A study of thermodynamics and kinetics pertinent to formation of PVDF membranes by phase inversion

Poly (vinylidene fluoride) (PVDF) is a family of hydrophobic fluoropolymers with a unique polarity that enables certain solvents to dissolve them. Because of the selective solubility, PVDF membranes can be fabricated by the phase inversion technique. In this work, the thermodynamic and kinetic aspects pertinent to membrane formation by the phase inversion process were investigated for different grades of PVDF polymers developed by Arkema Inc. The phase separation data (thermodynamics) and the rates of solvent evaporation and polymer precipitation (kinetics) were determined. The phase boundary determines the nonsolvent content that the polymer system can tolerate without causing precipitation, while the kinetics of polymer precipitation influences not only the ultimate structure of the membrane but also the gelation time required for the membrane to solidify during the wet step. As the polymer precipitation is due to exchange of solvent in the polymer system with the nonsolvent in the gelation bath, the polymer precipitation kinetics was characterized by the solvent-nonsolvent exchange rate and the additive leaching rate. The partial evaporation of solvent from the surface of nascent membrane in the dry step, which influences subsequent polymer precipitation in the wet step, was quantified by an empirical equation. (C) 2012 Elsevier B.V. All rights reserved.