Arabinoxylan/nanofibrillated cellulose composite films

Jasna S. Stevanic • Elina Mabasa Bergstrom •

Paul Gatenholm • Lars Berglund • Lennart Salmen

Received: 14 March 2012 / Accepted: 24 May 2012 / Published online: 16 June 2012

� Springer Science+Business Media, LLC 2012

Abstract There is an increasing interest in substituting

petroleum based polymer films, for food packaging appli-

cations, with films based on renewable resources. In many

of these applications, low oxygen permeability and low

moisture uptake of films are required, as well as high

enough strength and flexibility. For this purpose, rye ara-

binoxylan films reinforced with nanofibrillated cellulose

was prepared and evaluated. A thorough mixing of the

components resulted in uniform films. Mechanical, ther-

mal, structural, moisture sorption and oxygen barrier

characteristics of such films are reported here. Reinforce-

ment of arabinoxylan with nanofibrillated cellulose affec-

ted the properties of the films positively. A decrease in

moisture sorption of the films, as well as an increase in

stiffness, strength and flexibility of the films were shown.

From these results and dynamic FTIR spectra, a strong

coupling between reinforcing cellulose and arabinoxylan

matrix was concluded. Oxygen barrier properties were

equal or better as compared to the neat rye arabinoxylan

film. In general, the high nanofibrillated cellulose con-

taining composite film, i.e. 75 % NFC, showed the best

properties.

Introduction

Production of environmentally friendly packaging materi-

als, based on polymers from renewable resources, is

increasingly demanded. Films made of plant-derived non-

food polysaccharides, such as cellulose and hemicelluloses

(i.e. xylans and glucomannans), excluding storage poly-

saccharides, such as starch, may provide a potential alter-

native for today’s petroleum based packaging materials.

These polysaccharide polymers are hydrophilic, and have

good barrier properties against oils and fats, but are by its

nature less efficient as moisture and water vapour barriers

[1]. Also, oxygen barrier properties are good at low or

moderate relative humidities, particularly for xylans [2–4].

The mechanical strength of pure hemicellulose-based films

does however vary considerably and are generally con-

sidered to be too low without additives. One possibility to

enhance the performance of such hemicellulose films is by

reinforcing them with cellulose, thus initiating this study.

Arabinoxylans are a group of polysaccharides that can

be found in almost all annual plants, and many woody

plants, such as softwoods [5, 6]. The function of the ara-

binoxylans is to make up part of the matrix material in the

plant cell wall. The chemical structure of arabinoxylans

varies significantly depending on their source. The back-

bone of arabinoxylans is built up of (1 ? 4)-linked b-D-

xylopyranosyl units, which are further substituted with

varying degrees by a-L-arabinofuranosyl groups [7, 8].

Other substituents found in arabinoxylans are a-D-gluco-

pyranosyl uronic acid (or its 4-O-methyl ether), D-xylo-

pyranosyl and acetyl groups [7, 8]. Cereal cell walls, such

as from barley, husk, oat and rye, are rich in arabinoxylans

and therefore an attractive source since they represent non-

food fractions and a potential agricultural waste. In rye

(Secale cereale) endosperm arabinoxylan, the b-D-

J. S. Stevanic (&) � E. M. Bergstrom � L. Salmen

INNVENTIA AB, Fibre and Material Science, Box 5604,

SE-114 86 Stockholm, Sweden

e-mail: jasna.stevanic@innventia.com

J. S. Stevanic � L. Berglund � L. Salmen

Wallenberg Wood Science Center, The Royal Institute of

Technology, SE-100 44 Stockholm, Sweden

E. M. Bergstrom � P. Gatenholm

Wallenberg Wood Science Center, Department of Chemical and

Biological Engineering, Chalmers University of Technology,

SE-412 96 Gothenburg, Sweden

123

J Mater Sci (2012) 47:6724–6732

DOI 10.1007/s10853-012-6615-8

xylopyranosyl units are both mono- and di-substituted by

(1 ? 2)- and (1 ? 3)-linked a-L-arabinofuranosyl units

(Fig. 1). The ratio of mono and disubstituted xylopyranosyl

residues is 2:1 and the average arabinose to xylose (Ara/

Xyl) ratio is between 0.50 and 1.00 [3].

