Liquid crystalline elastomer
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Liquid crystalline elastomers (LCEs) are slightly crosslinked liquid crystalline polymer networks. These materials combine the entropy elasticity of an elastomer with the self-organization of the liquid crystalline phase. In liquid crystalline elastomers, the mesogens can either be part of the polymer chain (main-chain liquid crystalline elastomers) or they are attached via an alkyl spacer (side-chain liquid crystalline elastomers).
Due to their actuation properties, liquid crystalline elastomers are attractive candidates for the use as artificial muscles in soft or microrobots. These were already predicted by Pierre-Gilles de Gennes in 1975 and first synthesized by Heino Finkelmann. In the temperature range of the liquid crystalline phase, the mesogen's orientation forces the polymer chains into a stretched conformation. By heating the sample above the clearing temperature this orientation is lost and the polymer backbone can relax into (the more favored) random coil conformation which can lead to a macroscopic, reversible deformation. For good actuation, it is necessary to have a good alignment of the domains' directors before cross-linking. This can be achieved by: stretching of the prepolymerized sample, photo-alignment layers, magnetic or electric fields and microfluidics. Recently, Roach, et. al. described a way to align the LCE mesogens using room temperature 3D printing so that they can be used for 4D printed, multi-material smart structures.
Beside the thermal deformation of a sample, a light-responsive actuation can be obtained for samples with azobenzenes being incorporated in the liquid crystalline phase. The phase transition temperature of an azo-liquid crystalline elastomer can be reduced due to the trans-cis isomerization of the azobenzenes during UV-irradiation and thus the liquid crystalline phase can be destroyed isothermally. For liquid crystalline elastomers with a high azo-concentration, a light-responsive change of the sample's length of up to 40% could be observed.
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