Review: current international research into cellulose nanofibres
and nanocomposites
S. J. Eichhorn
Æ
A. Dufresne
Æ
M. Aranguren
Æ
N. E. Marcovich
Æ
J. R. Capadona
Æ
S. J. Rowan
Æ
C. Weder
Æ
W. Thielemans
Æ
M. Roman
Æ
S. Renneckar
Æ
W. Gindl
Æ
S. Veigel
Æ
J. Keckes
Æ
H. Yano
Æ
K. Abe
Æ
M. Nogi
Æ
A. N. Nakagaito
Æ
A. Mangalam
Æ
J. Simonsen
Æ
A. S. Benight
Æ
A. Bismarck
Æ
L. A. Berglund
Æ
T. Peijs
Abstract This paper provides an overview of recent
progress made in the area of cellulose nanofibre-based
nanocomposites. An introduction into the methods used to
isolate cellulose nanofibres (nanowhiskers, nanofibrils) is
given, with details of their structure. Following this, the
article is split into sections dealing with processing and
characterisation of cellulose nanocomposites and new
developments in the area, with particular emphasis
on applications. The types of cellulose nanofibres covered
are those extracted from plants by acid hydrolysis
(nanowhiskers), mechanical tre atment and those that occur
naturally (tunicate nanowhiskers) or under culturing con-
ditions (bacterial cellulose nanofibrils). Research high-
lighted in the article are the use of cellulose nanowhiskers
for shape memory nanocomposites, analysis of the inter-
facial properties of cellulose nanowhisker and nanofibril-
based composites using Raman spectroscopy, switchable
interfaces that mimic sea cucumbers, polymerisation from
the surface of cellulose nanowhiskers by atom transfer
radical polymerisation and ring opening polymerisation,
S. J. Eichhorn (&)
Materials Science Centre, School of Materials and the Northwest
Composites Centre, Grosvenor Street, Manchester M1 7HS, UK
e-mail: stephen.j.eichhorn@manchester.ac.uk
A. Dufresne
Grenoble Institute of Technology, The International School
of Paper, Print Media & Biomaterials (Grenoble INP Pagora),
BP65, 38402 Saint Martin D’He
`
res Cedex, France
M. Aranguren N. E. Marcovich
National Institute of Research in Science and Technology
of Materials (INTEMA), Universidad Nacional de Mar del Plata,
Av. Juan B. Justo 4302, B7608FDQ Mar del Plata, Argentina
J. R. Capadona
Rehabilitation Research and Development, Louis Stokes
Cleveland DVA Medical Center, 10701 East Blvd., Cleveland,
OH 44106, USA
S. J. Rowan
Department of Macromolecular Science and Engineering, Case
Western Reserve University (CWRU), 2100 Adelbert Road,
Cleveland, OH 44106, USA
C. Weder
Adolphe Merkle Institute, University of Fribourg, 1700 Fribourg,
Switzerland
W. Thielemans
School of Chemistry and Process and Environmental Research
Division, Faculty of Engineering, The University of Nottingham,
Nottingham NG7 2RD, UK
M. Roman S. Renneckar
Department of Wood Science and Forest Products, Virginia
Tech, 230 Cheatham Hall, 0323, Blacksburg, VA 24061, USA
W. Gindl
Department of Materials Science and Process Engineering,
University of Natural Resources and Applied Life Sciences,
BOKU-Vienna, Austria
S. Veigel
Wood K plus, Competence Centre for Wood Composites
and Wood Chemistry, Linz, Austria
J. Keckes
Department of Materials Physics, Erich Schmid Institute
of Materials Science, Austrian Academy of Sciences, University
of Leoben, Leoben, Austria
H. Yano K. Abe M. Nogi A. N. Nakagaito
Research Institute for Sustainable Humanosphere,
Kyoto University, Uji, Kyoto 611-0011, Japan
http://doc.rero.ch
Published in "Journal of Materials Science 45(1): 1-33, 2010"
which should be cited to refer to this work.
1
and methods to analyse the dispersion of nanowhiskers.
