Surface & Coatings Technology 205 (2011) 3356–3364

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Nitrogen plasma functionalization of low density polyethylene

Carmen López-Santos a,⁎, Francisco Yubero a,1, José Cotrino a,b,1, Agustín R. González-Elipe a,1

a Instituto de Ciencia de Materiales de Sevilla (CSIC-Univ. Sevilla), Avda. Américo Vespucio 49, 41092 Sevilla, Spainb Física Teórica, Departamento de Física Atómica, Molecular y Nuclear, Facultad de Física (Univ. de Sevilla), Avda. Reina Mercedes, 41012, Sevilla, Spain

⁎ Corresponding author. Tel.: +34 954489500; fax: +E-mail address: mclopez@icmse.csic.es (C. López-Sa

1 Tel.: +34 954489500; fax: +34 954460665.

0257-8972/$ – see front matter © 2010 Elsevier B.V. Adoi:10.1016/j.surfcoat.2010.11.038

a b s t r a c t

a r t i c l e i n f o

Article history:Received 28 July 2010Accepted in revised form 16 November 2010Available online 26 November 2010

Keywords:Nitrogen plasmaLDPE surfaceAmine groupsWettingAFM

Low density polyethylene (LDPE) films have been treated with different nitrogen containing plasmas with thepurpose of incorporating nitrogen functional groups on its surface and analyzing the changes experienced intheir surface tension. Effects of a dielectric barrier discharge (DBD) at atmospheric pressure and a microwavedischarge (MW) at reduced pressure are compared with those obtained by using an atom source suppliedwith N2 and mixtures Ar+NH3 as plasma gas. X-ray photoelectron spectroscopy (XPS) analysis has providedinformation about the chemical surface changes whereas the surface topography of the treated samples hasbeen examined by atomic force microscopy (AFM). Non-destructive depth profiles of oxygen and carbon havebeen obtained for the treated and one month aged samples by means of the non-destructive Tougaard'smethod of XPS background analysis. Generally, an oxygen enrichment of the deeper region of treated LDPEsurfaces has been observed. Chemical derivatization of the treated samples has shown that a DBD plasmawitha mixture of Ar+NH3 was the most efficient treatment for nitrogen and amine group functionalization. It isargued that the high concentration of NH* species in this plasma is themost important factor in enhancing thenitrogen functionalization of this polymer. It has been also found that the observed increase in hydrophilicityand surface tension cannot be attributed to the anchored nitrogen functional groups formed on plasmatreated LDPE. Differences in the plasma activation behaviour of LDPE and that of other polymers subjected tosimilar treatments are stressed.

34 954460665.ntos).

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© 2010 Elsevier B.V. All rights reserved.

1. Introduction

Most applications of polymeric materials require the modificationof their surface properties. Plasma techniques have demonstrated tobe quite efficient for this purpose, since they provide an efficientcontrol of wetting properties and improve surface adhesion [1–6]. In aplasma process, gas molecules are dissociated into ions, electrons, freeradicals and neutral species. The interaction of these species with thepolymer surface causes a series of chemical and/or physical changeswhich usually extend through a surface layer of up to 10 nm thickness[3,6,7]. Plasmas of nitrogen are usually employed to incorporatenitrogen functional groups like –NH2 on the surface of the treatedpolymers [7–12], aiming at modifying their bioactive or biocideactivity [13–15].

Due to their exceptional bulk properties (low density, flexibility,and high chemical resistance), polyethylene (PE) is very much usedfor industrial and medical applications, although its low surfaceenergy and the low or null concentration of functional groups on itssurface may result in incomplete adhesion and poor chemical re-activity [16]. Trying to get around these drawbacks, plasma activationof PE, particularly with regard to its nitrogen functionalization, has

been an active area of research during the last years [16–18]. Theseprevious works have shown that low pressure ammonia plasmas areefficient to incorporate surface amine groups, while simultaneouslyinducing other side effects like an increased hydrophilicity andchanges in roughness. Improving the efficiency of nitrogen plasmafunctionalization and describing the aging effects after plasmatreatment constitute a first motivation of the present work.

In addition, trying to understand the effect of the different speciesof the plasma in these surface functionalization processes, we proposea comparative study of the effect of different types of plasmas andgases on the surface functionalization of LDPE. Our study focuses onthe incorporation of nitrogen functional groups, particularly aminegroups and on the determination of the changes in the chemical depthdistribution through the first surface layers of the treated specimen. Arelated problem is to determine to which extent the chemical changesinduced by the nitrogen plasmas are the cause of the modification ofthe surface tension and wettability of the plasma treated polymers.This point, of great importance for the development of biomaterials, isstill controversial since no clear answer exists to the question ofwhether nitrogen functional groups or other side effects of the plasmatreatments are responsible for the induced increase in surfacehydrophilicity [19,20]. This work is the continuation of previous in-vestigations of our group where we have intended a similar approachwith polyethylene terephthalate (PET) and DLC thin films [21,22].Differences with respect to these two materials are highlighted here,

3357C. López-Santos et al. / Surface & Coatings Technology 205 (2011) 3356–3364

emphasizing the importance of chemical etching processes in thecontrol of the functionalization processes in LDPE.

