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Polymer 55 (2014) 4498e4513

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Polymer

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Compatibilization and property enhancement of poly(lacticacid)/polycarbonate blends through triacetin-mediated interchangereactions in the melt

Vu Thanh Phuong a, b, 1, 2, Maria-Beatrice Coltelli a, 1, Patrizia Cinelli a, c, 1, 3, Mario Cifelli d, 4,Steven Verstichel e, 5, Andrea Lazzeri a, *, 1

a University of Pisa, Department of Civil and Industrial Engineering, Largo Lucio Lazzarino, 1 e 56126 Pisa, Italyb Can Tho University, Department of Chemical Engineering, 3/2 Street, Can Tho City, Viet Namc National Interuniversity Consortium of Materials of Science and Technology (INSTM), Via G. Giusti, 9, 50121 Firenze, Italyd University of Pisa, Department of Chemistry and Industrial Chemistry, Via Risorgimento 35, 56126 Pisa, Italye Organic Waste Systems, Dok Noord 5, B-9000 Ghent, Belgium

a r t i c l e i n f o

Article history:Received 10 February 2014Received in revised form19 June 2014Accepted 20 June 2014Available online 28 June 2014

Keywords:Reactive extrusionPolycarbonatePoly(lactic acid)

* Corresponding author. Tel.: þ39 050 2217807; faxE-mail addresses: a.lazzeri@ing.unipi.it, rea.lazzeri

1 Tel.: þ39 050 2217807; fax: þ39 050 2217903.2 Tel.: þ84 710 3832663; fax: þ84 710 3838474.3 Tel.: þ39 050 2217825; fax: þ39 050 2217903.4 Tel.: þ39 050 2219249; fax: þ39 2219260.5 Tel.: þ32 (0)9 233 02 04; fax: þ32 (0)9 233 28 2

http://dx.doi.org/10.1016/j.polymer.2014.06.0700032-3861/© 2014 Elsevier Ltd. All rights reserved.

a b s t r a c t

Tetrabutylammonium tetraphenylborate (TBATPB) and triacetin were added during extrusion to meltblends of polylactic acid (PLA) and polycarbonate bisphenol A (PC) through a reactive compatibilizationapproach in order to enhance the materials' mechanical properties and thermal resistance. Dynamicmechanical thermal analysis revealed a new peak attributable to the glass transition temperature (Tg) ofthe PLA-PC copolymer at a temperature lower than the Tg typical of PC and higher than the Tg of PLA. Theresults of tensile tests, thermogravimetric analysis, differential scanning calorimetry, scanning electronmicroscopy, transmission electron microscopy, size exclusion chromatography, and NMR analysis for thecompatibilized and uncompatibilized blends were, on the whole, in agreement with the formation of thePLA-PC copolymer due to the action of the TBATPB and triacetin during the short extrusion time. Themechanical behaviour, morphology, and thermal properties of the PLA/PC compatibilized blends wereinvestigated as a function of composition, with the intention of broadening the utility of these biobased-blends. Finally, a general scheme for the reactions that occur during extrusion was proposed based on theexperimental results.

© 2014 Elsevier Ltd. All rights reserved.

1. Introduction

The development of new materials derived from renewablesources is a goal of high technological and environmental priority.In this context, polymers derived from agricultural sources, such asthe corn starch-derived polylactic acid (PLA) and its copolymers,are of great importance today [1,2]. Even though PLA, with its highelastic modulus and transparency compared to traditional plastics,is of great interest as a compostable material derived from arenewable resource, PLA-based polyesters remain limited due to

: þ39 050 2217903..pisa@gmail.com (A. Lazzeri).

5.

their high hydrolytic susceptibility, low thermal stability, brittle-ness, and low crystallization rate. The mechanical performance ofPLA-based products has been continuously improved through theincorporation of additives, but despite these technical advance-ments, there is still considerable interest in cost-effective methodsto enhance PLA properties [2]. Its relatively low glass transitiontemperature (Tg), around 60 �C, is a limitation for many applica-tions. Moreover, the possibility exists that further crystallizationcould occur at temperatures above the Tg, which would result in thedimensional instability of manufactured items under theirrespective operating conditions. These features preclude the use ofPLA-based materials in product categories such as automotiveparts, electrical equipment, and electronics, as well as durableconsumer goods.

In principle, PLA-based materials that are able to maintain theirmechanical properties in temperatures above the Tg and belowtheir melting temperatures can be obtained a) by an annealing stepallowing crystallization, either by reheating after moulding or

V.T. Phuong et al. / Polymer 55 (2014) 4498e4513 4499

through the use of nucleating and accelerating agents, or b) byphysical mixing with a second PLA-immiscible polymer componentwhich is characterized by a glass phase having a high Tg [3].

Annealing after moulding or increasing the crystallization rateby nucleation [4e5] results in the problem of long moulding times,with the annealing process requiring peculiar operations in order toavoid the crystallization-based deformations that result inshrinkage. Therefore, these methods create problems in terms ofcost and productivity.

In the physical mixing approach, the blending of PLA with pol-ycarbonate bisphenol A (PC), with its high Tg (above 150 �C), can bea successful strategy. In the patent literature, blends of PLA with PC[4�6] or other commercially available products [7�8] have beendescribed. PC has been widely used as an engineering plasticbecause of its high thermal stability and high tensile strength andelongation at break.

Extensive scientific work on the blending of PLA with differentpolymers has been performed, and the effects on characteristicssuch as biocompatibility or ductility have been studied. Toimprove the compatibility between two immiscible components, athird component is added as a compatibilizer or catalyst in mostcases. The compatibilizer can be either premade or formed in situduring melt blending. One successful application of this type ofreactive blending was the addition of peroxides to several PLAblends [9�14] with simultaneous melt blending, which resulted inthe formation of cross-linked and/or branched structures by het-eroradical coupling reactions. In a well-determined compositionalrange, the treatment resulted in improved blend compatibility.Other reactive compatibilization methods adopted in PLA blendsare principally based on the reactions between PLA's terminalfunctional groups (i.e. A-OH or A-COOH) and the complementaryfunctional groups (mainly epoxides) in the compatibilizers. Ex-amples of successfully used compatibilizers in this categoryinclude glycidyl methacrylate (GMA) [15�16], POE-g-GMA modi-fier [17], and other multifunctional polymers bearing epoxidegroups [18].

On the other hand, the blending of PC with polyamides orpolyesters can result in improved phase adhesion when a couplingagent [19�20] or a catalyst for interchange reactions (e.g. dio-ctadecyl phosphate [21,22], alkyl titaniums [23], tin(II) octoate [24],tetrabutyltitanate [25], and tetrabutylammonium tetraphenylbo-rate [26]) is used. As an example, PC has been used as a goodcomponent for blending with polyethylene terephthalate (PET). Inmany investigations [21�24,27�29], an interchange reactionapproach was selected to allow the formation of the PC-PETcopolymer during melt blending, thus resulting in some advanta-geous properties such as transparency, stable processing, highthermal resistance, and modulated crystallinity.

Several studies have investigated the blending of PLA and PC(Tg z 160 �C) to enhance thermal resistance [23�24,29�33].However, PLA and PC are immiscible, and adhesion between thetwo polymers is weak due to high interfacial tension. Therefore, asuitable compatibilization method is necessary. In these polymericblends, compatibilization agents of a polymeric nature have beenused to improve the interface adhesion between the two phases.Most studies used discontinuous blending methods, but the pro-cessing was lengthy (10e60 min) and conducted under conditionsthat were unsuitable for industrial use [61].

Kanzawa and Tokumitsu [35] added poly(butylene adipate-co-terephthalate) (PBAT) as a toughening agent to PLA/PC blends, butas a result, the thermal resistance was significantly decreased.Other studies on PLA/PC blends [29�32] have demonstratedslightly enhanced thermal resistance and interface adhesion be-tween the two components. Although catalysts and compatibilizershave been used to blend PLA and PC, little chemical evidence of the

nature of their interaction could be demonstrateddthe similarchemical functionalities in the polymers hinders the application ofspectroscopic techniques. Recently, Liu et al. [34] investigated theeffect of a catalyst on the interchange reactions of PLA and PC andtheir mechanism under a flow field, by preparing blends in adiscontinuous mixer and adopting a long blending time (1000 s).They identified the presence of the PLA-PC copolymer through amethod based on selective extraction and spectroscopic investi-gation. Moreover, they observed the appearance of a Tg that wasintermediate between those of PLA and PC via dynamic mechanicalthermal analysis (DMTA) [34].

From the preceding literature survey, it is evident that newcompatibilized biodegradable materials based on PLA and aromaticpolycarbonates would be useful, especially if they exhibit thermo-mechanical properties suitable for the production of materials fordifferent industrial sectors and can be prepared through rapidprocesses that are compatible with industrial extrusion.

