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Polymer Degradation and Stability 97 (2012) 1996e2001

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Polymer Degradation and Stability

journal homepage: www.elsevier .com/locate/polydegstab

Effect of organoclay content and molecular weight on cellulose acetatenanocomposites properties

Francisco J. Rodríguez a,*, Alejandro Coloma b, María J. Galotto a, Abel Guarda a, Julio E. Bruna a

aCenter for the Development of Nanoscience and Nanotechnology (CEDENNA), Food Packaging Laboratory (LABEN-CHILE), Department of Food Science and Technology,Faculty of Technology, University of Santiago de Chile, Santiago, ChilebDepartment of Agro-Industries, Faculty of Agricultural Science, National University of the Altiplano, Puno, Peru

a r t i c l e i n f o

Article history:Received 27 September 2011Received in revised form4 June 2012Accepted 5 June 2012Available online 15 June 2012

Keywords:Cellulose acetateCloisite30BTriethyl citrateNanocompositesFilms

* Corresponding author. Food Packaging Laboratoryof Food Science and Technology, Faculty of TechnoloChile, Santiago, Chile.

E-mail address: francisco.rodriguez.m@usach.cl (F

0141-3910/$ e see front matter � 2012 Elsevier Ltd.http://dx.doi.org/10.1016/j.polymdegradstab.2012.06.0

a b s t r a c t

Nanocomposites based on cellulose acetate (CA), a commercial organoclay (Cloisite30B) and triethylcitrate (TEC) were obtained using a solution casting method. Different nanocomposites were preparedaccording to different organoclay contents (2.5, 5.0, 7.5 and 10.0 wt.%) and molecular weight of celluloseacetate (Mn 30,000 and 50,000). The properties of the nanocomposites were evaluated by means ofopacity index, X-ray diffraction (XRD), differential scanning calorimetry (DSC), mechanical (modulus ofelasticity, tensile strength and elongation at break), scanning electron microscopy (SEM) and oxygentransmission rate (OTR) measurements. All obtained nanocomposites showed the intercalation ofpolymer inside the clay structure which was slightly favored for nanocomposites with CA 30,000.Important changes on the opacity index, mechanical properties, glass transition and melting tempera-tures, crystalline fraction, oxygen permeability and fracture morphology of nanocomposite films wereobserved according to the increase of organoclay content. On the other hand, intercalation level andoxygen permeability showed some differences with the molecular weight of cellulose acetate.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Food packaging is a continuously changing field, based on itsdevelopment. Increasingly demanding customers is the mostimportant reason for this dynamism. In this sense, safety andrespect for the environment are two very important trends to takeinto considerationwhilst developing new packaging systems. Thus,the active food packaging and nanocomposites have been recog-nized as the main areas of interest in the development of newpackaging systems [1,2].

Considering food industry as one of the highest consumers ofpetroleumebased plastic materials which are poorly biodegrad-able, the development of eco-friendly materials is one of thegreatest challenges which has become imperative to this industry.So, the development of plastic materials based on polymers fromnatural resources has risen as an important alternative to confrontthe environmental problems associated with traditional plastics[3]. Research on these materials has mainly been oriented tobiopolymers or their derivatives; however, applications of this kind

(LABEN-CHILE), Departmentgy, University of Santiago de

.J. Rodríguez).

All rights reserved.03

of polymers are limited due to their poor properties (e.g. brittle-ness, high gas permeability), preventing their use. Nevertheless, theuse of nanofillers to produce bionanocomposites has also risen asan important alternative to change these negative properties [4,5].In recent years, bionanocomposites based on polylactic acid [6,7],starch [8,9], polyhydoxyalkanoates [10,11], cellulose [12,13] andchitosan [14,15] have been studied with different nanofillers suchas carbon nanotubes, clays and metal oxides. Regarding clays,montmorillonite has been the most commonly employed claymineral in polymer composites [16]. To ensure good compatibilitybetween polymer and montmorillonite in composites, this layeredsilicate must be modified with organic molecules such as quater-nary ammonium salt with alkyl chains [17]. In this way, the orga-noclays structure favors the nanoparticleepolymer interactionsnecessary to produce nanocomposites.