The applications and new advances covered in this review
are the use of cellulose nanofibres to reinforce adhesives, to
make optically transparent paper for electronic displays, to
create DNA-hybrid materials, to generate hierarchical
composites and for use in foams, aerogels and starch
nanocomposites and the use of all-cellulose nanocompos-
ites for enhanced coupling between matrix and fibre. A
comprehensive coverage of the literature is given and some
suggestions on where the field is likely to advance in the
future are discussed.
Introduction to cellulose structure/property
relationships
Cellulose is probably one of the most ubiquitous and
abundant polymers on the planet, given its widespread
industrial use in the present age, but also in the past for
ropes, sails, paper, timber for housing and many other
applications. By far the most commercially exploited nat-
ural resource containing cellulose is wood. The word
‘material’ in fact derives from the Latin for ‘trunk of tree’.
Indeed, Chaucer writes in the ‘Parson’s Tale’ in 1390
‘‘For he that is in helle hath defaute of light material.
for certes, the derke light that shal
Come out of the fyr that evere shal brenne’’
showing quite clearly that the relationship between wood
and material was persisting into the Middle Ages.
Other plants also contain a large amount of cellulose,
including hemp, flax, jute, ramie and cotton. In addition to
these, there are non-plant sources of cellulose; for instance,
forms produced by bacteria and cellulose produced by
tunicates. Bacterial cellulose (BC) is produced by the gram-
negative bacteria Acetobacter xylinum (or Gluconacetob-
acter xylinum), which manifests itself under special cultur-
ing conditions as a fine fibrous network of fibres [1]. Tunicate
cellulose is produced by sea creatures (e.g. Microcosmus
fulcatus) in the form of rod-like near perfect crystals of the
material [2].
Since cellulose is classed as a carbohydrate (a sub-
stance containing carbon, hydrogen and oxygen), it is
necessary to point out that although this term applies to
a large number of organic compounds, cellulose is
unique in that it can be either synthesised from, or
hydrolysed to, monosaccharides [3]. The repeat unit of
the cellulose polymer is known to comprise two anhy-
droglucose rings joined via a b-1,4 glycosidic linkage
from this unit [4] (called cellobiose) as shown in Fig. 1.
In its native form cellulose is typically called cellulose-I.
This cellulose-I crystal form, or native cellulose, also
comprises two allomorphs, namely cellulose Ia and Ib
[5]. The ratio of these allomorphs is found to vary from
plant species to species, but bacterial and tunicate forms
are Ia and Ib rich, respectively [6, 7]. The crystal
structures of cellulose allomorphs Ia and Ib have been
determined with great accuracy, particula rly the complex
hydrogen bonding system [6, 7]. The hydrogen bond
network makes cellulose a relatively stable polymer,
which does not readily dissolve in typical aqueous sol-
vents and has no melting point. This network also gives
the cellulose chains a high axial stiffness [8]. Since high
stiffness is a desirable property for a reinforcement fibre
in a composite, the determination of the crystal modulus
of cellulose will be reviewed later.
Cellulose chains aggregate into the repeated crystal-
line structure to form microfibrils in the plant cell wall,
which also aggregate into larger macroscopic fibres. It is
this hierarchical structure that is essentially deconstructed
in order to generate cellulose nanofibres from plants.
BC and whiskers produced by tunicates already exist in
A. Mangalam
Department of Wood Science and Forest Products,
Virginia Tech, 248 Cheatham Hall, Blacksburg, VA 24061, USA
J. Simonsen
Department of Wood Science & Engineering, Oregon State
University, Corvallis, OR 97331, USA
A. S. Benight
Departments of Chemistry & Physics, Portland State University,
Portland, OR, USA
A. Bismarck
Department of Chemical Engineering, Polymer and Composite
Engineering Group (PACE), Imperial College London, London,
UK
L. A. Berglund
Department of Fibre & Polymer Technology, Wallenberg Wood
Science Centre, KTH, 10044 Stockholm, Sweden
T. Peijs
Centre for Materials Research, Queen Mary, University
of London, Mile End Road, London E1 4NS, UK
O
O
OH
O
OH
OH
OH
O
HO
OH
n
Fig. 1 The repeat unit of cellulose
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2
this form, making them desirable materials for niche
applications.
