the effects of different plasticizers on the properties of thermoplastic starch as solid polymer...

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The Effects of Different Plasticizers on theProperties of Thermoplastic Starch as SolidPolymer Electrolytes

Xiaofei Ma, Jiugao Yu,* Kang He, Ning Wang

Plasticizers with amide groups (urea and formamide) and polyols (glycerol, glucose, andsorbitol) were used to prepare thermoplastic starch (TPS) containing NaCl salt as solid polymerelectrolytes (SPE). Fourier Transform infrared (FT-IR) spectroscopy revealed that both plasti-cizers and Naþ could form the interaction with starch, and plasticizers containing amidegroups had stronger hydrogen bond-forming abilities with starch than polyols. These inter-actions prevented starch molecules from crystallizing again, indicated by X-ray diffraction(XRD). Scanning electronmicroscope (SEM) showed that starch granules were in amolten stateand a continuous phase of TPS was formed. Among TPS as SPE, formamide-plasticized TPS(FPTPS) had the largest elongation at break and lowesttensile stress. The conductance of TPSwas sensitive towater. TPS plasticized by solid plasticizers had thehigher sensitivity of the conductance to water con-tents at the low water contents (<0.1). The relation-ship of the conductance and water contents was inagreement with the second-order polynomial corre-lation when water contents were below 0.45. FPTPShad the best conductance as a whole. At the mediumwater content (0.2), the conductance of FPTPS con-taining NaCl was about 10�3 S � cm�1.

Introduction

In the last two decades, solid polymer electrolytes (SPE)

have been studied more because of their potential appli-

cation in solid state batteries, chemical sensors, and

electrochromic devices.[1] As the matrices of SPE, several

synthetic polymers have been developed and character-

ized such as poly(ethylene oxide) (PEO), poly(propylene

oxide), poly(acrylonitrile), poly(methyl methacrylate),

poly(vinyl chloride), poly(vinylidene fluoride), etc.[2]

X. F. Ma, J. G. Yu, K. He, N. WangSchool of Science, Tianjin University, Tianjin 300072, ChinaFax: (þ86) 22 27403475; E-mail: [email protected]

Macromol. Mater. Eng. 2007, 292, 503–510

� 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

However, many synthetic polymers are usually prepared

in the form of intractable films, gels, or powders that are

insoluble in most of the solvents. Others are cast out of

acidic aqueous or nonaqueous solvents such as ether,[3]

while the rapid growth of the industry has increased the

demand for injection moldable thermoplastics with no

environmental pollution.[4]

Polymer electrolytes obtained from natural polymers

such as starch, cellulose, chitosan, pectin, hyaluronic acid,

agarose, and carrageenan have attracted attention

in recent years because of their superior mechanical

and electrical properties.[5] Among them, starch is an

abundant, renewable, low-cost, and biodegradable natu-

ral polymer, both starch melt extrusion (thermoplastic

DOI: 10.1002/mame.200600445 503

X. F. Ma, J. G. Yu, K. He, N. Wang

504

processing)[6] and casting processing[7] are available, and

its use offers a promising alternative for the development

of new SPE materials. Amylopectin-rich starch is gelati-

nized with water on a hot-plate, mixed with glycerol

and LiClO4, cast onto Teflon plates. The obtained

starch–glycerol-LiClO4 films exhibit conductance of

around 10�5 S � cm�1.[1] Finkenstadt and Willett accurately

determined the moisture content of native starch using a

direct-current resistance technique,[8] and produced solid

ion-conducting materials with water, starch, and metal

halides.[3] Ma et al. prepare glycerol-plasticized thermo-

plastic starch (TPS) as the SPE, which is doped with alkali

metal chlorides (LiCl, NaCl, and KCl salts) by using melt

extrusion.[9]

In this study, two plasticizer styles are used to prepare

TPS with NaCl salt as SPE by melt extrusion; one is the

plasticizer containing amide groups such as urea and

formamide, and the other is the polyol such as glycerol,

glucose, and sorbitol. The different plasticizers can form

the different interactions with starch in TPS, which

restrains starch recrystallization, improves the movement

of starch molecules and enhance the ionic conductance.

According to molecular dynamics simulations by Wolf-

gang,[10] the Liþ ions are complexed to PEO through

approximately five ether oxygens of a PEO chain. The

mobility of the Li cations is related to the motions of the

complexing segments of the PEO chain. In view of TPS, Naþ

is complexed to polar groups in starch, such as the oxygen

of –C–O–H groups and C–O–C groups, which is indicated

by FT-IR. Consequently, the mobility of Naþ is related to

the motions of the complexing segments of starch chain.

