Composites Science and Technology 98 (2014) 72–78

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Composites Science and Technology

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Nanofibrillated cellulose reinforced acetylated arabinoxylan films

http://dx.doi.org/10.1016/j.compscitech.2014.04.0100266-3538/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author at: WWSC, Wallenberg Wood Science Center, ChalmersUniversity of Technology, Kemivägen 10, SE-412 96 Göteborg, Sweden. Tel.: +46317723407.

E-mail address: paul.gatenholm@chalmers.se (P. Gatenholm).

Agnes M. Stepan a,b, Farhan Ansari a,c, Lars Berglund a,c, Paul Gatenholm a,b,⇑a WWSC – Wallenberg Wood Science Center, Chalmers University of Technology, Kemivägen 10, SE-412 96 Göteborg, Swedenb Biopolymer Technology, Department of Chemical and Biological Engineering, Chalmers University of Technology, Kemivägen 10, SE-412 96 Göteborg, Swedenc Department of Fiber and Polymer Technology, Royal Institute of Technology, SE-10044 Stockholm, Sweden

a r t i c l e i n f o

Article history:Received 26 October 2013Received in revised form 5 April 2014Accepted 13 April 2014Available online 21 April 2014

Keywords:A. Flexible compositesA. NanocompositesB. Thermomechanical propertiesB. Transport propertiesBiobased composite

a b s t r a c t

In this study, acetylated rye arabinoxylan (AcAX) films were reinforced with nanofibrillated cellulosefrom spruce (NFC) ranging from 1 to 10 wt% of the total composition. Free-standing composite films werecasted without the use of any plasticizers. The homogeneous dispersion of NFC in the films was con-firmed with scanning electron microscopy. The ultimate strength and the Young’s modulus determinedby tensile tests increased from 65 MPa and 2190 MPa for neat AcAX films to 93 MPa and 3360 MPa for the10% composite films, respectively. The elongation to break of the 10% NFC composite film was a remark-able 10.5%. The moisture absorbed was still less than 8 wt% for the films with 10% NFC content at 97%relative humidity at room temperature, which is low for hemicellulose-based films. The addition ofNFC decreased the water permeability of the films at low NFC contents, which was studied in diffusioncells using radioactive labeled water. Thus NFC can be used in an unmodified form as reinforcement inAcAX films to prepare films or coatings that are more water and humidity resistant than neat hemicellu-lose-based films.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

There is increasing social and economical pressure that is drivingpackaging industries to turn towards more sustainable solutionsregarding their material resources. Plant polysaccharide basedfilms, particularly those from cellulose or hemicelluloses, have agreat potential to provide alternative biobased films to replace someof the non-renewable, fossil fuel based plastics [1–3]. Hemicellu-loses are the second most abundant plant biopolymer on earth aftercellulose. Pulp and paper processing and cultivation of cereal cropscontinuously produce an enormous amount of byproducts with ahigh hemicellulose content. These unutilized biomasses could be apotential future resource for hemicelluloses with adequate isolation[4–9]. Xylans are a group of hemicelluloses that have recentlygained significant attention due to their excellent film-forming abil-ities and unique oxygen barrier properties. These hemicelluloses,being hydrophilic polymers, are mostly water soluble, which resultsin the films being highly sensitive to water and moisture [10–14].Although the hydrophilic properties of xylans possibly contributeto their excellent oxygen barrier properties, the films have

limitations in applications due to their water sensitivity and needprotection against water and humidity. Derivatization of hemicellu-loses by for example acetylation can be used to prevent water solu-bility and reduce moisture sensitivity [12,15,16]. Acetylation of thexylans also results in increased flexibility and potentially also makesthe polysaccharides thermoplastic [16,17]. The mechanical proper-ties of the polymers can be optimized by an addition of fibers or fill-ers. Nanocellulose has been at the center of interest for use asreinforcement in composites due to its unique structural and phys-ical properties, such as high strength and stiffness combined withlow weight, biodegradability and renewability [18,19]. Further-more, due to its nano-scale dimensions, nanocellulose presents highlight transmittance when incorporated into films [20]. These prop-erties combined with its potential economic advantages can offerthe possibility to make lighter, strong materials with greater dura-bility [21]. The microfibrillated cellulose (MFC) is cellulose openedto its substructural microfibrils, which generally consist of elemen-tary fibril bundles in the order of tens of nanometers in thicknessand can be several micrometers in length [22]. MFC has morerecently been referred to in the literature as nanofibrillated cellu-lose (NFC) [23,24], in order to emphasize the scale of the fibril diam-eter. To achieve NFC with more uniform and smaller particle sizemechanical shearing can be combined with chemical and enzymaticpretreatments, which also contribute to lower processing energyand costs [25].

