Reinforcing Potential of Nanofibrillated Cellulose FromNonwoody Plants

Ashraf Chaker,1 Pere Mutje,2 Fabiola Vilaseca,2 Sami Boufi1*1University of Sfax, Facult�e des Sciences de Sfax, LMSE, BP 1171-3000 Sfax, Tunisia

2LEPAMAP Group, University of Girona, Campus Montilivi, 17071, Girona, Spain

In the present work the reinforcing potential of nanofi-brillated cellulose (NFC) from five different non-woodyplants, namely; abaca, sisal, hemp, jute and flax wasinvestigated. Nanocomposite materials were preparedby casting a mixture of NFC suspension and a polymerwaterborne latex dispersion, and their mechanical prop-erties in both linear and nonlinear ranges as well as theoptical properties of the ensuing films were analyzed.Irrespective of their origin, the incorporation of the NFCwithin the polymer matrix brings about a huge reinforc-ing effect above the glass transition. The percolationapproach has been shown to give a reasonably accurateprediction of the stiffness over the whole range of vol-ume fraction investigated. The evolution of the magni-tude of tan d vs. the NFC content was also analyzed anddiscussed in term of the effective interfacial thickness.The optical transparency of the nanocomposite film atdifferent NFC content was also studied and comparedaccording to the NFC origin. POLYM. COMPOS.,34:1999–2007, 2013. VC 2013 Society of Plastics Engineers

INTRODUCTION

During the last few decades, nanosized based cellulose

particles have become the focus of a rising interest in

both scientific and industrial communities [1–3]. The sus-

tainable character of cellulose, the relative ease of extrac-

tion of nanozised fibrils from cellulose fibers along with

their outstanding mechanical properties drive toward the

use of these new class of renewable nanofiller in nano-

composite materials [4].

Basically there are two families of nanosized cellulosic

fibrils, the first of them is the cellulose nanocrystals [5]

(CNCs) and the second one is microfibrillated cellulose

(MFC) also termed nanofibrillated cellulose [6] (NFC).

The former are rod-like shaped particles with typical

dimension ranging from 5 to 10 nm in diameter and from

100 to 500 nm in length. The basic approach in the isola-

tion of CNCs is the acid-catalyzed hydrolysis of the

amorphous phases of cellulose leaving the highly ordered

and regular rod-like nanocrystals. On the other side, NFC

with diameter in the range of 10–100 nm and lengths

within micronic scale might be viewed as bundles of ele-

mentary cellulose fibrils embedded in hemicelluloses

matrix. Each elementary fibril is composed of 30 to 40

extended cellulose chains with a square cross-section of

3–5 nm in size.

Among these two types of nanosized cellulose, NFC is

more easily-prepared at high yield. It has recently gained

much attention due to its potential use in many applications

such as reinforcement nanofiller for polymer matrices [6,7],in paper and board industry [8], and in high barrier packag-

ing materials [9]. In the field of material science, cellulose

nanofibers are of high interest as nanofillers to elaborate

nanocomposites with enhanced mechanical properties[10,11]. This meanfull improvement was explained by the

high aspect ratio of the nanofibers, their high crystallinity

degree and high Young’s modulus, as well as their aptitudeto set up entangled networks held through strong hydrogen

bonding. Additionally, as the width of the cellulose nanofi-

brils is lower than one-tenth of the visible light wavelength,

the optical transparency of the matrix is expected to be pre-served, as long as the nanofibrils aggregation is prevented

during the nanocomposite processing. This feature is of

extreme importance for transparent applications.

Although wood is the main source of cellulose fibers,

annual plants and agricultural residues may also constitute

a starting material to extract NFC. Compared to wood,

annual plants have several advantages, among which, one

can cite their high yield of cellulose, lower lignin content

compared to wood [12] and a short growing cycle. The

use of NFC from different non-woody plants has been the

subject of various research works [13–15], however, a

comparative analysis of their reinforcing potential has not

been undertaken. By using the same extraction procedure,

the reinforcing capacity of NFC from different origins

can be compared and related to their morphological

properties.

Correspondence to: Sami Boufi; e-mail: sami.boufi@fss.rnu.tn

DOI 10.1002/pc.22607

Published online in Wiley Online Library (wileyonlinelibrary.com).

