European Polymer Journal 45 (2009) 2996–3003

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Contents lists available at ScienceDirect

European Polymer Journal

journal homepage: www.elsevier .com/locate /europol j

Macromolecular Nanotechnology

Organically modified rectorite toughened poly(lactic acid):Nanostructures, crystallization and mechanical properties

Bo Li, Feng-Xia Dong, Xiu-Li Wang *, Juan Yang, De-Yi Wang, Yu-Zhong Wang *

Center for Degradable and Flame-Retardant Polymeric Materials (ERCEPM-MoE), College of Chemistry, State Key Laboratory of Polymer MaterialsEngineering, Sichuan University, 29 Wangjiang Road, Chengdu 610064, China

a r t i c l e i n f o

Article history:Received 19 May 2009Received in revised form 20 July 2009Accepted 15 August 2009Available online 22 August 2009

Keywords:Poly(lactic acid)NanocompositeCrystallizationMechanical properties

0014-3057/$ - see front matter � 2009 Elsevier Ltddoi:10.1016/j.eurpolymj.2009.08.015

* Corresponding authors. Tel.: +86 28 85410755; f(X.-L. Wang).

E-mail address: xiuliwang1@163.com (X.-L. Wan

a b s t r a c t

To improve the toughness of PLA, poly(lactic acid) (PLA)/organically modified rectorite(OREC) nanocomposites were prepared via the melt-extrusion method. A partially exfoli-ated and partially intercalated structure was confirmed by WAXD and TEM. The crystalli-zation behaviors of neat PLA and nanocomposite were studied by POM and DSC, and it wasfound that OREC had a great effect on the overall crystallization rate and spherulitic textureof PLA. The presence of OREC could toughen PLA greatly. For example, when 1 wt.% ORECwas added, the elongation at break of the nanocomposite was increased to 210%. Thetoughening mechanism was analyzed through the observation of the inner structure ofthe tensile test bar using SEM.

� 2009 Elsevier Ltd. All rights reserved.

1. Introduction

Poly(lactic acid) (PLA) is a biodegradable thermoplasticaliphatic polyester, which can be synthesized from renew-able feed stocks [1,2], therefore it has been paid muchattention as an alternative to conventional synthetic poly-mers owing to the shortage of petroleum resources andenvironmental concerns. However, PLA is rigid and brittlebelow the glass transition temperature (Tg) ranging from55 to 65 �C, and its applications in some areas are re-strained. Adding plasticizer is a popular method forincreasing the plastic deformation ability of semi-crystal-line polymers. Poly(ethylene glycol) [3], poly(propyleneglycol) [4], citrate esters [5], triacetine [6] and acetyl tri-Bu citrate (ATBC) [7] were found to be efficient plasticizersfor PLA. Although a great improvement in elongation atbreak can be attained by adding a large amount of plasti-cizer, the glass transition temperature (Tg), modulus andthe yield strength of materials decreased severely. In addi-tion, plasticizer molecules easily migrate to the material

. All rights reserved.

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g).

surface at higher temperatures, causing the deteriorationof material properties [8].

In recent decade, various inorganic nanoparticles such aslayered silicates, carbon nanotubes and cellulose whiskershave been chosen as additives to enhance the performanceof polymers. Of particular interest are nanocomposites con-sisting of organically modified layered silicate (OMLS)because they often exhibit remarkably improved mechani-cal and various other properties compared with those of vir-gin polymer. Various kinds of layered silicates such asmontmorillonite, talc, kaolin, mica and so on are employedto prepare PLA/OMLS for enhancing the performance ofPLA. Ogata et al. [9] first prepared the composites of PLAand OMLS by a solution casting method, and the modulusof the composites were slightly higher than that of neatPLA. Ray and Okamoto’s group [10–13] prepared a series ofPLA/clay nanocomposites via melt-extrusion, which exhib-ited remarkable improvements in dynamic mechanicalproperties, flexural properties, gas permeability, and biode-gradability compared to that of neat PLA.

