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Toughening Poly(lactic acid) by Melt Blending withPoly(ether-block-amide) CopolymerJianchen Cai1lowast Jinyun Jiang1 Zhaozhong Zhou1 Yun Ding2 Yang Zhang2 Fei Wang2 Cui Han3Jiang Guo4 Qian Shao3 Huayun Du4 Ahmad Umar5lowast and Zhanhu Guo4lowast

1College of Mechanical Engineering Quzhou University Zhejiang 324000 China2College of Mechanical and Electrical Engineering Beijing University of Chemical Technology Beijing 100029 China3College of Chemical and Environmental Engineering Shandong University of Science and TechnologyQingdao 266590 P R China4Integrated Composites Laboratory (ICL) Department of Chemical and Biomolecular EngineeringUniversity of Tennessee Knoxville TN 37996 USA5Department of Chemistry Faculty of Sciences and Arts Promising Centre for Sensors and Electronic Devices (PCSED)Najran University PO Box 1988 Najran 11001 Saudi Arabia

ABSTRACT

Melt blending with poly(ether-b-amide) copolymer (PEBA) was found to significantly enhance the toughnessof poly(lactic acid) (PLA) The miscibility morphology thermal behavior mechanical properties and rheologicalbehavior of the blends were investigated DMA and DSC analysis revealed that the PLAPEBA blends wereimmiscible A clear phase-separated morphology was observed from SEM results The addition of PEBAenhanced the cold crystallization of the PLA in the blends and accelerated the crystallization rate due to theinterfacial function between PLA and PEBA phases Significant enhancement of the toughness of PLA wasachieved by the incorporation of PEBA copolymer The optimum impact modification of the PLA was obtained byblending with 20 wt PEBA with an elongation at break of 300 as compared to 5 for pure PLA The impactstrength was ten times higher than that of the pure PLA SEM results showed that the interfacial cavitationfollowed by a massive shear yielding of the matrix contributed to the increased toughening effect in the blendsThe rheological measurement revealed an interaction between PLA and PEBA in the melt state

KEYWORDS Mechanical Properties Toughness Blending

1 INTRODUCTIONOver the past decade increasing research interest has beengrown in the biodegradable polymers prepared with renew-able sources aiming to alleviate solid waste disposal issuesand to reduce the usage of petroleum-based materialsespecially polymers1ndash3 Among these polymers poly(lacticacid) (PLA) is one aliphatic polyester manufactured com-pletely from renewable materials and has drawn increas-ing attention and wide studies Being degradable naturallyand compatible with other biomaterials improved mechan-ical properties and easy reshaping4 PLA has been widelystudied for use in biomedical applications such as surgicalsuture and drug delivery system and in electronic devicesRecently owing to its biomass origin good performanceand availability in the market at a reasonable price PLAhas been considered as a promising and ideal alternative to

lowastAuthors to whom correspondence should be addressedEmails cai198666126com umahmadnuedusa zguo10utkeduReceived 8 March 2017Accepted 3 May 2017

petroleum-based plastics in commercial applications suchas packaging fiber materials4ndash7 However PLA has onlyenjoyed limited success in replacing petroleum-based plas-tics in commodity applications today It exhibits brittle-ness and its fracture strain is only about 5 in the tensiletest which results in poor impact and tear resistance Theinherent brittleness of PLA has been the major deficiencythat hinders its large-scale applications in both commodityand biomedical areas68ndash22 Accordingly the developmentof PLA-based materials with high toughness has attractedwide attention in recent yearsBlending with other proper polymer has been jus-

tified as a promising way to improve the mechanicalproperties of PLA Up to now PLA has been meltprocessed with many flexible polymers to improve itstoughness and flexibility8ndash17 Some interesting and note-worthy results have been reported however most ofthe blends needed compatibilizers to improve their com-patibility to realize the desired mechanical properties17

Rubber toughening mechanisms developed on conven-tional thermoplastics have also been successfully applied

