Structural, rheological and dynamics insights of hydroxypropyl guar gel-like systems

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Structural,

rheological

and

dynamics

insights

of

hydroxypropyl

guar

gel-like

systems

Chiara

Berlangieri

a

_,

_Giovanna

_Poggi

a

_,

_Sergio

_Murgia

b

_,

_Maura

_Monduzzi

b

_,

_Luigi

_Dei

a

_,

Emiliano

Carretti

a,∗

a_Department_of_Chemistry_“Ugo_Schiff”_&_CSGI_Consortium,_University_of_Florence,_via_della_Lastruccia,_3,₅₀₀₁₉_Sesto_Fiorentino,_Florence,_Italy b_Department_of_Chemical_and_Geological_Sciences_and_CSGI,_University_of_Cagliari,_ss₅₅₄_bivio_Sestu,₀₉₀₄₂_Monserrato,_Cagliari,_Italy

a

r

t

i

c

l

e

i

n

f

o

Articlehistory: Received3October2017

Receivedinrevisedform9January2018 Accepted11February2018

Availableonline13February2018 Keywords:

Guargum Borax Hydrogel Cleaning

Culturalheritageconservation

a

b

s

t

r

a

c

t

Adynamic,rheological,andstructuralcharacterizationofaqueousgel-likesystemscontaining hydrox-ypropylguargum(HPG),boraxandglycerolispresentedinthispaper.Theroleofglycerol,whichis introducedasaplasticizerintheformulation,isinvestigatedbymeansof11_B_NMR_and1_H_NMR_PGSTE

measurementsinordertoclarifyitscontributiontothegelnetworkformationanditsinteractionwith borax,withwhomitformsacomplex.Theeffectofgelscomponentsontherheologicalbehaviourandon theactivationenergyrelatedtotherelaxationprocessofthesystemwasassessedbymeansofrheology. Theresultsobtainedsuggestthatthemechanicalpropertiesofthesegelscanbetunedandcontrolled bymodulatingtheformulationinawiderangeofcompositions.Moreover,astructuralcharacterisation hasbeenalsocarriedoutbymeansofSmallAngleX-rayScattering(SAXS)tohighlighttheroleofthe variouscomponentsonthemeshsizeofthenetwork.Thestructuralandmechanicalcharacteristicsof thesesystemssuggesttheirpotentialuseforapplicativepurposes.Inthisregard,oneofthegelsetup hasbeensuccessfullytestedascleaningagentonthesurfaceofaXIXstuccofragmentcomingfromthe LaFenicetheatre(Venice,Italy)fortheremovalofadirtlayercomposedbydustandparticulatedmatter originatedduringaﬁrein1996.

1. Introduction

Hydroxypropylguargum (HPG) is a derivativeof guargum inwhichseveralfreehydroxylsaresubstitutedbyhydroxypropyl groupsuponetheriﬁcation(Fig.1A).Itisanaturalpolysaccharide extractedfromtheseedsofCyamopsistetragolonoba,anditis com-posedbyalinearchainofmannoseunitslinkedby␤-1,4bonds. Thisbackboneisfunctionalizedbygalactoseresidues␣-1,6-linked, randomlyattached,formingshortside-branches.Itischeap, non-toxic,biocompatible,biodegradable,easyavailableandsolublein water,whereitformsviscoussolutionsevenatlowconcentrations [1–5].Forthesereasonsguargumhasseveralapplicationsinfood, paper,textile,pharmaceuticalandpetroleumindustries.

HPGtogetherwithsimilarpolysaccharideshasbeenrecently usedfortheformulationofversatileandwell-performingmaterials withvariousﬁeldsofapplication[1,6–10].Gel-likesystems con-tainingionicsurfactantsandmodiﬁedguarcovalentlycrosslinked

∗ Correspondingauthor.

E-mailaddresses:[email protected]ﬁ.it,emiliano.carretti@uniﬁ.it(E.Carretti).

havealsobeenproposedasagentsforremovingsolidparticles,able toperformagentlemechanicalsurfacecleaningofsensitive mate-rialssuchasceramic,wood,glassandmetals[11].Hydrogelsmade byguarandHPGcrosslinkedwithionssuchastitanateandborax arewidelyusedintheoilandgasindustry,mainlyasaproppant transportagentinhydraulicfracturingprocess[1,6–10].

Furthermore,theefﬁcacyofboraxascrosslinkerforpolymers bearing hydroxyl groups is well known (see Fig. 1B) [12,13]. The nature of this linkage is debated: according to the model proposedbyShibayamaforpolyvinylalcohol[14]andfor scleroglu-cans[15,16],mixedphysical/chemicallinkagesoccur;nevertheless, morerecentstudiessuggestthepresenceofchemical crosslinks onlybetweenboraxandpolymericchainswiththeformationof chemicalbridgesbytransientcovalentbonds[17,18].Moreover, beingthelifetimeofthesecovalentcrosslinksintheorderof sec-onds[13,19],theguar-boratenetworksareusuallycharacterized byself-healingproperties.