The conductance of TPS is mainly dependent on the

movement of the starch chain and Naþ concentration. The

applied properties of TPS polymer electrolytes such as

mechanical properties and the effect of water contents on

TPS conductance are also reported.

Experimental Part

Materials

Cornstarch was obtained from Langfang Starch Company. NaCl

and plasticizers (glycerol, glucose, sorbitol, formamide, and urea)

were purchased from Tianjin Chemical Reagent Factory, which

were analytical reagents and used without further purification.

Preparation of TPS Polymer Electrolytes with

Different Plasticizers

Cornstarch was dried at 120 8C for 2 h. Water and NaCl were

adequately mixed, and added with the plasticizer. The total

amount of plasticizer and water was around 30% by weight of the

total mixture (excluding NaCl salt). The obtained mixture was

blended (3 000 rpm, 2 min) with dried cornstarch by the use of



High Speed Mixer GH-100Y (Beijing Plastic Machinery Factory,

China), and then stored for 24 h. The mixtures were manually fed

into the single screw Plastic Extruder SJ-25(s) (Screw Ratio

L/D¼25:1, Beijing Plastic Machinery Factory). The screw speed

was 20 rpm. The die was a metal plate of 10 mm thickness with

eight holes of 3 mm diameter. The temperature profiles along the

extruder barrel (from feed zone to die) were 110–120–120–110 8Cfor GlyPTPS, 100–110–100–100 8C for glucose-plasticized TPS

(GluPTPS), 125–135–135–120 8C for sorbitol-plasticized TPS

(SPTPS), 125–135–130–120 8C for urea-plasticized TPS (UPTPS),

and 110–120–120–110 8C for formamide-plasticized TPS (FPTPS).

Characterization Techniques

FT-IR spectroscopy was measured with BioRad FTS3000 IR

Spectrum Scanner. The extruded TPS strips were pressured to

the transparent slices with the thickness of around 0.2 mm in the

Flat Sulfuration Machine, and tested by the transmittance

method.

The fracture surfaces of extruded TPS strips were performed

with scanning electron microscope (SEM) Philips XL-3, equipped

with an X-ray energy spectrometer (OXFORD Link ISIS300). TPS

strip samples were cooled in liquid nitrogen, and then broken. The

fracture surfaces were vacuum coated with gold for SEM.

The extruded cylindrical samples were piled up and com-

pressed at the temperature of 100 8C and the pressure of 10MPa in

the Flat Sulfuration Machine. The obtained slices were cut and

placed in a sample holder for X-ray diffractometry. X-ray dif-

fraction (XRD) patterns were recorded in the reflection mode in

angular range of 108–308 (2u) at the ambient temperature by a

BDX3300 diffractometer, operated at the CuKa wavelength of

1.542 A. The radiation from the anode, operating at 36 kV and

20 mA, monochromized with a 15-mm nickel foil. The diffract-

ometer was equippedwith 18 divergence slit, a 16-mmbeambask,

a 0.2-mm receiving slit, and a 18-scatter slit. Radiation was

detected with a proportional detector.

Samples of about 8� 2 cm2 in size were cut from the slices. The

Testometric AX M350-10KN materials testing machine was

operated and a crosshead speed of 10 mm �min�1 was used for

tensile testing (ISO 1184-1983 standard). The data were averages

of - five to eight specimens.

Volume resistivity measurements were performed on samples

of all the composites that were firstly compressed into thin sheets.

A Model ZC36 electrometer (SPSIC Huguang Instruments and

Power Supply Branch, China)was used for high resistivity samples

with 50 mm diameter and 0.5 mm thickness. For more conductive

samples (larger than 10�6 S � cm�1), strips with dimensions of

30� 5mm and 0.5 mm of thickness weremeasured using aModel

ZL7 electrometer (SPSIC Huguang Instruments and Power Supply

Branch) using a four-point test fixture.

The original water contents (dry basis) of TPS were determined

gravimetrically by drying small pieces of TPS at 105 8C overnight.

At this condition, the evaporation of the plasticizers was negli-

gible.[11] When TPS was stored at the different relative humidity

for a period of time, its water content was calculated on the basis

of its original weight, its current weight, and its original water

content.

DOI: 10.1002/mame.200600445

The Effects of Different Plasticizers on the Properties of Thermoplastic Starch . . .