A.M. Stepan et al. / Composites Science and Technology 98 (2014) 72–78 73

Nanocomposites have the advantage of attaining superior ther-mal, mechanical and barrier properties compared to the neatmatrix material, at reinforcement levels as low as 5% or less [26].In recent years, several publications have targeted the preparationof composites from hemicelluloses and NFC/MFC and improvedmechanical properties with different types of nanocellulose rein-forcement have been reported [27–30]. For example, strongerand stiffer films were produced from rye AX and bacterial nanocel-lulose, and from galactoglucomannans and MFC [28,30]. However,these films also have limitations in performance and processing.Brittleness and low heat distortion temperature, along with themoisture sensitivity of the materials, are among the major chal-lenges for these bio-nanocomposites. The potential of NFC net-works to resolve the brittleness problem was demonstrated forplasticized starch [31]. The thermoplastic properties of the acety-lated polysaccharides might provide a solution to some of theseissues, including moisture sensitivity [17,32].

Another important aspect of the polymer matrix in nanocellu-lose composite preparation is the hydrophobic/hydrophilic natureof the matrix. In composites the dispersion and good adhesionbetween the fibers and the polymer matrix highly depends onthe hydrophilicity of the matrix [15,33–35].

The aim of this study was to prepare all-plant-based compositefilms from AcAX reinforced with NFC. In particular, preparation offilms with improved thermal and mechanical properties and lowwater sensitivity was targeted. The contribution of NFC and AcAXcomponents to properties is discussed.

2. Experimental

Rye arabinoxylan (AX) was purchased from Megazyme (�95%purity, LOT 20601a, Ireland). The arabinose to xylose ratio was0.52 as determined by ion chromatography, published elsewhere[16]. Acetic anhydride (Ac2O) and pyridine were purchased fromSigma–Aldrich and hydrochloric acid (35%) from VWR Interna-tional S.A.S. (France). The dimethyl formamide (DMF) was pur-chased from VWR, Ethanol (95%) was purchased from Solveco.All chemicals were used without further purification.

2.1. Acetylation of the rye AX

Esterification of arabinoxylan was carried out based on experi-mental conditions described in detail elsewhere [16]. The drywhite powder of AX was suspended in formamide which was fol-lowed by the addition of pyridine. The reagent was acetic anhy-dride and the reaction was running on room temperature for30 h in total. The reaction was precipitated in ice-cold, dilutehydrochloric acid and the precipitate was washed with water,methanol and diethyl-ether. For AX from rye, the maximum degreeof substitution is 2, meaning that in average there are 2 availablehydroxyl groups per sugar monomer unit that can be substitutedwith acetyl groups. The degree of substitution is determined mostcommonly by 1H NMR by comparing the relative intensities of thesignals of the acetyl groups (at 2 ppm) and those of all carbohy-drate signals (3.0–5.6 ppm) [12]. The described acetylation methodhas been proven to result in a maximum degree of acetylation, 2.for AX according to literature measured using [16].

2.2. Preparation of NFC

NFC was prepared from never-dried softwood sulfite pulp(13.8% hemicelluloses and 0.7% lignin, Nordic paper – Sweden)according to a previously reported method [25]. The processincluded enzymatic pretreatment and mechanical beating of pulpfollowed by 8 passes through a Microfluidizer (Microfluidics,

USA), which gave a gel like suspension with ca. 2 wt% NFC in water.This preparation of nanofibers results in fibrils with a width of 15–30 nm and a length of several micrometer with a fraction of shorterand thinner (5–10 nm) nanofibers [25]. The crystallinity of theseNFC lies in the range of 50–70% [36].

