ORIGINAL PAPER
Electrically conductive nanocellulose/graphene compositesexhibiting improved mechanical properties in high-moisturecondition
Luong Nguyen Dang . Jukka Seppala
Received: 13 January 2015 / Accepted: 1 April 2015
� Springer Science+Business Media Dordrecht 2015
Abstract Nanofibrillated cellulose (NFC) has re-
ceived significant attention in materials science
recently because of its unique properties such as high
mechanical properties, high surface area, and appli-
cable rheology. NFC-based papers possess high me-
chanical strength and excellent oxygen barriers.
However, they exhibit poor mechanical properties in
high-humidity environments because of their hy-
drophilicity, thus narrowing its applications. In this
study, we demonstrated that an incorporation of
chemically reduced graphene oxide (RGO) sheets into
NFC paper resulted in significantly improved me-
chanical properties in high-humidity condition. Dy-
namic mechanical analysis showed that all NFC/RGO
composite papers containing graphene ranging be-
tween 1 and 10 wt% were not broken in an extreme
test condition at 80 �C and 80 % relative humidity.
Meanwhile, neat NFC paper was broken when the
temperature reached 50 �C. In addition, the tensile testdemonstrated that Young’s modulus of the NFC/RGO
composite paper was significantly higher than that of
neat NFC paper. Furthermore, the NFC/RGO com-
posite papers possessed high electrical conductivity,
which was proportionally increased as the graphene
loading content increased. The developed NFC/RGO
composite materials can find potential uses as con-
ductors, antistatic coatings, and electronic packaging,
especially where high moisture is present.
Electronic supplementary material The online version ofthis article (doi:10.1007/s10570-015-0622-2) contains supple-mentary material, which is available to authorized users.
L. Nguyen Dang � J. Seppala (&)
Laboratory of Polymer Technology, Department of
Biotechnology and Chemical Technology, School of
Chemical Technology, Aalto University, P.O. Box 16100,
00076 Aalto, Finland
e-mail: [email protected]
123
Cellulose
DOI 10.1007/s10570-015-0622-2
Graphical Abstract
Keywords Nanofibrillated cellulose � Grapheneoxide � Graphene � Nanocomposites
Introduction
Graphene and its derivatives have attracted significant
interest from material scientists worldwide owing to
their outstanding physical properties and widely possi-
blemodifications. For example, it has been reported that
many physical properties of graphene were measured
experimentally and exceeded those obtained by any
other material. For example, its exceptional mechanical
properties of 1 TPa of Young’s modulus and 130
GPa of ultimate strength (Lee et al. 2008) have been
widely considered as the highest mechanical properties
ever measured. High room-temperature electron mo-
bility of around 200,000 cm2 V-1 s-1 makes it a very
promising material in electronic applications such as
flexible displays and transistors (Mayorov et al. 2011).
The excellent gas barrier property of graphene could be
used in packaging materials (Bunch et al. 2008).
Among graphene derivatives, it has been widely
recognized that graphene oxide (GO), which is
produced via exfoliating graphite oxide, and its
reduced forms are the most promising reinforcements
for composites. Graphite oxide is commonly synthe-
sized by oxidizing graphite based on Hummers’
method (Hummers and Offeman 1958). It is notewor-
thy that graphite has a large annual global production
of over 1.1 million tons with a price in the order of
$825/ton in 2008 (Kim et al. 2010). Two advantages of
graphite oxide are its hydrophilic characteristic and its
abundant oxygen functionalities; thus, graphite oxide
can be readily exfoliated and dispersed in water,
creating a stable GO suspension. Therefore, GO is
compatible with many hydrophilic polymer matrices,
such as water-soluble polymers (poly(vinyl alcohol))
and hydrophilic polymers (cellulose) (Zhao et al.
2010). Additionally, the GO can be chemically/
thermally reduced to produce conductive graphene,
namely reduced graphene oxide (RGO), and its
conductivity can be easily controlled by the reduction
level.
Fabrication of electrically conductive and me-
chanically strong graphene-based composites, espe-
cially those from biodegradable and renewable
polymers, is of significant interest. Cellulose is known
as the most abundant polymer on earth and has an
estimated annual production of around 90 9 109
metric tons (Pinkert et al. 2009). It is renewable,
biodegradable, and can be transformed into various
useful materials. Among them, nanofibrillated cellu-
lose (NFC) has recently shown many practical appli-
cations in polymer composite areas where high
mechanical properties, high oxygen barriers, and/or
green materials are possible (Moon et al. 2011). NFC
is a nanomaterial whose fibril dimensions are a few
tens of nanometers in diameter and several mi-
crometers in length. It is produced and stored in the
form of suspensions in water in which the hydrogen
bonding between NFC fibrils and water molecules
plays a key role in the forming of a stable dispersion. It
is important to address that this high aspect ratio of the
NFC fibrils together and their abundant hydroxyl
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123
groups are beneficial for further chemical modifica-
tion/composite fabrication (Seppala 2012). Although
its utilization in polymer composites is still a new
research field, NFC has great potential for use as a
reinforcement in polymer composites, especially
biodegradable polymer composites (Siro and Plackett
2010). For example, it was reported that the me-
chanical properties of NFC-containing poly(e-capro-lactone) (PCL) nanopaper prepared via surface-
initiated ring-opening polymerization are much higher
than those of pure PCL (Boujemaoui et al. 2012). In
our previous study, NFC was used as substrate for the
deposition of polyaniline (PANi) via in situ polymer-
ization of aniline monomer in NFC suspension (Luong
et al. 2013). The developed NFC/PANi composite
paper showed good mechanical properties and high
electrical conductivity, attributed to the contributions
of the NFC and PANi components, respectively.
