European Polymer Journal 49 (2013) 3309–3317

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European Polymer Journal

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Miscibility and properties of poly(L-lactic acid)/poly(butyleneterephthalate) blends

0014-3057/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.eurpolymj.2013.06.038

⇑ Corresponding author. Tel.: +39 081 8675059; fax: +39 081 8675230.E-mail address: dilorenzo@ictp.cnr.it (M.L. Di Lorenzo).

Maria Laura Di Lorenzo ⇑, Paolo Rubino, Mariacristina CoccaConsiglio Nazionale delle Ricerche, Istituto di Chimica e Tecnologia dei Polimeri, c/o Comprensorio Olivetti, Via Campi Flegrei, 34, 80078 Pozzuoli, (NA), Italy

a r t i c l e i n f o a b s t r a c t

Article history:Received 7 February 2013Received in revised form 20 May 2013Accepted 28 June 2013Available online 12 July 2013

Keywords:Poly(L-lactide)Poly(butylene terephthalate)MiscibilityCompatibilityMorphology

Binary blends of poly(L-lactide) (PLLA) and poly(butylene terephthalate) (PBT) containingPLLA as major component were prepared by melt mixing. The two polymers are immisci-ble, but display compatibility, probably due to the establishment of interactions betweenthe functional groups of the two polyesters upon melt mixing. Electron microscopy analy-sis revealed that in the blends containing up to 20% of poly(butylene terephthalate), PBTparticles are finely dispersed within the PLLA matrix, with a good adhesion between thephases. The PLLA/PBT 60/40 blend presents a co-continuous multi-level morphology,where PLLA domains, containing dispersed PBT units, are embedded in a PBT matrix. Thevaried morphology affects the mechanical properties of the material, as the 60/40 blenddisplays a largely enhanced resistance to elongation, compared to the blends with lowerPBT content.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Poly(L-lactic acid) (PLLA) is a compostable polymer thatcan be derived from renewable resources [1]. Annual crops,like corn and sugar beets, are currently utilized as feed-stock in the commercial production of PLLA resin. Theyearly supply of PLLA is expected to increase rapidly inthe near future owing to the various attractive properties,such as high rigidity, biodegradability, and biocompatibil-ity. For a massive consumption of PLLA, however, consider-able efforts are required to overcome a few defects, whichinclude a slow crystallization rate and mechanical brittle-ness [2].

Numerous studies have been carried out with the aimto improve these drawbacks. Polymer blending is an effec-tive way of achieving a desirable combination of proper-ties, which are often absent in single polymers. However,a polymer blend often produces a material with poormechanical properties, as most polymer pairs are thermo-

dynamically immiscible with each other [3]. In order topromote compatibility between a given polymer pair, usu-ally a third component, miscible with both polymers isadded, or chemical reactions between the functionalgroups of the polymers are induced, which can lead tomodification of the interfaces of an immiscible blend [4].In polyester–polyester blends, such compatibilization canbe achieved upon processing, as the blend componentscan undergo chemical reactions at high temperatures inthe melt, like alcoholysis, acidolysis, or direct ester inter-change [5–7]. These reactions can result in formation ofblock or random copolymers [8], or simply promote com-patibilization of the blend through the establishment ofweak interactions between the functional groups of thetwo polyesters [9].

A number of polyester–polyester blends containingpoly(L-lactic acid) as major component have been investi-gated, and miscibility was found to depend on the specificpolymer pair. Immiscibility was reported for blends ofPLLA with poly(e-caprolactone) (PCL) and with poly(butyl-ene succinate) (PBS) [10,11], whereas partial miscibilitywas determined for blends with poly(tetramethylene adi-pate-co-terephthalate) (PTAT) [12]. In some cases, misci-

3310 M.L. Di Lorenzo et al. / European Polymer Journal 49 (2013) 3309–3317

bility was found to be controlled by the molecular charac-teristics of the components, like in binary blends of PLLAwith poly[(R)-3-hydroxybutyrate] (PHB): PLLA shows nomiscibility with high molecular weight PHB, whereas it ispartially miscible with low molecular weight PHB [13].