VC 2013 Society of Plastics Engineers

POLYMER COMPOSITES—2013

In a previous work a comparative investigation on the

nanofibrillation behavior of different non-woody plants

was carried out, and the extracted NFC were character-

ized [16]. In the present work, the reinforcing potential of

NFC extracted from abaca, sisal, hemp, jute and flax was

studied and correlated with the morphology of the NFC.

The optical transparency of the nanocomposite film at dif-

ferent NFC content was also studied and compared

according to the NFC origin.

MATERIALS AND METHODS

Materials

The non-woody plants were acquired from different

origins: Flax and Hemp were from France, Jute from

Bangladesh, Sisal from Tanzania and Abaca from Philip-

pines. The 2,2,6,6-tetramethyl-piperidine-1-oxyl-radical

(TEMPO), sodium bromide, sodium hypochlorite solution

(NaClO) and sodium chlorite (NaClO2) were purchased

from Sigma–Aldrich and used as received without further

purification. The polymer acrylic latex dispersion is a

commercial product from MPC-PROKIM.

Preparation of Cellulose Fibers

The extraction of the fibers from the non-woody plants

was carried out according to the following steps: the orig-

inal fibers were cut down to 2–3 cm length and cooked in

a digester with a solution of NaOH (16–17% in volume)

at a temperature of 165�C during 1–2 h, until a kappa

number of 7–8. The ensuing pulp was washed three times

with water and submitted to a bleaching treatment using

sodium chlorite (1.5% NaClO2) at pH 4 to remove the

residual lignin. Afterward, the pulp was dried before use.

Degree of Polymerization (DP)

The measurements were performed on dissolved fibers in

cupriethylendiamine (CED), based on the standard ISO

5351-1 method. The measurements of intrinsic viscosity gwere performed with an automatic viscosimeter. The

obtained intrinsic viscosities were converted into the respec-

tive values of DP according to the following equation [17]:

DP0:905 5 0:75g

TEMPO-Mediated Oxidation

The TEMPO-mediated oxidation was carried out fol-

lowing the procedure described elsewhere [18].

Fibrillation Process

The oxidized fibers were dispersed in water at 1–2

wt% consistency and pumped through a high pressure

homogenizer (NS1001L PANDA 2K-GEA). The homoge-

nization was conducted in two steps. First, the fiber sus-

pension was passed five to seven times at 300 bar until

the viscosity of the slurry increased, and then the fibrilla-

tion was pursued by 10 passes at a pressure of 600 bar.

The ensuing product was a high viscosity translucent gel.

The operating temperature was not controlled and reached

60–70�C when the pressure was raised to 600 Bar. The

fibrillation was performed under neutral pH.

Yield in Nanofibrillated Cellulose

A dilute suspension with about 0.1% of solid content

(Sc) was centrifuged at 4000 rpm for 20 min to separate

the nanofibrillated material (in supernatant fraction) from

the nonfibrillated or partially fibrillated ones, which settle

down. Then, the sediment fraction was dried to a constant

weight at 90�C in a halogen desiccator. The yield was

calculated from Eq. 1:

Yield %5 12weight of dried sediment

ðweight of diluted sample 3%ScÞ

� �3100

(1)

The results represent the average values of the three

replications.

AFM Observation

AFM images were obtained in the intermittent contact

mode at room conditions using a multimode scanning

probe microscope from Nanoscope IIIa electronics (Digital

Instruments). For the AFM observation, a drop of diluted

NFC suspension, with a solid content of about 0.02–0.05%

was then deposited onto freshly cleaved mica.

Nanocomposites Processing

A commercial latex (PROKIL S330P-MPC-PROKIM-

Tunisia) obtained by the copolymerization of styrene (35

wt%) and butyl acrylate (65 wt%) was used as a matrix.

The size of the polymer particles was around 140 nm and

the solid content 50 wt%. The glass–rubber transition

temperature (Tg) of the poly(S-co-BuA) copolymer was

about 25�C.

The NFC gel was mixed with the latex in order to

obtain nanocomposite films with weight fraction of cellu-

lose ranging from 0 to 15%. After stirring for 1h, the

mixture was cast in a Teflon mould and stored at 40�Cuntil water evaporation was completed. A transparent to

translucent film, depending on the NFC content, was

obtained with a thickness in the range of 300–400 lm.

