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Polymer Degradation and Stability 97 (2012) 2027e2036

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Polymer Degradation and Stability

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

Microstructure and nonisothermal cold crystallization of PLA composites basedon silver nanoparticles and nanocrystalline cellulose

E. Fortunati a,*, I. Armentano a, Q. Zhou b,c, D. Puglia a, A. Terenzi a, L.A. Berglund c, J.M. Kenny a,d

aMaterials Engineering Center, UdR INSTM, University of Perugia, Str. Pentima 4, 05100 Terni, Italyb School of Biotechnology, Royal Institute of Technology, SE-106 91 Stockholm, SwedencWallenberg Wood Science Center, Royal Institute of Technology, SE-100 44 Stockholm, Swedend Institute of Polymer Science and Technology, CSIC, Juan de la Cierva 3, 28006 Madrid, Spain

a r t i c l e i n f o

Article history:Received 18 November 2011Received in revised form9 March 2012Accepted 16 March 2012Available online 28 March 2012

Keywords:Poly(lactic acid)Cellulose nanocrystalsSilver nanoparticlesMechanical propertiesThermal properties

* Corresponding author.E-mail address: elena.fortunati@unipg.it (E. Fortun

0141-3910/$ e see front matter � 2012 Elsevier Ltd.doi:10.1016/j.polymdegradstab.2012.03.027

a b s t r a c t

Poly(lactic acid) (PLA) based high performance nanocomposites, were prepared using an innovativecombination of nanocrystalline cellulose and silver nanoparticles. Binary and ternary systems wereprepared by solvent casting process and their morphological, mechanical and thermal responses wereinvestigated. Pristine (CNC) and surfactant modified cellulose nanocrystals (s-CNC) and silver (Ag)nanoparticles were used, and the effect of cellulose crystal nano-dimension, cellulose modification, andthe combination of cellulose nanostructures with silver nanoparticles, was investigated. The importantindustrial problem of slow crystallization of PLA was addressed by the use of cellulose nanocrystals asbiobased nucleating agents and the nonisothermal cold crystallization behaviour of reinforced binaryand ternary systems was studied.

The presence of surfactant on the nanocrystal surface favoured the dispersion of CNC in the PLA matrixwhile the thermal investigations and the nonisothermal crystallization studies underlined the ability ofs-CNC to act as nucleation agent in both binary and ternary nanocomposites.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

The development and use of biodegradable plastics fromrenewable resources in different application fields, have beenstimulated by public concerns and interest for environmentalprotection. Polymer based nanocomposites are usually preparedfrom materials with dissimilar properties but most polymercomposites are difficult to recycle or substantial costs are incurredfor their disposal. Poly(lactic acid) (PLA) is a versatile biopolymermade from the renewable resources, and shows good potential forapplications in several industrial sectors including packaging,automotive and biomedical fields [1,2]. In particular, PLA hasa significant potential for the packaging industry because of thecombination of high transparency and stiffness, excellent print-ability, and it can be processed using readily available productiontechnologies. However, the thermal properties, toughness, watervapour and gas barrier properties of PLA are inferior to those ofconventional petroleum-based polymers [3,4]. One possiblestrategy to tailor the properties of PLA is combining the polymerwith reinforcing elements.

ati).

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Strong research efforts have been focused on the use of naturalfibres as reinforcements for polymer composites, because naturalfibres are renewable and abundant [5]. Among reinforcing mate-rials of natural origin, purified cellulose fibres with one nano-scaledimension have more potential as reinforcing fillers in polymermatrices due to their good mechanical properties with very highbending strength and stiffness [6,7]. Specifically, cellulose nano-crystals (CNC) have attracted significant attention during the lastdecade as potential nanoreinforcement in different polymers [7,8].

Cellulose nanocrystals (CNC) are typically a rigid rod-shapedmonocrystalline cellulose domain, 1e100 nm in diameter andfrom tens to hundreds of nanometers in length, depending on thespecies, cultivar and agronomical characteristics (e.g., plant matu-rity, characteristics of the soil) from which cellulose is extracted.CNC have a crystalline structure, a very high aspect ratio (thedistribution of length/diameter is centred around 70), and a largesurface area (ca. 150 m2 g�1) [9]. Cellulose nanocrystals can beproduced from acid hydrolysis of various natural cellulose fibressuch as cotton, cellulose fibres from lignocellulosic materials, andmarine animal tunicate [10e12]. Cellulose nanocrystals have bettermechanical properties than the majority of the commonly usedreinforcing materials. It has advantages of their exceptionallyunique characteristics such as biodegradability and biocompati-bility, high stiffness, and low density. The use of cellulose

E. Fortunati et al. / Polymer Degradation and Stability 97 (2012) 2027e20362028

nanocrystals as nanoreinforcement is an important field in nano-technology, but there are still many obstacles for their use. Firstly,cellulose nanocrystals are not fully commercially available.Secondly, their production is costly as it is time consuming and isstill affected by low yields. Thirdly, they are difficult to use insystems that are not water based due to their strong hydrogenbonding and this affects the production of nanocomposites based ina water insoluble polymer like PLA. So, cellulose nanocrystals haveto be transferred from water to an appropriate solvent in order toproduce and process PLA nanocomposite systems [12e15].

The incorporation of antimicrobial substances representsa promising way to control undesirable growth of microorganisms.Heavy metals have long been recognized for their broad-spectrumbiocide effects, being the most commonly used inorganic antimi-crobials. Among them, ionic silver is known to have the largestantimicrobial capacity, with long-term biocide properties, lowvolatility and low toxicity to eukaryotic cells. Thus, silver-basedantimicrobial fillers in polymer matrices base their antimicrobialactivity on a sustained release of silver ions [16,17].

