Thermostable and Redispersible Cellulose Nanocrystals with Thixotropic Gelation Behavior by a Facile Desulfation Process

As a ecofriendly and promising nanomaterial, cellulose nanocrystals (CNCs) have attracted significant interest. However, the intrinsic sulfate-modified surface and self-aggregation behavior of the most common acid-hydrolyzed CNCs with sulfuric acid lead to the low thermostability and poor redispersion of solid-state CNCs materials, which limit their practical applications. In this work, we propose a simple and effective strategy to not only remove the sulfates in CNCs but also resuscitate their dispersibility by a facile post-treatment process, induding an acid-triggering gelation, solvent exchange with ethanol, and supercritical CO2 drying. The resultant CNC powder exhibits a high thermostability, much higher than that of the common acidhydrolyzed CNCs and similar to that of the pristine cellulose. Moreover, the CNC powder can be readily redispersed in various solvents by a short ultrasonic treatment. Subsequently, as the CNC concentration increases, CNC gels spontaneously form and exhibit thixotropic behaviors. Taking advantage of these characteristics of the above desulfated CNC powder, we fabricated a CNC/Ag nanoparticle gel, magnetic CNC/Fe3O4 nanoparticle gel, pressure-sensitive polymer/CNC composite gel, and colorless and transparent poly(lactic acid)/CNC thermoplastic films. Therefore, the highly thermostable, redispersible, and easy-to-gel CNC powder has a huge potential as a building block for various stable, well-dispersed, and functional nanocomposite materials.