JPS PP 49(15) (2011)

Research Progress in Toughening Modification of Poly(lactic acid)

Hongzhi Liu, Jinwen Zhang

Composite Materials and Engineering Center, Washington State University, Pullman, Washington 99164

Correspondence to: J. Zhang (E-mail: [email protected])

Received 24 March 2011; revised 3 May 2011; accepted 6 May 2011; published online 31 May 2011

DOI: 10.1002/polb.22283

ABSTRACT: Renewable poly(lactic acid) (PLA) exhibits high

strength and stiffness. PLA is fully biodegradable and has received

great interest. However, the inherent brittleness of PLA largely

impedes its wide applications. In this article, the recent progress in

PLA toughening using various routes including plasticization,

copolymerization, and melt blending with flexible polymers, was

reviewed in detail. PLA toughening, particularly modification of

impact toughness through melt blending, was emphasized in this

review. Reactive blending was shown to be especially effective in

achieving high impact strength. The relationship between compo-

sition, morphology, and mechanical properties were summarized.

Toughening mechanisms were also discussed. VC 2011 Wiley Peri-

odicals, Inc. J Polym Sci Part B: Polym Phys 49: 1051–1083, 2011

KEYWORDS: blending; blends; copolymerization; plasticization;

polylactide; reactive blending; toughening; toughness

INTRODUCTION Increasing concerns over the environmentalimpact and sustainability of conventional polymer materialshave motivated academia and industry to devote consider-able efforts to the development of polymers from renewableresources. Among a few commercially available biobased orpartially biobased thermoplastic polymers, poly(lactic acid)(PLA) has undergone the most investigation. PLA is synthe-sized either through polycondensation of lactic acid(2-hydroxy propionic acid) or ring-opening polymerization oflactide (LA) (the dimer of lactic acids), as illustrated inFigure 1. The monomer, lactic acids, can be produced viabacterial fermentation using enzyme-thinned corn starch orsugar directly as carbon sources. Lactic acid is one of thesimplest chiral molecules and exists as the two stereoisomers: L- and D-lactic acid.

Advances in the polymerization technology have significantlyreduced the production cost and have contributed to makePLA economically competitive with petroleum-based poly-mers. PLA has attracted increasing interest in various mar-kets, such as packaging, textile, and automotive industries, asa very promising eco-friendly alternative to traditional petro-leum-based commodity polymers. Despite its numerousadvantages such as high strength and high modulus, theinherent brittleness significantly impedes its wide applica-tions in many fields. Compared with the general purposepolystyrene (PS), a mainstream thermoplastic widely used inmany industrial and home products, PLA not only has com-parable tensile strength and modulus but also exhibits verysimilar inherent brittleness (as shown in Table 1). Just asthe limitation of brittleness of PS led to the development ofrubber-modified high impact PS and its copolymers [e.g.,

acrylonitrile–butadiene–styrene copolymer, (ABS)] for advancedengineering applications, in recent years PLA toughening hasbecome the focus of numerous investigations. Many strategieshave been developed in the literature to improve the toughnessof PLA, including plasticization, copolymerization, addition ofrigid fillers, and blending with a variety of flexible polymers orrubbers.

It has been demonstrated that the variation in stereochemis-try, molecular weight, and crystallinity of pristine PLA canimprove its ductility and impact resistance to some extent.Nevertheless, such influences are usually marginal and theresulting increase of toughness properties is usually insuffi-cient to satisfy the requirement of most practical applica-tions. The detailed discussion regarding these factors can befound in a previous review entitled ‘‘Toughening Polylactide’’by Anderson et al.1 and in other related literature.2–5 There-fore, this review will mainly focus on toughening modifica-tion of PLA by plasticization, copolymerization, and moreindustrially practical melt-blending technologies.

PLASTICIZATION

Plasticizers are used not only to improve the processabilityof polymers but also to enhance flexibility and ductility ofglassy polymers. A preferred plasticizer for PLA should beone, which significantly lowers the glass transition tempera-ture (Tg) of PLA, is biodegradable, nonvolatile, and nontoxic,and exhibits minimal leaching or migration during aging. Theplasticizing efficiency of a plasticizer, which is usually eval-uated in terms of the depression in Tg and enhancement intensile toughness, depends on its miscibility with host

VC 2011Wiley Periodicals, Inc.

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polymers, molecular weight, and loading level. The close-ness of solubility parameters (d) and magnitude of interac-tion parameters (vT) between plasticizers and PLA as ahost polymer is usually used to evaluate the miscibilitybetween them, and thus provide a reference for the selec-tion of effective plasticizers.8–16 Generally, small moleculeplasticizers are more efficient than larger ones, especially inlowering Tg of the host polymer. The miscibility of a poly-mer with plasticizers from the same chemical familydecreases with increase in the molecular weight of theplasticizers, because mixing with low-molecular weightplasticizers has high entropy of mixing. To date, variousmonomers and oligomers have been investigated as poten-tial plasticizers for PLA. Among them, polyethylene glycol(PEG) and citrate esters are perhaps the most commoninvestigated plasticizers.

Monomeric PlasticizersWith 19.2 wt % of LA in PLA, Sinclair17 demonstrated thatthe elongation of the plasticized PLA increased to 536%, andthe corresponding elastic modulus and stress at breakdropped to 0.66 GPa and 29.2 MPa, respectively. Tg waslocated between 32 and 40 �C with LA concentration varyingfrom 15 to 20 wt %. Unfortunately, LA was reported to read-ily volatilize during melt processing because of its low boil-ing point. This study also reported plasticization of PLAusing oligomeric lactic acid (OLA) but relatively lower effi-ciency in lowering Tg was achieved relative to using LAmonomer.

Several citrate esters are commercial plasticizers for foodcontact films, including triethyl citrate (TEC), tributyl citrate(TBC), acetyltriethyl citrate (ATEC), and acetyltributyl citrate(ATBC). Labrecque et al.18 studied the plasticization of PLA

Hongzhi Liu received his Ph.D. degree in polymer chemistry and physics at the Institute of

Chemistry, Chinese Academy of Sciences, Beijing, China, in 2005. From 2006 to 2007, he was

a postdoctoral fellow in Seoul National University. From 2007 to 2008, he was a postdoctoral

researcher in Louisiana State University. Since 2008, he is now working as a post-doctoral

research associate at the Composite Materials and Engineering Center of Washington State

University. His current research interests focus on development and characterization of

biobased polymeric materials derived from renewable natural resources.

Jinwen Zhang received his Ph.D. in polymer science from University of Massachusetts

Lowell in 1997. Dr. Zhang is currently a tenured associate professor at the Composite

Materials and Engineering Center of Washington State University. For the past 14 years, Dr.

Zhang’s research has been focused on biobased polymer materials ranging from synthesis

of new renewable polymers, new processing techniques and application development.

FIGURE 1 Synthesis route of poly(lactic acid).

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using these citrate esters in extruded PLA films. All of theplasticized PLA compositions (up to 30 wt %) exhibited asingle Tg which was lower than that of neat PLA. The elonga-tion of PLA was improved on plasticization but the plasticiz-ing efficiency was higher for ATBC. The citrate plasticizersappeared more effective in enhancing the elongation whenits presence was in excess of 10 wt %. At a plasticizer con-tent of 20 wt %, the plasticized PLA showed a minimum of76% drop in yield strength compared to that (51.7 MPa) ofneat PLA. Yield strength further decreased below 10 MPawhen 20–30 wt % plasticizer was added. The loss of thoselow-molecular weight citrate plasticizers during processingwas also observed because of their relatively lower boilingpoints. In another study, Ljungberg and Wesslen12 demon-strated that both glycerin triacetate (GTA; also known as ‘‘tri-acetin’’) and TBC were more effective plasticizers for PLAthan the other three citrates (TEC, TBC, and ATEC) on thebasis of the extent of the Tg depression. Phase separationoccurred when the content of both plasticizers were inexcess of �25 wt %. Phase separation was also noted duringheat treatment of the plasticized PLA. An increase in thecrystallinity of PLA as a result of cold crystallization wasconsidered to be responsible for the phase separation. Withabout 15.6 wt % TBC in PLA, phase separation after thestorage for almost 30 days was also noted by Sierra et al.19

Murariu et al.8 studied the plasticization of PLA using threelow-molecular weight ester-type plasticizers, bis(2-ethyl-dhexyl) adipate (DOA), GTA, and ATBC. Size exclusionchromatography results revealed that molecular weight anddistribution of PLA were less affected by the amount andnature of the plasticizers used during melt blending. Thethermal stability of the plasticized PLA correlated with theamount and volatility of the plasticizer used. Differentialscanning calorimetry (DSC) analysis demonstrated that theaddition of 20 wt % GTA which had the lowest molecularweight and the lowest interaction parameter with PLAamong these three plasticizers resulted in the lowest Tg(�29 �C). PLA plasticized with 20 wt % DOA exhibited phaseseparation and a smaller decrease in Tg (�45 �C) butenhanced crystallization rate of PLA. Addition of up to 20 wt% plasticizer led to a gradual decrease in Young’s modulus

and increased ductility in the following order of efficiency:GTA > ATBC > DOA. The best notched impact performancewas seen in PLA plasticized with 20 wt % GTA, in whichspecimens could not be broken in notched impact testing. Bycomparison, addition of TBAC led to the least improvementin the impact strength among the three plasticizers, onlyinducing a 77% increase on an addition of 20 wt %. Table 2summarizes the molecular weight and solubility parameters(d) of some monomeric plasticizers and their interactionparameter (vT) with PLA, as well as their plasticizationeffects on PLA.

Oligomeric and Polymeric PlasticizersSmall molecule plasticizers usually evaporate during meltprocessing12,18,21,23 and also have a strong tendency tomigrate toward the surfaces during storage.11,13–16,23 Thedriving force of the migration is attributed to the depletionin the amorphous PLA phase due to enhanced crystallinity ofPLA in plasticized samples, and consequently, the ability ofPLA to accommodate the plasticizer diminished.9,11,12,15,16,23

Migration not only contaminates the food or beverage incontact with plasticized PLA but also causes plasticized PLAto regain part of the brittleness of neat PLA. The commonway to reduce migration and evaporation of plasticizers is toincrease the molecular mass of the plasticizer to an upperlimit where migration will be minimized while the miscibilitywith the matrix is still retained.13 In recent years, increasingattention has been paid to the utilization of oligomeric orpolymeric plasticizing agents for PLA.

Martino et al.21 compared the plasticizing effects of threecommercial adipates as potential plasticizers for PLA. At 10wt % of the plasticizer content, DOA resulted in much higherelongation (259%) of the plasticized PLA than the two poly-meric adipates (5% and 7%, respectively). At 20 wt % of theplasticizer content, however, both polyadipates resulted inmuch higher elongation (>480%) of the plasticized PLA withrespect to DOA (295%). Also, at 20 wt % DOA, lack of homo-geneity and significant release of plasticizer during process-ing were noted, while good compatibility with PLA andhigher plasticizing efficiency were observed for the othertwo polymeric adipates (especially polyadipate with the

TABLE 1 Comparison of Typical PLA Properties with Several Petroleum-Based Commodity Thermoplastic Resins

PLA PET PS HIPS PP

Tg (�C)a 55 75 105 – �10

Tensile strength @ break (MPa)a 53 54 45 23 31

Tensile modulus (GPa)a 3.4 2.8 2.9 2.1 0.9

Elongation @ break (%)a 6 130 7 45 120

Notched Izod IS (J/m)a 13 591 27 123 276 (i-PP)

Gardner impact (J)a 0.06 0.32 0.51 11.30 0.79

Cost ($/lb)b 1–1.5 0.70–0.72 0.99–1.01 1.01–1.03 1.15–1.17

PET: poly(ethylene terephthalate); PS: polystyrene; HIPS: high-impact polystyrene; PP: polypropylene; i-PP: isotactic polypropylene homopolymer;

IS: impact strength.a Data mainly cited from ref. 7.b Cost cited from ‘‘Plastics News’’, March 31, 2011 except PLA resin.



TABLE 2 Molecular Weight and Solubility Parameters (d) of Some Monomeric Plasticizers and their Plasticization Effects on PLA

Materials

Content

(wt %)

Tg (�C),DSC

TS

(MPa) E (MPa) e (%)

Notched

IS (kJ/m2)Name

Molecular

Weight

(g/mol) d (MPa)0.5 vT

LA17 Mn ¼ 144 – – 1.3 – 51.7 1993 3 –

17.3 – 15.8 820 288 –

19.2 32–40 29.2 658 536 –

25.5 – 16.8 232 546 –

PLA18 Mw ¼ 137,000 – – 100 59.1 51.7 – 7 –

TEC18 Mn ¼ 276 19.8 – 10 42.1 28.1 – 21.3 –

20 32.6 12.6 – 382 –

TBC18 Mn ¼ 360 18.8 – 10 40.4 22.4 – 6.2 –

20 17.6 7.1 – 350 –

ATEC18 Mn ¼ 318 19.6 – 10 50.8 34.5 – 10 –

20 30.0 9.6 – 320 –

ATBC18 Mn ¼ 402 18.7 – 10 25.4 17.7 – 2.3 –

20 17.0 9.2 – 420 –

PLA (aging for

10 days)19Mw ¼ 204,453 – – 100 58.01 41.69 3364 1.27 –

TBC (aging for

10 days)19Mn ¼ 360 19.612,13 – 12.44 41.12a 38.27 2542 2.14 –

12.99 37.61a 12.44 1248 >100.23 –

15.58 32.1a 6.39 53 >100.16 –

22.52 21.74a 9.84 55 >102.77 –

PLA (aging for

24 days)19Mw ¼ 204,453 – – 100 – 41.69 3364 1.27 –

TBC (aging for

24 days)19Mn ¼ 360 9.612,13 – 12.44 39.45b 41.95 2301 3.43 –

12.99 36.97b 19.97 1337 >99.98 –

15.58 30.37b 7.67 313 >100.03 –

22.52 33.95b 11.95 159 >100.08 –

PLA20 Mn ¼ 84,000 – – 100 60 66 3300 1.8 –

ATBC20 Mn ¼ 402 – – 10 41 50.1 2900 7 –

20 24 23.1 100 298 –

PLA8 Mn ¼ 74,500 20.1 – 100 62 66 6 2 1020 6 100 11 6 3 2.6 6 0.2

ATBC8 Mn ¼ 402.5 19.2 0.46 10 44 51 6 1 970 6 70 11 6 4 2.4 6 0.4

20 38 30 6 1 270 6 20 317 6 4 4.6 6 1.3

DOA8 Mn ¼ 370.6 17.6 1.33 10 45 29 6 2 720 6 90 36 6 5 2.6

20 45 21 6 1 670 6 120 78 6 33 28.7

GTA8 Mn ¼ 218.2 20.1 0.34 10 48 38 6 3 760 6 140 8 6 2 2.7 6 0.3

20 29 24 6 1 10 6 3 443 6 13 NB

PLA9 Mw ¼ 74,000 23.1 – 100 59.2 64.0 6 1.5 2840 6 50 3.0 6 0.3 –

DBS9 Mw ¼ 314 17.7 3.7 10 39.9 39.2 6 4.0 2000 6 80 2.3 6 0.2 –

20 �66.9/26.1 23.1 6 0.9 430 6 50 269.0 6 6.0 –

AGM9 Mw ¼ 316 18.5 1.5 10 45.8 52.1 6 4.0 2240 6 100 32 6 2.1 –

20 �65.8/24.3 27.1 6 3.1 35 6 5 335.0 6 2.3 –

PLA21 Mn ¼ 63,000 19.93 – 100 58.2 47 6 5 2000 6 200 6 6 2 –

DOA21 Mn ¼ 371 16.67 – 10 40.8 27 6 4 1600 6 100 259 6 64 –

20 40.1 17 6 1 1400 6 100 295 6 89 –

PLA22 Mn ¼ 81,000 – – 100 61 52 6 2 1800 6 150 6 6 1 2.6 6 0.2

TBC22 Mn ¼ 360 – – 20 20 20 6 1 9 6 1 320 6 20 NB

LA: lactide; TEC: triethyl citrate; TBC: tributyl citrate; ATEC: acetyl

triethyl citrate; ATBC: acetyl tributyl citrate; DOA: dioctyl adipate (also

known as ‘‘bis(2-ethyldhexyl) adipate’’); GTA: glycerin triacetate (also

known as ‘‘triacetin’’); DBS: dibutyl sebcate; AGM: acetyl glycerol mono-

laurate; TS: the maximum tensile strength; E: tensile modulus; e: tensileelongation at break; IS: impact strength; NB: no break.a Aging for 20 days.b Aging for 32 days.

lower molecular mass) at 20 wt % content. In another workby Martino et al.,11 the plasticization of amorphous PLAusing four commercially available adipates was alsoexplored. Each plasticizer was miscible with PLA until a criti-cal concentration was reached, which depended on the mo-lecular mass of the individual adipate. A remarkable increasein elongation was achieved when the concentration of plasti-cizer reached 10 wt %, whereas the decreases in elasticmodulus and tensile stress were noted for all the plasticizersinvestigated. It was shown that DOA and the polyadipatewith the highest molecular mass (GlyplastV

R

G206/7) wereless efficient plasticizers. The former showed some migrationat the concentrations higher than 10 wt %, while the lattereasily caused phase separation to occur because of the lowercompatibility with PLA matrix. It was evidenced that theother two polymeric adipates, GlyplastV

R

206/3 and GlyplastVR

206/5, were miscible with PLA at least for the compositionsranging from 5 to 20 wt %. The best plasticizing effectswere achieved with the polyadipate having lower molecularmass (GlyplastV

R

206/3), as it showed that Tg decreased from55.1 �C for the neat PLA to 28.3 �C for the PLA with 20 wt% of the plasticizer. The elongation at break increased up to250% and tear resistance increased by �135%. Meanwhile,the ultimate stress and elastic modulus decreased by �44%and �62%, respectively.

