Studies of Pyroelectric Dynamics of Polymers by means of Temperature Jump Spectroscopy

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A charge response is observed for a wide range of time in a poled ferroelectric polymer due to its pyroelectricity when its temperature is raised suddenly by applying a YAG laser pulse with a duration of several nanoseconds. This work is aimed to understand the dynamics of pyroelectricity by analyzing quantitatively the pyroelectric response curves. In addition to a YAG laser Surelite-I and a high speed digitizing oscilloscope TDS62O in our laboratory, we bought a second-harmonics-generating crystal. We also developed software to measure and anaylize the respons. As a result, the measurement in a range of 1 ns to 10 s became possible. We used a copolymer of vinylidene fluoride VDF with trifluoroethylene TrFE of content of 75/25 mol%. A film was obtained by casting from solution with dye. After poled by a high ac field, the film was subjected to a laser pulse, being connected to the oscilloscope. The pyroelectric response curves thus obtained show that there are several modes of response : the instantaneous mode, two oscillating modes and a gradual decreasing mode which is due to cooling of the sample. Oscillating modes result from the mechanical oscillation induced by the abrupt themal expansion. The thickness extensional and the length extensional mode oscillations are induced and are observed as oscillations of charge response as a result of coupling with piezoelectricity. The instantaneous mode corresponds to the pyroelectric primary effect which is only ca. 20% of total response. The response curves were also measured at elevated temperatures. The amount of the response increases as the temperature is raised up to just below the Curie temperature, and then decreases as the polarization vanishes. The oscillatory modes are found to vanish prior to the instantaneous mode at the Curie temperature.