As analysis tools we have used X-ray photoemission spectroscopy(XPS) to assess the surface chemical composition, water andiodomethane contact angle (WCA) measurements for estimation ofthe surface tension [23,24] and atomic force microscopy (AFM) for thecharacterization of the surface topography. These characterizationstudies have been carried out just after the plasma treatments or afterthe storage of the samples for one month (i.e., to verify their agingbehaviour). A very important issue is the assessment of the in-depthatom distribution after different plasma treatments. With this aim, wehave used the XPS peak shape analysis developed by Tougaard[25,26]. This analysis has provided a semiquantitative description ofthe oxygen/carbon depth distributions for both the “treated” and“aged” samples. This method has been previously used by our groupto determine the oxygen in-depth distribution in PET and LDPEexposed to different oxygen plasmas [27] and in PET treated withnitrogen plasmas [21]. In agreement with these previous works wehave found here that the changes in atom composition not only occurat the upmost surface layers but also extend to deeper zones up tosome tenths of angstroms.

2. Experimental

2.1. Materials

Commercial films of LDPE (0.5 mm thickness) from GOODFELLOW(Goodfellow Ltd, Cambridge) are exposed to the different plasmatreatments. Previously, the filmswere cleanedwith ethanol to removepossible surface contamination. After this cleaning treatment, we stilldetected some O and N atoms at the surface of the polymer. Most ofthese contamination atoms up to a level of 1% or less were readilyremoved from the surface by the exposure of the samples to a beam ofAr atoms of ~1 eV for 5 min under ultrahigh vacuum conditions (seethe following discussion). This proved the surface character of thesespecies and their easy removal by amild surface treatment. In the text,the samples after cleaning with ethanol will be designed as untreatedor original samples.

2.2. Plasma treatments

To get a deep insight into the role played by neutral, ion and atomspecies in the plasma activation of LDPE we have used three differentmethods of surface activation. These procedures include low pressureMW plasma, atmospheric pressure DBD plasma and a beam of neutralspecies containing nitrogen. A full account of the experimental detailscan be found in our previous publications [21,22,27].

In the low pressure MW plasma reactor, consisting of a quartz tubewith a funnel shape termination attached to a stainless-steel chamber, asurfatron [28] launcher at 60 W of microwave power generates adischargewith a typical density of ionic species of about 1011 cm−3 [29].N2 (17 sccm) or mixtures Ar+NH3 with Ar as majority component(40 sccm) and NH3 (1.2 sccm) as minority component are used asplasma gases, working at a pressure of 40 Pa. Before the plasmaactivation treatments, the discharges of the different gasmixtures weremaintained ignited for 30 min to ensure that no traces of air remain inthe reaction chamber. By opening a shutter, the polymer surfaces wereexposed to the plasma discharge for 1 min, a time that was sufficient toachieve steady state conditions.

The DBD reactor consists of two active stainless steel cylindricalelectrodes with a diameter of 1 cm and a plane parallel configuration.LDPE samples were placed on the bottom electrode, where it acted asa dielectric barrier. The distance between the upper metallic electrodeand the polymer was fixed at 1 mm. The discharge was generated byan AC high voltage power supply working at a typical voltage of 8 kVand a frequency of 1.5 kHz, with an applied power of 30 W. Formation

of small filaments was observed with an oscilloscope connected inparallel between the two electrodes [27]. At a submicron scale thesefilaments may be the cause of a certain surface heterogeneity.N2 (47 sccm) or mixtures of Ar (76 sccm) and NH3 (2 sccm) wereemployed as plasma gases. The purity of the plasma gas within thereactor was checked by analysis of the outlet gases by massspectrometry. LDPE surfaces were exposed to the plasma dischargeduring 1 min. It is expected that the density of ionic species in thissystem has an approximate value of 109 cm−3 [30].

Plasma treated samples were stored in a desiccator in air atroom temperature for different periods of time until their surfacecharacterization.

2.3. Treatments with neutral species

These treatments were made “in situ” in the same XPS device usedfor the analysis of the samples treated “ex-situ” by DBD and MWplasmas. The beam of neutral atoms/species is generated with anOxford OSPrey source (Oxford, UK) supplied with pure nitrogen orammonia at a pressure of 0.01 Pa. Details of the main operatingparameters of this source can be found in our previous publications[21,22,27]. From these previous analyses the dose of species comingfrom the source can be estimated in 5×1013atoms cm−2s−1. In thecase of ammonia, it was found by mass spectrometry that species ofhydrogen, nitrogen and different types of H and NHx neutral species(all of them designed as NHx species in the figures) impinge on thesurface of the polymer. The base pressure in this pre-treatmentchamberwas 2.7×10−6 Pa. Long exposure times of up to 15 minwereused in this experiment to ensure that the surface of samples reachthe steady state.