Ljungberg and Wesslen13,15 prepared two oligomeric plasti-cizers of different molecular weights (Mw ¼ 4,550 g/mol vs.63,600 g/mol) in terms of transesterification of TBC withdiethylene glycol, and investigated the effects of these TBC-based oligomers on thermal–mechanical and aging proper-ties of the extruded PLA films. Both of investigated TBColigomers did not lower the Tg of PLA as greatly as mono-meric TBC. But among the two oligomeric plasticizers, a rela-tively larger reduction in Tg was achieved with the oligomerhaving the lower molecular weight (Mw ¼ 4,550 g/mol). Par-tial phase separation occurred after the plasticized PLA with10–20 wt % of the TBC-oligomer was aged at ambient tem-perature for several weeks. The higher the molecular weightof the plasticizer, the lower the critical saturation concentra-tion, at which phase separation began to occur. Compared tothe TBC monomer, the morphological stability of the PLAblends with the oligomer having lower Mw was enhancedwhen the concentration of the oligomers was relatively low(i.e., 10–15 wt %). By reacting diethyl bishydroxymethyl mal-onate (DBM) with acid dichlorides and/or diamines, a seriesof DBM-oligoesters and DBM-oligoesteramides were synthe-sized with different molecular weights, respectively.14–16 Theoligomeric plasticizers resulted in a slightly smaller Tgdepression of PLA than the monomeric DBM. The compatibil-ity between PLA and the plasticizer and the enhancement inelongation were influenced by the molecular weight of theoligomer and the presence of polar amide groups that wereable to positively interact with the PLA chains. With 15 wt% of either DBM-oligomester or DBM-oligoesteramide basedon triethylene glycol diamine, the elongation increased toabove 200%, whereas the oligoesteramide based on polypro-pylene glycol diamine only showed an elongation of around20%. It was found that annealing of the plasticized PLA at

100 �C for 4 h promoted cold crystallization and phase sepa-ration, causing the plasticized PLA to regain the brittleness.On the contrary, physical aging at ambient temperaturerevealed that the enhanced flexibility and morphological sta-bility of the film plasticized with the oligomers could bemaintained.

LapolVR

, LLC recently introduced a commercial bioplasticizer/impact modifier, LapolTM, which was specifically designed forPLA. Lapol is a viscous liquid modifier and is claimed to be alactic acid-derived polymer.24 This liquid plasticizer com-prises both polyester plasticizing units and compatibilizingunits. It is therefore thought to be compatible and misciblewith PLA and other biopolymers up to 20% and does notrequire any additional compatibilizers. Compared to tradi-tional small molecule plasticizers, it is claimed that Lapolcan improve ‘‘flexibility’’ of PLA without considerably sacrific-ing the modulus at relative low concentrations (5–10%).

PEG, conventionally referred to poly(ethylene oxide) of lowmolecular weight (<20,000 g/mol), is a class of nontoxic,water-soluble, and crystalline polymer commercially availableover a broad range of molecular weights from 200 to 2 � 104

g/mol. The miscibility of PEG and PLA depends on molecularweight and content of PEGs.25–29 Lower molecular weightPEGs exhibit better miscibility with PLA and result in more ef-ficient reduction of Tg, which can lead to drastic improvementin ductility and/or impact resistance of PLA at low concentra-tions. Baiardo et al.20 investigated the thermal and mechanicalproperties of PLA plasticized with PEGs of different molecularweights from 400 to 10,000. It was shown that Tg invariablydropped to a certain plateau value with the addition of PEG,and this limit concentration ranged from 15 to 30 wt %,depending on molecular weight of PEGs. The concentration atwhich maximum elongation was achieved also varied with themolecular weight of PEG. When PEG10000 was used, 20 wt %was needed to achieve an elongation of 130%, while the simi-lar increase was obtained by 10 wt % in the case of the lowermolecular weight PEG400. In another report by Martin andAverous,30 PEG400 and OLA were found to be the most effi-cient plasticizers of amorphous PLA among various biocom-patible monomeric and oligomeric plasticizer, while glycerolwas the least efficient plasticizer.

Jacobsen and Fritz31 used PEG with a molecular weight of1500 g/mol (PEG1500), glucose monoester (DehydatV

R

VPA1726), and partial fatty esters (LoxiolVR

GMS95) to plasti-cize PLA and examined the influences of these plasticizerson tensile and unnotched Charpy impact resistance of injec-tion-molded PLA specimens. The significant improvement inboth elongation (180%) and impact resistance (nonbreakunder unnotched Charpy impact test condition) wasreported when 10 wt % PEG1500 was added, whereas inthe case of glucose monoester and partial fatty acid ester,elongation of PLA was improved but impact strength wasslightly decreased at all concentrations examined (i.e., 2.5–10wt %). No crazing was observed in the deformed tensilespecimens plasticized with 10 wt % of PEG1500, which wasdifferent from the ones with 10 wt % of glucose monoesteror partial fatty acid ester.



Pillin et al.9 investigated the thermal and mechanical proper-ties of PLA blends plasticized with PEGs (Mw ¼ 200, 400,1000 g/mol) or several other plasticizers that can be used infood packaging, such as poly(1,3-butanediol) (PBOH), dibutylsebacate (DBS), and acetyl glycerol monolaurate (AGM). Theexperimental results were further compared to the theoreti-cally predicted results. Among these plasticizers, PEGs werethe most efficient in reducing the Tg of PLA. For more than20 wt % plasticizers, all plasticized PLA blends exhibited alimit of miscibility and a plateau of Tg reached. Also, thermaland mechanical results were found to contradict with theprediction of miscibility through empirical interaction pa-rameters and Fox equations. For PEGs which should have op-timum miscibility with the PLA matrix according to the theo-retical predictions, macroscopic phase separation occurred ata certain PEGs concentration (20 wt % for PEG200 and 30wt % for PEG400). Nevertheless, the improvement of misci-bility was observed for the other three plasticizers that wereexpected to be less miscible with PLA. The authors attrib-uted this discrepancy to a more remarkably enhanced crys-tallization of PLA in the presence of PEGs. Results of tensiletests showed a strong decrease in modulus and stress atbreak for plasticizer content higher than 20 wt %. At higherplasticizer contents (�20 wt %), PEGs led to a lower elonga-tion of blends in comparison to the other plasticizers. Thus,the authors stated that PBOH, AGM, and DBS at a loadinglevel of 20–30 wt % were the more efficient according tothe mechanical requirements. Kulinski and Piorkowska32

studied the effects of different end groups (hydroxyl vs.methyl) of PEG on the plasticization of both amorphous andsemicrystalline PLA with plasticizer concentrations up to 10wt %. No marked effects induced by different end groups ofthe plasticizer were found and thermal and mechanical prop-erties were predominantly governed by the plasticizer con-tent. All plasticizers used enabled Tg depression andimproved the ability of the PLA to undergo cold crystalliza-tion. At the same plasticizer content, the amorphous plasti-cized PLA blends exhibited much higher elongation at breakthan the corresponding semicrystalline plasticized PLAblends. This difference was attributed to the reduced abilityof PLA to plastic deformation due to the crystallization na-ture of the latter. It was found that the plastic deformationof both neat and plasticized PLA was associated withcrazing.

Therefore, it was indicated that with lowering molecularweight and increasing concentration of PEGs, the crystalliza-tion temperature of PLA shifted to lower temperatures inparallel with the depression in Tg. At a certain PEG concen-tration depending on its molecular weight, the blends withPEGs would undergo a phase separation because of the slowcrystallization of PEGs during aging, thereby resulting ingradual embrittlement of the materials.25–29,33 Furthermore,because of the hydrophilic nature of PEG, leaching of PEGfrom the host polymer during contact with an aqueous envi-ronment was another drawback of the PEG plasticizers.26

To combat these aforesaid deficiencies, PLA-b-PEG blockcopolymers were synthesized and investigated as PLA plasti-

cizers.34 The plasticization behaviors of these compoundswere complicated by the dependence on the PEG blocklength. Some samples showed the microphase separationand crystallization of the PEG blocks, resulting in incompleteplasticization of the host polymer. In a separate strategy, thedirect copolymerization of L-LA with ethylene oxide wasreported to yield copolymers having a multiblock struc-ture.35 Solvent-casting films from blends of these copolymersand PLA exhibited improved modulus and yield stress aswell as comparable elongation with respect to the PLLA/PEGblend with an identical L-LA/EO (ethlene oxide) composition.The authors expected that leaching of these copolymer plas-ticizing agents was also greatly reduced when compared toPLLA/PEO blend. Although individual block sizes could becontrolled to a certain degree by manipulating the reactionconditions, all of the reported block copolymers exhibitedtwo melting transitions, suggesting that the blocks had suffi-cient length to undergo crystallization-induced microphaseseparation. In light of the above considerations, Bechtoldet al.36 synthesized alternating copolymers of lactic acid andethylene oxide poly(3-methyl-1,4-dioxan-2-one) (PMDO) as apotential macromolecular plasticizing agent for PLA. Themiscibility of PLA and PMDO was evidenced by a single Tgthat was well described by the Fox relationship of miscibleblends.

Poly(propylene glycol) (PPG) also has been attempted inplasticization of PLA. Unlike the semicrystalline PEGs, PPG isamorphous. McCarthy and Song33 compared the plasticiza-tion of PLA using PPG and epoxy-capped PPG (referred to asPPG-E) of similar molecular weights (720 g/mol vs. 640 g/mol). DSC results showed that both PPG and PPG-E weremiscible with PLA. The Tg of PLA decreased linearly withincreasing concentration of either plasticizer, with PPG-E dis-playing a higher depression effect than PPG. Both plasticizerswere very effective in improving tensile toughness. When theplasticizer content was above 15 wt%, the elongationincreased to more than 250% for all the blends. For PPG-Eat 15 wt % and for PPG at 15–20 wt %, the ductility of theblends were improved without sacrificing strength and stiff-ness. However, when the concentration of PPG-E was higherthan 20 wt %, the modulus of the blends decreased to therange typical of elastomers. After aging for 1 month, the me-chanical properties of the plasticized PLA did not changeremarkably. This result indicated that PPG and PPG-E couldprevent the physical aging embrittlement of PLA.

Subsequently, Kulinski and coworkers37,38 studied the plasti-cization of PLA using PPG with a nominal molecular weightof 425 g/mol (PPG425) and 1000 g/mol (PPG1000), to-gether with PEG600 (nominal Mw ¼ 600 g/mol) as a com-parison. The plasticized samples with both PPGs showed thedecrease in Tg and the enhanced ability of PLA to crystallizebut this effect induced by PPGs were relatively smaller whencompared with that of the PLA plasticized by PEG600. Asevidenced by the results, minor phase separation occurred inthe blend containing 12.5 wt % of PPG1000, suggesting thatthe miscibility of PPGs with PLA for PPG decreased with theincrease of PPG molecular weight. Unlike PEG600, however,



phase separation of PPG from amorphous PLA did not dete-riorate the drawability of the PLA materials. As oneexpected, increasing PPG content led to an increase in elon-gation and a decrease in yield stress. At the plasticizer con-tent of 12.5 wt %, the use of PPG425 resulted in the maxi-mum elongation (702%), which was significantly higher thanthat of neat PLA (64%). In addition, at higher contents ofPPGs (�7.5 wt %), the PLA samples exhibited strain-inducedcrystallization during deformation, whereas the evidences ofcrazing were noted in the deformed PLA samples containingthe lower PPG concentrations. For semicrystalline PLA plasti-cized with the same PPGs, it was found that the crystalliza-tion in the blends was accompanied by phase separation.38

Increasing the plasticizer concentration in the amorphousphase and annealing the blends at crystallization tempera-tures contributed to the phase separation. With an increaseof PPG content, yield stress decreased while the elongationincreased. PLA/PPG blends universally exhibited higher elon-gations than the corresponding PLA/PEG600 ones. At 12.5wt % of PPG content, the elongation values of the PLA/PPG425 and PLA/PPG1000 blends reached 105% and 65%,respectively, while in PLA/PEG blends, it decreased to only15% at PEG content above 10 wt %. Neat PLA yielded an av-erage elongation of �8%. The PLA/PPG1000 blends showedmost intense phase separation, and the formation of tinyPPG droplets. Based on morphological analysis, the authorsargued that tiny liquid pools of PPG facilitated local plastici-zation of PLA during plastic flow and had a positive effecton drawability, while solid inclusions of crystallizable plasti-cizers like PEG were undesirable as they deteriorated theblend drawability. Table 3 summarizes the molecular weightand solubility parameters (d) of some oligomeric or poly-meric plasticizers and their interaction parameter (vT) withPLA, as well as their plasticization effects on PLA.

Mixed PlasticizersWhile increasing the molecular weight of the plasticizer canslow down migration rate and thus improve morphologicalstability of PLA materials during storage, it also decreases itssolubility and plasticizing efficiency. Additionally, high-molec-ular weight plasticizers are more prone to phase separationbecause of low saturation concentrations of plasticizers. Touse the complementary advantages, the combination of smallmolecule plasticizers with polymeric or oligomeric ones wasalso attempted in the literature. Ren et al.39 used a mixture(1/1, w/w) of GTA and oligomeric poly(1,3-butylene glycoladipate) to plasticize PLA. Tg decreased from 59.7 �C forpure PLA to 37.4 �C for PLA containing 29 wt % mixed plas-ticizers. Tensile strength progressively decreased with anincrease of the total content of mixed plasticizer, while a sig-nificant increase in elongation occurred at the content ofabout 5–9 wt %. The blends were brittle with less than 5%plasticizers and were ductile with great than 9 wt % plasti-cizers. Lemmouchi et al.22 recently reported the plasticiza-tion of PLA using a mixture of TBC and a more thermallystable low-molecular weight poly(D,L-LA)-b-poly(ethyleneglycol) copolymer (PLA-b-PEG) with different moleculararchitecture (Table 4). The use of TBC alone was the most

effective in lowering Tg and enhancing elongation of PLA,while the use of PLA-b-PEG copolymers alone well maintainedtensile strength and modulus. Diblock copolymer (COPO1 orCOPO2) seemed to be slightly more efficient in decreasing Tgthan triblcok (COPO3) or star copolymers (COPO4). However,the combination of TBC and PLA-b-PEG copolymer (1/1, w/w) mixtures led to a medium level of depression in Tg andmore balanced mechanical properties, compared to the use ofan individual plasticizer. It was claimed that varying the struc-ture of copolymers allowed tailoring of the end-use perform-ance required for different targeted applications. Table 4 sum-marizes the molecular weight of some mixed plasticizers andtheir plasticization effects on PLA.

Other PlasticizersRecently, two phosphonium type ionic liquids (ILs) with dif-ferent anions, as shown in Figure 2, were evaluated as poten-tial plasticizers and/or lubricants for amorphous PLA.40 BothILs were found to lower the Tg of PLA and modify rheologicalcharacteristics as manifested by reduced viscosities, apparentphase separation, and lubrication effect. These effects weremuch more pronounced for the IL-1 containing a hydrophobicdecanoate anion, presumably as a result of higher overallcompatibility with the matrix with respect to the one contain-ing a hydrophilic BF4 anion. Nevertheless, thermogravimetricanalysis data showed that the presence of ILs had a catalyticeffect on PLA degradation. Mechanical properties of the IL-plasticized PLA materials were not reported.

Epoxidized soybean oil (ESO) has long been used as a plasti-cizer for PVC. Ali et al.41 studied the plasticizing effects ofESO on PLA and found that the plasticization efficiency wasrelatively low. For instance, PLA containing 20 phr ESO onlydisplayed an elongation of 38%, meanwhile, yield stress ofthe neat PLA decreased from 60 to 26 MPa.