Nanofibrillated cellulose (NFC) is a specifically pre-

pared cellulosic material composed of liberated semicrys-

talline nanosized cellulose microfibrils with a high aspect

ratio (i.e. length to with ratio). The lateral dimension of the

NFC nanofibrils and nanofibril aggregates is in the order of

5–20 nm, respectively [9] and, if further aggregated, up to

40 nm [10] (cf. for kraft pulp fibres, the lateral dimension

of microfibrils and microfibril aggregates is about 4 and

20 nm, respectively [11]), a value highly dependent upon

the preparation technique used. Most frequently, various

chemical and/or enzymatic pre-treatments before an

intensive mechanical fibrillation are used for preparation of

the NFC. The pre-treatments are introduced to purify the

origin material from other cell wall components, such as

hemicelluloses and lignin, but also to significantly reduce

the energy consumption during the microfibril liberation

process [12]. The NFC, at the time termed microfibrillated

cellulose (MFC), was first introduced by Turbak et al. [13]

and Herrick et al. [14]. An improved MFC was later pre-

pared [9, 15] using a combination of mechanical and

enzymatic pre-treatments, followed by a high pressure

homogenization. The NFC used in the present study was

prepared according to such a procedure outlined by Hen-

riksson et al. [15]. This NFC forms a stable hydrocolloidal

dispersion and forms a gel already at fairly low concen-

tration. The gel consists of a strongly entangled and dis-

ordered network of cellulose nanofibrils and nanofibril

aggregates. Filtration and drying can be used to form a

nanopaper structure [16] in the form of a NFC fibril net-

work of high tensile strength ([200 MPa), high stiffness

(*15 GPa) and relatively high strain-to-failure (6–10 %).

In addition, NFC from wood pulp is characterised by bio-

degradability, it is from renewable resources, and the

existing infrastructure for commercial wood pulping also

makes NFC attractive as an easily available resource to be

used as reinforcement component for biocomposites.

The goal of the present study was to investigate the

morphology, physical and mechanical properties of bio-

composite films, based on rye arabinoxylan (rAX) and

NFC, and to examine effects of addition of NFC.

Materials and methods

Preparation of rAX solution

A high molecular weight rAX (lot 20601a) was used

(Megazyme International Ireland Ltd); molar mass (Mw)

289,300 g/mol [17]. The arabinose/xylose (Ara/Xyl) ratio of

the rAX was 0.50. An aqueous arabinoxylan solution was

prepared by dissolving the rAX flour, 2 g, (pre-wetted with

95 % C2H5OH) in deionised water, heating it at 100 �C

under magnetic stirring for 10 min. The solution was

allowed to cool down under magnetic stirring to room tem-

perature and its concentration was adjusted to 2 % (w/v).

Preparation of nanofibrillated cellulose

NFC was prepared from a never-dried bleached sulphite

pulp based on Norwegian spruce (Picea abies) (kindly

provided by Nordic Paper Seffle AB, Sweden), containing

13.8 % hemicelluloses and 0.7 % lignin. A combined pre-

treatment, i.e. an enzymatic degradation and a mechanical

beating, was used, followed by a disintegration into NFC

by a homogenisation, i.e. passing several times through a

microfluidizer, as described by Henriksson et al. [15, 16]

The solid content of the NFC (deionised water dispersion)

was adjusted to 1.4 % (w/v). The charge density of the

NFC was 50 lequiv/g, and it contained 0.8 % arabinog-

lucuronoxylan and 2.5 % galactoglucomannan (most likely

a low Gal substituted fraction of galactoclucomannan).

This lower amount of hemicelluloses as compared to the

original pulp can be explained by the fact that much of the

galactoglucomannan has been released upon the liberation

of the cellulose microfibrils during the NFC preparation

process. The crystallinity of the NFC may be estimated to

around 8–12 % using NMR, based on previous studies [9],

where the NMR crystallinity value corresponds to an X-ray

crystallinity of ca. 50 %, due to the contribution of the

paracrystalline material.

Preparation of films

The films were prepared by mixing the NFC dispersion

with the rAX solution in different proportions, with the

rAX/NFC ratios of 100/0, 75/25, 50/50, 25/75 and 0/100

(see Table 1). No external plasticizer was added.

