Possible reasons that piezoelectricity has not been found in bulk polymer of polyvinylidene cyanide

The structure, energy, internal rotation, dipole moments and molecular polarizabilities of poly(vinylidene cyanide) (PVDCN) of α- and β-chain models were studied with density functional theory at B3PW91/6-31G(d) level. The effects of chain length on the polymer chain stabilities, chain conformations and electric properties were examined and compared with those of the piezoelectric PVDF in order to explore the possible reasons that the piezoelectricity has not been observed in the bulk polymer of PVDCN experimentally. The results show that the energy barriers (11.5 and 6.9 kJ/mol) of PVDCN in its α- → β-chain and β- → α-chain transitions are smaller than those (16.3 and 8.2 kJ/mol) of PVDF, which promises an easier transition between the two conformations. This may be one of the reasons that the preparation of the stable piezoelectric phase will be difficult. However, an extremely important feature of the ideal β-chain is that the PVDCN chain is curved with a radius of only about 8.5 Å, which is much smaller than that (30.0 Å) in PVDF and is smaller than that (12.6 Å) in polymethylvinylidenecyanide (PMVC). This suggests that the bulk polymer of PVDCN is difficult to be prepared into crystal phase, and the contribution of the dipole moment per unit to the total dipole will be cancelled if the chain is longer than 12 units (hemicycle). Therefore, the reasons that the piezoelectricity has not been found may be due to the orientation difficulties of chains and dipoles.