Nematic liquid crystal elastomers

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. 

J. Harden, M. Chambers, R. Verduzco, P. Luchette, J. Gleeson and S. Sprunt, Giant flexoelectricity in bent-core nematic liquid crystal elastomers, Appl. Phys. Lett. 96(10), 102907/1-3, (2010). doi 10.1063/1.3358391... 

Buguin, A., Li, M.-H., Silberzan, P., et al. (2006) Micro-Actuators When Artificial Muscles Made of Nematic Liquid Crystal Elastomers Meet Soft Lithography, J. Am. Chem. Soc., 128, 1088-9. 

Mraitazami R, Spillmann CM, Naciri J, Rama BR (2012) Enhanced thermomechanical properties of a nematic liquid crystal elastomer doped with gold nanoparticles. Sens Actuators A Phys 178 175-178... 

Wei RB, Zho LY, He YN, Wang XG, Keller P. Effect of molecular parameters on thermomechanical behavior of side-on nematic liquid crystal elastomers. Polymer 2013 54 5321-9. 

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. 

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. 

Bent-core liquid crystalline elastomer Bent-core nematic swollen in a liquid crystal elastomer Black lipid membranes Direct current Electroconvection Electrohydrodynamic Ferroelectric liquid crystal Hybrid-aligned nematic Human embryonic kidney Isotropic... 

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. 

However, it is well known that a mechanical deformation of a conventional, isotropic polymer network causes anisotropy. Under deformation the chain segments become oriented according to the symmetry of the external field and the state of order of the network can be characterized by an order parameter similar to that of nematic liquid crystals. Very early mechanical experiments on nematic polydomain elastomers actually demonstrate that a uniaxial deformation of a nematic elastomer converts the polydomain structure into a macroscopically xmi-formly ordered monodomain network [44]. This is shown in Fig. 2, where the opaque polydomain becomes optically transparent and converts into a monodomain... 

In conclusion, electric field effects in liquid crystals is a well-developed branch of condensed matter physics. The field behavior of nematic liquid crystals in the bulk is well understood. To a certain extent the same is true for the cholesteric mesophase, although the discovery of bistability phenomena and field effects in blue phases opened up new fundamental problems to be solved. Ferroelectric and antiferroelectric mesophases in chiral compounds are a subject of current study. The other ferroelectric substances, such as discotic and lyotropic chiral systems and some achiral (like polyphilic) meso-genes, should attract more attention in the near future. The same is true for a variety of polymer ferroelectric substances, including elastomers. 

Biggins JS, Terentjev EM, Warner M (2008) Semisoft elastic response of nematic elastomers to complex deformations. Phys Rev E 78 041704 1-4 Bladon P, Terentjev EM, Warner M (1993) Transitions and instabilities in liquid-crystal elastomers. Phys Rev E 47 R3838-R3840... 

Komp A, Ruhe J, Finkelmann H (2005) A versatile preparation route for thin fiee-standing liquid single crystal elastomers. Macnnnol Rapid Commun 26 813—818 Ktipfer J, Finkelmann H (1991) Nematic liquid single elastomers. Makromol Chem Rapid Commun 12 717-726... 

Petelin A, Copic M (2010) Strain dependence of the nematic fluctuation relaxation in liquid-crystal elastomers. Phys Rev E 82 011703 1-8... 

Godinho MH, Martins AE, Eigueirinhas JL (1998) Composite systems for display applications from cellulose elastomers and nematic liquid crystals. Opt Mater 9 226—229 Godinho MH, Canejo JP, Pinto LEV, Borges JP, Teixeira PIC (2009a) How to mimic the shapes of plant tendrils on the nano and microscale spirals and helices of electrospun liquid crystalline cellulose derivatives. Soft Matter 5 2772... 

Thisayukta J, Nakayama Y, Kawauchi S, Takezoe H, Watanabe J (2000) Distinct formation of a chiral smectic phase in achiral banana-shaped molecules with a central core based on a 2,7-dihydroxynaphthalene unit. J Am Chem Soc 122 7441-7448 Tschierske C, Dantlgraber G (2003) From antiferroelectricity to ferroelectricity in smectic mesophases formed by bent-core molecules. Pramana J Phys 61 455-481 Ungar G, Percec V, Zuber M (1992) Liquid crystalline polyethers based on conformational isomerism. 20. Nematic-nematic transition in polyethers and copolyethers based on l-(4-hydroxyphenyl)2-(2-R-4-hydroxyphenyl)ethane with R=lluoro, chloro and methyl and flexible spacers containing an odd number of methylene units. Macromolecules 25 75-80 Urayama K (2007) Issues in liquid crystal elastomers and gels. Macromolecules 40 2277-2288 Vorlander D (1908) About transparently clear, crystalline liquids. Rep Ger Chem Soc 41 2033-2052... 

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. 

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. 

Static mechanical properties in the vicinity of the nematic-isotropic transition in liquid single crystal elastomers (LSCEs) have been investigated [10, 11]. In Fig. 5 the deformation L/Lq (mon) is plotted as a function of the reduced temperature red- Here Lo(mon) denotes the length of the LSCE at the phase transition temperature of the nematic-isotropic phase transition and... 

Ultrasonic experiments using laser induced phonon spectroscopy have been performed in a nematic liquid single crystal elastomer [48]. The experiments reveal a dispersion step for the speed of sound and a strong anisotropy for the acoustic attenuation constant in the investigated frequency range (100 MHz -1 GHz). These results are consistent with a description of LCEs using macroscopic dynamics [54-56] and reflect a coupling between elastic effects and the nematic order parameter as analyzed in detail previously [48]. 

Kiipfer J, Finkelmann H. 1991. Nematic liquid single crystal elastomers. Makromol Chem Rapid Commun 12 717 726. 

In-plane switching Indium-tin-oxide Liquid crystal Liquid crystal display Liquid crystalline elastomer Nematic... 

The ability to form monodomain structures makes LC elastomers of interest for a number of optical applications where the optical axis of the polymer must be macroscopically and uniformly aligned. Kuepfer and Finkelmann have reported in detail the formation of what was termed nematic liquid single crystal elastomers [29], which are optically similar to organic or inorganic single crystals. 

A survey of recent studies of biaxial liquid crystals, in the context of the reported biaxial nematic phase in low molecular mass bent-core mesogens, with emphasize on the use of NMR spectroscopy is available. A discussion of orientational order parameters particularly in smectic-C and biaxial nematic phases and their determination by various techniques including NMR has been presented. The relation between molecular dynamics and biaxiality of nematic polymers and elastomers has been studied by H NMR spectroscopy. "" It was observed that the side-on attachment of the mesogens hinder the rotation of the molecules around their long axes and that in nematic polymers and elastomers phase biaxiality... 

Kundler, L Finkelmann, H., Strain-Induced Director Reorientation in Nematic Liquid Single Crystal Elastomers. Makromol. Rapid Commun. 1995,16, 679-686. 

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