O

OHOHOH

O

O

OHOHOH

O

O

OHOHOH

O OOO

OOH

O

OH

OO

OH n

Fig. 1 Chemical structure of

rye arabinoxylan

J Mater Sci (2012) 47:6724–6732 6725

123

In order to produce homogenous films, a thorough

mixing is necessary obtained by a homogenization and

further dispersion of the rAX/NFC mixtures in three steps:

(a) using an Ultra-Turrax� 9500 rpm for 5 min,

(b) exposing the mixture to magnetic stirring for 3 days,

and (c) once again using the Ultra-Turrax� 9500 rpm for

10 min; adopting method developed by Svagan et al. [10].

The resulting dispersions were exposed to vacuum under

magnetic stirring for 24 h to remove air bubbles. The

homogenized dispersions were then cast into Teflon

(PTFE) coated polystyrene petri dishes (Ø 8.4 cm). These

were left to evaporate at 23 �C and 52 % RH (relative

humidity) for 48 h. The thickness of the resulting films was

approximately 20 lm (measured using a micrometre screw

gauge).

Scanning electron microscopy (SEM)

SEM images of the films, brittle fractured in liquid N2,

were collected with a LEO ULTRA 55 FEG SEM (LEO

electron microscopy LTD, Cambridge, UK) equipped with

a field emission gun and an Inlens detector. The accelera-

tion voltage was set to 5 kV. Prior to the SEM analysis, the

films were glued to aluminium specimens, mounted with a

colloidal silver liquid and sputter coated with a thin layer

of gold.

Atomic force microscopy (AFM)

Surface roughness of the films was measured using a

Digital Instrument Nanoscope IIIa AFM (Digital Instru-

ment Inc., Santa Barbara, CA, USA), equipped with a type

G scanner and a lMasch ultrasharp nanocontact silicon

cantilever NSC15. Areas of 25 9 25 lm of the films were

analysed in tapping mode. The surface roughness was

expressed as mean roughness, Ra, and calculated accord-

ing to Eq. (1):

Ra ¼ 1

LxLy

ZLy

0

ZLx

0

f x; yð Þj jdxdy; ð1Þ

where Lx and Ly are the dimensions of the surface and

f x; yð Þ is the surface relative to the centre plane. The Ra is

the arithmetic average of the absolute values of the mea-

sured profile height deviation taken within the sampling

length and measured from the graphical centre line. It is the

mean value of the surface relative to the centre plane.

Dynamic vapour sorption (DVS)

Moisture sorption isotherms were obtained with a Surface

Measurement System Plus II Oven DVS (Surface Mea-

surement System Ltd, Rosemont Road, UK). 4 mg material

of each film was measured in adsorption, from 0 to 90 %

RH. At each step of 0, 20, 40 and 60 % RH the humidity

was kept constant for 300 min, at 80 % RH it was kept for

400 min, and at 90 % RH it was kept for 500 min. The

weight increase, registered by a microbalance, was recor-

ded as a function of the increase in humidity of the sur-

rounding air. This device generates a moist air by mixing

dry and water-saturated air streams at 30 �C. The results

were based on a single measurement and generated using a

DVS 32 Novideo software. The moisture uptake was

expressed according to Eq. (2):

Moisture uptake ¼ 100Wmoist �Wdry

Wdry

; ð2Þ

where Wmoist is the sample weight equilibrated at the

chosen relative humidity and Wdry is the weight of the dry

sample.

Tensile testing

Mechanical testing (stress–strain scanning) was carried out

on a Single Column Table Top Instron 5944 (Instron Ltd.,

Coronation Road, UK), with a load cell of 2 kN, operating

in tension mode. Film stripes cut to the size of 5 9 50 mm

(width 9 length), of 20 lm thickness, were used for test-

ing at 50 % RH and 23 �C. The applied loading rate was

10 % of the span length/min, where the initial span length

of films was 30 mm. Ten replicates from each film type

were tested. The Young’s modulus (E) was taken as the

initial slope of the linear part of the stress–strain curve. The

stress at break (rb) and strain at break (eb) were evaluated

based on the initial sample dimensions. Data were pre-

sented as average values from ten measurements.

