Cholesteric elastomer

Considerably less work has been done on discotic liquid-crystalline elastomers [189,190] and cholesteric elastomers [191]. The same seems to be true for smectic elastomers [192,193], even though some of them have the additional interesting property of being chiral [194,195]. 

Fig. 14 UV-VIS spectra of a cholesteric elastomer, based on a mixture of derivative 2 (44 wt %), monomer 1 (52wt%), and monomer 2 (3wt%), crosslinked at 32 °C. Arrows indicate the maximum position of the selective light-reflection band. (Reproduced from [233])...

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. 

Figure 24. Electromechanical response, U, vs. temperature for a compression of 25% of a cholesteric elastomer.

Figure 25. Relative fitted change of sample thickness of a cholesteric elastomer minus the relative fitted change of sample thickness of the racemic elastomer AL/Lf) as function of the electric field strength E (reproduced with permission from [72]).

Mao Y, Terentjev EM, Warner M. 2001. Cholesteric elastomers deformable photonic solids. Phys Rev E 64 041803. 

Different types of LC systems are found in elastomers. In nematic liquid crystals, the molecules have orientational but no positional order, their center of mass positions being randomly distributed. Most nematic elastomers are employed in uniaxial deformation. If the LC elements contain chiral groups, they are termed as cholesteric elastomers. Discotic nematic LC elastomers contain disk-shaped molecules that can be oriented in layers. Smectic LC elastomers form well-defined layers. 

Keywords Chain conformation Cholesteric Elastomer Liquid crystal Liquid crystal polymer Nematic Polymer networks Smectic... 

Mechanical fields can also be deployed to achieve a macroscopic orientation of the helicoidal z-axis of cholesteric elastomers. As discussed above, nematic side chain polymers with odd spacer length exhibit a locally prolate chain conformation... 

In the first part of this section we will discuss several strategies developed in the last decades to prepare LSCEs with a permanently stable orientation of the main axis, i.e., the director n for nematic elastomers, the helix axis for cholesteric elastomers, and the layer normal k for smectic polymer networks, respectively (Sect. 4.1). In some cases, however, alignment of the main axes is not sufficient to achieve a uniform orientation of the whole phase structure and more complex... 

The orientation achieved by the methods discussed in this section is sometimes, especially in the case of cholesteric elastomers, much better than for the thick films prepared by coupling to a mechanical field. 

Materials whose dielectric properties vary periodically in space are capable of localizing photons. Such photonic band gap materials, whose structure can be regarded as a distributed laser cavity, can also lase. Cholesteric liquid crystals can act as such lasers. In addition, cholesteric elastomers can allow the distributed cavity, and hence the lasing wavelength, to be tuned by mechanical strain. 

Schmidtke J, Kniesel S, Finkelmann H (2005) Probing the photonic properties of a cholesteric elastomer under biaxial stress. Macromolecules 38 1357-1363... 

Warner M, Terentjev EM, Meyer RB, Mao Y (2000) Untwisting ofa cholesteric elastomer by a mechanical field. Phys Rev Lett 85 2320-2323... 

The controversial discussions about the different theoretical models stimulated experimental investigations on cholesteric elastomers. In the next sections, different synthetical approaches are summarized for cholesteric elastomers. Thereafter, electromechanical investigations are reviewed. 

Another method to obtain cholesteric elastomers was applied by Meier [14]. The starting material is a nematic network that is swollen by a chiral low molar mass liquid crystal. Similar to conventional induced cholesteric phases, the concentration of the chiral dopant (or swelling solvent) determines the cholesteric pitch within the network. A linear relationship between... 

Figure 13.1. Cholesteric elastomer the initially unloaded sample exhibits a polydomain and is turbid (la). Compression causes a transition into a translucent monodomain (lb) (reproduced with permission from [14]).

Figure 13.2. Wavelength of the reflection Xr of two cholesteric elastomers as a function of uniaxial compression (reduced temperature Tjed = 0.956). The vertical dashed line indicates the threshold deformation 2. For X < 0.72, Xr decreases nearly aiSne with the macroscopic network dimensions (reproduced with permission from [18]).

Electromechanical experiments on cholesteric elastomers carried out by Meier in 1990 [17] seemed to confirm Brand s theoretical model. On compression of uniformly ordered networks along the helicoidal axis, an electric... 

In 1997, Chang et al. provided theoretical and experimental evidence that only a shear field produces piezoelectricity in cholesteric elastomers [16]. Applying a dynamic shear force perpendicular to the helicoidal axis, a nearly linear response is foimd between the piezoelectric voltage and the shear amplitude, as shown in Figure 13.3 for two frequencies. Only at larger shear displacement, the induced voltage tends toward saturation. 

Figure 13.3. Induced voltage from piezoelectricity as a function of the shear displacement with two frequencies produced by a cholesteric elastomer gel. The induced voltage saturates at larger shear displacement (reproduced with permission from [16]).

Note that the tilt does not necessarily require a layer structure, and the same concept can be employed to explain the piezoelectricity of the cholesteric liquid crystal. In this case, the shear along the helical axis leads to a tilt of the director toward the helical axis and a polarization normal to the shear plane. This "shear electricity" was predicted by Prosh and experimentally was verified in case of cholesteric elastomers, where the molecules are weakly cross-linked and thus can sustain elastic strains. ... 

Elastomeric LCP networks with the potential to show piezoelectric behavior were described by Zentel and co-workers [109, 110, 125, 773]. More recently, Meier and Finkelmann [774, 775] created piezoelectric cholesteric elastomers and described some of their properties. Polymers [776] and networks [126,777] with nonlinear optical responses were recently synthesized. Some of these polymers are liquid crystalline [776], We see no reason why mesogenic monomers and NLO responsive monomers may not soon be combined to form permanently crosslinked NLO LCP networks. 

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