Plasticized Polylactide/Clay Nanocomposites. I. The Roleof Filler Content and its Surface Organo-Modificationon the Physico-Chemical Properties

MIROSLAW PLUTA,1 MARIE-AMELIE PAUL,2 MICHAEL ALEXANDRE,2 PHILIPPE DUBOIS2

1Centre of Molecular and Macromolecular Studies, Polish Academy of Sciences, 90-363 Lodz, Poland

2Laboratory of Polymeric and Composite Materials, University of Mons-Hainaut, 7000 Mons, Belgium

Received 29 July 2005; revised 15 October 2005; accepted 15 October 2005DOI: 10.1002/polb.20694Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: Polylactide (PLA)-layered silicate nanocomposites plasticized with 20 wt % ofpoly(ethylene glycol) 1000 were prepared by melt blending. Three kinds of organo-modifiedmontmorillonites—Cloisite1 20A, Cloisite1 25A, and Cloisite1 30B—were used as fillersat a concentration level varying from 1–10 wt %. Neat PLA and plasticized PLA with thesame thermomechanical history were considered for comparison. Nanocomposites basedon amorphous PLAwere obtained via melt-quenching. The influence of both plasticizationand nanoparticle filling on the physicochemical properties of the nanocomposites wereinvestigated. Characterization of the systems was achieved by size exclusion chromatogra-phy (SEC), thermogravimetric analysis (TGA), thermally modulated differential scanningcalorimetry (TMDSC), X-ray diffraction (XRD), and dynamic mechanical analysis (DMTA).SEC revealed a decrease of the molecular weight of the PLA matrix with the filler content.Thermal behavior on heating showed one cold crystallization process in the reference neatPLA sample, while two cold crystallization processes in plasticized PLA and plasticizednanocomposites. The thermal windows of these processes tend to increase with the fillercontent. The crystalline form of PLA developed upon heating was affected neither by theplasticization nor by the type and content of Cloisite used. It was found that the series oforgano-modified montmorillonites with decreasing affinity to PLA is Cloisite1 30B,Cloisite1 20A, and Cloisite1 25A, respectively. The dynamic mechanical properties weresensitive to the sample composition. Generally, the storage modulus increased with thefiller content. Glassy PEG, well dispersed within unfilled PLA matrix, exhibited also areinforcing effect, since the storage modulus of this sample was higher than for unplasti-cized reference at temperature region below the glass transition of PEG. Moreover, lossmodulus of all plasticized samples revealed an additional maximum ascribed to the glasstransition of PEG–rich dispersed phase, indicating partial miscibility of organic compo-nents of the systems investigated. The magnitude of this mechanical loss was correlatedwith the filler content, and to some extent, also with the nanofiller ability to be intercalatedby polymer components. VVC 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 299–

311, 2006

Keywords: montmorillonite; organoclay; physical properties; plasticized nanocom-posites; polylactide

INTRODUCTION

Recently, an enormous interest has arisen fromnanocomposite materials, especially for thoseprepared from polymer matrix and nanoscale-

Correspondence to: M. Pluta (E-mail: mpluta@bilbo.cbmm.lodz.pl)

Journal of Polymer Science: Part B: Polymer Physics, Vol. 44, 299–311 (2006)VVC 2005 Wiley Periodicals, Inc.

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sized inorganic particles. Different polymermatrices as well as various kinds of nanopar-ticles, with respect of geometry and properties,have been considered–isotropic nanoparticlessuch as Al2O3 or ZnO1 but also nanoparticlescharacterized by higher aspect ratio such asnanotubes2 or layered fillers.3 More particularly,polymer-layered silicate (clay) nanocompositesrepresent an interesting class of materials due tothe variety of structural forms possible to obtain(intercalated, exfoliated, mixed), leading to sig-nificant improvement of the physicochemicalproperties at low filler content (from 1 to 5 wt %)in comparison with the neat polymer or micro-composite counterparts.4 Such an improvementof the properties has motivated the academic andindustrial communities to develop nanocompo-sites, playing on the composition or on the prepa-ration method. Three main preparation techni-ques can be distinguished5: blending the moltenpolymer matrix with the layered silicate, mixingthe polymer with the layered silicate in a com-mon solvent (followed by recovering of the nano-composite material by solvent evaporation or pre-cipitation in a nonsolvent), or in situ polymeriza-tion of the monomer in the presence of dispersednanoparticles.

This work deals with the preparation andcharacterization of nanocomposites preparedfrom plasticized polylactide (PLA) and differenttypes of organically surface-treated montmoril-lonites, that is 1-nm thick-layered aluminosili-cates with high aspect ratios (50–1000). Plasti-cized PLA-based nanocomposites have a chanceto find new application in the field of flexiblepackaging material. Furthermore, Cloisites ap-plied are available as commercial products. Themelt blending technique, in which shearing proc-esses are involved to disperse the filler within thepolymer matrix, was used as a suitable methodfor large-scale production. PLA was chosen as amatrix because of its interesting mechanicalproperties (comparable to atactic polystyrene6)and good processability on standard processingequipments. Beside, PLA can be produced fromannually renewable resources; it exhibits otheradvantageous features, including biocompatibil-ity and biodegradability in short time periods(ecological aspect).6 Moreover, the increasingavailability of PLA, due to the development oflarge scale production (for example, ref. 7),makes this polymer convenient for the produc-tion of numerous environmental-friendly dispos-able plastic articles.