90095010001050110011501200

0.2mol NaCl/100g starch

1146

1075

1013

1016

e

d

c

b

a

Wavenumber/cm-1

a: glycerolb: sorbitolc: glucosed: formamidee: urea

Tra

nsm

ittan

ce

10761148

Figure 2. FTIR spectra of TPS with different plasticizers at 0.2 molNaCl per 100 g starch: (a) Glycerol, (b) sorbitol, (c) glucose, (d)formamide, and (e) urea.

Results and Discussion

The Interaction

Native starch contained two different molecular struc-

tures: the linear (1,4)-linked a-D-glucan amylose and highly

(1,6)-branched a-D-glucan amylopectin. The process of

using thermal and mechanical energies to modify native

starch granules into TPS was well known.[12] The thermo-

mechanical process with water and plasticizer broke

starch granules, the hydrogen bonding, and crystal struc-

ture of native starch.

Formation of homogeneous TPS was a result of strong

interactions between starch and plasticizers. The analysis

of FTIR spectra of the blends enabled the interactions to be

identified.[13] FT-IR spectra for TPS with different plasti-

cizers are shown in Figure 1. In the fingerprint region of

FT-IR spectra of TPS, three characteristic peaks appeared

between 1 200 and 900 cm�1, attributed to C–O bond

stretching of starch.[14] The characteristic peak at

1 150 cm�1 in Figure 1 is ascribed to C–O bond stretching

of C–O–H group in starch while two peaks at 1 080 and

1 020 cm�1 are attributed to C–O bond stretching of C–O–C

group in the anhydroglucose ring. As shown in Figure 1,

those of TPS located at the different wave numbers,

because glycerol, sorbitol, glucose, urea, and formamide

had different hydrogen bond-forming abilities with both O

of C–O–H groups andO of O–C anhydroglucose ring groups

in starch. The lower the peak frequency of C–O group in

starch was, the stronger the interaction between starch

and plasticizers was.[15] At three characteristic peaks,

FPTPS and UPTPS were situated lower than GlyPTPS,

GluPTPS, and SPTPS. Therefore, plasticizers containing

amide groups had stronger hydrogen bond-forming abi-

lities with starch than polyols. This also was consistent

with chemical computation by the hydride density

functional method B3LYP.[16] On the other hand, GluPTPS

90095010001050110011501200

Tra

nsm

ittan

ce

e

d

c

b

a

1015

10771149

1018

1078

1151

Wavenumber/cm-1

no NaCl

a: glycerolb: sorbitolc: glucosed: formamidee: urea

Figure 1. FTIR spectra of TPS with different plasticizers: (a) gly-cerol, (b) sorbitol, (c) glucose, (d) formamide, and (e) urea.



had the largest wave numbers at around 1150 and

1 080 cm�1 as shown in Figure 1, which revealed that

glucose had the weakest hydrogen bond-forming ability

with starch among these plasticizers.

During the thermoplastic process, the plasticizers could

form the hydrogen bonds with starch, take the place of the

strong action between hydroxyl groups of starch mole-

cules. The effect of NaCl on the interaction between starch

and plasticizers was also researched with FT-IR, here. As

shown in Figure 2, these characteristic peaks of TPS plasti-

cizedwith plasticizers containing amide groups situated at

lower wave number than TPS plasticized with polyols.

After NaCl was added into TPS, urea or formamide could

still form stronger interaction with starch than glycerol,

glucose, or sorbitol, and starch molecular movement was

more flexible in UPTPS or FPTPS than GlyPTPS, GluPTPS, or

SPTPS as SPE. The flexible starch molecules would be apt to

improve the conductance of TPS.

Compared with the situations of three characteristic

peaks in Figure 1, those in Figure 2 shifted lower in each

TPS after NaCl was added. For example, NaCl introduction

made C–O bond stretching of C–O–C group in the

anhydroglucose ring shift from 1018 to 1 016 cm�1 for

GlyPTPS, GluPTPS, and SPTPS, while from 1015 to

1 013 cm�1 for FPTPS and UPTPS. It meant that NaCl could

also form the interaction with starch, which made the

peaks shift to lower wavenumber.

