T Dufour

J Minnebo

S Abou Rich

E C Neyts

A Bogaerts

F Reniers

Understanding polyethylene surface functionalization by an atmospheric He/O2 plasma through combined experiments and simulations Authors

High density polyethylene surfaces were exposed to the atmospheric post-discharge of a radiofrequency plasma torch supplied in helium and oxygen. Dynamic water contact angle measurements were performed to evaluate changes in surface hydrophilicity and angle resolved X-ray photoelectron spectroscopy was carried out to identify the functional groups responsible for wettability changes and to study their subsurface depth profiles, up to 9 nm in depth. The reactions leading to the formation of C-O, C=O and O-C=O groups were simulated by molecular dynamics. These simulations demonstrate that impinging oxygen atoms do not react immediately upon impact but rather remain at or close to the surface before eventually reacting. The simulations also explain the release of gaseous species in the ambient environment as well as the ejection of low molecular weight oxidized materials from the surface.

I. Introduction

The wettability of polymer surfaces is of great importance for various industrial applications: deposition of stable coatings, pattern formation in soft lithography, biocompatibility of coated implants and smart surfaces that can respond to physico-chemical changes of the surrounding medium, such as temperature, pH, [START_REF] Shimomura | Bottom-up strategy of materials fabrication: a new trend in nanotechnology of soft materials[END_REF][START_REF] Vandencasteele | Plasma-modified PTFE for biological applications: correlation between protein-resistant properties and surface characteristics[END_REF][START_REF] Hegemann | Plasma treatment of polymers for surface and adhesion improvement Ioniz[END_REF].

It is well established that the wetting properties of polymer surfaces result mostly from chemical interactions and surface roughness, the latter being also considered as mechanical interlocking, in the case of adhesion phenomena [START_REF] Marshall | A review of adhesion science[END_REF][START_REF] Cai | Improvement of surface wettability and interface adhesion of poly-(p-phenylene terephthalamide) by incorporation of the polyamide benzimidazole segment[END_REF][START_REF] Rich | In-depth diffusion of oxygen into LDPE exposed to an Ar-O2 atmospheric post-discharge: a complementary approach between AR-XPS and TOF-SIMS techniques Surf[END_REF][START_REF] Rich | LDPE films treated by an atmospheric Ar-O2 postdischarge: functionalization, etching, degradation and partial recovery of the native wettability state[END_REF][START_REF] Dufour | Competitive and synergistic effects between excimer VUV radiation and O radicals on the etching mechanisms of polyethylene and fluoropolymer surfaces treated by an atmospheric He-O2 post-discharge[END_REF]. In the case of polyethylene, the wettability surface properties are poor due to the low PE surface energy, its chemical inertness and its smooth surface morphology. They can, however, be enhanced by exposing the polymer to a treatment that induces morphological changes by etching and roughening processes, as well as chemical bond cleavage. This last mechanism can lead to: (i) the production of free radicals, which can interact with the environment to produce surface active species, such as polar groups [START_REF] Svorcik | Modification of surface properties of high and low density polyethylene by Ar plasma discharge[END_REF]; (ii) the release of low mass stable degradation products, such as the low molecular weight oxidized materials (LMWOM) [START_REF] Truica-Larasescu | VUV-induced nitriding of polymer surfaces: comparison with plasma treatments in nitrogen[END_REF]; (iii) the formation of a large number of double bonds and extensively cross-linked structures.

Among the treatments already used, irradiation treatments (laser, ion beam, electron beam, UV, X-rays and γ -rays) lead to dramatic modification of polymer surfaces in terms of thickness and structure [START_REF] Pignataro | Physico-chemical modifications of solid surfaces by interaction with ion beams[END_REF]. Svorcik et al. [START_REF] Svorcik | Structure and biocompatibility of ion beam modified polyethylene[END_REF] induced structural changes of polymer surfaces exposed to an energetic ion beam of 40 keV Ar + ions. They proved the production of oxidized structures and conjugated double bonds in the surface, determining a significant oxidation depth as high as 120 nm, using infrared, ultravioletvisible spectroscopies and Rutherford backscattering. However, the fluence of the Ar + ions had to be limited to prevent the prevalence of deoxygenation processes. As an alternative, Truica-Larasescu et al. [START_REF] Truica-Larasescu | VUV-induced nitriding of polymer surfaces: comparison with plasma treatments in nitrogen[END_REF] managed to incorporate high concentrations of bonded nitrogen and oxygen (almost 10 %) on low density polyethylene (LDPE) surfaces by VUV irradiation in ammonia. The surface energy of LDPE was strongly increased and was even higher than the value obtained in the case of a N2 plasma treatment, although the surface became partially damaged due to significant changes in the surface energy.

In contrast, the treatment of polymer by cold plasmas encountered a great success as the modifications could be applied to a lesser depth and under milder conditions [START_REF] Sanchis | Surface modification of low density polyethylene (LDPE) film by low pressure O2 plasma treatment[END_REF][START_REF] Lehocky | Plasma surface modification of polyethylene Colloids Surf[END_REF]. As a result of a low-pressure Ar plasma treatment, Svorcik et al. [START_REF] Svorcik | Modification of surface properties of high and low density polyethylene by Ar plasma discharge[END_REF] obtained texturized high-density polyethylene (HDPE) films with a subsurface oxidation depth of about 25 nm. Carbonyl, carboxyl and amide groups were detected with C=C bonds either in aromatic or in aliphatic groups. Sanchis et al. [START_REF] Sanchis | Surface modification of low density polyethylene (LDPE) film by low pressure O2 plasma treatment[END_REF] exposed LDPE surfaces to a low pressure O2 plasma and showed an improvement in the surface wettability for short exposure times, by promoting the formation of polar groups (mainly carbonyl, carboxyl and hydroxyl groups). Longer exposure times caused slight abrasion on LDPE films evidenced by a small increase in the surface roughness. By means of attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) and X-ray photoelectron spectroscopy (XPS), Lehocky et al. [START_REF] Lehocky | Plasma surface modification of polyethylene Colloids Surf[END_REF] evidenced the formation of carbonyl, carboxyl, ether and peroxide functionalities resulting from an oxidative (oxygen and air) atmospheric pressure RFplasma treatment of HDPE.

Using an atmospheric He-O2 post-discharge generated by a radiofrequency (RF) plasma torch can be regarded as a promising approach offering several advantages: (i) the surface properties of the material can be modified without altering its bulk properties (Young modulus, yield strength, etc.), (ii) plasma treatments are environmentally friendly (no need of chemical reactants commonly used in multiple step processes), (iii) they are dry processes, (iv) working at atmospheric pressure prevents the use of expensive pumping systems and (v) working in the postdischarge regime instead of the discharge ensures a milder treatment than usually attained in low pressure discharges. As observed by Abou Rich et al. [START_REF] Rich | In-depth diffusion of oxygen into LDPE exposed to an Ar-O2 atmospheric post-discharge: a complementary approach between AR-XPS and TOF-SIMS techniques Surf[END_REF] from micrographs obtained by scanning electron microscopy (SEM), LDPE surfaces exposed to a dielectric barrier discharge (DBD) show micro-scale protrusions while the latter appear negligible in the case of an atmospheric post-discharge treatment, for similar plasma conditions.

In the present paper, we study the compositional modifications of the (sub)surface of HDPE exposed to a He or a He-O2 postdischarge by investigating the effects of the treatment time and of the gap (i.e. the distance separating the plasma torch from the polyethylene surface). X-ray photoelectron spectroscopy (XPSchemical surface composition and chemical bonds) and dynamic water contact angle (WCA -hydrophilicity) measurements are performed. The increase in wetting properties achieved by plasma treatment is typically lost over time due to a slow loss of the surface functionalization until a partial recovery of the native wettability. This so-called ageing effect is also studied here. However, the fundamental interpretation of the reaction mechanisms occurring at the post-discharge/polyethylene interface remains difficult in our case, essentially because of the atmospheric pressure and the interaction between the postdischarge and the environment. The experimental work is therefore complemented by simulations in which we only focus on the main reactive species experimentally evidenced in He-O2 atmospheric flowing post-discharges, namely the oxygen radicals [START_REF] Dufour | Competitive and synergistic effects between excimer VUV radiation and O radicals on the etching mechanisms of polyethylene and fluoropolymer surfaces treated by an atmospheric He-O2 post-discharge[END_REF][START_REF] Dufour | Chemical mechanisms inducing a dc current measured in the flowing post-discharge of an RF He-O2 plasma torch Plasma Sources[END_REF]. Atomistic simulations of oxygen atoms impinging on and reacting with a polyethylene surface are carried out, as well as simulations of oxygen atoms already diffused into the (sub)surface layer, in order to highlight the mechanisms occurring at the interface. Furthermore, the plausible reaction pathways between oxygen atoms and polyethylene chains are proposed by calculating their activation barriers with nudged elastic band (NEB) calculations.

