Articlepubs.acs.org/JACSPolymerCompositionandSubstrateIn?uencesontheAdhesiveBondingofaBiomimetic,Cross-LinkingPolymer
CristinaR.Matos-Perez,?JamesD.White,?andJonathanJ.Wilker*,?,??DepartmentofChemistry,PurdueUniversity,560OvalDrive,WestLafayette,Indiana47907-2084,UnitedStates?SchoolofMaterialsEngineering,PurdueUniversity,NeilArmstrongHallofEngineering,701WestStadiumAvenue,WestLafayette,Indiana47907-2045,UnitedStates
ABSTRACT:Hierarchicalbiologicalmaterialssuchasbone,seashells,andmarinebioadhesivesareprovidinginspirationfortheassemblyofsyntheticmoleculesintocomplexstructures.Theadhesivesystemofmarinemusselshasbeenthefocusofmuchattentioninrecentyears.Severalcatechol-containingpolymersarebeingdevelopedtomimicthecross-p96linkingofproteinscontaining3,4-dihydroxyphenylalanine330(DOPA)usedbyshell?shforstickingtorocks.Manyofthesebiomimeticpolymersystemshavebeenshowntoform3asurfacecoatingsorhydrogels;however,bulkadhesionisdemonstratedlessoften.Developingadhesivesrequiresaddressingj/12designissuesincluding?ndingagoodbalancebetweencohesiveandadhesivebondinginteractions.Despitethegrowingnumber01.ofmussel-mimickingpolymers,therehasbeenlittlee?orttogeneratestructure?propertyrelationsandgaininsightsonwhat01 :chemicaltraitsgiverisetothebestglues.Inthisreport,weexaminethesimplestofthesebiomimeticpolymers,poly[(3,4-ioddihydroxystyrene)-co-styrene].Pendantcatecholgroups(i.e.,3,4-dihydroxystyrene)aredistributedthroughoutapolystyrene | 21backbone.Severalpolymerderivativeswereprepared,eachwithadi?erent3,4-dihyroxystyrenecontent.Bulkadhesiontesting02showedwheretheoptimalmiddlegroundofcohesiveandadhesivebondingresides.Adhesiveperformancewasbenchmarked ,92againstcommercialgluesaswellasthegenuinematerialproducedbylivemussels.Inthebestcase,bondingwassimilartothat yaobtainedwithcyanoacrylate“KrazyGlue”.Performancewasalsoexaminedusinglow-(e.g.,plastics)andhigh-energy(e.g.,M :metals,wood)surfaces.Theadhesivebondingofpoly[(3,4-dihydroxystyrene)-co-styrene]maybethestrongestofreported)beWmusselproteinmimics.Theseinsightsshouldhelpustodesignfuturebiomimeticsystems,therebybringinguscloserto( developmentofbonecements,dentalcomposites,andsurgicalglues.etaD noitac■INTRODUCTION
tohigh-energysurfacesviametalchelation,14?18individualilbuAdhesivesplayaprominentroleineverydaylife,beingusedinmetal?ligandbonds,16,19nonspeci?cadsorption,18P manyindustriesincludingaerospace,automobilemanufactur-?or23hydro-gen-bonding.18,20Oxidative21,22andenzymatic21cross-ing,housinglinkingmayalsobeinvolved.
