SURFACE AND INTERFACE ANALYSISSurf. Interface Anal. 29, 276–283 (2000)

Surface characterization and ageing ofultraviolet–ozone-treated polymers using atomicforce microscopy and x-ray photoelectronspectroscopy†

D. O. H. Teare, C. Ton-That and R. H. Bradley*Materials Surfaces and Interfaces Group, School of Applied Sciences, The Robert Gordon University, St Andrew Street,Aberdeen AB25 1HG, UK

Ultraviolet–ozone (UVO) treatment of poly(ethyleneterephthalate) (PET) films and polystyrene (PS) dishesof up to 10 min exposure has been studied. Surface polarity, oxygen chemisorption and topographical changewere analysed by contact angle measurement, x-ray photoelectron spectroscopy and atomic force microscopy.Studies of the treated surface reveal the chemistry behind the increasing oxygen content. The oxidationprocess is shown to proceed via different mechanisms for the two polymers. Polyethyleneterephthalateappears to undergo a Norrish-type chain depolymerization reaction, whereas PS undergoes a much morerandom chain scission attack. Atomic force microscopy analysis shows an increase in the surface roughnesswith increasing exposure to UVO for both polymers, with grains of low-molecular-weight oxidized material(LMWOM) forming at the surface. This material can be removed by washing. Surfaces that remain afterwashing have a higher concentration of oxygen species than the native surfaces. Analysis of aged surfacesshows that for oxidized PET a relaxation process occurs, lowering the levels of surface oxygen. This appearsto occur due to the diffusion of LMWOM into the PET bulk. Relaxation of the oxidized PS surface is lessthermodynamically favourable owing to the apolar nature of the PS. Copyright 2000 John Wiley & Sons,Ltd.

KEYWORDS: XPS; AFM; PET; polystyrene; ultraviolet-ozone; surface modification

INTRODUCTION

Ultraviolet–ozone (UVO) treatments, initially consideredas a surface cleaning method,1 have been shown recentlyto modify the surface chemistry and improve the wettingcharacteristics of natural and synthetic polymers. Pretreat-ment of polymer surfaces is often necessary due to thelow intrinsic activities of polymer surfaces. Such treat-ments are useful in a range of technological applicationsin order to increase the surface energy of many organicpolymers for the substrate to perform its desired task. Thisis of importance in areas such as composites (e.g. adhesionbetween polymer and epoxy2), printability, wettability andbiocompatibility3 (e.g. tissue culture polystyrene is typi-cally plasma modified), where the bulk properties of thepolymer (such as optical clarity, mechanical durability andcost) are desirable.

Pretreatments of polymers are often oxidative, per-formed in either the gaseous or liquid phase in orderto introduce a range of functionalities at the surface,thus increasing the polar surface free energy. Previoussurface modification methods that have been applied topolymers have included chromic acid,4 corona,5 flame,6

* Correspondence to: R. H. Bradley, Materials Surfaces and Inter-faces Group, School of Applied Sciences, The Robert Gordon Univer-sity, St Andrew Street, Aberdeen AB25 1HG, UK.

† The results in this paper were presented at ECASIA’99, 4–8October 1999, Seville, Spain.

radiofrequency (r.f.)7 and remote plasmas,8 gamma irradia-tion9 and electron beam modification.10 A common prob-lem with these methods is that reproducibility and controlmay not be simple. They often involve the handling ofsophisticated apparatus or use reactive gases and chemi-cals, none of which makes them suitable for point-of-usemodification of polymer materials and also adds to thecost of producing the desired surface. A principal advan-tage of the UVO method is that it can be applied underambient conditions as a continuous treatment method, orin small batches with a very high degree of control.

