Toward a computational and experimental model of a poly-epoxy surface

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Toward a computational and experimental model of a

poly-epoxy surface

Thomas Duguet, Camille Bessaguet, Maëlenn Aufray, Jérôme Esvan, Cédric

Charvillat, Constantin Vahlas, Corinne Lacaze-Dufaure

To cite this version:

Thomas Duguet, Camille Bessaguet, Maëlenn Aufray, Jérôme Esvan, Cédric Charvillat, et al.. Toward

a computational and experimental model of a poly-epoxy surface. Applied Surface Science, Elsevier,

2015, vol. 324, pp. 605-611. �10.1016/j.apsusc.2014.10.096�. �hal-01105225�

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Identification number: DOI : 10.1016/j.apsusc.2014.10.096

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http://dx.doi.org/10.1016/j.apsusc.2014.10.096

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http://oatao.univ-toulouse.fr/

Eprints ID: 12279

To cite this version:

Duguet, Thomas and Bessaguet, Camille and Aufray, Maëlenn and Esvan,

Jérôme and Charvillat, Cédric and Vahlas, Constantin and Lacaze-Dufaure,

Corinne Toward a computational and experimental model of a poly-epoxy

surface. (2015) Applied Surface Science, vol. 324 . pp. 605-611. ISSN

01694332

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Toward

a

computational

and

experimental

model

of

a

poly-epoxy

surface

Thomas

Duguet

∗

_,

_Camille

_Bessaguet,

_Maëlenn

_Aufray,

_Jérôme

_Esvan,

_Cédric

_Charvillat,

Constantin

Vahlas,

Corinne

Lacaze-Dufaure

CIRIMAT,CNRS–UniversitédeToulouse,4,alléeEmileMonso,BP-44362,31030ToulouseCedex4,France

Keywords: Epoxy Amine Surfacescience AFM XPS DFT

a

b

s

t

r

a

c

t

Amodelpoly-epoxysurfaceformedbythereactionofDGEBAandEDAisstudiedbythecombination ofexperimentsandDFTcalculations.Aspecialsynthesisprotocolispresentedleadingtotheformation ofasurfacethatissmooth(Sa<1nm),chemicallyhomogeneous,andthatpresentsalow-defectdensity (0.21mm−2),asshownbyAFMcharacterizations.Then,XPSisusedforthedeterminationoftheelemental andfunctionalgroups’surfacecomposition.DFTallowstheidentificationandassignmentofindividual bondscontributionstotheexperimental1score-levelpeaks.Overall,wedemonstratethatsuchamodel sampleisperfectlysuitableforauseasatemplateforthestudyofpoly-epoxysurfacefunctionalization.

1. Introduction

Poly-epoxypolymersarewidelyimplementedinthreefamilies ofapplications:adhesives,paints,andcompositematerials[1].The latters,suchasepoxy/Cfiberscompositesareincreasinglyfoundin awealthofdevicesandpartsinthefieldsofleisure(skis,rackets, boats,golfclubs,etc.),ortransports,aeronauticsandspace(cars, aircrafts,satellites,etc.),tonamebutafew.Thesecomposite mate-rialspossessstiffnessandYoung’smodulusthatcomparewellwith metallicalloysbutwithamuchlowerchemicalreactivityand den-sity.Therefore,theyallowmassreductionandalargeincreaseof partsdurability.

Replacementofmetallic or ceramicparts bypolymersoften requiressurfacefunctionalizationinordertoacquireoptical, elec-trical,magnetic,biomedical,esthetic,orchemicalproperties.The maindrawbackwhenitcomestocoatortograftthesurfaceof polymer-basedcompositescomesfromtheverylowsurfaceenergy of such materialsoncepolymerized.This leadstoa poor wett-abilityrenderingpaintingorgluingdifficult,andresultinginpoor adhesion.Thesurfaceenergy ofpoly-ether etherketone (PEEK) orpoly-epoxyisapproximately40–50mJ/m2 _to_be_compared_to

∗ Correspondingauthor.Tel.:+330534323439.

