ContentslistsavailableatScienceDirect
Carbohydrate
Polymers
jo u r n al h om ep age :w w w . e l s e v i e r . c o m / l o c a t e / c a r b p o l
Rheology
of
Laponite-scleroglucan
hydrogels
R.
Lapasin
a,∗,
M.
Abrami
b,
M.
Grassi
a,
U. ˇSebenik
caUniversityofTrieste,EngineeringandArchitectureDepartment,PiazzaleEuropa,I-34127,Trieste,Italy bUniversityofTrieste,LifeSciencesDepartment,CattinaraHospital,StradadiFiume447,TriesteI-34149,Italy cUniversityofLjubljana,FacultyofChemistryandChemicalTechnology,Veˇcnapot113,1000Ljubljana,Slovenia
a
r
t
i
c
l
e
i
n
f
o
Articlehistory:
Received31October2016
Receivedinrevisedform15March2017 Accepted21March2017
Availableonline23March2017
Keywords: Laponite Scleroglucan Blendingeffects Hydrogels Nanoparticles
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BothLaponiteandscleroglucancanfindseveralapplicationsinvariousfields(fromindustrialto biomed-icalone)invirtueoftheirpeculiarfeaturesandrheologicalpropertiesdisplayedinaqueousphases. StructuralstatesofLaponitedispersionsstronglydependonconcentrationandionicstrength.When attractiveandrepulsiveinterparticleinteractionsaresoeffectivethattheyleadtoarrestedstates (attrac-tivegelorrepulsiveglass),therheologicalbehaviorofthedispersionundergoesasharptransition,from quasi-Newtoniantomarkedlyshearthinningandviscoelastic.Conversely,scleroglucansolutions grad-uallychangetoweakgelswithincreasingpolymerconcentration.Thepresentworkisconcernedwith aqueousLaponite-scleroglucanmixedsystems,obtainedaccordingtodifferentpreparationmodes,and isaimedatexamininghowmuchthecontentandproportionofbothcomponentsaffecttheviscoelastic andflowpropertiesofthemixedsystem.
©2017ElsevierLtd.Allrightsreserved.
1. Introduction
Bothscleroglucan,aneutralbiopolymer,andLaponite,a
syn-theticclay,haveseparatelyfoundnumerousapplicationsinvarious
industrialfieldsaswellasinpharmaceuticalandbiomedicalareas,
byvirtueofthestructuralfeaturesandrheologicalpropertieswhich
aredisplayed,when theyaredissolvedordispersedin waterat
sufficientlyhighconcentration.
Scleroglucanisanonionicpolysaccharidesecretedexocellularly
byfilamentousfungiofthegenusSclerotium.Itsprimarystructure
consistsofalinearbackboneof(1,3)--linkedd-glucopyranosyl
residues bearing a single (1,6)--linked d-glucopyranosyl unit
everythreesugarresiduesofthemainchain(Rinaudo&Vincendon,
1982).Bothinaqueoussolutionandinthesolidstate,
scleroglu-canadoptsahighlyordered,rigid,triplehelicaltertiarystructure
(triplex),whichconsistsofthreeindividualstrandscomposedofsix
residuesinthebackboneperturn.Thethreestrandsofthetriplex
areheld together by interstrand hydrogenbondsat the center
ofthetriplex.The(1→6)-linked-d-glucopyranosylsidegroups
protrudefromtheoutsideofthetriplex,sopreventing
intermolec-ular aggregationand polymer precipitation (Bluhm, Deslandes,
∗ Correspondingauthor.
E-mailaddresses:rlapasin@alice.it,romano.lapasin@dia.units.it
(R.Lapasin),MICHELA.ABRAMI@phd.units.it(M.Abrami),mario.grassi@dia.units.it (M.Grassi),Urska.Sebenik@fkkt.uni-lj.si(U. ˇSebenik).
Marchessault, Pérez,&Rinaudo, 1982; Fari ˜na,Si ˜neriz,Molina & Perotti,2001;Palleschi,Bocchinfuso,Coviello,&Alhaique,2005;
Yanaki&Norisuye,1983).Thetriplexconformationisdestabilized
onlyindimethylsulfoxideorstrongalkalineconditionsandis
char-acterizedbya highrigiditywhich isresponsibleofthepeculiar
propertiesexhibitedbyaqueousscleroglucansolutionsinawide
pHrangeandevenatrelativelyhightemperatures.Asofttransition
fromsoltoweakgelpropertieswithincreasingpolymer
concen-trationcanbedetectedinbothcontinuousandoscillatoryshear
tests(Grassi,Lapasin,&Pricl,1996;Lapasin,Pricl,&Esposito,1990).
Astriplexclusteringincreasesleadingtotheformationofa
three-dimensionalhydrogelnetwork,shearthinningbehaviorbecomes
moremarkedandultimatelyanapparentyieldstresscanbe
indi-viduatedfromflowcurves,thixotropicresponsesbecomesmore
andmoreevident,andaprogressivetransitionisobservedinthe
mechanicalspectrawithprevailingelasticityoverthewhole
fre-quencywindow.
Duetothemarkedshearthinning behaviorof itshydrogels,
scleroglucanisusedasthickenerandsuspendingagenttoimpart
adequaterheologicalpropertiesandimprovestabilityofdisperse
systemsinseveralindustrialsectors(Lapasin&Pricl,1995).
Non-ionicpolymersusuallyshowonlyslightinteractionswithnonionic
andcationicsurfactantsandcanbeconvenientlyemployedforthe
preparationofstablecosmeticO/Wemulsionssincebothpolymer
andsurfactantmixturescancontributeindependentlyand
posi-tivelytothestabilizationofthedispersedphase(Bais,Trevisan,
Lapasin,Partal,&Gallegos,2005).Scleroglucanisnottoxicanddoes
notalterbloodorlivingtissues;whenappliedtoskinoreyesit
doesnotcausesensitization.Inaddition,itbelongstothegroup
ofbiologicalresponsemodifiers,whichhavebeenattributedwith
antitumoreffectsinmanycases(Bohn&BeMiller,1995).Allthese
characteristicsareveryimportant,especiallywhenpreparingboth
pharmaceuticalandcosmeticemulsions.
Amongbiopolymers,scleroglucananditsderivativesappearto
beparticularlywellsuitedfortheformulationofhydrogelmatrices
forsustaineddrugreleaseofbioactivemolecules,becauseoftheir
peculiarfeaturessuchashighbiocompatibility,biodegradability,
bioadhesivity,chemicalandthermalresistanceandgood
mechan-icalproperties(Coviello,Grassi,Lapasin,Marino,&Alhaique,2003;
Covielloetal.,2005;Grassi,Lapasin,Pricl,&Colombo,1996;Grassi etal.,2009;Matricardi,Onorati,Coviello,&Alhaique,2006;Vi ˜narta, Franc¸ois,Daraio,Figueroa,&Fari ˜na,2007).Inthefieldofenhanced
oilrecovery (EOR) scleroglucan is assessed as environmentally
friendlyviscosifyingagentinvirtueofitsnotoxicityand
biodegrad-ability. Indeed, it is quite suitable for formulating water based
drillingmuds tobeemployed in harsh environmentsowingto
itsgoodstabilitytowardshighsalinities,temperaturesand
alka-line conditions, moderate interactions with surfactants and its
robust sheartolerance (Baba Hamed &Belhadri,2009; Gallino,
Guarneri,Poli,&Xiao,1996;Kulawardanaetal.,2012;Sveistrup, vanMastrigt,Norrman,Picchioni,&Paso,2016).
Ontheotherhand,Laponiteiscurrentlyusedasarheology
mod-ifierinvarioustechnologicalapplications,suchassurfacecoatings,
ceramicglazes,paints,homecareandpersonalcareproducts,as
wellasfilmformingagentandinpharmaceuticaland
nanocom-positeformulations.
