Definition of the Subject
Anisotropic (liquid crystalline) elastomers and gels bring together, as nowhere else, three important ideas: orientational order in amorphous soft materials, responsive molecular shape, and quenched topological constraints. Acting together, they create many new physical phenomena that are briefly reviewed in this article. Classical liquid crystals are typically fluids of relatively stiff rod molecules with long-range orientational order. Long polymer chains, with incorporated rigid anisotropic units, can also form orientationally ordered liquid crystalline phases. By contrast with free rigid rods, these flexible chains change their average molecular shape, from isotropic spherical to ellipsoidal, when their component rods align. Linking the polymer chains together into a network fixes their relative topology, and the polymer melt (or solution) becomes an elastic solid – an elastomer (or gel). Radically new properties arise from the ability to change average...
Abbreviations
- Elastomer:
-
Nominally equivalent to “rubber,” this defines a weakly cross-linked network of polymer chains which retain high thermal mobility of the strands between cross-links. In this case the entropic effects dominate the response of such networks. Entropic rubber elasticity arises when such a network percolates the whole macroscopic system and the cross-links constrain it such that it remembers its equilibrium shape and responds elastically to deformations. In this sense, an elastomer is contrasted to a polymer glass: The latter can refer to a network so densely cross-linked that the individual chain segments have no significant mobility or a polymer system below its structural glass transition. The shear elastic modulus of elastomers is usually in the range 10–1000 kPa, the estimate arising from the thermal energy kT per network strand, while the glass modulus is usually 0.1–10 GPa, as in most solids.
- Gel:
-
This word is used in many different contexts, in all cases referring to a soft object that is capable of retaining its shape in ambient conditions against, e.g., gravity. The latter condition distinguishes a gel from a “sol” or a nominal liquid. The softness is a relative concept; usually it refers to the modulus at or below the “human” scale, around 1–100 kPa, but there are many examples of even softer gels. In this article, a gel is contrasted to an elastomer, referring to a cross-linked network of polymer chains which is swollen by a good solvent. In this case the effective shear modulus becomes very low; nevertheless, the system remains elastic as long as the integrity of percolating network remains intact.
- Quenched constraints:
-
The term quenched (as opposed to annealed) refers to objects or systems that are prevented from exploring their full phase or conformational space during thermal motion. In elastomers and gels, network cross-links are the quenched objects, constraining the polymer chains connecting to them. The effect of randomly quenched constraints is profound in many physical systems, as local thermodynamic equilibrium has to be established among the mobile (annealed) elements while observing the constraints, the random local realization of which is determined by external factors, such as the preparation history.
- Liquid crystal:
-
This refers to a group of anisotropic phases with incomplete translational order (which would represent a crystalline lattice). Classical examples are the nematic phase, a fluid with no translational order at all, but with a uniaxial orientational order of anisotropic molecules, and smectic or lamellar phases, which in addition to orientational order also have a one-dimensional density modulation (a stack of parallel layers). Liquid crystallinity is a state of spontaneous, equilibrium anisotropy (breaking of orientational symmetry), in contrast to anisotropy induced by external factors such as electric/magnetic field or mechanical deformation.
- Shape memory:
-
Strictly, any elastic material “remembers” its equilibrium shape and returns to it after deformation. The term shape memory was introduced to distinguish materials that could be made to preserve their deformed state, until a trigger (e.g., heating above a transition temperature) induces the return to the original equilibrium shape. In shape-memory alloys, this is due to the martensitic transition; in shape-memory polymers, the deformed state can be fixed by a glass transition or partial crystallization. Liquid crystal elastomers and gels have a reversible or equilibrium shape memory in that the shape of the body is determined by the current state of order at any given temperature.
