Structure and piezoelectric properties of poly(vinylidene fluoride) studied by density functional theory

The internal rotation, geometry, energy, vibrational spectra, dipole moments and molecular polarizabilities of poly(vinylidene fluoride) (PVDF) of α- and β-chain models were studied by density functional theory at B3PW91/6-31G(d) level. The effects of chain lengths and monomer inversion defects on the electric properties and vibrational spectra were examined. The results show that the tgtg′ conformation angle between g and g′ is about 55° and the ttt conformation is a slightly distorted all-trans alternating planar zigzag with ±175° repeating motif. The average distance between adjacent monomer units in the β-PVDF is 2.567 Å. The energy difference between the α- and β-chains is about 10 kJ/mol per monomer unit. The dipole moment will be affected by chain curvature (with a radius of about 30.0 Å for ideal β-chain) and by defect concentration other than localization. The chain lengths and defects will not significantly affect the mean polarizability. The calculations indicated that there are some additional characteristic vibrational modes that may help identification of the α- and β-phase PVDF.