Effects of Sample Processing and High-Energy Electron Irradiation Conditions on the Structural and Transitional Properti
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EXPERIMENT The PVDF (x) - TrFE (l-x), copolymers powder having x = 50 mol % was supplied by Solvay and Cie, Bruxelles. The unstretched films were prepared either by hot pressing followed by slow cooling. The stretched films were prepared by uniaxially stretching the quenched hot pressed or solution cast unstretched films up to 5 times to their initial length at 25'C. In order to enhance the crystallinity, the stretched films were annealed under the clamped condition at 140 0 C for 16 hours. The electron irradiation was carried out in a nitrogen atmosphere with 2.55 MeV electrons at 95°C for several irradiation doses ranges from 40 to 100 Mrad. The polarization hysteresis loops were measured using a Sawyer-Tower technique. An external electric field was applied in the form of triangular waveform with a frequency 10 Hz and amplitude 150 MV/m. The DSC measurements were taken at a scanning rate 10°C/min under a nitrogen atmosphere using differential scanning calorimeter (TA instrument, model no. 2010).
The X-ray pattern were taken at room temperature (20'C ) using Scintag diffractrometer (model PAD-V) with Ni filtered CuKax radiation. The IR measurements were carried out in the range of 4000 to 400 cm' spectral region using Bio-Rad (model no. FfS 175) FTIR spectrophotometer. The measurement on the crosslinking factor was performed by measuring the gel content of irradiated films using the American standard test method (D2765-95) [4].
Unstretched (A)
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Non-irradiia~ted
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Figure 1: X-ray diffraction scans of (A) unstretched (B) stretched, P(VDF-TrFE) 50/50 mol % copolymer films irradiated with 0 different doses at 95 C, using 2.55 MeV electrons.
RESULTS AND DISCUSSIONS X-ray Results Figures 1 (A) and (B) show X-ray patterns obtained from unstretched and stretched films irradiated for different doses, respectively. Consistent with the earlier studies [5,6], before irradiation, the unstretched films exhibit two peaks at 20 =18.790 (4.72 A ) and 19.280 (4.59 A). The peak at 4.72 A is due to the hexagonal packing of 3/1-helical chains generated due to the presence of TG and TG' defects and second is due to similarly packed trans-planar chains. However, the stretched films show only one peak at 4.68 A spacing and therefore show that the stretching not only transform the chain segments containing 3/1 helical conformation but also pack more closely and more regularly the chain segments that already are in the trans-planar conformation [6]. After 40 Mrad of irradiation, only one peak is observed at a lower angle for both the unstretched and stretched films and thus clearly indicates the expansion of the lattice due to the introduction
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of defects in the crystalline phase during the irradiation. This is responsible for the observed change in the polarization hysteresis from a typical normal ferroelectric hysteresis loop to a slim polarization loop (figur
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