Usually, the modulus of the PLA/clay nanocompositescould be improved, whereas the elongation at breakdecreased as compared to neat PLA. Hasook et al. [14] pre-pared PLA/clay nanocomposites by melt compounding and

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found that the Young’s modulus increased, whereas thetensile strengths and elongations decreased. In general, itis believed that the concurrent property improvements innanocomposites come from interfacial interaction betweenthe polymer matrix and OMLS. A few weight percent ofwell dispersed OMLS with a layer thickness of the orderof 1 nm and a very high aspect ratio (e.g., 10–1000) cancreate much more surface areas due to the polymer/fillerinteraction. It can be deduced that different layered sili-cates with different aspect ratios would result in differentproperties of the nanocomposites.

Rectorite (REC) [15,16] is a regularly interstratified claymineral of dioctahedral mica-like layer (non-expansible)and dioctahedral smectite-like layer (expansible) in a 1:1ratio, its aspect ratio and interlayer distance are larger thanthose of MMT. The special structure was propitious to formintercalated/exfoliated nanocomposites which contributedto the enhancement of polymer properties. However, therehave been few papers that focus on the polymer/REC nano-composites up to now.

In this study, PLA composites with organically modifiedrectorite (OREC) were prepared by melt-extrusion, and thetoughness of PLA were improved obviously by addingOREC. The effect of organic/inorganic ratio on the crystalli-zation behavior and mechanical properties of the nano-composite was investigated. In addition, the tougheningmechanism was analyzed through observing the innerstructure of the fractured tensile test bars.

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2. Experimental

2.1. Materials

NatureWorks� PLA (4032D), a weight-average molecu-lar weight (Mw) of 260 kDa and polydispersity of 1.76determined by gel permeation chromatography (GPC),was used after dried under vacuum at 60 �C for 24 h. Dif-ferential scanning calorimeter (DSC) determined its glasstransition temperature (Tg) and melting point (Tm) to beca. 60 �C and 167 �C, respectively. The organic rectorite(OREC) with a d-space of 3.8 nm prepared by replacingCa2+ with octadecyl dimethyl benzyl ammonium cationthrough ion exchange reaction (cation-exchange capacityof 40 mequiv/l00 g) was provided by Hubei ZhongxiangRectorite Mine Exploitation Ltd. (Wuhan, China). ORECwas screened with a sieve of 500-mesh and dried undervacuum at 80 �C for 24 h before using.

2.2. Preparation of PLA/OREC (PLAOR) nanocomposites

PLA (pellet form) and OREC (powder form) were pre-mixed in a high-blender at 400 rpm for 5 min and subse-quently was melt-extruded by a co-rotating twin-screwextruder (CTE 20, Coperion Keya Machinery Co. Ltd., Nan-jing, China) with a L/D ratio of 44, operated at 170–190 �Cwith the screw speed of 90 rpm. Nanocomposites with 0.5,1, 2, 3, 5 wt.% OREC were manufactured, respectively. Here-after, the product were abbreviated as PLAORn, n means theweight percent of OREC in the composites. The neat PLA wasalso prepared according to the same procedure. The ex-

truded strands were pelletized by a pelletizer (LQ-300, Taiz-hou Xiangxing Plastic Machine Co., Taizhou, China) anddried under vacuum at 60 �C for 24 h before using.

2.3. Characterization

2.3.1. WAXDX-ray diffraction (WAXD) analyses were performed on a

DX-1000 X-ray diffractometer (Dandong, China) with Cu Karadiation (wavelength, k = 0.154 nm), operated at 40 kV and25 mA with a scanning rate of 0.06 �/sec.

2.3.2. TEMTransmission electron microscopy (TEM) was per-

formed on a Tecnai20 TEM (FEI, America) with a 120 kVaccelerating voltage. The pressed sheets were sectionedinto roughly 50–100 nm thin sections at ambient temper-ature with an ultramicrotome equipped with a diamondknife and without staining.

