thermoplastic starch plasticized by glycerol as solid polymer electrolytes

Full Paper DOI: 10.1002/mame.200600261 1407

Summary: Melt extrusion can be used to prepared GPTPSmixed with alkali metal chlorides (LiCl, NaCl and KCl) as apotential solid polymer electrolytes. SEM reveals that excesssalt results in the congregation of salt crystals, which affectsthe mechanical properties and conductivities of GPTPS.FT-IR spectroscopy shows that alkali metal ions can inter-act with starch, which restrains starch recrystallization, asproved by XRD. The introduction of salts improves theelongation at break, but reduces the tensile stress of GPTPS.Both salt content and water content have an effect on theconductance of GPTPS. The introduction of salts increasesthe conductance by a factor of 1–2, while the conductanceincreases about 5 orders of magnitude as the water contentincreases from 0.05 to 0.55 wt.-%. The relationship of con-ductance versus water content can be well described by asecond-order polynomial, with R2> 0.96. The type of salthas no obvious effect on the correlation between the con-ductance and water contents.

SEM micrographs of native starch granules and fracturedsurfaces of GPTPS filled with different types and amounts ofalkali metal chlorides per 100 g starch.

Thermoplastic Starch Plasticized by Glycerol as

Solid Polymer Electrolytes

Xiaofei Ma, Jiugao Yu,* Kang He

School of Science, Tianjin University, Tianjin 300072, ChinaE-mail: [email protected]

Received: July 3, 2006; Revised: August 23, 2006; Accepted: August 23, 2006; DOI: 10.1002/mame.200600261

Keywords: extrusion; solid polymer electrolyte; starch; thermoplastics

Introduction

Solid polymer electrolytes (SPEs) have been extensively

studied in the last two decades because of their potential in

many technological areas, including solid-state batteries,

chemical sensors and electrochromic devices.[1] Currently,

several synthetic polymer matrices have been developed

and characterized; these include poly(ethylene oxide) (PEO),

poly(propylene oxide), poly(acrylonitrile), poly(methyl

methacrylate), poly(vinyl chloride), poly(vinylidene fluor-

ide), poly[(vinylidene fluoride)-co-(hexafluoro propyl-

ene)].[2] Many synthetic polymers are usually fabricated

as intractable films, gels, or powders because they are

insoluble in most solvents. Others are prepared by casting

out of acidic aqueous or nonaqueous solvents such as

ether,[3] while the rapid industrial development has increa-

singly demanded injection moldable thermoplastics and no

environmental pollution.[4]

Macromol. Mater. Eng. 2006, 291, 1407–1413

Polymer electrolytes obtained from natural polymers,

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

agarose and carrageenan, have attracted attention in recent

times because of their superior mechanical and electrical

properties.[5] Among them, starch is an abundant, renew-

able, low-cost, and biodegradable natural polymer; it pro-

vides both starch melt extrusion (thermoplastic processing)

and casting processing options; and its use offers a pro-

mising alternative for the development of new SPE mate-

rials. Lopes et al gelatinized amylopectin-rich starch

with water on a hotplate.[1] The solution was combined

with glycerol, mixed with LiClO4, cast onto Teflon plates,

and allowed to dry. The starch/glycerol/LiClO4 films

exhibited conductance of around 10�5 S � cm�1. Finken-

stadt et al. studied the accurate determination of the mois-

ture content of native starch using a direct-current resis-

tance technique,[6] and prepared thermoplastic starch films

not by adding plasticizers but by adding water doped

� 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1408 X. Ma, J. Yu, K. He

Figure 1. SEMmicrographs of (a) native starch granules and fractured surfaces of GPTPS filled with differenttypes and amounts of alkali metal chlorides per 100 g starch: (b) 0.2 mol LiCl, (c) 0.2 mol NaCl, (d) 0.2 molKCl, (e) 0.6 mol LiCl, (f) 0.6 mol NaCl, (g) 0.6 mol KCl.

