XPS analysis of PE and EVA samples irradiated at different γ-doses

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XPS analysis of PE and EVA samples irradiated at

different γ-doses

Samuel Dorey, Fanny Gaston, Sylvain R.A. Marque, Benjamin Bortolotti,

Nathalie Dupuy

To cite this version:

Samuel Dorey, Fanny Gaston, Sylvain R.A. Marque, Benjamin Bortolotti, Nathalie Dupuy. XPS

analysis of PE and EVA samples irradiated at different γ-doses. Applied Surface Science, Elsevier,

2018, 427, pp.966 - 972. �10.1016/j.apsusc.2017.09.001�. �hal-01666619�

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ContentslistsavailableatScienceDirect

Applied

Surface

Science

j o u r n a l ho me p ag e :w w w . e l s e v i e r . c o m / l o c a t e / a p s u s c

Full

Length

Article

XPS

analysis

of

PE

and

EVA

samples

irradiated

at

different

␥-doses

Samuel

Dorey

a,∗

_,

_Fanny

_Gaston

a,b,c,∗∗

_,

_Sylvain

_R.A.

_Marque

c,d

_,

_Benjamin

_Bortolotti

e

_,

Nathalie

Dupuy

b

a_Sartorius_Stedim_FMT_S.A.S,_Z.I._Les_Paluds,_Avenue_de_Jouques_CS91051,₁₃₇₈₁_Aubagne_Cedex,_France b_Aix_Marseille_Univ,_CNRS,_IRD,_Avignon_Université,_IMBE_UMR_7263,13397,_Marseille,_France c_Aix_Marseille_Univ,_CNRS,_ICR,_Case_551,₁₃₃₉₇_Marseille_Cedex_20,_France

d_Vorozhtsov_Novosibirsk_Institute_of_organic_chemistry_Ofﬁce_312,₉_Prospect_Academican_Laurentiev,₆₃₀₀₉₀_Novosibirsk,_Russia e_{STMicroelectronics}_Rousset_SAS,_ZI_Rousset,_Avenue_Celestin_Coq,₁₃₁₀₆_Rousset_Cedex,_France

a

r

t

i

c

l

e

i

n

f

o

Articlehistory: Received21June2017

Receivedinrevisedform24August2017 Accepted1September2017

Availableonline21September2017

Keywords: ␥-irradiation Polyethylene Polyethylene-vinylacetate XPS

a

b

s

t

r

a

c

t

Theprincipalplasticmaterialsusedforthefluidcontactandstorageinthebiopharmaceuticalindustry aremainlymadeupofsemi-crystallinepolymers,polyolefins,PVC,SiloxaneandPET.Thepolyethylene (PE)andthepolypropylene(PP)areoftenusedasfluidcontactinmulti-layermaterialslikefilms.As onesterilisationwayofsingle-useplasticdevicesusedinmedicalandpharmaceuticalfieldscantake placevia␥-irradiation,theeffectofsterilizationonplasticsmustbeinvestigated.Theirradiationprocess leadstotheproductionofradicals,whichcangeneratechangesinthepolymerstructureandonthe polymersurface.Itiswellknownthatthepresenceofoxygenwithfreeradicalsprecedethegeneration ofperoxidespeciessocalledROS(reactiveoxygenspecies)whicharehighlyreactive.Thepurposeof thisworkistoinvestigatethe␥-raysimpactonthesurfaceofPE(polyethylene)andEVA (polyethy-lenevinylalcohol)basedfilmswhenionizedatdifferentdoses.X-rayPhotoelectronSpectroscopy(XPS) wasappliedtodeterminethesurfacecompositionsofthepolymerstohighlightthedifferentchemical moietiesgeneratedduringthe␥-irradiationprocessandtomonitorthepotentialpresenceoftheROS.

