Computer simulation of structure and ferroelectric phase transition of vinlidene fluoride copolymers. 5. Influence of orientational disorder of dipole moments and domain walls on phase transitional behavior

摘要

Molecular dynamics (MD) simulations have been performed in order to clarify the mechanism of ferroelectric phase transition in vinylidene fluoride-trifluoroethylene (VDF-TrFE) copolymers. Influence of orientational disorder of CF_2 dipoles and domain walls on the ferroelectric phase transition behavior has been investigated for VDF 50 mol.% copolymer system. Two types of MD unit cells were constructed by packing the 36 and 16 infinitely long chains, each of which consisted of random sequences of five VDF and five TrFE monomer units under the 3D periodic boundary conditions. For the models with all the CF_2 dipoles arrayed in parallel, the trans-to-gauche conformational change occurred at 450-500 K. Introduction of orientational disorder of one or two chains to the model, in which the dipoles are oriented into the opposite direction along the b axis, caused the reorientation of these dipoles to the original parallel direction at low temperature and the trans-to-gauche conformational change at 450 K. However, by making the dipole packing more irregular, the trans-to-gauche conformational change was found to occur quite easily even below the room temperature. An existence of the domain boundary, which was made in-between the domains consisting of the parallel packing of dipoles, was also found to cause similar structural changes. In this way, the irregular dipole array and/or the domain boundary were found to induce the trans-gauche structural transformation more easily at lower temperature, different from the models of regular dipole arrangements.