Compatibility Characterization of Poly(lactic acid)/Poly(propylene carbonate) Blends

XIAOFEI MA, JIUGAO YU, NING WANG

School of Science, Tianjin University, Tianjin 300072, China

Received 18 July 2005; revised 26 September 2005; accepted 26 September 2005DOI: 10.1002/polb.20669Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: The compatibility of poly(lactic acid) (PLA)/poly(propylene carbonate) (PPC)blends was investigated with Fourier transform infrared (FTIR) spectroscopy, differ-ential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and tensiletesting. The PLA/PPC blends were prepared over the whole composition range. FTIRspectroscopy revealed that there were several specific interactions between the chainsof PLA and PPC: the interaction between C��H and O¼¼C�� and C¼¼O���O¼¼C orC¼¼O���O��C dipole–dipole interactions. Moreover, PLA and PPC were compatible.DSC indicated that PLA and PPC were partially miscible but compatible to someextent because of the similar chemical natures of the blend components. TGA showedthat the compatibility of PLA and PPC enhanced the thermal stability of PPC in theblends. As calculated by the Horowitz–Metzger equation, the activation energy fordecomposition (Et) of PPC in PLA/PPC (70/30) was 200.6 kJ/mol, whereas Et of purePPC was only 56.0 kJ/mol. A study of the mechanical properties versus the composi-tion and the strain versus the stress illustrated that there was good compatibilitybetween PLA and PPC, and the phase inversion of the PLA/PPC system occurredbetween 70 and 60 wt % PLA in the PLA/PPC blends. The Pukanszky model gavecredit to very strong interfacial adhesion between PLA and PPC. VVC 2005 Wiley Periodi-

cals, Inc. J Polym Sci Part B: Polym Phys 44: 94–101, 2006

Keywords: blends; compatibility; extrusion; poly(lactic acid); poly(propylene carbonate)

INTRODUCTION

Environmental concerns and a shortage of petro-

leum resources have driven efforts aimed at the

bulk production of biodegradable materials.1

Poly(lactic acid) (PLA) is a linear aliphatic poly-

ester produced from renewable resources and is

readily biodegradable. PLA is synthesized by the

ring-opening polymerization of lactides and lac-

tic acid monomers, which are obtained from the

fermentation of sugar feed stocks.2 PLA is a

thermoplastic, high-strength, high-modulus poly-

mer.3 PLA homopolymers have a glass-transition

temperature (Tg) and melting temperature of about

55 and 175 8C, respectively. They require processing

temperatures in excess of 185–190 8C. At these

temperatures, unzipping and chain-scission re-

actions leading to a loss of molecular weight, as

well as thermal degradations, are known to

occur. Consequently, PLA homopolymers have a

very narrow processing window. The most widely

used method for improving PLA processability is

based on melting point depression by the random

incorporation of small amounts of lactide enan-

tiomers of the opposite configuration into the poly-

mer (i.e., adding a small amount of D-lactide to

L-lactide to obtain poly(D-lactide and L-lactide)

(PDLLA). The melting point depression is accom-

panied by a significant decrease in the crystallin-

ity and crystallization rates.3 However, the low

Correspondence to: J. G. Yu (E-mail: makong2001@hotmail.com)

Journal of Polymer Science: Part B: Polymer Physics, Vol. 44, 94–101 (2006)VVC 2005 Wiley Periodicals, Inc.

94

deformation at break of PLA still limits its ap-

plication.2

One potential method of overcoming this diffi-culty is blending PLA with other thermoplastics.Numerous studies on the miscibility of polymerblend systems have also been devoted to PLA/biode-gradable polyester blends. Polycaprolactone (PCL)is reactively blended with PLA.4 Mechanical prop-erty measurements indicate that the elongation ofthe PLA/PCL blends is improved significantly, anda synergism is observed for certain compositions(PLA/PCL ¼ 80/20 or 20/80). Poly(3-hydroxybuty-rate-co-3-hydroxyhexanoate) [P(3HB-co-3HHx); con-centration of 3-hydroxyhexanoate ¼ 13.4 mol %]has the potential to increase the toughness of PLAdramatically, thereby enlarging the design space ofsustainable materials achievable by the blending ofP(3HB-co-3HHx) with PLA. Infrared (IR) spectraprovide insights into the submolecular-level interac-tions and miscibility of this polymer blend system.5

