Instant Locking of Molecular Ordering in Liquid Crystal Elastomers by Oxygen‐Mediated Thiol–Acrylate Click Reactions

Abstract

Liquid crystal elastomers (LCEs) with intrinsic anisotropic strains are reversible shape-memory polymers of interest in sensor,a ctuator,a nd soft robotics applications. Rapid gelation of LCEs is required to fix molecular ordering within the elastomer network, whichi se ssential for directed shape transformation. Ah ighly efficient photo-cross-linking chemistry,b ased on two-step oxygen-mediated thiol-acrylate click reactions,a llows for nearly instant gelation of the mainchain LCE network upon exposure to UV light. Molecular orientation from the pre-aligned liquid crystal oligomers can be faithfully transferred to the LCE films,a llowing for preprogrammed shape morphing from two to three dimensions by origami-(folding-only) and kirigami-like (folding with cutting) mechanisms.T he new LCE chemistry also enables widely tunable physical properties,i ncluding nematic-toisotropic phase-transition temperatures (T N-I ), glassy transition temperatures (T g ), and mechanical strains,w ithout disrupting the LC ordering.Reconfigurable soft materials have gained much attention [1] because of their potential in aw ide range of applications, including energy storage devices, [2] smart windows, [3] microfluidics, [4] optics, [5] flexible electronics, [6] artificial muscles, [7] and soft robots. [8] To realize complex and arbitrary shapes that can transition from two-(2D) to three-dimensions (3D), it is essential to program the global shape transformation with precisely controlled local deformation, in terms of both the magnitude and orientation of the mechanical strain. Anisotropic materials,such as liquid crystal elastomers (LCEs), are smart materials that can deform selectively and reversibly when exposed to heat, light, and electric and magnetic fields. [9] Theu nique shape adaptivity and reversible shape-memory effect of LCEs originate from the molecular ordering within the network during sample preparation;f or example,b y application of am echanical loading, [10] magnetic field, [11] or surface alignment, [12] followed by cross-linking.More complex mechanical responses can be obtained by preprograming LC orientation in locally defined monoliths, thus generating localized elasticity. [9b,12b,13] To do so,i ti s critical to faithfully transfer molecular ordering from the prealigned LC monomers (LCMs) or oligomers to the LCE

Molecularly‐Engineered, 4D‐Printed Liquid Crystal Elastomer Actuators

“…While printing, the ink is photocrosslinked by radical reaction of the thiol‐terminated oligomers with a trifunctional vinyl crosslinker (1,3,5‐triallyl‐1,3,5‐triazine‐2,4,6(1H,3H,5H)‐trione (TATATO)). This synthetic approach is inspired by several prior reports that utilize thiol‐ene reactions to synthesize LCEs . Nonetheless, this approach is distinct in that it is designed specifically to be compatible with 3D printing and in that the thiol‐ene reaction is used for both chain extension and crosslinking which allows for the generation of LC inks (and the resulting LCEs) with highly tunable physical properties .…”

Polymer Network Structure, Properties, and Formation of Liquid Crystalline Elastomers Prepared via Thiol–Acrylate Chain Transfer Reactions

“…In the case of thiol-Michael addition reactions, the reactions are fast and readily amenable to patterning via 3D printing but have not yet been shown to be conducive to surface-enforced alignment. More recently, a simple, radical-mediated thiol–acrylate photopolymerization reaction has been detailed to prepare LCEs. − This approach to preparing LCEs assimilates the amenability to surface-enforced alignment with the speed, simplicity, and spatiotemporal control of a photopolymerization reaction …”

Seamless multimaterial 3D liquid-crystalline elastomer actuators for next-generation entirely soft robots

“…Here, we choose an unequimolar base-catalyzed thiol-acrylate reaction due to its rapidity and low sensitivity to oxygen inhibition. To fabricate multimaterial LCE soft robots, in this paper, we first prepare six different acrylate group-rich LCE films with different chemical components (namely, pre-LCE 1, pre-LCE 2, pre-LCE 3, pre-LCE 4, pre-LCE 5, and pre-LCE 6) based on thiol-acrylate chemistry, which has been widely used to construct LCEs in the past (23,24,28,(30)(31)(32)(33)(34)(35). Pre-LCE 1, whose starting materials are shown in Fig.…”