|Liquid-Crystalline Elastomers (LCEs) are materials which combine the entropic properties of a crosslinked polymer melt with the enthalpic properties of a liquid-crystalline state of order. LCEs show unique characteristics: visco-elasticity and order at the same time in one system. The elastic and the viscous properties come from the crosslinking and friction of the polymer chains, respectively, while the orientation comes from the mesophase which keeps the polymer backbone aligned.
LCEs behave as normal polymer networks or rubbers when no energy-storing mesophase is present. This state of disorder can be induced by means of temperature, light, electric or magnetic fields. Thermally, the change in shape of LCEs can easily reach 300% in the direction of the director n when aligned, and all the enthalpy stored by the mesophase is released allowing the crosslinked polymer chains to move and adopt a random coil conformation. This effect is a reversible process, where change in length is directly proportional to the change of the local order parameter S.
The external stimulus-induced local disorder can be achieved when shape-changing molecules are incorporated in the LCE matrix. These compounds are able to absorb light, rearrange themselves in a new shape and thus disturb the mesophase. This results in the molecules that are keeping the order no longer being able to sustain the retractive force from the polymer backbone, and the material contracting, to exert an actuating force. In the same way, in order to disturb the mesophase, nanoparticles which absorb light or reorient under electromagnetic fields can be dispersed and new actuation modes can be achieved.
But, how does a stimulus-responsive side-chain LCE behave? And a main-chain LCE? What about nematics or smectics? Is a different kind of actuation, besides the common retractive force, possible? To answer these questions, new chemistry needs to be developed, together with new physics to understand the systems, and new applications need to be created.|