Sci Adv Mater 2017 Vol 9 No 9 1947-2935201791683010 doi101166sam20173121 1683



Toughening Poly(lactic acid) by Melt Blending with Poly(ether-block -amide) Copolymer Cai et alARTICLE

to biopolymers18 Blending with suitable elastomeric mate-rials provides an effective method to improve the tough-ness of PLA1319ndash23 Poly(ether-block-amide) copolymer(PEBA) is a kind of important commercial thermoplas-tic elastomers with unique physical and processing prop-erties It is a segmented block copolymer consisting ofan aliphatic polyamide as the hard block a polyether asthe soft block and a di-acid serving as the joint betweenthe two blocks PEBA offers a wide range of excellentperformances and has been used in many fields such asantistatic sheets or belts and packaging2425 More inter-estingly PEBA has a good biocompatibility and finds wideuses in a variety of medical applications such as short-termimplantation in humans and virus-proof surgical sheet-ing Recently it is announced by Arkema Company thatthis type polymer can be made from renewable materialsincluding castor oil which could reduce fossil-based mate-rial consumption by 29 and CO2 equivalent emissionsby 3224 As a result PEBA is an ideal blending con-stituent for PLA to prepare new environmentally benignmaterial for uses in both biomedical and commodity appli-cations Meanwhile PEBA has a high impact resistanceeven at temperature as low as minus40 C since the polyether-rich phase has a much lower glass transition temperatureand thus PEBA can be used as an impact modifier for thebrittle thermoplastic polymer2425 Consequently it is quitereasonable to expect that the introduction of PEBA intoPLA may effectively enhance the flexibility and toughnessof PLA To our knowledge the study of PLAPEBA blendshas not been reported so farIn the present work the blending PLA with a kind of

PEBA was reported to improve the flexibility and tough-ness of PLA The miscibility and morphology of the blendwere studied The effects of the presence of PEBA on thethermal behaviors mechanical properties and rheologicalproperties of the PLA in the blends were also thoroughlyinvestigated

2 MATERIALS AND METHODS21 MaterialsThe Nature works PLA 4032D (weight-average molec-ular weight (Mw) 207 kDa polydispersity index 174(GPC analysis) was obtained from Nature Works LLCUSA The PEBA was provided by Elf Atochem Inc hav-ing a trade name of Pebax 2533 which comprises ofhard poly(lauryl lactam) (PA 12) segment (12 wt) softpoly(tetramethylene ether) glycol (PTMO 84 wt) seg-ment and adipic acid as the linkage (36 wt) The weightpercentages of the segment and linkage were determinedfrom 1H NMR spectra25

22 Preparation of the BlendsPrior to blending all the materials were dried overnightunder vacuum PLA at 60 C and the PEBA at80 C respectively Blends of PLAPEBA of varying

compositions were prepared on the Haake Batch IntensiveMixer (Haake Rheomix 600 Germany) 50 mL blends wasbatch processed at 180 C and a screw speed of 60 rpmfor 8-min mixing until the viscosity had reached a nearlyconstant value Pure PLA was also processed followingthe same procedures used for the blends for comparison

23 CharacterizationsDynamic mechanical analysis (DMA) was carried outwith a Diamond DMA dynamic mechanical analyzer(PerkinElmer American) in a tensile mode The specimenswith dimensions of 30times10times1 mm3 were used The heat-ing temperature was controlled from minus100 to 140 C theheating rate was 3 Cmin and the frequency was one HzThe morphology of the blends was observed by a

scanning electron microscope (SEM XL30 ESEM FEGFEI Co) The samples were prepared by breaking afterprocessed in liquid nitrogen Then the specimen wasmounted on an aluminum stub using a conductive paintand finally sputtered with gold prior to fractographicexamination The different zones of the specimens aftertensile tests and fracture surfaces obtained from notchedIzod impact test were also characterized by SEMThe differential scanning calorimetry (DSC) measure-

ments were carried out on a TA Q100 DSC instrumentunder N2 atmosphere The samples were heated to 190 Cat a heating rate of 20 Cmin maintained at 190 C for5 min before cooling down to minus100 C at a rate of 80and 2 Cmin The second heating scan was from minus100 to190 C at a heating rate of 10 Cmin from which the glasstransition temperature (Tg) cold crystallization tempera-ture (T prime

c ) and melting temperature (Tm) were determinedWide angle X-ray diffraction analysis (WAXD) tests

were done on a Rigaku Dmax 2500 V PC X-ray diffrac-tometer The measurements were operated at 40 kV and20 mA from 2ndash40 at a 2 scan rate of 4minTensile properties were tested at room temperature with

an Instron 1121 testing machine (Canton MA) accordingto GBT10403-2006 (China) The specimens were pre-pared by compression molding into sheets with a thicknessof one mm then being cut into a dumbbell shape withgauge dimensions of 20 mmtimes4 mmtimes1 mm A crossheadrate of 10 mmmin was used The data reported were themean and standard deviation from five determinationsThe notched Izod impact strength was tested with a