Dynamic mechanical analysis (DMA)

Dynamic mechanical testing (humidity scanning) was car-

ried out using a Perkin-Elmer DMA 7e (PerkinElmer Corp.,

Norwalk, CT, USA), operating in tension mode. Film stripes

cut to the dimensions of 3 9 15 mm (width 9 length), of

Table 1 Codes and descriptions of rAX, NFC and composite films

thereof; % is percent by weight

Code Description

100rAX 100 % Arabinoxylan

25NFC 75 % Arabinoxylan ? 25 % nanofibrillated cellulose

50NFC 50 % Arabinoxylan ? 50 % nanofibrillated cellulose

75NFC 25 % Arabinoxylan ? 75 % nanofibrillated cellulose

100NFC 100 % Nanofibrillated cellulose

6726 J Mater Sci (2012) 47:6724–6732

123

20 lm thickness, were first conditioned at 5 % RH for 3 h;

(the hysteresis was negligible due to the low moisture con-

tent of ca. 3 at 5 % RH). The samples were then scanned in

the range of 5–95 % RH at a speed of 1 % RH/10 min at a

temperature of 30 �C, a speed low enough to maintain

equilibrium conditions during scanning [18]. The desired

relative humidities were achieved by mixing dry air and

water-saturated air using a Wetsys humidity generator

(Setaram Instrumentation, Caluire, France). The static load

was adjusted to be equal to 140 % of the dynamic load, the

amplitude was set to be constant at 5 lm corresponding to a

deformation of 0.033 %, at a frequency of 1 Hz. Three

replicates from each film type were tested. The storage

modulus (E’) and the loss tangent (tan d) were recorded using

a Pyris DMA 28 software (PerkinElmer Corp., Norwalk, CT,

USA).

Dynamic mechanical testing (temperature scanning)

was carried out using a Perkin-Elmer DMA 7e (PerkinEl-

mer Corp., Norwalk, CT, USA), operating in tension mode.

Film stripes cut to the dimensions of 5 9 20 mm

(width 9 length), of 20 lm thickness, were first condi-

tioned at a relative humidity of 0 % RH in a He atmo-

sphere, a temperature of 30 �C for 30 min and then

scanned in a range of 30–300 �C at a speed of 2 �C/min.

The static load was adjusted to be equal to 120 % of the

dynamic load, the amplitude was set to be constant at

3 lm, at a frequency of 1 Hz. Three replicates from each

film type were tested. The storage modulus (E0) and the

loss tangent (tan d) were recorded using a Pyris DMA 28

software (PerkinElmer Corp., Norwalk, CT, USA).

Dynamic FTIR spectroscopy

Dynamic FTIR (Fourier Transform InfraRed) spectra were

recorded on a Varian 680-IR spectrometer (Varian Inc.,

Santa Clara, CA, USA) in transmission mode. A liquid

nitrogen cooled MCT (Mercury Cadmium Telluride)

detector was used and the IR radiation was polarized by a

KRS5 wire grid polarizer at 0� in relation to the stretching

direction. An optical filter was added after the polarizer to

reduce the spectral range (3950–700 cm-1). The interfer-

ometer was run in step-scan mode at a phase modulation

frequency of 400 Hz (the moving mirror oscillates around

each step point with a frequency of 400 Hz). Thin sheets

with a dimension of 10 9 20 mm (width 9 length) and of

20 lm thickness were mounted between two parallel jaws

in a specially constructed polymer modulator (PM-100).

The PM-100 was placed in a temperature control system

(TC-100) (MAT-Manning Applied Technology Inc., Troy,

ID, USA). A modular humidity generator (MHG-32)

(Projekt Messtechnik, Ulm, Germany) was connected to

the TC-100. Spectra were recorded with a resolution of

8 cm-1; one scan per measurement. To ensure the linear

deformation during the collection of the dynamic FTIR

spectra, the amplitude of the applied sinusoidal strain was

less than 0.3 % of the sample length at a frequency of

16 Hz. The dynamic changes in the IR spectrum were

divided into two orthogonal spectra: the in-phase spectrum

indicating immediate changes or elastic responses and the

out-of-phase spectrum representing the time-delayed

changes or viscous responses using Varian Resolutiom Pro

5.1 software (Varian Inc., Santa Clara, CA, USA). The

resulting interferograms were Fourier transformed and a

Norton–Beer medium apodization function was used. The

in-phase and out-of-phase spectra obtained were base-line

corrected at 1800 cm-1. They were then converted into a

phase spectrum and a magnitude spectrum, where the

converted magnitude spectrum was normalised to 1.0 at

1160 cm-1. New in-phase and out-of-phase spectra were

calculated from these. Two parallel measurements were

made at 50 % RH and 30 �C. The average in-phase and

out-of-phase spectra were then calculated.

