Effect of pulse repetition rate and number of pulses in the analysis of polypropylene and high density polyethylene by nanosecond infrared laser induced breakdown spectroscopy

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ContentslistsavailableatSciVerseScienceDirect

Applied

Surface

Science

j o ur na l ho me p age :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

Effect

of

pulse

repetition

rate

and

number

of

pulses

in

the

analysis

of

polypropylene

and

high

density

polyethylene

by

nanosecond

infrared

laser

induced

breakdown

spectroscopy

Flavio

O. Leme

a

_,

_Quienly

_Godoi

a,b

,

Paulo

H.M.

Kiyataka

c

,

Dario

Santos

Jr.

d

,

José

A.M.

Agnelli

e

,

Francisco

J. Krug

a,∗

a_Laboratório_de_Química_Analítica_“Henrique_Bergamin_Filho”,_Centro_de_Energia_Nuclear_na_Agricultura,_Universidade_de_São_Paulo,_Av._Centenário_303, 13416-000Piracicaba,SP,Brazil

b_Departamento_de_Química,_Universidade_Federal_de_São_Carlos,_Rod._Washington_Luís,_km_235,_13565-905_São_Carlos,_SP,_Brazil c_Centro_de_Tecnologia_de_Embalagens,_Instituto_de_Tecnologia_de_Alimentos,_Av._Brasil_2880,_13070-178_Campinas,_SP,_Brazil

d_Departamento_de_Ciências_Exatas_e_da_Terra,_Universidade_Federal_de_São_Paulo,_Rua_Prof._Artur_Riedel_275,_09972-270_Diadema,_SP,_Brazil e_Departamento_de_Engenharia_de_Materiais,_Universidade_Federal_de_São_Carlos,_Rod._Washington_Luís,_km_235,_13565-905_São_Carlos,_SP,_Brazil

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received28July2011 Receivedinrevisedform 25November2011 Accepted25November2011

Available online 6 December 2011

Keywords:

LIBS

Laserinducedbreakdownspectroscopy Laserpulserepetitionrate

Craterscharacterization Highdensitypolyethylene Polypropylene

a

b

s

t

r

a

c

t

Pulserepetitionratesandthenumberoflaserpulsesareamongthemostimportantparametersthatdo affecttheanalysisofsolidmaterialsbylaserinducedbreakdownspectroscopy,andtheknowledgeoftheir effectsisoffundamentalimportanceforsuggestinganalyticalstrategieswhendealingwithlaserablation processesofpolymers.Inthiscontribution,theinfluenceoftheseparametersintheablatedmassandin thefeaturesofcraterswasevaluatedinpolypropyleneandhighdensitypolyethyleneplatescontaining pigment-basedPbCrO4.Surfacecharacterizationandcratersprofilewerecarriedoutbyperfilometryand

scanningelectronmicroscopy.Area,volumeandprofileofcraterswereobtainedusingTaylorMap soft-ware.AlaserinducedbreakdownspectroscopysystemconsistedofaQ-SwitchedNd:YAGlaser(1064nm, 5ns)andanEchellespectrometerequippedwithICCDdetectorwereused.Theevaluatedoperating con-ditionsconsistedof10,25and50laserpulsesat1,5and10Hz,250mJ/pulse(85Jcm−2_),₂

␮sdelaytime

and6␮sintegrationtimegate.Differencesinthetopographicalfeaturesamongcratersofbothpolymers

wereobserved.Thedecreaseintherepetitionrateresultedinirregularcratersandformationofedges, especiallyinpolypropylenesample.Thedifferencesinthetopographicalfeaturesandablatedmasses wereattributedtotheinfluenceofthedegreeofcrystallinity,crystallinemeltingtemperatureandglass transitiontemperatureintheablationprocessofthehighdensitypolyethyleneandpolypropylene.It wasalsoobservedthattheintensitiesofchromiumandleademissionsignalsobtainedat10Hzweretwo timeshigherthanat5Hzbykeepingthenumberoflaserpulsesconstant.

1. Introduction

LIBS has been established as an important analytical tool whichallowsdirectanalysisofsolidswithoutorwithminimum samplepreparation. Thistechnique useslaserablationfor sam-plingandsubsequentmeasurementoftheemissionintensityof UV–visible radiation from excited atoms and/or ions in laser-induced microplasma [1,2]. Laser ablation promotes the direct samplingirrespectiveoftheformofthematerial.Theablation pro-cessdependsontheinteractionoflaserpulseswiththesample surfaceandresultsincraterformationinsolids[3].