Itiswellknownthatglycerolincreasestheﬂexibilityof poly-mericmaterialsbyincreasingtheinterchainspacing;duetothis andtoitslargeavailability,glycerolisusedasplasticizerfor poly-mericmaterialsin manyﬁelds,as foodpackaging[20,21] orto

https://doi.org/10.1016/j.colsurfb.2018.02.025

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Fig.1. (A)Chemicalstructureofhydroxypropylguar(HPG).(B)Crosslinkingreactionleadingtotheformationofagelatornetwork.

improvetheplasticityofproteinandpolysaccharidesbasedﬁlms [20,21].Itcanalsoleadtoanincreaseoftheelongationatbreakof materials,resultinginsystemsthatcanresisttoacertainstretching beforebreaking[20,21].

In this paper, a dynamic, rheological, and structural charac-terizationofaqueousgel-likesystemscontainingHPG,boraxand glycerolis presented.In thelast decades,severalpapers focus-ingonthestudyofthemechanicalpropertiesandtheapplicative performancesofHPGaqueoussolutions[22]andgel-likesystems [8–10,23,24]havebeenpublished.However,toourknowledge,a systematicstudyofthesimultaneousroleofbothboraxand glyc-erolonthestructuralandonthemechanicalpropertiesofaqueous HPGbasedgels, whichisthemain subjectofthis paper,is still lacking.Theroleofglycerolinthecrosslinkingmechanismandits interactionswithboraxwerestudiedheresystematicallyforthe ﬁrsttimebyusingacombinedapproachbasedonboth11_B_NMR, 1_H_NMR_PGSTE_and_rheological_{measurements.}_In_particular

rheo-logicalinvestigationsallowedtoobtaininformationontheeffect ofgelscomponentsontheviscoelasticbehaviour,ontheactivation energyrelatedtotherelaxationprocessofthesystemandonthe criticalpolymericconcentration,abovewhichanextended three-dimensionalpolymernetworkisformed.Furthermore,information abouttheroleofallthecomponentsonthestructureofHPG-based systems,wereobtainedbyaSmallAngleX-rayScattering(SAXS) investigation.

Thisapproachallowedtoidentifyseveralofthekeyfactorsthat controlthenatureofthematerialsandtheresultsobtained sug-gestthatthemechanicalpropertiesofthesegel-likefluidscanbe tunedbymodulatingtheircomposition.Theextendedstructural andmechanicalcharacterizationofthesegel-likesystemsshows thattheirpropertiescanbewidelycontrolledandtuned,bytuning theconcentrationofthecomponents.Thisfeaturehasimportant applicativeimplicationsmainlyin thefieldof CulturalHeritage conservationwheretheHPG/glycerol/boraxsystemscanbeused fortheselectiveandgentlecleaningofpaintedsurfacesof histor-icalandartisticinterest.Inthisregard,apreliminaryapplicative testwascarriedoutonasamplecollectedfromastuccodecoration ofLaFenicetheatre,inVenice.Agel-likeformulationwas success-fullyusedfortheremovalofadarkpatinaresultedfromthefire thatalmostdestroyedthetheatrein1996.

2. Materialsandmethods 2.1. Chemicals

Hydroxypropylguargum (HPG,Mw=90KDa) witha substi-tutionlevelof1.2waskindlyprovidedbyAmetechSrl;glycerol (Merck,purity99%)andsodiumtetraboratedecahydrate,(Sigma Aldrich,purity99.5–100%)wereusedasreceivedbythesuppliers.

2.2. Samplespreparation

Sampleswithoutplasticizerwerepreparedthroughthe follow-ingprocedure:HPGwasplacedinwaterandthenstirredandheated at65◦Ctofavoursolubilization.Anaqueoussolutionofboraxat 4%wasthenaddedandthesamplewasstirredwithaVortexto obtaina homogenousgelation.For samplescontainingthe plas-ticizer,HPGwasaddedtoaglycerolaqueoussolution.Inallthe samplescontainingborax,theratiopolymer/boraxwasﬁxedto4/1 asreportedinliteraturefortheformulationofcrosslinkedsystems [10,23],exceptfortheseriesinwhichthecrosslinkereffectwas investigated.

2.3. Rheologicaltests

OscillatoryshearmeasurementswerecarriedoutonaTA Instru-ment HybridRheometer DISCOVERY HR-3, using a plate–plate geometry(FlatPlate40mmdiameter)andaPeltierfortemperature control.Theminimumgapbetweentheplatesatzeroradial posi-tionwas500␮m.Thecellwasclosedbyloweringtheheadtothe measuringpositioninthezaxisforcecontrolledmode;the max-imumsqueezingforcewas1.0N.Beforestartingtheexperiments, thesampleswerelefttoequilibratefor30minat25◦C.Frequency sweepmeasurementswerecarriedoutwithinthelinear viscoelas-ticrange(5%strain),determinedbymeansofanamplitudesweep test(see Fig.S1).Thestorageand lossmoduli(G’ andG”)were measuredoverthefrequencyrangeof0.001–100Hz.

