Liquid crystalline elastomer network

In recent years, the behaviour of liquid crystalline polymers including elastomers has been a subject of considerable interest 104,105). It is known that small molecule liquid crystals turn into a macroscopic ordered state by external electric or magnetic fields. A similar behaviour seems to occur for liquid-crystalline polymer networks under mechanical stress or strain. 

Cross-linked liquid crystalline polymers with the optical axis being macroscopically and uniformly aligned are called liquid single crystalline elastomers (LSCE). Without an external field cross-linking of linear liquid crystalline polymers result in macroscopically non-ordered polydomain samples with an isotropic director orientation. The networks behave like crystal powder with respect to their optical properties. Applying a uniaxial strain to the polydomain network causes a reorientation process and the director of liquid crystalline elastomers becomes macroscopically aligned by the mechanical deformation. The samples become optically transparent (Figure 9.7). This process, however, does not lead to a permanent orientation of the director. 

For an experimental check-up of the theoretical considerations about liquid-crystalline elastomers in a mechanical field, Fin-kelmann and coworkers [107, 123] studied, in nematic networks, the evolution of the order parameter and of the transition temperature as a function of the stress. The observed results are in full agreement with the predictions of the Landau-de Gennes theory, since an increasing clearing temperature as well as an increasing order parameter are observed with increasing stress. From their results, it was possible to estimate the crosscoupling coefficient U (see Sec. 3.1.1) between the order parameter and the strain of a nematic elastomer [123]. 

In the preceding chapters the synthesis properties of linear liquid crystalline polymers are described, where different approaches exist to obtain the liquid crystalline state rod-like or disc-like mesogenic units are either incorporated in the polymer backbone or are attached as side groups to the monomer units of the main chain. Following conventional synthetic techniques these linear polymers can be converted to polymer networks. Compared to low molar mass liquid crystals and linear liquid crystalline polymers, these liquid crystalline elastomers exhibit exceptional new physical and material properties due to the combination and interaction of polymer network elasticity with the anisotropic liquid crystalline state. 

Finkelmann reported synthesis of a novel cross-linked smectic-C main-chain liquid-crystalline elastomer that was formed by polycondensation of vinyloxy-terminated mesogens, tetramethyldi-siloxane, and pentamethyl-pentaoxapentasilicane. The introduction of the functional vinyloxy group allows the synthesis of well-defined networks with good mechanical properties due to ehmination of side reactions as in the case of vinyl groups [60]. 

Theoretical models for other systems, such as star, branched, and ring polymers, random and alternating copolymers, graft and block copolymers are discussed in the book by Mattice and Suter [1]. Block copolymers are discussed in Chap. 32 of this Handbook [2]. Theories of branched and ring polymers are presented in the book by Yamakawa [3]. Liquid-crystalline polymers are discussed in the book by Grosberg and Khokhlov [4], and liquid crystalline elastomers in the recent book of Warner and Terentjev [5]. Bimodal networks are discussed by Mark and Erman [6,7]. Molecular theories of filled polymer networks are presented by Kloczkowski, Sharaf and Mark [8] and recently by Sharaf and Mark [9]. 

The main objective of this chapter is to review and outline the research studies and perspectives on liquid crystalline elastomers and LC anisotropic networks, with emphasis on recent interesting innovations on network-stabilized ferroelectric LC (FLC) gels, discotic columnar networks, and self-assembly hydrogen-bonded LC network. We will also present hybrid networks based on ladderlike polysiloxanes that have been developed in our group as advanced functional film materials. 

The first example of a neat liquid crystalline elastomer appeared in the literature in 1981 (Finkelmann et al.) [7] and was based upon side-chain LCPs. Semi-flexible main-chain-based LC networks were also reported later in 1986 by Zental and Reckert [8]. 

The liquid crystalline elastomers simultaneously exhibit properties associated with low molar mass LCs and standard elastomers. Therefore, the mechanical and optical properties of such networks are anisotropic below the clearing point Tc) and also are dependent upon stress/strain field caused by mechanical deformation. 

Because of their known structures, such model elastomers are now the preferred materials for the quantitative characterization of rubberlike elasticity. The properties of PDMS networks have been of interest to a variety of groups. " Such specific cross-linking reactions are also useful in the preparation of some of the liquid-crystalline elastomers,discussed in chapter 3. 

In 1975 P.G. de Gennes recognized that the interplay between liquid crystalline order and the macromolecular network stmcture generates new physical properties that also resemble those of biological systems and muscles [3]. In the following years Otto Lehmann s early ideas were actually demonstrated with liquid crystalline elastomers although at that time the basic concepts of macromolecular chemistry were still unknown. 

Just as for biological beings, the liquid crystalline phase structure and simultaneously the functionality of liquid crystalline elastomers are strictly limited to a defined temperature regime. Similar to low molar mass liquid crystals and LC polymers this temperature regime is determined by the chemical constitution of the polymer networks. For the synthesis and investigation of liquid crystalline elastomers the basic concepts of liquid crystals, LC polymers, and polymer networks have to be brought together. 

Brehmer M, Zentel R (1994) Liquid crystalline elastomers- characterization as networks. Mol Cryst Liq Cryst Sci Technol Sect A-Mol Cryst Liq Cryst 243 353-376. doi 10.1080/ 10587259408037775... 

Gebhard E, Zentel R (2000) Ferroelectric liquid crystalline elastomers, 1 - Variation of network topology and orientation. Macromol Chem Phys 201(8) 902-910. doi 10.1002/ (SICI)1521-3935(20000501)201 8<902 AID-MACP902>3.0.CO 2-9... 

The coupling between the properties of conventional polymer networks and the properties of chiral liquid crystalline phases results in interesting, new opto- and electromechanical effects of the chiral liquid crystalline elastomers, as demonstrated by theoretical considerations and experiments. Knowledge about these new materials is still in its infancy. But the properties analyzed so far for these elastomers indicate promising aspects for application and are the basis for the new syntheses of optimized chiral liquid crystal networks. 

A final comment concerns the possibility of using networks instead of linear pofymers. In the case of networks synthesized using 4-vinylpyridine and divinylbenzene, the phase behavior is not changed in an essential way until the amount of divinylbenzene exceeds 1.0 mol%, where the network structure severely starts to suppress nanostructure formation [131]. As in the case of liquid-crystalline elastomers, using networks has the potential advantage of elasticity and orientability [132] (see also [133]). 

Liquid crystalline polymers can be crosslinked to form a network, or an elastomer, while retaining liquid crystallinity. This section is devoted to both the liquid crystallinity and the rubber elasticity of the crosslinked polymers. 

Small molecular mass liquid crystals do not respond to extension and shear stress. Liquid crystalline polymers may exhibit a high elastic state at some temperature due to the entanglements. However, the liquid crystalline network itself is an elastomer, showing rubber elasticity. In the presence of external stress, liquid crystalline networks deform remarkably and then relax back after the release of stress. The elasticity of liquid crystalline networks is more complicated than the conventional network, such as the stress induced phase transition, the discontinuous stress-strain relationship and the non-linear stress optical effect, etc. 

The liquid crystalline polymer has since developed far beyond imagination that a decade ago. The liquid crystalline polymer family has so far included the main chain-, side chain-, and crosslinked- (i.e. network or elastomer) types, and their solutions and gels. The liquid crystal phases cover nematic, cholesteric and smectics. Although the science of the liquid crystalline polymer is not fully mature, it has attracted significant research interests and has already made tremendous progress. As investments and human resources continue, the liquid crystalline polymer is expected to have an even brighter future. 

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