E-mailaddress:thomas.duguet@ensiacet.fr(T.Duguet).

approximately500mJ/m2_for_aluminum._Moreover,_the_polar

com-ponent(duetoHbonding)isaslowas6–7mJ/m2_which_inhibits

theuseofsimplefunctionalizationprotocols[2–4].Hence,alarge number ofparticularprotocolshasbeendescribed orpatented, wheretheincreaseofreactivityandroughnessissought.A selec-tion amongst the wealth of publications can be foundin Refs.

[5–16].

Such protocols or methods that have been used until now remainempiricaldespitetheresultingimprovementofthetargeted properties and/or the extension of thedurability of the mate-rial. Therefore,the needexists toaccess thebasicmechanisms whichcontrolthesurfacefunctionalizationofpolymersandto con-trolthemsoastoachieve satisfactoryfunctionalpropertiesand adhesion.Bysubscribinginthis perspective,ourapproachaims atdescribingthenucleationandgrowthofmetallicthinfilmson polymersurfaces,by usinganintegrated methodwhereallthe elementarymechanismsaretakenintoaccount.Thefirststepin this frame–objectof thepresent study–is toobtaina model ofthepolymersurface,bothexperimentalandtheoretical,atthe atomic/molecular level. Such a model will serve as a template forfurthersurfacetreatments,includingpretreatments, molecu-largrafting,orapplicationoffilmsandcoatings.Itisworthnoting that,totheauthors’knowledge,nosuchatheoreticalsurfacemodel exists,mostlikelybecauseofstructuraldisorderandalackof exper-imentalinputs.

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Regardingourobjectives,specificationsofsuchanexperimental modelpolymersurfaceinclude:

•A100%polymerizationaftercuringtobecomparablewith calcu-lations,wheretotalpolymerizationisassumed.

•Alowsurfacearithmeticroughness,namelyRa<1nmtomake

surethatwecanobservenano-islandsornano-clustersofagiven thinfilm.Otherwise,theywouldbehinderedbyroughness. •Averylowdefectdensitytoavoidheterogeneousnucleationat

defects.

•Chemical homogeneityto make surethat calculationmodels wherehomogeneityisassumedarerepresentativeofthetracked chemicalreactivity.Alsotomakesurethatchemicalcomposition isindependentontheanalyzedsurfaceareacorrespondingtoa givenprobesize.

Ourexperimentalapproachisbasedonthemethoddescribed in [17,18] for forming model poly-epoxy surfaces. It consists in the polymerization of the poly-epoxy in an Ar gloves box at ambient temperature for at least 24h, followed by a post-curingatelevatedtemperature(polymer-dependent).Guetal.[17]

synthesizesamplesfromastoichiometricmixtureofDGEBA+ 1,3-di(aminomethyl)-cyclohexane,withasmallamountoftoluenefor decreasingviscosityandfavoringanhomogeneousstirring(7min). Samplesarethenstoredfor24hatambienttemperature,and post-curedfor2hat130◦_C_in_an_air_furnace._{Characterizations}_of_the

freesurfacesareperformedbyatomicforcemicroscopy(AFM)in Tapping®_mode._Surface_roughness_and_phase_contrast_are

deter-mined.ItisshownthatsamplessynthesizedinanArgloveboxshow alowersurfaceroughnessthanthosepreparedinambient condi-tions,andthattheyarehomogeneousincomposition.Kansowet al.[18]useasimilarmethodwiththeaimofcharacterizingthe for-mationofAl,Cu,Ag,andAufilmsbyphysicalvapourdeposition. DGEBAreactswithdiethylenetriamineinlowexcessat55◦_C_under

controlledatmosphere,beforeitisleftfor48hatambient temper-ature.Atthisstep,polymerizationrateisabout75%.Completionis achievedbypostcuringfor1hat120◦_C._Surface_roughness_is_about

1nm.

