Linear Rheology of Guar Gum Solutions.docx

Linear Rheology of Guar Gum Solutions.docx

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LinearRheologyofGuarGumSolutions

LinearRheologyofGuarGumSolutions

RolandH.W.Wientjes,*MichelH.G.Duits,RobJ.J.Jongschaap,andJorritMellema

RheologyGroup,DepartmentofAppliedPhysics,UniversityofTwente,（memberTwenteInstituteofMechanicsandJ.M.BurgersCentre）,P.O.Box217,7500AEEnschede,TheNetherlands

ReceivedJune20,2000;RevisedManuscriptReceivedOctober17,2000

ABSTRACT:

Wehaveinvestigatedthelinearviscoelasticbehaviorofguargumsolutionsasafunctionoffrequency,temperature,polymerconcentration,andmolecularweight.Thiswasdonetosortouttheimportanceofdifferentrelaxationmechanismslikereptationorthebreakupofphysicalbonds.Inthekilohertzregime,Rousebehaviorisobserved.Atlowerfrequencies,twostoragemodulusplateauzoneswerefound,indicatingtwoadditionalrelaxations.Oneisoperativebetween1and100Hzandgivesrisetoaverybroadrelaxationspectrum,evenformonodisperseguar.Describingthedependenciesoftherelaxationtimeandlow-shearviscosityonconcentrationandmolecularweightwithpowerlawsresultedinunusuallyhighcoefficients.Thesecondrelaxationbecomesmanifestbelow0.01Hzandhasnotbeenearlierreported.Herethetemperaturedependenceisverystrongwhereasallotherdependenciesareweak.Analyzingtheexperimentswithexistingmodelsfortransientpolymernetworksrevealedthatatbestapartialdecriptionoftheexperimentaldependenciescanbeobtained.Itwasconcludedthatatleasttwodifferentrelaxationmechanismsmustplayarole,classicalreptationnotbeingoneofthese.Bestoverallpredictionswereobtainedwithamodelassumingtwotypesofassociations.However,alsothepictureofstarpolymer-likestructuresheldtogetherviabondswithalonglifetimecouldgivecomparablepredictions.Forafurtherdistinctionbetweenthesemechanisms,moreinformationaboutthemesoscopicstructureisneeded.

10.1021/ma001065pCCC:

$19.00©2000AmericanChemicalSociety

PublishedonWeb12/01/2000

1.Introduction

Galactomannansarewater-solublepolysaccharidesfoundintheseedendospermofavarietyoflegumes.Theyconsistofa（1^4）-linked/3-D-mannopyranosylbackbonepartiallysubstitutedatO-6witha-D-galac-topyranosylsidegroups.6Onegalactomannanwhichiswidelyusedasanindustrialhydrocolloidisguargumwhichhasamannose:

galactoseratioof1.55.Inconnectiontoitsuseasathickenerinfoodproducts,severalresearchgroupshaveinvestigatedtherheologyofguargumsolutions.8,9,23,26,27

InarheologicalstudyperformedbyRoss-Murphy,28wherestartshearbehaviorandthevalidityoftheCox-Merzrulewereinvestigated,itwasconcludedthatguargumsolutionsbehavelikeanentangledsolution,asdescribedbyDoiandEdwards.7ThisconclusionwasdrawndespiteearlierobservationsofRichardsonandRoss-Murphy26whonotedtheonsetofatransitionatlowshearrates（0.01s-1）,althoughtheNewtonianlow-shearplateauhadalreadybeenreached.Robinsonetal.27mentionedastrongnonlineardependenceofthespecificviscosityuponconcentration.Fromthisitwasconcludedthatnotonlypurelytopologicalentanglements,butalsospecificattractivepolymer—polymerinteractionsmustplayarole.IndicationsforthiswerealsoobtainedbyGoycooleaetal.15andbyGidleyetal.,13whoattributedacrucialroletothea-D-galactosesidegroupsintheprocessofnetworkcross-linkingbysemihelix—helixaggregation.

Fromthisshortoverview,itisclearthattherheologicalbehaviorofguargumsolutionsisstillincompletelyunderstoodandthatspecificpolymer—polymerinteractionsmightplayaroleintheobservedrheologicalbehavioraswellasreptationphenomena.Tosortouttheimportanceofdifferentrelaxationmechanisms,wehavesystematicallyinvestigatedthelinearviscoelasticbehaviorasafunctionoffrequency,concentra

tion,temperature,andmolecularweight.TosupportinterpretationwehavecharacterizedthemolecularpropertiesbyusingGPC,intrinsicviscositymeasurements,andseveralmicroscopytechniques.Inthispaper,wewillcomparethelinearviscoelasticbehaviorwithpredictionsfromexistingmicrorheologicalmodelsthattakeintoaccounttopologicalconstraintsand/orphysicalbonds.

Thepaperisfurtherorganizedasfollows.Insection2,thepreparationofthesolutions,themicroscopiccharacterizationsandtherheologicalmeasurementtechniquesarediscussed.Insection3,theexperimentalresultsareshown.Theseresultswillbecomparedtorheologicalmodelsinsection4,followedbyadiscussioninsection5,afterwhichconclusionswillbedrawninsection6.

