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Role of Nano-Sized TiO2 on Mechanical and Thermal Behavior of Starch/Poly (vinyl alcohol) Blend Films*

Z.Hejri1*,A.Ahmadpour2,A.A.Seifkordi1,S.M.Zebarjad3

1-DepartmentofChemicalEngineering,ScienceandResearchBranch,IslamicAzadUniversity,Tehran,I.R.Iran

2-DepartmentofChemicalEngineering,FerdowsiUniversityofMashhad,Mashhad,I.R.Iran3-DepartmentofMaterialsScienceandMetallurgy,FerdowsiUniversityofMashhad,Mashhad,I.R.Iran

(*)Correspondingauthor:za_hejri@yahoo.com(Received:30 Nov. 2012 and Accepted: 27 Dec. 2012)

Abstract:A novel Starch/Poly (vinyl alcohol)/nano-Titanium dioxide (ST/PVA/nano-TiO2) biodegradable nanocomposite film was prepared by homogeneously dispersed TiO2 nanoparticles in different ratios of starch/PVA-based materials, via a solution casting method. Glycerol was used as plasticizer. The mechanical and thermal properties of films were studied using tensile and perforation strength tests and thermogravimetric analysis (TGA). In order to determine the effective parameters on mechanical properties of prepared films, a general full factorial experimental design approach was used. ST/PVA/TiO2 nanocomposites were also characterized by scanning electron microscopy (SEM). The results of mechanical analysis showed that ST/PVA films with higher content of PVA had much better mechanical properties. In thermal analysis it was found that addition of TiO2 nanoparticles have considerable effects on the thermal stability of the films. This enhancement of thermal stability can be attributed to an improvement in the interfacial adhesion and compatibility between the nano- TiO2 and matrix, due to the treating effect of plasticizer and film forming process. SEM micrographs, taken from the fracture surface of samples, illustrated a quite uniformly dispersion of TiO2 nanoparticles in the matrix. Moreover, addition of PVA to the composition of film made it softer and more flexible. Keywords: Nanocomposite, Biodegradable, Nano-TiO2, Mechanical properties, Thermogravimetric analysis.

Int. J. Nanosci. Nanotechnol., Vol. 8, No. 4, Dec. 2012, pp. 215-226

1.INTRODUCTION

The main types of plastics that are currently usedwidespread throughout the world are derived fromnon-renewablepetroleumresources.Becauseofvastapplications, the disposal of waste plastics createsserious problem of environmental pollution. Forthis reason, thedevelopmentofbiodegradablefilmshasbeenregardedasmoreenvironmentallyfriendlymaterials for packaging.They have the potential toreplacethesyntheticconventionalnon-biodegradable

polymers.Among biopolymers, starch has receivedconsiderableattentioninmanyapplicationsbecauseofitsbiodegradability,availabilityandlowcost.However,thepoormechanicalpropertiesandhydrophilicnatureof starchprevent itsuse inwidespreadapplications;hence its modification is necessary. Blending withotherbiodegradablesyntheticpolymericmaterials isacommonmethodtoimprovemechanicalpropertiesand reduce water sensitivity of starch. Starch/Poly(vinylalcohol)blendfilmsareoneofthemostpopularbiodegradable polymers, and are widely used in

* This papar is selected from 4th International Congress on Nanoscience and Nanotechnology (ICNN 2012)