On the other hand, considering that cellulose is the mostabundant naturally available polysaccharide, its use as rawmaterialto produce eco-friendly nanocomposites for food packaging couldbecome promising to this industry. Studies applied to cellulose arecentered on cellulose derivatives, such as methyl cellulose (MC),cellulose acetate (CA) and cellulose acetate butyrate (CAB), becausethese present better processability than pristine cellulose. Rimdusitet al. [18] prepared methyl cellulose nanocomposites with mont-morillonite, by means of a chemical crosslinking using a solvent

F.J. Rodríguez et al. / Polymer Degradation and Stability 97 (2012) 1996e2001 1997

casting method. In this study, X-Ray diffraction (XRD) and trans-mission electron microscopy (TEM) analyses evidenced the exfoli-ation of clay by methyl cellulose. Consequently, these authorsobserved an important effect of crosslinking agent on mechanicaland thermal properties of films. Other studies based on celluloseacetate butyrate (CAB) showed the intercalation of the polymerinside the structure of a layered silicate which was confirmed bythe increase in the interlayer distance between layers in the silicate.In this study, incorporation of layered silicate produced importantchanges on the transparency of the CAB films [19]. On the otherhand, Park et al. reported the effect of a compatibilizer [20] anda plasticizer [21] in the exfoliation process of commercial organo-clay using a melt technique in CA nanocomposites. Likewise,Wibowo et al. [22] used CA, triethyl citrate (TEC) as a plasticizer andorganoclays to produce an intercalated nanocomposite which evi-denced important changes on the mechanical properties. In ourprevious work, we reported the effect of different clays modifiedwith organic ammonium salts on the cellulose acetate nano-composite properties [23]. Here, a commercial organoclay (Cloisi-te30B) displayed the best performance to produce nanocompositeswhich was explained due to presence of polar groups (eCH2CH2OH)in the ammonium component in the organoclays that favored theinteractions between cellulose acetate and this organoclay.

Then, based on the high potential that eco-nanocompositespresents to the development of new materials in the foodpackaging area, our studies have been oriented to developnanocomposites based on cellulose acetates and organoclays.Thus, the aim of the present work was oriented to developcellulose acetate nanocomposites with different organoclaycontents using a solution casting technique. Accordingly, thisstudy evaluated the effect of cellulose acetate of differentmolecular weight (Mn 30,000 and 50,000) on the properties ofthe nanocomposites. The nanocomposites elaborated werecharacterized by opacity index, X-Ray diffraction (XRD), tensiletest, differential scanning calorimetry (DSC) and oxygenpermeability.

2. Experimental

2.1. Materials

A commercial organoclay (Cloisite30B) was providedby Southern Clay Products, Inc. Cellulose acetates with molec-ular weight of w30,000 (39.8 wt.% acetyl) and of w50,000(39.7 wt.% acetyl), and triethyl citrate (>99%) were supplied byAldrich.

2.2. Preparation of cellulose acetate nanocomposites

The nanocomposites were prepared according to a previouslyreported procedure [23]. Cellulose acetate nanocomposites con-sisted of variable contents of Cloisite30B (0, 2.5, 5.0, 7.5 and10.0 wt.%) and 15 wt.% TEC. Nanocomposites were prepared bymeans of solution casting method using acetone as solvent. 5 g ofcellulose acetate and TEC were dissolved in 75 mL of acetone undervigorous stirring for 1 h at room temperature. The correspondingweight of Cloisite30B was dispersed in 25 mL of acetone andsonicated for 30 min at room temperature. Then, the celluloseacetate solution was added on organoclay suspension undervigorous stirring and the mixture was stirred for 60 min at roomtemperature. After that 30 mL of mixture was added on Petric disc(19 cm) and dried at 40 �C in oven for 4 h. Finally, the films wereremoved from the glass disc and stored in polyethylene bags toavoid contamination.

2.3. Characterization of nanocomposites

2.3.1. Morphology and structure2.3.1.1. X-Ray diffraction. X-Ray Diffraction (XRD) analysis werecarried out in a Siemens Diffractometer D5000 (30 mA and 40 kv)using CuKa (l¼ 1.54 Å) radiation at room temperature in a 2q rangeof 2e10� at 0.02�/sec. Interlaminar distances were calculated usingBragg’s Law d ¼ l/2sinq [24].

2.3.1.2. Opacity index. The opacity index of nanocomposite filmswas determined in a UVeVis spectrophotometer ShimadzuUVmini-1240. The films were cut into a rectangular piece(1 � 4.5 cm) and placed on the sample compartment of the spec-trophotometer and the absorbance was determined at 600 nm. Theopacity index was calculated as the quotient of the absorbancevalue divided by film thickness in mm [25].

2.3.1.3. Scanning electronic microscopy. The SEM micrographs ofnanocomposite films were obtained from a JSM-5410 JeolScanning Microscope with accelerating voltage at 10 kV. Thesamples were previously fractured under liquid nitrogen and thecross-section coated with gold palladium using a SputteringSystem Hummer 6.2.