II. Experimental setup II.1. Material

In this work, large 1mm thick foils of HDPE were purchased from Goodfellow. The foils were cut into 80 × 15 mm 2 sized samples and subsequently cleaned in pure iso-octane and then in pure methanol. After this cleaning procedure, the samples were exposed to the plasma post-discharge.

II.2. Plasma source features

The sample surfaces were exposed to the post-discharge of an RF atmospheric plasma torch (AtomfloTM 400 LSeries, SurfX Technologies) [START_REF] Gronet | Self-cleaning protective coatings for use with photovoltaic cells Patent US[END_REF]. The controller of this plasma source included an RF generator (27.12 MHz), an auto-tuning matching network and a gas delivery system with two mass-flow controllers to regulate the helium and oxygen gases supplying the plasma source. Helium (carrier gas) was used with the flow rate fixed at 15 L.min -1 , in combination with oxygen (reactive gas) with flow rates ranging from 0 to 200 mL.min -1 . The RF power coupled into the plasma source was fixed at 120 W. As shown in Figure 1, the resulting gas mixture enters through a tube connected to a rectangular housing in which the gas was homogenized by crossing two perforated sheets. The gas then flows around the upper electrode and passes through a slit in the center of the lower electrode. A plasma is struck and maintained between these electrodes by applying an RF power to the upper electrode while the lower electrode is grounded. The slit has an aperture length of 20 mm and a width of 0.8 mm. A thorough background characterization of the mechanisms governing this flowing postdischarge has previously been published [START_REF] Dufour | Chemical mechanisms inducing a dc current measured in the flowing post-discharge of an RF He-O2 plasma torch Plasma Sources[END_REF].

A robotic arm was integrated to the plasma torch, enabling the treatment of large samples located downstream. In all our experiments, the plasma source was only moved in one direction along a scan length of 100 mm at a velocity of 25 mm.s -1 . The gap was varied between 1 mm and 25 mm.

II.3. Diagnostics

A drop shape analyzer (Krüss DSA 100) was employed to measure dynamic contact angles of water drops deposited onto HDPE samples. Advancing and receding WCAs were measured by growing and shrinking the size of a single drop on the HDPE surface, from 0 to 15 μL at a rate of 30 μL.min -1 [START_REF] Strobel | An essay on contact angle measurements Plasma Process[END_REF]. The WCA plotted in this article correspond to the advancing water contact angles (aWCA); rWCA are not plotted for the sake of clarity but also because they are known to be insensitive to the low-energy surface components. Moreover, they are not easily measurable in the case of our plasma-treated surfaces due to the already low values of the advancing WCA [START_REF] Mundo | Comments regarding an essay on contact angle measurements Plasma Process[END_REF].

To evaluate the chemical composition at the surface of the samples, XPS analyses were performed (Physical Electronics PHI-5600). The base pressure in the analytical chamber was ≈ 10 -9 mbar. Spectra were acquired using the Mg anode (Mg Kα line at 1253.6 eV) operating at 300 W. Wide spectra were used to determine the chemical elements present at the HDPE surface [START_REF] Briggs | Practical Surface Analysis, Auger and X-ray Photoelectron Spectroscopy[END_REF]. Narrow-region spectra were used for the chemical study of the C 1s and O 1s peaks and high-resolution spectra were measured to evidence the nature of the bonds between C and O. The values of the pass-energies, number of accumulations, time/step and energy/step parameters are reported in Table 1. The elemental composition was calculated after removal of a Shirley background line and using the sensitivity coefficients: SC = 0.205 and SO = 0.63 [START_REF] Wagner | Handbook of X-ray Photoelectron Spectroscopy[END_REF]. The data were acquired with the AugerScan software and the peak fittings were achieved with the Casa XPS software, with FWHM set to 1.5 eV for all the spectral components. The resulting compositions must be taken as indicative and are used only for comparison between the different plasma treatments (with/without O2). They do not reflect the absolute surface composition [START_REF] Vandencasteele | Plasma-modified polymer surfaces: characterization using XPS[END_REF]. In our case, several analyzed regions on the same sample always led to the same relative compositions. To carry out the angle resolved X-ray photoelectron spectroscopy (ARXPS) measurements, the take-off angle (TOA), defined between the analyzer axis and the surface, was varied by steps: 15°, 25°, 35°, 45°, 55°, 65°, 75°. The angular acceptance and the number of angular channels were 60° and 120°, respectively. In a film of slightly oxidized polyethylene, the photoelectron emitted by an O 1s atom presents a kinetic energy of ≈ 721 eV corresponding to an attenuation length (λ) of 2.5 nm. In the case of a C 1s atom, this electron kinetic energy is estimated to ≈969 eV, which corresponds to λ = 3.1 nm [START_REF] Tanuma | Calculations of electron inelastic mean free paths: V. Data for 14 organic compounds over the 50-2000 eV range[END_REF]. For TOA = 90°, the average XPS analysis depth from which ≈ 63 % of the signal originates could be calculated by considering a single attenuation length. But it is also well established that 95 % of the information obtained by XPS comes from within three attenuation lengths of the surface (3λ). For this reason, the analysis depth (z) is calculated from equation (1), according to which z = 2.8λ for TOA = 75°, while z = 0.8λ for TOA = 15° [START_REF] Settle | Handbook of Instrumental Techniques for Analytical Chemistry[END_REF][START_REF] Watts | An Introduction to Surface Analysis by XPS and AES[END_REF]. Table 2 represents the analysis depths (nm) calculated from equation ( 1) by considering that the intensity on the top surface (with TOA equal to 15°) is the reference intensity, since the XPS analyzer limits are 15° and 75° [START_REF] Chan | Polymer Surface Modification and Characterization[END_REF][START_REF] Jama | Characterization of polyethylene surfaces heated after plasma fluorination Surf[END_REF]:

Wide spectra

Narrow region spectra

High

TOA

𝑧(𝑛𝑚) = 3. 𝜆. sin(𝑇𝑂𝐴) (1)

III. Molecular dynamics and nudged elastic band simulations

Molecular dynamics (MD) is a simulation technique allowing one to follow the time evolution of a set of atoms through space. Interatomic interactions are governed by the interatomic potential used. The negative gradient of this potential yields the forces on the atoms, while their motion is calculated by solving the set of coupled equations of motion using the velocity Verlet algorithm. The simulations in this work utilize the reactive force field (ReaxFF), using parameters for hydrocarbon oxidation, developed by Chenoweth et al. [START_REF] Chenoweth | ReaxFF reactive force field for molecular dynamics simulations of hydrocarbon oxidation[END_REF]. In ReaxFF, several partial energy functions contribute to the total system energy, as given by [START_REF] Van Duin | ReaxFF: a reactive force field for hydrocarbons[END_REF] 𝐸 = 𝐸 𝑏𝑜𝑛𝑑 + 𝐸 𝑜𝑣𝑒𝑟 + 𝐸 𝑢𝑛𝑑𝑒𝑟 + 𝐸 𝑣𝑎𝑙 + 𝐸 𝑝𝑒𝑛 +𝐸 𝑡𝑜𝑟𝑠 + 𝐸 𝑐𝑜𝑛𝑗 + 𝐸 𝑉𝑑𝑊𝑎𝑎𝑙𝑠 + 𝐸 𝐶𝑜𝑢𝑙𝑜𝑚𝑏 [START_REF] Vandencasteele | Plasma-modified PTFE for biological applications: correlation between protein-resistant properties and surface characteristics[END_REF] These terms account for the bond energy between two connected atoms (Ebond), the additional energy caused by the overcoordination (Eover) or under-coordination (Eunder) of an atom, the valence angle energy between triplets of atoms (Eval), the penalty energy associated with two double bonds sharing an atom in a valency angle (Epen), the torsional angle energy between quadruplets of atoms (Etors), the energy of conjugation effects in unsaturated systems (Econj), the van der Waals interaction energy between all pairs of atoms (EvdWaals) and the Coulomb interaction energy between all pairs of atoms (ECoulomb), respectively.

The simulations of a polyethylene system were carried out using the LAMMPS simulation package [START_REF] Plimpton | Fast parallel algorithms for short-range molecular dynamics[END_REF]. Periodic boundary conditions were imposed in all directions. The time step was set to 0.25 fs. The polyethylene structures used in the simulations are built from branched chains that extend through the periodic boundaries in the x and y directions. The structure size was set to 30 Å × 30 Å × 20 Å in the x, y and z directions, respectively. The minimum chain length was set to ten carbon atoms, while the branching parameter was chosen such that a side chain of at least one ethylene unit was placed on approximately one out of every ten carbon atoms of the primary chain. Furthermore, a few ether and alcohol functional groups were added to represent a native oxygen content of 1.2 wt.%. The final polyethylene structures had a density varying from 0.91 to 0.93 g.cm -3 and contained around 2150 atoms. A representative example of a simulated PE structure is shown in Figure 2. In the case of subsurface oxidation, the PE structure fills the whole simulation box to represent a portion of the subsurface, whereas in the case of oxygen atom impact, the simulation box is extended by 30 Å in the z direction, while the positions of the atoms in the bottom 2 Å are kept fixed to mimic the rigidity of the deeper layers. As a result, the PE surface is free to expand into the vacuum at the top of the structure and an interface layer of about 5 Å is formed. Additionally, NEB simulations [START_REF] Henkelman | A climbing image nudged elastic band method for finding saddle points and minimum energy paths[END_REF] were performed on a system consisting of a decane molecule and a single oxygen atom, to calculate the potential energy barriers for some plausible reaction pathways. Decane was chosen as a model for polyethylene because (i) it is large enough to represent a single polyethylene chain and (ii) it is small enough to allow reasonable simulation times.