Ultraviolet and ozone photooxidation was first recog-nized as a potential polymer surface treatment in theearly 1980s.11 – 13 Since then, there have not been manystudies into this potentially very useful and applicabletechnique. The UVO treatment relies upon the com-bined effects of UV light and ozone, producedin situfrom a gas-phase photodissociation of molecular oxy-gen. The individual and combined effects of UV lightcombined with ozone have been studied for a vari-ety of surfaces, including polyethylene (PE), polypropy-lene (PP), poly(ethyleneterephthalate) (PET), poly(etherether ketone) (PEEK) and polystyrene (PS). Ultravio-let–ozone treatment causes the surface energy of thepolymers to increase through the breaking of the poly-mer chain by insertion of oxygen-containing functionalgroups. It was found that the oxygen levels on UVO-treated polymer surfaces increase quite markedly afterrelatively short exposure times (<5 min) compared withother UV treatments.2,14 – 18 The wettability of PET15,16 and

Copyright 2000 John Wiley & Sons, Ltd. Received 4 November 1999Revised 30 December 1999; Accepted 30 December 1999

SURFACE CHARACTERIZATION OF UV–OZONE-TREATED POLYMERS 277

PS17 surfaces increases with increasing oxygen content onthe surface.

What has not really been examined in depth hasbeen the simultaneous changes in topography that arerelated to surface treatments. In this study we investi-gated the correlation between surface oxygen chemistryand topographical changes of PET and PS surfaces by con-tact angle measurement, x-ray photoelectron spectroscopy(XPS) and atomic force microscopy (AFM). The stud-ies will lead to a greater understanding of the changes instructure and chemistry that occur on the modified poly-mer surfaces. Identification of the major functional groupsthat form, as well as observing the changing roughness ofthe surface, will facilitate control of the treatment. Addi-tionally we have made a pilot study into the mechanicalproperties of the oxidized polymers using scanning probemicroscopy.

EXPERIMENTAL

The PET films (ICI, pure, unfilled and untreated biaxi-ally oriented melinex ‘O’ grade) and the internal surfacesof 60 mm untreated PS cell culture dishes (LP Italiana,Milan, Italy) have been modified oxidatively using UVO.The treater contains a high-intensity low-pressure quartzfused mercury vapour grid lamp that emits UV lightacross a wide range of wavelengths, including 184.9 nmand 253.7 nm, which excite molecular oxygen to formozone and also photosensitize polymer surfaces.18,19 In thisstudy PET and PS substrates were treated at a distanceof 3 cm from the lamp; results are reported for expo-sure times within the range 0–10 min under atmosphericconditions after warming up the UV lamp for 30 min.The relative humidity inside the treater was<15% underoperating conditions and the nominal treatment temper-ature was 363 K. Polymer films exposed to the thermaleffect without UVO present did not show any chemicaldegradation or topography changes of the surfaces thatwere discernible by AFM or by XPS. Similar chemistryto that observed by us has resulted from UVO treatmentsat reduced temperatures.16 It has been reported also thatwater vapour can affect the modification18 by increasingoxygen incorporation. However, this only occurs at shortexposure times and the treatment level in this study thatcorrelates to the reports is<20 s of exposure. Other thanensuring that a series of modifications were performedunder the same atmospheric conditions, no conditioningof the air in the treater was deemed necessary.

Samples were analysed within 1 h of treatment, unlessstated otherwise. Aged samples were wrapped in cleanaluminium foil and stored in the laboratory at 293 K forup to 2 months. For washing, samples were agitated gentlyin HPLC-grade water (Millipore, resistivity 18 M� Ð cm)for 5 min and dried at laboratory temperature.