In this work, PLA/PC compatibilized blends were prepared by aprocess of reactive extrusion in the molten state in the presence oftriacetin (TA) and tetrabutylammonium tetraphenylborate(TBATBP). The extrusion conditions such as temperature and timewere selected to promote the occurrence of interchange reactionsbetween the polymers. The procedure afforded compatibilizedblends which were assessed with respect to their mechanicalproperties, morphology, and thermal and biodegradation behav-iours as a function of composition. Compatibilized and uncompa-tibilized blends were compared to understand structureepropertyrelationships and develop a general reaction mechanism on thebasis of molecular weight and spectroscopic evidence. Such bio-based polymers and composites would broaden the potential util-ity of these renewable materials.

2. Experimental

2.1. Materials

Poly (L-lactic) acid was purchased from NatureWorks LLC, hav-ing a nominal average molecular weightMw ¼ 199,590 Da (Ingeo™2003D Extrusion Grade) and a density of 1.24 g/cm3. The poly-carbonate of bisphenol A (Iupilon S3000) with a density of 1.20 g/cm3 and average molecular weight Mw ¼ 20 kDa was purchasedfrom Mitsubishi Engineering Plastics. Triacetin (TA, also known asglycerin triacetate or 1,2,3-triacetoxypropane) and tetrabuty-lammonium tetraphenylborate (TBATPB, CAS #15522-59-5) werepurchased from Aldrich Chemicals. The chemical structures ofthese raw materials are presented in Scheme 1.

2.2. Processing

After the starting polymers (PLA and PC) were dried at 60 �C and133 Pa for 4 days, they were mechanically mixed at room temper-ature for about 10 min in different ratios with a high speed mixer.Then, the triacetin and TBATPB were added and mixing wascontinued for another 10 min using the same equipment. Theresulting mixtures were processed with a MiniLab II Haake™Rheomex CTW 5 conical twin-screw extruder (Thermo ScientificHaake GmbH, Karlsruhe, Germany). Mixing was conducted at210 �C and 230 �C with a screw speed of 100 rpm for a recirculatingtime of 1 min. After extrusion, the molten materials were trans-ferred through a preheated cylinder to the Haake™ MiniJet II miniinjection moulder (Thermo Scientific), to obtain Haake type-IIIspecimens that were used for measurements and analysis. Thespecimens were placed in plastic bags and vacuum sealed to pre-vent moisture absorption.

Scheme 1. Chemical structures of raw materials.

V.T. Phuong et al. / Polymer 55 (2014) 4498e45134500

2.3. Characterization methods

Tensile tests were performed at room temperature, at a cross-head speed of 10 mm/min, by means of an Instron 4302 universaltesting machine (Canton MA, USA) equipped with a 10 kN load celland interfaced with a computer running the TestWorks 4.0 soft-ware (MTS Systems Corporation, Eden Prairie, MN, USA).

Differential scanning calorimetry (DSC) measurements werecarried out to investigate the thermal behaviour of the materialswith a TA Q200 instrument (TA Instruments, Newcastle, DE, USA)with nitrogen as the carrier gas and indiumused for calibration. Thesamples were first heated from�100 �C to 250 �C at 10 �C/min, andthen cooled to �100 �C at 20 �C/min. Then, the second heating wasinvestigated using the same conditions as the first heating.

Size exclusion chromatography (SEC) analysis was performedwith a Jasco PLUS system consisting of a PU-2029 pump, CO-2063column oven set at 80 �C, RI-2031 differential refractometer, andUV-2077 UV detector, fitted with two PL-Gel Mixed D columns.Column calibration was performed with narrow distributionpoly(styrene) standards. A 4 mg/mL solution of the polymer in THF(0.1% w/V) was filtered through a 0.2 mm membrane syringe filter,and 20 mL solution was injected using a 1 mL/min flow rate.

13C NMR spectra were acquired on a Bruker DRX400 spec-trometer in deuterated chloroform. The excitation pulse for 13C was

calibrated at 30 �C, and the repetition time was 1.5 s. Proton irra-diation was applied before each scan to enhance the nuclearOverhauser effect (NOE) and during the 1 s FID acquisition forheteronuclear decoupling.

Thermogravimetric analysis (TGA) was performed under a flowof nitrogen gas at a scan speed of 10 �C/min, from room tempera-ture to 1000 �C, using a TGA 1000 instrument (Rheometric Scien-tific Inc., USA).

DMTA was carried out on a Gabo Eplexor® 100N (Gabo Qual-imeter GmbH, Ahlden, Germany). Test bars were cut from thetensile bar specimens (size: 20 � 5 � 1.5 mm) and mounted in atensile geometry. The temperature used in the experiment wasvaried from �100 �C to 170 �C at a heating rate of 2 �C/min andfrequency of 1 Hz.

The morphology of the composites was studied by scanningelectronmicroscopy (SEM) using a JEOL JSM-5600LV (Tokyo, Japan),by analysing cryofractured sample surfaces, previously sputteredwith gold.

The transmission electron microscopy (TEM) study was per-formed with a JEOL 1210 operating at 120 kV. The samples weretrimmed with a Leica ULTRACUT E ultramicrotome room using adiamond-trimming knife and then ultra-thin sectioned with thesame ultramicrotome using a diamond knife. The section thicknesswas nominally 70 nm (setting). TEM micrographs were obtained inrepresentative areas of the samples at 3000� magnification.

Aerobic biodegradability tests were carried out under controlledcomposting conditions according to ISO 14855. The test materialswere examined in the form of granulates. The composting inoc-ulumwas derived from an organic fraction of municipal solid wastethat was aerated and stabilized under pilot-scale compostingconditions over a period of more than 20 wk. The compost wassieved to remove particles over 5 mm in size, and the fine fractionwas then used as the inoculum. Control reactors contained only thisinoculum without test material. The reactors were placed in anincubator without light at 58 ± 2 �C and continuously aerated.During biodegradation, microorganisms present in the inoculumconverted carbon in the reference or test material into CO2. The gasleaving each individual reactor was analysed at regular intervals forCO2 and O2 content, and the gas flow rate was measured. Thebiodegradation percentage was determined as the percentage ofthe carbon in the starting reference or test material that was con-verted into CO2. Continuation of the biodegradation test waspossible since sufficient O2 supply was present in the reactorheadspace. The tests were fully complete after 150 d.

3. Results and discussion

3.1. The effect of processing conditions

Preliminary studies to assess the effect of the TA/TBATPB addi-tion on the reactive extrusion of PLA and PC blends were carried outon a PLA40/PC60 composition. The different blend compositionsand conditions are listed in Table 1

The extrusions were conducted at 210 �C and 230 �C at a100 rpm screw speed, for a recirculating time of 1 min. Thematerialwas then injection moulded: the mould temperature was 50 �C, theinjection time was 20 s, and the injection pressure was 790 mmHg.Afterwards, the moulded samples were annealed at 80 �C and1 mm Hg for 48 h. The samples were tested by DMTA and tensiletests.

Blend 1, obtained without the addition of TBATPB catalyst or TA,exhibited a tensile strength of 54.6 MPa and an elongation at breakof 5.1% after extrusion at 210 �C. The analogous mixture, Blend 5,extruded at 230 �C, afforded a higher elongation at break (96%).This means that the higher extrusion temperature resulted in

Fig. 2. Stressestrain curves of PLA40/PC60 blends prepared at 230 �C.

Table 1Compositions and tensile properties of PLA40/PC60 blends.

Blends TBA-TPB(wt%)

Triacetin(wt%)

Extrusiontemperature(�C)

Tensilestrength(MPa)

Young'smodulus(GPa)

Elongationat break(%)

Blend 1 210 54.6 ± 0.8 2.97 ± 0.08 5.1 ± 0.3Blend 2 5 210 63.1 ± 0.9 3.2 ± 0.1 99 ± 6Blend 3 0.2 210 51 ± 1 3.07 ± 0.09 2.3 ± 0.2Blend 4 0.2 5 210 65.5 ± 0.9 3.1 ± 0.1 46 ± 4Blend 5 230 62 ± 1 3.03 ± 0.09 96 ± 5Blend 6 5 230 65.1 ± 0.8 3.25 ± 0.08 101 ± 6Blend 7 0.2 230 63.9 ± 0.9 3.1 ± 0.1 47 ± 5Blend 8 0.2 5 230 68.6 ± 0.9 3.2 ± 0.1 35 ± 3

V.T. Phuong et al. / Polymer 55 (2014) 4498e4513 4501

transformations that led to improved compatibility. Blend 2, inwhich TAwas used, displayed very ductile behaviour; its elongationat break of 98.7% indicates its plasticization. Blend 3, with onlyTBATPB, was even more brittle than Blend 1, with an elongation atbreak of 2.3% (Fig. 1). This means that TBATPB is not active as acatalyst at this temperature (210 �C), probably as a consequence ofthe short extrusion time. Blend 4, which contained both TBATPBand TA, showed significantly improved mechanical properties overBlend 3, with a tensile strength of 65.5 MPa (corresponding to a20.0% increase over the simple mechanical blend), while main-taining an excellent value for elongation at break (46.5%), attestingto the synergism of the TBATPB and TA.