Figure 2. Side view of the polyethylene structure, as used in the MD simulations for oxygen atom impact (left) and for subsurface layer oxidation (right), with carbon, hydrogen and oxygen atoms represented in grey, white and red, respectively.

IV. Experimental results

IV.1. Influence of the treatment time

The influence of the treatment time on the HDPE samples exposed to the plasma torch can be evaluated in terms of the number of scans. The modifications in surface hydrophilicity as a function of the number of scans are reported in Figure 3 for a torch-tosubstrate distance (gap) of 1 mm, a scan velocity of 25 mm.s -1 , an RF power of 120 W and flow rates of 15 L.min -1 and 200 mL.min -1 for helium and oxygen, respectively. Whatever the exposure time, the surface hydrophilization seems instantaneous since a single scan enables a decrease of the aWCA from 90° (i.e. native WCA) to 35°. Treatments performed with more scans do not significantly influence the hydrophilicity of the surface since the WCA values remain more or less constant, and always lower than 40°. This WCA characterization was complemented by XPS measurements. In Figure 4, the oxygen surface concentration increases from 20 % obtained for a single scan to 32 % after five scans, and then remains more or less constant. For comparison, the native HDPE subsurface was slightly oxidized (almost 3 %; also indicated in Figure 4). To verify whether the oxidation of the HDPE surface corresponds to a functionalization of the surface, the C 1s peak in the XPS measurements was decomposed into several components corresponding to the C-C, C-O (i.e.] ether), C=O (i.e. carbonyl) and O-C=O (i.e. carboxylic) chemical bonds identified at 285.0 eV, 286.5 eV, 288.0 eV and 289.1 eV binding energies [START_REF] Dufour | Competitive and synergistic effects between excimer VUV radiation and O radicals on the etching mechanisms of polyethylene and fluoropolymer surfaces treated by an atmospheric He-O2 post-discharge[END_REF], respectively. In Figure 5, the relative content of these chemical groups is plotted versus the number of scans. The fraction of C-C groups drops from almost 100 % to about 65 % after ten scans and these bonds are replaced by oxygenated groups which increase from a few percents (< 5 %) to approximately 10-15 % after the same number of scans. Subsequently, the fraction of C-C groups remains more or less constant at a value of 60%, while the fractions of C-O, C=O and C-O=O groups also remain more or less constant with values around 15%, 5-10 % and 12-16 %, respectively.

IV.2. Depth profiles of the functional groups

Besides the information about the fractions of these functional groups determined for a TOA fixed at 75°, it is also interesting to investigate their depth profile, to evaluate which functional groups are the most anchored onto the HDPE surface. Figure 6 shows the oxygen depth profiles, both for an untreated and a treated HDPE sample (after five scans). In the case of the untreated (native) HDPE surface, the oxygen concentration drops from almost 5 % to 3 % in the first 8 nm, while for the treated surface, the oxygen concentration decreases from 33 % to 29 %. The main feature is obviously a significant difference in the O-concentration between the treated and the untreated sample. However, in both cases, the variations in relative O-concentration between 0 and 8 nm remain lower than 5 %. These very low variations may be attributed to a too high O2 flow rate (200 mL.min -1 ), which immediately saturates the subsurface of the polymer, although experiments performed with a flow rate of only 20 mL.min -1 give the same results. An O2 flow rate comprised between 1 and 5 mL.min -1 may be required to observe significant variations in O-concentration but they cannot be experimentally obtained since the mass flow meters integrated to the plasma source present a too low accuracy (10 mL. min -1 in step). In Figure 7, the depth profiles of the functional groups are plotted for the same samples as in Figure 6. The profiles of the oxygenated functional groups from the topmost surface until a depth of 9 nm remain almost constant while presenting some local variations. The latter ones could result from randomly oriented distributions of the oxygenated functions. In the untreated sample, the dominant fraction is the C-C bonds (90 % and more), while the remaining fraction (< 10 %) is attributed to C-O and C=O bonds. For the treated surface, the fraction of C-C bonds is still around 55-60%, but the C-O, O-C=O and C=O bonds are now also clearly visible. The most important of these functional groups are C-O and O-C=O (slightly below 20 %), while the C=O bonds have a lower concentration of around 10 %, analogous to the results shown in Figure 5 above.

IV.3. Influence of the gap

The gap (i.e. the distance separating the plasma torch from the polyethylene surface located downstream) is a critical parameter for the treatment of polymer surfaces with a plasma torch. On one hand, if the gap is too small, the polymer surface can reach its melting point in the case of a long treatment time due to the postdischarge heating. On the other hand, if the gap is too large, the interaction of the post-discharge with the environment air is enhanced, thus giving rise to new reactions with new impurity sources, with the risk of limiting the treatment efficiency. Figure 8 indeed shows that the advancing WCA values increase from about 30-40° for a gap of 1 mm, to values around 60°-80° when the gap rises to 25 mm. Hence, they approach the threshold of 90°, corresponding to the advancing WCA of a native HDPE surface. This indicates indeed that a larger gap reduces the treatment efficiency. Furthermore, it is clear from Figure 8 that the aWCA values depend on whether oxygen is mixed with the carrier gas (helium) or not. The addition of 200 mL.min -1 of oxygen to the 15 L.min -1 of helium gas represents a variation of only 1.3% in the global gas flow rate, but the differences in aWCA values measured at a gap of 1mm are about 10° (i.e. 30° versus 40°), and the difference becomes even more significant at a gap of 25 mm, i.e. 82° (without oxygen) compared to 66° (with oxygen). Hence, the treatment with a small fraction of oxygen in the helium gas flow gives rise to clearly lower aWCA values, indicating that more oxygen-containing functional groups are built in, as expected. Plotting the variations in oxygen surface concentration versus the gap is therefore relevant to verify if this surface hydrophilization is correlated with the grafting of oxygenated functional groups. Figure 9 indicates a significant drop in the oxygen concentration when the gap is increased from 1 to 25 mm. Indeed, in the case of a He-O2 post-discharge, the oxygen concentration drops from about 30 % at a gap of 1 mm, towards 8 % at a gap of 25 mm; in the pure He post-discharge, the oxygen concentration drops from 21 % to 5 %, hence approaching the value of 3 %, characteristic for the native HDPE surface. This indicates again that the treatment becomes less efficient for larger gaps. The fact that the He-O2 treated surface shows higher oxygen concentrations is logical, and consistent with the results from Figure 8.

The depth profiles of the C-C, C-O, C=O and O-C=O functional groups are plotted in Figure 10 for a gap of 1 mm and 10 mm. The fractions of these functional groups remain again more or less constant as a function of depth, as was also illustrated in Figure 7. Furthermore, when comparing the results obtained for the two different gaps, it is clear that the fraction of the C-O and C=O groups are fairly independent of the gap (i.e. around 15-20 % and 7-10 %, respectively), while the formation of O-C=O groups seems to be enhanced for the smallest gap (1 mm); as a consequence, the fraction of C-C bonds is somewhat lower in the case of a 1mm gap.

IV.4. Effect of ageing time

It is clear that the atmospheric plasma treatment increases the amount of polar groups on the polyethylene surface, but in order to determine whether this increase is irreversible or not, a dedicated study was carried out to investigate the evolution of the surface wettability over time. The plasma-treated films were subjected to ambient air ageing (70 % relative humidity (RH) and 293 K) to evaluate their hydrophobic recovery time. The samples were not rinsed after plasma treatment and before WCA measurements. Their chemical compositions are plotted as a function of depth in the surface in Figure 11 for a freshly treated film and a film treated one month ago. Figure 11 shows that the oxygen concentration is fairly constant as a function of depth, as is also illustrated above in Figure 6, but a decay in the oxygen concentration from 32 % to 24 % is observed after one month, indicating that the surface hydrophilicity slightly drops again after the plasma treatment. In Figure 12.a, the fractions of the functional groups are plotted as a function of depth, for both the freshly treated film and the film treated one month earlier. It is clear that the fraction of C-C bonds increases, whereas the fractions of O containing bonds decrease with ageing time, but to a different extent. For example, at a depth of 2.4 nm, the fraction of C-C groups increases from 55 % to 66 %, the fractions of C-O and C=O groups drops only slightly from 19 % and 10 % to about 18 % and 6 % respectively, while the fraction of O-C=O groups decreases more significantly from 17 % to 9 %. For the sake of clarity, the variations in the fraction of functional groups are also plotted in Figure 12.b versus the depth as measured by XPS. Hence, it is clear that the fractions of C-O and C=O groups drop only by a few per cent (to maximum 5 %) over one month, irrespective of the depth, while the fraction of O-C=O groups shows a decrease between 7 % and 10 % on the whole subsurface, and the fraction of C-C groups rises considerably, especially at larger depths (i.e. around 10 % at the 2 mm depth, but up to 17 % at the 8 mm depth).