The surface chemical compositions of the polymer sur-faces have been characterized using a Kratos Axis HSi 5channel-imaging x-ray photoelectron spectrometer usingmonochromated Al K radiation (1486.6 eV) operated at150 W in a residual vacuum of<4 ð 10�9 Torr withthe analyser in fixed transmission mode. Charge neu-tralization was used for all samples with the standardoperating conditions for insulator surfaces:�2.8 V biasvoltage;�1.0 V electron filament voltage;C1.9 A fila-ment current. All spectra were acquired in hybrid mode,

using both the electrostatic and the magnetic lenses.Elemental surface compositions (at.%) were calculatedfrom survey spectra measured at a pass energy of 80 eV,with the error for the peak area analysis estimated atš5 at.%, deduced from elemental composition analysis ofpoly(tetrafluoroethylene) (PTFE). Detailed surface chemi-cal information was obtained by analysis of the C 1s peakenvelopes at the higher resolution pass energy of 20 eV.Chemical shift data are charge referenced to the centre ofthe C–C/C–H peak at 284.7 eV for PET and 285.0 eV forPS.20 Peak analysis was carried out using Kratos softwareand also version 1.5 of The Spectral Data Processor (XPSInternational), giving data with a reproducibility ofš5%,which allows an accurate intercomparison of data trends.Peaks were defined using linear background subtraction,with end points being averaged over 3–5 data points.

Topography changes resulting from UVO treatment ofthe polymers have been studied using a Digital Instru-ments (DI) Nanoscope SPM IIIa under ambient condi-tions. All surfaces were imaged in tapping mode usingsilicon tips. Imaging was performed in amplitude detec-tion mode with feedback set at¾70% of the resonancedriving amplitude. In this mode, the cantilever is made tooscillate at high frequency and briefly contacts the samplesurface. No wearing of the surfaces was observed whenimaging in tapping mode. Image analysis has been carriedout using DI version 4.23r3 software. Multiple images ofall UVO-oxidized surfaces were acquired, enabling statis-tical analysis of data.

Contact angle determination has been made by pho-tographing a 20µl drop of pure water on the polymersurface and determining the contact angle directly fromthe print. The size of the polymer sample was typically1.5 cm2. All UVO-treated polymer surfaces were washedin pure water for 5 min prior to analysis. This was toensure consistency of results, because water-soluble mate-rial present in the UVO-exposed surface would affect themeasured wettability of the surface. As a consequence,no contact angle data of the unwashed polymer surfacesare given.

RESULTS

X-ray photoelectron spectroscopy of PET

Survey spectra of all UVO-PET surfaces demonstrate thepresence of only carbon and oxygen, the native surfacehaving an oxygen concentration of¾26 at.%. The C 1speak of untreated PET matches that of pure PET.20 Thelevels of chemisorbed oxygen as a function of treatmenttime are shown in Fig. 1, which includes data for thewashed and the aged surfaces at the same treatment. Thehighest oxygen level reached after treatment is 39 at.%,which agrees with other reported UVO data.15 The data forthe washed surfaces show that¾50% of this is removed bywashing. From the point of divergence of the two curvesthis occurs immediately after the start of UVO exposure,suggesting that the formation of loosely-bound oxidizedmaterial occurs early in the process.

The chemistry of the oxidation process has been studiedby analysis of the C 1s peak envelopes, a typical peak-fitted example of which is shown in Fig. 2(a) for atreatment time of 360 s (after washing). The envelopes

Copyright 2000 John Wiley & Sons, Ltd. Surf. Interface Anal. 29, 276–283 (2000)

278 D. O. H. TEAREET AL.

Figure 1. Oxygen concentration for UVO-treated PET after treat-ment, washing and ageing for 1 month.

Figure 2. Carbon 1s XPS spectrum of: (a) washed UVO-PET after360 s; (b) washed UVO-PS after 300 s of exposure.

from the oxidized surfacescan be resolved into fivecomponents.The major peakin the spectrumat 284.7eVis due to C–C/C–H groups.Thereare four other majorpeaksdue to C–OR, R2C O and RO–C O (esterandcarboxylicacidgroups)at respectiveshiftsof 1.5,3.0,4.0and4.5eV, andalso�–�Ł satellitesat shiftsof 5–8 eV.21