1. Introduction

Theprincipalplastic materialsusedfor thefluidcontact and storageinthebiopharmaceuticalindustryaremainlymadeupof semi-crystallinepolymers,polyolefins,PVC,SiloxaneandPET.The polyethylene(PE)andthepolypropylene(PP),whichbelongtothe familyofpolyolefins,entermainlyinthemanufactureofone-layer films,covers,stoppersandaremoreandmoreusedasfluid con-tactinmulti-layermaterialslikefilms. Asonesterilisationway ofsingle-useplasticdevicesusedinmedicalandpharmaceutical fieldscantakeplaceviaionizingradiation[1,2],theeffectof ster-ilizationonplastics(i.e.thepolymersandtheiradditives)mustbe investigated.Irradiationprocessingsuchas␥-irradiationofplastic single-usemedicaldevicesmayaffectchemicalandphysical prop-ertiesoftheplasticmaterials[3,4].Theirradiationprocessleadsto theproductionofradicals,whichcangeneratechangesinthe poly-merstructureandonthepolymersurface[5].Itiswellknownthat

∗ Correspondingauthor.

∗∗ Correspondingauthorat:SartoriusStedimFMTS.A.S,Z.I.LesPaluds,Avenuede JouquesCS91051,13781AubagneCedex,France

E-mailaddresses:samuel.dorey@sartorius.com(S.Dorey),

fanny.gaston@sartorius.com(F.Gaston).

thepresenceofoxygenwithfreeradicalsprecedethegeneration ofperoxidespeciessocalledROS(reactiveoxygenspecies)which arehighlyreactive.Multilayerfilmscouldbethepredominant ele-mentstoconstituteflexibleplasticbagswhichcancontainproteins orotheractivepharmaceuticalingredients(API).Themodification ofthefilmsurfacepropertiesandsomeby-productsthereof,can thusleadtothemodificationofsensibleproteinsproneto oxida-tion[6,7]duetothepresenceofROS.Surprisinglythepresence ofperoxideisalwaysreportedbyindirectmethodseitherusing enzyme,orcolorimetry,dies,etc.[8–16].

In parallel, high-energy irradiation leads to a succession of chemicalreactions,whichultimatelyleadtoeitheranincreaseor adecreaseinthemolecularweightofapolymer.Theinitial pro-cesseswhichoccurwhenahigh-energyphotoninteractswithan organicpolymerarereasonablywellestablishedanddonotdepend essentiallyonthechemical structure of thematerial.However, theseprimaryprocessesleadtoacascadeoffurtherreactions,the natureofwhichdependssensitivelyonthenatureofthepolymeric material.

Intheevaluationofradiationresistanceofplastics,changesin thephysico-mechanical,thermal, optical,physico-chemical, and otherpropertiesneedtobeinvestigated[17].Itshouldbenoted thatradiationdoesnotaffectthepropertiesofallpolymersinthe sameway,ortothesamelevel,andwhenselectingapolymerfor

http://dx.doi.org/10.1016/j.apsusc.2017.09.001

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aparticularapplicationtheeffectofirradiationshouldbe consid-ered.Thepurposeofthisworkistoinvestigatethe␥-raysimpacton thesurfaceofseverallotsofcommercialPE(polyethylene)andEVA (polyethylenevinylacetate)basedﬁlmswhenionizedatdifferent doses.ContactanglemeasurementsandXPS(X-rayphotoelectron spectroscopy)methodscanbeusedtoanalysesurfacepropertiesof PEandEVA[18–20].X-rayphotoelectronspectroscopy(XPS)was appliedaswelltodeterminethesurfacecompositionsoftheEVA copolymers[21–23].Inthiscurrentstudy,ﬁlmsurfaceanalysisare performedbyXPStohighlightthedifferentchemicalmoieties gen-eratedduringthe␥-irradiationprocessandtomonitorthepotential presenceoftheROS.

2. Materialsandmethods 2.1. Filmsamples

Thetwomultilayerfilmsstudiedinthisworkare:PEfilmand EVAfilm.ThePEfilmhasthefollowingstructure:PE/EVOH/PE,with athicknessofabout400␮m.TheEVAfilmsampleiscomposedof ethylenevinylacetate(EVA)andethylenevinylalcohol(EVOH): EVA/EVOH/EVA,withathicknessofabout360␮m.Twobatchesof EVAfilmandthreebatchesofPEfilmareinvestigated.Forthesake ofclarifty,theresultsofthethirdlotofPEfilmarenotdescribedas itdisplaysverysimilarresults.