It is well accepted that themechanical and physical properties ofthe semi-crystalline polymers are governed by the supermolecularmorphology, which in turn is controlled by the crystallizationprocess. The properties of semi-crystalline polymers such as PLAdepend on the crystallization behaviour of the polymer. Hence, thestudy of the kinetic of crystallization is necessary for optimizing theprocess conditions and establishing the structure-property corre-lations in polymers. Hitherto the melt and/or cold crystallizationbehaviour of neat PLA [18], PLA copolymers [19], PLA stereocomplex[20] and PLA/other polymers blend [21], has been investigatedextensively by both isothermal and nonisothermal methods.

This research is focused on poly(lactic acid) (PLA) based highperformance nanocomposites produced using an innovativecombination of nanocrystalline cellulose and silver nanoparticles asantimicrobial agent. Binary and ternary systems were prepared bysolvent casting process and their morphological, mechanical andthermal responses were investigated taking into account the effectof cellulose crystal nano-dimension, cellulose modification, and thecombination of cellulose nanostructures with silver nanoparticles.The important industrial problem of slow crystallization of PLAwasaddressed by the use of cellulose nanocrystals as biobased nucle-ation agents and the nonisothermal cold crystallization behaviourof binary and ternary systems was studied.

2. Experimental

2.1. Materials

Poly(lactic acid) (PLA) 3051D, with a specific gravity of1.25 g cm�3, a molecular weight (Mn) of ca. 1.42 � 104 g mol�1, anda melt flow index (MFI) of 7.75 g/10 min (210 �C, 2.16 kg) wassupplied by Nature Works�. PLA pellets were dried in a vacuumoven at 98 �C for 3 h as suggested in material datasheet.Commercial silver (Ag) nanopowder, P203, with a size distributionranged from 20 to 80 nm, was purchased from Cima NanoTech(Corporate Headquarters Saint Paul). Microcrystalline cellulose(MCC, dimensions of 10e15 mm) was supplied by Sigma Aldrich.

2.2. Nanocrystal synthesis and modification

Microcrystalline cellulose (MCC) was hydrolyzed in sulphuricacid hydrolysis (64% (wt/wt)) at 45 �C for 30 min following therecipe used by Cranston and Gray [22]. Immediately following theacid hydrolysis, the suspension was diluted 20-fold with deionizedwater to quench the reaction. The suspension was centrifuged at4500 rpm for 20 min to concentrate the cellulose and to remove

excess aqueous acid. The resultant precipitate was dialyzed againstwater for 5 days and an ultrasonic treatment by means of a tipsonicator (Vibracell, 750) for 5 min was performed. The resultantcellulose nanocrystal aqueous suspension was approximately 0.5%(wt/wt) by weight and the yield was ca. 20%. Mixed bed ionexchange resin (Dowex Marathon MR-3 hydrogen and hydroxideform) was added to the cellulose suspension for 48 h and thenremoved by filtering through hardened ash less filter paper(Whatman 541). CNC were neutralized by addition of 1.0% (v/v) of0.25 mol l�1 NaOH [23]. The synthesized nanocrystals wereexamined by transmission electron microscope (TEM, Philips Tec-nai 10) operated at an acceleration voltage of 80 kV. A droplet ofdilute CNC suspension (0.1 wt%) was deposited on a bacitracin-pretreated surface of a carbon-coated grid.

Cellulose nanocrystals were modified with an acid phosphateester of ethoxylated nonylphenol. The CNC modified with surfac-tant (designed as s-CNC) were prepared by adding the Beycostat AB09 (CECCA S.A.) surfactant, to the water suspension containingnanocrystals in portion of 4/1 (wt/wt) using an estimated weight ofCNC directly after the hydrolysis process [12]. This high content ofsurfactant is necessary to re-disperse the cellulose nanostructuresin organic solvent after the freeze-drying process [12]. The pH ofthe suspension was then adjusted to 8.5 using the same 0.25 wt%NaOH solution as above.

After freeze-drying, chloroform was directly added to thecellulose powder (both pristine CNC and s-CNC) forming 1 wt%suspensions. In order to improve the dispersion of the cellulose inchloroform, the suspensions were exposed to sonication (Vibracell,750) for 1 min in an ice bath. The microstructure of unmodified andsurfactant modified cellulose nanocrystals and their morphologyafter the dispersion in the organic solvent, were investigated bymeans of field emission scanning electron microscope (FESEM,Supra 25-Zeiss). Few drops of the different suspensions were caston to silicon substrate, vacuum dried for 2 h and gold sputteredbefore the analysis. Photographs of CNC or s-CNC solution weretaken in order to visually evaluate the effect of surfactant on thestability of the system.

The morphological features of the modified nanocrystals wereinvestigated using atomic force microscopy (AFM). Few droplets ofdilute CNC suspension (0.1 wt%) were deposited on a silicon waferand analyzed in a tapping mode with a scanning probe microscope(Nanoscope IV, Multimode TM from Digital Instruments). Heightand phase images were obtained under ambient conditions witha typical scan speed of 0.5e1 line s�1, using a scan head witha maximum range of 100 mm � 100 mm.

2.3. PLA nanocomposite film preparation

PLA nanocomposite films were prepared by the solvent castingmethod [24]. PLA (2 g) was dissolved in 20 ml of chloroform withvigorous stirring at room temperature (RT). The dissolved solutionwas poured onto a glass Petri dish with a diameter of 9 cm, andthen allowed to dry for about 24 h at RT. For the preparation of thenanocomposite films, a predetermined amount of s-CNC suspen-sion in chloroform was mixed with the previously prepared PLAsolution. The solutions were stirred for 4 h before they were castonto the glass Petri dish and dried. The resultant films witha thickness of ca. 200 mmwere peeled from the glass Petri dish afterdrying.

