Effect of temperature on the formation of microporous PVDF membranes by precipitation from 1-octanol/DMF/PVDF and water/DMF/PVDF systems

The effects of precipitation temperature on the morphology and crystal structure of PVDF membranes formed in a wet phase inversion process were studied. The liquid-liquid demixing line (binodal) and gelation phase boundaries of the systems were determined over the temperature range 25-85 degrees C. As the temperature was raised, the crystallization-induced gelation line was found to approach and/or intersect the binodal, leading to a favored situation for liquid-liquid demixing to dominate the initial stage of precipitation. This was well demonstrated in the cellular asymmetric morphology as observed by SEM and AFM imaging. The top surface of this membrane was composed of stacks of crystalline lamellae, with interlamellar distance estimated to be 13-20 nm by AFM. At lower temperatures (e.g., 25 degrees C), the membrane solution precipitated into a uniform morphology composed of spherical crystallites that exhibited the "beta" form (type I) crystal structure, as was evidenced by the WAXD analysis. By contrast, when PVDF was precipitated at higher temperatures (e.g., 65 degrees C) from 1-octanol/DMF, the formed membrane became largely in the "alpha" form (type II) crystal structure.