Effect of Green Solvents in the Production of PVDF-Specific Polymorphs

Fluoropolymers constitute a unique class of materials with a combination of interesting properties that has attracted significant attention from material researchers over the past few decades. Generally, these polymers have high thermal stability, improved chemical resistance, and low surface tension because of the low polarizability and the strong electronegativity of the fluorine atom, their small van der Waals radius and the strong C-F bond. Because of its combination of good properties and processability, poly(vinylidene fluoride) (PVDF) has been widely used as a membrane material, especially for new membrane operations, including membrane contactors in membrane distillation, membrane-assisted crystallization and membrane condensers. These membrane operations are strongly influenced by the membrane's surface properties (energy, topography, morphology, etc.). The probability that properties of membranes also change as a function of their polymorphism is high. There are still challenging issues, beyond the relationship between polymorphism and mechanical strength (?-phase PVDF has some specific properties such as polarity and higher mechanical strength compared with ?-phase) and the correlation between polymorphism and membrane morphology/surface chemistry/hydrophobicity; i.e., surface-related properties are of interest for use in membrane contactors. The present work focuses on the effect of the interactions between PVDF chains and solvent molecules on the structure of PVDF obtained as experimental films by solution casting. We performed molecular dynamics simulations to probe the affinity of solvent molecules toward PVDF molecular chains and specifically versus ?-PVDF. The theoretical results confirmed that specific solvents such as methyl-5-(dimethylamino)-2-methyl-5-oxopentanoate (commercially available as Rhodiasolv PolarClean) can influence the formation of a ?-phase due to specific hydrogen bridges with the PVDF chains, whereas others like acetyl tributyl citrate (ATBC) cannot affect the ?-phase due to the lack of interactions. These outputs give useful insights on the relationship between PVDF crystalline forms and membrane preparation conditions and can offer interesting perspectives for the design of novel advanced membranes, particularly for new membrane operations.