The areasof the carbonbondedto oxygenpeaksincreasewith UVO exposuretime. Significantpeakbroadeningisseenin the longer exposedUVO-PET spectra.This canbe attributed to changesin the molecular environment,e.g.ˇ-shifting of theC–C/C–H peakdueto juxtapositionnext to an RC–O O group. Evaluationof the relativeareasof thesepeaksleadsto Fig. 3. Note that the datadisplayedshow the changein the relative areasof eachpeakin theC 1senvelope.Thedatashowthatat treatmenttimes of <60 s the oxidation processproceedsvia theformationof CORandRO–C O groups,althoughsomeR2C O groups are also formed. At longer treatment

times RO–C O groups becomethe dominant species.Saturationof C–OR occurs rapidly (after 40 s), afterwhich the proportionactually falls but the proportionofRO–C O continuesto riseandreachesa maximumafterabout 240 s of exposure.Figure 4 shows that the lossof oxygen due to water washingof the surfacesoccursby removalof materialmainly consistingof RO–C Oand R2C O groups (i.e. those that require the mostpolymer chain scission for formation). Residual stableoxygenlevelsare¾29–31at.%but theRO–C O speciesis now lessdominant.A gradualincreasein unsaturatedoxygencomponentsis observedup to a treatmenttime of120 s, after which the levelshavereachedequilibrium.

Analysisof the relative intensitiesof the �–�Ł shake-up satellites from the C 1s peak envelopesreveals adecreasein intensitywith treatmenttime. Comparisonofthedatafor unwashedandwashedsurfacesshowsa smallincreaseon washing; however, the shake-upintensityremainslower than that of the native polymer. Analysisof the UVO-PET surface by angle-resolvedXPS didnot reveal significant differencesto the above studies,suggestingthat the depth of UVO modification reacheddeeperinto the PSsurfacethan that probedby XPS (i.e.>10 nm). Otherstudies6 haveshownthatUVO treatmentof polymers penetratesmuch deeper(up to 1 µm hasbeensuggested)thanthemoresurface-specificplasmaandcoronaoxidativemodifications.

The washedUVO-PET sampleswere reanalysedafterallowing the surfacesto age for over 1 month. Levelsof RO–C O drop, whereasthoseof C–OR, C–C/C–Hand�–�Ł shake-upsatellitesrise.Thedataareconfirmedby surveyspectra,which showlower oxygenlevels.The

Figure 3. Change in C 1s peak areas of the oxygen functionalgroups for UVO-PET immediately after treatment.

Figure 4. Change in C 1s peak areas of the oxygen functionalgroups for UVO-PET after washing.

Surf. InterfaceAnal. 29, 276–283 (2000) Copyright 2000JohnWiley & Sons,Ltd.

SURFACE CHARACTERIZATION OF UV–OZONE-TREATED POLYMERS 279

surfaces, however, remain in an oxidized state whencompared to the native PET surface.

X-ray photoelectron spectroscopy of PS

Only oxygen and carbon were detectable in all of theUVO-PS spectra. The C 1s peak of the native PS matchesthat of pure PS.20 The levels of chemisorbed oxygen asa function of treatment time are shown in Fig. 5, whichalso includes data for the washed and aged surfaces atthe same treatment levels. The maximum oxygen levelreached is 36 at.%, which agrees with other reportedUVO treatment,17 and the data for the washed surfacesshow that¾30% of this is removed by washing. From thepoint of divergence of the two curves, this removal of theoxidized surface occurs after¾40–60 s of UVO exposure,by which time the oxygen levels are at 15–20 at.%.

