Far infrared and submillimetre studies of the ferroelectric phase transition in (VF2-F3E) copolymers

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Abstract
Polarized transmission spectra of oriented (VF2-F3E) copolymer film were measured in the 8–500 cm−1 range for the VF2/F3E composition of 100/0, 80/20, 70/30 and 60/40 as a function of temperature in both ferroelectric and paraelectric phases. The spectra were evaluated to obtain the dielectric response function and absorption coefficient. In comparison with pure PVF2, the lowest-frequency mode at 73cm−1 splits in the copolymer samples into two components at ∼51–61 and ∼ 115 cm−1 which are assigned to Brillouin zone centre and boundary (Z-point) librational modes around the polymer chain axis with respect to the Brillouin zone of the ideal PVF2 crystal. The lower mode of this pair disappears in the paraelectric phase in agreement with selection rules for regular 50/50 copolymer. Higher-frequency modes do not strictly obey the selection rules because of compositional and dynamical disorder. In the submillimetre range, 8–30 cm−1, additional relaxation (over-damped oscillation) was found which is assigned to IR active standing acoustic waves within the lamellar crystallites broadened by their thickness distribution. Differences between the absorption in crystalline and amorphous phase are quantitatively evaluated for an unoriented sample.