Naoyuki and Yoshiharu6 revealed that the structureof poly(3-hydroxybutyric acid) (P3HB)/PLA blendsis strongly dependent on the molecular weight ofthe poly(L-lactide) (PLLA) component. Blends ofP3HB with PLA of high weight-average molecularweight (Mw) values (>20,000) show two phases inthe melt at 200 8C, whereas blends of P3HB withPLA of low Mw values (<18,000) are miscible in themelt over the whole composition range. PLA andpoly(hydroxy ester ether) are also miscible.7

Poly(propylene carbonate) (PPC) is also a bio-degradable aliphatic polycarbonate. In the pres-ence of a heterogeneous catalyst system, propy-lene oxide and carbon dioxide produce a polymerinvolving the regular alternating copolymerPPC.8,9 PPC is an amorphous polymer and has achemical structure similar to that of PLA. Thechemical structures of PLA and PPC are shownin Figure 1. In this study, the compatibility ofmelt-mixed blends of PLA/PPC in the completecomposition range was characterized mainlywith Fourier transform infrared (FTIR) spectro-scopy, differential scanning calorimetry (DSC),scanning electron microscopy (SEM), thermogra-vimetric analysis (TGA), and tensile testing.

EXPERIMENTAL

Materials

PLA was obtained from Natureworks LLC (UnitedStates). The general molecular weight average wasabout 160,000–220,000. The concentration of the D-(�)-isomer was 12.0 6 1.0%. PPC was obtainedfrom the Inner Mongolia Meng Xi High-Tech Group(China). The general molecular weight average wasabout 70,000–110,000 (weight-average molecularweight/number-average molecular weight¼ 4.3).

Blend Preparation

The blends were mixed in an SJ-25(s) single-screw plastic extruder (Axon Australia Pty, Ltd.,Australia) with a screw diameter of 30 mm anda length-to-diameter ratio of 25:1. The tempera-ture profile along the extruder barrel was 110/115/120 8C, and the temperature at the die was90 8C. The screw speed was 20 rpm. The diewas a round sheet with 3-mm-diameter holes.

FTIR Spectroscopy

FTIR spectra were obtained at a 2-cm�1 resolu-tion with a Bio-Rad FTS3000 IR spectrum scan-ner. Typically, 64 scans were signal-averaged toreduce spectral noise. The extruded thermoplas-tic starch (TPS) strips were pressured into trans-parent slices with a thickness of around 0.2 mmin a flat sulfuration machine tested by the trans-mission method.10

DSC

A Netzsch DSC 204 differential scanning calorime-ter was employed to measure Tg of the blends. Thespecimens were heated from 0 to 100 8C at a rate of10 8C/min. The midpoints of the transitions in thetraces recorded in the heating scan were taken asthe values of Tg. The specimen weight was 8–10 mg.

TGA

TGA of the blends were measured with a ZTY-ZP-type thermal analyzer. The sample weightvaried from 10 to 15 mg. The samples wereheated from room temperature to 500 8C at aheating rate of 15 8C/min.

Mechanical Testing

Samples [8 cm � 3-mm diameter (/)] were cut fromthe extruded strips. A Testometric AX M350-10KNmaterials testing machine, operated at a crossheadspeed of 50 mm/min, was used for tensile testingFigure 1. Chemical structures of PLA and PPC.

COMPATIBILITY CHARACTERIZATION 95

(ISO 1184-1983 standard). An average of five toeight bars was recorded for every sample.

SEM

The fracture surfaces of the samples were stud-ied with a Philips XL-3 scanning electron micro-scope operated at an acceleration voltage of20 kV. The samples were cooled in liquid nitro-gen and then broken. The fracture surfaces werevacuum-coated with gold for SEM.

RESULTS AND DISCUSSION

FTIR

The interaction of polymer blends can be identi-fied with FTIR spectra. If two polymers formcompletely immiscible blends, there are no ap-

preciable changes in the FTIR spectra of theblends with respect to the coaddition of eachcomponent.11 However, if two polymers are com-patible, a distinct chemical interaction (a hydro-gen-bonding or dipolar interaction) exists be-tween the chains of one polymer and those ofthe other, causing the IR spectra for the blendto change (e.g., band shifts and broadening).12

As a result, FTIR can identify segment interac-tions and provide information about the phasebehavior of polymer blends.

Figure 2 shows the FTIR spectra of PLA/PPCblends at room temperature in several specificstretching regions. The peak band wave num-bers and assignments for PLA and PPC IR spec-tra are listed in Table 1.

In the 3700–3100-cm�1 region, the absorptionof PPC and PLA is negligible because of therareness of terminal O��H groups. With the ad-

Figure 2. FTIR spectra of PLA/PPC blends.

