Anisotropic Colloidal Micromuscles from Liquid Crystal Elastomers

Abstract

Monodomain liquid crystal elastomers (LCEs) are new materials uniquely suitable for artificial muscles, as they undergo large reversible uniaxial shape changes, with strains of 20-500% and stresses of 10-100 kPa, falling exactly into the dynamic range of a muscle. LCEs exhibit little to no fatigue over thousands of actuation cycles. Their practical use has been limited, however, owing to the difficulty of synthesizing components, achieving consistent alignment during cross-linking across the whole material and often a high nematic-isotropic phase transition temperature. The most widely studied method for LC alignment involves mechanical stretching of the material during one of two cross-linking steps, which makes fabrication difficult to control and lends itself mainly to samples that can be easily grasped (with sizes of the order of mm). In this article, we describe a method of adapting the LCE synthesis to microscale objects, achieving monodomain alignment with a single cross-linking step, and lowering the cycling temperature. LCE precursor droplets are embedded in and then stretched in a polymer matrix at high temperature. Confinement of the uniaxially stretched droplets maintains the alignment achieved during stretching and allows us to eliminate one of the cross-linking steps and the variability associated with it. Adding a comonomer during the polymerization leads to lowering of the nematic-to-isotropic transition temperature (58 °C), significantly expanding the range of potential applications for these micromuscles. We demonstrate reversible thermal switching of the micromuscles in line with the largest strain changes observed for side-chain LCEs and a differential scanning calorimetry characterization of the material phase transitions. The method demonstrates the parallel fabrication of many microscale actuators and is amenable to further scale-up and manufacturing.

Recent progress in shape memory polymer: New behavior, enabling materials, and mechanistic understanding

“…Despite their technological potential, shape changing microparicles had been limited to those exhibiting only nonprogrammable shape changing behavior [137,138].…”

Liquid Crystal Elastomer Actuators from Anisotropic Porous Polymer Template

“…In this method, fast reaction rate of vinyl groups and a constant load to fix the alignment of LC mesogens are required that inherently limits its practical applications. In addition, electromagnetic fields and photopatterning have been demonstrated to be effective ways to fabricate LCEs, although they are difficult for scale‐up production . The utilization of LC cells coated with mechanical rubbed polymer films (polyvinyl alcohol or polyimide) as an active layer inducing the alignment of LCs to fabricate LCEs is the most widely accepted approach.…”

Nano-Zirconium Tungstate Reinforced Liquid Crystalline Thermosetting Composites with Near Zero Thermal Expansion

“…Liquid crystalline elastomers (LCE) represent a special class of SMPs that are defined by a reversible LC phase transition and a unique coupling between LC mesogens and polymer networks [96][97][98][99] . They exhibit reversible shape change when exposed to external stimuli, such as heat 96,[100][101][102][103][104][105] , light [106][107][108][109][110][111][112] , or magnetic field [113][114] , which makes them excellent candidates for artificial muscles, sensors, lithography substrates, and shape memory materials. A number of LCEs with different LC phases and network structures have been synthesized and characterized, including nematic main-chain, smectic mainchain, nematic side-chain, and smectic side-chain LCEs.…”