TimeTemperatureSuperposition(TTS)procedurewascarried out to observethe temperature-dependence of the mechanical propertiesoftheviscoelasticmaterials.Intheseexperiments,the Storagemodulus(G’)andLoss modulus(G”)weremonitoredas a functionoffrequencyin thelinearviscoelasticregionat vari-oustemperaturesbymeansofisothermalfrequencysweeptests. TTSprincipleimpliesthatcurvesobtainedatvarioustemperatures canbesuperimposedbyshiftingthedataintheverticaland/or horizontaldirections;allthecurvescanbecombinedinasingle mastercurvechoosingareferencetemperature(T0).Allthe

sys-temswereanalysedbykeeping25◦Casthereferencetemperature. The shape ofthemaster curvedoesnot changewith tempera-ture,butthecurveappearstobeonlyshifted[23,25].Themaster curvegeneratedbythesoftware(TRIOSDataAnalysis,TA Instru-ment)representsthetimeresponseofthematerialatthereference temperature.Thesoftwarecananalysetheshiftfactorsusingthe ArrheniusEq.(1)tocalculatetheActivationEnergyoftherelaxation process: lnaT=

−E_R

1_{T −}_T1 0

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water at 25◦C using a Bruker Advance 300MHz (7.05T) spec-trometerattheoperating frequenciesof300.13and96.29MHz, respectively.11_B_NMR_chemical_shifts_were_referenced_with_respect

toanexternalBF3–OEt2standardat0.0ppm.

AstandardBVT3000variabletemperaturecontrolunitwithan accuracyof±0.5◦_C_was_used._{Self-diffusion}_{coefﬁcients}_were

deter-minedusingaBrukerDIFF30probe suppliedbya BrukerGreat 1/40amplifierthatcangeneratefieldgradientsupto1.2Tm−1. Thepulse-gradientstimulatedecho(PGSTE) sequencewasused [26,27].Self-diffusioncoefficients wereobtainedbyvaryingthe gradientstrength(g)whilekeepingthegradientpulselength(ı) andthegradientpulseintervalsconstantwithineach experimen-talrun.ThedatawerefittedaccordingtotheStejskal-TannerEq. (2):

I/I0=exp(-Dq2t) (2)

whereIandI0arethesignalsintensitiesrespectivelyinthepresence

andabsenceoftheappliedfieldgradient,q=gıistheso-called scatteringvector(beingthegyromagneticratiooftheobserved nucleus),t=(−ı/3) is thediffusiontime, isthedelay time betweentheencodinganddecodinggradients,andDisthe self-diffusioncoefficienttobeextracted.Errorsontheself-diffusion coefficientswereestimatedaround 2%onthebasis ofrepeated measurements.Areasofthe11_B_NMR_resonances,_assuming_they

camera(Kratky-type)equippedwitha position-sensitive detec-tor (OED 50M) containing 1024 channels of width 54mm. Cu K␣radiation(␭=1.542Å)wasprovidedbyanultrabrilliantpoint micro-focusX-raysource(GENIX-Fox3D,Xenocs,Grenoble), oper-ating at a maximum power of 50W (50kV and 1mA). The sample-to-detectordistancewas281mm.Thespacebetweenthe sampleandthedetectorwaskeptundervacuumduringthe mea-surementstominimizescatteringfromtheair.Scatteringcurves wereobtainedintheq-rangebetween0.01and0.54Å−1.Gel sam-ples were placed into 1mm demountable cells having Kapton ﬁlmsaswindows.Thetemperaturewasmaintainedat25±0.1◦C byaPeltiercontroller. Theeffectofthebeamontothesamples duringthecollectionofSAXScurveswasevaluatedbycollecting somespectraatdifferentacquisitiontimesandnodifferenceswere observed,indicatingthatthesampleswerenotdamaged bythe beam.AllSAXScurvesweremodelledaccordingtoaLorentzian model(Eq.(3)).

I (q) = I0

1_{+ (qL)}2+bkg (3)

where I0 is thescatteringintensityatq=0,L isthecorrelation

lengthwhichcorrespondsinasemi-dilutedsolutiontothe aver-agedistancebetweenneighbouringentanglementpointsandbkg isaq-independentinstrumentalbackgroundterm.[28–30]

Fig.2.A)FrequencysweepmeasurementsonHPG/watersamplebefore(blackcurves)andafterboraxaddition(graycurves).TheratioHPGwt%/Boraxwt%wasﬁxedat4/1. Theoscillationamplitudewas5%.G’(䊏),G”(䊉).(B)Frequencysweepmeasurementsonsampleswithincreasingamountofglycerol,HPG3wt%andaratioHPGwt%/Borax wt%ﬁxedat4/1.Theoscillationamplitudewas5%.G’(䊏),G”(䊉).(C)Storagemodulusvaluesat1Hzplottedagainstglycerolconcentration.(D)Entanglementdensity(␳E)

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3. Resultsanddiscussion

3.1. Formulationandcharacterization

ThedynamicmechanicalpropertiesofHPG-basedsystemswere investigatedbymeansoffrequencysweeposcillationtests.They provideinformationaboutthedependenceofthestoragemodulus (G’)andofthelossmodulus(G”)onthefrequencyoftheapplied shearperturbation.Fig.2Ashowsthetrendofthestoragemodulus G’andthelossmodulusG”ataconstantstrainof5%asafunction oftheappliedshearperturbation.Thepolymerconcentrationin bothsampleswas3wt%whiletheratioHPGwt%/Boraxwt%was ﬁxedat4/1asreportedinliteraturefortheformulationofsimilar crosslinkedsystems[10,23].

