Elastomers liquid crystal

Structural and synthetic aspects of these materials were discussed earlier (Section 7.4.3), and reviews are available from key workers in the field. The crucial phenomenological bonus offered by the network architecture is due to stress-optical effects. Thus, partial or macroscopic 

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. 

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Shahinpoor, M., Elastically-activated artificial muscles made with liquid crystal elastomers. Proceedings of SPIE 7th Annual International Symposium of Smart Structures and Materials, EAPAD Conf, 3987, pp. 187-192, 2000. 

M. Warner and E.M. Terentjev, Liquid Crystal Elastomers, Oxford University Press, New York, 2003. 

Knight, D. P., and Vollrath, F. (2002). Biological liquid crystal elastomers. Philos. Trans. R Soc. Lond. B Biol. Sci. 357, 155-163. 

The reversible shape change in molecular materials was found for the first time in 2001 by using azobenzene-containing liquid crystal elastomers [39]. Figure 23.6... 

Figure 23.6 Contraction fraction of azobenzene-containing liquid crystal elastomers, (Lo - f-O/f-o, at 298 K against the time upon irradiation with ultraviolet light and in the dark, to and L, represent lengths of the elastomers in the initial state and after the time, respectively.

Defining permanent memory of macroscopic global alignment in liquid crystal elastomers... 

Electrostrictive polymers have a spontaneous electric polarization. Electrostriction results from the change in dipole density of the material. These polymers contain molecular or nanocrystaUine polarizations that align with an applied electric field. PVDF copolymers with nano-sized crystalline domains, electrostrictive graft copolymers, and liquid crystal elastomers fall under this category. 

Fig. 1.7 Actuation mechanism of a liquid crystal elastomer. The application of an electric field results in the realignment of intrinsically polarized liquid crystal mesogens. The mesogens are either grafted to elastomer chains or incorporated within them. The elastomer chains prevent the free flow of the mesogens and couple their motion to bulk stresses and strain...

Warner M, Terentjev M (2003) Liquid crystal elastomers. Oxford Science Publications, Oxford... 

Thomsen DL, Keller P, Naciri J, Pink R, Jeon H, Shenoy D, Ratna BR (2001) Liquid crystal elastomers with mechanical properties of a muscle. Macromolecules 34 5868... 

Li MH, Keller P (2006) Artificial muscles based on liquid crystal elastomers. Philos Trans R Soc A 364 2763... 

Shenoy DK, Thomsen DL, Srinivasan A, Keller P, Ratna BR (2002) Carbon coated liquid crystal elastomer film for artificial muscle applications. Sens Actuators A 96 184... 

Finkelmann H, Shahinpoor M (2002) Electrically controllable liquid crystal elastomer-graphite composite artificial muscles. Proc SPIE 4695 459... 

Chambers M, Einkelmann H, Remskar M, Sanchez-Ferrer A, Zalar B, Zumer S (2009) Liquid crystal elastomer-nanoparticle systems for actuation. J Mater Chem 19 1524... 

Spillmann CM, Ratna BR, Naciri J (2007) Anisotropic actuation in electroclinic liquid crystal elastomers. Appl Phys Lett 90 021911... 

While there are not many NMR investigations of the isotropic-nematic transition in sidechain liquid crystal elastomers [20, 21], they turned out to be crucial in answering the question whether strained LCEs show a first order phase transition with a classical two phase region or whether there is a continuous change of the local degree of nematic order, since one is in a state beyond the critical point. As already mentioned earlier in this section this question could not be settled conclusively with the mechanical and optical investigations described above. 

Camacho Lopez M, Finkelmann H, Palffy Muhoray P, Shelley M. 2004. Fast liquid crystal elastomer swims into the dark. Nat Mate 3(5) 307 310. 

Barnes NR, Davis FJ, Mitchell GR. 1989. Molecular switching in liquid crystal elastomers. Mol Cryst Liq Cryst 168 13 25. 

Buguin A, Li MH, Silberzan P, Ladoux B, Keller P. 2006. Micro actuators when artificial muscles made of nematic liquid crystal elastomers meet soft lithography. J Am Chem Soc 128 1088 1089. 

Kiipfer J, Finkelmann H. 1994. Liquid crystal elastomers influence of the orientational distribution of the crosslink s on the phase behaviour and reorientation processes. Macromol Chem Phys 195 1353 1367. 

Warner M, Terentjev EM. 2003. Liquid Crystal Elastomers. UK Oxford University. 

Xie P, Zhang R. 2005. Liquid crystal elastomers, networks and gels advanced smart materials. J Mater Chem 15 2529 2550. 

Yusuf Y, Ono Y, Sumisaki Y, Cladis PE, Brand HR, Finkelmann H, Kai S. 2004b. Swelling dynamics of liquid crystal elastomers swollen with low molecular weight liquid crystals. Phys Rev E 69 021710. 

Fig. 3.13. Relative temperature dependences for the BC nematic lODClPBCP fluid monomer, for a BC nematic swollen in a calamitic liquid crystal elastomer (BCN-LCE) and a BC nematic elastomer (BCLCE). The flexoelectric coefficient of an LCE is also shown (note that it is not distinguishable from the horizontal axis at the present scale due to the three orders of magnitude difference).

The frequency dependences of the bend fiexoelectric coefficients were also measured for the same BC nematic fluid monomer, BC nematic swollen in a calamitic liquid crystal elastomer (BCN-LCE) and for the bent-core nematic elastomer (BCLCE) as shown in Fig. 3.14. One can see that for each material the fiexoelectric effect was found to be zero below 1 Hz, then the response increases abruptly up to 2 Hz and then decreases slightly. The apparent absence of the response below 1 Hz is probably due to screening by free ions. The slow decrease of the fiexoelectric coefficient at higher / is not yet clear. We assume, however, that it is not a measurement error, because 5CB showed a constant value in this frequency range. 

R. Verduzco, P. Luchette, S.H. Hong, J. Harden, E. DiMasi, P. Palfly-Muhoray, S.M. Kilbey II, S. Sprunt, J.T. Gleeson and A. JakU, Bent-core liquid crystal elastomers, J. Mater. Chem. 20(39), 8488-8495, (2010). doi 10.1039/C0JM01920H... 

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