Structure and electric properties of poly(vinylidene fluoride-tetrafluoroethylene) copolymer studied with density functional theory

The internal rotation, geometry, energy, vibrational spectra, dipole moments and molecular mean polarizabilities of poly(vinylidene fluoride-tetrafluoroethylene) (P(VDF-TeFE)) of α- and β-chain models were studied with density functional theory at B3PW91/6-31G(d) level and compared with those of poly(vinylidene fluoride) homopolymer and P(VDF-TrFE) copolymer. The electric properties, chain conformation and stability of the copolymer influenced by the chain length and TeFE content were examined. Based on the internal rotation curves of P(VDF-TeFE) dimer models (H[CH₂CF₂-CF₂CF₂]H and H[CF₂CH₂-CF₂CF₂]H), the conformational angles, relative stabilities of α- and β-conformation, 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 thermodynamically and the β → α transition in P(VDF-TeFE) is more difficult than that in PVDF. Thus the copolymer should be in favor of preventing the piezoelectric phase from depolarization. The ideal β-chains are curved with a radius of about 30 Å, which is very close to those in both PVDF and P(VDF-TrFE). Similar to P(VDF-TrFE), the α-chain P(VDF-TeFE) containing 0.50 mole fraction of TeFE is also a helical structure. However, the α-chain with 0.33-0.20 mole fraction of TeFE are almost linear in structure, which might be responsible for enhancing crystallinity of the copolymer. The contribution of average dipole moment per monomer unit in the β-chain is affected by the chain curvature and TeFE content, and there is a weakly parabolic dependence on the VDF content. The chain length and TeFE content will not significantly affect the mean polarizability per monomer unit. The calculations show that there are some characteristic vibrational modes that may be used in the identification of the α- and β-phase P(VDF-TeFE) with different TeFE contents.