2.2. -Irradiation

AllfilmsamplesofPEand EVA filmshave beenpraparedin specificpackaging(PE)tobeirradiatedatroomtemperaturein a60_Co_␥-source_providing_a_dose_rate_of_8–13_kGy/h,_as_given_by

Synergy Healthcompany (Marseille, France). The sampleshave been ␥-irradiated at doses of 30 (±1), 50 (±1), 115 (±2) and 270(±5)kGy.Asterilizationcyclecorrespondsapproximatelyto 25kGy. Desired doseis obtainedbyseveral sterilizationcycles, includinganon-controlledwaitingtimeinnon-controlledstorage conditionsbetweeneachcycle.Theimpactofthe␥-irradiationis assessedfrommodiﬁcationsocurringbetweenirradiatedsamples andnon-sterilizedamples,whichcorrespondto0kGysamplesin thedocument.

2.3. XPS

TheXPSspectrawerecarriedoutwithaKratosAxisNova spec-trometerusingamonochromaticAlK(alpha)source(15mA,15kV). XPScandetectallelementsexcepthydrogenandhelium,probes thesurfaceofthesampletoadepthof5–10nanometres,andhas detectionlimitsrangingfrom0.1to0.5atomicpercentdepending ontheelement.

Theinstrumentworkfunctionwascalibratedtogiveabinding energy(BE)of83.96eVfortheAu4f7_/

2lineformetallicgoldand

thespectrometerdispersionwasadjustedtogiveaBEof932.62eV fortheCu2p3_/

2 lineofmetalliccopper.TheKratoscharge

neu-Fig.1.XPS spectraofEVAﬁlmatdifferentabsorbeddoses.OnlyXPSspectra recordedonbatch1arepresentedastheyareidenticaltobatch2.

tralizersystemwasusedoneachsamples.Theeffectivenessofthe chargeneutralizationwastunedbymonitoringtheFullWidthat HalfMaximumoftheC1speakofadventitiouscarbon.The quan-tiﬁcationsarecalculatedfromrelativesensibilityfactors(RSF)from eachelement,givenbythemanufacturer.Measurementsare per-formedwithaprecisionof5%forthemajorelementsand10%for theminorelements.

Surveyscananalyseswerecarriedoutwithananalysisareaof 300×700␮m,apassenergyof160eVandadwelltimeof100ms. Highresolutionanalyseswerecarriedoutwithananalysisareaof 300×700␮m,apassenergyof10eVandadwelltimeof500ms. Spectrahavebeenchargecorrectedtothemainlineofthecarbon 1sspectrum(adventitiouscarbon)ﬁxedat284.8eV.

SpectrawereanalysedusingCasaXPSsoftware(version2.3.16). 3. Resultsanddiscussion

3.1. EVAﬁlm

Fig.1showsthesurfaceXPSspectraforpristineEVAfilmand␥ ray-irradiatedfilmsatdifferentirradiationdoses.Allspectrapoint outthepresenceofoxygenandcarbonatomsandnoother ele-menthasbeenfoundabovethedetectionlimit.Accordingtothe literature[24,25],thepeakatapproximately285eVisassignedto C1sphotoelectronslikelycorrespondingtoC C,C O,andO C O bondsonthesurfaceofsample.ThepresenceofO1sat532eVin thepristinesampleisdirectlyattributedtothepresenceoftheester moietyintheEVApolymer.Afterirradiation,itisclearlyobserved intheXPSspectrathattheO1speakintensitydecreasewith ␥-irradiationdosesupto50kGyandincreaseafterwards.Thepeak O1sassignedtothepresenceofC OorC O24,pointsoutsurface changesasoxidationduringirradiationunderaircondition.To esti-matechangeinoxygencontentinthesurfaceoftheEVAfilmupon

Table1

PercentageofcarbonandoxygeninEVAﬁlmafterdifferentirradiationdoses.

Irradiationdose(kGy) Relativecomposition Relativecomposition

batch1 batch2

C1s(%Atomic Concentration)

O1s(%Atomic Concentration)

RatioO/C C1s(%Atomic

Concentration) O1s(%Atomic Concentration) RatioO/C 0 92.9 7.1 7.6% 94.1 5.9 6.3% 30 94.3 5.7 6.0% 94.2 5.8 6.1% 50 94.1 6.0 6.4% 94.3 5.7 6.0% 115 94.0 6.0 6.4% 93.6 6.4 6.8% 270 93.1 6.9 7.4% 92.9 7.1 7.6%

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Table2

AssignmentofthepeaksdetectedinC1sXPSspectra26–32andVariationofthedifferentchemicalmoietyrelativeconcentrationaccordingtothe␥-irradiationdoses.Carbon markedwithanasterismisthetargetonewhenseveralcarbonsarewritten.

