Non-Electrical Poling in Novel Ferroelectric Polymers
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107 Mat. Res. Soc. Symp. Proc. Vol. 600 ©2000 Materials Research Society
energy poling".) When the dipoles of the polymers can rotate or move thermally, the molecule only located near both surfaces should orient for attaining the energy minimum. If the polymers show ferroelectric behavior, the dipole orientation of the polymer should be as far as the bulk state because of the high cooperativity of the dipoles. In crystals, the dipole orientation is governed by crystallization factors, such as nucleation and crystal growth, therefore it may be difficult to orient bulk dipoles. On the other hand, in an amorphous material with a polar aggregation, it would be possible to get the dipole orientation up to a certain thickness. In this paper, we will follow up the possibility of surface energy poling in polymers showing ferroelectric behavior. We has been found recently several ferroelectric polymers, such as polythioureas [10], polycyanophenylenesulfides,[8] polyurethanes [11,12] and poly(vinylfluoride-trifluoroethylene)[ 13], having a large dipole moment, and a large dielectric relaxation strength ( more than 40). Experimental Ferroelectric polymers ware obtained by polymerization methods previously reported.[713] Figure 1 shows the chemical structure of these polymers. Surface energy was estimated from advancing contact angles of water, glycerol, formamide, diiodidomethane and tricrecyl phosphate on the film surfaces poled. The contact angle was measured at 20'C using an Erma contact anglemeter with a goniometer, model G-I. From the contact angle data the surface energy was estimated according to Kaelble's method [14] In electrical poling, polymer films (surface energy : 33-45 mJ/m 2) were coated on indium tin oxide(ITO) glass slide or Aluminum coated glass slide with higher surface energy (more than 40mJ/m 2 ) and were given the upper electrode by aluminum evaporation. (Fig.2 a) In surface energy poling, the upper electrode was changed to 10xtm Teflon FEP film with lower surface energy (15 mJ/m 2) as shown in Fig.2 b-I and b-2. The sample were heated up to Tp(surface poling temperature), usually above Tg (glass transition temperature in the amorphous polymers) or Tc (ferro-paraelectric transition temparature in the crystalline polymers), and cooled down slowly ( cooling rate 2-3°C/min) to room temperature. Dielectric measurements were carried out in vacuum using a Hewlett-Packard HP-4285 LCR meter. Thermally stimulated depolarization current(TSC) and pyroelectric response for poled samples were recorded simultaneously from the current through the electrode irradiated by a pulsed semiconductor laser(670 nm, 3mW, 10Hz) upon heating(3°C/min). The absolute value of the pyroelectric constant was determined from the reversible TSC by heating and cooling near room temperature. Piezoelectric constant (do) was obtained from the thickness strain (Laser Doppler Vibrometer ONO SOKKI LV3 100, laser spot 0.1 mm) induced by the AC field (1 kHz).
Results and Discussion I Thermal and Dielectric properties of Ferroelectric Polymers
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