ThepotentialoftheLIBSforthecharacterizationandchemical mappingofsurfaceshasbeendemonstrated[4–16].LIBSpresents

∗Correspondingauthor.Tel.:+551934294648;fax:+551934294774.

E-mailaddress:[email protected](F.J.Krug).

theadvantages ofperforminganalysisin fewseconds, allowing directsamplingfromanymaterialirrespectiveofitsconductive sta-tusinairatmosphericpressure,simplifyingtheanalyticalsequence byminimizingand/oravoidingsamplepreparationsteps,and pre-sentingnolimitationsorminimumrestrictionsregardingtosample

size[4,16].

In a review paper, Vadillo and Laserna emphasize that the introductionofcompactandreliablesolidstatelasersand tech-nologicaldevelopmentinmultidimensionalintensifieddetectors haveenabledtheseekingofnewanalyticalnichesforLIBS,where their advantages could be explored. In addition to the above-mentionedadvantages,thecapabilityofLIBSforspotanalysis,line scan, depth-profiling, areaanalysisand compositional mapping withasingleinstrumentintheair,underatmosphericpressure, makesthetechniqueausefultoolforsurfaceanalysisandchemical mappingindifferentmatrices[4].DepthprofilesbyLIBSanalysis, forexample,weredemonstratedindifferentmatricessuchasbrass,

doi:10.1016/j.apsusc.2011.11.122

Open access under the Elsevier OA license.

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258 (2012) 3598–3603 3599

steels,siliconwafers,zincandironfoils[7,8,10,17,18].LIBSallows toperformscansinthesamplesurfaceatdifferentdepths,making theevaluationofthespacialdistributionofelementspossible.From thescanninginformation, itis possibletoacquirethechemical mappingoftheanalytes.Chemicalmapsalsoallowtoevaluate con-taminantsindifferentplacesofthesamesample.Thesepossibilities maketheapplicationofLIBSveryinterestingforqualitycontrolin theindustry[4].Severalstudieshaveassessedtheapplicationof LIBSinthechemicalmappingofdifferentsampletypes[6,12–15]. Thepossibilityof performingthree-dimensional multielemental chemicalmapsofnon-flatsurfacesbyLIBSwasdemonstratedby Nicolasetal.[16].Surfacecharacterizationisanothertask show-ingthepotentialandversatilityofLIBS.However,theapplication topolymersisachallengingtopicconsideringthegreatnumber ofpolymerswithdifferentphysicalproperties.Thelaser-sample interactiondepends not onlyon thephysical properties of the samplebutonlaserparameterssuchaswavelength,energy,and repetitionrateaswell.Thus,thecharacterizationoftopography, volume,diameteranddepthofcratersisanattractivewayto under-standtheablationprocess[19,20].

Opticalmicroscopeperfilometryisausefultoolforlasercraters characterizationandhasbeenappliedtocratersformedinmetal samples(Cu,Al,andPb)afterlaserablationwithfemto,pico,and nanosecondlaserpulses[3].

Theeffectsofthenumberofpulses,pulserepetitionrateand fluenceduringlaserablationofpolymethylmethacrylate(PMMA) werestudiedusingaKrF@248nmexcimerlaseratpulse repeti-tionrateof2and10Hz[21].Inthisstudyaperfilometerwasused forcraterscharacterization.Accordingtotheauthors,thethermal effectsbecomemorepronouncedwhentherepetitionrateishigh enoughtosignificantlyshortenthepulse–pulserelaxationtime,or whentheamountoflaserpulsesislargeenoughtoproduceheat accumulation.Inaddition,craterswithsmootherandlessporous bottomwerealsoobservedwiththeincreaseofthepulserepetition rateandthenumberofpulses.

Theefficiencyoflaserablationofpaintwasinvestigatedwith nanosecondpulsedNd:YAG@532nmlaserasafunctionof repe-titionrate,laserfluenceandpulseduration[22].Thebestablation efficiencywasobtainedwiththehighestrepetitionrate.Theheat accumulationwitheachpulseina thermalconfinementregime duetothehighrepetitionrateallowedincreasesintheablation efficiency[22].Althoughalargenumberofstudiesonlaser abla-tionhadbeencarriedout,fewworksarerelatedtopolymers.In theanalysisoforganicswithLIBSunderatmosphericpressure,the interactionbetweenplasmaandtheambientairismuchmore sen-sitivewhencomparedwiththeanalysisofmetallicsamplesdueto thefactthatthemediatobeanalyzedarecomposedofthesame elementalsubstances.Forinstance,carbon,oxygen,nitrogen,and hydrogenareomnipresentandprovidethecommonbasisofthe mostorganicmaterials[23].Adetailedunderstandingofthelaser ablationinducedbytheplasmaisacrucialsteptowardtheincrease ofLIBSapplicationforthesesamples[24].

