Recently, Professor Wang Yonggui, a young teacher from the team led by Academician Li Jian and Professor Xie Yanjun of NEFU, made innovative achievements in the field of intelligent assembly of cellulose nano-units, and published the paper entitled Multifunctional Reversible Self-assembled Structures of Cellulose-derived Phase Change Nanocrystals, with the Northeast Forestry University as the first complete unit, in the internationally renowned academic journal Advanced Materials (IF2019: 27.398). The first author of the paper was Professor Wang Yonggui, and the corresponding authors were Professor Wang Yonggui and Professor Zhang Kai from the University of Göttingen, Germany. Academician Li Jian and Professor Xie Yanjun made guidance and suggestions on the completion of the paper.  The relevant research was sponsored by the National Natural Science Foundation of China and other projects.

Self-assembly is a ubiquitous phenomenon in nature, which can endow corresponding substances with specific structures and functions.  The preparation of the functional materials based on self-assembly of nanoparticles, especially the smart materials with responsive reversibility, has always been a research hotspot in the field of material science. The solid-liquid phase change material is a temperature-sensitive material that can realize the reversible transformation of crystallization and melting through temperature control. The research of solid-liquid phase change materials mainly focuses on improving heat storage efficiency and stabilizing structure etc.  There are few reports on the use of the temperature-sensitive reversibility of the phase change structure to control the characteristics of the assembly of nano-units.

Professor Wang Yonggui from the Northeast Forestry University and Professor Kai Zhang from the University of Göttingen collaborated to modify cellulose nanocrystals (CNCs) and introduce 10-undecenoyl groups as flexible spacers to give the nano-units movement under solid-phase conditions at room temperature.  At the same time, through click chemistry, a phase change structure unit containing an octadecane chain was introduced at the end of the flexible spacer to synthesize core-shell cellulose nano-units (C18-UCNCs) with a phase change structure. The phase change structure of the shell endows the C18-UCNCs with a significant phase change energy storage effect. The latent heat of phase change is about 40J/g, which has the same endothermic and heat preservation performance as stearic acid.  At the same time, the unreacted cellulose crystal core ensures the thermal stability of its structure.  C18-UCNCs show good thermal imaging characteristics, and can realize thermal imaging on the surface of different substrates such as glass and metal.

In addition, the synergy of the flexible spacer and the phase change structure unit makes C18-UCNCs have intelligent assembly characteristics.  After the film is formed by casting, a sheet-like self-assembled structure is formed on the surface of the film. The self-assembled structure has heat-sensitive reversible performance and self-repair performance under solid-phase conditions. After the heat treatment, the self-assembled structure disappears. After cooling and standing at room temperature for a few hours, the self-assembled structure will regenerate. It can still maintain good reversibility through 100 heating-cooling cycles. At the same time, along with the disappearance and regeneration of the self-assembled structure, the surface hydrophobicity, transparency and bifolding properties of the film undergo corresponding reversible transformations. What is more surprising is the self-repairing performance of the self-assembled structure. After the self-assembled structure of the surface layer of the C18-UCNCs film is destroyed or removed by physical scratching, it can be regenerated from the underlying substrate by simple heating treatment. This research provides a new strategy for the intelligent reversible assembly of nano-cells under solid-phase conditions.

Link to the paper: https://doi.org/10.1002/adma.202005263