Dynamic Mechanical Thermal Analysis (DMTA)

The dynamic mechanical analysis (DMA) was con-

ducted in tension mode using a PYRISTM Diamond DMA

2000 POLYMER COMPOSITES—2013 DOI 10.1002/pc

(Perkin-Elmer, Waltham, MA) equipment. Temperature

scans run from 250�C up to 100�C at a heating rate of

2�C min21, a frequency of 1 Hz and amplitude of 10 lm.

The storage (E0) and the loss (E00) modulus, as well as the

loss factor tan d 5 (E00/E0), were measured as a function

of the temperature. Sample dimensions were about 20

mm (length), 10 mm (width), and 0.3–0.5 mm

(thickness).

Tensile Tests

The nonlinear mechanical behavior of the films was

analyzed using an Instron testing machine in tensile

mode, with a load cell of 100 N at a strain rate of

5 mm�min21 and at 25�C temperature. The specimens

were obtained using a cutting device.

Transparency Measurement

The transparency of neat acrylic film and nanocompo-

site films were measured at wavelengths from 200 to 800

nm using a UV–visible spectrometer (Lambda 35, Perkin-

Elmer.). Transmission spectra of the films were recorded

using air as reference.

RESULTS AND DISCUSSION

Morphological and Characteristic Features of NFC FromNonwoody Plant

The morphology of the NFC extracted from the differ-

ent annual plant was analyzed by AFM observation, from

which the width and length was estimated. As shown in

Fig. 1, all of the NFC samples showed nanosized fibrils

with a width in the range of 10–50 nm, and exhibited a

high potential to build up an entangled network through

hydrogen bonding. However, if we compare the aspect

and the morphology of the NFC obtained from the differ-

ent plants, two different groups can be distinguished: The

first one encompasses sisal and abaca, both of which

show high fractions of individualized thin fibrils with a

wide distribution in widths close to 20 nm and relatively

short lengths in the range of 400 nm up to 1 lm. The sec-

ond category includes NFC from hemp, jute and flax,

with fibril diameters in the range of 30–100 nm and

lengths exceeding several microns.

The second aspect worth to note is the difference in

the yield of fibrillation according to the fibers origin. On

the basis of our previous study [16], this difference was

attributed to the difference in their hemicellulose content

after the delignification and the bleaching processes; the

higher the hemicellulose content, the higher the yield in

nanofibrillated material is.

Dynamic Mechanical Thermal Analyses (DMTA) of theNanocomposite Film

To investigate the reinforcing potential of the NFC

from the non-woody plants, nanocomposite films with

nanofiller loading ranging from 1 up to 10 wt% were pre-

pared by solvent casting, and their mechanical behavior

analyzed by DMTA. The temperature dependence of the

storage modulus E0 at 1Hz for the unfilled matrix and the

nancomposite films with different NFC content is shown

in Fig. 2a. The effect of the inclusion of NFC into the

polymer matrix is different depending on the temperature

domain. A huge enhancement in the modulus is observed

above the glass transition (Tg) when the polymer matrix

is in the rubbery state, while the increment in modulus is

much more modest below the Tg. This behavior is com-

mon to nanocomposite materials based on nanosized cel-

lulose. In fact, when the matrix is in the glassy domain,

the fairly small difference between the modulus of the

glassy matrix (close to 2 GPa) and the NFC network (in

the range of 10–15 GPa) may be the reason for the lower

stiffening effect observed below glass transition. On the

other hand, the huge reinforcement effect above Tg is

explained by the formation of a stiff rigid network ensu-

ing from the nanofibrils entanglement and from the nano-

fibrils’ bonded area strongly interacting through hydrogen

bonds [6,10]. However, the setting up of this network is

tributary of the good dispersion of the cellulose nanofiller

within the polymer matrix. The selection of the casting

mode to prepare the nanocomposite film ensures good

nanofibrils dispersion and prevents their aggregation dur-

ing the film-formation process.

