1. IntroductionDuring the past decade, the issue of sustainabilityhas been high on the European Union (EU) agenda,encouraging academia and industry to develop sus-tainable alternatives thus aiming to preserve resourcesfor future generations. The successful promotionand use of biological, renewable materials for theproduction of packaging materials will satisfy anumber of the EU objectives [1]. To date, packag-ing materials have been, to a large extent, based onnon-renewable materials. The only widely usedrenewable packaging materials are paper and boardwhich are based on cellulose, the most abundant

renewable polymer world-wide. However, majorefforts are under way to identify alternative non-food uses of agricultural crops and the productionof packaging materials, based on polymer fromagricultural sources, could become a major use ofsuch crops [2–4].Indeed such alternative bio-based packaging mate-rials have attracted considerable research and devel-opment interest for long time [1, 4] and in recentyears the materials are reaching the market. Amongall bio-based biodegradable polymers studied, poly(lactic acid) (PLA) appears to be one of the mostattractive polymers commercially available, because

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Properties of poly(lactic acid) nanocomposites based onmontmorillonite, sepiolite and zirconium phosphonateK. Fukushima1*, A. Fina1, F. Geobaldo2, A. Venturello2, G. Camino1

1Department of Applied Science and Technology, Polytechnic of Turin, V.le Teresa Michel 5, 15121 Alessandria, Italy2Department of Applied Science and Technology, Polytechnic of Turin, Corso Duca degli Abruzzi 24, 10129 Torino, Italy

Received 18 April 2012; accepted in revised form 22 June 2012

Abstract. Poly(lactic acid) (PLA) based nanocomposites based on 5 wt.% of an organically modified montmorillonite(CLO), unmodified sepiolite (SEP) and organically modified zirconium phosphonate (ZrP) were obtained by melt blending.Wide angle X-ray scattering (WAXS) and scanning electron microscopy (SEM) analysis showed a different dispersion leveldepending on the type and functionalisation of nanoparticles. Differenctial scanning calorimetric (DSC) analysis showedthat PLA was able to crystallize on heating, and that the addition of ZrP could promote extent of PLA crystallization,whereas the presence of CLO and SEP did not significantly affect the crystallization on heating and melting behaviour ofPLA matrix. Dynamic Mechanical Thermoanalysis (DMTA) results showed that addition of all nanoparticles brought con-siderable improvements in E! of PLA, resulting in a remarkable increase of elastic properties for PLA nanocomposites. Themelt viscosity and dynamic shear moduli (G!,G") of PLA nanocomposites were also enhanced significantly by the presenceof CLO and SEP, and attributed to the formation of a PLA/nanoparticle interconnected structure within the polymer matrix.The oxygen permeability of PLA did not significantly vary upon addition of SEP and ZrP nanoparticles. Only addition ofCLO led to about 30% decrease compared to PLA permeability, due to the good clay dispersion and clay platelet-like mor-phology. The characteristic high transparency of PLA in the visible region was kept upon addition of the nanoparticles.Based on these achievements, a high potential of these PLA nanocomposites in sustainable packaging applications could beenvisaged.

Keywords: nanocomposites, PLA, montmorillonite clay, sepiolite, zirconium phosphonate

eXPRESS Polymer Letters Vol.6, No.11 (2012) 914–926Available online at www.expresspolymlett.comDOI: 10.3144/expresspolymlett.2012.97