1,2construction,woodproducts,packaging,andlabeling.WorldwiderevenuegeneratedbyadhesivestoppedIncorporatingDOPAandanalogousreactivegroupssuchas$40billionin2010.3Newrolesforspecialtyadhesiveswillbecatechol(i.e.,1,2-dihydroxybenzene)intopolymersisbeingfoundoncewecandevelopthematerialsindemandforpursuedforavarietyofapplications.This?eldisexpandingapplicationssuchassurgicaladhesives,orthopediccements,andrapidly,especiallydentalglues.Marinebiologycanprovideinspirationforthe24inthepast5years,withmanylaboratoriescontributing.Musseldesignofsuchmaterials.Thenaturaladhesivesystemofmarine25?27mimeticpolymersarebeinggeneratedfrompolypeptides,musselsisreceivinggrowinginterestinthecontextof52polyamides,28polyacrylates,17,29?35polyethyleneglycols,36?polystyrenes,53?59andpolyur-biomimetics.Theseshell?sha?xthemselvesethanes.60Thesepolymersareenablingthedevelopmentof4,5towetrocksbyassemblingacross-linkedmatrixofproteins.Essentialtotheimagingagents,48nanoparticlecross-linkingchemistryoftheseproteinsisthe3,4-dihydrox-31shells,44,48,61elastomers,30,33,59resins,58,62coacervates,hydrogels,36?38,42,43surfacetreat-yphenylalanine(DOPA)residue.4,5Severalproteinshavebeenments,27,40,49,52isolatedfrommusseladhesiveplaques,eachwithDOPA?47,50,51antibacterialcoverings,51,63andantifoulingcoatings.34,35,45Asubsethaveshowntheabilitytocomprisingbondtwosubstratestogether.25,26,29?33,36?42,53,54,604,5between3and30%ofthetotalaminoacidcontent.AmechanismwehaveWhereasacoatingrequiresonlyadhesivebondingtothe3+proposedfortheformationofmusseladhesiveinvolvesFetemplatingDOPAresiduessurfaceofinterest,bulkgluesalsoneedthepresenceofcohesivefollowedbyredoxchemistrytogenerateradicals.6?13Reactivityforces.Thesecohesiveinteractionsarerequiredtoformtheoftheseradicalsmaybringaboutprotein?proteincouplingformajorityofthematerialandreachbetweensubstratestoyielda
cohesivebondingwithinthebulkmaterialandprotein?substratelinkagesforsurfaceadhesivebonding.12,13Alter-Received:April8,2012natively,orperhapscomplementary,isdirectbindingofDOPA
Published:May14,2012
?2012AmericanChemicalSociety
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Downloaded by DONGHUA UNIV LIB on September 9, 2015 | http://pubs.acs.org JournaloftheAmericanChemicalSocietyArticlefunctionalglue.Toomuchcohesion,however,willresultinadihydroxystyrene)-co-styrene]foundlapshearbulkadhesionathardenedmaterialwithoutsigni?canta?nityforasurface.upto1.2±0.5MPa.53Over1MPa(～145poundspersquareLikewise,toomuchadhesivebondingwillcomeattheexpenseinch(psi))canbeconsideredintherealmofhigh-strengthofcohesion,andthebulkmaterialwillnotexist.Thisbalanceofbondingand,onceachieved,willenabledevelopmentofcohesionandadhesioncanbeelusive,withnowaytopredictapplicationsinseveral?elds.1,2Ofcourse,evenstrongerwhereanoptimalinterplaymayreside.
bondingisoftendesired.
DespitethegrowingnumberofsyntheticsystemsmimickingSeveralfactorsin?uencetheperformanceofanadhesive,aspectsofmusseladhesiveproteins,therehavebeenfewincludingthesubstratetype,surfacepreparation(e.g.,rough-detailedandsystematicstudiestoillustratewhichaspectsoftheness),cureconditions(e.g.,temperature,time,humidity),polymersgiverisetothegreatestbulkadhesion.Inparticular,solvent,concentration,andviscosity.2Beyondsuchformulationperformanceenhancementswillarisefromunderstandinghowissues,anappealingchemicalaspecttoexploreisthatofthepolymercompositiondictatesfunction.Inotherwords:polymercomposition.Byvaryingtheratioof3,4-dihydrox-Howmuchpendantcatecholshouldapolymercontaininorderystyrene:styrenewithinpoly[(3,4-dihydroxystyrene)-co-styr-toachievethestrongestbulkbonding?Toanswerthisquestion,ene],wecangainaccesstoafamilyofadhesivecopolymersweembarkeduponastructure?propertystudyinwhichthewithvarieddegreesofcross-linking.Thistypeofsystematicrelativecontributionsofcohesionandadhesioncouldbestudyhasnotbeencarriedoutindetailwithanyothermusselchangedsystematicallybyalteringthepolymercomposition.mimeticpolymersystem.BondingperformancedescribedTheresultinginsightswillshowwhereonemight?ndthebelowwasexaminedonanarrayoflow-tohigh-energyhighest-performingbiomimeticmaterial.
