All-atom molecular dynamics study on the non-solvent induced phase separation: Thermodynamics of adding water to...

The change in the thermodynamics when adding water in poly(vinylidene fluoride) (PVDF)/N-methyl-2-pyrrolidone (NMP) solution is studied from all atom molecular dynamics (MD) simulations. This is done by estimating the free energy of mixing of PVDF/NMP solution with increasing volume fraction of water (ϕ_w) using an appropriately chosen thermodynamic cycle and the Bennett acceptance ratio method. The MD calculations predict the thermodynamic phase separation point of water/NMP/PVDF to be at ϕ_w = 0.08, in close agreement with the experimental cloud point measurement (ϕ_w = 0.05). Examining the enthalpic and entropic components of the free energy of mixing reveals that at low concentrations of water, the enthalpy term has the most significant contribution to the miscibility of the ternary system, whereas at higher concentrations of water, the entropy term dominates. Finally, the free energy of mixing was compared with the Flory-Huggins (FH) free energy of mixing by computing the concentration-dependent interaction parameters from MD simulations. The FH model inadequately predicted the miscibility of the PVDF solution, mainly due to its negligence of the excess entropy of mixing.