In general, plasticization has been demonstrated to be verysuccessful in improving the flexibility and ductility of PLA inthe literature. However, there are still some problems associ-ated with this method. Typically, relatively high percentageof plasticizers (15–20 wt %) are required to provide a re-markable reduction in Tg, adequate ductility or tensile tough-ness of the PLA matrix. The significant improvement in elon-gation is usually accompanied by substantial reductions instrength and modulus (even up to three orders of magni-tude). Moreover, an excessive incorporation of plasticizertends to result in the phase separation because of the satu-ration of plasticizer in the amorphous phase of PLA. In addi-tion, there seems to exist a competition between plasticiza-tion efficiency and the kinetics of aging or unfavorable coldcrystallization in the plasticized PLA. The more the materialis plasticized, the larger the increase in chain mobility andthe faster the cold crystallization process. It is therefore im-perative to find an optimal balance at which PLA is suffi-ciently flexible for the desired application without the occur-rence of overly fast cold crystallization.

COPOLYMERIZATION

Copolymerization has been extensively investigated as apowerful means to obtain polymer materials with properties



TABLE3MolecularWeightandSolubilityParameters

(d)ofSomeOligomericorPolymericPlasticizers

andTheirPlasticizationEffects

onPLA

Materials

Content

(wt%)

Tg(�C),

DSC

TS

(MPa)

E(M

Pa)

e(%

)

Notched

IS(kJ/m

2)

Name

Molecular

Weight

(g/m

ol)

d(M

Pa)0.5

v T

PLA30

Mv¼

49,000

––

100

58

–20506

44

96

2–

OLA30

––

–10

37

–12566

38

326

4–

20

18

–7446

22

2006

24

–

PEG40030

Mw¼

400

––

10

30

–14886

39

266

5–

20

12

–9766

31

1606

12

–

M-PEG

30

Mw¼

400

––

10

34

–15716

51

186

2–

20

21

–11246

33

1426

19

–

PLA20

Mn¼

84,000

––

100

60

66

3300

1.8

–

PEG40020

Mw¼

400

––

10

23

32.5

1200

140

–

20

19

15.6

500

71

–

PEG150020

Mw¼

1500

––

10

42

46.6

2800

5–

20

20

21.8

600

235

–

PEG10K20

Mw¼

10,000

––

10

42

48.5

2800

2.8

–

––

20

34

22.1

700

130

–

PLA9

Mw¼

74,000

23.1

–100

59.2

64.0

61.5

28406

50

3.0

60.3

–

PBOH9

Mw¼

2100

21.3

2.3

10

47.6

56.3

61.9

23506

50

3.0

60.1

–

20

�48.5/30.1

30.2

61.1

3506

20

302.5

632.0

–

PEG2009

Mw¼

200

23.5

0.0

10

35.8

30.0

64.1

17006

100

2.0

60.6

–

PEG4009

Mw¼

400

22.5

0.1

10

37.1

39.0

63.0

19206

53

2.4

60.3

–

20

�50.2/18.6

16.0

60.3

6306

20

21.2

62.3

–

PEG10009

Mw¼

1000

21.9

0.5

10

40.2

39.6

65.0

19706

120

2.7

60.3

–

20

�62.7/22.4

21.6

60.4

2906

50

200.0

612.5

–

PLA

withlow

stereoregularity

28

Mw¼

160,000

––

100

58

536

222006

50

146

1–

PEG800028

Mw¼

8000

––

10

36

236

19506

30

2006

10

–



TABLE3(Continued)

Materials

Content

(wt%)

Tg(�C),

DSC

TS

(MPa)

E(M

Pa)

e(%

)

Notched

IS(kJ/m

2)

Name

Molecular

Weight

(g/m

ol)

d(M

Pa)0.5

v T

15

30

166

16306

20

2606

10

–

20

21

56

11806

20

3006

20

–

30

9–

56

15006

20

–

30(agingfor6h)

–16

0.2

406

54006

10

–

30(agingfor30h)

14a

26

60.2

1006

10

3406

10

–

30(agingfor120h)

–76

0.3

2206

20

3006

20

–

30(agingfor500h)

22a

96

0.3

3706

20

2506

10

–

30(agingfor1800h)

27a

96

0.3

4006

20

2406

20

–

PLA

withlow

stereoregularity

29

Mw¼

190,000

––

100

60

686

225006

200

36

0.5

–

PEG800029

Mw¼

8000

––

10

39

266

19006

50

1806

10

–

20

21

46

0.5

1506

20

2606

20

–

30

12

–206

23006

30

–

30(agingfor2h)

–2.2

60.3

806

52506

20

–

30(agingfor10h)

–3.8

60.3

1406

10

2306

20

–

30(agingfor24h)

–4.5

60.6

1506

10

2206

10

–

30(agingfor120h)

–5.0

60.2

1706

15

1706

20

–

30(agingfor720h)

–7.0

60.3

2406

10

1606

10

–

30(agingfor3000h)

–7.5

60.3

2506

10

1506

15

–

PLA33

Mn¼

11,800

––

100

61

62.8

2249

4.5

–

PPG72033

Mw¼

720

––

10

51

52.2

1820

4.2

–

20

38

39.9

1296

260

–

PPG640-E

33

Mw¼

640

––

10

42

50.2

2170

4.2

–

20

26

28.5

4.4

250

–

AgedPLA

for1month

33

Mn¼

11,800

––

100

–65.6

2338

4.0

–

AgedPPG720for1month

33

Mw¼

720

––

10

–54.9

1827

4.3

–

20

–33.1

1404

260

–



TABLE3(Continued)

Materials

Content

(wt%)

Tg(�C),

DSC

TS

(MPa)

E(M

Pa)

e(%

)

Notched

IS(kJ/m

2)

Name

Molecular

Weight

(g/m

ol)

d(M

Pa)0.5

v T

AgedPPG640-E

for1month

33

Mw¼

640

––

10

–54.1

2081

3.9

–

20

–28.2

9.9

260

–

PLA37

Mw¼

108,000

––

100

55.7

41.4

61.5

–646

42

–

PPG42537

Mw¼

530

––

10

33.1

21.0

61.5

–5246

66

–

PPG100037

Mw¼

1123

––

10

34.0

23.1

60.9

–4736

111

–

PEG60037

Mw¼

578

––

10

31.3

18.5

61.2

–4276

42

–

PLA21

Mn¼

63,000

19.93

–100

58.2

476

520006

200

66

2–

GlyplastVR

206/2

21

Mn¼

1532

21.91

–10

39.5

346

216006

100

56

1–

20

25.4

256

42006

100

4856

65

–

GlyplastVR

206/7

21

Mn¼

2565

22.87

–10

42.1

366

217006

200

76

5–

20

30.6

286

25006

100

4916

34

–

LapolVR10824

Mn¼

30,000–40,000

––

5–

77–84

2160–2313

160–200

–

–10

–57–59

1700–1786

180–210

–

PEG

35

Mn¼

18,500

––

11

–14.5

62.8

4236

20

2406

21

–

LA-co-PEG

35

Mn¼

17,600

––

20

–24.1

63.1

7106

21

2046

18

–

PLA22

Mn¼

81,000

––

100

61

526

218006

150

66

12.6

60.2

COPO122

Mn¼

650

––

20

29

216

17906

180

1706

10

1.6

60.6

COPO222

Mn¼

1000

––

20

26

256

13006

50

2206

20

8.3

62.5

COPO322

Mn¼

1050

––

20

36

306

117006

100

1306

20

1.9

60.6

COPO422

Mn¼

1750

––

20

35

246

211506

150

1706

10

1.9

60.7

LA:lactide;OLA:oligomericlacticacid;PEG:poly(ethylene

glycol),thesubsequentnumberrepresents

itsnominalmolecularweight;

M-PEG:poly(ethyleneglycol)

monolaurate;PBOH:poly(1,3-

butanediol);PPG:poly(propyleneglycol),thesubsequentnumberrepresents

itsnominalmolecularweight;

PPG-E:epoxycapped

poly(propyleneglycol);GlyplastVR206/2

and

GlyplastVR206/7:tw

o

kindsofcommercialpolymericadipateswithmolecularweightof1532and2565g/m

ol,respectively;COPO1andCOPO2:tw

okindsofAB-typeblockcopolymers

ofDLLA

andeitherPEG350mono-

methyletherorPEG750monomethylether,

thatis,PDLLA-b-PEG350(10/4,molarratioofD,L-LA

monomerto

PEG350usedin

thefeed)andPDLLA-b-PEG750(10/4,molarratioofD,L-LA

monomerto

PEG750usedin

thefeed);COPO3:ABA-typeblockcopolymerofPDLLA

andPEG400,thatis,PDLLA-b-PEG400-b-PDLLA

(10/2,molarratioofD,L-LA

monomerto

PEG400usedin

thefeed);COPO4:3-

star-(PEG-b-PDLLA)blockcopolymer(10/1.3,molarratio

of

D,L-LA

monomerto

3-star-PEG

usedin

thefeed);COPO5:4-star-(PEG-b-PDLLA)blockcopolymer(10/1,molarratioof

D,L-LA

monomerto

4-star-PEG

usedin

thefeed);TS:themaxim

um

tensilestrength;E:tensilemodulus;e:

tensileelongationatbreak;IS:im

pactstrength;NB:nobreak.

aTgmeasuredfrom

E00in

DMA.



unattainable by homopolymers. Properties including tensileand impact performances of a copolymer can be tailored in aversatile way by manipulating the architecture of the mole-cule, sequence of monomers, and composition. Copolymeriza-tion of PLA can be conducted either through polycondensa-tion of lactic acid with other monomers (or polymersegments) or ring-opening copolymerization (ROC) of LAwith other cyclic monomers (or polymer segments). Becausethe latter synthesis route gives a more precise control ofchemistry and higher molecular weight of copolymers, it ismore widely used to improve the toughness or flexibility ofPLA in the literature. The polymerization can be ionic, co-ordination, or free radical depending on the type of catalystsystem involved.42,43 Figure 3 shows the chemical structuresof some of the reported comonomers and block segmentsused for PLA toughening in the literature. According to thedifference in molecular architecture, the resulting copoly-mers can be mainly classified into the following categories.

Linear Random CopolymersHomopolymers of e-caprolactone (CL) and trimethylene car-bonate (TMC), that is, poly(CL) (PCL) and poly(TMC)(PTMC), are two biodegradable polyesters and are highlyductile. The Tgs of PCL and PTMC are approximately �60and �20 �C, respectively. The excellent flexibility of the PCLand PTMC homopolymers prompted CL and TMC to be themost used comonomers to copolymerize with LA in achiev-ing tough copolymers.

Effects of the comonomer ratio on the thermal and mechani-cal properties of the poly(CL-co-L-LA) and poly(CL-co-D,L-LA) copolymers were examined by Hiljanen-Vainio and co-workers.44 The monomer ratio was varied from 80/20 to40/60 (w/w). The physical characteristics of resultingcopolymers ranged from weak elastomers to tough thermo-plastics as a function of CL/LA ratio and type of LA mono-mer in the copolymerization. Compared with the PLLA orPDLA homopolymers, the copolymers exhibited larger elon-gation (>100% for most copolymers) but lower tensile mod-ulus and strength. The copolymers containing L-LA hadgreater tensile strength than those containing D,L-LA due tothe crystalline nature of the former. Grijpma et al.45 also syn-thesized high-molecular weight copolymers of L-LA and CLby ROC. It was found that the copolymers (L-LA/CL ¼ 1/1,mole ratio) exhibited a tensile strength of 34 MPa and anelongation as high as 500%. In addition, it was shown thatthe ROC temperature (110 �C vs. 80 �C) influenced

TABLE 4 Molecular Weight of Some Mixed Plasticizers and their Plasticization Effects on PLA22

Materials

Content

(wt %)

Tg (�C),DSC TS (MPa) E (MPa) e (%)

Notched

IS (kJ/m2)Name

Molecular

Weight (g/mol)

PLA Mn ¼ 81,000 100 61 52 6 2 1800 6 150 6 6 1 2.6 6 0.2

TBC Mn ¼ 360 20 20 20 6 1 9 6 1 320 6 20 NB

COPO1 Mn ¼ 650 20 29 21 6 1 790 6 180 170 6 10 1.6 6 0.6

COPO2 Mn ¼ 1000 20 26 25 6 1 300 6 50 220 6 20 8.3 6 2.5

COPO3 Mn ¼ 1050 20 36 30 6 1 1700 6 100 130 6 20 1.9 6 0.6

COPO4 Mn ¼ 1750 20 35 24 6 2 1150 6 150 170 6 10 1.9 6 0.7

COPO1/TBC – 10 42 40 6 2 2000 6 110 4 6 1 2.7 6 0.2

20 24 17 6 1 9 6 1 260 6 20 6.4 6 1.9

COPO2/TBC – 10 – 27 6 2 1480 6 80 140 6 20 2.4 6 0.2

20 26 24 6 1 19 6 5 260 6 10 NB

COPO3/TBC – 10 – 37 6 6 1 1850 6 200 4 6 1 –

20 25 16 6 1 16 6 7 300 6 20 –

COPO4/TBC – 10 41 39 6 2 2000 6 100 4 6 1 2.5 6 0.2

20 27 22 6 1 150 6 65 250 6 10 5.5 6 0.8

COPO5/TBC – 10 47 37 6 1 1950 6 150 4 6 1 2.5 6 0.2

20 23 20 6 1 400 6 140 260 6 20 3.8 6 1.1

TBC: tributyl citrate; COPO1 and COPO2: two kinds of AB-type diblock

copolymers of PDLLA and either PEG350 monomethyl ether or PEG750

monomethyl ether, that is, PDLLA-b-PEG350 (10/4, molar ratio of D,L-LA

monomer to PEG used in the feed) and PDLLA-b-PEG750 (10/4, D,L-LA

monomer to PEG molar ratio used in the feed); COPO3: ABA-type tri-

block copolymer of DLLA and PEG400, that is, PDLLA-b-PEG400-b-

PDLLA (10/2, molar ratio of D,L-LA monomer to PEG used in the feed);

COPO4: 3-star-(PEG-b-PDLLA) block copolymer (10/1.3, molar ratio of

D,L-LA monomer to PEG used in the feed); COPO5: 4-star-(PEG-b-PDLLA)

block copolymer (10/1, molar ratio of D,L-LA monomer to PEG used in

the feed); TS: the maximum tensile strength; E: tensile modulus; e: ten-sile elongation at break; IS: impact strength; NB: no break.

FIGURE 2 Chemical structures of ionic liquids (IL-1 and IL-2).



mechanical properties of the resulting copolymers.46 Thehigher copolymerization temperature resulted in lower yieldstress and tensile modulus but higher elongation, which wasattributed to the less blocky copolymer formed at the higherpolymerization temperature.

Grijpma et al.3,47 further compared the influences of comono-mer content and the mode of sample preparation (i.e., as-polymerized vs. compression-mold) on mechanical propertiesof L-LA/CL or LA/TMC copolymers. At low CL content whenthe Tg was still well above room temperature, the unnotchedDynstat impact strength of the copolymers differed slightlybut yield stress, crystallinity, melting temperature, and Tgdecreased with increasing CL content. It was not until the Tgof the materials approached room temperature (�10 mol %CL) that the Dynsta impact strength began to increase con-tinuously with comonomer content and high impact tough-ness and ductility (>100%) were obtained. Under these

compositions, however, the copolymers showed low modulusand yield stress. For a given poly(L-LA-co-CL) composition,the as-polymerized samples had higher impact strength thanthe compression-molded ones. However, the situation wasdifferent when L-LA was copolymerized with TMC. Inaddition to the high impact strength achieved at high TMCcontent (�30 mol %) at which the Tg approached room tem-perature, a very sharp maximum impact strength (34 kJ/m2)at 1.0 mol % concentration was also noted. At such low TMCcontent (�1 mol %), the tensile properties of the L-LA-co-TMC copolymer were hardly affected and remained as highas those of the as-polymerized homo-PLLA. Similar but lessdrastic enhancement in toughness by copolymerization withTMC was reported by Ruckenstein and Yuan.48 The copoly-mer of L-LA and TMC (15 wt % TMC) showed the elongationof �15% (vs. �6% for pure PLLA) and tensile toughness of7 MJ/m3 (vs. 2.5 MJ/m3 for pure PLLA). However, when the

FIGURE 3 Chemical structure of cyclic comonomers and block segments used to toughen PLA via copolymerization route.



TMC content increased to 32 wt %, the copolymer displayeda rubbery behavior, and the elongation and tensile toughnessincreased significantly to 375% and 105 MJ/m3, respectively.

In addition to the CL and TMC comonomers, b-methyl-d-valerolactone (MV) is also used to ring-copolymerize withLA.49 The Tg of the copolymers gradually decreased withincreasing MV content. When the content of MV was higherthan 20 mol %, the copolymer became amorphous. At 8 mol %MV, the elongation reached 680% and tensile strength was37.8 MPa. With MV content ranging from 10 to 21 mol %, theelongation varied between 530% and 900%. Tensilestrength did not change considerably within the range8–15 mol % L-LA unit content. Table 5 summarizes thereported mechanical properties for these above linearrandom PLA copolymers.