It has been shown that PLA-clay systems pre-pared by solution mixing and then casting8

exhibited improved mechanical properties, evenif the filler, arranged in the form of tactoids con-sisting of several silicate monolayers, did notlead to the formation of a real nanocomposite(neither complete intercalation nor exfoliation).On the other side, the PLA-clay nanocompositesprepared by blending of the molten polymerwith organo-modified clays (�3 wt %) exhibitedintercalated structure and increased storage mo-dulus compared with the unfilled polymermatrix.9–11 Intercalated morphologies have alsobeen found in PLA-clay nanocomposites pre-pared by extrusion.12–16 In these latter works,different properties, including mechanical, ther-mal, biodegradability, and permeability havebeen investigated.

It is known that PLA is not flexible enoughand breaks down at rather low deformation. Fill-ing PLA with layered silicates leads to nanocom-posites of even higher stiffness. This feature maybe undesired for some end-use applications of thenanocomposites. A way for reducing the rigidityof PLA-based materials consists in its plasticiza-tion that can be achieved simply by blending thePLA matrix with low molecular weight additivesor another miscible polymer, characterized by alow Tg. The best plasticizing effect in PLA isobtained with the monomer lactide itself.6 Otherorganic molecules have been tested as possibleplasticizers for PLA : oligo(e-caprolactone),12 cit-rate esters,17 poly(ethylene glycol) (PEG),11,18

glucose monoesters and partial fatty acidesters,19 glycerol and oligomeric lactic acid,20 tri-acetine, acetyl tributyl citrate, and acetyl triethylcitrate.21

In this study, we have considered PEG-plasti-cized PLA nanocomposites filled with layered sili-cates organo-modified with different ammoniumcations (namely, the commercially available Cloi-site1 20A, Cloisite1 25A, and Cloisite1 30B, seeexperimental). Owing to the organo-modification,the clays are intercalated with alkylammoniumcations bearing long alkyl chains, which increasethe interlayer spacing and can improve the com-patibility of layered silicates and the polymermatrix. As a result, chains of polymer matrix canintercalate the layered organo-clay and may fur-ther favor exfoliation, that is the dispersion ofindividual platelets upon blending under shear.In all the considered nanocomposites, PLAmatrix was plasticized by 20 wt % PEG with Mn

¼ 1000 (PEG1000). A simple blend of PLA and

300 PLUTA ET AL.

PEG1000 was also prepared as a reference compo-sition. The attention was focused on three maintopics: (1) the effect of the filler concentration (0,1, 3, 5, and 10 wt %), (2) the nature of its organo-modification as well as (3) the effect of plasticiza-tion on the basic physicochemical and thermalcharacteristics of the prepared nanocomposites.Molecular parameters were determined by size-exclusion chromatography (SEC). The type ofnanocomposites was established by X-ray diffrac-tion (XRD). Viscoelastic measurements (DMTA)were used to study the mechanical response andrelaxation processes in relation to the compositionand temperature. Thermal behavior on heatingwas determined using thermally modulated differ-ential scanning calorimetry (TMDSC). Finally, themost efficient filler for the modification of plasti-cized PLA was defined via the magnitude of theintercalation and the extent of the thermome-chanical property changes.

EXPERIMENTAL

Materials

Poly(L,L-lactide) containing 100% of L,L-lactideunits (abbreviated PLA) (from Galactic S.A., Mn

¼ 81,800, Mw/Mn ¼ 1.9) was used as the matrix.Poly(ethylene glycol) with a molecular weight of1000 (PEG1000) (from Sigma–Aldrich, Flukadiv.) was selected as the plasticizer of the PLA.Three organically treated clays supplied bySouthern Clay Products (Gonzales, TX) wereused in this study: Cloisite1 20A (modified withdimethyl di(hydrogenated tallowalkyl) ammo-nium cations), Cloisite1 25A (modified withdimethyl-2-ethylhexyl(hydrogenated tallowalkyl)ammonium cations), and Cloisite1 30B (modifiedwith methyl-bis(2-hydroxyethyl) tallowalkyl am-monium cations). Description of the organo-modi-fied montmorillonites used in this study is givenin Table 1.

Nanocomposite Preparation

Prior to the preparation, PLA was dried at 60 8Covernight under reduced pressure and storedunder vacuum in the presence of a humidityabsorbent. The clays were dried at 40 8C for 4 hunder reduced pressure. PEG1000 was used asreceived. Melt blending of PLAwith clay particlesand PEG1000 was carried out in the presence of0.3 wt % of Ultranox 626 stabilizer (General Elec-tric Co.). The components were loaded simultane-ously into an internal mixer (Brabender OHG)and blended at a rotation speed of 20 rpm during4 min. and then at 60 rpm for 3 min. The process-ing temperature was set at 185 8C; however, itincreased to about 195 8C as a result of shearing.Nanocomposites containing 1, 3, 5, and 10 wt %(relative to the inorganic content) of organo-modified montmorillonite Cloisite1 20A, Cloisite1