As an attempt to improve the above mentioned draw-backs of poly(L-lactic acid), we propose to blend it with an-other polyester, namely poly(butylene terephthalate)(PBT). Poly(butylene terephthalate) is a widely used poly-ester with a very high crystallization rate, and it crystal-lizes at temperatures higher than the melting point ofPLLA [14]. Therefore PBT crystals in a PLLA/PBT blendmay in principle promote nucleation of PLLA, enhancingits crystallization kinetics. PLLA was reported to be fullymiscible with the lowest homologue of PBT, poly(ethyleneterephthalate) [15], whereas contradictory results were re-ported for blends with poly(trimethylene terephthalate)(PTT): Zou et al. claimed full miscibility for PLLA/PTTblends [16], whereas Lin and Cheng reported completeimmiscibility of this polymer pair [17]. It is likely that, sim-ilarly to PLLA/PHB blends, miscibility of PLLA with PTT isaffected by the molecular characteristics of the specificpolymer grades.

Binary blends of PLLA with PBT have been poorly de-tailed in the literature. To our knowledge, only an investi-gation on chain extension effects of para-phenylenediisocianate on miscibility and biodegradation of PLLA/PBT blends appeared in the literature [18]. Copolyestersbased on PBT and PLLA units, as well as core-sheath conju-gate fibers, where PLLA is the sheath and PBT is the core,have also been prepared and their properties discussed asfunction of composition [19–22].

The aim of the study detailed in this contribution is todefine miscibility, phase structure and mechanical proper-ties of PLLA/PBT blends containing PLLA as the major com-ponent. The influence of PBT on crystallization kinetics ofPLLA is presented and discussed in a forthcomingmanuscript.

2. Experimental part

2.1. Materials

Poly(buthylene terephthalate) (PBT) with melt flow in-dex 20 g/min was purchased from Sigma–Aldrich Corp.Poly(L-lactic acid) (PLLA), grade name 4032D, containing1.5% D-isomer, was provided by NatureWorks LLC.

Before melt mixing, PBT was dried in a vacuum oven at120 �C for 16 h, and PLLA in vacuum oven at 80 �C for 4 h.

2.2. Blend preparation

PLLA/PBT blends were prepared by melt mixing in aBrabender-like apparatus (Rheocord EC of Haake Inc.) at250 �C and 60 rpm for 6 min. Binary blends with PLLA/PBT 100/0, 90/10, 80/20, 60/40 wt/wt were prepared.

3. Preparation of compression-molded sheets

PLLA/PBT blends were compression-molded with a Col-lin Hydraulic Laboratory Forming Press P 200 E at a tem-

perature of 250 �C for 2 min, without any appliedpressure, to allow complete melting. After this period, aload of about 0.5 ton was applied for 2 min, then the sam-ple was cooled to room temperature in about 3 min bymeans of cold water circulating in the plates of the press.

3.1. Calorimetry

The thermal properties of PLLA/PBT blends were inves-tigated using a Mettler DSC 822e calorimeter, Mettler-Tole-do, Inc, equipped with a liquid-nitrogen accessory for fastcooling. The calorimeter was calibrated in temperatureand energy using indium. Dry nitrogen was used as purgegas at a rate of 30 ml/min during all measurements andthermal treatments.

A small piece of compression-molded PLLA/PBT blend,weighting about 10 mg, was encapsulated in standard alu-minum 40 ll pans, then heated at a heating rate of 20 �C/min for conventional DSC analyses.

Temperature-modulated calorimetry (TMDSC) mea-surements were conducted using a sinusoidal oscillationwith a temperature amplitude of 0.25 �C, an underlyingheating rate of 0.5 �C/min, and a modulation period of60 s. Sample mass was reduced to about 3 mg. TMDSCraw data were analyzed by approximating the modulatedsignal with a Fourier series, and the reversing signals wereobtained from the first harmonics of the Fourier series [23–25].

A fresh sample was used for each DSC or TMDSC analy-sis. All the measurements were repeated three times to im-prove accuracy.