*Corresponding author, e-mail: kikku.fukushima@gmail.com© BME-PT

of its biodegradability, ease of processing, trans-parency and price. In general, commercial PLAgrades are copolymers of poly (L-lactic acid) andpoly(D,L-lactic acid), which are produced from L-lactides and D,L-lactides respectively. The ratio ofL-enantiomers to D,L-enantiomers is known toaffect the properties of PLA [5, 6], i.e. whether thematerials are semicrystalline or amorphous. Thereis increasing interest in using PLA for disposabledegradable plastic articles; however, there are prop-erties such as flexural properties, gas barrier proper-ties, high melt viscosity and melt strengh/’elasticity’during processing, that are often not good enoughfor some end-use applications, such as blow mold-ing [7, 8]. To improve the physical properties ofPLA, especially in terms of thermomechanical sta-bility, addition of different fillers (nanoparticles) inPLA was explored [2, 9–11]. Most of the literatureregarding nanocomposites is devoted to lamellarlayered silicates, in particular organically modifiedmontmorillonites due to their ability to significantlyenhance several polymer physical properties ascompared to unmodified layered silicate clays,including gas barrier, flame retardancy, thermal sta-bility and influence on the polymer biodegradationrate [2, 12–14]; however, needle like phyllosilicates(sepiolites) and zirconium phosphate are alsoreported in literature [15–21].Sepiolite is a layered hydrated magnesium silicatecharacterized by a needle like morphology based onalternated blocks of tunnels in the fibre direction[16] and very high surface area (BET 374±7 m2/g)[23] as compared to layered phyllosilicates (BET82±1 m2/g) [23, 24]. The addition of sepiolite hasbeen reported to lead an improvement of the mechan-ical properties in various polymers, such as poly(vinylidene fluoride) and poly(methyl methacry-late) [25, 26], poly(styrene butadiene) block copoly-mers [27], ethylene-propylene (EPM) compounds[28] and natural rubber [29]. Based on the fewpapers on PLA, Poly (#-caprolactone) (PCL), PPand Poly (butylene terephthalate) (PBT) [15, 16],the possibility of an unmodified sepiolite to welldisperse in polymers by melt blending was evi-denced and mainly attributed to the large concentra-tion of surface silanols, spaced every 0,5 nm alongthe length of sepiolite needle [30–32], that are eas-ily available for coupling reactions with local polar-ity on polymer chains.

Zirconium bis(monohydrogen orthophosphate)monohydrate, ($-Zr(HPO4)2(H2O) ($-ZrP)), has alayered structure with many interesting properties,such as high density of grafted organic modifiers,high thermal stability [33], possible achievement oflarge aspect ratios, high elastic modulus and thepotential to delaminate and become intercalatedwithin the polymer [17, 18]. PLA/zirconium phos-phate composites has been studied and showedintumescent flame-retardant properties [34]. How-ever, to the best of our knowledge, no studies onlayered zirconium phosphonate PLA nanocompos-ites have been reported for possible packagingapplications.The aim of this work is to improve melt viscosity,thermo-mechanical and gas barrier properties ofPLA by mixing it with organically modified mont-morillonite, unmodified sepiolite and modified zir-conium phosphonate, and thus producing final PLAnanocomposites with properties able to enlarge thePLA application fields.

2. Experimental section2.1. MaterialsThe poly(lactic acid) – PLA, 2002D –, averagemolecular weight 121 400 g/mol, ratio 96% L-lac-tide to 4% D-lactide units, MFR 6.4 g/10 min, wasa commercial grade supplied by NatureWorks, Min-neapolis Minnesota, USA. One commercial mont-morillonite modified with a ditallow, dimethylammonium salt – CLOISITE 20A – was supplied bySouthern Clay, Cheshire, UK, commercial unmodi-fied sepiolite – PANGEL S9 – supplied by Tolsa,S.A, Madrid, Spain and $-carboxyl-ethane zirco-nium phosphonate – ZrP 102 – (ultrapure >%99%)were supplied by Prolabin & Tefarm, Perugia, Italy.The characteristics of the nanoparticles used in thiswork are listed in Table 1.Prior to the melt blending, polymer matrix wasdried at 70ºC under vacuum for 4 h to achieve aresidual moisture less than 190 ppm. Nanoparticleswere dried at 100°C under vacuum for 10 h for CLOand SEP; and 4 h for ZrP to achieve residual mois-ture less than 250 ppm. Nanocomposites were pre-pared at 5% filler loading by melt blending using aMicroextruder DSM Micro 15ml Twin Screw Com-pounder, with a mixing time of 5 min, at 180°C innitrogen flow. The mixing was performed at two dif-ferent rotor speeds: 60 rpm in the loading step and

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100 rpm during mixing. Sheets were obtained bycompression molding in a hot-plate hydraulic pressat 190°C and allowed to cool (ca. –10°C/min) toroom temperature under pressure (60 Kgf/cm2).Morphological and thermo-mechanical characteri-zations were made on compression moulded0.6 mm films, melt rheological measurements werecarried out on compression moulded 1 mm films,optical transparency analyses were made on com-pression moulded 40–50 µm films and oxygen gasbarrier analyses were performed on compressionmoulded 320 µm films (for PLA/ZrP) and on chillroll extruded 150–160 µm films (for PLA, PLA/CLOand PLA/SEP). These last films were moulded at180°C, passing the specimens by a chill roll with150 µm opening of several slits and allowed to coolto room temperature. In the case of PLA andPLA/CLO chill roll films, previous to the film cool-ing process to room temperature, these were biaxi-ally stretched above PLA glass transition tempera-ture (above 60°C) to obtain films with 50–70 µmthick.