The NFC-water suspension was diluted 10 times with DMF andthe suspension was thoroughly homogenized with an Ultra Turrax(model D125 Basic, IKA, Germany) for 10 min at 10,000 rpm. Thesuspension was then centrifuged at 4754g for 20 min. This processwas repeated 4 times to ensure efficient exchange from water toDMF.

2.3. Preparation of films

Preweighed amounts of DMF suspension of NFC and dry AcAXpowder were mixed and further diluted to 20 ml with DMF for easydissolution. Suspensions were prepared with different NFC con-tents (1, 3, 5, 10 wt% of total dry matter). The suspensions weresubjected to magnetic stirring for 2 days (which was found to bethe optimum stirring time), followed by mechanical mixing withUltraturrax (model D125 Basic IKA, Germany) until homogeneityat 12,000 rpm. They were then degassed in vacuum and cast inglass petri dishes. The films were dried for 24 h at 60 �C and thenfor 4 h under vacuum at 40 �C. The films were moved to a condi-tioning room with 50% relative humidity (RH) and 23 �C for 2 days.These samples were then subjected to further analysis.

2.4. Contact angle and critical surface energy measurements of filmcomponents

The static contact angles of a neat AcAX film and a neat NFC film(which was prepared by filtration of solution until dry) weremeasured with an NRL C.A. Goniometer from Ram e-hart (model100-00 230) with a lamp from LEP (model 990018) with 3 differentliquids (ethylene glycol, diiodomethane and deionized water). Adrop of liquid was placed on the surface of the films, and the con-tact angle was read after 30 s of contact. 5–10 measurements weremade on each sample, and average values were calculated. Basedon the contact angle data, the total surface tensions of AcAX andNFC were calculated by determining their polar and dispersivecomponents with the SCA202 v.4.2.3 (Dataphysics Instruments)software based on the Owens, Wendt, Rabel and Kaelble method[37].

2.5. Mechanical properties

The tensile properties of the films with varying contents of NFCwere evaluated on an Instron 5944 with a 500 N load cell. Rectan-gular specimens with the dimensions 40 � 4 � 0.05 mm weretested with a gauge length of 20 mm at a strain rate of 10%/min.Prior to testing, the samples were conditioned at 23 �C and 50%RH for 2 days, and the tests were run at these conditions. Averageproperty data from 4 to 6 specimens for each composition arereported. Modulus was determined from the slope of the curvebelow 0.2% strain. Strain was determined from the grip positioncalibrated for machine compliance effects. Tensile strength wasdetermined from the maximum tensile stress based on the initialcross section area.

To predict the modulus, a simple rule of mixture was employedbased on the modulus of a random in plane NFC nano paper (np)and the neat AcAX [38]. The weight fractions were first convertedto volume fractions using a density of 1 for AcAX and 1. 5 for NFC inthe following equation:

Ec ¼ Enp � Vnp þ EAcAX � VAcAX ð1Þ

74 A.M. Stepan et al. / Composites Science and Technology 98 (2014) 72–78

where Ec, Enp, EAcAX refer to the modulus of composite, nanopaperand acetylated xylan, respectively and Vnp, VAcAX refer to the volumefraction of nano fibrils and acetylated xylan respectively. The use ofnanopaper model is a simplification but the purpose is to make arough estimate of reinforcement mechanism.

A DMA-Q800 (TA Instruments) operating in tensile mode wasused to measure dynamic mechanical properties. The sample sizewas 15 � 5.7 � 0.05 mm, cut by a die with parallel razor blades.The measuring frequency was 10 Hz with an amplitude of 5 lm.Temperature scans were made after an initial drying step (ramp30–80 �C with a rate of 2 �C/min, holding at 80 �C for 30 min)and conditioning section (isothermal status at 30 �C for 45 minafter cooling to 30 �C at the same rate). The temperature scan cov-ered the interval of 30–250 �C with a heating rate of 2 �C/min. Inthe case of sample failure, the experiment was stopped earlier.Two or three parallel measurements were averaged per sample.

The samples were also subjected to isothermal tests at 25 �Cand ramping of the surrounding RH from 0% to 90% at a rate of1%/10 min using the same DMA instrument with the humiditychamber set-up. The samples were mounted with an approximatedistance between the clamps of 1 cm; the thickness and widthwere the same as in the temperature scan measurements. An iso-thermal conditioning step at 25 �C and 0% RH for 4 h was donebefore testing the samples. The preload was set at 0.1 N with125% force track, and a 1 Hz frequency was used with constantamplitude of 5 lm. At least two parallel measurements were aver-aged per sample.

