DENSITY FUNCTIONAL THEORY STUDY OF STRUCTURE AND ELECTRIC PROPERTIES OF POLY(VINYLIDENE CYANIDE TETRAFLUOROETHYLENE) COPOLYMER ULTRATHIN FILM

Abstract

The geometry, energy, internal rotation barrier, vibrational spectra, dipole moments, and molecular polarizabilities of poly(vinylidene cyanide‐tetrafluoroethylene) (P(VDCN‐TeFE)) of α‐ and β‐chain models were studied with density functional theory at B3PW91/6‐31G(d) level. The effects of chain length and the tetrafluoroethylene (TeFE) content on the copolymer chain stabilities, conformations, electric properties, and vibrational spectra were examined and compared with those of the poly(vinylidene fluoride‐tetrafluoroethylene) (P(VDF‐TeFE)) copolymer and the polyvinylidene cyanide (PVDCN) homopolymer to explore whether the ultrathin film of P(VDCN‐TeFE) possesses an expected good piezoelectricity or not. The results show that the β‐conformation is more stable than the α‐conformation, the β→α transition is more difficult, and the α→β transition is easier in the copolymer than those in the homopolymer, predicting that a good piezoelectricity in the copolymer is hopeful. The contribution of average dipole moment per monomer unit in the β‐chain is affected by the chain curvature and VDCN content. The alternate P(VDCN‐TeFE) with 50 mol% TeFE has better piezoelectric properties than the alternate P(VDF‐TeFE). The mean polarizability decreases with increasing TeFE content. This work also predicted some characteristic vibrational modes that may be used in identification of the α‐ and β‐P(VDCN‐TeFE) with different TeFE content.