surfaces:poly(tetra?uoroethylene)(PTFE,commonnameforInane?orttogainstraightforwardchemicalinsightsandalsotheDuPontproductTe?on),poly(vinylchloride)(PVC),tokeepfuturescale-upinmind,ourmimicsofmusseladhesivepolishedaluminum,sandedsteel,andwood.Polymerp9proteinsarekeptassimpleaspossible.TheDOPAaminoacidcompositionturnsouttobeamajorfactordictatingbonding63canbestrippeddowntoonlyacatecholgrouppendantfromaperformance.Thisstudypresentsthesynthesis,character-303polymerizableole?n,hencethechoiceof3,4-dihydroxystyreneization,andbulkadhesionofseveralpolymers.Weareexcitedaj/1(Figure1).Tominimizestructuralandthermalperturbationstoreportthatthestrongestbondingofthesepolymersdisplays201tothehostpolymerresultingfromthismonomer,polystyreneadhesion“onparwiththatofcommercialproductssuch.01waschosentorepresentaproteinbackbone(Figure1).Styrene :ioiscommerciallyavailableandeasytopolymerizeonlargescales.d | 2Afurtheradvantageforthesestudiesisthatpolystyrene153alone0doesnotexhibitanyappreciablebondingcapability.The■
KrazyGlue”,albeitwithverydi?erentadhesionchemistry.
asEXPERIMENTALSECTION
Styrene2 ,9targetcopolymeristhuspoly[(3,4-dihydroxystyrene)-co-styr-puriand3,4-dimethoxystyrenemonomerswerepurchased2 yene],showninFigure1.
inhibitors.?edwithaMcommercialDetailsaluminaareprovidedcolumnsinourforearlierremovalofpolymerizationand :)wasbeW(GPC)usedtoanhydrouscollectNMRgrade.spectra.AVarianreport.53SolventswereGelInova-300MHzspectrometer( systemdataetawereandTHFwereobtainedeluent.PolystyreneusingaPolymerpermeationchromatographyGPCLaboratoriesstandardsPL-GPC20D n(DSC)usedoitcalorimeter.
dataforinstrumentwereobtainedcalibration.withaDiTA?erentialInstrumentsscanning(Varian,DSCQ2000calorimetryInc.)acilbupolymers.SynthesisInofatypicalPoly[(3,4-dimethoxystyrene)-polymerization,2.86mLco(24.9-styrene]mmol)Co-ofP styrenetoreactionaround-bottomand3.70mL(25.0?askmmol)with30of3,4-dimethoxystyrenemLofwereaddedbutyllithiumwascooledto?78°C,and,afteranhydrous10min,0.17toluene.mLofThestirredFigureallowedunderwasanargonaddedatmospheredropwise.Thefor8solutionhturnedorange,wasn-These1.proteinsMusselareadhesivemimickediscomprisedwithsyntheticofDOPA-containingpolymersbyproteins.placingPolymerizationtowarmtoroomtemperatureatover?7812°C,handofthenwaspendantFurtherpossiblecatecholgroupsalongapolymerchain.Oneofthesimplesttheadditionwasof～quenched100mLofbyadditionof～1mLofmethanol.reaction.polystyrenemimicsrepresentsispoly[(3,4-dihydroxystyrene)-theproteinbackboneandcoDOPA-styrene],isrepresentedinwhichleastpolymer.Afterisolationby?coldltering(?20and°C)dryingmethanolunderprecipitatedby3,4-dihydroxystyrene.
precipitationthreeroundsunreactedwithmethanolofdissolution(～inchloroform(～15vacuum,mL)andatCopolymerswerepreparedbyatwo-stepsyntheticroutestyrenemonomers.Yieldof100poly[(3,4-dimethoxystyrene)mL)wereusedtoremove33-co-developedinourlaboratorypreviously.53Wehavealsomadeppmcationicversionsofthesecross-linkingpolymers.54Polymer-peaks),(broad,67]waspolymer4.4g,33backbone),mmol,66%.1HNMR(CDCl3):δ0.6?2.3Synthesis6.0?7.43.4?3.8ppm(broad,methoxyofppmPoly[(3,4-dihydroxystyrene)-(broad,aromatic).