2.4. Analysis methods

The surface tension and wetting properties of the treated polymerwere determined by measuring the wetting contact angles (WCA) ofwater and iodomethane [23,24]. The roughness was assessed by AFMand the chemical and in-depth compositions by XPS before and afterthe polymer was exposed to the different treatments or after severalaging times. Here, only a brief account about these experimentaldetails will be provided. A more thorough description can be found inRefs. [21,22,27].

XPS spectra were obtained with a PHOIBOS-100 5MCD (SPECS)electron spectrometer with an un-monochromatized Mg Kα radiationasexcitation source. The spectrawere acquiredatnormal take-off anglesin the constant pass energymode fixed at a value of 20 eV. The workingpressurewas about 5×10−9 mbar. For calibration of the binding energy(BE) scale, a value of 285.0 eVwas assigned to the C1s signal componentcorresponding to C–H and C–C bonds [31]. The average surfacecomposition was obtained from the area behind the C1s, O1s and N1speaks corrected with their corresponding sensitivity factors.

The concentration of amino groups incorporated onto the treatedsurfaces was estimated by vapour phase chemical derivatizationaccording to the method of Chevallier et al. [32] as used by us inprevious works [21,22,27]. The standard procedure consisted of theexposure of the treated samples to chlorobenzaldehyde vapours for aperiod of 5 h and a temperature of 50 °C. Under these conditions,preliminary test experiments showed that the developed Cl2p signalreached its maximum intensity and the residual N1s signal intensity ashape that did not change after longer reaction times. We thereforeconcluded that most –NH2 have reacted after the standard derivati-zation treatment.

The following specific reaction involving amino groups occurs atthe surface:

Cl–C6H4–CHO þ NH2–½R�→Cl–C6H4–CH ¼ N–½R� þ H2O ð1Þ

Fig. 2. O1s photoemission spectra of LDPE subjected to the indicated surface activationtreatments by plasmas and beams of neutral species (black line) and for these filmsafter storage for one month (gray dotted line).

3358 C. López-Santos et al. / Surface & Coatings Technology 205 (2011) 3356–3364

so that the chlorine concentration can be correlated with the amountof amino groups that have reacted according to Eq. (1) by using theexpression [33]:

½ΝΗ2� =½Cl�

½C�−7½Cl� : ð2Þ

XPS peak shape analysis using the QUASES software was applied toanalyze the in-depth distribution of C and O atoms within the upmostsurface regions of the samples before and after plasma treatments[34–36]. Wide scans of C1s and O1s peaks including their inelasticbackgroundwere studied. However, the relatively low intensity of thenitrogen signal did not allow to carry out a similar analysis with thispeak.

Static WCA measurements of the contact angle of deionized andbidistilled water and iodomethane droplets (5 μl) were made by theYoung method with a CAM100 instrument (KSV Instruments Ltd,Finland). The mean value of a minimum of seven measurements oneach examined surface is taken as its WCA value. Polar and non-polarcontributions to the surface tension were estimated according to theOwens–Wendt–Kaelble approximation [37].

Changes on the treated surface topography were studied with aNanotec AFM microscope and a Dulcinea electronics in tapping mode.AFM images were taken with high frequency levers and analyzed bythe WSxM software. The roughness is expressed through the RMSparameter.

3. Results

3.1. Standard XPS analysis of functional groups after plasma and beamtreatments

Chemical changes on the treated LDPE surface were monitored byXPS analysis. Figs. 1–3 present the C1s, O1s and N1s spectra of samplesjust after the different treatments and after their storage for onemonth.Meanwhile, Table 1 presents the surface composition determined withthis technique for LDPE subjected to the different plasma and beamtreatments.

The C1s spectra of the untreated LDPE consist of a peak associatedto the –C–H/–C–C bonds at 285.0 eV of binding energy and a smallshoulder at higher binding energies [3,31,38]. Untreated LDPEpresents some oxygen and nitrogen impurities on its surface thatmust be attributed to the surface contamination of the polymer during

Fig. 1. C1s photoemission spectra of LDPE subjected to the indicated surface activationtreatments by plasmas and beams of neutral species (black line) and for these filmsafter storage for one month (gray dotted line).

its manufacture and posterior manipulation and cleaning procedure inthe laboratory [8]. This contamination generates the observed shoulderin the C1s spectra. This shoulder increased slightly in intensity after thetreatment of the untreated samples with the different plasmas. Such anincrease has been usually attributed to the surface incorporation ofoxygen and/or nitrogen functional groups characterized by differentbinding energies: C–N/C–O (285.9 eV), C≡N (286.7 eV), N–CjO/CjO(287.6 eV) and N–C(jO)–N/O–CjO (288.9 eV) [31,39–42]. Because ofthe superposition in this spectral region of the possible contribution ofquite different functional groups of carbon bonded to oxygen and/ornitrogen, no fitting analysis will be intended here.