Binary nanocomposites with 1 wt% or 5 wt% of s-CNC andternary nanocomposite films with 1 wt% of Ag and 1 wt% or 5 wt%of s-CNC were prepared. For comparison, PLA/CNC nanocompositefilms with 1 wt% and 5 wt% of unmodified CNC and ternary systemswith also 1 wt% of Ag nanoparticles were prepared following thesame procedures as described above. The material formulations are

E. Fortunati et al. / Polymer Degradation and Stability 97 (2012) 2027e2036 2029

shown in Table 1. All systemswere placed in a vacuum oven at 40 �Cfor two weeks in order to remove all remaining chloroform.

2.4. Characterization methods

Dynamic-mechanical thermal analysis (DMTA) was performedwith an ARES N2 instrument (Reometric Scientific). The followingparameters were fixed for DMTA measurements: constantfrequency of 1 Hz, strain amplitude of 0.1%, temperature range of25 �Ce100 �C, heating rate of 3 �C min�1 and gap distance of20 mm. The strain amplitude was selected after preliminary strainsweep test in order to maintain the linear viscoelasticity regimeduring the DMTA measurements. The samples were prepared bycutting strips from the films with awidth of 5 mm and five sampleswere characterized for each material formulation. Storage (G0) andloss (G00) moduli, tand values and the glass transition temperatures(Tpeak) were evaluated.

The mechanical behaviour of neat PLA, binary and ternary PLAnanocomposite systems was evaluated by tensile tests, performedon rectangular probes (100 � 10 mm2) on the basis of UNI ISO 527[25] with a crosshead speed of 5 mmmin�1, a load cell of 500 N andan initial gauge length of 50 mm. Average tensile strength (sb) andelongation at break (εb), yield stress and strain (sY, εY) and elasticmodulus (E) were calculated from the resulting stress-strain curves.The measurements were done at RT and at least five samples foreach formulation were tested.

Differential scanning calorimeter (DSC, Mettler Toledo 822/e)measurements were performed in the temperature rangefrom�25e210 �C, at 2, 5, 7 and 10 �Cmin�1, performing two heatingandone cooling scans.Melting and cold crystallization temperaturesand enthalpies (Tm, Tcc and DHm, DHcc) were determined from thesecond heating scan and glass transition temperature (Tg) werealso measured. The crystallinity degree was calculated as:

c ¼ 1�1�mf

��DHm � DHcc

DHm0

�*100 (1)

where DHm, is the enthalpy for melting, DHm0 is enthalpy ofmelting for a 100% crystalline PLA sample, taken as 93 J g�1 [26] and(1�mf) is the weight fraction of PLA in the sample.

In the nonisothermal cold crystallization process, the sampleswere first heated from room temperature to 210 �C at the selectedheating rates and held for 5 min to eliminate the previous thermalhistory. Then, the samples were cooled to RT and again heated to210 �C at the predetermined rates. The exothermal curves of heatflow as a function of temperature were recorded to analyze thenonisothermal cold crystallization process of the neat PLA and itsnanocomposites. All the experiments were carried out undernitrogen flow.

Table 1Material formulations.

Material PLA (wt%) CNC (wt%) s-CNC (wt%) Ag (wt%)

PLA 100 - - -Binary systemsPLA/1Ag 99 e e 1PLA/1CNC 99 1 e e

PLA/1s-CNC 99 e 1 e

PLA/5CNC 95 5 e e

PLA/5s-CNC 95 e 5 e

Ternary systemsPLA/1CNC/1Ag 98 1 e 1PLA/1s-CNC/1Ag 98 e 1 1PLA/5CNC/1Ag 94 5 e 1PLA/5s-CNC/1Ag 94 e 5 1

The microstructure of PLA nanocomposite films was investi-gated by field emission scanning electron microscope, FESEM,Supra 25-Zeiss. Cryo-cross sections of the nanocomposites weresputtered with gold and then analyzed.

3. Results and discussion

3.1. Cellulose nanocrystal properties and their dispersion in organicsolvent

In order to evaluate the hydrolysis process and the effect oncellulose structure properties, a morphological characterizationwas conducted by means of a transmission electron microscope.Fig. 1 shows a TEM image of cellulose nanocrystals. The CNC areindividualizedwith typical dimensions ranging from 100 to 200 nmin length and 5e10 nm in width [8,27]. TEM allowed for detailedinspection of the individual cellulose crystals aided with staining toenhance the contrast. The dispersion and self-ordering propertiesof cellulose crystals are restricted to aqueous suspensions ordispersions in a few organic solvents with high dielectric constantsuch as DMSO or ethylene glycol [28]. The main reason for suchrestriction is the electrostatic character of the suspension stability.The difficulty to disperse cellulose structures in organic solvent,strongly limits their applications. Moreover, during the freeze-drying process the crystals tend to agglomerate and, as a conse-quence, their redistribution in organic solvent is difficult. In thepresent research, after the hydrolysis process, the two finalsuspensions, containing unmodified crystals (CNC) and surfactantmodified cellulose (s-CNC) in solution, were freeze-dried and thecellulose powders were re-dispersed in chloroform. The freeze-drying step is critical for the production of nanocomposites withuniform CNC dispersion since the nanocrystals tend to agglomerateinto flakes during the freeze-drying due to strong hydrogen bondsformation as the water sublimate [8]. Therefore, one possiblesolution to avoid agglomeration would be to use a surfactant thatwill cover the CNC and thereby hinder the nanocrystals fromagglomeration. This strategy to employ a surfactant to avoidaggregation and favour the dispersion in organic solvent, as chlo-roform, was early used by Heux et al. to disperse crystals fromcotton and tunicin in toluene [12]. Moreover, in order to determineif crystals are well isolated in solutions, flow birefringence is oftenused and Heux et al. previous reported the capability of surfactanttreated crystals to show flow birefringence in organic solvents. Thisproperty is related to the surfactant possibility to hinder hydrogenbonding between the cellulose nanocrystals during the freeze-drying and further to aid the distribution of cellulose in organicsolvent as chloroform.

