Electrical properties at very low frequencies of alpha PVDF subjected to thermal cycling

Abstract

Very low frequency dielectric spectra (103-10-4 Hz) show space charge build-up in alpha PVDF under the influence of a low field as the temperature is raised to 80 degrees C or above. Preheating changes very little the medium frequency alternating conductivity when gold electrodes are used, whereas with aluminium electrodes the conductivity increases substantially as excess fluorine collects under the electrodes. Experimental results are interpreted through a two-carrier (H+ and F-) conduction model based upon three assumptions: (i) an increase in H+ ion concentration during the first heating: (ii) partial deep trapping of H+ ions at high temperature; (iii) field-induced extraction of F- ns from the bulk leading to H+ enhancement of pre-exponential mobility factor. Orders of magnitude are proposed for H+ and F- ion activation energies.

Peculiarities of high-temperature dielectric relaxation in vinylidene fluoride – hexafluoropropylene copolymers

“…But we should not exclude changes in charge carriers' mobility. For example, it was shown earlier, that in case of Al electrode, near-electrode layers in fluorine polymers are characterized by formation of new functional groups [18,19] and thus at high-temperatures it can lead to some features in the dielectric spectra [20]. If in case of Ag such changes are not detected, then for Al one can expect partial bounding of F-containing ions at metal-polymer boundaries.…”

Dielectric relaxation in vinylidene fluoride–hexafluoropropylene copolymers

“…22,94 From the other hand, H þ and F À ions, which appear from dissociation of HF molecules, should be carriers in considered polymers. 95 It is supposed that HF molecules are generated at high temperatures by the dehydrofluorination reactions…”

“…It is possible to explain only by the supposition that namely fluorine ions can perform active interaction with Al. 95,96 Correlation diagrams for a-transition are shown in Figure 10. The dependences obtained from temperature changes of s dc , tand m , and M 00 m are shown separately on an insert.…”

Peculiarities of dielectric relaxation in poly(vinylidene fluoride) with different thermal history

“…The nature of ionic impurities is usually associated with remains of the polymerization process products, which is emulsion in our case. However, one should take into account that in such systems (particularly at high temperatures) dehydrofluorination reactions accompanied by formation of HF molecules are possible [18,19]. It is also known that similar products can be formed in PVDF in fields of high strength [20,21].…”

“…For the purpose of its more detailed description, the process of electrode polarization should be taken into account [8]. In the case of Al electrodes, one should not exclude chemical interactions forming new compounds at the polymer-metal boundary [19,23].…”