2.2. Characterization techniquesWide Angle X-Ray Spectra (WAXS) were recordedusing a Thermo ARL diffractometer X-tra 48, atroom temperature in the range 1–30º (2&) (stepsize = 0.02º, scanning rate = 2 s/step) by using fil-tered CuK$ radiation (' = 1.54 Å).Scanning electron microscopy (SEM) was carriedout on surfaces obtained after sectioning with amicrotome diamond knife to avoid large surfaceroughness sputtered with gold, using a LEO 1400 VPSeries (Carl Zeiss, Oberkochen, Germany) equippedwith energy dispersive spectroscopy (EDS).Transmission electron microscopy (TEM) analyseswere performed with a high-resolution equipment.

Ultrathin sections of about 100 nm thick were cut atroom temperature with a microtome equipped witha diamond knife and placed on a 200-mesh coppergrid. Differential Scanning Calorimetry (DSC) testswere carried on a DSC Q100 TA Instruments (NewCastle, DE, USA) under nitrogen atmosphere at ascanning rate of 10°C/min, sample size 3–5 mg inaluminium pans. Thermal history of samples waserased by a preliminary heating cycle at 10°C/minfrom –20°C to +280°C. The glass transition temper-ature (Tg), cold crystallization temperature (Tcc),melting temperature (Tm), cold crystallizationenthalpy ((Hcc) and melting enthalpy ((Hm) weredetermined from second heating scan at 10°C/min.Dynamic-Mechanical Thermal Analysis (DMTA)was performed on compression moulded 6%)20%)0.6 mm3 films, using a DMA Q800 TA Intruments(New Castle, DE, USA) in tension film clamp. Thetemperature range analysed was from +20 to +80ºC,at a heating rate of 2ºC/min, 1 Hz frequency, instrain controlled mode, 15 micron of amplitude andstatic loading = 125% dynamic loading. At leasttwo samples of each material were tested and theaverage value of these parameters for each materialwas calculated and reported. The estimated experi-mental error based on the Storage Modulus and TanDelta deviations between repeated tests was ca.10%.Melt rheological measurements were performed onARES instrument (New Castle, DE, USA) with atorque transducer capable of measurements over therange of 0.02 to 200 g*cm. Dynamic oscillatoryshear measurements were performed by applying atime dependent strain of +(t) = +0 sin(,t) and theresultant shear stress is -(t) = +0 [G!*sin(,t) +G"*cos(,t)], with G! and G" being the storage andloss modulus, respectively. Measurements were con-

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Table 1. Characteristics of nanoparticles

HT: hydrogenated linear alkyl chains: C8–18a)Organic modifier content ca. 38 wt% according to CLOISITE 20A technical data sheet.b)Functional formula taken from reference [2].c)Functional formula taken from reference [22].d)Functional formula taken from reference [35].

Nanoparticle type Commercial name Modifier structure Functional formula of nanoparticle Notation in text

Montmorillonitea),b) CLOISITE 20AMx(Al4–xMgx)Si8O20(OH)4M: monovalent cation; x: degree of isomor-phous substitution (between 0.5 and 1.3)

CLO

Sepiolitec) PANGEL S9 None Si12O30Mg8(OH)4·(H2O)4·8H2O SEP$-carboxyl-ethaneZirconium phosphonated) ZrP 102 CH2CH2COOH Zr(O3P(CH2)2COOH)2 ZrP

ducted by using a set of 25 mm diameter parallelplates with a sample thickness of 1 mm. The strainamplitude was fixed to 1% for PLA/CLO and 10%for neat PLA and PLA/SEP systems, in order toobtain reasonable signal intensities even at low fre-quency (,) to avoid the non linear response. Foreach type of material the limits of linear viscoelas-ticity were determined by performing strain sweepsat series of fixed ,s. The dynamic viscoelasticcurves were obtained by using the principle of time-temperature superposition and shifted to a commonreference temperature of 190°C which was chosenas the most representative of a typical processingtemperature of PLA.Oxygen gas barrier properties were measured oncompression moulded films of 5 cm2)320 µm forPLA/ZrP, on chill roll moulded films of 50 cm2)150–160 µm for PLA/SEP and on chill roll mouldedfilms of 50 cm2)50–70 µm for PLA and PLA/CLO,using a permeabilimeter MOCON OX-TRAN 2/21(Minneapolis, USA). The experimental conditionswere 23°C, 760 mm Hg pressure, 0% relative humid-ity and with permeant concentration of 100%. Twosamples were tested for each type of material andaverage results of permeability (normalised on thethickness) are presented.Optical transparency of PLA and PLA/compositeswas obtained using a UV-vis spectrometer CARY500 (California, USA) on compression moulded5 cm2)40–50 µm. The analysis was performancein transmittance mode from 380–780 nm wave-length, UV-vis scan rate of 600 nm/min and DoubleBeam Mode. The cumulative transmission over vis-ible spectra was measured to three samples of eachmaterial and the average value of these measure-

ments was reported for each material type. The esti-mated experimental error was ca. 10%.