Star and Linear Block CopolymersGrijpma et al.47 prepared a block copolymer using L-LA witha rubbery L-LA/CL (50/50 mol/mol) copolymer segment.With 34 wt % rubber block, the copolymer displayed anelongation of 1500% and did not fracture during Charpyimpact test. However, tensile strength decreased to 30.8MPa.

Using multifunctional alcohol as an initiator for ring-openingpolymerization, star-shaped AB block50 and ABA blockcopolymers4,50 were also synthesized by Grijpma and co-workers, respectively. In the above formulations, A is LAblock; B is a TMC, a TMC/CL (50/50 in mole ratio) or a CL/d-valerolactone (VL) (60/40 in mole ratio) rubber block. Itwas found that all three rubber blocks adequately toughen

PLA at concentrations higher than 15 wt %.50 While TMC/CL- or CL/VL-toughened star-shaped block copolymersexhibited significantly higher Dynstat unotched impactstrengths than TMC-toughened star-block copolymers, highertensile strength was achieved for the latter. This was attrib-uted to the relatively high Tg of the TMC rubber block. Thestar-shaped block copolymer with 17 wt % TMC rubbermerely exhibited an unnotched impact strength of 13.4 kJ/m2, while the copolymers with 15 wt % TMC/CL or CL/VLrubber was even nonbreakable in the Dynstat impact test.Overall, compared to neat PLA, the star-shaped block copoly-mers with TMC/CL rubber block exhibited much higher val-ues in both ductility and impact strength with relativelysmall reductions in modulus and acceptable tensile strength.In addition, it seemed that the preparation route (bulk poly-merization in the melt vs. polymerization in the solvents) aswell as the molecular weight of star-shaped copolymers wascrucial for obtaining good ultimate mechanical properties.

Grijpma et al.4 showed that the TMC rubber block (Mn ¼65.1�103 g/mol) in the tri-block copolymers was also effec-tive in toughening PLA. With varying weight content of TMCrubber from 10.9 to 21.4 wt %, the elongation increasedfrom 135% to 210%. Similar to the corresponding PLA/PTMC blends, tensile strength of the tri-block copolymersdecreased while impact strength increased with rubber con-tent. However, elongation was much higher for triblockcopolymers. The tri-block of PLA–PTMC–PLA containing 21wt % TMC had a comparable notched Izod impact strength(66.7 J/m vs. 52–63 J/m) and tensile strength (36.9 MPa vs.39.2 MPa) to the corresponding blend. But with molecular

TABLE 5 Summary of Reported Mechanical Properties for Some Linear Random PLA Copolymers

System

Comonomer/LA

Composition TS (MPa) E (MPa) e (%)

TT

(MJ/m3)

Unnotched

Dynstat

IS (kJ/m2)

PLLA (Dry)44 100/0 46.5 6 1.3 1720 6 50 2.9 6 0.2 – –

L-LA-co-CL (dry)44 40/60 (w/w) 3 6 0.56 30 6 7 87 6 28 – –

PDLLA (dry)44 100/0 34.5 6 1.4 1930 6 80 2.0 6 0.0 – –

D,L-LA-co-CL (dry)44 40/60 (w/w) 0.08 6 0.01 2.8 6 3.7 >100 – –

L-LA-co-CL (synthesized at 110 �C)46 50/50 (mol/mol) 18.2 84 480 – –

L-LA-co-CL (synthesized at 80 �C)46 50/50 (mol/mol) 9.0 5.2 880 – –

L-LA-co-CL45 50/50 (mol/mol) 34 – 500 – –

L-LA-co-CL3 85.5/14.5 (mol/mol) 30 – – – –

L-LA-co-TMC3 99/1 (mol/mol) – – – – 34

PLLA48 100/0 61 1400 6 2.5 –

L-LA-co-TMC48 85/15 (w/w) 62 1480 15 7 –

68/32 (w/w) 33 650 375 105 –

L-LA-co-MV49 92/8 (mol/mol) 37.8 – 680 – –

90/10 (mol/mol) 40.8 530 – –

18/15 (mol/mol) 42.9 – 800 – –

81/19 (mol/mol) 23.5 – 880 – –

79/21 (mol/mol) 10.8 – 900 – –

TS: the maximum tensile strength; E: tensile modulus; e: tensile elongation at break; TT: tensile toughness; IS: impact strength.



weight of 20.1 � 103 g/mol, the tri-block copolymer contain-ing 21 wt % TMC block led to lower yield strength (24.4MPa) but higher elongation (280%). This result was attrib-uted to the plasticizing effect of low-molecular weight TMCblocks. The tri-block copolymer with 20 wt % TMC/CL rub-ber block (Mn ¼ 46.5 � 103 g/mol) was much tougher dur-ing the impact test. No fracture during the Dynstat unotchedimpact test and a notched Izod impact strength of 446 J/mwere observed. Tensile tests showed an yield strength of41.4 MPa and an elongation of 120%. Also, its tensile andimpact properties were superior to those of the correspond-ing PLA/P(TMC/CL) blend. In contrast, a triblock copolymercontaining 20 wt % TMC/CL with relatively lower molecularweight (Mn ¼ 41.5 � 103 g/mol) in the rubber block had aslightly inferior yield strength (35.1 MPa) but a clearly lowervalue in elongation (50%) and unotched Dynstat impactstrength (only 6.6 kJ/m2). The much inferior impact strengthin this case was attributed to the too small rubber domainto optimally toughen PLA matrix at the high strain rate.

Haynes et al.51 copolymerized L-LA with commercial perfluoro-polyether oligomers (PFPE). The fluoropolyether segments

improved ductility, optical clarity, and melt processability whilereduced surface energy and water wettability. In contrast to thecorresponding physical blend of PLLA and PFPE, the copolymersdid not showmacrophase separation but exhibited higher opticalclarity and elongation. With 5 wt % PFPE block, the novel ABAtri-block copolymer film exhibited a dramatic increase in elonga-tion (>300% vs. 10–15% for neat PLLA). Tensile strength andmodulus of the copolymers were slightly lower than that of thePLLA homopolymer. Table 6 summarizes reported mechanicalproperties for the above star or block PLA copolymers.

Graft CopolymersGraft copolymerization is a convenient method to impart apolymer with unique properties and is generally performedin a separate reaction step. Toughening modification of PLAhas been also attempted by graft copolymerization. Jing andHillmyer52 described the synthesis of a novel bifunctionalmonomer consisting of a LA substituted with a norbornenemoiety. Ring opening of matathesis polymerization (ROMP)of this bifunctional monomer and 1,5-cyclooctadiene (COD)in a molar ratio of 3/97 yielded a rubbery statistical copoly-mer with pendant LA rings (PCOD/2). Subsequent ring-

TABLE 6 Summary of Reported Mechanical Properties for Some Star or Block PLA Copolymers

System

Comonomer/LA

Composition

rmax

(MPa) E (MPa) e (%)

TT

(MJ/m3)

IS

Notched

Izod (J/m)

Unnotched

Dynstat (kJ/m2)

L-LA-b-(L-LA/CL) (synthesized at 100 �C)47 70.3/29.7 (wt/wt) 29.7 – 90 – – –

L-LA-b-(L-LA/CL) (synthesized at 100 �C)47 67.4/32.6 (w/w) 29.8 – 150 – – –

L-LA-b-(L-LA/CL) (synthesized at 110 �C)47 70/30 (w/w) 24.9 – 180 – – –

L-LA-b-(L-LA/CL) (synthesized at 120 �C)47 70.6/29.4 (w/w) 30.7 – 250 – – –

L-LA-b-(L-LA/CL) (synthesized at 140 �C)47 66/34 (w/w) 30.8 – 1500 – – –

PDLLA4 100/0 56.8 – – – 5.0 6 0.1 41

Tri-block D,L-LA-b-(TMC)-b-D,L-LA

Mn(TMC) ¼ 20.1 � 103 g/mol479/21 (w/w) 24.4 – 280 – – –

Tri-block D,L-LA-b-(TMC)-b-D,L-LA

Mn(TMC) ¼ 65.1 � 103 g/mol489/11 (w/w) ~40 – 135 – ~5 –

79/21 (w/w) 36.9 – 210 – 66.7 No break

Tri-block D,L-LA-b-(TMC/CL)-b-D,L-LA,

Mn(TMC/CL) ¼ 41.5 � 103 g/mol480/20 (w/w) 35.1 – 50 – – 6.6

Tri-block D,L-LA-b-(TMC/CL)-b-D,L-LA,

Mn(TMC/CL) ¼ 46.5 � 103 g/mol480/20 (w/w) 41.4 – 120 – 446 No break

PLLA51 100/0 – – 10–15 – – –

PFPE-b-L-LA-b-PFPE51 95/5 (w/w) – – >300 – – –

D,L-LA-star-(TMC/CL)50a 79/21 (w/w) 34.5 – 290 – – No break

D,L-LA-star-(TMC/CL))b50 85/15 (w/w) 21.6 – 29 –– – No break

79/21 (w/w) 25.9 – 625 – – No break

D,L-LA-star-(CL/d-VL))c50 85/15 (w/w) 22.1 – 400 – – No break

D,L-LA-star-TMCc50 94/6 (w/w) 55.0 – 4 – – 3.6 6 0.2

89/11 (w/w) 50.9 – 180 – – 4.4 6 0.2

83/17 (w/w) 47.5 – 230 – – 13.4 6 0.5

rmax: The maximum tensile stress; E: tensile modulus; e: tensile elonga-

tion at break; TT: tensile toughness; IS: impact strength.a Polymerization in toluene.

b Polymerization in CH2Cl2.c Bulk polymerization in the melt.



opening transesterification polymerization (ROTEP) of D,L-LA monomer in the presence of the rubberyPCOD/2 yieldeda mixture composed of PLA graft copolymer and PLAhomopolymer. Unlike the opaque physical blend of PLA andpoly (COD), this in situ-synthesized PLA blend containing20 wt % rubbery PCOD/2 was translucent and exhibited aunique nanophase separation. This PLA blend displayedhigher elongation (65% vs. 5%) and tensile toughness(16 MJ/m3 vs 2 MJ/m3) than the PLA homopolymer butlower strength (24 MPa vs. 44 MPa).

Recently, Theryo et al.53 further adopted a ‘‘grafting-from’’(polymer with functional groups which initiate the polymer-ization of monomer) approach to synthesize another graftcopolymer of LA and COD. ROMP of COD with 5-norbornene-2-methanol was first conducted to obtain the pendant pri-mary hydroxyl groups statistically distributed along a rub-bery backbone (resulting block copolymer was referred as‘‘PCN’’), followed by ROTEP of LA initiated at those hydroxylsites. The graft copolymer containing only 5 wt % rubberybackbone was transparent and exhibited about 18-foldincrease in elongation (238% vs. 13%) and about 13-foldincrease in tensile toughness (95 MJ/m3 vs. 7 MJ/m3) withrespect to the neat PLA, respectively. Meanwhile, the tensilemodulus (1.86 GPa vs. 2.03 GPa) and yield strength (64.8MPa vs. 67.9 MPa) were only slightly lower than that of theneat PLA. Unfortunately, impact performance was absent inboth of the above studies. Table 7 summarizes the reportedmechanical properties for both grafted PLA copolymers.

Crosslinked CopolymersIntroduction of an appropriate level of crosslinking to PLAcould also impart the simultaneous enhancements in tensileand impact strengths. Crosslinked PLA materials have beensynthesized either by (1) copolymerization of LA with a mul-tifunctional monomer or by (2) introducing a crosslinkablemoiety into the polymer backbone and then performingpostpolymerization crosslinking modifications.

By bulk copolymerization of LA with small amounts of tetra-functional spiro-bis-dimethylene-carbonate (spiro-bis-DMC),the chemically crosslinked PLA samples were obtained.3,54

By copolymerizing L-LA copolymer with 0.2–0.3 mol %spiro-bis-DMC, even the occurrence of nonfracture in theunnotched Dynstat impact test was observed and tensilestrength essentially increased to a limiting value (ca., 70MPa) compared to 59.5 MPa of the neat PLLA. The authorsargued that the increased interconnectivity of PLA molecular

chains accounted for the simultaneous enhancement in ten-sile and impact strengths. The reinforcing effect in tensilestrength was also observed for the crosslinking of L,D-LAcopolymers with spiro-bis-DMC.54 But with incorporation ofsimilar contents of the crosslinker, the unnotched impactstrength of L,D-LA copolymers was less improved or evenreduced, depending on the D-LA content in the PLA copoly-mers.3 The higher impact strength of crosslinked PLLA wasattributed to the higher network strength of the networks dueto the presence of not only chemical crosslinks but also physi-cal crosslinks. By using the same copolymerization approach,another tetra-functional monomer, 5,50-bis(oxepane-2-one)(5,50-BO) as a crosslinker was also used to copolymerize withL-LA.55 It was found that the optimal mechanical properties ofthe crosslinked PLLA were obtained at relatively low polymer-ization temperatures and short reaction times with the cross-linker concentration close to 1.0 mol %. The Dynastat impactstrength of crosslinked bulk-polymerized PLLA containing1.00 mol % of 5,50-BO was 24 kJ/m2 compared to 14 kJ/m2 ofthe linear PLLA. Meanwhile, the corresponding tensilestrength was increased from 55 to 61 MPa.

By functionalizing telechelic star-shaped poly(CL/D,L-LA)oligomers with methacrylate anhydride followed by chemicalcrosslinking of the double bonds using dibenzoyl peroxide(DBPO) as a crosslinking agent, Helminen et al.56 obtainedcrosslinked PLA copolymers with a wide range of elasticproperties. At a fixed DBPO content (0.5 wt %), tensile prop-erties of the crosslinked copolymers were found to remark-ably change with the monomer compositions and coinitiator(pentaerythritol) content. At the CL/D,L-LA molar ratio of30/70 (mol/mol), the elongation reached 190%, while ten-sile strength (9.72 MPa) and modulus (5.2 MPa) were verylow. Table 8 summarizes reported mechanical properties forsome crosslinked PLA copolymers.

In summary, it should be pointed out that the majority ofpublications in this area either did not report impact proper-ties at all or only reported unnotched impact strength.Because the energy to initiate a crack is emphasized in theunnotched test, the results of unnotched impact strengthmay not be valid for the comparison between materials andsamples. In addition, the reproducibility of unnotched impactstrength is usually not high. Although a broad spectrum ofmechanical properties of PLA materials seemed achievableby manipulating the copolymerization, unfortunately, none ofthese copolymerization processes is currently economicallyviable.

TABLE 7 Summary of Reported Mechanical Properties for Some Grafted PLA Copolymers

System

Comonomer/LA

Composition rmax (MPa) E (MPa) e (%) TT (MJ/m3)

PDLLA53 100 44 – 5 2

D,L-LA-g-(PCOD/2)53 80/20 (w/w) 24 – 65 16

PDLLA52 100/0 67.9 6 1.3 2.03 6 0.07 13 6 4 7 6 2

D,L-LA-g-PCN52 95/5 (w/w) 64.8 6 2.0 1.86 6 0.09 238 6 43 95 6 23

rmax: The maximum tensile stress; E: tensile modulus; e: tensile elongation at break; TT: tensile toughness.



MELT BLENDING WITH FLEXIBLE POLYMERS

Melt blending of polymers is a much more economic and con-venient methodology than synthesizing new polymers toachieve the properties unattainable with existing polymers.Toughening is usually an integral part of blend design, espe-cially for those blends involving rigid polymers. PLA has beenblended with various polymers for different purposes. In thisreview, the discussion of blending of PLA is only limited tothe literatures with the specific purpose of PLA toughening. Avariety of biodegradable and nonbiodegradable flexible poly-mers have been used as toughness modifiers for PLA.

Biodegradable Polymer ModifiersAliphatic Polyesters and Their CopolyestersPolycaprolactone. PCL is a biodegradable polyester andpossesses excellent flexibility and ductility. Its chemicalstructure is shown in Figure 4. Blending of PCL and PLA hasbeen extensively investigated in the past years. However, thesimple melt blending of PLA and PCL usually leads to amarginal improvement in toughness because of theirimmiscibility.57,58

The use of small molecule reactive additives during compound-ing has been demonstrated to be an effective way to improvethe compatibility between PLA and PCL. Wang et al.59 used tri-phenyl phosphate (TPP) as a catalyst or coupling agent in thepreparation of PLA and PCL blends. The addition of 2 phr TPPto PLA/PCL (80/20, w/w; PCL used with Mn ¼ 80,000 g/mol)blend during melt blending resulted in higher elongation(127% vs. 28%) and tensile modulus (1.0 GPa vs. 0.6 GPa) butlower tensile strength at break (33 MPa vs. 44 MPa). The bal-ance between degradation of molecular weight and the forma-tion of copolymer was thought to govern the final mechanicalproperties of the blends. Reaction time and molecular weightof PCL used were found to have remarkable effects on mechani-cal properties of the blends. Higher molecular weight PCL (Mn

¼ 80,000 g/mol) and medium reaction time (15 min) pro-moted the largest improvement in elongation.

