Available online at www.sciencedirect.comCOMPOSITES

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Composites Science and Technology 68 (2008) 268–273

SCIENCE ANDTECHNOLOGY

Glycerol plasticized-starch/multiwall carbon nanotube compositesfor electroactive polymers

Xiaofei Ma, Jiugao Yu *, Ning Wang

School of Science, Tianjin University, Tianjin 300072, China

Received 8 January 2007; received in revised form 8 February 2007; accepted 8 March 2007Available online 23 March 2007

Abstract

As the potential electroactive polymers, glycerol plasticized-starch (GPS)/multiwall carbon nanotube (MWCNT) composites wereprepared by casting. Scanning and transmission electron microscopy and X-ray diffraction proved that the MWCNTs were dispersedwell in the GPS matrix. The introduction of MWCNTs restrained starch re-crystallization, improved the tensile strength and Young’smodulus, but reduced the toughness of the nanocomposites. The electrical conductivity was sensitive to the presence of water. The con-ductivity versus water content relationship could be described with a second-order polynomial. The composites exhibited a low electricalpercolation threshold of 3.8 wt% MWCNTs loading and the conductivity of the composite containing 4.75 wt% MWCNTs reached100 S/cm, which was almost independent of water contents.� 2007 Elsevier Ltd. All rights reserved.

Keywords: A. Starch; A. Carbon nanotubes; B. Electrical properties; A. Electroactive polymers; E. Casting

1. Introduction

As a new class of materials, electroactive polymers(EAPs) have the potential to be used for applications likebiosensors, environmentally sensitive membranes, artificialmuscles, actuators, corrosion protection, electronic shield-ing, visual displays, solar materials, and components inhigh-energy batteries [1]. Currently, several synthetic poly-mer matrices have been developed and characterized thatinclude poly(ethylene oxide) (PEO), poly(propylene oxide),poly(acrylonitrile), poly(methyl methacrylate), poly(vinylchloride), poly(vinylidene fluoride), poly(vinylidene fluo-ride-hexafluoro propylene), etc. [2]. Many synthetic poly-mers are usually prepared in the form of intractablefilms, gels, or powders that are insoluble in most solvents.

EAPs obtained from natural polymers such as starch,cellulose, chitosan, pectin, hyaluronic acid, agarose and

0266-3538/$ - see front matter � 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.compscitech.2007.03.016

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

carrageenan, have attracted attention in recent times [1].Among of them, starch is an abundant, renewable, low-cost and biodegradable natural polymer. Both melt extru-sion and casting are available for starch, and its use offersa promising alternative for the development of new EAPmaterials. Lopes et al. [3] gelatinize amylopectin-rich starchwith water on a hotplate. The solution is combined withglycerol, mixed with LiClO4, cast onto Teflon plates, andallowed to dry. The starch–glycerol–LiClO4 films exhibitconductivity of around 10�5 S/cm. Finkenstadt et al. [4]study the accurate determination of the moisture contentof native starch using a direct-current resistance technique,and prepare thermoplastic starch films doped with metalhalides to produce solid ion-conducting materials [5].Starch based-materials are potential to become EAP, butthe mechanical properties and the electrical conductivitymust be improved.

On the other hand, the extraordinary mechanical andelectrical properties of carbon nanotubes make them out-standing materials to blend with polymers to preparepotentially multifunctional nanocomposites [6]. The

X. Ma et al. / Composites Science and Technology 68 (2008) 268–273 269

dispersion of carbon nanotubes in solvents or polymers atthe aid of a surfactants or a copolymer is an importantmethod without containing chemical reaction. Carbonnanotubes are dispersed well in high density polyethylene(HDPE) [7], poly(propylene) (PP) [8], PEO [6], bisphenol-A polycarbonate (PC) [9], epoxy resin [10], polyaniline[11], polyurethane [12], poly(ethylene terephthalate)(PET) [13], and so on.

In order to improve the mechanical and electrical prop-erties of starch-based materials, the multiwall carbon nano-tube (MWCNT) is doped into glycerol-plasticized starch(GPS) matrix to prepare GPS/MWCNT composites asEAPs by casting in this study. The dispersion of MWCNTsin GPS matrix is studied by scanning electron microscopy(SEM), transmission electron microscopy (TEM) andX-ray diffraction (XRD). The mechanical, electrical prop-erties and the effect of water content on the electrical con-ductivity of composites are also researched here.

2. Experimental section

2.1. Materials

The purified MWCNTs with an average diameter of10 nm used in this work were provided by Department ofChemical Engineering, Tsinghua University, and synthe-sized from ethylene and propylene gas via catalytic\(Fe/Al2O3 as the catalyst) chemical vapor deposition[14]. The nanotubes were purified by the methods of Zouet al. [15]. The MWCNTs were treated by immersing in

Fig. 1. TEM micrograph for MWCNTs (a), GPS filled with 2.85 wt% MWC

3 mol/l nitric acid and refluxing for 6 h, subsequentlywashed with distilled water until the pH of the solutionapproached 7. Cornstarch was obtained from LangfangStarch Company. Glycerol and sodium dodecylsulfate(SDS) were purchased from Tianjin Chemical ReagentFactory, which were analytical reagents and used withoutfurther purification.