2.3.3. POMPolar optical microscopy (POM) was carried out with a

microscope (Nikon ECLIPSE LV 100POL) equipped with ahot stage (INSTEC STC 200). Samples weighing 3–5 mgwere melted on glass slip with cover slip to form 20–50 lm thick films. Each specimen was heated to 200 �Con a hot stage and held at that temperature for 5 min, thenwas cooled to the desired crystallization temperatures at arate of 60 �C/min. For studying the effect of crystallizationtemperature on the crystallization behavior of samples,110, 120 and 130 �C were set as the observation crystalli-zation temperature. The hot stage was calibrated with amelting point standard to 0.2 �C accuracy. As soon as thesample was cooled to desired temperature (Tc), observa-tion was started immediately and was continued. In addi-tion, photographs were taken by a digital camera.

2.3.4. DSCThermal transition behaviors of the samples were

investigated using differential scanning calorimeter (DSC)(TA Q200) calibrated using indium. All experiments werecarried out under a nitrogen atmosphere. The specimenswere heated to 200 �C and maintained at that temperaturefor 5 min to eliminate their thermal history; subsequentlythey were cooled to �30 �C at a rate of 5 �C/min, and thenwere heated to 200 �C at a rate of 5 �C/min. The crystalliza-tion temperature during cooling (Tc1), the enthalpies ofcrystallization during cooling (DHc1), the glass transitiontemperature (Tg), the crystallization temperature duringheating (Tc2) and the melting temperature (Tm) for all sam-ples are recorded and presented in Table 1.

2.3.5. Mechanical properties testMechanical properties test of the composites was per-

formed with a universal test machine (CMT4104, ShenzhenSANS Testing Machine Co., China) and conducted at 25 �Cusing a cross-head rate of 5 mm/min. The test bars werecut from films (thickness was about 0.5 mm) which wereprepared by compression molding at 200 �C for 5 min with10 MPa pressure. Data reported were the mean values of fivedeterminations and the standard deviation were calculated.

Table 1Thermal transition behaviors and crystallinity of neat PLA and PLAOR.

Samples Clay content (wt.%) Tc1a (�C) DHc1

b (J/g) Tg (�C) Tc2c (�C) Tm (�C) DHm (J/g) vc (%)

PLA 0 95 3.5 60 101 167 31.3 34PLAOR0.5 0.5 115 33.8 62 – 163, 169 38.2 41PLAOR1 1 94 14.4 61 97 167 32.2 35PLAOR2 2 94 12.8 60 98 166 31.6 35PLAOR3 3 95 15.3 61 98 167 31.7 35PLAOR5 5 96 19.9 61 96 167 31.0 35

a The crystallization temperature determined from the cooling scan.b The enthalpies of crystallization during cooling.c The crystallization temperature determined from the heating scan.

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2.3.6. SEMScanning electron microscopy (SEM) images were re-

corded with a FEI Inspect F instrument operated at 10 kV.The tensile break surface of neat PLA, PLAOR1 and PLAOR5were observed. In addition, to explore the tougheningmechanism, the longitudinally cryo-fractured surface ofthe PLAOR1 test sample was investigated. All fracture sur-faces were coated with gold prior to examination.

3. Results and discussion

3.1. Microstructure of PLAOR nanocomposites

Conventionally, the microstructure of polymer/claynanocomposites is characterized by WAXD and TEM. Fig. 1shows the WAXD patterns of OREC and PLA/OREC compos-ites in the range of 2h = 2–10�. A strong peak belonging tothe OREC powder was found at 2h = 2.30�, which illustratedthat the mean interlayer space of the (0 0 1) plane (d001) forthe OREC was 3.85 nm, larger than that of organic MMT.During the melt-extrusion with PLA, the OREC was interca-lated into the PLA matrix under high shear force, and thed001 space was farther enlarged. As a result, all the d001 peaksof OREC in the composites were broaden and shifted to asmall angle lower than 2h = 2�, which could not be observedin the WAXD patterns.

