COMPOSITES

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Composites Science and Technology 66 (2006) 2360–2366

SCIENCE ANDTECHNOLOGY

Properties of biodegradable poly(propylene carbonate)/starchcomposites with succinic anhydride

Xiaofei Ma, Jiugao Yu *, Ang Zhao

School of Science, Tianjin University, Tianjin 300072, China

Received 12 July 2005; received in revised form 26 October 2005; accepted 25 November 2005Available online 10 January 2006

Abstract

Biodegradable composites of poly(propylene carbonate) (PPC) reinforced with granular cornstarch are prepared in a single screwextruder. The effects of succinic anhydride (SA) on the morphology, thermal properties, as well as mechanical properties of PPC/starchcomposites, are investigated. Scanning electron microscope (SEM) shows that starch surface becomes coarse, the interface is not clear andcompatibility is increased when SA is added. Fourier transform infrared (FT-IR) Spectroscopy reveals that SA can improve the interactionbetween PPC and starch. Thermogravimetric analysis (TGA) results show that SA leads a significant improvement of thermal stability forPPC/starch composites. Mechanical testing illustrates that SA can increase mechanical properties of PPC/starch composites. The yieldstresses of PPC/starch composites without SA and with SA are, respectively, 19.20 and 22.94 MPa. SA enhances the properties ofPPC/starch composites, which are all ascribed to the improvement of the interaction between PPC and granular starch at the existenceof SA.� 2005 Elsevier Ltd. All rights reserved.

1. Introduction

Much effort [1–3] has recently been made to develop bio-degradable materials because of the worldwide environ-ment problems resulted from petroleum-derived plastics.Many renewable resource-based biopolymers such asstarch plastics, cellulose plastics [4], polylactides, polyhy-droxyalkanoates (bacterial polyesters) [5], and soy-basedplastics [6,7] have been investigated to alternate conven-tional non-degradable or incompletely degrading syntheticpolymers (e.g. polyolefin) in the application scopes of one-off materials [8]. Poly(propylene carbonate) (PPC) is a bio-degradable aliphatic polycarbonate. At the existence ofheterogeneous catalyst system, propylene oxide and carbondioxide produce a polymer involving the regular alternat-ing copolymer PPC [9,10], which can be used as adhesives,solid electrolytes, polyols, photoresists, barrier materials,flexibilizers and plasticizers [11]. However, such materials

0266-3538/$ - see front matter � 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.compscitech.2005.11.028

* Corresponding author. Tel.: +86 22 27406144; fax: +86 22 27403475E-mail address: maxiaofei@tju.edu.cn (J. Yu).

are generally easy to decompose under the influence of heatand catalysts, which is resolved by end-capping using dif-ferent active agents. [12] And their mechanical propertiesneed to be improved by blending with other polymers toform polymer composites [11].

Starch is one of the promising raw materials for the pro-duction of biodegradable plastics [13]. Starch is renewableand biodegradable from a great variety of crops. Since1970s starch has been incorporated into polyethylene inorder to increase the biodegradability. Biodegradable com-posites of poly(propylene carbonate) (PPC) reinforced withunmodified cornstarch have been studied by Peng et al. [14]and Ge et al. [15]. In this paper, succinic anhydride (SA) asan active agent is introduced to PPC/starch composites. Onthe one hand, it is expected that SA can end-cap PPC toimprove thermal stability. On the other hand, succinic anhy-dride is prone to react with hydroxyl groups in starch, intro-duce ester groups into starch and improve the compatibilitybetween PPC matrix and starch. In this paper, the morphol-ogy, FT-IR, thermal properties and mechanical propertiesof PPC/starch composites are investigated. SA increases

X. Ma et al. / Composites Science and Technology 66 (2006) 2360–2366 2361

the interaction between PPC and starch, which results in theimprovement of starch dispersion, thermal stability andmechanical properties in PPC/starch composites.

2. Experimental section

2.1. Materials

PPC is obtained from State Key Polymer Physics andChemistry Laboratory, Changchun Institute of AppliedChemistry, China. Its molecular weight measured byGPC is 130,000 and Mw/Mn = 4.3. Cornstarch (11% mois-ture) was obtained from Langfang Starch Company(Langfang, Heibei, China). Succinic anhydride (SA) is pur-chased from Tianjin Chemical Reagent Factory (Tianjin,China). SA is analytical reagent.

