Piezoelectricity elastomers

ZenteP has drawn attention to the possibility of creating piezoelectric elastomers by the combination of ferroelectric comb architecture (Section 7.6.1.3) within a cross-linked system. The ferroelectric polarization of such a material should be modified by the application of mechanical force to which the macroscopic ordering is vulnerable. That would offer the possibility of transforming a mechanical signal into an electrical response, and the elastomer would behave as a piezoelement. 

In addition, some liquid-crystalline elastomers are ferroelectric (possess spontaneous electric polarization) [196,197], or piezoelectric (become electrically... 

Crosslinked LC elastomers (Figure 19d) are very promising for piezoelectric and ferroelectric applications, and also as non-linear optic materials. The first synthetic step to such materials is the preparation of usual side chain or combined LC copolymers doped with a small part of side chains containing a polymerizable >C=C< double bond at the end (Figure 23 shows a particular example of a crosslinkable LC polymer64). The copolymer can be further photocrosslinked, giving an elastic polymer film which reveals... 

Roy SS, Lehmann W, Gebhard E, Tolksdorf C, Zentel R, Kremer F (2002) Inverse piezoelectric and electrostrictive response in freely suspended FLC elastomer film as detected by interferometric measurements. Molec Cryst Liq Cryst 375 253... 

X-ray measurements on LC elastomers have shown [6-8] that the reversible transition between a chiral smectic C phase with and without a helical superstructure can be induced mechanically. The helix untwisted state corresponds in this case to a polar ferroelectric monodomain. The piezoelectricity arising from this deformation of the helical superstructure (which does not require a complete untwisting) has been demonstrated [9] for polymers cross-linked by polymerization of pendant acrylate groups (Figure 15). 

Figure 14. Idealized presentation of the orientation process, which leads to piezoelectricity in chiral smectic C elastomers (only the mesogens are shown) [28], P macroscopic polarization). The deformed states with a partially unwound helix (left and right) are prepared from the ground state with a helical superstructure (middle) by mechanical forces. 0 and direction of the spontaneous polarization in and out of the plane of drawing, respectively.

Theoretical investigations by Brand [ 135] and Brand and Pleiner [136] predicted that a monodomain liquid-crystalline elastomer exhibiting a cholesteric or a chiral smectic C phase should display piezoelectric properties due to a modification of the pitch of the helix under strain. So, a piezoelectric voltage should be observed across the sample when a mechanical field is applied parallel to the helicoidal axis. In this description, the crosslinking density is supposed to be weak enough to allow the motion of the director, and deformations of the sample (compression, elongation, etc.) are assumed to be much smaller than those that should lead to a suppression of the helix. The possibility of a piezoelectric effect do not only concern cholesteric and chiral smectic C phases, but was also theoretically outlined for more exotic chiral layered systems such as chiral smectic A mesophases [137]. 

Preceding the reports on elastomers, piezoelectricity in chiral smectic C phases of low-molar weight molecules or of polymers has usually been observed. The special property is that the system possesses macroscopic electrical polarization without an external field, so it is classified as ferroelectric. 

From these theoretical and experimental works, it emerges that both chiral smectic C and cholesteric elastomers are piezoelectric and can lead to a piezoelectric voltage comparable to that of classical piezoelectric crystals, such as quartz. Thus they can be used potentially as piezoelectric elements, which can be produced in any shape needed. 

Bent-core liquid crystal elastomers have shown to exhibit large values of flexoelectricity as many as three orders of magnitude larger than liquid crystal elastomers containing rod-shaped molecules [44]. These high responses are attributed to a piezoelectric phenomenon. Liquid crystal elastomers combine elasticity and flexibility inherent to rubbers and the optical and electrical properties of liquid crystals, and are promising materials for applications such as electrooptics, flexible electronics, and actuator technologies for biomedical applications. 

As mentioned above, LC elastomers, especially LSCE with monodomain order, anisotropic LC networks, and gels, may be optimized to make a kind of smart material for molecular switching and piezoelectric or pyroelectric sensors, because of their sensitivity to environmental conditions (electric or stress fields, temperature, and radiation, etc.) and memory effect. Other applications include wave-guide, polarizers, optical filters, alignment, and compensation films for LCD displays. 

Cross-linked polymeric liquid crystals offer a wide variety of unique and in-tere.sting properties. Because of the interaction between the mesogens and the network backbone in liquid crystal elastomers, mechanical deformations can align the director, and these materials are piezoelectric. Industrial applications of liquid crystalline thermosets are driven by additional properties such as toughness, a tunable coefficient of thermal expansion, ferroelectricity, and nonlinear optical properties. Reviews on this topic are given by Barclay and Ober [4] and by Warner and Terentjev [5]. 

FIGURE 27 Piezoelectric coefficient, for a ferroelectric LC elastomer on heating (closed symbols) and cooling (open symbols), versus temperature. (Data point values from Ref. 114.)... 

Schoenfeld, A., Kremer, F., Vallerien, S. U., Poths, H.. and Zentel, R., Collective and molecular relaxations in polymeric ferroelectric liquid crystals and experimental proof of piezoelectricity in chiral. smectic C-elastomers, Ferroelectrics, 121, 69-77 (1991). 

Brehmer, M., Zentel, R., Wagenblast, G., and Siemensmeyer, K., Ferro- and piezoelectric LC elastomers. 23th Freiburg Meeting on Liquid Crystals, Freiburg, Germany, March 23-25, 1994, p. 07. 

Finkelmann, H and Eckert. T., Piezoelectricity of Sf- -liquid single crystal elastomers, Material Research Society 1996 Spring Meeting, San Franci.sco, April 8-12, 1996, Abstracts, 18.6. 

Fig. 16. Electrostriction of a ferroelectric LC-elastomer (43). Big diagram Thickness variation Ah as a function of the applied ac voltage (/ac- Interferometric data were obtained at the fundamental frequency of the electric field (piezoelectricity, first harmonic -t) and at twice the frequency (electrostriction, second harmonic o). Sample temperature 60°C. Inset Electrostrictive coefficient a (-I-) versus temperature. At the temperature where the non-cross-linked polymer would have its phase transition Sc -Sa (about 62.5 0, the tilt angle of 0° is unstable. That is why the electroclinic effect is most effective at this temperature. An electric field of only 1.5 MV/m is sufficient to induce lateral strains of more than 4%.

As pointed out already in Section 2.5.5, low-molecular weight ferroelectric liquid crystals (FLCs) and FLCPs are attracting a lot of interest because of their potential for electro-optical applications. The polymers offer new possibilities, e.g., as elastomers for piezoelectric elements or by copolymerization [77, 78, 105] due to the formation of intrinsic mixtures between SmC mesogenic units and other comonomers. This leads to FLCPs combining several material properties which might be utilized for colored displays in the case of comonomers containing chromophores. For the differentiated evaluation of such copolymers with reference to the possible exploitation of nonlinear optical (NLO) properties, the interplay of the different orientation tendencies of the side-chain functionalities is of crucial importance [36,106]. 

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