2.ExperimentalSection

2.1.Materials.Guargum（Meyhall）waspurifiedfromacommercialflourusingamodificationofthemethodofMcClearyetal.4Crudeguargum（10g）wastreatedwith200mLofboiling,aqueous80%ethanolfor10min.Theobtainedslurrywascollectedonaglassfilter（no.3）andwashedsuccessivelywithethanol,acetone,andether.Thismaterialwasaddedto1Lofdemiwaterandallowed1htohydrate.Itwasthenstirredwithafoodblender（125W）,homogenized（1min）,andcentrifugedat2300gfor15min.Thesupernatantwasprecipitatedintwovolumesofcoldacetone.Afterredissolvinginhotwater,thepolymersolutionwasultracentrifugedat82000gfor1.5hatroomtemperature.Thesupernatantwasprecipitatedwithtwovolumesofethanol.Theprecipitatewascollectedonaglassfilter（no.4）andwashedwithethanol,acetone,andetherbeforefreeze-drying.Thisleadtoalmostmonodispersepurifiedguargum.Onlyonebatchofguarwaspurifiedinthislaboriousway,togetamonodispersesystem.Solutionsofthismaterialwerepreparedbyaddingknownweightsofthedryguartotwicedistilledwater,andallowingittohydrateforextendedperiods（severaldays）toensurethatthesamplehadcompletelydissolved.Thiswasdoneata

Table1.MolecularWeightsforSamplesPreparedviaProceduresIandII

guar

Mw（kD）

Mw/Mn

HM

1048

1.02

150

1400

1.1

90

1000

1.5

30

350

1.7

temperatureof277Kforconcentrationsbetween0.4and2.0%（w/w）.WewillrefertothispurificationanddissolvingmethodasprocedureI.

ForthepurposeofcharacterizationwithMarkHouwinkplots,threeguargumswithdifferentmolecularweights（Meyhall）werepurifiedwithalesslaboriousbutotherwisesimilarprocedure.Herecrudeguargum（10.0g）wassuspendedin1Lofdemiwaterandstirredwithafoodblender（125W）for1min.Theobtainedsolutionwasplacedinarefrigeratorfor24handthencentrifugedat22000^for5h.Then,800mLoftheobtainedsupernatantwasprecipitatedintwovolumesofcoldacetone.Theprecipitatewascollectedonaglassfilter（no.4）andwashedwithethanol,acetone,andether.Thesoobtainedpurifiedguargumwasfreeze-driedanddissolvedasdescribedinprocedureI.ThisprocedureiscalledprocedureII.

Usingtheseproceduresledtoverylongdissolvingtimes.Toshortenthis,athirdpurificationanddissolvingprocedurewasused.Inthisprocedurefourdifferentguargums（Meyhall）withdifferentmolecularweightswerepurifiedbyadding10.0gto400gacetatebufferofpH4.66（Merck）.Theslurrywashomogenizedfor75swithafoodblender（500W）andcentrifugedat22000^for5hatroomtemperature.Thesupernatant（typicalconcentration2%w/w）wasusedasastocksolutionfromwhichlowerconcentrationswereobtainedviadilution.ThisisprocedureIII.SeveralcontrolexperimentsrevealedthattherheologicalbehaviorwasnotsignificantlychangedonswitchingfromprocedureIItoIII.

2.2.MolecularCharacterization.ThemolecularweightsofthepurifiedguargumsusingprocedureIandIIweredeterminedbyGPC-MALLS-RI（multianglelaserlightscattering）.Theguarwasdissolvedina50mMphosphatebufferwithpH8.0toaconcentrationof0.1%（w/w）andfilteredthrougha0.45^mfilterpriortoinjection.AttheexitoftheGPCcolumnthe（instantaneous）valuesofMandRgweredetectedonline.TheresultsaresummarizedinTable1.

Themannose/galactoseratioofguarHM/150/90/30wasdeterminedbyHPLCafterhydrolysisofthepolymertobe1.59±0.05.

Molecularweightsoftheguargums,obtainedviaprocedureII,wereobtainedfromintrinsicviscositymeasurements.TheseexperimentsweredonewithanUbbelohdecapillaryviscometer（Scott,type53201/0A）.

Fromthereducedviscositymeasurements,intrinsicviscositieswereobtainedusingtheHugginsequation:

n-1

nred=1^Y~=[n]+Ac[n]2c

（1）

Herecistheconcentrationguargum,nredisthereducedviscosity,ysisthesolventviscosityand[y]istheintrinsicviscosity.hcistheso-calledHugginscoefficientwhichisapolymerconstantwhichusuallyliesintherange0.5-0.8.TheHugginscoefficientwasdeterminedforallthecurvesandturnedouttobeconstantwithintheexperimentalerrorrange:

0.55±0.05,whichiswithintheexpectedrange.WiththeobtainedintrinsicviscositieswemadeaMark-Houwinkplot（Figure2）andfoundtheMark-HouwinkconstantsAMHandatobe（6.7±1.1）x10-7L/gand1.05±0.01.ThisrelationwasusedlaterasacalibrationcurvetodeterminethemolecularweightsfromintrinsicviscositymeasurementsforthesamplesmadebypurificationprocedureIII.

Thecriticalconcentrationwhereoverlapbetweenpolymerchainsstartstooccur（c*）andtheintrinsicviscositiesforthe,viapreparationprocedureIIIobtainedsamples,wereobtained

Figure1.Reducedviscositymeasurements:

（a）guar30;（♦）guar30duplo;（•）guar90;（+）guar150.

Figure3.Determinationofc*ofguar150:

（•）Ubbelohdemeasurements;（O）ContravesLowShear40measurements.

Table2.[n],Mwandc*fortheSamplesPreparedviaProcedureIII

guar

[n]（L/g）

Mw（kD）

c*（g/L）

150

1.2