Laponite is a synthetic hectorite manufactured by
pro-cessing combined salts of sodium, magnesium and lithium
along with sodium silicate with an empirical chemical
for-mula Na+0.7[(Si
8Mg5.5Li0.3)O20(OH)4]−0.7.Laponite nanoparticles
arerigiddisc-shapedcrystalswithathicknessof1nm,anaverage
diameterof30nm,abulkdensityof2.65·103kg/m3.Eachplatelet
iscomposedofanoctahedralmagnesiasheetthatissandwiched
betweentwotetrahedralsheetsofsilica.Theisomorphic
substi-tution of divalent magnesium by monovalent lithium causes a
deficiencyofpositivecharge,whichisbalancedbysodiumatoms
residingintheinterlayerspace.Whendispersedinaqueousmedia
thesodium ionsare released,leading toa permanent negative
chargedistributiononbothoppositefacesofLaponitedisks.
Pos-itivechargesontherimareduetotheprotonationprocessofthe
localhydroxidegroups.
Becauseofallthesepeculiarplateletfeatures,aqueousLaponite
dispersions can display a variety of structural conditions and,
consequently,ofrheologicalpropertiesfordifferentparticle
con-centrationsand ionicstrengthvalues.Markedshearthinningor
plasticbehaviorandhighlyelasticresponsesareexhibitedwhen
interparticle(attractiveorrepulsive)interactionsaresoeffectiveto
generatearrestedstatesofvariousnature(attractivegel,Wigneror
repulsiveglasses).Numerousinvestigationshavebeenaddressed
todefine thestatediagram ofaqueous Laponitedispersions in
theionic strength vs clay concentration plane,i.e. to
individu-atethedifferentregionsofisotropicliquids,disordered(gelsand
glasses),ordered(nematicphases),flocculatedstates(Cummins,
2007;Gabriel,Sanchez,&Davidson,1996;Jabbari-Farouji,Tanaka, Wegdam, &Bonn, 2008; Levitz, Lécolier, Mourchid, Delville, & Lyonnard,2000;Mourchid,Lécolier,VanDamme,&Levitz,1998;
Mourchid,Delville,Lambard,Lécolier,&Levitz,1995;Mongondry, Tassin,&Nicolai,2005;Ruzicka&Zaccarelli,2011;Ruzicka,Zulian, &Ruocco,2004;Ruzicka,Zulian,&Ruocco,2006;Tanaka,Meunier, &Bonn,2004; Tanaka,Jabbari-Farouji,Meunier, &Bonn,2005).
Thevariouscontradictionsemergingfromthecomparisonofthe
proposedLaponitedispersionstatediagramsarepartlyapparent
andcanbemainlyascribedtodifferentagingtimesand,
secon-darily,todifferentprotocolsofsamplespreparation orLaponite
type(Ruzicka&Zaccarelli,2011).Indeed,timeelapsedafter
dis-persionpreparationplaysaparamountrolesinceevenverylow
Laponite concentration dispersions can undergo aging up to a
finalarrestedstate.Regardingthedifferenttimeevolutionofthe
sol/arrested statetransition in salt-freeaqueous systems (ionic
strength≈2×10−4M), two distinct non-ergodicstates (geland
repulsiveglass)havebeenindividuatedatlowerandhigher
con-centrations,respectively(Ruzickaetal.,2004).Inthesetworegions,
theattractive(rim–face)andrepulsive(face–face,rim–rim)
elec-trostaticinteractionsbetweenplateletsare,respectively,dominant
intheformationandstabilityofthearrestedstructure.Atverylow
concentration(below1.0wt%)theslowgelationprocess,
originat-ingfromtheattractiveinterparticleinteractionsandtherelevant
clustering, is followed by an extremely slow phase separation
between clay-poor and clay-rich phases on the year timescale
(Ruzickaetal.,2011).Atevenhigherconcentration(above3wt%)
theformation of nematic microdomainswaspostulated onthe
basis of birefringence measurements between crossed
polariz-ers(Mourchidetal.,1998).Uponincreasingionicstrengthabove
10−4M,therole ofelectrostaticrepulsionsdecreasesinfavorof
attractiveinteractionsbetweentheoppositelychargededgesand
facesoftheclayplatelets.Accordingly,theamplitudeofthegel
regionincreasesandthegelationtimestronglydecreases.Athigh
saltconcentrations,theenergybarriertoparticleaggregationis
stronglyreducedandphaseseparationintheformoflarge
aggre-gatesoccurs.
Severalstudies have beencarried out onaqueous
Laponite-polymer systems,obtainedbydispersing nanoparticlesin
poly-meric matrices oradding polymer toLaponite dispersions.The
scenarios drawn by the structural conditions of these mixed
systems may be even more complex than those exhibited by
the corresponding simple systems (water-Laponite and
water-polymer)owingtovarious possibleinteractionmodes between
plateletsand polymerchains.Thebalanceof different
competi-tivemechanismsrelatedtopolymeradsorption(sterichindrance,
changeinsuperficialcharge,depletion, bridging,increase inthe
solutionviscosity)canassistorhindertheagingdynamicsandthe
formationofafinalarrestedstate(glassorgel),stronglydepending
onthepolymerconcentrationand molecularweight.If
superfi-ciallyadsorbed,shortflexible polymerchainscanprovidesteric
stabilizationowingtoexcludedvolumeeffectsbetweenpolymer
segments,thusinhibitingorslowingdowntheaggregationprocess
when edge-to-face attractive interactions are potentially
domi-nant.Athighermolecularweights,adsorbedpolymerchainsmay
belongenoughtobridgebetweenparticles,sopromoting
parti-cleclusteringortheformationofanassociativenetwork.Beyond
particlesurfacesaturation,depletionforcescanpromoteparticle
aggregationinthepresenceoffreenonadsorbedchainsin
solu-tion. As polymer concentration increases,interactions between
adsorbed layers and free chains can result in re-stabilization
ofclustersand, abovetheoverlapconcentration,cluster–cluster
attractionsincreasesobecominglongranged.Allthesestructural
effectshavebeenpostulatedforLaponitedispersionscontaining
poly(ethyleneoxide)(PEO)whichcanbeconsideredaparadigmatic
exampleofplateletdispersionsinneutralpolymersolutions,from
dilutetoconcentratedregime(Baghdadi,Sardinha,&Bhatia,2005;
Baghdadi,Jensen,Easwar,&Bhatia,2008;Kishore,Chen,Ravindra, &Bhatia,2015;Mongondry,Nicolai,&Tassin,2004;Zulian,Ruzicka, &Ruocco,2008;Zulian,AugustoDeMeloMarques,Emilitri,Ruocco, &Ruzicka,2014).
As several other complex systems, Laponite–PEO systems
exhibit intriguing macroscopicbehavior by varyingthe applied
shearconditions,sincevariousstructuralchangescanbeproduced
ondifferentlengthscalesforarangeofclayandPEO
2008; DeLisi etal.,2008;Lapasin&Maiutto, 2006; Lin-Gibson, Kim, Schmidt, Han, &Hobbie, 2004; Maiutto, 2005; Morariu & Bercea,2015;Malwitz,Butler,Porcar,Angelette,&Schmidt,2004;
Pozzo&Walker,2004;Schmidt,Nakatani,Butler,Karim,&Han, 2000;Zebrowski,Prasad,Zhang,Walker,&Weitz,2003).Besides
breakdownofparticleaggregates,plateletsorientation,anisotropic
phase distribution or separation, which have been observed in
aqueousLaponitedispersions,alsoareversible‘shakegel’
forma-tioncanoccuroveranarrowcompositionrange,closetosurface
saturation.Such a shear-induced gelation can be ascribed to a
polymer–clayadsorption–desorptionmechanismwiththe
forma-tionofunstablepolymerbridges.