2.3.2. Thermo-mechanical properties2.3.2.1. Differential scanning calorimetry. Differential ScanningCalorimetry (DSC) analyses were conducted with a Mettler DSC-822e calorimeter. Specimens weighing 5e10 mg were sealed inaluminum pans (40 mL) and heated from 25 �C to 300 �C at a rate of10 �C/min under the purge of dry nitrogen.

2.3.2.2. Thickness. The nanocomposite films thickness weremeasured using a Mitutoyo Absolute ID-C112 digimatic indicatorwith absolute encoder, specially devised for plastic film thick-ness measurements, with a resolution of 0.001 mm and a rangeof 12.7e0.001 mm. The digimatic indicator was equipped witha 10 mm diameter flat point. These measurements (at least five)were carried out at various locations on the films and then anaverage value was calculated for each film sample.

2.3.2.3. Mechanical properties. Tensile strength, elongation atbreak, and modulus of elasticity for each material weremeasured at room temperature with a Zwick Roell model BDO-FB 0.5 TH Tensile Tester, according to ASTM D-882. Strips(16.5 � 2.5 cm) of nanocomposite films were cut using a diecutter and kept at 25 �C and 50%RH for 48 h before the test.Analyses were carried out with a 1 kN load cell. The initial gripseparation was 10 cm and the crosshead speed used was 50 mm/min. Results are the average of 6 specimens for each nano-composite film.

2.3.3. Oxygen barrier2.3.3.1. Oxygen transmission rate. The OTR of films was determinedwith an Oxygen Permeation Analyzer (MOCONOX-TRAN�MS2/20),equipped with a Coulox� oxygen sensor with a sensitivity of 0.1[cc/(m2 day atm)]. Measurements were carried out at 23 �C and 0%RHuntil a steady-state oxygen transmission rate was achieved. Outputvalues were expressed as the oxygen transmission rate in [cc/(m2 day)].

2.3.4. Statistical analysisA randomized experimental design was used. The data analyses

were carried out using a Statgraphics Plus 5.1 software usinganalysis of variance and Fisher’s LSD test. Differences wereconsidered significant at p < 0.05.

Table 1Results of characterization of cellulose acetate nanocomposites by XRD and DSC.

MolecularweightCA

Organoclaycontent(wt.%)

2q (�) Interlaminardistancea

(nm)

Tg(�C)b

Tm(�C)b

DHm

(J/g)bXc

(%)c

30,000 0.0 ns e 134 175 0.68 1.1630,000 2.5 4.00 2.21 119 156 0.53 0.9030,000 5.0 4.02 2.20 117 156 0.47 0.8030,000 7.5 3.96 2.23 110 151 0.39 0.6630,000 10.0 3.96 2.23 101 136 0.30 0.5150,000 0.0 ns e 130 172 0.62 1.0550,000 2.5 4.22 2.09 116 156 0.53 0.9050,000 5.0 4.20 2.10 111 154 0.45 0.7750,000 7.5 4.16 2.12 110 154 0.37 0.6350,000 10.0 4.08 2.16 110 150 0.28 0.48

a Based on Bragg’s Law.b From the second scan.c Based on DHf (100%) ¼ 58.8 J/g.

2 4 6 8 10

(e)

(d)

(c)

(b)

(a)

In

te

ns

ity

(a

.u

)

2 theta (degree)

Fig. 2. X-ray diffraction patterns of cellulose acetate nanocomposites based on CA50,000. a) Pure Cloisite30B, b) 2.5 wt.%, c) 5.0 wt.%, d) 7.5 wt.% and e) 10.0 wt.%.

F.J. Rodríguez et al. / Polymer Degradation and Stability 97 (2012) 1996e20011998

3. Results and discussion

3.1. Morphology and structure

3.1.1. X-Ray diffractionTo determine the type of nanocomposite obtained, the different

films were analyzed by XRD. Table 1 summarizes the XRD resultsbased on maximum diffraction peak observed for the differentnanocomposites. All nanocomposites showed a d001 diffractionplane which was centered at 2q ¼ 4.0� corresponding to inter-laminar distances around 2.2 nm (Figs. 1 and 2). Consideringa d001 ¼ 1.85 nm for pure Cloisite30B, a clear intercalation ofcellulose acetate inside the clay structure could be evidenced.

2 4 6 8 10

(e)

(d)

(b)

(c)

In

ten

sity (a.u

)

2 theta (degree)

(a)

Fig. 1. X-ray diffraction patterns of cellulose acetate nanocomposites based on CA30,000. a) Pure Cloisite30B, b) 2.5 wt.%, c) 5.0 wt.%, d) 7.5 wt.% and e) 10.0 wt.%.