The study of cellulosic nanofibres as a reinforcing phase
in nanocomposites started 15 years ago [2]. Since then a
huge amount of literature has been devoted to cellulose
nanofibres, and it is becoming an increas ingly topical
subject. Different descriptors of these nanofibres are often
referred to in the literature. These include ‘‘nanowhiskers’’
(or just simply ‘‘whiskers’’), ‘‘nanocrystals’’ or even
‘‘monocrystals’’. These crystallites have also often been
referred to in literature as ‘‘microfibrils’’, ‘‘microcrystals’’
or ‘‘microcrystallites’’, despite their nanoscale dimensions.
The term ‘‘whiskers’’ is used to designate elongated crys-
talline rod-like nanoparticles, whereas the designation
‘‘nanofibrils’’ should be used to designate long flexible
nanoparticles consisting of alternating crystalline and
amorphous strings.
1
In essence, the princi ple reason to utilise cellulose
nanofibres in composite materials is because one can
potentially exploit the high stiffness of the cellulose
crystal for reinforcement. This can be done by breaking
down the hierarchical structure of the plant into individ-
ualised nanofibres of high crystallinity, therefore reducing
the amo unt of amorphous material present. Since plant
fibres are hierarchically fibrous it is possible to do this,
yielding a fibrous form of the material (nanowhiskers,
nanofibrils), which due to their aspect ratio (length/
diameter) and therefore reinforcing capabilities are
potentially suitable for composite materials. A high aspect
ratio to the fibres is desirable as this enables a critical
length for stress transfer from the matrix to the rein-
forcing phase. This will be discussed in more detail once
the mechanical properties of cellulose nanofibres have
been presented.
It is however not clear what the true crystal modulus of
cellulose is, nor whether this stiffness is really obtainable
from plants, bacteria or tunicates. Establishing a true
value of the crystal modulus of cellulose sets an upper
limit to what is achievable in terms of reinforcing
potential.
The crystal modulus of cellulose was first determined
in 1936 by Meyer and Lotmar [9] using a theoretical
model and bond stiffness constants derived from spec-
troscopic measurements. They obtained a value of *120
GPa, which is close to values that were later
experimentally confirmed for this property [10, 11].
Despite this prediction, Meyer and Lotmar used an
incorrect structure for cellulose, and so when this was
corrected by Lyons, and a value of 180 GPa was obtained
[12]. Lyons however used an incorrect term in his
mathematical expression for bond angle bending. This
was rectified by Treloar [13], who reported a modulus of
56 GPa. This value is now consider ed to be too low,
probably due to the lack of intramolecular hydrogen
bonding in Treloar’s cellulose structure [13]. Sakurada
et al. [10] reported a value of 138 GPa for the crystal
modulus of cellulose, which was determined using X-ray
diffraction of deformed fibre bundles. This paved the way
for many more measurements and determinations of the
crystal modulus of cellulose using X-ray diffraction [11,
14] and theoretical approaches [15–18], all of which have
obtained values in the range 100–160 GPa. A more recent
determination of the cellulose crystal modulus using
inelastic X-ray scattering (IXS) reported a value of 220
GPa [19]. Such a high crystal modulus for cellulose has
not been reported before, but this may be due to the fact
that the assumption of uniform stress in the crystals of
cellulose, a basic assumption for most crystal modulus
determinations, is not correct [19, 20]. This high value for
the crystal modulus does however call into question the-
oretical approaches, and since they are consistent with
experiment it may be that a more modest value is
appropriate. Nevertheless, this value of the modulus of
crystalline cellulose is quite large compared to other
materials, especially if its comparatively lower density is
taken into account. The moduli of a number of commonly
used engineering materials are reported in Table 1. Also
reported are the specific moduli (modulus/density), which
show that the specific modulus of crystalline cellulose
exceeds engineering materials such as steel, concrete,
glass and aluminium. It is worth pointing out that cellu-
lose has obvious disadvantages compared to traditional
engineering materials; for instance, moisture absorption
and swelling, and enzymatic degradability to name but
two. It is worth pointing out that the microfibrils com-
prising plants do not swell themselves, as it is not ener-
getically favourable for water to penetrate the bulk
material.