Fig. 2 showed the TEM micrographs of PLAOR0.5,PLAOR1 and PLAOR5 nanocomposite respectively, in

Fig. 1. WAXD patterns of OREC and PLAOR nanocomposites.

which the gray areas represented the OREC layers andthe bright areas were PLA matrix. In Fig. 2(a0 and b0),the layers of OREC were intercalated into the PLA matrix,and even some fragments of OREC layers were exfoliatedfrom the silicate crystallites as a result of the high shearstress in the processing. On the contrary, a large amountof aggregations of clay layers were found in Fig. 2 (c andc0), and the degree of order of layer stacking was observ-ably enhanced compared with that of PLAOR0.5 andPLAOR1. All these results demonstrated that the interca-lated and exfoliated OREC coexisted in the PLA matrixwhen its content was 0.5 and 1 wt.%. However, there

Fig. 2. TEM images of PLAOR0.5 (a and a0), PLAOR1 (b and b0) and PLAOR5(c and c0).

Fig. 3. WAXD patterns of neat PLA and PLAOR nanocomposites.

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were some agglomerations in OREC when 5 wt.% ORECwas added into PLA.

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3.2. Crystallization behavior of PLAOR nanocomposites

Fig. 3 shows the WAXD patterns of neat PLA and PLA/OREC composites in the range of 2h = 10–40�. The neatPLA exhibited major diffraction peaks at 2h = 16.5�, 18.7�,ascribed to the (2 0 0) or (1 1 0) and (2 0 3) planes, respec-tively, indicating PLA was the typical orthorhombic crystal[17,18]. For all nanocomposites, the diffraction peaks cor-responded to PLLA were not changed indicated the ORECdid not alter the crystal structure of the PLLA matrix.

Fig. 4. POM photos of neat PLA (a, a0 and a0 0), PLAOR0.5 (b, b0 and b0 0), PLA

POM was used to illustrate the difference of the crystalmorphologies between neat PLA and PLAOR nanocompos-ites. Fig. 4 presents the POM images of neat PLA, PLAOR0.5,and PLAOR5 completely crystallized at 110, 120, and130 �C, respectively. As shown in Fig. 4, the spheruliteswere formed in both neat PLA and nanocomposites afterisothermal crystallization, and spherulite sizes systemati-cally increased with the increase of crystallization temper-ature. When 0.5 wt.% OREC was introduced in PLA matrix,the spherulite became smaller and texture became finer.This result indicated that even a minor weight percentageof OREC played a significant nucleation role on the PLA ma-trix. As the clay content was increased to 5 wt.%, thespherulites became dim and showed unclear boundarycompared with that of neat PLA indicated that the spheru-lites were less ordered. The similar results were also foundby Nam et al. [19].

For study the effect of crystallization temperature onthe spherulite growth rate, the spherulite radiuses weremeasured at five different times and then was linear fitted.Fig. 5 shows the spherulite growth rate as a function of iso-thermal crystallization temperature. From the figure wecan see clearly that the spherulite growth rate of nanocom-posites increased dramatically especially when the crystal-lization temperature was raised from 110 �C to 120 �C.With the crystallization temperature was further elevatedto 130 �C, the spherulite growth rate of PLAOR0.5 was in-creased slightly while that of PLAOR5 began to decrease.As we know, the spherulite growth rate of a neat polymerincreases by increasing crystallization temperature upto a peak value, and then it starts to decrease byfurther increasing the crystallization temperature [20]. Asthe investigated crystallization temperature range

OR5 (c, c0 and c0 0) crystallized at 110, 120, and 130 �C, respectively.

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Fig. 5. Spherulite growth rate as a function of isothermal crystallizationtemperature (110, 120 and 130 �C) for neat PLA, PLAOR0.5, and PLAOR5.