Oxygen permeability

Oxygen transmission rate (OTR) was measured according

to an ASTM F—1927-07 standard method, using a Mocon

Ox-Tran� 2/21, ML Master Base Control Module, (Mo-

con� 7500 Boone Avenue North, Minneapolis, Minnesota,

USA) operating with a Mocon’s WinPermTM permeability

software. The measurements were done at controlled con-

ditions of 50 % RH and 23 �C, using a coulometric

detector. Duplicates from each film type of 20 lm thick-

ness with an area of 5 cm2 were tested. Oxygen perme-

ability (OP) was calculated from the film thicknesses and

expressed as cm3 lm/m2 day kPa.

Results and discussion

Film formation and morphology

Upon casting from aqueous dispersions, both the rAX and

the NFC (i.e. 100rAX and 100NFC, respectively) were able

to form cohesive films without the use of any plasticiser

added. In the case of the composite films (i.e. 25NFC,

50NFC and 75NFC), cohesive films were also produced.

With the higher amount of NFC added, the films became

successively less transparent and more opaque, while the

100NFC film seems as slightly less opaque than the 75NFC

film, as illustrated in Fig. 2. All the films were homogenous

without any visible particles present, as inspected by eyes.

A similar trend was observed regarding surface rough-

ness (AFM measurements, Table 2), where an increase was

noted with the higher amount of NFC added. This

increased surface roughness may contribute to the

J Mater Sci (2012) 47:6724–6732 6727

123

successive decrease in transparency of the films. The

100NFC film displayed a lower surface roughness than that

of the 75NFC film, in line with the slightly higher trans-

parency of the pure NFC film.

The morphology of the films was studied by SEM. In

Fig. 3, SEM micrographs of the fracture surfaces, obtained

by breaking nitrogen frozen films, are shown. The 100rAX

film displayed a continuous and homogenous structure. The

composite films all showed somewhat more aggregated

structures, which became less homogeneous and more

layered at higher NFC content. The 25NFC film remained

quite homogeneous in structure with visible and dispersed

bundles of NFC embedded in the matrix of rAX. The

50NFC film displayed a more heterogeneous structure. The

75NFC film showed a more layered structure with thick

NFC lamellae coated by thinner layers of rAX. The

100NFC film showed a clear lamellar structure with the

lamellae composed of a random in-plane network of cel-

lulose NFC. This lamellar structure of NFC films, com-

posed of a fibrous network, has previously been reported by

several authors [10, 16, 19].

Moisture sorption isotherms

The moisture sorption behaviour of films is an important

characteristic of hemicellulose films. The sorption iso-

therms of the films, the 100rAX, 100NFC and composite

films, all displayed a sigmoidal shape (Fig. 4), normally

reported for hydrophilic materials. This shape of the curves

is characteristic for systems with strong polymer–polymer

and polymer–solvent interactions, i.e. a type II isotherm

[20]. At the higher relative humidities, the molecular

interactions between cellulose and/or hemicellulose mole-

cules become partly replaced by interactions with water

molecules. In general, the water molecules are adsorbed

around polar groups (i.e. –OH groups) in the amorphous

regions of the molecules.

The NFC film displayed the lowest moisture adsorption,

reflecting the crystalline nature of the microfibrillar cellulose.

However, the present NFC contains a substantial amount of

disordered cellulose as well as some hemicelluloses. The

100rAX film showed the highest moisture content, reflecting

the highest amount of water sorption sites available. The

composite films showed intermediate moisture sorption iso-

therms, where the moisture sorption isotherm successively

decreased with increasing amount of NFC present.

If the moisture sorption data of the composite films are

compared with the expected moisture uptake based on the

rule of mixture (i.e. calculated on the basis of the moisture

uptake contribution of each component) (Fig. 5), it is

apparent that the composite films adsorb less moisture than

expected. This indicates that strong permanent H-bonds

and other interactions were created between the xylan and

the cellulose NFC, decreasing the number of available sites

for moisture sorption. A larger difference was seen for the

50NFC film, as compared with the other two composite

films (i.e. 75NFC and 25NFC). This may be explained by a

better mixing of the two components (i.e. xylan and cel-

lulose) in this film (cf. Fig. 3).