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packaging applications. Jayasekara et al., [1],blendedthePVAwithstarchandglyceroltoformpolymerfilms.Theymodified thesurfaceof thefilmsby treatmentwith chitosan and observed improvements in someproperties. Zhai et al.,[2],showedthatthepropertiesofstarch-basedplasticsheetssuchasflexibilityandwetstrengthwasimprovedfurtherbyincorporatingPVAintostarch-basedsheetsunderionizingradiation.Rayet al., [3],preparedstarch/PVAfilmsplasticizedwithglycerolandcharacterizedfortheirphysicomechanicalandmorphologicalproperties.Zhouetal.,[4],foundthat the surface modification, considerably reducedthesurfacehydrophiliccharacterofthethermoplasticstarch/PVAfilms,enhancedthefilm’swaterresistanceand also increased tensile strength and Young’smodulusbutdecreasedelongationatbreakofthefilms.PVA is a biodegradable synthetic polymer whichhas theadvantagesofgoodfilmformation, strongadhesiveness, high thermal and mechanicalpropertiesanditmaybetheonlysynthesizedpolymerwhosebackbonemainlycomposedofc-cbond,thatis absolutely biodegradable [5]. Nevertheless, themechanical properties andwater resistance of thestarch/PVAfilmarestill lower than thoseofotherpolymers made from petroleum. Many studieshave indicated that nano-materials can improvethe performance of polymeric materials such as plastics.Avellaetal.,[6],preparedstarch/clayfilms.Theyobservedagoodintercalationofthepolymericphaseintoclayinterlayergalleries,togetherwithanincreaseofmechanicalparameters,suchasmodulusand tensile strength. Tang et al., [7], developedseveral starch/ poly (vinyl alcohol) /nano-silicondioxide biodegradable blend films and discoveredthe mechanism for improving the properties of astarch-based biodegradable film using nano-SiO2. Deanetal.,[8],preparedthermoplasticstarch/poly(vinylalcohol)montmorillonite(Na-MMT)micro-and nanocomposites and observed improvementsin properties of films.Yoon et al., [9], found thatthe mechanical properties and water resistanceof starch/PVA composite filmswere improved upto 70–400% by the addition of nano-sized poly(methylmethacrylate-co-acrylamide).Inthiswork,thestarch/PVAblendfilmwaspreparedby a castingmethod, and nano-TiO2 was used toimprove the properties of starch/PVAblendfilms.

Titaniumdioxideisanontoxicmaterialandhasbeenapplied inenvironmental treatmentssuchaswaterandairdisinfectionbecauseofitsuniquepropertiessuchasstrongphotocatalyticactivityandchemicalstability [10]. TiO2 which promotes peroxidationof the polyunsaturated phospholipids ofmicrobialcellmembranes iswidelyusedasaphotocatalyticdisinfecting and self-cleaning material forsurface and coatings industry [11]. A kind ofphotodegradable polystyrene-TiO2 nanocomposite film was synthesized by Zan et al., [12]. TheyshowedthatPS/TiO2nanocompositefilmscouldbeefficiently photocatalytically degraded under UVilluminationinair.Theyobservedthatbigcavitieswereformednotonlyonthesurface,butalsoinsidethefilms.Thefilmstructurewasdestroyedandthechalking phenomenon took place, whereas onlysmall cavities and cracks were observed on thesurfaceofthepurepolystyrenefilm.Thisindicatesthat TiO2 nanoparticles will greatly enhance thephotocatalytic degradation of the polystyrenematerial.ChawengkijwanichandHayataetal.,[13], developedaTiO2powder-coatedpackagingfilmandverified its ability to reduceE.coli contaminationonthesurfaceofsolidfoodproducts.To the best of our knowledge, any researchconcentratedonthermalandmechanicalpropertiesofbiodegradable ST/PVApolymersmodified by nano-TiO2 have not yet been reported. The starch/PVAblendfilmswerepreparedbyacastingmethod,anddifferentcontentsofnano-TiO2wereusedtoimprovethepropertiesofpreparedfilms.Themainobjectiveof this paper is to investigate the mechanical andthermalpropertiesofresultednanocomposites.Starchcommonlyexists intheformofgranuleswithabout15-45%crystallinity.Toproducethermoplasticstarch(TPS) without granular structure, plasticizers havetobe incorporated,because theycanformhydrogenbonds with starch, replacing the strong intra- andintermolecular hydrogen bonds in starch, resultingin plasticization [14]. Generally, polyols are usedas plasticizers forTPS.After some studies glycerolwasselectedasplasticizerforcornstarchtopreparethermoplasticstarch.Theamountofplasticizershouldbehighenoughtoobtainasuitableflexiblefilm.Ourinitialtestsshowedthat30wt.%glycerolgivesgoodmechanicalandthermalpropertiesinpreparedfilms.

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2.MATERIALSANDMETHODS

2.1. Materials

Corn starch (ST)was provided by Sigma-Aldrich(amylose content 27%) and poly (vinyl alcohol)(PVA) by Merck (molecular weight 72000 g/mole). Nano-TiO2 powder (anatase-phase crystalstructure with average particle size of about 25nm)was suppliedbyNanolin,Germany.Glycerol(GLY)(99%purityfromMerck)wasemployedasplasticizer. The distilled and deionizedwater wasusedinexperiments.

