Novel toughened polylactic acid nanocomposite: Mechanical, thermaland morphological propertiesHarintharavimal Balakrishnana, Azman Hassana,*, Mat Uzir Wahita, A.A. Yussufa,Shamsul Bahri Abdul RazakbaEnhanced Polymer Research Group, Department of Polymer Engineering, Faculty of Chemical and Natural Resources Engineering, Universiti Teknologi Malaysia, Johor, MalaysiabMalaysian Rubber Board, Rubber Research Institute of Malaysia Experiment Station, Sungai Buloh, Selangor, Malaysiaarti cle i nfoArticle history:Received 29 September 2009Accepted 4 February 2010Available online 8 February 2010Keywords:NanocompositesPolylactic acidLinear low density polyethyleneMechanical propertiesThermal analysisabstractTheobjectiveofthestudyistodevelopanoveltoughenedpolylacticacid(PLA)nanocomposite. Theeffectsoflinearlowdensitypolyethylene(LLDPE)andorganophilicmodiedmontmorillonite(MMT)on mechanical, thermal and morphologicalproperties of PLA were investigated. LLDPE toughened PLAnanocomposites consisting of PLA/LLDPE blends, of composition 100/0 and 90/10 with MMT content of2 phr and 4 phr were prepared. The Youngs and exural modulus improved with increasing content ofMMTindicatingthatMMTiseffectiveinincreasingstiffnessofLLDPEtoughenedPLAnanocompositeeven at low content. LLDPE improved the impact strength of PLA nanocomposites with a sacrice of ten-sileandexuralstrength. ThetensileandexuralstrengthalsodecreasedwithincreasingcontentofMMT in PLA/LLDPE nanocomposites. The impact strength and elongation at break of LLDPE toughenedPLA nanocomposites also declined steadily with increasing loadings of MMT. The crystallization temper-ature and glass transition temperature dropped gradually while the thermal stability of PLA improvedwith addition of MMT in PLA/LLDPE nanocomposites. The storage modulus of PLA/LLDPE nanocompositesbelowglasstransitiontemperatureincreasedwithincreasingcontentof MMT. X-raydiffractionandtransmission electron microscope studies revealed that an intercalated LLDPE toughened PLA nanocom-posite was successfully prepared at 2 phr MMT content. 2010 Elsevier Ltd. All rights reserved.1. IntroductionBiopolymers are expected to be an alternative for conventionalplastics due to the limited resources and soaring petroleum pricewhich will restrict the use of petroleum based plastics in the nearfuture. PLA has attracted the attention of polymer scientist recentlyas a potential biopolymer to substitute the conventional petroleumbased plastics. Apart from being in the category of biodegradablepolymer, PLA has wide applications in biomedical eld due to itsbiocompatibilitycharacteristics. Recent studies andndings onPLA had proven that the biopolymer has good mechanical proper-ties, thermalplasticityandbiocompatibility, andisreadilyfabri-cated, thus being a promising polymer for various end-useapplications [1]. However, PLA, similar to polystyrene, is a compar-atively brittle and stiff polymer with low deformation at break [2].One main task is to modify these properties in such a way that PLAis able to compete with other more exible commodity polymerssuchaspolyethylene, polypropylene, polyethyleneterephthalateor polyvinyl chloride.Previousresearchershaveshownthatadditionofplasticizerssuch as polyethylene glycol (PEG), glucosemonoesters and partialfattyacidesters hadsuccessfullyovercomethebrittleness andwiden PLAs application [24]. Jacobsen et al. [2] have revealed thatplasticizationhadsignicantlyimprovedtheelongationatbreakand impact strength of PLA. However, plasticization of PLA had sac-riced the inherent stiffness of the material limiting its applicationfor structural uses. Thus, the development of an impact modier issignicantlyrequiredtoimprovethetoughnessof thematerialwithout extensively forfeiting the stiffness of PLA.Recent achievements in nanocomposite technology have fueledthe need