Influence of cellulose nanocrystal surface functionalization on the bending response of cellulose nanocrystal/ionic liquid soft actuators

Abstract

This work reports the development of renewable cellulose nanocrystal (CNC) and ionic liquid (IL) hybrid materials for bending actuator applications. For this purpose, cellulose nanocrystals with different surface charges (neutral, positive and negative) were prepared and increasing amounts of the IL 2-hydroxy-ethyl-trimethylammonium dihydrogen phosphate ([Ch][DHP]) (10 and 25 wt%) were incorporated into the CNC hosting matrix. The morphology of the samples was evaluated, proving that both surface charge and IL incorporation do not affect the characteristic layered structure of the CNC. Atomic force microscopy results reveal a sea-island morphology in the hybrid films, where CNC bundles are surrounded by [Ch][DHP]-rich regions. An increase in the electrical conductivity is observed upon IL incorporation into the CNC matrix, regardless of the CNC surface charge. The highest electrical conductivity values are observed for IL/CNC (+) 25 wt% with an electrical conductivity of 3.18 × 10⁻⁵ ± 2.75 × 10⁻⁷ S cm⁻¹ and IL/CNC (−) 10 wt% (1.26 × 10⁻⁵ ± 5.92 × 10⁻⁶ S cm⁻¹). The highest bending displacement of 2.1 mm for an applied voltage of 4.0 Vpp at a frequency of 100 mHz was obtained for the IL/CNC (+) 25 wt% composite, demonstrating the suitability of cellulose to develop soft actuators.