XPS studies of the ageing of plasma-treated polymer surfaces

SURFACE AND INTERFACE ANALYSISSurf. Interface Anal. 29, 56–64 (2000)

XPS studies of the ageing of plasma-treatedpolymer surfaces

R. W. Paynter*INRS Energie et Materiaux, 1650 boulevard Lionel-Boulet, Varennes, Quebec, Canada J3X 1S2

Polystyrene and polyethyleneterephthalate (PET) coupons were exposed to r.f. plasmas containing helium,oxygen and nitrogen. The modified surfaces were characterized by x-ray photoelectron spectroscopy shortlyfollowing plasma treatment and after storage in air for about 360 days. The oxygen concentration inpolystyrene treated with plasmas for at least 30 s was found to tend to a value of 12–14% following storage.Polystyrene exposed to a helium plasma took up additional oxygen during storage, mainly in CO, O–C Oand O–C O groups. Polystyrene exposed to a nitrogen/hydrogen plasma lost nitrogen over time but tookup oxygen in single, double and multi-atom bond configurations. Polystyrene exposed to an oxygen/heliumplasma lost oxygen over time, especially from pendant groups in which carbon was bonded to three oxygenatoms. The evolution of polystyrene and PET samples exposed to combinations of oxygen and nitrogenplasmas is also discussed. Copyright 2000 John Wiley & Sons, Ltd.

KEYWORDS: XPS; plasma; polystyrene; PET

INTRODUCTION

Although the use of d.c., r.f. and microwave plasmas forthe modification of polymer surfaces is rapidly becom-ing a mature technology,1 – 4 research continues into theevolution of the treated surfaces with time. One com-monly observed and troublesome phenomenon is the dete-rioration of the beneficial surface properties (such ashydrophilicity) upon storage in air. The problem is com-plex because of the variety of processes that may beinvolved, e.g., the diffusion and reaction of free radicals5,6

created by exposure to plasma and its attendant UV radia-tion, the adsorption of polar7 and non-polar contaminantsand the diffusion of adatoms8,9 and re-orientation of polargroups10 – 12 away from the surface.

Polyethyleneterephthalate (PET) is a polymer widelyused in commerce and industry. Because PET containsoxygen, we have conducted parallel studies on polystyrene(PS), which contains no oxygen but which, like PET,contains aromatic carbon rings. We have reported anx-ray photoelectron spectroscopy (XPS) study of the sur-face modifications brought about by the exposure of PSand PET to r.f. plasmas containing helium, oxygen andnitrogen.13 In that paper we noted a negative correlationbetween the advancing water contact angle and the con-centration of electronegative (oxygen and nitrogen) atomsin the surface.

This paper reports the results of a re-evaluation, aftera storage period of some 360 days, of a subset of thepolymer samples prepared for that study. In the previ-ous work, the surface chemical changes resulting fromexposure of the PS and PET samples to helium, nitro-gen and oxygen plasmas were evaluated. Emphasis was

* Correspondence to: R. W. Paynter, INRSEnergie et Materiaux,1650 boulevard Lionel-Boulet, Varennes, Quebec, Canada J3X 1S2.

Contract/grant sponsor: Natural Sciences and Engineering ResearchCouncil of Canada.

placed on evidence for the formation of amide groups onpolymers exposed to both oxygen and nitrogen plasmas.In this paper, we concentrate on the changes in the surfacecomposition, following storage in air, of the PS and PETsamples exposed to nitrogen and oxygen plasmas in com-bination. In order to seek an understanding of the changestaking place, the PS samples exposed to just one oxygen,helium or nitrogen plasma were also re-evaluated.

EXPERIMENTAL

Coupons were cut from PS and PET films obtained fromGoodfellow Corporation, washed with ultrasonic agitationin isopropanol, rinsed in methanol and dried. The XPSanalyses performed on freshly prepared samples showedthe PS to be slightly oxidized at 97.8% carbon and 2.2%oxygen, with 73.3% carbon, 0.06% nitrogen and 26.6%oxygen being observed for PET.