Macromol. Mater. Eng. 2006, 291, 1407–1413 www.mme-journal.de � 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Thermoplastic Starch Plasticized by Glycerol as Solid Polymer Electrolytes 1409

850900950100010501100115012001250

d

c

b

a

1149

1016

1014

1016

1018

1078

1076

1076

10771149

1149

1151

Tra

nsm

ittan

ce

Wavenumber/cm-1

Figure 2. FT-IR spectra of TPS with different amounts of LiClper 100 g starch: (a) 0, (b) 0.2, (c) 0.4 (d) 0.6 mol LiCl.

with metal halides to produce solid ion-conducting

materials.[3]

In this study, glycerol-plasticized thermoplastic starch

(GPTPS), mixed with alkali metal chlorides is prepared

as a solid polymer electrolyte. Glycerol as a plasticizer is

added because the incorporation of plasticizers into poly-

mer electrolyte can reduce its crystallinity, improve the

movement of polymeric molecules, and enhance the ionic

conductance.[7] Melt extrusion is also introduced here

instead of casting as it is more convenient and efficient.

The interaction between salts and starch, the salts dis-

persion in thermoplastic starch (TPS), and the effect of

salts on starch recrystallization are studied, which are

related to the conductance of GPTPS. The applied proper-

ties of GPTPS polymer electrolytes such as mechanical

properties and the effect of water on GPTPS conductance

are also researched here.

Experimental Part

Materials

Cornstarch (25 wt.-% amylose and 75 wt.-% amylopectin)was obtained from Langfang Starch Company. Alkali metalchlorides (LiCl, NaCl, and KCl salts) and glycerol werepurchased from Tianjin Chemical Reagent Factory; they wereanalytical reagents and used without further purification.

b

a

Preparation of GPTPS Polymer Electrolytes

Cornstarch was dried at 120 8C for 2 h. Water and alkali metalchlorides were adequately mixed, and added to glycerol. Theobtained mixture was blended (3 000 rpm, 2 min) with driedcornstarch by use of a high speed mixer GH-100Y (BeijingPlastic Machinery Factory, China), and then stored for 24 h.The mixtures were manually fed into the single screw plasticextruder SJ-25(s) (Screw ratio length:diameter¼ 25:1, BeijingPlastic Machinery Factory, China). The screw speed was20 rpm. The temperature profile along the extruder barrel was110, 120, 120, 110 8C (from feed zone to die). The die was ametal plate 10 mm thick with eight holes of 3 mm diameter. Inthe composition of GPTPS polymer electrolytes, dried starch,water, and glycerol were 100, 20 and 20 g, respectively, whilealkali metal chlorides varied, with 0, 0.2, 0.4, 0.6, 0.8, and1.0 mol.

3025201510

e

dc

2 theta (deg.)

Figure 3. The X-ray diffraction patterns of corn starch and TPSstored for one month in airtight containers. (a) native starch;(b) TPS; (c) TPSþ 0.2 mol LiCl per 100 g starch; (d) TPSþ0.2 mol NaCl per 100 g starch; (e) TPSþ 0.2 mol KCl per 100 gstarch.

Characterization Techniques

GPTPS strip samples were cooled in liquid nitrogen, and thenbroken. The fracture surfaces were vacuum coated with goldand then examined by scanning electron microscopy (SEM),using a Philips XL-3 (FEI, Hillsboro, OR), with an accelera-tion voltage of 20 kV.

FT-IR spectroscopy was carried out with a BIO-RADFTS3000 IR spectrum scanner (Hercules, USA). By using aflat pressure machine (Beijing Plastic Machinery Factory,

Macromol. Mater. Eng. 2006, 291, 1407–1413 www.mme-journal.de

China), the extruded GPTPS strips were applied to thetransparent slides and had a thickness of around 0.2 mm. Thetransmittance method was used.

The extruded GPTPS strips were compressed at 10 MPawith a flat pressure machine. The slices were placed in asample holder for X-ray diffractometry. X-ray diffractionpatterns were recorded in the reflection mode in the angularrange 10–308 (2u) at the ambient temperature by a BDX3300diffractometer (Peking University, China), operated at theCu Ka wavelength of 1.542 A. The radiation from the anode,operating at 36 kVand 20 mA, monochromized with a 15 mmnickel foil. The diffractometer was equipped with 18 diver-gence slit, a 16 mm beam bask, a 0.2 mm receiving slit, and


1410 X. Ma, J. Yu, K. He

a 18 scatter slit. Radiation was detected with a proportionaldetector.

