Computer modeling and molecular dynamics of polarization switching in the ferroelectric films PVDF and P(VDF-TrFE) on nanoscale

In this paper, molecular models are used to investigate and analyze the structure and polarization of polyvinylidene fluoride (PVDF) and poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) Langmuir–Blodgett (LB) nanofilms, depending on the structure and composition of the monomers of their polymer and copolymer chains. Quantum-mechanical calculations and modeling, as well as molecular dynamics (MD) simulations based on semi-empirical quantum-chemical methods (such as PM3), show that the energy of the studied PVDF and P (VDF-TrFE) molecular structures, and their polarization switching proceed by homogeneous switching mechanism in the framework of the phenomenological theory of Landau–Ginzburg–Devonshire (LGD) in the linear approximation of low values of the electric field. The magnitude of the resulting critical coercive field is within the $E_C\sim0.5\dots2.0$ GV/m, which is consistent with experimental data. It is also found that the uniform polarization switching mechanism of the polymer chains PVDF and P (VDF-TrFE) is due to the quantum properties of the molecular orbitals of the electron subsystem: the applied electric field induces a gradual shift of the electron "clouds" density (electron polarizability), which in turn causes a gradual shift of the nuclear cores, in accordance with the principle of minimum total energy of the system, and this leads eventually, when it reaches a critical point (bifurcation) — to overturn of the entire chain and a sharp decrease in the total energy of the total system to its energetically more favorable state. This is clearly seen in both the polarization hysteresis loops, and the total energy changes. In this case, the turnover chain time, obtained by molecular dynamics within semi-empirical quantum-chemical PM3 approach in a limited Hartree–Fock approximation, when approaching this critical point, increases sharply, tending to infinity, which corresponds to the theory of LGD.