2.2. Composite preparation

Starch samples were dried at 115 �C for 4 h to elimi-nate water. PPC is blended (3000 rpm, 2 min) with starchor/and SA by use of High Speed Mixer GH-100Y (madein China). The composites are mixed in the single screwPlastic extruder SJ-25(s) (Axon Australia Pty. Ltd., Aus-tralia) with a screw diameter of 30 mm and a length todiameter ratio of 25:1. The temperature profile alongthe extruder barrel was 110/115/120 �C, and the tempera-ture at the die is 90 �C. The screw speed is 20 rpm. Thedie is a round sheet with the diameter 3 mm holes. Theoptimum content of SA is 1 wt/gross of PPC and starch.The superfluous SA can result the degradation of PPCduring the processing.

2.3. Fourier transform infrared (FT-IR) spectroscopy

FT-IR spectra are obtained at 2 cm�1 resolution withBIO-RAD FTS3000 IR Spectrum Scanner. Typically, 64scans are signal-averaged to reduce spectral noise. Theextruded TPS strips are compressed to the transparentslices with the thickness of around 0.2 mm in the Flat Sulf-uration Machine, tested by the transmission method [16].

2.4. Scanning electron microscope (SEM)

The cryo-fractured surfaces of extruded PLA/PPC com-posites are performed with Scanning Electron MicroscopePhilips XL-3 (FEI Company, Hillsboro, Oregon, USA),operating at an acceleration voltage of 20 kV. The compos-ites are cooled in liquid nitrogen, and then broken. Thefracture surfaces are vacuum coated with gold for SEM.

2.5. Thermogravimetric analysis (TGA)

The thermal properties of the composites are measuredwith a ZTY-ZP type thermal analyzer. The sample weightvaries from 10 to 15 mg. Samples are heated from the roomtemperature to 500 �C at a heating rate of 15 �C/min.

2.6. Mechanical testing

Samples 8 cm · B3 mm in size are cut from the extrudedstrips. The Testometric AX M350-10KN Materials TestingMachines operates and a crosshead speed of 100 mm/minis used for tensile testing (ISO 1184-1983 standard).

3. Results and discussion

3.1. Morphology

The morphology structure of polymer composites is avery important characteristic because it ultimately deter-mines many properties of the polymer composites, suchas thermal stability and mechanical properties. In the pres-ent study, starch functioned as the filler because the gran-ular structure of starch is retained after extrusion and ishomogeneously dispersed in the PPC matrix (shown inFig. 1). There are the smooth surface of corn starch andthe distinct interfacial appearance between corn starchand PPC. Therefore, their interfacial tension is large andtheir interfacial adhesion is low, which agrees with recentresults of Liu et al. [17] and Sailaja et al. [18] for polyeth-ylene and granular starch composites.

The morphology of cryo-fractured surfaces is shown inFig. 1. At the existence of SA (Fig. 1(c) and (d)), the dis-tinction between corn starch and PPC is not as clear as thatof the composites without SA (Fig. 1(a) and (b), and thesurface of corn starch becomes coarse. These characteris-tics are typical of compatibility, suggesting the occurrenceof good interaction between starch and PPC.

PPC is an aliphatic polycarbonate, which is hydropho-bic. Starch is a multi-hydroxyl polymer with three hydroxylgroups per monomer, which is hydrophilic. SA improvesthe interfacial adhesion between PPC and corn starch,and the improved interfacial adhesion results in increasedcompatibility. The improved interfacial adhesion attributesto the strong chemical and physical interaction. Besides theend-capped reaction between PPC and SA, the chemicalinteraction presumably results from reaction of hydroxylgroups in starch with anhydride groups in SA [19] underthe extrusion conditions of high temperature and highshear. This reaction makes starch surface become coarse,as shown by arrows in Fig. 1(d). The strong physical inter-actions occur between PPC and SA modified starch,between SA end-capped PPC and starch, between SAend-capped PPC and SA modified starch. When SA is sit-uated at the interface between starch and PPC and inter-acted with both, the interfacial tension is reduced andcompatibility is increased.

3.2. FT-IR

The interaction of polymer composites can be identifiedby the FT-IR spectra. On the basis of the harmonic oscil-lator model the reduction in force constant f can be repre-sented by [16]

Fig. 1. SEM micrographs of cryo-fractured PPC/starch and PPC/starch/SA composites: (a) PPC/starch (70/30), 500·; (b) PPC/starch (70/30), 2000·; (c)PPC/starch/SA (70/30/1), 500· and (d) PPC/starch/SA (70/30/1), 2000·.

2000 1900 1800 1700 1600 1500

1745.58

1747.5

1749.43

1741.43

1739.5

Tra

nsm

ittan

ce

Wavenumber (cm-1)

e

d

c

b

aa: PPCb: PPC/starch 70/30c: PPC/starch 50/50d: PPC/starch/SA 70/30/2e: PPC/starch/SA 50/50/2

Fig. 2. The FT-IR spectra of PPC and PPC/starch composites.

