MEASUREMENT OF THE INTERNAL ELECTRIC-FIELD IN A FERROELECTRIC COPOLYMER OF VINYLIDENE FLUORIDE AND TRIFLUOROETHYLENE USING ELECTROCHROMIC DYES

This paper presents a measurement of the internal electric field induced by poling in a feroelectric copolymer of 75 mol % vinylidene fluoride and 25 mol % trifluorethylene (VDF-TrFE copolymer) using the electrochromism of dyes, 4-(dimethylamino)-4'-nitrostilbene (DANS) and 2-methyl-4-nitroaniline (MNA), dissolved in the copolymer. As a result of poling, the absorption intensity decreases, the peak position of the original band shifts to longer wavelength, and the width of the band slightly increases. These changes in the absorption spectra of DANS and MNA after removal of the poling field have been interpreted to assess the internal electric field E(i) created by the oriented crystallite dipoles in the VDF-TrFE copolymer. E(i) values were calculated from an analysis of the differential spectrum after and before poling, using theoretical expressions involving the first and second derivatives of the original spectrum. E(i) values increase with increasing remanent polarization P(R), achieving a value of 3.4-3.9 MV/cm at P(R) = 6.1-mu-C/cm2 after poling at a maximum electric field of 1.15 MV/cm. In the process of thermal annealing after poling, E(i) increases slightly in the vicinity of 80-degrees-C and above this temperature gradually decreases until an abrupt decrease occurs near 135-degrees-C corresponding to the total loss of polarization at the Curie temperature. Furthermore, a new absorption band arises and its intensity increases upon poling at higher electric field or upon thermal annealing after poling. This new absorption band is ascribed to that of a protonated species of the dye which implies that HF is generated upon poling and thermally dissociates into H+ and F-. The intensity ratio of the new to the original band increases with increasing annealing temperature up to 80-degrees-C but decreases above 80-degrees-C, suggesting that the generated HF is lost from the sample film above 80-degrees-C. Therefore, the slight increase of E(i) in the vicinity of 80-degrees-C may be due to a loss of ionic countercharges at crystal-amorphous interfaces. Somewhat higher E(i) and higher P(R) are measured for a sample annealed prior to poling.