Similar intercalation levels were reported by Park et al. [21] whencellulose acetate was mixed with Cloisite30B using a melt process.It can be also observed the interlaminar distance was not affectedby the organoclay content; however, all nanocomposites based onCA 30,000 presented greatest interlaminar distances. This perfor-mance would indicate an easier intercalation of the celluloseacetate with low molecular weight than with high molecularweight due to its smallest molecular volume. Effect of molecularweight of a polymer on the intercalation level was observed ina previous study, in which organoclays based on montmorilloniteand chitosan with different molecular weights were used. In thisstudy, organoclays based on chitosan with low molecular weights(320 kDa) showed a highest intercalation level in comparison witha polymer with a highmolecular weight (640 kDa) [26]. In this way,steric hindrance would be the key to explain the differences in theobserved interlaminar distances.

3.1.2. Opacity indexAbsorbance measurements have been reported as a simple

method to determine opacity index in plastic films [25,27] andnanocomposites [28]. Table 2 shows the opacity index values of the

Table 2Opacity index values of the nanocomposite films.

Organoclay content (wt.%) Opacity index

CA 30,000 CA 50,000

2.5 0.470 � 0.030a 0.303 � 0.058a

5.0 0.666 � 0.081b 0.746 � 0.062b

7.5 0.709 � 0.054b 0.934 � 0.085c

10.0 0.866 � 0.076c 1.257 � 0.098d

Means followed by the same letter are not significantly different (p< 0.05) based onthe analysis of variance and Fisher’s LSD test.

Fig. 3. SEM images of different nanocomposites based on CA 30,000 (aee) and CA 50,000 (fej) and different contents of Cloisite30B. a) and f) 0 wt.%, b) and g) 2.5 wt.%, c) and h)5.0 wt.%, d) and i) 7.5 wt.% and e) and j) 10.0 wt.% of Cloisite30B.

F.J. Rodríguez et al. / Polymer Degradation and Stability 97 (2012) 1996e2001 1999

different nanocomposites obtained. When the Cloisite30Bconcentration increased from 2.5 wt.% to 10 wt.% in the CA filmmatrix, an important increase in the absorbance was observed,believed to be associated with a decrease in the amount of lightpassing through the nanocomposite film. Thus the addition ofnanofiller resulted in a decrease in the transparency of thematerial,given by the interaction of the light with the inorganic structureembedded within the polymer matrix. There was an increase in theopacity index when the molecular weight of CA increased from30,000 to 50,000 at different organoclay contents except 2.5 wt.%. Ithas been reported that higher degree of intercalation or exfoliationof clay by polymeric matrices decrease the opacity property due toa minor interaction between light and the layers of clay [29,30]. So,the lowest opacity index for CA 30,000 nanocomposites can beexplained due to its higher intercalation than CA 50,000.

3.1.3. Scanning electron microscopySEM images of all nanocomposites are presented in Fig. 3. The

fractures look more brittle and no plasticity of matrix polymer canbe seen with the increase of the content of Cloisite30B. Similarfracture morphologies were observed by Petersson et al. [19] incellulose acetate butyrate/layered silicate nanocompositesprepared by solution casting method.

3.2. Thermo-mechanical properties

3.2.1. Differential scanning calorimetryTable 1 also displays the results of DSC analysis to the nano-

composites obtained. An increase of Cloisite30B content inside thepolymer matrix produced a decrease in all of the evaluated

Table 3Results of mechanical characterization of cellulose acetate nanocomposites.

Organoclaycontent (wt.%)

CA 30,000

Thickness (mm) Modulus of elasticity(N/mm2)

Tensile strength(N/mm2)

Elongaat brea

0 52.0 � 3.2a 1068.4 � 192.7a 53.4 � 4.6a 16.8 �2.5 48.8 � 4.4a 1715.4 � 69.0b 51.6 � 6.0a 9.8 �5.0 55.5 � 1.6b 1741.5 � 105.2b 47.0 � 2.1a 4.4 �7.5 57.8 � 1.7b 1928.3 � 86.7c 41.5 � 1.7b 3.8 �10.0 58.0 � 3.5b 1963.6 � 145.9c 34.4 � 4.7c 1.9 �

Means followed by the same letter are not significantly different (p < 0.05) based on the

parameters, that is to say, glass transition (Tg), melting point (Tm),melting enthalpy (DHm) and crystalline fraction (Xc). These resultsconfirmed the plasticizing effect of the organic component(ammonium surfactant) from commercial organoclay would bereleased by the introduction of cellulose acetate chains inside theorganoclay structure when the nanocomposites are formed [23].No difference between CA 30,000 and CA 50,000 was evidenced.

