Vibrational spectroscopic study of the ferroelectric phase transition in vinylidene fluoride-trifluoroethylene copolymers: 1. Temperature dependence of the Raman spectra

Abstract

Raman spectral changes in the ferroelectric phase transition have been measured for a series of vinylidene fluoride-trifluoroethylene (VDF-TrFE) copolymers with VDF molar contents of 73, 52, 37 and 0%. In the vicinity of the transition temperature, the trans band at 845 cm−1 decreases in intensity and the gauche band at 800 cm−1 increases. Such an intensity exchange between the trans and gauche bands has been found to become more diffuse and incomplete over a wider temperature region as the VDF content decreases. By coupling these Raman spectral data with the X-ray diffraction data (00l meridional reflections), the structural regularity in the gauche-type crystalline phase has been compared among these copolymer samples. For the copolymer with higher VDF content the regular TGTḠ conformation with a little residue of TT sequences exists in the high-temperature phase; while for the sample with lower VDF content, some trans sequences are still included in the polymer chains even in the high-temperature phase and the structure is in a more disordered state. This may be due to the high probability of interrupting the VDF monomer sequences by an intrusion of TrFE units and the resulting stability of the trans form relative to the gauche form for these lower VDF copolymer samples.