To compare the reinforcing potential of the different

NFC, the evolution of the storage modulus E0 at 70�C(about Tg130�C) versus the NFC loading of the nano-

composite films is plotted in Fig. 3. The results show a

FIG. 1. AFM images of the NFC from different origins used in the

present work. [Color figure can be viewed in the online issue, which is

available at wileyonlinelibrary.com.]

DOI 10.1002/pc POLYMER COMPOSITES—2013 2001

significant rose in the storage modulus, in line with the

strong reinforcing capacity of the as-prepared NFC. For

instance, at 70�C, nanocomposite containing 5 wt% NFC

from sisal exhibits a storage modulus around 60 MPa,

which represents a 450-fold enhancement over that of the

neat matrix (0.133 MPa).

The establishment of interconnected networks has been

put forward as a plausible mechanism to account for the

huge increase in the modulus, exceeding those modeled

by mean-field results e.g., Halpin–Kardos [19]. The con-

cept of percolation in cellulose rod-like cellulose nano-

crystals (CNC) based nanocomposite has been

successfully adopted to account for the outstanding rein-

forcing effect at low concentration of CNC (below 10

wt%) in rubbery-like polymer [20]. In the present work,

the concept of percolation was adopted in order to check

whether this concept could be extended to NFC based

cellulose nanofiller. On the basis of this concept, the elas-

tic tensile modulus Ec of the composite can be expressed

by the following equation:

Ec5122w1w/ð ÞEsEr1 12/ð ÞwE2

r

12/ð ÞEr1 /2wð ÞEs

(2)

where w can be written as:

w50 For / < /p (3)

w5/:/2/p

12/p

!b

For / � /p(4)

w, / and b are the volume fraction of percolated net-

work, the total volume fraction of the nanofiller and the

FIG. 2. Evolution of (a) the storage tensile modulus, E0, (b) loss modulus, and (c) tangent of the loss angle,

tan d, versus temperature at 1 Hz for nanocomposites based on NFC extracted from abaca fibers. [Color fig-

ure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

FIG. 3. Evolution of the storage modulus versus NFC content at 70�Cfor nanocomposite films prepared from NFC obtained from the different

plant and acrylic latex: comparison between the experimental data

(filled), and predicted data according to the Percolation model (—).

[Color figure can be viewed in the online issue, which is available at

wileyonlinelibrary.com.]

2002 POLYMER COMPOSITES—2013 DOI 10.1002/pc

critical exponent, respectively. Es and Er are the modulus

of the cellulose network and the matrix, respectively. The

modulus of the cellulose network Er, which differs from

that of isolated nanofibrils, was determined from DMTA

analysis using thin sheet films of neat NFC obtained by

casting a suspension of NFC in a Teflon mould followed

by drying at 40�C. The Er values for the different NFC

are reported in Table 2, from which one can note a higher

value for abaca NFC. A possible reason of the stiffer net-

work for abaca NFC reside on the higher hemicellulose

content that contributed to enhance the adhesion between

nanofibers in the dried state, leading to an improvement

in the stiffness, and strength of the network. The impor-

tance of hemicelluloses for the paper strength was pointed

by several publications, where it was shown that fibers

with higher hemicellulose content showed higher stiffness

and tensile strength compared to pulp with lower hemicel-

lulose content [21].

The percolation volume threshold (/p) can be deter-

mined by applying a power law function to the E0 versus

NFC loading according to Eq. 5, as predicted from the

percolation theory [22,23].

E0

//

/2/p

12/p

!b

For / > /p(5)

Applying this concept to the NFC of different origins,

the percolation threshold /p was adjusted to the value

giving the best linear regression for E0=/ vs./2/p

12/p

� �on a

log–log scale (Fig. 3: see inset). For the different nano-

composite, the model provides a good linear fitting with a

correlation regression higher than 0.99 and with a value

of /p comprised between 0.25 and 0.4 depending on the

NFC origin (Table 2). Furthermore, from the linear

regression, the critical exponent coefficient was deter-

mined, and found to be within the range of 1.6–1.8,

which is in agreement with the predicted value of 1.6–2

for a 3D percolated network [24].