The trend was confirmed by comparing the stressestrain curvesof the blends prepared at 230 �C (Fig. 2). Asmentioned above, Blend5, the simplemechanical mixture, shows a rather ductile behaviour,with a tensile strength of 62 MPa and an elongation at break of96.4%; but both Blends 6 and 7 show increased yield strengths of65.1 and 63.9 MPa as well as elongations at break of 100.7 and46.9%, respectively. This indicates that both TBATPB and TA arechemically active in the system at this temperature. The best me-chanical performance is shown by Blend 8, wherein both TA andTBATPB were added during the extrusion, showing a tensilestrength of 68.6 MPa (an increase of 12.1% with respect to themechanical mixture), while maintaining a very good elongation atbreak of 35.3%. Thus, the synergistic action of the TBATPB and TA inpromoting the compatibility of the PLA/PC system is thusconfirmed.

Dynamic mechanical thermal analysis was carried out in thetemperature range�100 to 250 �C at a heating rate of 2 �C/min anda frequency of 1 Hz. From the point of view of dynamic mechanical

Fig. 1. Stress-strain curves of the PLA40/PC60 blends prepared at 210 �C.

properties, the traces obtained by plotting tan d versus temperaturerevealed a new peak that did not appear in the case of the simplephysical mixture of the two homopolymers. This new peakappeared at a Tg lower than that of PC, and may be related to thepresence of PC-blocks in a copolymer. In Fig. 3, for blends extrudedat 210 �C, the newpeaks can be seen for Blend 2 (TA only) and Blend4 (TBATPB/TA) at 123.5 and 113.5 �C, respectively, whereas it wasnot present for Blend 1 (mechanical mixture) or Blend 3 (TBATPBonly).

In Fig. 4, a similar superimposition is shown for the blendsprepared at 230 �C. Blend 6 (containing TA only) and Blend 7

Fig. 3. DMTA trends for PLA40/PC60 blends obtained at 210 �C.

Fig. 5. Comparison of DMTA trends for PLA, Blend 1, and Blend 4.

V.T. Phuong et al. / Polymer 55 (2014) 4498e45134502

(containing TBATPB/TA) show the new peak at 128 �C, whereas themechanical blend (Blend 5) exhibits a peak associatedwith the Tg ofthe PC phase at 160 �C. Blend 7, in which only TBATPB was used,reveals a peak at a slightly lower temperature, 155 �C, which can beprobably be explained by the occurrence of interchange reactionsleading to PC blocks with lengths shorter than in the mechanicalblend. The DMTA data are consistent with the fact that PLA and PCare immiscible. In addition, the PLA-rich phase exhibits bothamorphous and crystalline phases. The Tg values of both PLA(50e70 �C) and PC (155�160 �C) are not, in general, much differentfrom typical values of the two polymers, but a new peak appears atintermediate temperatures (110�130 �C) for the copolymer, inagreement with the results of Liu et al. [34] These authorsdemonstrated the presence of a PLA-PC copolymer with bothsimilar DMTA results, showing the appearance of a new peak, andselective extraction techniques followed by NMR analysis.

In Fig. 5, we compare non-annealed samples of pure PLA, Blend1, and Blend 4, to show that both blends have amuch higher storagemodulus across and beyond the Tg of PLA. Also, the modulus of PLAtends to increase starting from about 80 �C to 120 �C, where itreaches a new maximum. This is associated with the partial crys-tallization of the PLA phase. A similar phenomenon is observed forBlend 1, although in a limitedway, whereas it is not visible for Blend4. This can be explained by the presence of the PC-PLA copolymer inthis material. The PLA blocks that are probably present in thecopolymer are shorter than in pure PLA or in the mechanicalmixture with PC. Moreover, they are surrounded by rigid PC blocksthat strongly limit molecular mobility and hinder the crystalliza-tion process. As proof of this statement, we can observe an increasein themodulus of Blend 4 above 160 �C, which corresponds to the Tgof the PC blocks, up to 200 �C, where the Young's modulus of the

Fig. 4. DMTA trends for PLA40/PC60 blends obtained at 230 �C.

blend reaches a new maximum. This can be explained by consid-ering that the PC-blocks are beyond their Tg, and hence, the PLAblocks recover their mobility and are able to crystallize.

The TGA and derivative thermogravimetric (DTG) curves ofPLA40/PC60 (Blends 1�4) with and without catalyst at 210 �C arereported in Fig. 6a and b, respectively, and the corresponding dataare reported in Table 2.

The degradation of pure PLA occurs in a single step starting at280 �C through a final temperature of 427 �C, with a DTG peaktemperature (Tmax) at 356 �C. The Tmax is the temperature at themaximum rate of weight loss, that is, the decomposition temper-ature. Song et al. [39] observed that PC exhibited two-stagedegradation behaviour in air. The main stage started at 435 �Cand ended at about 700 �C, with 21.3% solid residue and a Tmax of509 �C. A shoulder peak appeared before the main peak at 419 �C.This trend showed that complex chemical reactions take placeduring the first stage of thermal oxidative degradation, includingthe free radical chain scission of the isopropylidene linkage and thebranching and crosslinking reactions of molecular segments inbetween oxygen atoms during the reaction in inert atmosphere. Infact, Lee and Jang [40,41] suggested that the initial steps in thethermal oxidative degradation of PC are the oxidative cleavage ofhydrogen from the isopropylidene linkage and then carbonecarbonscission, followed by the hydrolysis and alcoholysis of carbonate.Oxygen may facilitate the free radical branching reaction via theformation of peroxides, and a highly cross-linked structure withdiaryl ester; ether, and unsaturated carbonaceous bridges is formedabove 440 �C.

PLA40/PC60 blends with and without different catalysts pro-cessed at 210 �C exhibited two stages of degradation, attributablefirst to PLA and then to PC. The onset of degradation in the blendbegan at the temperature typical of PLA and finished at 547 �C,before the typical end-point temperature for PC. This thermal

Fig. 6. Thermal analysis of the PLA40/PC60 blends obtained at 210 �C by (a) TGA and(b) DTG.

Fig. 7. Thermal analysis of the PLA40/PC60 blends obtained at 230 �C by (a) TGA and(b) DTG.

V.T. Phuong et al. / Polymer 55 (2014) 4498e4513 4503

behaviour is quite similar to those of the blends that were pro-cessed at 230 �C, as shown in Fig. 7a and b. Although the Tmax of PLAin both cases did not change in the presence of TA/TBATPB, that ofPC changed from 509 �C in the pure polymer to 455 �C in theblends. This phenomenon might possibly be explained by the factthat the degradation product of PLA could have facilitated thethermal degradation of PC. At both processing temperatures, theDTG peaks for PLA degradation in the blends are broader than in

Table 2TGA and DTG results of PLA40/PC60 blends obtained at different processingtemperatures.

Formula T_10% (�C) T_50% (oC) T_70% (

�C) Tmax Solid residue

(%) at 800 �C

PLA 320.6 347.8 355.5 356.1 1.8Blend 1 346.3 389.5 430.4 358.7 8.9Blend 2 325.4 373.6 411.8 341.3 7.9Blend 3 341.1 385.3 423.0 353.5 8.0Blend 4 332.2 377.4 413.9 351.5 7.7Blend 5 335.7 379.0 412.1 349.7 7.4Blend 6 329.5 376.5 428.7 353.2 8.2Blend 7 330.7 372.6 397.0 356.2 7.3Blend 8 329.2 371.4 419.7 345.5 8.6PC 439.4 506.1 528.5 508.9 21.3

T_x% ¼ temperature at which x% of weight loss was achieved.Tmax ¼ temperature corresponding to the highest degradation rate.

pure PLA, meaning that the thermal resistance of the materialsincreases in the presence of the polycarbonate. More specifically,Blends 2, 4, 6, and 8, all containing TA, appeared to have a slightpeak from 120 to 220 �C, with a Tmax of 180 �C. This peak was farfrom the boiling and degradation temperatures of TA, 260 and312 �C, respectively [42]. As the onset of this peak is near the boilingtemperature of acetic acid, we suggest that the TA molecule maydecompose or react in the presence of PLA and PC, resulting in theevolution of acetic acid or other by-products. It is possible that theformation of the PLA/PC copolymer is favoured by processing at230 �C with TA and TBATPB because reactions leading to acetic acidevolution occur.