3.2. Thermogravimentry

Thermal decomposition of PBT-PLLA blends was ana-lyzed using a Perkin Elmer Pyris Diamond TG/DT analyzer.A small piece (about 10 mg) of each blend was placed inplatinum open sample pans and heated from room tem-perature to 800 �C at 20 �C/min. High purity nitrogen wasfluxed through the furnace at a flow rate of 50 mL/min.

3.3. Scanning electron microscopy

Morphological analysis of cryogenically fractured PLLA/PBT blends was performed using a FEI Quanta 200 FEGenvironmental scanning electron microscope (ESEM)(Eindhoven, The Netherlands) in low vacuum mode, usinga Large Field Detector (LFD) and an accelerating voltage of10–20 kV. Before analysis, the samples were sputtered-coated with Au–Pd alloy using Baltech Med 020 SputterCoater System and mounted on aluminum stubs by meansof carbon adhesive disks.

3.4. Optical microscopy

Morphology of PLLA/PBT blends was investigated byoptical microscopy, using a Zeiss polarizing microscopeequipped with a Linkam TMHS 600 hot stage and a ScionCorporation CFW-1312C Digital Camera. A small piece ofeach blend was squeezed between two microscope slides,

0 200 400 6000

50

100

Wei

ght (

%)

Temperature (°C)

100/0 90/10 80/20 60/40

Fig. 1. Weight remaining in PLLA/PBT blends upon heating at 20 �C/minin nitrogen atmosphere.

M.L. Di Lorenzo et al. / European Polymer Journal 49 (2013) 3309–3317 3311

then inserted in the hot stage. The thickness of thesqueezed sample was lower than 10 lm.

3.5. Fourier-transform infrared analysis

FTIR spectra of PLLA/PBT blends were recorded at roomtemperature by means of a Perkin Elmer Spectrum 100 FT-IR spectrometer, equipped with an attenuated total reflec-tance accessory (ATR). The scanned wavenumber rangewas 4000–650 cm�1. All spectra were recorded at a resolu-tion of 4 cm�1, and 16 scans were averaged for eachsample.

3.6. Gel permeation chromatography

Gel permeation chromatography (GPC) measurementswere performed using a Waters 150C instrument equippedwith a Polymer Laboratories evaporative detector and twolinear columns. Tetrahydrofuran was used as eluent sol-vent at a flux of 1 mL/min; 12 polystyrene standards rang-ing between 3,000,000 Da and 580 Da were used forcalibration. The samples were dissolved in tetrahydrofuranat a concentration of 1 mg/mL at room temperature.

4. Results and discussion

Poly(L-lactic acid) is known to undergo thermal degra-dation during processing, which is usually conducted at210–240 �C [26–30]. In order to blend PLLA with PBT, themixing temperature was raised to 250 �C, so that bothpolymers are in the melt state and have sufficient mobilityto ensure intimate mixing. Such high temperature, how-ever, is known to induce degradation of the biodegradablepolyester, which results in a decreased molecular mass[26–30]. In order to determine the extent of thermal degra-dation occurring during processing, gel permeation chro-matography (GPC) was performed.

GPC analysis revealed that molecular mass of the as re-ceived PLLA chips is Mn = 147.000 and Mw = 225.000, witha polydispersity Mw/Mn = 1.53. Melt processing at 250 �Cfor 6 min results in a noteworthy reduction of molecularmass to Mn = 104.000 and Mw = 172.000, i.e. the originalaverage chain length is reduced to about 70% of the initialvalue after high temperature treatment. The increasedpolydispersity (Mw/Mn = 1.65) indicates the occurrenceof random chain scission upon melt mixing [31]. The dim-inution of molecular mass upon processing at 250 �C is inline with literature data on usual processing conditionsof poly(L-lactic acid), that report a reduction of Mn to 64–90% of the initial value for processing at temperatures inthe range 210–240 �C, with a noteworthy influence notonly of temperature but also of processing time [32,33].The decreased length of PLLA chains after melt mixing isto be taken into account upon discussion of miscibilitywith PBT, which in principle may be affected by molecularmass of the blend components, as detailed above.