Tensile properties

Figure 6 shows average stress–strain curves of the different

films tested at 50 % RH and 23 �C (see Table 3). The

100rAX film showed the lowest stiffness, strength and

ductility of all films. The Young’s modulus was 3.1 GPa, a

higher value than previously reported (i.e. 2.5 GPa at 30 �C

[21] and 1.8 GPa at 23 �C [3]), which can be explained by

the use of an improved homogenisation procedure in the

present study. The composite films showed gradual

Fig. 2 Optical images of films, from left to right: pure rye arabinoxylan film (100rAX), composite films: 25NFC, 50NFC, 75NFC, and pure

nanofibrillated cellulose film (100NFC)

Table 2 Mean roughness (Ra) of rAX, NFC and composite films

thereof

Code 100rAX 25NFC 50NFC 75NFC 100NFC

Surface

roughness,

Ra (nm)

56 103 139 221 189

6728 J Mater Sci (2012) 47:6724–6732

123

increase in stiffness, strength and strain-to-failure with

increased NFC reinforcement content. The 100NFC film,

itself, had lower strength, stiffness and strain-to-failure

than the 75NFC film.

The low mechanical properties for the 100NFC film

were most likely due to high porosity, since NFC films are

fibrillar network structures. One could also observe

lamellar separation in the SEM micrographs, see Fig. 3.

When smaller denser regions were sampled for DMA

measurements, much higher Young’s modulus was mea-

sured at 50 % RH (i.e. 11.5 GPa). This was more in line

with previously reported data on homogenous films with

19 % porosity, showing mechanical strength properties of

approximately 15 GPa Young’s modulus, 200 MPa tension

strength and 7 % strain-to-failure [16]. With rAX, as an

amorphous polymer [3, 21] present in the structure, a more

uniform stress distribution results in the better strength

properties.

Moisture sensitivity

The effect of relative humidity (humidity scans–absolute

moduli, E0 and loss tangent, tan d) on moduli and

mechanical damping of the films are shown in Fig. 7. With

100rAX 25NFC 50NFC

2 µm

75NFC 100NFC

Fig. 3 SEM micrographs of fracture surfaces (obtained by breaking nitrogen frozen films) of films; pure rye arabinoxylan film (100rAX),

composite films: 25NFC, 50NFC, 75NFC, and pure nanofibrillated cellulose film (100NFC); scale bar is 2 lm

0

10

20

30

9060300Mo

istu

re u

pta

ke (

w/w

dry

bas

is)

(%)

Relative humidity (%)

25NFC50NFC

100rAX

100NFC75NFC

Fig. 4 Water vapour sorption isotherms of rye arabinoxylan film,

nanofibrillated cellulose film and composite films made thereof;

recorded at 30 �C

-0.1

0.1

0.3

0.5

0.7

0 20 40 60 80

mo

istu

re u

pta

ke_c

al. a

nd

mo

istu

re u

pta

ke_e

xp. (

%)

Relative humidity (%)

25NFC50NFC

75NFC

Dif

fere

nce

bet

wee

n

Fig. 5 Difference between calculated moisture uptake and measured

moisture uptake of the composite films versus RH

Table 3 Average values of Young’s modulus (E), stress at break (rb) and strain at break (eb)

Code Young’s modulus, E (GPa) Stress at break, rb (MPa) Strain at break, eb (%)

100rAX 3.1 ± 0.2 62 ± 3 4.0 ± 0.9

25NFC 4.8 ± 0.7 108 ± 17 6.0 ± 1.1

50NFC 6.3 ± 0.4 135 ± 31 6.4 ± 2.4

75NFC 7.3 ± 0.5 143 ± 15 7.2 ± 1.2

100NFC 6.6 ± 0.5 107 ± 14 5.7 ± 1.9

J Mater Sci (2012) 47:6724–6732 6729

123

increasing relative humidity, the stiffness of the films

decreased, as expected for these hydrophilic materials,

while the mechanical damping (tan d) increased (above

85 % RH the increase was strong). The 100NFC film

showed the highest storage modulus and the lowest values

for tan delta. The 100rAX film showed the lowest storage

modulus and the highest tan delta values. With increasing

amounts of the NFC, the composite films showed increased

storage modulus and data for the mechanical damping

successively decreased. The 100NFC film showed a drop in

the storage modulus from 13–7.7 GPa (5–90 % RH),

which may be due to both fibril softening and decreased

interfibril interactions. The change of the tan d value with

RH was negligible. The pure 100rAX film was strongly

sensitive to moisture with the storage moduli dropping

from 6.8 to 0.9 GPa (5–90 % RH), reflecting the amor-

phous and hygroscopic nature of rAX.