Inthiswork,theeffectsofpulserepetitionrateandnumberof pulsesonthetopographicalfeaturesofthecratersinLIBSanalysis ofhighdensitypolyethylene(HDPE)andpolypropylene(PP)were studiedbyscanningelectronmicroscopy(SEM)andperfilometry.

2. Experimental

2.1. LIBSinstrumentation

ExperimentswerecarriedoutwithaQ-switchedNd:YAGlaser (Brilliant,Quantel,France)at1064nm,generating5nspulsesof (365±3)mJ in a 6mm diameter beam with quality factor M2

smallerthan2,at1,5,and10Hzrepetitionrate.Thelaserpulses

werefocusedonthetestsamplebyaconvergentlenswith2.54cm diameterand20cmfocallength(EdmundOptics,USA).Theplasma emittedradiationwascollectedusingafusedsilicalens(i.e.80mm focal length) and collimated into a spectrometer fiber (1.5m, 600␮m core) matching its numerical aperture using a lens of

50mmfocallength(LLAInstrumentsGmbH,Germany).Theoptical axisofthecollectingsystemwasapproximately25◦_from_the_laser

axis.

A model ESA 3000 spectrometer (LLA Instruments GmbH, Germany) equipped with echelle optics and focal length of 25cm with aperture of 1:10 was used, which provides a 24.5mm×24.5mm flat image plane. This was selected as a compromise between resolution in the wavelength range of 200–780nmwithresolvingpowerrangingfrom10,000to20,000. Thelineardispersionperpixelrangesfrom5pmat200nmto19pm at780nm.ThedetectorisanICCDcamera,comprisedaKodakKAF 1001CCDarrayof1024×1024pixelsfullframe(24␮m×24␮m)

and a microchannel plateimage intensifierof 25mm diameter coupledtoa UV-enhancedphotocathode. Theimagesignalsare digitalized in dynamic range of 16 bits and further processed byacomputer.ThedarkcurrentoftheICCDwasautomatically subtracted from the measured spectral data. The instrumental parametersweredefinedelsewhere[25]andconsistedof18cm lens-to-sampledistance(f=20cm),250mJperpulse(85Jcm−2_),

2␮sdelaytimeand6␮sintegrationtimegate.

2.2. LIBSanalysis

PolymerplateswereproducedwithHDPEandPPcontaining2% (w/w)ofPbCrO4basedpigment(PigmentRed104;CASNumber:

12656-85-8).Polymerswereweightedand mixedwithpigment andthemixturewasinjectedusingaBattenfeldmodel350Plusat 200◦_C_and₇₀_bar._A₂₃_s_injection_cycle_followed_by₁₇_s_cooling_was

used.Afterpreparation,thedensitiesofHDPEandPPwithPbCrO4

basedpigmentwere1.056and1.002gcm−3_,_{respectively.}

Testsampleswereobtainedfromeachpolymerplateby manu-allycuttingpartsofthesampleintheformofflatdisks(e.g.3.0cm). Theselectedtestsamplewasplacedinthesampleholderofthe ablationunit,whichwasassistedbyatwo-axismanualcontrolled translationstagethatmovedintheplaneorthogonaltothelaser direction.Inordertodisplacetheambientairatmospherefromthe samplesurface,alaminarstreamofargonwascontinuouslyfedinto thesampleholderassemblyfrombelow,flowingat1.0Lmin−1_.

LIBSspectrawerecollectedaccordingtothefollowing proce-dure:aseriesofaccumulatedspectrawasobtainedafter10,25and 50laserpulsespersiteat1,5and10Hzrepetitionrate.The aver-ageoffiveaccumulatedspectracollectedfrom5sitesonthepellet surfacewasusedastherepresentativespectrum.Foreach poly-mer,LIBSdatatreatmentwasbasedonthreerepresentativespectra (n=3).ESAWINsoftware,NISTAtomicDatabase[26]andaroutine developedatMATLAB®_version_7.0_(MathWorks,_Inc.,_Natick,_USA) softwarewereusedfordataacquisitionanddatatreatment.