The thermal stability of PLLA and of PLLA/PBT blendswas analyzed by thermogravimetric analysis in nitrogenatmosphere. Fig. 1 shows the normalized weight loss forthe analyzed samples at a heating rate of 20 �C/min, as a

function of temperature. PLLA undergoes a one-stage deg-radation process, with a complete decomposition of thematerial at 370 �C, in agreement with literature data [32].The addition of PBT induces an additional degradation stepin the blends in the temperature range 350–420 �C. This isthe temperature range of degradation of PBT commonly re-ported in the literature [33]. Similar to PLLA, also PBT has asingle mass loss step, occurring at higher temperaturesthan PLLA, with some residue of about 5–10% of the initialmass remaining at 500 �C [33]. The entity of the seconddegradation step, as well as the amount of residue remain-ing at high temperature are proportional to PBT content inthe blend, and no sizeable variation in the temperature ofthe degradation steps occurs, which indicate that PBT doesnot affect the thermal stability of the blend, nor its thermaldegradation process.

To gain information about the phase structure and mor-phology of the PLLA/PBT blends, scanning electron micros-copy analyses were performed. Fig. 2a illustrates thecryogenically-fractured surfaces of compression-molded,plain PLLA, which appears quite smooth, as expected. Theaddition of PBT results in phase-separated blends, withthe development of PBT particles dispersed within thepoly(L-lactic) matrix, as seen in Fig. 2b and c, that presentthe SEM micrographs of the 90/10 and 80/20 blends,respectively. Some degree of compatibility between theblend components can be discerned from these pictures.In fact, many dispersed droplets are pulled out upon thecryogenically fracture process, whereas some other parti-cles remain attached to the matrix. It is likely that someinteraction between the functional groups of PLLA andPBT, established during melt mixing, provides compatibili-zation of the blends, as discussed below.

The fractured surface of the 60/40 blend, displayed inFig. 2d, present quite large dispersed granules. Again, mostparticles seem to well adhere to the matrix, as also seen inFig. 2b and c. In some cases, the dispersed areas appear notsmooth. Such morphology is illustrated in Fig. 3, which de-tails a further enlargement of the SEM micrograph of thefractured PLLA/PBT 60/40 blend shown in Fig. 2d. The en-larged granule presents a number of embedded particlesthat are well fixed to the overall structure. It is worth to

Fig. 2. SEM micrographs of cryogenically fractured surfaces of the PLLA/PBT blends: (a) 100/0; (b) 90/10; (c) 80/20 and (d) 60/40.

Fig. 3. SEM micrograph of cryogenically fractured surface of the PLLA/PBT60/40 blend.

65013001950260032503900Wavenumber (cm-1)

Abso

rban

ce

a

b

c

d

e

Fig. 4. FTIR spectra of PLLA/PBT blends: (a) 100/0; (b) 90/10; (c) 80/20;(d) 60/40 and (e) 0/100.

3312 M.L. Di Lorenzo et al. / European Polymer Journal 49 (2013) 3309–3317

note that only few areas with detailed multi-level struc-ture (like the one shown in Fig. 3) can be observed in thecryogenically fractured surface of the 60/40 blend, as seenin Fig. 2d. This is to be linked to the adhesion between thephases, which determines the fracture in each specific re-gion of the blend.

To detect possible establishment of interactions be-tween the functional groups of PLLA and PBT during meltmixing, Fourier transform infrared analysis was performed.Fig. 4 shows the FTIR spectra of PLLA/PBT blends, a magni-fication of the carbonyl region is reported in Fig. 5. The

spectrum of neat PLLA, Fig. 5a, presents a strong absorptionband centered at 1748 cm�1, due to amorphous carbonylvibration. The absorption peak due to crystalline carbonylvibration of PLLA, expected at �1755 cm�1 [34,35], cannotbe identified since the polymer is amorphous, as detailedbelow. In the spectrum of pure PBT, Fig. 5e, there is astrong and sharp peak at 1708 cm�1, which is attributedto the stretching vibrations of crystalline carbonyl groups[36]. The FTIR spectra of PLLA/PBT blends present thetwo major carbonyl stretching bands due to carbonylvibration of PLLA and PBT, respectively, and the intensityratio of these two bands changes with the compositionratio.