2.6. Moisture sorption

The nanocomposite films were dried in a vacuum oven for2 days at 40 �C. The films were subsequently conditioned at 25 �Cin an air-tight vessel containing saturated salt solutions. The saltswere lithium chloride (Ridel-de Haen 31407), magnesium chloridehexahydrate (Aldrich 208337), magnesium nitrate hexahydrate(Sigma–Aldrich F63079), sodium chloride (Ridel-de Haen 31434)and potassium sulfate (Sigma–Aldrich R31270), targeting a gradi-ent of relative humidities of 11%, 33%, 54%, 75% and 97%, respec-tively, at ambient temperature according to ASTM E104-85. Thevacuum oven dried films were first conditioned at the lowest rela-tive humidity (11%) and then moved successively to the highestrelative humidity (97%). The equilibrium water content was mea-sured gravimetrically on a balance and calculated as the weightof the water in the sample at equilibrium compared to the totalweight of the sample. Each measurement point is an average of 3or 4 individual measurements.

2.7. Water permeability

Water permeability measurements were performed in diffusioncells using 3[H]-labeled water as the diffusing tracer, as describedin literature [39]. The composite films were placed between thedonor and acceptor chamber (with 15 ml of Milli-Q water in them),where the donor chamber contained the labeled water (10 ll). Thechambers were stirred at 50 rpm with a rotating table (EdmundBühler 7400, Germany). The donor concentration was taken to beconstant during the experiments due to the large difference in con-centration of the donor and acceptor chambers. Measurementswere run until 10% of the donor chambers mass permeatedthrough the films to avoid errors caused by possible back flow.The thickness of the films was measured on 5 points prior to themeasurement, and further calculations were based on the aver-ages. Samples of 500 ll were taken from the acceptor chambersand immediately replaced by 500 ll of Milli-Q water at 1, 2, 3and 6 h and 1, 3 and 7 days. Each of the samples was mixed with3 ml of scintillation liquid (Ultima Gold�), and the radioactivity

of the samples was analyzed with a liquid scintillation analyzer(Tri-Carb B2810TR, Perkin Elmer, USA). The radioactivity measuredin the acceptor chamber was assumed to be directly proportionalto the diffused mass. Using Ficks first law, the mass transfer ratethrough a film in steady state can be calculated on the basis ofthe following equation [40]:

dm=dt ¼ APðcd � caÞ=h ð2Þwhere cd and ca are the concentrations of the 3[H]-labelled water inthe donor and acceptor chambers, respectively, A is the area of thefilm, h is the average film thickness and P is the permeability. Themass transport (dm/dt) was determined from the linear slope ofthe accumulated radioactivity versus the time plot. The permeabil-ity was calculated as follows:

P ¼ ðdm=dt � hÞ=ðA � DcavÞ ð3Þ

where Dcav is the average concentration difference between thedonor and acceptor chambers. The permeability is therefore pre-sented in (m2/s) units. 3–4 Replicas of the films were measured,and average permeabilities are presented with standard deviations.

2.8. Scanning electron microscope (SEM)

The cross section of the samples fractured during the tensiletests were observed in a Field-Emission Scanning Electron Micro-scope (Hitachi S-4800, Japan) after sputtering the surface with agold–palladium layer for 20 s.

3. Results and discussion

3.1. Preparation of films

During the solvent exchange from water to DMF, no significantagglomeration of the NFC could be detected visually and a suspen-sion with an appearance similar to that of NFC in water wasobtained. When added to the NFC-DMF suspension, AcAX was dis-solved in DMF without causing any visual agglomeration. Exten-sive magnetic stirring and shearing by Ultraturrax was used toimprove homogeneity of the mixture. The suspensions were clearbut slightly yellowish, which is characteristic of AcAX solution inDMF, and had an increasing haze as the concentration of NFCincreased. Films of homogeneous appearance were obtained,although there was a decreased transparency for films with higherconcentrations of NFC.

