To build the ultrathin film with a thickness as low as 8.5 micrometers in the present work, Gan et al. partially removed lignin (delignification) and hemicellulose from natural balsa wood. They generated a highly porous material, which retained most of the cellulose in the cell walls, followed by increasing the density of treated wood by hot pressing for a thickness reduction of 97 percent. The densely packed wood cell wall structure combined with highly aligned cellulose fibers, contributed to superior tensile strength and high Young's modulus. The research team used industry-based cutting methods to develop a meter-long natural balsa wood film in the lab to reveal the material's potential for large-scale manufacture via a top-down approach.
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To understand the material's mechanical properties, the research team conducted mechanical tensile tests. Ultrathin wood showed greatly improved mechanical behavior compared with natural wood, with increased fracture strength of up to 342 MPa and Young's modulus of 43.65 GPa. These values indicated an almost 20-times improvement in tensile strength and 35-times enhancement in Young's modulus compared to natural wood.
The scientists were keen to understand the underlying mechanisms. For this, they used SEM observations and demonstrated a porous microstructure with numerous wood channels in the natural wood slice after tensile tests. The feature made it easier to pull loosely assembled wood during tension; explaining the naturally low fracture strength observed. In contrast, wood cell walls within the synthetic ultrathin wood film formed hydrogen bonds between the firmly compressed cellulose nanofibers after densification; requiring higher energy to be pulled apart.
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