for new knowledge and ndings in the eld of polymernanocomposites resulting in the development of respective poly-mer nanocomposites; polyamide6/polypropylene [5,6], polyamide66[7], polypropylene[8], polycaprolactone[9], polystyrene[10]andnaturalrubber[11]. Forthepastfewyears, theeldofPLAnanocomposites [12,13] based on layered silicates, such as MMT,has gainedits popularityamong scientist andindustrials. Thenanoscaledistributionofsuchhighaspectratiollersbringsupsome large improvements to the polymer matrix interms of0261-3069/$ - see front matter 2010 Elsevier Ltd. All rights reserved.doi:10.1016/j.matdes.2010.02.008*Corresponding author. Address: Department of Polymer Engineering, Faculty ofChemical and Natural Resources Engineering, Universiti Teknologi Malaysia, 81300Skudai, Johor, Malaysia. Tel.: +60 7 5537835; fax: +60 7 5581 463.E-mail address: azmanh@fkkksa.utm.my (A. Hassan).Materials and Design 31 (2010) 32893298ContentslistsavailableatScienceDirectMaterials and Designj our nal homepage: www. el sevi er . com/ l ocat e/ mat desmechanical, re retardant, rheological, gas barrier and opticalproperties, especiallyatlowclaycontent(assmallas1 wt.%)incomparison with conventional microcomposites (>30 wt.% ofmicroller). In order to reach this nanoscale distribution, the natu-rallyhydrophilicclayllerhastobeorganicallymodiedtobemore compatible with the organic polymer matrix.Interestingly, the distribution of nanoclay in PLA is proven to bewell dispersed without introduction of compatibilizing agents [13].This is due to the interaction of hydrogen-bondings betweenammoniumgroupsintheorganicsurfactantoftheorganoclaywith the carbonyl group of PLA chain segments contributes to thisprocess[14]. TherearealsostronginteractionsbetweenthePLAhydroxyl end groups and the MMT platelet surfaces or the hydro-xyl groups of the ammonium surfactant in the organically modiedMMT reported in previous study by Jiang et al. [15]. Regardless ofthe improvements achieved in the development of PLA nanocom-posites, the polymers brittleness had become more inherent. Thishad limited its application for structural applications.Similar brittleness problems hadbeensolvedbefore byre-searches with the approach of introduction of elastomeric materi-als into a brittle nanocomposite system. Wahit et al. [5] hadexamined the ethylene octene copolymer toughening of polyamide6/polypropylene nanocomposites in terms of impact strength, duc-tilebrittle transition temperature, and tensile properties. Limet al.[8] had successfully developed the toughened polypropylene nano-composites with poly(ethylene-co-octene), PP/POE and studied itsmorphology, thermal andmechanical behaviour. However, onlyseveralstudieshadbeenconductedrecentlyinthedevelopmentof plasticizedPLAnanocomposites[1620] but, themechanicalpropertiesof thesenanocomposites werenot studiedindetail.Thellen et al. [18] had investigated the inuence of MMT layeredsilicate on plasticized PLA blown lms and concluded that the plas-ticizedPLA/MMTnanocompositesdidnot seehighlysignicantenhancements with addition of MMT but the toughness is at leastmaintainedinthenanocompositesunlikeotherlledpolymericsystems. They found that the plasticization effect reduced the brit-tlenessofthenanocompositesandthebreakthroughwill widenPLAs applications.Although the addition of plasticizers such as PEG will overcomePLAsbrittleness, however, it comeswithasacriceof stiffnesswhichis alsoimportant for structural applications. Plutaet al.[19] showed that the addition of PEG into MMT lled PLA had sig-nicantly sacriced the stiffness of the nanocomposite. The storagemodulus (E0) at room temperature, resembling the stiffness of PLAnanocomposite, had decreased from 700 to 450 MPa with additionof20 wt.