Theoretically, our greatest challenge is to circumvent the descriptionof thedisordered/amorphous structure andto limit thenumberofatoms.Tothatend,westartwithasmall macro-moleculemadefromthereactionofbisphenolAdiglycidylether (DGEBA)withethylenediamine(EDA) (61atoms).Even for this moderately complex system, the analysis of the experimental core-levelXPSspectrum isnot trivialand canleadtoincorrect conclusions.Thehelpofaccuratetheoreticaltoolsisthusneeded anddensity-functionaltheory(DFT)isusuallyusedforcomputing XPScore-levelshiftsinthecaseofsmallorganicorinorganic sys-tems.Theapplicationofthistheoreticalmethodtolargesystems, e.g.polymers,isachallengebutitisestablishedthatexperimental spectraaredirectlyrelatedtotheelectronicstatesobtainedfrom calculationsonsmallermodelmolecules.Forinstance,Endoetal. presentedacomprehensiveanalysisoftheXPSC1sspectrafor poly-mersusingthenegativeoftheenergyofmolecularorbitals[19,20]. Morerecently,theyusedthe‘transitionstate’theory[21]forthe calculationofthecoreelectronbindingenergies[22,23]. Follow-ingthisworkandinafirstapproach,wecomputethemolecular orbitalsenergiesonmodelmoleculesaspreliminaryinputforthe assignmentofexperimentalXPSspectraoftheinvestigated poly-mer.

WecomplementtheseresultsinthedifferentDGEBA+EDA sys-tembyimplementingamoredetaileddescriptionofsurfacesby AFMandXPScharacterizationscomplementedbyDFTcalculations. Thepaperisorganizedasfollows.Experimentalandcomputational

detailsaregiveninSection2,followedbyresultsinSection3. Con-clusionsandperspectivesarepresentedinSection4.

2. Experimentalandcomputationaldetails

2.1. Synthesis

We use a stoichiometric mixture of DGEBA (DER 332, Dow Chemicals, n=0.03) and EDA (analytical grade, purity>99.5%, SigmaAldrich).ThemassofDGEBA(mDGEBA)isfixedto5g.The

massofEDAmEDAisthusdeterminedfollowingEq.(1).

mDAE=fDGEBA

fDAE

×MDAE×mDGEBA MDGEBA

=0.43g (1) where MDGEBA is themolar mass(348.52g/mol) ofthis DGEBA

and fDGEBA is its functionality (2), and MEDA is themolar mass

(60.10g/mol)andfDAEisthefunctionality(4)oftheEDA.Weassume

thatnoetherificationoccurs.

Themixtureisthenmechanicallystirred(inanArglovebox whenspecified)for7minbeforeitispouredintodifferentmolds ordepositedasathindropletonaluminumfoil.Polymerizationis thenallowedfor48hatambienttemperature,followedbyapost curingof2hat140◦_C._For_roughness_comparison,_we_consider_the

followingpoly-epoxysurfacesformed:

-Atfreesurfaces,surfacesref.eitherepoxyAir_or_epoxyArgon_.

-Attheinterfacewitha1cm×1cm×0.2cmsiliconemold,itself moldedonaSiwaferfortransferringatomicflatness.Interfaces ref.SiOSi/epoxyAirorSiOSi/epoxyArgon.

-Attheinterfacewitha1cm×1cm×0.2cmsiliconemold,itself molded on polystyrene (PS). Interfaces ref. SiOPS/epoxyAir or

SiOPS/epoxyArgon.

-Bymechanicalpolishinguptoa¼mmwithdiamondpaste. Sur-facesref.polishedAir_.

Interfaces formedin the same molds but in air or Ar show different roughnesses (shown hereafter). This is the reason whySiOSi/epoxyAir andSiOSi/epoxyArgon,andSiOPS/epoxyAir and

SiOPS/epoxyArgonaredifferentiated.

2.2. Bulkcharacterizations

Differentialscanningcalorimetry(DSC)isusedforthe determi-nationoftheglasstransitiontemperature(Tg)ofthepoly-epoxy

underinvestigation.WeuseaDSC204PhoenixSeries(NETZSCH) coupledwithaTASC414/4controller.Theapparatusiscalibrated againstmeltingtemperaturesofIn,Hg,Sn,Bi,andZn,applyinga +10◦_/min_temperature_ramp._Samples_are_placed_in_aluminum

cap-sules.Massismeasuredwithanaccuracyof±0.1mg.Wechooseto reporttheonsetTg-onsettemperature.