Microscopy

SEM micrograph of native starch granules and fractured

surface of TPS plasticized with different plasticizers at

0.2 mol per 100 g starch are shown in Figure 3. As solid

www.mme-journal.de 505


Figure 3. SEMmicrograph of native starch granules (a) and fractured surface of TPS plasticizedwith different plasticizers at 0.2mol NaCl per100 g starch (b) glycerol-TPS-NaCl, (c) sorbitol-TPS- NaCl, (d) glucose-TPS-NaCl, (e) formamide-TPS-NaCl, and (f) urea-TPS-NaCl)

506

plasticizers, glucose, sorbitol, and urea were dissolved in

water or melted to a liquid state by the application of heat

and pressure during the melt extrusion. Plasticizers in

liquid state or aqueous solution were known to enter

starch granular interior, disrupt intermolecular and intra-

molecular hydrogen bonds and made the native starch



plastic. During the melt extrusion, native starch granules

[shown in Figure 3(a)] were physically broken up into

small fragments and melted. A continuous phase of TPS

[shown in Figure 3(b–f)] was formed by the action of

shear, high temperature, pressure, water, plasticizers, and

NaCl salt. In view of plasticization, glucose was not as

DOI: 10.1002/mame.200600445


good as the other plasticizers. As marked with arrows in

Figure 3(d), some starch granules, which were not comp-

letely molten, were embedded in the continuous TPS

phase. This could relate to the weakest hydrogen bond-

forming ability with starch among these plasticizers,

indicated by FT-IR spectra. Figure 3(f) reveals that there

were many white dots in the micrograph of UFTPS,

which were indicated as NaCl grains by X-ray energy

spectrometer. It meant that a small quantity of NaCl

separated out from UPTPS at the low water content.

Figure 5. The X-ray diffractograms showing retrogradation ofGlyPTPS, GluPTPS, SPTPS, FPTPS, and UPTPS containing 0.2 molNaCl per 100 g starch as SPE stored at RH50% for 25 d.

XRD

Native starch was commonly of about 15–45% crystal-

linity, and starch-based materials were susceptible to

aging and starch recrystallization. TPS plasticized with

glycerol was thought to tend to recrystallization after

being stored for a period of time, which would worsen

mechanical properties of TPS, generally embrittle TPS.[15]

On the other hand, starch recrystallization would restrict

the movement of starch molecules and ionic conductivity.

Native cornstarch was of the A-style crystallinity.[17]

In the melt extrusion, plasticizers and water molecules

entered into starch granules, and replaced starch inter-

molecular and intramolecular hydrogen bonds and des-

tructed the crystallinity of starch. There were no obvious

starch crystals in newly made TPS.[13] The newly made TPS

was stored at RH50% for 25 d, the XRD patterns of which

are shown in Figure 4. When TPS were stored at RH50% for

25 d, TPS plasticized by formamide or urea [Figure 4(d and

e)] showed no obvious starch crystal peaks (the pointed

peaks at 22.38 was related to the urea crystallinity), while

GlyPTPS, GluPTPS, and SPTPS prone to starch recrystal-

lization, showed a VH style crystal peak.[18] Urea and

Figure 4. The X-ray diffractograms showing retrogradation ofGlyPTPS, GluPTPS, SPTPS, FPTPS, and UPTPS stored at RH50%for 25 d.



formamide could restrain starch recrystallization, because

they could form more strong and stable hydrogen bonds

with starch than polyols, and then prevent starch

molecules from interacting and crystallizing again.

The newly made TPS with 0.2 mol NaCl per 100 g starch

as SPEwere stored at RH50% for 25 d, and tested. Their XRD

patterns are shown in Figure 5. At the existence of NaCl,

there was no starch recrystallization even in TPS plasti-

cized with polyols. The addition of NaCl could restrain

starch recrystallization, because NaCl could form the

interaction with starch according to FT-IR analysis, which

prevented starch molecules from crystallizing again. This

restraint of starch recrystallization was in favor of starch

chain mobility and ion conductivity.

Mechanical Properties

The newly made TPS samples containing different NaCl

contents and plasticizers were immediately enveloped

aftermelt extrusion and stored for oneweek in the airtight

container before testing. The effect of NaCl contents and

plasticizers on tensile strength and elongation at break of

TPS is shown in Figure 6. On increasing NaCl contents for

each TPS, the elongation at break firstly increased to the

maximumat 0.2mol NaCl per 100 g starch, then decreased.

However, the tensile stress changed a little at NaCl

contents of 0.6, 0.8, and 1.0 mol per 100 g starch, where the

elongation at break did not changemuch either. This could

be related to the congregation of superfluous NaCl when

NaCl contents exceeded 0.6 mol per 100 g starch. This

congregation would not improve the effective NaCl

contents. On the whole, the introduction of NaCl improved

the elongation at break, but decreased the tensile stress.