Two transition points could be noted, a secondary

transition around 20 % RH and a main transition [22] at

around 80 % RH. The secondary transition was most

apparent for the 100rAX film and may be attributed to the

motion of the arabinose substituent [23]. The main transi-

tion at 80 % RH was attributed to the moisture-induced

glass transition of the xylan [24–26]. It is notable that

already with an addition of 25 % NFC (25NFC film), the

indication of the glass transition almost disappeared in the

data. This may be due to possible interactions between

NFC and arabinoxylan, limiting the moisture-induced

mobility of the xylan chains [7].

In Fig. 8, the relative softening behaviour (humidity

scans–relative moduli, E0) is illustrated for the 100rAX,

100NFC and composite films. Even the reinforcement with

the lowest amount of the less hygroscopic NFC (25NFC

film) showed considerably reduced moisture sensitivity.

The low moisture sensitivity of the NFC results in that the

reinforcing effect being substantial above the Tg of the

arabinoxylan, i.e. above 80 % RH, is similarity with effects

on composites above the matrix Tg [27, 28].

Thermal properties

The thermomechanical properties (temperature scans–

absolute moduli, E0 and loss tangent, tan d) are shown in

Fig. 9. With increasing temperature the stiffness decreased,

and the mechanical damping increased for all films. The

pure 100rAX film showed a distinct glass transition (Tg) at

200 �C. It was previously reported that dry hemicelluloses

from wood typically have a Tg between 150 and 220 �C

(the span depends on differences in chemical composition,

configuration, side groups and molecular weight), while

dry cellulose may show a Tg of amorphous parts between

200 and 250 �C (the span depends on the degree of crys-

tallinity) [29]. For the pure NFC studied here, as well as for

the 75NFC film, no transitions were noted in this temper-

ature range. For the composite films the Tg shifted to higher

temperatures with increasing amount of reinforcing NFC.

0

40

80

120

160

0 1 2 3 4 5 6 7 8

Str

ess

(MP

a)

Strain (%)

25NFC

50NFC

100rAX

75NFC

Fig. 6 Stress–strain curves of rye arabinoxylan film (100rAX) and

composite films of rye arabinoxylan reinforced with nanofibrillated

cellulose: 25NFC, 50NFC, 75NFC; recorded at 50 % RH and 23 �C

0

0.2

0.4

0.6

0.1

1

10

100

5 20 35 50 65 80

Tan

δ

log

Sto

rag

e m

od

ulu

s (G

Pa)

Relative humidity (%)

50NFC

100rAX

100NFC75NFC

25NFC

Fig. 7 Humidity scans of rye arabinoxylan film, nanofibrillated

cellulose film and composite films made thereof; recorded at 1 Hz and

30 �C

0

20

40

60

80

100

5 20 35 50 65 80

Rel

ativ

e S

tora

ge

mo

du

lus

(%)

Relative humidity (%)

50NFC

100NFC75NFC

25NFC

100rAX

Fig. 8 Relative storage modulus of rye arabinoxylan film, nanofibr-

illated cellulose film and composite films made thereof, as a function

of relative humidity; recorded at 1 Hz and 30 �C

6730 J Mater Sci (2012) 47:6724–6732

123

This, again, points to the strong interactions between the

cellulose and xylan.

Molecular interactions

Figure 10 illustrates dynamic FTIR spectra of the 75NFC

film in the wavenumber region of 3950–700 cm-1, show-

ing both the in-phase spectrum representing the elastic

response of the components and the out-of-phase spectrum

representing the viscous response of the components. The

signals occurring in the in-phase spectrum are much more

intense than in the out-of-phase spectrum indicating that, at

50 % RH, the film responded mostly elastically. Three

absorption vibrations are highlighted in the in-phase

spectrum, two cellulose signals at 1160 cm-1 (asymmetric

C–O–C bridge stretching vibration) and at 1425 cm-1 (C–

OH bending vibration of the CH2–OH group attached at

Glc unit) [30, 31], and an arabinoxylan signal at

1460 cm-1 (CH2 symmetric bending on the Xyl ring) [32–

34]. The cellulose signal at 1160 cm-1 was used for nor-

malization of the spectra. The signal at 1425 cm-1 is

selective for cellulose, while the signal at 1460 cm-1 is

selective for arabinoxylan, due to the fact that the func-

tional groups absorbing IR radiation are unique for

respective components. The elastic response from both

components indicates that the two components are strained

simultaneously with the applied sinusoidal perturbation.

This shows that strong interactions between the arabin-

oxylan and the cellulose exist in the composite films.