V. Calculation results

According to the experiments, the He-O2 plasma treatment introduces a significant amount of oxygen functional groups on the HDPE surfaces. However, the processes contributing to this result are less evident. Therefore, MD simulations and NEB calculations were performed to obtain a better understanding of the interactions between polyethylene and oxygen atoms, which are assumed to be the main plasma species responsible for surface oxidation.

V.1. MD simulations

First, the reactivity of oxygen atoms upon impact with the polyethylene surface was investigated. The oxygen atoms impinge on the surface with a thermal energy of 0.025 eV, which is a realistic assumption for a post-discharge setup as used in the experiments. In these simulations, the oxygen atoms do not react immediately upon impact with a polyethylene surface (i.e. within the simulation time of 2 ps). Almost 90 % of the impinging oxygen atoms, however, are not reflected from the surface either, but remain at the surface or diffuse slightly into it. These results indicate that the oxygen atoms react only some time after they reach the surface and that they can readily diffuse into the surface layer before reacting.

Second, we also studied the reactivity of oxygen atoms in the subsurface region. Two different polyethylene structures, representing the subsurface, were modified by placing oxygen atoms into them, representing 10 and 15 wt.% oxygen. Within a simulation time of 2 ns at 300 K, 7 % and 17 % of the oxygen atoms had reacted with the polyethylene for the two different polyethylene structures, respectively, while 47 % and 36 % had recombined to form oxygen molecules. However, the formation of new functional groups, as evidenced by the XPS measurements, was not observed within this short timescale. It is clear from the simulations that the reactivity of the polyethylene increases significantly at the site of an initial bond breaking reaction. This leads to the hypothesis that the activation energy for the first bond breaking reaction is higher than for subsequent reactions.

V.2. NEB calculations

In order to verify this hypothesis and to predict which reaction pathways are important for the introduction of functional groups to the polyethylene surface, NEB calculations were performed to calculate the activation energy of the most plausible reactions between an oxygen atom and a decane molecule (see Figure 13).

Reaction (1) depicts the oxidative cleavage of a carbon-carbon bond in a pristine polyethylene chain, reaction (2) illustrates the carbon-oxygen bond breaking of an ether group, and reactions (3) and ( 4) show the carbon-carbon bond breaking near an ether group. These reactions, in combination with the hydrogen abstraction reactions ( 5) and ( 6) at the polyethylene chain or at an alcohol group, respectively, are considered as the initial reactions in this work. Subsequently, reactions ( 7)-( 15) can take place. In reaction [START_REF] Rich | LDPE films treated by an atmospheric Ar-O2 postdischarge: functionalization, etching, degradation and partial recovery of the native wettability state[END_REF], a formaldehyde molecule is split from the chain by a carbon-carbon bond breaking event, while in reactions ( 8) and ( 13), an aldehyde and a ketone, respectively, are formed by a hydrogen abstraction. Further reactions of ketones are not shown in Figure 13, as the activation energies are similar to those of aldehydes and moreover the formation of a ketone was not observed during the simulations. When an aldehyde is produced, it can react with another oxygen atom to form a carboxylic acid, as shown in a single step in reaction [START_REF] Truica-Larasescu | VUV-induced nitriding of polymer surfaces: comparison with plasma treatments in nitrogen[END_REF], or in two steps in reactions [START_REF] Svorcik | Modification of surface properties of high and low density polyethylene by Ar plasma discharge[END_REF] and [START_REF] Pignataro | Physico-chemical modifications of solid surfaces by interaction with ion beams[END_REF]. However, the intermediate product formed in reaction [START_REF] Svorcik | Modification of surface properties of high and low density polyethylene by Ar plasma discharge[END_REF] is not very stable and can easily dissociate, as depicted in reaction [START_REF] Svorcik | Structure and biocompatibility of ion beam modified polyethylene[END_REF], to split the oxidized carbon atom from the chain. Finally, reactions ( 14) and ( 15) depict the hydrogen abstraction from a second decane molecule, by an oxygen radical attached at the end or in the middle of the chain, respectively, to form an alcohol.

Figure 13. Activation energies in eV for reaction pathways between an oxygen atom and a decane molecule or a derivated compound.

The values obtained from the NEB calculations confirm that an initial carbon-carbon bond breaking reaction increases the reactivity of the polyethylene chain at that site. For example, the potential energy barriers for the first and second carbon-carbon bond breaking (reactions (1) and ( 7)) are 0.45 eV and 0.10 eV, respectively. When comparing the potential energy barrier for reaction [START_REF] Shimomura | Bottom-up strategy of materials fabrication: a new trend in nanotechnology of soft materials[END_REF] with the barrier obtained from discrete Fourier transform (DFT) calculations [START_REF] Gindulyte | Can hydrocarbon chains be disrupted by fast O(3P) atoms?[END_REF], our calculated energy barrier is quite low (0.45 versus 1.95 eV), but the relative order of the energy barriers, at least for reactions (1) and ( 5), was found to be consistent with DFT simulations. For the other reactions, no DFT calculations could be found in the literature. From the calculated activation energy and the chemical structure of polyethylene, it is expected that the most important initiation reaction is the hydrogen abstraction (reaction ( 5)). However, this reaction was only observed a single time during the MD simulations. Instead, the carbon-carbon bond breaking (reaction (1)) was observed as the prevalent initiation reaction. Since NEB calculations are static simulations and MD simulations are dynamic, this discrepancy in reactivity can be attributed to the dynamic steric hindrance for the hydrogen abstraction, due to the rotation of the methylene group, away from the approaching oxygen atom.

For subsequent reactions, the activation energy is lower for a second carbon-carbon bond breaking (reaction [START_REF] Rich | LDPE films treated by an atmospheric Ar-O2 postdischarge: functionalization, etching, degradation and partial recovery of the native wettability state[END_REF]) than for a hydrogen abstraction at the activated site (reaction [START_REF] Dufour | Competitive and synergistic effects between excimer VUV radiation and O radicals on the etching mechanisms of polyethylene and fluoropolymer surfaces treated by an atmospheric He-O2 post-discharge[END_REF]). In the MD simulations, reaction [START_REF] Rich | LDPE films treated by an atmospheric Ar-O2 postdischarge: functionalization, etching, degradation and partial recovery of the native wettability state[END_REF] was indeed found to occur more often, with the result that a formaldehyde molecule was split from the chain, which was then further oxidized to carbon dioxide. When reaction (8) did occur, the aldehyde was quickly further oxidized through the addition of another oxygen atom (reaction ( 9)) and subsequently split from the chain via reaction [START_REF] Svorcik | Structure and biocompatibility of ion beam modified polyethylene[END_REF], resulting in the loss of the functional group. According to the calculated activation energies, the aldehyde could as well be oxidized to a carboxylic acid (reaction [START_REF] Truica-Larasescu | VUV-induced nitriding of polymer surfaces: comparison with plasma treatments in nitrogen[END_REF]), but this reaction mechanism was not observed in the MD simulations, which may have been due to the limited timescale.

Finally, the formation of alcohol groups was not observed through the proposed reaction mechanisms ( 15) and ( 16) involving a hydrogen abstraction, despite their relatively low activation energy. This can (in part) be explained by the fact that these reactions involve two decane molecules, which represent two different polyethylene chains. As such, the probability for these reactions to occur is low due to geometrical constraints on the reactive oxygen radical, which thus imposes an entropic barrier on this reaction.

VI. Combining the results from experiments and simulations

The results from the MD simulations and NEB calculations correlate very well with the experimental results presented in Figure 5. The initial reactions (1)-( 6) between oxygen atoms and polyethylene are the main production channels of either hydroxyl radicals (which can either recombine with other radicals or be released to the ambient atmosphere (reactions ( 5) and ( 6))), or oxygen radicals, which are connected with a single bond to the polyethylene chain (reactions (1), ( 2), (3), ( 4)).