The percolation threshold for the different NFC was

found to be within the range of 0.25–0.4 vol% which is

lower than the one commonly found for CNCs, the value

of which exceeds 1 vol%. This lower value is likely the

consequence of the higher aspect ratio of NFCs compared

to CNCs, although the former are prone to curvature and

curling effect. Indeed, the percolation threshold is known

to be inversely proportional to the aspect ratio of the dis-

persed objects [25]. Moreover, if we compare the four

types of NFC, one can note a higher value of /p for

abaca and sisal compared to that of flax and hemp. This

disparity is likely the consequence of a difference in the

NFC morphology. Indeed, referring to the AFM observa-

tions, a lower length for NFC from abaca and sisal was

noted compared to those from hump and flax.

From Fig. 3, one can also note a good agreement

between the experimental values of the storage modulus

E0 and those calculated on the basis of the Percolation

model (Eq. 2) using the corresponding Er. This confirms

again that the filler–filler interaction is the key parameter

that account for the huge reinforcing effect of NFC based

nanofiller. The validity of the percolation approach is also

indicative of the good dispersion of the nanofiller within

the host matrix.

Analysis of the a Relaxation

For all the nanocomposites as well as the neat matrix,

the tan d plot the exhibits a well-defined relaxation (arelaxation) around 28–34�C, associated to the cooperative

motions of long chain sequences (Fig. 2b). The inclusion

of NFC resulted in a continuous drop of the magnitude at

the maximum of tan d, along with a shift towards lower

temperatures of the peak of tan d. However, the half-

height width of the relaxation a did not undergo signifi-

cant change with the content of NFC.

We should note that the shift toward lower temperature

of the tan d peak is likely a simple coupling effect and

did not reflect a shift in the glass transition. This hypothe-

sis is supported by the invariability in the position of the

maximum position of E00 vs. temperature and also by

DSC analysis (result not shown) confirming a constant

position of the glass transition. In fact the inclusion of

nanoparticle in polymer matrix can induce a substantial

deviation in Tg relative to the bulk polymer; decreasing

when polymer–nanofiller interfacial interaction is absent

and increasing when attractive interaction between the fil-

ler and the polymer is present [26,27]. Accordingly, the

insensitivity of the Tg position to the NFC content might

be suggestive that interaction between the cellulose nano-

filler and the acrylic matrix is likely to occur, at least to

some extent. Actually, given the presence of the ACOOA

TABLE 1. Physical characteristics of the NFC used in the present

work.

Sisal Abaca Jute Hemp Flax

Yield of fibrillation (%) 95 88 85 78 69

Hemicellulose content (%) 20 14 13 8 6

Crystalline index of NFC (%) 61 60 66 78 78

Intrinsic viscosity (ml g21) 697 1128 704 725 636

DP 1008 1716 1020 1053 911

NFC width (nm)a 10–20 10–30 20–50 20–40 50–100

aDetermined from AFM observation.

TABLE 2. The critical exponent, the percolation threshold and the

modulus of the cellulose network.

Plant b /p (vol%) R2 Er (GPa)

Abaca 1.8 0.3 0.996 17

Sisal 1.62 0.4 0.995 9.4

Flax 1.78 0.25 0.997 9.3

Hemp 1.65 0.25 0.997 9

DOI 10.1002/pc POLYMER COMPOSITES—2013 2003

group within the polymer backbone, a possible polar

interaction between the surface hydroxyl groups of the

NFC and the polymer matrix is not excluded.

In nancomposite, the damping properties (tan d) which

measure the energy dissipation of the material provide

useful indications about the changes in the molecular

mobility of the polymer matrix induced by the nanofiller

inclusion. The dependence of the maximum tan d vs.

NFC content may be highlighted from the evolution of E0

and E00 vs. NFC content at different temperature, namely

lower than Tg, around Tg and above Tg (Fig. 4). Below

Tg, both of E0 and E00 remained roughly constant and

independent from the NFC content with E0>E00, and a

low damping which is a consequence of the glass-like

behavior of the matrix. Around Tg, the polymer matrix

reaches the maximum viscoleastic properties that is a con-

sequence of the release of large scale molecular motion

responsible of the huge dissipation effect: E00 is higher

than E0 and a maximum in tan is observed. The inclusion

of NFC within the polymer matrix brings about a continu-

ous increase of both E00 and E0 with a greater effect on

E0, leading to a prominent drop in the magnitude of tan d.