The observed increase in the C1s shoulder intensity is accompa-nied by a significant incorporation of oxygen at the surface as revealedby the O1s spectra in Fig. 2 and the surface atomic compositionsgathered in Table 1. The incorporation of oxygen functional groupsduring the nitrogen plasma treatment of LDPE must be attributed tothe effect of excited species of oxygen always present in conventionalplasma reactors. These species have been previously characterized byoptical emission spectroscopy (OES) analysis in a previous work [22].The data in Fig. 2 and Table 1 reveal that DBD/N2 and MW/N2 plasmasincorporate greater amounts of oxygen than the other treatments. TheO1s signals obtained after the different treatments present a quite

Fig. 3. N1s photoemission spectra of LDPE subjected to the indicated surface activationtreatments by plasmas and beams of neutral species (black line) and for these filmsafter storage for one month (gray dotted line)

Table 1Atomic percentages in LDPE subjected to the indicated treatments. In parenthesis,percentage of chlorine determined after derivatization.

Sample/treatment

1 h (%) (±0.5%) 1 month (%) (±0.5%)

C O N C O N

LDPE untreated 93.4 4.5 2.1MW N2 65.3 30.1 4.7(0.7) 70.1 27.7 2.2DBD N2 62.8 32.1 5.1(0.5) 73.4 25.3 1.3Nx species 86.9 7.7 5.4(0.0) 89.1 6.3 4.6MW Ar+NH3 81.7 14.3 4.0(0.5) 83.7 12.7 3.6DBD Ar+NH3 66.7 23.2 10.1(1.5) 67.3 24.3 8.4NHx species 85.5 9.9 4.6(0.8) 88.6 8.5 2.9

originalMW N2

DBD N2

Nx species

MW Ar+NH3

DBD Ar+NH3

NHx species

0

1

2

3

NH

2 (%

)

- NH2 concentration

Fig. 4. Amine concentration of LDPE surface subjected to the indicated surfaceactivation treatments by plasmas and beams of neutral species (black) and for thesefilms after storage for one month (gray).

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broad shape and maxima at BE positions ranging from 531 to 533 eV.In this BE region there can be different contributions attributed to O–C, CjO or N–CjO groups [31,43]. Due to the poor resolution of thepeak shape no detailed analysis of these spectra will be intended here.

Incorporation of nitrogen onto the surface during these treatmentsis evidenced by the development of an XPS signal with a BE comprisedbetween 399 eV and 400 eV (Fig. 3). From the evaluation of the areaof these peaks, the atomic percentages of nitrogen summarized inTable 1 are obtained. The width of the recorded signals lies between2.4 and 2.5 eV (in contrast with the C1s main peak at 285.0 eV char-acterized by a width of ca. 1.6 eV) and suggests that the obtainedspectra are the contribution of several types of functional groups.According to literature, nitrogen species with BEs around this energyvalue can be attributed to C–N/C–NH2 and C N/N–C O groups at398.9 eV and 400.2 eV, respectively [31,41,44]. Since no clearshoulders/bands can be identified in the experimental spectra nofitting analysis has been intended here. Nonetheless, a qualitativeassessment of the functional groups is possible by following theevolution of the centroid of the experimental band [9]. XPS signal witha BE around 398.9 eV suggests formation of amine-like species. Onthe other hand, bands appearing at higher BEs would indicate theformation of imide, amide and other related nitrogen species. The N1sspectra in Fig. 3 evidence a certain shift towards 398.9 eV after thetreatments with MW Ar+NH3 or DBD N2 plasmas and the exposureto the beam of neutral species. It is thus likely that in these samplesthe relative concentration of amine groups is higher. In addition, onthe DBD N2 plasma treated sample the broadening of the peaktowards higher BEs suggests the formation of –NO like species char-acterized by a BE of 401.5 eV [41]. According to the spectra in Fig. 3and the data in Table 1, DBD Ar+NH3 plasma is the most efficienttreatment for nitrogen incorporation in LDPE, where a N/C ratio of 15%is reached. Owing to the large width of the obtained signal, it is likelythat various chemical forms of nitrogen are formed in this sample.

According to Fig. 3 and Table 1, after aging, the treated LDPE surfacesundergo important losses of nitrogen, especially for the samples treatedwith nitrogen plasmas. The oxygen concentration in these samples alsodecreased after one month of storage. By contrast, it is noticeable thatthe nitrogen concentration at the surface is little affected in the agedsamples treated with the Ar+NH3 plasmas. A similar behaviour isobtained with the samples treated with the neutral beams of N or NHx.