1
For the sake of clarity and consistency the term ‘‘nanowhiskers’’
will be used to describe material hydrolysed from plants, and
‘‘nanofibrils’’ for material extracted by mechanical means or from
native sources such as bacterial cellulose. The term ‘‘nanofibres’’ will
be used as a general descriptor of both these sub-forms of
reinforcement.
Table 1 Moduli of engineering materials compared to cellulose
Material Modulus
(GPa)
Density
(Mg m
-3
)
Specific modulus
(GPa Mg
-1
m
3
)
Reference
Aluminium 69 2.7 26 [279]
Steel 200 7.8 26 [279]
Glass 69 2.5 28 [279]
Crystalline
cellulose
138 1.5 92 [10]
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The crystalline modulus of cellulose is hard to achieve
in reality for a micron-sized fibre. Plant fibres often have
moduli well below the crystalline value. Typical values of
the modulus of a range of cellulose fibres are given in
Table 2. When the density of the fibres is taken into
account and specific modulus is determined (assuming a
density of cellulose of 1.5 Mg m
-3
), then the values
approach those of glass and other engineering solids. Many
authors have published values for the modulus of plant
fibres, some closer to the crystal modulus of cellulose
quoted in Table 1. It is however difficult to obtain an
accurate modulus for plant fibres, given their often irreg-
ular and variable cross-sections and the pres ence of voids
in the form of lumens. It is beyond the scope of this article
to fully review this aspect of natural fibre mechanics, but it
is acknowledged that higher values than those quoted in
Table 2 have been reported. Plant fibres are known to have
variable mechanical properties, and another reason for
extracting nanofibers and nanofi brils from the cell wall of
plants is that they are thought to have more consistent
properties.
One way therefore to obtain fibres that have a modulus
that approaches that of pure crystalline cellulose is to break
down the structure of the plant into the ele mentary nano-
fibrils, or crystals (nanowhiskers), that make up the fibre.
Another approach is to source material that already has
these structural forms. Two examples of this latter
approach are to use microbial or BC, or to take whiskers of
cellulose from an animal source (such as tunicates, a sea
creature). BC fibrils are produced by a family of bacteria
referred to as G. xylinum, under special culturing condi-
tions [21]. The fibrils are generally in the form of a fine
non-woven mesh or network, and have been reported to
have moduli in the range 78–114 GPa [22, 23]. Some of the
first reports of the use of BC for composite materials
appeared in the mid-1990s [24, 25], but there has recently
been a resurgence of this research area. Notable examples
of this come from Japan at Kyoto University [ 26 ] and from
the UK at Imperial College [27–30], both of whom have
contributed to this article.
Microfibrillated cellulose (MFC), where fine nano-sized
fibrils are extracted from plants by mechanical processing
and/or homogenisation, was first reported in the early
1980s [31]. Since then, and in more recent times, a large
number of papers have been publishe d on this topic, the
full scope of which is beyond this review although an
overview of the physical properties of nanofibrils from this
source will be given in the section ‘‘An overview of cel-
lulose whisker and nanofibre properties (Grenoble Institute
of Technology (INPG), Interna tional School of Paper,
Grenoble, France)’’. Notabl e recent examples of research
into these materials have been by groups in Japan at Kyoto
University [32], in Sweden at KTH [33], in the USA at
Virginia Tech [34], and in Austria at BOKU, all of whom
have made contributions to this artic le.
