Linear Rheology of Guar Gum Solutions.docx
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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
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