2.2.Preparationofthefilms

BlendedStarch/PVAgelswerepreparedbythermalgelatinizationofmixturesuspensionsonamagneticstirrerusingdifferentstarchandPVAconcentrations.SomeoffilmswerefilledwithTiO2 nanoparticles accordingtothecorrespondingrunofexperiments.The levels of each componentwere selected afterpreliminarytests.Atfirst,aqueousST/PVAsolutionswerepreparedbydissolvingPVAin60mLdistilledwaterat80forabout6h.AftercompletedissolvingofPVA,starchwasmixedandafterthat,glycerolata30wt.%ratiobasedontotaldryweightofstarchand poly (vinyl alcohol)was added.Thismixturewasstirredforabout120minuntil40mLvolumeof solution was left.(water should be adequately

available in order to facilitate the PVAdissolvingand starch gelatinization completely, but at theend, the amountofwater shouldbeminimized inordertogetalessstickyandeasydrying).TheTiO2 nanoparticlesweredispersedinthematrixusingabath sonicator for one hour.These solutionswerethen poured onto glass petri dishes, placed on aleveled flat surface and dried under atmosphereconditions. The fully dried films were peeledawayfromtheglassplatesandstoredat20°C.Theseriesofstarch/PVA/nano-TiO2 formulations along withplasticizedcornstarchandPVA,whichwerepreparedforthisstudy,aresummarizedinTable1.

2.3.Filmcharacterization

2.3.1. Moisture contentMoisture content of samples was determinedmeasuringweight lossoffilms,upondryinginanovenat105C.Sampleswereanalyzedatleasttwiceand resultswereexpressedaspercentofmoisturecontent of samples. Percentage of moisture wascalculatedbasedontheoriginalweightofspecimenasfollows:

(1)

Where:w1=Originalweightofspecimenw2=Weightofspecimenafterovendrying

Table1: Composition of experimental samplesSample Starch

(Wt.%)Polyvinyl alcohol

(PVA)(Wt.%)

Glycerol(wt.%)(basedontotaldryweight

ofstarchandPVA)

Tio2nanoparticle(wt.%)(basedontotaldryweightof

starchandPVA)

Set 1 60 40 30 0Set 2 60 40 30 2.5Set 3 60 40 30 5Set 4 50 50 30 0Set5 50 50 30 2.5Set6 50 50 30 5Set7 40 60 30 0Set 8 40 60 30 2.5Set9 40 60 30 5Set 10 100 0 30 0Set 11 0 100 30 0

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2.3.2. Thickness measurementsThickness of the films was determined using adigitalmicrometer(Mitutoyo,Japan)at10randompositionsofthefilmandthemeanvaluewasreportedand used in the calculations of the mechanicalproperties.

2.3.3. Mechanical tests Tensile and perforation strength tests wereperformedusingauniversaltensilemachinemodelZwick/Z250system(Germany).Tensiletestswereperformed according to the ASTM D882(2004),[15],method;filmswerecutinto120×30mmstrips,with the gauge length (i.e., the distance betweenthetwoclamps)setat50mm,andthetensilerate(cross head speed) was kept 3 mm/min. Stress(MPa) as a function of strain (%) and maximumforce at break (N), tensile strength (calculated bydividing maximum force by film cross-section inMPa)andE-modulus(Young’smodulus,slopeofstress-strain curve, N/mm2) were reported by thesystem software. Moreover, elongation at breakof thefilms (deformationdividedby initial lengthand multiplying by 100, %) were measured. Inperforation test, samples with diameters of 120mmwerefixedontheholderwithaholeof40mmdiameter.A2KNloadcellwithperforationrateof3mm/minwasperpendicularlymoved to thefilmsurfaceuntiltheprobepassedthroughthefilm.Thecurves of force (N) as a functionof displacement(mm) and the tolerated maximum force againstperforation were automatically recorded by thesystemsoftware.

2.3.4. Scanning Electron Microscopy (SEM)The morphology, nano-particle dispersion andrupture mechanism of films were studied byscanning electron microscopy (SEM). Analysiswere performed using an electron microscopemodel LEO 1450 VP (Germany) equipped withEDXmodel7353(UK),withanominalresolutionof133EV,andanaccelerationvoltageof35kV.Allimagesweretakenwithanoperatingvoltageat10kVand250, 1000, 2000, 5000, 10000 and15000magnifications. All the specimens were sputter-coatedwithgold-caladiumalloyinasputtercoatermodelCS7620in2min.

