Influence of Polyvinylidene Fluoride Polymorphs in Membrane-Assisted Crystallization: NaCl Crystals Nucleation and Growth

Abstract

Membrane-assisted crystallization is a new unit operation for separation and/or promotion of pure crystals formation, where microporous hydrophobic membranes are used not as selective barriers but to pr omote water vapor transfer between phases inducing supersaturation in solution. Polyvinylidene fluoride (PVDF) is one of the most utilized polymers due to its excellent combination of properties and processability and has been used in a wide plethora of applications in membrane technology including membrane crystallization. PVDF has different polymorphs and this property is important in membrane technology because different phases might significantly affect the final membrane properties and performances, e.g., on membrane fouling and membrane wetting. A clear correlation between the dominating crystalline phases in PVDF and the performance in membrane contactor applications, and in particular in membrane crystallization is still missing and necessary. In this work, molecular dynamics simulation was utilized to analyze the relationship and influence of different polymorphs of PVDF membranes on crystals formation via membrane crystallization process. Atomistic simulations were used to study the crystal nucleation and growth of sodium chloride in contact with amorphous, α and β PVDF hydrophobic polymer surfaces at a supersaturated concentration of salt. Predictions about the ability of α and β polymorphs to influence the nucleation and growth of salts were obtained. The results specified the crystals (as cluster) size distributions, the size of critical nuclei, and the nucleation rate. The amorphous PVDF led to the formation of smaller but more regular clusters in reference to the other samples; α PVDF produced much more inhomogeneous crystals and of small dimensions; βPVDF produces crystals of larger dimensions than α PVDF.