XXIV ICTAM, 21-26 August 2016, Montreal, Canada
ANOMALOUS SCALING LAW OF MECHANICAL PROPERTIES OF CELLULOSE
NANOPAPER:
DEFEATING THE CONFLICT BETWEEN STRENGTH AND TOUGHNESS
Shuze Zhu,1,# Zheng Jia,1,# Hongli Zhu,2,# Sepideh Parvinian, 2 Yuanyuan Li,2Oeyvind Vaaland,2 Liangbing Hu,2
Teng Li1,a
1
Department of Mechanical Engineering, University of Maryland, College Park, MD 20742
2
Department of Materials Science and Engineering, University of Maryland, College Park, MD 20742
#
These authors contributed equally to this work.
Summary A long-standing challenge in material design is to overcome the conflict between strength and toughness, as they are generally
mutually exclusive. To address this challenge, we rationally design cellulose-based nanopaper and investigate the dependence of their
mechanical properties on constituent cellulose fiber size. Surprisingly, we find that both the strength and toughness of the nanopaper increase
simultaneously (40 and 130 times, respectively) as the average diameter of constituent cellulose fibers decreases from 27 micrometers to 11
nanometers, suggesting the promising potential toward an anomalous but highly desirable scaling law: The smaller, the stronger AND the
tougher. There are abundant opportunities to utilize the fundamental bottom-up strategy to design a novel class of functional materials that are
both strong and tough.
The mechanical properties of materials are inherently tied to the material
structure, ranging from atomic structure to grains, fibrils, and layers. There
have been tremendous efforts in recent years to develop materials with higher
strength, by using smaller scale material structures (down to nms). For example,
by decreasing the grain size of crystalline metals (thus more grain boundaries),
dislocation motions (thus plasticity) are more restricted, leading to a higher
strength. On the other hand, such treatments also minimize the possible
mechanisms (e.g., crack-tip blunting) to relieve local high stress, resulting in
lower toughness. The atomic scale origins of high strength of a material, e.g.,
Fig. 1. An anomalous but desirable strong directional bonding and limited dislocation mobility, are also essentially
scaling law of mechanical properties the roots for brittleness and low toughness of the material. In short, the wellrequires defeating the conventional recognized scaling law of “The smaller, the stronger” comes with a price of
conflict of strength vs. toughness.
sacrificing toughness (Fig. 1).
Despite the consensus on mutual exclusiveness of strength and toughness, a significant gap exists between engineering
practice and scientific considerations. So far, there exists only rather limited success in attaining both strength and
toughness, which often involve material-specific, complicated or expensive synthesis processes and thus are hardly
applicable to other materials. A general and feasible mechanism to address the conflict between strength and toughness still
remains elusive.
To gain insight toward a generally applicable solution to the challenge of attaining both strength and toughness, we
investigate the mechanical properties of cellulose-fiber-based paper, with constituent building block size ranging from 10s
mm down to 10 nm, from which we unveil a highly desirable but hard-to-achieve scaling law of mechanical properties of
cellulose-based nanopaper that is anomalous to conventional understanding: both the strength and toughness of the
nanopaper increase simultaneously (40 and
130 times, respectively) as the size of the
constituent cellulose building blocks decreases
(from a diameter of 27 µm to 10 nm). These
stimulating results suggest the promising
potential toward a new and highly desirable
scaling law: the smaller, the stronger AND the
tougher.
Cellulose is the most abundant biopolymer
on earth and has long been used as the
sustainable building block for paper. Wood
fibers are the main natural source of cellulose
and have an intrinsically hierarchical structure
(Fig. 2a). A 20~50 µm thick wood fiber Fig. 2. (a) Hierarchical structure of wood biomass and the characteristic of
comprises thousands of nanofibrillated cellulose fibrils. Note the rich inter-chain hydrogen bonds between
cellulose (NFC, 5-50 nm in diameter), each of neighboring cellulose molecular chains. (b) Single cellulose chain repeat
which can be disintegrated into finer unit. Note the six hydroxyl groups (red solid circles) in each repeat unit.
a)
Corresponding author. Email: [email protected]
elementary fibrils consisting of cellulose molecular chains.
Cellulose molecule is a linear chain of ringed glucose molecules, with a repeat unit (Fig. 2b) comprising two
anhydroglucose rings (C6H10O5) linked through C-O-C covalent bond. Rich hydroxyl groups in cellulose molecule (six in
each repeat unit) enable facile formation of hydrogen bonds, both intra-chain and inter-chain.
Fig. 4. Intriguing results from our pilot study: an anomalous scaling law of strength and toughness of cellulose-based
nanopaper. (a) Schematic of cellulose nanopaper, made of a network of NFC. Inset shows HR-TEM image of a ~10nm
NFC. (b) Stress-strain curves of cellulose paper made of NFC of various diameters. As the NFC diameter decreases, both
tensile strength and ductility of the cellulose paper increases, leading to an anomalous scaling law shown in (c): The
smaller, the stronger AND the tougher. (d) Further reveals that the tensile strength scales inversely with the square root of
NFC diameter. (e) Optical (inset) and SEM images of a CNT film made of network of CNTs. (f) Lack of inter-CNT
hydrogen bonds, the CNT film is shown to have a much lower tensile strength, and toughness than the cellulose nanopaper,
although the constituent CNTs and NFCs have similar diameter (~10nm).
Motivated by the unique molecular feature and attractive mechanical properties of cellulose, we rationally designed
highly transparent, mechanically strong nanopaper and investigate the dependence of mechanical properties of nanopaper on
constituent NFC size. Surprisingly, we revealed the simultaneous and significant increase of both the tensile strength and
toughness of nanopaper as the constituent NFC diameter decreases, a highly desirable but hard-to-achieve scaling law in
advanced materials design (Fig. 4a-d). To further shed insight on the anomalous scaling law of strength and toughness of the
cellulose nanopaper, we also fabricated CNT films of comparable thickness that are made of network of CNTs with
diameter similar with NFC fibers (Fig. 4e). It turns out that the strength and toughness of cellulose nanopaper are also much
higher (9 and 195 times, respectively) than those of the CNT films (Fig. 4f).
The above results suggest the following mechanistic understanding on the exceptional mechanical properties and
anomalous scaling law of cellulose nanopaper: while the increase in tensile strength of cellulose nanopaper is attributed to
reduced intrinsic defect size as the constituent NFC size decreases (the smaller, the stronger); the simultaneous increase in
toughness essentially results from the significant increase of hydrogen bonding among NFC fibers as their diameter
decreases (and thus their surface area increases).
CONCLUSIONS
We conducted a first-of-its-kind pilot study on the mechanical properties of cellulose-fiber-based paper, with constituent
building block size ranging from 10s mm down to 10 nm, from which we reveal a fundamental bottom-up strategy to achieve
both high strength and toughness that is generally applicable for a wide range of other material building blocks.
References
[1] H. Zhu, S. Zhu, Z. Jia, S. Parvinian, Y. Li, O. Vaaland, L. Hu, T. Li, Anomalous scaling law of strength and toughness of cellulose nanopaper, Proceedings
of the National Academy of Sciences of the United States of America (PNAS), 112(29), 8971-8976 (2015).