CEAST impact machine according to GB1843-93 (China)The rheological measurements were carried out on a

PHYSICA MCR 300 instrument (Stuttgart Germany) Thefrequency sweep was carried out under nitrogen at 180 Cusing 25 mm geometry with the sample gap being set as06 mm An initial strain sweep was tested to select thelinear viscoelastic region of the materials The strain wasfixed at 5 and the angular frequency range was from005 to 100 rads The samples were pressed into 1 mmthick at about 190 C The data was obtained by using theMCR300 software

1684 Sci Adv Mater 9 1683ndash1692 2017



Cai et al Toughening Poly(lactic acid) by Melt Blending with Poly(ether-block -amide) Copolymer

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3 RESULTS AND DISCUSSION31 Miscibility of PLAPEBA BlendsDMA was firstly employed to assess the miscibility ofthe polymer blends and the results Figures 1(a) and (b)The observed Tan peak at about 63 C Figure 1(a)corresponds to its glass transition of the neat PLA Forpure PEBA an intense Tan peak was observed at aboutminus70 C corresponding to the glass transition of PTMOsegments in PEBA copolymer25 At the upside of this Tanpeak a minor shoulder was observed and was assignedto the partial cold crystallization and melting behavior ofthe PTMO phase No clear glass transition of PA12 phasewas observed because the PA12 segments were shorterand possibly further mixed with some of the PTMO mate-rial giving rise to a broadening and a downward shiftin its Tg

25 As for the blends the Tan curves mainlyexhibited two distinct Tan peaks corresponding to theglass transition of the initial products in the blends It wasnoticed that the Tg values of both components in theblends are almost independent of the blend compositionThis indicates that these two components in the blendwere immiscible Figure 1(b) shows the storage modulus

Fig 1 DMA graphs of PLA blended with PEBA elastomer at variousconcentrations (a) Tan versus temperature (b) storage modulus versustemperature

curves of the samples In consistence with the Tan curveno obvious change of the transition temperature of PLAcan be observed for the blends due to the immiscibil-ity of the two components The Eprime of PLAPEBA blendsdecreased with increasing the PEBA content in the blendsat room temperature Moreover the temperature at whichthe storage modulus started to increase arising from thecold crystallization of PLA shifted to a lower tempera-ture compared with that of the neat PLA with the addi-tion of PEBA This phenomenon strongly suggested thatthe incorporation of PEBA elastomer enhanced the cold-crystallization ability of PLA and therefore decreased thecold-crystallization temperature of PLA in the blendFigure 2 presents the SEM micrographs of PLAPEBA

blends The PEBA phase domains are preferentially dis-persed as spheres in the continuous PLA matrix withsmooth and distinct interfaces The approximate diame-ters of the dispersed PEBA phases are 03ndash15 m with5ndash20 wt PEBA With further increasing the content ofPEBA there was a corresponding increase in the PEBAparticles size due to the coalescence It is well known thatthe interfacial adhesion of blend was closely related to itsmiscibility The clear phase separation further testified thatthe two components have a poor miscibility

32 Thermal and Crystallization Behaviors ofPLAPEBA Blends

Both PLA and PEBA are semi-crystalline polymersAccordingly it is important to study the influence of theexisting PEBA on the crystallization of PLA To studythe presence of the PEBA on the cold crystallization ofPLA the samples were first quenched from the crys-tal free melt at a cooling rate of 80 Cmin by DSCFigure 3 shows the cooling DSC thermograms at a cool-ing rate of 80 Cmin for neat polymer and its blends aftermelted at 190 C for 5 min During the cooling scan noexothermic peak corresponding to the crystallization of thePLA component was observed in all the samples indicat-ing that the PLA was primarily amorphous when beingcooled from melt at 80 Cmin However pure PEBAwas still able to crystallize at this cooling rate and twoexothermic peaks corresponding to the crystallization ofthe PA12 segments and the PTMO segments of PEBAwere observed Figure 4 shows the second heating ther-mal scans As for the neat PEBA two separate meltingpeaks were observed and were attributed to the melting ofthe two blocks25 The endothermic peak at around 10 Cwas attributed to the melting behavior of PTMO crys-tal in PEBA copolymer The melting behavior of PA12crystal occurred at about 135 C and a minor endother-mic peak could be observed Neat PLA exhibits a specificheat increment at about 61 C ascribed to the glass-rubbertransition of PLA At higher temperature an exothermalpeak at 134 C corresponding to the cold crystallizationof PLA was observed The melting of these crystallizeddomains occurred at 167 C The thermal behaviors of

