Self-folding behavior of bilayer films actuated by liquid crystal elastomers

Liquid crystal elastomers (LCEs) films enable thermally responsive shape change. Mesogenic segments in the elastomeric network can be aligned into crystalline domains via mechanical deformation of the film. If subjected to a second-stage UV cure in the deformed shape, the crystalline domains are retained upon release of the external load. This induces a temporary shape in the LCE. Subsequent heating of the LCE above the nematic-to-isotropic temperature disorders the liquid crystals, and strain energy stored in the elastomer causes the LCE to return to the undeformed. In a reversible manner, the LCE returns to the temporary shape when cooled. In this work, we use thermally responsive LCE films applied to passive thin films, such as mylar and Kapton. The passive film enhances the mechanical strength and stiffness of the film but prevents alignment of the LCE crystalline domains through stretching. Instead, these bilayer films are restricted to folding deformations, wherein the LCE layer is used to induce thermally responsive shape change. We study the effects of layer thickness ratios on the reversibly self-folding bilayer films. The LCE films are synthesized directly on the passive film layer using a two-stage thiol-acrylate Michael addition and photopolymerization (TAMAP) reaction in which the first stage is a thermal cure, and the second stage is a UV cure. We demonstrate the reversible shape change between flat and folded states, and quantify the shape change in terms of the shape fixity and recovered flatness ratios. Potential applications for this system are actuators for deployable structures and soft robotics.