Anomalous scaling law of strength and toughness of cellulose nanopaper

Abstract

The quest for both strength and toughness is perpetual in advanced material design; unfortunately, these two mechanical properties are generally mutually exclusive. So far there exists only limited success of attaining both strength and toughness, which often needs material-specific, complicated, or expensive synthesis processes and thus can hardly be applicable to other materials. A general mechanism to address the conflict between strength and toughness still remains elusive. Here we report a first-of-its-kind study of the dependence of strength and toughness of cellulose nanopaper on the size of the constituent cellulose fibers. Surprisingly, we find that both the strength and toughness of cellulose nanopaper increase simultaneously (40 and 130 times, respectively) as the size of the constituent cellulose fibers decreases (from a mean diameter of 27 μm to 11 nm), revealing an anomalous but highly desirable scaling law of the mechanical properties of cellulose nanopaper: the smaller, the stronger and the tougher. Further fundamental mechanistic studies reveal that reduced intrinsic defect size and facile (re)formation of strong hydrogen bonding among cellulose molecular chains is the underlying key to this new scaling law of mechanical properties. These mechanistic findings are generally applicable to other material building blocks, and therefore open up abundant opportunities to use the fundamental bottom-up strategy to design a new class of functional materials that are both strong and tough.strength | toughness | scaling law | cellulose | hydrogen bond

Toughening mechanisms in cellulose nanopaper: the contribution of amorphous regions

“…The effect of microvoids on the mechanical properties of all-cellulose fibreboard was also reported by Arévalo and Peijs (2016) where denser fibreboard resulted in higher mechanical properties. Recently, Zhu et al (2015) proposed that the high toughness of cellulose nanopaper was caused by breaking and reformation of hydrogen bonding during inter-fibre slippage. The hypothesis was supported by results of atomistic simulation.…”

A Strong, Tough, and Scalable Structural Material from Fast‐Growing Bamboo

“…Furthermore, the two key structural properties—strength and toughness—tend to be mutually exclusive in structural material design, in which strong materials are usually brittle, whereas tough materials are frequently weak . To address this challenge, tremendous efforts have been devoted in developing bioinspired structural materials that can offer the combination of both strength and toughness at low density (i.e., lightweight) while also manufactured at high volume and low cost …”

“…In other word, the closer packing made possible by the removal of the hemicellulose/lignin matrix allows the formation of new hydrogen bonds. The highly aligned state of the fibers amplifies their effects due to collective synergy of molecular interlocking, leading to stiffening and effective energy dissipating mechanisms . Consequently, the total energy needed to fracture the densified bamboo is much higher than that needed to fracture natural bamboo.…”

Cellulose‐Nanofiber‐Enabled 3D Printing of a Carbon‐Nanotube Microfiber Network

“…Figure S3c (Supporting Information) shows the typical stress–stain curve of the CNT–NFC paper. A pure CNT film was also prepared, which has a tensile strength of 31 MPa . This value is much lower than that of the CNT–NFC film, indicating the reinforcing effect of NFC.…”

“…For comparison, we also simulate the sliding of the top and bottom CNT layers relative to the middle NFC layer in the CNT–NFC–CNT not aligned model. Recent studies on tensile failure of cellulose nanopaper (made of a network of NFC nanofibers) and cellulose–graphene oxide hybrid fiber reveal that, due to the facile formation nature of hydrogen bonds, the inter‐fiber sliding involves a cascade of events of forming, breaking, and reforming of hydrogen bonds in between neighboring NFC nanofibers. Each hydrogen bond breaking dissipates energy.…”