ThemechanicalbehaviourofHPG/watersystemwas investi-gatedbeforeandaftertheadditionofboraxthatactsascrosslinker. Borate anions induce the formation of covalent intermolecular junctionsbetweenthepolymerchainsthroughadi-diol condensa-tionreaction.Thisresultsinamorerigidnetworkandinadecrease oftheviscouscharacterofthesamples:thesecovalentcrosslinks, together with theintramolecular and intermolecular hydrogen bondinginteractionsthatoccurbetweenthehydroxylgroupsof theHPGchains,leadtoanincreaseoftheelasticbehaviourandof systemsstiffness.Fromtherheologicalpointofview,theincrease ofboraxdeterminestheincreaseofthesolidlikebehaviourofthe system,withahugeincreaseintheStoragemodulusG’asclearly indicatedbythefrequencysweepcurvesreportedinFig.S2.

To modify the system’s stiffness, tuning the mechanical behaviourofthesample,glycerolwasintroducedasplasticizer, anditsroleintheviscoelasticpropertiesoftheformulationwas systematicallyinvestigatedbymeansofoscillatingrheologicaltest. Frequencysweeptestswerecarriedoutonsystemscontaining dif-ferent glycerolconcentrations, whilekeeping HPGat3wt%and boraxat0.75wt%.ThetrendoftheStorageandoftheLoss mod-uliG’andG”,ataconstantstrainof5%(inthelinearviscoelastic range),asafunctionofthefrequencyisreportedinFig.2B,where thechangeoftheelasticityofthesystemsinducedbytheaddition ofglycerolcanbeobserved.InFig.2C,theStoragemodulus(G’)at 1Hzisplottedasafunctionofglycerolconcentration.Thepresence oftwodifferentregimesisevident:atglycerolconcentrationslower than1wt%,aprogressiveincreaseofthestiffnessofthesystemis observedindicatingastructuringroleofthisadditive.Abovethis threshold,theplasticizingeffectofglycerolprevailsasindicated bythedecreaseofG’.Fig.2Dshowsthetrendoftheentanglement density␳Easafunctionoftheglycerolcontent.Theentanglement

densityisequaltokTG’1HzwherekistheBoltzmanconstant,Tis

theabsolutetemperatureandG’1Hzisthevalueoftheelastic

mod-ulusG’at1Hz[28].Theincreaseof␳Eatlowglycerolcontent(up

to1wt%)indicatesanincreaseofthecomplexityofthenetwork

conﬁrmingthestructuringroleofglycerolatlowconcentrations.A highercontentofthisplasticizerinducesaprogressivedecreaseof theentanglementdensity.

Systems containing differentglycerol amounts were deeper investigatedasafunctionoftemperaturebetween25◦Cand60◦C bymeansoftheTimeTemperatureSuperposition(TTS)approach, toobservethetemperature-dependenceofviscoelasticproperties oftheseﬂuidsand,accordingtoEq.(1),tocalculatethe Activa-tionEnergyassociatedtotherelaxationprocess.InFigs.S3–S7the isothermalfrequencysweepmeasurementsandthemastercurves for samplescontainingHPG(3wt%)/borax(0.75wt%)/water and increasingamountsofglycerolarereported.TheActivationEnergy trendisreportedinFig.3A:weobservedthattheEaprogressively decreasesasglycerolconcentrationincreasesabove1wt% conﬁrm-ingthatabovethisthresholdthedrivingfactoristheplasticizing roleofglycerolthatcauseadecreaseoftheEaoftheinvestigated systems.

Withtheaimofclarifyingtheroleofglycerolinthe crosslink-ingmechanism,anextensive11_B_NMR_study_was_carried_out_on

samplescontainingincreasingamountofglycerol.Thereare sev-eralpapersinliteratureabouttheinteractionbetweenboraxand hydroxyl groups[31andrefs therein]Severalofthem focuson crosslinkedsystemssuchasgel-likematerialsbasedonpoly(vinyl alcohol) (PVA)[18,32] and onborax-galactomannan based gels [12].