96 MA, YU, AND WANG

dition of PPC, the peak band of PLA in the Aregion, ascribed to the ��CH�� stretching vibra-tion, changes greatly when the PPC concentra-tion varies from 0 to 100%; this suggests somechanges in the intermolecular interactions dueto blending. The electron attraction of ester inPLA and carbonic ester in PPC lowers the den-sity of the electron cloud of methine hydrogen,making an interaction possible between C��Hand O��C�� or between C��H and O¼¼C��.

Both PLA and PPC have a strong carbonylstretching absorption in the B region. With theaddition of PPC, the C¼¼O peak of PLA at 1749cm�1 shifts to a lower wave number (by 11 cm�1)when the PPC concentration increases from 0 to100%. The interaction formation reduces thestretching vibration frequency of the carbonylbond and gives rise to a shift to a lower frequency,as extensively studied for poly(vinylphenol)/PCLblends.13 This means that carbonyl groups alsotake part in the interaction between PLA andPPC and result in the blueshift of the C¼¼O vibra-tion of PLA. One reason is the interactionbetween C��H and O¼¼C��, as mentioned previ-ously. Another reason is C¼¼O���O¼¼C orC¼¼O���O��C dipole–dipole interactions, similar tothe dipole–dipole C¼¼O���Cl��C interaction sug-gested by Allard and Prudhomme14 for PCL/chlorinated polypropylene blends.

According to Fei et al.,15 the ��O��C��O��stretching vibration appears at 1223 cm�1, whichdeviates from the i line in Figure 2 with decreasingPPC contents. The C and D regions in Figure 2,separately related to the ��C��O�� stretchingvibration in the ��CH��O�� group and ��O��C¼¼O group, show that the ��C��O�� stretchingvibration in the ��CH��O�� group, appearing at17 cm�1, blueshifts from pure PLA to PPC,whereas the main peak of the ��C��O�� stretchingvibration in the ��O��C¼¼O group also has a 17-cm�1 blueshift with decreasing PPC contents. TheC¼¼O���O��C dipole–dipole interactions and theinteraction between C��H and O��C�� exist.

In the FTIR spectra of PLA/PPC blends, theshifting and broadening of some listed bands for

PLA and PPC indicate that there are severalspecific interactions between the chains of PLAand PPC, which are compatible.

DSC

Figure 3 shows the DSC traces of PLA/PPCblends. A DSC analysis of the blends clearly in-dicates two Tg values between PLA and PPC.The spectra are commonly observed for partiallymiscible polymer pairs.

There is a difference in the convergence ofthe main Tg between PLA and PPC. Tg of PLAchanges slightly from 57 to 54 8C for the PLA/PPC (30/70) blend, being practically independentof the nominal composition, as shown in Table 2,whereas Tg of PPC shifts greatly from 22 to43 8C for the PLA/PPC 70/30 blend. For the 70/30 PLA/PPC composition, the difference betweenthe Tg values of PLA and PPC is the lowest.The DSC results support the view of a partiallymiscible blend in the temperature regime inwhich this method is applied. This is supportedby the considerable component Tg convergencein the blend, particularly that of PPC. This indi-cates that PLA/PPC blends are partially misci-ble but compatible to some extent because of the

Table 1. Peak Band Wave Numbers (cm�1) and Assignments for PLA and PPC IR Spectra

Assignment ��CH����C¼¼OCarbonyl ��O��C��O��

��C��O��in ��CH��O��

��C��O��in ��O��C¼¼O

PLA (cm�1) 2997, 2949 1749 — 1182 1127, 1082, 1044PPC (cm�1) 2986 1738 1223 1165 1124, 1065Symbol A region B region i line C region D region

Figure 3. DSC traces of PLA/PPC blends.

COMPATIBILITY CHARACTERIZATION 97

similar chemical natures of the blend compo-nents; this is similar to the results for polycar-bonate/copolyester blends.16