A wide variety of nanocomposite hydrogel systems can be
preparedbyaddingLaponitenanoparticlestochemicalor
phys-icalhydrogels.Thepresenceofnanoparticlescanservetoimpart
uniquemechanicalandotherphysicalpropertiestopolymer
hydro-gels,whichcanleadtomanydiversetechnologicalapplications,
mostlyinbiotechnologicalareas.Physicalgelsmadeof
biopoly-mers,especially if unmodified, maybe quiteinteresting in the
preparationofcontrolled/sustaineddrugreleasesystemsandother
biomedical devices also in virtue of their biocompatibity,
non-toxicity,biodegradabilitycharacteristics.Physicalhydrogelsderive
fromweakinterchainassociations due tochain entanglements,
hydrogenbonds,ionicbondsorstrongvanderWaalsinteractions
betweenchains,hydrophobicinteractionsorcrystallitesbringing
togethertwoormoremacromolecularchains.Theadded
nanopar-ticlescanbesimplyentrappedanddistributedwithinthehydrogel
network,asindividualunitsoraggregates,thusactinglikeother
solidfillersdispersedinpolymericmatrices.Particleadsorptionto
orinteractionswithpolymerchainscanalterthenetwork
archi-tecture,evenincreasingitsconnectivitywithadditionalinterchain
physicalcrosslinks,thusleadingtodiverseextentsofmutual
inter-penetrationbetweenthetwocomponentsand heterogeneityat
differentlengthscales,uptophaseseparation.
Aspreviously mentioned, aqueous Laponitedispersions
dis-playaveryrichphasebehaviorthat includessol,gel,glass,and
nematicstates, dependingonclayconcentration,ionic strength
andtimeofaging.Additionofioniccomponentsgenerally
intensi-fiestheiragingandresultsinadecreaseinthetimeoftransition
intoarrestedstates.Theadsorptionofcationicspeciesonplatelet
surfaceinduceadditionalattractionbetweenthemowingtothe
enhancementofhydrophobicinteractions,thusfavoringparticle
aggregationandreducingsedimentationstabilityofLaponite
dis-persions,asdetectedinthecaseofCTAB(cetyltrimethylammonium
bromide)addition (Savenkoet al., 2013).Homopolymer
hydro-gelsmadefromcationicbiopolymerarelessfrequentthanthose
obtainedfromanionic polyelectrolytessuchas xanthan,gellan,
welan,carrageenan,alginate,pectin,carboxymethylcellulose,etc.
Theexactnatureof theinteractionsbetweenLaponiteparticles
and anionicpolymersdepends subtly onthe detailed structure
ofthepolymer:type,numberandseparationoftheanionicsites,
flexibilityandhydrophobicityofthebackbone,branchingdegree
andaveragemolecularweight(Fitch, Jenness,&Rangus, 1991).
Electrostatic attractionsbetween anionic groups and positively
charged edges of Laponite particles could increase the
stabil-ityoftheclaydispersiononconditionthat therepulsiveforces
betweenplateletfacesremainalmostunchanged.Atsufficiently
highpolymerconcentration,whentheneutralizingcations
(usu-allysodium,potassium,calcium) effectivelyscreenthenegative
chargesonLaponiteparticles,thisenhancestheattractive
inter-actionsbetween thepolymer and can result in changes in the
dynamicsofagingandthestateofthesystem.Thehighnumerical
densityofLaponiteparticlesmakespossibletheformationofdense
fractalaggregatesduetotheinterparticleconnectionsestablished
bypolymerchains,andeventuallyofagelnetworkstructure,as
observedinthecaseofpolystyrenesulfonateadditiontoLaponite
glassdispersion(Savenko,Bulavin,Rawiso,&Lebovka,2014).
Rhe-ological studiesperformedonLaponite/carboxy-methylcellulose
blendshaverevealedstrongsynergisticeffects whichhavebeen
ascribedtomultipleparticle-chaininteractionswhilelesssynergy
havebeenobservedbetweenLaponiteandxanthangum,which
hasbulkytrisaccharidechain(Fitchetal.,1991).Therheological
responsesexhibitedbyLaponitedispersionsinaqueousxanthan
matriceswithaddedNaClstronglydependonconcentrationsand
proportionofthetwocomponents(Maiutto2005;Lapasin,2016).
AsxanthanconcentrationincreasesindiluteLaponitedispersions,
theattractiveparticle-chain interactions giveorigintosynergic
effects,alsocontributingtotheconnectivityofthepolymer
net-workwhenitisformed.AsLaponiteconcentrationincreases,the
contributionoftheparticlegelnetworkbecomespredominantover
thatoftheaddedpolymer.Recently,arheologicalstudyhasbeen
performedtoexaminetheeffectsofLaponiteadditiontoalginate
solutions,inparticulartoanalyzethephysicalgelationduetothe
electrostaticinteractionsbetweentheLaponiteplateletsand
algi-natechains(Davila&d’Ávila,2017).
Amongnon-ionicpolysaccharidesscleroglucanandits
chem-ically analogous schizophyllan occupy a quite special position
invirtueoftheirtriplehelicaltertiarystructure(triplex),which
makepossibletheformationofahydrogelnetworkatsufficiently
high polymer concentration. Aqueous concentrated
Laponite-scleroglucan blends appear to be worthy of consideration for
biomedical applications, in particular for the preparation of
hydrogel-basedreleasesystems.Laponiteadditiontoscleroglucan
weakgelcouldcontributetobettercontrolthedrugdiffusionand
releasekineticsaswellastoimprovethemechanicalpropertiesof
thematrix.Startingfromthesepracticalperspectives,thepresent
workhasbeenaddressedtostudytherheologicalpropertiesof
aqueous Laponite-scleroglucan mixed systems, even differently
prepared,sufficientlyconcentratedtoexhibitmarkedviscousand
elasticresponses,and,inparticular,toexaminehowmuchthe
con-tentandproportionofbothcomponentsaffecttheviscoelasticand
flowpropertiesofthemixedsystems.
2. Experimental
2.1. Materialsandsamplepreparation
ScleroglucanoActigumCS11ofaveragemolecularweight(MW)
1.2×106wasprovidedbyCargillInc.(USA)andusedasreceived
withoutfurtherpurification.Laponite® XLG (providedby
Rock-woodAdditivesLtd,UK)isasyntheticpurifiedhectoritewithalow
heavymetalscontentandhighsurfacearea(BET370m2/g).
The most concentrated (2wt%) Scleroglucan solution and
Laponitedispersionwerepreparedbygraduallyaddingthe
poly-merpowderorclaypowdertodistilledwaterundermechanical
stirring.The othersimple systemsof lowerconcentrationwere
obtainedbydilutionofthesemothersystemswithdistilledwater
undermagneticagitation.
Laponite-scleroglucanmixedsystemswerepreparedaccording
twodifferentmodalities:1)(freshoraged)simplesystemshaving
thesameweightpercent(2wt%)ofscleroglucanorLaponitewere
blendedindifferentproportions(3:1,1:1,1:3)undermagnetic
agi-tation;2)Laponitewasdispersedundermixinginascleroglucan
matrix(2wt%)toobtaindifferentsolidconcentrations(0.5,1and
2wt%).
Aftertheirpreparationallthesystemswerekeptinfridgeat
temperatureof8◦Cfortheprogrammedtimeintervalbeforebeing
Table1
RBCandCrossparametersofscleroglucansystems.
C (wt%) 0.25 0.50 1.00 1.50 2.00 RBC 0 (Pas) 0.25 18.4 610 7980 28500 c (Pa) 0.072 0.999 3.91 18.8 28.6 m (−) 1.25 2.31 3.92 9.97 10.4 Cross 0 (Pas) 0.17 15.8 690 9770 26600 ˙c (s−1) 0.48 0.0112 0.0027 0.0017 0.0011 n (−) 0.59 0.70 0.81 0.91 0.92
2.2. Instrumentandexperimentalprocedures
Experimentaltestswereperformedat25◦Cusingthecontrolled
stressrheometerHaakeMarsIIIequippedwithPeltiertemperature
controlsystemandusingthecone–plategeometryC60/1◦
(diame-ter60mm,angle1◦)or,alternatively,thecrosshatchedplate–plate
geometryPP35Ti(diameter35mm).Aglasssolventtrapcoverwas
usedtopreventwaterevaporation.
