Abstract
Inspired by plant movements driven by the arrangement of cellulose, we have fabricated nanopapers of nanofibrillated cellulose (NFC) showing actuation under pH changes. Bending was achieved by a concentration gradient of charged groups along the film thickness. Hence, the resulting nanopapers contained higher concentration of charged groups on one side of the film than on the opposite side, so that pH changes resulted in charge-dependent asymmetric deprotonation of the two layers. Electrostatic repulsions separate the nanofibers in the nanopaper, thus facilitating an asymmetric swelling and the subsequent expanding that results in bending. Nanofibrillated cellulose was modified by 2,2,6,6-tetramethylpiperidin-1-yloxyl radical (TEMPO) oxidation at two reaction times to get different surface concentrations of carboxylic acid groups. TEMPO-oxidized NFC was further chemically transformed into amine-modified NFC by amidation. The formation of graded nanopapers was accomplished by successive filtration of NFC dispersions with varying charge nature and/or concentration. The extent of bending was controlled by the charge concentration and the nanopaper thickness. The direction of bending was tuned by the layer composition (carboxylic acid or amine groups). In all cases, a steady-state was achieved within less than 25 s. This work opens new routes for the use of cellulosic materials as actuators.
Highlights
In plant cell walls, cellulose fibrils are embedded in a highly swellable matrix consisting of hemicelluloses, pectins, structural proteins and/or lignins
The cellulose microfibrils are well aligned along the awn axis in the cap while they are randomly oriented at the ridge
Olszewska et al [6] observed that cationic cellulose nanofibrils swelled at low pH due to the protonation of the amine groups, and they concluded that the higher the charge of nanofibrillated cellulose (NFC), the more water uptake
Summary
Cellulose fibrils are embedded in a highly swellable matrix consisting of hemicelluloses, pectins, structural proteins and/or lignins. Olszewska et al [6] observed that cationic cellulose nanofibrils swelled at low pH due to the protonation of the amine groups, and they concluded that the higher the charge of nanofibrillated cellulose (NFC), the more water uptake. Kuang et al [9] took advantage of the dehydration in cellulose for fabricating soft actuators consisting of aligned NFC onto a passive layer. In this case, bending was driven by asymmetric water evaporation, and actuation was tuned by temperature and nanofiber alignment. We investigated the induced actuation behavior by the fabrication of asymmetric NFC nanopapers containing different charge concentrations along the film thickness.
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