AssignmentofthepeaksdetectedinC1sXPSspectra

Bindingenergy 285.2eV 286.8eV 287.8eV 289.2eV 290.2eV

Assignment CC,CH R O C* O C O R C* O(COCH3) COOH

Chemicalformsrelativeconcentration(%)

Batchnumber 1 2 1 2 1 2 1 2 1 2 ␥-irradiationdoses (kGy) 0 92.8 92.7 3.3 3.4 0.4 0.4 3.3 3.4 0.2 0.1 30 92.9 92.7 3.1 3.3 0.5 0.6 3.0 3.3 0.5 0.2 50 92.8 92.8 2.9 3.1 0.8 0.6 2.8 3.1 0.7 0.4 115 93 92.7 2.9 2.8 0.7 1.0 2.6 2.8 0.8 0.7 270 92 92.5 3.2 2.7 0.9 1.3 2.6 2.7 1.2 0.9

Fig.2.a)C1sXPSspectraofEVAﬁlmirradiatedatdifferent␥-doses,b)exampleof decompositionoftheC1sspectrumrecordedonsampleirradiatedat270kGy.

irradiation,thelevelofoxidationwasfurtherquantitatively calcu-latedandisshowninTable1.Withanincreaseofabsorbeddoses, thelevelofsurfaceoxidationchangesgradually(Table1,Table2

andFigs.2and3).

Fig.2ashowstheC1sXPSspectraofEVAﬁlmsirradiatedwith different␥-doses.Accordingtotheliterature[20,22,24,26–32],one majorpeakat285eVisattributableto CH2 groupsinexpected

environments(Table2).Twominorpeaksat289eVand286.6eV areassignedtotheester COORspecies.Thispeakisexpectedinthe freshﬁlmbecauseoftheacetatemoietyinthepristinepolymer.On

closerscrutiny,asmallpeakonthehighbindingenergysideofthe CHpeakisapparent.Uponcurveﬁtting(Fig.2b),thissmallpeakis situatedat287.8eVandisascribedtocarbonylC Ospecies[26,27]. Fortheirradiatedsurfaces,achemicalshiftintheC1sspectrahas occurred,whichindicatesthatoxygencontainingcompoundsare produced.CarboxylgroupsareformedontheEVAsurfacesthrough thebondscissionbytheirradiationofO2 monomerionswitha

ﬁngerprintat290eVaffordingabroadeningofthepeakat289eV aswell.The290eVpeakintensitydueto COOgroupsproduced bytheEVAdegradationgrowsinintensityduring␥-rayexposure (Table2).

Allmoiety creationsand modiﬁcations observedin C1s XPS spectraarecorroboratedlikewisewiththeO1sXPSspectra(Fig.3a). Curveﬁttingallowshighlightingthereofallthedifferent chemi-calmoietiesduringC1sXPSspectradecomposition(Fig.2b).An increaseofthe␥-irradiationdosecausesanincreaseofgeneration ofketones,aldehydesand acidsdirectlyrelated tothe degrada-tionoftheestergroupsfromtheEVApolymer.Thisobservation agreeswithotherresultsobtainedusingATR-FTIRtechnics[33]. Unfortunately,hydro(gen)peroxideshavenotbeendetected.

TherelativecontentsofCCandCHarestableupto270kGy, informing us that the polymer structure is not deteriorated. Acetategroupdecreasesandacidgroupsincreasewiththe␥-doses whateverthelots. TheC O andCOOHrelative contents evolve simultaneously.One may presume that the acetate moieties is transformedintocarboxylmoieties.Noperoxidehasbeendetected, whereasperoxidemustbetherootcauseofthegenerationof car-boxylmoieties.