Such a C-O bond is indeed the most abundant functional group according to the XPS measurements (see Figure 5). Since the MD simulations have shown that the oxygen radicals are almost exclusively located at the end of chain, reactions [START_REF] Rich | LDPE films treated by an atmospheric Ar-O2 postdischarge: functionalization, etching, degradation and partial recovery of the native wettability state[END_REF] and ( 8) seem the most important for further oxidation, both leading to the formation of a carbonyl bond. One could therefore expect the increase of the carbonyl bonds to be correlated with a decrease of the ether groups. This is however not observed in Figure 5, since the plasma torch continuously provides O radicals to the polyethylene chains, thus simultaneously producing ether groups according to reactions (1)-(4). In the case of reaction [START_REF] Rich | LDPE films treated by an atmospheric Ar-O2 postdischarge: functionalization, etching, degradation and partial recovery of the native wettability state[END_REF], the formaldehyde molecule that is produced can be further oxidized to a volatile carbon dioxide molecule in multiple steps, as observed in the MD simulations (as explained above) and disappear from the surface. In the case of reaction [START_REF] Dufour | Competitive and synergistic effects between excimer VUV radiation and O radicals on the etching mechanisms of polyethylene and fluoropolymer surfaces treated by an atmospheric He-O2 post-discharge[END_REF], the formed aldehyde is not very stable in the oxidative environment during plasma treatment and can react with another oxygen atom, leading to the formation of a carboxylic acid as suggested in reaction [START_REF] Truica-Larasescu | VUV-induced nitriding of polymer surfaces: comparison with plasma treatments in nitrogen[END_REF], or again to the loss of the functional group (reactions ( 9) and ( 12)). Therefore, the very low activation energy of reaction (10) of 0.09 eV can explain why the XPS measurements show more O-C=O (i.e. carboxyl) than C=O (carbonyl) groups in Figure 5. Indeed, most of the carbonyl groups are expected to be aldehydes, which are likely to be further oxidized to carboxyl groups. Reaction [START_REF] Pignataro | Physico-chemical modifications of solid surfaces by interaction with ion beams[END_REF] could also explain the formation of carboxyl groups, which however depends on reactions ( 8) and ( 9), but the associated activation energy is quite high (1.00 eV). This explanation also accounts for the large difference in the fraction of carboxyl groups when varying the gap, as observed in Figure 10. When the gap is reduced from 10 mm to 1 mm, the amount of oxygen atoms that reach the surface increases, meaning that the surface oxidation also increases. This in turn gives rise to a higher reaction rate of all the reactions that require a new oxygen atom, of which the most important for the surface composition (i.e. for the production of a new functional group) are reactions (1), ( 8) and [START_REF] Truica-Larasescu | VUV-induced nitriding of polymer surfaces: comparison with plasma treatments in nitrogen[END_REF]. The formation of ketones by reaction ( 13) is less important because most of the oxygen radicals are located at the end of a chain, and the formation of carboxylic groups by reaction [START_REF] Pignataro | Physico-chemical modifications of solid surfaces by interaction with ion beams[END_REF] is not likely because of its high activation energy. If we assume that these three reactions are all dependent in first order on the concentration of oxygen atoms, the fraction of C-O and C=O bonds will remain constant, while the fraction of O-C=O bonds will increase when the gap is reduced, which is exactly what was measured experimentally (see Figure 10).

Another correlation between experiment and simulation can be drawn from the ageing study presented in Figure 12, albeit less straightforward. A mechanism commonly invoked to explain the hydrophobic recovery over time is a reorientation of the polar groups into the bulk of the polymer. According to Gerenser [START_REF] Gerenser | XPS studies of in situ plasma modified polymer surfaces[END_REF], LMWOMs coming from chain scissions during the plasma treatment could slowly diffuse into the bulk material upon the ageing time, thus contributing to a decrease of the concentration of oxygen on the topmost layers of the film. However, Figure 12 shows that the loss of functional groups is almost uniform in depth. This suggests that the ageing does not take place by a rearrangement of functional groups from the surface to the bulk, because in that case, the concentration of functional groups should decrease in the first few nm, in favor of the deeper layers. Instead, the concentration of functional groups decreases more in the deeper layers than in the top layers of the surface. This measurement is (strong) evidence for an upward movement of the functional groups, which can be explained by the creation of LMWOMs during the plasma treatment that do not diffuse in depth. These small fragments are likely to be volatile, and thus will migrate to the surface and subsequently be lost to the environment. This result can be correlated with the works of Payner [START_REF] Paynter | Angle-resolved XPS study of the effect of X radiation on the aging f polystyrene exposed to an oxygen-argon plasma Surf[END_REF] on polystyrene surfaces exposed to an Ar-O2 plasma, where added oxygen to the surface during the plasma treatment appears not to diffuse into the bulk over time (92 h). Also, the work of Weon [START_REF] Weon | Effects of thermal ageing on mechanical and thermal behaviors of linear low density polyethylene pipe[END_REF] evidences the depletion of antioxidant originally added to LDPE after a thermal ageing of 6000 h.

Our explanation for the ageing process is supported by the simulations. As mentioned above, reactions [START_REF] Rich | LDPE films treated by an atmospheric Ar-O2 postdischarge: functionalization, etching, degradation and partial recovery of the native wettability state[END_REF] and [START_REF] Svorcik | Structure and biocompatibility of ion beam modified polyethylene[END_REF] were frequently observed during the MD simulations, leading directly to the formation of LMWOMs, containing a carbonyl and a carboxyl group, respectively. These fragments can remain trapped in the polyethylene subsurface and be slowly released over time. Indeed, Figure 12 shows a greater loss for carbonyl and carboxyl groups, as opposed to alcohol or ether bonds. This line of thought also applies to fragments consisting of several carbon atoms, which are the result of multiple bond breaking reactions on the same polyethylene chain. Such fragments are likely to contain oxygen functional groups at the ends of the fragment, which will most likely be carbonyl or carboxyl groups as a result of reactions ( 8) and [START_REF] Truica-Larasescu | VUV-induced nitriding of polymer surfaces: comparison with plasma treatments in nitrogen[END_REF], respectively.

VII. Conclusion

The treatment of HPDE samples by a helium-oxygen postdischarge leads to a higher oxygen concentration in the (sub)surface of the sample and therefore also in higher concentrations of oxygenated functional groups, namely C-O, C=O and O-C=O. This functionalization was studied by ARXPS to determine their depth profiles. In the case of an O2 flow rate of 200 mL.min -1 , the treatment time only slightly influences the depth profiles while they are more affected by a gap variation from 1 to 25 mm. The ageing study showed that while the concentration of ether groups remains constant, while the concentration of carbonyl and carboxylic groups drops. The formation of low molecular weight oxidized materials such as formaldehyde could explain the instability of the surface over time, while the rearrangement of chains at the surface was found to be unlikely, based on the ARXPS measurements.

Surface analysis (XPS and WCA) experiments and simulations showed a good correlation and allow one to further understand the mechanisms occurring at the interface between the postdischarge and the polymer. The reactions that were observed in the MD simulations, together with their activation energies as calculated by NEB simulations, can be used to explain the differences in concentration between the different functional groups as measured by XPS. The reactions which are of particular importance for the formation of functional groups at the polyethylene surface are the C-C bond breaking (reaction (1)), the hydrogen abstraction to form an aldehyde (reaction (8)) and the oxidation of an aldehyde to a carboxylic acid (reaction [START_REF] Truica-Larasescu | VUV-induced nitriding of polymer surfaces: comparison with plasma treatments in nitrogen[END_REF]).

Figure 1 .

 1 Figure 1. Cross sectional diagram of the RF-plasma source (Patent US7329608 SurfX Technologies [17]), with the entire setup at the left and the detail of the plasma source at the right.

Figure 3 .

 3 Figure 3. Variation of the aWCA of HDPE surfaces exposed to a He-O2 post-discharge versus the number of scans for (He) = 15 L.min -1 , (O2) = 200 mL.min -1 , PRF = 120 W, gap = 1 mm.

Figure 4 .

 4 Figure 4. Variation of the oxygen surface concentration of HDPE surfaces exposed to a He-O2 post-discharge versus the number of scans for (He) = 15 L.min -1 , (O2) = 200 mL.min -1 , PRF = 120 W, gap = 1 mm, TOA = 75°.

Figure 5 .

 5 Figure 5. Relative content of C-C, C-O, C=O and O-C=O chemical bonds measured by XPS as a function of treatment time expressed as numberof scans, with (He) = 15 L.min -1 , (O2) = 200 mL.min -1 , PRF = 120 W, gap = 1 mm, TOA = 75°.

Figure 6 .

 6 Figure 6. Depth profile of the oxygen concentration in the subsurface of both an untreated HDPE sample, and a treated sample (after five scans), with the same experimental conditions as reported in Figure5.

Figure 7 .

 7 Figure 7. Depth profiles of the C-C, C-O, C=O and O-C=O chemical bonds for a native and a plasma-treated HDPE surface (after five scans), with the same experimental conditions as reported in Figure5.

Figure 8 .

 8 Figure 8. Variation of the aWCA of HDPE surfaces exposed to a He postdischarge, with and without the addition of oxygen, versus the gap between 1mm and 25 mm, for (He) = 15 L.min -1 , (O2) = 0 or 200 mL min -1 , PRF = 120 W, 5 scans.

Figure 9 .

