Rapidly switchable water-sensitive shape-memory cellulose/elastomer nano-composites

Abstract

We report a new phenomenon in which the reversible formation and disruption of a cellulose nano-whisker (CNW) percolation network in an elastomeric thermoplastic polyurethane (TPU) matrix leads to an unprecedentedly rapidly switchable shape-memory effect (SME) that may be activated by water. The materials have been fully characterized to investigate the SME phenomenon using a number of different experimental techniques including cyclic tensile deformation, dynamic mechanical analysis, FTIR and polarized Raman spectroscopy. A model is developed in which it is shown that exposure to water allows breakup of the CNW percolation network so that the flexible elastomer matrix can be deformed to the desired shape. The CNW percolation network reforms after drying to provide a fixing force for the temporary shape. The entropy elasticity of the TPU matrix then enables rapid shape recovery when the CNW percolation network is disrupted again during wetting. This completely athermal water-sensitive SME mechanism is totally different from traditional ones, in which the water or other solvents are used as plasticizers to lower the glass transition temperature of shape memory polymers, so as to allow triggering of the shape recovery at room temperature or lower. The reported work provides a novel and effective strategy to achieve rapidly switchable shape recovery in a material by a simple wetting process and fixing through an easily applicable programmed drying process.