Figure 2(b) shows the C 1s spectra of UVO-PS for atreatment time of 300 s (after washing). The envelopesfrom the oxidized surfaces can be resolved into six com-ponents. The major peak in the spectrum contains twocomponents: one at 285.0 eV due to C–C/C–H; anda second peak due to C–C/C–H that isˇ-shifted by0.4–0.6 eV due to juxtaposition to carbons bonded tooxygen. There are four other shifted peaks due to C–OR,ˇ-shifted C–OR, R2C O, and RO–C O at respectiveshifts of 1.5, 2.0, 3.0 and 4.5 eV, as well as�–�Ł satel-lites at shifts of 5–8 eV. The -shifted peak for C–ORhas been shown to occur in some polymers22 due to neigh-bouring RO–C O groups. The area of this peak increaseswith oxidation. Another contribution to this peak is pos-sibly due to epoxide groups, which may form on theoutermost surface in particular. The peak broadening andconsiderable overlap of areas seen in Fig. 2(b) presentproblems in resolving individual peaks in this type ofhighly oxidized polymer. Evaluation of the relative areasof these peaks leads to Fig. 6, which shows that at treat-ment times of<60 s the oxidation process proceeds viathe formation of C–OR and R2C O groups, althoughsome RO–C O are also formed. At longer treatmenttimes RO–C O groups become the dominant species.Saturation levels of C–OR and R2C O are reachedafter 120 s, but the proportion of RO–CO continuesto rise. Figure 7 shows that the loss of oxygen due towashing of the surfaces occurs by removal of materialrich in RO–C O and R2C O groups (again, those thatrequire the most polymer chain scission to form, simi-lar to PET). Residual stable oxygen levels are¾20–25

Figure 5. Oxygen concentration for UVO-treated PS after treat-ment, washing and ageing for 1 month.

at.% but the dominant oxygen speciesis now C–OR.A gradualincreasein all componentsis observedup toa treatmenttime of 180 s, after which the levels havereachedsaturation.Note that the changesin peak areafor UVO-PS are greaterthan thoseof UVO-PET. Thisis due to PET already containing oxygen before UVOtreatment.

Comparisonof the relative intensities of the �–�Ł

shake-upsatellitesfrom the C 1s peak envelopesagainrevealsa decreasein intensitywith treatmenttime,similarto that seenfor PET. This indicates that some of theoxidationproceedsby ring openingandoxidationor evenby ablation of the pendantaromatic rings. Comparisonof the data for unwashedand washedsurfacesshowsasmall differencebetweenthe respectivelevels,indicatingthat washingrevealsfresh polymer,however,the shake-up intensityremainslower thanthatof thenativepolymer.Again, analysisof theUVO-PSsurfaceby angle-resolvedXPS did not reveal significant differencesto the abovestudies,andnor did XPS imagingrevealany localizationof chemicallydistinct regions.

The washedUVO-PS surfaceswere reanalysedafterallowing the surfacesto agefor over 1 month.The datashowa slow restorationbackto the nativepolymerstate.Levelsof RO–C O andR2C O drop very slightly, theC–OR level doesnot changewhereasthoseof C–C/C–Hand�–�Ł shake-upsatellitesrise;thesedataareconfirmedby surveyspectrashowingoxygenlevelsthataremoreorlessstable(seeFig. 5). The surfacesremain in a stableoxidizedstateover the periodstudiedwhencomparedtothe nativePET surface.

Figure 6. Change in C 1s peak areas of the oxygen functionalgroups for UVO-PS immediately after treatment.

Figure 7. Change in C 1s peak areas of the oxygen functionalgroups for UVO-PS after washing.

Copyright 2000JohnWiley & Sons,Ltd. Surf. InterfaceAnal. 29, 276–283 (2000)

280 D. O. H. TEAREET AL.

Atomic force microscopy of PET

Changes in PET topography due to UVO treatment areseen in the 2µm ð 2 µm images of Fig. 8. The trendis for increased grain size and surface roughness as thetreatment time is increased. The topography of the nativePET appears comparatively smooth, consisting of small(<50 nm) sized grains or clusters. The UVO treatment onthe PET surface increases grain size, as seen in Fig. 8.The 6 min treatment produced the largest grain size of¾200 nm in diameter. The increase in grain size correlateswith the increases in surface roughness. An increase of themean roughness,Ra, from 0.8 nm on native PET to 5 nmoccurred after 6 min of treatment (see Fig. 9).