Fourier transform infrared spectroscopy, FTIR (Frontier, PerkinElmerequippedwithaNIRTGSdetector),isperformedin transmissioninthe4000–8000cm−1_range.₁₆_scans_are_collected

for each analysis witha resolution of 4cm−1_._We _monitor_the

characteristic epoxy band (combination band of the –CH2 of

theepoxy group) at 4530cm−1 _with_increasing _{polymerization}

time,andafterpostcuringtreatment.Thereferencebandisthe combinationbandofC Cwitharomatic CHat4623cm−1 _[24]_.

Peakareasarethenusedforcalculatingtheconversionrate(XeNIR)

ofepoxygroups,followingEq.(2). XeNIR=1− Aepoxy/Areference

t=t Aepoxy/Areference

_t=0 (2)

whereAepoxy andAreference arethepeak areasoftheepoxyand

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Fig.1.Modeldimer(1DGEBA+1EDA).

2.3. Surfacecharacterizations

Surface roughness and viscoelastic homogeneity are deter-mined by AFM (Agilent Technologies model 5500) in ambient conditions.Theformerisperformedincontactmodewithtipsof spring constantkapprox. 0.292N/m,whereas thelatteris per-formedinTapping® _mode_with_tips_of_k₌_25–75_N/m_(AppNano).

Scanningrateis2mm/s.Imagesareprocessedwiththesoftwares Gwyddionversion2.19[25]andPicoImage(AgilentTechnologies). SurfaceroughnessparametersfollowtheGeometricProduct Spec-ificationsISO25178.Saisthearithmeticroughness,Sqistheroot

meansquareroughness,andSzisthetotalroughness(maximum

peak-to-valley),determinedbyprocessingtheAFMimages. XPSanalysisisperformedusingaThermoelectronKalpha appa-ratus.PhotoemissionspectraarerecordedusingAl-Karadiation (h=1486.6eV)fromamonochromatizedsource.TheX-rayspot diameter onthe sample surface is 400mm. The pass energy is fixedat30eVfor narrowscan and170eVforsurveyscans.The spectrometerenergycalibrationwasperformedusingtheAu4f7/2

(83.9±0.1eV)and Cu2p3/2 _(932.8_±_0.1_eV)_{photoelectron}_lines.

ThebackgroundsignalisremovedusingtheShirleymethod.Atomic concentrations are determined from photoelectron peak areas usingtheatomicsensitivityfactorsreportedbyScofield[26]and takingintoaccountthetransmissionfunctionoftheanalyzer.This functionwasdeterminedatdifferentpassenergiesfromAg3dand AgMNNpeakscollectedonasilverstandard.Finally,photoelectron peaksareanalyzedanddeconvolutedusingaLorentzian/Gaussian (L/G=30)peakfitting.

2.4. Calculations

WeusedthemodelmoleculeshowninFig.1thatresultsofthe additionofoneDGEBAandoneEDAmolecule.

The geometry of the model molecule was optimized at the B3LYP/6-31G* level of theory using the Gaussian 03 software package[27].

3. Results

Bulkcharacterizationsareperformedonsamplespolymerized underambientconditions.DSCisusedforthedeterminationof

Tg-onset.Temperaturerampsaredoubledforeachsampleinorder

toensurethatthereisnophysicalagingandtoverifythat polymer-izationiscomplete.ForallsamplesTg-onset=113±1◦_C._We_assume

thatTg-onsetisnotdifferentafterpolymerizationintheArglovebox

(nobulkcharacterizationforthesesamples).

Wethenmonitorthepolymerizationratewithreactionduration byfollowingthegradualdecreaseoftheepoxypeakareabyFTIR, andcalculatingtheconversionrateusingEq.(2).Resultsareshown inFig.2.

