Structure and electric properties of Poly(vinylidene cyanide-tetracyanoethylene) copolymer predicted with density functional theory

Abstract

The geometry, energy, internal rotation barrier, dipole moments, and molecular polarizabilities of poly(vinylidene cyanide-tetracyanoethylene) (P(VDCN-TeCN)) of α- and β-chain models were studied with DFT at B3PW91/6-31G(d) level. The effects of chain length and TeCN content on the copolymer chain stabilities, chain conformations, and electric properties 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 P(VDCN-TeCN) possess an expected good piezoelectricity or not. Based on the internal rotation potential curves of P(VDCN-TeCN) dimer models (H[CH₂C(CN)₂–C(CN)₂C(CN)₂]H and H[C(CN)₂CH₂–C(CN)₂C(CN)₂]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 stability of the β-conformation increases and the β→α transition in P(VDCN-TeCN) is more difficult than that in PVDCN. The energy difference per monomer unit between the β- and α-chains decreases with increasing TeCN content. The contribution of average dipole moment per monomer unit in the β-chain is affected by the chain curvature and TeCN content, and there is a weakly parabolic dependence on the VDCN content. For the same chain length, the calculations show that the dipole moment contribution per monomer unit in the P(VDCN-TeCN) with 0.5 molar fractions TeCN is smaller than either the β-chain PVDCN or the β-chain P(VDCN-TeFE). The chain length does not produce a significant change in the mean polarizability for either the α- or the β-P(VDCN-TeCN); however, the value increases with increasing the TeCN content in P(VDCN-TeCN)s.