Hierarchical structure of wood fibers and the characteristic of cellulose fibrils. Note the rich interchain hydrogen bonds among neighboring cellulose molecular chains.
Paper made of cellulose fibers is tougher and stronger the smaller the fibers get. For a long time, engineers have sought a material that is both strong (resistant to non-recoverable deformation) and tough (tolerant of damage). “Strength and toughness are often exclusive to each other,” said Teng Li, associate professor of mechanical engineering at UMD. “For example, a stronger material tends to be brittle, like cast iron or diamond.”
The UMD team pursued the development of a strong and tough material by exploring the mechanical properties of cellulose, the most abundant renewable bio-resource on Earth. They made papers with several sizes of cellulose fibers – 30 micrometers to 10 nanometers. The paper made of 10nm-thick fibers was 40X tougher and 130X stronger than regular notebook paper, which is made of cellulose fibers a 1000X larger. “These findings could lead to a new class of high performance engineering materials that are both strong and tough, a Holy Grail in materials design,” said Li.
An anomalous scaling law of strength and toughness of cellulose nanopaper. (A) Schematic of cellulose nanopaper, made of a random network of CNF fibers. (Inset) High-resolution transmission electron microscopy (HRTEM) image of an ∼11-nm CNF fiber. (B) Stress–strain curves of cellulose paper made of cellulose fibers of various mean diameters. As the cellulose fiber diameter decreases from micrometer scale to nanometer scale, both tensile strength and ductility of the cellulose paper increases significantly, leading to an anomalous scaling law (C): the smaller, the stronger and the tougher. (D) Reveals that the ultimate tensile strength scales inversely with the square root of cellulose fiber diameter.
BENEFIT: Cellulose fibers can easily form many H bonds. Once broken, they can reform on their own: ie ‘self-healing’ quality. The smaller the cellulose fibers, the more H bonds/ sq area ie paper made of very small fibers can both hold together better and re-form more quickly, which is the key for cellulose nanopaper to be both strong and tough.
They did a similar experiment using carbon nanotubes similar in size to the cellulose fibers. The CNTs’ had much weaker bonds, so under tension they did not hold together as well. Paper made of carbon nanotubes is weak, though individually nanotubes are arguably the strongest material ever made. One possible future direction for the research is the improvement of the mechanical performance of carbon nanotube paper.
http://imechanica.org/files/PNAS-2015-Zhu-1502870112%20plus%20SI.pdf
http://phys.org/news/2015-07-stronger-tougher-paper-metal.htmljCp
Atomistic simulations to demonstrate the hydrogen bond breaking and reforming events among cellulose molecular chains. (A) Simulation model of acellulose bundle contains seven cellulose molecular chains. Top view only shows the middle three chains for visual clarity. (B) Variation of total potential energy as a function of the sliding displacement of the center cellulose chain out of the bundle. (Insets) Clearly shown are the hydrogen bond breaking and reformingevents (dotted circles), each of which dissipates energy. (C) Relative cellulose chain-sliding, during which a series of hydrogen bond breaking and reforming events happen (in boxed region) when neighboring hydroxyl groups come close to each other
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