25A, and Cloisite1 30B were compounded, re-spectively. 20 wt % PEG1000-plasticized PLAwithout nanofiller was prepared as well. Fromthese materials, 0.5-mm thick samples were pre-pared by compression molding at 185 8C and thenquenching between two aluminum sheets at 0 8C.All nanocomposites considered in this work arespecified in Table 2. In this Table, the abbrevia-tion of pN10B30 for instance refers to plasticizednanocomposites (pN) based on 10 wt % Cloisite1

30B (10B30) and having a composition PLA:PEG1000:Cloisite1 30B in the weight ratio of70:20:10. Melt-quenched neat PLA and PLA plas-ticized with 20 wt % of PEG1000 are denoted asPLA and pPLA, respectively. In time-precedingcharacterization, all samples were stored inclosed plastic bags at lowered temperature to4 8C, that is below the melting temperature ofPEG component (Tm(PEG)) and well below theglass-transition temperature of PLA (Tg(PLA)),to inhibit structural reorganization ascribed tothe aging effects known to occur at ambient tem-perature.18,22,23

Table 1. Characteristics of the Organo-Modified Montmorillonites

MontmorilloniteType Ammonium Cation

OrganicFraction(wt %)

InterlayerSpacing(nm)

Cloisite1 20A (Hydrogenated-C18-C16-C14)2-Nþ(CH3)2 26.0 2.36

Cloisite1 25A (Hydrogenated-C18-C16-C14)-Nþ

(CH3)2[CH2-CH(C2H5)-C3H9]29.2 2.04

Cloisite1 30B (C18-C16-C14)-Nþ(C2H4OH)2CH3 20.1 1.84

PLASTICIZED PLA/CLAY NANOCOMPOSITES 301

Characterization

The number–average molecular weight (Mn) andpolydispersity index (Mw/Mn) of the neat PLA andPLA extracted from the nanocomposites weredetermined by SEC. These experiments wereaimed at quantifying the effect of additives onchanges of Mn upon thermal processing. All thesamples were dissolved in chloroform and filteredoff to eliminate the clay when present. Residualcatalyst was removed by liquid–liquid extractionwith a 0.1 M HCl aqueous solution, and PLA wasrecovered by precipitation of the chloroform solu-tion from cold methanol at 4 8C. SEC measure-ments were carried out in tetrahydrofuran (THF;sample concentration: 2 wt %) with a PolymerLaboratory (PL) liquid chromatograph equippedwith a PL-DG802 degazer, an isocratic HPLCpump (LC1120, PL; flow rate ¼ 1 mL/min), aBasic-Marathon autosampler from PL, a PL-RI re-fractive index detector, and four columns: a Plgel10-lm guard column (50 � 7.5 mm2) and threePlgel 10-lm mixed-B columns (300 � 7.5 mm2).Molecular weights and molecular weight distribu-tions were calculated by reference to a PS stand-ard calibration curve, with the Khun-Mark-Hou-wink equation for poly(L-lactide) in THF: Mn(PLA)¼ 0.4055 � Mn(PS)

1.0486.24 It should be noticedthat this procedure may lead to some loss of lowmolecular weight fraction of extracted PLA.

The thermal behavior was measured usingThermally Modulated DSC 2920 (TMDSC) fromTA Instruments. The TMDSC technique is anenhancement to conventional DSC whereby thetotal heat flow is separated into reversible events(i.e. the glass transition, melting) and nonreversi-ble components (contains kinetic events such ascold crystallization, crystal perfection and reorgan-ization, curing, and decomposition reactions).25

Measurements were performed with a ‘‘heatingonly profile’’ at a ramp of 3 8C/min, with a modula-tion period of 40 s and a modulation temperature

amplitude of 0.318 8C according to the manufac-turer principles25 and own experience with thePLA samples investigated. For the PLA, the glass-transition temperature (Tg(PLA)) and the enthalpyof transitions (DHc, DHm) connected with struc-tural transformations of exothermic and meltingnatures were determined. Conventional DSC char-acterization of the PEG1000, taken as the plasti-cizer of PLA, was performed using heating/coolingrun at a rate of 5 8C/min. Baseline calibration wasperformed according to the experimental require-ments.

The structure of nanocomposites as well asdevelopment of the crystalline structure of thePLA upon heating was investigated using h–hgoniometer of Siemens Diffractometer D5000with Ni-filtered Cu Ka radiation (k ¼ 0.154 nm).The measurements were performed at the sameexperimental conditions for all systems usingcomparable sample thickness (i.e. comparablescattering volume) for the purpose of comparisonand qualitative analysis of the recorded diffrac-tion intensities.

Dynamic mechanical properties of all sampleswere measured with an MkIII DMTA apparatus(Rheometric Scientific, Inc.) in a dual-cantiliverbending mode. The dynamic storage and lossmoduli (E0 and E@) were determined at a con-stant frequency of 1 Hz as a function of tempera-ture from �90 to 150 8C at a heating rate of3 8C/min.