Washing the UVO-PET films in water decreases themean roughness (see Fig. 9). The mean surface roughnessincreases almost linearly with the treatment time for the

Figure 8. The 2 µm ð 2 µm AFM images of: (a) UVO-PET after120 s; (b) UVO-PET after 360 s.

Figure 9. Mean surface roughness against exposure time datafor UVO-PET, washed UVO-PET and washed UVO-PS from AFMmeasurements.

unwashedsamplesbut it reachesa limiting value of¾1.5 nm for the washedsamples.Small grains, whichare seenon the native PET surface,are not completelyremovedon washingbut do diminish in size,suggestingthat thereis somelow-molecular-weightmaterialalreadypresentasa surfacefilm.

Atomic force microscopyof PS

The topographychangesthatoccurasa functionof UVOtreatmentareshownby the 10 µmð 10 µm AFM imagesin Fig. 10 for 60 and 180 s, respectively.Note that theimagesare a different size to the PET imagesin Fig. 8.This is to enable the optimum scale for depicting thesurfacefeatures.However,the roughnessdatashown inFig. 9 arefor 2 µmð2 µm areasthatareidenticalto thoseanalysedfor PET to enablemeaningfulcomparison.Thenative PS dishesare relatively smoothwith few surfacefeaturesanda root-mean-square(RMS) surfaceroughnessof ¾1.7 nm. The UVO treatment of PS induces theformationof surfacegrains,asseenin Fig. 10(a).Cross-sectionalanalysisrevealsthatthesegrainshaveaheightof20–30 nm, with lateraldimensionsof 300–400 nm. Thelargestgrainswere observedat ¾60 s of treatment.Thesurfacegrainsthen diminish in size for longer treatmenttimes.For treatmentsgreaterthan120 s thereareonly afew small grainson the UVO-PSsurfaces[Fig. 10(b)].

Figure 9 showsdata for the washedUVO-PS dishes.Washing removesall the grains formed on the treatedsurfacesby the treatment.The RMS roughnessand sur-face areasare averagesof five 2 µm ð 2 µm areasatdifferent locationson the surface.The RMS roughnessonly increasesslightly for longer treatmenttimes.

Figure 10. The 10 µmð 10 µm AFM images of: (a) UVO-PS after60 s; (b) UVO-PS after 180 s.

Surf. InterfaceAnal. 29, 276–283 (2000) Copyright 2000JohnWiley & Sons,Ltd.

SURFACE CHARACTERIZATION OF UV–OZONE-TREATED POLYMERS 281

Contact angle

The results of the contact angle determination (shown inFig. 11) show a decrease in the water contact angle afterUVO exposure. The surface energy of UVO-PET doesnot appear to change significantly with longer exposure,whereas the UVO-PS surface energy reaches a minimumcontact angle after 240 s of exposure. It is apparent thatthe polar surface free energy of both polymers is similarat oxygen saturation.

DISCUSSION

Ultraviolet–ozone treatment produces reactive gaseousspecies such as atomic oxygen and ozone. The atomicoxygen is very reactive and is present in two forms:excited or singlet-state O(1D); and ground- or triplet-stateO(3P).18 The UVO reaction produces a quantum yieldfor ozone production of 0.5.19 Oxygen(1D) reacts withpolymer chains through a concerted insertion reaction intoC–H and C–C bonds to form C–OR groups. Oxygen(3P)reacts through abstracting hydrogen from the polymer toleave a carbon radical, which can react with molecularoxygen, ozone and other radicals. This latter reactiontends to form more highly oxidized functionalities suchas carbonyl and carboxyl groups, and can also lead tocross-linking reactions between polymer chains.

