Abstract
Liquid crystalline elastomers (LCEs) exhibit a number of remarkable physical effects, including a uniquely high-stroke reversible mechanical actuation triggered by external stimuli. Fundamentally, all such stimuli affect the degree of liquid crystalline order in the polymer chains cross-linked into an elastic network. Heat and the resulting thermal actuation act by promoting entropic disorder, as does the addition of solvents. Photo-isomerization is another mechanism of actuation, reducing the orientational order by diminishing the fraction of active rod-like mesogenic units, mostly studied for azobenzene derivatives incorporated into the LCE composition. Embedding nanoparticles provides a new, promising strategy to add functionality to LCEs and ultimately enhance their performance as sensors and actuators. The motivation for the combination of nanoparticles with LCEs is to provide better-controlled actuation stimuli, such as electric and magnetic fields, and broad-spectrum light, by selecting and configuring the appropriate nanoparticles in the LCE matrix. Here we give an overview of recent advances in this area with a focus on preparation, physical properties and actuation performance of the resultant nanocomposites.
Highlights
Smart materials, which are capable of converting external stimuli into mechanical responses, have long been recognized as an exciting and high-impact research area with widespread applications [1,2].Several different types of responsive materials have been developed so far, with varying mechanical properties and response mechanisms
Some systematic work has been done with carbon reprocessed liquid crystal elastomers (LCEs), where the pre-formed mono-domain LCEs were swollen in a solution containing carbon nanoparticles; evaporating the solvent from this solution leaves a thin conducting carbon layer on the surface of the LCE
We found that Carbon nanotubes (CNTs) can impede the polymerization process, and that a catalysis system optimized for pure LCEs must be adjusted when CNTs are included; in our experience, Karstedt’s catalyst (Platinum(0)-1,3-divinyl1,1,3,3-tetramethyldisiloxane) is more effective than the often-used Pt(COD)Cl2 when CNTs are part of the first gelation step
Summary
Smart materials, which are capable of converting external stimuli into mechanical responses, have long been recognized as an exciting and high-impact research area with widespread applications [1,2]. Other drawbacks of direct heating are the difficulty of creating a heat supply that would change the surrounding temperature of LCEs at a very high speed, and the difficulty of creating a heat stimulus that would be entirely localized around the LCE component (so that adjacent actuating features could be stimulated independently) Even if such a heating mechanism were available, it would be difficult to incorporate it into a practical device. Some systematic work has been done with carbon reprocessed LCEs, where the pre-formed mono-domain LCEs were swollen in a solution containing carbon nanoparticles; evaporating the solvent from this solution leaves a thin conducting carbon layer on the surface of the LCE Such a layer allows resistive heating in an electric field, resulting in electrical actuation with little loss of mechanical response. The review concludes with some relevant challenges and perspectives of this field to stimulate further discussions on design, fabrication and performance, and research interest in these nanocomposites
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