Experimentsareperformedfrom15minto11520min(8days) aftermixingofthereactants.Theconversionrateincreasesslowly in the first hours and reaches an asymptote between 24 and 48h.Themaximumconversionatambienttemperatureis84%for

t≥48h.Theonlymeanforachievingacompletepolymerization is to set the sample at a temperature above its glass transi-tion.Thepostcuringtreatment(140◦_C,₂_h)_leads_to_a_complete

Fig.2.Epoxygroupconversionrateasafunctionofpolymerizationoveran8-day periodoftime.Dashedlineindicatesthatpolymerizationiscompleteafterpost curingat140◦_C_for₂_h.

polymerization(>98%,takingintoaccounttheFTIRspectrometer sensitivity)illustratedbythedashedlineinFig.2.

Thedifferentsurfacesthatweconsiderarethencharacterizedby AFMover3mm×3mmsurfaceareaimagesinordertodetermine roughnessparameters.ResultsaresummarizedinFig.3.

Roughnessofthefreesurfacesisreduced bythreeordersof magnitudewhenpolymerizationisperformedintheArglovebox. Under Ar, Sa and Sq do not exceed 1.5nm, except for sample

SiO_Si/epoxyArgon_,_for_which_these_two_values_are_4.9_nm_and_6.8_nm,

respectively.ThelatterisnotacceptablefortheAFMobservation of metallicnanoislands or clusters that wetarget, in therange of1–20nmindiameter[18].Inordertotransferatomicflatness tothemolds,andthentotheSiOSi/epoxysurfaces,wemold

sili-conemoldsagainstSiwaferoragainstPS.Intheseconditions,the lowestroughnessisagainobtainedwhenthesurfacesareformed underAratmosphere,andissimilarbetweenSiandPSprocesses. Somehow,atmospherealsoplaysaroleregardingroughnessatthe substrate/polymerinterface.However,aroughnessaslowasatthat ofthefreeepoxyArgon_surface_is_not_achieved,_indicating_that

mold-ingintheseconditionsisnotwellsuitedforourpurpose.Finally,the roughnessparametersofthepolishedsurfacearequitelowbutAFM imagesshowmanyscratcheswherenucleationmaypreferentially occur.Sincewewanttoavoidheterogeneousnucleationinorder tocomparenucleationwithadsorptionenergiesatthemolecular level,polishingisabandoned.

Fig.3.Roughnessparametersdeterminedbyimageprocessingon3mm×3mm sur-facescharacterizedbyAFMincontactmode.Apolynomialofdegree2isusedin ordertocorrectimagecurvature.

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Fig.4.AFMimagesoftheepoxyAir_(a_and_b)_and_epoxyArgon_(c_and_d)_surfaces._Left_column_shows_topographic_images_after_a_polynomial_of_degree₂_correction,_and_right

columnshowsdeflectionimages(orphasecontrast).

Fig.4showsaselectionofAFMimagesoftheepoxyAir_(a_and

b)andepoxyArgon_(c_and_d)_surfaces,_obtained_in_Tapping®_mode.

Rightcolumn(Fig.4aandc)correspondstothesurfacetopography andleftcolumn(Fig.4bandd)tothedeflectionofthecantilever,i.e. tothephasecontrast.WhereasTapping®_mode_leads_to_different

apparentvaluesofroughnesscomparedtocontactmode, rough-nessisagainlowerontheepoxyArgon_surface,_as_can_be_noticed_on

thecontrastscale,ontheright-handsideoftheimages.However, bothsurfacesarequiteflatandexhibitaverylowphasecontrast. Themeasureofphasecontrastprobesthelocalviscoelastic prop-ertiesthatweassumetobeanindicationofchemicalhomogeneity inthenanometerrange.Finally,Fig.4canddischosenonpurpose inordertoillustratethepresenceofdefects,intheformofapprox. 50nm-in-diametertroughs.ThedensityshowninFig.4isnot rep-resentative(overestimated).Athoroughcountoveratotal90mm2 surfaceareagivesadefectdensityequalto0.21mm−2.