 9 Figure 9. Variation of the oxygen surface concentration of HDPE surfaces exposed to a He post-discharge, with and without the addition of oxygen, versus the gap between 1 and 25 mm, for (He) = 15 L.min -1 , (O2) = 0 or 200 mL.min -1 , PRF = 120 W, 5 scans, TOA = 75°.

Figure 10 .

 10 Figure 10. Depth profile of the C-C, C-O, C=O and O-C=O chemical bonds for plasma-treated HDPE surfaces with a gap of (a) 1mm (b) 10 mm. In both cases, (He) = 15 L.min -1 , (O2) = 200 mL.min -1 , PRF = 120W, 5 scans.

Figure 11 .

 11 Figure 11. Chemical surface composition of a HDPE sample obtained by XPS the same day as its plasma treatment ((He) = 15 L.min -1 , (O2) = 200 mL.min -1 , PRF = 120 W, 5 scans, gap = 1 mm) and one month later.

Figure 12 .

 12 Figure 12. (a) Depth profiles of the functional groups measured the same day as the plasma treatment and one month later. (b) Variations in the depth profiles over one month. The experimental conditions are the same as inFigure 11.

Table 1 . Experimental conditions used for the ARXPS measurements

 1

	-resolution
	spectra

Table 2 . Sampling depth analysis (nm).

 2

		15°	25°	35°	45°	55°	65°	75°
	z (nm)	O 1s 2.0 C 1s 2.4	3.3 3.9	4.4 5.3	5.5 6.5	6.4 7.5	7.1 8.3	7.6 8.9

J. Phys. D: Appl. Phys., 2014, Vol. 47, Issue 22, 224007 (12 pp), http://dx.doi.org/10.1088/0022-3727/47/22/224007

VIII. Acknowledgments

This work was part of the IAP/7 (Inter-university Attraction Pole) program 'PSI-Physical Chemistry of Plasma Surface Interactions', financially supported by the Belgian Federal Office for Science Policy (BELSPO). This work was carried out in part using the Turing HPC infrastructure at the CalcUA core facility of the Universiteit Antwerpen (UA), a division of the Flemish Supercomputer Center VSC, funded by the Hercules Foundation, the Flemish Government (department EWI) and the UA.

IX. References


S Abou Rich

T Dufour

P Leroy

L Nittler

J-J Pireaux

F Reniers

LDPE surface modifications induced by atmospheric plasma torches with linear and showerhead configurations Authors

Low density polyethylene (LDPE) surfaces have been plasma-modified to improve their nanostructural and wettability properties. These modifications can significantly improve the deposition of subsequent layers such as films with specific barrier properties. For this purpose, we compare the treatments induced by two atmospheric plasma torches with different configurations (showerhead vs linear). The modifications of LDPE films in terms of chemical surface composition and surface morphology are evidenced by X-ray photoelectron spectroscopy, water contact angles measurements and atomic force microscopy. A comparison between the two post-discharge treatments is achieved for several torch-tosubstrate distances (gaps), treatment times and oxygen flow rates in terms of etching rate, roughening rate, diffusion of oxygen into the subsurface and hydrophilicity. By correlating these results with the chemical composition of the postdischarges, we identify and compare the species which are responsible for the chemical surface functionalization, the surface roughening and etching.

I. Introduction

Polymers play an essential role in everyday life due to the remarkable diversity of their properties and to successful research aimed at manufacturing products with desired mechanical and chemical properties, tailored for each specific use [START_REF] Siracusa | Biodegradable polymers for food packaging: a review[END_REF]. Among them, polyethylene is a non-polar polymer without specific chemical functionalities and used in a broad spectrum of applications such as food packaging, medical, automotive, aerospace and electronics fields [2][3][4]. In that respect, low density polyethylene (LDPE) can be modified to enhance its wettability properties in order to improve the adherence of subsequent deposited films such as barrier layers for food packaging applications [5].

Plasma treatment is known as a very effective approach to improve the hydrophilicity of polymer surfaces without altering their intrinsic bulk properties towards oxygen and water vapor permeation, since only the outermost atomic layers are modified [6][7]. One may prefer using a plasma operating at atmospheric pressure rather than at low pressure as the constraints linked to a costly high-vacuum system are avoided. Furthermore, the implementation of the process in a continuous production line becomes easier. More specifically, atmospheric flowing postdischarges are particularly suitable for the treatment of soft matter such as polymers since maintaining milder conditions than those of a plasma treatment where the same material would be placed between the two electrodes. Most of the electrons and ions are indeed neutralized in the post-discharge which then mostly supplies excited species and long lifetime radicals.

The activation mechanisms of polyethylene surfaces using plasma treatment are fairly well mastered [8][9][10]. However, the understanding of the diffusion, etching and grafting mechanisms at the plasma/polymer interface is still not well puzzled out, particularly when using atmospheric plasmas with O2 as reactive gas [11][12].

In previous studies [13][14][15] we have corroborated the necessity of using oxygen to improve the hydrophilicity of LDPE and we have suggested an anisotropic etching mechanism occurring in an atmospheric flowing Ar-O2 or He-O2 post-discharge. For instance, we have shown that an Ar-O2 flowing post-discharge treatment induces a decrease in aWCA as a function of treatment time, until a plateau around 40° is reached after 30 s. This plateau, suggesting a saturation of the oxygenated functions (three polar functionalities, i.e. C-O, C=O, and O-C=O) on the surface has lead us to study the diffusion of oxygen into the bulk of the film (subsurface) [13]. We consider here as "subsurface" the sample's region located immediately below the outer surface and that is not in direct contact with the gaseous species of the plasma phase. Etching rates (between 2.7 nm/s and 7.3 nm/s) specific to the Ar-O2 plasma treatment conditions have been determined from mass losses measurements (between 10 µg/cm 2 and 60 µg/cm 2 ) performed as a function of treatment time [14]. A stronger hydrophilicity is obtained for smaller gaps (lower than 15 mm) where -according to optical emission spectroscopy (OES) results -the more energetic species could break bonds and either eject fragments from the surface or create surface radicals [14]. The ageing study has shown that the aWCA of the modified surfaces increases towards a common threshold of 83° after 30 days of storage, thus indicating a partial recovery of the native wettability state (aWCA = 94°).

In the case of a pure He(O2) post-discharge treatment applied to high-density polyethylene samples, same aWCA values as low as 25° have been obtained, whatever the O2 flow rate [15]. The O radicals have been considered as the main reactive species responsible for this wettability since in the pure He post-discharge, we detected mostly O radicals, OH radicals and He metastable atoms while in a He-O2 post-discharge, O radicals, O2 metastable species and O2 + ions were evidenced. In this article, we compare the wettability and nanostructural properties of LDPE surfaces after their exposure to two atmospheric post-discharges generated by two different commercial RF plasma torches [16][17][18][19][20][21]. The first presents a "linear" configuration and uses helium as carrier gas while the second presents a "showerhead" configuration and is supplied with argon. The modifications of LDPE surfaces are investigated by X-ray electron spectroscopy (XPS), drop shape analysis to measure advancing water contact angles (aWCA), atomic force microscopy (AFM) and mass losses measurements. Those results are correlated with the chemical compositions of the post-discharges to identify and compare the gaseous species participating to the surface modifications mechanisms. A comparison between the two plasma treatments is achieved for several torch-to-substrate distances (gaps), treatment times and oxygen flow rates in terms of hydrophilicity, etching rate, roughening rate and diffusion of oxygen into the LDPE subsurface.

II. Experimental setup II.1. Materials & gases

Transparent LDPE films (37 % crystalline) provided by PackOplast-Belgium are used, with a thickness of 40 µm, a surface of 20  20 mm² and a density of 0.93 g/cm 3 . The helium, argon and oxygen gases are provided by the company Air Liquide, under the label Alphagaz TM 1 which certifies the following specifications: H2O < 3ppm vol., O2 < 2 ppm vol. and CnHm < 0.5 ppm vol.

II.2. Plasma sources

The LDPE films are treated using two plasma sources from SurfX Technologies LLC: the Atomflo TM 400L-Series (referenced here as "linear plasma torch") [START_REF]Patent US7329608 Surfx Technologies, Method of processing a substrate[END_REF] and the Atomflo TM -AH-250D (referenced here as "showerhead plasma torch") [START_REF]Patent US20060156983 Surfx Technologies, Low temperature, atmospheric pressure plasma generation and applications[END_REF].

The two plasma devices present common features, namely an RF generator (27.12 MHz), an auto-tuning matching network and a gas delivery system with two mass-flow controllers to regulate the carrier gas (helium or argon) and the reactive gas (here oxygen) supplying the post-discharge. According to the manufacturer's handbooks, the linear torch must be supplied with helium and the showerhead plasma torch with argon as carrier gas, to ensure proper operation. In each configuration, the upper electrode is biased to the RF voltage while the lower is grounded.