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(110–130 �C), the growth rate of PLA linear decreased withthe increase of crystallization temperature. Nam et al. alsofound that the growth rate of PLA decreased in the temper-ature range 120–140 �C [19]. When the OREC was added,the intercalated clay layers acted as a nucleating agent thattriggered the formation of spherulites and the partiallyexfoliated clay layers played a template role which acceler-ated the spherulite growth rate [21]. Therefore, when thecrystallization temperature was higher than 120 �C, thespherulites of nanocomposites developed faster than thatof neat PLA. On the country, at lower crystallization tem-perature (Tc < 120 �C), the spherulites of nanocompositesgrew slower than that of neat PLA. This may be due tothe fact that the dispersed clay layers hindered thechain-folding of local PLA at lower temperature.

The DSC cooling and second heating thermograms for allsamples are presented in Fig. 6. The data obtained from DSCare listed in Table 1. It was found that the crystallizationenthalpies during cooling (DHc1) of all the nanocompositeswere higher than that of neat PLA, especially for PLAOR0.5who’s the crystallization temperature during cooling (Tc1)and the crystallization enthalpy were remarkably higher

Fig. 6. DSC traces of neat PLA and PLAOR nanocomposite

than that of other samples. It had been demonstrated thatthe intercalated clay could act as a nucleation agent andthe partially exfoliated layers could played a template rolefor the crystallization of PLA [21]. Both of them may havethe synergistic effect for PLAOR0.5, and these enhancedthe crystallization of PLA greatly. Therefore, PLAOR0.5 wasfully crystallized and showed a big exothermal peak duringthe previous cooling scan. When OREC content was furtherincreased this synergistic effect was disappeared indicatedthere were the optimal addition amount of OREC for improv-ing the crystallization ability of PLA nanocomposite.

Because PLAOR0.5 already fully crystallized during theprevious cooling scan from its second heating thermogram,the cold crystallization peak could not be found. Two over-lapping endothermic melting peaks were presented in thesecond heating scan of PLAOR0.5. And these can be due tothe less perfect crystals had enough time to melt and reor-ganized into crystals with higher structural perfection,which re-melted at higher temperature during the slowDSC scanning [22]. Low-temperature crystals had the samestructure as the high-temperature ones, but with smallerlamellar thickness.

As well as PLA and PLAOR1–5 were concerned, theyexhibited cold crystallization exothermal peak (Tc2) ob-served at 101–96 �C during the subsequent heating scansindicating that they did not fully crystallize during the pre-vious cooling at 5 �C/min. When the temperature wasabove the glass transition temperature, PLA and PLAOR1–5 underwent extensive reorganization and formed moreperfect crystals. Beside this, the cold crystallization tem-peratures (Tc2) of these nanocomposites were lower thanthat of neat PLA. From Table 1 it can be found that theTgs and Tms of PLAOR1–5 were almost as same as that ofPLA. These results implied that the OREC played a hetero-geneous nucleation role, and enhanced the crystallizationability of PLA. However, the addition of clay did not signif-icantly alter the overall crystal structure of PLA.

By integrating the area under the endothermic region ofthe DSC curves and then subtracting the extra heat absorbedby the crystallites formed during melting, the meltingenthalpy (DHm) of all samples was calculated, and at thesame time the degree of crystallinity (vc) was estimated

s at a cooling and second heating rate of 5 �C/min.

Fig. 7. Stress–strain curves of (a) neat PLA, (b) PLAOR1, and (c) PLAOR5.

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by considering the melting enthalpy of 100% crystalline PLAas 93 J/g [23]. The data of vc in Table 1 showed that the crys-tallinity of PLA was greatly enhanced when 0.5 wt.% ORECwas added. Further increased the OREC content, the crystal-linity was invariable compared with that of pure PLA. Thisdemonstrated again that there was synergistic effect be-tween the nucleation of intercalated OREC and template roleof partially exfoliated OREC for PLAOR0.5 which enhancedthe crystallization of PLA greatly.