160016501700175018001850Wavenumber (cm -1)

Abso

rban

ce

a

b

c

d

e

Fig. 5. FTIR spectra of PLLA/PBT blends: (a) 100/0; (b) 90/10; (c) 80/20;(d) 60/40 and (e) 0/100. Magnification of the carbonyl region.

M.L. Di Lorenzo et al. / European Polymer Journal 49 (2013) 3309–3317 3313

In order to evaluate possible interaction between PLLAand PBT, the blend components of PLLA/PBT 60/40 wereseparated by selective extraction. PBT is insoluble in chlo-roform, which is instead a good solvent for PLLA [37]. Asmall piece of the PLLA/PBT 60/40 blend was ground andimmersed in CHCl3 upon continuous stirring for three daysat room temperature. The insoluble fraction was recovered,washed with CHCl3 and dried at ambient temperature for72 h. The FTIR spectrum of the insoluble fraction is re-ported in Fig. 6 and compared to the 60/40 blend, as wellas to the plain components. The FTIR spectrum of the insol-uble fraction is characterized by the presence of an absorp-tion band centered at 1708 cm�1, typical of the crystallinecarbonyl stretching of PBT chains, and also of an absorptionband centered at 1758 cm�1, due to crystalline carbonylvibration of PLLA units. This result suggests that some PLLAchains remain non-extracted from the blend after pro-longed exposure to chloroform. The decreased solubility

160016501700175018001850

Wavenumber (cm -1)

Abso

rban

ce

PBT

PLLA/PBT 60/40

PLLA

Insoluble fraction

Fig. 6. FTIR spectra of PLLA/PBT 60/40 blend, fraction insoluble inchloroform, PLLA and PBT. Magnification of the carbonyl region.

of PLLA chains may be ascribed to the establishment ofinteractions between the functional groups of the twopolyesters during melt mixing, which makes the interact-ing PLLA molecules non-soluble in CHCl3. It can be hypoth-esized that the interacting PLLA and PBT chains are locatedat the interfacial regions of the blend, promoting compati-bilization of the components, which results in the good dis-persion and adhesion of the phases, probed by SEManalysis.

To gain further information on composition of thephases, the temperature-dependent morphology of thePBT/PLLA blends was investigated by optical microscopy.Fig. 7 illustrates the optical micrographs of the PLLA/PBT80/20 blend in the melt at 250 �C (Fig. 7a) and after coolingfrom 250 �C to room temperature at 4 �C/min (Fig. 7b).Phase separation of the components is evident in Fig. 7a,which displays small droplets homogeneously distributedwithin the matrix. Upon cooling at 4 �C/min, both PLLAand PBT crystallize, although in different temperatureranges [14,38]. A very high number of small spherulites ap-pear, which are almost indistinguishable upon opticalmicroscopy analysis, due to the high nucleation density.The 90/10 blend displays a similar morphology (notshown) both in the melt state and after crystallization,with minor differences in the size and number of dispersedparticles in the melt, as well as in the dimension of thecrystallites after completion of solidification of thematerial.

A different morphology is instead exhibited by thePLLA/PBT 60/40 blend, whose optical micrographs gainedat various temperatures upon cooling from the melt at4 �C/min are presented in Fig. 8. In the melt at 250 �C largeareas containing dispersed particles, all included in a co-continuous interconnected structure, appear, as shown inFig. 8a. At 210 �C some crystallites become visible in theco-continuous phase, due to crystallization of PBT chains,as seen in Fig. 8b. As the temperature decreases to 200 �C(Fig. 8c), crystallization of PBT is completed in the co-con-tinuous structure, and some birefringent spots also becomevisible in the embedded areas, due to growth of PBT crys-tals. Further lowering of the temperature leads to crystalli-zation of PLLA in the dispersed regions, as shown in Fig. 8d,which illustrates the optical micrographs of the 60/40blend upon cooling from the melt at 80 �C.