In the case of the linear plasma torch (Figure 1a), the gas mixture (helium with/without oxygen) enters through a tube connected to a rectangular housing (55 mm  20 mm  80 mm). Inside, the gas is homogenized through two perforated sheets, then flows down around the left and right edges of the upper electrode and passes through a slit in the center of the lower electrode. The plasma is lighted and maintained between these electrodes by applying a RF voltage to the upper electrode while the lower electrode is grounded. Due to the rectangular geometry of the slit presenting a length of 20 mm and a width of only 0.8 mm (Figure 1c), this plasma torch is commonly described as a "linear plasma torch" with an output area of 16 mm 2 .

The second plasma torch (Figure 1b) consists of two parallel circular electrodes perforated by 126 holes, each one with a critical diameter of 0.6 mm. A 126-holes circular metallic mesh is located parallel and downstream to the two electrodes to homogenize the flowing post-discharge. The gas flow (argon with/without oxygen) is oriented perpendicularly to the two electrodes. As these holes are arranged to form a circular pattern of 20 mm in diameter (Figure 1d), this torch is commonly called "showerhead plasma torch". The output area of this source is evaluated to 35.6 mm² (126.πR²).

Figure 1. Schematic diagrams of the (a) linear plasma torch (supplied in helium) and (b) showerhead plasma torch (supplied in argon). Pictures of the (c) He post-discharge and (d) Ar post-discharge taken below each torch. Schematic tridimensional profiles of the (e) He post-discharge (linear torch) and (f) Ar post-discharge (showerhead torch). Pictures of a copper plate after exposure to (g) the He post-discharge and (h) the Ar post-discharge.

For all the experiments presented in this article, the plasma torches are used about 10 min after their ignition to reach a stable regime and especially a constant gas temperature. Each of these plasma sources is mounted on a robotic arm enabling its motion back and forth along a single direction and thus the treatment of large surface samples located downstream [START_REF] Dufour | [END_REF]. In our study, we have set the scanning length (LS) equal to 150mm and the scanning velocity (vs) to 50 mm/s. The distance between the plasma torch and the upper surface of the LDPE sample is called gap and has been varied between 2 and 30 mm. The number of scans (NS) achieved by the robotic arm has been calculated to obtain the same treatment time with the two torches. For instance, a treatment time of 60 s has been achieved with the linear plasma torch for 150 scans while 75 scans were required with the showerhead plasma torch.

Each of these plasma sources generates a flowing post-discharge with specific features. The characteristic width () of the postdischarge corresponds to its dimension parallel to the scanning direction for a specific value of the gap. Thus, for a gap of 2 mm,  is evaluated to 10 mm for the linear plasma torch and to 20 mm for the showerhead plasma torch, as indicated in Figures 1e and1f. These values have been evaluated by placing a copper plate 2 mm downstream from the He and Ar flowing post-discharge supplied in oxygen and by measuring the dimensions of the two oxidized patterns as depicted in Figures 1g and1h. A 10  25 mm 2 rectangular pattern and a  .(20 mm) 2 /4 circular pattern have been obtained with the linear and showerhead plasma torches respectively. The area of the post-discharge in contact with the sample () depends also on the gap and is represented in Figures 1e and1f. With a gap fixed at 2 mm,  is equal to 250 mm 2 and 314 mm 2 for the He and the Ar plasma torches respectively. As the RF power applied to the two torches is 90 W, the surface plasma power density estimated on  is 0.36 W/mm 2 for the linear plasma torch and 0.28 W/mm 2 for the showerhead plasma torch. As explained in earlier works [START_REF] Dufour | [END_REF], this difference can be considered as negligible since plasma power variations over this range does not induce significant surface modification in terms of aWCA, mass losses, roughness or chemical surface composition [13]. Those values are reported in Table 1. Another feature of these flowing post-discharges concerns the flow rates which have to vary on similar ranges while respecting the manufacturer limitations, as reported in Table 2. The vector gas flow rates have been fixed at 15 L/min in the case of helium and 33.4 L/min in the case of argon so as to maintain a velocity of 15 m/s at the slit in both configurations for the sake of comparison (see Table 1). The O2 flow rate has been varied from 0 to 40 mL.min -1 (helium post-discharge) and from 0 to 80 mL.min -1 (argon post-discharge) so as to keep the same flux (0.94 L/min/mm 2 ) in both configurations, as reported in Table 1. The influence of the reactive gas can therefore be estimated from one torch to the other based on the volume fraction of oxygen (O2), i.e. the ratio of the O2 flow rate by the carrier gas glow rate. For instance, the value O2 = 0.04/15 = 2.6.10 -3 for the He-O2 post-discharge with 40 mL/min of O2 can be compared with the value O2 = 0.08/33.4 = 2.6.10 -3 obtained for the Ar-O2 post-discharge with 80 mL/min of O2.

Parameters & Variables Unit

Linear plasma torch

Showerhead plasma torch

Parameters

Parameters

II.3. Diagnostics

The XPS measurements have been performed with a PHI 5600 photoelectron spectrometer, operating at 300 W with a Mg K Xray source (1253.6 eV), under a vacuum of 9.10 -9 Torr, with an angular acceptance of 60° and a number of angular channels of 120. Pass energies of the survey spectra and high-resolution spectra have been fixed at 93.90 eV and 23.5 eV respectively. The take-off angle (TOA) of the photoelectrons is 45° with respect to the sample normal axis. The C 1s core level at 285.0 eV has been used to calibrate the binding energy scale. The surface elemental composition has been calculated after removal of a Shirley background by using the following sensitivity coefficients: SC = 1.00 and SO = 2.85.

To carry out ARXPS (Angle Resolved X-ray Photoelectron Spectroscopy) measurements, the TOA defined between the axis of the photoelectron collection lens and the sample plane has been varied by steps of 10° as follows: 15°, 25°, 35°, 45°, 55°, 65°, 75° [13,25] . In a film of slightly oxidized polyethylene, the photoelectron emitted by an O 1s atom presents a kinetic energy of ≈ 721 eV corresponding to an attenuation length (λ) of 2.5 nm.

In the case of a C 1s atom, this electron kinetic energy is estimated to ≈ 969 eV, which corresponds to λ = 3.1 nm [26]. For TOA = 90°, the average XPS analysis depth from which ≈ 63 % of the signal originates could be calculated by considering a single attenuation length. But it is also well established that 95 % of the information obtained by XPS comes from within three attenuation lengths of the surface (3λ). For this reason, the analysis depth (z) is calculated from the following equation, according to which z = 2.8λ for TOA = 75°, while z = 0.8λ for TOA = 15° [START_REF]Settle Handbook of Instrumental Techniques for Analytical Chemistry[END_REF][START_REF] Watts | An Introduction to Surface Analysis by XPS and AES[END_REF]. The Table 3 represents the analysis depths (nm) calculated from the equation by considering that the intensity on the top surface (with TOA equal to 15°) is the reference intensity, since the XPS analyzer limits are 15° and 75° [START_REF] Chan | Polymer Surface Modification andCharacterization[END_REF][START_REF] Jama | [END_REF]:

𝑧(𝑛𝑚) = 3𝜆. sin(𝑇𝑂𝐴) TOA 15° 25° 35° 45° 55° 65° 75° z (nm)
O 1s 2.0 3.3 4.4 5.5 6.4 7.1 7.6 C 1s

2.4 3.9 5.3 6.5 7.5 8.3 8.9 Water contact angle (WCA) measurements have been performed using a drop shape analyzer (Krüss DSA 100) in an air-conditioned room and using milli-Q water as working liquid. Advancing (aWCA) and receding (rWCA) angles have been measured by depositing and withdrawing a droplet of 5 µL on the surface. In this article, each value of dynamic WCA corresponds to the average of 5 drops measurements, randomly deposited onto the sample surface.

The mass losses of the plasma-treated LDPE films have been evaluated by employing a Sartorius BA110S Basic series analytical balance, characterized by a 110 g capacity and 0.01mg precision.

The surface roughness has been evaluated using atomic force microscopy (AFM). The device is a Dimension 3100 AFM using a Nanoscope IIIa controller equipped with a phase imaging extender, from Digital Instruments operating in the Tapping-Mode (TM-AFM). Standard silicon tips (Tap300Al, BudgetSensors) with a 42 N/m nominal spring constant and a 300 kHz nominal resonance frequency have been used. All images (5  5 µm² scanning area) have been recorded in air at room temperature with a scan velocity of 1 Hz. Except a second order polynomial function background slope correction, no further filtering has been performed. From these flattened corrected data, the root-meansquared roughness (Rrms) and the maximum topographic height are determined on the flattened 5  5 µm² images.

III. Results

The treatments of LDPE surfaces induced either by a He (with/without O2) or an Ar (with/without O2) flowing postdischarge are compared in four dedicated sections dealing with the influence of the gap (g), the treatment time (t), the addition of reactive gas (O2) and the diffusion of functional groups in the subsurface.