Bibliography
Abramchuk SS, Khokhlov AR (1987) Molecular theory of high elasticity of the polymer networks with orientational ordering of links. Dokl Akad Nauk 297:385
Adams JM, Warner M (2005a) Elasticity of smectic-A elastomers. Phys Rev E 71:021708
Adams JM, Warner M (2005b) Soft elasticity in smectic elastomers. Phys Rev E 72:011703
Annaka M, Tanaka T (1992) Multiple phases of polymer gels. Nature 355:430
Bhattacharya K (2003) Microstructure of martensite. Oxford University Press, Oxford
Bladon P, Terentjev EM, Warner M (1994) Deformation-induced orientational transitions in liquid crystal elastomers. J Phys II 4:75
Brehmer M, Zentel R, Giesselmann F, Germer R, Zugemaier P (1996) Coupling of liquid crystalline and polymer network properties in LC-elastomers. Liq Cryst 21:589
Broer DJ, Lub J, Mol GN (1995) Wide-band reflective polarizers from cholesteric polymer networks with a pitch gradient. Nature 378:467
Camacho-Lopez M, Finkelmann H, Palffy-Muhoray P, Shelley M (2004) Fast liquid-crystal elastomer swims into the dark. Nat Mater 3:307
Chang C-C, Chien L-C, Meyer RB (1997a) Electro-optical study of nematic elastomer gels. Phys Rev E 56:595
Chang C-C, Chien L-C, Meyer RB (1997b) Piezoelectric effects in cholesteric elastomer gels. Phys Rev E 55:534
Cicuta P, Tajbakhsh AR, Terentjev EM (2002) Evolution of photonic structure on deformation of cholesteric elastomers. Phys Rev E 65:051704
Cicuta P, Tajbakhsh AR, Terentjev EM (2004) Photonic bandgaps and optical activity in cholesteric elastomers. Phys Rev E 70:011703
Clarke SM, Terentjev EM (1998) Slow stress relaxation in randomly disordered nematic elastomers and gels. Phys Rev Lett 81:4436
Clarke SM, Tajbakhsh AR, Terentjev EM, Remillat C, Tomlinson GR, House JR (2001a) Soft elasticity and mechanical damping in liquid crystalline elastomers. J Appl Phys 89:6530
Clarke SM, Tajbakhsh AR, Terentjev EM, Warner M (2001b) Anomalous viscoelastic response of nematic elastomers. Phys Rev Lett 86:4044
Courty S, Tajbakhsh AR, Terentjev EM (2003a) Phase chirality and stereo-selective swelling of cholesteric elastomers. Eur Phys J E 12:617
Courty S, Tajbakhsh AR, Terentjev EM (2003b) Stereo-selective swelling of imprinted cholesterics networks. Phys Rev Lett 91:085503
Cviklinski J, Tajbakhsh AR, Terentjev EM (2002) UV-isomerisation in nematic elastomers as a route to photo-mechanical transducer. Eur Phys J E 9:427
Finkelmann H, Benne I, Semmler K (1995) Smectic liquid single-crystal elastomers. Macromol Symp 96:169
Finkelmann H, Kim ST, Munoz A, Palffy-Muhoray P, Taheri B (2001a) Tunable mirrorless lasing in cholesteric liquid crystalline elastomers. Adv Mater 13:1069
Finkelmann H, Nishikawa E, Pereira GG, Warner M (2001b) A new opto-mechanical effect in solids. Phys Rev Lett 87:015501
Gebhard E, Zentel R (1998) Freestanding ferroelectric elastomer films. Macromol Rapid Comm 19:341
Golubović L, Lubensky TC (1989) Nonlinear elasticity of amorphous solids. Phys Rev Lett 63:1082
Hikmet RAM, Kemperman H (1998) Electrically switchable mirrors and optical components made from liquid-crystal gels. Nature 392:476
Hogan PM, Tajbakhsh AR, Terentjev EM (2002) UV-manipulation of order and macroscopic shape in nematic elastomers. Phys Rev E 65:041720
Ilmain F, Tanaka T, Kokufuta E (1991) Volume transition in a gel driven by hydrogen-bonding. Nature 349:400
Kim ST, Finkelmann H (2001) Cholesteric liquid single-crystal elastomers (LSCE) obtained by the anisotropic deswelling method. Macromol Rapid Commun 22:429
Kishi R, Shishido M, Tazuke S (1990) Liquid-crystalline polymer gels: anisotropic swelling of poly(gamma-benzyl L-glutamate) gel crosslinked under a magnetic field. Macromolecules 23:3868
Kishi R, Suzuki Y, Ichijo H, Hirasa H (1997) Electrical deformation of thermotropic liquid-crystalline polymer gels. Mol Cryst Liq Cryst 294:411
Kundler I, Finkelmann H (1995) Strain-induced director reorientation in nematic liquid single-crystal elastomers. Macromol Rapid Commun 16:679
Kundler I, Finkelmann H (1998) Director reorientation via stripe-domains in nematic elastomers. Macromol Chem Phys 199:677
Küpfer J, Finkelmann H (1991) Nematic liquid single-crystal elastomers. Macromol Rapid Commun 12:717
Kutter S, Terentjev EM (2001) Tube model for the elasticity of entangled nematic rubbers. Eur Phys J E 6:221
Legge CH, Davis FJ, Mitchell GR (1991) Memory effects in liquid-crystal elastomers. J Phys II 1:1253
Lehmann W, Gattinger P, Keck M, Kremer F, Stein P, Eckert T, Finkelmann H (1998) The inverse electromechanical effect in mechanically oriented SmC*-elastomers. Ferroelectrics 208:373
Li MH, Keller P, Li B, Wang XG, Brunet M (2003) Light-driven side-on nematic elastomer actuators. Adv Mater 15:569
Lubensky TC, Terentjev EM, Warner M (1994) Layer-network coupling in smectic elastomers. J Phys II 4:1457