3.2. Formation of amino groups determined by surface derivatization

The incorporation of amino surface groups can be assessedspecifically by means of the exposure of the plasma treated LDPE tovapours of chlorobenzaldehyde. According to Chevallier et al. [32] thesurface concentration of chlorine after reaction (1) enables an es-timation of the surface concentration of amino groups before reaction.In general, per each reacted amino group seven carbon atoms anda chlorine atom of the chlorobenzaldehyde molecule becomeincorporated onto the LDPE surface. This fact was verified as thesurface concentration of nitrogen atoms decreased and that of carbonatoms increased after derivatization.

Percentages of chlorine atoms shown in parenthesis in Table 1indicate that the plasma treatments at atmospheric pressure, inparticular the DBD Ar+NH3, produce the maximum concentration of–NH2 surface groups, followed by the MW plasma treatments and theexposure to the beam of nitrogen/hydrogen neutral species. Bycontrast, no chlorine atoms could be detected on the LDPE sampleexposed to the beam of nitrogen atoms despite the relatively highconcentration of incorporated nitrogen.

Fig. 4 shows the obtained –NH2 concentrations formed on eachsample: The reported values have been obtained according toexpression (2). A maximum amine concentration, close to 2% of allsurface atoms, is reached after the DBD Ar+NH3 plasma treatment.Meanwhile, concentrations higher than 1% were obtained after thetreatments with the MW N2 plasma and the beam of NHx species. Bycontrast, the efficiency for amine incorporation was smaller after thetreatment with the DBD N2 and MW Ar+NH3 plasmas. Since noamine could be detected on the sample exposed to the beam ofnitrogen atoms, we must conclude that this treatment produces othertypes of nitrogen species and/or amine groups which are not exposedat the external surface. After storage for one month, most activatedLDPE surfaces lost all the amine groups from the surface, with theexception of the ammonia plasma treated samples where the aminegroup concentration was still higher than 0.5 at.%.

3.3. Oxygen and carbon in-depth distributions from XPS peak shapeanalysis

According to the Tougaard's concepts it is possible to examine theatomic in-depth concentration profiles by XPS peak shape analysis toestimate atom depth distributions [34,35]. This technique has beenpreviously used by us to study oxygen depth profiles in PET and LDPEexposed to different plasmas of oxygen [27] or nitrogen [21]. Inagreement with other studies in the literature [23], the obtainedresults showed that oxygen can penetrate up to several nanometersinto the outmost surface layers of the plasma activated polymers. Wehave applied the XPS peak shape analysis to the oxygen and carbonphotoemission signals to estimate their in-depth distribution in theactivated LDPE. We also tried to apply this method to determinenitrogen depth profiles, but the low intensity of the N1s signal and theabsence of significant differences in the backgrounds behind the N1speak precluded the use of this technique to assess depth distributionprofiles of this element.

The method is based on the simulation of the inelastic lossesinduced by the transport of photoelectrons through a given material[34,35]. To do that, it is considered that a primary spectrum F(E)obtained just after excitation by the X-rays transforms into themeasured spectrum J(E) because of the energy losses underwent bythe primary electrons during their transport through the analyzed

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material. For the untreated LDPE sample, it is assumed that the F(E)carbon and oxygen functions result from a homogeneous in-depthdistribution of these elements in such a way that the probabilityfor electronic energy losses leading to the experimental J(E) isrepresented by the polymer inelastic scattering cross sectionscollected in the QUASES software package [36].

As an example, Fig. 5 shows the differences in the shape of thebackgrounds and the derived depth profiles wide scan spectracorresponding to the O1s signal of LDPE before and after being exposedto the DBD Ar+NH3 plasma. The spectra have been normalized to theintensity of the elastic O1s peaks to clearly see the changes in theinelastic backgrounds. The depth profiles obtained with the QUASESsoftware are also represented in this figure. The background of theplasma treated LDPE presents a higher intensity than the untreatedsample. The oxygen depth profile of the untreated LDPE is taken as flat,which is obtained after the better simulation of its XPS inelasticbackground.Meanwhile, the oxygen distribution after plasma treatmentcauses a generalized increase in the oxygen concentration with anenrichment at the inner regionsof the surface region, at least through thedepth probed by the background analysis (i.e., approximately 6–10 nm).