Byapplyinganincreasingsequenceofconstantstresssegments
tothesamplesandmeasuringthecorrespondingshearrate(˙)the
steadyflowbehaviorwasdetermined.Thestresswaskeptconstant
untiltherelativevariationoftheshearratesatisfiedthefollowing
constraint,(˙/˙)/t≤0.05,orthesegmentduration(t)wasno
longerthanthecutoffvalueof100s.Oscillatorystresssweeptests
wereperformedat1Hzinordertodefinethelinear
viscoelastic-ityrange.Allthefrequencysweepmeasurementswereperformed
withinthelinearrange.
2.3. Modelsfordatacorrelation
Usingshearrateasindependentvariable,experimentalviscosity
datacanbecorrelated withgoodapproximationwiththeCross
equation(Cross,1965):
=∞+ 0−∞
1+ (˙)n (1)
where0and∞aretheasymptoticvaluesoftheviscosityat
zeroandinfinite shearrates,respectively,isthecharacteristic
timeandnmeasurestheshearratedependenceofviscosityinthe
powerlawregion.Alternatively,ifshearstressisassumedas
refer-encevariable,asatisfactorydatafittingcanbeprovidedonlybythe
Roberts–Barnes–Carew(RBC)model,alsointhecaseofstrongshear
thinningorapparentlyplasticbehavior,whentheinitialgradual
decreaseintheviscosityisfollowedbyitsdramaticdrop(Roberts,
Barnes,&Carew,2001):
=’∞+
’ 0−’∞
1+
/c m (2)wherethecriticalstressclocatesthetransitionregionbetween
theformershearthinningandthelatteronewhiletheexponent
mrulestherateofviscositydecreasetherein.Inthecaseofplastic
behavior(highmvalues),theapparentyieldstressisgivenbyc.
TheRBCmodelrepresentsamodifiedversionoftheEllisequation
(Ellis,1927),moreflexiblethantheoriginalone,sincetheoriginal
parameters0and∞arenowsubstitutedbyfunctions(’
0 and
’
∞)oftheshearstressasfollows:
’0= 0 1+
/1 p (3) ’ ∞=∞ 1+/2 s (4)where1,2,pandsareadjustableparametersbesidesthe
asymp-toticviscosityvalues0and∞.
Theexperimentaldatafromoscillatorytests,namelyfrequency
sweep, can be described quite satisfactorily with the classical
generalized Maxwell(GM)model(orMaxwell-Wiechertmodel)
composedofanelasticspringandMaxwellelementsinparallel.
Actually,fourMaxwellelementsweresufficient todescribethe
mechanicalspectraqualitativelywell.
TheGMequationsfordescribingthefrequencydependenceof
theviscoelasticmoduliare:
G=Ge+
4 i=1 Gi2 iω 2 1+2 iω2 (5) G=4 i=1 Giiω 1+2 iω2 (6)whereGeistheequilibriummodulus(→0),whileGiandiare
therelaxationmodulusandthecorrespondingrelaxationtimeof
theithMaxwellelement,respectively.Inordertoreducethe
corre-lationdegreebetweentheadjustableparameters,theminimization
procedurewasperformedbyadoptingthefollowingrecurrent
con-straintfortherelaxationtimes:i+1=10i.
3. Resultsanddiscussion
3.1. Simplesystems
The first section regards the rheological characterization of
aqueoussimplesystems,containingonlyscleroglucanorLaponite
atdifferentconcentrationsandisaimedatdrawingthenecessary
backgroundfortheanalysisofmixedsystems.
3.1.1. Scleroglucansystems
Theshearthinningcharacterofaqueousscleroglucansolutions
increaseswithincreasingpolymerconcentrationCandabove1wt%
thebehaviorbecomesapparentlyplasticwithasignificantviscosity
dropconfinedinanarrowstressinterval,astypicalforweak
poly-mergels.Fig.1Areportstheexperimentaldatainalogviscosity-log
stressplotfor polymerconcentrationsbetween0.25 and2wt%,
togetherwiththeflowcurvescalculatedwiththeRBCmodel.The
increaseinpolymerconcentrationleadstotheprogressive
shift-ingoftheflowcurvestowardshigherviscosityandstressvaluesas
wellasamoremarkedviscositydropintheshearthinningregion.
TheseeffectscanbequantitativelyevaluatedfromtheRBC
param-etersreportedinTable1.Thezero-shearrateviscosity0andthe
criticalstresscincreasewithpolymerconcentrationaccordingto
scalinglaws.Theirexponentsare5.6and2.88,respectively.The
abruptincreaseofthemvalueabove1%underlinesthetransition
toapparentlyplasticbehavior.
Asatisfactory fittingqualityis alsoobtainedusingtheCross
equationforviscosity-shearratecorrelation.Theestimated0
val-uesdonotdiffersignificantlyfromthosegivenbytheRBCmodel
(seeTable1).Thevaluesofthecriticalshearrate ˙c(equaltothe
reciprocalofthecharacteristictime)andofthepowerlaw
expo-nentnclearlyindicatehowmuchthepolymerconcentrationaffects
Fig.1. A)Flowcurvesofscleroglucansystemsatdifferentpolymerconcentrations (from0.25to2wt%);B)Straindependenceoftheviscoelasticmoduliforthree scle-roglucansystems(1,1.5and2wt%);C)Mechanicalspectraofscleroglucansystems atdifferentpolymerconcentrations(0.5,1,1.5and2wt%).
Thesol-geltransitioninducedbyincreasingpolymer
concentra-tioncanbemoreproperlyindividuatedbyexaminingthechanges
intheviscoelasticresponsesobtainedfromboth stressand
fre-quencysweeps.Fig.1Billustrates thestraindependence ofthe
viscoelasticmodulifor threedifferentconcentrations.Above1%
deformation,theborderofthelinearviscoelasticregimeismarked
byaninitialincreaseofthelossmodulusGfollowedbya
subse-quentdecreaseinstronglynonlinearconditions.ThisLAOS(Large
AmplitudeOscillatoryShear) behavior characterizedby a weak
strainovershoothasbeenclassifiedastypeIII(Hyun,Kim,Ahn,&
Lee,2002)andistypicalofweakgels,suchasxanthan,andseveral
otherweaklystructuredfluidsaswell.
Evenmoreevidentarethechangesofthemechanicalspectraas
polymerconcentrationincreases(seeFig.1C).Below1wt%the
lin-earviscoelasticresponseistypicalofordinarypolymersolutions
witha crossover conditiondetectable within the experimental
window.Conversely,atscleroglucanconcentrationshigherthan
1wt%itresemblesthoseofotherweakpolymericgels(Lapasin,
2015; Lapasin & Pricl, 1995): G exceeds G over the whole
rangeexplored,andtheprofilesofG()andG()arenearly
par-allelwithaslightfrequency dependence.Using thegeneralized
Maxwellmodelfordatafitting,theseconcentrationeffectsresult
inaprogressivetransitionoftherelaxationtimespectrumfroma
wedge-typetowardsabox-typedistributionowingtheincreasing
weightoflongerrelaxationtimes.
3.1.2. Laponitesystems
Anincreaseinnanoclayconcentrationgivesorigintoasharp
transitioninboththeflowpropertiesandthelinearviscoelastic
behaviorofaqueousLaponitesystems.Below0.75%dilute
disper-sionsarealmostNewtonianorslightlyshear thinningand their
viscoelasticpropertiesarepracticallyundetectablewith
conven-tional instruments and methods. At higher concentrations, the
shearbehaviorisdecisivelyplasticsinceadramaticviscositydrop
(ofseveraldecades)isconfinedwithinaverynarrowstressinterval
(seeFig.2A),andthemechanicalspectrumistypicalofgelsorother
arrestedstates,withanevidentprevalenceofGoverG(morethan
oneorderofmagnitude)alongthewholefrequencywindow(see
Fig.2B).Asexpected,stresssweeptestsshowedthatconcentrated
Laponitedispersions(above0.75%)exhibitaLAOSbehavioroftype
III.