2.3.5. Thermogravimetric Analysis (TGA)Thermogravimetric analysis (TGA) techniques,usedtocharacterizethethermalstabilityofpreparedfilms,usingthermalanalysisinstrument(Shimadzu,Japan). Film samples were cut into small flakesweighingabout6-7mg,thetemperaturerangewasfrom 20 to 1000°C at heating rate of 10°C/minundernitrogenatmosphere.

2.4.Statisticalanalysis

Ageneralfullfactorialdesignwasusedforanalyzingresulteddata anddetermining the influenceof thetwo independent variables (PVA and nano-TiO2 concentrations), at three levels (40, 50, 60wt.%forPVAand0,2.5and5wt.%fornano-TiO2) on mechanical properties of starch film. The factorsand their levels have been selected according topreliminarytests.Designexpertsoftwareversion8was used for analysis of variance (ANOVA).Thesignificancelevelusedwas0.05.

3.RESULTSANDDISCUSSION

3.1.Moistureandthicknessmeasurements

Moisturecontentofsampleswasfrom0.12to0.15wt.%basedontheoriginalweightofspecimenandtheiraveragethicknesswasabout200μm.

3.2.Mechanicalpropertiesofthefilms

As packaging materials may be subjected tovariouskindsofstressduringuse,determinationofmechanicalpropertiesisnecessary.Theinteractionbetween different components has an importantrole indifferentpropertiesof thefilms, especiallyin mechanical properties. Different quantitiesof starch and PVA, and consequently differentextents of H-bonding in the films, besides oftheir various interactions with the plasticizer andTiO2 nanoparticles, lead to different mechanicalbehaviorofsamples.Also,itshouldbenoticedthathomogeneous matrix of films is a good indicatoroftheirstructuralintegrity,andconsequentlygoodmechanical properties; so a complete mixing andusing of sonication for adequate dispersion ofnanoparticles in the based polymers is necessary.

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On theother hand, the thermoplastic starch (TPS)is obtained after disruption and plasticization ofstarch macromolecules by heating in the presence of waterandplasticizerssuchasglycerol.Thepreviousstudiesalsohaveshownthatadditionofplasticizerisnecessarytoimprovebiopolymerfilmproperties,specially the mechanical ones. Furthermore, the effect of plasticizer on mechanical behavior ofplasticized films depends on its concentration.Filmswithlowerplasticizercontenthaverigidandbrittlestructureandarenotflexibleenoughforfilmformation [10].Inthiswork,afterdoingwidetestsoneffectivecontentofglycerolon themechanicalproperties of films, a 30 wt. % concentration,basedon totaldryweightof starchandPVA,wasconsidered.

Thedependencyoftensileandperforationstrength,E-modulus, toughness and elongation at break ofthefilmsonthePVAandnano-TiO2 content of the ST/PVA/TiO2nanocompositeswerestudied.The dependence of the elongation at break (E.B.%)onthePVAandnano-TiO2contentofthefilmsis shown in Figure (1a). The elongation at breakoftheblendfilmsincreasesalongwithanincreasein more flexible PVA molecules. This result issimilar to what proposed by Dean et al., [8], in starch/PVA/montmorillonite nanocomposite. Themaximum amount of elongation at break for thefilmscontaining40%PVAis198.34%,ratherthanthat of films with 50 % PVA which is 244.48%.Themagnitudepromotesupto366.7%whenPVAcontent reaches to 60 %. Tensile and perforation

8  

behavior of plasticized films depends on its concentration. Films with lower plasticizer content have

rigid and brittle structure and are not flexible enough for film formation [10]. In this work, after doing

wide tests on effective content of glycerol on the mechanical properties of films, a 30 wt. %

concentration, based on total dry weight of starch and PVA, was considered.

                        Fig. 1. Effect of nano-TiO2 content on: (a) elongation at break (E. B.) (b) tensile strength (c)toughness (d)perforation strength

The dependency of tensile and perforation strength, E- modulus, toughness and elongation at break

of the films on the PVA and nano-Tio2 content of the ST/PVA/TiO2 nanocomposites were studied.