co-styrene].Treat-izationofstyreneand3,4-dimethoxystyreneyieldedpolymersmentforwhichtheratioofmonomersinthe?nalpolymerswaspolymerswithgenerallyare?ectionofthestartingfeed.53Thestyreneand3,4-wasaccordingBBr3andtoanouracidicpreviousworkupmethods.yielded53thecatechol-containingdihydroxystrenemonomersdistributethroughoutthecopoly-styreneaccomplishedmerstatisticallyorrandomly,therebyprovidingasuitablemethane](4.4g,by33dissolvingmmol)poly[(3,4-dimethoxystyrene)Atypicaldeprotection33%-co-67%modelforhowDOPAcooled53residuesarelocatedwithinmusseladhesiveproteins.Therelativelysimplesynthesisallowsdropwiseto(DCM)0°underanargonin50.0atmosphere.mLofanhydrousThereactiondichloro-wasaccesstolargequantitiesofpolymer,upto～20gperreactionandinanacademiclaboratory.Ourinitiale?ortwithpoly[(3,4-followedstirredoverC,overnight10and,min.after(～The10min,BBr3(1.2mL,13mmol)wasadded12solutionh).Thepolymerwaswarmedwastotreatedroomwithtemperature1%ene)byanaqueousworkupmmol,toobtain82%).poly[(3,4-dihydroxystyr-HCl33%-co-styrene67%](3.6g,27Lossofthe1HNMR
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feed(%)polymerobserved(%)3,4-dimethoxystyrenestyrene3,4-dimethoxystyrenestyreneMnMwPDITg(°C)0100010032300384001.210659559537500488001.3103991109039800500001.21001585158540700487001.2932278198140900545001.3975050267449600658001.3675050336757500842001.5625149425850575617001.26053
47
36
64
32700
43800
1.3
68
methoxy0.6peaksindicatedcompletedeprotection.1Hdimethoxystyrenecontentofeach?nalpolymerwassimilararomatic).
?2.3ppm(broad,polymerbackbone)and6.0?NMR7.4ppm(CDCl(broad,3):δtothatplacedinthefeed.Table1providesmolepercentdataAdhesionStudies.Substratesforlapsheartestingwerepreparedforeachmonomerinthefeedversusthatfoundintheisolatedbypolymers.Fortargetinglowcatecholpolymers(e.g.,<15%),thecmcuttingeachmaterialintorectangularpieces,8.89cmlong3,4-dimethoxystyrenemonomercontentinthe?nalpolymeradherendwide.Awasare?ectionofthatinthestartingfeed.Whentargeting60612.22centeredcmfromholeof0.64cmdiameterwasdrilledinto×each1.25highercatecholderivatives,themonomerratioinfeedneededp9followedT6,andmirrorpolishedoneend.withAluminumMibrono.was3and0.318Mibrocmthick,typetoincludealittlemore3,4-dimethoxystyrenethanthecatechol63deionizedbymolepercentdesiredforthe?nalpolymer(Table1).
303adherends,water,washing30minwitheach,hexanes,andair-driedethanol,acetone,no.and5polishthentoThelastfourentriesofTable1servetoillustratetheaj/1(0.318testingand0.318thencmwashedthick,weresandedwith50overnight.gritsandpaperThepriorsteelvariabilityobservedwhenrepeatingagivensynthesis.With50?201RidoutcmPlasticsthick)andPTFEwith(0.953ethanol,cmacetone,thick)wereandobtainedhexanes.PVCfrom53%ofthe3,4-dimethoxystyrenemonomerinthefeed,.01polymerswereobtainedwithbetween26and42%incorpo- :iotoRedration.Thisinconsistencymayberelatedtowatercontentofd | that1.27oakcmthick,was(SanpurchasedDiego,atCA).
alocalhardwarestoreand,aftercuttingtheliquidmonomer.The3,4-dimethoxystyrenewascolumn-210adhesionof220grithadsandpaper.asurfaceTheroughnesswoodapproximatelyadherendswereequivalentcutandtopuri?edpriortoeachpolymerizationreaction.Weobserved2 ,9alongthathigherincorporationoftenresultedwhenthetimebetween2 ysubstratesthestrength8.89cmwasedgemeasuredoftheadherend.parallelWatertothewoodgrain,runningpuri?cationandpolymerizationwasminimized.
aMseveralhoakmayadherendshaveoccurredbeforeversusduringtheadhesivelossfromcure.theseMassingwoodofGPCandDSCwerealsocarriedoutinordertofurther :)brevealedatroomtemperature,22hat55°afterC,andatypical1curetreatmentof1characterizetheisolatedcopolymers.Giventhecross-linkingeWandadhesivenatureofthesepolymers,GPCandDSCdata( et5544Lapshearanaverageadhesion4.12%measurementsdecrease(e.g.,fromh10.1atroomto9.68temperatureg).