From the data in Table 2, the temperatures for obtaining 10% and50% degradation (T10%, T50%) of all the blends at 230 �C are quitesimilar. Beyond that, the decomposition temperatures of the blendsat 70% degradation (T70%) are much affected by the presence ofTBATPB, as can be seen clearly in the expansion in Fig. 7a. Theslightly increased thermal resistance of the blend obtained at230 �C with TA and TBATPB, as evidenced in the expansion inFig. 7a, with respect to mechanical blend, may indicate that somespecific interactions or chemical reactions have occurred betweenPLA and PC, as observed in the case of poly(vinyl chloride)/ethylene-vinyl acetate blends [43]. This result is in agreementwith the evidence from the DMTA and tensile tests.

On the basis of these mechanical and thermal results, a pro-cessing temperature of 230 �C was selected for further studies in

Table 3Mechanical properties of physical and catalyzed PLA/PC blends having differentcompositions.

Blends Tensile strength(MPa)

Young's modulus(GPa)

Elongationat break (%)

PC 57.2 ± 0.8 2.25 ± 0.06 84.4 ± 4.3PLA30PC70 55.3 ± 0.3 2.86 ± 0.07 125 ± 3.3PLA30PC70CATa 58 ± 0.6 3.01 ± 0.09 110 ± 5.3PLA40PC60 55.2 ± 0.7 2.99 ± 0.17 126 ± 3.4PLA40PC60CAT 60.9 ± 0.7 3.22 ± 0.05 126 ± 3.6PLA60PC40 56.1 ± 0.7 2.98 ± 0.03 3.3 ± 0.5PLA60PC40CAT 51.9 ± 2.4 3.33 ± 0.12 2.0 ± 0.12PLA70PC30 51.1 ± 1.7 3.18 ± 0.08 2.3 ± 0.4PLA70PC30CAT 49.1 ± 1.6 3.34 ± 0.06 1.8 ± 0.1PLA80PC20 52.5 ± 2.3 3.38 ± 0.19 2.0 ± 0.1PLA80PC20CAT 48.9 ± 1.1 3.38 ± 0.09 1.65 ± 0.1PLA 60.4 ± 0.26 3.54 ± 0.12 4.1 ± 0.5

a CAT ¼ TA/TBATPB.

Fig. 8. Young's modulus of the blends with different PLA/PC compositions. Comparisonbetween experimental data and models' predictions.

V.T. Phuong et al. / Polymer 55 (2014) 4498e45134504

which mechanical, thermal, morphological, and biodegradationproperties were investigated as a function of polymeric composi-tion in the presence and absence of TA/TBATPB.

3.2. Thermal and mechanical properties of PLA/PC blends over thefull compositional range

The tensile properties of the PLA/PC blends as a function ofcomposition are reported in Table 3. For comparison, the tensilestrength, Young's modulus, and elongation at break of pristine PCare 57.2 MPa, 2.25 GPa, and 84.4%, respectively (Table 4).

As the amount of PLA increases, the Young's modulus of theblends increases up to 3.54 GPa, i.e. the Young's modulus of purePLA. The tensile strengths of the blends did not change, as thetensile strengths of pure PLA and PC are quite similar. Conversely,the elongations at break of the blends decreased from 126% for thePLA40/PC60 blend to 3.3% for the PLA60/PC40 blend (near the valueof 4.1% for pure PLA). This change can be explained by a phaseinversion, occurring at a composition of ~50/50 PLA/PC, from aphase morphology in which PC is the matrix and PLA the dispersedphase, to the opposite condition in which PLA is the continuousphase and PC is the discontinuous phase. It was worth noting thatthe elongation at break for the PLA40/PC60 blend reaches 126%,which is significantly higher than that of pure PC (84%), evidencingthe strong toughening effect by the PLA domains in these blends.

Fig. 8 presents the Young's modulus values for blends withdifferent PLA/PC compositions that were processed in the presenceof TA and TBATPB (indicated with the suffix CAT) or as physicalblends. The experimental data appear to fit well with Barentsen'smodel [37] (Appendix A) at the two extremes of the compositionalrange. In the middle range, the Young's modulus of the blends is

Table 4Thermal analysis of blends by DSC testing.

Samples Tg (�C) Tonset (�C) Tc (�C) (�C) DHc (J/g) DHm (J/g) Xc (%)

PLA 59.1 97.3 110 26.6 31.7 5.5PLA80PC20 60.2 105.5 116.8 16.7 19.3 2.8PLA80PC20-CAT 48.8 94.6 108.5 18.9 24.5 6.0PLA40PC 60 58.8 109.2 125 2.1 3.4 1.4PLA40PC60-CAT 48.7 95.4 114.5 1.7 3.4 1.8PC 147.4

Tg ¼ PLA glass transition temperature.Tonset ¼ temperature at which the beginning of crystallization occurs.Tc ¼ peak temperature of crystallization.DHc ¼ crystallization enthalpy.DHm ¼ melting enthalpy.Xc ¼ crystallinity.Tc ¼ peak temperature of crystallization.

substantially higher thanwhat is predicted by Barentsen's model. Abetter fit can be obtained with themodel proposed by Veenstra andco-workers [38], wherein the higher Young's modulus values canbe explained by the occurrence of a co-continuous phase distri-bution [44]. Moreover, the Young's modulus values approach theparallel model (Appendix A) and appear to be isotropic, whichmeans that in these blends, both phases fully contribute to theblend modulus in all directions. These results follow the generalphase behaviour of polymer blends [45] and were confirmed by themorphological analysis (see Figs. 16 and 17). The big difference inthe elongation at break between PLA40/PC60 and PLA60/PC40 in-dicates that a phase inversion occurs between these two compo-sitions. Above this critical composition, the elastic modulus followsthe trend predicted for a rigid PLA-richmatrix phasewith dispersedPC domains; that is why the experimental data are fitted againwithBarentsen's model, and the elongation at break rapidly decreases tothe values characteristic of pristine PLA.

In Fig. 8, for the droplet/matrix morphologies, equation (5)(Appendix A) was used for as high as 20 wt% PLA when PLA (thestiff component) was the matrix, whereas from 80 to 100 wt%,where the stiff PLA component is theminor phase, equation (6) wasapplied. In the case of co-continuous morphologies, when the stiff

Fig. 9. DSC thermograms (2nd scan) for PLA/PC blends prepared with and without TA/TBATPB.

Fig. 10. DMTA trends of physical PLA/PC blends after annealing at 80 �C for 24 h.

Fig. 11. DMTA trends of PLA/PC blends processed at 230 �C and containing TA/TBATPBafter 24 h annealing at 80 �C.

V.T. Phuong et al. / Polymer 55 (2014) 4498e4513 4505

PLA component dominated (~50e60 wt% PLA), equation (7) wasused, whereas in the range 20e50 wt% PLA, equation (8) wasapplicable. With these calculations, the experimental data and thetheoretical models fit in a quite satisfactory way.

The Young's modulus values of the PLA/PC binary blends werewell fitted by Veenstra's model [38], as can be seen in Fig. 8.Nevertheless, at all compositions, the Young's modulus of theblends prepared in the presence of catalyst is always higher thanthe elastic modulus for the corresponding physical blends, as can benoted by comparing the data in Table 3. The Young's modulus of theblends decreases for PC contents ranging from 0 to 30 wt%, andthen does not change much from 30 to 40 wt%, suggesting that thematerials start to reach a co-continuous phase morphology, whichis similar to the behaviour observed for physical blends.

It is interesting to observe that the experimental data for theYoung's modulus of the blends prepared with TA/TBATPB exceedthe theoretical predictions. Also, the compositional range for fullco-continuity seems to be widened, from 20 to 60 wt% PLA for thephysical blends to 20�80 wt% PLA for the blends obtained inpresence of the catalysts. This can be explained by the decrease inPLA molecular weight, resulting in reduced melt elasticity [59] andreduced melt viscosity. The former can allow the formation ofelongated PLA domains at low PLA weight fractions. The latter canresult in a change in the viscosity ratio resulting in the increase ofPC domains at high PLA weight fractions.

The thermal behaviours of the materials were investigated byDSC. Fig. 9 shows the thermograms of the second scans at a heatingrate of 10 �C/min.