As mentioned above, the enhancement in E0 is the result

of the network formed by the entangled cellulose nanofi-

brils. The increase in E00 is also an indirect consequence

of the change in the viscoleastic properties of the material

from a liquid-like to a solid-like behavior as the content

of the nanofiller exceeds the percolation threshold

[28,29].

The loss factor might be also analyzed in terms of the

contribution of the filler, the polymer matrix and the

interface pondered by their corresponding volume fraction

[30,31] as follows:

tan dc5/f tan df1/itan di1/ptan dp (6)

where subscripts c, f, i, and p stand for the nanocompo-

site, filler, interface, and polymer matrix, respectively.

This expression is a simple rule of mixtures taking

account of the filler, the polymer matrix and the inter-

phase which may be a source of new damping mecha-

nisms. Indeed, given the nano-scale of the nanofiller, the

addition of even a small volume fraction of nanoparticles

introduces a large amount of interfacial area with proper-

ties being different than those of the polymer matrix. This

led to the formation of an immobilized polymer layer sur-

rounding the filler particles, the presence of which indi-

rectly increases the effective filler volume fraction in the

nanocomposite and causes a mechanical coupling effect

between the matrix and the filler. Assuming the nanofibri-

lar cellulose to be perfectly elastic, and taking account of

the interfacial thickness of the interphase DR, Eq. 6 may

be rewritten with the introduction of a correction parame-

ter P [32]:

tan dc5tan dp 12P:/fð Þ (7)

where P5 11DR

R

� �2

(8)

Taking into account Eq. 8 and the diameter of NFC,

the effective interfacial thickness (DR) was calculated for

NFC from abaca. As shown in Table 3, there is a general

decreasing trend of DR with respect to the volume frac-

tion, presumably indicating that as the volume fraction

increases, there is a greater chance in the overlap between

the interfacial thicknesses of adjacent cellulose nanofib-

ers. Additionally, the pronounced aptitude of cellulose

fibrils to self-interact through hydrogen bonding forming

hard entangled network contributed to reduce the fraction

of the available interfacial area.

Tensile Properties of Nanocomposite Films

The nonlinear tensile mechanical properties of the

nanocomposite films with different content of NFC were

studied at room temperature. A ductile polymer matrix

with a neat Tg around 210�C was chosen in order to

reach the limit strength without premature breaking of the

sample due to the excessive rigidity. Moreover, in all

cases, three samples of each specimen were tested and

average values reported. Typical stress versus strain

curves with different NFC content is plotted in Fig. 5.

FIG. 4. Change in E0 and E00 at 0, 30, and 70�C vs. NFC content:

(NFC were from sisal). [Color figure can be viewed in the online issue,

which is available at wileyonlinelibrary.com.]

TABLE 3. Summary of DMTA results for nanocomposite prepared

from sisal NFC.

NFC content Tga (in �C) tan d L DR (nm)

0 21 2.39 22 –

1 21 1.17 20 95

2 21 0.75 24 74

3 21 0.51 20 63

5 21 0.4 19 47

7 20.6 0.35 18 39

10 20 0.27 20 31

aPosition corresponding to the maximum of E00.

2004 POLYMER COMPOSITES—2013 DOI 10.1002/pc

To compare the reinforcing potential of the different

NFC, the evolution of the tensile modulus and tensile

strength vs. the NFC content are plotted in Fig. 6a and b,

respectively. It can be observed that both of the tensile

modulus and the tensile strength dramatically increased

with the inclusion of the NFC. For instance, with 10 wt%

NFC from abaca, tensile strength went from 0.36 to 9

MPa which is a more than 25 fold higher than that of the

neat matrix. Even a loading as low as 1 wt%, which is

only 0.6% by volume, brought about seven fold enhance-

ments in the tensile strength over that of the neat matrix.

When we compare the reinforcing potential of the different

NFC, one can note that NFC from flax gives rise to the highest

level of tensile strength, though they exhibit the lowest yield in

nanofibrillated material (NFC yield about 70%). On the other

hand, NFC from sisal, being the most fibrillated (yield about

95%), imparted the lowest level of strength, regardless of the

NFC content. For jute and abaca, the strength is roughly the

same being at a level intermediate between flax and sisal. This

behavior seems unexpected if we consider that the high rein-

forcing aptitude of NFC arises from their nanosized scale.