Semba et al.60 used dicumyl peroxide (DCP) during blendingto promote reactive compatibilization of the PLA/PCL blendsunder different melt-compounding conditions (internal mixervs. twin-screw extruder). The compression-molded film ofthe uncompatibilized PLA/PCL (70/30, w/w) blend displayed

TABLE 8 Summary of Reported Mechanical Properties for Some Crosslinked PLA Copolymers

System

Comonomer/LA

Composition

rmax

(MPa) E (MPa) e (%)

Unnotched

Dynstat IS (kJ/m2)

PLLA (synthesized at 110 �C for 230 h)55 100/0 55 – – 14

PLLA-cross-5,50-BO(synthesized at 110 �C for 230 h)55

99.9/0.10 (mol/mol) 54 – – 18

99.8/0.20 (mol/mol) 54 – – 10

99.65/0.35 (mol/mol) 52 – – 11

99.5/0.50 (mol/mol) 58 – – 23

99/1 (mol/mol) 61 – – 24

98.5/1.5 (mol/mol) 62 – – 14

97/3 (mol/mol) 47 – – 5

PLLA (synthesized at 130 �C for 72 h)55 100/0 28 – – 5

PLLA-cross-5,50-BO(synthesized at 130 �C for 72 h)55

99.9/0.10 (mol/mol) 45 – – 11

99.8/0.20 (mol/mol) 49 – – 14

99.65/0.35(mol/mol) 2 – – 16

99/1 (mol/mol) 56 – – 14

98.5/1.5 (mol/mol) – – – 16

97/3 (mol/mol) 48 – – 6

PLLA3 100 59.5 – – 12.7–13.5

PLLA-cross-spiro-bis-DMC3 99.8/0.2 (mol/mol) 67.2 – – No break

99.48/0.52 (mol/mol) 68.1 – – 36.9–44.0

PDLLA (1.0 mol % D-LA)3 100 65.2 – – 26.3

PDLLA (1.0 mol % D-LA)-cross-spiro-bis-DMC3 99.28/0.72 (mol/mol) 67.0 – – 24.1–27.0

PDLLA (5.0 mol % D-LA)3 100 56.9 – – 9.1

PDLLA (5.0 mol % D-LA)-cross-spiro-bis-DMC3 99.8/0.2 (mol/mol) 68.5 – – 8.4

99.35/0.65 (mol/mol) 68.0 – – 7.6

PDLLA-cross-CL56a 70/30 (mol/mol) 9.72 6 1.32 5.21 6 0.16 190 6 20 –

rmax: The maximum tensile stress; E: tensile modulus; e: tensile elonga-

tion at break; IS: impact strength.

a 1 phr pentaerythritol was used as a coinitiator.



an elongation of only 15% compared to 3.6% of the neat PLA.When 0.1–0.2 phr DCP was added during blending of the PLA/PCL blend, the elongation of the resulting blend film was dramat-ically increased to the maximum 130% with yielding and neck-ing observed during deformation. Further addition of DCPbeyond the optimum amount had an opposite effect on elonga-tion. For the compression-molded film samples, tensile modulusand tensile stress at break were independent of the DCP concen-tration but linearly decreased with increasing PCL content.Atomic force microscopy observation revealed that the diameterof the dispersed PCL domains decreased with increasing DCPcontent. Injection-molded specimens exhibited a similar trend oftensile properties as the compressed films. As for the impactstrength (notched Izod test), addition of 0.3 phr DCP duringblending resulted in the PLA/PCL (70/30, w/w) blend with an

impact strength of 2.5 times more than that of neat PLA. In con-trast, addition of DCP to PLA alone did not alter mechanicalproperties. It was considered that the crosslinking between PLAand PCL in the presence of DCP accounted for the improvedinterfacial adhesion. It was also found that tensile propertieswere not dependent on feeding procedure, but addition of DCPvia the splitting feeding method resulted in a higher reverse Izodimpact strength than feeding at once through the main hopper.61

Based on the high reactivity of isocyanate groups reactingwith end hydroxyl or carboxylic groups, Takaya et al.62,63

improved the compatibility of PLA and PCL using lysine trii-socyanate (LTI) as a compatibilizer. Compatibility of PLA andPCL was also improved, resulting in the reduction of size ofPCL spherulites. Impact fracture toughness was markedlyimproved by increasing LTI content, which was attributed tothe strengthening structure of the blend as a consequence ofcrosslinking reactions. In another study, Harada et al.64 sys-tematically compared the compatibilizing effects of LTI withfour other reactive processing agents (Fig. 5) on the PLA/PCL (80/20, w/w) blends. Addition of 0.5 phr of each reac-tive agent resulted in an increase in the unnotched Charpyimpact strength in the order of LTI > LDI (lysine diisocya-nate) > Duranate TPA-100 [1,3.5-tris(6-isocyanatohexyl)-

FIGURE 4 Chemical structure of poly(e-caprolactone) (PCL).

FIGURE 5 Chemical structures of five reactive processing agents used in the PLA/PCL blends.



1,3,5-triazinane-2,4,6-trione] > Duranate 24A-100 [1,3,5-tris(6-isocyanatohexyl)biuret] > Epiclon 725 (trimethylolpro-pane triglycidyl ether). Among four isocyanates used, LTIinduced the superior unnotched Charpy impact strength ofthe PLA/PCL blend (nonbreakable). The presence LDI orTPA-100 moderately increased the impact strength of thePLA/PCL blend (64 kJ/m2 and 58 kJ/m2). However, the addi-tion of Epiclon 725 did not improve the impact strength (17kJ/m2) of the binary blend. With 0–10 wt % PCL in theblends, the unnotched impact strength increased graduallywith LTI concentration. However, with 20 wt % PCL in theblend, the addition of only 0.15 phr LTI led to the nonbreakduring the unnotched Charpy impact test. With 0.5 phr ofLTI, the notched Charpy impact strength and ultimate strainreached 17.3 kJ/m2 and 268%, respectively, while tensilestrength was well maintained with respect to the binaryPLA/PCL blend (47.3 MPa vs. 55.4 MPa). It was assumedthat the reaction of isocyanates group with both terminalhydroxyl and carboxylic groups of polyesters accounted forimproved compatibility at the PLA/PCL interfaces and thusthe increases in the physical properties.

Poly(butylene succinate) and Their Copolyesters. Poly(butylene succinate) (PBS, Fig. 6) and copolyesters are com-mercialized under the trade name BionolleV

R

(Grade 1000 se-ries). PBS and copolymers have low Tgs and are highly flexi-ble. In addition to PBS, other PBS-based copolyesters, suchas poly(butylene succinate-co-adipate) (PBSA; e.g., BionolleV

R

3000 series) and poly(butylene succinate-co-L-LA) (PBSL;e.g., GS PLAVR grade), have been used to toughen PLA.58,65–68

Blends of PLA with these polymer modifiers are immiscible.Except notable increases in flexibility and elongation, signifi-cant improvement of impact toughness was seldom observedor only achieved at very high concentrations of the modi-fiers. In some studies, a third component as a compatibilizerwas incorporated to improve compatibility.

Harada et al.69 studied the melt blending of PLA and PBSand their reactive compatibilization using LDI and LTI. With-out compatibilization, the PLA/PBS binary blend (90/10, w/w) displayed a slightly higher elongation and almost thesame unnotched Charpy impact strength (18 kJ/m2) com-pared with neat PLA. Even with PBS increased to 20 wt %,the impact strength still showed little change. However, onaddition of 0.5 wt % LDI or 0.15 wt % LTI, elongation of thePLA/PBS (90/10, w/w) blend was increased to more than150%. It was found that the magnitude of impact strength ofthe blends was independent of the molecular weight of PBSbut was affected by concentrations of LTI and PBS. For theblends with 10–15 wt % PBS content, the impact strengthwas sharply increased with addition of LTI and saturated at

50–70 kJ/m2. Addition of LTI as low as 0.15 wt % signifi-cantly increased the impact strength of the PLA/PBS (80/20,w/w) blend, and the unnotched samples were not brokenduring the impact test. In contrast, even with addition of LDIto 0.5 wt %, the impact strength of PLA/PBS (80/20, w/w)blend only increased to 31 kJ/m2. The results implied thatLTI was the more effective reactive processing agent toincrease the toughness of the PLA/PBS blends. Also, on addi-tion of 0.15 phr LTI into the PLA/PBS (90/10, w/w) blend,the size of dispersed PBS particles was significantly reducedand further increasing the content of LTI or PBS did not alterthe size of PBS markedly.

Vannaladsaysy et al.70 investigated the effects of LTI on frac-ture behavior of the PLLA/PBSL blend. Similar to the PLA/PBS blend, the incorporation of LTI into the PLLA/PBSLblend effectively improved the compatibility between PLLAand PBSL, resulting in the suppression of spherulite forma-tion of PBSL and the formation of a firm structure consistingof entanglements of PLLA and PBSL molecules and thereforehigher energy dissipation during the initiation and propaga-tion of crack growth.

As DCP was successfully used to compatibilize the PLA/PCLblends,60,61 it was also incorporated to induce in situ compa-tibilization of the PLLA/PBS (80/20, w/w) blend by Wanget al.71 The uncompatibilized blend showed much higherelongation than PLLA (250% vs. 4%) but only slightly highernotched Izod impact strength (2.5 kJ/m2 vs. 3.7 kJ/m2).Addition of 0.1 phr DCP greatly increased the impactstrength of the blend to 30 kJ/m2. Both strengths and mod-uli invariably decreased with increasing DCP content. It wasfound that the addition of DCP led to a reduction in the sizeof the PBS domains and improved interfacial adhesionbetween the PLLA and PBS phases. The toughening effect ofthe blends was considered to be related to the debonding-initiated shear yielding.

Polyhydroxyalkanoates and Their Copolyesters. Dependingon the pendent alkyl chain length, bacterial polyesters, poly-hydroxyalkanoates (PHAs), possess a wide array of mechani-cal properties ranging from stiff thermoplastics to elastomers(Fig. 7). According to the carbon atom numbers of the alkylchains, PHAs are roughly divided into three classes, that is,short-chain-length PHA (scl-PHAs) with carbon atom num-bers of monomers ranging from C3 to C5, medium-chain-length PHA (mcl-PHAs) with carbon atom numbers of mono-mers ranging from C6 to C14, and long-chain-length PHA(lcl-PHAs) with carbon atom numbers of monomers of more

FIGURE 6 Chemical structure of poly (butylene succinate) (PBS).

FIGURE 7 The general structure of polyhydroxyalkanoates

(PHAs).



than C14.72 mcl-PHAs are less crystalline and elastomer-like,depending on their side-chain compositions. Thus, they havebeen used as modifiers to toughen PLA.

Lee and McCarthy73 used poly(3-hydroxy octanoate) (PHO)modified with hexamethylene diisocyanate (HDI) to melt-blend with PLA in a torque rheometer. DSC results suggestedthat the PLA/modified PHO blends were immiscible over theentire composition range. Elongation of PLA was only slightlyincreased at the modified PHO content less than 80 wt %.On the contrary, tensile strength and modulus were signifi-cantly reduced with the incorporation of modified PHO.

NodaxTM developed by Procter and Gamble Co., is a family ofPHA copolymers of 3-hydroxybutyrate and a small amountof mcl 3-hydroxyalkanoate comonomers (Fig. 8).74,75 Nodaand coworkers75 melt-blended PLLA with a poly(3-hydroxy-butyrate-co-3-hydroxyhexanoate) copolymer (i.e., NodaxH6,containing 5 mol % 3-hydroxyhexanoate (3-HH) unit). In thePLA/NodaxH6 (90/10, w/w) blend, tensile toughness was10 times more than that of neat PLA and elongation was>100%. When NodaxH6 content was less than 20 wt % inthe blends, its crystallization in the blends was largely re-stricted and thus NodaxH6 was dispersed as rubbery amor-phous droplets in PLA. Furthermore, it was interesting thatthe inclusion of these small amounts of PHA did not compro-mise the optical clarity of PLA itself.

Schreck and Hillmyer76 investigated the impact toughness ofblends of PLLA with a NodaxH6 containing 7 mol % 3-HH.The PLLA/NodaxH6 (85/15, w/w) blend demonstrated atwofold increase in notched Izod impact strength (44 J/m)compared with that of PLLA (22 J/m). In an attempt to pro-mote interfacial adhesion and hence increase impact per-formance, 5 wt % PLLA-b-NodaxH6 block copolymer wasadded to the binary blend. However, no positive effect wasnoted.

Poly(propylene carbonate). Poly(propylene carbonate)(PPC) (Fig. 9) is a biodegradable amorphous polymerproduced from propylene oxide/carbon dioxide copolymer-ization. Ma et al.77 prepared the PLA/PPC blends and

investigated their tensile properties. Elongation of the blendsmonotonically increased with PPC content and exceeded200% at PPC content of more than 30 wt %. Meanwhile, ten-sile strength and modulus decreased linearly with increasingPPC content. From the analysis of mechanical properties, theauthors concluded that there was good compatibilitybetween PLA and PPC.

Aliphatic–Aromatic CopolyestersPoly(butylene adipate-co-terephthalate) (PBAT) is a fully bio-degradable aliphatic–aromatic copolyester and its chemicalstructure is illustrated in Figure 10. PBAT is commerciallyavailable under the tradename of EcoflexV

R

(BASF Co.). PBATpolymer is said to be able to biodegrade in a few weeks inthe presence of naturally occurring enzymes. PBAT is a ther-moplastic with properties similar to those of low-density PEbut has high mechanical properties. In view of its high flexi-bility and ductility (elongation > 700%) and excellent biode-gradability, PBAT is thus considered a good choice for tough-ening of PLA without compromising the biodegradability offinal materials. Currently, PBAT/PLA blends are being com-mercially produced by BASF Co. under the trademarkEcovioV

R

for film and extruded foam applications.

Jiang et al.78 first reported the PLA/PBAT blends in the liter-ature and detailed the morphology, tensile properties, andtoughening mechanism. The PLA/ PBAT blend was immisci-ble and blending was conducted using a corotating twin-screw extruder. Even without use of compatibilizers, PBATwas evenly dispersed in PLA with an average particle size atthe level of 0.3–0.4 lm. The elongation of neat PLA wasmerely 3.7%; however, with only 5 wt % PBAT, the blendexhibited an elongation of �115%. When PBAT content wasincreased to 20 wt %, elongation of the blend increased tomore than 200%. On the other hand, tensile strength andmodulus decreased monotonously with PBAT content. With20 wt % PBAT, tensile strength decreased by 25% from 63(neat PLA) to 47 MPa, while modulus decreased by 24%from 3.4 (neat PLA) to 2.6 GPa. It was revealed that thedebonding-induced shear yield was responsible for the re-markable high extensibility of the blends. Because of weakinterfacial adhesion in the blends, impact toughness was

FIGURE 9 Chemical structure of poly(propylene carbonate)

(PPC).

FIGURE 8 The general structure of PHA copolyesters.

FIGURE 10 Chemical structure of poly(butylene adipate-co-terephthalate) (PBAT).



only slightly improved. For example, the impact strength ofthe blend with 20 wt % PBAT was only 4.4 kJ/m2 comparedwith that of 2.6 kJ/m2 for neat PLA.

To improve the compatibility between PBAT and PLA, Zhanget al.79 used a random terpolymer of ethylene, acrylate ester,and glycidyl methacrylate (referred as ‘‘T-GMA’’) as a reactivecompatibilizer in melt compounding. With the addition ofonly 2 wt % T-GMA, both ultimate strain and notchedCharpy impact strength of PLA/PBAT blends were increasedwithout decreasing tensile strength compared to the uncom-patibilized binary blends. For the PLA/PBAT (70/30, w/w)blend with 1–3 wt % T-GMA, the notched impact strengthreached 30–40 kJ/m2, approximately two times that of theuncompatibilized binary blend.

Elastomers and RubbersPolyurethane Elastomer. Thermoplastic poly(ether)ur-ethane (PU) is a biodegradable elastomer and possessesgood low-temperature properties. Li and Shimizu80 blendedPLA with PU elastomer. It was demonstrated that PU mark-edly improved the impact toughness of PLA materials whenits content was above 10 wt % in the blends. Elongation andimpact strength continuously increased with PU content.Compared with the neat PLA, the PLA blend with 30 wt %PU had a lower tensile strength (31.5 MPa vs. 65 MPa), agreater elongation (363% vs. 4%) and higher unnotchedimpact strength (315 J/m vs. 64 J/m). PLA/PU blend wasfound to be a partially miscible system, and PU was dis-persed in PLA with domain sizes at the submicrometer scale.Based on scanning electron microscopy (SEM) analysis oftensile and impact-fractured surfaces, matrix shear yieldinginitiated by debonding at the matrix/particle interface wasconsidered to be responsible for the improved toughness.