The r.f. plasma reactor in which the polymer couponswere exposed has been described elsewhere.14 The sam-ples were held at 5 cm from the powered electrode andexposed to a plasma excited at 13.56 MHz for timesranging up to 16 min. The operating conditions were asfollows: plasma power 15 W, gas flow rate 100 sccm, gaspressure 200 mTorr. Three working gases were employed:helium, 10% oxygen/90% helium and 33% nitrogen/67%hydrogen (pre-mixed and used as-received from Praxair).Oxygen and nitrogen plasmas are known to produce amore hydrophilic surface, and the helium plasma wasused to evaluate the possibility of the incorporation ofoxygen and nitrogen from the ambient. The oxygen wasmixed with helium to reduce the risk of combustion withpump oil, and hydrogen was mixed with the nitrogen inan attempt to saturate the carbon–nitrogen bonds formedon the polymer surfaces.

Following the plasma treatment, the samples weretransferred in air for XPS analysis. The delay between

CCC 0142–2421/2000/010056–09 $17.50 Received 27 June 1999Copyright 2000 John Wiley & Sons, Ltd. Revised 30 September 1999; Accepted 30 September 1999

AGEING OF PLASMA-TREATED POLYMER SURFACES 57

preparation of the sample and the initial XPS analysiswas typically <5 min. The XPS analyses were per-formed in a VG 220iXL spectrometer by means of non-monochromatic Al K x-radiation in the ‘large area’ lensmode, without additional charge compensation. The sur-face composition was obtained from measurement of theareas of the C 1s, N 1s and O 1s peaks, obtained at 20 eVpass energy in CAE mode, at a photoemission angle of 0°

(i.e. normal to the sample surface.)Curve-fitting was performed on the C 1s peaks follow-

ing subtraction of a ‘Shirley’-type background. A Gaus-sian–Lorentzian cross-product was employed for the peakshape, and the least-squares fitting halted when the com-puted chi-squared value converged to six significant digits.Only the component peak separations were constrained,with each component peak having an identical peak shapeand width.

The C 1s envelopes obtained from both the polystyreneand PET samples were fitted with six component peaks.For the polystyrene samples, the positions of the fivepeaks in addition to the dominant ‘C–C’ carbon compo-nent were fixed atC1.5,C2.9,C4.3,C5.0 andC6.6 eVwith respect to the ‘C–C’ component. In terms of car-bon–oxygen bonds, these components can be assignedto C–O, C O, O–C O, O–CO–O and the�Ł �‘shake-up’ peak, respectively. For the PET samples,the positions of the additional components were fixedat C1.59, C2.45, C3.96, C5.90 andC7.24 eV withrespect to the dominant ‘C–C’ component. In bothcases the component peak shift values were calculatedas the average of the binding energy shifts of compo-nent peaks obtained from freely fitting all of the C 1speak envelopes acquired. The peak fitting is thereforenot intended to be a definitive identification of the car-bon–heteroatom bonding states present, because it doesnot allow for the presence of carbon–nitrogen bondsexplicitly.

The samples were stored in closed PS sample boxesin unfiltered laboratory air. A second XPS analysis wasperformed after about 360 or 490 days, as noted inthe figure captions. The experimental parameters wereidentical to those employed in the first round of XPSanalyses. The apparent composition was compared withthe measurements taken immediately following exposureof the samples to plasma.

RESULTS

Figure 1 shows the O 1s, N 1s and fitted C 1s peaksobtained from a PS sample 5 min after exposure to anitrogen/hydrogen plasma for 4 min. The binding energyscale has been adjusted in order to place the principalcomponent peak in the C 1s envelope at 284.6 eV. Theoxygen and nitrogen peaks are broad and appear to beslightly asymmetric. The carbon peak is highly asymmet-ric, exhibiting a spectrum of functionalities introduced bythe plasma treatment. Although the PS surface must con-tain carbon–nitrogen bonds to account for the quantityof nitrogen observed in this case (11.9% at a 0° pho-toemission angle), because of the broadness of the C 1senvelope we have chosen to interpret the carbon spectraby forcing a fit to six nominal carbon–oxygen function-alities as described above. We are therefore obliged to

Figure 1. Core-level spectra obtained from a polystyrene sampleexposed for 4 min to a nitrogen/hydrogen plasma: (a) O 1s;(b) N 1s; (c) fitted C 1s.

treat oxygen and nitrogen atoms as equivalent in theirinfluence on binding energy shift in order to discuss theobserved changes in peak shape. Attempts to includecarbon–nitrogen functionalities explicitly (e.g. C–N atC1.2 eV, C N at C2.36 eV and C N at C3.4 eV)destabilized the fitting procedure.