The dependence of the elongation at break (E.B. %) on the PVA and nano-TiO2 content of the films

is shown in Fig. (1a). The elongation at break of the blend films increases along with an increase in

more flexible PVA molecules. This result is similar to what proposed by Dean et al., [8], in

starch/PVA/montmorillonite nanocomposite. The maximum amount of elongation at break for the

(a) 

0

2

4

6

8

10

12

0 1 2 3 4 5

Tens

ile S

tren

gth

(MPa

)

nano- TiO2 percent

40 % PVA50 % PVA60 % PVA

(b)

0

100

200

300

400

0 1 2 3 4 5

E. B

. (%

)

nano- TiO2 percent

40 % PVA50 % PVA60 % PVA

(a)

0

1

2

3

4

5

6

0 2 4

Perf

orat

ion

Str

engt

h (M

Pa)

nano-TiO2 Percent

40 % PVA

50 % PVA

60 % PVA

(d)

0

500

1000

1500

2000

2500

3000

0 1 2 3 4 5

Toug

hnes

s (N

.mm

)

nano- TiO2 percent

40 % PVA50 % PVA60 % PVA

(c)

Figure1: Effect of nano-TiO2 content on: (a) elongation at break (E. B.) (b) tensile strength (c) toughness (d) perforation strength

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)a(

)c(

)e(

)b(

)d(

)f(

Figure2: SEM images of fracture surface of films formulated as: (a) ST/PVA (60:40) composite film, (b) ST/PVA/TiO2 (60 :40:2.5) nanocomposite film, (c) ST/PVA/TiO2 (60 :40: 5) nanocomposite film, (d) ST/PVA (40:60)

composite film, (e) ST/PVA/TiO2 (40 :60:2.5) nanocomposite film, (f) ST/PVA/TiO2 (40 :60: 5) nanocomposite film.

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strength of films also increase slowly along withan increase in PVA content (Figures 1b, d). Thetensile strength of packaging film must be morethan3.5MPa,accordingtoconventionalstandards[17].Thus, thevalueofover6MPa forST/PVA/TiO2nanocompositesisanappropriatevalueforitsuse as a disposable packaging film.Moreover, asitisseeninFigure(1c),toughnessofthefilmshasimprovedbyadditionofPVA.Toughnessisdefinedas the ability of a material to absorb energy before fracture. Statistical analysis, also shows that theinfluenceofPVAcontentonmechanicalpropertiesof the films is significant (P<0.05). We attributetheseresultstogoodmechanicalpropertiesofPVAanditssuitableinteractionwithstarch,inpresenceofglycerolinthematrix.ST/PVA film with 40:60 ratio containing 5 wt.%TiO2 has the best toughness and elongation atbreak at about 2529.5 and 366.7 compare to thesame film without TiO2 nanoparticles which is2501.1and332.9indicatinganimprovementequalto1.14and10.16%respectivelyinthepresenceofTiO2. However statistical analysis of mechanicalresults shows that the addition of nano-TiO2 has notasignificanteffectonmechanicalpropertiesofthefilms(P>0.05).WithrespecttothismatterthatTiO2 hasn’t anynegative influenceonmechanicalpropertiesoftheST/PVAfilm,itwillbepossibletouseitforimprovingothervaluablepropertiessuchasantibacterial,degradability,thermalandsoon.

3.3.SEManalysisoffilms

The SEM micrographs of the cross section ofsome blend films are shown in Figures 2a to 2f.Thenanoparticlewillreinforcethepolymerifitisdispersedproperly.UsingofultrasonicmixinghasledtoquiteuniformdispersionofTiO2 nanoparticles inside the starch/PVA matrix. SEM micrographs,taken from the fracture surface of samples, illustrate thatstarchgranulesarecompletelydisruptedandacontinuousphaseisobtained,alsoEDXanalysisofsamplesshowedthepresenceofTitanium,OxygenandCarbonelements.Micrographsof thefracturesurfaceofsets1,2and3areshowninFigures2a,b and c.As it is seen, there is a hard breakdowninsampleswith60wt.%starchandthesesampleshaveapproximatelyflatandbrittlefracturesurface;

whereas in samples6, 7 and9 that contain lowercontentofstarch(40wt.%)aquitesoftbreakdownis happened (Figures 2d, e and f) and stretchedfibers in film texture are observable. In fact, theadditionofPVAtothecompositionoffilmmakesitsofterandmoreflexible. 3.4.Thermalanalysis