The existence of highly crystalline cellulose nanowhis-
kers has been known for some time. They can be extracted
from plant material via a controlled acid hydrolysis, which
more readily hydrolyses the amorphous regions of the
cellulose, leaving high aspect ratio (length to diameter
ratio) crystals of pure cellulose. The first report of cellulose
crystals, produced in solution, was by Ranby and Noe in
1961 [35]. This was followed by the first report of the
production of cellulose nanowhi skers by acid hydrolysis
[36]. Nanowhiskers of cellulose can also be extracted from
the mantle of tunicates, a sea creature [37]. Tunicate
nanowhiskers are reported to have moduli of *140 GPa
[17], but acid hydrolysed nanowhiskers are thought to have
much lower moduli (50–100 GPa) [38]. The first report of
the use of cellulose nanowhiskers in composite materials
was by Favier et al. in 1995 [2
]. They investigated the
percolation of nanowhiskers extracted from tunicates.
Since then a large numb er of groups have reported on the
use of cellulose nanowhiskers and their use in composites,
some of whom have contribute d to this article; namely
from Argentina at INTEMA, within the UK at the Uni-
versity of Manchester and the University of Nottingham,
and from the USA at Case Western Reserve University,
Virginia Tech and at Oregon State University.
The relative mechanical advantage of using cellulose
nanofibres (nanowhiskers, nanofibrils) over conventional
fibres is best shown graphically. Figure 2 shows Halpin-
Tsai micromechanical predictions for unidirectional poly-
propylene matrix composites filled with 50 vol% of uni-
directional cellulose fibres as a function of different fibre
aspect ratios and Young’s moduli. The Halpin-Tsai model
[39, 40] is a short-fibre composite model which predicts all
the elastic constants of composite materials as a function of
the aspect ratio of the filler when the constituent properties
and the volume fractions of the two phases (matrix and
reinforcement) are known.
Table 2 Mechanical properties of some common plant fibres, namely,
Young’s modulus, specific Young’s modulus, breaking strength and
breaking strain
Fibre type Young’s
modulus
(GPa)
Specific Young’s
modulus
(GPa Mg
-1
m
3
)
Breaking
strength
(GPa)
Breaking
strain (%)
Flax 27.0 18.0 0.81 3.0
Jute 25.8 17.2 0.47 1.8
Hemp 32.6 21.7 0.71 2.2
Ramie 21.9 14.6 0.89 3.7
Data taken from Morton and Hearle [280] with the conversion from N
tex
-1
to GPa being made using a density of cellulose of 1.5 Mg m
-3
from Table 1
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4
The Halpin-Tsai equation can be written as
E
c
E
m
¼
1 þ fgu
f
1 gu
f
ð1Þ
with
g ¼
E
f
E
m
1
E
f
E
m
þ f
ð2Þ
where E
c
, E
f
and E
m
are respectively the composite, rein-
forcement and matrix Young’s moduli; u
f
is the filler
volume fraction and f a shape factor. The shape factors
relative to fibres reinforcement has been chosen to be
(0.5s)
1.8
, in accordance with a previous study [41], where s
is the aspect ratio. For a more extensive introduction to the
model, the reader is ref erred to the relevant scientific lit-
erature [39, 40]. The model supposes a perfect interface
between matrix and fibre, but does not account for fibre–
fibre interactions, which can take place in high loading
cellulose composite/nanocomposites (i.e. percolated net-
works). Young’s modulus of cellulose fibres can vary
according to the sourc e, and fibre dimensions. A modulus
of 40–60 GPa is usually found for natural bast fibres like
flax and hemp (see Table 2), while it potentially increases
up to 80 GPa for single cells [42] and certainly in the range
of 100–140 GPa for nanofibrils and nanowhiskers [17, 23].
Due to the intrinsic higher performances of nano- sized
fillers, cellulose nanowhiskers are predicted to enhance
stress transfer and therefore the final composite modulus
(more than 3-fo ld) when compared with traditi onal micron-
sized cellulose fibres. It is clear from these data why the
recent interest in studying nano-cellulose composite has
occurred. Nevertheless, such effects can only be realised
for fibres of a high-enough aspect ratio. Cellulose nanofi-
bres with an aspect ratio smaller than 10 would not have
any major benefits when compared with conventional
micron-sized filaments. Only nanofibres with aspect ratios
bigger than 50 can guarantee an efficient reinforcement
effect. For aspect ratios larger than 100, Young’s moduli
reach a plateau, which correspond to the upper-limit case
for reinforcement. For exampl e, single flax fibres, which
are around 25 mm long and 20 lm thick, will have an
aspect ratio of 1250, which is well above the critical value.