However, referring to AFM observation, it can be noted that,

compared to sisal, NFC from flax exhibits a higher potential to

form a dense entangled network, probably due to the differ-

ence in the length of the cellulose fibrils. This hypothesis sup-

ported by AFM observation, may also explain the higher

percolation threshold noted for sisal compared to flax or hemp,

i.e., 0.4 vol% for sisal against 0.25 vol% for flax and 0.2 vol%

for hemp.

Optical Properties of the Nanocomposite Films

According to Rayleigh and Mie scattering law, the

scattering loss through the optical pathway in a composite

material depends mainly on the relative size (d/k) and the

refractive index of the dispersed phase, with respect to

the refractive index of the surrounding medium. Given

the fact that the refractive index of NFC (1.58) did not

entirely match with that of the host matrix (1.48), the

transparency of the NFC based nanocomposite film will

governed by the light scattering from the dispersed NFC.

For this reason, the critical factor likely to control the

transparency degree of the nanocomposite film will be

the size of the cellulose nanofibrils within the polymer

matrix. Considering the width of the cellulose fibrils

being lower than several tens of nanometers, and assum-

ing the aggregation during the drying process is pre-

vented, we could expect a high degree of transparency of

the nanocomposite film.

The optical transparency of the nanocomposite films with

200–300 lm in thickness were analyzed by transmittance

measurement in the visible wavelength range. To avoid the

effect of the variation of the film thickness, the film trans-

mittance was normalized to a 200 lm thickness using the

Beer–Lambert law. To compare the optical transparency of

the nanocomposites according to the NFC content and origin,

the transmittance value at 700 nm was used as an indicator.

From Fig. 6b, one can note that the transmittance value

remained higher than 80% as the NFC content is lower than

7 wt%, which is indicative of a good transparency. At the

NFC content exceeds 10%, the transmittance undergo a sig-

nificant drop to about 74–65% according to the NFC origin.

FIG. 5. Typical stress–strain curve for nanocomposite films based on

NFC extracted from sisal fibers.

FIG. 6. Evolution of the (a) tensile modulus and (b) tensile strength of

the nanocomposite film with the NFC content. [Color figure can be

viewed in the online issue, which is available at wileyonlinelibrary.com.]

DOI 10.1002/pc POLYMER COMPOSITES—2013 2005

The lower transparency observed in flax NFC nanocompo-

sites at contents exceeding 7 wt% is probably the conse-

quence of the higher amount of partially fibrillated material

(about 30%) with size being within the microns scale, and

also due to their relatively large width ranging from 20 up to

100 nm (Fig. 1).

On the other hand, although NFC from abaca was the

most fibrillated and displayed the smallest width, their

nancomposite film exhibited the lowest transparency. This

unexpected result might result from the fibrils clustering

during the water evaporation and the coalescence of the

polymer particles.

CONCLUSION

NFC is known to convey a huge reinforcing potential

when it is incorporated in a polymer matrix in a way that

nanofibers aggregation is avoided and the hydrogen inter-

action among the cellulose nanofiller is promoted.

In the present work, the reinforcing potential of NFC

from abaca, hump, flax, and sisal was investigated. Nano-

composite films were prepared by casting and evaporating

a mixture of NFC suspension and acrylic latex dispersion.

The visual observation of the prepared nanocomposite

film has shown a highly transparent material up to NFC

content of 7%, which is a good indication that nanofibers

aggregation was prevented during film-formation.

The DMTA study has indicated a noticeable enhance-

ment in the storage modulus of the nanocomposites above

the glass transition, which agrees with the well-known

behavior of the cellulose based nanofiller. The glass tran-

sition temperature (Tg) of the polymer was not signifi-

cantly influenced by the incorporation of the cellulose

nanofibers.

The huge enhancement in the stiffness was attributed

to the formation of a rigid entangled network held by

strong hydrogen bonds which favor the stress transfer

from the matrix to the nanofiller network. The formation

of this network was assumed to be governed by a percola-

tion approach which has been found to fit adequately to

the experimental data over the whole range of volume

fraction investigated.

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[Color figure can be viewed in the online issue, which is available at

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