Copyright 2000 John Wiley & Sons, Ltd. Surf. Interface Anal. 29, 56–64 (2000)

58 R. W. PAYNTER

Polystyrene exposed to a helium plasma

Figure 2 shows the apparent concentrations of oxygen andnitrogen (evaluated by XPS and therefore on a hydrogen-free basis) as a function of helium plasma time, evalu-ated immediately following the treatment and 360 dayslater. Following storage, a slight increase in the nitro-gen concentration and a marked increase in the oxygenconcentration are in evidence.

Figure 3 shows the results of peak-fitting the C 1senvelope obtained at a 0° photoemission angle imme-diately following the plasma treatment [Fig. 3(a)] and360 days later [Fig. 3(b)]. Immediately post-plasma, itcan be seen that the component atC1.5 eV is the domi-nant polar component, representing carbon singly bondedto one oxygen or nitrogen atom. The intensity of this com-ponent scales with the plasma duration. After an appar-ent residence time of 8 s, the component atC2.9 eVstarts to increase in intensity, indicating carbon atomsdoubly bonded to an oxygen or nitrogen atom. At 16 sof plasma time one observes the growth of a compo-nent peak atC4.3 eV, which is indicative of carbonatoms bonded to two oxygen/nitrogen atoms. For plasmatimes up to 16 s, the shake-up component is observedto decrease in intensity, indicating disruption of the ben-zene rings.

Comparing the C 1s peak shape at 360 days with thatobtained immediately post-plasma, the relative intensityof the component atC1.5 eV has not changed very much.However, the intensities of the components atC2.9,C4.3andC5.0 eV have all increased, which is an indication thatthe increase in oxygen concentration noted above is due tothe formation of additional polar groups in which carbon ismultiply bonded to, or bonded to multiple, oxygen atoms.The intensity of the shake-up peak has further decreasedat 360 days.

In summary, the PS samples exposed to the heliumplasma exhibit a clear increase in oxygen content fol-lowing storage, which is principally the consequenceof an increasing concentration of CO and O–C Ofunctionalities.

Figure 2. Apparent nitrogen and oxygen concentration deter-mined by XPS as a function of plasma time for polystyrenecoupons exposed to a helium plasma, measured 5 min and360 days after treatment.

Figure 3. Result of peak-fitting the C 1s envelope obtained ata 0° photoemission angle from polystyrene samples exposedto a helium plasma: (a) immediately post-plasma; (b) followingstorage for 360 days.

Polystyreneexposedto a 33% nitr ogen/67%hydrogenplasma

Comparingthe apparentcompositionof the PS surfaceimmediatelyfollowing exposureto the nitrogen/hydrogenplasmawith that determinedfollowing storage,the XPSresults show that whereas the nitrogen concentrationapparentin the surfacedecreasedover time [Fig. 4(a)],the oxygenconcentrationincreased[Fig. 4(b)]. The dataweretakenfollowing storagefor 489 days,exceptfor thesamplesexposedfor 30 and 60 s, which were evaluatedfollowing a shorterstorageperiodof 374days.Theconti-nuity in thenitrogenconcentrationcurvesuggeststhat theapparentcompositionof thesurfacechangedlittle betweenthe two setsof measurements.

The evolution of the C 1s peakshape(Fig. 5) is lessstraightforwardthan in the caseof a helium plasma.ThecomponentatC1.5 eV showsan initial rapid growth andthen decreasesin intensity beforeonceagain increasingin intensity for very long plasmatimes. The maximumintensity for this componentis obtainedwith a plasmaduration of 2 min. The componentat C2.9 eV exhibitsa rapid increasein intensity with plasmaduration,up to4 min, and then a more gradualincreaseup to 16 min.The componentat C4.3 eV shows a more monotonicincreasein intensity with plasma duration. The effect

Surf. InterfaceAnal. 29, 56–64 (2000) Copyright 2000JohnWiley & Sons,Ltd.