RESULTS AND DISCUSSION

Size Exclusion Chromatography

PLA is reported to easily degrade upon melt proc-essing and it can loose even up to 80% of its initialmolecular weight, depending on its grade and theprocessing conditions.26 It is known that thehydrolytic degradation mechanism is dominant

Table 2. Codes and Compositions of the Nanocomposites

Composition by wt %PLA/PEG/Clay

Clay Nature/Sample Abbreviation

Cloisite1 20A Cloisite1 25A Cloisite1 30B

79:20:1 pN1A20 pN1A25 pN1B3077:20:3 pN3A20 pN3A25 pN3B3075:20:5 pN5A20 pN5A25 pN5B3070:20:10 pN10A20 pN10A25 pN10B30

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up to 215 8C, and above, nonhydrolytic degrada-tion becomes equally important.27 Therefore, itwas interesting to control the molecular charac-teristic (Mn and Mw/Mn) of PLA chains extractedfrom the so-prepared nanocomposite materials.The results are shown in Table 3 (with an accu-racy of 10%). Unprocessed PLA is characterizedby aMn equal to 81,800. As revealed by SEC, uponmelt processing, PLA plasticized by 20 wt %PEG1000 degraded noticeably (Mn drops by�33%). In turn, the Mn of PLA extracted fromplasticized nanocomposites is even more decrea-sed and shows dependence on the clay content;the weight loss reaching up to 50% at maximalfilling (10 wt %). However, the PLA degradation,according to the Mn decrease, seems to be rela-tively less dependent on the type of organo-modifi-cation of the clay surface. Several factors such asmechanical blending and shearing processes dur-ing compounding of PLA with clay, also the claycontent, and the presence of hydroxy groups onthe clay surface (upon organo-modification) cancontribute to the Mn reduction. The chemicalpurity of PLA matrix, for example, the presence ofresidual catalysts can also play a role in the re-duction of molecular weight upon melt processing.

Nanostructure by X-Ray Diffraction

The nanocomposite morphology is considered asa specific form of dispersion of the layered sili-cates within the polymer matrix combined withthe intercalation and/or exfoliation of the filler.These phenomena are generated by shearingprocesses during melt blending and prove to bedependent on compatibility of the polymer matrixand organo-modified surface of the clay. To deter-mine the nanostructure in the relation to thefiller type and its content, an XRD study hasbeen performed.

Figure 1(a–c) presents diffractograms recordedfrom 18 to 108 of 2h for plasticized nanocompo-sites with increasing content of the filler : Cloi-

site1 20A, Cloisite1 25A, and Cloisite1 30B,respectively. At low 2h angle region, all investi-gated nanocomposites are featured by two dis-tinct diffraction peaks, whose intensity increaseswith the filler content. The larger diffractionpeak localized at lower value of 2h (slightly above28) is indicative for the intercalation of the lay-ered silicate with the matrix constituents. Thesecond diffraction peak seen at 2h � 58 corre-sponds with the second registry (002). Theincrease of the interlayer distance (Dd)—definedas a difference between interlayer spacing in thenanocomposite and in the corresponding organo-modified clay—depends on the clay type and onits content, as far as nanocomposites filled withCloisite1 20A and Cloisite1 25A are concerned[Dd values are given in Fig. 1(a–c)]. For thesetwo organo-clays, the increase of the interlayerdistance Dd becomes smaller with the filler con-tent increase: below 1.60 nm for the materialscontaining Cloisite1 20A and below 1.34 nm forthe Cloisite1 25A-based nanocomposites. In con-trast, for the nanocomposites containing Cloi-site1 30B, the increase of the interlayer distanceis independent on the filler content used and it isthe largest with Dd ¼ 1.96 nm. This indicatesthat Cloisite1 30B is more prone to easily inter-calate plasticized PLA matrix than Cloisite1 20Aand Cloisite1 25A. Probably, prolongation ofblending time will lead to disappearance of con-centration dependence of the Dd value for thetwo later Cloisites.

Concerning the competition between PLAchains and PEG1000 molecules in the intercala-tion process of the layered silicate, the compari-son of the gallery space increases for the plasti-cized nanocomposite pN3A25 of value Dd¼ 1.25 nm, and for nonplasticized nanocompositecontaining the same amount of Cloisite1 25A, forwhich Dd ¼ 1.10 nm,9 tends to indicate the possi-ble cointercalation of PEG1000 with PLA in thisparticular clay. Since, for Cloisite1 20A andCloisite1 25A, Dd is varying with the relative

Table 3. Molecular Characteristics of PLA Chains Extracted From Nanocompositesas Determined by SEC

Sample Mn Mw/Mn Sample Mn Mw/Mn Sample Mn Mw/Mn

pN1A20 52,700 1.6 pN1A25 56,400 1.7 pN1B30 60,300 1.8pN3A20 38,400 1.7 pN3A25 62,900 1.7 pN3B30 39,400 1.8pN5A20 37,400 1.8 pN5A25 n.d. n.d. pN5B30 n.d. n.d.pN10A20 28,400 1.7 pN10A25 42,200 1.7 pN10B30 28,650 1.9

n.d, not determined.

PLASTICIZED PLA/CLAY NANOCOMPOSITES 303

amount of clay, one can assess that, depending onthe nanocomposite composition, the amount ofintercalated PLA and PEG1000 may vary from

one sample to the other. Interestingly, this Ddvariation is not observed for Cloisite1 30B. Thismight arise from a better polarity matchingbetween the PEG1000 and the bis(hydroxyl)functionalized ammonium cation of this particu-lar organo-clay, stabilizing the plasticizer withinthe clay galleries.