Sci Adv Mater 9 1683ndash1692 2017 1685




Fig 2 SEM images of cryofractured surface of PLAPEBA blend with a PLAPEBA weight compositions of (a) 5 (b) 10 (c) 20 and (d) 30 wtPEBA

the blends mainly showed the thermal transition of thePLA and the melting of the PTMO segment in the PEBAcopolymer while the melting of the PA12 crystal couldnot be clearly observed because the small concentration ofPA12 segment in the blend and overlapped with the transi-tion of PLA As shown in Figure 4 in consistence with theDMA and SEM results no obvious change of Tg and Tmof components was observed This suggests that the twocomponents were completely immiscible Interestingly themelting peak of PLA showed a transition from single peakto two separate peaks Generally multiple melting peaksof semi-crystalline polymers are attributed to two mainreasons melt-recrystallization and dual crystal structures

Fig 3 The cooling DSC thermograms at a cooling rate of 80 Cminfor neat polymer and its blends after melted at 190 C for 5 min

To further reveal the fact WAXD patterns were determinedfor the samples subjected to the same thermal treatment asDSC Figure 5 shows the WAXD patterns of PLA and thePLAPEBA blends As shown in Figure 5 the characteris-tic reflection peaks of crystal PLA kept almost the sameposition regardless of the composition variation indicat-ing that the PLA unit cell never changed Therefore thebimodal melting peaks of the blends must be resulted fromthe melt-recrystallization During the slow DSC scans theless perfect crystals got enough time to melt and reor-ganize into crystals with higher structure perfection andremelt at higher temperature Similar phenomena were alsoreported in other PLA blends1421ndash23

Fig 4 The second heating DSC thermograms for PLA PEBA and theblends





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Fig 5 The WAXD patterns of PLA and the PLAPEBA blends

However the cold-crystallization exothermic peak of theblends was dramatically different from neat PLA A tinybroad exothermic peak at 134 C was observed for neatPLA this indicated that PLA has a rather low cold crys-tallization capability Compared with the neat PLA thecold crystallization temperature (Tcc) of PLA in the blendswas evidently found to shift towards a lower tempera-ture region and the width of exothermic peaks becamenarrower than that of neat PLA These indicated that theaddition of PEBA promoted the cold crystallization ofPLA which agreed well with the DMA results Moreoverit can be seen that a further increase in the PEBA contenthad little effect on the position of the exotherm corre-sponding to the cold crystallization of PLA in the blendsIn previous studies similar phenomena were also reportedin other immiscible PLA blends such as PLAPCL andPLAPBAT blends1013 The enhancement of cold crys-tallization of the PLA in the blends may be inducedby the interface between the phase-separated domains inthe immiscible blends The chains of the polymer seg-ments near the interface will be different from that ofthe segments in the bulk at two aspects due to the pres-ence of the interface in the multiphase systems Firstlyan enrichment of chain ends the shorter chains of thecomponents and small third-molecule at the vicinity ofthe interface will improve the chain mobility of the poly-mer segments near the interface2627 Recently Roth andTorkelson pointed out that in the nanostructure or quasi-nanostructured immiscible polymer blend an enhancementof chain mobility near interface might occur causing alower glass transition temperature28 Secondly an enhance-ment of orientation ordering of polymeric coils may occurnear the interface in the blends Several researches hadfound that the enhanced segmental mobility near the freesurface might result in an enhanced local ordering ofthe the near-surface chain262930 Such localized align-ment could act as a precursor to become crystalliza-tion In poly(dimethylsiloxane) composites Dollase andhis coworkers suggested that the regions of non-random