Mao Y, Warner M (2000) Theory of chiral imprinting. Phys Rev Lett 84:5335
Matsui T, Ozaki R, Funamoto K, Ozaki M, Yoshino K (2002) Flexible mirrorless laser based on a free-standing film of photopolymerized cholesteric liquid crystal. Appl Phys Lett 81:3741
Matsuyama A, Kato T (2002) Nematic ordering-induced volume phase transitions of liquid crystalline gels. J Chem Phys 116:8175
Meier W, Finkelmann H (1990) Piezoelectricity of cholesteric elastomers. Macromol Chem Rapid Commun 11:599
Mitchell GR, Davis FJ, Guo W (1993) Strain-induced transitions in liquid-crystal elastomers. Phys Rev Lett 71:2947
Olmsted PD (1994) Rotational invariance and goldstone modes in nematic elastomers and gels. J Phys II 4:2215
Pelcovits RA, Meyer RB (1995) Piezoelectricity of cholesteric elastomers. J Phys II 5:877
Roberts PMS, Mitchell GR, Davis FJ (1997) A single director switching mode for monodomain liquid crystal elastomers. J Phys II 7:1337
Schmidtke J, Stille W, Finkelmann H (2003) Defect mode emission of a dye doped cholesteric polymer network. Phys Rev Lett 90:083902
Schönstein M, Stille W, Strobl G (2001) Effect of the network on the director fluctuations in a nematic side-group elastomer analysed by static and dynamic light scattering. Eur Phys J E 5:511
Shibayama M, Tanaka T (1993) Volume phase-transition and related phenomena of polymer gels. Adv Polym Sci 109:1
Stenull O, Lubensky TC (2004) Anomalous elasticity of nematic and critically soft elastomers. Phys Rev E 69:021807
Stenull O, Lubensky TC (2005) Phase transitions and soft elasticity of smectic elastomers. Phys Rev Lett 94:018304
Stenull O, Lubensky TC (2006) Soft elasticity in biaxial smectic and smectic-C elastomers. Phys Rev E 74:051709
Tabiryan N, Serak S, Dai X-M, Bunning T (2005) Polymer film with optically controlled form and actuation. Opt Express 13:7442
Tajbakhsh AR, Terentjev EM (2001) Spontaneous thermal expansion of nematic elastomers. Eur Phys J E 6:181
Tanaka T (1978) Collapse of gels and critical endpoint. Phys Rev Lett 40:820
Terentjev EM (1993) Phenomenological theory of non-uniform nematic elastomers: free energy of deformations and electric field effects. Europhys Lett 23:27
Terentjev EM (1995) Density functional model of anchoring energy at a liquid crystalline polymersolid interface. J Phys II 5:159
Terentjev EM, Warner M (1999) Piezoelectricity of chiral nematic elastomers. Eur Phys J B 8:595
Terentjev EM, Warner M, Bladon P (1994) Orientation of liquid crystal elastomers and gels by an electric field. J Phys II 4:667
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
Urayama K, Okuno Y, Nakao T, Kohjiya S (2003) Volume transition of nematic gels in nematogenic solvents. J Chem Phys 118:2903
Urayama K, Arai YO, Takigawa T (2005a) Volume phase transition of monodomain nematic polymer networks in isotropic solvents. Macromolecules 38:3469
Urayama K, Kondo H, Arai YO, Takigawa T (2005b) Electrically driven deformations of nematic gels. Phys Rev E 71:051713
Vallerien SU, Kremer F, Fischer EW, Kapitza H, Zentel R, Poths H (1990) Experimental proof of piezoelectricity in cholesteric and chiral smectic C* phases of LC-elastomers. Macromol Chem Rapid Comm 11:593
Verwey GC, Warner M (1997) Compositional fluctuations and semisoftness in nematic elastomers. Macromolecules 30:4189
Verwey GC, Warner M, Terentjev EM (1996) Elastic instability and stripe domains in liquid crystalline elastomers. J Phys II 6:1273
Warner M, Terentjev EM (2007) Liquid crystal elastomers, 2nd edn. Oxford University Press, Oxford
Warner M, Gelling KP, Vilgis TA (1988) Theory of nematic networks. J Chem Phys 88:4008
Warner M, Bladon P, Terentjev EM (1994) ‘Soft Elasticity’ - Deformations without resistance in liquid crystal elastomers. J Phys II 4:93
Warner M, Terentjev EM, Meyer RB, Mao Y (2000) Untwisting of a cholesteric elastomer by a mechanical field. Phys Rev Lett 85:2320
Yu Y, Nakano M, Ikeda T (2003) Directed bending of a polymer film by light - miniaturizing a simple photomechanical system. Nature 425:145
Yusuf Y, Ono Y, Sumisaki Y, Cladis PE, Brand HR, Finkelmann H, Kai S (2004) Swelling dynamics of liquid crystal elastomers swollen with low molecular weight liquid crystals. Phys Rev E 69:021710
Zubarev ER, Talroze RV, Yuranova TI, Vasilets VN, Plate NA (1996) Influence of crosslinking conditions on the phase behavior of a polyacrylate-based liquid-crystalline elastomer. Macromol Rapid Commun 17:43
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Terentjev, E.M. (2015). Anisotropic Networks, Elastomers, and Gels. In: Meyers, R. (eds) Encyclopedia of Complexity and Systems Science. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-642-27737-5_20-2
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