Fig. 1. TEM image of cellulose nanocrystals.

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The microstructure and agglomeration effects of CNC and s-CNCnanocrystals are reported in Fig. 2. The formation of aggregates isobserved for unmodified cellulose nanocrystals as shown in Fig. 2a.After modification with the surfactant, a good dispersion isobserved for the cellulose nanocrystals suspension in chloroform(Fig. 2b). The nanocrystals are well individualized without flakes[8]. The ability of the samples to form stable suspensions or gels inchloroform was qualitatively evaluated analyzing the sedimenta-tion of the suspensions in order to establish the effect of modifi-cation on the dispersion and stability in the organic solvent. Aftertransferring the suspensions into the testing tubes, photographswere taken at 24 h. The visual observations of CNC and s-CNCsuspensions (insert Fig. 2a and b respectively) confirm this result.The appearance of the dispersion differed depending on the pres-ence of the surfactant. In the case of unmodified CNC solution,a cloudy-white dispersion is observed, whereas in modified s-CNCdispersion, a transparent solution is obtained. The transparency ofs-CNC based solution indicates that the dimensions of a major partof cellulose particles in the suspension are below the limit for light-scattering. This confirms the results obtained by FESEM charac-terization of the re-dispersed suspensions after freeze-drying[29]. AFM analysis of the cellulose nanocrystals showed to bea good alternative to electron microscopy, without any limitationsregarding contrast and resolution.

The AFM image of modified cellulose nanocrystals re-dispersedin chloroform and deposited on silicon is reported in Fig. 2c whilethemorphology profile is shown in Fig. 2d. The results about crystalshape and size are in agreement with the TEM observations con-ducted for the pristine CNC obtained after the hydrolysis (Fig. 1)highlighting that no particular morphological modificationsoccurred during the dispersion in the organic solvent. In this case,however, it is also possible to estimate the thickness of the crystalsby measuring the height difference between the silicon surface andthe cellulose (Fig. 2d). Height scans show several individual cellu-lose crystals with a 15e20 nm height difference between the siliconsubstrate and the cellulose.

Fig. 2. Microstructure of freeze-dried cellulose nanocrystals without surfactant (a) and w(b) respectively). AFM image and profile of deposited surfactant modified s-CNC (c,d).

3.2. Nanocomposite mechanical characterization

Dynamic-mechanical thermal analysis (DMTA) provided infor-mation on mechanical behaviour, molecular relaxations as well asinteractions taking place in the produced materials as thetemperature is varied. The storage modulus (G0) as a function oftemperature and the tand peak for some selected formulations areshown in Fig. 3 while G0 at 25 �C and 65 �C and the glass transition(Tpeak) values for all the produced materials are reported in Table 2.Fig. 3a presents the DMTA results for PLA, PLA/1s-CNC and PLA/5s-CNC binary systems in order to deeply evaluate the effect ofdifferent contents of surfactant modified s-CNC on PLA properties.It can be seen that the surfactant modified nanocrystals are able toimprove the storage modulus of PLA in the region under the Tg forboth PLA/1s-CNC and PLA/5s-CNC binary systems. Moreover, attemperatures where the PLA matrix softened, the reinforcing effectof the cellulose nanocrystals at 5 wt% remains evident due to theirability to restrict the motions of the PLA chains, while a decreasedG0 value respect to PLA matrix is detected for PLA/1s-CNC. A similarbehaviour is also noticed for the binary systems based on unmod-ified CNC (Table 2) except for the PLA/1CNC nanocomposite thatshows a reduction of 21% in storage modulus value at 25 �C respectto PLA. This result can be explained by the different dispersion ofcellulose structures obtained during the compounding. The s-CNCcrystals are better dispersed in the PLA matrix also in the case of5 wt% and form looser networks with increased surface areacompared to the tight CNC flakes. The Tpeak values presented inTable 2 for binary systems based on CNC and s-CNC at differentcontents, support this assumption. The cellulose nanocrystals at5 wt% maintain the tand peak values of PLA while a slight shift atlower temperature, 5 �C, for PLA/1CNC and PLA/1s-CNC is detectedindicating a minor hindering of the segmental motions of the PLAmatrix. The increase in the storage modulus values and in the tandpeak temperature for the systems based on surfactant modified s-CNC indicate a high level of interaction between the PLAmatrix andthe surfactant that is able to hinder the crystallization process in

ith surfactant (b). Visual observations of CNC and s-CNC suspensions (insert (a) and

Fig. 3. Storage modulus curves and tand peaks from DMTA analysis for binary (a) and ternary (b) PLA nanocomposites.

E. Fortunati et al. / Polymer Degradation and Stability 97 (2012) 2027e2036 2031

PLA [27]. Fig. 3b presents the DMTA results for PLA, PLA/1s-CNC/1Ag and PLA/5s-CNC/1Ag ternary systems. All ternary nano-composites increase storagemodulus in the region under the Tg andthe PLA/5s-CNC/1Ag shows the highest value with about 43%increase respect to PLA due to the combination of cellulose struc-tures with silver nanoparticles [8]. Moreover, in ternary systemsbased on s-CNC, the tand peaks are maintained at higher temper-atures respect to the unmodified CNC based nanocomposites. TheDMTA study showed that the incorporated crystals were able tohinder the motions of the PLA chains in the matrix and thedispersion of cellulose structures inside the matrix was of greatimportance since it governed the available surface area of thecrystals.