2362 X. Ma et al. / Composites Science and Technology 66 (2006) 2360–2366

Df ¼ fb � fnb ¼lðm2

b � m2nbÞ

4p2ð1Þ

where l = m1m2/(m1 + m2) corresponded to the reducedmass of the oscillator, m the oscillating frequency and f

the force constant. The subscripts b and nb denote bondedand non-bonded oscillators, respectively. The reduction offorce constant brought about by some interaction is di-rectly related to the frequency (or wave number) shift ofstretching vibrations. Thus, the lower the wave numbercorresponding to absorption peak, the stronger is thehydrogen bond interaction between polymer composites.

Fig. 2 shows the spectra of PPC and PPC/starch compos-ites without or with SA at room temperature in the carbonylstretching region. PPC had a strong carbonyl stretchingabsorption at about 1750 cm�1. With the increase of starchcontent in PPC/starch composites without SA, the absorp-tion peak shifts towards lower wave number. For 70/30PPC/starch composite, the wave number corresponding toabsorption peak is 1747.5 cm�1, about 2 cm�1 lower thanthat for pure PPC. For 50/50 PPC/starch composite, thewave number corresponding to absorption peak is1745.58 cm�1, about 4 cm�1 lower than that for purePPC. Because starch is a multi-hydroxyl polymer with threehydroxyl groups per monomer, the shift of carbonyl stretch-ing absorption to lower wave number is ascribed to theinteraction between carbonyl groups of PPC and hydroxylgroups of starch by hydrogen bonding. [14,15] At the exis-tence of SA, with the increasing of starch content in PPC/starch/SA composites the absorption peak shifts moretowards lower wave number. For 70/30 PPC/starch/SA

composite, the wave number of absorption peak is1741.43 cm�1, 8 cm�1 lower than that for pure PPC. For50/50 PPC/starch/SA composite, the wave number ofabsorption peak is 1739.5 cm�1, about 10 cm�1 lower thanthat for pure PPC. Obviously, SA can improve the interac-tion between PPC and starch.

3.3. Thermogravimetric analysis (TGA)

Thermogravimetric analysis (TGA) is performed for thecomposites, where the weight loss due to the volatilizationof the degradation products is monitored as a function of

X. Ma et al. / Composites Science and Technology 66 (2006) 2360–2366 2363

temperature. The thermogravimetric (TG) and derivativethermogravimetric (DTG) curves of pure PPC, SA end-capped PPC, PPC/starch and PPC/starch/SA in air at aheating rate of 15 �C/min are shown in Figs. 3 and 4,respectively. The decomposed temperature, Tmax is thetemperature at maximum rate of weight loss, i.e., the peaktemperature shown in Fig. 4. It can be seen that the degra-dation of end-capped PPC takes place at higher tempera-

0 100 200 300 400 500 6000

20406080

100

Mas

s Lo

ss (

%)

Temperature (oC)

PPC

020406080

100

PPC capped with SA

020406080

100

PPC/starch

020406080

100

PPC/starch/SA

Fig. 3. Thermogravimetric curves for PPC and PPC/starch composites ata heating rate of 15 �C/min in air.

0 100 200 300 400 500 600

-2.5-2.0-1.5-1.0-0.5

PPC

DT

G

Temperature (oC)

-2.5-2.0-1.5-1.0-0.5

PPC capped with SA

-2.0

-1.5

-1.0

-0.5

PPC/starch/SA

PPC/starch

-1.5

-1.0

-0.5

Fig. 4. Derivative thermogravimetric curves for PPC and PPC/starchcomposites at a heating rate of 15 �C/min in air.

Table 1Thermal gravimetric parameters of PPC and PPC/starch composites

PPC, Tmax (K) Starch, Tmax (K)

PPC 528.85 –PPC/SA 561.24 –PPC/starch 504.79 606.88PPC/starch/SA 561.24 612.75

ture than that of uncapped PPC. Peng et al. [12] reportthat PPC is easily decomposed to cyclic carbonate. Liet al. [20] have found that PPC pyrolysis obeys two-steppyrolysis mechanism: main chain random scission andunzipping. During end capping, nucleophilic terminalhydroxyl groups are replaced with less reactive groupings.Although the potential for depolymerisation via chain scis-sion is not affected by the transformation, due to inhibitionof the formation of five-member rings in the chain end afterend capping, end-initiated depolymerisation via chainunzipping is prevented. For end-capped PPC, randomchain scission dominates the whole degradation process.The addition of SA improves the thermal stability of PPC.