chain conformations on the intermediate length scales nearthe interface may play an important role in the early stagesof crystallization even if the orientation ordering of theseregions does not match that of the lowest energy crystallinephase26 These regions may offer a different pathway forthe crystallization by allowing the system to bypass itskinetic barriers that might delay the crystallization In asemicrystalline polymer such as poly(ethylene terephtha-late) (PET) an enhanced mobility at the surface in tandemwith localized alignment was found to lead to a lower coldcrystallization temperature and increased kinetics at thefree surface3132 These results had important implicationsfor our understanding of the behaviors of polymer sys-tems near interfaces In immiscible blends the influence ofthe interface between the phase-separated domains on thecrystallization should be considered seriously1026 Takinginto account of the immiscibility for the PLA and PEBAin the blends the sharp interface between the PLA andPEBA may play a favorable role in the cold crystallizationof PLA in the blendsAs we have demonstrated above the added PEBA elas-

tomer improved the cold crystallization of PLA This resultis important both for the end-use and for the manufacturingof PLA since in its amorphous form its low glass transi-tion temperature and the slow crystallization rate will limitits processability and thus its applications Consequentlythe non-isothermal crystallization behaviors of PLA andits PEBA blends were further exploited The nonisother-mal crystallization of the PLAPEBA blends was studiedby DSC at a cooling rate of 2 Cmin after melted at190 C for 5 min Figure 6 depicts the DSC thermogramsat a cooling stage When the cooling rate was 2 Cmina clear exothermic peak from the crystallization of PLAwas seen at sim98 C for the neat PLA Pure PEBA exhib-ited three exothermic peaks one peak at about minus11 Cwhich was attributed to the crystallization of PTMO seg-ments in PEBA and another two peaks at high temper-ature which were associated with the crystallization of

Fig 6 The cooling DSC thermograms at a cooling rate of 2 Cmin forneat polymer and the blends after melted at 190 C for 5 min





PA12 segments in PEBA25 As shown in Figure 6 the DSCcooling scan of the blends reveals all the three exothermicpeaks corresponding to the crystallization of the compo-nent polymers No obvious changes were observed for thecrystallization temperature (Tc) of PTMO segments in theblends compared with neat PEBA As for the crystalliza-tion of PLA in the blends the Tc of PLA changed slightlyfrom 98 C for neat PLA to 96 C for the blends and thenalmost constant as the PEBA content increased The inter-pretation of the crystallization of PA12 segments in PEBAwas difficult because the small concentration of PA12 seg-ment in the blends overlapped with the glass transitionof PLA However it should be noted that the exothermicpeaks of PA12 segments in the blends shifted a little toa higher temperature compared to pure PEBA This resultwas because that the crystals of PLA might act as hetero-geneous nucleating agents for the PA12 For a completelyimmiscible blend crystallization is expected to occur sep-arately for each of the melt phases There was no sig-nificant change in the crystallization temperatures of thecomponent polymers indicating that the components crys-tallized separately from melting state in the blends due tothe immiscibility between these two components

33 Mechanical Properties of PLAPEBA BlendsFigure 7 displays the typical tensile strain-stress curves ofthe samples The average values of the tensile stress andelongation at break were provided in Figure 8 Neat PLA isa very rigid polymer and deforms in a brittle fashion iewithout obvious yield and low elongation at break onlysim4 However the ductility of PLA was improved by theintroduction of the PEBA elastomer The blend underwentdistinct yielding and considerable cold drawing after theyielding This reflects a transition of the fracture behav-ior from brittle to ductile fracture As shown in Figure 8blending a small amount of PEBA was able to consid-erably improve the flexibility of PLA without reducingits tensile strength apparently When 10 wt PEBA was

Fig 7 Typical tensile stressndashstrain curves of PLA and PLAPEBAblends

Fig 8 Mechanical properties of PLA with various PEBAconcentrations

added the elongation at break reached 188 increased byabove 40 times over pure PLA while the yielding stressremained 48 MPa slightly lower than neat PLA The elon-gation at break increased initially from 4 for neat PLAto above 300 for the blend as the PEBA content wasfurther increased to 20 wt and then decreased to 200as the amount of PEBA in the blend increased to 30 wtThe tensile strength and modulus of the PLAPEBA blendsdecreased with increasing the PEBA content as expecteddue to the incorporation of a soft elastomeric phase to thePLA matrixThe toughening mechanisms of PLAPEBA blends were