Figure 4. (a) Nitrogen concentration versus plasma time forpolystyrene exposed to a nitrogen/hydrogen plasma, evaluatedimmediately and following storage for 374 or 489 days.(b) Oxygen concentration versus plasma time for polystyreneexposed to a nitrogen/hydrogen plasma, evaluated immediatelyand following storage for 374 or 489 days.

of the nitrogen plasma on the shake-upcomponentiscomparableto that of helium.

Comparingthe C 1s peakshapesafter374 or 489 dayswith thoseobtainedimmediatelypost-plasma,oneobservesanoverall increasein intensityin thecomponentsatC1.5andC4.3eV andin thecomponentatC2.9eV for shorterplasmadurations.For the longer plasmadurations,thecomponentatC5 eV is betterdevelopedfollowing storage.Theadditionaloxygentakenupduringstorageis thereforepresentin single,doubleandmulti-atombondconfigura-tions.A modestdecreasein the shake-upintensity is alsoobserved.

The PS samples exposed to the nitrogen/hydrogenplasmaexhibit a loss of nitrogen and a gain in oxygenfollowing storage.The net result in the C 1s peakshapeis an increasein intensity of the C–X, X –C X andX –CX–X groups(whereX denoteseitheran oxygenora nitrogenatom).

Polystyreneexposedto a 10% oxygen/90%heliumplasma

Comparingthe apparentcompositionof the PS surfaceimmediately following exposureto the oxygen/helium

Figure 5. Result of peak-fitting the C 1s envelope obtained ata 0° photoemission angle from polystyrene samples exposedto a nitrogen/hydrogen plasma: (a) immediately post-plasma;(b) following storage for 374 or 489 days.

plasmawith that determinedfollowing storagefor 376 or492days,theXPSresultsshowanapparentlossof oxygenandslight increasein nitrogenwith storage(Fig. 6), which

Figure 6. Oxygen and nitrogen concentrations versus plasmatime for polystyrene exposed to an oxygen/helium plasma,evaluated after 1 day and after 376 or 492 days.

Copyright 2000JohnWiley & Sons,Ltd. Surf. InterfaceAnal. 29, 56–64 (2000)

60 R. W. PAYNTER

is contrary to that observed on the surface exposed to anitrogen/hydrogen plasma.

Immediately post-plasma, the C 1s peak shape is rel-atively constant no matter what the plasma duration[Fig. 7(a)]. One can hypothesize that the reaction kineticsare so rapid that the initial increases from zero intensity toa plateau value take place within 2 s for all but theC5 eVpeak. Surprisingly, this peak, representing carbon atomsbound to three oxygen atoms, grows to exceed the inten-sity of the component atC4.3 eV, which represents carbonatoms bound to two oxygen atoms. One can hypothesizethat the establishment of plateau values for the componentpeak intensities is indicative of an equilibrium betweenoxidation of the polymer surface and chemical erosion ofthat surface by the oxygen/helium plasma.

Following storage for 376 or 492 days [Fig. 7(b)],the intensity of theC5 eV component (O–CO–O) hasdecreased by 80%, restoring the expected ranking order ofcomponent peak intensity. The intensities of theC1.5 eV(C–O) andC4.3 eV (O–C O) components remain rela-tively unchanged, the intensity of theC2.9 eV component(C O) has decreased a little and, once again, the shake-upcomponent has lost intensity.

The PS samples exposed to the oxygen/helium plasmaexhibit a loss of oxygen and a modest gain in nitrogen

Figure 7. Result of peak-fitting the C 1s envelope obtained at a0° photoemission angle from polystyrene samples exposed toan oxygen/helium plasma: (a) 1 day post-plasma; (b) followingstorage for 376 or 492 days.

following storage.Theoxygenlossappearsto comeprin-cipally from carbonatefunctionalities.