By a systematic analysis of the XPS photoelectron spectral shapeaccording to the aforementioned methodology, carbon and oxygendepth profiles have been studied for all samples. Fig. 6 shows theseprofiles for LDPE just after being subjected to the different treatmentsand after their storage for one month. For LDPE subjected to both DBDandMWplasmas, the profiles reveal a relative increase of oxygen in thewhole upmost surface regionwith a tendency to concentrate in the innerpart of the probed zone. Except for the case of the DBD Ar+NH3 plasmatreatment, this distribution is accompanied by a decrease in the relativeconcentration of carbon from the surface topmost layers. It is importantto note that because of the incorporation of nitrogen in the treatedsamples (cf. Table 1) the oxygen and carbon profiles do not necessarilysum 100 at.%. When the samples were stored for one month, theirconcentration profiles tend to flatten, although the relative concentra-tion of oxygen was still much higher than in the untreated sample.

The oxygen and carbon depth profiles of LDPE exposed to thebeams of neutral species depict a different behaviour. In general, wecan appreciate in Fig. 6 that very little oxygen becomes incorporatedin the treated surfaces and that the oxygen concentration relativelydecreases at the most external surface when LDPE was exposed to thenitrogen species but remains almost constant through the wholethickness when LDPE was exposed to the beam of NHx species.

Fig. 5. Normalized O1s wide simulated spectra of the untreated LDPE and of thissample treated with the DBD Ar–NH3 plasma and aging treated sample with theircorresponding depth profiles.

Meanwhile, the carbon profile is almost flat through the entire probeddepth. Similarly to the case of LDPE exposed to the different plasmas,the profiles in Fig. 6 for the beam treated polymer recover theiroriginal flat shape after aging for one month. We must also note thatthese results are not contradictory with the quantitative evaluation ofthe oxygen content reported in Table 1, since the values in this tableare averaged percentages corresponding to the whole samplethickness from which the elastic photoelectrons are ejected (theestimated mean free path of the O1s photo-electrons in polymers isabout 23 Å [22]).

3.4. Surface topography of plasma treated LDPE

Plasma chemical functionalization of polymers is usually accom-panied with a modification in their surface topography. AFM analysisof the plasma treated samples is carried out to appreciate thesechanges. Fig. 7 shows images corresponding to LDPE before and afterbeing subjected to the different surface activation treatments. Lineprofiles taken along the marked lines are included in the figure. Thedifferent surface topographies observable in the reported images andtheir different color scales clearly support that roughness is stronglydependent on the type of plasma treatment. The RMS valuessummarized in Table 2 confirm these differences.

It can be assessed from the RMS values in Table 2 that this polymeris rather flat and that the roughness noticeably increases for thenitrogen plasma treated samples as comparedwith those treated withammonia plasmas. Samples treated with the DBDN2 plasma develop asurface topography with well defined valleys or holes whoseformation can be related with the impingement on the surface ofmicrofilaments which are typical of this type of discharge [45].Meanwhile, the DBD Ar+NH3 plasma induces the formation of aseries of small agglomerates on the surface. Strobel et al. [45] havereported that DBD plasmas of oxygen produce similar surface featuresthat they attributed to the agglomeration of low molecular weightpolymeric fragments resulting from the partial oxidation of theamorphous component of the polymer. In a previous publication, wehave reported a similar behaviour for LDPE exposed to a DBD O2

plasma [27] and for PET exposed to DBD N2 or Ar+NH3 plasmas. Incomparison, MW Ar+NH3 plasmas and the beam of neutral speciesare less aggressive as it can be appreciated by examining the resultingLDPE surface topography. A further assessment of the topographicchanges is provided by the Bearing plots reported in Fig. 8, where it isclearly appreciable that the Nx species and the DBD N2 plasmas arequite efficient in inducing the formation of high surface features.

3.5. Surface tension of plasma and beam treated LDPE surfaces

The hydrophilic state of the LDPE surface changes after the dif-ferent plasma/beam treatments, as well as with the aging time. Fig. 9shows the evolution of the water wetting angles represented as arelative change with respect to the value of the untreated sample(θi=92°). The obtained values for the plasma treated samples revealnegative changes that amounted to roughly 50% of θi. The decreasewas slightly smaller after the MW Ar+NH3 plasma treatment (i.e.approximately −33%). For all plasma treated samples the wettingangle recovered partially with the aging time although, after onemonth, this parameter was still smaller than in the untreated sample.This figure also shows that the wetting angle for LDPE subjected to theNHx and N beam treatments only changes by 16% and 5%, respectively,and that these samples completely recover their original state afteraging for one week.

The changes of the surface tension estimated by means of themethod of Owens–Wendt–Kaelble [37] from the static contact anglesof water and iodomethane are also presented in Fig. 9. The surfacetension increased after the different plasma treatments (from about28 mNm−1 for the untreated LDPE to around 50 mNm−1), whereas

Fig. 6. Calculated depth profiles of oxygen and carbon for LDPE subjected to the different plasma and beam treatments. Continuous full gray and black color profiles correspond to theuntreated and plasma and beam treated samples, respectively, while the dashed profiles correspond to the samples aged for one month.