3.3. Tensile test

The mechanical behaviour of neat PLA and PLA nanocompositeswas also evaluated by tensile test and the results are reported inTable 3. All the nanocomposite formulations show Young’smodulus higher than neat PLA (670 MPa) except the PLA/1CNCsystem that shows a decreased modulus. Meanwhile, the additionof only 1 wt% of s-CNC in PLA results in maintaining a Young’smodulus value close to that of PLAmatrix but in a drastic increase ofthe deformation at break (115%) with respect to pure PLA. It isimportant to observe that the PLA film shows a ductile behaviourwith higher elongation at break than expected values. The mainreason of this behaviour is due to the processing methodology usedin this work. Rhim et al. [30] demonstrated that PLA processed bysolvent casting shows a ductile behaviour with very high elonga-tion at break due to the residual solvent present in thematerial thatact as plasticizer. This effect is maintained and amplified in thenanocomposites loaded with 1 wt% of both unmodified and

Table 2Results from DMTA measurements of PLA and PLA nanocomposite films.

Material G0 [MPa] @ 25 �C G0 [MPa] @ 65 �C Tpeak [�C]

PLA 2800 � 100 280 � 40 52.5 � 0.5Binary systemsPLA/1Ag 1700 � 100 140 � 10 45.1 � 0.3PLA/1CNC 2200 � 200 170 � 20 46.7 � 0.1PLA/1s-CNC 3100 � 300 140 � 10 47.6 � 0.3PLA/5CNC 3500 � 200 370 � 10 50.0 � 0.4PLA/5s-CNC 3300 � 200 360 � 20 52.3 � 0.5Ternary systemsPLA/1CNC/1Ag 3500 � 100 200 � 30 45.7 � 0.7PLA/1s-CNC/1Ag 2600 � 200 250 � 20 48.0 � 0.2PLA/5CNC/1Ag 2900 � 200 400 � 50 42.6 � 0.4PLA/5s-CNC/1Ag 4000 � 200 190 � 40 50.8 � 0.3

surfactant modified cellulose nanocrystals. Moreover, an increasein tensile strength (53%) for PLA/1s-CNC is measured, highlightingthe reinforcement effect exerted by modified cellulose nano-crystals. The main reason for this behaviour is associated with thenanostructure characteristics resulting from more efficient disper-sion and alignment of the functionalized nanocrystals as discussedin the following morphological section. A more evident enhance-ment in Young’s modulus is detected for PLA nanocompositesloadedwith 5wt% of cellulose structures both in the case of pristineCNC (1.2 GPa) and modified s-CNC (1.1 GPa). However, when thefiller concentration increases at 5 wt%, the effect of the fillerbecomes prevalent and the materials show a brittle behaviour [8].The lowering of elongation at break in polymers with the additionof reinforcement at certain content, is a common trend observed inthermoplastic composites. The elongation at break is affected bythe volume fraction of the added reinforcement, the dispersion inthe matrix, and the interaction between the reinforcement and thematrix [31]. In our case, cellulose nanocrystals cause substantiallocal stress concentrations and then failure at reduced strain values.In addition, although the modification of cellulose nanocrystalsimproved their dispersion degree, probably the interfacial adhesionbetween PLA and cellulose nanocrystals was not enhanced [32].

A similarmechanical behaviour respect to the binary compositesis detected for ternary systems producedwith both 1wt% and 5wt%PLA/1CNC/1Ag and PLA/1s-CNC/1Ag systems that show drasticincrease of deformation at break in comparison to pure PLA; but inthis latter case, the combination of silver nanoparticles ensureda positive effect in Young’s modulus value. Moreover, although PLA/5CNC/1Ag and PLA/5s-CNC/1Ag are characterized by higher Young’smodulus with respect to the ternary systems loaded with 1 wt% ofcellulose structures, they also show a significant reduction of theelongation at break suggesting that 1 wt% content of cellulose is

Table 3Results from tensile test of PLA and PLA nanocomposite films.

Material sY [MPa] εY [%] sb [MPa] εb [%] EYoung [MPa]

PLA 20.2 � 1.0 6.2 � 0.7 18.7 � 0.6 170 � 50 670 � 10Binary systemsPLA/1Ag 32.0 � 2.8 4.5 � 0.2 22.9 � 1.5 290 � 50 1141 � 50PLA/1CNC 9.6 � 1.3 6.6 � 0.4 14.7 � 1.0 270 � 20 390 � 40PLA/1s-CNC 23.4 � 1.0 10.1 � 0.9 28.8 � 2.2 360 � 24 680 � 30PLA/5CNC 21.4 � 1.0 2.3 � 0.2 20.8 � 1.0 3.9 � 0.7 1200 � 30PLA/5s-CNC 27.4 � 1.6 3.1 � 0.1 31.3 � 2.4 4.9 � 0.1 1100 � 40Ternary systemsPLA/1CNC/1Ag 29.5 � 2.7 6.2 � 0.7 21.1 � 1.5 270 � 50 780 � 50PLA/1s-CNC/1Ag 20.0 � 0.8 4.4 � 0.2 14.4 � 1.7 250 � 50 810 � 30PLA/5CNC/1Ag 19.4 � 1.0 2.2 � 0.1 20.8 � 0.7 3.6 � 0.1 1000 � 50PLA/5s-CNC/1Ag 39.2 � 2.8 4.2 � 0.7 27.3 � 2.4 10 � 1 1200 � 50

E. Fortunati et al. / Polymer Degradation and Stability 97 (2012) 2027e20362032

a more adequate amount in order to obtain a well-balancedmechanical behaviour in both elastic and plastic regions useful forthe further practical application of these nanocomposite films.