weremosteasilyobtainedfromtheprotectedpoly[(3,4-aD Copolymermaterialsdimethoxystyrene)-co-styrene)]species.Thisapproachpre-noitaddedventedbothadhesionontothehighsurfaceareaGPCcolumnacilsolventtoeachsolutionstestingadherend.insystem1:1equippedwerewithconducteda2000onNanloadInstroncell.Next,acetone/DCM15μLof(0.3cross-linkingg/mL,22.5solutionμL)wereandcross-linkingduringthehigh-temperatureDSCexperiment.buPequivTheGPCdata,showninTable1,providedmolecularweight overlappedofwhencross-linkernotaddingperthecatecholcross-linker)group.wasTheaddedtodeliver0.33(ordistributionsforthecopolymers.Anionicpolymerization2).hatEach55°assemblyat1.25×1.25cminalapshearcon?adherendsguration(FigurewereC,andthenwas1allowedtocurefor1hatroomtemperature,22yieldedconsistentnumber-averagemolecularweightsintherangeof～32000?58000foreachderivative.Useofa1:35quantifyingFigure2showsratioofn-BuLiinitiator:monomersforallpolymerizationscrossheadadhesion.arepresentativehcoolingatroomtemperature.
Theextensionversusforceplotusedforhelpedkeepmolecularweightssimilar.Polydispersityindicesbeginsmovesuptobeginearlyloadingregionofthethesample.traceisOnce?atwhilethe(PDIs)allfellbetween1.2and1.5.AnionicpolymerizationwasbondusedheretoachievelowPDIvalues.RadicalpolymerizationsThebreakage.tobestressed,Adherendsarisewereisseenpulleduntilapartthesuddenataratedrop,theof2indicatingbondmm/min.mayalsobesuitableforthesynthesisofpoly[(3,4-forcemaximumbondingforceinNewtonswasrecorded.Finaladhesivedihydroxystyrene)-co-styrene].Whenpreparingcationicderiv-failure,inatives,nitroxide-mediatedradicalpolymerizationworkedwell.54meters.inmegapascalsNewtons,bywastheobtainedmeasuredbyareadividingofadhesivethemaximumloadatwasDSCindicatedthattheglasstransitiontemperatures(TbarstestedForatheminimumpolymerofcompositionstudiesinFigureoverlap3,eachinsamplesquareshiftedlowerwithincreasing3,4-dimethoxystyreneintheg)adhesivesshowing20times,averaged,andreportedwitherrorin±1standarddeviation.Thecomparisonstocommercialpolymerandlessstyrene.Table1showsTg=106°Cforaaveraged,100%polystyrene.Introductionof3,4-dimethoxystyreneTensileandTablesreported2andwith3wereerroreachbarstestedaminimumof10times,droppedtheTgvaluesgraduallytoward60°Cfor42%3,4-aluminumadhesiondimethoxystyrene/58%styrenecopolymer.Themethoxy■
rodsoftests1cmwerediameter.
carriedoutshowinginananalogous±1standardmannerdeviation.usinggroupsof3,4-dimethoxystyrenemaydisruptpolymerorder,RESULTSANDDISCUSSION
thusresulting64inthesedecreasedTEachpoly[(3,4-dimethoxystyrene)-gvaluesrelativeto100%polystyrene.co-styrene]PolymerSynthesisandCharacterization.Ninepoly-derivativeshowedaTmersofvariedcompositionwerepreparedinordertoexaminegbelowthatof100%polystyrene(T106°C)andabovethatof100%poly(3,4-dimethoxystyrene),g=thein?uenceofcatecholcross-linkingchemistryuponadhesionfoundearliertobeTstrength.Accordingto1HNMRspectroscopy,the3,4-g=53°C.53Foreverypolymeronlyonetransitiontemperaturewasobserved.Thesesinglethermal
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Downloaded by DONGHUA UNIV LIB on September 9, 2015 | http://pubs.acs.org JournaloftheAmericanChemicalSocietyArticleeventsindicateastatisticalorrandomcopolymerinwhichthethanFe3+3,4-dimethoxystyrenemonomersaredistributedthroughout53whencross-linkingpoly[(3,4-dihydroxystyrene)-co-styrene].Musselsdonothaveaccesstoreagentssuchasthehostpolystyrenechain.65Bycontrast,multiplethermal(IOeventswouldhaveindicatedsegregatedphasesorblocks.654)?.Nonetheless,thegoalhereistoachievestrongadhesion,hencethechoiceof(IOThermogravimetricanalysis(TGA,5°C/min)ofatypicalPeriodatehasbeendeprotected23usedtoinducecross-linking4)?forcross-linking.inDOPA-～containing25peptidesandsyntheticpolymerscontainingDOPA.Thetetrabutylammoniumsaltof(IObeing200°polymershowed7.5%masslossbetween～50andsubjectedC.WhenthetothesameconditionspolymerwasusedexaminedforadhesionbyTGAexperi-aftersolubilityinthesameorganicsolventsas4)?wasusedhereforthecatechol-ments(DCM/acetonesolvent,1hroomtemperature,22hatcontainingcopolymers.Whenthecopolymerswerecross-55°C,1hroomtemperature),8.2%masslosswasnoted.Thislinked,a3:1catechol:(IOresultindicatesthatagreatdealofadditionalsolventdoesnot4)?ratiowasemployed.ThisratiopreservesthatofDOPA:FeproposedtoexistinFe(DOPA)appeartopersistwithinthepolymeraftercuring.Generallycomplexes3speaking,thissyntheticapproachprovidescontroloverthe10,12,13,68,69withinmusseladhesiveplaquesandthreads.polymercomposition,molecularweight,anddistributionofBulkadhesionstrengthwasmeasuredforeachofthependantcatecholgroupsthroughoutthepolymerchain.