The crystallinity of the materials was calculated using the for-mula Xc ¼ ðDHm=fPLA=DH0

mÞ*100%, where DH0m is the theoretical

melting heat of 100% crystalline PLA (93 J/g) [47]., fPLA is the vol-ume fraction of PLA and DHm is the melting enthalpy. Upon thesecond scanning, the melting of pure PLA showed a double peak,

which means that there are two types of crystalline phases in thePLA. According to several studies, the crystalline phase of PLA de-pends on the processing temperature for crystallization [46,48,49].Therefore, the disordered a0 and ordered a phases of the PLAcrystalline phases are formed at low (Tc < 100 �C) and high(Tc � 120 �C) temperatures, respectively. In our case, at a heatingrate of 10 �C/min, 100 �C < Tc � 120 �C, PLA crystallization occurredin the range of the disorder-to-order phase transition, in accor-dance with investigations of Zhang and co-workers [47].

The Tgs of pure PLA and the PLA in the blends did not differ andremained near 59.1 �C, even in different compositions. It decreasedto 48.8 �C in the presence of TA/TBATPB in blends containing 20 and60 wt% PC. These results indicate that TA is not only involved inmelt reactions, but also plays the role of a plasticizer in the PLA[50,51]. Moreover, the crystallization temperature peak of the PLAin the blends is higher than in pure PLA because of the steric hin-drance of the PC domains on the crystallization of PLA. Similar tothe preceding explanation for the changes in Tg, the action of TA/TBATPB leads to a decrease in the crystallization temperature peakof the PLA in the blends because of TA plasticization. This increasesthe percentage of crystallinity for all compositions and favours theformation of the disordered a' crystalline phase.

More specifically, the Xc of PLA in PLA80/PC20-CAT is slightlyhigher than the crystallinity percentage of pure PLA, but it issignificantly increased with respect to the Xc of the correspondingphysical blend, in accordance with the important role of the TA/TBATPB in favouring PLA crystallization. However, at high PC con-tent, the crystallization of PLA is quite low, as shown in the ther-mograms of the PLA40/PC60 blends. Melting peaks are not evident,so the materials can retain their mechanical properties at highertemperatures.

The data from the DMTA of the blends without a catalyst areshown in Fig. 10, after annealing at 80 �C for 24 h. The first peak on

Fig. 12. DMTA of the PLA/PC blends (a) of different compositions with TA/TBATPB at 230 �C. (b) Comparison of pure PLA, mechanical, and catalysedcatalysed blends.

V.T. Phuong et al. / Polymer 55 (2014) 4498e45134506

the tan d curve is the relaxation of pure PLA at 69 �C. The tan d curveusually indicates the relaxation processes of a polymer. The majorrelaxation process in PLA is associated with the Tg [52].

Above the Tg of PLA, the storagemodulus of the blends increaseswith increasing PC content. The blends obtained without additivesshowed two peaks at 69 and 160 �C, corresponding to the Tgs of PLAand PC, respectively. This indicates the presence of separated PLAand PC phases, in agreement with the SEM, TEM, and mechanicalproperties results. Furthermore, the height of the tan d peak isassociated with the mobility of the amorphous regions in thepolymer [53]. As the amount of PC increases, the PLA peakbroadens, and the height of the tan d peak decreases. Corre-spondingly, the PC peak is sharpened and heightened because of itsimproved mobility.

In the presence of catalyst (Fig. 11), the blends exhibit Tg ¼ 60 �Cfor PLA, with a reduction similar to that observed by DSC analysis.Additionally, a new peak occurs between the Tg values of PLA andPC. Similarly to the explanations given above, this result is the mostimportant evidence for the formation of a PLA-PC copolymer. In the

Fig. 13. Experimental DMTA-based Tg values of non-crystallized copolymer blends as afunction of PC content, and comparison with different models.

crystalline condition, the middle peak on the tan d curve does notchange significantly as the amount of PC varies. These results aredifferent with respect to those shown in Fig. 12, inwhich the blendswere not annealed at 80 �C for 24 h before testing. This dissimilaritycan be explained by the consequence of the presence of anincreased crystalline fraction of PLA in the former blends, reducingthe mobility of the polymer chains, and thus affecting the relaxa-tion of the new copolymer. In fact, the lowest peak is associatedwith one of the PLA80-PC20 blend, showing high crystallinity.

In Fig. 12A, the DMTA results for pure PLA are shown. Above theTg, the storage modulus shows a minimum at about 80 �C and then

Fig. 14. SEC analysis of PLA/PC blends: superimposition of chromatograms obtained byrefractive index detection.

Fig. 15. 13C NMR spectra of (a) pure PLA and (b) pure PC, and (c and d) PLA80/PC20 without and with TA/TBATPB respectively.

V.T. Phuong et al. / Polymer 55 (2014) 4498e4513 4507

modulus increases, reaching a maximum at 120 �C. This impliesthat the crystallization of the PLA phase proceeded during theDMTA run (low scan rate). The decreasing storage modulus of PLAin the blends starts and ends at lower temperatures than in theoriginal PLA due to TA plasticization.

Fig. 12B shows that, in the case of the PLA40.PC60-CAT andPLA30.PC70-CAT blends, the storage modulus does not decreasein the temperature zone typical of PLA crystallization, which iscompletely different from the PLA40/PC60 physical blends. This isevidence to confirm that the TA/TBATPB addition results in theformation of chemical links between PLA and PC, so the chain

mobility of PLA becomes limited and crystallization does notoccur. This is a significant point for the application of PLA,because the material can maintain its mechanical properties up to100 �C.

Moreover, the temperatures of the middle peaks or Tgs attrib-utable to the PLA-PC copolymer change as a function of PC contentin the non-annealed samples (Fig. 13), in agreement with the pre-dictive models of Tgs for polymer blends suggested by GordonTaylor [54] and for complex polymer blends or copolymer systemssuggested by Kwei and Kalogeras/Brostow [55,56]. These modelsare formulated as follows: GordoneTaylor

Fig. 16. SEM micrographs for PLA80/PC20 (a) without and (b) with TA/TBATPB.

V.T. Phuong et al. / Polymer 55 (2014) 4498e45134508

Tg ¼ 41Tg;1 þ kGTð1� 41ÞTg;241 þ kGTð1� 41Þ

(1)

Kwei

Tg ¼ 41Tg;1 þ kKwð1� 41ÞTg;241 þ kKwð1� 41Þ

þ q41ð1� 41Þ (2)

Kalogeras/Brostow

Tg ¼ 41Tg;1 þ ð1� 41ÞTg;2 þ 41ð1� 41Þ½a0 þ a1ð241 � 1Þþ a2ð241 � 1Þ� (3)

where 41 is the volume fraction of component 1, Tg,1 and Tg,2 are theglass transition temperatures of component 1 and 2 respectivelyand kGT, kKw, q, a0, a1, and a2 are empirical parameters of equations(1)e(3).

By increasing the amount of PC, the Tg of the PLA-PC copolymersdecreases slightly and does not change from 40 to 60wt% PC; abovethis value, it increases. Therefore, this complex behaviour of Tg doesnot fit the model of GordoneTaylor when applied to a binarypolymer with a simple interaction in the amorphous state. In thatequation, Tg versus the weight fraction follows a linear or hyper-bolic function. In addition, the crystallization temperature of PLA isquite similar to the Tg of the PLA-PC copolymer. Thus, the crystal-lization process will affect the transition temperature of the PLA-PC

copolymer. Concerning this peculiar phenomenon, Kalogeras ex-plains that, in terms of microstructure, such intriguing variationsmay be partly attributed to the different types of segregation in thecopolymer and their relative contributions to the overall structure.In fact, different kinds of segregation may occur: (a) inter-lamellarsegregation (the amorphous new copolymer resides in the inter-lamellar region within the lamellar stack); (b) inter-fibrillarsegregation (the amorphous chains are placed outside thelamellar stacks of the PLA crystalline component(s), but are stilllocated within the spherulite); and/or (c) inter-spherulitic segre-gation (the amorphous phase is expelled from the lamellar stacksand resides at the inter-spherulitic region of the PLA component)[56]. By considering these complexities, Kwei and Kalogerasextended the original equation of the GordoneTaylor model. Theirresults were applied and fit perfectly with the experimental datafor the new PLA/PC blend copolymer obtained in the presence ofTA/TBATPB. Therefore, the variations in Tg of the new copolymer areaffected by the content of the PC, as well as by the extent of PLAcrystallization, as shown in Figs. 11 and 12.

3.3. Structure analysis

An investigation about the macromolecular weights of the PLA/PC blends was carried out by SEC, using both refractive index (RI)and ultraviolet (UV) detectors. A superimposition of the chro-matograms obtained by the RI detector is shown in Fig. 14. Thechromatographic curves obtained by RI detection consisted of apeak at a long elution time (i.e. at lower molecular weight) due tothe PC, with a shoulder at a shorter time due to the PLA. In Fig. 14,the PLA shoulder intensity is clearly much lower than that for thePC, even in the PLA-based blends. This observation suggests thatthe PLA chain lengthwas significantly reduced in the presence of TAand TBATBP.