Recently, there has been interest in using NFC to
fabricate films and papers through casting or filtering
the suspension, where the water is removed so that a
solid cellulose fibril network is formed. For example,
cellulose nanopapers with high toughness and con-
trolled porosity were prepared by a vacuum filtration
NFC suspension (Henriksson et al. 2008).
Significant improvements in polymer nanocompos-
ites whose graphene derivatives have been used
recently as reinforcements have motivated several
studies on NFC/graphene composite. Several interest-
ing studies investigated how graphene and NFC
combine to make composites (Laaksonen et al. 2011;
Malho et al. 2012; Luong et al. 2011). In these
composites, significant improvements in mechanical
properties were found for all combinations (Laakso-
nen et al. 2011; Malho et al. 2012; Luong et al. 2011),
together with high electrical conductivity (Luong et al.
2011). However, to our knowledge there have been no
studies on the mechanical property of cellu-
lose/graphene composites in high moisture conditions.
It is commonly supposed that NFC paper is hy-
drophilic and thus exhibits high sensitivity to mois-
ture, especially at high temperature, consequently
showing poor mechanical properties in this condition.
In this study, we showed that a combination of reduced
graphene oxide (RGO) and NFC in a well-controlled
manner produced composite materials with sig-
nificantly improved mechanical properties in high-
moisture conditions compared to neat NFC paper.
Dynamic mechanical analysis showed that the NFC/
RGO composite paper was able to sustain extreme
testing conditions, conducted from 15 to 80 �C at
80 %RH. It should be stressed that at 80 % RH, 80 �Cis the highest temperature that can be reached
according to the recommendation for our DMA
equipment (TA Instruments Q800). Meanwhile, the
neat NFC paper was broken when the temperature
reached 50 �C in the same humidity condition. We
believe that the developed NFC/RGO composites
could potentially be used in many applications
including antistatic packages, electromagnetic shield-
ing, and sensors. In addition to mechanically strong
composite papers, it is worth mentioning that various
stable NFC/RGO suspensions with different graphene
contents are easily prepared and scalable; these may be
used to fabricate other forms of materials including
electrically conductive cellulose aerogels and conduc-
tive inks.
Experimental section
Materials
Graphite flakes (particle size\ 200 lm), sulfuric acid
(98 %), hydrochloric acid (36 wt%), potassium per-
manganate (99? %), sodium nitrate (99.5 %), and
hydrazine hydrate were purchased from Sigma-
Aldrich Co. Ammonia solution (28 %) was supplied
by VWR Co. Hydrogen peroxide (30 %) was obtained
from Merck. Nanofibrillated cellulose suspension
(1.39 wt%) was provided by UPM Corp; (Helsinki,
Finland) with the product name UPM Fibril Cellulose.
The NFC fibrils were mostly 20–30 nm in diameter
and several micrometers in length. The material was
produced by mechanical disintegration of bleached
birch pulp, which was pretreated with a Voith refiner
prior to fibrillation with an M7115 fluidizer from
Microfluidics Corp. (Newton, MA, USA) (Paakko
et al. 2007). Deionized (DI) water was used in all
experiments.
Methods
Preparation of graphene oxide suspension
Graphite was oxidized by a modified Hummers
method (Hummers et al. 1958). A volume of 250 ml
concentrated sulfuric acid (95–97 %) was added to a
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123
1-L flask equipped with a magnetic stirrer, which was
immersed in an ice bath. Then, 10 g of graphite flakes
(flake size of *200 lm) and 5 g of sodium nitrate
were supplied into the sulfuric acid container. Subse-
quently, 30 g potassium permanganate was slowly
added to the above mixture so that the temperature was
kept below 20 �C. After this, the temperature was
raised to 35 �C and kept at this temperature for 1.5 h.
This mixture was cooled down to room temperature
and left over night. Next, 200 ml of water was added
slowly for 2 h; then 50 ml hydrogen peroxide was
injected into the reaction mixture; 500 ml of DI water
was added to dilute the reaction mixture. The mixture
was washed with 1.5 l of 5 wt% HCl solution via
vacuum filtration. Finally, the product was washed
with water by centrifugation (five times) and dialyzed
again in DI water until reaching a neutral pH. Graphite
oxide powder was obtained after drying. A graphene
oxide (GO) suspension was prepared by ultrasonic
treatment of the graphite oxide in water (1 g/400 ml)
for 30 min (output power of 100 W). The sonicated
mixture was centrifuged at 4000 rpm for 20 min to
remove the precipitate. The solid content of graphene
oxide in the dispersion wasmeasured to be 2 mg ml-1.
Preparation of NFC/graphene oxide
and NFC/graphene composite papers
The NFC suspension and GO dispersion were me-
chanically mixed in various proportions so that solid
contents of graphene oxide of 1, 3, 5, and 10 wt%were
obtained, which was compared to that of solid NFC.