Thermal Properties

The thermal properties were determined forPEG1000 plasticizer, neat PLA, and pPLA-basedcompositions. Figure 2 shows conventional DSCthermograms obtained for PEG1000 on coolingscan followed by the heating one at a rate of 5 8C/min. No clear glass transition can be observed,even though low temperature (down to �80 8C)has been reached. This can be explained by thehigh crystallinity of this sample. The value of theglass-transition temperature for PEG1000(Tg(PEG)) was determined to be �61.0 8C.28 As isseen in Figure 2 PEG1000 crystallizes on coolingfrom the melt between 20 and 30 8C (with thepeak of the exotherm at Tc ¼ 27.5 8C); then, dur-ing the heating run, it melts from about 20 to40 8C (with the peaks at Tm1 ¼ 28.6 8C and at

Figure 1. (a) X-ray diffractograms for plasticizednanocomposites containing 1, 3, and 10 wt % of Cloi-site1 20A, respectively, (b) X-ray diffractograms forplasticized nanocomposites containing 1, 3, 5, and 10wt % of Cloisite1 25A, respectively, and (c) X-ray dif-fractograms for plasticized nanocomposites containing1, 3, 5, and 10 wt % of Cloisite1 30B, respectively.

Figure 2. Conventional DSC thermograms duringcooling from the melt followed by a heating scanrecorded at a rate of 5 8C/min for PEG1000 taken asreceived.

304 PLUTA ET AL.

Tm2 ¼ 35.6 8C). These transitions, close to theambient temperature, justified storage of thesamples at 4 8C.

Figure 3(a–c) presents TMDSC thermogramsobtained for plasticized PLA nanocomposites con-taining 1, 3, 5, and 10 wt % Cloisite1 25A,respectively, as well as for plasticized PLA andneat PLA for the sake of comparison. Since othernanocomposites containing Cloisite1 20A or Cloi-site1 30B exhibit very similar thermograms com-pared with Figure 3(a–c), their TMDSC charac-terizations are not included. Figure 3(a) showstotal heat flow thermograms, equivalent to con-ventional DSC thermograms. Neat PLA is char-acterized by a glass transition (Tg(PLA)) close to50 8C, a cold crystallization exotherm at Tc2

(87.0 8C), a premelting crystallization at Tc3

(158.6 8C), and finally, a melting endotherm atTm equal to 173.4 8C. For the plasticized samplepPLA (without filler), the total thermogram ismore complex. Indeed, for this sample, the glasstransition appears at lower temperature (around23.4 8C), and contrary to neat PLA, it is immedi-ately followed by a weak exothermic peak at Tc1

(33.8 8C), at which, interestingly, the plasticizershould already be molten. At a slightly highertemperature, a very weak and broad endothermappears that partially compensates the low tem-perature shoulder of the cold crystallization peakat Tc2 (around 88.3 8C). Therefore, the resultantcold crystallization peak for pPLA sample ap-pears at higher Tc2 than those observed for neatPLA and for the plasticized nanocomposites asdiscussed below. It is also seen that plasticizationdecreases Tm by �5 8C. In turn, the total thermo-grams for the plasticized nanocomposites are fea-tured by the absence of a weak endothermbetween Tc1 and Tc2 seen only for pPLA and bythe shift of the cold crystallization peak towardhigher Tc2 with the filler content.

Figure 3(b,c) shows separated TMDSC thermo-grams, the reversing signal (R), and nonreversingsignal (NR), respectively. The reversing signalclearly shows jump characteristic for the glasstransition of the PLA matrix Tg(PLA). The values

Figure 3. TMDSC thermograms for plasticizedPLA-based nanocomposites containing 1, 3, 5, and 10wt % Cloisite1 25A, respectively, and for plasticizedPLA and neat PLA: (a) total heat flow signals, (b)reversing heat flow signals, and (c) nonreversing heatflow signals. The thermograms are related to the totalmass of PLA systems.

PLASTICIZED PLA/CLAY NANOCOMPOSITES 305

of Tg(PLA) determined from the reversing signalare gathered in Table 4. In turn, the NR signal foreach plasticized PLA matrix reveals one additionalthermal event, in comparison to the correspondingtotal thermogram, that is the premelting crystalli-zation process at Tc3 from 130 to 150 8C [compareFig. 3(a) and (b)]. Insufficient deconvolution of theR and NR signals at Tc2 region does not allow anyfurther interpretation on this transition.

In Table 4 are gathered calorimetric parametersderived from the total thermogram (Tc1, Tc2, Tm,