The use of the UV detector at 254 nm should provide evidencefor the linkage of PLA to PC. In fact only PC is detectable by thisdetector, and the presence of linked PLA chains having the molec-ular weight of pure PLA should result in the presence of a shoulderat a lower retention time. However, a shoulder could not be dis-cerned, thus indicating the likelihood that only short PLA chains arelinked to PC. On the basis of chromatograms examination, themolecular weight of PC is not significantly influenced by the pro-cessing, and that the amount of formed copolymer would be lowand have a molecular weight similar to PC.

Themolecular weight data referred to polystyrene standards arereported in Table 5. PC-PLA copolymer formation was anywaydemonstrated by the data obtained with the UV detector. In fact,although the Mn of PC was similar or slightly decreased withrespect to the pure PC, the Mw values slightly but significantlyincreased, and the dispersity index Id also increased in all theblends obtainedwith the TA/TBATPB additives. In particular, theMwof PLA60/PC40-CAT was 68300 (Id ¼ 2.22), whereas the corre-sponding value for PLA60/PC40 was 53,800 (Id ¼ 1.75). In the latterblend, only a very slight decrease in themolecular weight of PC wasobserved with respect to pure PC due to chain scission. In contrast,in the presence of TA/PBATPB, as was evidenced in PLA/PBAT blendsobtained in the presence of a transesterification catalyst [61], thedata reveal the presence of a macromolecular fractionwith a highermolecular weight in the PC phase, most likely due to the linking ofPLA chains or PC branching. However, the latter hypothesis is indisagreement with the DMTA results in which a peak at a tem-perature higher than that of PC should have been revealed. Hence,the results can be explained only by considering the formation ofthe PLA-PC copolymer, in good agreement with the studies by Liuet al. [34].

Fig. 17. TEM micrographs for: PLA80/PC20 (a) without and (b) with TA/TBATPB; and PLA60/PC40 (c) without and (d) with TA/TBATPB.

V.T. Phuong et al. / Polymer 55 (2014) 4498e4513 4509

To investigate PLA chain scission in the blends, the theoreticalMn (Mnth) was calculated by considering the Mn values of pure PLAand PC as obtained from the analysis via RI detection. The calcu-lated values are significantly higher than the experimental onesobtained via RI detection. The difference between the theoreticaland the experimental values, DMn, was also calculated, and it wasfound to increase as the PLA content in the blend increased. Thesedata are in good agreement with the observations from the chro-matograms: the molecular weight of PLA is significantly reducedduring processing and to a greater extent than that of PC. The dataset for the blend obtained without TA/TBATPB showed that theoccurrence of PLA chain scission cannot be exclusively ascribed tothe action of the catalyst, as it occurred to a slight minor extent inthe absence of the TBATPB and TA. Moreover, after simplifying byneglecting the effect of copolymer formation on the molecularweight, the molecular weight of the PLA in all the blends wascalculated by considering equation (4):

MnPLAcalc ¼MnRI �MnUV$xPC

xPLA(4)

where MnRI is the number average weight determined by RIdetection, MnUV is the number average molecular weight deter-mined by UV detection (considered the Mn of PC), and xPC and xPLAare the mole fractions of PC and PLA, respectively. The calculatedvalues are reported in the last column of Table 5. The results

confirm the occurrence of chain scission in PLA, especially in blendswith a higher PLA content.

The 13C NMR spectra of the two pure polymers are rather simple(Fig. 15a). Although small impurity peaks are present, the signalassignments are quite straightforward and account for therepeating units of PLA (a) and PC (b).

In Fig. 15b, 13C NMR data for the physical blend (c) are reportedas well as for a PLA80/PC20 blend of the two polymers obtained inthe presence of TA/TBATPB (d). Upon examination, no particulardifferences between the two samples can be noted. The signalmarked with the asterisk (*) in spectrum (d) is due to residualacetone. At this stage, no evidence of copolymerization is apparentin spectrum (d). The main signals from the starting polymers are allpresent and their intensity is in fair agreement with the weightratios in both spectra (c) and (d). Other smaller signals are visible,but they are also present in spectra (a) and (b) of the startingpolymers. Because of the many similar functional groups, thedetection of processing products by 13C NMR could be difficultbecause of the overlapping of signals.

The spectral differences observed between the spectra of thepure polymers and blends obtained with catalysts were almostnegligible. On the other hand, the copolymer would be a minorcomponent in the presence of the predominant pure polymers, andwould show identical spectral signals. Although a similar compar-ison was carried out by examining 1H NMR spectra, they could notprovide further evidence of copolymer formation for the samereasons.

Table 5Mn and Mw data obtained by SEC analysis onto PLA/PC blends by using an RI and an UV detector, theoretical Mn and calculated Mn of PLA.

Samples Mnath DMn

b RI detector UV detector MnPLA calc

Mn Mw Id Mn Mw Id

PLA 135,000 0 135,000 207,600 1.54 e e e 135,000PC 32,900 0 32,900 56,200 1.74 29,600 56,700 1.92 e

PLA80PC20-CAT 83,300 47,100 36,200 87,100 2.41 23,700 62,100 2.62 49,200PLA60PC40-CAT 60,300 26,400 33,900 78,900 2.32 30,700 68,300 2.22 42,500PLA40PC60-CAT 47,200 15,700 31,500 65,800 2.09 26,200 54,400 2.07 64,100PLA60PC40 60,300 25,100 35,200 73,000 2.08 30,700 53,800 1.75 47,400

a Mnth ¼ theoretical Mn, calculated by keeping into account composition.b DMn ¼ Mnth�Mn (Mn obtained with RI detector was considered).

V.T. Phuong et al. / Polymer 55 (2014) 4498e45134510

An NMR investigation was carried out by Liu et al. [34] on a PC-PLA copolymer after separation by selective extraction. Theobserved spectral differences were slight, even though the purepolymers that had not participated in interchange reactions wereremoved from the blends. The results are probably in agreementwith a block structure in the PLA-PC copolymer, because the con-centration of PC units linked to PLA units, which would result in theslight NMR spectral change, is quite low in this case, and probablyundetectable due to both sensitivity and peak overlap.

Mechanistically, the PLA chains in this process are broken downin the presence of TA/TBATPB, and then react with PC. This is in

Scheme 2. Proposed general scheme of reactions between PLA and PC i

good agreement with prediction made on the basis of the work ofPenco et al. [60], who investigated PC/PMMA blends compatibilizedby TBATPB. By also considering the literature review by L�opez andZiebaa [57,58], the mechanism of the reaction between PLA and PCin the presence of TA and the interchange reaction catalyst TBATPBwas proposed (Scheme 2).

TA, which is the main blend component based on molar fractionand themostmobile chemical species in the system, reacts with theOH groups in PLA and PC under the melting conditions of extrusionand the catalytic influence of TBATBP to produce molecules with 1,2, or 3 hydroxyl groups (glycerol in the latter case). The chain

n the presence of TA and the interchange reaction catalyst TBATPB.

Fig. 18. Biodegradation tests of the PLA/PC blends without (a) and with (b) TA/TBATPB.

V.T. Phuong et al. / Polymer 55 (2014) 4498e4513 4511

scission of PLA occurs through the involvement of these activatedmolecules derived from TA. PLA having a linear or branchedstructure and a lower molecular weight is thus formed. PLAdegradation also occurs during processing at high temperature inthe presence of residual humidity, and results in the increasedconcentration of both hydroxyl and terminal carboxylic groups.These can react with TA, resulting in the evolution of acetic acid, inagreement with the TGA results.

The interchange reaction of modified PLA with PC can produceblock copolymers with variable-length PLA blocks and branchingpoints. The occurrence of branching creates an irregular structurewhich is probably responsible for the peculiar thermal properties ofthe blends. The presence of TA is fundamental as it facilitates therapid formation of the PLA-PC copolymer, thus favouring theoccurrence of compatibilization under laboratory extrusion condi-tions (60 s).

3.4. Morphology

The morphologies of the blends were investigated by SEM.Fig. 16 displays two micrographs obtained for the PLA80/PC20blend in the presence (A) or absence (B) of TA/TBATPB. In theformer, phase separation is evident and the PC domains in the PLAmatrix can be observed. The large spaces noticed at the edges ofdomains between the PC and PLA are attributable to low adhesion.The dimensions of the PC domains were variable, in the range1e3 mm.

Nevertheless, in the blend having the same polymeric compo-sition but in the presence of TA/TBATPB, there were no spaces be-tween the PLA domains and the PC matrix; rather, there were onlylight boundaries between the two phases. A new interlayer be-tween the domain and the matrix could be observed, in goodagreement with the evidence for PLA-PC copolymer formation. Thedomains seemed flatter and larger than in the physical blends,measuring about 5e7 mm. These findings agree with the TEM re-sults presented in Fig. 17. This phenomenon can be explained by adecrease in the surface tension of the PC due to the formation of thePLA-PC copolymer.