%PEG. Thus, anewtoughenerwasneededtoovercomethebrittleness nevertheless signicantlyreducingthestiffness.LLDPE could become the best impact modier for PLA as it is costeffective and easy processibility. The introduction of LLDPE as animpact modier in polymers had been studied by previousresearchers[2123]. PreviousstudybyAndersonetal. [21]whoperformed melt blending of PLA and LLDPE in an effort to toughenPLA showed that semicrystalline PLA has relatively good miscibil-itywithLLDPE, hencethesetwomaterialsarechosenasmatrixandimpactmodier. Theyalsoreportedasignicantincreaseofimpact strength in semicrystalline PLA with the addition of LLDPEandstatedthat theyhadsuccessfullytoughenPLAwithLLDPE.However, the researchers had only investigated the effect of20 wt.% LLDPE with PLA and the study focused on the compatibilityof LLDPE in semicrystalline PLA.To the best of our knowledge, no systematic studies havebeendonesofar toinvestigatethepropertiesof LLDPEtough-enedPLAnanocomposites. Therefore, theobjectiveofthispaperistoinvestigatethemechanical, thermal andmorphologyprop-ertiesofLLDPEtoughened PLAnanocompositeswith comparisontoPLAnanocomposites withadditionof MMTat various con-tents. Although it seems that the use of LLDPE, a non-biodegrad-able polymer, as an impact modier for PLAnanocompositesmay defeatthepurposeofdevelopingagreen polymer,thepo-tential applicationfromthisresearchisforutilizationofPLAinstructural applications especially when petroleumbased com-modityplasticsbecomesmorescarceandexpensive.2. Experimental2.1. MaterialsPolylactic acid (PLA; molecular weight, Mw: 220 kDa; Mn:101 kDa)wasobtainedfromBiomer, Krailing, Germany(productnameBiomerL9000). LLDPEofinjectionmoldinggradewasob-tained from Titan Polyethylene, Johor, Malaysia (TitanexLi 5011,Melt Mass FlowRate (MFR) of 50 g/10 min and density of0.926 g/cm3), montlorillonite(MMT)obtainedfromNanocorInc.Arlington Heights IL, USA (Nanomer 1.30TC) organically modiedwith octadecylamine with mean dry particle size of 1622 lm.2.2. Nanocomposites preparationsPLA and LLDPE pellets were dried at 40 C and MMT at 100 Crespectivelyfor24 hpriortocompounding. ThenanocompositesaccordingtoTable1werecompoundedbysimultaneousaddingof all components to Brabender Plasticoder PL 2000 counter-rotat-ingtwin-screwextruder. PLAandLLDPEblendswerepreparedwith weight percentage while MMT was added as parts per hun-dred(phr)intheblends. Thebarreltemperatureproleadoptedduring compounding of all blends was 190 C at the feed section,decreasingto175 Cat thediehead. Thescrewrotationspeedwas xed at 35 rpm. The extruded materials were injection moldedinto standard tensile, exural and Izod impact specimens by usingaJSW(Muroran, Japan)ModelNIOOBIIinjection-mouldingma-chine with a barrel temperature of 165190 C. All test specimenswereallowedtoconditionunderambientconditionsforatleast48 h prior to testing.2.3. Mechanical analysisTensile test was carried out according to ASTM D638 using anInstron (Bucks, UK) 5567 under ambient conditions with crossheadspeeds of 5 mm min1. Flexural test was done according to ASTMD790 by AG-5kNE Shimadzu universal testing machine underambient conditions with crosshead speed of 3 mm min1. Izod im-pact tests were carried out on notched impact specimens accordingtoASTM256, byusingaToyoseiki(Tokyo, Japan)impacttestingmachine under ambient conditions. Five specimens of each formu-lation were tested and the average values reported.2.4. Thermal analysisThemeltingandcrystallizationbehaviour of theblendsandnanocomposites were studied under nitrogen atmosphere by dif-ferential scanning calorimetry (DSC) (PerkinElmer DSC-6), usingTable 1Designations of material and their compositions.Designations Compositions PartsPLA PLA 100PLA/M2 PLA/MMT 100/2PLA/M4 PLA/MMT 100/4P90/L10 PLA/LLDPE 90/10P90/L10/M2 PLA/LLDPE/MMT 90/10/2P90/L10/M4 PLA/LLDPE/MMT 90/10/43290 H. Balakrishnan et al. / Materials and Design 31 (2010) 32893298810 mg sample sealed