3. Results and discussion3.1. Morphology3.1.1. Wide Angle X-ray analysis (WAXS)The WAXS pattern of PLA is characterized by abroad amorphous halo with maximum approxi-mately at 2& = 17º (not shown here). These resultsindicate that PLA matrix was not able to crystallizeduring the film cooling process (ca. 10°C/min),thus obtaining a completely amorphous structurefor the polymer matrix at room temperature.A similar broad WAXS halo with maximum at ca.2& = 17º was observed for PLA/CLO and PLA/ZrP,indicating an amorphous structure of the polymermatrix in these specimens. This result suggests thatthe addition of these nanoparticles does not inducepolymer crystallization under the conditions of filmcooling carried out here.The most significant features are indeed encoun-tered in the lower angle range, which gives indica-tion of the nanoparticles interlayer distance. Fig-ure 1a and 1b represent the WAXS patterns ofcomposites of PLA with CLO and ZrP, comparedwith the pristine nanoparticles. CLO is character-ized by two diffraction peaks at 2& = 3.5 and 7.2°respectively (Figure.1a); the diffraction peak at 2& =3.5° correspond to the crystalline plane (001) [11],whereas the weak peak at 7.2°, corresponding to aninterlayer distance of 1.24 nm, is likely to be relatedto a low content of montmorillonite silicates layerswithout organic modifier insertion [11].The PLA/CLO nanocomposite exhibited two dif-fraction peaks at 2.4° corresponding to an interlayer

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Figure 1. WAXS patterns of PLA based composites with 5% (a) CLO and (b) ZrP

spacing of 3.71 nm, and a secondary diffractionpeak, (d002), at 5.0° [8, 11]. An increase in the inter-layer spacing from 2.45 nm in the nanoclay filler to3.71 nm in the PLA nanocomposite gave credit tosome intercalation occurring during the componentblending above the melting temperature of the poly-mer matrix. This finding showed the possibility ofpenetration of the PLA chains within the nanoclaygalleries on shearing at high temperature.This intercalation of polymer chains in the CLO sil-icate layers can be obtained thanks to chemicalaffinity between the polymer and the clay, origi-nated from the hydrogen bonding between the car-bonyl groups of the main chain of PLA moleculesand the hydroxyl groups belonging to CLO surfacesilicate layers [2, 11, 15]. The presence of the organicmodifier between CLO layers could also make pos-sible the delamination of CLO silicate layers inPLA/CLO hybrid by high mixing torques duringthe melt blending process, despite, strong chemicalinteractions between the organic modifier mole-cules and PLA matrix would not be expected, con-sidering the non-polar structure of this organicmodifier [8].Figure.1b shows the WAXS patterns of the pristineZrP, showing two characteristic peaks at 2& = 6.70and 13.5°, which are attributed to the (002) planefor organo-modified ZrP and pristine ZrP, respec-tively. Indeed, the main WAXS peak at 6.70° (d =1.30 nm) indicates greater separation of the ZrP lay-ers by the presence of the carboxyl-ethane groups[20, 21, 35], whereas the weaker WAXS peak at13.5° corresponds to the interlayer d-spacing ofinorganic $-ZrP as an impurity [19, 35, 36].

The WAXS patterns of PLA/ZrP hybrid do notshow signs of intercalation of PLA into ZrP platelets,likely due to the insufficient interlayer thickness,which is significantly lower in ZrP as compared toCLO. Indeed, the presence of the sharp peaks at dif-fraction angles correspondent to ZrP confirms that,in these compounding conditions, the stacking ofparallel zirconium phosphate sheets is not signifi-cantly modified.No information is delivered by WAXS on disper-sion of needle-shaped sepiolite in PLA, given thelack of periodic stacking of pristine nanoparticles.