Based on the optical microscopy data, the dispersedphase of the 60/40 blend seems to be mainly constitutedby PLLA, whereas the co-continuous region, as well as thedispersed droplets within the embedded phase, appear tobe mainly constituted by PBT chains.

The thermal properties of the PLLA/PBT blends wereanalyzed by differential scanning calorimetry. DSC plotsof the compression-molded blends are illustrated inFig. 9. Plain PLLA curve displays a step, centered around62 �C, due to glass transition (Tg), an exothermic transitionascribed to cold crystallization upon heating, and a finalmelting endotherm with a small shoulder in the low tem-perature side due to growth of two different crystal modi-fications [39]. By subtracting the area under theexothermic peak from that under endothermic peak it isevidenced that neat PLLA after film pressing is fully amor-phous; this result is also confirmed by the heat capacity

(a) (b)

Fig. 7. Optical micrographs of the PLLA/PBT 80/20 blend: (a) 250 �C, parallel polarizers and (b) room temperature, crossed polarizers.

(a) (b)

(d)(c)

Fig. 8. Optical micrographs of the PLLA/PBT 60/40 blend during cooling at 4 �C/min: (a) 250 �C (parallel polarizers); (b) 210 �C (crossed polarizers); (c)200 �C (crossed polarizers) and (d) 80 �C (crossed polarizers).

3314 M.L. Di Lorenzo et al. / European Polymer Journal 49 (2013) 3309–3317

step at the glass transition, which corresponds to fullyamorphous PLLA [40].

Addition of PBT results in a shift to lower temperaturesof the cold-crystallization event of PLLA, due to nucleatingeffect of PBT crystals, detailed in the second article of thisseries [41], and in the appearance of a second endotherm,centered around 225 �C, due to fusion of PBT crystals.Moreover, the DSC traces of PLLA/PBT blends exhibit twoconsecutive weak endothermic and exothermic events at140–160 �C, immediately before PLLA main melting peak.These multiple transitions are well detailed in the litera-ture and are linked to the existence of different crystalstructures in PLLA that lead to the appearance of multiplemelting peaks [39,42].

Crystallinity of PBT does not depend on blend composi-tion: integration of the endothermic peaks, and normaliza-tion with respect to blend composition, discloses a heat offusion of PBT chains of 45 ± 3 J/g for the three blends con-taining PBT, which suggests that PLLA does not signifi-cantly affect crystallization of PBT. Some slight increasesof crystallinity of PLLA upon addition of PBT can insteadbe measured by DSC. The difference between the areas un-der the endothermic and exothermic peaks raises from 0 J/g in plain PLLA to 1.7 J/g in the 60/40 blend, and the in-crease is proportional to PBT content in the blend. Consid-ering that the melting enthalpy of PLLA is 93.6 J/g [43], theweak variation in the heat of fusion of the blends corre-sponds to an increase of PLLA crystallinity from 0% to 3%,

50 100 150 200

Hea

t flo

w ra

te

Temperature (°C)

100/0 90/10 80/20 60/40

1 W

/g^ exo

Fig. 9. Heat flow rate plots of PLLA/PBT compression-molded blends uponheating at 20 �C/min.

M.L. Di Lorenzo et al. / European Polymer Journal 49 (2013) 3309–3317 3315

which is close to the experimental uncertainty associatedto DSC analysis [44]. As detailed above, FTIR analysis re-veals that in the blends the PLLA chains, whose functionalgroups interact with the functional groups of PBT, aresemicrystalline. The degree of interaction between thefunctional groups of the two polyesters depends on theirratio in the blend, which may be linked to the weak in-crease of crystallinity of PLLA upon variation of blendcomposition.

The influence of blend composition on the glass transi-tion behavior is difficult to asses by DSC analysis at con-ventional heating rates, since the blend components havevery close glass transition temperatures [40,45]. To sepa-rate close transitions, a lower heating rate should be used,but at very low scanning rates, conventional DSC analysisusually has a poor signal to noise ratio. In order to improveoutput quality, temperature-modulated DSC (TMDSC) wasused. TMDSC analyses are conducted at much lower under-lying scanning rate, and provide a highly improved preci-sion in the analysis of reversing events, like the glasstransition, since all error signals not occurring within themodulation frequency are eliminated [46].