into aluminum pans. The temperature wasraised from 30 to 250 C at a heating rate of 10 C/min, and after aperiod of 1 min it was swept back at 10 C/min.Thermogravimetryanalysis(TGA), onaMettlerToledomodeltga/sdta851instrument, wasperformedatarateof 10 C min1under a nitrogen atmosphere in order to examine the thermal deg-radation behaviour of the organic components in the PLA and PLA/LLDPE blends.The storage modulus (E0), as a function of temperature (T), weredeterminedbydynamicmechanical thermal analysisbyusingaPerkinElmer DMA7einstrument. DMAspectraweretakeninthe three point bending mode, at a frequency of 1 Hz, over a broadtemperature range (T = 30150 C) at a programmed heating rateof 5 C min1. Sampleswithdimensionsof 15 9 3mm3, pre-pared by injection-moulding, were used for analysis.2.5. Morphological studiesX-ray diffraction (XRD) analysis was carried out on a Siemens(Berlin, Germany) D5000 X-ray diffractometer. The diffraction pat-terns were recorded with a step size of 0.02, from 2h = 2.0 to 10.0.The interlayer distances (d-spacing) of the MMT in the nanocom-positeswerederivedfromthepeakpositions(d001reections)in the XRD scans, according to the Bragg.d spacing nk= sinhp1where n is an integer, hp is the diffraction angle giving the primarydiffractionpeak, andkistheX-raywavelength. Intheseexperi-ments, k = 0.15147 nm (Cu Ka) and n = 1 were used.For transmission electron microscopy (TEM), the samples weretrimmed into trapezium shape of 1 mm2blocks. The sample wasthen mounted onto aLeica EM FC6cryo-ultramicrotome for sec-tioning with the temperature set at 135 C. Sections were cut atthickness of 7080 nm. A copper grid with propanol were used topick up the sections from the frozen chamber and deposited ontoa copper grid. The grid was then viewed under a TEM model LEOL-IBRA 120 (Carl Zeiss) at 120 kV. The micrographs were then takenusing soft imaging system software.3. Results and discussion3.1. Mechanical analysisFigs. 1 and 2 show the effect of MMT content on Youngs mod-ulus and exural modulus of PLA and PLA/LLDPE nanocomposites.ItcanbeobservedthatthestiffnessofbothPLAandPLA/LLDPEnanocomposites increased steadily with increasing content ofMMT. Theadditionof 4 phrofMMTtoPLAandPLA/LLDPEhadincreasedtheYoungs modulus of thenanocomposites by10%and 20%, respectively. Furthermore, the exural modulus for bothPLAandPLA/LLDPEnanocompositesalsohadincreased18%and20%, respectively, with incorporation of 4 phr of MMT. These nd-ingsareconsistentwithpreviousstudies[12,15]whichreportedsimilarenhancementintermsof stiffnesswithincorporationofMMT. Leeet al. [12] observedabout 40%increment inYoungsmodulusfor PLAnanocompositesscaffoldcomparedtopristinePLAwith5.79 vol.%additionof MMT. Another study by Jianget al. [15] alsoreportedanincreaseinYoungs modulus from3500 to 5000 MPa in PLA/MMT nanocomposites with 7.5 wt.% con-centration of MMT. The improvement in stiffness was due to thereinforcement effect of the rigid inorganic MMT which constrainsthe molecular motion of PLA chains. In addition, the existence ofhydrogenbondings interactions between PLA hydroxyl endgroups and the MMT platelets surfaces and/or the hydroxyl groupsof the ammoniumsurfactant in the organically modies clayresulting in enhanced interaction between PLA and MMT with im-proved rigidity. The PLAs chain movement was also suppressed byMMT tethering and gallery connement [15].The improvement in the modulus of PLA/MMT and PLA/LLDPE/MMT nanocomposites may also be caused by the intercalation andexfoliation of MMT layers in PLA (refer to morphological analysis).When the clay particles are dispersed in intercalated and/or exfo-liatedform, itcouldleadtoahigheraspectratioof thesilicatelayer, and a larger interfacial area. Both of the higher aspect ratioand interfacial area will make stress transfer to the silicate layersmore effective, and