Plastic pollution poses a huge threat to the ecological environment and human health. From the perspective of sustainable development, cellulose is expected to become excellent due to its renewable and biodegradable characteristics, excellent mechanical properties, and adjustable surface chemistry. Plastic alternatives. In the practical application of cellulose, due to the inherent amphiphilic nature of its molecule, cellulose will cause uncontrollable deformation and decrease in mechanical properties under certain environmental humidity. Although the stability of cellulose-based materials caused by humidity is a general concern, it is still unclear how moisture affects cellulose on multiple scales and the corresponding mechanical response and deformation mechanism, which makes the performance of cellulose-based materials It is difficult to maintain, which seriously hinders its practical application.
Professor Wu Hengan, Academician Yu Shuhong, Associate Researcher Zhu Yinbo of the University of Science and Technology of China and others have revealed the humidity-mediated interface and related deformation behavior in multi-level nano cellulose (CNC) through multi-scale simulation and experimental research. The results show that water molecules as interfacial media can simultaneously strengthen and toughen CNC within a proper relative humidity range. The competition between water molecules and hydrogen bonds (HBs) captured at the CNC interface will trigger the formation of the sliding interface and the inelastic deformation of the CNC. The author discussed in detail the strain hardening stage and deformation mechanism at the molecular level, and combined experiments to verify the inelastic deformation behavior on the macro scale. This work shows that the humidity-regulated interface can improve the mechanical properties of CNC, which is of great significance for obtaining high-perf ormance cellulose-based materials through interface design. The research was published on ACS Nano as a paper entitled "Strengthening and Toughening Hierarchical Nanocellulose via Humidity-Mediated Interface.

[The role of hydrogen bond] The mechanical behavior of CNC on the nanoscale is the starting point for the bottom-up design of cellulose-based materials. In general, cellulose Iβ is a more stable crystal phase and dominates in plants. Due to the different exposure of hydrophobic groups, it has two main characteristic crystal faces (Figure 1). The (010) plane exhibits exposed -OH groups, and the (100) plane exhibits buried -OH groups. Therefore, the author performed first-principles calculations to prove the difference in HBs caused by intercalated water molecules. Density functional theory calculations show that embedded water molecules can significantly enhance the interface strength of CNC, and the average strength of CNC-water-CNC HBs is much higher than that of CNC-CNC HBs. At the same time, compared with a simple CNC interface, CNC with interface water molecules can form more CNC-water-CNC HBs and a denser HBs network.

Figure 1 is a schematic diagram of the layered structure based on CNC. Its fracture behavior depends on humidity [humidity-mediated interface sliding and inelastic deformation] Changes in interface properties (such as interface strength and interface toughness) will significantly affect the deformation and mechanical properties of the material. Therefore, the authors performed MD simulations to study the mechanical response of CNCs with humidity-mediated interfaces (Figures 2, 3). The MD simulation results of uniaxial stretching show that the stress-strain curve of CNC with humidity- mediated interface exhibits linear elasticity in the first stage, and shows extremely high tensile stress and large fracture strain in the second stage. Strain hardening characteristics. Compared with the first linear elastic stage, the zigzag stress-strain curve of the second stage can be on average a linear stage with a lower slope, similar to the bilinear tensile stress-strain curve of cellulose-based m icrofibers and membranes. In addition, the author discovered disordered cellulose chains on the CNC interface during the interface slip process of humidity adjustment.

Figure 2 Mechanical properties obtained by MD simulation

Fig. 3 The influence of humidity-mediated interface on CNC deformation behavior [Mechanical properties under different humidity] Finally, the author conducted mechanical experiments on CNC film under different humidity to study the influence of humidity on the mechanical properties of CNC film (Fig. 4 ). For 30%≤humidity≤50%, after the CNC strain hardening stage, the linear elastic stage causes the fracture strain to increase greatly, which is consistent with the stress-strain curve and interface behavior in the MD simulation. When the humidity is greater than or equal to 60%, the elastic modulus and strength of CNC are significantly reduced. This is mainly due to excessive watermolecules at the interface, which weakens the strength of the interface and hinders the load transfer ability. Within a suitable humidity range, the strength and toughness of CNC are significantly improved. The increase in tensile strength is attributed to the increase in CNC-water- CNC HBs, while the increase in fracture toughness and strain is attributed to the strain hardening stage caused by the humidity-adjusted interface slip.

Figure 4 Summary of the mechanical properties and fracture surface of CNC films under different humidity: The author has a basic understanding of the deformation behavior and mechanical properties of nanocellulose through multi-scale simulation. Simulations and experiments show that the humidity-mediated interface can promote the enhancement of the mechanical properties of nanocellulose, which reveals the importance of the interface design based on hydrogen bonds in cellulose-based materials, so that the bottom-up design has all the advantages. Cellulose-based materials requiring mechanical properties provide a promising strategy .

Source of information: Frontiers of Polymer Science
molecules at the interface, which weakens the strength of the interface and hinders the load transfer ability. Within a suitable humidity range, the strength and toughness of CNC are significantly improved. The increase in tensile strength is attributed to the increase in CNC-water- CNC HBs, while the increase in fracture toughness and strain is attributed to the strain hardening stage caused by the humidity-adjusted interface slip.

Figure 4 Summary of the mechanical properties and fracture surface of CNC films under different humidity: The author has a basic understanding of the deformation behavior and mechanical properties of nanocellulose through multi-scale simulation. Simulations and experiments show that the humidity-mediated interface can promote the enhancement of the mechanical properties of nanocellulose, which reveals the importance of the interface design based on hydrogen bonds in cellulose-based materials, so that the bottom-up design has all the advantages. Cellulose-based materials requiring mechanical properties provide a promising strategy .

Source of information: Frontiers of Polymer Science

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