2.2. Preparation of GPS/MWCNT composites

MWCNT aqueous solution was prepared at the aid ofSDS, according to a reported method by Zhang [16]. Indetail, 100 ml solution containing 0.5 g MWCNTs and0.5 wt% SDS based on H2O was sonicated for 2 h and thencentrifuged at 4000 rpm for 20 min. The MWCNTs weresuspended in the aqueous solution, whereas the remainderis deposited. The morphology of MWCNTs was shown inFig. 1a.

Five grams starch and 1.5 g glycerol were added into theaqueous solution with MWCNTs of 0.19 wt%. The solu-tions were mixed with strong stirring and heated for40 min in 75 �C water bath. The obtained solution wascasted onto a polystyrene tray, with the length of 20 cmand the width of 15 cm. The cast solutions were dried at80 �C for 1 h in the oven and then at room temperaturefor 2–12 h and then in a climate-controlled container at20 �C and 50% relative humidity (RH) for 24 h. Theobtained films with the thickness of 0.5 mm were precondi-tioned in a climate chamber at 20 �C and 50% RH for atleast 48 h prior to all testing.

NTs (b) and SEM for GPS filled with 2.85 wt% MWCNTs (c) and (d).

270 X. Ma et al. / Composites Science and Technology 68 (2008) 268–273

2.3. Scanning electron microscopy (SEM)

SEM was carried out with Philips XL-3. GPS/MWCNTcomposites were cooled in liquid nitrogen, and then bro-ken. The fracture surfaces were vacuum coated with goldfor SEM.

2.4. Transmission electron microscopy (TEM)

Sample preparations of MWCNTs and GPS/MWCNTcomposites for TEM testing were different. The suspen-sion of MWCNTs was dropped on the copper grid, driedin the air, and tested for TEM. The samples of GPS/MWCNT composites were sliced in liquid nitrogen withthe Reichert-Jung Utracut E extrathin slicer. The slices(the thickness of 50–70 nm) were spread on copper gridfor TEM testing. The samples were performed withTEM JEM-1200EX, operating at an acceleration voltageof 80 kV.

2.5. X-ray diffraction (XRD)

GPS/MWCNT composites were placed in a sampleholder for XRD. XRD patterns were recorded in the reflec-tion mode in angular range 10–30� (2h) at the ambient tem-perature 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 a15 lm nickel foil. The diffractometer was equipped with1� divergence slit, a 16 mm beam bask, a 0.2 mm receivingslit and a 1� scatter slit. Radiation was detected with a pro-portional detector.

2.6. Mechanical properties

Samples were cut from the composite films. The Testo-metric AX M350-10KN materials testing machine wasoperated and a crosshead speed of 10 mm/min was usedfor tensile testing (ISO 1184-1983 standard). Here the ten-sile strength, the elongation at break, Young’s modulusand energy break were tested. Energy Break was expressedas the areas below the stress–strain curves of GPS/MWCNT composites. The data was averages of 5–8specimens.

2.7. Electrical conductivity

Volume resistivity measurements were performed onsamples of all composites that were firstly compressed intothin sheets. A Model ZC36 electrometer (SPSIC HuguangInstruments & Power Supply Branch, China) was used forhigh resistivity samples with 50 mm diameter and 0.5 mmthickness. For more conductive samples (larger than106 S/cm) strips with dimensions of 30 mm · 5 mm and0.5 mm thickness were measured using a Model ZL7 elec-trometer (SPSIC Huguang Instruments & Power SupplyBranch, China) using a four-point test fixture.

2.8. Water content

In order to analyze the effect of water contents on elec-trical conductivity, the samples were stored in closed cham-bers over several materials at 20 �C for several days. Theused materials were dried silica gel, substantive 55.01%H2SO4 solution, substantive 35.64% CaCl2 solution, NaClsaturated solution and distilled water, providing relativehumidities (RH) about 0%, 25%, 50%, 75% and 100%,respectively. The original water contents (dry basis) ofTPS were determined gravimetrically by drying smallpieces of TPS at 105 �C overnight. At this condition, theevaporation of the plasticizers was negligible [17]. WhenTPS was stored for a period of time, its water contentwas calculated on the base of its original weight, its currentweight and its original water content. Water contents werethe weight ratios of water and dried samples.