The pH of the mixture was adjusted to 10 using
ammonia solution (25 wt%). For chemical reduction
of GO, hydrazine was added, and the mixture was
heated to 95 �C for 2 h. The effective reduction of
hydrazine and ammonia was observed by the color
change from the light yellow of the NFC/GO suspen-
sion to the dark color of the NFC/reduced graphene
oxide (NFC/RGO). NFC/RGO composite papers were
subsequently fabricated by vacuum filtration of the
reaction product through a cellulose ester porous
membrane (U47 mm). The mixture was washed three
times with DI water to remove impurities. The NFC/
RGO composite was dried at room temperature for
48 h, then peeled off from the filter membrane. The
NFC/RGO composite paper was finally dried at 60 �Cfor 24 h. Preparation of NFC/GO composite papers
was carried out through filtrating the suspensions
without the chemical reduction step.
Measurements
Atomic force microscope (AFM)
AFM images of the GO sheets and NFC fibrils were
taken from drop-cast dispersion of GO and NFC on
silicon substrates using a Dimension 5000 (Veeco
Inc.) in tapping mode with silicon probes.
Tensile properties
Tensile testing of the paper samples with thicknesses
of around 20 9 0.03 9 5.3 cm3 (length 9 thick-
ness 9 width) was measured using Instron 4204
universal testing equipment with a test speed of
2 mm min-1. The relative humidity of 50 % and
temperature of 23 �C were kept during the measure-
ment. At least five specimens were used for each
sample in the test.
Electrical conductivity measurement
Electrical conductivity was characterized with a four-
point probe method from Jandel Engineering Ltd.
(Jandel RM3000). Sheet resistance (Rs, ohms per
square) and thickness (t, cm) were used to calculate the
specific resistivity, q = Rs 9 t and the corresponding
conductivity, r = 1/q (S cm-1).
Structural morphology observation
The morphology of the fractured surfaces of the
sample after tensile test was observed by a scanning
electron microscopy (SEM, Zeiss Sigma VP) at 3 kV.
The exposed cross-sectioned surfaces were coated
with a thin layer of gold/palladium by sputtering to
promote conductivity before SEM observation.
X-ray diffraction (XRD)
XRD of the samples was carried out on an X’Pert PRO
Alpha-1 (PANalytical) with Cu K_alpha1 radiation
(k = 0.154 nm), and data were collected in the 2h of
5–50� with a scanning speed of 3o min-1. Radiation
conditions were 45 kV and 40 mA.
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123
Dynamic Mechanical Analysis (DMA) with humidity
control
(1) Measurement at ambient humidity
DMA measurement was carried out using a TA
Instruments Q800. Samples for testing of 30microns in
thickness were cut into 20 mm 9 5.3-mm rectangular
pieces. The nitrogen environment for the test was
supplied from the liquid nitrogen tank equippedwith the
DMA instrument. The temperature was conditioned at
-20 �C for 5 min. Then, it was heated from -20 to
120 �Cwith a heating rate of 3 �C/min and amplitude of
25 lm. A frequency of 1 Hz was used for all tests.
(2) Measurement at 80 % of humidity
DMA measurements with controlled humidity condi-
tions were studied on the TA Instruments Q800
equipped with a humidity chamber (Kep Technologies
Wetsys Setaram Instrumentation), which provides the
humidity inside the measuring chamber. The two
tension clamps inside the measuring chamber fixed the
samples, then a humidity of 80 % was introduced. The
samples were conditioned in this humidity for 90 min
at 15 �C before the measurement was started. The
samples were tested over temperature ranges of
15–80 �C with a heating rate of 0.5 �C/min and
amplitude of 25 lm. A frequency of 1 Hz was used for
all tests.
Thermogravimetric analysis (TGA)
TGA thermograms of the composite were analyzed in
a TA instrument (Q500) with a temperature range of
25–750 �C and a heating rate of 10 �C min-1 under a
nitrogen atmosphere.
Results and discussion
The NFC herein was produced by combining enzy-
matic hydrolysis and mechanical disintegration, which
leads to the formation of cellulose nanofibrils with
higher aspect ratio compared with those obtained by
hydrolysis and mechanical treatment (Paakko et al.
2007). Figure 1 shows the schematic for the fabrica-
tion of the NFC/RGO composite paper based on GO
dispersion and NFC suspension. The homogeneous
dispersion of GO sheets in NFC fibril networks may be
attributed to hydrogen bonding between their oxygen
functionalities. After the chemical reduction by
hydrazine, the color change of GO from light yellow
to black can be used as an indicator for the formation
of reduced GO (RGO). The RGO sheets remained
dispersed in the mixture without any observable
aggregation after a few months at ambient conditions.
This stabilization is due to the electrostatic stabiliza-
tion by the ammonia and excess hydrazine in the
mixture after reaction (Li et al. 2008). The NFC/RGO
Fig. 1 Schematic showing the fabrication of NFC/RGO composite paper via mixing of the two NFC and GO suspensions, followed by
chemical reducing of GO and vacuum filtrating of the forming NFC/RGO suspension
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123
composite paper containing up to 30 wt% RGO can be
easily prepared by vacuum filtration; it is flexible and
bendable, as seen in Fig. 2.