TGA

TGA has been performed for the blends, and theweight loss due to the volatilization of the deg-radation products has been monitored as a func-tion of temperature, as shown in Figure 4. T5%,which is the temperature corresponding to 5%weight loss determined from Figure 4, is sum-marized in Table 3. T5% of the PLA/PPC blendsincreases from 202 to 351 8C as the concentra-tion of PLA increases from 0 to 100 wt %. PPCshows low thermal stability. Dong et al.17 re-ported that PPC is easily decomposed to cyclic car-bonate at a temperature of only about 180 8C.Meng et al.18 found that PPC pyrolysis obeysa two-step pyrolysis mechanism: main-chainrandom scission and unzipping. The addition ofPLA improves the thermal stability of PPC.Derivative thermogravimetry (DTG) curves forPLA/PPC blends in Figure 5 show the Tmax val-ues of PPC and PLA in PLA/PPC blends, whichalso are listed in Table 3. Here Tmax is the tem-perature at the maximum rate of weight loss,that is, the decomposition temperature. Tmax ofPPC and PLA in blends is located between thoseof pure PPC and PLA. Tmax of PPC in the PLA/PPC (70/30) blend reaches 551.55 K, in compari-son with 528.85 K for pure PPC. This Tmax

improvement of PPC is ascribed to the compati-bility of PLA and PPC in blends. However, Tmax

of PLA in PLA/PPC blends decreases withincreasing contents of PPC. Tmax of PLA de-creases from 675.95 K for pure PLA to 570.35 Kfor the PLA/PPC (30/70) blend. The degradationproducts of PPC possibly facilitate the thermaldegradation of PLA.

The activation energy of decomposition, Et, ofthe polymer blends can be calculated from the

TGA curves by the integral method proposed byHorowitz and Metzger with eq 1:19

ln½lnð1� aÞ�1� ¼ Eth=RT2 max ð1Þ

where a is the decomposed fraction, T is temper-ature, h is T � Tmax, and R is the gas constant.From the plots of ln[ln(1 � � a)�1] versus h,which are shown in Figure 6, Et can be deter-mined from the slope of the straight line of theplots. As demonstrated in Table 3, the Et valuesof both PLA and PPC in blends are raised asthe PLA concentration increases from 0 to 70%.Et of PPC in PLA/PPC (70/30) is 200.6 kJ/mol,whereas Et of pure PPC is only 56.0 kJ/mol.In view of Et, PLA (228.8 kJ/mol) in PLA/PPC(70/30) is even more thermally stable than purePLA (213.9 kJ/mol). This is attributed to thespecific interactions between the chains of PLAand PPC, which are similar to poly(vinyl chlor-ide)/ethylene–vinyl acetate copolymer blends.20

Although the degradation products of PPCfacilitate the thermal degradation of PLA atlower temperatures, the specific interactionsbetween the chains of PLA and PPC improvethe activation energy of PLA decomposition.

Mechanical Properties

Figure 7(A) illustrates the yield-stress/composi-tion relationship for the PLA/PPC blends. Somestudies have suggested that the yield behaviorof polymer blends is affected by the interfacialadhesion.21–23 Pukanszky and coworkers21–23

proposed upper and lower values for the yieldstress in cases of extreme values of interfacial

Table 2. Tg’s of PLA/PPC Blends

PLA/PPC

Tg (8C)

PLA PPC

100/0 57 —70/30 56 4350/50 55 3330/70 54 290/100 — 22

Figure 4. Thermogravimetric curves for PLA/PPCblends at a heating rate of 15 8C/min in air.

98 MA, YU, AND WANG

adhesion. When the interfacial adhesion isstrong enough for stress transfer to occurbetween two phases, the yield stress obeys thelaw of mixtures (the upper value):

rb ¼ r1/1 þ r2/2 ð2Þ

where b is the blend, r is the yield stress andsubscripts 1 and 2 refer to component 1 (PLA)and component 2 (PPC), respectively.

In the case of a lack of interfacial adhesion,the dispersion of the minor component only re-sults in a reduction of the load-bearing crosssection of the matrix. The yield stress in thencalculated with eq 4 (the lower value):

r0b ¼ rm1� /d

1þ 2:5/d

ð3Þ

where superscript 0 denotes zero interfacialadhesion, m is the matrix or continuous phase,and d is the dispersed phase. Figure 7(A) com-pares the experimental data (full line and fullcircles) with the predictions for extreme inter-

facial adhesion. The dotted line has been plot-ted with eq 2 (strong interfacial adhesion),whereas the dashed–dotted line has been plottedwith eq 3 (no interfacial adhesion). The PLA/PPC blends have a significant positive devi-ation above 30 wt % PPC, whereas the PLA/PPC blends basically follow the additive rulespredicted by eq 2 below 30 wt % PPC. ThePukanszky model gives credit to very stronginterfacial adhesion between PLA and PPC.