Polystyreneexposedto a 10% oxygen/90%heliumplasma for 30 s and then to a 33% nitr ogen/67%hydrogenplasma

Comparingthe apparentcompositionof the PS surfaceimmediatelyfollowing exposureto the plasmasequencewith that determinedfollowing storage for 365 days,nitrogenappearsto belostovertimeandtheconcentrationof oxygenin the surfaceappearsto increase(Fig. 8).

As notedabove,exposureto theoxygen/heliumplasmaalone [zero nitrogenplasmatime in Fig. 9(a)] producesmore O–CO–O groups than O–C O groups initially,a situation that is reversedupon storage[Fig. 9(b), zeronitrogenplasmatime]. Exposureto a nitrogen/hydrogenplasmafollowing a 30 s exposureto an oxygen/heliumplasma eliminates the excessO–CO–O groups evenbefore storage[Fig. 9(a)]. The componentat C1.5 eVpeaksin intensityat a nitrogenplasmadurationof 1 min,whereasthe componentsat C2.9 and C4.3 eV exhibitan increasein intensity within the 2–4 minute plasmadurationinterval.

Following storage,the componentatC2.9 eV is obser-ved to decreasein intensity for longer nitrogen plasmadurationsand the componentat C4.3 eV increasesinintensity for shorternitrogenplasmadurations.The com-ponentat C1.5 eV remainsrelatively stableand thereissomeloss in intensity in the shake-uppeak.

The PS samplestreatedwith the oxygen plasmaandthenthe nitrogenplasmaexhibit a lossof nitrogenbut anincreasein theoxygenconcentration.For shorternitrogenplasmadurationsthe concentrationof O–C O groupsincreasedon storage,and for longer nitrogen plasmadurationsthe concentrationof C X groupsdecreasedonstorage.

Polystyreneexposedto a 33% nitr ogen/67%hydrogenplasma and then to a 10% oxygen/90%helium plasma for 30 s

The XPSstudiesshow(Fig. 10) that the apparentoxygenconcentration,which is initially higherthanin thesample

Figure 8. Nitrogen and oxygen concentrations as a functionof nitrogen plasma time for polystyrene exposed to an oxy-gen/helium plasma for 30 s and then to a nitrogen/hydrogenplasma, evaluated immediately and after 365 days.



Figure 9. Result of peak-fitting the C 1s envelope obtained at a 0°

photoemission angle from polystyrene samples exposed to anoxygen/helium plasma for 30 s and then to a nitrogen/hydrogenplasma for various times: (a) immediately post-plasma; (b) fol-lowing storage for 365 days.

Figure 10. Nitrogen and oxygen concentrations versus nitrogenplasma time in polystyrene exposed to a nitrogen/hydrogenplasma and then to an oxygen/helium plasma for 30 s, evaluatedimmediately and after storage for 365 days.

treated with oxygen first (cf. Fig. 8), remains stableover the 365 day storageperiod, whereasthe nitrogenconcentrationagainappearsto decrease.

Figure 11. Result of peak-fitting the C 1s envelope obtained ata 0° photoemission angle from polystyrene samples exposed toa nitrogen/hydrogen plasma for various times and then to anoxygen/helium plasma for 30 s: (a) immediately post-plasma;(b) following storage for 365 days.

With the nitrogen/hydrogenplasmaprecedingthe oxy-gen/heliumplasma,the intensityof theC5 eV componentof the C 1s peak[Fig. 11(a)] remainshigh. Additionally,the intensity of the C4.3 eV componentequalsthat ofthe componentatC2.9 eV immediatelypost-plasma.ThecomponentatC1.5 eV is lessintensethan in the caseinwhich the nitrogenplasmawas appliedafter the oxygenplasma[cf. Fig. 9(a)].

Following storage[Fig. 11(b)], there is some loss inintensity in theC1.5 andC4.3 eV componentsat longernitrogenplasmadurations.The intensity of theC2.9 eVcomponentremainsrelatively stable,whereastheC5 eVcomponentis reducedin intensityby¾80%.Theintensityof the shake-upcomponentis alsoreduced.

ThePSsamplestreatedwith anitrogenplasmaandthenan oxygenplasmaexhibit a modestloss of nitrogenfol-lowing storage,particularlyfrom groupsin which carbonatomsareboundto threenitrogenand/oroxygenatoms.

