Ferroelectric Transition in Vinylidene Fluoride and Trifluoroethylene Copolymers Studied by NMR Method II. 52 and 65 mol% Vinylidene Fluoride Copolymers

Abstract

NMR derivative line shapes f′(ΔH) in vinylidene fluoride (VDF)/trifluoroethylene (TrFE) copolymers with VDF contents of 52 and 65 mol% were measured as a function of the alignment angle γ between the drawn axis and magnetic field over a temperature range between 20 and 130°C. The fine structure of f′(ΔH) and the motional narrowings of the line widths in the each copolymer were investigated by comparing them with those for VDF 72 mol% copolymer. In the ferroelectric phase, f′(ΔH)’s in VDF 65 and 72 mol% copolymers consisted of three components of narrow, intermediate and broad line widths (ΔHs, ΔHm, and ΔHb), ΔHm and ΔHb which were due to the TrFE and VDF groups, respectively; f′(ΔH) in the VDF 52 mol% copolymer had two components of ΔHs, and ΔHb. The value of ΔHb was almost equal to ΔHm in 65 and 72 mol% copolymers. When VDF content decreased, the flip-flop motion of TrFE groups in the VDF 72 mol% copolymer changed into rotational motion around the chain axis. Near the Curie temperature, Tc two kinds of trans VDF group motion around the chain axis and TrFE group motion coordinate with the VDF rotational motion disappeared in the VDF 52 mol% copolymer. At the Tc, ΔHb discontinuity for each copolymer was observed, but the thermal hysteresis of ΔHb vs. temperature decreased as VDF content decreased, and was not significant in the VDF 52 mol% copolymer. These findings suggest that the ferroelectric phase transition is of the first order above VDF 65 mol%, and of the first order close to the second order for VDF 52 mol%.