Cross-Linked LC Elastomers

In order to combine LC properties and rubber elasticity a variety of LC elastomers were prepared [8]. In these systems a reor- 

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Cross-linked LC elastomers from combined LC polymers were mostly obtained by means of the hydrosililation reaction shown in Figure 7 [3-7, 10]. Later, a thermal or photochemical polymerization of acrylates was used [9, 11]. As can be seen from Figure 7, this cross-linking process, which... 

An interesting feature of polymer nematics was discovered in studying the orientation of some polymethacrylate polymers and cross-linked LC elastomers based on polysiloxanes [57]. Most nematic polymers form an optically positive, uniaxial, homeotropic structure under the effect of a mechanical Held such polymers have a positive birefringence (An > 0), like most low-molecular-weight liquid crystals. 

Scheme 4. Cross-linking of LC elastomers by radical polymerization of pendant acrylate groups.

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 15. Logarithmic plot of the piezosignal for an LC elastomer with the phase sequence g 1 SmC 81 I [19]. The cross-linking was done according to Scheme 4.

As already mentioned, four types of chemical structures are known for ferroelectric LC polymers side-chain polymers, main-chain polymers, combined ones, and cross-linked polymers, i.e., ferroelectric LC elastomers. All of them contain rodlike mesogenic moieties (Fig. la) but not disklike ones (Fig. lb). From the first publications to date, most research work has been carried out on side-chain polymers about 240 side-chain FLCPs versus 5 main-chain FLCPs [10]. Synthesis of the two other types of polymers under discussion is often related to the procedures used for side-chain FLCPs e.g., preparation of cross-linkable side-chain polymers is usually the first step in the synthesis of ferroelectric LC elastomers. That is the reason for the prior discussion of the synthesis of side-chain FLCPs. 

FIGURE 10 Synthetic pathway to cross-linked FLCPs (ferroelectric LC elastomers). 

As mentioned earlier, ferroelectric LC elastomers were first synthesized by Zentel et al. [94-96] the synthesis and properties of LC elastomers are reviewed by Gleim and Finkelmann [97]. One of the synthetic routes to ferroelectric LC elastomers is preparation of side-chain polymers (Fig. 10) or combined polymers (Fig. 12b) containing active groups in side chains, with further cross-linking. The ultraviolet (UV) light-induced radical polymerization of acrylamide or acrylate active groups has been used in the former case [83,95,98] and hydrosilation addition in the latter case [94]. 

LC-elastomers (see Fig. 4a) have been investigated in detail (4-8). Although the liquid crystalline phase transitions are nearly unaffected by the network, the network retains the memory of the phase and director pattern during cross-linking... 

Mechanical Properties of Ferroelectric LC-Elastomers. The cross-linking reactions of a series of copolymers analogous to polymer P2, but differing in the amoimt of cross-linkable groups, were studied by ftir spectroscopy (17). These measurements show a decrease of the acrylamide double bond on irradiation. Conversions between 60 and 84% were observed. The imcertainty of the conversion, however, is high because only very few double bonds are present in pol5nner P2 and they are visible in the infrared spectrum at rather low intensity. 

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%.

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