3. Results and discussion

3.1. The dispersion

Dispersion of the MWCNTs in the GPS matrix was oneof the key elements to the electrical conductivity andmechanical properties of the GPS/MWCNT composites.The morphology and the degree of dispersion of theMWCNTs in the GPS matrix were studied using a combi-nation of TEM and SEM. Fig. 1a indicated that the carbonnanotubes were 10 nm in outer diameter and about 3–5 nmin inner diameter. As shown in Fig. 1b, the MWCNTsappeared to be typically well dispersed as the single nano-tube in the GPS matrix because few compact aggregatescould be detected. The MWCNTs seemed to be well wettedby the GPS, and this suggested good adhesion betweenGPS and the MWCNTs. Fig. 1c showed the SEM of typi-cal cryo-fractured surfaces of GPS with 2.85 wt%MWCNT at 10,000· magnifications. The wirelikeMWCNTs could be clearly identified and were uniformlydispersed throughout the cross section, indicating the for-mation of an isotropic, three-dimensional nanotube net-work in the host GPS matrix. It was essential to obtainGPS/MWCNT composites with isotropic electrical con-ductivity and mechanical properties. On the other hand,as shown in Fig. 1d, native starch granules were brokenup, and a continuous phase of GPS matrix formed onthe action of the hot water and glycerol.

3.2. XRD

GPS was prepared with different MWCNT contents,stored in the airtight containers for one week, and testedwith XRD. As shown in Fig. 2a, the XRD pattern ofMWCNTs exhibited a sharp (002) Bragg reflection atabout 2h = 25.7�, which was derived from the orderedarrangement of the concentric cylinders of graphitic car-bon [18]. This peak was absent in Fig. 2c–e for theXRD patterns of GPS/MWCNT composites, which was

10 15 20 25 30

edcb

aInte

nsity

(cps

)

2 theta (degree)

Fig. 2. The X-ray diffraction patterns of MWCNTs and GPS/MWCNTcomposites stored for one week in the airtight container (a) MWCNTs, (b)GPS, (c) GPS-0.95 wt% MWCNTs, (d) GPS-2.85 wt% MWCNTs, and (e)GPS-3.8 wt% MWCNTs.

0 1 2 3 4 52

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elon

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n at

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tren

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)

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0 1 2 3 4 5120

160

200

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0.6

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1.2

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Ene

rgy

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.m)

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Pa)

MWCNT content (wt%)

Fig. 3. The effect of MWCNT contents on mechanical properties of GPS/MWCNT composites.

X. Ma et al. / Composites Science and Technology 68 (2008) 268–273 271

the further evidence for efficient dispersion of theMWCNTs in GPS matrix [19]. In the gelation processing,glycerol and water molecules entered into starch granules,and replaced starch intermolecular and intramolecularhydrogen bonds and destructed the crystallinity of starch.There was no obvious starch crystallinity in new-madeGPS [20]. However, GPS was thought to tend to re-crys-tallization after being stored for a period of time [21].As shown in Fig. 2b, V-style starch crystallinity [22]appeared again in GPS without MWCNTs, while therewas no obvious starch crystallinity in GPS with MWCNTsin Fig. 2c–e. The addition of MWCNTs could restrainstarch re-crystallization, because the MWCNTs couldform the interaction with starch according to Stobinskiet al. [23], and the good dispersion of MWCNTs in GPSmatrix spatially prevented starch molecules from moving,interacting and crystallizing again. This result was consis-tent with the paper of Angellier et al. [24], which revealedthat the filler in nanometer-scale reduced the mobility ofpolymer chains and led to a considerable slowing downof the re-crystallization of TPS. The amorphous regionof starch was advantageous for the electrical conductivityof GPS/MWCNT composites, because the re-crystalliza-tion of starch could spatially demolish the good dispersionof MWCNT in GPS.

3.3. Mechanical properties

GPS/MWCNT composites were enveloped in a climatechamber at 20 �C and 50% RH for one week prior tomechanical test. The mechanical properties of the GPS/MWCNT composites were measured as a function ofMWCNT contents and were shown in Fig. 3. The tensilestrength and Young’s modulus increased as the contentof MWCNTs was increased up to 4.75 wt%. However,both the elongation at break and energy break decreased.

With the increasing of MWCNT content, the interac-tions between the MWCNTs were improved, and crack

propagation was inhibited, which resulted in the increasedtensile strength and Young’s modulus. Contrarily, it illus-trated that there were interfacial adhesion betweenMWCNTs and GPS, otherwise, it would result in prema-ture composite failure because the reinforcing nanotubessimply pulled out of the matrix without contributing tothe strength or stiffness of the material.

The toughness of the GPS/MWCNT composite wasreduced and addition of the MWCNTs yielded increasinglybrittle samples. The good dispersion of MWCNTs in GPSmatrix spatially restrained the slippage movement amongstarch molecules, so low loadings of MWCNTs signifi-cantly decreased both elongation at break and energybreak.