3.1.2. Scanning electron microscopy (SEM)All formulations were observed by SEM on sur-faces to study the dispersion and distribution ofnanoparticles. SEM micrographs at low magnifica-tion (not shown here) reveals no significant pres-ence of agglomerated (>%10 µm) inorganic nanopar-ticles regardless the composition. In PLA/CLO avery uniform distribution of small stacks of silicatelayers can be observed in the polymer matrix, asshown in Figure.2a. The presence of clay layers isindeed highlighted by the cracks observable on dia-mond cut surfaces, which are not present in pristinePLA when prepared in the same way. Indeed, suchcracks are attributed to delamination between poly-mer matrix and nanoclay stacks. TEM images (Fig-ure.2b) confirmed the presence of both isolatednanoclay layers and stacks of a few clay layers.In the case of PLA/SEP, nanofibres emerging fromthe polymer surface are clearly visible in SEMmicrographs (Figure.3). Diameter of these fibresare in the range of 10 to 100 nm, therefore are

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Figure 2. Scanning electron micrograph (a) and transmission electron micrograph (b) for PLA + 5% CLO

assigned to single sepiolite needles and some bun-dles of a few individual sepiolite needles.The addition of 5% ZrP (Figure.4) reveals the pres-ence of ZrP micro-particles fairly distributed in thepolymer surface. However, poor adhesion of the

microparticles is evidenced by the presence of holesleft by detachment of some ZrP particles duringmirotome cutting. Interestingly enough, SEM micro-graphs show that ZrP powder presents a plate-likestructure and regular sheets, which can be related tothe presence of very sharp WAXS peaks both in pris-tine ZrP and in the composite.

3.2. Thermal analysis (DSC)Crystallisation of PLA nanocomposites was studiedby means of DSC measurements. In Figure.5, cool-ing and second heating DSC plots for PLA andcomposites are reported. On cooling at a rate of10°C/min (Figure 5a), the pure PLA does not exhibitany exothermic peak, evidencing that no crystal-lization takes place. Crystallization on cooling ofPLA is not significantly affected by the addition ofthe nanoparticles, as evidenced by the identical DSCthermograms, confirming that all materials addressedhere are not able to crystallise at cooling rates of10°C/min or higher, in agreement with the WAXSanalysis which evidenced fully amorphous struc-tures.Figure.5b reports the DSC thermograms of the sec-ond heating curves of PLA and PLA/nanocompos-ites. PLA partially crystallizes on heating at about130°C (cold crystallization) giving a crystallinephase which melts with a peak endotherm at about154°C [15, 37] (see Table 2).Figure.5b and Table 2 show that the addition ofCLO and SEP does not significantly affect the crys-tallization and melting behaviour of PLA, since nosignificant variations of cold crystallization temper-ature (Tcc), enthalpy of cold crystallization (/Hcc),

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Figure 3. Scanning electron micrographs at 50 kX of mag-nification for PLA + 5% SEP. The cracks observedin the polymer matrix corresponds to matrixdegradation under electron beam irradiation

Figure 4. Scanning electron micrographs for PLA + 5% ZrP

Figure 5. DSC thermograms of PLA and PLA/composites at (a) cooling and (b) second heating. Plots are vertically shiftedfor the sake of clarity.

melting temperature (Tm), melting enthalpy (/Hm)and glass transition temperature (Tg) were observed.Addition of ZrP particles neither leads importantvariations in the Tcc, Tm and Tg of PLA matrix, how-ever, increases of the enthalpy of cold crystalliza-tion (ca. 9 J/g) and of the melting enthalpy of neatPLA (ca. 10 J/g) could be detected by the presenceof these particles (see Figure.5b and Table 2). Theseincreases of /Hcc and /Hm obtained by ZrP basedcomposites indicate that the addition of these parti-cles promotes extent of crystallization of the PLAon heating, which is most likely related to the regulardistribution of ZrP micronic particles (previouslyverified through WAXS and SEM analysis) able toact as nucleating agents of PLA crystallization.It is also possible that a higher dispersion level ofCLO and SEP particles into the PLA as compared toZrP, could interfere with polymer molecules crys-tallization instead of enhancing it. In general, it hasbeen reported the possible nucleating effect of welldispersed organoclay layers on polymer crystalliza-tion of PLA [2, 9–11]; however, it has been alsoreported that high interactions between polymermatrix and nanoscale particles, can provoke thatsome polymer chains can be attached to the claylayers, and then partially immobilized and hinderedfrom taking part in the flow process and their crys-tallization process [8, 38]. Accordingly, in additionto acting as nucleation agents, the silicates particlesof CLO and SEP could also become retardant ofcrystallization, acting as physical hindrance if thereare important interactions between polymer matrixand clay, and consequently obtaining insignificantchanges in the crystallization and melting processof PLA.