The reversing heat flow rate of PLLA/PBT blends, mea-sured by TMDSC at the underlying heating rate of 0.5 �C/

30 40 50 60 70

100/0 90/10 80/20 60/40 0/100

Rev

hea

t flo

w ra

te0.

02 W

/g

Temperature (°C)

^ exo

Fig. 10. Reversing heat flow rate plots of PLLA/PBT compression-moldedblends.

min, is shown in Fig. 10 and compared to the plain poly-mers. Plain PLLA presents a sharp glass transition, whichextends from 57 to 62 C. The slight shift in Tg, comparedto the data shown in Fig. 9, is due to the lower underlyingheating rate used in TMDSC analysis. PBT has a muchbroader glass transition, which starts from about 28 �Cand is completed around 55 �C. The step associated to theglass transition of PBT is much smaller than that of PLLA,due to the considerable initial crystallinity of PBT, accom-panied by vitrification of the rigid amorphous fraction,probed for both PLLA and PBT, which also contributes tothe decreased heat capacity jump at Tg [39,47]. ThePLLA/PBT 90/10, 80/20 and 60/40 blends display a sharpstep in the reversing heat flow rate curve, whose tempera-ture range perfectly overlaps the Tg step of plain PLLA. Aweaker step can also be discerned in the reversing heatflow rate plot of the PLLA/PBT 60/40 blend. This step occursat temperatures below the Tg of PLLA and above the glasstransition of plain PBT. In the blends with lower PBT con-tent (90/10 and 80/20) the glass transition of PBT dispersedphase cannot be determined, if present, because of the lowamount of PBT in the analyzed composition range. More-over, PBT crystallinity is quite high, which, coupled withvitrification of the rigid amorphous fraction, contributesto the weak step at Tg [47].

The shape and temperatures of glass transition of theblends provide information on miscibility of PLLA andPBT. In the analyzed composition range, the blends displayphase-separation, as probed by the optical and electronmicroscopy analyses detailed above. The invariance ofPLLA glass transition in the blends, compared to the plainpolymer, shows that the continuous phase of the 90/10and 80/20 blends is made of pure PLLA. In the 60/40 blend,the glass transition associated to PLLA overlaps that of theplain biodegradable polyester phase, in terms of both tran-sition temperature and width, which also indicates that thedispersed phase of the 60/40 blend is made of pure PLLA.Conversely, the weak and somewhat broad step in thereversing heat flow rate plot of the 60/40 blend is locatedat temperatures closer to the Tg of PLLA than that of PBT.At this composition, phase inversion occurs in the blends,since PBT becomes the continuous phase, and the blendspresents a co-continuous multi-level morphology, asshown above. The varied morphology, coupled with chem-ical interaction between the functional groups of the twopolyesters, may affect mobility of the phases, and resultin a shift of glass transition temperature with respect tothe expected values [48], as reported in the literature forother polymer pairs. In immiscible blends of isotactic poly-propylene and atactic polystyrene (iPP/aPS), the glass tran-sition of aPS was reported to vary with iPP content, due tointerface interactions between the two polymers [48,49].Similar increase in Tg was probed in poly(ethylene tere-phthalate)/bisphenol A polycarbonate (PET/PC) blends,caused by the presence of a rigid, glassy polycarbonate ma-trix domain when PET goes through its glass transition,which produces a friction at the interface/wall betweenthe two phases [50]. Analogous effects were also probedin immiscible multi-block copolymers, where the immisci-ble components are linked by chemical bonds to nano-phase-separated layers, and the structure and mobility of

3316 M.L. Di Lorenzo et al. / European Polymer Journal 49 (2013) 3309–3317

each phase affect the other, causing shifts of the glass-tran-sition temperatures [51].