NaCl could form the interaction with starch and weaken



1.00.80.60.40.20.00

50

100

150

200

250

300

Elo

ngat

ion

at b

reak

/%

NaCl content (mol per 100g starch)

a)

b)

GlyPTPS GluPTPS SPTPS FPTPS UPTPS

1.00.80.60.40.20.00

1

2

3

4

5

6

7

8

Ten

sile

str

engt

h /M

Pa

NaCl content (mol per 100g starch)

GlyPTPS GluPTPS SPTPS FPTPS UPTPS

Figure 6. The mechanical properties of TPS containing differentplasticizers and amounts of NaCl per 100 g starch. a) Elongationat break. b) Tensile strength.

508

the interaction of starch molecules, and the slippage

movement among starch molecules was facile.

Plasticizers also had a great effect on tensile stress and

elongation at break of TPS. SPTPS had the best tensile

strength (8.02MPa) and lowest elongation at break (25.3%),

while FPTPS had the largest elongation at break (136%) and

lowest tensile stress (1.51 MPa). Although tensile stress

and elongation at break of TPS changed much with the

increasing in NaCl contents, the orders were basically

invariable at the same NaCl contents, i.e., the order

of tensile strength was SPTPS>GluPTPS>UPTPS>

GlyPTPS> FPTPS, and the order of elongation at break

was FPTPS>GlyPTPS>UPTPS>GluPTPS> SPTPS.

Conductivity

As shown in Figure 7, the conductance of TPS was very

much dependent on water contents. The conductance of



TPS increased about 5 orders of magnitude from 0.05 to

0.55 water contents. The effect of water contents on the

conductance at the low water content (from 0.05 to 0.30)

was more obvious than one at the high water content

(from 0.30 to 0.55). The conductance of TPS increased about

4 orders of magnitude from 0.05 to 0.30 water contents,

while it increased only about 1 order of magnitude from

0.30 to 0.55 water contents. On the one hand, water could

form the interactionwith starch, weaken the interaction of

starch molecules, and improve the movement of starch

chain. It was advantageous to the transference of Naþ in

TPS. On the other hand,water could dissolve NaCl, decrease

salt congregation, and increase the effective ion numbers.

TPS with different Naþ contents and plasticizers exhi-

bited the similar relationship of conductance versus water

contents. A second-order polynomial was used to describe

the relationship well when water contents were below

0.45. The model gave a good agreement (R2> 0.96) for

water contents between 0.05 and 0.55. The binomial

correlation of the conductance (y) and water contents (x)

was supposed as y¼B2x2þB1xþB0. The second-order

polynomial correlation between the conductance (y) and

water contents (x) of TPS are listed in Figure 7, which

revealed that with the increase in NaCl contents the

conductance increased, but the increment was very

limited. On the other hand, superfluous NaCl resulted in

the congregation, which could not improve the ion

concentration and conductance of TPS, so TPS containing

above 0.6 mol NaCl per 100 g starch are not listed here.

Plasticizers had a great influence on the conductance. As

shown in Figure 7, FPTPS and UPTPS had higher conduc-

tance than GlyPTPS, GluPTPS, and SPTPS at the contents of

0.2, 0.4, and 0.6 mol NaCl per 100 g starch. Plasticizers

containing amide groups could form stronger interaction

with starch than polyols, therefore, the interaction of

starch molecules was more weakened, the movement of

starch chain was more flexible, and the transference

of Naþ was more facile in FPTPS and UPTPS than in

GlyPTPS, GluPTPS, and SPTPS. Among these TPS, FPTPS had

the best conductance as a whole. At the medium water

content (0.2), the conductance of FPTPS containing NaCl

was about 10�3 S � cm�1.

