Some Advances in Liquid Crystal Elastomers: From Crosslinks Affected Ordering to Carbon Nanoparticles Enabled Actuation
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1005-Q04-06
Some Advances in Liquid Crystal Elastomers: From Crosslinks Affected Ordering to Carbon Nanoparticles Enabled Actuation Slobodan Zumer1,2, Martin Chambers1, George Cordoyiannis1,3, Heino Finkelmann4, Zdravko Kutnjak1, Andrija Lebar1,4, Maja Remskar1, and Bostjan Zalar1 1 Jozef Stefan Institute, Ljubljana, Slovenia 2 Phyics Department, University of Ljubljana, Ljubljana, Slovenia 3 Department of Physics and Astronomy, Katholieke Universiteit Leuven, Leuven, Belgium 4 Institute for Macromolecular Chemistry, Albert-Ludwigs-Universit‰t, Freiburg, Germany
ABSTRACT Liquid crystal elastomers (LCE) exhibit a combination of elasticity and mesogenic ordering, yielding large thermally stimulated changes in shape. These LCE systems although well characterised, still yield open questions in the nature of how the crosslinking affects the LCE phase transition. Therefore calorimetry and deuteron-nuclear magnetic resonance were used to study the isotropic-nematic phase transition of uniformly ordered LCE. We observed that the density of crosslinkers strongly affects the nematic-isotropic phase transition. The observed spread critical transitions are explained with a dispersion of local mechanical fields that yields a weakly disordered orientational state composed of regions that exhibit temperature profiles of the nematic order parameter ranging from first order to supercritical. On increasing crosslinking density, the predominantly first order thermodynamic response transforms into a predominantly supercritical one. Additionally, to illustrate the response of these actuating systems, it was demonstrated that a LCE can be electrically heated. The insulating LCE network was reprocessed using conducting nanoparticles dispersed in a solvent with high LCE swelling capability. This results in a low electrical resistivity surface layer of LCE network with a high concentration of conducting nanoparticles. The reprocessing allows the effective resistivity of a LCE film to be reduced from highly insulating values to values useable for electrical actuation. This layer in addition withstands large changes in geometrical shape both in contraction and expansion. Utilizing a resistive ìJouleî heating effect, the reprocessed system exhibits an indirect electromechanical effect characterised by a 150% length change that can be cycled for more than 10,000 times.
INTRODUCTION Liquid crystalline elastomers are materials composed of cross linked polymer networks with either main-chain or side-chain mesogenic units. The combination of the near ideal entropic elasticity of polymer networks with the orientational ordering of anisotropic, nematogenic molecules yields interesting mechanical, optical, electrical and thermal properties [1]. Among the many interesting phenomena, the most unusual include spontaneous shape change at phase transitions, strain-induced orientational transitions leading to new organized morphologies, and soft elasticity. Such multifunctional materials are flexible like simple polymers but their elastic modulus and the
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