AFM images of NFC, GO, and the NFC/RGO
composite with 10 wt% RGO are presented in Fig. 2.
The average thickness of the GO sheets along the line
1 showed an average value of around 0.9 nm, which
can be regarded as the thickness of a single-layer GO.
The thickness of a graphene sheet is reported to be
approximately 0.34 nm (Stankovich et al. 2006a, b).
The higher thickness of one GO sheet reported here is
due to the presence of various oxygen functionalities,
such as epoxy and hydroxyl groups on the basal planes
and carboxyl groups in the edges (Fig. S1). The AFM
image of neat NFC demonstrates entangled networks
of high aspect ratio cellulose nanofibrils with di-
ameters ranging between few nm to tens of nm and
length of micrometers. As seen, the NFC/RGO
composite consists of both graphene sheets and NFC
nanofibrils, and they fuse together, which may indicate
good adhesion in the composite. The good dispersion
can also be seen clearly in the SEM images in Fig. 2b
when the top and cross-sectional surfaces of the NFC/
RGO composite with 30 wt% graphene loading were
imaged.
By the chemical reduction using hydrazine, the sp2
structure of GO was partly restored. Indirect evidence
for the reduction of GO is the increase in electrical
conductivity as seen in Table 1. The improvement in
conductivity is due to the efficient reduction of GO to
conductive graphene, RGO. The measurement showed
that NFC, GO, and NFC/GO papers were not electri-
cally conductive, while NFC/RGO composites were
conductive, and the conductivity of the composite
papers was getting higher with the increase of RGO
contents. The conductivity of NFC/RGO composite
paper with 1 wt% was 7.3 10-2 S m-1, which is
already much higher than the value for the electrostatic
Fig. 2 AFM images of
NFC (left), graphene oxide
(middle), and NFC/RGO
composite with 30 wt%
RGO (a). Thickness of theGO sheet along line 1 was
around 0.9 nm. Digital
images of NFC/RGO
composite paper containing
10 wt% RGO and SEM
images taken at the top and
cross-sectional surfaces (b)
Table 1 Electrical conductivity of NFC/RGO composite pa-
pers with 1, 3, 5, and 10 wt% graphene loadings
Sample Electrical conductivity (S m-1)
NFC Insulator
GO Insulator
NFC/RGO (1 wt%) 7.3 9 10-2
NFC/RGO (3 wt%) 0.9
NFC/RGO (5 wt%) 3.4
NFC/RGO (10 wt%) 15.4
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level, which is 10-8 S m-1 (Stankovich et al. 2006a,
b). The conductivity value suggests that the composite
may be used for some specific applications requiring
the electrostatic dissipation. The conductivity is in-
creased progressively with the increase in graphene
content. When the graphene content increased to 10
wt%, a high value of conductivity at 15.4 S m-1 was
obtained. The high conductivity is attributed to the
good connection between graphene sheets. The pur-
pose of using hydrazine/ammonia as reducing agent is
to stabilize the formed NFC/RGO suspension in
addition to its excellent reducing capability for GO.
It has been shown that hydrazine is so far one of the
most powerful reducing agents for GO (Chua and
Pumera 2014). In our study, we see that with the use of
hydrazine as a reducing agent in combination with
ammonia as a pH controller, the stability of the NFC/
RGO suspension could be controlled easily. It is also
worth noting that the excess hydrazine is easily
removed by filtration and/or solvent evaporation
during film sample preparation.
Figure 3 shows the effect of the GO and RGO
content on the mechanical properties of the composite
papers. The neat NFC paper shows a tensile strength of
172 MPa and Young’s modulus of 5.3 GPa. Incorpo-
ration of both GO and RGO displayed significant
improvement in the mechanical properties. Tensile
strengths of NFC/GO and NFC/RGO composite
papers with 1 wt% were measured to be 201 and
192 MPa, which are roughly 17 and 12 %, respec-
tively, higher than that of neat NFC paper. However,
when the contents of GO and RGO increased further,
the tensile strength of both composite papers started to
decrease slowly, probably due to the aggregation of
graphene sheets. It is worth noting that the increase in
Young’s modulus was even more pronounced than in
tensile strength for the NFC/RGO composite papers.
Meanwhile, the Young’s modulus of the GO-
Fig. 3 Mechanical properties including (a) tensile strength, (b) Young’s modulus, and (c) elongation at break of the neat NFC, NFC/
GO, and NFC/RGO composite papers with 1, 3, 5, and 10 wt% graphene contents
Cellulose
123
containing sample decreased gradually. For example,
the addition of 1 wt% RGO significantly improved the
Young’s modulus to 9.4 GPa, which is increased by
1.8 fold compared to the NFC. Meanwhile, the
Young’s modulus of NFC/GO composite decreased
to 4.5 GPa. Similarly to the tensile strength, when
more graphene was added to the composite, the
Young’s modulus became lower. Additionally, the
elongation at break of the NFC/GO composites was
quite similar to that of neat NFC paper. Meanwhile,
the NFC/RGO composites showed a significant de-
crease in elongation at break, which could be due to
the weakening of the hydrogen bonding in the
reduced-GO-containing composites upon chemical
reduction using hydrazine. In our previous study, we
reported that ANFC/RGO composite papers showed
an improvement in tensile strength that was more
pronounced than in the case of NFC/RGO composite
paper in this study; however, the Young’s modulus is
not as high as in the case of NFC/RGO composite
paper (Luong et al. 2011). A plausible explanation of
these phenomena could be the difference in hydrogen
bonding in ANFC/RGO and NFC/RGO composite
systems. Another reason may be the formation of
covalent bonding between ANFC and RGO, which
does not happen in the NFC/RGO composites.
