluoride) membranes by immersion precipitation in an alcohol bath

Microporous poly(vinylidene fluoride) (PVDF) membranes were prepared by isothermal immersion precipitation of PVDF/N,N-dimethylformamide (DMF) casting dopes in different alcohol baths. The formed membranes can be divided into two groups, based on the morphology and crystal type of the globular particles constituting the membrane matrix. The particles in the first group of membranes appear like a hydrangea flower composed of petal-like crystal lamellae. It was formed when precipitation was carried out in a lower-carbon alcohol bath, e.g. 1- or 2-propanol. On the other hand, when higher alcohol bath such as 1-butanol or 1-octanol bath was used as the coagulant, the formed membranes (group II) comprised compact globules whose surfaces were covered with granular identities of ca. 30 nm dia. XRD and FTIR-ATR analyses of the membranes indicate that group I membranes contain both alpha- and gamma-type crystals, whereas group II membranes contain only gamma-type crystals. Transformation of PVDF chains from gamma- to alpha- conformation due to dipole rotation during membrane formation was delineated by F-19 NMR spectroscopy. The crystallinities of the membranes were determined both by diffraction peak deconvolution and differential scanning calorimetry. Both methods indicated that the group I membranes had a crystallinity of ca. 66%, which was 5% lower than that of the group II membranes. (C) 2016 Elsevier B.V. All rights reserved.