2.3. Perfilometryandscanningelectronmicroscopy(SEM)

Topographicalinformations of craters were obtainedwith a FormtracerSV-C525perfilometer(TaylorHobsonPrecision, Eng-land).Volume,areaandprofileofeachcraterwerecalculatedwith TaylorMapsoftwaretools.

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3. Resultsanddiscussion

Figs.1and2showtypical3Dimagesofcratersformedonthe

surfaceofHDPEandPP.Thelaserfluenceatthetargetsurfaceforall testsamples(i.e.plates)was85Jcm−2_,_which_was_defined

experi-mentallyinapreviousworkdealingwithpolymerictoysanalysis [25].Theimagesshowthatwidthanddepthofthecraterincrease withthenumberofpulsesandrepetitionrateforbothpolymers. Themassofmaterialremovedduringtheablationwithdifferent

numberofpulsesandrepetitionratewasestimatedfromthecrater volumecalculated byTaylorMap® _software._Table₁_shows _the ablatedmassesin HDPEandPPplatesafter10,25and 50laser pulseswithrepetitionratesof1,5and10Hz.Theestimatedablated masseswerebasedontheaveragecratervolumesat5differentsites inthesametestsample.

Theperfilometricimages(Figs. 1and2)ofboth polymersat 1Hzlaserrepetitionrateshowirregularcraterformationandlarger varianceinthemassremovalwhentheminimumnumberof10

Fig.1. Perfilometricimagesofcratersafter10,25and50laserpulsesatpulserepetititonratesof1,5and10HzinPP.

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Table1

AblatedmassesinHDPEandPPplatesafter10,25and50laserpulsesat1,5and10Hz repetitionrates.Uncertaintiesarerepresentedby±1estimatedstandarddeviation (n=5craters).

Laserpulses Ablatedmass(␮g)

HDPE PP

1Hz

50 15±2 14±2

25 8±2 11±2

10 3±1 3±1

5Hz

50 46±4 47±7

25 23±2 32±2

10 9±2 16±3

10Hz

50 88±6 80±8

25 43±5 61±5

10 22±3 33±4

laserpulseswasapplied.For25and50consecutivelaserpulsesthe

resultingcraterspresentedmoreuniformgeometry.Smalledges

aroundthecraterforbothpolymerscanbeobserved,whichwere

probablyduetothethermaleffectscausedbytherelativelylarge

timeintervalbetweenpulses(100–1000ms),whichresultsin

cool-ing.Consequently, theablationsurface becomes quiteirregular

aftertheendoftheablationprocess.Irregularandporousablated

surfacewasalsoobservedinPMMA[21].Accordingtotheauthors,

theseeffectswererelatedtothermaleffectsduringtheplasma for-mationandaffectedbythelaserwavelength,laserpulserepetition rateandthenumberoflaserpulses.

Inthepresentcase,highcoefficientsofvariation(>35%)inthe calculatedablatedmasseswereobservedforcratersformedwith 10laserpulsesat1HzinbothHDPEandPPplates.Whencraters wereformedbyapplying25and50pulsesatthesamerepetition rate,thisuncertaintydecreasedto20%,whichwasexpecteddueto abettersamplingprocess.

Ontheotherhand,cratersweremuchmoreuniformby apply-ing10,25or50laserpulsesat5and10Hzandthecoefficientsof variationofmassremovalwerebetween18%and5%,respectively. ThesmalledgesaroundthecraterswereobservedinHDPE sam-plesbyapplying10and25laserpulsesat5Hzand10laserpulses at10Hz.InthePPplates,edgeswereobservedinthesurroundings ofallcraters.

Theablatedmasseswithrepetitionratesof5and10Hz(Table1) showa considerabledifferencebetweentheHDPE andPPafter 10and25laserpulses.ThemassremovalfromPPwas consider-ablylargerthantheoneremovedfromHDPE.For50laserpulses, non-significantdifferencesbetweentheablatedmassesfromboth polymerswereobserved.Theformationofedgesanddifferences incrater’sgeometryinfunctionofpolymercompositionwerealso observedbyGodoietal.[25].

Thermaleffectsoftheablationprocessweremorepronounced forPPthanHDPE,andmoreuniformcratersbeingobservedinHDPE (Fig.3).TheperfilometriesoftheablatedsitesinbothPPandHDPE plates,obtainedafter50laserpulsesat10Hz,showedthatcraters’ depthwereapproximately420and550␮m,respectively.