Since cellulose nanowhiskers generally have lower aspect
ratios, typically between 10 and 30, there is a need for
longer nanofibres of this type.
This article contains contributions to the field of cellu-
lose nanocomposites in the areas of processing and char-
acterisation and applications and new advances in the
subject. It is intended that a flavour of current research
taking place internationally will be given, rather than a
general overview of the area of research. For other reviews
of cellulose nanocomposites, the reader is referred to an
article by Samir et al. [43] and another by Kamel [44 ].
Before each research contribution is reported, a detailed
overview of nanowhisker and nanofibril properties will be
given by Alain Dufresne at INP, Grenoble, France, who is a
pioneer in this research area.
An overview of cellulose whisker and nanofibre
properties (Grenoble Institute of Technology (INPG),
International School of Paper, Grenoble, France)
As already mentioned, native cellulose present in macro-
scopic fibres, like for instance plant fibres, consists of a
hierarchical structure. This hierarchical structure is built up
by smaller and mechanically stronger entities consisting of
native cellulose fibrils. These fibrils interact strongly and
aggregate to form the natural or native cellulose fibres. The
lateral dimension of these fibrils depends on the source of
the cellulose but it is typically of the order of a few
nanometres. The fibrils contain crystalline cellulosic
domains but also noncrystalline domains located at the
surface and along their main axis. The noncrystalline
domains form weak spots along the fibril. These fibrils
display high stiffness and are therefore suitable for the
reinforcement of nanocomposite materials.
There are numerous methods to prepare nanofibres from
natural cellulose fibres. The properties of these nanofibres
will now be outlined in more detail. One method consists of
submitting plant fibres to strong acid conditions combined
with sonication. It leads to the hydrolysis of noncrystalline
domains, and rod-like nanofibres called cellulose nano-
whiskers result from this treatment. The dimensions of these
resultant nanowhiskers depend on the source of the cellu-
lose, but their length generally ranges between 100 and
300 nm. Some typical transmission electron microscope
1 10 100 1000
0
10
20
30
40
50
60
70
40 GPa
60 GPa
80 GPa
100 GPa
120 GPa
E
c
)aPG(
Aspect ratio (L/d)
φ
v
=50 vol%
140 GPa
Fig. 2 Model plots of the Halpin-Tsai equation (Eq. 1) for a range of
fibre moduli showing the predicted composite modulus (E
c
)asa
function of the aspect ratio of the fibre reinforcement. The model
assumes a unidirectional composite sample, with no fibre–fibre
interactions and a polypropylene matrix
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5
![Fig. 25 SEM micrographs of interfacial failure between PLA and sisal fibre without bacterial cellulose (left) and with bacterial cellulose attached (right). Reproduction of images from [27] with permission from Wiley ( Wiley 2008)](/figures/fig-25-sem-micrographs-of-interfacial-failure-between-pla-3pvs88jq.webp)
![Fig. 12 Reaction scheme for the labelling of cellulose nanowhiskers with fluorescein50-isothiocyanate and image showing aqueous suspensions of (A) cellulose nanowhiskers (0.8 wt%) and (B) FITC-labelled cellulose nanowhiskers (0.5 wt%). Reproduction of images a and b from [161] with permission from American Chemical Society ( American Chemical Society 2007)](/figures/fig-12-reaction-scheme-for-the-labelling-of-cellulose-1ys76bem.webp)
![Fig. 20 DLS cycling data showing repeatable DNA duplex formation and melting with temperature. Reproduction of image from [219] with permission from American Chemical Society ( American Chemical Society 2009)](/figures/fig-20-dls-cycling-data-showing-repeatable-dna-duplex-1tbn8ogv.webp)