The ratio between the viscosity of the dispersed phase and thatof the matrix, hD/hM, in the PLA/PC 80/20 blend is higher than 1, asthe PC (i.e. the dispersed phase) is highly viscous in the melt at230 �C (a lower temperature than usually employed for processing).The occurrence of chain scission in the PLA when TA and TBATPBare added results in an increase in the viscosity ratio, inducing anincrease in the dispersed-phase diameter, in good agreement withWu's theory [63]. The increase in the diameter of the PC domains isthus the consequence of chain scission occurring in the PLA. Hence,as the amount of PC in the physical blends increases, the domainsizes also increase, as shown on Fig. 17c for PLA60/PC40. This factwas applied to model the mechanical properties of the blend.Furthermore, there seem to be some small sub-inclusions insidethe PC domains due to the rare coalescence of PLA particles insidethe highly viscous PC phase. In the presence of TA/TBATPB in thePLA60/PC40 blend, the chain scission leads to an increase in theviscosity ratio and the collapse of the dispersed particles in thecontinuous PC phase, because the percolation threshold is over-come. In fact, the viscosity ratio affects the co-continuity rangewidth, which usually becomes narrower for a viscosity ratio closerto 1 [63,64]. Since an increase in the viscosity ratio due to PLA chainscission is obtained in the PLA/PC blends, an enlargement of the co-continuity range reasonably occurs. The PLA60/PC40 blend is thusco-continuous, and this kind of morphology is in agreement withits high elongation at break, typical of the PC polymer. Therefore,this blend shows properties similar to those of PC, although the PCcontent is only 40% by weight.

3.5. Biodegradation

The average biodegradation (%) of PLA, PC, and physical blendgranulates are shown in Fig. 18a. Although pure PC is not biode-gradable, pure PLA starts to degrade after 18 days, and proceeds to50 wt% after 55 d and 100 wt% after 80 d. After 150 d, the degra-dation was 138 wt%. Biodegradation percentages above 100% areexplained by a synergistic effect known as priming, which occurs ifthe compost inoculum in the test reactor produces more CO2 thanthe compost inoculum in the control reactors. This results in a netCO2 production that is not produced exclusively from the test itemand, in the case of readily degradable products, in a measuredbiodegradation percentage exceeding 100%.

In the physical blends, the final percentage of degradation issimilar to the percentage of PLA in the blend. Depending on the PCcontent, the onset of degradation was delayed and the finalbiodegradation percentage reduced. Interestingly, bi-sigmoidalcurves were observed for all the PLA/PC physical blends. Thetimes at which the inflection points in the curves occur areincreased as the amount of PC in the blends increases.

The PLA/PC blends containing TA/TBATPB (Fig.18b) did not showbi-sigmoidal trends and the start of degradation was anticipated.The former observation can be the consequence of interchangereactions between PLA and PC and the latter observation can beattributed to the lower molecular weight of PLA due to chainscission, which would favour biodegradation. As a result, thepresence of TA and TBATPB could initially increase the speed ofdegradation, but decrease the final biodegradation percentage. The

V.T. Phuong et al. / Polymer 55 (2014) 4498e45134512

formation of linkages between the PLA and PC blocks due tointerchange reactions and the presence of the branching pointsintroduced by TA insertion [62] results in a lower final biodegrad-ability of the material.

4. Conclusions

The preparation of PLA and PC blends obtained by extrusionwith andwithout TA/TBATPBwas investigated. The addition of bothTA and TBATPB at 230 �C resulted in improved compatibilitythrough the formation of a PLA-PC copolymer. In fact, a tan d trendwith a new peakwas obtained by DMTA analysis. The newpeakwassituated between the Tg values of PLA and PC. The Tg of the copol-ymer as a function of blend composition was consistent with themodels of Kwei [55] and Kalogeras [56].

The tensile characterization of physical PLA/PC blends showedthat the Young's modulus was improved as the PLA content wasincreased. The maximum elongation at break was obtained as thecontent of PC reached the percolation threshold, allowing theachievement of a co-continuous phase morphology. More specif-ically, the data fit well with two different predictive models. As thecatalyst was added, the Young's modulus of the materials increasedas a result of both the improvement of compatibility and chainscission in the PLA phase, enabling the achievement of a co-continuous phase morphology for lower PC contents.

DSC and DMTA confirmed that, as the content of PC increased,the crystalline fraction of the materials was reduced. More spe-cifically, in the PLA40/PC60eCAT blend, the typical decrease instorage modulus due to the overcoming of Tg followed by the in-crease due to crystallization was not observed, suggesting themaintenance of mechanical stability at temperatures higher thanthe Tg of PLA and the disabling of the crystallization process. Theeffect is probably attributable to the links formed between PLAand PC, which reduce PLA mobility. These advantageous propertiesbroaden the temperature range for applications of current mate-rials based on PLA.

Investigations of the reactions occurring in themelt through SECand NMR analysis were consistent with the evident PLA chainscission on one hand and the formation of a low amount of PLA-PCmultiblock copolymer, probably partially branched, on the other.The presence of TA was fundamental as it allowed the rapid for-mation of the PLA-PC copolymer, thus favouring the compatibili-zation under laboratory extrusion conditions (60 s).

In addition, the SEM and TEM analyses confirmed the effects ofTA/TBATPB on the morphology of the blends. The interaction be-tween the domains and the matrix was improved as the adhesionbetween the PC and PLA phase increased, whereas the sizes of thedispersed domains increased in agreement with the improvedtendency toward co-continuity because of PLA chain scission.

Chain scission was also responsible for the anticipated onset ofbiodegradation in the blends obtained with TA and TBATPB incontrast to the physical blends. Nevertheless, the final biodegra-dation percentage was lowered mainly by the addition of PC andslightly by the presence of TA/TBATPB. The biodegradation behav-iour was in good agreement with the reactivity and phasemorphology deductions. In general, the final percentage ofbiodegradation in all the blends is quite similar to the percentage ofPLA in the blend. Therefore, the presence of PC does not appear tobe detrimental to PLA biodegradation.

Acknowledgement

The authors gratefully acknowledge the financial support of theFORBIOPLAST (Forest Resource Sustainability through Bio-Based-Composite Development) project e Contract No. 212239-FP7-

KBBE, funded by the European Commission under the 7th Frame-work Programme (FP7) (http://www.forbioplast.eu). The authorswish also to acknowledge Mrs Irene Anguillesi for performingDMTA tests and Prof. Valter Castelvetro and Dr. Sabrina Bianchi forSEC tests and helpful discussion.

Appendix A. Theoretical analysis

The Young's modulus of binary blends will change as theamount of polymers varies due to the different effects on the me-chanical properties of constituent polymers and the phase behav-iour of blends. In the literature, there are several studies thatattempt to predict the mechanical properties of binary polymerblends [35,36]. In particular Davies proposed for the modulus ofblends with dispersed morphologies the following expression [36]:

E1=5 ¼ 41E1=51 þ 42E

1=52 (5)

where E1 and E2 are the elastic moduli of the two components andf1 and f2 are the volume fractions.

Another model to describe droplet/matrix blends, was proposedby Barentsen [37] who extended the combination of parallel andseries elements as proposed by Takayanagi for two-dimensionalgeometries to a three-dimensional arrangement.

The elastic modulus of polymer blends with a droplet/matrixmorphology when the dispersed particles are evenly distributed inthe matrix can be estimated with the Barentsen’s model whichconsiders a series model of parallel parts (Ea) or parallel model ofserial linked parts (Eb):

Ea ¼ Eml2Ed þ

�1� l2

�Em

ð1� lÞl2Ed þ�1� lþ l3

�Em

(6)

Eb ¼�1� l2

�Em þ l2EmEd

lEm þ ð1� lÞEd(7)

The modulus of the blend (Ea or Eb) is expressed as a function ofa volume fraction (fd ¼ 1�fm ¼ l3), the modulus of the dispersedphase (Ed), and the modulus of the matrix (Em).

Nevertheless in polymer blends, the morphology of the mate-rials will be different as the proportions of each component vary.The morphology of the blends, which can show a changing phasebehavior from phase-separation, to the formation of co-continuousphases and even phase inversion, will have a significant effect onthe final properties and makes the prediction of such models,which do not consider the type of phase morphology, not fullyreliable. In fact, both the Davies's and Barentsen's models, basedonly on the properties of the individual separated phases, do not fitwell with the experimental data for co-continuous blends.

