Density functional theory study of the structure and electric properties of poly(vinylidene cyanide trifluoroethylene) copolymer ultrathin film

The geometry, energy, internal rotation barrier, vibrational spectra, dipole moments and molecular polarizabilities of poly(vinylidene cyanide-trifluoroethylene) (P(VDCN-TrFE)) 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 trifluoroethylene (TrFE) content on the copolymer chain stabilities, chain conformations, electric properties and vibrational spectra were examined and compared with those of the poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) copolymer and the polyvinylidene cyanide (PVDCN) homopolymer to explore whether the ultrathin film of P(VDCN-TrFE) possess an expected good piezoelectricity or not. Based on the internal rotation potential curves of P(VDCN-TrFE) dimer models (H(CH2C(CN)2-CFHCF2)H and H(C(CN)2CH2-CF2CFH)H), the conformational angles, relative stabilities of α- and β-conformations and the transition energy barriers of β→α and α→β were discussed. The results show that the β-conformation is more stable than the α-conformation and the β→α transition in P(VDCN-TrFE) is more difficult than that in PVDCN. The energy difference per monomer unit between β- and α-chain P(VDCN-TrFE) copolymers with a variety of TrFE contents is smaller than that in PVDCN homopolymer, suggesting that α→β conformational change would be easier in the copolymer than in the homopolymer. The ideal β-chain of P(VDCN-TrFE) alternate copolymer with 50 mol% TrFE has a radius of about 16 Å, which is almost twice as large as PVDCN (8.5 Å) and the dipole moment contribution per monomer unit in the copolymer is even larger than the alternate P(VDF-TrFE) copolymer. The energy difference per monomer unit between the β- and α-chains decreases with increasing TrFE content. The contribution of average dipole moment per monomer unit in the β-chain is affected by the chain curvature and TrFE content, and there is a parabolic dependence on the VDCN content. The mean polarizability decreases with increasing TrFE content slightly. This work also predicted that there are some characteristic vibrational modes that may be used in identification of the α- and β-P(VDCN-TrFE) with different TrFE content.