Theconcentrationeffectsontherheologicalpropertiesof
scle-roglucansystemsaremoregradualandunaffectedbystoragetime,
whereasthesharpsol-geltransitionofLaponitesystemsdepends
ontimeelapsedsincesamplepreparationandprogressivelyshifts
tolowerclayconcentrationthresholdwithincreasingagingtime.
Previousconsiderationsarereferredtoexperimentaldata
deter-minedonagedsystems(9daysafterpreparation).
Fig.3Ashowshowtheflowcurveofthe2%Laponitedispersion
graduallychangesitsshapewithincreasingagingtime.Afterfew
daysthebehaviorbecomesplasticandthetransitionfromthelow
shearNewtonianplateautotheshearthinningregionshiftsalong
bothaxestowardshigherviscositiesandstresses.Thesechanges
canbeconvenientlymeasuredthroughtheprogressiveandparallel
increaseofthetwoRBCparameters,0 andc,asillustratedin
Fig.3B.Thechangefromshearthinningtoplasticbehavioriseven
betterrecognizablefromthetimeevolutionoftheparameterm,
whichreacheshighvalesafterfewdays.
Asdiscussedpreviously,timeelapsedafterpreparationplays
an important role in the structural reorganization of Laponite
nanoparticles,thusstrengtheningthegelbehaviorofconcentrated
dispersionsorfavoringtheevolutiontowardafinalarrestedstateat
lowerconcentrations.Accordingly,agingtimeaffectsalsothe
sol-geltransition,shiftingthethresholdconcentrationtowardslower
values.
3.2. Laponite-scleroglucansystems
Thissectionisdedicatedtomixedsystems,prepared
accord-ingtodifferentproceduresandcontainingbothscleroglucanand
Fig.2.A)FlowcurvesofagedLaponitesystemsatdifferentclayconcentrations(from0.5to2wt%)(testsperformed9daysaftersamplepreparation);B)Mechanicalspectra ofagedLaponitesystemsatdifferentclayconcentrations(from0.75to2wt%)(testsperformed9daysaftersamplepreparation).
Fig.3.A)FlowcurvesofLaponitesystem2wt%obtainedfromtestsperformedatdifferentstoragetimes(2–18daysaftersamplepreparation)(symbols:experimentaldata, solidlinescalculatedfromtheRBCmodel);B)RBCparametersrelatedtotheflowbehaviorofLaponitesystem2wt%obtainedfromtestsperformedatdifferentstoragetimes (2–18daysaftersamplepreparation):zero-shearviscosity0(filledcircles),criticalstressc(opencircles),parameterm.
rheologicalresponses,immediatelyaftertheirpreparationorafter
enoughelapsedtime.Theexperimentaltestshavebeenaimedat
examininghowmuchthecontentand proportionofboth
com-ponentsaffecttheviscoelasticand flowpropertiesofthemixed
system,alsoinviewofpracticalapplicationsinthebiomedicalfield,
whereaqueoushydrogelmatricescanbeprofitablyusedto
for-mulatedrugdeliverysystemsinvirtueofappropriatestructural
featuresandrheologicalbehavior.
3.2.1. Blendsoffreshsimplesystems
Scleroglucan-Laponite blends have beenobtained bymixing
fresh simple systems under mechanical agitation in different
scleroglucan-Laponiteproportions(3:1,1:1,1:3).Thescleroglucan
andLaponitesystemshave beenpreparedatthesamepolymer
andclayweightpercentconcentration(2wt%),respectively,and
blendedtwodaysaftertheirpreparation.Experimentaltestshave
beenrepeatedlyperformedonmixedandsimplesystemsduring
Fig.4.A)Flowcurvesofscleroglucan-LaponiteblendspreparedfromfreshsimplesystemsandwithdifferentLaponitefractionwL(0,0.25,0.5,0.75,1)andmeasuredafter
twodaysaging(symbols:experimentaldata,solidlinescalculatedfromtheRBCmodel);B)Mechanicalspectraofscleroglucan-Laponiteblendspreparedfromfreshsimple systemsandwithdifferentLaponitefractionwL(0,0.25,0.5,0.75,1)andmeasuredaftertwodaysaging(symbols:experimentaldata,solidlinescalculatedfromtheGM
model).
Flowcurvesandmechanicalspectradeterminedaftertwodays
ofagingarecomparedinFig.4.Allthesystemsexhibitaplastic
behaviorandtheneachofbothRBCparameters,0orc,canbe
equivalentlyusedtodescribetheblendingeffectsonshearbehavior
owingtotheirinterrelationship.Indeed,achangeinmixingratio
leadstosimilarshiftsoftheflowcurvealongthetwoaxes.After
twodaysofagingtheLaponitedispersionischaracterizedbythe
lowestvaluesofbothparametersamongtheexaminedsystems,
whilethescleroglucancurveisplaced atintermediateviscosity
andstresslevels.Themaximum0andc(yieldstress)valuesare
attainedformixedsystems(1:1and1:3)withequalproportions
ofthetwocomponentsorprevailingclaycontentinthemixture.
EquivalentresultsareobtainedusingtheCrossparameter0whose
valuesdonotdifferappreciablyfromthoseofthecorresponding
RBCparameter.
Thus,thecomparisonoftheflowcurvesoffersafirstevidence
ofthesynergisticeffectsofpolymer-clayblending.Anotherproof
canbeobtainedfromlinearviscoelasticbehaviors.Allthe
mechan-icalspectraarecharacterizedbythepredominanceofthestorage
modulusand itsslightornegligible frequencydependence (see
Fig.4B).Theblendingeffectslookquitesimilartothoseevinced
fromflowbehaviors.Indeed,bothmixedsystems1:1and1:3show
an extended plateau with highG values, one order of
magni-tudehigherthanG.TheGMmodelprovidesaquitesatisfactory
fit to dataand itsparameters are certainly suitable toanalyze
thechangesassociatedwithblendingratio.However,theycanbe
replacedbytheviscoelasticmoduli,measuredatagivenfrequency
(1Hz),whichcanservethesamepurpose,evenmoreconveniently,
sincetheshapeoftherelaxationtimespectrumdoesnotchange
significantlywiththeblendingratioandonlyquantitativeeffects
areobserved.
ThefollowingFiguresshowhowtheviscousand viscoelastic
parameterschangewithincreasingagingtime.Fig.5Aillustrates
thedependenceofthezero-shearrateviscosityonwL,which
rep-Fig.5.A)Zero-shearviscosity(0)obtainedfromtestsperformedatdifferentstoragetimes(from2to15days)forscleroglucan-Laponiteblendspreparedfromfreshsimple
systemsandwithdifferentLaponitefractionwL(0,0.25,0.5,0.75,1);B)Viscoelasticmoduli(at1Hz)obtainedfromtestsperformedattwodifferentstoragetimesfor
Fig.6.A)Excessfunctionsforzero-shearviscosityandviscoelasticmoduliofblendspreparedfromagedsimplesystemsandwithdifferentLaponitefractionwLafter15days
ofaging;B)Excessfunctionsforzero-shearviscosityandviscoelasticmoduliofblendspreparedfromfreshsimplesystemsandwithdifferentLaponitefractionwLafter
15daysofaging.
resentstheweightfractionofsimpleLaponitedispersionpresent
inthemixedsystem.Thereported0valueshavebeencalculated
withtheCrossequationfromdataobtainedafterdifferentaging
times(2,5,8,11,15days).Fig.5Bregardstheviscoelasticmoduli
(G1Hz,G1Hz)measured2and15daysafterpreparation.The
syn-ergisticeffectsofblendingcanbeclearlyevincedbytheprofilesof
0,G1Hzand,toalesserextent,G1Hz,particularlytwodaysafter
preparation.Increasingagingtimeleadstosignificantincrements,
inparticularof0,forthesimpleLaponitesystemandevenforthe
mixedsystemwithprevailingLaponitecontent.