EpoxyArgon_is_selected_as_the_best_candidate_for_an

experimen-talmodelsurfaceofpoly-epoxy.Thus,weinvestigateitssurface chemicalcompositionbyXPSandusetheoutputofDFT calcula-tionsforpeakidentificationsandbindingenergyassignments.A firstobservationismadeonfreesurfacesofsamplessynthesizedin siliconemolds(i.e.thatwerenotincontactwiththemold).Survey spectrashowastrongSi2pcontributionat101.8±0.1eV,whichis characteristicofsiloxanegroups[28].Itrepresentsalargeamount ofadsorbedsiliconeonthesurface(approx.8at.%).Consequently, epoxyAr_samples_are_now_synthesized_on_Al_foil_(and_silicone_is

banishedfromtheglovebox).Thesignificantthicknessofthe poly-epoxycoupons(1mm)ensuresthatAldoesnotdiffuseuptothe freesurface,sincethemeasuredinterphasesdonotexceed300mm

[29].

TheXPSsurveyspectrumofepoxyArgon _surfaces_polymerized

onaluminumfoilshowneitherSinorotherelementsthantheone expectedinthepolymerorfromadsorbedmoleculesfromtheair. Atomiccompositionofthesurfaceisdeterminedby1speaksfitting, repeatedatdifferentx–ycoordinatesonthesamplesurface.We determinethefollowingsurfacecomposition:

81.5at.%C,1.8at.%N,and16.7at.%O

Theresult is slightly differentfromthebulk composition of the poly-epoxy, where the basic motif is made of 2 DGEBA (2×21C+2×4Oatoms)moleculesfor1EDA(2C+2Natoms) molecule,resultinginabulkcompositionof:81.5at.%C,3.7at.%N, and14.8at.%O.Whereasthecompositionofthesurfaceshowsa similarcarboncontent,itisricherinoxygenandpoorerin nitro-genthanthebulk.Thisisanindicationofamildsurfaceoxidation thatmayoccurinthecourseofpost-curing,whenpolymerization isnotyetcomplete(post-curingstartsat85%polymerizationrate). Itisquestioningthoughthatthecarboncontentisapparentlynot affectedaswell.

Inordertofurtherinvestigatethesurfacechemistryofthemodel poly-epoxysurface,molecularorbitalsextracteddirectlyfromDFT resultsarestudied.Table1showsthebindingenergiesof1s elec-tronsinvolvedinthedifferentbondsofthemodeldimer.Thedimer ismadeof1DGEBAand1EDAthatvirtuallybondedthrough1 epoxy/1amineprotonreaction.Therefore,thereareafew discrep-anciesbetweentheexperimentalfully-polymerizedsamplesand themodeldimer.Theyareenlightenedbythegraycoloringofthe linescorrespondingtosecondaryandprimaryamines(allshouldbe ternary)andtotheepoxygroup(nomoreepoxyringsinthe100% polymerizedsample).Thebindingenergiesshownarethe nega-tivevalueofthemolecularorbitalsenergies.Therefore,absolute valuesarenotcorrectbecause(i)XPSbindingenergiescorrespond toamulti-stepprocesswherephotoelectronsinteractwiththe cre-atedholes,withthematrixandwiththeirimagebeforeandafter extractionintovacuum,(ii)temperatureisnotconsidered,(iii)of thelimitationofKohn–Shamorbitalenergiesasreflectinginitial stateeffects[30].Nevertheless,chemicalshiftscanbeusedifone considerthelatterprocessesconstantinagivenenergydomain.

Aminimummeanchemicalshiftof0.2eVistechnically observ-ablewithourXPSapparatus.Therefore,wediscriminatephenyl groupsfrom CH3groups,andC OH&partoftheC O Cbonds

fromtheotherC O Cbonds.ThankstothesupportofDFTresults, weuse5contributionstotheC1speakdeconvolutionand2 con-tributionstotheO1speakdeconvolution.ThefinefittingoftheC 1sandO1sspectraareshowninFig.5.N1sspectrumisnotshown becauseitexhibitsonlyonecontributionforC Nbondscentered

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Table1

MolecularorbitalsinvolvingO,N,andC1satomicorbitalsfromDFTcalculationsonthemodelDGEBA–EDAdimer.Correspondingelectronicbindingenergies((−1)×orbital energy),andmeanchemicalshiftsforthegivenbond.Grayedcellsdonothaveacounterpartintheexperimentalfully-reticulatedpoly-epoxy.