The enhancement in mechanical properties of the
composites with the addition of graphene material was
also studied by dynamic mechanical analysis (DMA),
and the result is shown in Fig. 4. Herein, we performed
the DMA measurement at two different conditions. In
the first test was carried out from -20 to 120 �C at
ambient conditions (Fig. 4a–c); in the second test it
was carried out at 80 % relative humidity (Fig. 4d–f).
The aims of the first and second tests are to study the
mechanical properties of the material at normal
conditions and a high-humidity environment, respec-
tively. Figure 4a shows the storage modulus (E0) ofneat NFC and composites with different RGO contents
as a function of temperature, which was varied
between -20 and 120 �C. All the composites showed
higher E0 values compared to those of the neat NFC
paper. For example, at-20 �C, the E0 of the NFC was
13.5 GPa, which was much lower than that of the
composite with 10 wt% RGO having a value of 15.2
GPa. As the temperature increased, all the composite
papers displayed a gradual decrease in E0, showingsimilar behavior to NFC. At 120 �C, the E0 of the
composite with 10 wt% RGO was measured to be 11.2
GPa, while the neat NFC showed a much lower value
of 9.3 GPa.
As is known, NFC is a hydrophilic material. Thus,
the mechanical properties of the NFC-based paper are
moisture sensitive. This phenomenon was seen in a
weakening of the mechanical property when the
samples were tested in high-humidity conditions. This
behavior is clearly demonstrated in Fig. 4d when the
NFC sample was measured in high-humidity condi-
tions of 80 % RH and temperature ranging from 15 to
80 �C. As presented in Fig. 4d, the storage modulus of
all NFC/RGO composite paper was higher than that of
the neat NFC sample with graphene ranging between 1
and 10 wt%. Incorporation of RGO sheets into the film
would result in increased hydrophobicity of the sample,
thus preventing water absorption. As seen, the storage
moduli of the composite filmswere higher than those of
the neat NFC film. The improvement in storage
modulus indicates that the composites become stiffer
with the addition of the graphene sheets. The observed
reinforcement is again indicative of graphene’s ability
to reinforce and enhance stiffness into the composite
material. For example, when theNFCwas incorporated
with RGO at 1 wt%, the storage modulus increased
from 3328 to 4379 MPa at 25 �C. As the temperature
increased, the storage modulus displayed a gradual
decrease for all samples. It is worth noting that when
the temperature reached 50 �C, the neat NFC sample
was broken and thus the measurement was stopped,
which could be explained by the weakening of
hydrogen bonds between NFC fibrils. Meanwhile, at
this point, all the composites with the presence of RGO
still strongly remained, and even they were me-
chanically strong until the end of the test, which was
at 80 �C. At 50 �C and 80 % RH, a storage modulus of
2474 MPa was measured for the composite containing
1 wt% RGO and 3193 for the one with 10 wt% RGO.
When the temperature increased to the maximum
temperature of 80 �C, which is the maximum tem-
perature that can be reached at 80 % RH for with the
DMA equipment (as recommended in the manual by
the manufacture), the composites kept storage moduli
of 1835, 1951, 2273, and 2653 MPa for the composites
with 1, 3, 5, and 10 wt% graphene loadings, respec-
tively (see more in the Supporting Information). This
result demonstrated the effective reinforcement by
graphene in NFC paper. The reinforcing ability of
graphene sheets in the NFC paper could be attributed to
the high mechanical properties of the graphene itself
Cellulose
123
and the uniform distribution throughout the composite
paper (see SEM picture in Figs. 5, 6). Moreover, the
interaction between the cellulose nanofibrils and
functionalized graphene sheets is probably due to the
hydrogen bonding between them, thus playing a vital
role in efficient loading transfer.
The loss moduli and tand of the neat NFC and NFC/
RGO composites tested in the temperature ranging
between -20 and 120 �C are presented in Fig. 4b, c,
demonstrating that the RGO sheets in the composites
restrict the slippage of the NFC fibrils. This is
demonstrated by the shifting of the glass transition
Fig. 4 Dynamic moduli and tand of the NFC and NFC/RGO
composite papers as a function of graphene content. a-c The testwas carried out with temperature ranging between -20 and
120 �C and d-f with 80 % RH and temperature varying from 15
to 80 �C. It is noted that the neat NFC sample was broken in the
test with 80 % RH when the temperature reached 50 �C
Cellulose
123
Fig. 5 SEM images of neat NFC, NFC/GO, and NFC/RGO composite papers with 1, 3, 5, and 10 wt% graphene loadings. All pictures
were taken at the fracture surfaces of specimens after the tensile testing. All scale bars are 2 lm
Fig. 6 SEM images taken at the top surfaces of neat NFC, NFC/GO, and NFC/RGO composite papers with 1, 3, 5, and 10 wt%
graphene loadings. All scale bars are 1 lm
Cellulose
123
of the composites to the right when more RGO was
added to the composite systems. In addition, the loss
modulus was lower for the composites compared to
that of NFC film. These observations demonstrate the
reinforcement of RGO in the composites, which
complements the tensile results. However, the DMA
test carried out in humidity chamber as shown in
Fig. 4e–f demonstrates more complicated behavior of
the composites in a high-moisture environment.