3.3. Dynamic-mechanical thermal analysis(DMTA)

The dynamic-mechanical experiments of PLA andnanocomposites are reported in Figure.6, Figure.7

and Table 3, showing the temperature dependenceof Storage Modulus (E!) and tan%0 for all the studiedmaterials. A general enhancement of E! below Tgwas observed compared to neat PLA: at 20°C, E!increases of ca. 60, 35 and 45% were obtained forPLA/CLO, PLA/SEP and PLA/ZrP, respectively(Table 3). Higher enhancements of E! were observedabove Tg: at 65°C increases of ca. 150, 120 and170% were obtained for PLA/CLO, PLA/SEP andPLA/ZrP, respectively.The dynamo-mechanical results show improve-ments in E! with the addition of all nanoparticles,indicating that addition of particles into PLA matrixresults in a remarkable increase of elastic propertiesfor PLA composites, more noticeable at higher tem-peratures (above 60°C), which means an increase ofthermal-mechanical stability for the composites athigh temperatures [38]. The increase of E! for com-posites with temperature compared to that of neatPLA may be attributed to a restriction in the move-ments of the polymer chains above Tg [2, 38]. How-

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Table 2. DSC data on PLA and PLA/nanocompositesobtained by second heating curves

a)Temperatures quoted are peak temperatures

Sample Tcca)

[°C]!Hcc[J/g]

Tma)

[°C]!Hm[J/g]

Tg[°C]

PLA 130 2 154 2 61PLA + CLO 131 2 154 2 61PLA + SEP 131 3 154 3 61PLA + ZrP 129 11 154 12 60

Figure 6. Temperature dependence of storage modulus (E!)for PLA and PLA/composites

Figure 7. Temperature dependence of tan0 for PLA andPLA/composites

ever, given the very low storage modulus values ofneat PLA above Tg, these enhancement in modulusabove this temperature have limited practical inter-est, absolute values for E! being below 50 MPa forall composites.This improved strength has also been observed forother polymer/clay nanocomposites, depending onthe degree of dispersion of clay in the polymermatrix [13, 15]. In the case of PLA nanocompositesin this study, all composites present E! improve-ments both below and above glass transition tem-perature, explained by strong interactions betweenthe particles and PLA molecules originated fromhydrogen bonding between the carbonyl groups ofPLA and the hydroxyl groups belong to the struc-ture of CLO, SEP and ZrP (see Table 1). Indeed,such interactions have been previously verified byother researchers in biopolymer nanocompositesbased on similar nanoparticles rich of surface silanolgroups (Si–OH) [21, 39, 40]. In this case, they havecorroborated the possible spontaneous assembly ofthe biopolymer matrix and nanoparticles throughhydrogen-bonding interactions, by the observedinfrared perturbation of the O–H stretching vibra-tions assigned to the surface silanol groups. Interest-ingly enough, similar increases of E! were obtainedupon the addition of ZrP into PLA as compared toCLO and SEP (even higher at 65°C), despite its lowdispersion level into the PLA matrix according toWAXS and SEM analysis, and probably attributedto the high crystallinity of ZrP particles as well as tothe very regular distribution of ZrP micronic parti-cles along all the polymer matrix (see Figure.1band 4).The presence of the particles does not show signifi-cant shift of tan0 (Figure.7 and Table 3) for all com-posites compared to pure PLA, in agreement withDSC analysis.