Molecularorbital Bindingenergy(Hartree) Bindingenergy(eV) Meanchemicalshift(±0.1eV) Bond

O1s −19.177 521.8 +0.8 C O C

−19.170 521.6 C O C

−19.165 521.5 +0.6 Epoxy

−19.145 520.9 Ref. O H

N1s −14.324 389.8 +0.3 Secondaryamine

−14.316 389.5 Ref. Primaryamine

C1s −10.249 278.9 +2.0 C O C −10.249 278.9 C O C −10.246 278.8 C O C −10.244 278.7 C O C −10.239 278.6 +1.8 C O C −10.239 278.6 C O C −10.238 278.6 C OH −10.212 277.9 +1.0 C N −10.209 277.8 C N −10.207 277.7 C N −10.205 277.7 QuaternaryC C −10.186 277.2 +0.2 Phenyl −10.185 277.1 Phenyl −10.185 277.1 Phenyl −10.183 277.1 Phenyl −10.182 277.1 Phenyl −10.181 277.0 Phenyl −10.181 277.0 Phenyl −10.181 277.0 Phenyl −10.180 277.0 Phenyl −10.176 276.9 Phenyl −10.173 276.8 0.0 CH3 −10.173 276.8 Ref. CH3

Fig.5.XPSfinespectraofC1sandO1s.Spectraarefittedwithcontributionsderived fromDFTcalculationsonthemodeldimer.

at399.2eV.BindingenergyscaleoftheC1sspectrumstartswith the–CH3contributionfixedat284.4eV.Then,meanchemicalshifts

extractedfromtheenergydifferencebetweenmolecularorbitalsof DFT(seeTable1)areusedforhigher-binding–energycontributions (284.4+0.2,+1.0,+1.8,+2.0eV).

Thefilledareashowstheenvelopeofthefittingcurve.Thereis anexcellentmatchingwithbothOandC1sexperimental spec-tra. Again,calculationsensurethat contributions arereal; even C N,forinstance,whichisburiedinthetailsofneighboring con-tributions.Inordertoconsolidatetheseresults,wenowdiscuss fittingwithregardstothefunctionalgroupcompositionshownin

Table2.

Experimentalatomiccompositionsinfunctionalgroupsare con-sistent.Forinstance,whereoneO1sorbitaloftheC O Cbonds showsacompositionof15.5at.%,twoC1sorbitalsoftheC O C bondsshowanapproximatelydoubledcomposition of30.5at.% (28.0 plus the contribution of C O C at 286.2eV of about 3.7 (C 1sC O C, C OH)– 1.2 (O1sC OH)=2.5at.%). Similarly, N 1sand C 1scompare wellin termsof composition inthe C N bonds(1.8vs.1.3at.%).Finally,thelastcolumnofTable2shows theexpected compositionin functional groups ina poly-epoxy wheretheDGEBA:EDAratioequals2:1.Forinstancethenumber of C 1sin phenylgroups is calculated as follows:2 DGEBA×2 phenyls/DGEBA×6Catoms=24C1s.Overall,onecanfind4C1s in–CH3,24C1sinphenyls,2C1sinC N,4C1sinC OH,4C

1sinC O C286.2eV,4C1sinC O C286.4eV,2N1sinC N,4O

1sinC OH,and4O1sinC O C.Thereforethetotalnumberof considered1sorbitalsis52.Weobservelargediscrepancies con-cerningthephenylbondsconcentrationandtheoxygenatedbonds C OHandC O Cconcentrations,a differencethatwasalready mentionedwhenconsideringtheelementalatomiccomposition. Therearetwopossibilitiesforexplainingthesedifferences:either thesurfaceisoxidizedandoxygenatedbondscontributetotheC 1sandO1ssignalsatneighboringbindingenergies,orthepolymer isorientedinsuchawaythatC O Cbondsemergeatthesurface.

