Elastomer liquid-crystalline

Liquid crystalline elastomers (LCEs) are composite systems where side chains of a crystalline polymer are cross-linked. Their mesogenic domains can be ordered nematically and undergo a phase transition to a disordered state at a temperature well above the glass-transition temperamre (Tg) of the polymer. Although the phase transition is thermally driven, LCEs demonstrate electrical conductivity and thus can be electrically stimulated." Ratna" has reported contractions of nearly 30% due to the phase transition of acrylate-based LCEs. 

Ratna, B.R., Liquid crystalline elastomers as artificial muscles Role of side chain-backbone coupling. Proceedings of SPIE 8th Annual International Symposium of Smart Structures and Materials, EAPAD... 

Both low molecular weight materials [145] and polymers [146,147] can show liquid crystallinity. In the case of polymers, it frequently occurs in very stiff chains such as the Kevlars and other aromatic polyamides. It can also occur with flexible chains, however, and it is these flexible chains in the elastomeric state that are the focus of the present discussion. One reason such liquid-crystalline elastomers are of particular interest is the fact that (i) they can be extensively deformed (as described for elastomers throughout this chapter), (ii) the deformation produces alignment of the chains, and (iii) alignment of the chains is central to the formation of liquid-crystalline phases. Because of fascinating properties related to their novel structures, liquid-crystalline elastomers have been the subject of numerous studies, as described in several detailed reviews [148-150]. The purpose here will be to mention some typical elastomers exhibiting liquid crystallinity, to describe some of their properties, and to provide interpretations of some of these properties in molecular terms. 

Side-Chain liquid-crystalline elastomers 9.3.1 Some general aspects... 

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]. 

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

Finally, some liquid-crystalline elastomers exhibit interesting photonic effects [200,201]. Of particular importance are non-linear optical properties. These involve interactions of light with the elastomer in a way that some of the characteristics of the incident light change, specifically its phase or frequency (including frequency doubling or frequency mixing) [202,203]. 

It can be safely predicted that applications of liquid crystals will expand in the future to more and more sophisticated areas of electronics. Potential applications of ferroelectric liquid crystals (e.g. fast shutters, complex multiplexed displays) are particularly exciting. The only LC that can show ferroelectric property is the chiral smectic C. Viable ferroelectric displays have however not yet materialized. Antifer-roelectric phases may also have good potential in display applications. Supertwisted nematic displays of twist artgles of around 240° and materials with low viscosity which respond relatively fast, have found considerable application. Another development is the polymer dispersed liquid crystal display in which small nematic droplets ( 2 gm in diameter) are formed in a polymer matrix. Liquid crystalline elastomers with novel physical properties would have many applications. 

Hence, we are still at the very beginning of thermoelastic and thermomechanical investigations of liquid crystalline elastomers. 

Beside the synthesis of linear polysiloxanes, hydrosilylation was also nsed to obtain liquid crystalline elastomers. This can be performed in two ways, either a one pot reaction involving a diolefln together with the functionalized mesogen or a two step process involving first the synthesis of a linear polysiloxane, bearing adequate functions for a further crossfinking. For example, a photosensitive group such as benzophenone could be introduced (Fig. 5, x and y are theoretical values). In order to ensure a full conversion in silane, the time of reaction was increased here to 65 h [17]. 

Photochemical preparation of liquid crystalline elastomers with a memory of the aligned cholesteric phase... 

The following protocols (6-10) describe the synthesis of some cholesterol-based acrylates and their photopolymerization in an aligned cholesteric phase. The protocols utilize a modification of a system previously described by Shannon. 5 6 ip ie absence of a diacrylate comonomer, the cholesteric phase produced initially on copolymerization is not stable and reverts to a smectic phase on a single cycle of heating and cooling. In the presence of the diacrylate the first-formed phase is stable. This is one example of how crosslinking can stabilise the liquid crystal phase in liquid crystalline elastomers, others include, the so-called, polymer-stabilized liquid crystals and those described in the later protocols. 

The following examples describe two different approaches to developing liquid crystalline elastomers. The hrst two examples, developed at Reading, utilize the acrylate-based polymer described in Protocol 4 the hnal technique, invented in Freiburg utilizes siloxane-based polymers. This latter process is particularly useful where high levels of orientation are required (and of course room temperature liquid crystalline phases). 

Imprinting chiral structure in liquid crystalline elastomers... 

Fig. 9.6 Imprinting chiral structure in liquid crystalline elastomer, polymer 6 (CBZ6), chiral dopant 23 (CB15) and cross-linking agent 24 (MDI).

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. 

Fig. 9.7 Liquid crystalline elastomer (LCE) (a) polydomain sample (A = 1.0) (b) sample aligned by a mechanical field (A = 1.4) (A = L/Lq, L = length of the sample, Lq = length ofthe sample without load).

Patel, S. I. Memory Effects in Liquid Crystalline Elastomers PhD Thesis University of Reading 2001. 

Brand HR, Finkelmann H (1998) Physical properties of liquid crystalline elastomers. In Demus, Goodby, Gray, Spiess (eds) Handbook of liquid crystals VIII, vol. 3, chap V, pS277. Wiley-VCH, UK... 

Wermter H, Finkelmann H (2001) Liquid crystalline elastomers as artificial muscles. e-Polymers 13 1, http //www.e-polymers.org... 

Lehmaim W, Hartmann L, Kremer F, Stein P, Finkelmann H, Kruth H, Diele S (1999) Direct and inverse electromechanical effect in ferroelectric liquid crystalline elastomers. J Appl Phys 86 1647... 

Leister N, Lehmann W, Weber U, Geschke D, Kremer F, Stein P, Fiiikelmann H (2000) Measurement of the pyroelectric response and of the thermal diffusivity of microtomized sections of single crystalline ferroelectric liquid crystalline elastomers. Liq Cryst 27 289... 

Skupin H, Kremer F, Shilov SV, Stein P, Finkelmann H (1999) Time-resolved FTIR spectroscopy on structure and mobility of single crystal ferroelectric liquid crystalline elastomers. Macromolecules 32 3746... 

The rheological properties of liquid-crystalline elastomers strongly depend on the mes-omorphous order [95, 98, 102, 113-118], even for strains that are too small to induce macroscopic orientation of the mesogens (strain amplitude <10%). This is mirrored by the thermal evolution of the storage modulus (Fig. 30), defined for very small... 

The mechanical orientability is the most prominent property of liquid-crystalline elastomers. Above a threshold stress (Sec. 3.3.1), small strains (about 20%-40%) are... 

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]. 

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