3.4. Melt rheologyThe rheological properties of polymer nanocom-posites are directly related to their melt behaviorduring processing, such as extrusion or injectionmoulding. Moreover, rheology offers a mean toassess the level of filler dispersion in nanocompos-ites directly in the molten state, given that rheologi-cal properties of particle-filled materials are sensi-tive to the structure, particle size, shape and interfacecharacteristics of the dispersed phase [2].The rheological behavior was studied for the PLAnanocomposites containing well dispersed particles(CLO and SEP); the linear dynamic viscoelasticcurves for these materials as well as neat PLA areshown in Figure.8. Both G!(,) and G"(,) moduli ofnanocomposites increase at all frequencies as com-pared to PLA. At high frequencies, the values ofG!(,) and G"(,) of neat PLA are practically unaf-fected by the presence of nanoparticles. However,at low frequency, the presence of nanoparticlesCLO and SEP induces an increase of G!(,) andG"(,) compared to neat PLA (see Figure.8a, b).Indeed, at , <%10 rad*s–1 for PLA/CLO and , <0.05 rad*s–1 for PLA/SEP, both G!(,) and G"(,)display a significantly diminished frequency depend-ence as compared to PLA. The lower frequencydependence and the higher absolute values of thedynamic moduli observed for PLA/CLO comparedto PLA suggest the formation of a ‘network’ struc-ture in the molten state [41]. This is explained bythe presence of highly dispersed silicate layers ofCLO which are incapable rotating freely and hence,by imposing small ,, the relaxations of the struc-ture are almost completely prevented. This type ofprevented relaxation due to the highly geometricconstrains of dispersed silicate layers leads to themarked presence of the pseudo-solid like behaviorfor PLA/CLO system [41]. Similar results areobtained for PLA/SEP, but only observing theachievement of a spatially linked structure deter-mining G!(,) plateau for , <%0.05 rad*s–1, probablydue to its lower dispersion level as compared toPLA/CLO in accordance with SEM results and/ordue to the different geometrical features comparedto lamellar clay, i.e. needles vs platelets.The complex viscosity [|1*|(,)] curves are alsoshown in Figure.8c; it is possible to observe at low

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Table 3. E! and tan0 values of PLA and PLA/composites at20 and 65°C. Estimated experimental error ±10%

Sample E" at 20°C[MPa]

E" at 65°C[MPa]

T at tan#max[°C]

PLA 2429 13 64PLA + CLO 3815 32 65PLA + SEP 3264 28 64PLA + ZrP 3529 35 65

, region (, <%10 rad*s–1) that the neat PLA showsNewtonian behavior while PLA/CLO (, <%10 rad*s–1)and PLA/SEP (, <%0.05 rad*s–1) exhibit a consider-able increase of the viscosity with decreasing ,,especially in the case of PLA/CLO. This last behav-ior is associated to a solid-like behavior commentedabove, especially for CLO. By increasing the shearrates, shear thinning behavior was observed forCLO and SEP materials, indicating that these nano -particles do not significantly influence viscosity at

higher frequencies, reaching [|1*|(,)] values analo-gous to those found for neat polymer.

3.5. Oxygen barrier propertiesThe O2 gas permeability values of pure PLA andseveral composites are presented in Figure.9, show-ing that oxygen permeability of PLA does not varysignificantly, taking into account the experimentalerror, upon addition of SEP and ZrP particles. Onlyaddition of 5.wt% CLO leads to a decrease of thePLA oxygen permeability (by ca. 30%).The higher oxygen barrier properties of CLO nano -composites as compared to PLA can be explainedby the concept of the tortuous path, given to its highclay dispersion, according to WAXS and SEM analy-sis, and its clay platelets like morphology whichmaximize the permeant path length due to the largelength to width ratio [2]. In the case of SEP, the lackof barrier effect is likely related to its needle likemorphology, which cannot play a considerable bar-rier role of oxygen molecules through the bulkmaterial as compared to platelets morphology ofCLO. In the case of ZrP based material, the low dis-persion level of lamellar nanoparticles into PLA isassumed to cause the lack of significant effects onpermeability.For food packaging applications is widely commonthe use of Polystyrene (PS), Polypropylene (PP) andPolyethylene terephthalate (PET), given their goodmechanical and oxygen barrier properties in thisindustrial field. The typical oxygen barrier values ofthese polymers are listed in Table 4; comparing theseresults with those obtained for neat PLA (see Fig-ure 9), this results to be an alternative to PS and PP,in terms of oxygen barrier properties. The additionof CLO can decrease the oxygen permeation of neat

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Figure 8. Reduced frequency dependence of (a) storagemodulus [G!(,)], (b) loss modulus [G"(,)], and(c) complex viscosity [|1*|(,)] at 190°C of neatPLA and PLA/composites

Figure 9. Oxygen gas permeability of PLA and PLA/com-posites, measured at 23°C and 0% relative humidity

PLA; however, this decrease is not sufficient toreach the low permeability of PET. The deep under-standing of chemical and physical interactionbetween CLO, SEP and ZrP particles and the poly-mer could result in the optimization of the melt blend-ing conditions and clay content in order to obtainpolymer nanocomposites with the reduced oxygenpermeation required for materials with high oxygenbarrier properties such as PET, contributing to thedevelopment of new bio-based polymer compositesfor food packaging applications.