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Table2

ResultsoftheC,O,andN1sdeconvolutions;peakbindingenergy:BE,chemicalshiftimposedafterDFTresults;heightincountspersecond:CPS;fullwidthathalf-maximum: FWHM;peakarea;scofieldrelativesensitivityfactor:RSF;atomicfraction:at.%;andthecompositionexpectedfromthemodelpolymerwithaDGEBA:EDAratioof2:1.

Name PeakBE(eV) Chemicalshift(eV) Height(CPS) FWHM(eV) Area(CPSeV) ScofieldRSF At.% 2DGEBA:EDAmotif(at.%)

C1s CH3 284.4 0.0 2249.74 1.06 2578.62 1 6.5 7.7

C1sphenyl 284.6 0.2 11580.87 1.3 16279.01 1 40.8 46.2

C1sC N 285.4 1.0 328.14 1.41 500.77 1 1.3 3.8

C1sC O C,C OH 286.2 1.8 1009.93 1.36 1488.52 1 3.7 C OH:7.7+C O C:7.7 C1sC O C 286.4 2.0 7147.92 1.44 11172.06 1 28.0 7.7

C1sshakeup 291.2 n/a 349.91 1.32 501.28 1 1.3 n/a

N1sC N 399.2 n/a 808.92 1.28 1195.36 1.8 1.8 3.8

O1sC OH 532.0 0.0 669.53 1.73 1255.76 2.93 1.2 7.7 O1sC O C 532.9 0.9 9494.3 1.53 15728.75 2.93 15.5 7.7

Ifweassumethatamildoxidationoccurredinthecourseof sam-plepreparation,itmaybeassignedtosub-stoichiometricgroups, suchasamines(1.3–1.8vs.3.8at.%expected)andphenyls(40.8vs. 46.2at.%expected).Inthatcasedeconvolutionmaybeimproved bysubstitutingorimplementingadditionalcontributionsthatwe arenotabletoidentifynow.

4. Conclusions

Weselectedanepoxy-aminesystemwhichpermitsitsuseas bothanexperimentalandacomputationaltemplateforfurther sur-facetreatments.DGEBAandEDAmixedinstoichiometricratioand slowlypolymerized(48h)inanArgloveboxleadtotheformation ofapoly-epoxypolymerizedata rateof85%.Total polymeriza-tionisachievedbypost-curingat120◦_C_for₂_h._Such_a_poly-epoxy

exhibitsa glass transitiontemperature onset of 113±1◦_C.

Dif-ferent substrates and atmospheres were tested and compared intermsofsurfaceroughness.Thelowestroughness(arithmetic roughness=0.2nm,peak-to-valley=1.5nm)isobtainedatthefree surfacethatpolymerizedunderAratmosphere.AFMobservations revealthat,inadditiontothehighsmoothness,thedefectdensityof thesurfaceislowenoughtoavoiddefectdrivenundesirable nuclea-tion.Additionally,phasecontrastisalmostnullwhichindicatesthat thesurfaceischemicallyhomogeneous.Atomiccompositionsfrom XPSsurveyspectraatdifferentpositionsconfirmthisresult.Fine XPSspectraoverC,O,andN1scorelevelsareanalyzedinview oftheDFTcalculationsresults.Theoreticalbindingenergy chemi-calshiftsallowanexcellentfittingoftheexperimental1sspectra. Alimitationhasbeenemphasizedconcerningthecompositionsin chemicalgroups:themaindiscrepancyconcerningamuchlarger compositioninC O Cthantheonetheoreticallyexpectedfrom theperfectpolymermodel.Inanearfuture,wewilldedicateour effortstotheimprovementof(i)thepoly-epoxynetworkmodelby allowingalargernumberofatomsandbyusingmolecular dynam-icscomputationstofreezethestructureatgiventemperatures,and (ii)ofthecore-levelbindingenergiescalculationsusingthe gener-alizedtransitionstatemethod[21]thatallowsabettertreatmentof theXPSphotoemissionprocess.Finally,theperspectivesfor exper-imentalworkwillbetheformationofthinmetallicfilmsandthe mechanisticdescriptionofnucleationandgrowth.

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