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the exposure to the beam of neutral species only induced littlechanges in the value of this magnitude. Storage of the samples for longperiods of time leads to a recovery of the surface tension to reach thevalue of the untreated LDPE.

4. Discussion

4.1. Nitrogen functionalization and type of plasma/beam activation

The treatment of LDPE with nitrogen and ammonia plasmasproduces the incorporation of new nitrogen functional groups onto its

surface, according to the following efficiency order: DBD Ar+NH3NDBD N2NMWN2NMW Ar+NH3. A comparable efficiency fornitrogen incorporation is also found for the neutral beams of N andNHx species. A significant difference between DBD atmosphericplasmas and low pressure MW plasmas is that, in the former case,neutral active species are more abundant than in the latter. Theopposite happens with regard to the concentration of ionized species[46–49]. This has been effectively verified with our experimental set-ups in previous characterization studies by optical emission spectros-copy (OES) [22]. In this previous work we also found that NH* speciesare preferentially formed in the DBD and, to a lesser extent, in MW

Fig. 8. Bearing plots of the AFM images of the LDPE surface subjected to the indicatedtreatments.

Fig. 7. AFM images of the LDPE surface subjected to the indicated treatments.

3362 C. López-Santos et al. / Surface & Coatings Technology 205 (2011) 3356–3364

ammonia-containing plasmas, while N2* and N2+ species form in the

DBD and, preferentially, MW nitrogen plasmas. In agreement withprevious works [48,49], these findings indicate that NH* species,majority in the DBD Ar+NH3 plasmas, are more efficient than othernitrogen species for the nitrogen functionalization of polymers. Theefficiency of neutral NH* species for this purpose is further supportedby the results here with the beams of neutral species. In this ex-periment the absence of ions or other excitation factors like UV

Table 2RMS values of LDPE subjected to the indicated treatments.

Sample/treatment RMS/nm (1 μm×1 μm)

LDPE untreated 2.1MW N2 3.9DBD N2 15.4Nx species 3.3MW Ar+NH3 1.6DBD Ar+NH3 5.0NHx species 2.1

photons, quite abundant in conventional plasmas, supports theactive role of the neutral NH* species in the surface functionalizationprocesses.

In addition, the derivatization experiments (cf. Table 1) indicatethat ammonia containing plasmas, particularly those based on DBDprocesses, are highly efficient for the incorporation of amine groupsonto the surface of LDPE. In comparisonwith previous results, it is alsorelevant that amine incorporation on LDPE is significantly moreefficient than on PET exposed to the same type of nitrogen plasma/beam treatments [21]. Since amine formation on the surface of poly-mers occurs through C–H bond breaking and hydrogen abstractionmechanism [49], the higher efficiency for amine incorporation foundin LDPE indicates that these basic processes are more favoured on ahighly hydrogenated polymer like LDPE than in others like PET with ahigh concentration of oxygen in its structure.

4.2. Incorporation of oxygen, depth profiles and aging processes

Previous OES analysis of the MW and DBD plasmas used in thepresent work showed that small peaks due to O* and OH* species aredetected under our working conditions [22]. Thus, the increase inoxygen surface concentration on the plasma treated LDPE is to berelated with the presence of impurity species of oxygen whoseformation is practically unavoidable in most practical plasma reactors.This assumption is supported by the fact that the beam activationexperiments, carried out under ultrahigh vacuum conditions, do notproduce any significant increase in the surface concentration of thiselement (cf. Table 1). For LDPE surfaces treated with MW and DBDplasmas, an additional factor contributing to the incorporation ofoxygen functional groups can be the irradiation with UV photons thatcan penetrate deeper into the material and contribute to someoxidation reactions through the breaking of C–C and C–H bonds andthe generation of very reactive radicals. The oxygen depth profilesreported in Fig. 6 agree with this view as they show a significantenrichment of oxygen in the inner surface layers of the plasma treatedpolymers. This is also supported by the results of the treatments withthe beams of neutral species which do not produce an equivalentenrichment in this element in the topmost surface layers (in the caseof the treatment with the nitrogen beam, oxygen is even depletedfrom the topmost surface layers as evidenced by the correspondingdepth profile in Fig. 6). These results contrast with the effect of similarplasma and beam treatments on PET where, in all cases, oxygenenrichment occurred in the upmost surface layer [21]. This differenceclearly proves that the polymer structure has a clear influence on boththe mechanisms of surface functionalization and in the diffusionprocesses of foreign species into the activated materials.

Depth profiles have also been used to account for the evolution ofthe atom distribution upon aging. The analysis of the aged LDPEsamples reveals a homogenization of the depth distribution which

Fig. 9. (Top) Relative change of the water contact angle of LDPE subjected to the indicated plasma and beam treatments and then aged for the indicated period of time (θi=92°).(Bottom) Surface tension of LDPE just after its surface activation by plasma and beam treatments and after storage for one month.