Recently, NatureWorks81 reported the toughening of PLAusing a PCL-based PU elastomer produced by Dow ChemicalCompany, PellethaneTM 2102-75A. With 30% of this elasto-mer, the notched Izod impact strength and elongation of theresulting PLA blend were increased to 769 J/m and 410%(27 J/m and 10% for neat PLA), respectively. Meanwhile,tensile yield strength of the blend was reduced by 32% withrespect to neat PLA.

Biodegradable Polyamide Elastomer. Zhang et al. used abiodegradable thermoplastic polyamide elastomer (PAE) totoughen PLA.82 This PAE was a block copolymer consisting apolyamide-12 (22 wt %) block as the hard segment and apolytetramethylene oxide block (78 wt %) as the soft seg-ment. SEM revealed that PAE indeed showed good interfacialcompatibility with PLA. PAE was dispersed in the PLA matrixuniformly and the size of PAE domains was at the submicro-scale. The incorporation of 5 wt % PAE into PLA resulted ina significant increase in elongation (161.5% vs. 5.1%), withlittle change in tensile modulus (1.5 GPa vs. 1.8 GPa) andstrength (48.1 MPa vs. 46.8 MPa) compared with neat PLA.With further addition of PAE up to 20 wt %, the elongationat break was further increased to 184.6%, whereas the ten-sile strength of the blend was markedly reduced by �49%compared to neat PLA. Interestingly, the PLA/PAE blends

exhibited a shape memory behavior after high deformation.For the blend containing 10 wt % PAE, the deformed speci-mens after stretching to 100% were able to restore to theoriginal shape within 3–8 s after heating at 80–90 �C andretained 92% of the original mechanical properties.

Hyperbranched PolymersHBPs possess a globular molecular architecture, cavernousinteriors, and a large number of peripheral end groups. HBPhas low hydrodynamic volume and viscosity and may have alevel of good solubility or miscibility with other polymers.Therefore, HBPs have high potential for the use as modifiersin a variety of industrial applications.83

HBPs have been recently used by several groups to modifyproperties of PLA. Zhang and Sun84 investigated mechanicalproperties and crystallization behavior of the hydroxyl-termi-nated HBP-modified PLA. Neat PLA exhibited a tensilestrength of 57.6 MPa and elongation of 3.33%. PLA contain-ing 2 wt % HBP displayed a tensile strength of 70.8 MPaand an elongation of 5.16%. However, tensile strengthdecreased with increasing BHP but elongation remainedabout the same until 8 wt % BHP. Bhardwaj and Mohanty85

developed HBP-modified PLA blends through reactive blend-ing of PLA, HBP, and polyanhydride (PA). During melt proc-essing, the hydroxyls of HBP would react with the anhydridegroups to crosslink in the PLA matrix. Compared with neatPLA, the PLA/HBP/PA (92/5.4/2.6, w/w) blend exhibitedimproved elongation (48.3% vs. 5.1%) and tensile toughness(17.4 MJ/m3 vs. 2.6 MJ/m3). However, tensile modulus andstrength of the blend decreased from 3.6 GPa to 2.8 GPa and76.6 MPa to 63.9 MPa, respectively. Lin et al.86 used a biode-gradable aliphatic hyperbranched poly(ester amide) as amodifier for PLA. PLA blends showed gradual increase inelongation with HBP content without a severe loss in tensilestrength. The elongation of the blend with 20 wt % HBPreached 50%, more than 10-fold over that of neat PLA (ca.,3.7%). Within 10 wt % content of HBP, the blend exhibited asomewhat increase in yield strength, as compared to neatPLA. Impact-fractured surfaces also demonstrated the changeof fracture mode from brittle to ductile failure with the addi-tion of HBP. Similarly, Zhang et al.87 reported the use of abiodegradable hyperbranched poly(ester amide) with aro-matic rings to modify the brittleness of PLA. By increasingHBP content from 2.5 to 10 wt %, the blend exhibited aslight increase in tensile strength but a remarkable increasein elongation.

Soybean Oil DerivativesRecently, Robertson et al.88 studied toughening of PLA usinga polymerized soybean oil derivative, polySOY, which wasprepared by crosslinking the double bonds of soybean oilmolecules using a free radical crosslinking agent or oxygenunder heating. A block copolymer, poly(isopropene-b-L-LA),was added as a compatibilizer. The elongation and tensiletoughness of the PLA/polySOY blends were four and sixtimes greater than those of unmodified PLLA, respectively.

Gramlich et al.89 used a conjugated soybean oil (CS) deriva-tive to toughen PLA through reactive blending. First, a



terminal-functionalized PLA was prepared by ring-openingpolymerization of L-LA using N-2-hydroxyethylmaleimide(HEMI) as an initator and Sn(Oct)2 as a catalyst. The malei-mide-terminated PLA (HEMI-PLLA) was then melt-blendedwith CS. It was demonstrated that the Diels-Alder reactionbetween the maleimide of HEMI-PLLA and the conjugateddouble bond of CS resulted in interfacial compatibilizationbetween the two immiscible components. Blends of reactiveHEMI-PLLA and 5 wt % CS resulted in an elongation ofmore than 17-fold at break that of neat HEMI-PLLA, as wellas a more than 133% increase in elongation compared tothe similar nonreactive PLA blend with 5 wt % CS. As the Tgand crystallinity of the PLA component was not significantlydifferent from that of PLA homopolymer, the authors arguedthat the toughening of the blends did not originate fromplasticization.

Nonbiodegradable Polymer ModifiersWhile it is desirable for researchers to continue pursuingviable eco-friendly solutions to address the brittleness prob-lem of PLA materials, blending PLA with nonbiodegradablebut readily available petroleum-based thermoplastic poly-mers to modify the properties of PLA materials has gainedmomentum in recent years. NatureWorks81,90 reported PLAblends with various commercial nonbiodegradable polymersin its Technology Focus Reports, such as ABS, acrylic impactmodifiers, thermoplastic polyester elastomers, styrenic blockcopolymers, and polycarbonate (PC). Some of such blendsare also commercially available.91,92 Although it may not bea long-term solution, it provides an economic and viablemeans to meet the need of consumers. Usually, the majorityof modifiers tend to be thermodynamically immiscible withPLA due to the lack of favorable interactions. To improve thecompatibility between the modifier and matrix, a third com-ponent is added as a compatibilizer in most cases. The com-patibilizer can be either premade or in situ formed duringmelt blending. For the latter, the rationale of reactive compa-tibilization is principally based on the reactions between endfunctional groups (i.e., AOH or ACOOH) of PLA and othercomplementary functional groups (mainly epoxide groups) ofthe compatibilizers. As a result, improved interfacial adhe-sion and hence fine dispersion are achieved. Until now, vari-ous types of rubbery modifiers have been used to toughenPLA. A few super-toughned PLA blends (notched Izod impactstrength > �530 J/m)93 have been successfully prepared interms of melt blending.

Poly(ethylene-co-octene)Poly(ethylene-co-octene) (POE) is a thermoplastic polyolefinelastomer (TPO) and has been attempted in PLA toughening.POE and PLA are immiscible and have no strong interactionsat the interface. Ho et al.94 prepared a series of POE-g-PLAcopolymers as premade compatibilizers. The graft copoly-mers were synthesized by reacting terminal hydroxyl groupsof PLA with maleic anhydride-functionlized POE (POE-MAH)using 4-dimethylaminopyridine as a catalyst. It was demon-strated that the copolymers significantly improved the com-patibility of the PLA/TPO (80/20, w/w) blend. The size ofthe dispersed POE particles was substantially reduced with

the addition of the compatibilizers until the equilibrium par-ticle size was achieved at a certain critical concentration. Asthe concentration of POE-g-PLA copolymer increased, elonga-tion and tensile toughness initially increased but then beganto decline when the compatibilizer concentration was above2.5 wt %. However, the presence of POE-g-PLA copolymerdid not affect tensile strength or modulus markedly. It wasfound that the POE-g-PLA copolymers with long PLA seg-ments resulted in higher elongation and tensile toughness.This work also showed that a POE-g-PLA copolymer wasmore efficient than POE-MAH to compatiblize the PLA/POE(80/20, w/w) blend.

In another study, Su et al.95 used GMA-grafted POE (mPOE)as a toughener of PLA. Both elongation and notched Charpyimpact strength invariably increased with mPOE content.The uncompatibilized PLA/POE (85/15) blend exhibited animpact strength of only 19.4 kJ/m2. In contrast, whenunreactive POE was replaced by mPOE in the blend, theimpact strength reached 29.8 kJ/m2, more than seven timesthat of neat PLA (4.0 kJ/m2). With further addition of mPOEto 45 wt %, the impact strength increased to 54.7 kJ/m2. Atthe same time, both strengths and modulus suffered from agreat loss because of the addition of an excessive amount ofrubbery POE.

Acrylonitrile–butadiene–styrene CopolymerNatureWorks81 recently reported commercial tougheningagents for PLA in a Technology Focus Report available in itswebsite. They identified BlendexTM 338, an ABS resin con-taining 70% butadiene rubber, as an effective tougheneramong various impact modifiers. With 20% BlendexTM 338,the blend achieved a notched Izod impact strength of 518 J/m and an elongation of 281%. In contrast, neat PLA exhib-ited impact strength of 26.7 J/m and an elongation of 10%.As generally expected, tensile yield strength of the blend wasdecreased from 62 MPa for neat PLA to 43 MPa.

To enhance the compatibility between PLA and ABS, Li andShimizu96 introduced styrene/acrylonitrile/GMA copolymer(SAN-GMA) as a reactive compatibilizer and ethyltriphenylphhosphonium bromide (ETPB) as a catalyst during meltblending. Fourier transform infrared (FTIR) analysis revealedthat the epoxy group of SNA-GMA reacted with PLA endgroups under the mixing conditions and that addition ofETPB accelerated the reactions. It was also found that reac-tive compatibilization led to a remarkable decrease in thesize of dispersed ABS domains. The compatibilized PLLA/ABS blends exhibited improved impact strength and elonga-tion but slight reductions in modulus and tensile strength.For instance, adding 5 phr SAN-GMA to the PLLA/ABS (70/30, w/w) blend increased elongation from 3.1% to 20.5%and impact strength from 63.8 to 81.1 kJ/m2. By furtherincorporating 0.02 phr ETPB, the elongation and impactstrength of the blend increased to 23.8% and 123.9 kJ/m2,respectively.

Poly(ethylene-co-glycidyl methacrylate)Oyama97 studied toughening of PLA using poly(ethylene-co-glycidyl methacrylate) (EGMA). It was shown that when the



lower molecular weight PLA (L-PLA) and screw speed of 200rpm during melt blending were used, the blend with 20 wt% EGMA had a much higher elongation (above 200%) rela-tive to neat L-PLA (5%). But the notched Charpy impactstrength of the blend was slightly increased, merely twotimes that of neat L-PLA (Fig. 11). Interestingly, much higherimpact strength can be achieved after the injection-moldedspecimens of the L-PLA/EGMA (80/20, w/w) blend wereannealed at 90 �C for 2.5 h. After annealing, the impactstrength was significantly increased to 72 kJ/m2, about 50times that of neat L-PLA. Also, the measurable improvementin strength and modulus of the blend was accompanied by asignificant decrease in elongation at break. With the highermolecular weight PLA (H-PLA) as a matrix, such positiveeffect of annealing on impact strength appeared relativelyless prominent. Based on DSC and wide-angle X-ray diffrac-tion data, the author argued that the crystallization of thePLA matrix played a key role in such significant improve-ment, although the effects of annealing on phase morpholo-gies and interfacial adhesion were not elucidated.

PolyethyleneWith PLLA-b-PE diblock copolymers as a compatibilizer,Anderson et al.98,99 melt-blended PLA with linear low den-sity PE (LLDPE) at a fixed PLA/LLDPE ratio (80/20, w/w).Addition of PLLA-b-PE block copolymers into the binaryblend resulted in improved interfacial adhesion and finerdispersion of LLDPE in PLA matrix, as evidenced by SEM.The tacticty of the PLA matrix (amorphous vs. semicrystal-line), molecular weight of the PLLA block (5 kg/mol vs. 30kg/mol) in the PLLA-b-PE block copolymers, and its content(0–5 phr) were found to have a remarkable effect in deter-mining the magnitude of ultimate notched Izod impactstrength.

For the binary blend of amorphous PLA (a-PLA) and LLDPE,only a minor increment in the impact strength was observedwith respect to neat a-PLA (34 J/m vs. 12 J/m). By adding 5wt % of the block copolymer with the molecular weight ofthe PLLA block below its critical entanglement molecular

weight (Mc ¼ 9 kg/mol), that is, PLLA-b-PE (5-30), the com-patibilized blend exhibited almost comparable impactstrength to the uncompatibilized binary blend (36 J/m vs. 34J/m). With the addition of 5 wt % of the block copolymerhaving the molecular weight of PLLA block above its Mc, thatis, PLLA-b-PE (30-30), however, the impact strength wasdrastically increased to 460 J/m. This difference was attrib-uted to the superior ability of the block copolymer with thelong PLLA block to suppress the coalescence of dispersedphase. The situation was somewhat different in the case ofthe semicrystalline PLA (PLLA) matrix. Even without thePLLA-b-PE block copolymers, the PLLA/LLDPE blends exhib-ited significantly higher impact strength than that of thePLLA homopolymer (350 J/m vs. 20 J/m) despite a largestandard deviation in impact strength values. The adhesiontest gave an indication of the superior adhesion for thePLLA/LLDPE interface to the PLA/LLDPE interface. With theaddition of the PLLA-b-PE block copolymers, the impactstrength was further increased to 510 J/m for use of 5 wt %PLLA-b-PE (5-30) and 660 J/m for use of 5 wt % PLLA-b-PE(30-30), respectively. The authors proposed that the tactictyeffects on either the entanglement molecular weight of PLAor miscibility degree of PLA matrix with LLDPE phaseaccounted for the difference between the two binary blend.

The dependence of impact toughness as well as LLDPE parti-cle size on the amount of block copolymer was also exam-ined. It was found that only 0.5 wt % of the block copolymerwas sufficient to achieve the optimum impact toughness.With increasing amounts of PLLA-b-PE (30-30) block copoly-mer in the PLLA/LLDPE (80/20, w/w) blends, the dispersedLLDPE particle size was gradually reduced. At the block co-polymer amount of 3 wt %, the size of the dispersed LLDPEparticles began to level off at around 1.0 lm. As one of theimportant parameters determining ultimate final impact

FIGURE 11 Notched impact strength of PLAs and PLA/EGMA

blends [C: complete break, P: partial break]. From Oyama, Poly-

mer, 2009, 50, 747–751, VC Elsevier, reproduced by permission.

FIGURE 12 Relationship between matrix ligament thickness

(MLT) and impact resistance for: 80:20 PLLA/LLDPE binary

blend (open circles); 80:20:5 PLLA/LLDPE/PLLA–PE(5–30) (black

circles); 80:20:5 PLLA/LLDPE/PLLA-b-PE(30–30) (grey circles);

80:20 (w/w) PLA/LLDPE binary blend (open squares); and

80:20:5 (w/w) PLA/LLDPE/PLLA-b-PE(30–30) (grey squares).

From Anderson et al., J. Appl. Polym. Sci., 2003, 89, 3757–

3768, VC Wiley Periodicals, Inc., reproduced by permission.



toughness,100,101 the matrix ligament thickness (MLT) wasfurther calculated with relation to impact resistance of theblends (Fig. 12). It was found that as the MLT decreased, theimpact toughness increased and the critical MLT for PLAtoughening was found to be approximately 1 lm.

At a fixed composition of compaibilized PLA/PE/PLLA-b-PE(80/20/5, w/w), notched Izod impact properties of theblends was also found to be highly dependent on the dis-persed PE phase properties.99 The flexible LLDPE tended toresult in blends with the high levels of toughness. On thecontrary, the stiff high-density polyethylene (HDPE), even inthe case of the ternary blends with a MLT of less than 1 lm,the maximum impact strength obtained was noticeably lower(64 J/m). Also, the level of interfacial adhesion needed toachieve maximum toughening varied with the PE used. Useof LLDPE, which relieve impact stresses by cavitation,required higher interfacial adhesion than use of HDPE, whichwas likely to dissipate energy by the debonding at the parti-cle–matrix interface.

