Polypyrrole-decorated cellulose nanocrystal fillers in liquid crystal elastomers for multi-stimuli response†

Abstract

Near-infrared (NIR) liquid crystal elastomer (LCE) materials are a class of soft actuators capable of absorbing light and converting it into heat energy, resulting in anisotropic shape changes. The rapid response, high deformation and tunability of NIR-responsive LCEs make them emerging materials in the fields of soft robotics, artificial organs, and mechanical actuators. In this study, we proposed a functionalized filler system, where one component absorbs NIR light, while the other enhances the mechanical properties of LCEs without compromising their elasticity. Specifically, polypyrrole (PPy), a conjugated polymer, was used as a photothermal agent to absorb NIR light (808 nm). To improve the mechanical strength of LCEs, cellulose nanocrystals (CNCs) were functionalized with PPy, forming CNCs–PPy fillers, which were incorporated into LCEs. Four different weight percentages (0.01–0.04 wt%) of CNCs–PPy fillers were prepared and integrated into LCEs to obtain composite films. The addition of CNCs–PPy enhanced the mechanical stability of LCEs, with LCE/CNCs–PPy(0.02 wt%) exhibiting the highest mechanical strength (2.08 MPa) and a total elongation of 107%. Moreover, this composite had impressive weight lifting capability, lifting and holding a maximum weight of 350 g under IR illumination—1182 times its weight. All the composites retained liquid crystalline properties and demonstrated good thermal stability. The composites were analyzed for their multi-stimuli response (thermal and light). Overall, LCE/CNCs–PPy(0.02 wt%) had superior mechanical stability, while all the LCE/CNCs–PPy(0.02–0.04 wt%) composites exhibited reversible actuation in response to thermal and IR light stimuli. The composite films displayed consistent behavior over multiple cycles and in aging experiments thus demonstrating their durability and reliability LCE/CNCs-PPy materials have great potential for application in the field of artificial muscles.