As shown in Fig. 4A, the 11_B _NMR _spectrum _of _the _HPG

polymer/boraxsysteminwaterischaracterizedbyoneintense res-onanceat11.60ppmandasmallresonanceemergingat5.55ppm, beingthedownfieldresonanceduetotheborate/boricacid molec-ularspecies, and theothertothe cross-linkedborate [33].The additionofglycerol(0.5wt%)causesstrongalterationsoftheNMR spectrum,namelyadownfieldshiftofthefreeboronspecies res-onanceat13.88ppm,andtheappearanceofanewNMRsignalat 9.77ppmthatcanbeassignedtotheformationofaglycerol-borate complex(seebelow).Adramaticincreaseofthecross-linkedborate specieswithrespecttothefreeborate/boricacid,asdeterminedby theareaoftheNMRresonancesignalsat13.88and5.55ppm(see TableS1),isalsoobserved.Withincreasingglycerolconcentration, wenoticeaprogressivedecreaseoftheamountofnon-complexed boronspecieswhoseNMR signalundergoesasignificant down-field shift, whereas no significant chemical shift changes are observedforthetwocomplexedBoratespecies[33].Conversely, theamountof glycerol-boratecomplex increases,whereas that ofHPG-crosslinkedincreasesupto2wt%ofglycerol.Forglycerol contenthigherthan2wt%onlytheamountofglycerol-borate com-plexincreasessuggestingaveryhighstabilityofthisnewcomplex. Remarkably,thesignificantdownfieldshiftofthenon-complexed boronspeciesindicatesachangeintheborate/boricacidratio,with anincreaseintheunchargedspecies[33].Suchaphenomenonwas

Fig.3. (A)ActivationEnergyvaluesobtainedfromrheologicalmeasurements()andGlycerol-BoratecomplexAreaobtainedfrom11_B_NMR_measurements₍₎_are_plotted

againstglycerolconcentration.Allsamplescontain3wt%ofHPGand0.75wt%ofborax.(B)Storagemodulusvaluesat1Hzobtainedfromrheologicalmeasurements(䊏)and HPG-CrosslinkedBorateAreaoftheresonancepeaksobtainedfrom11_B_NMR_measurements₍₎_are_plotted_against_glycerol_{concentration.}

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Fig.4.(A)11_B_NMR_stacked_plot_spectra_of_the_{polymer/water}_system_containing

3wt%ofHPG,0.75wt%ofboraxandincreasingamountsofglycerol(indicatedas wt%).Peaksattribution:non-complexedboron(1),glycerol-boratecomplex(2), HPG-crosslinkedborate(3).(B)1_H_NMR_stacked_plot_spectra_of_the_{glycerol/water}

systemcontainingincreasingamountsofborax(wt%).Thestarindicatesanew resonanceattributedtotheglycerol-boratecomplex.

previouslysuggestedtoberelatedtothelessattractive environ-menttoborateionsoriginatedbytheadditionoforganicliquids [18,34].

In other words, the addition of glycerol deeply affects the borate/boric acid equilibrium since the B(OH)4− anion is

pref-erentiallyinvolvedin theformationof boththeglycerol-borate complexandtheHPG-crosslinkedspecies.Clearlytheadditionof glycerol,eveninsmallamount,mayalsoalterthesolvent proper-tiessuchas,forinstance,thedielectricconstant.NMRdataallow ustodeeperunderstandthemeaningofrheologicaldata.From Fig.3Bweobservehowtheadditionofglycerolupto1%induced anincreaseofentanglementdensityresultinginanincreaseof sys-temelasticbehaviour,whileastrongdecreaseofG’occurredat higherconcentrations.Inthesamegraph,wecanseethatthearea ofthecrosslinkedborate,measuredby11_B_NMR,_follows_the_same

trend,indicatingthatafractionofboraxisnotanymoreactingasa crosslinkerinthepolymericnetwork.

Thisbehaviourmaybeascribedtoa possibledouble role of glycerol in the crosslinking mechanism. On one hand, at low concentrations,glycerolmolecules seem toact asa structuring agent,inducingthecrosslinkingofboraxandimprovingthe elas-ticbehaviourofthesystem.Ontheotherhand,atconcentration higher than 1wt%, glycerolseems tobehave mainly asa plas-ticizer,reducing thedensityof thecovalentboratebridges and thencausingadecreaseinthesamplestiffness.Thetrendoftan␦ (wheretan␦=G”/G’)at1Hz,reportedinFig.S8,conﬁrmsachange intheviscoelasticbehaviourfor HPG/H2O/boraxbased systems

uponincreasingtheamountofglycerol.Inparticular,above1%,an increaseofG”modulusisobserved,resultinginlessrigidsamples.

in11_B_NMR_measurements₍_Fig.₃_B)._The_two_phenomena,_which

contributetothevariationofG’,seemtoproceedinthesame direc-tionuptoaglycerolconcentrationbetween1%and2%,thenthe formationoftheglycerol-boratecomplexbecomesfavouredover theboraxcrosslinkingmechanism.

To conﬁrm the glycerol-borate complex formation, glycerol aqueoussolutionswereinvestigatedbeforeandafterborax addi-tionmeasuringtheself-diffusioncoefﬁcientDofglycerolby1_H

PGSTENMR.Intheanalysedsamplesglycerolwasmaintainedat afixedconcentrationindeuteratedwater,whiletheamountof boraxwasincreased.AsreportedinTableS2thestronginteraction betweenglycerolandborateinducesanevidentdecreaseinthe glycerolmotionsthatcanbealreadyobservedatlow glycerol-to-boratemolecularratio,andbecomesmoresignificantastheborate concentrationincreases.Interestingly,upontheadditionofborax anewdownfieldresonancestartedtogrow(seeFig.4B).Itisworth noticingthatboraxin(deuterated)waterdoesnotshowany1_H