subsequently improve the mechanical proper-ties of the formed nanocomposites. The dramatic improvement ofmodulus providedbytheexfoliatednanocomposites structureson PLA/MMT nanocomposites had also been reported by previousresearchers[24,25]. Thedegreeofdispersionandinterlayerdis-tancebetweenthesilicatelayerswill beinvestigatedfurtherinthe morphological analysis part of this paper (refer to morpholog-ical analysis).Figs. 1and2alsoillustratetheeffectofMMTonthetensilestrength and exural strength of PLA and PLA/LLDPE nanocompos-ites. It can be observed that tensile strength and exural strengthofPLAnanocompositeshaddecreasedgraduallywithincreasingcontent of MMT. The addition of 4 phr of MMT had decreased thetensile strength and exural strength by 10% and 25%, respectively.The exural strength of PLA/LLDPE nanocomposites also droppedgradually withincreasingcontentofMMT, witha15%reductionat 4 phr of MMT compared to PLA/LLDPE. The drop in tensile andexural strengthof thenanocompositessuggeststhattheMMTlayershadaggregatedtoact asmaterial aws, whichtriggeredbrittleresponseandearlymaterial failureinthetensiletesting.Previous study on PLA nanocomposites by Shyang and Kuen [26]Fig. 1. The effect of MMT content on Youngs modulus and tensile strength of PLAand PLA/LLDPE nanocomposites.Fig. 2. The effect of MMT content on the exural modulus and exural strength ofPLA and PLA/LLDPE nanocomposites.H. Balakrishnan et al. / Materials and Design 31 (2010) 32893298 3291also showed that the exural strength of PLA decreased when theMMTloadingsmorethan1 wt.%. ThiswasduetothehighMMTcontent, which leads to agglomeration in the polymeric material.Further analysis on the morphology of PLA and PLA/LLDPE nano-composites will be able to justify the decrease observed.Interestingly, no signicant changes were observed for the ten-sile strength of PLA/LLDPE nanocomposites with addition of MMT.The addition of MMT into PLA/LLDPE did not seem to cause a fur-ther drop in tensile strength which previously caused by the blend-ing of LLDPE into PLA. The addition of LLDPE into PLA haddecreased the tensile strength of the neat PLA by 28% (Fig. 1). Fur-thermore,theexuralstrengthhadregistered afurtherdecreasewith increasing addition of MMT. These results could be attributedto the difference in stress orientation between tensile (stretching)and exural (bending). This shows that the orientation and disper-sion of MMT platelets in the nanocomposites plays a vital role inthe determination of strength.The effect of MMT content on the impact strength for both PLAand PLA/LLDPE nanocomposites are shown in Fig. 3. It can be seenthat the impact strength for both PLA and PLA/LLDPE gradually de-creased with incorporation of MMT. The impact strength droppedby 13% and 30% at 4 phr of MMT for PLA and PLA/LLDPE nanocom-posites, respectively. It is interesting to note that PLA/LLDPE nano-composites containing 4 phr of MMT and 10 wt.% of LLDPE had asimilar impact strength value to that of neat PLA. This implies thatincorporation of 10 wt.% of LLDPE is able to compensate the loss inimpact strength caused by the presence of 4 phr MMT producing ananocomposite with balanced stiffness and toughness.Shyang and Kuen [26] showed that the agglomeration of MMTat high loading levels (>1 phr) may induce local stress concentra-tions in the composites. Thus, when subjected to impact loading,PLAandPLA/LLDPEnanocomposites failedinabrittlemanner,henceresultinginlower impact strengths. Ontheother hand,the blending of 10 wt.