3363C. López-Santos et al. / Surface & Coatings Technology 205 (2011) 3356–3364

now approaches the flat profiles of the untreated sample. A similareffect was observed in PET, even if in that case the initial situation wascharacterized by an enrichment of oxygen in the topmost surfacelayers [21]. We believe that in the two cases the rearrangement of thepolymer chains is the cause of the changes in the concentration offunctional groups through the external layers of the treated polymers[16,17].

4.3. Hydrophilicity of surfaces and amine functional groups

The changes in chemical composition and surface roughness (cf.Fig. 7) resulting from the different plasma/beam treatments of LDPEshould have a definitive influence into the modification of the surfaceenergy of this material. An important point in this regard is todetermine whether the amine and/or the oxygen functional groupsincorporated onto the surface are responsible for the modification ofthe surface energy after plasma treatments. This point is controversialas most plasma activation experiments aiming at the aminefunctionalization of polymer surfaces end up with the simultaneousincorporation of oxygen functional groups [42]. A related openquestion is the solubility of amine groups in water and othersolvents. For example, Girard-Lauriault et al. [50] have shown thatseveral N-containing plasma polymers can become soluble in waterand other polar solvents such as ethanol, if a high amine concentrationis achieved on their surface. This effect has been related with the highsurface energy of these functionalized materials towards polarsolvents and to a relatively low degree of surface polymerisationand cross-linking. An exception to that behaviour was that of N-richpolyethylene which was insoluble in water and alcohols.

According to the WCA results in Fig. 9, the samples exposed to thebeam of neutral species of nitrogen do not modify significantly theirWCA through variations in the polar component of the surfacetension, even if a relatively high concentration of nitrogen and aminegroups can be detected on their surface. This supports that the newoxygen functional groups incorporated onto the surface during the

conventional plasma treatments are mainly responsible for thismacroscopic modification of the surface properties. Although furtherstudies are still necessary (e.g., to discard the effect of the differentsurface topographies on the WCA variations or to ensure that theamine groups on the surface are not affected/removed when thepolymer is brought in contact with the water droplets used for WCAmeasurements [50]), we can conclude that, contrary to what has beenpreviously claimed in literature [16,19,41,42], nitrogen functionalgroups, including amine groups, do not induce a significant surfacehydrophilicity. In a similar line of reasoning, it can be tentativelyproposed that the partial solubility of nitrogen plasma treatedpolymers reported by Girard-Lauriault et al. [50] can be due to thesimultaneous incorporation of oxygen functional groups on thepolymer surfaces.

5. Conclusions

The previous results and discussion have shown that nitrogen andammonia plasma activation of LDPE are effective to incorporatenitrogen surface groups onto its surface and that Ar+NH3 plasmas,particularly based on DBD processes, are the most efficient for thispurpose. Surface functionalization of LDPE in conventional low pres-sure or atmospheric plasma reactors produces not only the incorpo-ration of nitrogen surface groups, but also an increase of the oxygenconcentration in the outmost surface regions of the treated polymers.Background shape analysis has been used to assess depth distributionprofiles in plasma and beam treated LDPE. This analysis has revealedthat the oxygen distribution through the upmost surface regionsseems to be dependent of the structure and composition of the poly-mer [21,27]. In LDPE the topmost surface appears relatively depletedin oxygen with respect to inner zones of the treated material.Incorporation of nitrogen functional groups without increasing theoverall concentration of oxygen in the surface region is possible underultrahigh vacuum conditions using a source of neutral species (i.e. Nand N–Hx). The obtained results with the beam treated samples

3364 C. López-Santos et al. / Surface & Coatings Technology 205 (2011) 3356–3364

suggest that nitrogen species (among other amine groups, deter-mined by derivatization) do not significantly affect the surface tensionof functionalized polymers and that the main reason for the decreasein WCA found after plasma treatment has to be attributed to thepresence of some oxygen functional groups formed at the externalsurface. In comparison with previous nitrogen functionalizationexperiments carried out with PET [21], the present investigation hasshown that, after being subjected to the same treatments, theconcentration of nitrogen and amine groups formed on LDPE ishigher. A comparative assessment based on the analysis of the oxygendepth profiles also shows that the upmost surface layers of PETbecome enriched in oxygen during the plasma treatments, a resultthat contrasts with that for the same treatments in which oxygenpenetrates efficiently in the inner layers of LDPE and becomesdepleted from its surface.

Acknowledgements

We thank the Ministry of Science and Education of Spain (projectsMAT 2007-65764, MAT2010-18447 and CONSOLIDER INGENIO 2010-CSD2008-00023) and the Junta de Andalucía (projects CTS-5189 andP07-FQM-03298) for financial support.

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