3.2. PEﬁlm

RadiationeffectonthesurfaceofPEfilmfromthreebatcheswas studiedbyXPSspectra;resultsononebatcharepresentedand dis-cussedastheyarestrictlyidenticalforotherbatches.Fig.4shows thesurfaceXPSspectraforpristinefilmand␥ray-irradiatedfilmsat differentirradiationdoses.Allspectraindicatethepresencecarbon atomsandsomeofthemexhibitthepresenceofoxygenatoms.The peakatapproximately285eVisassignedtoC1sphotoelectronsand thepeakat532eVisassignedtoO1sphotoelectrons[22,24,26–30]. TheabsenceofO1sat532eVinthepristinesampledirectly indi-catesthattherenooxidationoccurredduringthermalprocessing ofthePEfilm.

Therelativeelementalcompositionbetweencarbonandoxygen isgiveninTable3.

TherelativecontentsofCarestableupto115kGyanddecrease slightlyat270kGy,informingusthatthepolymerstructureisnot deteriorated.

Fig.5ashowstheC1sXPSspectraofPEﬁlmsirradiatedwith dif-ferent␥-doses.Onesinglepeakat284.8eVisascribedto (CH2)n

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Fig.3.a)O1sXPSspectraofEVAﬁlmirradiatedatdifferent␥-doses,b)exampleofdecompositionoftheO1sspectrumrecordedonthenon-irradiated(NS)sample,c) exampleofdecompositionoftheO1sspectrumrecordedontheirradiatedsampleat115kGy.DirectdecompositionofoxygenO1sspectracannotbedoneclearly.

Table3

PercentageofcarbonandoxygeninPEﬁlmafterdifferentirradiationdoses.

␥-irradiationdose(kGy) Relativecomposition

Batch1 Batch2

C1s(%AtomicConcentration) O1s(%AtomicConcentration) C1s(%AtomicConcentration) O1s(%AtomicConcentration)

0 100 0 100 0

30 100 0 99.9 0.1

50 99.9 0.1 99.8 0.2

115 99.5 0.5 99.4 0.6

270 98.1 1.9 98.4 1.6

spectrahasoccurred,whichindicatesthatoxygencontaining com-poundsareproduced[37].Oncloserscrutiny,asmallshoulderon thehighbindingenergysideoftheCHpeakisapparent.Uponcurve ﬁtting(Fig.5b),thissmallpeakisat∼285.2eVandmaybeascribed toC O(H)species[38],ortosp2 _carbons_[39]_._In_another_paper,

decompositionoftheC1sspectraofLDPEsamplesleadstoCsp2_at

∼284.9eV,andtoCsp3at285.2eV[40].

DuetotheoverlappingofsomeO1scomponents,allmoiety generationsandmodiﬁcationsobservedinC1sXPSspectradonot directlyﬁtwiththeO1sXPSspectra(Fig.6).Carboxylgroupsare formedonthePEsurfacesthroughthebondscissionbythe irra-diationofO2monomerions.Anincreaseofthe␥-irradiationdose

causesanincreaseofoxygenatedcompoundswhiledistinguishing

betweenketones,aldehydesandacidsisnotdirectlypossibleby curveﬁttingduetotheweakpeakintensities.Thisindicatesthe oxidationoccursinaweakproportioninaPEﬁlmsupporting pre-viouslyreportedresults[41].Besidesnohydro(gen)peroxidehas beendetected.

3.2.1. Formationmechanismsofoxygenatedspecies

Severalmechanismsareconsideredtoaccountfortheformation ofoxygenatedspecies.SomeofmechanismsproposedinScheme1

arereportedintheliterature[42–44].Carboxylicacidisgenerated throughamanifoldbasedonalkylperoxylradical.Interestingly,the formationsofketoneandaldehydeareinvolvedinroutesA3,A4,

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Scheme1.Routesdescribingtheformationofoxidizedproducts.

Fig.4. XPSsurveyspectraofPEﬁlmatdifferentabsorbeddoses.Onlyonebatchis representedasothersareidentical.

B,whichmayaccountforthenon-symmetricalpeakobservedin

Fig.5a.

Thealkylradicalsareobtainedafter␥-irradiationofPE,which are scavenged by the molecular oxygento afford an alkylper-oxylradicalA.The intermediateA abstractsa H-atomfromits surroundingtoaffordthehydroperoxydeBoritreactsby inter-molecularcouplingtoaffordatetra-oxideG.