3.2.2. Mechanical propertiesRegarding mechanical characterization, nanocomposite films

were analyzed by means of tensile tests. As shown in Table 3,modulus of elasticity and elongation at break were the mostaffected mechanical properties when the organoclay content wasincreased in the CAmatrix. On the other hand, a minor effect on thetensile strengthwas observed. For the nanocomposites based on CA30,000 and 50,000 with a 10 wt.% of organoclay the tensilemodulus increased 84% and 60%, the tensile strength decreased 40%and 14% and elongation decreased 89% and 62%, respectively. Theseresults have shown that nanocomposite films become more brittlewith the addition of Cloisite30B, which is in agreement with SEMimages of these materials. Therefore, this effect can be observed tohave a greater importance for nanocomposites with CA 30,000.Here, the different intercalation levels of CA 30,000 and 50,000inside the organoclay structure could explain these differences.

3.3. Oxygen barrier

3.3.1. Oxygen transmission rateOxygen permeability of nanocomposites films was determined

by means of oxygen transmission rate (OTR) measurements. Fig. 4

CA 50,000

tionk (%)

Thickness (mm) Modulus of elasticity(N/mm2)

Tensile strength(N/mm2)

Elongationat break (%)

3.6a 56.8 � 3.8a 1123.8 � 188.4a 48.0 � 7.3a 12.2 � 3.3a

2.0b 59.8 � 5.9a 1127.4 � 204.2a 43.7 � 1.0a 9.7 � 1.3a

0.3c 58.3 � 7.4a 1577.5 � 171.4b 46.8 � 5.5a 6.7 � 1.5b

0.2c 61.8 � 7.0a 1660.6 � 43.4b 41.9 � 4.2a 5.5 � 1.4b

0.4c 66.6 � 9.5b 1797.4 � 169.5b 39.7 � 1.9a 4.6 � 0.4b

analysis of variance and Fisher’s LSD test.

0,0 2,5 5,0 7,5 10,0

200

400

600

800

1000

1200

1400

1600

1800

2000 CA 30,000CA 50,000

OT

R (cm

3

/m

2

d

ay)

Cloisite30B (wt. %)

Fig. 4. Effect of Cloisite30B content on oxygen transmission rate of cellulose acetatenanocomposites based on CA 30,000 and 50,000.

F.J. Rodríguez et al. / Polymer Degradation and Stability 97 (2012) 1996e20012000

collects the OTR values of all the elaborated nanocomposites. Animportant reduction in OTR values in agreement with the increasein nanofiller content in the CA matrix can be noticeably observed.An increase in tortuous path with the increase in the nanofillercontent would explain these results [31]. This tortuous path wasshown to be quite effective for the nanocomposites with 5 wt.% ofCloisite30B where the OTR values were reduced by about half forboth CA 50,000 and 30,000 nanocomposites. A further increase inthe Cloisite30B content reduced the oxygen permeability, but tomuch less extends. In addition, nanocomposites based on CA50,000 showed the lowest OTR values. These results are consistentwith a study where CO2 permeability in membranes based onpoly(methyl methacrylate) was reduced with the increase ofmolecular weight of the polymer. This reductionwas explained dueto polymers with high molecular weight favors stronger intra-molecular interactions than those of low molecular weightswhich reduce inter-chain alignment and, subsequently, increasingthe gas permeability [32].

4. Conclusions

All the elaborated nanocomposites have shown the intercalationof cellulose acetate within the organoclay structure, however, theintercalation level was affected by the molecular weight of CA.Plasticizing effect of the organic component from commercialorganoclay was observed on all nanocomposites. Significantchanges in the modulus of elasticity and elongation at break wereobserved with the Cloisite30B content. Therefore, importantchanges on the oxygen permeability of the nanocomposites wereevidenced with the changes in the organoclay content and themolecular weight of cellulose derivative.

Acknowledgments

The authors thank to Comisión Nacional de Investigación Cien-tífica y Tecnológica, CONICYT, for the financial support from Pro-grama Bicentenario de Ciencia y Tecnología (Project PDA-22),Programa de Financiamiento Basal para Centros Científicos y Tec-nológicos de Excelencia (Project FB0807) and Fondo Nacional deDesarrollo Científico y Tecnológico (Project FONDECYT de Inicia-ción 11100389).

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