Increasing the polar surface energy of PET and PSby UVO modification is accompanied by oxidation androughening. The AFM images clearly show that thisroughening is evident in the formation and growth ofgrains on the polymer surfaces with longer UVO expo-sure. These grains are principally made of low-molecular-weight oxidized material (LMWOM). This material is amobile, loosely bound layer consisting of molecules con-taining a high proportion of carbonyl and carboxyl orester groups. The proportion of these functionalities growswith exposure time for both polymers when compared tohydroxyl and ether groups as further oxidation occurs. Onwashing the surface, AFM reveals the grains diminishingand XPS reveals oxygen levels decreasing as LMWOM isremoved. As UVO polymers still exhibit a relatively highcontact angle it is unlikely that this rearrangement is dueto water-induced swelling. What remains after washing isa functionalized, oxidized surface with increased rough-ness. Other AFM studies of polymers modified by UVOhave reported similar topographical changes.23,24

Figure 11. Change in water contact angles for UVO-PET andUVO-PS after washing.

Thesurfaceof UVO-PETundergoesphotocleavageviaNorrish I and II 25 reactionsof the estergroup, probablyby insertionof O(1D). TheAFM imagesandXPSspectrafor the washedsurfacesshow immediate formation ofLMWOM, which supportsthis.Furtheroxidationon otherparts of the chain occurswith extendedexposure.Thisaccountsfor the significant rise in the number of freeacid groupsseenin the XPS data.Somedegradationofthearomaticpartof PETis inevitableasthechromophorefunctionalitiesin PETabsorbUV, excitingthe� electronsin thephenylring. Ring openingandreactionwith atomicoxygen(O(3P))to formphenolichydroxylandcarboxyl15,25

groupsandablationof volatileoxidizedaromaticmoleculesare two possibilities.The retentionof significant �–�Ł

transitionsin eventhemosthighly oxidizedsurfaceswouldsuggestthatmostof thephenylgroupsremainintact.Afterabout 4 min of exposureto UVO a steadystate in thelevel of oxygenincorporationis reached.This is probablydueto equilibrium betweenoxidationandlossof volatileLMWOM by thermaldesorption.

Thewashingoff of LWMOM is accompaniedby a lossin grain size and a decreasein the XPS peak intensityfrom acid and estergroups.The washedsurfaceretainsa higher degreeof oxidation than the native surface.Allimagesweretakenusingtappingmode;however,we haveobservedthat using AFM in contact mode at elevatedloadingforceresultsin theLMWOM clustersmovingasaresultof the appliedforce.Sampledamagewasobservedto besignificantat forcesof >50 nN for UVO-PETfilms.Figure 12 is an image of a UVO-PET surfacetreatedfor 360 s. The centreof the image has beenscratchedrepeatedlyby draggingan AFM tip in contactmode atan appliedforce of ¾120 nN acrossthe surface.As canbe seen,the LMWOM clusterson the surfacehavebeenremoved,suggestingthat the LWMOM film is mobile.The native PET surfacesare not rearrangedin the sameway becausethere is no mobile layer to deform on thesurface.24 The fluidity of this materialwill impart to it asurfacetensionthatcausesaggregationto form thegrains.

Furtherageingof theUVO-PETsurfaceresultsin somerestorationof the surfacechemistry to the native state.This can be attributed to reorientationof the oxidizedpolymer chainsat the surfaceand also diffusion into thebulk in order to decreasethe polar surfacefree energy.The diffusion of polar molecules is likely through apolymer suchas PET due to the polar oxygenchemicalfunctionalitiesin the chain.

Figure 12. Poly(ethyleneterephthalate) treated with UVO for360 s. The central area has been scratched in contact modeusing a silicon nitride tip; applied force ¾120 nN.

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282 D. O. H. TEAREET AL.

The LMWOM on the surface of UVO-PS is formed bya more random photochemical attack on the chain. It isnot possible to tell whether the point of attack is on thealiphatic or aromatic sections of the molecule. Althoughthe �–�Ł shake-up satellite peak drops in intensity, itis still present in the most oxidized polymer surface,suggesting that some aromaticity is retained. Again, athigh UVO exposure a steady state in the level of oxygenincorporation is achieved, due to equilibrium betweenoxidative attack and loss of volatile LMWOM. The initialincrease is of C–OR groups, followed by R2C O andRO–C O. This mirrors the findings of MacManus18 withUVO polypropylene. They proposed that this order ofoxidation is due to the initial reaction being insertion ofO(1D) into the chain to form C–OR groups. The rate ofthis reaction slows down as more carbonyl and carboxylgroups are formed due to oxidation following hydrogenabstraction by O(3P). Our results here provide furtherevidence to suggest that this is why the more highlyoxidized functional groups dominate at extended exposuretimes, and also why the majority of the LMWOM consistsof these functional groups.