poly[(3,4-dihydroxystyrene)-co-styrene]variants.Figure3plotsBulkAdhesionStrengths.Cross-linkingcanoftenadhesionasafunctionofthemolepercentpendantcatecholenhancetheadhesivebondingofpolymers.Toomuchcross-monomerineachpolymer.At0êtechol,the100%linking,however,maybecounterproductiveandgenerateapolystyreneexhibitedverylittlebondingat0.6±0.3MPa.hardenedmaterialwithouttheabilitytobondsurfaceswell.Everycatechol-containingpolymershowedmoreadhesionthanThereisnoeasywaytopredictwherethis“sweetspot”of100%polystyrene,rangingfromonlyslightlymorewiththe5%optimalcross-linkingresides.Consequently,wepreparedthecatecholpolymerat0.8±0.3MPato～3MPaforthe33%p9familyofcopolymersshowninTable1,eachwithvariedcatecholandhighercopolymers.Ingeneral,increasingthe633pendantcatecholcontenttobringaboutdi?erentdegreesofcatecholcontentbroughtaboutincreasedadhesionuptothe03cross-linking.
pointof～33%.Furtheradditionofcatecholintothepolymersaj/1Generalinsightsonthebulkadhesivebondingofthesedidnotenhanceadhesion.Perhapscatecholintherangeof33 1polymersweregainedbymeasuringthelapshearadhesionofmaximizedadhesionwithoutreachingintotherangeoftoo.01mirrorpolishedaluminum(Figure2).Lapshearbondingisthemuchcross-linkingbeingadetrimenttofunction.
:iomostwidelyusedgeneralmethodforquantifyingadhesion.1,2d | 2Althoughstandarddeviationsmayappeartobelarge,other10adhesioncon?gurationssuchastensileorpeeltendtobe2 ,9worse.Aluminumisahigh-energysurfaceandacommon2 ysubstratefortheaviationandautomotiveindustries.2WeaM mirror-polishedthealuminumtomaketheadhesionmore:)bechallenging.
W( etaD noitacilbuP Figurecontent3.inAdhesionaseriesstrengthofpoly[(3,4-dihydroxystyrene)-asafunctionofthependantco-styrene]catecholcopolymers.linkedwithperiodatePerformance(inblue)ofthearepolymershown.Adhesivealone(inbondingred)andisincross-alapshearcon?gurationwithaluminumsubstrates.
Figuresubstrates2.Lapandpulledshearbonding.tofailure.PolymerMaximumgluesadhesionareplacedisindicatedbetweenbytwothepeakoftheextension(inmillimeters)versusforce(inNewtons)plot.
Reactionsofthecopolymerswith(IO4)?oftenbroughtaboutsigni?cantincreasesinadhesion.DatainFigure3showthat,uptoAdhesionofeachpolymerwasexaminedbothwithand?33êtechol,everypolymerbondedmorestronglywithouttheadditionofacross-linkingagent.Iron-induced,6?13with(IO4)relativetotheanalogouspolymeralone.With5êtechol,forexample,adhesionofthepolymeraloneat0.8±simpleoxidative,21,22andenzymatic22,23cross-linkinghavebeen0.3MPajumpedseveral-foldto3±1MPawith(IOproposedtobeusedbymarinemusselswhenproducingtheircatechol,however,adhesiondroppedin4)?.Beyond33adhesiveplaques.OurpriorstudieswithDOPA-containingconspicuousfashion.Adhesionof(IOproteinsshowedthatadhesioncouldbeincreasedwithFe3+.660.1MPa.
