MOLECULAR MODEL OF POLARIZATION SWITCHING AND NANOSCALE PHYSICAL PROPERTIES OF THIN FERROELECTRIC LANGMUIR-BLODGETT P(VDF-TrFE) FILMS

ABSTRACT

This paper reports computer simulations (accompanied by nanoscale measurements) performed in order to understand the molecular mechanism of the polarization switching in the polyvinydilene flurouride and its copolymer with trifluoroethylene [P(VDF-TrFE)], using quantum-chemical methods based on the HyperChem 7.5 software. The simulated data for P(VDF-TrFE) were calculated for different copolymer contents (70:30), (60:40) and for pure PVDF. The calculated values of the dipole moment and average polarization of the molecular chains show well defined hysteresis under varying electric field with polarization saturated at known data ∼0.1…0.14 C/m². The calculated coercive fields (for model of the rotation of molecular chains) are consistent with experimental data for thin P(VDF-TrFE) (70:30) films, prepared by Langmuir-Blodgett (LB) technique (E _c = 5–18 MV/cm). The piezoresponse force microscopy (PFM)) measurements of the local hysteresis loop (at fixed tip position) at same LB samples demonstrated clear ferroelectric switching. The molecular dynamic simulation performed by HyperChem 7.52 tool for the molecular model of 4 free polymer chains slab provides very fast rotation (relaxation) time ∼0.4 ps. The analysis show that this model is most close to thick LB films, while in thin LB films the strong electrostatic dipole interaction with substrate must be taken into account.

ACKNOWLEDGMENTS

Work was supported by INTAS-05–1000008-8091 and RFBR 05–02-16871. V. S.B. and D. A. K thank to FCT (Portugal) for SFRH/BPD/22230/2005 and SFRH/BD/22391/2005.