Thermogravimetric analysis was performed toexamine the thermal stability of pure corn starch,purePVA,andplasticizedstarch(100wt.%starch+ 30 wt.% glycerol), plasticized PVA (100 wt.%polyvinyl alcohol + 30 wt.% glycerol), ST/PVAcompositeandST/PVA/TiO2nanocompositefilms.The thermal stability of a polymeric materialdepends on the inherent characteristics of theconstituentsaswellasonthemolecularinteractionsbetween the different macromolecules [18]. Thethermal stability of the prepared films is affectedby starch to PVA ratio and TiO2 nanoparticles content. The various proportions of starch/PVAandTiO2content,sodifferentextentofH-bondingbetween the molecules and incorporation ofTiO2 nanoparticles have influenced the thermaldegradation behavior of the samples. Ray, et al., [18], alsoobtained the same results that indicatedintercomponent H-bonding between starch, Poly(vinyl alcohol) andglycerol enhances the thermalstabilityofthefilms.Comparedwiththeorganicmaterials,theinorganicmaterials have better thermal stability, due totheir configuration characteristics. Therefore, theintroduction of inorganic particles would greatlyimprove the thermal stability of organic polymermaterials [19].Weightlossandtherateofdegradationofsets1to11alongwithpurecornstarchandPVAareshownbyTGAandDerivativeThermoGravimetry(DTG)curves in figures 3 to 6. A considerable shift intheheightandpositionofthepeaksdependingontheproportionof different constituents infilms isobservable.Figure3showsweightlossofpurecornstarchandPVApowdersandthecurvesrelatedtoset10and11filmsareshowninFigure4.Asitisseen,purestarchanditsplasticizedfilm(set10),havealowerthermalstabilityrelativetoPVAanditsfilm.Pure

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starchshowsasharpandstrongpeakat311.89˚C,indicatingaveryhighdegradationrate,whilepurePVA shows a comparatively low intensity peaksindicatinglowerrateofdegradation.Comparisonoftheirplasticizedfilmsshowsthesameresult.

Figure3:TGA and DTG thermograms of pure PVA and Starch powder

Weight loss and the rate of degradation of sets 1to 3 along with plasticized corn starch and PVAfilms (sets 10 and 11) are shown in Figures (4a,b). Itcanbeseenthat,decompositiontemperatureofplasticizedcornstarchandPVAfilmsimprovesconsiderably in blend films of starch / PVA inpresence of glycerol and nano -TiO2. Thus thethermaldegradationtemperatureoftheblendfilmsis higher than that of plasticized starch and PVA.

The reason is related to new H-bonding betweenthe molecules of starch, PVA and glycerol andincorporation of TiO2 nanoparticles. Thermaldegradation of the plasticized starch (sets 10) isoccurred in approximately two-step degradationprocess (Figure 4a).

Figure4:TGA and DTG thermograms of ST/PVA (60:40) based films along with plasticized starch

and PVA films

Ithasaveryslightdegradation ratebelow230ºC,above this temperature, degradation process takesplace very rapidly until 344˚C, after this point,degradation continues with lower rate to lessthan 520ºC. This result is also confirmed by thepresence of two peaks (in 279.38 and 484.31˚C)inDTGcurve(Figure4b).Inaddition,asitisseen

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in Figures (4a, b), plasticized PVA (set 11) has afive stage degradation process and therefore hasfivepeaks (in181.58,270.94,410.07,703.76and720.38˚C).Itcanbeseenthatpositionandintensityofpeaksinstarch/PVAblendfilms(sets1,2and3)isconsiderablydifferentrelativetoplasticizedstarchandPVAfilms(sets10and11),asthemainpeakofblendfilmstakesplaceinhighertemperatureswithlowerintensities.

Figure5: TGA and DTG thermograms of ST/PVA (50:50) based films

Figure 5 shows a comparison between sets 4, 5and6.ThreesampleshavethesameproportionofstarchandPVA,butamountofTiO2 nanoparticles

isvariedinthreelevelsof0,2.5and5wt.%.Asitisseen,thereisnotaconsiderableshiftinthepeakpositions,buttherateofdegradationdecreasedwithadditionofTiO2nanoparticles.Thereasonisrelatedto adequate incorporation of nano-TiO2 particles withmatrixmoleculesRayetal.,[18],hadobtainedthesameresultsforstarch/poly(vinylalcohol)andbentoniteclayfilms.