NMRsignal.Therefore,thenewsignalwasattributedtotheglycerol boratecomplex,andimplicitlyaccountsforitshighstability,being theﬁngerprintofastablecomplexratherthanalooseassemblyin fastexchangebetweentheboundandfreeform.Inaddition,the formationoftheglycerolboratecomplexisconﬁrmedbythe pro-gressivebroadeningofthewidthathalfhighandtheconsequent coalescenceoftheNMRsignalsobservedinthe1_H_NMR_spectra_as

theboraxconcentrationisincreased(Fig.4B).Indeed,suchalossof resolutionisrelatedtothemorerapidspin–spinrelaxationrates, inturn,causedbya)thereducedmobilityexperiencedbythe glyc-erolmoleculesbecauseoftheformationofthecomplexwithborate ions,andparticularlybyb)theeffectofthequadrupolarrelaxation oftheboronnucleusonthenearbynuclei.

After the dynamical characterization of systems containing increasing amount of glycerol, we focused on the role of the polymerinourformulations.TounderstandtheeffectofHPG con-centrationonthemechanicalpropertiesoftheHPGbasedsystems, frequencysweepmeasurementswerecarriedoutonaseriesof samplesprepared withdifferentamountof HPG (Fig. 5), while keepingtheHPG/boraxratioat4/1andglycerolat3.5wt%.This valuewasselectedinviewofapotentialapplication[18].

Fig.5AindicatesthattheincreaseinHPGconcentrationdoesn’t signiﬁcantlyaltertheshapeoftheshearmodulialthoughitsvalue increasesuponincreasingtheHPGconcentration,asalsoindicated by thedecrease in thetan␦ values (see Fig. S9). Moreover,for HPGconcentrationslowerthat3wt%,twodifferentregimescanbe observed(Fig.5A):atω>ωc(whereωcisthecrossoverfrequency betweentheG’andG”curves),G’>G”,meaningthattheelastic behaviourprevails;atω<ωcwhereG’<G”,theviscouscharacteris

predominant.Fig.5AalsoshowsthatuponincreasingtheHPG con-centrationthecrossoverbetweenG’andG”curvesshiftstowards lowerfrequencyvalues,indicatinganincreaseofthemean relax-ationtimethatisequalto1/ωc.Inparticular,themeanrelaxation timemovesfrom1.6sforthesampleHPG1%to28.6sforHPG2%. Thesevaluesareusuallyrelatedtostickyreptationdynamics[28]. Furthermore,foraHPGcontenthigherthan3wt%,thecrossover betweenG’andG”movestonotinstrumentallyaccessible frequen-cies,indicatinganincreaseofthesolid-likecharacterofthesystem. MoreovertheincreaseofG’observedbyincreasingtheHPG con-centrationindicatesanincreaseinthecomplexityofthestructure oftheinvestigatedsystemsandanincreaseinthestrengthofthe 3Dnetworkduetotheincreaseofthespatialdensityofthe

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borate-Fig.5. (A)FrequencysweepmeasurementsonsampleswithHPG(from1to4%),glycerol3.5%andtheratioHPGwt%/Boraxwt%ﬁxedat4/1.Theoscillationamplitudewas 5%.G’(䊏),G”(䊉).(B)Storagemodulusvaluesat1HzasafunctionofHPGconcentration.(C)Entanglementdensity(␳E)valuesasafunctionofHPGconcentration.

modulatedcross-linking(e)betweentheHPGchains[35].Fig.5B

reportsthetrendoftheelasticmodulusG’at1Hzasafunction oftheHPGconcentration;thegraphshowsthatG’(1Hz)isnearly constantupto2wt%ofHPG,whileabovethisthreshold(indicated asC*),thedrasticincreaseinG’(1Hz),thatreachesavalueof600Pa at4wt%HPG,indicatestheformationofanextended3Dpolymer network.Thisisalsoconﬁrmedbythetrendoftheentanglement density␳EvaluethatstronglyincreasesforaHPGcontenthigher

than2wt%(Fig.5C).

Thereis a considerable literature onthedependence ofthe G modulus onpolymer concentration in solution [35 and refs therein].Inmostcases,thedependenceofGisdescribedbyapower law:G∝␾n₍₄₎_where_␾;_is_the_polymer_{concentration}_expressed

aswt%[36].Forrigidnetworkshavingaﬁbrillarymorphologythe exponentnis strictlyrelatedtothefractal dimensionDFofthe

objectconnectingatthejunction(5)[37–39].

G’∝n ₍₄₎

G’∝(3+DF)/(3−DF) ₍₅₎

InFig.S10thetrendoftheG’valuesat1Hzareplottedona log/logscaleasafunctionofthepolymerconcentration.Fromthe obtainedslope(n),afractaldimensionof1.6iscalculated,andthis suggests,accordinglytoJonesandMarquestheory[38],anetwork withobjectsconnectedbyrigidjunctions,forwhichanenthalpic elasticityoccurs[37].

TherheologicalpropertiesofaselectedsystemcontainingHPG 3%/borax0.75%/glycerol3.5%werealsoinvestigatedasafunctionof temperaturebetween25◦Cand60◦C,todeepenthe temperature-dependenceofviscoelasticpropertiesoftheseﬂuidsandtoobtain the activation energy (Ea) associated to the relaxation process throughtheTTSapproach(Eq.(1)).Thefrequencysweepcurves andthemastercurvesforsamplesinvestigatedarereportedinFigs.

