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 stru cture of PVDF membranes formed in a wet phase inversion process were studie d. 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 wa s found to approach and/or intersect the binodal, leading to a favored situ ation for liquid-liquid demixing to dominate the initial stage of precipita tion. This was well demonstrated in the cellular asymmetric morphology as o bserved by SEM and AFM imaging. The top surface of this membrane was compos ed 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 mem brane 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 precipitate d at higher temperatures (e.g., 65 degrees C) from 1-octanol/DMF, the forme d membrane became largely in the "alpha" form (type II) crystal structure.