Hydrogenated Styrene-b-butadiene-b-styrene CopolymerRecently, Hashima et al.102 toughened PLA using hydrogen-ated styrene-b-butadiene-b-styrene copolymer (SEBS) and areactive EGMA. The PLA/SEBS/EGMA (70/20/10, w/w)blend achieved a notched Izod impact strength of 92 kJ/m2

and an elongation of 185%. After annealing the samples at80 �C for 48 h, impact strength and elongation decreaseddramatically to 32 kJ/m2 and 100%, respectively. The nega-tive effect of annealing on the impact strength was alsoobserved in the binary and quaternary blends. However, nodetailed explanation for the reduction of impact toughnesswas given in this study.

By incorporating 40 wt % PC in the ternary blends, the heatdeflection temperature and aging resistance were improvedwithout severe deteriorations in impact toughness and duc-tility. Transmission electron microscopy (TEM) observationrevealed that PC and SEBS were separately dispersed in thePLA matrix. For the PLA/PC/SEBS/EGMA (40/40/15/5, w/w) blend, the maximum notched impact strength attained

was about 60 kJ/m2. The authors attributed the outstandingtoughness and aging resistance of the quaternary alloy tothe negative pressure of SEBS that dilated the bicontinuousPLA/PC matrix to enhance the local segment motions. Thechart of the above development in notched Izod impactstrength in the above PLA blends is briefly outlined inFigure 13.

A Novel Reactive Blend Systems Involving Dual ReactionsThe majority of the above modifiers, when being used aloneor in combination with a compatibilizer, proved to be fairlyeffective in enhancing tensile toughness and ductility of PLA.However, as for impact strength, especially in the notchedstate, these modifiers either had little effects or only intro-duced modest improvement. Even though a few supertoughPLA blends have been reported in the literature,97–99,102 acomprehensive understanding of the relationship betweentoughness and morphology is still lacking.

Liu et al.103 introduced a novel PLA ternary blend systemconsisting of an ethylene/n-butyl acrylate/GMA terpolymerelastomer (EBA-GMA) and a zinc ionomer of ethylene/metha-crylic acid copolymer (EMAA-Zn). In the reactive ternaryblend system, simultaneous vulcanization (crosslinking) ofEBA-GMA and interfacial reactive compatibilization betweenPLA and EBA-GMA took place. Figure 14 shows the influenceof extrusion temperature and EBA-GMA/EMAA-Zn (i.e., rub-ber/ionomer) weight ratio (total 20 wt % in the blends) onnotched Izod impact strength and elongation of the blends. Aremarkable dependence of impact strength on extrusion tem-perature was found.103 The ternary blends prepared at 185�C only exhibited similar impact strength to that of binaryblends, less than threefold that of the neat PLA control. Incontrast, a tremendous toughening effect was observed inthe ternary blends prepared at 240 �C. Furthermore, suchimprovement was more pronounced when the weight ratio

FIGURE 13 Summarized stream for the development of super-

tough 4 component alloy. From Hashima et al., Polymer, 2010,

51, 3934–3939, VC Elsevier, reproduced by permission.

FIGURE 14 Notched Izod impact strength (solid line) and strain

at break (%) (dash line) of PLA/EBA-GMA/EMAA-Zn (80/20-x/x,

w/w) blends as a function of weight content of added EMAA-

Zn under 240 �C versus 185 �C. From Liu et al., Macromole-

cules, 2010, 43, 6058–6066, VC American Chemical Society,

reproduced by permission.



of EBA-GMA/EMAA-Zn �1. Especially the ternary blend with15 wt % EBA-GMA showed impact strength of 860 J/m,approximately 35 times that of neat PLA. It is noteworthythat the remarkable enhancement in impact strength wasaccompanied by high elongation (>200%). Evidently, thisresult was different from the results by Oyama97 in the studyof PLA/EGMA (80/20, w/w) binary blend, which exhibitedthe similar toughness after annealing of the molded speci-mens but substantially lower strain at break (�35%).

TEM observations revealed that the dispersed domains inthe ternary blends displayed a unique ‘‘salami’’-like phasestructure. When the rubber content was higher than the ion-omer content, this substructure was the occluded ionomerinside the rubber droplets which were dispersed in the PLAmatrix (Fig. 15). In this case, the interface at the rubberdroplet/PLA matrix exhibited good wetting and the blendsexhibited high impact strength.104 When there was more ion-omer than rubber in the blends, however, phase inversionoccurred in the substructure during blend compounding.104

Consequently, the substructure turned out to be the rubberoccluded inside the ionomer droplets. It was found that inthe latter case the wetting at the ionomer droplet/PLA ma-trix interface became poor and the particle size of the dis-persed phase was relatively larger. As a result, the impactstrength of the ternary blends decreased rapidly.

Unlike other reactive rubber toughening which onlyinvolved reactive compatibilization at the rubber/matrixinterface,95–97,102 both reactive interfacial compatibilization

and rubber crosslinking reactions simultaneously took placein this ternary blend system. Torque and dynamic mechani-cal analysis (DMA) data demonstrated that increasing EMAA-Zn content led to a faster vulcanization and progressivelyhigher crosslink level of the EBA-GMA phase.104 FTIR spectrasuggested that the variation in the EBA-GMA/EMAA-Zn ratiodid not remarkably change the extent of compatibilizationbetween PLA and EBA-GMA. Figure 16 proposes the possiblereaction scheme that accounts for the remarkable dependenceof impact strength on blending temperature.103 At 185 �C,moderate curing reactions took place between the carboxylgroups of EMAA-Zn and epoxy groups of EBA-GMA under thecatalysis of Zn2þ ions, but the compatiblization reactionsbetween the epoxy groups of EBA-GMA and hydroxyl groupsof PLA were not significant. Hence, like many other soft poly-mer toughened PLA blends, the resulting ternary blend dis-played high ductility but only limited improvement in impactstrength. At 240 �C, not only the degree of curing of the EBA-GMA rubber was greatly increased but also the compatibiliza-tion reactions between the rubber and PLA phases were sig-nificantly enhanced. Therefore, the resulting interface wasable to stabilize premature crack propagation at the earlystage of impact test having a high-strain rate before massivematrix shear yielding took place.

By correlating dispersed particle size with notched Izodimpact toughness, an optimum particle size range (ca. 0.7–0.9 lm) for PLA toughening was identified in the PLA/EBA-GMA/EMA-Zn (80/20-x/x, w/w) blend system, as shown in

FIGURE 15 TEM micrographs of PLA/EBA-GMA/EMAA-Zn (80/20-x/x) ternary blends with varying EMAA-Zn content: (a) 0 wt %; (b)

5 wt %; (c) 15 wt %; and (d) 20 wt %. At 5 wt % EMAA-Zn (b), dark EMAA-Zn was encapsulated in grey EBA-GMA; at 15 wt %

EMAA-Zn (c), grey EBA-GMA was occluded in dark EMAA-Zn. From Liu et al., Macromolecules, 2011, 44, 1513–1522, VC American

Chemical Society, reproduced by permission.



Figure 17.104 Likewise, the optimum particle size has alsobeen widely reported for other thermoplastic matrices con-taining a variety of rubbers, such as semicrystalline nylon-6(PA6: 0.2–0.5 lm),105–107 amorphous nylon (a-PA: 0.2–0.5lm),108 PMMA (0.2–0.3 lm),109–111 PVC (0.2 lm),112 poly(-styrene-co-acrylonitrile) (SAN: 0.75 lm),113 and PS (0.8–2.5lm).93,113. Wu correlated rubber particle diameter withchain structure parameter of the matrix and claimed that theoptimum particle size for toughening decreased as the ma-trix becomes less brittle.92 Because PLA exhibited relativelyhigher intrinsic brittleness (characteristic chain ratio as ameasure of chain flexibility, C1 ¼ 9.5–11.8114–116 dependingon the L/D LA ratio) than other matrices (e.g., C1 ¼ 6.2 forPA6, C1 ¼ 5.4 for a-PA, C1 ¼ 7.6 for PVC, C1 ¼ 8.2 forPMMA, C1 ¼ 10.6 for SAN, and C1 ¼ 10.8 for PS),93,117 thisoptimum particle size for the toughened ternary PLA systemseemed reasonable. By correlating tensile toughness withdispersed particle diameter in PLLA/CS binary blends, Gram-lich et al.89 also reported the similar range of optimum parti-cle diameter (i.e., 0.5–0.9 lm) for toughening PLA.

In addition, the deformation mechanism of these blends wasanalyzed in terms of electron microscopic observation of theimpact-fractured surfaces.104 SEM fractographic observation

revealed that perceptible matrix plastic deformation onlyoccurred in the ternary blends where there was more rubberthan ionomer, for example, the PLA/EBA-GMA/EMAA-Zn(80/15/5 w/w) blend. TEM micrographs of the subfracturesurface were further performed to identify the micromechan-ical deformation process (Fig. 18). For the binary PLA/EBA-GMA (80/20, w/w) blend, it was found that only tinydebonding around relatively larger particles was observedwithout internal cavitation [Fig. 18(a)]. Also, at the highermagnification, the existence of minute fibrillated crazes pass-ing through other neighboring particles was also noted [Fig.18(b)]. Therefore, the debonding in the PLA/EBA-GMA bi-nary blend was unable to trigger the massive matrix plasticdeformation required for high impact toughness. This situa-tion was similar for the ternary PLA/EBA-GMA/EMAA-Zn(80/5/15, w/w) blend, in which the debonding at the ion-omer/PLA interfaces prevailed. On the contrary, cavitationinside the grey EBA-GMA phase was noted in the PLA/EBA-GMA/EMAA-Zn (80/15/5, w/w) blend. Therefore, the evi-dence suggested that the high impact toughness observedfor some of the ternary blends was attributed to the lowcavitation resistance of dispersed particles in conjunctionwith suitable interfacial adhesion.

Commercial Impact Modifiers for PLAIn recent years, several series of commercial impact modi-fiers for biopolymers (especially PLA) have been launched.These modifiers are either linear elastomers of low Tg orcrosslinked core–shell polymers. The core–shell modifierstypically consist of a low Tg rubbery core encapsulated by aglassy shell that has a good interfacial adhesion with the ma-trix polymer. When well-dispersed, these modifiers act asnanoscale or microscale rubbery domains that dissipate me-chanical energy and retard or arrest initiation and

FIGURE 16 Proposed reactions during reactive blending pro-

cess, together with schematic phase morphologies of the PLA/

EBA-GMA/EMAA-Zn ternary blends prepared at 185 and 240�C, respectively. More PLA molecules were grafted at the inter-

faces and the higher crosslinking degree inside EBA-GMA

domains was achieved for the ternary blend prepared at 240�C. From Liu et al., Macromolecules, 2010, 43, 6058–6066,

VC American Chemical Society, reproduced by permission.

FIGURE 17 Notched Izod impact strength of PLA/EBA-GMA/

EMAA-Zn (80/20-x/x) blends (240 �C, 50 rpm) with total content

of both modifiers fixed at 20 wt % as a function of weight aver-

age particle diameter (dw). From Liu et al., Macromolecules,

2011, 44, 1513–1522, VC American Chemical Society, reproduced

by permission.



propagation of microcracks through the polymer. Thesemodifiers were reported to bring varying magnitudes oftoughening effects to PLA. For the toughened PLA to retaingood clarity, small particles with refractive indices similar tothat of PLA are desired. To better match the refractive indexof PLA matrix, low-Tg acrylates such as ethyl acrylate orbutyl acrylate, are used to replace butadiene of the rubbercore in the cores–shell impact modifier.

SukanoVR

PLA im Series. To overcome the inherent brittle-ness of PLA, Sukano Co. has developed a patented impactmodifier (SukanoV

R

PLA im S550) based on elastomer, and itwas targeted for transparent applications (e.g., packaging).118

The special feature of this unique impact modifier, which hasbeen optimized for use with FDA approved, biodegradablePLA, is that it does not impair the transparency or heat sta-bility of PLA. At a concentration of just 4%, impact resist-ance of PLA was improved by a factor of 10, so preventingcracks and splinters in the PLA sheet and film during cuttingor stamping. Furthermore, in addition to its compostabilityand excellent transparency, SukanoV

R

PLA im S550 wasclaimed to be highly cost-effective in comparison to similarproducts on the markets. Recently, another new transparentimpact modifier, SukanoV

R

PLA im S555, was also launchedby Sukano Co.119

OnCapTM BIO Impact Series. In 2010, PolyOne introduceda new transparent impact modifier (OnCapTM BIO Impact T)for PLA.120,121 It is a masterbatch containing a specific elas-

tomer. This modifier was said to improved impact propertiesin PLA while maintaining the desired transparency at thesame time. Tear resistance of PLA was also improved withaddition of OnCap BIO Impact T.

Recently, Scaffaro et al.122 compared toughening effects ofOnCapTM BIO Impact T and SukanoV

R

PLA im S550, on PLA.Both modifiers were immiscible with PLA but SukanoV

R

PLAim S550 displayed a more homogeneous dispersion in thePLA matrix. It was found that neither impact modifiersbrought obvious increase in elongation to PLA. The maxi-mum Izod impact strength was achieved by using 8 wt %SukanoV

R

PLA im S550 (141 J/m), while the impact strengthonly increased to 124 J/m even with addition of OnCapTM

BIO Impact T.

BiomaxVR Strong Series. BiomaxVR

Strong 100 and 120 aretwo commercial modifiers for PLA from DuPont Company.Both modifiers are said to be ethylene–acrylate copolymersand are designed to improve the toughness of PLA in pack-aging and industrial applications with minimal impact ontransparency.123,124 BiomaxV

R

Strong 100 is designed for non-food applications and BiomaxV

R

Strong 120 for food packag-ing applications. It was claimed that addition of only 2 wt %BiomaxV

R

Strong to PLA, either amorphous or semicrystallinePLA, resulted in a significant increase in impact strength.With 5 wt % Biomax Strong or less, the blends maintainedcontact clarity similar to that of the clarified PP. Furtherincreasing the loading level of BiomaxV

R

Strong in the range

FIGURE 18 TEM micrographs of stress-whitening zone: (a) PLA/EBA-GMA (80/20) binary blend, low magnification (�7500); (b) PLA/

EBA-GMA (80/20) binary blend, high magnification (�30,000) at the localized area; (c) PLA/EBA-GMA/EMAA-Zn (80/15/5) ternary

blend; (d) PLA/EBA-GMA/EMAA-Zn (80/5/15) ternary blend. From Liu et al., Macromolecules, 2011, 44, 1513–1522, VC American

Chemical Society, reproduced by permission.



of 5–15% resulted in blends with different degrees of trans-lucence, similar to that of the unclarified PP. Also, both modi-fiers improved the cutting and trimming performance ofPLA.

Murariu et al.125 studied toughening effects of BiomaxStrongV

R

100 on PLA and high-filled PLA/b-calcium sulfateanhydrite (AII) composites. Notched Izod impact strength ofPLA with 5 and 10 wt % BiomaxV

R

Strong 100 increasedfrom 2.6 kJ/m2 of the neat PLA to 4.6 and 12.4 kJ/m2,respectively. Elongation was above 25% for the blend with10 wt % of the impact modifier, while tensile strength andmodulus of PLA gradually decreased with addition of theimpact modifier. Addition of 5 and 10 wt % of the impactmodifier to the PLA/AII (70/30, w/w) composite alsoincreased their impact strength to 4.5 and 5.7 kJ/m2, respec-tively. Impact strength slightly decreased with furtherincrease of the filler loading to 40 wt % but remained higherthan that of both the unmodified composites and the neatPLA. On the other hand, for the PLA composites with 40%filler, tensile strength and elongation markedly decreasedwith inclusion of the impact modifier.

Zhu et al.126 studied the films of the PLA blends containingeither BiomaxV

R

Strong 100 or SukanoVR

PLA im S550 as atoughener. It was shown that the modulus decreased withincreasing concentration of the former modifier but was rela-tively independent of the concentration of the latter tough-ener. The maximum elongation was 255% for the former ata 12 wt % loading and 240% for the latter at a 8 wt % load-ing, while elongation of neat PLA was about 90%. For agiven composition, the latter modifier gave a clearer filmthan BiomaxV

R

Strong 100, but the clarity of films decreasedwith concentration for both tougheners.

Afrifah and Matuana127 compared the toughening effects ofBiomaxV

R

Strong 100 on semicrystalline and amorphous PLA.BiomaxV

R

Strong 100 achieved superior toughening on semi-crystalline PLA over amorphous PLA. With 40 wt % of thetoughener, the notched Izod impact strength of the semicrys-talline PLA increased from 16.9 J/m of pure PLA to 248.4 J/m. In addition, the presence of 15 wt % BiomaxV

R

Strong 100lowered the brittle-to-ductile transition temperature of PLA,as revealed by the notched Izod impact test data of thefreezed specimens under the designated temperatures.

