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: jukka.seppala@aalto.fi

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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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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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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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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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

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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

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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.

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