Figure6: TGA and DTG thermograms of ST/PVA (40:60) based films

Acomparisonbetweensets7,8and9,isshowninFigure 6. It can be seen that, in 40:60 proportionof starch andPVA, thefilm containing 2.5wt.%

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of TiO2 nanoparticles (set 8) indicates a lowerdegradation rate andhasabetter thermal stabilityrelativetosets7and9.TheresulteddatashowsthattheadditionofPVAinthematrixincreasesthedegradationtemperatureofcomposites;alsotheadditionofTiO2 nanoparticles to ST/PVA improves its degradation temperature.It can be seen from the Figs. 4 to 6 that for allcomposites, temperature of main peak is higher thanplasticizedstarchandPVAones.Decomposition temperatures at 10, 50 and 90 %mass loss (Td (-10)),Td(-50) andTd(-90)), for sets 1 to 11,aresummarizedintable2.Asitisseen,inallofblendfilms,decompositiontemperaturesat10,50and90%masslossimprovesbyadditionofTiO2 nanoparticles. Dai et al., [19], obtained similarresultsforthermoplasticstarchandmontmorillonitenanoparticles. For entire prepared films,decomposition temperatures show a considerableincreasebyadditionofTiO2nanoparticlesto2.5wt.%,butinsomesamplesadecreaseindecompositiontemperatures for addition of TiO2 nanoparticles from2.5wt.%to5wt.%TiO2isobserved.Sets7,8and9allhavethesamematrix(40%starchand60%PVA)whilesets8and9haveadditional2.5and5wt. % nano -TiO2 particles loadings respectively.Asit isseen,Decompositiontemperaturesat10%mass loss (Td(-10)) increases from186.72˚C inset7(withoutTiO2nanoparticles)to193.62˚Cinset8(with2.5wt.%TiO2nanoparticles),andreachesto217.41˚Cinset9(with5wt.%TiO2 nanoparticles). Also it is observable that, when the nano - TiO2 contentvariesfrom2.5to5%,theTd(-50)andTd(-90)

ofthefilmdecrease,buttheyarestillmuchhigherthanthatoftheset7(containinganynanoparticles).Comparison between decomposition temperaturesofsets10and11intable2,showsthatplasticizedPVA(set11),indespiteofitslowerTd(-10)andTd(-50) comparedtoplasticizedstarch(set10),haveamuchmoreTd(-90)andlowerdegradationrate, indicatingits higher thermal stability.Totally, the curves of Figures 3 to 6 confirm thatthe thermaldegradationbegan tooccuronlyafterthematerialshaveabsorbedcertainamountsofheatenergy.Theheatinitiatedthedegradationprocessesand the breaking down of the matrix structureby causing molecular chain ruptures. Moreover,the addition of nano-TiO2 improves the thermalresistanceofstarch/PVAbasedcomposites.

4.CONCLUSIONS

In this work, mechanical and thermal propertiesof biodegradable starch/polyvinyl alcohol/TiO2 nanocompositefilmswerestudied.Allthefilmswereobtainedbysolutioncastingmethod.NanocompositefilmswerepreparedbyhomogeneouslydispersingTiO2nanoparticlesindifferentratiosofstarch/PVAmixtures.Concentrationofglycerolwasfixedat30wt.%,basedontotaldryweightofstarchandPVA.TheresultsshowedthattheadditionofPVAinfilmstructure, will improve its thermal, mechanicaland morphological properties significantly.Alongwith the addition of PVA chains,which aremoreflexible than the starch molecules, film rigidity

Table2: Decomposition temperatures at 10, 50 and 90 % mass loss of films

Sample Td(-10) Td(-50) Td(-90)

Set 1Set 2Set 3Set 4Set5Set6Set7Set 8Set9Set 10Set 11

187.56205.83218.45183.67207.63209.64186.72193.62217.41224.88155.20

316.31319.04320.78310.67318.40314.92321.14328.97325.96307.71273.83

528.55538.85544.37531.79546.57558.36535.48553.40547.75478.25671.52

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was decreased and its elongation was increased.Statistical analysis of obtained experimental datashowed that, mechanical properties of the filmswere not enhanced by addition of nano-TiO2 significantly. However, Set 9 sample was foundto have the best tensile and perforation strength,toughness, and elongation at break compare toother samples. Thermogravimetric analysis ofprepared films indicated that, thermal stability ofST/PVA/nano-TiO2nanocompositeswasimprovedbyadditionofPVAandTiO2 nanoparticles.

ACKNOWLEDGMENTS

We are grateful to IranNanotechnology InitiativecouncilforpartialfinancialsupportandtoChemicalEngineeringResearchandMechanicallaboratoriesofFerdowsiUniversityofMashhadfortheirhelpsinperformingthermalandmechanicalexperiments.

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