Samples of 8 cm� 3 mm in size were cut from the extrudedstrips, and pressure was applied with the flat pressure machineat 100 8C. The Testometric AX M350-10kN materials testingmachine (The Testometric Company Ltd, Rochdale, UK) wasoperated and a crosshead speed of 10 mm �min�1 was used fortensile testing (ISO 1184-1983 standard). The final data werethe averages of values from five to eight specimens.

Volume resistivity measurements were performed on sam-ples of all composites that were firstly compressed into thinsheets. A Model ZC36 electrometer (SPSIC Huguang Instru-ments & Power Supply Branch, China) was used for highresistivity samples of diameter 50 mm and thickness 0.5 mm.For more conductive samples (larger than 106 S � cm�1) stripswith dimensions of 30� 5 and 0.5 mm3 thickness weremeasured using a Model ZL7 electrometer (SPSIC HuguangInstruments & Power Supply Branch, China) using a four-point test fixture.

In order to analyze the effect of water contents on electricalconductivity, the samples were stored in closed chambers overseveral materials at 20 8C for several days. The materials usedwere dried silica gel, substantive 55.01% H2SO4 solution,substantive 35.64% CaCl2 solution, NaCl saturated solution,and distilled water, providing relative humidities of about 0,25, 50, 75, and 100%, respectively. The original watercontents (dry basis) of TPS were determined gravimetricallyby drying small pieces of TPS at 105 8C overnight. Underthese conditions the evaporation of the plasticizers was negli-gible.[8] When TPS was stored for a period of time, its watercontent was calculated on the base of its original weight, itscurrent weight, and its original water content.

Figure 4. The mechanical properties of GPTPS samples con-taining different types and contents of alkali metal chlorides.

Results and Discussion

Microscopy

SEM micrographs of native starch granules and fractured

surface of GPTPS filled with different alkali metal chlo-

rides (LiCl, NaCl, and KCl salts) and contents are shown

in Figure 1. During the melt extrusion, native starch

granules [shown in Figure 1(a)] were physically broken up

into small fragments and molten. A continuous phase of

thermoplastic starch (TPS) [shown in Figure 1(b–g)] was

found to form on the action of shear, high temperature,

pressure, water, glycerol, and alkali metal chlorides. As

shown in Figure 1 (b), (c), and (d), LiCl, NaCl, and KCl

could completely be dissolved in water contained in TPS at

the low salt content (0.2 mol per 100 g starch), so no salt

crystals were observed. However, at the high salt content

(0.6 mol per 100 g starch), superfluous salts could be partly

dissolved in water contained by GPTPS, then would

congregate for LiCl [revealed by the dark shadow in

Figure 1(e)] or become crystals for NaCl and KCl [revealed

by the white crystals in Figure 1(f) and (g)]. Both this

congregation and these crystals would definitely affect the

mechanical properties and conductivity of GPTPS.


FT-IR

Ma et al studied the interaction between starch and plasti-

cizers in TPS, which could be identified by the FT-IR

spectra, and thought that the lower the peak frequency the

stronger was the interaction.[9] Starch was composed of

two different polysaccharides: the linear (1, 4)-linked a-

D-glucan amylose and highly (1, 6)-branched a-D-glucan

amylopectin. As shown in the fingerprint region of the



FT-IR spectrum of GPTPS (Figure 2), three characteristic

peaks appeared between 1 200 and 900 cm�1, attributed to

the C–O bond stretching of starch.[10]

The characteristic peak near 1 150 cm�1 (Figure 2) was

ascribed to the C–O bond stretching of C–O–H groups in

starch, where the peakwas shifted from 1 151 cm�1 (GPTPS

without LiCl ) to 1 149 cm�1 (GPTPS with LiCl). This

illustrated that OH groups of starch interacted with Liþ. As

the LiCl content increased from 0.2 to 0.6 mol per 100 g

starch, the fact that there was no shift of this peak meant that

0.2 mol Liþ was enough for there to be an interaction with

OH groups of 100 g starch, while the rest of OH groups

would not form the interaction with OH groups of 100 g

starch.