3.4. Morphological investigation

Fig. 4 shows FESEM images of the cryo-fractured surfaces ofpure PLA and binary and ternary PLA nanocomposite films. In thePLA/5CNC system (Fig. 4a) the cellulose structures seem poorlydistributed. The nanocrystals remained present in flakes as iden-tified earlier. At higher magnifications (Fig. 4a, insert) these flakeshave a compact structure (arrows) and a poor interfacial adhesionto the PLA matrix is obtained. The cellulose nanocrystals appear tobe better distributed in the PLA/s-CNC nanocomposite for both theformulations at 1 wt% and 5 wt% of s-CNC (Fig. 4c,b respectively).These systems contain smaller agglomerates which are moreevenly distributed in the material compared to PLA/CNC basedcomposites. The agglomerates seem less compact and with goodinterfacial adhesion to the PLA and this effect indicate a welldistribution of single cellulose nanocrystals in the PLA. A similarmorphology is detected also for PLA/5s-CNC/1Ag ternary systems(Fig. 4d). These results are in agreement with the discussed DMTAand tensile test analysis. Moreover, the PLA nanocomposites basedon modified s-CNC appear to be very porous. Foam was generatedduring processing of the materials containing surfactant and it wastherefore believed that air was trapped in the materials [27].

3.5. Nonisothermal crystallization behaviour

The nonisothermal crystallization and melting behaviour of thePLA nanocomposites were investigated by DSC. The nonisothermalcold crystallization thermograms corresponding to the second

Fig. 4. Microstructure of fracture surface of PLA/5CNC (a), PLA/5s-CNC (b) and P

heating scan at various heating rates of neat PLA, PLA/1s-CNC, andPLA/5s-CNC binary systems, and PLA/1s-CNC/1Ag ternary nano-composites are shown in Fig. 5. The thermograms for the secondheating show the glass transition temperature (Tg), the cold crys-tallization exotherm (Tcc) and the melting endotherm (Tm) of thePLA matrix indicating that the cooling conditions applied were notable to produce the complete crystallization of the PLA. Withincreasing the heating rate, the Tcc shifts gradually to highertemperatures due to the thermal activation while above a heatingrate of 7 �C min�1, both the exothermic and endothermic peaksweaken significantly and are hard to be observed. Under this fastheating conditions, the amorphous PLA chains (Fig. 5a) do not haveenough time to well self-adjust, resulting in reduced cold crystal-lization and the subsequent multi melting behaviour. The presenceof multiple melting peaks in PLA could be related to the formationof different crystal structures [33] or to lamellar populations withdifferent perfection degrees [34,35]. A similar thermal behaviourcan be observed for nanocomposite systems (Fig. 5bed).

Table 4 shows the calorimetric parameters measured at thesecond heating scan performed at different heating rates (4) for PLAand PLA binary and ternary composites. It can be seen that thedegree of crystallinity of neat PLA is 6%, if measured at low heatingrate and it is nearly independent on the heating rate if lower than5 �C min�1. The significant decrease in the degree of crystallinity ofneat PLA is due to the rapid loss of crystallizability when annealedat heating rates higher than 7 �C min�1. The introduction ofmodified s-CNC at 1 wt% or 5 wt% produce an increase in the degreeof crystallinity, particularly evident at low heating rates. This effectis more evident in PLA ternary composites and the PLA/5s-CNC/1Agsystem shows the highest value of crystallinity degree at all studiedheating rates highlighting the positive effect of s-CNC modificationand their synergic combination with silver nanoparticles. The

LA/1 s-CNC (c) binary systems and of PLA/5s-CNC/1Ag (d) ternary system.

Fig. 5. Nonisothermal cold crystallization thermograms of neat PLA (a), PLA/1s-CNC (b) and PLA/5s-CNC (c) binary system and PLA/1s-CNC/1Ag (d) ternary sample from the secondheating scan at various heating rates.

Table 4Thermal properties of PLA and PLA nanocomposites at the second heating scan.

Samples 4 [�C min�1] Tg [�C] DHm [J g�1] Tm [�C] T0m [�C] DHcc [�C] Tcc [J g�1] c [%]

PLA 10 55.0 � 2.0 2.8 � 0.5 144.0 � 1.5 e 2.3 � 0.3 115.5 � 0.8 0.6 � 0.17 58.4 � 2.0 2.7 � 0.4 146.2 � 2.0 e 2.1 � 0.3 114.8 � 0.9 0.7 � 0.15 58.4 � 0.3 16.6 � 1.0 144.7 � 0.2 141.8 � 0.2 10.5 � 0.8 113.5 � 0.2 6.5 � 0.42 57.6 � 0.4 13.9 � 2.0 149.3 � 0.1 144.3 � 0.1 8.3 � 0.9 123.5 � 0.2 6.0 � 1.0

PLA/1s-CNC 10 57.0 � 2.0 0.3 � 0.1 149.7 � 0.1 e 0.3 � 0.1 125.8 � 1.0 0.9 � 0.27 57.1 � 0.1 4.0 � 0.4 149.7 � 0.2 e 0.2 � 0.1 125.8 � 0.5 4.3 � 0.35 53.2 � 1.0 30.6 � 0.2 144.7 � 0.5 137.5 � 0.2 22.4 � 0.4 100.7 � 0.4 8.9 � 0.42 48.7 � 1.0 40.1 � 0.4 144.7 � 0.1 135.7 � 0.7 27.9 � 0.8 93.6 � 0.7 13.4 � 0.7

