Synthesis of Programmable Main-chain Liquid-crystalline Elastomers Using a Two-stage Thiol-acrylate Reaction

Abstract

This study presents a novel two-stage thiol-acrylate Michael addition-photopolymerization (TAMAP) reaction to prepare main-chain liquidcrystalline elastomers (LCEs) with facile control over network structure and programming of an aligned monodomain. Tailored LCE networks were synthesized using routine mixing of commercially available starting materials and pouring monomer solutions into molds to cure. An initial polydomain LCE network is formed via a self-limiting thiol-acrylate Michael-addition reaction. Strain-to-failure and glass transition behavior were investigated as a function of crosslinking monomer, pentaerythritol tetrakis(3-mercaptopropionate) (PETMP). An example non-stoichiometric system of 15 mol% PETMP thiol groups and an excess of 15 mol% acrylate groups was used to demonstrate the robust nature of the material. The LCE formed an aligned and transparent monodomain when stretched, with a maximum failure strain over 600%. Stretched LCE samples were able to demonstrate both stress-driven thermal actuation when held under a constant bias stress or the shape-memory effect when stretched and unloaded. A permanently programmed monodomain was achieved via a second-stage photopolymerization reaction of the excess acrylate groups when the sample was in the stretched state. LCE samples were photo-cured and programmed at 100%, 200%, 300%, and 400% strain, with all samples demonstrating over 90% shape fixity when unloaded. The magnitude of total stress-free actuation increased from 35% to 115% with increased programming strain. Overall, the two-stage TAMAP methodology is presented as a powerful tool to prepare main-chain LCE systems and explore structure-property-performance relationships in these fascinating stimuli-sensitive materials.

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 .…”

Facile Anisotropic Deswelling Method for Realizing Large‐Area Cholesteric Liquid Crystal Elastomers with Uniform Structural Color and Broad‐Range Mechanochromic Response

“…We propose that we do in fact benefit from anisotropic deswelling, but it is realized in a very much simplified manner compared to the original Finkelmann approach, as illustrated schematically in Figure . A key element is, we believe, the gel formation during the initial click polymerization of acrylates and thiols in the precursor mixture: it appears that the gel forming when using this chemistry adheres so strongly to the substrate that it cannot shrink in the lateral dimension as the solvent evaporates, as evidenced from the unchanged surface area of the sample during drying (see Figure b). This restricts the deswelling to take place only in the vertical () direction as the solvent evaporates, causing an in‐plane orientation of the liquid crystal director and thus a vertical helix axis, that is, along .…”

“…Here, we report an elegantly simple, highly reproducible, and easily scalable method to prepare uniformly colored large‐area CLCE films that show robust, rapid, and reversible mechanochromic response to stretching and compression under loads typical of those delivered by a human, using only off‐the‐shelf commercial chemicals. Our method is based on the two‐stage thiol‐acrylate Michael addition and photopolymerization reaction reported by Yakacki et al for making non‐chiral nematic main‐chain LCEs, the move to CLCEs being achieved by the introduction of the diacrylate chiral mesogen LC756 ( Figure a; see Section and Supporting Information for more details). While we also apply anisotropic deswelling to control the helix orientation, our modification allows us to carry out the procedure without centrifugation and near room temperature, simply casting the CLCE precursor solution onto a substrate, where it reacts to yield a CLCE film with uniform selective reflection color in the visible spectrum.…”

Thiol‐acrylate main‐chain liquid‐crystalline elastomers with tunable thermomechanical properties and actuation strain

“…Recently, our group introduced a two‐stage thiol‐acrylate Michael addition‐photopolymerization (TAMAP) methodology for the first time in mesomorphic systems to prepare nematic monodomain main‐chain LCEs . Several recent examples of using TAMAP methodology to prepare main‐chain LCEs have been performed …”