Young’s modulus of polymer blends mainlydepends on the modulus of each constitutivecomponent and blend composition. It is alsosomewhat affected by the interracial interac-tions and changes in the phase morphology.21

Figure 7(B) shows that this general behavior isconfirmed by the PLA/PPC blends. When PPC isthe continuous phase at high PPC concentra-tions (>40 wt %), PLA/PPC blends show a posi-tive deviation in the modulus with respect tothe weight average values (dotted line). Whenthe PPC concentration is less than 30 wt %,PLA becomes the continuous phase in PLA/PPCblends, which show a negative deviation in the

Figure 5. DTG curves for PLA/PPC blends at aheating rate of 15 8C/min in air.

Figure 6. Plots of ln[ln(1 � a)�1] versus h for thedetermination of Et. The straight line is the linear fitof the data points.

Table 3. Thermogravimetric Parameters of PPC and PLA Blends

PLA/PPC(w/w)

T5%

(8C)PPC

Tmax (K)PLA

Tmax (K)Slopeof Eq 1

PPC Et

(kJ/mol)PLA Et

(kJ/mol)

0/100 202 528.85 — 0.0241 56.0 —30/70 229 534.85 570.35 0.0589 140.1 159.350/50 243 550.55 580.25 0.0598 150.7 167.470/30 256 551.55 589.05 0.0793 200.6 228.8100/0 351 — 675.95 0.0563 — 213.9

COMPATIBILITY CHARACTERIZATION 99

modulus. The interfacial adhesion is compara-tively strong when PPC is the continuous phase.The phase inversion of the two-phase systemoccurs between 30 and 50 wt % PPC.

As shown in Figure 7(C), the break stress ofPLA/PPC blends well follows the additive law(dotted line) at high PPC contents. The beneficialeffect of the PLA/PPC interfacial adhesion couldalso account for the negative deviation observedin the dependence of the break stress on the PPCcontent [Fig. 7(C)]. Indeed, the break stress ofPLA is much higher than that of PPC, so the con-tinuous PPC phase is reinforced by PLA as effi-ciently as the interfacial adhesion is high.

The break energy shows a remarkable posi-tive deviation that corresponds to a synergismover more than 85% of the composition range[Fig. 7(D)]. The break energy is known to resultfrom a complex interplay of several experimen-tal parameters, such as the phase morphology,relative modulus of the phases, chain structure,and interfacial adhesion. The continuous PPCphase results in a lower yield stress and thusfavors the matrix yielding, which requires moreenergy to break the materials.24

As shown in Figure 8, the stress–strain curvesof PLA/PPC blends have similar rubber plateaus.

Both the number and height of the rubber pla-teau vary with the PLA concentration. At a lowPLA concentration (15 wt %), there is only oneplateau. When the PLA concentration reaches 30wt %, double plateaus appear. The first one isrelated to the tensile strain of the PPC matrixwith a low height, whereas the second one is re-lated to the tensile strain of PLA with a highheight. Because of the good compatibility, the PPCmatrix induces PLA (dispersion phase) yielding.

Figure 7. Mechanical properties versus the composition for PLA/PPC blends: (A)yield stress, (B) Young’s modulus, (C) break stress, and (D) break energy.

Figure 8. Strain–stress curves of PLA/PPC blends.

100 MA, YU, AND WANG

With increasing PLA concentration, the heightof the plateaus increases. When the PLA concen-tration reaches 60 wt %, the second plateau isobviously shortened. When the PLA concentra-tion reaches 70 wt %, only one plateau exists,and the strain descends rapidly to 35%. Herephase inversion of the two-phase system occurs,as shown in Figure 9. The PLA matrix issmoother than the PPC matrix. The PLA matrixhas a much lower tensile strain but strongerstress than the PPC matrix.

From the results of the mechanical propertiesversus the composition and the strain versus thestress, it can be concluded that there is good com-patibility between PLA and PPC, and phase in-version of the PLA/PPC system occurs between70 and 60 wt % PLA in PLA/PPC blends.

CONCLUSIONS

FTIR, DSC, TGA, and mechanical propertiesillustrate that PLA and PPC are partially misci-ble but are compatible to some extent because ofthe similar chemical natures of the blend com-ponents. The compatibility of PLA and PPCenhances Tg and Tmax of PPC in blends. Thereare several specific interactions between thechains of PLA and PPC. Between 70/30 and 60/40 PLA/PPC compositions, the phase inversionof the PLA/PPC system occurs.

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Figure 9. SEM micrographs of cryofractured PLA/PPC blends: (a) 70/30 PLA/PPCand (b) 60/40 PLA/PPC.

COMPATIBILITY CHARACTERIZATION 101