PLA/5s-CNC 10 50.0 � 1.0 7.2 � 0.8 153.3 � 0.2 147.0 � 0.4 4.4 � 0.7 123.9 � 0.7 3.1 � 0.27 47.0 � 2.0 25.9 � 0.2 149.2 � 0.2 136.6 � 0.5 17.2 � 0.8 115.2 � 0.9 10.0 � 0.55 49.2 � 1.0 30.6 � 0.2 150.3 � 0.5 131.0 � 0.5 21.5 � 0.5 116.0 � 0.5 11.9 � 0.82 49.6 � 1.0 38.5 � 0.9 151.9 � 0.8 146.3 � 1.0 29.6 � 0.5 104.1 � 0.7 10.2 � 0.5

PLA/1s-CNC/1Ag 10 57.0 � 0.3 1.1 � 0.1 149.7 � 0.1 e 0.3 � 0.1 125.8 � 2.0 0.9 � 0.17 56.5 � 0.4 4.5 � 0.5 149.2 � 0.5 e 1.7 � 0.1 126.4 � 0.9 3.0 � 0.15 56.3 � 0.3 11.5 � 0.9 154.7 � 0.5 148.9 � 2.0 10.4 � 0.4 125.6 � 0.7 1.2 � 0.32 45.8 � 3.0 36.2 � 0.5 146.3 � 0.7 138.0 � 0.9 23.3 � 0.5 91.06 � 1.0 14.2 � 0.6

PLA/5CNC/1Ag 10 47.9 � 2.0 1.2 � 0.1 145.7 � 0.5 e 0.7 � 0.1 120.8 � 0.7 0.7 � 0.27 59.6 � 2.0 3.5 � 0.2 149.3 � 0.2 e 1.2 � 0.1 126.9 � 0.5 2.7 � 0.35 58.0 � 0.5 9.2 � 0.3 148.9 � 0.5 e 7.1 � 0.5 124.6 � 1.0 2.4 � 0.42 45.2 � 1.0 42.5 � 0.4 147.7 � 0.5 142.6 � 0.7 21.4 � 1.0 91.0 � 1.0 24.1 � 0.5

PLA/5s-CNC/1Ag 10 47.2 � 3.0 34.4 � 3.0 146.2 � 0.5 135.2 � 2.0 19.9 � 2.0 108.4 � 1.0 16.6 � 1.07 53.8 � 2.0 4.1 � 1.0 148.3 � 0.6 133.4 � 1.0 1.2 � 1.0 129.0 � 1.0 4.6 � 0.15 53.6 � 1.0 12.5 � 1.0 152.9 � 1.0 147.5 � 2.0 3.1 � 0.2 123.6 � 1.0 10.7 � 0.52 43.2 � 3.0 44.3 � 0.2 147.8 � 2.0 139.8 � 1.0 23.9 � 0.2 85.5 � 2.0 23.4 � 0.5

E. Fortunati et al. / Polymer Degradation and Stability 97 (2012) 2027e2036 2033

Fig. 6. DSC curves from the second heating scan for PLA and PLA binary and ternary systems at 2 �C/min and 5 �C min�1 heating rate.

Fig. 7. Relative crystallinity curves versus time and Avrami plots for PLA (a,c) and PLA/5s-CNC/1Ag (b,d) systems.

E. Fortunati et al. / Polymer Degradation and Stability 97 (2012) 2027e20362034

E. Fortunati et al. / Polymer Degradation and Stability 97 (2012) 2027e2036 2035

difference in the dispersion state of pristine CNC and modified s-CNC in the PLA matrix represents an important factor on thedifference crystallization behaviour of both binary and ternarysystems [36]. With addition of cellulose structures and silvernanoparticles the cold crystallization temperature, Tcc, increasesmonotonously respect to neat PLA value at heating rates higherthan 5 �C min�1. However, a decrease of Tcc at 2 �C min�1 respect tothe PLA is detected for all studied nanocomposites.

In the heating scan, Tcc exothermic peaks with very low inten-sity, also at low heating rates, are observed for the pure PLA (Fig. 5a)and PLA/CNC nanocomposite samples, indicating a rather lowcrystallization capability. As for the PLA/s-CNC binary samples(Fig. 5b,c), and for the ternary PLA/1s-CNC/1Ag, the crystallizationpeaks has relatively higher intensity as a result of enhanced crys-tallizability. The nucleation effect is remarkably enhanced whenhomogeneous cellulose nanocrystals are dispersed in PLA anda good interaction with silver nanoparticles is achieved [32].

Table 5Avrami kinetic parameters for the nonisothermal crystallization of PLA and PLAnanocomposites.

Samples 4 [�C min�1] n K [min�n] s1/2 [min]

PLA 10 1.97 6.18E-05 2.127 2.65 1.55E-06 2.425 3.76 1.51E-10 6.212 3.40 1.78E-10 11.56

PLA/1CNC 10 1.82 2.69E-04 1.387 2.51 1.74E-06 3.145 2.70 1.26E-07 5.402 3.11 1.41E-08 6.60

PLA/1s-CNC 10 1.87 1.23E-04 1.847 2.15 2.88E-05 2.105 2.84 1.00E-07 4.542 2.87 7.94E-08 6.27

PLA/5CNC 10 2.12 3.31E-05 2.007 2.33 1.32E-06 2.705 2.42 1.91E-06 4.602 3.23 4.37E-10 11.63

PLA/5s-CNC 10 1.90 1.82E-04 0.807 1.84 2.75E-04 1.455 2.12 2.04E-05 2.542 2.93 4.17E-09 10.65

PLA/1s-CNC/1Ag 10 2.14 3.47E-05 1.917 2.78 7.59E-07 2.515 2.83 1.10E-07 4.432 2.95 8.91E-09 8.56

PLA/5CNC/1Ag 10 2.32 6.61E-06 2.697 2.38 3.80E-06 2.935 2.93 3.63E-08 5.132 3.02 4.90E-09 8.23

PLA/5s-CNC/1Ag 10 2.52 2.34E-06 2.507 2.63 5.25E-07 3.565 2.66 4.17E-07 3.782 2.98 1.82E-08 5.65

3.6. Nonisothermal crystallization kinetic

In order to understand the effect of the addition of unmodifiedand surfactant modified cellulose nanocrystals and their combi-nation with silver nanoparticles on the bulk crystallization rate ofPLA, the nonisothermal crystallization kinetics for neat PLA and PLAnanocomposites at different formulations was also studied. Fig. 6shows the nonisothermal cold crystallization curves at heatingrates of 2 �C min�1 (Fig. 6a) and 5 �C min�1 (Fig. 6b) for neat PLAand its nanocomposites. Clearly, when cellulose structures andsilver nanoparticles are introduced, the exothermic peaks shift tolow temperatures gradually while remain constant or increases athigh rates (10 �C min�1).