The surface energies of the AcAX and NFC were determined toinvestigate the possible interactions between the AcAX and NFC.According to the Owens, Wendt, Rabel and Kaelble method, thesurface tension and its polar and dispersion components can beestimated on the basis of contact angle of at least two liquids withknown polar and dispersion components against the solid [37].

The key equation of the method originates from the combina-tion of the surface tension equation and the Young equation:

ð1þ cos hÞ � r1

2ffiffiffiffiffiffirD

1

q|fflfflfflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflfflfflffl}

y

¼ffiffiffiffiffiffirP

S

q|ffl{zffl}

m

�

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffirP

1

qffiffiffiffiffiffirD

1

qvuuut|fflfflfflffl{zfflfflfflffl}

x

þffiffiffiffiffiffirD

S

q|ffl{zffl}

b

ð4Þ

where h is the contact angle of the liquid against the solid, and r isthe surface tension of the liquid (l) and solid (s), where D and Psuperscripts indicate the dispersion and polar components,respectively.

In a linear regression of the plot of y against x, rSP can be

obtained from the square of the slope of the curve m and rSD from

the square of the ordinate intercept b.The linear regression was fitted using SCA202 v.4.2.3 software

on data points of 5 different liquids (ethylene glycol, formamide,

A.M. Stepan et al. / Composites Science and Technology 98 (2014) 72–78 75

diiodomethane, glycerol, water) for both AcAX and NFC with anacceptable correlation coefficient (R2). As indicated in Table 1, AcAXis less polar and more dispersive than the NFC, as expected. How-ever, the total surface tensions of the AcAX and NFC are similar.The surface tension of the liquids used is from literature [41,42].

Small differences in the surface tensions of materials will resultin a low interfacial surface energy, which is a requirement for ahigh work of adhesion. Strong adhesion may promote the reinforc-ing effect of the filler (NFC), since high work of adhesion will facil-itate good stress-transfer through the interface. A similarity in thesurface tension of two components may prevent extensive agglom-eration of the individual components, facilitating good dispersionof the NFC phase in the AcAX solution.

3.2. Mechanical properties

Fig. 1 shows the appearance and representative stress–straincurves of the different composite films tested at 50% RH and23 �C, and Table 2 summarizes the average values and the standarddeviations of the measurements. The Young’s modulus E increasedwith the addition of 1% NFC to the AcAX, and increased furtherwith increasing NFC content: from ca. 2200 MPa for the neat AcAXfilm to over 3300 MPa for the 10% NFC composite. The strength ofthe films also gradually increased with increasing NFC content,reaching an average of more than 90 MPa for the 10% NFC films.These properties are similar to those of other water sensitive com-posite films reported in the literature [27]. Most interestingly, thefilms with a low amount of NFC (1 wt%) had a strain to failure com-parable to the neat AcAX. Although the strain to failure decreasedupon increasing NFC content, the film with 10 wt% NFC fractured at10.5% strain, which is quite remarkable in comparison to workwith similar nanocomposites previously reported [27–30]. The

Table 1Surface tensions of the testing liquids and for AcAX and NFC.

Material Dispersivecomp. (mJ/m2)

Polar comp.(mJ/m2)

Total surfacetension (mJ/m2)

R2

Ethylene glycola 30.9 16.8 47.7Formamidea 39 19 58Diiodomethanea 50.8 0 50.8Glycerola 37 26.4 63.4Watera 21.8 51 72.8AcAX 9.4 37.8 47.2 0.72NFC 13.0 36.0 49.0 0.88

a Refs. [41,42].

Fig. 1. Representative tensile stress–strain curves of neat and NFC reinforced AcAXfilms and their appearance (in inset).

characteristic ductile behavior of NFC nanopaper is due to nanofi-ber slippage [31]. Apparently, this plasticity mechanism is pre-served in the composite material.

The modulus was predicted based on a rule of mixtures approach,see Table 2. It was assumed that composite modulus scales withnanopaper modulus. The agreement with experimental data is good.As a first approximation, nanopaper and matrix moduli can be usedto predict composite modulus, although this is a simplification.These results indicate favorable polymer matrix characteristics,fairly good dispersion and efficient stress transfer at the interface.

