Liquid structuring in fluoropolymer solutions induced by water

The water-repellent nature of ferro/piezoelectric fluoropolymers such as PVDF and P(VDF-TrFE) gives rise to a rich solution phase behavior, in particular, because these materials are typically processed from hygroscopic solvents. Water, either deliberately added or condensing into an evaporating fluoropolymer solution from the processing environment, leads to structuring through liquid-liquid phase separation (demixing) into a polymer-rich and a polymer poor phase. Using ternary Flory-Huggins theory in combination with models for liquid-vapor mass transfer, as well as diffusive mass transport, we reveal that the phase-separated morphology is likely determined during the early stages of demixing. Hence, the point of demixing in composition space determines the structure size. In the case of electrospinning of fluoropolymer fibers, this point can be controlled by the deliberate addition of water to the spinning solution. In the case of film casting, one may exert control by tuning the humidity of the processing environment.