4)?with100%poly-styrenewasnegligible,at0.1±Additionofironmayhaveenhancedadhesionofpoly[(3,4-Thesedataindicatethatthe“sweetspot”foroptimaldihydroxystyrene)-co-styrene]53andaDOPA-containingpro-adhesionresidesatthecross-linkedpolymerof～33êtecholtein67somewhat,butthee?ectswereminimal.Periodate,and～67%styrene.Whenthecatecholcontentgoesover～33%(IO4)?,isastrongoxidantanddidleadtostrongeradhesion
and(IO4)?isadded,adhesionsu?ers.Mostlikely,toomuch
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Downloaded by DONGHUA UNIV LIB on September 9, 2015 | http://pubs.acs.org JournaloftheAmericanChemicalSocietyArticlecross-linkingisbiasingthesystemtowardextracohesionwithinpoly[(3,4-dihydroxystyrene)-co-styrene]bondsaluminummorethebulkmaterialattheexpenseofsurfaceadhesiveattachment.stronglythanwhiteglueandasstronglyasacyanoacrylateglue,Withlesscatecholthan～33%,notenoughcross-linkingisalthoughepoxyadheredthemost.Interestingly,eventhoughavailableandbulkadhesionhasnotbeenmaximized.
poly[(3,4-dihydroxystyrene)Inour?rstreportofpoly[(3,4-dihydroxystyrene)-co-styrene]33%-co-styrene67%]adherescompa-rablytocyanoacrylate,thechemistrydi?ersdramatically.copolymers,themaximumadhesiondeterminedfroma3.4%Cyanoacrylateadhesivesareappliedtosurfacesintheformcatecholcopolymerwith(IOofa?owingmonomerfollowedbypolymerization.Bycontrast,thatsigni?cant4)?was0.9MPa.53Datapresentedhereshowimprovementscanbemadetothebiomimeticadhesiveisdepositedontothesubstratealreadyadhesiveperformancefromasystematicstudyofpolymerpolymerizedandisthencross-linked.Whereascyanoacrylatecomposition.Relativeto100%polystyreneat0.6±0.3or0.1±doesnothaveanyspeci?cchemistryforbindingsurfaces,the0.1MPawith(IO4)?,incorporationofmusselmimeticcatecholgroupsofpoly[(3,4-dihydroxystyrene)-co-styrene]canchemistrybroughtaboutadhesiontothispolymerthat,bringaboutboththisneededsurfaceadhesionandcross-linkingotherwise,doesnotdisplayanystrongbondingproperties.withinthebulk.
ComparisonofPolymerCatecholContenttoDOPAinAdhesiononDi?erentSubstrates.OfthemyriadMusselAdhesiveProteins.SeveralDOPA-containingmusselparametersin?uencingadhesion,theverysubstrateontofootproteins(Mfp’s)havebeenisolatedfromtheadhesivewhichthematerialbondsmaybeoneofthemostsigni?cant.plaquesofthisshell?sh.TheDOPAcontentofeachproteinSubstratescanrangefromlow-energyplasticstohigh-energycanvarywithseveralfactors,themostprominentofwhichismetals.Thesurfacescanbesmoothorrough.Generallythetimeofyear.Thatsaid,ofalltheaminoacidsineachspeaking,roughenedsurfacesofhighenergytendtobetheprotein,DOPA71comprisesroughly10?15%ofMfp-1,702?4êsiestforstrongadhesion,allowingbothchemicalinteractionsofMfp-2,25%ofMfp-3,725%ofMfp-4,7330%ofMfp-5,74andmechanicalinterlockingbetweentheglueandthesurface.p9and3.5%ofMfp-6.74MusseladhesiveplaquesareconstructedSmoothplasticsare,classically,themostchallengingsubstrates633fromahierarchyofproteins.Contactingthesurfacedirectlyareforadhesion.Oncethestrongestadheringvariantofpoly[(3,4-03Mfp-3,72Mfp-5,74andMfp-6.74Aprotectivecoatingoverthedihydroxystyrene)-co-styrene]wasidenti?edforbondingaj/1wholeplaqueiscomprisedofMfp-1.75Thebulkadhesivepolishedaluminum(Figure3),performancewasassessedon201plaque,73abovethesurfaceandbelowthecoating,isMfp-271andothersubstrates.Inadditiontoaluminum,PTFE,PVC,sanded.01Mfp-4.Ourmoststronglyadheringbiomimeticcopolymersteel,andredoakadherendsweremachined.Thesesubstrates :iocontainstheequivalentof～33%DOPA.Thisvaluemostprovidearangeofsurfaceenergies,roughness,andindustriald | 2closelyresemblesMfp-3(～25%DOPA)andMfp-5(～30%applications.Pairsofeachsubstratewerejoinedtogetherusing10DOPA).Inmusselplaques,thesetwoproteinsmayonlybepoly[(3,4-dihydroxystyrene)2neededtoprovideadhesiveinteractionswiththesurface.The33%-co-styrene ,9aswellaswith67%],bothwithandwithout(IOotherproteinsareavailableforcohesion.ComparedtoDOPA4)?cross-linking,threecommercial2 yglues.