investigated through observing the morphology of differ-ent necking regions of the tensile-tested specimens bySEM Figure 9 shows the SEM microstructures of the10 wt PEBA samples after the tensile testing TheseSEM micrographs were taken at different locations of thenecked down region representing different deformationstages of the sample during the stretching Debonding ofthe round PEBA particles from PLA matrix under ten-sile stress was clearly observed at the initial stage of thestretching Figure 9(b) When the sample was subjectedto the tensile stress the domains of PEBA acted as thestress concentrators because of different elastic propertiesof the PEBA from that of the PLA The stress concentra-tion resulted in a high triaxial stress in the PEBA domainsand the debonding occurred at the particle-martrix inter-face due to insufficient interfacial adhesion The interfacialcavitation led to a relief of the triaxial stress state of thePLA matrix around the voids thus creating a stress statebeneficial for the initiation of multiple matrix shear yield-ing As the yielding of the matrix happened the stress wasthen applied to the PEBA domains Then with the debond-ing progresses the orientation of both the matrix and thedispersed domains occurred Figure 9(d) Similar toughen-ing mechanism was also founded in other PLA blends inwhich the improvement in toughness was achieved1421ndash23

The increase in the elongation at break observed forthe PLAPEBA blends was concomitant with a significantincrease in the impact strength of PLA as the concentration





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Fig 9 SEM micrographs of the blend with 10 wt PEBA at different locations of the necked down region during tensile testing

of PEBA in the blends was increased Figure 10 com-pares the impact strength of neat PLA with that of theblends at room temperature All the blends clearly showeda higher impact strength than that of pure PLA A rapidincrease in impact strength was obtained at room temper-ature when more than 10 wt PEBA was added Signifi-cant improvement can be observed for blends with 20 wtPEBA about ten times bigger compared with neat PLAThe toughening effect of PLAPEBA blends was studiedfurther with the fracture surface of the impact specimenFigure 11 Pure PLA showed a smooth and featurelessfracture surface without much deformation indicating atypical brittle fracture behavior On the fracture surfacesof the blend with 5 wt PEBA multiple fracture surfacesreplaced the single fracture lines Some fibrils were dis-cernible on the fracture surfaces The toughening effect in

Fig 10 Impact strength of PLAPEBA blends as function of the weightfraction of PEBA

this case remained moderate The surface of the blend with10 wt PEBA became rougher and rougher with moreand longer fibrils threads found a clear evidence of theductile fractures With 20 wt PEBA significant plasticdeformation of PLA matrix was evident indicating a shearyielding The PEBA particles could not be clearly definedfrom the matrix PLA The corresponding amount of plas-tic deformation was high and effectively dissipated thefracture energy which resulted in highly improved impactstrengths at room temperature

34 Rheological Properties of PLAPEBA BlendsFigure 12 shows the storage modulus (Gprime) loss modu-lus (Gprimeprime) and complex viscosity (lowast) of neat polymers andPLAPEBA blends as a function of frequency at 180 CThere was a well developed dependence of the dynamicmodulus on the PEBA contents at high frequencies boththe Gprime and Gprimeprime decreased with the increase of the PEBAcontent However it can be clearly seen from Figure 12(a)that for low frequencies even at medium-frequency regionthe Gprime of the blends for each composition is larger thanthose for the pure phases and increases when the dispersedphase concentration increases Generally in immiscibleblends the enhancement of elasticity can be attributed tothe relaxation process of the dispersed phase droplets whenslightly sheared When the dispersed phase concentrationincreased the diameter of the dispersed phase increasedand the relaxation process of the dispersed phase becamelonger leading to an increase of the storage modulus3334

Such non-terminal behavior has also been observed onother PLA based blend3536 But it should be pointed outthat in the PLAPEBA blends besides long relaxation





Fig 11 SEM images of the impact-fracture surface of the PLAPEBA blends with a PEBA weight percent of (a) 0 (b) 5 (c) 10 (d) 20 and(e) 30 wt

Fig 12 (a) The storage modulus Gprime (b) the loss modulus Gprime and (c) complex viscosity of neat PLA PEBA and the blends at 175 C





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process of the dispersed phase the interaction between thetwo phases will be an important reason for the enhancedGprime at low frequency

