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Anomalous scaling law of strength and toughness of cellulose nanopaper, Zhu, et al., Li, PNAS, 2015

Novelty/impact/significance:

A cellulose-based nanopaper that shows a scaling law of a simultaneous, substantial increase of strength and toughness as the cellulose fiber sizes decrease from 27 µm to 11 nm is developed. The mechanism includes the reduced intrinsic defect size and, more importantly, the repeated (re)forming of hydrogen bonding, a design/optimization strategy applicable to wide materials systems.

Scientific question:

Looking for a general mechanism to address the conflict between strength and toughness for advanced materials design.

Key of how:

The reduced intrinsic defect size (through an analytical model) and the repeated break and formation of hydrogen bonds among cellulose molecular chains (by atomistic and fiber-scale simulations) are the key of the scaling law.

Fiber diameter decrease → surface area (per unit volume) increase, the contact area between neighboring fibrils and thus effective region of hydrogen bonding increase → degree/quantity of effective hydrogen bond breaking and reforming for toughening increase.

Major points (useful info):

1. The conflict of strength and toughness exists in almost all engineering materials, since the origin of strength lies in the strong bonding between atoms and thus a high difficulty to deform, which, however, is the key for toughness.

2. Toughening mechanisms can be intrinsic (ahead of the crack, plasticity) and extrinsic (reduce crack driving force, crack deflection, bridging). But simultaneously attaining strength and toughness via a general and feasible mechanism is rare.

3. Through fabricating a cellulose nanopaper with varying fiber diameter and associated experimental and theoretical studies, a new scaling law (smaller for stronger and tougher) suggesting bottom-up design and wide applicability is developed.

Cellulose (in fibrous form) is most abundant and has high mechanical properties, which are due to the rich hydrogen bondings.

4. Cellulose nanopapers (CNP) with different fiber diameters (27 µm, 28, 20, 11 nm) and random entanglement are made from wood-derived cellulose fibers. Tensile tests show that CNP with 11 nm fiber show substantially higher strength and toughness than CNP with 27 µm fiber. (It seems that the toughness increase from 0.13 to 11.68 MJ/m3 is less than 130 times).

Building a continuum mechanistic model of a cellulose fiber with preexisting defect reveals that the strength is inversely proportional to the square root of fiber diameter, and the higher strength is due to the reduced intrinsic defect size with fiber diameter decrease.

5. Control groups of carbon nanotube (CNT) films (CNT diameter 11 nm) show much lower strength and toughness compared with CNP. (It might be good to including how to appropriately determine the cross sections, and thus the strength, modulus and toughness, of the porous, cellulose nanopapers (if I did not miss any information), as these are important to exclude the uncertain effects of porosity.)

6. It is proposed that the simultaneous increase of toughness with strength of CNP is due to the increase in hydrogen bonding interactions between cellulose fibrils with diameter decrease. Cellulose molecules slide while breaking and reforming of hydrogen bonds occur, which dissipates a large amount of energy and increases toughness. The CNT films do not have such mechanism as CNTs only have weaker van der Waals forces.

7. Atomistic at the molecular level confirms this mechanism: the potential energy v.s. the pulling-out displacement shows a zigzag fluctuation profile corresponding to the cycles of hydrogen bond stretching, breaking, and reforming during loading. Structural observations also corroborate this.

Molecular dynamics simulations at the fiber-level (bundles of molecules/tubes) of CNP and CNT films further confirm the mechanism, by examining the parallel and perpendicular slidings. Cellulose fibers show that zigzag fluctuation of potential energy matches well with hydrogen bonding energy (dominant effect). Perpendicular sliding show higher total potential energies which is due to severe bending of bottom cellulose fiber.

The smaller fibril diameter, the more contact area between fibrils for hydrogen bonding, and thus the higher degree of hydrogen bond breaking and reforming for toughening and strengthening.

 

The atomistic and fiber-level simulations are very beautiful and clear, explaining the fundamentals well.

It would be no easy to execute the scaling law, since making nanomaterials with precise control in dimension and quality and enabling the mechanism of hydrogen bonding at molecular origin is mostly beyond the existing know-how of current engineering techniques. But the near future with better instrument to implement the mechanism demonstrated by this work is exciting.

The writing is clear, the finding is novel and significant, and the analysis is comprehensive and convincing, all demonstrating the feature of high-quality research: it looks simple, interesting & impressive, despite the not-simple process of working out the new idea.

Here is the fulltext: https://www.pnas.org/content/112/29/8971#ref-37

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