Solid-state 19F MAS and 1H→19F CP/MAS NMR study of the phase-transition behavior of vinylidene fluoride–trifluoroethylene copolymers: 2. semi-crystalline films of VDF 75% copolymer

Abstract

The changes in the crystalline structure and molecular mobility in the semi-crystalline vinylidene fluoride (VDF) and trifluoroethylene (TrFE) copolymer P(VDF75/TrFE25) caused by the ferroelectric→paraelectric phase transition were analyzed using variable temperature solid-state 19F magic angle spinning (MAS) and 1H→19F cross polarization/MAS nuclear magnetic resonance spectroscopy in the temperature range of 42–129 °C. The conformational exchange between trans and gauche at the VDF and VDF–TrFE head-to-head linkage gradually increases at temperatures above 77 °C, and the conformational exchange at the head-to-tail linkage has a key role in the ferroelectric→paraelectric phase transition. The anomalous decrease in the amorphous peak intensities and the presence of a T1ρF value similar to that of the crystalline domain at 122 °C indicate that cooperative motion occurs in both phases just above the transition temperature (Tc). The amorphous domain is assimilated by the crystalline domain upon lowering the temperature from 129 to 85 °C, which significantly increases the crystallinity to below Tc, as indicated by the shape of the spectra, and identical T1ρF and T1F values for the two domains at 85 °C. Changes in the crystalline phase structures and molecular mobility of a semi-crystalline P(VDF75/TrFE25) have been analyzed using solid-state 19F MAS and 1H→19F CP/MAS NMR spectroscopy. The conformational exchange motion at the head-to-tail linkage has a key role for the ferroelectric→paraelectric phase transition. The anomalous decrease in the signal intensity of the amorphous peaks and its T1ρF value similar to the crystalline domain at 122 °C indicate the existence of cooperative motion occurring in both phases just above the transition temperature (Tc).