The SEM images shown in Fig. 2 present the fracture surface of aneat and a 10% NFC composite after tensile testing. Substantialplastic deformation is apparent at submicron scale for the neatAcAX (Fig. 2a). This is also the case for 10% NFC composite, whereindividual nanofibers are apparent (Fig. 2b). The NFC appears to bewell-dispersed in the AcAX.

3.3. Dynamic mechanical analysis

DMTA and RH-DMA scans were performed on the films in orderto determine the dependence of material properties on tempera-ture and relative humidity.

Fig. 3a presents the storage modulus as a function of tempera-ture for the neat AcAX and the composite films. Neat AcAX filmis stable up to ca. 120 �C after which the storage modulus startsto decrease rapidly. This drop is associated with the molecularmotions related to the transition from the glassy to the rubberyphase. The addition of 10% NFC to AcAX significantly increasesthe glassy storage modulus of the films from about 3000 MPa to3900 MPa. There was no significant difference between the storagemodulus of the composites on room temperature (under the Tg)containing higher loading of NFC, once a percolating network isalready formed (which is between 1% and 5% for NFC) (Table 2).However, if we compare the storage moduli above glass transitiontemperatures, we see a difference of about a magnitude in the stor-age modulus of the composites. On the other hand, it is interestingto note that the NFC not only increases the storage modulus butalso substantially shifts the onset of the rubbery transition tohigher temperatures and reduces the drop in storage modulus(slope of the curve after transition starts). The films with 5% and10% NFC were strong enough to exhibit the glass transition as wellas higher temperature softening, while the films with lower NFCcontent (1%, 2% and 3%) failed during the transition to rubberyphase. The greater stability of the high NFC content films may beattributed to the formation of a percolating network of the fibrils.Here data indicate network formation and distinct storage modu-lus plateau for 10% NFC. Other authors have estimated networkformation to take place somewhere between 1% and 5% NFC forsimilar reinforcement systems [43]. The values of the glass transi-tion temperature (Tg) were estimated by the peak position of theloss modulus curves (Fig. 3b/Table 2). The addition of smallamounts of NFC increased the Tg (from 125 �C of neat AcAX to139 �C in 1% films), but a subsequent increase in NFC content didnot seem to have any significant impact on the Tg and it stayedapproximately the same (within experimental errors).

The effect of the surrounding relative humidity on the compos-ites during DMA measurements was also evaluated. The depen-dence of the storage modulus on the RH is displayed in Fig. 3c. Itis interesting to note that, even though the films with higher NFCcontent absorb more moisture in humid conditions (as discussedlater), the reinforcing effect is still significant at very high humidity(90% RH). Interestingly, the films with 1% and 2% NFC have thesmallest drop in the storage modulus from 0% to 90% RH, evenlower than the neat AcAX film (about 20.5%, 21% and 29% respec-tively) (Fig. 3d). Nano reinforcements in composites can have a sig-nificant effect on the material properties, even at very low content

Table 2Summary of mechanical, thermal and moisture sorption properties of films.

Films wt%NFC

NFC vol.fraction (%)

Tensile strength(MPa)

Modulus(MPa)

Predicted modulusa

(MPa)True strain tofailure (%)

Storage modulus (MPa)(34 �C)

Tg

(�C)Permeability(10�12 m2/s)

AcAX 0 65.1 ± 4.2 2160 ± 80 – 21.0 ± 3.1 3010 ± 160 125 4.42 ± 1.081% NFC 0.7 72.5 ± 5.6 2390 ± 140 2240 19.2 ± 6.4 3040 ± 260 139 2.43 ± 0.373% NFC 2.0 75.5 ± 7.9 2540 ± 170 2390 17.3 ± 5.0 3460 ± 380 140 2.47 ± 0.515% NFC 3.4 74.7 ± 10.8 2880 ± 180 2540 13.0 ± 3.7 3820 ± 350 140 3.05 ± 0.4910% NFC 6.9 90.1 ± 7.4 3330 ± 220 2940 10.5 ± 2.2 3850 ± 140 144 4.45 ± 0.51

a See equation in the experimental description of mechanical properties. It is assumed that composite modulus scales with nanopaper modulus (see experimental).