3.2.2. Blendsofagedsimplesystems
Agingstabilityisdecisivelyhigherifblendsarepreparedfrom
agedsimplesystems.Themixtureparametersaresimilartothose
exhibitedbyblendsoffreshsimplesystemsafter15daysofaging
(Fig.6).Here, thelogarithmsof 0,G1Hz and G1Hz arechosen
asthereferencequantitiesforexaminingtheblendingeffects.In
analogywiththecorrelationequations,oftenusedfordynamicor
kinematicviscositiesofNewtonianliquidsmixtures(Grunberg&
Nissan,1949;McAllister,1960),linearadditivity isassumed for
suchquantitiesinabsenceofpolymer-clayinteractionsand,
con-sequently,positiveornegativesynergisticeffectsaremeasuredby
theexcessfunctions.i.e.deviationsfromthelinearmixingrule:
Y=Yexp−Yid (Y=log0,logG,logG) (7)
Yid=YS+wL(YL−YS) (8)
wherethesuffixesexpandidstandformeasuredvalueandvalue
predictedbythelinearmixingrule,respectively,andthevalues
ofthesimplescleroglucanandLaponitesystems(YsandYL)are
linearlycombinedthroughtheweightfractionwLtocalculatethe
idealreferencevalue.
ItcanbeobservedinFig.6thatblendingeffectsonviscosityand
moduliaresimilar,especiallyforblendspreparedfromfreshsimple
systems.log0,logGandlogGdeviationsfromthelinearmixing
ruleshowcomparableprofilesindependenceofblendcomposition.
Forbothsystemstheeffectofblendingismaximalwhenblends
arerich in Laponiteand diminisheswithdecreasing
nanoparti-clescontent,inparticularif0 eGareconsidered.Therefore,it
maybeconcludedthatthemodeofpreparationandstorage
condi-tionsdoaffectblendrheologicalpropertiessignificantly,depending
ontheLaponiteamount.Moreover,interestingconclusionsmay
bedrawnbyobservingviscosityratios(0,R),obtainedby
divid-ing0valuesofblendsbythoseofcorrespondingsimplesystems,
whichcontainjustonecomponent(scleroglucan orLaponite)in
thesameconcentrationoftheblend.InFig.7Athezero-shear
vis-cosityprofilesofblendspreparedfromagedsimple systemsare
compared withthoseof aqueousLaponitedispersionsand
scle-roglucansolutionshavingequalconcentrationofpolymerorclay.
InFig.7Btheviscosityratios0,R ofblendsandrelative
viscosi-ties0,r ofthecorrespondingsimplesystemsareplottedvsboth
Laponiteandscleroglucanconcentrations.Itcanbenoticedthatthe
contributionofLaponitetotheviscosityofscleroglucanmatrices,
measuredby0,R,progressivelyincreaseswithincreasingLaponite
amountand/orwithdecreasingpolymerconcentrationinblend.At
thesametimethecontributionofthepolymertotheviscosityof
thesystemgraduallydiminishes.
3.2.3. ScleroglucanhydrogelsaddedwithLaponitenanoparticles
TheadditionofLaponitetoscleroglucanhydrogel(2wt%)
pro-ducesonlyquantitativeeffectsonitsviscousandlinearviscoelastic
behaviors,whichdonotchangequalitativelyandremainsimilarto
thoseofatypicalphysicalpolymergel.AsLaponiteconcentration
increases,theshapeofthemechanicalspectradoesnotchange
sig-nificantly.FittingdatatothegeneralizedMaxwellmodelshows
thattheequilibriummodulusGeincreasesandalsothe
contribu-tionoftherelaxationtimespectraslightlyincreases,maintaining
thesameshape(box-typedistribution).Alltherheological
param-etersincreasewithincreasingLaponiteconcentration.Theeffect
tendstodiminishprogressively,when,despitetheirhigh
concen-tration,nanoparticles,distributedwithinthepolymernetwork,are
nolongerabletodevelopanextendedaggregationstructure,
com-parabletothatofthecorrespondingsimpleLaponitesystem.The
ratiosofrheologicalparameters,obtainedbydividingmeasured
rheologicalparametersofblendsbythoseofthesimple
scleroglu-cansystem,whichareshowninFig.8,supporttheseobservations.
Quitesimilarincrementsareobservedforthecriticalstresscand
Fig.7. A)0valuesofblendspreparedfromagedsimplesystemscomparedwiththoseofthecorrespondingscleroglucansolutionsandLaponitedispersions(withequal
concentrationofpolymerorclay);B)Viscosityratios(0,R)ofblends(referredtoscleroglucanorLaponitesimplesystems)(filledsymbols)andrelativeviscosities(0,r)of
scleroglucansolutionsandLaponitedispersions(opensymbols)vsLaponiteandscleroglucanconcentrations(CsandCL).
Fig.8. A)Viscosityratios(0,R)ofblendsobtainedbyLaponiteadditiontoscleroglucanhydrogel(2wt%)independenceofLaponiteconcentrationCL;B)Storageandloss
moduliratios(GRandGR)ofblendsobtainedbyLaponiteadditiontoscleroglucanhydrogel(2wt%)independenceofLaponiteconcentrationCL.
Asitcanbenoted,theincreaseofviscosityissignificantand
greaterthanthoseofthecorrespondingmoduliparameters.Indeed,
forthemaximalLaponiteaddition(2wt%)thezeroshearviscosityis
seventimeshigherthanthatofthepolymericmatrix,whileitis70%
higherthanthatoftheaqueousLaponitedispersionat2wt%.
How-ever,itshouldbealsounderlinedthatthe0,R,valueofthemixed
system,obtainedbyLaponiteadditiontothescleroglucanhydrogel
matrix,issignificantlylower(forabout8ordersofmagnitude)than
the0,rvalueoftheLaponitedispersion(seepreviousfigure).The
rheologicalresponsesofthehydrogelwith2wt%Laponiteadded
aremuchclosetothoseexhibitedbyblendsoffreshoragedsimple
systemswith1:3scleroglucan-Laponiteproportioninspiteoftheir
lowercontentsofbothcomponents.
It maybeassumed, that byadding Laponitetoa previously
formedscleroglucanhydrogel,theindividualclaynanoparticles,
althoughcapableofaggregation,aredistributedwithinthemeshes
oftriplexnetwork,thuscontributingtotherheologicalresponseof
thesystemnotdifferentlyfromotherparticulatefillers.
4. Conclusions
Thestudycarriedouthasshownthattheblendingofmature
Laponitedispersionswithweakgelsofscleroglucancangivehybrid
systemsofrheologicalpropertiessuperiortothosepredictableby
applyingthelinearmixingruleforcorrespondingsimplesystems.
Thesynergistic effects previously explainedin theintroduction
areparticularlyevidentforLaponiterichblends.Forexample,for
blendswithLaponite/scleroglucanweightratio3:1themeasured
rheologicalparameters(zeroshearviscosityandviscoelastic
mod-uli)werefromtwotothreetimeshigherthanthosepredictedby
themixingrule.
Differenttypesandgradesofhydrogelstructuresareobtained
byvaryingtheratioofthetwocomponents.Forinvestigatedblends
theprogressiveincreaseofthecontentofLaponiteparticles
cor-respondstoanequalreductionofthescleroglucanamount.The
increase of the Laponite amount in a blend, beyond a critical
ofnanoparticleaggregateswhoserheologicalcontributioncannot
onlycompensatebut alsoovercomethenegativeeffects dueto
decreasingpolymercontribution.
On theother hand, byadding Laponite nanoparticlesinto a
gelmatrixofscleroglucan(2wt%)significantincreasesinviscosity
andviscoelasticmoduliareobtained.However,theseincremental
effectsdiminishwithincreasingnanoclayaddition.
The rheological responses of such mixed system having a
Laponitecontentof2wt%areonlyslightly higherthanthoseof
thecorrespondingsimpledispersionofLaponite(2wt%)inwater.
Theyarealsoverysimilartothoseofblendsobtainedfromsimple
systemsofLaponiteandscleroglucanwithblendingratio3:1
(over-allamountofLaponiteandscleroglucanrepresents2wt%,freshor
maturedblends),whichcontainlessLaponiteandscleroglucan.