DHc1, DHc2, and DHm) and from the reversible sig-nal (Tg(PLA)) for all considered samples. Calori-metric parameters characterizing very weak ther-mal events occurring between Tc1 and Tc2 for pPLAsample [Fig. 3(a)] and at Tc3 for all samples [Fig.3(a,c)] are not accounted in Table 4 for the sake ofclarity. Plasticization reduces Tg(PLA) from 49.7 8Cfor neat PLA to 23.4 8C for pPLA; however, thisdecrease is slightly smaller (2–5 8C) for plasticizednanocomposites without clear relation to the fillertype and related content. This stiffening action oforgano-clay particles toward plasticized PLAmatrix is also reflected by a shift toward highertemperature of the cold crystallization processes atTc1 and at Tc2 about 3–4 8C. This decrease in plasti-cization could be due to the inability of a part ofPEG1000 to interact with PLA because it is alreadyinteracting with the organo-clays (either interca-lated within or adsorb at their surface). Enthalpies

for these two cold crystallization processes aresomewhat higher (with exception of pN1A20) thanthose calculated for the unfilled pPLA sample (9.0and 19.9 J/gPLA for DHc1and DHc2, respectively).Consequently, Tm of plasticized nanocomposites isslightly higher (with exception of pN10B30) thanin case of unfilled pPLA. It is worth to note that theenthalpy of the cold crystallization of neat PLA(DHc2 ¼ 34.3 J/gPLA) slightly exceeds the sum ofthe cold crystallization enthalpies (DHc1 and DHc2)of the PLA-based nanocomposites (with exceptionof pN10B30), while for the melting enthalpy of thesamples compared above, the relation is opposite,that is DHm for all plasticized systems is higherthan for neat PLA. This observation is more likelyexplained by the fact that the aforementioned val-ues were determined from the total heat flow,which therefore do not take into account the contri-bution from the premelting crystallization effect(Tc3), as revealed by the nonreversing signal [Fig.3(c)]. Combination of all these observations clearlyindicates that the presence of organo-clays tends toinduce a better ability for the plasticized PLA tocrystallize upon heating.

Structural Reorganization

The study of structural reorganization of PLAaccompanying thermal transitions during TMDSCmeasurements were completed by XRD experi-

Table 4. Calorimetric Parameters Derived From the Total Heat Flow (Tc1, DHc1, Tc2, DHc2, Tm, and DHm) andthe Glass-Transition Temperature (Tg) Determined From the Reversing Signal for Nanocomposites, PlasticizedPLA and Neat PLA, as Determined by TMDSC (3 8C/min, Modulation Period 40 s, and Amplitude 0.318)

Sample Tg (8C)

1st Cold Crystallization Peak 2nd Cold Crystallization Peak Melting Peak

Tc1 (8C) DHc1 (J/gPLA) Tc2 (8C) DHc2 (J/gPLA) Tm (8C) DHm (J/gPLA)

PLA 49.7 – – 87.0 34.3 173.4 47.5pPLA 23.4 33.8 9.0 88.3 19.9 168.5 58.3pN1A20 28.1 45.2 9.8 82.9 17.4 170.0 56.4pN1A25 28.3 45.3 10.6 83.7 20.5 170.0 58.0pN1B30 25.5 45.7 9.5 83.1 23.8 169.8 58.3pN3A20 28.2 45.7 11.3 85.0 21.3 169.2 55.8pN3A25 25.8 44.8 11.6 85.5 23.4 170.3 58.2pN3B30 27.2 45.1 9.4 84.2 22.3 168.9 56.1pN5A25 28.5 45.7 11.3 84.5 22.6 169.4 57.5pN5B30 26.3 47.5 10.7 84.2 18.7 168.5 56.6pN10A20 27.6 47.2 12.9 85.8 18.1 168.5 55.6pN10A25 28.5 55.1 12.1 84.8 18.7 170.1 55.5pN10B30 28.2 49.6 16.3 86.8 23.3 167.2 61.5

Tg, glass-transition temperature determined from the reversing signal of the TMDSC; Tc1, Tc2, temperatures of the cold crys-tallization peaks; DHc1, DHc2, enthalpies of cold crystallization normalized to the unit mass of PLA matrix; Tm, temperature ofthe melting peak; DHm, enthalpy of melting normalized to the unit mass of PLA matrix.

306 PLUTA ET AL.

ments performed for all samples at selected temper-atures [indicated by arrows in Fig. 3(a)]. Figure 4presents representative diffractograms for nano-composite pN10A25 recorded at 20, 60, 130, and156 8C as well as for the comparative samples, thatis pPLA and neat PLA, recorded at 20 8C (thusbelow both Tg(PLA) and Tm(PEG)). The diffracto-gram of neat PLA shows (at 20 8C) a broad maxi-mum around 2h � 178 what confirms the amor-phous structure of the starting sample. However,for the pPLA sample, a sharpening of the peakaround 168 is observed at 20 8C. This indicates thatplasticized polylactide is susceptible to some struc-tural ordering immediately after melt-quenchingprocedure (even if the sample is hold below theambient temperature). Consequently, this poorlyordered phase undergoes melting upon heatingbetween Tc1 and Tc2 [cf. Fig. 3(a)]. However, thisstructural ordering is considerably reduced in plas-ticized nanocomposites as the diffractogram for thepN10A25 exhibits (at 20 8C) amorphous spectrumwith peaks ascribed only to the organo-clay particles(at 2.938, 5.698, 8.698, and 19.988 in Fig. 4). In thissample, a poorly ordered phase develops at Tc1 [Fig.3(a)] and its presence is reflected by small peak at2h � 168 seen on diffractogram taken at 60 8C (Fig.4). Diffractograms recorded at higher temperaturesfor plasticized nanocomposites, at 130 (above Tc2)and at 156 8C (close to Tc3), are similar and showcrystalline peaks at 2h of 14.78, 16.58, 18.88, and22.28 attesting for the pseudo-orthorombic crystal-line form of PLA matrix.29,30 This observationshows that the crystalline form developed because

of the cold crystallization at Tc2 and then because ofthe premelting crystallization at Tc3 is the same.Furthermore, no contribution from the crystallinephase of the PEG component is observed (if meas-ured below Tm(PEG)), the strongest diffractionpeaks of crystalline PEG1000 being found at 2h of19.38, 23.58, and 26.58.

