Review: current international research into cellulose nanofibres and nanocomposites

doi:10.1007/S10853-009-3874-0

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

OH

O

OH

O

HO

OH

n

Fig. 1 The repeat unit of cellulose

http://doc.rero.ch

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]

http://doc.rero.ch

3

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

http://doc.rero.ch

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

http://doc.rero.ch

5

Review: current international research into cellulose nanofibres and nanocomposites

Summary (1 min read)

Processing and characterisation of cellulose nanocomposites

Applications and new advances in cellulose nanocomposites

Discussion and conclusions

Figures (26)

Citations

References

"Review: current international resea..." refers methods in this paper

Related Papers (5)