The viscoelastic response of the blends in low fre-quencies (low shear rates) can be used for evaluating ofthe interfacial interaction between phases Because at lowshear rates the effect of flow induced molecular orienta-tion on the viscosity and elasticity becomes much less37

As shown in Figure 12(c) at low frequency the increase inthe viscosity of the blends was also observed correspond-ing to the storage modulus The experimental viscositydata were compared with the additive effect of the linearmixing rule

lowast12 = 1

lowast1 +2

lowast2 (1)

where 1 and 2 are the weight ratio of the components 1and 2 respectively and lowast

1 and lowast2 are the viscosities of

the two components According to this equation Utrackidivided viscosity-composition curve of the blends intothree types positive deviation blend (PDB) negative devi-ation blend (NDB) positivendashnegative deviation38 It wasevident from the inset of Figure 11(c) that the experimentalviscosity data of the blends exhibited a positive deviationat low frequency which heavily suggested that there mightbe some interactions between PLA and PEBA under theexperimental conditions Though the blends were immis-cible there might be some interactions between thesetwo components at the interface in the melt state It wasreported that the hydrogen bonding between the hard andsoft segments of the PEBA was dissociated at higher tem-perature above the melting point of the hard segments39

It would be propitious to form an interaction between theNndashH group in the hard segments of PEBA and the car-bonyl groups of the PLA The interaction between thesetwo phases will reduce the possibility of the interlayer slipand increase the formation of associative network whichmay result in the yield stress

4 CONCLUSIONSSignificant improvements in the flexibility and toughnesswere achieved in PLA by melt mixing it with a bio-compatible thermoplastic poly(ether-b-amide) copolymerDMA and DSC results indicated that the two compo-nents were immiscible A clear phase-separated morphol-ogy with PEBA dispersed in the PLA matrix was observedfrom the SEM results Remarkably by incorporating asmall percentage of PEBA elastomer into PLA the failuremode changed from the brittle fracture of the neat PLA tothe ductile fracture of the blends as demonstrated by ten-sile test and the SEM micrographs of impact-fracture sur-face An elongation at break of 300 was achieved for theblends with 20 wt PEBA as compared to 5 for the neatPLA without significantly sacrificing the tensile strengthof PLA matrix A substantial increase in the notched Izodimpact strength was also observed with increasing the con-tent of PEBA in the blends The cold crystallization of

PLA was promoted by the presence of PEBA in the blendsdue to the interfacial function between the phases whileno obvious influence of PEBA on the crystallization ofPLA from the melting state was observed In addition theviscosity and melt elasticity increased at low frequencywith increasing the concentration of PEBA indicating aninteraction between PLA and PEBA in the melt state Thisprovides an opportunity to make high performance mul-tifunctional polymer nanocomposites for different appli-cations including electromagnetic interference shieldingwater treatment and various sensing40ndash51

Acknowledgment This work was supported byZhejiang Provincial Natural Science Foundation of Chinaunder Grant No LQ16E030008 the Quzhou Science andTechnology Plan Project No 2016Y002 and the SpecialFunds for the Construction of teaching staff in QuzhouUniversity

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to biopolymers18 Blending with suitable elastomeric mate-rials provides an effective method to improve the tough-ness of PLA1319ndash23 Poly(ether-block-amide) copolymer(PEBA) is a kind of important commercial thermoplas-tic elastomers with unique physical and processing prop-erties It is a segmented block copolymer consisting ofan aliphatic polyamide as the hard block a polyether asthe soft block and a di-acid serving as the joint betweenthe two blocks PEBA offers a wide range of excellentperformances and has been used in many fields such asantistatic sheets or belts and packaging2425 More inter-estingly PEBA has a good biocompatibility and finds wideuses in a variety of medical applications such as short-termimplantation in humans and virus-proof surgical sheet-ing Recently it is announced by Arkema Company thatthis type polymer can be made from renewable materialsincluding castor oil which could reduce fossil-based mate-rial consumption by 29 and CO2 equivalent emissionsby 3224 As a result PEBA is an ideal blending con-stituent for PLA to prepare new environmentally benignmaterial for uses in both biomedical and commodity appli-cations Meanwhile PEBA has a high impact resistanceeven at temperature as low as minus40 C since the polyether-rich phase has a much lower glass transition temperatureand thus PEBA can be used as an impact modifier for thebrittle thermoplastic polymer2425 Consequently it is quitereasonable to expect that the introduction of PEBA intoPLA may effectively enhance the flexibility and toughnessof PLA To our knowledge the study of PLAPEBA blendshas not been reported so farIn the present work the blending PLA with a kind of