Although the loss moduli of the composites show
higher values compared to those of neat NFC
(Fig. 4e), the tand in Fig. 4f indicates a similar trend
in which RGO sheets are considered reinforcement
components. This DMA result could be explained by
two simultaneous phenomena happening because of
the presence of RGO sheets, which are reinforcing and
lubricating effects of RGO sheets in the composites.
This means that RGO sheets probably increase the
slippage of NFC fibrils at a certain level, but reinforce
NFC via hydrogen bonding interactions between NFC
fibrils and RGO sheets. Taking these two DMA tests
together, we may conclude that at normal conditions
the reinforcing effect of RGO sheets in NFC/RGO
composite is dominant, while at high-moisture condi-
tions (80 % RH in this study), the reinforcing and
lubricating effects would contribute simultaneously to
the final mechanical properties.
It has been reported that the graphene materials
improve the thermal stability of polymer composite
relative to the pure polymer itself (Potts et al. 2011).
Graphene can act as barrier against water and oxygen
diffusion (Novoselov et al. 2012). Thus, it would be
expected that the incorporation of graphene materials
into the polymer could enhance the corresponding
barrier properties. In this study, we clearly pointed out
the enhancement in the case of NFC composites where
in graphene oxide and reduced graphene oxide mate-
rials were incorporated (see Table S2 in the Support-
ing Information). For the NFC/GO composite papers,
in the testing, the storage modulus was higher than
those of the NFC/RGO composite samples with the
composite containing less than 10 wt% graphene
content. The higher storage modulus could be due to
the stronger adhesions between NFC and GO than the
ones of NFC and RGO. However, it should be noted
that with 10 wt% GO content, the composite was
broken when the temperature reached 80 �C, probablydue to the high amount of GO, thus promoting water
absorption. Meanwhile, this is not the case with NFC/
RGO composite film. This could be due to the
enhanced hydrophobicity caused by RGO. The storage
modulus of the sample in the DMA test with a high-
humidity environment exhibited a different result
compared to the one in tensile testing. To the best of
our knowledge, this is the first study on the effect of
graphene on the mechanical properties in a high-
humidity environment in a polymer composite, espe-
cially in cellulose/graphene composites.
The fracture surface morphology of the NFC and
composites is shown in Fig. 5. The morphology of the
specimens, after being fractured by the tensile test, can
be used to study the interfacial interaction between the
graphene and cellulose phases in the composites. The
NFC sample shows a highly fibrous network structure
consisting of ultrafine cellulose fibrils. Besides, all
composite samples exhibited uniform and rough
fracture surfaces, revealing strong interfacial adhesion
and thus good compatibility between the two compo-
nents. However, it can be seen that when the graphene
content increased, the surface become more compact,
especially in the case of RGO-containing composites.
This morphology could be used to explain why the
Young’s moduli of NFC/RGO composites were higher
than those of neat NFC.
From the cross-sectional surfaces of the composites
in Fig. 5, we cannot see graphene sheets, which could
be due to the complete exfoliation of graphene in the
NFC matrix. Furthermore, we observe the differences
in morphology between NFC/GO and NFC/RGO
composites paper in the SEM images taken at the top
surface of the papers (Fig. 6). As seen, NFC/GO
composites show a smooth surface without any aggre-
gations of GO, and they look similar to that of neat
NFC, which could be due to the high flexibility of GO
sheets, thus allowing them to be flattened together with
the NFC fibrils by the action of vacuum filtrating in the
preparation step. It was impossible to see the GO sheets
when the NFC/GO composite papers were analyzed in
both the top and cross-sectional surfaces, again con-
firming the good compatibility of the NFC and GO
components and high flexibility of GO sheets. This
could be used to explain why NFC/GO composites
showed higher strain at break compared to that of the
NFC/RGO composites. In contrary, for the NFC/RGO
composite papers containing 3, 5, and 10 wt% RGO
loadings, we can see the RGO sheets that are assembled
and fused together, showing rough surfaces. This
morphology of NFC/RGO composites could be
Cellulose
123
attributed to the hydrophobic property of reduced
graphene oxide. Combining SEM observations at the
top and cross-sectional surfaces, we can conclude that
the graphene sheets are well arranged parallel to the
papers’ surfaces; the arrangements were induced by the
vacuum filtrating. This structure explains to the high
mechanical properties of the composite papers.