76 A.M. Stepan et al. / Composites Science and Technology 98 (2014) 72–78

(below 5%) [26]. On the other hand, the neat NFC adsorbs morewater than the AcAX matrix. In 1% and 2% films, the effect of addi-tional water uptake caused by the NFC probably remains negligi-ble. For the higher NFC content, films have a gradually increasingloss in the storage modulus from 0% to 90% RH as the NFC contentincreases. Finally, the film with 10% NFC has a drop in the storagemodulus of 27% from 0% to 90% RH, close to the neat AcAX film.Since the NFC adsorbs more moisture, this causes higher moisturecontent in the films. Moisture may not only plasticize NFC andmatrix components, but also reduce stress transfer in the NFC net-work composite. This can lead to disrupted fibril-fibril bondinginside the NFC fibril bundles and also in the junction points ofthe NFC network. An interfacial matrix-NFC slippage is also pro-posed in literature when discussing NFC reinforced composites[36]. This can lead to a larger drop in the storage modulus withincreasing RH for films with a higher NFC content.

3.4. Moisture sorption

The water sorption curves of the neat and reinforced films aredisplayed in Fig. 4 by showing the moisture uptake as a functionof relative humidity. Here the curves follow an expected trend,showing a direct dependence of the moisture uptake on the NFCcontent in the films. However, since the highest total adsorbedmoisture was still less than 8%, the difference in the water contentof the samples at lower humidities was insignificant, or in the rangeof measurement errors. It could be noted that the biggest step in thewater sorption behavior was between the films with 0% and 1% NFCcontent. These moisture sorption isotherms are categorized as typeIII isotherms according to IUPAC [44]. A type III isotherm isdescribed as hydrophobic or low hydrophilic material with weakadsorbate and adsorbent interactions. Moisture sorption kineticsof NFC/polysaccharides was analyzed for the case of amylopectin-rich starch, where the NFC resulted in reduced sorption [45].

Fig. 2. SEM images of fracture surface of (a) neat AcAX film

In the present case, NFC increases moisture sorption to a limitedextent but contributes substantial mechanical reinforcement.

3.5. Water permeability

During the permeability measurements, the steady state of thesystem was indicated by a linear character of the curve without alag phase. As a percolating network of NFC is suggested, at certainNFC concentrations, it could be expected that the NFC would facil-itate higher water permeability through this hydrophilic intercon-nected network. However, with respect to the standard deviationsof the measurements, no significant difference can be observed inthe water permeability when comparing the neat films to the filmsreinforced with 10% of NFC (Table 2). On the other hand, the filmscontaining 1%, 3% and 5% NFC had a significantly lower averagepermeability value than the neat film. The permeability of the filmswith 1, 3 and 5% NFC was also significantly lower than the 10% NFCreinforced film. This was statistically confirmed using 95% confi-dence interval with ANOVA using least square difference and thesoftware Statgraphics Centurion XV. At low NFC content, fibrilsmay decrease swelling due to their reinforcement effect and thusdecrease permeability. At high NFC content, it is possible thatliquid water may diffuse more rapidly along the path of the perco-lated NFC network, and permeability increases. The speculationwith respect to improvements at low NFC content is that there isstrong molecular NFC/AcAX interaction at the interface in the formof hydrogen bonding as well as non-polar interactions. This inter-action is supported by the strong increase in AcAX Tg at low NFCcontent, where the mobility of the AcAX molecule is decreaseddue to the proximity of the rigid cellulose surface. A similar phe-nomenon has been observed with MFC reinforced poly(lactic acid)(PLA), polyhydroxybutyrate-co-valerate (PHBV) and polycaprolac-tones (PCL) films [40]. In the work of Sanchez-Garcia and co-workers, the PLA biocomposite samples with an MFC content of

and (b) AcAX films with 10% NFC. (Scale bar is 1 lm).