PEBA was reported to improve the flexibility and tough-ness of PLA The miscibility and morphology of the blendwere studied The effects of the presence of PEBA on thethermal behaviors mechanical properties and rheologicalproperties of the PLA in the blends were also thoroughlyinvestigated

2 MATERIALS AND METHODS21 MaterialsThe Nature works PLA 4032D (weight-average molec-ular weight (Mw) 207 kDa polydispersity index 174(GPC analysis) was obtained from Nature Works LLCUSA The PEBA was provided by Elf Atochem Inc hav-ing a trade name of Pebax 2533 which comprises ofhard poly(lauryl lactam) (PA 12) segment (12 wt) softpoly(tetramethylene ether) glycol (PTMO 84 wt) seg-ment and adipic acid as the linkage (36 wt) The weightpercentages of the segment and linkage were determinedfrom 1H NMR spectra25

22 Preparation of the BlendsPrior to blending all the materials were dried overnightunder vacuum PLA at 60 C and the PEBA at80 C respectively Blends of PLAPEBA of varying

compositions were prepared on the Haake Batch IntensiveMixer (Haake Rheomix 600 Germany) 50 mL blends wasbatch processed at 180 C and a screw speed of 60 rpmfor 8-min mixing until the viscosity had reached a nearlyconstant value Pure PLA was also processed followingthe same procedures used for the blends for comparison

23 CharacterizationsDynamic mechanical analysis (DMA) was carried outwith a Diamond DMA dynamic mechanical analyzer(PerkinElmer American) in a tensile mode The specimenswith dimensions of 30times10times1 mm3 were used The heat-ing temperature was controlled from minus100 to 140 C theheating rate was 3 Cmin and the frequency was one HzThe morphology of the blends was observed by a

scanning electron microscope (SEM XL30 ESEM FEGFEI Co) The samples were prepared by breaking afterprocessed in liquid nitrogen Then the specimen wasmounted on an aluminum stub using a conductive paintand finally sputtered with gold prior to fractographicexamination The different zones of the specimens aftertensile tests and fracture surfaces obtained from notchedIzod impact test were also characterized by SEMThe differential scanning calorimetry (DSC) measure-

ments were carried out on a TA Q100 DSC instrumentunder N2 atmosphere The samples were heated to 190 Cat a heating rate of 20 Cmin maintained at 190 C for5 min before cooling down to minus100 C at a rate of 80and 2 Cmin The second heating scan was from minus100 to190 C at a heating rate of 10 Cmin from which the glasstransition temperature (Tg) cold crystallization tempera-ture (T prime

c ) and melting temperature (Tm) were determinedWide angle X-ray diffraction analysis (WAXD) tests

were done on a Rigaku Dmax 2500 V PC X-ray diffrac-tometer The measurements were operated at 40 kV and20 mA from 2ndash40 at a 2 scan rate of 4minTensile properties were tested at room temperature with

an Instron 1121 testing machine (Canton MA) accordingto GBT10403-2006 (China) The specimens were pre-pared by compression molding into sheets with a thicknessof one mm then being cut into a dumbbell shape withgauge dimensions of 20 mmtimes4 mmtimes1 mm A crossheadrate of 10 mmmin was used The data reported were themean and standard deviation from five determinationsThe notched Izod impact strength was tested with a

CEAST impact machine according to GB1843-93 (China)The rheological measurements were carried out on a

PHYSICA MCR 300 instrument (Stuttgart Germany) Thefrequency sweep was carried out under nitrogen at 180 Cusing 25 mm geometry with the sample gap being set as06 mm An initial strain sweep was tested to select thelinear viscoelastic region of the materials The strain wasfixed at 5 and the angular frequency range was from005 to 100 rads The samples were pressed into 1 mmthick at about 190 C The data was obtained by using theMCR300 software





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Documents

Toughening Poly(lactic acid) by Melt Blending with Poly ... in pdf/Toughening Poly(lactic acid) by Melt Blending...even at temperature as low as −40 Izod impact test were also characterized