The intermediate B reacts either by the fragmentation of the peroxidic bond (route A2) with loss of hydroxyl radical and formation of alkoxyl radical C [42], or by the H-atom abstraction(routeA1)atthe␣-positionofhydroperoxydegroup (Bond Dissociation Energy, BDE(CH3CH2COOH)∼406kJmol−1)

affording hydroperoxyalkyl radical. Then the latter fragments into the ketone D and a hydroxyl radical [42–44]. Abstraction (route A4) of the H-atom at ␣-position of the oxyl radical C (BDE(C2H5CH(O ˙)CH3)∼49kJmol−1) may occur to afford the

ketoneD.Morelikelytooccuristhe␤-fragmentationofC(route A3,BDE(CH3CH2-CH(CH3)O•)∼18kJmol−1)releasinganaldehyde

Fig.5.a)C1sXPSspectraofPEﬁlmirradiatedatdifferent␥-doses,b)exampleof decompositionoftheC1sspectrumrecordedonsampleirradiatedat50kGy.

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Fig.6. O1sXPSspectraofPEﬁlmirradiatedatdifferent␥-irradiationdoses.

(E)andanalkylradical.TheketoneDandthealdehydeEcouldbe furtheroxidizedintocarboxylicacidsF.

InparalleltotherouteA,theintermediateAreacts alsoby self-coupling(routeB)toaffordthetetra-oxideG,whichcollapses eitherinketoneDandalcohol[45] (routeB1)orbyO Obond cleavagereleasingO2intheintermediateradicalC(routeB2)[46].

4. Conclusion

Intheevaluationofthe␥-raysimpactonPE(polyethylene)and EVA(polyethylenevinylalcohol)ﬁlmswhenionizedatdifferent doses,changesinthephysico-chemicalfeaturesonsurfacehave beeninvestigatedbyXPS(X-ray PhotoelectronSpectroscopy)to highlightthedifferentchemicalmoietiesgeneratedduringthe ␥-irradiationprocess.

IntheEVAfilm,thechemicalgroupscorrespondingtothe car-boxylicacidsandaldehyde/ketonesincreasewiththe␥-irradiation dose.Theacetatemoietydecreasesonfilmsurfacewhen increas-ing␥-irradiationduetothelysisoftheacetategroupoftheEVA polymerinprofittothecarboxylate,aldehyde/ketonesmoieties. TheshiftoftheC1sandO1speaksindicatesaswellanoxidationof thefilmsurfaceoccurs.Afterall,itseemsthehighestdose inves-tigatedinthatpaper,i.e.270kGy,leadsaswelltothecleavageof carboxylates.

Inopposite,theXPSanalysisofthePEfilmdoesnotrevealahigh contentofoxygenwhen ␥-irradiatingthesamples.Asymmetric shapesofC1sandO1speakslinkedtoweakintensityinXPSspectra onPEfilmsdonotallowclearlyidentifyingthegeneratedchemical groups. Previousstudysuggests thatcarboxylic acidsand alde-hyde/ketonesaregenerated41_._The_films_surface_study_investigated

byXPSdidnotallowdetectingthehydro(gen)peroxides gener-atedduringthe␥-irradiationprocess.Nevertheless,thehydro(gen) peroxide(s)mustbepresenttoinduceoxidationofthepolyoleﬁn chainsleadingtothegenerationoftheoxygenatedspecies.

The radiationdoes not affectthe properties of allpolymers inthesameway,ortothesamedegree,even thoughpolymers investigatedhereareallpolyolefinbased.Itmeanstheeffectof irradiationshouldbeconsideredwhenselectingapolymerfora particularapplication.However,theextentofthemodifications issuedby␥-raysshouldbeseenasminorasnewchemical moi-etiesmainlybroughtbyoxygenatedspeciesrepresent<1%ofthe pristinemoietiespresentaftertheplasticsproduction.A stabilisa-tionoftheoxidisedspeciemoietiesonfilmsurfacewouldindicate

thathydro(gen)peroxidehaswhollyreactedguarantyinganinert ﬁlmsurface.

Acknowledgements

FGthanksSartoriusStedimBiotechforPhDgrant.NDandSRAM arethankfultoAMUandCNRSforsupport,andtoSartoriusStedim Biotechforfunding.

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