Structural correlation between crystal lattice and lamellar morphology in the ferroelectric phase transition of vinylidene fluoride-trifluoroethylene copolymers as revealed by the simultaneous measurements of wide-angle and small-angle X-ray scatterings

Intimate correlation has been detected between the crystal structure change and the morphological change of stacked lamellae in the ferroelectric phase transition of vinylidene fluoride-trifluoroethylene (VDF-TrFE) copolymers on the basis of simultaneous measurement of wide-angle (WAXS) and small-angle X-ray scattering (SAXS) patterns in the heating process. The VDF content of copolymers investigated was 65 and 73 mol%. For both of the copolymers, a cold-drawn sample showed the WAXS pattern of the regular low-temperature phase and the four-points SAXS pattern corresponding to the stacking structure of lamellae tilted by about 30° from the draw axis. When the sample was heated above the Curie transition point Tc, the low-temperature phase changed to the conformationally-disordered paraelectric high-temperature phase and the stacked lamellae reoriented into the direction closer to the draw axis. It was also found that the thickness and long period of the lamellae increased remarkably above Tc. These large changes in lamellar stacking mode occurred in parallel with the trans-to-gauche conformational change of the molecular chains in the crystal lattice. In this way, the morphological change was found to relate intimately with the crystal structural change, both of which are considered to be caused commonly as a result of the trans-gauche conformational change and the translational motion of the thermally-activated molecular chains along the chain axis.