Starch can accelerate the thermal decomposition ofPPC. As shown in Fig. 4 and Table 1, the introductionof starch decreases Tmax from 528.85 K of pure PPC to504.79 K of PPC in PPC/starch composites. However, SAincreases the decomposed temperatures of both PPC andstarch in PPC/starch/SA composites, compared to PPC/starch composites. And the decomposed temperature ofPPC in PPC/SA (561.24 K) is just equal to the decomposedtemperature of PPC in PPC/starch/SA (561.24 K).

Activation energy of decomposition, Et, of the polymercomposites can be calculated from the TGA curves by theintegral method proposed by Horowitz and Metzger using[21]

ln½lnð1� aÞ�1� ¼ Eth=RT 2max ð2Þ

where a is the decomposed fraction, Et is the activation en-ergy of decomposition, Tmax is the temperature at maxi-mum rate of weight loss, h is (T � Tmax), and R is thegas constant. From the plots of ln[ln(1 � a)�1] versus h,which are shown in Fig. 5, the activation energy (Et) fordecomposition can be determined from the slope of thestraight line of the plots. Et of both PPC and starch in com-posites are listed in Table 1.

As demonstrated in Table 1, Et of both PPC and starchin composites are raised when SA is added. Et of PPC inPPC/SA is 116.3 kJ/mol, while Et of pure PPC is 56 kJ/mol. In view of Et, PPC (120.5 kJ/mol) in PPC/starch/SAis more thermally stable than pure PPC (56 kJ/mol) andPPC (45.3 kJ/mol) in PPC/starch. SA also improves Et ofstarch from 34.3 kJ/mol in PPC/starch to 57.4 kJ/mol inPPC/starch/SA. At the existence of SA, the improvementof thermal stability for both PPC and starch is attributedto the specific interactions between the chains of starchand SA end-capped PPC.

PPC Starch

The slope Et (kJ/mol) The slope Et (kJ/mol)

0.0250 56.0 – –0.0444 116.3 – –0.0214 45.3 0.0112 34.30.0460 120.5 0.0184 57.4

-60 -50 -40 -30 -20-4.0

-3.5

-3.0

-2.5

-2.0

-1.5

theta (K)

PPC: y=-1.7166+0.0250x

-60 -50 -40 -30 -20-3.5

-3.0

-2.5

-2.0

-1.5

-1.0

PPC in PPC/SA: y=-0.7095+0.0444x

theta (K)

0 5 10 15 20 25 30 35 40-1.5

-1.0

-0.5

0.0

0.5

1.0

PPC in PPC/starch: y=-0.6327+0.02138x

theta (K)80 85 90 95 100 105 110 115 120

-1.0

-0.5

0.0

0.5

1.0

1.5

Starch in PPC+starch: y=-0.6887+0.0112x

theta (K)

-35 -30 -25 -20 -15 -10 -5 0 5-2.5

-2.0

-1.5

-1.0

-0.5

0.0

PPC in PPS/starch/SA: y=-0.4912+0.0460x

ln[ln

(1-a

) -1]

ln[ln

(1-a

) -1]

ln[ln

(1-a

) -1]

ln[ln

(1-a

) -1]

ln[ln

(1-a

) -1]

ln[ln

(1-a

) -1]

theta (K)-35 -30 -25 -20 -15 -10 -5 0 5

-1.0

-0.5

0.0

0.5

1.0

1.5

Starch in PPC/starch/SA: y=0.5970+0.0184x

theta (K)

Fig. 5. Plots of ln[ln(1 � a)�1] versus h for determination of the decomposition activation energy Et. Straight line is the linear fit of the data points.

0 50 100 150 200 250 300 3500

2

4

6

8

10

12

14

16

18

c

b

a

a: PPCb: PPC/SA (100/1)c: PPC/SA (100/2)

Str

ess(

%)

Strain(%)

Fig. 6. The effect of SA contents on the stress–strain curves of PPC/SAcomposites.

2364 X. Ma et al. / Composites Science and Technology 66 (2006) 2360–2366

3.4. Mechanical properties

Fig. 6 shows the effect of SA contents on mechanicalproperties. With the increasing of SA content, the breakstrain of PPC decreases. And the break stress is increasedform 9.78 to 10.26 MPa at 1 wt% SA content, and thendecreases to 6.53 MPa at 2 wt% SA content. The superflu-ous SA will result the degradation of PPC during the pro-cessing. Therefore, the content of SA is fixed at 1 wt% SAbased on the gross of PPC and starch.

Interfacial interaction between the fillers and matrix isan important factor affecting the mechanical properties ofthe composites. Thus, theoretical tensile yield strengthand ultimate tensile strength of the composites are mod-elled for the cases of adhesion and no adhesion betweenthe filler particles and matrix.