Polymerlaser ablation involvesphotothermal and/or photo-chemicalprocesses,dependingonthenatureofthepolymersand thelaserparameters[27–29].AccordingtoPhametal.[30]the

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220.6 220.5 220.4 220.3 220.2 220.1 0 10000 20000 30000 40000 50000 60000 E m ission Intensit y (ar b . unit) Wavelenght (nm) 5 Hz 10 Hz Pb II 220.353 HDPE 205.8 205.7 205.6 205.5 205.4 205.3 0 6000 12000 18000 24000 30000 Emissi on Int ens ity ( a rb . u n it ) Wavelenght (nm) 5 Hz 10 Hz Cr II 205.552 HDPE 220.6 220.5 220.4 220.3 220.2 220.1 0 7000 14000 21000 28000 35000 E m issi on Int e ns ity (arb. u n it) Wavelenght (nm) 5 Hz 10 Hz Pb II 220.353 PP 205.8 205.7 205.6 205.5 205.4 205.3 0 3000 6000 9000 12000 15000 E m iss ion In ten s it y (a rb . un it) Wavelenght (nm) 5 Hz 10 Hz Cr II 205.552 PP

Fig.4.PbII 220.353andCrII205.552nmLIBSemissionlinessignalsfromHDPEcontaining2000±40mgkg−1_Cr_and ₆₉₈₀

±200mgkg−1 _Pb,_and_from_PP_with

1420±20mgkg−1_Cr_and₅₆₅₀

±120mgkg−1_Pb_after₂₅_laser_pulses_at₅_and₁₀_Hz.

photothermalprocessinvolvestheabsorptionofphotons,followed byrelease of thesephotonsinto thepolymer matrix.This pro-cessinducesarapidtemperatureriseinthebulkmaterialleading tothethermaldecomposition ofthepolymer. Accordingtothe authors,ifthevibrationalenergyattainsaparticularfluence thresh-old,thechemical bondsin thepolymer willbreak,resulting in a phenomenon knownasphotofragmentation.These fragments typically occupya larger volume compared tothe surrounding materialandleadtotheforwardedejectionofablatedmaterial.

Inthepresentstudy,onlythephotothermalprocesstakesplace becausetheexperiments werecarried outwithlaser operating intheinfrared (1064nm),whichis incontrast withthe photo-chemicalprocessthatpromotesbreakingofchemicalbondswith nsUVpulses,ascommentedelsewhere[30].Therefore,themain propertiesthatmayinfluencetheablationprocessaredegreeof crystallinity,crystallinemeltingtemperature(Tm)andglass

tran-sitiontemperature(Tg).

Crystallinityinpolymersreferstoanarrangementofmolecules thatdenoteuniformityandcompactnessofthemolecularchains andcanbeattributedtotheformationofsolidcrystalshavinga definitegeometricform[31].Thus,thecrystallineregionisthepart ofthepolymerthathasthesefeatures.Incontrast,theamorphous regionisthepartofthepolymerthathasarandomstructureand, consequently,doesnothaveamolecularorderdefined.Degreeof crystallinityisdefinedastheratioofthecrystallineregionandthe amorphousregionofthepolymer.

TheTmisthetemperatureatwhichthecrystallineregionofthe

polymerdisintegratesandmerges.Inthistemperature,boththe liquidandthesolidphaseshavethesameGibbsfreeenergyandthe polymericchainbecomeliquidwithoutestablishedorder[32,33]. TheTgistheapproximatemidpointofthetemperaturerangeover

whichtheprimaryglasstransitiontakesplace.Tgisrelatedtothe

stiffnessofthemolecularsegments,degreeofcross-linking, entan-glementandcrystallizationkinetics[31,32].

PPandHDPEhaveasimilarstructureandtheonlydifferenceis thatPP’schemicalstructurehasaCH3grouplinkedtooneofthe

carbonsofthemonomer.PPandHDPEhavelinearstructuresbut, duetothepresenceoftheCH3group,PPhasahelicalconfiguration

withanglesof120◦_among_the_methyl_groups._The_presence_of_the

CH3 groupisresponsibleforthedifferencesinthepropertiesof

bothpolymers,mainlydensity,degreeofcrystallinity,Tm,andTg.