Polyethyleneterephthalateexposedto a 10% oxygen/90% helium plasma for 30 s and then to a 33%nitr ogen/67%hydrogenplasma

The XPS results(Fig. 12) showthat the apparentoxygenconcentrationtendsto a plateauvaluearound25%.This is


62 R. W. PAYNTER

Figure 12. Nitrogen and oxygen concentrations versus nitrogenplasma time in polyethyleneterephthalate exposed to an oxy-gen/helium plasma for 30 s and then to a nitrogen/hydrogenplasma, evaluated immediately and after 356 days.

lower thanthe theoreticalvalueof 28.6%for virgin PETand lower even than the value observedexperimentally(26.6%). The apparentnitrogen concentrationis againobservedto decreaseover the 365 day storageperiod.

With respect to the C 1s peak shape [Fig. 13(a)],increasingthe nitrogenplasmadurationtendsto increase

Figure 13. Result of peak-fitting the C 1s envelope obtained ata 0° photoemission angle from polyethyleneterephthalate sam-ples exposed to an oxygen/helium plasma for 30 s and then toa nitrogen/hydrogen plasma for various times: (a) immediatelypost-plasma; (b) following storage for 356 days.

Figure 14. Nitrogen and oxygen concentrations versus nitrogenplasma time for polyethyleneterephthalate exposed to a nitro-gen/hydrogen plasma and then to an oxygen/helium plasma for30 s, evaluated immediately and after 356 days.

the intensity of the C2.45 eV component(representingcarbon doubly bonded to oxygen or nitrogen) at theexpenseof the componentsat C1.59 and C3.96 eV(representingthe C–O and O–C O carbonspresentinthe formal PET structure).

Following storage,the major changeis a lossof inten-sity in thecomponentatC1.59eV [Fig. 13(b)], represent-ing groupsin which carbonatomsare boundto a singlenitrogenor oxygenatom.

Polyethyleneterephthalateexposedto a 33%nitr ogen/67%hydrogenplasma and then to a 10%oxygen/90%helium for plasma 30 s

The XPS studies show that over the 365 day storageperiod, oxygen in excessof 25–26% appearsto be lost(Fig. 14) and the apparentnitrogen concentrationalsodecreases.

With thenitrogenplasmaprecedingtheoxygenplasma,the C 1s componentpeak at C3.96 eV is more intensethan the peak at C1.59 eV [Fig. 15(a)], which is con-trary to thatobservedwith theorderof treatmentreversed[cf. Fig. 13(a)]. In conformity with the observationreportedin the previoussection,nonethe less,the inten-sity of thecomponentatC1.59eV is observedto decreasewith increasingnitrogenplasmadurationandtheintensityof theC2.45eV componentincreases.

Following storage [Fig. 15(b)], the intensity of theC1.59eV componentis relatively stable.The othercom-ponentsdecreasein intensity, particularly the compo-nent peak at C3.96 eV, which representscarbonatomsbound to multiple oxygen and/or nitrogen atoms,espe-cially X –C X groups.

DISCUSSION

It is interesting to note that, upon storage, those PSsamplesexposedto plasmasfor at least 30 s tend toan apparentoxygen concentrationin the neighbourhoodof 12–14%, regardlessof the compositionof the plasmagas. Therefore,those samplestreatedwith plasmasnotcontaining oxygen, that initially introduce (presumably



Figure 15. Result of peak-fitting the C 1s envelope obtained ata 0° photoemission angle from polyethyleneterephthalate sam-ples exposed to a nitrogen/hydrogen plasma for various timesand then to an oxygen/helium plasma for 30 s: (a) immediatelypost-plasma; (b) following storage for 356 days.

parasitic)oxygen in concentrationsinferior to 12–14%,see the apparentoxygen concentrationrise over time.Conversely,thosesamplesinto which oxygen has beenintroducedat concentrationsin excessof 12–14%exhibitan apparentlossof oxygenover time.

To a lesserextent,the sameis true of nitrogen.Thosesamplestreatedwith nitrogenplasmasexhibit anapparentlossof nitrogenwith time, whereasthosesamplestreatedonly with helium or oxygenexhibit a modestincreaseinthe apparentnitrogenconcentration.

