Nanostructured Composites Based on Liquid-Crystalline Elastomers

Dynamics of the photo-thermo-mechanical actuations in NIR-dye doped liquid crystal polymer networks

We describe photo-thermo-mechanical actuation and its dynamics in thin films of a liquid crystal networks (LCN) under near infrared (NIR) illumination through experiments and simulations. Splay aligned films of different thicknesses (25 μm to 100 μm) were obtained by crosslinking a mixture of mono-functional and bi-functional liquid crystal monomers. The NIR-driven thermo-mechanical actuation was achieved by adding an NIR dye to the monomer mixture. The absorption of incoming radiation by the dye molecules raises the local temperature of the film causing an order-disorder (nematic-isotropic) transition, thereby resulting in a macroscopic shape change. We have investigated the effect of film thickness, NIR laser power and dye concentration on the tip displacement of the films in a cantilever configuration. The experimental findings and finite element simulation results are in reasonably good quantitative agreement. Despite using lower NIR powers than typically employed, the films show high actuation and large displacements. After achieving saturation in actuation, the films exhibit a flutter behavior which is discussed in light of the observed overshoot in the tip displacement for certain intensities and thicknesses. Finally, using a solar simulator, we also show the visible light response of the film.

Actuation performance of a liquid crystalline elastomer composite reinforced by eiderdown fibers

Liquid crystalline elastomer (LCE) materials have been developed and investigated for several decades. One important obstacle, which impedes the practical industrial application of LCE materials, is their modest robustness as actuator materials. In this work, we developed a LCE composite which was fabricated by incorporating eiderdown fibers into a polysiloxane-based main-chain LCE matrix. The eiderdown fibers were used as the flexible reinforcement phase suitable for the shape-morphing performance of LCE materials upon being stimulated. Due to the long fiber property, specific structure and surface characteristics of the eiderdown fibers, they constructed a reinforcement network in the LCE matrix and formed tight interfacial adhesion with the matrix. The LCE composite demonstrated enhanced actuation mechanical properties and robust actuation performance. Its actuation blocking stress and modulus were increased due to the reinforcement effect of the eiderdown fibers. The tensile strength and the performance of anti-fatigue failure under repeated actuation cycles and high loadings were greatly improved due to the crack-resisting effect and bridging effect of the eiderdown fibers. While other properties, such as the liquid crystalline phase structure, the stimulus deformation ratio, phase transition temperature of the LCE matrix, etc., did not deteriorate or change due to the high flexibility, thermal stability and chemical stability of the eiderdown fibers.

Mechanically programmed 2D and 3D liquid crystal elastomers at macro- and microscale via two-step photocrosslinking

Liquid crystal elastomers (LCEs) are a unique class of active materials with the largest known reversible shape transformation in the solid state. The shape change of LCEs is directed by programming their molecular orientation, and therefore, several strategies to control LC alignment have been developed. Although mechanical alignment coupled with a two-step crosslinking is commonly adopted for uniaxially-aligned monodomain LCE synthesis, the fabrication of 3D-shaped LCEs at the macro- and microscale has been rarely accomplished. Here, we report a facile processing method for fabricating 2D and 3D-shaped LCEs at the macro- and microscales at room temperature by mechanically programming (i.e., stretching, pressing, embossing and UV-imprinting) the polydomain LCE, and subsequent photocrosslinking. The programmed LCEs exhibited a reversible shape change when exposed to thermal and chemical stimuli. Besides the programmed shape changes, the actuation strain can also be preprogrammed by adjusting the extent of elongation of a polydomain LCE. Furthermore, the LCE micropillar arrays prepared by UV-imprinting displayed a substantial change in pillar height in a reversible manner during thermal actuation. Our convenient method for fabricating reversible 2D and 3D-shaped LCEs from commercially available materials may expedite the potential applications of LCEs in actuators, soft robots, smart coatings, tunable optics and medicine.