Solubility and H-bonding of poly(vinylidene fluoride) copolymers in carbonyl liquids: Experiment and molecular simulation

Abstract

H-bond formation among fluoropolymers and carbonyl solvents was studied by microcalorimetry and quantum chemistry simulation. Typically, fluorine atoms in organic molecules depress their ability to molecular interactions with solvents. Meanwhile, it is not the case if fluoropolymer is dissolved in carbonyl solvent. Among other organic solvents carbonyl-containing liquids are characterized by very large exothermic effects of dissolution of vinylidene fluoride copolymers which are typical for the systems with electron-donor-acceptor complexes. The enthalpy of mixing in the entire concentration range was determined for P(VDF-co-HFP) and P(VDF-co-CTFE) solutions in ethyl acetate and Flory-Huggins parameter was evaluated. It was −1.31, which means very strong interaction among solvent and vinylidene fluoride monomer units. This interaction was modeled by quantum chemistry semi-empirical AM1, PM3, PM5 and density functional B88-PW91, B88-LYP, D-VWN calculations for the complexes between acetone or ethyl acetate and vinylidene fluoride tetramer. The heat of formation of the complex from its components was found around −18 kJ/mol. The binding site of the complex was a macrocyclic structure which comprised H-bonds between carbonyl and methylene hydrogens of vinylidene fluoride backbone, and weaker H-bonds between fluorine atoms and methyl hydrogen. The analysis of molecular orbitals of the complex reveals extended electron delocalization across the binding center.