Viscoelastic Properties

The temperature dependencies of the storagemodulus E0 and loss modulus E@ for the nanocom-posites containing Cloisite1 20A and for pPLAand neat PLA samples are shown in Figure 5(a,b),

Figure 4. XRD diffractograms for nanocompositepN10A25 and reference samples, that is neat PLAand pPLA, recorded at different temperatures as indi-cated within the figure.

Figure 5. (a) E0 versus temperature for nanocompo-sites containing Cloisite1 20A (1, 3, and 10 wt %) andfor pPLA and neat PLA samples. (b) E@ versus tem-perature for nanocomposites containing Cloisite1 20A(1, 3, and 10 wt %) and for pPLA and neat PLA sam-ples.

PLASTICIZED PLA/CLAY NANOCOMPOSITES 307

respectively. Analogous dependencies for the sys-tems containing Cloisite1 25A and Cloisite1 30B,including samples pPLA and PLA, are shown inFigures 6(a,b) and 7(a,b), respectively.

Storage Modulus

Neat PLA shows a gradual decrease of E0 with thetemperature increase from �95 to �50 8C; then, itrapidly drops because of the glass transition andreaches a minimal value around 75 8C. By furtherincreasing the temperature, E0 increases achiev-ing a maximal value around 100 8C. This reflectsan enhancement of the sample rigidity resulting

from the cold crystallization process detected bythe TMDSC and XRD. Above 100 8C, the E0 dropsagain because of softening and melting of the PLAmatrix. The influence of the plasticizer on E0 curvedepends on the temperature region. At very lowtemperature, glassy PEG reinforces PLA matrixas demonstrated by the E0 value displayed by thepPLA sample (this value is even higher than forneat PLA). Above the glass transition of PEG1000(Tg(PEG) �61.0 8C), E0 of pPLA sample rapidlydecreases reaching minimal values located atlower temperature range (50–80 8C) as comparedwith neat PLA. Then E0of pPLA increases becauseof the cold crystallization process in the tempera-

Figure 6. (a) E0 versus temperature for nanocompo-sites containing Cloisite1 25A (1, 3, 5, and 10 wt %)and for pPLA and neat PLA samples. (b) E@ versustemperature for nanocomposites containing Cloisite1

25A (1, 3, 5, and 10 wt %) and for pPLA and neatPLA samples.

Figure 7. (a) E0 versus temperature for nanocompo-sites containing Cloisite1 30B (1, 3, 5, and 10 wt %)and for pPLA and neat PLA samples. (b) E@ versustemperature for nanocomposites containing Cloisite1

30B (1, 3, 5, and 10 wt %) and for PLAp and PLAsamples.

308 PLUTA ET AL.

ture region characteristic for the neat PLA. Above100 8C, it decreases finally. Plasticized nanocom-posites exhibit a similar behavior. However, theirstorage modulus increases with the filler contentand seems to be influenced by the molecularweight of the polymer matrix. The stiffening effectis especially observed at lower temperature and itis preserved in the positive temperature rangewhere the PLA matrix undergoes recrystalliza-tion. In Table 5 are compared the E0 values for allsamples, determined at selected temperatures,that is at �75 8C (below Tg(PEG)), at 0 8C(between Tg(PEG) and Tg(PLA)), at 75 8C (atwhich the material exhibits the highest suscepti-bility for deformation), and at 100 8C (above thecold crystallization process at Tc2). From thesedata, it can be observed that, at �75 and 0 8C, E0

systematically increases with the filler content atleast up to 5 wt %. This increase is the most pro-nounced for the nanocomposites filled with Cloi-site1 20A. At 10 wt % loading, the E0 increaseappears lower than at 5 wt %, most probably dueto the reduced molecular weight of the PLAmatrix of the formed nanocomposite (Table 3).

Loss Modulus

Figures 5(b) and 7(b) show the temperature de-pendencies of the mechanical loss (E@) for thesame set of samples as considered earlier. Theneat PLA sample does not reveal any loss maxi-

mum in temperature region below its Tg(PLA).Actually, the Tg(PLA) value is determined inDMTA at a maximum of the E@ localized at51 8C, that is at nearly the same temperature asthose determined from the reversing signal ofthe TMDSC measurement (49.7 8C, Table 4).Consequently, the loss maximum (E@) connectedwith the glass transition of all plasticized sam-ples is shifted toward lower temperature. How-ever, the plasticized polylactide (pPLA sample)exhibits additional enhancement of the E@ in abroad temperature range, displaying a maxi-mum around �25 8C, that is well above Tg(PEG).This maximum in E@ is ascribed to the glasstransition of the PEG-rich dispersed phase andindicates that the polymer system is partiallymiscible. Interestingly, the magnitude of the dis-cussed mechanical losses is increased withinplasticized nanocomposites. Generally, higherfiller content leads to higher mechanical loss(note that this relation is not fulfilled for samplespN1B30, pN10B30, and pN10A25). Moreover,the E@ maximum at Tg(PEG) region is betterdeveloped for these nanocomposites in which theintercalation effect proved to be more pro-nounced, that is for those containing Cloisite1

