Welding complex-shaped actuators from dynamic liquid crystal elastomers

Abstract

Liquid crystal elastomers (LCEs) are promising materials for constructing and programming soft actuators and small-scale soft robotic systems due to their exceptional stimuli-responsive properties. However, fabricating complex-shaped LCE actuators with controlled shape transformations remains challenging. Herein, we present a welding-based strategy for fabricating light-responsive, multicomponent LCEs with complex shape morphing capabilities while preserving the distinct functional responses of individual components. This approach leverages dynamic disulfide bonds incorporated into surface-aligned, chain-extended LCEs, which enables robust adhesion between LCE segments without disrupting their molecular orientation during welding. The resulting structures seamlessly integrate differently oriented LCE segments, enabling diverse shape-morphing and establishing a platform for weldable, arbitrarily aligned, and complex-shaped LCE actuators.

Fig. 1. Disulfide-containning, chain-extended LCEs. (a) Chemical composition of the liquid crystal monomer mixture and schematic illustration of the disulfide bond metathesis. (b) Schematic illustration of the dynamic bond exchange due to disulfide metathesis. (c) Normalized absorption spectra of planar-aligned LCE₀ and LCE₃₀ films (thickness 5 μm) (d) DSC curves of LCE₀, LCE₂₀, and LCE₃₀ during the second cooling cycle (10 °C min⁻¹). (e) Tensile stress–strain curves of splay-aligned LCE₀, LCE₂₀, and LCE₃₀ at room temperature.

Fig. 2. Actuation performance. (a) Bending of splay-aligned LCE₃₀ strip at different temperatures and corresponding photographs (insets). r, radius of the bending arc. (b) Curvature change of a splay-aligned LCE₃₀ strip as a function of irradiation intensity (385 nm) and corresponding photographs (insets). (c) Thermally induced contraction of planar-aligned LCE₃₀ during one heating–cooling cycle. (d) Thermally induced twisting of twist-aligned LCE₃₀ and corresponding photographs (insets). (e) Light-induced heating kinetics of LCE₃₀ upon illumination with different intensities (385 nm). (f) Light-induced heating kinetics (385 nm, 240 mW cm⁻²) of planar-aligned LCEs with 4,4′-dithiodianiline, cystamine, and 4,4′-diaminodiphenylmethane (30 mol-% with respect to RM82) used as chain extenders/crosslinkers. Strip sizes in all experiments: 12 × 2 × 0.1 mm³.

Fig. 3. Welding of the disulfide-containing LCEs. (a) Photographs of welding of LCE₃₀. After welding, the sample can withstand a 25 g weight without rupture. One LCE is stained by thermally diffusing disperse red 1 into the LCE for better visualization. (b) ¹H NMR spectra of (1) 4,4′-dithiodianiline, (2) cystamine, (3) a mixture of 4,4′-dithiodianiline and cystamine, and (4) a mixture of 4,4′-dithiodianiline and cystamine after heating to 80 °C for 2 h in DMSO-d6. (c) Tensile stress–strain curves of splay-aligned LCE₃₀ upon welding at different temperatures. (d) Welding efficiency of LCE₀, LCE₂₀, and LCE₃₀ (welding conditions: 80 °C, 5 min). (e) Tensile stress–strain curves of LCEs with cystamine and 4,4′-diaminodiphenylmethane crosslinkers. (f) Welding efficiency and (g) Young's moduli of planar-aligned the 4,4′-dithiodianiline-, cystamine-, and 4,4′-diaminodiphenylmethane-containing LCEs, by stretching along the director axis. (h) Adhesion lap–shear testing for LCE₃₀, where the shear strength is plotted against the displacement. Inset: The measurement setup.

Fig. 4. Shape morphing after welding. Schematic diagram of (a) twisted, (b) splayed, and (c) planar alignment and photographs of actuation before and after cutting, pasting, and welding at 80 °C for 5 min. (d) Thermal contraction of welded, planar-aligned LCE₃₀ during one heating–cooling cycle. (e) The deformation reversibility of a bending LCE₃₀ strip after welding upon cyclic light excitation. Light: 385 nm, 120 mW cm⁻².

Fig. 5. Versatile shape transformations in welded LCE strips. (a) Examples of different actuator structures prepared by welding, including “S”-shape, wavy, round, and petal shapes and their heat-induced deformations. (b) Complex actuator assemblies by welding splayed, twisted, and planar-aligned LCE strips together. Snapshots of a strip ring rolling towards (c) and away from (d) the light source. Schematic illustration and photograph of a ring obtained through welding the two ends of a splay-aligned LCE strip (middle). The LCE is made by fixing its heat-induced bending direction towards the inner (c) or outer (d) surface. Irradiation conditions: 385 nm, 160 mW cm⁻².