Oxygen barrier properties

The oxygen permeability of the neat rAX and its composite

films showed very low values at 50 % RH (Fig. 11). The

composite films with higher amount of the NFC showed

even lower values than that of the pure rAX. This

decreased permeation of oxygen of the composite films,

50NFC and 75NFC, are most likely due to the low per-

meability of NFC itself. The oxygen permeability of the

100rAX film in this study was 1.0 cm3 lm/m2 day kPa,

which was lower than values earlier reported by Hoije et al.

(i.e. 2.0 cm3 lm/m2 day kPa for the same type of the

material measured under similar conditions of 50 % RH

and 23 �C [3]). This improvement may be explained by the

enhanced film preparation procedure adopted here, where

the dispersions were homogenised before casting. The

oxygen permeability of the films was in the range of poly

ethylene vinyl alcohol (EVOH), which is commercially

used as a barrier plastic, having an oxygen permeability of

0.1 cm3 lm/m2 day kPa at 0 % RH which increases to

12 cm3 lm/m2 day kPa at 95 % RH [35, 36].

0

0.3

0.6

0.9

1.2

0.01

0.1

1

10

100

30 120 210 300

Tan

δ

log

Sto

rag

e m

od

ulu

s (G

Pa)

Temperature (°C)

25NFC

50NFC

100rAX

100NFC75NFC

Fig. 9 Temperature scans of rye arabinoxylan film, nanofibrillated

cellulose film and composite films made thereof; recorded at a

frequency of 1 Hz

-0.4

0

0.4

0.8

1.2

700170027003700

No

rmal

ized

dyn

amic

res

po

nse

Wavenumber (cm-1)

In-phase

Out-of-phase 1160

1460

1425

Fig. 10 Dynamic FTIR spectra of the 75NFC film recorded at 0�polarization, 50 % RH and 30 �C; the in-phase spectrum (thin line)

indicates elastic responses and the out-of-phase spectrum (thick line)

indicates viscous responses

0.82 0.79 1.08 0.97 0.852

0.1

12

3

10

15.6

0

4

8

12

16

75NF

C

50NF

C

25NF

C

100rAX

MF

C

rAX

EV

OH

(0%R

H)

EV

OH

(95%R

H)

PV

DC

PE

T

polyester

OP

[(c

m3 µ

m)/

(m2 d

ay k

Pa)

]

Fig. 11 Oxygen permeability of rye arabinoxylan film and its

composite films; literature values for MFC (carboxymethylated)

[19], rAX [3], EVOH (poly ethylene vinyl alcohol) [36], PVDC

(polyvinylidene chloride) [37], polyester [36] and PET (poly(ethyl-

ene-terephthalate)) [37]; all tests done at 23 �C and 50 % RH, if not

specified differently

J Mater Sci (2012) 47:6724–6732 6731

123

Conclusions

This study demonstrates that, with the addition of NFC to

rAX, it is possible to produce composite films with greatly

improved mechanical, thermal, moisture sorption and

excellent oxygen barrier properties. Thorough mixing of

NFC with rAX resulted in homogenous composite films.

In general, stiffer, stronger and more ductile films were

produced when the NFC was added to the arabinoxylan;

with a Young’s modulus of up to 7.2 GPa, a tensile

strength of up to 143 MPa and a strain-to-failure of up to

7.3 %. The addition of the NFC to the arabinoxylan films

caused a shift of the Tg to higher temperatures than the

200 �C of the pure xylan. This indicates a substantial xylan

interphase region with reduced molecular mobility. A

strong coupling between cellulose and arabinoxylan was

also indicated. The moisture sorption and the associated

moisture-induced softening were both reduced with

increased NFC reinforcement content. With the reinforce-

ment, the oxygen barrier properties of the arabinoxylan

films were also improved, showing an oxygen permeability

of 0.8 cm3 lm/m2 day kPa at 50 % RH in a film composed

of 50–75 % NFC by weight.

Acknowledgements The Knut and Alice Wallenberg Foundation are

gratefully acknowledged for funding through the Wallenberg Wood

Science Center. The authors thank Dr Aihua Pei for assistance in pro-

curing the NFC and for fruitful discussions on the film casting tech-

niques, Anders Martensson for providing the SEM micrographs and

AFM measurements, Anne-Mari Olsson for performing the DVS and

Therese Johansson for performing O2 permeability measurements.

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