As reported in Table 4, the presence of cellulose nanocrystalscombined with silver can promote the cold crystallizationphenomenon, they may act as the additional active substrates forthe heterogeneous nucleation, leading as a result to the formationof more defect ridden crystalline lamella and less ordered crystalsof the PLA matrix. Such imperfections in the crystalline structuremay also explain the lower melting point observed on nano-composites, which is more evident at lower heating rates.

The relative degree of crystallinity (Xt), as a function of crystal-lization temperature (T), is defined as:

Xt ¼ZT

T0

ðdHc=dTÞdT=ZTN

T0

ðdHc=dHÞdT (2)

where T0 and TN represent the onset and the end of crystallizationtemperature, respectively. The Avrami equation [37] can be used todescribe nonisothermal crystallization:

XðtÞ ¼ 1� expð�ktnÞ (3)

log½ � lnð1� XðtÞÞ� ¼ nlogt þ logk (4)

where X(t) is the relative crystallinity at the crystallization time, t; nis the Avrami exponent correlated with the nucleation mechanismand crystal growth dimensions; k is the overall kinetic constantdepending on the geometry of the growing crystalline phase. Thevalues of log[�ln(1 � X(t))] were plotted versus log t, and theAvrami exponent n and the crystallization rate constant k werecalculated from the slope and intercept of the linear fit. The Avramiequation rarely describes the whole crystallization process and isusually valid at the primary crystallization stage [38]. Moreover, inour case, parameters n and k have quite different physical meaningsbecause the temperature changes constantly during nonisothermal

crystallization [36]. Thus, the initial linear portions of the plotswere analyzed. Fig. 7a,b shows a typical relative crystallinity curvesand Fig. 7c,d reports the corresponding Avrami plots (Eq. (4)) forneat PLA and PLA/5s-CNC/1Ag ternary system respectively. It isnoted that a good linearity (Fig. 7c,d) was obtained, so this theoryseems to be useful to appropriately explain the nonisothermalcrystallization of PLA nanocomposite systems.

The results obtained from Avrami plots are listed in Table 5. Forall the studied formulations, k increases with heating rate. PLAgives a value of about 2.65 < n < 3.40 under heating rates lowerthan 7 �C min�1. This value is in accordance with that reported inthe literature [39] suggesting a spherulitic growth from nucleiinitiated at time zero or with plate-like growth fromnuclei initiatedover time. The n value of 1.97 indicates that the primary crystalli-zation stage of PLA may correspond to a two-dimensional, circular,diffusion-controlled growth of nucleation at the high heating rateof 10 �C min�1 [36]. With the addition of cellulose structures, nvalue decreases significantly especially in the case of surfactantmodified cellulose crystals. This indicates that s-CNC may act asnucleation agent somewhat, further confirming the resultsobserved on DSC curves [32]. Moreover, this effect is remarkableenhanced in ternary systems due to the positive combination ofcellulose structures and silver nanoparticles.

Half-time of crystallization (s1/2) is defined as the time at whichthe extent of crystallization reaches 50% and it is calculated asfollows [18].

s1=2 ¼�ln2k

�1=n

(5)

Generally, the s1/2 value increased with increasing of heatingrate. The s1/2 values of PLA nanocomposites both binary and ternary

E. Fortunati et al. / Polymer Degradation and Stability 97 (2012) 2027e20362036

systems are lower than those of pure PLA for all the heating ratesand PLA/5s-CNC/1Ag sample shows the half-time of crystallizationlowest value, indicating rapid bulk crystallization rate by theaddition of well-dispersed surface modified cellulose nanocrystalscombined with silver nanoparticles in the PLA matrix.

4. Conclusions

PLA nanocomposite films based on a combination of cellulosenanocrystals and silver nanoparticles were successfully producedin this work. Cellulose nanocrystals were synthesized by sulphuricacid hydrolysis and the hypothesis to modify crystal surface withsurfactant as a means to improve the dispersion of cellulose inorganic solvent and consequently in the PLAmatrix, was confirmed.

The presence of surfactant on the CNC surface favoured thedispersion of cellulose in the PLA matrix. Tensile tests underlinedthat the addition of only 1 wt% of s-CNC in PLA promoted a drasticincrease of deformation at break while DSC investigations showedincreased values of crystallinity in the nanocomposites based ons-CNC, highlighting the positive influence of surfactant on thenucleation process. Moreover, the nonisothermal crystallizationstudies, underlined the effect of cellulose nanocrystal content andmodification on the crystallization process. The combination ofcellulose nanocrystals and silver nanoparticles in PLA matrix offersperspectives to realize multifunctional ternary composites withhigh performance and antimicrobial response suitable for filmfood-active packaging materials.

Acknowledgements

The authors gratefully acknowledge the financial support fromthe National Consortium of Materials Science and Technology(INSTM). Authors wish to thank the Institute of Polymer Scienceand Technology, CSIC, Spain, and Dr. Laura Peponi for AFMmeasurements.

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