3.4. Electrical conductivity

Because starch was hydrophilic, water sensitivity was animportant criterion for many practical applications ofstarch-based materials. As shown in Fig. 4a, the electricalconductivity of GPS/MWCNT composite was very depen-dent of water content. GPS/MWCNT composites with dif-ferent MWCNT contents exhibited the similar relationshipof the conductivity versus water content, which could be

0.0 0.1 0.2 0.3 0.4 0.5 0.6-11

-10

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CNT0%: y=-9.39+19.95x-16.43x2 R2=0.99

CNT0.95%: y=-7.28+12.95x-9.76x2 R2=0.99

CNT1.9%: y=-5.95+10.37x-8.74x2 R2=0.99

CNT2.85%: y=-5.02+7.88x-6.76x2 R2=0.98

CNT3.8%: y=-2.47+1.40x-1.04x2 R2=0.97

CNT4.75%: y=-0.14+0.17x-0.07x2 R2=0.90

log

Con

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ivity

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cm)

Water contents

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-8

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duct

ivity

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Fig. 4. The electrical conductivity of GPS with different MWCNTcontents. (a) The effect of water contents on the electrical conductivityof GPS with different MWCNT contents. (b) The conductivity of GPSfilled with various MWCNT contents at 0 water content.

272 X. Ma et al. / Composites Science and Technology 68 (2008) 268–273

described well with a second-order polynomial. The bino-mial correlation of the conductivity (y) and water content(x) was supposed as y = B2x2 + B1x + B0. The model gavea good agreement (R2 > 0.97) except the GPS/MWCNTcomposite with 4.75 wt% MWCNTs, which more accordedwith the line fit (y = �0.14 + 0.13x, and R = 0.94). Thesecond-order polynomial correlations of GPS/MWCNTcomposites were listed in Fig. 4a. The conductivity ofGPS without MWCNTs increased about 5 orders of mag-nitude when water content varied from 0 to 0.6. The con-ductivity of the GPS/MWCNT composite with 4.75 wt%MWCNTs changed less with the increasing of water con-tent. As the MWCNT contents of the GPS/MWCNT com-posites were increased, the sensitivity of the conductivity towater was reduced. It was obvious that both the monomialcoefficient B1 and the binomial coefficient B2 approachedmore to the zero with the increasing of MWCNT contents.Water could form the interaction with starch, weaken theinteraction of starch molecules and improve the movementof starch chain [21]. It was advantageous to improve theconductivity of the matrix [25]. However, the introductionof MWCNTs and good dispersion of MWCNTs in GPS

spatially restrained the movement of starch chain even athigh water content, so the effect of water content on theconductivity was weakened.

The addition of SDS definitely had an influence on theconductivity of the composites, especially when GPS con-tained water. In order to eliminate the effect of SDS andwater on the conductivity of GPS/MWCNT composites,the conductivity at water content (x = 0) was calculatedfrom the listed second-order polynomial correlations inFig. 4a and b showed the relationship between the conduc-tivity and the MWCNT contents at water content (x = 0).Apparently, the conductivity was improved by increasingMWCNTs. At very low content of MWCNT(<2.85 wt%), the conductivity gradually increased asincreasing nanotube content. However, the conductivityof the composite containing 4.75 wt% MWCNTs increasedto 100 S/cm from that of 2.85 wt% MWCNTs of 10�5 S/cm. This stepwise change in conductivity was a result ofthe formation of an interconnected structure of MWCNTsand could be regarded as an electrical percolation thresh-old. In the other words, at about 3.8 wt% MWCNT load-ing, a very high percentage of electrons were permitted toflow through the sample at applied electric filed due tothe creation of the interconnecting conductive channels,which had been highlighted using an upright bar inFig. 4b. As shown in Fig. 4a, the conductivity of the com-posites was affected much by water contents until an inter-connected structure of MWCNTs formed at above 3.8 wt%MWCNT loading. The formation of an interconnectedstructure of MWCNTs spatially restrained the movementof starch chain.

4. Summary

GPS/MWCNT composites as potential EAP was pre-pared by casting method. MWCNTs were dispersed wellin GPS matrix. The introduction of MWCNTs improvedthe tensile strength, Young’s modulus and the electricalconductivity. As shown by the binomial correlation ofthe conductivity and water contents, the introduction ofMWCNTs weakened the dependence of the electrical con-ductivity on water content, even eliminated above the elec-trical percolation threshold of 3.8 wt% MWCNT loading.The electrical conductivity of the composite containing4.75 wt% MWCNTs increased to 100 S/cm, which wasalmost independent of water contents.

As a natural biopolymer, starch would be a promisingalternative for the development of new EAP materials,which had a wide variety of potential applications suchas antistatic plastics, biosensor, artificial muscles, corrosionprotection, electronic shielding, environmentally sensitivemembranes and solar materials.

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