% of LLDPE into PLA nanocomposites had im-provedtheimpact strength. Theimpact strengthof PLA/LLDPEnanocomposites hadimprovedby53%and21%at loadings of2 phrand4 phrMMT, respectively, comparedtoPLAnanocom-posites.Nevertheless, theblendingofLLDPEintoPLA/MMTnanocom-posites in order to increase the toughness had scaried the stiff-ness of the nanocomposites system. The Youngs modulusdecreased about 20% with blending of LLDPE in PLA with MMT con-centration of 2 phr. The blending of LLDPE has increased the mobil-ity of PLAchains allowing it to deformeasily. Although theenhanced mobility of PLA chains is an advantage in terms of impactstrength, it however, led to a reduction of rigidity of the nanocom-posites. It can therefore be said the incorporation of the LLDPE hasimproved the toughness of the nanocomposites but at the expenseof stiffness and strength. On the other hand, the addition of MMTledtoasubstantial improvement instiffness inbothPLAandPLA/LLDPE nanocomposites.3.1.1. Overall mechanical propertiesOne of the most important aspects of materials development inthermoplasticsengineeringistoachieveagoodcombinationofproperties and processability at a moderate cost. As far as mechan-ical properties are concerned, the main target is to strike a balanceof stiffness and toughness. Fig. 4 shows the effect of MMT on stiff-ness and toughness of PLA and PLA/LLDPE nanocomposites. As pre-viously discussed, the incorporation of the LLDPE has improved thetoughnessofthenanocompositesbutattheexpenseofstiffnessand strength. The addition of MMT led to a substantial improve-mentinstiffnessinbothPLAandPLA/LLDPEnanocomposites. Itcan be observed that PLA/LLDPE/M4 has a balanced impactstrengthandexural moduluscomparedtootherformulations.PLA/LLDPE/M4 is also seen to inherit enhanced stiffness and tough-ness compared to the neat PLA.3.2. Morphological analysis3.2.1. X-ray diffraction (XRD)XRDpatterns of MMT, PLA, andPLA/LLDPEnanocompositesareshowninFig. 5and6. The(0 0 1) diffractioninneat MMTwas registered at 2h3.5 which corresponded to a d-spacing of2.48 nm(d-spacing and diffraction angle h is related throughBraggs relation: k = 2d sin h, where k is the wavelength of X-ray).The corresponding d-spacings of MMT in PLAand PLA/LLDPEnanocomposites are tabulated in Table 2.It is interesting to note that the XRD patterns of PLA nanocom-posites with 2 phr MMT content (Fig. 5) do not show (0 0 1) peak at2h2.5. This indicates the presence of interlayer distance whichat least larger than4.8 nmor withnoregular periodicity[25].The absence of this peak in the Fig. 5 also suggests that the parallelform of stacking of the MMT was totally disrupted. This also showsthescatteringanddispersionoftheMMTnanolayerswithinthePLA matrix with the formation of exfoliated nanostructure similartopreviouslyreportedstudies[12,14,25]. TheobservationformXRD is consistent with TEM images taken for PLA nanocompositeswith 2 phr of MMT in Fig. 8b which represents an exfoliated nano-composite, where no agglomeration of MMT observed.Fig. 5 also illustrates a small bulge for (0 0 1) at 2h2.5 with4 phr addition of MMT in PLA nanocomposite. The weak peak rep-resents theinterlayer spacingof stackedMMTlayers within-creased d-spacing from2.48 to 3.47 nm(see Table 2). Theincrement intheinterlayer spacingof MMTisanevident thatPLApolymerchainswereintercalatedinbetweenthegalleryofFig. 3. The effect of MMT content on the impact strength and elongation at break ofPLA and PLA/LLDPE nanocomposites.Fig. 4. Determination of balanced properties based on exural modulus and impactstrength of PLA/LLDPE blends.3292 H. Balakrishnan et al. / Materials and Design 31 (2010) 32893298MMT. The increase in interlayer spacing of MMT may be due to theorganicmodicationof MMTwhichprovidesthepossibilityforPLAchainstodiffusebetweenthelayersduringprocessing[26].Thediffusionof PLAchainsintoMMTlayersdirectlyincreasestheintergallerydistanceandreducestheelectrostaticattractionbetweenadjacent platelets. However, TEMimages (Fig. 8c) re-vealed dark patches of MMT which shows agglomeration of MMT.Jiang et al. [15] showed that a good dispersion of nanoparticlewas achieved only when the ller concentration was