Therefore,itmaybeconcludedthathydrogelsofcomparable
rheologicalbehaviorcanbepreparedbyblendingsimpleLaponite
andscleroglucanaqueoussystems,indifferentratiosandby
dif-ferentmodesofpreparation,orbyaddingLaponitepowderintoa
gelmatrixofscleroglucan.Theseobservationsopenupinteresting
prospectsofusinghydrogelsbasedonLaponiteandscleroglucan
inthebiomedicalfield,inparticularforthepreparationof
con-trolledreleasesystems,andhintatpossibleimprovementswith
respecttoperformanceofscleroglucanhydrogels.Despitethe
com-parableviscousand viscoelasticpropertiesof suchgelsystems,
itisreasonabletoassumethattheirstructuralcharacteristics,if
examinedondifferentlengthscales,aresubstantiallydissimilar.
Consequently,alsothediffusivity ofan activesubstancewithin
thecomplexgelstructurewithdifferentdegreeofheterogeneity
and interpenetrationofstructures of Laponiteandscleroglucan
shouldbesignificantlyaffected.Forthcomingstructural
investiga-tions,using low-fieldNMRtestsand studiesontheactivedrug
release,willallowustobetterassesstheopportunitiesforuseof
suchhybridsystemsinthefieldofdrugdeliveryand,possibly,to
optimizetheformulationoftheLaponite/scleruglucanhydrogel.
Acknowledgements
Theauthorsacknowledgethefinancialsupportfromthe
Slove-nianResearchAgency(researchcorefundingNo.P2-0191).
References
BabaHamed,S.,&Belhadri,M.(2009).Rheologicalpropertiesofbiopolymers drillingfluids.JournalofPetroleumScienceandEngineering,67,84–90. Baghdadi,H.A.,Sardinha,H.,&Bhatia,S.R.(2005).Rheologyandgelationkinetics
inLaponitedispersionscontainingpoly(ethyleneoxide).JournalofPolymer SciencePartB:PolymerPhysics,43,233–240.
Baghdadi,H.A.,Jensen,E.C.,Easwar,N.,&Bhatia,S.R.(2008).Evidencefor re-entrantbehaviorinLaponite–PEOsystems.RheologicaActa,47,121–127. Bais,D.,Trevisan,A.,Lapasin,R.,Partal,P.,&Gallegos,C.(2005).Rheological
characterizationofpolysaccharide–surfactantmatricesforcosmeticO/W emulsions.JournalofColloidandInterfaceScience,290,546–556. Bluhm,T.L.,Deslandes,Y.,Marchessault,R.H.,Pérez,S.,&Rinaudo,M.(1982).
Solid-stateandsolutionconformationofscleroglucan.CarbohydrateResearch, 100,117–130.
Bohn,J.A.,&BeMiller,J.N.(1995).(1→3)--d-Glucansasbiologicalresponse modifiers:Areviewofstructure-functionalactivityrelationships.Carbohydrate Polymers,28,3–14.
Coviello,T.,Grassi,M.,Lapasin,R.,Marino,A.,&Alhaique,F.(2003).
Scleroglucan/borax:Characterizationofanovelhydrogelsystemsuitablefor drugdelivery.Biomaterials,24,2789–2798.
Coviello,T.,Palleschi,A.,Grassi,M.,Matricardi,P.,Bocchinfuso,G.,&Alhaique,F. (2005).Scleroglucan:Aversatilepolysaccharideformodifieddrugdelivery. Molecules,10,6–33.
Cross,M.M.(1965).Rheologyofnon-Newtonianfluids:Anewflowequationfor pseudoplasticsystems.JournalofColloidScience,20,417–423.
Cummins,H.Z.(2007).Liquid,glass,gel:ThephasesofcolloidalLaponite.Journal ofNon-CrystallineSolids,353,3891–3905.
Daga,V.K.,&Wagner,N.J.(2006).Linearviscoelasticmastercurvesofneatand Laponite-filledpoly(ethyleneoxide)–watersolutions.RheologicaActa,45, 813–824.
Davila,J.L.,&d’Ávila,M.A.(2017).Laponiteasarheologymodifierofalginate solutions:Physicalgelationandagingevolution.CarbohydratePolymers,157, 1–8.
deBruyn,J.R.,Pignon,F.,Tsabet,E.,&Magnin,A.(2008).Micron-scaleoriginofthe shear-inducedstructureinLaponite–poly(ethyleneoxide)dispersions. RheologicaActa,47,63–73.
DeLisi,R.,Gradzielski,M.,Lazzara,G.,Milioto,S.,Muratore,N.,&Prévost,S.(2008). AqueousLaponiteclaydispersionsinthepresenceofpoly(ethyleneoxide)or poly(propyleneoxide)oligomersandtheirtriblockcopolymers.TheJournalof PhysicalChemistryB,112,9328–9336.
Ellis,S.B.(1927).Thesis.USA:LafayetteCollege,Pa.Citedin:Matsuhisa,S.,&Bird R.B.(1965).Analyticalandnumericalsolutionsforlaminarflowofthe Non-NewtonianEllisfluid.AmericanInstituteofChemicalEngineersJournal,11, 588–595.
Fari ˜na,J.I.,Si ˜neriz,F.,Molina,O.E.,&Perotti,N.I.(2001).Isolationand physicochemicalcharacterizationofsolublescleroglucanfromSclerotium rolfsii.Rheologicalproperties,molecularweightandconformational characteristics.CarbohydratePolymers,44,41–50.
Fitch,F.R.,Jenness,P.K.,&Rangus,S.E.(1991).Rheologicalstudyofblendsof Laponiteandpolymerthickeners.InR.A.F.Williams,&N.C.deJaeger(Eds.), Advancesinmeasurementandcontrolofcolloidalprocesses(pp.292–310). Oxford:Butterworth-Heinemann.
Gabriel,J.-C.P.,Sanchez,C.,&Davidson,P.(1996).Observationofnematic liquid-crystaltexturesinaqueousgelsofsmectiteclays.TheJournalofPhysical Chemistry,100,11139–11143.
Gallino,G.,Guarneri,A.,Poli,G.,&Xiao,L.(1996).Scleroglucanbiopolymer enhancesWBMperformances.InSPEannualtechnicalconferenceandexhibition (pp.105–119).http://dx.doi.org/10.2118/36426-ms
Grassi,M.,Lapasin,R.,Coviello,T.,Matricardi,P.,DiMeo,C.,&Alhaique,F.(2009). Scleroglucan/borax/drughydrogels:Structurecharacterisationbymeansof rheologicalanddiffusionexperiments.CarbohydratePolymers,78,377–383. Grassi,M.,Lapasin,R.,&Pricl,S.(1996).Astudyoftherheologicalbehaviorof
scleroglucanweakgelsystems.CarbohydratePolymers,29,169–181. Grassi,M.,Lapasin,R.,Pricl,S.,&Colombo,I.(1996).Apparentnon-fickianrelease
fromascleroglucangelmatrix.ChemicalEngineeringCommunications,155, 89–112.
Grunberg,L.,&Nissan,A.H.(1949).Mixturelawforviscosity.Nature,164,799–800. Hyun,K.,Kim,S.H.,Ahn,K.H.,&Lee,S.J.(2002).Largeamplitudeoscillatoryshear
asawaytoclassifythecomplexfluids.JournalofNon-NewtonianFluid Mechanics,107,51–65.
Jabbari-Farouji,S.,Tanaka,H.,Wegdam,G.H.,&Bonn,D.(2008).Multiple nonergodicdisorderedstatesinLaponitesuspensions:Aphasediagram. PhysicalReviewE,78,061405.
Kishore,S.,Chen,Y.,Ravindra,P.,&Bhatia,S.R.(2015).Theeffectofparticle-scale dynamicsonthemacroscopicpropertiesofdisk-shapedcolloid–polymer systems.ColloidsandSurfacesA:PhysicochemicalandEngineeringAspects,482, 585–595.