Another two peaks at 1 080 and 1 020 cm�1 were attri-

buted to C–O bond stretching of the C–O–C group in the

anhydroglucose ring. These two peaks shifted towards

lower wavenumbers as the LiCl content increased from 0 to

0.4 mol per 100 g starch. This meant that Liþwas also able

to interact with the oxygen in the C–O–C groups of starch.

However, when the LiCl content reached 0.2 mol per 100 g

starch, the peak at 1 014 cm�1 shifted to 1 016 cm�1 while

the peak at 1 076 cm�1 did not move. This could relate

to the congregation of excess LiCl in GPTPS, which

essentially decreased the LiCl contents, so the interaction

between starch and Liþ was contrarily weakened when

LiCl was available in excess.

A similar situation appeared in the spectra of GPTPS

containing NaCl and KCl. The interaction between alkali

metal ions and starch would influence starch recrystalliza-

tion, mechanical properties, and the conductance of GPTPS.

X-Ray Diffraction

Native starch commonly existed in a granule structure with

about 15–45% crystallinity,[11] and starch-based materials

were susceptible to aging and starch recrystallization. TPS

plasticized with glycerol was thought to tend to recry-

stallization after being stored for a period of time, which

would worsen mechanical properties of TPS, generally

embrittling TPS.[12] On the other hand, starch recrystalli-

zation would restrict the movement of starch molecules

and ionic conductivity.

Li+ Li+

Figure 5. Schematic of the segmental motimatrix.[15] The circles represent the ether oxy


GPTPS was prepared with alkali metal chlorides, and

stored in airtight containers for one month. The X-ray dif-

fraction patterns are shown in Figure 3. Native cornstarch had

the A-style crystallinity,[13] shown in Figure 3(a). In the melt

extrusion, plasticizer and water molecules entered into starch

granules, and replaced starch intermolecular and intramole-

cular hydrogen bonds and destructed the crystallinity of

starch. There were no obvious starch crystals in freshly made

TPS.[14] However, as shown in Figure 2(b), A-style starch

crystallinity appeared again in GPTPS without alkali metal

chlorides when stored for one month, while there was no

starch crystallinity in GPTPS with alkali metal chlorides in

Figure 3(c–e).

The addition of alkali metal chlorides could restrain

starch recrystallization, because alkali metal ions could

interact with starch, as shown by FT-IR analysis, and then

prevent starch molecules from interacting and crystallizing

again. This restraint of starch recrystallization was in favor

of starch chain mobility and ion conductivity.

Mechanical Properties

Figure 4 respectively displayed the effect of salt styles and

contents on mechanical properties of GPTPS samples,

which were immediately enveloped after melt extrusion

and stored for one week in an airtight container before

being tested. The tensile stress and elongation at break

for GPTPS without salts were respectively 2.45 MPa

and 113%. Figure 4 revealed that both tensile stress and

elongation at break were affected by the salt styles and

contents.

For GPTPS containing LiCl, as LiCl content increased

the tensile stress firstly decreased then increased, and the

lowest tensile stress was 0.76 MPa at a LiCl content of

0.6 mol per 100 g starch, while the elongation at break

increased as LiCl content increased, with the highest value

being 300% at the LiCl content of 0.8 mol per 100 g starch.

For GPTPS containing NaCl, when NaCl was added, the

tensile stress halved and the elongation at break increased

to 176%. As the NaCl content increased from 0.2 to 1.0 mol

per 100 g starch, both tensile stress and elongation at break

changed less than for GPTPS containing LiCl.

Li+

on assisted diffusion of Liþ in the PEOgen atoms of PEO.


1412 X. Ma, J. Yu, K. He

For GPTPS containing KCl, as KCl content increased

the elongation at break firstly increased then decreased,

and the largest onewas 166% at the KiCl content of 0.4 mol

per 100 g starch, while the tensile stress basically decrea-

sed as KCl content increased.

