Moisture-responsive films of cellulose stearoyl esters showing reversible shape transitions

Moisture-responsive materials are gaining greater interest for their potentially wide applications and the readily access to moisture. In this study, we show the fabrication of moisture-responsive, self-standing films using sustainable cellulose as starting material. Cellulose was modified by stearoyl moieties at first, leading to cellulose stearoyl esters (CSEs) with diverse degrees of substitution (DSs). The films of CSE with a low DS of 0.3 (CSE(0.3)) exhibited moisture-responsive properties, while CSEs with higher DSs of 1.3 or 3 (CSE(1.3) and CSE(3)) not. The CSE(0.3) films could reversibly fold and unfold as rhythmical bending motions within a local moisture gradient due to the ab- and desorption of water molecules at the film surface. By spray-coating CSE(3) nanoparticles (NPs) onto CSE(0.3) films, moisture-responsive films with non-wetting surface were obtained, which can perform quick reversible bending movements and continuous shape transition on water. Furthermore, bilayer films containing one layer of CSE(0.3) at one side and one layer of CSE(3) at the other side exhibited combined responsiveness to moisture and temperature. By varying the thickness of CSE(0.3) films, the minimal bending extent can be adjusted due to altered mechanical resistances, which allows a bending movement preferentially beginning with the thinner side.