Abstract
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 (CSE0.3) exhibited moisture-responsive properties, while CSEs with higher DSs of 1.3 or 3 (CSE1.3 and CSE3) not. The CSE0.3 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 CSE3 nanoparticles (NPs) onto CSE0.3 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 CSE0.3 at one side and one layer of CSE3 at the other side exhibited combined responsiveness to moisture and temperature. By varying the thickness of CSE0.3 films, the minimal bending extent can be adjusted due to altered mechanical resistances, which allows a bending movement preferentially beginning with the thinner side.
Highlights
Moisture-responsive materials are gaining greater interest for their potentially wide applications and the readily access to moisture
We show the first moisture-responsive, self-standing and transparent films using derivatives of sustainable cellulose, cellulose stearoyl esters (CSEs)
Cellulose stearoyl esters (CSEs) with different degrees of substitution (DSs) of 0.3, 1.3 and 3 were synthesized via two distinct synthesis routes, either heterogeneously with cellulose suspended in pyridine or homogeneously with cellulose dissolved in DMAc/LiCl before the chemical modification (Scheme S1)
Summary
Moisture-responsive materials are gaining greater interest for their potentially wide applications and the readily access to moisture. During the alteration of the environmental moisture content, i.e. relative humidity, a particular part of the biological systems reversibly absorbs or releases the moisture During this process, a mechanical deformation takes place, with the goal to perform a desired function such as directed complex motions[1,4]. Still no successful fabrication of moisture-responsive devices from pristine cellulose-derived materials without any other additives has been reported, such as self-standing films. This fact on the one hand limits the application of cellulose and on the other hand addresses new challenges for the development of cellulose-based compounds, which requires the precise control on polar and non-polar moieties
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