Modeling of switching and piezoelectric phenomena in polyvinylidenefluoride (PVDF)

The molecular models of ferroelectric polymer polyvinylidene fluoride (PVDF) film, consisting from one, two chains [-CH2-CF2-]_n and for PVDF unit cell are investigated in this work. HyperChem 7.5/8.0 for all modelings as for quantum calculations as well for molecular mechanics and molecular dynamic simulations (MD) were used. The first-principle approach is applied to the switching and kinetics of this model. Kinetics of polarization switching show a homogeneous critical behavior with a critical point at Landau-Ginzburg-Devonshire (LGD) coercive field E = E_C. Two types of behavior were established for 2 PVDF chains: simultaneous and sequential rotation in low and high electric field. For a one-chain model we obtained a hysteresis loop for PVDF with an LGD intrinsic coercive field of E_C ~ 1 GV/m, while for a two-chain model E_C ~ 2 GV/m). These data are related with arisen of the PVDF piezoelectric effects, especially of the negative piezoelectric coefficients. Both PVDF molecular chains and a unit cell of crystalline β-phase PVDF were modeled. Molecular modeling and first principles calculations show that the piezoelectric coefficient d33 has negative values in the range d₃₃ ~ -16.5..-33.5 pC/N (pm/V), corresponding to known data, and allowing us to explain the reasons for the negative sign of the piezoresponse. For value of ε ~ 5, we obtained a value of another oriented coefficient d31 ~ +15.5 pC/N. This computational study is corroborated by measured nanoscale data obtained by atomic force and piezoresponse force microscopy (AFM / PFM).