3.3. Mechanical properties and toughening mechanisms

As an example of semi-crystalline glassy polymer, neatPLA typically fails in a tensile test with strain not morethan 10%. The failure process results from the initiationand propagation of a few large crazes which soon fractureunder the relatively high level of stress [24]. However, theaddition of OREC significantly improves the tensile proper-ties of the PLA, as is shown in Table 2. In the presence of 0.5wt.% clay, the elongation at break of the nanocomposite in-creased from 7.9% to 58.7%, and reached the maximum of209.7% with 1 wt.% clay loading. Then the elongation atbreak decreased with continuous increasing clay content.Till the amount of clay content went up to 5 wt.%, the elon-gation at break dropped to around 25%, indicated thatthere were phase separation between agglomerated clayand PLA matrix induced premature material failure.

From Table 2 it can be found that the tensile strength atyield of the nanocomposites decreased compared with thatof neat PLA. This maybe caused by the absence of stronginteraction between PLA matrix and clay layers, which re-sulted in lots of cavities appeared at lower tensile stressand subsequent a premature yielding. In addition, the ten-sile modulus gradually increased with the increasing clayloading as shown in Table 2. The enhancement of the ten-sile modulus of nanocomposites can be attributed to tworeasons. Firstly, the clay layers are rigid and have highermodulus than PLA matrix. The modulus of PLA would beenhanced after the PLA matrix being filled with the rigidinorganic filler. Secondly, after being kneaded with neatPLA, the clay was intercalated and exfoliated into the PLAmatrix, and this induced the surface area of clay exposedto the polymer was significantly enlarged. The polymermatrix was physic-absorbed and adhered on the clay lay-ers surface which would make the materials stiffened [25].

Fracture behavior of the specimen in the tensile testschanged from brittleness of neat PLA to ductile fractureof nanocomposites. This was demonstrated in the stress–strain curves as shown in Fig. 7. Neat PLA was very rigid

Table 2Mechanical properties of neat PLA and PLAOR nanocomposites.

Samples Claycontent(wt.%)

Tensilemodulus(GPa)

Tensilestrength atyield (MPa)

Elongationat break (%)

Neat PLA 0 1.1 ± 0.1 68.8 ± 0.6 7.9 ± 0.8PLAOR0.5 0.5 1.2 ± 0.1 55.8 ± 1.9 58.7 ± 9.0PLAOR1 1 1.3 ± 0.1 58.7 ± 1.0 209.7 ± 25.7PLAOR2 2 1.3 ± 0.1 54.1 ± 2.6 106.1 ± 28.1PLAOR3 3 1.3 ± 0.2 46.1 ± 1.4 47.9 ± 3.8PLAOR5 5 1.5 ± 0.1 36.8 ± 3.6 25.2 ± 3.8

and brittle and showed a distinct yield point with subse-quent failure immediately upon the tensile load. PLAOR5showed a continuous strain after distinct yielding withthe stress remaining almost constant. A stress whiteningphenomenon induced by large amount of crazes was ob-served easily during the tensile test. Crazing is a dilativeprocess and involves localized plastic deformation [26].Extensive crazing of the nanocomposites resulted in largerstrain-at-break and higher toughness than those of neatPLA. Surprisingly, it was noticed that PLAOR1 showed ini-tial strain softening after yielding and then underwentcontinuously cold drawing which meant the necked-downregion prolonging under stress. The stress–strain curve be-yond the yield point showed a combination of strain soft-ening and cold drawing, and there was a competitionbetween PLA chain orientation and crack formation. Hence,a drop in stress with increasing strain can be observed eas-ily. Beyond the 20% of strain, a necking phenomenon ap-peared and only cold drawing dominated at a relativelyconstant stress. This suggested that large energy dissipa-tion occurred during the PLA chain orientation.