On the whole, the introduction of salts decreased the

tensile stress, but the tensile stress increased a little at salt

contents of 0.8 or 1.0 mol per 100 g starch. The crystals of

excess salt acted as the granular reinforcement of GPTPS.

On the other hand, the introduction of salts improved the

elongation at break by different means. Alkali metal ions

could interact with starch and weaken the interaction of

starch molecules, and the slippagemovement among starch

molecules was easy.

Figure 6. The effect of water contents (wt.-%) on the conduc-tance of GPTPS with different amounts of KCl per 100 g starch:(a) 0, (b) 0.2, (c) 0.4, (d) 0.6, (e) 0.8 mol KCl.

Conductivity

The ionic motion of a lithium ion in the PEO matrix is

shown in Figure 5.[15] In views of GPTPS, alkali metal ions

were mainly complexed to polar groups in starch, such as

oxygen atoms of C–O–H groups and C–O–C groups.

Alkali metal ions were also complexed to glycerol. How-

ever, glycerol was only 20 wt.-% of starch. Consequently,

the mobility of the alkali metal ions was mainly related to

the motion of the complexing segments of the starch

chains. The conductance of GPTPS was mainly dependent

on the movement of the starch chains and the concentration

of alkali metal ions. Here water acted as both a plasticizer,

which made the starch chains flexible, and a solvent, which

made alkali metal salt exist as the ions.

As shown in Figure 6, the conductance of GPTPS was

very dependent of water contents. The conductance of

GPTPS increased about 5 orders of magnitude as the water

content increased from 0.05 to 0.55 wt.-%. The effect of

water content on the conductance at low water content

(from 0.05 to 0.30 wt.-%) was more obvious than at high

water content (from 0.30 to 0.55 wt.-%). The conductance

of GPTPS was increased about 4 and 1 orders of magnitude

at these two ranges of water contents, respectively. On

the one hand, water interacted with starch, weakened the

interaction of starch molecules, and improved the move-

ment of the starch chain. This was prone to the transference

of alkali metal ions in GPTPS. On the other hand, water

could dissolve alkali metal salts, decrease salt congregation

or crystals, and increase the effective ion numbers.

GPTPS with different alkali metal ion styles and con-

tents exhibited a similar relationship between conductance

and water content. A second-order polynomial was suffi-

cient to describe the relationship. The model gave good

agreement (R2> 0.96) for water contents between 0.05 and

0.55 wt.-%. The second-order polynomial correlation

between the conductance (y) and water content (x) of

GPTPS were obtained from Figure 6 and are listed in

Table 1.


For GPTPS with the same salt and water content, salt

contents had a great influence on the conductance. As

shown in Figure 6, the introduction of salts increased the

conductance by 1–2 orders of magnitude. As the salt con-

tent increased, the concentration of alkali metal ions



Table 1. The second-order polynomial correlation between the conductance (y) and water contents (x/wt.-%) of GPTPS from Figure 6.

Salt content LiCl NaCl KCl

moles per 100 g starch

0 (a) y¼�16.43x2þ 19.95x�9.34R2¼ 0.989

0.2 (b) y¼�21.19x2þ 20.35x �7.16 y¼�22.86x2þ 22.84x �7.77 y¼�31.06x2þ 25.64x �7.83R2¼ 0.996 R2¼ 0.993 R2¼ 0.988

0.4 (c) y¼�18.37x2þ 19.03x �6.82 y¼�17.25x2þ 19.37x �7.11 y¼�25.22x2¼ 23.34x �7.41R2¼ 0.992 R2¼ 0.986 R2¼ 0.992

0.6 (d) y¼�16.45x2þ 18.47x �6.50 y¼�16.91x2þ 19.02x �6.92 y¼�22.92x2þ 22.04x �7.04R2¼ 0.991 R2¼ 0.979 R2¼ 0.979

0.8 (e) y¼�18.49x2þ 19.51x �6.40 y¼�17.11x2þ 19.00x �6.72 y¼�22.35x2þ 22.01x �7.03R2¼ 0.987 R2¼ 0.969 R2¼ 0.983

increased, and the conductance increased. However, super-

fluous salts resulted in the congregation of or crystals of

salt, which could not improve the ion concentration and

conductance of GPTPS. As shown in Figure 6, the effect of

salt contents on the conductance of GPTPS was not

obvious when the salt content was above 0.6mol per 100 g

starch. Furthermore, there was a similar second-order

polynomial correlation between the conductance (y) and

water content (x) between GPTPS with 0.6 mol KCl per

100 g starch and GPTPS with 0.6 mol KCl per 100 g starch,

as revealed in Table 1.