Kulawardana,E.U.,Koh,H.,Kim,D.H.,Liyanage,P.J.,Upamali,K.,Huh,C.,etal. (2012).RheologyandtransportofimprovedEORpolymersunderharsh reservoirconditions.InSPEimprovedoilrecoverysymposiumhttp://dx.doi.org/ 10.2118/154294-ms
Lapasin,R.,&Maiutto,A.(2006).RheologyofaqueousLaponite-polymermixtures. InAERC2006:3rdannualEuropeanrheologyconference(p.2006).
Lapasin,R.,&Pricl,S.(1995).Rheologyofindustrialpolysaccharides:Theoryand applications.London:Chapman&Hall,London.
Lapasin,R.,Pricl,S.,&Esposito,P.(1990).Rheologicalbehaviourofscleroglucan aqueoussystemsinthesolutionandgeldomains.InR.E.Carter(Ed.),Rheology offood,pharmaceuticalandbiologicalmolecules(pp.122–132).LondonandNew York:ElsevierAppliedSciences.
Lapasin,R.(2015).Rheologicalcharacterizationofhydrogels.InP.Matricardi,F. Alhaique,&T.Coviello(Eds.),Polysaccharidehydrogels:Characterizationand biomedicalapplications(pp.83–137).Singapore:PanStanfordPublishing. Lapasin,R.(2016).ProprietàreologichedidispersionidiLaponiteinmatrici
acquosedixantano.PantaRei,14,7–19.
Levitz,P.,Lécolier,E.,Mourchid,A.,Delville,A.,&Lyonnard,S.(2000).Liquid-solid transitionofLaponitesuspensionsatverylowionicstrength:Long-range electrostaticstabilisationofanisotropiccolloids.EurophysicsLetters,49, 672–677.
Lin-Gibson,S.,Kim,H.,Schmidt,G.,Han,C.C.,&Hobbie,E.K.(2004).Shear-induced structureinpolymer–claynanocompositesolutions.JournalofColloidand InterfaceScience,274,515–525.
Maiutto,A.(2005).MasterThesis.UniversityofTrieste,FacultyofEngineering. Malwitz,M.M.,Butler,P.D.,Porcar,L.,Angelette,D.P.,&Schmidt,G.(2004).
Orientationandrelaxationofpolymer-claysolutionsstudiedbyrheologyand small-angleneutronscattering.JournalofPolymerScienceB:PolymerPhysics, 42,3102–3112.
Matricardi,P.,Onorati,I.,Coviello,T.,&Alhaique,F.(2006).Drugdeliverymatrices basedonscleroglucan-alginate-boraxgels.InternationalJournalof
Pharmaceutics,316,21–28.
McAllister,R.A.(1960).Theviscosityofliquidmixtures.AmericanInstituteof ChemicalEngineersJournal,6,427–431.
Mongondry,P.,Nicolai,T.,&Tassin,J.F.(2004).Influenceofpyrophosphateor polyethyleneoxideontheaggregationandgelationofaqueousLaponite dispersions.JournalofColloidandInterfaceScience,275,191–196.
Morariu,S.,&Bercea,M.(2015).ViscoelasticpropertiesofLaponiteRDdispersions containingPEOwithdifferentmolecularweights.RevueRoumainedeChimie, 60,777–785.
Mourchid,A.,Delville,A.,Lambard,J.,Lécolier,E.,&Levitz,P.(1995).Phasediagram ofcolloidaldispersionsofanisotropicchargedparticles:Equilibrium properties,structure,andrheologyofLaponitesuspensions.Langmuir,11, 1942–1950.
Mourchid,A.,Lécolier,E.,VanDamme,H.,&Levitz,P.(1998).Onviscoelastic, birefringent,andswellingpropertiesoflaponiteclaysuspensions:Revisited phasediagram.Langmuir,14,4718–4723.
Palleschi,A.,Bocchinfuso,G.,Coviello,T.,&Alhaique,F.(2005).Molecular dynamicsinvestigationsofthepolysaccharidescleroglucan:Firststudyonthe triplehelixstructure.CarbohydrateResearch,340,2154–2162.
Pozzo,D.,&Walker,L.(2004).Reversiblesheargelationofpolymer–clay dispersions.ColloidsandSurfacesA:PhysicochemicalandEngineeringAspects, 240,187–198.
Rinaudo,M.,&Vincendon,M.(1982).13CNMRstructuralinvestigationof scleroglucan.CarbohydratePolymers,2,135–144.
Roberts,G.P.,Barnes,H.A.,&Carew,P.(2001).Modellingtheflowbehaviourof veryshear-thinningliquids.ChemicalEngineeringScience,56,5617–5623. Ruzicka,B.,&Zaccarelli,E.(2011).AfreshlookattheLaponitephasediagram.Soft
Matter,7,1268–1286.
Ruzicka,B.,Zulian,L.,&Ruocco,G.(2004).Routestogelationinaclaysuspension. PhysicalReviewLetters,93,258301.
Ruzicka,B.,Zulian,L.,&Ruocco,G.(2006).MoreonphasediagramofLaponite. Langmuir,22,1106–1111.
Ruzicka,B.,Zaccarelli,E.,Zulian,L.,Angelini,R.,Sztucki,M.,Moussaid,A.,etal. (2011).Observationofemptyliquidsandequilibriumgelsinacolloidalclay. NatureMaterials,10,56–60.
Savenko,V.,Bulavin,L.,Rawiso,M.,Loginov,M.,Vorobiev,E.,&Lebovka,N.I. (2013).SedimentationstabilityandagingofaqueousdispersionsofLaponitein
thepresenceofcetyltrimethylammoniumbromide.PhysicalReviewE,88, 052301.
Savenko,V.,Bulavin,L.,Rawiso,M.,&Lebovka,N.I.(2014).Agingofaqueous Laponitedispersionsinthepresenceofsodiumpolystyrenesulfonate. UkrainianJournalofPhysics,59,589–595.
Schmidt,G.,Nakatani,A.I.,Butler,P.D.,Karim,A.,&Han,C.C.(2000).Shear orientationofviscoelasticpolymer–claysolutionsprobedbyflow birefringenceandSANS.Macromolecules,33,7219–7222.
Sveistrup,M.,vanMastrigt,F.,Norrman,J.,Picchioni,F.,&Paso,K.(2016).Viability ofbiopolymersforenhancedoilrecovery.JournalofDispersionScienceand Technology,37,1160–1169.
Tanaka,H.,Meunier,J.,&Bonn,D.(2004).Nonergodicstatesofchargedcolloidal suspensions:Repulsiveandattractiveglassesandgels.PhysicalReviewE,69, 31404–31410.
Tanaka,H.,Jabbari-Farouji,S.,Meunier,J.,&Bonn,D.(2005).Kineticsof
ergodic-to-nonergodictransitionsinchargedcolloidalsuspensions:Agingand gelation.PhysicalReviewE,71,21402–21412.
Vi ˜narta,S.C.,Franc¸ois,N.J.,Daraio,M.E.,Figueroa,L.I.C.,&Fari ˜na,J.I.(2007). Sclerotiumrolfsiiscleroglucan:Thepromisingbehaviorofanatural
polysaccharideasadrugdeliveryvehicle,suspensionstabilizerandemulsifier. InternationalJournalofBiologicalMacromolecules,41,314–323.
Yanaki,T.,&Norisuye,T.(1983).Triplehelixandrandomcoilofscleroglucanin dilutesolutions.PolymerJournal,15,389–396.
Zebrowski,J.,Prasad,V.,Zhang,W.,Walker,L.M.,&Weitz,D.A.(2003).Shake-gels: Shear-inducedgelationofLaponite/PEOmixtures.ColloidsandSurfacesA: PhysicochemicalandEngineeringAspects,213,189–197.
Zulian,L.,Ruzicka,B.,&Ruocco,G.(2008).Influenceofanadsorbingpolymeron theagingdynamicsofLaponiteclaysuspensions.PhilosophicalMagazine,88, 4213–4221.