In a co-continuous blend, the dispersed phase does not consistof separate particles in the matrix phase, but it is interconnectedand forms elongated domains, which extend throughout the ma-trix. To visualize co-continuity, Veenstra and co-workers proposedthat the dispersed phases consist of three orthogonal bars ofpolymer 1 embedded in a unit cube where the remaining volume isoccupied by component 2. Repeating this unit cube in 3D showsthat component 2 has the same framework as component 1, i.e.both the components are interconnected. In a manner similar toBarentsen, modulus relations for a series model of parallel parts (Ec)and for a parallel model of serial-linked parts (Ed) can be derived[38] as:

V.T. Phuong et al. / Polymer 55 (2014) 4498e4513 4513

Ec ¼�a4 þ2a3b

�E21 þ2

�a3bþ3a2b2 þ ab3

�E1E2 þ

�2ab3 þ b4

�E22�

a3 þ a2bþ2ab2�E1 þ

�2a2bþ ab2 þ b3

�E2

(8)

Ed ¼ a2bE1 þ�a3 þ 2abþ b3

�E1E2 þ ab2E22

bE1 þ aE2(9)

where a is related to the volume fraction of component 1 by3a2�2a3 ¼ f1 and b is related to the volume fraction of component2 by b ¼ 1�a.

Equation (6) will be applied when the stiff component domi-nates and equation (7) as the stiff component is the minor phase.The same arguments can be used for the parallel model of serial-linked parts (Eq. (8)) and the series model of parallel-linked parts(Eq. (7)) that were derived for co-continuous blends.

References

[1] Johansson C, Bras J, Mondragon I, Nechita P, Plackett D, Simon P, et al. Bio-Resources 2012;7:2506e52.

[2] Metha R, Kumar V, Bhunia H, Upadhyay SN. J Macromol Sci C 2005;45:325e49.

[3] Zhang LH. J Polym Sci B 2011;49:1051e83.[4] Young Nam J, Sinha Ray S, Okamoto M. Macromolecules 2003;36:7126e31.[5] Li H, Huneault MA. Polymer 2007;43:6855e66.[6] Kido T, Yoshimura M, Yoka K. Japan Published patent application 1995,

109413.[7] Takehara A, Onoki T, Kageyama F. Japan Published patent application 2007,

131795.[8] Ishihara J, Kuyama H, Ozeki E, Ishitoku T, Tanaka M, Sakamoto N. European

patent application 1999, EP0896013.[9] Robeson LM. In: Utracky LA, editor. Polymer Blend Handbook. Netherlands:

Kluwer Academic Publisher; 2003. pp. 1167e200 [Chapter 17].[10] Takeshi K, Katsuhisa T. J Appl Polym Sci 2011;121:2908e18.[11] Semba T, Kitagawa K, Ishiaku US, Hamada H. J Appl Polym Sci 2006;101:

1816e25.[12] Semba T, Kitagawa K, Ishiaku US, Kotaki M, Hamada H. J Appl Polym Sci

2007;103:1066e75.[13] Wang R, Wang S, Zhang Y, Wan C, Ma P. Polym Eng Sci 2009;49:26e33.[14] Coltelli M-B, Bronco S, Chinea C. Polym Degrad Stabil 2010;95:332e41.[15] Zhang N, Wang QF, Ren J, Wang L. J Mater Sci 2009;44:250e6.[16] Kumar M, Mohanty S, Nayak SK, Parvaiz MR. Bioresour Technol 2010;101:

8406e15.[17] Su ZZ, Li QY, Liu YJ, Hu GH, Wu CF. Eur Polym J 2009;45:2428e33.[18] Corre YM, Duchet J, Reignier J, Maazouz A. Rheol Acta 2011;50:613e29.[19] Tjong SC, Meng YZ. Mater Res Bull 2004;39:1791e801.[20] Liang GG, Cook WD, Tcharkhtchi A, Sautereau H. Eur Polym J 2011;47:

1578e88.[21] Marchese P, Celli A, Fiorini M. Macromol Chem Phys 2002;203:695e704.[22] Marchese P, Celli A, Fiorini M, Gabalgi M. Eur Polym J 2003;39:1081e9.

[23] Wilkinson AN, Cole D, Tattum SB. Polym Bull 1995;23:751e7.[24] Yu T, Ren J, Gu S, Yang M. Polym Inter 2009;58:1058e64.[25] Warth H, Chen S, Wang Y, Li H. US patent 2010, US2010/0081739A1.[26] Penco M, Lazzeri A, Phuong TV, Cinelli P. International patent application

2012, WO2012025907A1.[27] Zhang GY, Ma JM, Cui BX, Luo XL, Ma DZ. Macromol Chem Phys 2001;202:

604e13.[28] Denchev Z, Sarkissiva M, Radusch HJ, Luepke T, Fakirow S. Macromol Chem

Phys 1998;199:215e21.[29] Aravind I, Pionteck J, Thomas S. Polym Test 2012;31:16e24.[30] Cho JK, Lee HT, Ha DH, Jung CD. US patent 2012, US8133943B2.[31] Lee JB, Lee YK, Choi GD, Na SW, Park TS, KimWN. Polym Degrad Stab 2011;96:

553e60.[32] Wang Y, Chiao SM, Hung TF, Yang SY. J Appl Polym Sci 2012;125:402e12.[33] Yao M, Deng H, Mai F, Wang K, Zhang Q, Chen F, et al. Exp Polym Lett 2011;5:

937e49.[34] Liu C, Lin S, Zhou C, Yu W. Polymer 2013;54:310e9.[35] Kanzawa T, Tokumitsu K. J Appl Polym Sci 2011;121:2908e18.[36] Davies WEA. J Phys D 1972;4:1325e39.[37] Barentsen WM. The Netherlands: Eindhoven University of Technology; 1972.

PhD thesis.[38] Veenstra H, Verooijen CJP, Lent JJB, Dam VJ, Boer PA, Nijhof HJA. Polymer

2000;41:1826e71.[39] Song L, He Q, Hu Y, Chen H, Liu L. Polym Degrad Stab 2008;93:627e39.[40] Lee LH. J Polym Sci A 1964;2:2859e73.[41] Jang BN, Wilkie CA. Polym Degrad Stab 2004;86:419e30.[42] Phuong TV, Lazzeri A. Compos Pt A 2012;43:2256e68.[43] Monteiro EEC, Thaumaturgo C. Comp Sci Technol 1997;57:1159e65.[44] Potschke P, Paul DR. J Macromol Sci C 2003;C43:87e141.[45] Williemse RC, Speijer A, Langeraar AE, Boer PD. Polymer 1999;40:6645e50.[46] Fischer EW, Sterzel HJ, Wegner G. Kolloid-Zu Z-Polym 1973;251:980e90.[47] Zhang JM, Duan YX, Sato H, Tsuji H, Noda I, Yan S, et al. Macromolecules

2005;38:8012e21.[48] Cho TY, Strobl G. Polymer 2006;47:1036e43.[49] Zhang JM, Tashiro K, Tsuji H, Domb JA. Macromolecules 2008;41:1352e7.[50] Ljungberg N, Wesslen B. J Appl Polym Sci 2001;86:1234e77.[51] Yamane H, Sasai K. Polymer 2003;44:2569e75.[52] Liu X, Dever M, Fair N, Benson RS. J Environ Polym Degrad 1997;5:225e35.[53] Silverajah VS, Ibrahim AN, Yunus WZW, Hassan HA, Woei CB. Int J Mol Sci

2012;13:5878e98.[54] Gordon M, Taylor JS. J Appl Chem 1952;2:493e500.[55] Kwei TK. J Polym Sci Lett 1984;22:307e13.[56] Kalogeras JM, Brostow W. J Polym Sci B 2009;47:80e95.[57] L�opez DE, Goodwin AG, Bruce DA, Lotero E. Appl Catal A 2005;295:97e105.[58] Zieba A, Drelinkiewicz A, Chmielarz B, Matachowskia L, Stejskalc J. Appl Catal

A 2010;387:3e25.[59] Coltelli MB, Harrats C, Aglietto M, Groeninckx G. Polym Eng Sci 2008;48:

1424e33.[60] Penco M, Sartore L, Della Sciucca S, Di Landro L, D’Amore A. Macromol Symp

2007;247:252e9.[61] Coltelli MB, Toncelli C, Ciardelli F, Bronco S. Polym Degrad Stab 2011;96:

982e90.[62] Quynh TM, Mitomo H, Nagasawa N, Wada Y, Yoshii F, Tamada M. Eur Polym J

2007;43:1779e85.[63] Wu S. Polym Eng Sci 1987;27:335.[64] Chuaia CZ, Almdal K, Lyngaae-Jorgensen J. Polymer 2003;44:481e93.