S3,S6,S11andS12,whiletheactivationenergyvaluesforsamples withandwithoutglycerolandboraxarelistedinTableS3.

TheadditionofboraxtotheHPGdispersioninwater(sample 2andsample1,respectively)leadtoanincreaseoftheEavalue, confirmingtheabilityofboraxinincreasingthesystems’stiffness (seeTableS3).Theadditionofglyceroltosample2determineda significantdecreaseoftheEavalue(sample3).Itisworthnoting thatboththeformulationscontainingglycerol(samples3and4), arecharacterizedbyactivationenergyvaluesthataremuchlower thanthoseofthecorrespondingsystemsthatdon’tcontain glyc-erol(system2and1,respectively).Asamatteroffact,over2wt%, glycerolcreatesastablecomplexwithborate,resultinginlessrigid systems,asshownbyNMRmeasurements.NMRresultsarealso confirmedbythecomparisonbetweenEavaluesofsamples3and 4:theadditionofboraxtoasystemcontainingglyceroldoesnot increasesignificantlytheactivationenergyoftherelaxation pro-cess,differentlyfromwhathappenedforsamples2and1.Thisis duetothefactthatborateispartiallysequesteredbyglyceroland itisnotentirelyinvolvedinthecrosslinkingofHPG.

SAXScurveshavebeencollectedonHPGaqueousbinary solu-tions before and after the addition of the plasticizer and the crosslinkertobetterunderstandtherole ofthecomponentson thestructureofthestudiedsystems.InFig.6,SAXSproﬁlesand thebestﬁtting curvesaccordingtoEq. (3) arereported.Fitting resultsarereportedinTable1.Asreportedinliterature[30],in asemi-dilutedpolymericsolution,themeshsize(indicatedbythe ScreeningLength(L))correspondstotheaveragedistancebetween neighbouringcrosslinkingpointsofthe3Dpolymericnetwork.

Forwhatconcernsamples1and2,theadditionofboraxdoesnot signiﬁcantlychangethemeshsizeoftheHPGaqueousdispersion, eveniftheentanglementdensityincreasesofalmosttwoorderof magnitude.Thisindicatesthatintheabsenceofglycerol,the inter-actionsoccurringbetweentheHPGchainsarethedrivingfactor indeterminingthemeshsizeofthenetwork,whilethevalueof␳E

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Fig.6. ComparisonbetweenSAXScurvessamplescontainingHPG/water(1),HPG/borax/water(2),HPG/glycerol/borax/water(3)andHPG/glycerol/water(4).

Table1

LorentzparametersassociatedwiththebestﬁtstotheSAXScurvesandtheentanglementdensity␳EforsamplescontainingHPGandHPG/glycerolaqueousdispersion,with andwithoutboraxascrosslinker.

SAMPLE COMPOSITION ScalefactorI0 ScreeningLengthL(Å) Bkg E(1/m3₎

1 HPG3wt%inwater 0.19±0.03 14.5±0.4 0.03 4.1×1020

2 HPG3wt%/borax/water 0.31±0.04 15.2±0.3 0.04 95×1021

3 HPG3wt%/glycerol3.5wt%/borax/water 0.30±0.06 24.5±0.6 0.05 80×1021

4 HPG3wt%/glycerol3.5wt%inwater 0.34±0.07 28.9±0.7 0.04 1.6×1020

mainlydependsuponthepresenceofthecrosslinkingagent.Onthe

contrary,inthepresenceofglycerol(sample3and4),boraxleads

toasigniﬁcantdecreaseinthescreeninglengthofthepolymeric

network.Moreover,theadditionofglycerolleadstoahugeincrease

ofthemeshsize(comparesample1withsample4andsample2

withsample3),conﬁrmingtheabilityofthisadditivetointerpose

amongHPGchains,reducingtheentityoftheinteractionsbetween

theHPGmonomers(mainlyduetoHbonds),thereforeactingasa

plasticizer.

3.2. Applicationtest

Inthelastdecades,severaladvancedandinnovativesystemsfor

theconservationofculturalheritagehavebeenproposed[40–42].

Inparticularhydroandorganogelshavebeenproposedasafeasible alternativetoovercomethelimitsoftraditionalmethodsforthe selectiveremovalofpatinasfromsurfacesofartisticandhistorical interest[43–47].

The characterization of HPG/glycerol/borax gel-like systems showsthattheirpropertiescanbetuned, evendramatically,by varyingtheconcentrationofthecomponents.Fromthe applica-tivepointofviewthisisaveryattractivefeatureofthesesystems. Therefore,apreliminaryapplicativetestusingagel-like

formula-tionwithHPG(3wt%),borax(0.75wt%),andglycerol(3.5wt%)was carriedout.Thissystemwaschosenbecauseitcanbestretched withoutbreaking(seeFig.S13),whichisanappealingfeaturefor theapplication on curved surfaces. Moreover,the valueof the intrinsicelasticmodulusofthissystemallowsitspeelingfromthe cleanedsurfaceinonestep[18].Itisworthnotingthatthis formu-lationhasapHbetween8and9,whichiscompletelycompatible withmostofobjectsofhistoricalandartisticinterest.

