Insights on Water Dynamics in the Hygromorphic Phenomenon of Biopolymer Films.

Water-responsive biopolymer thin films with engineered matrix characteristics can accomplish desirable shape changing properties such as self-folding. Self-folding response of chitosan film is experimentally characterized by its total folding time and rate of folding. Here, atomistic simulation is employed to investigate the molecular mechanism responsible for modified self-folding behavior observed in nanoparticle reinforced chitosan films. The nanocomposite system is solvated with water content varying from 10% to 100% of total mass of the system. The free volume available for diffusion of water molecules is affected by the flexibility of glycosidic linkages present in chitosan chains. The increase in mobility of water molecules with increase in water content decides the rate of folding. A separate molecular system is modeled with confined region between nanoparticles densified with chitosan chains and water molecules. The thickness of confined region is determined from the critical distance of influence of nanoparticles on water molecules. The adsorption of water on nanoparticle surface and relaxation of chitosan chains are responsible for increased total folding time with nanoparticle concentration. This simulation study, complemented with experimental observations provides a useful insight into the designing of actuators and sensors based on the phenomenon of hygromorphism.