30B and Cloisite1 20A. This response actuallyconfirms that the plasticizer molecules readily‘‘cointercalate’’ the organo-modified layered sili-cate, together with PLA. In other words, higherfiller contents allow for entrapping largeramounts of PEG molecules within the silicategalleries. This contributes, respectively, to theenhancement of the plastic response of the inter-calated regions to the deformation in the temper-ature range around Tg(PEG). Contrary, underabsence of plasticizer, the mechanical loss ofPLA-based nanocomposites is lower than that ofunfilled PLA15 in the considered temperaturerange. The latter observation supports the inter-pretation done earlier.

CONCLUSIONS

PLA-based nanocomposites plasticized with 20wt % PEG1000 were prepared by melt blending.Three types of plasticized nanocomposites con-taining from 1 to 10 wt % of different organo-modified layered silicates, that is Cloisite1 20A,Cloisite1 25A, and Cloisite1 30B, were consid-ered and compared with plasticized PLA andneat PLA as references. Plasticizing was appliedas a method for reducing the brittleness of the

Table 5. Values of the Storage Modulus Determinedat Selected Temperatures by DMTA (1 Hz, 3 8C/min)

Sample

Value of the Storage Modulus E0

(MPa) Determined at IndicatedTemperature

�75 (8C) 0 (8C) 75 (8C) 100 (8C)

PLA 1055 771 9 63pPLA 1261 567 10 83pN1A20 1751 870 9 48pN3A20 2867 1060 27 145pN10A20 2666 876 29 70pN1A25 1544 567 10 102pN3A25 1850 836 15 103pN5A25 1864 800 27 121pN10A25 1732 856 37 56pN1B30 1020 440 10 108pN3B30 1654 760 12 119pN5B30 2087 858 21 150pN10B30 1782 511 31 110

PLASTICIZED PLA/CLAY NANOCOMPOSITES 309

PLA matrix. For the same reason, the melt-quenching was used to promote the formation ofamorphous PLA matrices in all samples.

It is shown that plasticization reduces theTg (PLA) by about DTg ¼ 26.3 8C in case of un-filled PLA, while for the PLA-based nanocompo-sites, the reduction was somewhat smaller, DTg

� 21 8C. Both PEG and PLA seem to intercalatewithin the layered silicate galleries (semiexfolia-tion can not be excluded). The way PLA andPEG1000 intercalate seems to be dependent ofthe organo-modification of the selected clay.TMDSC and XRD analyses have demonstratedthat the physical organization of the investigatedsamples was thermally unstable. Indeed, thestructure of neat PLA undergoes transformationthrough a cold crystallization process into thecrystalline form classified as the a modification.Plasticized PLA and plasticized nanocompositesrevealed the occurrence of some additional struc-tural transition at lower temperature, slightlyabove Tg(PLA), connected with semiordering ofthe structure. This process is shifted towardhigher temperature with the filler content in-crease and it is completed by the cold crystalliza-tion process responsible for the formation of thecrystalline PLA phase. The presence of the plasti-cizer as well as filling with different type and con-tent of clays does not modify the crystalline evolu-tion of the PLA matrix.

Viscoelastic measurements revealed for theneat PLA one strong maximum of the mechanicalloss (E@) ascribed to the glass transition. Theglass transition of PLA matrix determined fromthe E@ maximum and from the reversing signalin TMDSC were comparable. In the case of plasti-cized PLA, an additional E@maximum assigned tothe glass transition of the PEG-rich dispersedphase was found (negative temperature region).This mechanical loss also occurred within plasti-cized nanocomposites and exhibited a tendencyto increase with the filler content. This increasewas somewhat correlated with the intercalationmagnitude, giving an evidence on the ability ofthe PEG molecules to penetrate the silicate gal-lery. The storage modulus of the nanocompositesexceeds those for the neat PLA, while dispersedPEG acts as a reinforcing agent for PLA in thelow-temperature region, below Tg(PEG).

The structural evolution of the plasticizedPLA-based nanocomposites as already pointedout in this study has been the object of a moredetailed investigation actually carried out on amuch longer time period, that is over three years.

Such an aging study is reported in the secondpart of this series.23

Contract grant sponsor: Poland State Committee forScientific Research; contract grant number: 7 T08E027 19. XRD measurements were carried out by M.P.at the Max-Planck-Institut fur Polymerforschung,Mainz (Germany) in Prof. T. Pakula lab and weremade possible by the grant 7T0 8E 027 19. M.-A. Paulthanks the F.R.I.A. (Fonds pour la Recherche Industri-elle et Agricole) for her Ph.D. grant. M.A. and Ph.D.are very grateful for the financial support from‘‘Region Wallonne’’ and European Community (FEDER,FSE) in the frame of ‘‘Pole d’Excellence Materia Nova’’.LMPC thanks the Belgian Federal Government Officeof Science Policy (STC-PAI 5/3). CMMS and LMPCthank the Polish Academy of Science and the FondsNational de la Recherche Scientifique (FNRS, Belgium)for a grant allowing traveling between the researchlaboratories.

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