aMproteins,poly[(3,4-dihydroxystyrene)-co-styrene]isaseem-DataprovidedinTable3showhoweachadhesiveperformed :)beinglysimplersystem,butonethatmustbringaboutbothonthedi?erentsurfaces.AftertheexperimentsofTable2withWcohesionandadhesion.PriortoseeingthedatainFigure3,wealuminum,thesubstratewaschangedtosandedsteel.Here( etacouldnothavelookedattheproteinsequencesandpredictedpoly[(3,4-dihydroxystyrene)thepolymercompositiongivingrisetothestrongestbonding.33%-co-styrene67%]performedcom-D nparablytowhiteglue,butthecyanoacrylateandepoxywereoitComparisonstoCommercialGlues.At7±1MPa,thestrongest.Interestingly,onsandedsteelthebiomimeticacilmaximumadhesionofcross-linkedpoly[(3,4-dihydroxystyr-copolymerdisplayedsimilaradhesionstrengthbothaloneandbuPene)33%-co-styrene67%]onaluminumisquiteappreciable.Wewhencross-linkedwith(IO soughttobenchmarkthisperformanceagainstestablishedsteel4)?.Herewemaybeseeingane?ectfromFe3+ofthesurfaceintroducingcross-linkingcommercialadhesivesunderidenticalconditions.Thesubstrate,chemistrytothepolymersandenhancingbulkbonding.Forquantityofglue,andcureconditions(e.g.,timeandPVC,performanceofpoly[(3,4-dihydroxystyrene)temperature)wereheldconstant.Threeofthemostcommon33%-co-styr-eneclassesofadhesiveswerechosenforcomparison:apoly(vinyl67%]withperiodatewassostrongthatthesubstrate,itself,failedpriortotheadhesivejoint.Likewise,thecyanoacrylateacetate)whiteglue(PVA,Elmer’sGlue-All),anethylalsobrokePVCandoutperformedbothwhiteglueandepoxy.cyanoacrylate(KrazyGlue),andatwo-partepoxy.ResultsareWhenperiodatewasleftoutoftheformulation,poly[(3,4-showninTable2.Weareexcitedtoreportthatcross-linkeddihydroxystyrene)strongly33%-co-styrenethan67%]joinedPTFEasstronglyasepoxyandmorewhitegluebutnottothesameTable2.AdhesivePerformanceofPoly[(3,4-degreeascyanoacrylate.
dihydroxystyrene)33%-co-styrene67%]CopolymerComparedOakprovidedthestrongestbondingforpoly[(3,4-dihydrox-toCommercialGluesaystyrene)33%-co-styrene67%]whencross-linked,at10±1MPa.Bothwhiteglueandepoxywereweaker,althoughthecyanoacrylateappearedtobethestrongest.Theporousnatureadhesive
adhesion(MPa)
strength
ofwoodmayallowforpenetrationandmechanicalinterlocking,poly(vinyltherebyexplainingthehighadhesionforpoly[(3,4-dihydrox-All)
acetate)(PVA,whiteglue,Elmer’sGlue-4±1ystyrene)ethylcyanoacrylate(KrazyGlue)7±133%-co-styrene67%].Thisresultpromptedustomeasuretheadhesionofacommercialwoodglue(TitebondII,Franklinepoxy(Loctite,HenkelCorp.)11±2International)underidenticalconditions.Interestingly,poly-biomimeticpolymeralone
3±1[(3,4-dihydroxystyrene)biomimeticpolymerwith(IO33%-co-styrene67%]appearstohave4)?
7±1
performedsimilarlytothepurchasedwoodglueat9±2MPa.aBondingwascarriedoutonaluminumsubstratesinalapshearThesecommercialadhesiveshavebene?ttedfromdecadesofcon?guration.
industrialformulationstudiesinwhichparameterssuchcure
9502
dx.doi.org/10.1021/ja303369p|J.Am.Chem.Soc.2012,134,9498?9505
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