In thecaseof PET,theconcentrationof nitrogenatomsin the surfaceis observedto decreasefollowing storage.

In all casesin which PS was exposedto plasma,the�Ł � ‘shake-up’ peak,characteristicof intact benzenerings, lost intensity during storage.We do not feel thatthe variety of changesin the C 1s peakshapesobservedis supportiveof the notion that the decreasein the inten-sity of the shake-upcomponentis due to the adsorptionof aliphaticcontaminationover time. Onemustaskif theexposureto x-rays during the initial XPS analysisinflu-encedthe evolutionof the samplesurfaceduring storage.A washedPS coupon,not exposedto any plasma,wasanalysedby XPS and then again following storagefor360 days.It canbe seenin Table1 that the modification

Table 1. Apparent composition and C 1s peak-fitting sum-mary for a washedpolystyrenesample,not exposedto any plasma, examined by XPS and then againfollowing storagefor 360 days

Intensity of Intensity ofC1.5 eV C 1s C6.6 eV C 1s

Oxygen Nitrogen component component(at. %) (at. %) (% of C 1s peak) (% of C 1s peak)

Virgin sample 2.2 NDa 3.5 3.4Sample followingstorage for 360 days 3.0 0.2 4.4 3.0

a ND = not detected.

in thesamplesurfacebroughtaboutby the initial analysisis minor whencomparedto the changesseenin the sam-ples exposedto plasmas.It would appearthat the initialexposureto plasmas,with the attendantexposureto ions,neutralsand UV radiation, causeslatent damageto thepolymerthat takesseveralmonthsto manifestitself in thesurfacecompositionandstructure.

In the original study madeof thesepolymersshortlyafter exposureto plasma,we plotted the polar carbonfraction obtained from peak-fitting the C 1s envelopeagainst the .O C N//C ratio obtained from the O 1s,N 1s and C 1s peak intensities(Fig. 16). Thosepointsthat lie on the line of unity sloperepresentsituationsinwhich, on average,eachnitrogenor oxygenatom bondsto just one carbonatom. Points lying to the left of theline representsituationsin which,on average,oxygenandnitrogenatomsbond to more than one carbonatom andpointson theright of theline representsituationsin which,on average,carbonatomsbondedto oxygenor nitrogenatomsbond to more than one hetero-atom.Plotting thedata obtainedfollowing storagefor 360 days (Fig. 17)indicatesan increasedtendencyfor the carbonatomstoassociatewith more than one oxygen and/or nitrogenatom. Further studieswill be required to elucidatetheeffect, if any, of this consolidationon surfacepropertiessuchasthe watercontactangle.

Figure 16. The fraction of the C 1s envelope associated withpolar carbon to oxygen/nitrogen bonds (determined by peakfitting) versus the .OCN//C ratio (determined from the elementalpeak intensities) for the polymers exposed to plasmas containingoxygen, nitrogen and helium, evaluated immediately followingthe plasma treatment.


64 R. W. PAYNTER

Figure 17. The fraction of the C 1s envelope associated withpolar carbon to oxygen/nitrogen bonds (determined by peakfitting) versus the .OCN//C ratio (determined from the elementalpeak intensities) for the polymers exposed to plasmas containingoxygen, nitrogen and helium, evaluated about 360 or 490 daysafter the plasma treatment.

CONCLUSION

The apparentsurfacecomposition,determinedby XPS,of PS and PET samplesexposedto plasmascontaininghelium,nitrogenandoxygenhasbeenobservedto evolvefollowing storagein air. The observedchangesin surfacecompositionwere found to be in excessof the changesobservedon a control sample not exposedto plasma.Peak-fittingthe C 1s envelopeallowed the characteriza-tion of the less-stablesurfacechemicalgroups in eachcase.No overall patternemerged from this study; evolu-tion of the surfacechemicalcompositiondependeduponthe plasmagas employed,and even the order of appli-cationof oxygenandnitrogenplasmashada discernableinfluence.

Acknowledgement

Partof this work was supportedby a grant from the Natural SciencesandEngineeringResearchCouncil of Canada.

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XPS studies of the ageing of plasma-treated polymer surfaces

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