wti

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rchprepared by casting. Scanning and transmission electron microscopy and X-ray diraction proved that the MWCNTs were dispersedwell in the GPS matrix. The introduction of MWCNTs restrained starch re-crystallization, improved the tensile strength and Youngsmodulus, but reduced the toughness of the nanocomposites. The electrical conductivity was sensitive to the presence of water. The con-

biosensors, environmentally sensitive membranes, articial

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

a promising alternative for the development of new EAP

the 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

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

Available online at www.sciencedirect.com

Composites Science and Technolo

COMPOSITESmuscles, 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 uoride), poly(vinylidene uo-ride-hexauoro propylene), etc. [2]. Many synthetic poly-mers are usually prepared in the form of intractablelms, gels, or powders that are insoluble in most solvents.

materials. Lopes et al. [3] gelatinize amylopectin-rich starchwith water on a hotplate. The solution is combined withglycerol, mixed with LiClO4, cast onto Teon plates, andallowed to dry. The starchglycerolLiClO4 lms exhibitconductivity of around 105 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 lms doped with metalhalides to produce solid ion-conducting materials [5].Starch based-materials are potential to become EAP, butductivity 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 like

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 oersGlycerol plasticized-starch/multifor electroac

Xiaofei Ma, Jiuga

School of Science, Tianjin U

Received 8 January 2007; received in revisedAvailable onlin

Abstract

As the potential electroactive polymers, glycerol plasticized-sta0266-3538/$ - see front matter 2007 Elsevier Ltd. All rights reserved.doi:10.1016/j.compscitech.2007.03.016all carbon nanotube compositesve polymers

Yu *, Ning Wang

rsity, Tianjin 300072, China

m 8 February 2007; accepted 8 March 20073 March 2007

(GPS)/multiwall carbon nanotube (MWCNT) composites were

www.elsevier.com/locate/compscitech

gy 68 (2008) 268273

SCIENCE ANDTECHNOLOGY

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 diraction (XRD). The mechanical, electrical prop-erties and the eect of water content on the electrical con-ductivity of composites are also researched here.

2. Experimental section

2.1. Materials

The puried MWCNTs with an average diameter of10 nm used in this work were provided by Department of

3 mol/l nitric acid and reuxing 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 purication.

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 temperature

X. Ma et al. / Composites Science and Technology 68 (2008) 268273 269Chemical 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 puried by the methods of Zouet al. [15]. The MWCNTs were treated by immersing inFig. 1. TEM micrograph for MWCNTs (a), GPS lled with 2.85 wt% MWCfor 212 h and then in a climate-controlled container at20 C and 50% relative humidity (RH) for 24 h. Theobtained lms 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 lled with 2.85 wt% MWCNTs (c) and (d).

an2.3. 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 dierent. 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 5070 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 diraction (XRD)

GPS/MWCNT composites were placed in a sampleholder for XRD. XRD patterns were recorded in the reec-tion mode in angular range 1030 (2h) at the ambient tem-perature by a BDX3300 diractometer, 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 diractometer 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 lms. 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, Youngs modulusand energy break were tested. Energy Break was expressedas the areas below the stressstrain curves of GPS/MWCNT composites. The data was averages of 58specimens.

2.7. Electrical conductivity

Volume resistivity measurements were performed onsamples of all composites that were rstly 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-

270 X. Ma et al. / Composites Sciencetrometer (SPSIC Huguang Instruments & Power SupplyBranch, China) using a four-point test xture.2.8. Water content

In order to analyze the eect 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 35 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 magnications. The wirelikeMWCNTs could be clearly identied 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 dierent 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 reection atabout 2h = 25.7, which was derived from the orderedarrangement of the concentric cylinders of graphitic car-

d Technology 68 (2008) 268273bon [18]. This peak was absent in Fig. 2ce for theXRD patterns of GPS/MWCNT composites, which was

propagation was inhibited, which resulted in the increasedtensile strength and Youngs modulus. Contrarily, it illus-trated that there were interfacial adhesion between

0 1 2 3 4 5120

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

anthe further evidence for ecient 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. 2ce. 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-

10 15 20 25 30

edcb

aInte

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s)

2 theta (degree)

Fig. 2. The X-ray diraction 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.

X. Ma et al. / Composites Sciencetent with the paper of Angellier et al. [24], which revealedthat the ller 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 Youngs 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 crack0 1 2 3 4 52

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d Technology 68 (2008) 268273 271MWCNTs 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 stiness 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 signi-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

an0.0 0.1 0.2 0.3 0.4 0.5 0.6-11

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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

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)272 X. Ma et al. / Composites Sciencedescribed well with a second-order polynomial. The bino-mial correlation of the conductivity (y) and water content(x) was supposed as y = B2x

2 + B1x + B0. The model gavea good agreement (R2 > 0.97) except the GPS/MWCNTcomposite with 4.75 wt% MWCNTs, which more accordedwith the line t (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 monomialcoecient B1 and the binomial coecient 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

the creation of the interconnecting conductive channels,

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Fig. 4. The electrical conductivity of GPS with dierent MWCNTcontents. (a) The eect of water contents on the electrical conductivityof GPS with dierent MWCNT contents. (b) The conductivity of GPSlled with various MWCNT contents at 0 water content.which had been highlighted using an upright bar inFig. 4b. As shown in Fig. 4a, the conductivity of the com-posites was aected 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, Youngs 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, articial muscles, corrosionprotection, electronic shielding, environmentally sensitivemembranes and solar materials.