It was believed that the main part of fracture energyconsumption was due to making plastic zone or stresswhitening zone in front of pre-crack [27]. In order to inves-tigate the morphology variation of PLA and nanocompos-ite, the tensile fracture surface of samples was observedby SEM. Fig. 8 shows the tensile fractured surface of neatPLA, PLAOR1, and PLAOR5. It was clear that the tensile frac-tured surface of neat PLA (Fig. 8(a)) was extremely flat,indicating the brittle failure of PLA under tensile loading.Oppositely, the tensile fractured surface of PLAOR5(Fig. 8(c)) was relative rough and lacked ductile tearing,and at the same time large voids could be found. This re-vealed that serious agglomeration of OREC resulted inphase separation between PLA matrix and OREC. The for-mation of these large voids were considered as the coales-cence of neighboring small cavities, caused by ORECdebonding from PLA matrix. The big voids not only re-sulted in severe decrease of the strength, but also inducedcracks, which finally triggered catastrophic failure undertensile loading. Considerable highly orientated ligamentsand surface roughness were appeared on the tensile

Fig. 8. SEM micrographs of (a) neat PLA, (b) PLAOR1, and (c) PLAOR5.

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fractured surface of PLAOR1 (Fig. 8(b)). The highly orien-tated ligaments were owing to the deformation of PLA ma-trix, and suggested that the failure mode changed frombrittle fracture to ductile one. Beside this, these stresswhitening ligaments revealed that the crack propagationabsorbed considerable strain energy before failure.

In order to explore the toughening mechanisms, thetensile specimen of PLAOR1 was cryo-fractured longitudi-nally after tensile test, and three locations with differentstress states were selected to be observed by SEM as

Fig. 9. SEM micrographs taken from the different locations of the cryo-fracturedthe measurement locations A, B, and C; (b) morphology in region A; (c) mmagnifications.

shown in Fig. 9. Fig. 9(b) reflected the interior structureof the location without undergoing stretch. Lots of tinycavities produced by pulling clay layers out of the PLA ma-trix were uniformly distributed on the fracture surface(Fig. 9(b)). Fig. 9(c) exhibited the interior structure of stresswhitening region, in which oval cavities, caused by thedebonding of OREC particles from PLA matrix under tensilestress, were observed easily. Argon et al. had demonstratedthat the toughening mechanism was governed by the cav-itation process or debonding [28].

longitudinally of PLAOR1 after the tension test. (a) Schematic diagram oforphology in region B; (d and e) morphology in region C at different

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In addition, it was believed that separation of particlesfrom a matrix during the tensile drawing facilitated theplastic deformation of the matrix, which was reflected ina decreased yield stress [29]. Well dispersed OREC layerscould result in more uniform distribution of pores formingduring the drawing, and was beneficial for a larger strainprior to fracture.

Fig. 9(d) and (e) are micrographs of the internal structureof the necked-down region at different magnifications. Itwas clear that large amount of highly oriented ligaments al-ready formed along the tensile direction, which indicatesthat much energy had already been dissipated by the liga-ments. In addition, these ligaments acted as strong barriersbetween cavities which prevented the cavities to coalesceand propagate into big crazes under uniaxial tension, whichwould induce a favorable toughening effect. Thus, the con-clusion can be drawn that a small amount of OREC propelledthe formation of a great amount of ligaments which contrib-utes to the significant toughness enhancement of PLA.

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4. Conclusion

A series of PLA/OREC nanocomposites with differentOREC amount were prepared by melt-extrusion. A partiallyexfoliated and partially intercalated structure was con-firmed by WAXD and TEM. The intercalated clay acted asa heterogeneous nucleating agent and the exfoliated layersplayed a template role for crystallization of PLA. Both ofthem accelerated the crystallization rate of PLA. Interest-ingly, in contrast with neat PLA, ductility of materialswas dramatically enhanced with slight sacrifice of tensilestrength in the presence of 1 wt.% OREC loading, and thetensile modulus increased in accordance with increase ofclay content. From the SEM photos it could be deducedmany ligaments derived from the debonding of OREC par-ticles from PLA matrix acted as strong barriers betweencavities which prevented the cavities to coalesce and prop-agate into big crazes under uniaxial tension that made PLAtoughened.

Acknowledgments

This work was supported financially by the NationalScience Fund for Distinguished Young Scholars (50525309)and the National Science Foundation of China (50173016)as well as the Program for New Century Excellent Talentsin University (NCET-06-0791).

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