TGA is used to study the thermal performance of
NFC/RGO composite papers with the addition of
graphene. The TGA thermograms and derivatives of
the neat NFC and NFC/RGO composites at different
graphene contents are presented in Fig. 7. For com-
parison, temperatures at 5 and 50 % weight loss (T5%and T50%) were evaluated. The T5% and T50% values of
the neat NFC were 245 and 333 �C, respectively,
while they were 255 and 336 �C for the NFC/RGO
containing 1 wt % RGO. Clearly, adding 1 wt% RGO
to NFC, the T5% was increased 10 �C, but the T50% did
not improve much. Furthermore, when RGO contents
were 3, 5, and 10 wt%, T5% values for the composite
papers were very close. Meanwhile, the T50% im-
proved to 338, 339, and 341 �C for the composites
with 3, 5, and 10 wt% RGO, respectively, which
means that for the composite containing 10 wt% RGO
content the T50% was 8 �C higher than that of neat
NFC. Other studies have shown the improvement in
thermal stability in graphene oxide-containing
cellulose materials (Kim et al. 2011; Han et al.
2011). For the cellulose/graphene composites (Mah-
muodian et al. 2012), the improvement in thermal
properties was also indicated by the higher thermal
decomposition temperatures, which have been ob-
served in our study.
As seen in the XRD in Fig. 8, the peak of graphite at
26.5� corresponded to the layer-to-layer distance of
3.36 A´. After the oxidation, GO shows a peak at 12.4�;
this was calculated to be 6.14 A´for the d-spacing,
which is significantly larger than that of pristine
graphite. This larger d-spacing is due to the presence
of oxygen functionalities such as epoxy and hydroxyl
on the basal planes and carboxyl on the edges of the
graphene sheets (FTIR, Fig. S1). For NFC/GO com-
posites at different contents, the (001) peak corre-
sponding to GO is not observed, which indicates that
GO sheets are exfoliated uniformly in the NFC matrix.
The XRD pattern of RGO shows a broad trace with no
clear view of the characteristic peaks. In addition, in the
NFC/GO and NFC/RGO composites at all graphene
material contents, the characteristic peak of the RGO
disappears, which can be attributed to the uniform
dispersion of RGO sheets in the NFC. Two character-
istic peaks at 16.5� and 22.2� fromNFC are attributed to
the typical profile of the cellulose I allomorph (Ya-
mashiki et al. 1990).
Fig. 7 TGA thermograms and their derivatives of NFC and NFC/RGO composite papers containing different contents of graphene
loadings
Cellulose
123
Conclusion
In summary, nanocellulose papers reinforced with
graphene have been successfully prepared via the
filtration of the NFC/RGO suspensions. The devel-
oped NFC/RGO composites showed enhanced me-
chanical, electrical, and thermal properties compared
to the neat NFC. Young’s moduli of the NFC/RGO
composite paper were much higher than those of neat
NFC as studied by DMA tests. Especially, the DMA
test showed the remarkable improvement in me-
chanical performance of the composite papers in
high-humidity environments. In addition, the electri-
cal conductivity could be controlled by the amount of
graphene loading.
Acknowledgments The authors acknowledge the Laboratory
of Inorganic Chemistry of Aalto University for access to the
X-ray diffraction equipment and Dr. Markus Valkeapaa for his
assistance with the measurements. This work made use of the
facilities of the Nanomicroscopy Center at Aalto University
(Aalto-NMC). The work was partly carried out as part of the
project Tailoring of Nanocellulose Structures for Industrial
Applications (NASEVA) funded by the Finnish Funding
Agency for Technology and Innovation (TEKES). Dr. Steve
Spoljaric at Aalto University is acknowledged for the valuable
discussion of the DMA results.
References
Boujemaoui A, Carlsson L, Malmstrom E, Lahcini M, Berglund
L, Sehaqui H, Carlmark A (2012) Facile preparation route
for nanostructured composites: surface-initiated ring-
opening polymerization of e-caprolactone from high sur-
face-area nanopaper. ACS Appl Mater Interfaces
4(6):3191–3198. doi:10.1021/am300537h
Bunch JS, Verbridge SS, Alden JS et al (2008) Impermeable
atomic membranes from graphene sheets. Nano Lett
8:2458–2462. doi:10.1021/nl801457b
Chua CK, Pumera M (2014) Chemical reduction of graphene
oxide: a synthetic chemistry view point. Chem Soc Rev
43:291–312. doi:10.1039/c3cs60303b
Han D, Yan L, Chen W et al (2011) Cellulose/graphite oxide
composite films with improved mechanical properties over
a wide range of temperature. Carbohyd Polym 83:966–972.
doi:10.1016/j.carbpol.2010.09.006
HenrikssonM, Berglund LA, Isaksson P, Lindstom T, Nishino T
(2008) Cellulose nanopaper structure. Biomacromolecules
9:1579–1585. doi:10.1021/bm800038n
Hummers WS Jr, Offeman RE (1958) Preparation of graphitic
oxide. J Am Chem Soc 80(6):1339. doi:10.1021/ja015
39a017
Kim H, Abdala AA, Macosko CW (2010) Graphene/polymer
nanocomposites. Macromolecules 43(16):6515–6530.
doi:10.1021/ma100572e
Kim CJ, Khan W, Kim DH et al (2011) Graphene oxide/cellu-
lose composite using NMMO monohydrate. Carbohyd
Polym 86:903–909. doi:10.1016/j.carbpol.2011.05.041
Laaksonen P, Walther A, Malho JM et al (2011) Genetic engi-
neering of biomimetic nanocomposites: diblock proteins,
graphene, and nanofibrillated cellulose. Angew Chem Int
Edit 50:8688–8691. doi:10.1002/anie.201102973
Lee C, Wei XD, Kysar JW, Hone J (2008) Measurement of the
elastic properties and intrinsic strength of monolayer gra-
phene. Science 321:385–388. doi:10.1126/science.