3.6. Optical transparencyIn polymer nanocomposites, it has been reportedthat film transparency is an effective index for pro-viding information on the size of dispersed particlesin the polymer matrix, typically observing aggre-gate domains larger than visible wavelength couldobstruct light, leading to translucent or opaquefilms [21], while highly dispersed nanoparticles in atransparent polymer matrix result in optically clearnanocomposites in visible light given that the sizeof particles is smaller than the wavelength [2].All films of neat PLA and PLA/composites obtainedby compression moulding were optically transpar-ent. Indeed, Table 5 shows the cumulative transmis-sion over visible spectra (380–780 nm wavelength)on neat PLA and composites; these results showthat the absorption in the visible region, taking intoaccount the experimental error, is not significantlyaffected by the presence of any particle, thereforethe characteristic transparency of PLA film of 50 µm

thick is not considerable reduced in the presence ofthese particles.

4. ConclusionsPoly(lactic acid) (PLA) based nanocomposites pre-pared by adding 5 wt% filler content of organicallymodified montmorillonite (CLO) and unmodifiedsepiolite (SEP) were obtained by melt blending.WAXS and SEM analysis showed a good disper-sion of CLO and SEP into the polymer matrix. PLAcontaining ZrP showed a limited dispersion on PLAmatrix as compared to CLO, likely due to its signif-icant lower interlayer thickness, leading to forma-tion of a polymer microcomposite.DSC analysis showed that neat PLA and compos-ites presented no tendency to crystallise on cooling.Nevertheless, it was observed that PLA was able tocrystallize on heating, and that the addition of ZrPcan promote the extent of crystallization of the PLAon heating, which can be explained by the regulardistribution of ZrP micronic particles able to act asefficient nucleating agents for PLA crystallization.The dynamo-mechanical results showed improve-ments in E! with the addition of all particles, result-ing in a remarkable increase of elastic properties forPLA composites, especially at temperatures close tothe glass transition temperature. These E! increaseswere associated to possible interactions between theparticles and PLA molecules originated from hydro-gen bonding between the carbonyl group of PLAand the hydroxyl groups belong to the structure ofCLO, SEP and ZrP. Interestingly, similar increasesof E! were obtained upon the addition of ZrP intoPLA as compared to CLO and SEP, despite its lowdispersion level into the PLA matrix, possiblyexplained by the high crystallinity of ZrP particlesas well as to the very regular distribution of ZrPmicronic particles along all the polymer matrix.The melt viscosity and dynamic shear moduli ofnanocomposites were also enhanced significantlyby the presence of CLO and SEP. The dynamic shearstorage modulus, G!, and loss modulus, G" of thesenanocomposites, exhibited less frequency depend-ence than pure PLA at a low frequency range. Theseresults could be attributed to important interactionsbetween PLA and nanocomposites, especially CLO,forming an interconnected structure within the PLAmatrix.

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Table 4. Oxygen permeation of PS, PP, PET films, valuestaken from Massey [42]

Reference Oxygen permeation[cc$mm/(m2$day$atm)]

Polystyrene (PS) 117–157Polypropylene (PP) 59–102Polypropylene oriented film (PPof) 59–63Polyethylene terephthalate (PET) film 3

Table 5. UV-visible transmission average results of PLAand PLA/nanocomposites from 380–780 nm wave-length. Experimental error 10%.

Sample Absorbance[A/%m]

Transmission[%T/50 %m]

PLA 0.0081 39PLA + CLO 0.0101 31PLA + SEP 0.0066 43PLA + ZrP 0.0096 33

The oxygen permeability of PLA did not signifi-cantly vary upon addition of SEP and ZrP particles.Only addition of CLO leaded to an importantdecrease of the PLA oxygen permeability. Opticaltransparency measurements showed the transparencyof PLA in the visible region not to change signifi-cantly with the presence of particles, thus preserv-ing the good optical properties typical of PLA.The improvements of viscous and/or elastic proper-ties for PLA composites as well as the similar trans-parency and improved gas barrier properties couldprovide to these composites an opportunity in thefield of packaging. Even more, it is reasonable toexpect that by controlling the nanoparticle contentand processing conditions, PLA based nanocom-posites films with improved properties could beobtained for various purposes of industrial applica-tion.

AcknowledgementsThe authors would like to thank gratefully Dr. Simona Cec-cia and Emilia Giofreddi for their great support and guid-ance throughout the measurements and analysis of rheolog-ical tests. This work was partially supported by the researchproject NAMATECH ‘Nano-materials and -technologiesfor intelligent monitoring of safety, quality and traceabilityin confectionery products’ in the frame of Piedmont RegionConverging technologies call 2007.

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