AfterthefirethatalmostdestroyedLaFenicetheatre(Venice) in1996,artworksanddecorationsthatbelongedtothebeautiful historicalbuildingwerealteredbyadarkcoating,resultedfrom dustandparticulatemattergeneratedfromthefire.Thestuccoof LaFenicetheatreisarough,porousmaterial,ascanbeseenfrom Fig.7A.Thefirstattempttoremove thesurfacepatinawas car-riedoutbyfollowingatraditionalprocedurebasedontheuseof acottonswabsoakedwithdemineralizedwater. Inthatway,it waspossibletoobtainthesofteningofthedirtlayer,whichwas thenabsorbedbytheporousmatrix,resultinginanunsatisfactory cleaningofthedecoration(Fig.7B).Asecondcleaningtestwasthen carriedoutbymeansofagel-likeHPG-systemthat,accordingto theWashburnequation,duetoitshighviscosity,allowedthe min-imizationofthepenetrationofthewaterintotheporoussupport. TheHPG/glycerol/boraxsystemwasappliedonthestuccosurface

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Fig.7.Detailofthesampleregionbefore(A),during(B)andafter(C)theapplicationasdescribedinthetext.Comparisonbetweenthecleaningperformanceofawetcotton swab(1)andoftheHPGbasedsystem(2).

withaspatulaandleftonthesurfaceforabout1min(seeFig.7B); aftertheapplication,thesystemwasremovedinonestepthrougha simplepeelingaction.Thecomparisonbetweenthecleaningaction ofoursystemandthatofneatwaterappliedbymeansofawet cottonswabcanbeobservedinFig.7C.Thepromisingresultsof thispreliminarytestsuggestapossibleapplicationofthis formula-tionfortheselectivecleaningofthesurfacemakingthesegel-like systemsparticularlyinterestingintheﬁeldofrestoration.

4. Conclusions

Arheological,dynamic,andstructuralcharacterizationofwater basedgel-likesystemscontaininghydroxypropylguargum(HPG), borax,andglycerolisreported.Fromrheologicalmeasurements, we observed that the addition of borax to aqueous HPG dis-persionsleadtosystemswithincreasedstiffness,beingboraxa well-knownefﬁcientcrosslinkerforhydroxyl-richpolymers.With theaimof tuningthemechanical properties ofHPG/borax gel-likesystems, variableamounts of glycerolwere addedand the overall effect onthe systems was studied.Surprisingly, at low concentration(below1%),glycerolactsastructuringagent, increas-ing thestorage modulus.Above 1%,it behavesas a plasticizer, asitmightbeexpected.Wethendecidedtodeepentheroleof glycerol in network formation and its interactions with borax, using11_B_NMR_and1_H_NMR_PGSTE._In_particular,_from11_B_NMR,

theformationofaglycerol-boratecomplexwasobserved.When glycerolishigherthan1%,thecomplexformationactasa driv-ingforceandoccursattheexpensesofthecrosslinkingbetween boraxandpolymerchains,resultinginthedecreaseofthe Stor-agemodulus, as observed fromrheological measurements.The formationofastableglycerol-boratecomplexwasconﬁrmedby

1_H _PGSTE _NMR _{measurements,} _from _which _we _calculated _the

diffusionof glycerolin boraxaqueous solutions. Theactivation energiesoftherelaxationprocessobtainedbyTimeTemperature Superposition(TTS)allow usto calculatethe fractal dimension of theinvestigated systems that resultedto bethe onetypical ofanetworkwithobjectsconnectedbyrigidjunctions.A struc-turalcharacterizationcarriedoutbySAXSexperimentsprovided insight about the mesh size, which is inﬂuenced by both the presenceofboraxandglycerol.Apreliminaryapplicativetestfor theremovalofadarkpatinafromastuccodecorationbelonging toLaFenicetheatre (Venice)wasthenperformed.The promis-ingresultsobtainedin thesepreliminaryexperimentssuggesta possiblefuture applicationof theseformulationsin theﬁeld of restoration.

Conﬂictofinterest None.

Funding

This work was supportedby Centerfor colloid and surface science and by the European Union’s Horizon 2020 research and innovation programme under grant agreementNo 646063 [NanorestartProject].SergioMurgiaandMauraMonduzzithank Fondazione Banco di Sardegna and Regione Autonoma della Sardegna(ProgettiBiennalidiAteneoAnnualità2016,Fondazione Sardegna CUP F72F16003070002 and F72F16003060002). All authors greatefully thankthe friend Prof. Piero Baglioni for its friendshipanditsconstantfundamentalcontributiontotheﬁeld ofSoft Matterand thenanotechnologiesappliedtotheCultural HeritageConservation.

AppendixA. Supplementarydata

Supplementarydataassociatedwiththisarticlecanbefound, intheonlineversion,athttps://doi.org/10.1016/j.colsurfb.2018.02. 025.

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