Solid and liquid plasticizers also had different effects on

the sensitivity of TPS conductance to water contents. As

shown in Figure 7, the conductance of GluPTPS, SPTPS, and

UPTPS was more sensitive to water contents at the low

water contents (<0.1) than TPS plasticized by liquid plasti-

cizers, i.e., GlyPTPS and FPTPS. At a certain water content,

the sensitivity of the conductance (y) to water contents (x),

i.e., the effect of dx (the change of water contents) on dy

(the change of the conductance) could be expressed with

differential equation as follows dy/dx¼ 2B2 xþB1. There-

fore, at the low water content (<0.1), the sensitivity of the

conductance to water contents was approximatively

DOI: 10.1002/mame.200600445


Figure 7. The effect of water contents on the conductance of TPS with different plasticizers and amounts of NaCl per 100 g starch.

denoted asmonomial coefficient B1. According to the listed

second-order polynomials in Figure 7, the monomial

coefficients of GluPTPS, SPTPS, and UPTPS were larger

than those of GlyPTPS and FPTPS at 0.2, 0.4, and 0.6 mol

NaCl per 100 g starch. For example, the monomial

coefficients of GluPTPS, SPTPS, and UPTPS were, respec-

tively, 29.40, 28.35, and 25.70 at 0.4 mol NaCl per 100 g

starch, while those of GlyPTPS and FPTPS were only 19.37

and 19.42, respectively. This phenomenon was more

obvious at 0.4 and 0.6 mol NaCl per 100 g starch than

the one at 0.2 mol NaCl per 100 g starch. At the low water

contents, solid plasticizers would separate out from TPS,

which would not form the interaction with starch. As a

result, the movement of starch chain was restrained, and

the conductance was decreased for TPS plasticized by solid

plasticizers. When water contents of TPS were increased,

solid plasticizers would dissolve and form the interaction

with starch again. Therefore, for TPS plasticized by solid

plasticizers, the high sensitivity of the conductance to

water contents was ascribed to the separation of solid

plasticizers from TPS at the low water contents.



Conclusion

Both the interactions between plasticizers and starch and

the interaction between Naþ and starch played an impor-

tant role on the mechanical properties and the conduc-

tance of TPS as SPE. Because plasticizers containing amide

groups had stronger hydrogen bond-forming abilities with

starch than polyols, formamide restrained starch recrys-

tallization, made starch chain mobility flexible and made

the transference of ions facile. FPTPS had the best elonga-

tion at break and lowest tensile stress. FPTPS had the best

conductance as a whole. At the medium water content

(0.2), the conductance of FPTPS containing NaCl was

about 10�3 S � cm�1. The conductance of TPS was water

sensitive. TPS plasticized by solid plasticizers had the high

sensitivity of the conductance to water contents at the low

water contents, which was ascribed to the separation of

solid plasticizers. A second-order polynomial was suffi-

cient to describe the relationship of conductance (y) versus

water contents (x) with a good agreement (R2> 0.96) when

water contents were below 0.45.



510

The further study on TPS as SPE was necessary. The

mixture of several plasticizers and the introduction of

carbon black and carbon nanotube were considered to

improve themechanical properties and the conductivity of

TPS as SPE. TPS would be a promising alternative for the

development of new SPE materials, which had a wide

variety of potential applications such as biosensor,

artificial muscles, corrosion protection, electronic shield-

ing, environmentally sensitive membranes, visual dis-

plays, solar materials, and components in high energy

batteries.[19]

Received: November 18, 2006; Revised: January 14, 2007;Accepted: January 15, 2007; DOI: 10.1002/mame.200600445

Keywords: extrusion; solid polymer electrolyte; starch; thermo-plastics

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Eng. 2002, 287, 395.[5] V. L. Finkenstadt, Appl. Microbiol. Biotechnol. 2005, 67, 735.[6] A. P. Mathew, A. Dufresne, Biomacromolecules 2002, 3, 1101.[7] A. Rindlav-Westling, M. Stading, P. Gatenholm, Biomacromo-

lecules 2002, 3, 84.[8] V. L. Finkenstadt, J. L. Willett, Carbohydr. Polym. 2004, 55,

149.[9] X. F.Ma, J. G. Yu, K. He,Macromol.Mater. Eng. 2006, 291, 1407.[10] H. M. Wolfgang, Adv. Mater. 1998, 10, 439.[11] X. F. Ma, J. G. Yu, Y. B. Ma, Carbohydr. Polym. 2005, 60, 111.[12] J. J. Guan, M. A. Hanna, Biomacromolecules 2004, 5, 2329.[13] X. F. Ma, J. G. Yu, Starch 2004, 56, 545.[14] X. F. Ma, J. G. Yu, J. Appl. Polym. Sci. 2004, 93, 1769.[15] A. Pawlak, M. Mucha, Thermochim. Acta 2003, 396, 153.[16] X. F. Ma, J. G. Yu, Carbohydr. Polym. 2004, 57, 197.[17] J. J. G. Van Soest, S. H. D. Hulleman, D. de Wit, J. F. G.

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DOI: 10.1002/mame.200600445

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