PPhasalowerdensitythanHDPEduetothestericeffectcaused byCH3 groups,whichmakethepackingofmacromoleculesless

effectiveforPP.Thisfactis alsorelatedtodifferencesinTm,Tg

and the degreeof crystallinity of both polymers.For PP,Tm is

between165and175◦_C _and_T_g_between₄_and₁₂◦_C._For_HDPE,

Tmis135◦ CandTgis−120◦C.Thedegreeofcrystallinityvaries

between50%and70% forPPandupto95%for HDPE[34].The degreeofcrystallinitycanbedeterminedbyseveralexperimental techniques.ThemostcommonlyusedareX-raydiffraction, differ-entialscanningcalorimetry,densitymeasurementsandinfrared spectroscopy.However,imperfectionsinthecrystallinestructure arenoteasilydistinguishedfromtheamorphousphaseandthese techniquesmaybeaffectedtoadifferentextentbyimperfections [35].Disagreementsamongresultsofquantitativemeasurements ofcrystallinityarefrequentlyencountered[35],anditiscommon tofindvaluesbetween50%and70%forPPandfrom80%upto95% forHDPE[34].

The resultsfoundherein suggest that thedifferences in the degreeofcrystallinity,TmandTgarecrucialintheablation

pro-cessof PP and HDPE. As aforementioned,the interactionof ns laserpulsesat1064nmwiththepolymerresultsinthe absorp-tionofphotons,generatinga fastincrease inlocaltemperature. The molecules acquire thermal energy sufficient to move the chainsand,consequently,tobreakthechemicalbondsresultingin photofragmentation.Duetothelowerdegreeofcrystallinityand higherTgof thePPcompared toHDPE,itsamorphousregionis

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258 (2012) 3598–3603 3603

at5and10HzishigherthanoftheHDPE.When50laserpulsesare applied,thedifferenceinthemassremovalisminimized.At1Hz nodifferencesinthemassremovalwereobserved,mostprobably duetothelargetimeintervalbetweenpulses.

Theformationofedgesmorepronounced aroundPPcraters, suggestthatoccurahighereffectoftheTmintheablation

pro-cessofPPcomparedtoHDPE.Asthetemperatureintheborders oftheplasmaisrelativelylow,andthePPhashigherTmcompared

withHDPE,theformationofpronouncededgesinPPplatesoccurs becausethetemperatureofthebordersofplasmaisnotenoughfor completevaporizationofthematerial.Fig.4showstheemission peakprofilesforCrandPbobtainedbyLIBSinHDPEandPPafter 25laserpulsesat5and10Hz.Itcanbeobservedthattheintensities obtainedatlaserrepetitionrateof10Hzaretwotimeshigherthan thoseat5Hz.Theseresultsareconsistentwiththemassremoval duringtheablationprocess,asshowninTable1.

4. Conclusions

PerfilometryandSEMareusefultoolsforlasercraters character-ization.Theimagesallowfeaturesevaluationandthemassremoval ofHDPEandPPcanbecalculatedthroughdataobtainedby per-filometry.Smalledgesaroundthecratersandhighcoefficientsof variationin themass removalwereobservedat1Hz repetition rateinbothpolymerplates.For 25and50laserpulses remark-ableimprovementsineitherthegeometryofcratersaswellasin thecoefficientsofvariationoftheablatedmasseswereobserved. HDPEandPPshowedsignificantdifferencesinmassremovalwith 10and25laserpulsesat5and10Hzrepetitionrate,butwith50 pulsessimilarresultswereobtained.

Resultsindicatethatthemainpropertiesthatinfluencethe abla-tionprocessinpolymersarethedegreeofcrystallinity,theTmand

theTg.Duetheremarkabledifferencesbetweentheseproperties

forbothpolymers,thethermaleffectsweremorepronouncedin PPand,consequently,theperfilometricimagesoftheablatedsites showedcraterswithpronouncededges.Thehigherdegreeof crys-tallinityandthelowerTmandTgarethepossiblereasonstoexplain

whytheformationofsmalledgeswerenotobservedin craters formedinHDPEplateswhen25and50laserpulsesat10Hzwere applied.

Acknowledgements

AuthorsarethankfultoConselhoNacionaldeDesenvolvimento CientíficoeTecnológico(CNPq140879/2008-0and 305913/2009-3)and Fundac¸ãodeAmparoàPesquisadoEstadodeSãoPaulo (FAPESP07/01052-3)forgrantsandfinancialsupport.Authorsare alsogratefultoProf.YounnesMessaddeq(IQ-UNESP)forhelping inperfilometricanalysis,toProf.JezW.B.Braga(IQ-UNB)forthe MatlabroutineandtoLizZanchetta(EP-USP)forhelpingwithSEM analysis.

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