10

100

1000

(a)

Stor

age

Mod

ulus

(MPa

)

Temperature (ºC)

AcAX 1% NFC 3% NFC 5% NFC 10% NFC

0

20

40

60

80

100

120

140

160

180

200

220

(b)

Loss

Mod

ulus

(MPa

) 0% NFC (AcAX) 1% NFC 3% NFC 5% NFC 10% NFC

0 20 40 601500

2000

2500

3000

3500

4000

4500

(c) (d)

Stor

age

Mod

ulus

(MPa

)

RH (%)

AcAX 1% NFC 3% NFC 5% NFC 10% NFC

60 80 100 120 140 160 180 200 220 240 260

Temperature (ºC)60 80 100 120 140 160 180 200 220 240 260

80 100 0 2 4 6 8 100

5

10

15

20

25

30

35

Tota

l Los

s of

Sto

rage

Mod

ulus

from

0-9

0%R

H (%

)

NFC content (%)

Fig. 3. (a) DMTA – storage modulus of neat and NFC reinforced AcAX films as a function of the temperature; (b) DMTA – loss modulus of neat and NFC reinforced AcAX filmsas a function of the temperature; (c) DMA-RH of neat and NFC reinforced AcAX films; (d) total loss of storage modulus of neat and NFC reinforced AcAX films (0–90% RH).

0 20 40 60 80 1000

2

4

6

8

Moi

stur

e co

nten

t of f

ilms

(wei

ght%

)

RH (%)

(AcAX) 0% NFC 1% NFC 3% NFC 5% NFC 10% NFC

Fig. 4. Moisture sorption isotherms of neat and NFC reinforced AcAX films at roomtemp.

A.M. Stepan et al. / Composites Science and Technology 98 (2014) 72–78 77

1% showed water barrier improvements and for the case of the 4%,5% and 10% MFC content the permeability was increased. The PHBVand PLC composites with 1%, 2%, 4% and 5% MFC showed a decreasein water permeability, and the 10% MFC loading had comparable orincreased permeability values compared to the neat materials.Thus the behavior of PCL (and PHBV) is in agreement with ourresults. Finally, it is interesting to note that the AcAX and its com-posites have comparable water permeabilities compared to MFCreinforced biocomposites of PHBV and PCL [40]. However, it isimportant to point out that the water permeability measurements

in the mentioned study were carried out with a slightly differentexperimental setup than ours. Thus comparing absolute valuesfrom this reference to our work is not straight-forward, althoughtrends are comparable with our work.

4. Conclusions

This work demonstrates all plant based water repellent thermo-plastic films from acetylated arabinoxylan reinforced with NFC. Ithas been shown that even 10% NFC can be dispersed without largeagglomerates in the AcAX matrix. After solvent exchanging thewater suspension of the NFC to DMF, the NFC still formed stablesuspension, and it was a key step to be able to mix it with the dis-solved matrix. In this way, the reinforced films could be preparedby film casting. The addition of an increasing content of NFCstrongly affected the mechanical properties. The tensile strain atbreak gradually decreased with increasing NFC content, leavingthe 10% NFC composite still with an elongation to break of 10.5%,which is notable among fully biobased composites. The stiffnessand the stress at break increased with increasing amount of NFC,resulting in a strong biobased film at 10% NFC content. Therewas a strong effect on apparent Tg already at 1% by weight ofNFC. This indicates favorable dispersion and strong interfacialinteraction, in particular at the lowest NFC contents. The issue ofmoisture sensitivity in hemicellulose films has been addressed bythe present use of acetylated xylan. The moisture uptake of thefilms at 97% RH and room temperature was below 8%, even forthe films with the highest (10%) NFC content, reflecting lowermoisture sensitivity than most hemicellulose films reported. Dur-ing water permeability tests, the films containing 1% and 3% NFC

78 A.M. Stepan et al. / Composites Science and Technology 98 (2014) 72–78

had a significantly lower average permeability value than the neatfilm and the ones reinforced with 10%. On the basis of the results,we can conclude a successful reinforcement of AcAX with NFC. Thissuggests that the current composites film concept has potential ina future bio-based packaging industry, since their water resistancein combination with their mechanical properties among fully bio-based composites is outstanding. Meanwhile potential heat pro-cessing of the AcAX matrix is also a promising feature of thecomposite.

Acknowledgements

The authors would like to acknowledge the Knut and Alice Wal-lenberg Foundation for financing this research, carried out in theWallenberg Wood Science Center. Dr. Gilbert Carlsson from SP inStockholm, Sweden is acknowledged for help with surface energycalculations and Dr. Christian Müller from Chalmers, Sweden isacknowledged for fruitful discussions related to this research.

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