It seemed that salt style had no obvious effect on the

correlation between the conductance and water content.

Water could complicate the effect of salt style on the

conductance, because NaCl, KCl, and LiCl had different

solubility in water, and water could form the interactionwith

starch and weaken the interaction between alkali metal ions

and starch.

Conclusion

GPTPS containing alkali metal chlorides (LiCl, NaCl, and

KCl salts) could be used as solid polymer electrolytes,

prepared by melt extrusion. In GPTPS, alkali metal ions

could interact with the oxygen of both O–H and C–O–C

groups in starch. This interaction could effectively restrain

starch recrystallization, make starch molecules facile, im-

prove the tensile elongation at break, and increase alkali

metal ion conductivity. An increase in salt content could

increase the conductance of GPTPS, but superfluous salts

resulted in the congregation of or crystals of salts, which

definitely affected mechanical properties and conductivity

of GPTPS. The appropriate salt content was 0.6 mol per

100 g starch. The conductance of GPTPS was water sensi-

tive. The relation between conductance (y) and water

content (x) could be described by a second-order poly-

nomial.


It is necessary to further study TPS as a solid polymer

electrolyte in the melt extrusion method. Because the

plasticizers containing amide groups can make starch

molecules more flexible than glycerol,[16] the effect of these

plasticizers (such as formamide and urea) on TPS conducti-

vity should be researched in detail. The introduction of

carbon black and carbon nanotubes into TPS should also

improve the mechanical properties and the conductivity of

TPS as solid polymer electrolytes. As the new SPE mate-

rials from natural biopolymers, TPS will have promising

applications such as in biosensors, artificial muscles,

electronic shielding, environmentally sensitive membranes,

visual displays, solar materials, and components in high-

capacity batteries.[17]

[1] L. V. S. Lopes, D. C. Dragunski, A. Pawlicka, Electrochim.Acta 2003, 48, 2021.

[2] A. M. Stephan, Eur. Polymer J. 2006, 42, 21.[3] V. L. Finkenstadt, J. L. Willett, J. Polym. Environ. 2004, 12,

43.[4] R. Andrews, D. Jacques, M. Minot, T. Rantell, Macromol.

Mater. Eng. 2002, 287, 395[5] V. L. Finkenstadt,Appl.Microbiol. Biotechnol. 2005, 67, 735.[6] V. L. Finkenstadt, J. L. Willett, Carbohydr. Polym. 2004, 55,

149.[7] M. Forsyth, M. Garcia, D. R. MacFarlane, Solid State Ionics

1996, 86–88, 1365.[8] X. F.Ma, J. G.Yu, Y. B.Ma,Carbohydr. Polym. 2005, 60, 111.[9] X. F. Ma, J. G. Yu, J. Appl. Polym. Sci. 2004, 93, 1769.[10] J. M. Fang, P. A. Fowler, J. Tomkinson, Carbohydr. Polym.

2002, 47, 245.[11] H. F. Zobel, Starch-Starke 1998, 40, 44.[12] J. J. G. Van Soest, N. Knooren, J. Appl. Polym. Sci. 1997, 64,

1411.[13] J. J. G. Van Soest, S. H. D. Hulleman, D. de Wit, Ind. Crop.

Prod. 1996, 5, 11.[14] X. F. Ma, J. G. Yu, Starch-Starke 2004, 56, 545.[15] W. H. Meyer, Adv. Mater. 1998, 10, 439.[16] X. F. Ma, J. G. Yu, Carbohydr. Polym. 2004, 57, 197.[17] V. L. Finkenstadt, J. L. Willett,Macromol. Symp. 2005, 227,

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thermoplastic starch plasticized by glycerol as solid polymer electrolytes

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