Computer simulation of structure and ferroelectric phase transition of vinylidene fluoride copolymers

Molecular chain motions in the ferroelectric phase transition of vinylidene fluoride (VDF)–trifluoroethylene (TrFE) copolymers have been studied by using molecular dynamics (MD) technique. The MD calculation was made for the crystal structures of VDF 50 and 70 mol% copolymers. In the ferroelectric phase or the low temperature phase, the parallel arrangement of the CF2 dipoles of the zigzag chains were found to be the energetically stable packing structure of the chains. The slightly deflected trans-zigzag chains were found to librate with relatively large amplitudes around their chain axes. In the non-polar paraelectric phase or the high temperature phase above the phase transition temperature, the molecular chains were found to take the statistically-disordered conformations of TG and T3G. The molecular motion became very vigorous by coupling of the rotational motion with the exchange motion between the trans and gauche conformations. These simulations were found to be consistent with the experimental data of solid-state NMR, IR, Raman, and X-ray diffraction. The transition temperatures calculated for VDF 50 and 70 mol% copolymers were about 425 and 480 K, respectively, corresponding relatively well to the experimental data.