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

The addition of SDS denitely had an inuence on theconductivity of the composites, especially when GPS con-tained water. In order to eliminate the eect 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] Stephan AM. Review on gel polymer electrolytes for lithium batteries.Eur Polym J 2006;42:2142.

[3] Lopes LVS, Dragunski DC, Pawlicka A, Donoso JP. Nuclearmagnetic resonance and conductivity study of starch based polymerelectrolytes. Electrochim Acta 2003;48:20217.

[4] Finkenstadt VL, Willett JL. A direct-current resistance technique fordetermining moisture content in native starches and starch-basedplasticized materials. Carbohydr Polym 2004;55:14954.

[5] Finkenstadt VL, Willett JL. Electroactive materials composed ofstarch. J Polym Environ 2004;12:436.

[6] Chatterjee T, Yurekli K, Hadjiev VG, Krishnamoorti R. Single-walled carbon nanotube dispersions in poly(ethylene oxide). AdvFunct Mater 2005;15(11):18328.

[7] Zhang QH, Rastogi S, Chen DJ, Lippits D, Lemstr PJ. Lowpercolation threshold in single-walled carbon nanotube/high densitypolyethylene composites prepared by melt processing technique.Carbon 2006;44(4):77885.

[8] Sarno M, Gorrasi G, Sannino D, Sorrentino A, Ciambelli P, VittoriaV. Polymorphism and thermal behaviour of syndiotactic poly(pro-pylene)/carbon nanotube composites. Macromol Rapid Commun2004;25(23):19637.

[9] Eitan A, Fisher FT, Andrews R. Reinforcement mechanisms inMWCNT-lled polycarbonate. Compos Sci Technol 2006;66(9):116273.

[10] Allaoui A, Bai S, Cheng HM. Mechanical and electrical properties ofa MWNT/epoxy composite. Compos Sci Technol 2002;62(15):19938.

[11] Mottaghitalab V, Spinks GM, Wallace GG. The inuence of carbonnanotubes on mechanical and electrical properties of polyanilinebers. Synthetic Met 2005;152(13):7780.

[12] Kuan HC, Ma CCM, Chang WP. Synthesis, thermal, mechanical and

[13] Li ZF, Luo GH, Wei F, et al. Microstructure of carbon nanotubes/PET conductive composites bers and their properties. Compos SciTechnol 2006;66(78):10229.

[14] Wang Y, Wei F, Luo GH, Yu H, Gu GS. The large-scale productionof carbon nanotubes in a nano-agglomerate uidized-bed reactor.Chem Phys Lett 2002;364(56):56872.

[15] Yaobang Zou, Yongcheng Feng, Lu Wang, Xiaobo Liu. Processingand properties of MWNT/HDPE composites. Carbon 2004;42:2717.

[16] Zhang QH, Lippits DR, Rastogi S. Dispersion and rheologicalaspects of SWNTs in ultrahigh molecular weight polyethylene.Macromolecules 2006;39(2):65866.

[17] Ma XF, Yu JG, Ma YB. Urea and formamide as a mixed plasticizerfor thermoplastic wheat our. Carbohydr Polym 2005;60:1116.

[18] Zhou O, Fleming RM, Murphy DW, Chen CH, Haddon RC,Ramirez PA. Doping carbon nanotubes. Science 1994;263:17447.

[19] McNallya T, Potschkeb P, Halleyc P, Murphyc M, Martinc D, BelldSEJ. Polyethylene multiwalled carbon nanotube composites. Polymer2005;46(19):822232.

[20] Ma XF, Yu JG. The eects of plasticizers containing amide groups onthe properties of thermoplastic starch. Starch-Starke 2004;56(11):54551.

[21] Van Soest JJG, Knooren N. Inuence of glycerol and water contenton the structure and properties of extruded starch plastic sheetsduring aging. J Appl Polym Sci 1997;64:141122.

[22] Van Soest JJG, Hulleman SHD, de Wit D, Vliegenthart JFG.Crystallinity in starch bioplastics. Ind Crop Prod 1996;5:1122.

[23] Stobinski L, Tomasik P, Lii CY, Chan HH, Lin HM, Liu HL. Single-walled carbon nanotubeamylopectin complexes. Carbohydr Polym2003;51(3):3116.

[24] Angellier H, Molina-Boisseau S, Dole P, Dufresne A. Thermoplastic

X. Ma et al. / Composites Science and Technology 68 (2008) 268273 273rheological properties of multiwall carbon nano tube/waterbornepolyurethane nanocomposite. Compos Sci Technol 2005;65(1112):170310.starch-waxy maize starch nanocrystals nanocomposites. Biomacro-molecules 2006;7:5319.

[25] Meyer WH. Polymer electrolytes for lithium-ion batteries. Adv Mater1998;10:43948.

Glycerol plasticized-starch/multiwall carbon nanotube composites for electroactive polymersIntroductionExperimental sectionMaterialsPreparation of GPS/MWCNT compositesScanning electron microscopy (SEM)Transmission electron microscopy (TEM)X-ray diffraction (XRD)Mechanical propertiesElectrical conductivityWater content

Results and discussionThe dispersionXRDMechanical propertiesElectrical conductivity

SummaryReferences