The topographical images show that the roughening ofthe UVO-PS surface is a seemingly random process, withgrains of LMWOM forming at irregular intervals. Thismakes a true quantitative measure of the surface rough-ness difficult. The grains are ‘swallowed’ up at longerexposure times, probably due to an increased fluidizationof the surface with the formation of further LMWOM.Distinct grains, as seen with PET, are not observed. Thissuggests that the LMWOM is of a much more fluid natureand the clusters that form quickly coalesce and form ahomogeneous layer of oxidized material.

Washing the surface with water again removes a pro-portion of the LMWOM. From the point of divergence ofthe unwashed and the washed UVO-PS curves in Fig. 5the formation of this material occurs after 40–60 s. Adifference with PET was that LMWOM was formed (andthus removed on washing) immediately. It is apparentfrom Figs 6 and 7 that the majority of the PS LMWOMremoved by washing consists of unsaturated (R2C O andRO–C O) groups, whereas less highly oxidized speciessuch as hydroxyls and water-insoluble higher-molecular-weight oxidized material remain. This has been reportedby other groups.7,15,17 Topographical images of the washedUVO-PS surface show increasing roughness, due to theoxidative attack on the polymer compared to the nativesurface.

The oxidized UVO-PS did not age as significantly as theUVO-PET. Some restoration of the surface was evident,

but apart from losing a fraction of the R2C O andRO–C O groups the UVO-PS surface showed greaterstability than the UVO-PET. Diffusion of oxidized speciesthrough the bulk of PS will be a lot slower, due tothe non-polarity of the chain. This surface relaxation,therefore, will be a lot less thermodynamically favourablethan for PET.

Using AFM and by calibrating the cantilever tip itis possible to obtain an estimation of the hardness ofa surface. Further experiments with the AFM probingthe stiffness of both UVO-PET and UVO-PS reveal sig-nificant softening of the surfaces with increasing expo-sure. Measurements of pull-off forces from force–distancecurves also show an increase in the tip–sample adhe-sion of the treated surfaces. With UVO-PET it is evidentthat the grains are softer and stickier than the inter-grain regions. Frictional force microscopy with chemicallymodified tips also shows a similar result with UVO-PSsurfaces.26

CONCLUSIONS

The PET and PS surfaces undergo considerable reorga-nization with exposure to the UVO source. The surfaceroughness, polar surface energy and grain size increasewith exposure time. Oxygen chemisorption reaches a sat-uration level after¾2 min of exposure, but the surfaceroughness and free energy continue to increase. It isknown that UVO treatment causes scission and oxida-tion of the polymer chain.2,14 – 17 In this instance the for-mation of low-molecular-weight oxidized molecules willenable the surface of the polymer to become more fluid-like in nature and cause clusters of these molecules toform. With PET the longer the exposure to UVO, themore oxidized the surface becomes and larger clusters areformed. The XPS results indicate that the point of oxi-dation is predominantly the ester section of the polymerchain. With PS the polymer surface becomes more flu-idized and the roughened surface smoothes out at longUVO exposure times. In both polymers the LWMOMconsists predominantly of carboxylic acid and ester func-tionalities. The oxidation is seen to proceed in two stages:insertion of oxygen to form ether and hydroxyl groups;and—eventually the dominant stage—formation of unsat-urated carbon to oxygen bonds. The UVO-PS surfaces aremore stable to rearrangement with time than the UVO-PET surfaces, due to the non-polar nature of the underly-ing bulk polymer.

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