Cellulose nanocrystals reinforced polyvinyl alcohol hydrogel with thermal, mechanical, and ionic responsive behavior

Conventional polyvinyl alcohol (PVA) hydrogels with isotropic polymer networks usually lack selective responses to external stimuli, which significantly limits their applications in smart devices. In this work, a series of double network (DN) composite PVA hydrogels reinforced with cellulose nanocrystal (CNC) were prepared by in situ photopolymerization of functional monomers and freeze-thawing (F-T). The incorporation of CNC and the DN structure resulted in the high tensile strength of the hydrogel (1.6 MPa), which was 6.7 times higher than that of the pure PVA hydrogel. The prepared hydrogels also showed multi-stimuli response behaviors, as well as unique optical anisotropy because of the orientation and birefringence of the introduced CNC. Results showed that a rapid and reversible color change gradually from blue to green, yellow, and red was observed when the hydrogel was stretched from 0 to 220 %. Similar results were found when the hydrogel was immersed in NaCl solutions of different concentrations. Additionally, the equilibrium swelling ratio (ESR) of the hydrogels in water notably decreased from 1125.5 to 730.0 % as the temperature increased from 0 to 60 °C. These properties make the prepared PVA hydrogels promising materials in environmental monitoring, soft mechanical stress, and temperature sensors.