Temperature dependence, ferroelectric polymers

The introduction of a polymer network into an FLC dramatically changes phase and electro-optic behavior. Upon addition of monomer to the FLC, the phase transitions decrease and after polymerization return to values close to that observed in the neat FLC. The phase behavior is similar for the amorphous monomers, HDD A and PPDA. The electro-optic properties, on the other hand, are highly dependent on the monomer used to form the polymer/FLC composite. The ferroelectric polarization decreases for both HDDA and PPDA/FLC systems, but the values for each show extremely different temperature dependence. Further evidence illustrating the different effects of each of the two polymers is found upon examining the polarization as both the temperature and LC phase of polymerization are changed. In PPDA systems the polarization remains fairly independent of the polymerization temperature. On the other hand, the polarization increases steadily as the polymerization temperature of HDDA systems is increased in the ordered LC phases. 

Vinylidene fluoride-trifluoroethylene (VF2-F3E) copolymers exhibit a ferroelectric-paraelectric phase transition, the first such case found for a synthetic polymer. In this transition, the electric polarization and piezoelectric constant of the film disappear above the Curie point (Tcurie)- The temperature dependence of the dielectric constant, , obeys the so called Curie-Weiss law ... 

The temperature dependence of the dielectric constant of P2 is shown in Figures 12.9 and 12.10. The dielectric constant (e) suddenly increased in the temperature region of the SmC phase. This is the typical ferroelectric behavior of FLC, because the dielectric constant of the ferroelectric material is inversely proportional to the absolute temperature according to the Curie-Weiss law, i.e. e = C(T — Tq), where C is the Curie constant and Tq is the Curie temperature. At the same time, the dielectric constant increased with decrease in frequency. This tendency is due to the high viscosity of the LC conjugated polymer. 

In summary, polythiophene derivatives were synthesized by introducing a fluorine-containing chiral LC group into the side-chains. It was found that the effective conjugation length of the main chain increased after the spontaneous orientation of the LC side-chains. One of the polymers (P2) showed an enantiotropic SmC phase, which was characteristic of ferroelectricity. The LC temperature region of the SmC phase was as broad as 20 °C. This is the first case where the SmC phase has been observed in polythiophene derivatives. The ferroelectric behavior was confirmed by examining the temperature dependence of the dielectric constant of the polymer. 

Nalwa H, Fukada E (eds) (1995) Ferroelectric polymers. Marcel dekker. New York Newman B et al (1980) The piezoelectricity of poly(vinyhdene fluoride). J Appl Phys 51 5161 Omote et al (1997) Temperature dependence of elastic, dielectric, and piezoelectric properties of single crystalline films of vinylidene fluoride trifluoroethylene copolymer. J Appl Phys 81 2760... 

The development of active ceramic-polymer composites was undertaken for underwater hydrophones having hydrostatic piezoelectric coefficients larger than those of the commonly used lead zirconate titanate (PZT) ceramics (60—70). It has been demonstrated that certain composite hydrophone materials are two to three orders of magnitude more sensitive than PZT ceramics while satisfying such other requirements as pressure dependency of sensitivity. The idea of composite ferroelectrics has been extended to other appHcations such as ultrasonic transducers for acoustic imaging, thermistors having both negative and positive temperature coefficients of resistance, and active sound absorbers. 

PVDF is mainly obtained by radical polymerisation of 1,1-difluoroethylene head to tail is the preferred mode of linking between the monomer units, but according to the polymerisation conditions, head to head or tail to tail links may appear. The inversion percentage, which depends upon the polymerisation temperature (3.5% at 20°C, around 6% at 140°C), can be quantified by F or C NMR spectroscopy [30] or FTIR spectroscopy [31], and affects the crystallinity of the polymer and its physical properties. The latter have been extensively summarised by Lovinger [30]. Upon recrystallisation from the melted state, PVDF features a spherulitic structure with a crystalline phase representing 50% of the whole material [32]. Four different crystalline phases (a, jS, y, S) may be identified, but the a phase is the most common as it is the most stable from a thermodynamic point of view. Its helical structure is composed of two antiparallel chains. The other phases may be obtained, as shown by the conversion diagram (Fig. 7), by applying a mechanical or thermal stress or an electrical polarisation. The / phase owns ferroelectric, piezoelectric and pyroelectric properties. 

It has been found experimentally that the spontaneous polarization Ps of side chain liquid crystalline polymers is more or less the same as that of small molecular mass liquid crystals, i.e., so far as the chemical formula of the side group of a side chain liquid crystalline polymer is the same as that of a small molecular mass liquid crystal. This phenomenon illustrates that either the side groups in the side chain liquid crystalline polymer or the small molecular mass ferroelectric liquid crystal in their Sc phase are aligned in same way. The dependence of Ps on temperature for the three small molecular mass ferroelectric liquid crystals LI, L2 and L3 and their polymer counterparts PI, P2 and P3 are depicted in Figure 6.40. 

MV/m) than that of solid ferroelectrics (usually a few MV/m or smaller). The ferroelectric properties depend sensitively upon the details of sample preparation, for example the use of melt quenching or melt extrusion, the annealing temperature, or the details of the poling procedure. Polymer ferroelectrics are useful for soft transducers. 

The tilt angle 0 is a true order parameter of the ferroelectric C phase. With some exceptions [170], its temperature behavior is analogous to that for low molecular mass compoimds. Three examples are shown in Fig. 7.30. First, we notice a rather smooth 6 T) dependence, smoother than the dependence 9(T) oc (Tc a — predicted by theory. This may be accounted for by the polydispersity of polymer liquid crystals which results in blurred phase transition regions. For compound (7.vii) studied in [170]... 

To summarize the ferroelectric and piezoelectric properties of the discussed polymers, some important ferroelectric and piezoelectric parameters are tabulated in Table 4. As discussed in the previous sections, the ferroelectric and piezoelectric properties of polymeric and polymeric composite systems depend on various factors, such as crystallinity, pohng conditions, glass transition temperature, and before and after electrical poling treatments (electrical, mechanical, and thermal treatments). In addition to the factors mentioned above, for composite systems, laminates or blends, fraction of constituents, and interfacial polarization are also important. Therefore, the... 

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