Electron Irradiated p(VDF-TrFE) Copolymers for Use in Naval Transducer Applications
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77 Mat. Res. Soc. Symp. Proc. Vol. 600 C 2000 Materials Research Society
Chemical Engineering using a homebuilt spectrometer tuned to the 19F Larmor frequency of 254.0 MHz. A Chemagnetics 3.2 mm variable temperature solids probe capable of a maximum spinning speed of 25 kHz was used to achieve high resolution magic angle spinning (MAS). RESULTS The DSC thermogram (heating and cooling cycles) of a p(VDF-TrFE) film previously annealed for 1 hour at 1400 C before irradiation is shown in figure 1. 1.2-
CRYSTALLIZATION
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The thermogram exhibits a Curie transition at about 1000 C that represents the complex solid-solid transformation from an ordered ferroelectric P3-phase into a more disordered, paraelectric ct-phase 3. The ferroelectric P-phase consists of ordered, all-trans monomer units where the fluorine atoms are arranged on one side of the packed polymer chains, giving rise to a large permanent dipole 4. In the paraelectric at-phase, the introduction of gauche bonds forces the carbon-fluorine dipoles to rotate so that the net polarization of the packed crystal cells is practically zero. Figure 2 shows typical DSC traces of p(VDF-TrFE) when subjected to electron irradiation with 1.2 MeV electron energy.
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Temperature ('C)
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What is particularly striking in figure 2 is the disappearance of the Curie transition and the broadening and shifting to lower temperature of the melt endotherm. The disappearance of the Curie transition on irradiation is consistent with the loss of ferroelectricity in the irradiated films. The loss of ferroelectricity suggests also the loss of long-range ordered crystal domains due to the formation of structural defects. Interestingly, the sample irradiated with the 20 Mrad dose still displays a vestige of the original melt endotherm. Figure 3 shows TGA weight loss curves for an unirradiated and several irradiated films. 120
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40 Mrads
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Temperature (*C)
Initially, the irradiated samples are less thermally stable, suggesting chain scission into lower molecular weight (and hence, more volatile) polymer chain fragments. At higher temperatures, the irradiated samples degraded to a lesser extent than the unirradiated sample. This observation suggests the presence of a higher crosslink density in the irradiated samples. Thus, on the basis of the TGA evidence it appears that chain scission is accompanied by formation of crosslinks during irradiation. Ultraviolet spectra of an unirradiated and several irradiated are shown in figure 4. The unirradiated film
shows a slight rise in absorption in the region from 200 nm to 360 nm. Irradiation, however, produces a broad, but distinct absorption in thi
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