The results of tensile tests on PLLA/PBT blends are sum-marized in Table 1. Plain PLLA displays a Young’s modulusof 2.6 GPa and an elongation at break of about 3%, in agree-ment with literature data [52–54]. The addition of 10% or20% of PBT does not affect remarkably the mechanicalproperties of the material, since the small diminution ofboth modulus and elongation at break appear as minorvariations that almost lay within the experimental error.Conversely, the PLLA/PBT 60/40 blend shows sizably differ-ent mechanical properties, as it can sustain a much higherelongation before rupture (160%), although the elasticmodulus is comparable to plain PLLA and to the 90/10and 80/20 blends. As probed by the optical microscopyanalysis discussed above, the PLLA/PBT 60/40 blend dis-plays a co-continuous morphology where the PLLA parti-cles are embedded in a PBT matrix, whereas in the otheranalyzed blend compositions, the matrix is made by PLLA.

The mechanical properties of crystallizable polymerblends depend markedly on composition, morphologyand on the level of phase segregation that result from crys-tallization and potential phase separation processes in theamorphous phase. In the case of compatible blends or mul-ti-component polymer systems presenting amorphousphase separation besides the crystalline phase, the large-strain properties such as the elongation at break are linkedto the adhesion between the phases present in the mate-rial. Conversely, the low-strain properties, like Young’smodulus, are much less dependent on compatibilizationof the components, being mostly additive [55]. In otherwords, in non-miscible or partially miscible polymer pairs,where, besides the crystalline phase, two or more amor-phous phases are present, good properties at rupture areexpected if domains are small in size and well dispersedin the phases. The optical and scanning electron micro-graphs of the PLLA/PBT blends, illustrated in Figs. 2, 6and 7, show that the two amorphous phases have good le-vel of homogeneity. Some degree of interaction betweenthe functional groups of PBT and PLLA was also probedby the FTIR analysis discussed above, which provides com-patibilization of the components. The good adhesion be-tween the phases results in practically constantmechanical properties for PLLA/PBT blends containing upto 20% PBT, as the main mechanical parameters are mostlydetermined by the properties of the PLLA matrix. The 60/40blend, instead, displays a remarkably different morphol-ogy, with a co-continuous PBT phase that embeds PLLAparticles, which, in turn have a multi-level structure, asprobed in Figs. 3 and 7. The enormously increased resis-tance to rupture is to be linked to the varied matrix, as

Table 1Mechanical parameters of PLLA/PBT blends: Young’s modulus (E), elonga-tion at break (er), stress at break (rr).

PLLA/PBT E (GPa) er (%) rr (MPa)

100/0 2.6 ± 1 2.9 ± 0.5 51 ± 590/10 2.5 ± 1 2.6 ± 0.4 52 ± 580/20 2.4 ± 1 2.4 ± 0.4 53 ± 560/40 2.4 ± 1 160 ± 20 33 ± 3

PBT is much more ductile than PLLA and can sustain elon-gation before rupture >200% when subjected to stress–strain tests [56].

5. Conclusions

Poly(L-lactic acid) and poly(butylene terephthalate) arenot miscible. Melt mixing induces the establishment ofinteractions between the functional groups of PLLA andPBT, which provide compatibilization of the blend and re-sult in a good dispersion and adhesion of the phases. Thetensile properties of polymer blends are very sensitive tointerfacial adhesion [57,58]. A poor interface behaves as aflaw, and the failure initiates at the interface, which resultsin low tensile strength and elongation at break. This is notthe case of PLLA/PBT blends, where the in situ compatibili-zation attained upon melt mixing ensures favorablemechanical response when the blends are subjected todeformation.

Some peculiarities were highlighted in the morphologyof the blends. At low PBT content, the PLLA matrix embedsfinely dispersed PBT particles. When PBT content is raisedto 40%, the blend presents a co-continuous multi-levelmorphology, where PLLA domains, containing dispersedPBT units, are embedded in a PBT matrix. The varied mor-phology affects the mechanical response, mostly due to thevaried matrix of the material.

Acknowledgements

Ms. Maria Cristina Del Barone and Dr. Barbara Immirziof ICTP-CNR are gratefully acknowledged for their valuableassistance in SEM analyses and GPC measurements,respectively.

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