1157996
Li D, Muller MB, Gilje S, Kaner RB, Wallace GG (2008)
Processable aqueous dispersions of graphene nanosheets.
Nat Nanotech 3:101–105. doi:10.1038/nnano.2007.451
Luong ND, Pahimanolis N, Hippi U, Korhonen JT, Ruoko-
lainen J, Johansson L-S, Nam JD, Seppala J (2011)
Fig. 8 XRD spectra of the neat NFC, GO, and NFC/GO composites with 1, 3, 5, and 10 wt% GO content (a); XRD spectra of the NFC,
RGO, and NFC/RGO composites with 1, 3, 5, and 10 wt% RGO content (b)
Cellulose
123
Graphene/cellulose nanocomposite paper with high
electrical and mechanical performances. J Mater Chem
21:13991–13998. doi:10.1039/C1JM12134K
Luong ND, Korhonen JT, Soininen AJ, Ruokolainen J, Jo-
hansson LS, Seppala J (2013) Processable polyaniline
suspensions through in situ polymerization onto nanocel-
lulose. Eur Polym J 49(2):335–344. doi:10.1016/j.
eurpolymj.2012.10.026
Mahmuodian S, Wahit MU, Imran M et al (2012) A facile ap-
proach to prepare regenerated cellulose/graphene nano-
platelets nanocomposite using room-temperature ionic
liquid. J Nanosci Nanotechno 12:5233–5239. doi:10.1166/
jnn.2012.6351
Malho JM, Laaksonen P, Walther A et al (2012) Facile method
for stiff, tough, and strong nanocomposites by direct ex-
foliation of multilayered graphene into native nanocellu-
lose matrix. Biomacromolecules 13:1093–1099. doi:10.
1021/bm2018189
Mayorov AS, Gorbachev AV, Morozov SV, Britnell L, Jalil R,
Ponomarenko LA, Blake P, Novoselov KS, Watanabe K,
Taniguchi T, Geim AK (2011) Micrometer-scale ballistic
transport in encapsulated graphene at room temperature.
Nano Lett 11:2396–2399. doi:10.1021/nl200758b
Moon RJ, Martini A, Nairn J, Simonsen J, Youngblood J (2011)
Cellulose nanomaterials review: structure, properties and
nanocomposites. Chem Soc Rev 40:3941–3994. doi:10.
1039/C0CS00108B
Novoselov KS, Fal’ko VI, Colombo L, Gellert PR, SchwabMG,
Kim K (2012) A road map for graphene. Nature
490:192–200. doi:10.1038/nature11458
Paakko M, Ankerfors M, Nykanen A, Ahola S, Ostergerg M,
Ruokolainen J, Laine J, Larsson PT, Ikkala O, Lindstrom T
(2007) Enzymatic hydrolysis combined with mechanical
shearing and high pressure homogenization for nanoscale
cellulose fibrils and strong gels. Biomacromolecules
8(6):1934–1941. doi:10.1021/bm061215p
Pinkert A, Marsh KN, Pang S, Staiger P (2009) Ionic liquids
interaction with cellulose. Chem Rev 109(12):6712–6728.
doi:10.1021/cr9001947
Potts JR, Dreyer DR, Bielawski CW, Ruoff RS (2011) Gra-
phene-based polymer nanocomposites. Polymer
52(1):5–25. doi:10.1016/j.polymer.2010.11.042
Seppala J (2012) Nanocellulose: a renewable polymer of bright
future. Express Polym Lett 6(4):257. doi:10.3144/
expresspolymlett.2012.28
Siro I, Plackett D (2010) Microfibrillated cellulose and new
nanocomposite materials: a review. Cellulose 17:459–494.
doi:10.1007/s10570-010-9405-y
Stankovich S, Dikin DA, Dommett GHB, Kohlhaas KM, Zim-
ney EJ, Stach EA, Piner RD, Nguyen SBT, Ruoff RS
(2006a) Graphene-based composite materials. Nature
442:282–286. doi:10.1038/nature04969
Stankovich S, Piner RD, Nguyen SBT, Ruoff RS (2006b)
Synthesis and exfoliation of isocyanate-treated graphene
oxide nanoplatelets. Carbon 44(15):3342–3347. doi:10.
1016/j.carbon.2007.02.034
Yamashiki T, Matsui T, Saitoh M, Matsuda Y, Okajima K,
Kamide K (1990) Characterization of cellulose treated by
the steam explosion method. Part 3: effect of crystal forms
(Cellulose I, II, and III) of original cellulose on changes in
morphology, degree of polymerization, solubility and
supramolecular structure by steam explosion. Brit Polym J
22:201–212. doi:10.1002/pi.4980220305
Zhao X, Zhang Q, Chen D (2010) Enhanced mechanical prop-
erties of graphene-based poly(vinyl alcohol) composites.
Macromolecules 43(5):2357–2363. doi:10.1021/ma902
862u
Cellulose
123
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