ParaloidTM BPM Series. The former Rohm & Hass Co. intro-duced the ParaloidTM BPM-500 acrylic-based impact modifierfor PLA resin. This modifier is a free-flowing white powderand is specially designed to improve impact properties with-out sacrificing the transparency of the product.128 It wasclaimed that improved impact properties were obtained withthe addition levels as low as 3 wt %. At a 5 wt % loading,the dart drop impact strength of extruded sheets wasincreased by threefold with respect to neat PLA. In addition,PLA modified with BPM-500 also showed a marked improve-ment in cutting, slitting, and flexibility. Because of the combi-nation of the nanoscale particle size and the excellent dis-persability in PLA, this modifier has a minimal effect on theclarity of the PLA films. With addition up to 5 wt %, the

haze measured on a 15 mil extruded sheet was increased to�6% compared with 3–4% for neat PLA. In 2009, DowChemical Company launched a new acrylic impact modifierin this series to impart toughness and maintain clarity ofPLA.129 It was claimed that ParaloidTM BPM-515 offered thesame benefits as ParaloidTM BPM-500 but with higher effi-ciency with a loading level as low as 1 wt %.129,130 The hazemeasured on a 15-mil extruded sheet was less than 6% withaddition up to 3 wt % ParaloidTM BPM-515.

BiostrengthTM Series. Three grades of core–shell impactmodifiers for PLA, BiostrengthTM 130, 150, and 200, werelaunched by Arkema. These modifiers are white powdersand are suggested to be added at 2–6 wt % in PLA. BothBiostrengthTM 130131 and BiostrengthTM 200132 are acryliccore–shell impact modifiers which are designed to increasestoughness of PLA and retain adequate transparency.

BiostrengthTM 150 is a methyl methacrylate–butadiene–sty-rene-type core–shell impact modifier for opaque applicationsand is said to be especially effective in durable injectionmolding applications requiring high ambient and low tem-perature durability.133,134 Cygan et al.135 compared theeffects of BiostrengthTM 130 and BiostrengthTM 150 onimpact strength and clarity of the resulting PLA blends. Itwas demonstrated that compared to BiostrengthTM 130, Bio-strengthTM 150 provided somewhat higher impact Gardnerimpact properties but much higher haze. In 2010, Arkemaintroduced another new clear acrylic core–shell impact modi-fier (BiostrengthTM 280) and recommended its use in PLAapplications that require toughness and high transparency.136

A small amount of BiostrengthTM 280 impact modifier incor-porated in PLA during extrusion turned the resulting sheetfrom brittle to ductile, allowing easier manufacturing andmore durable end use properties of the thermoformedpackage.

Other Rubbery ModifiersIshida et al.137 studied the toughening of PLA using four rub-bers: ethylene–propylene copolymer, ethylene–acrylic rubber,acrylonitril–butadiene rubber (NBR), and isoprene rubber(IR). Izod impact testing revealed that toughening was onlyachieved with the use of NBR which exhibited smaller parti-cle size (3–4 lm) than the other three in the blends. In ac-cordance with the morphological analysis, the interfacial ten-sion between PLA and NBR phases was the lowest. Themore polar structure of NBR was considered to be responsi-ble for the better toughening effect. Tensile propertiesshowed that NBR and IR without internal crosslinks pos-sessed a high ability to induce plastic deformation beforebreak as well as high elongation properties.

Ito et al.138 investigated fracture mechanism of neat PLA andPLA blends toughened with an acrylic core–shell modifier.The acrylic modifier was composed of a crosslinked alkyl ac-rylate rubber core and PMMA shell, and the particle size wasin the range of 100–300 nm. Plane strain compression test-ing of PLA clearly showed strong softening after yielding.Because the stress for craze nucleation was close to that ofyield stress, brittle fracture occurred for neat PLA. Addition



of the acrylic modifier significantly lowered the yield stressand formed many microvoids. Release of strain constraint bymicrovoiding and decrease of yield stress led to the relaxa-tion of stress concentration, and the toughness wasimproved moderately. Table 9 summarizes reported mechani-cal properties of some of highly toughened PLA blends pre-pared via melt-blending.

OTHER APPROACHES

Addition of Rigid FillersUsually, most of the fillers increase the stiffness of PLA mate-rials with little benefit to toughness.140,141 By in situ poly-merization of L-LA in the presence of 36 wt % carbon fiber,Grijpma et al.47 found that the unnotched strength (Dynstat)was increased from 12.1 kJ/m2 of as-polymerized PLA to62.8 kJ/m2. PLA with low viscosity-average molecular weightgave low impact strength.

To overcome the negative effect of rubber toughening on thestiffness of PLA blends, Xia et al.142 recently introduced aspecially engineered mineral-EMforceTM Bio calcium carbon-ate (CaCO3) with a aspect ratio of 5.42 for PLA reinforce-ment. Interestingly, besides linearly increasing flexural mod-ulus with the loading level, this mineral filler also provided amoderate improvement in room-temperature crack initiationenergy (�20 J) and notched Izod toughness (>120 J/m) atthe 30 wt% loading. On the contrary, PLA composites filledwith other mineral fillers such as mica, talc, and groundCaCO3, did not exhibit superior toughness to that of the neatPLA (�3 J and �40 J/m). The moderate toughening effect ofEMforceTM Bio CaCO3 on the PLA matrix was also identifiedin the NatureWorks’ Technology Focus Report.140 With 30 wt% of the filler, the dart impact strength, unnotched andnotched Izod impact strength were increased to 27 J, 294 J/m, and 123 J/m, respectively, with respect to that of theunmodified PLA being 4 J, 235 J/m, and 37 J/m.

Combination of Flexible Polymer and Mineral FillerToughening of PLA by incorporation of a flexible polymer isusually accompanied by sacrifices in strength and modulus.On the contrary, addition of mineral fillers generally leads toincreased modulus but also reductions in elongation andimpact strength in most cases. Therefore, in an attempt toachieve balanced overall properties, PLA ternary compositescontaining both flexible polymer and rigid inorganic fillerswere recently studied.

Chen et al.143 studied the inclusion of organically modifiedclay in PLLA/PBS blends. The compatibility of clay and poly-mer was found to be critical for the property enhancementof resulting composites. Addition of 10 wt % ClositeV

R

25A tothe PLLA/PBS (75/25, w/w) blend increased tensile modu-lus from 1.08 GPa to 1.94 GPa but decreased elongationfrom 71.8% to 3.6% which was even lower than that of neatPLLA (6.9%). In contrast, the use of an epoxy-functionalizedorganoclay (TFC) at the same amounts not only retainedhigh tensile modulus but also increased elongation to 118%.Chen et al.144 also noted the similar compatibilizing effect ofTFC on the PLA/PBSA (75/25 w/w) blends, but the increase

in elongation (46% vs. 5%) in this case was not as much asthat in the PLA/PBS blends above.

Li et al.145 recently reported PLA/clay/core–shell rubber ter-nary composites. The core–shell rubber impact modifier wasParaloidTM EXL 2330, a polybutylacrylate (core)–polymethyl-methacrylate (shell)-based material from the former Rohmand Haas Co. With 20% EXL 2330 and 5 wt % Cloisite 30Bclay, the notched Izod impact strength increased from 2.2 kJ/m2 of neat PLA to 5.2 kJ/m2, whereas the tensile modulusonly decreased from 1.81 MPa of neat PLA to 1.79 MPa.Elongation showed little change, being 7% compared to6.6% of neat PLA. However, the tensile strength suffered asignificant drop from 61.0 MPa of neat PLA to 43.8 MPa.

Hasook et al.146 investigated effects of PCL molecular weighton mechanical properties of ternary PLA/PCL/organicallymodified clay (OMMT) composites. Optimal mechanical prop-erties were achieved with a PLA/PCL/OMMT (90/5/5, w/w)ternary composite in which the weight-average molecularweight of PCL was 40,000 g/mol. This ternary composite dis-played increases in strength, modulus, and elongation of 19,9, and 53%, respectively, with respect to neat PLLA.

Jiang et al.147 compared effects of OMMT and nanosized pre-cipitated calcium carbonate (NPCC) on mechanical propertiesof PLA/PBAT/nanofiller ternary composites. Mechanical test-ing demonstrated that the composites containing OMMTexhibited higher tensile strength and modulus but lowerelongation, compared with the ones containing NPCC. When25 wt % of the PLA was replaced by maleic anhydride-grafted PLA (PLA-g-MA), the elongation of the ternary com-posites was substantially increased, possibly as a result ofimproved dispersion of the nanoparticles and enhancedinterfacial adhesion. Among these composites, PLA/10 wt%PBAT/2.5 wt %OMMT with 25 wt % of PLA being PLA-g-MA demonstrated the best overall properties with 87%retention of tensile strength of pure PLA, slightly highermodulus and significantly improved elongation (16.5 timesthat of neat PLA). Teamsinsungvon148 also reinforced PLA/PBAT blends using microsized precipitated CaCO3 andachieved similar toughening effects on PLA/PBAT (80/20,w/w) blends.

Formation of Semi-Interpenetrating NetworkSemi-interpenetrating network (SIPN) is defined as a blendin which one or more polymers are crosslinked and one ormore polymers are linear or branched.149 Yuan and Rucken-stein150 toughened PLA by forming a network of thermoset-ting PU in the matrix. The PU was prepared using PCL diolsand triols and toluene-2,4-diisocyanate and its degree ofcrosslinking was controlled by altering the diol/triol ratio.The toughening effect was influenced by crosslinking degreeof PU. No crosslinking or excessive crosslinking both tendedto lower the toughening effect. With 5 wt % of properlycrosslinked PU, an optimum tensile toughness of 18 MJ/m3

could be achieved compared with 1.6 MJ/m3 of pure PLA.Elongation of the toughened PLA increased to �60%, mean-while, yield strength, tensile strength, and Young’s modulusdecreased by �26, �30, and �22%, respectively. Optimum



TABLE9Summary

ofReportedMechanicalPropertiesofSomeHighly

ToughenedPLA

BlendsPreparedvia

Melt-Blending

Modifiers

Compatibilizers

TS

(MPa)

TM

(GPa)

e(%

)NotchedIS

Note

Type

Content

(wt%)

Type

Content

EGMA97

20

––

40

1.35

>200(10mm/m

in)

5–10kJ/m

2(Charpy)

As-m

olded;tw

in-screw

extruder(210

� C,200rpm)

20

––-

42.8

1.6

12(10mm/m

in)

72kJ/m

2(Charpy)

Annealedat90

� Cfor2.5

h;

twin-screw

extruder

(210

� C,200rpm)

SEBS102

20

EGMA

10wt%

––

185(5

mm/m

in)

92kJ/m

2(Izo

d)

As-m

olded;tw

in-screw

extruder

(200

� C,150rpm)

20

EGMA

10wt%

––

100(5

mm/m

in)

32kJ/m

2(Izo

d)

Annealingat80

� Cfor48h;

twin-screw

extruder

(200

� C,150rpm)

LLDPE99a

20

PLLA-b-PE

(30–30)b

5phr

24.3

61.1

1.326

0.11

316

18(10mm/m

in)

7606

50J/m

(Izo

d)

Batchmixer(190

� C,50rpm)

TPU

81

30

––

42.2

n.a.

410(50mm/m

in)

769J/m

(Izo

d)

–

ABS81

20

––

43.4

n.a

281(50mm/m

in)

518J/m

(Izo

d)

–

ABS139

20

––

39

2.4

5.0

251J/m

(Izo

d)

–

GMA-g-POE95

45

––

36.3

60.6

–2826

27

54.7

kJ/m

2(Charpy)

Batchmixer(180

� C,50rpm)

PBAT79

30

T-G

MA

3wt%

––

–30–40kJ/m

2(Charpy)

Twin-screw

extruder(160–185)

PBS71

20

DCP

0.1

phr

49.3

60.9

–2496

40(20mm/m

in)

30.0

62.7

kJ/m

2(Izo

d)

Batchmixer(170

� C,50rpm)

PCL64

20

LTI

0.5

phr

47.3

–268(5

mm/m

in)

17.3

kJ/m

2(Charpy)

Twin-screw

extruder

(190

� C,300rpm)

EBA-G

MA/EMAA-Zn103

15/5

––

35.8

60.1

2.206

0.03

2086

11(5

mm/m

in)

8606

64J/m

(Izo

d)

Twin-screw

extruder

(240

� C,50rpm)

TS:Tensile

stress;TM:tensile

modulus;EGMA:ethylene-co-glycidylmethacrylate);

SEBS:

hydrogenated

styrene-b-butadiene-b-styrene

copolymer;

LLDPE:linearlow-density

polyethyl-

ene;PLLA-b-PE:poly(L-lactide)-polyethyleneblockcopolymer;

TPU:polyesterpolycaprolactone-

based

polyurethane

elastomer(PellethaneTM

2102–75A);

ABS:acrylonitrile–butadiene–styrene

terpolymercontaining

70%

butadienerubber(BlendexTM

338);

GMA-g-POE:glycidylmethacry-

late

grafted

poly(ethylene-co-octene);

PBAT:poly(butyleneadipate-co-terephthalate)(EcoflexVR),

supplied

by

BASF

Co.;

T-G

MA:a

random

terpolymerofethylene,acrylic

ester,

and

glycidyl

methacylate

(GMA),

LotaderVR

from

Arkema

Co;PBS:poly(butylene

succinate);

DCP:dicumyl

peroxide;PCL:poly(e-carbonate);

LTI:lysinetriisocyanate;EBA-G

MA:ethylene/n-butylacrylate/

glycidylmethacrylate

terpolymer(66/28/5,w/w

);EMAA-Zn:zinc

ionomerofethylene/m

etha-

cryalicacid

copolymer.

aEthylene/octanecopolymer(Engage8100)with13.2

mol%

octene,suppliedbyDow

Chemical

Co.

b‘‘30–30’’denoted

both

PLLA

and

PE

blockswith

amolecularweightof30kg/m

oland

30kg/

mol,respectively.



toughness was thought to result from the balance betweenthe compatibility of the semi-interpenetrating PU networkwith PLA and the stiffness of this network.

OrientationIf molecular orientation could be introduced convenientlyand economically through processing techniques, it may pro-vide a facile route to toughen PLA without compromising itstensile properties.

Bigg151 demonstrated that biaxial orientation of PLA induceda 5–10 fold increase in elongation with enhanced tensilestrength. The considerable increase in tensile toughness forthe initially amorphous PLA was attributed to strain-inducedcrystallization during orientation. Grijpma et al.152 showedthat drawing of an injection-molded amorphous PLA sampleat temperatures below the Tg of the polymer increased ten-sile strength from 47 MPa of the unoriented PLA to 73.3MPa. At the same time, notched Izod impact strength wasincreased from 1.6 to 52.0 kJ/m2. Molecular orientationcould be also manipulated via shear-controlled orientationduring a nonconventional injection-molding (SCORIM), inwhich a macroscopic oscillating shear force was applied toorientate the solidifying polymer melt. A Charpy impactstrength of 21.3 kJ/m2 was obtained, as compared with 15.1kJ/m2 for the sample molded by conventional injection mold-ing technique (CIM). It was noted that the SORIM processdecreased the molecular weight of PLA slightly more thanthe CIM process. The degree of molecular orientation wasnot uniform throughout the specimen cross-section with thehighest degree of orientation in the shell layers.

Recently, Ghosh et al.153 investigated the effects of operativeparameters of SCORIM and compared the results of themolded PLLA samples prepared by SCORIM and CIM. PLLAmolded by SCORIM demonstrated tensile toughness andstrength which were 641 and 134% that of PLLA molded byCIM, respectively, without sacrificing modulus. The high duc-tility achieved by SCORIM was attributed to the preferentialmolecular orientation in the core sections. The orientation inthe core was more pronounced at low mold temperaturesand increased with increasing shearing time.

CONCLUSIONS

Due to the inherent rigidity of PLA chains, crazing deforma-tion was favored over shear yielding in the case of neatPLA.93 The brittle fracture behavior of PLA in tensile andimpact testing has been associated with the crazing deforma-tion mechanism through which the polymer fails.3,138 Variousmethods have been used to improve the toughness of PLA.Blending with polymeric tougheners has proved to be aneconomic and effective means to toughen PLA. The toughen-ing effects of PLA blends are complicated by many variables,including size, volume fraction, substructure and inherentproperty of the dispersed phase, and interfacial adhesion.

It has been demonstrated that reactive blending is more effec-tive in improving the toughness of PLA blends, particularlyimpact strength. In some cases, supertough PLA blends havebeen successfully achieved.97–99,102,103 In most of these

blends, however, the achievement of superior impact tough-ness relies on the addition of a large amount of nonbiodegrad-able petroleum-based polymers (�20 wt %), which compro-mise the integral biodegradability and compostability of thePLA materials. In addition, the significant improvement inimpact toughness was usually accompanied by a great loss(30–50%) in strength and stiffness. Thus, how to greatlyenhance impact toughness while minimizing the reductions instrength and stiffness of the PLA materials still remains achallenge. Furthermore, the roles of tacticty and crystalliza-tion (e.g., degree of crystallinity and crystalline structure) ofPLA matrix in PLA toughening are not well understood yetuntil now and are thus worth more attentions.

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