In the case of no adhesion, the interfacial layer cannottransfer stress. The tensile strengths of the compositescan be predicted using Nicholais–Narkis models [22]

0 10 20 30 40 500

2

4

6

8

10

12

14

16

18

0

60

120

180

240

300

360

Bre

ak Strain

(%)

Bre

ak

Str

ess

(MP

a)

starch weight content (%)

0 10 20 30 40 50

800

1200

1600

2000

2400

Yo

ungs

Mo

dulu

s (M

Pa)

starch weight content (%)

Fig. 8. The effect of starch weight contents on mechanical properties ofPPC/starch composites.

X. Ma et al. / Composites Science and Technology 66 (2006) 2360–2366 2365

rc ¼ rmð1� aUbf Þ ð3Þ

where Uf, rc and rm are volume fraction of filler, and ten-sile strengths of the composite and matrix, respectively. Inthe Nicholais and Narkis model, parameters a and b are theconstants related to filler–matrix interaction and geometryof the filler, respectively. The value of a less than 1.21 rep-resents good adhesion for composites containing sphericalfillers. In the absence of adhesion for the composites, Eq.(4) becomes

rc=rm ¼ ð1� 1:21U2=3f Þ ð4Þ

This model is based on the assumption that the decrease oftensile strength is due to the reduction in effective cross-sec-tion area caused by the spherical filler particles. If perfectadhesion were present between PPC and starch granules,the loading stresses would be transferred to the starch,and no reduction in effective surface area would result.

The densities of cornstarch and PPC used in the workare 1.40 and 1.30 g/cm3, respectively. The weight fractionof filler is transferred to volume fraction. The experimentaland theoretical curves are plotted in Fig. 7.

From the figure it can be seen that the experimentalvalue of PPC/starch is higher than that calculated by Eq.(4). This indicates that there is the adhesion with somedegree between PPC and starch granules. And experimen-tal curve of PPC/starch/SA is much higher. SA improvesthe degree of adhesion between PPC and starch granules.This result also proves that SA can enhance the interactionbetween the PPC chains and the surface of starch granule.

When starch content is 28.4% (volume fraction), i.e., 30wt%, the yield stress ratio of composite/matrix reaches thepeak at 1.18 for PPC/starch composites without SA and1.41 PPC/starch composites with SA. The yield stressesof PPC/starch composites without SA and with SA are,respectively, 19.20 and 22.94 MPa.

The effect of starch weight contents on break strain,break stress and Young’s Modulus of PPC/starch compos-

0.0 0.1 0.2 0.3 0.4 0.50.2

0.4

0.6

0.8

1.0

1.2

1.4

theoretical curve by Eq.2experimental curve of PPC/starchexperimental curve of PPC/starch/SA

Yie

ld s

tres

s ra

tio o

f com

posi

te/m

atrix

starch volume fraction (%)

Fig. 7. The effect of starch volume fraction on yield stress ratio of PPC/starch composites and PPC.

ites are shown in Fig. 8. At the starch weight content rangefrom 0% to 50%, both break stress and Youngs Moduluscurves of PPC/starch/SA composites are higher than onesof PPC/starch composites. And at the starch weight con-tent range from 10 to 50%, break strain curve of PPC/starch/SA composites is also higher than one of PPC/starch composites. These all illustrates that SA can increasemechanical properties of PPC/starch composites becauseSA enhances the interaction between PPC and starchgranule.

4. Summary

SEM has proved that SA reduces interfacial tension andimproves the interfacial compatibility between PPC matrixand dispersed starch granules. As revealed by FT-IR, thewave number of absorption peak for 50/50 PPC/starch/SA composite is about 6 cm�1 lower than that for 50/50PPC/starch composite. SA obviously improves the interac-tion between PPC and starch, which results in the improve-ment for thermal stability and mechanical properties ofPPC/starch composites. In view of Et, PPC (120.5 kJ/mol) in PPC/starch/SA is more thermally stable than

2366 X. Ma et al. / Composites Science and Technology 66 (2006) 2360–2366

PPC (45.3 kJ/mol) in PPC/starch. SA also improves Et ofstarch from 34.3 kJ/mol in PPC/starch to 57.4 kJ/mol inPPC/starch/SA. At starch weight content range from 10to 50%, break strain, break stress and Youngs Modulusof PPC/starch/SA composites are all higher than ones ofPPC/starch composites. At 28.4% volume fraction starch,the yield stresses of PPC/starch composites without SAand with SA, respectively, reach the vertex at 19.20 and22.94 MPa.

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