Browsing by Author "Legrand, J. F."

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N.m.r. study of the ferroelectric phase transition in a 70/30 mol% copolymer of vinylidene fluoride (VF2) and trifluoroethylene (TrFE)
(1985-07) Legrand, J. F.; Schuele, Paul J.; Schmidt, V. Hugo
Nuclear magnetic resonance (n.m.r.) studies of 19F nuclei in a 70/30 mol% random copolymer of vinylidene fluoride and trifluoroethylene were performed at 9.14 MHz and 20.0 MHz. The free induction decays (FIDs) were analysed in terms of two T2 components attributed to the amorphous and crystalline portions of the polymer. The changes in crystallinity as well as the effects of the ferroelectric transition were observed during cycles of heating and cooling between 20°C and 140°C. The crystalline component of the FID lengthens by a factor of 2 at 100°C on heating and decreases by this factor at 60°C on cooling, thus exhibiting the thermal hysteresis of this ferroelectric transition. The spin-lattice relaxation was also investigated. From measurements at 9.14 MHz the observed longitudinal relaxation time T1 appears to be dominated by the dynamics of the amorphous phase and exhibits no anomaly through the phase transition. However, from measurements at 20 MHz, well defined minima of T1 were observed, which are associated with the ferroelectric transition (especially after repeated annealing of the samples). Results are discussed in terms of the crystalline phase structure, which appears dynamically disordered above the ferroelectric phase transition. An analogy is considered with the plastic phase transitions encountered in molecular crystals.
Neutron Scattering Studies of the Ferroelectric Transition in P (VDF-TrFE) Copolymers
(1990-09-01) Legrand, J. F.; Frick, B.; Meurer, B.; Schmidt, V. Hugo; Bee, M.; Lajzerowicz, J.
In P(VDF-TrFE) random copolymers with compositions from 60/40 to 80/20 mol%, the ferroelectric phase transition exhibits a strong first order character and is accompanied by a large expansion of the crystal lattice. In the ferroelectric structure, the polymer chains have an all-trans conformation and are close-packed parallel to one another in a pseudo-hexagonal lattice. According to the m2m symmetry, the spontaneous polarization Py is perpendicular to the chain axis 2. In the paraelectric phase, the structure is disordered : the chains undergo continuous conformational changes, they have a statistical Cm symmetry and the expanded lattice is hexagonal (6/m point group)