III.1. Influence of the treatment time

The wettability state of a polyethylene surface exposed to a cold plasma generally depends on the treatment time. Figure 2a illustrates the variation of the aWCA versus the treatment time for a gap of 2 mm and 90W of plasma power. A decrease from 94° (untreated LDPE sample) to 37° and 31° is evidenced with the showerhead (Ar) and linear (He) plasma torches respectively. These measurements can be correlated with the O/C ratio where O and C correspond to the relative surface composition of oxygen and carbon determined from the O 1s (532 eV) and C 1s (285 eV) peaks measured by XPS. Figure 2b shows a slight increase in the O/C ratio with the treatment time, thus indicating a gradual enrichment of the surface in oxygen functionalities. The O/C ratios are slightly higher when using the linear plasma torch with helium and as a result the wettability as well. Whatever the plasma torch used, it is noticeable that the O/C ratio tends to reach a plateau for treatment times higher than 30 seconds. The mass losses seem to vary linearly with the plasma exposure time, since a linear fit is obtained with a correlation coefficient of 0.985. As the density of LDPE (LDPE) is 0.93 g/cm 3 , mass losses (expressed in µg/cm 2 ) can easily be converted into an etching rate (expressed in nm/s) defined as:

𝑒 𝑟𝑎𝑡𝑒 = 1 𝜌 𝐿𝐷𝑃𝐸 . ( 𝑚𝑎𝑠𝑠 𝑙𝑜𝑠𝑠 𝑡 )
This averaged rate is evaluated to 9.0 nm/s and 13.9 nm/s after a treatment induced by the linear and showerhead plasma torches respectively.

Rrms values are reported as a function of the treatment time in Figure 3b. The Rrms values are always much higher than the Rrms of the native surface which is only 2.0 ± 0. 0.977, thus allowing the expression of an averaged roughening rate for these treatment times defined as the ratio of the roughness by its corresponding treatment time. This rate is equal to 0.94 nm/s (linear plasma torch with He) and 1.24 nm/s (showerhead plasma torch with Ar). AFM images of an untreated LDPE surface and several plasma-treated LDPE surfaces for different treatment times are also shown in Figure 3c.

III.2. Influence of the gap

The gap is a parameter of interest to determine (i) the critical value beyond which the treatment is no more efficient and (ii) whether impurity sources from the ambient air could contaminate the surface of the treated samples. Figure 4a introduces the values of aWCA as a function of the gap comprised between 2 and 30 mm using the linear and showerhead plasma torches. The two trends are similar since the aWCA increase from 30° to 94° with the linear plasma torch and from 40° to 94° with the showerhead plasma torch. The plateau at 94° obtained for gaps higher than 15 mm corresponds to the native LDPE aWCA and therefore to the fact that the treatment is no more efficient. Below the critical gap of 15 mm, an offset of approximately 8° appears between the two sets of measurements, indicating a treatment more efficient if achieved with the linear plasma torch.

Whatever the configuration of the plasma torch, Figure 4b indicates a decrease in the O/C ratio versus the gap, hence indicating that more oxygen functionalities are grafted on the surface for the shorter gaps. In accordance with Figure 4a, a slight offset between the two curves arises for gaps equal or lower than 15 mm, meaning that the linear plasma torch (pure He) allows a better oxidation of the LDPE surface and therefore a better hydrophilicity state than the showerhead plasma torch (pure Ar). The existence of a plateau for gaps higher than 15 mm is also consistent with the plateau from Figure 4a. To investigate topographical changes of the plasma-treated surfaces, AFM imaging in tapping mode has been achieved as well as mass losses measurements on the treated samples. The Figure 5a introduces the mass losses of the plasma-treated samples and their corresponding etching rate as a function of the gap. For gaps comprised between 2mm and 30 mm, the etching rate decreases from 7.4 nm/s to approximately 0.7 nm/s in the case of the linear plasma torch, while in the showerhead configuration a decrease from 13.9 nm/s to 0.9 nm/s is depicted. It appears that no material has been removed for gaps higher than 15 mm. These etching rates can be compared to the roughening rates calculated by expressing the ratio of Rrms by the treatment time, as shown in Figure 5b where both the roughness and the roughening rate are plotted as a function of the gap. Consistently with the trends depicted in Figure 5a for a gap comprised between 2 and 30 mm, the roughening rate decreases from 1.2 nm/s to 0.1 nm/s (He post-discharge) and from 1.6 nm/s to 0.1 nm/s (Ar postdischarge). Tridimensional AFM images of an untreated LDPE surface and several plasma-treated LDPE surfaces are shown in Figure 5c for different gaps. discharge. However, in absence of this reactive gas, the two wettability states are slightly different, even if much lower than the native wettability state (94°). The O/C ratios from Figure 6b are consistent with the results presented in Figure 6a: the O/C ratios are almost constant and turning around 0.360 when O2 is injected in the plasma torch, while they are slightly lower if no O2 is mixed with the carrier gas: about 0.354 with pure He (in the linear configuration) and 0.336 with pure Ar (in the showerhead configuration).

III.3. Influence of the oxygen volume fraction

Figure 5. (a) Mass losses and etching rate, (b) Root mean squared roughness (Rrms) and roughening rate (c) Tridimensional AFM images (5*5µm²) vs. gap between 2nm and 30nm, for treatments achieved with the linear (He) and showerhead (Ar) plasma torches (t = 30s, PRF = 90W).

Figure 7a shows the etching rate (and mass losses) of the sample after its exposure to the post-discharge as a function of the volume fraction of oxygen. The increase of O2 from 0 to 2.6.10 -3 enhances the etching rate from 9.0 nm/s to 29.5 nm/s (linear plasma torch) and from 13.9 nm/s to 37.3 nm/s (showerhead plasma torch). This raise is correlated with the increase in the roughening rate shown in Figure 7b where on the same O2 range, it increases from 0.94 nm/s to 1.86 nm/s (linear plasma torch) and from 1.24 nm/s to 1.89 nm/s (showerhead plasma torch). The Ar post-discharge treatment resulting from the showerhead configuration makes the surface of the films rougher than the alternative treatment.

IV. Discussion

The discussion is elaborated in four sections: first we remind the main species generated by the two torches on the basis of our previous works achieved using optical emission spectroscopy. Then, a correlation between roughening rate and etching rate is presented, followed by a discussion on the relation between wettability and surface activation. Finally, the subsurface functionalization is discussed through the ARXPS measurements.

IV.1. Reminder on the main species detected by OES in the linear and showerhead plasma torches

The pure He flowing post-discharge generates highly-excited states of He, He metastables, O and OH radicals. Although some VUV are known to be emitted by plasma, the effect of these radiations can be considered as negligible for the treatment of polymers due to the ambient air oxygen absorption [15]. In the pure Ar post-discharge, the main species are highly-excited states of Ar but also OH and O radicals [33-34-35]. Also, simulations performed by Atanasova et al. on the showerhead plasma torch show the presence of Ar2 + ions in the post-discharge few millimeters away from the grounded electrode [36]. Even if these ions could not be directly detected with optical emission spectroscopy, at least a positive current of +200 µA was evidenced in the post-discharge by placing a metallic plate 1 mm downstream of the flow. This current being positive, it represents a flow of positively charged gaseous species that could result from Ar2 + ions. O + , N + , O2 + , N2 + or even Ar + ions might participate to this positive current but as no emission from them could be detected by optical emission spectroscopy, they might be involved in non-radiative processes [33].

Mixing O2 with the He carrier gas consumes the He metastable species to produce O2 + and N2 + ions through Penning ionization reactions but also enhances the production of O radicals and O2 metastables [15]. Mixing O2 with the Ar carrier gas leads to the consumption of the Ar metastable species through reactions different from Penning ionizations of O2 since no emission of O2 + ions could be evidenced. Also, the production of O radicals is strongly enhanced. As the positive current measured in the postdischarge decreases with the O2 flow rate, a consumption of the Ar2 + ions seems plausible.

IV.2. Roughening rate & etching rate

The previous results of roughening and etching rates have been synthesized in Table 5 and correlated with the emission of O radicals and O2 metastable species, considering post-discharges with/without oxygen. When no oxygen is mixed with the carrier gas, both the etching and roughening rates become stronger using the showerhead plasma torch. According to the simulations of Atanasova et al. [36], the most suitable hypothesis would lie on the presence of the Ar2 + ions responsible for a physical sputtering.

On the contrary, no He2 + band although observable in the visible range could have been detected. This assumption is sustained by the measurement of positive currents 1 mm away from the plasma torch which are close to 20 µA and 200 µA for the linear and showerhead configurations respectively.

Mixing oxygen (O2 = 2.6.10 -3 ) with helium in the linear plasma torch enhances the roughening and etching rates. The same scenario occurs for the showerhead plasma torch when O2 is added to Ar. Raisings of these rates seem directly depend on the O radicals production. Indeed and as previously stated, a rise in the O2 flow rate supplying any of the plasma torch increases its dissociation into O radicals. The Table 5 indicates for O2 = 2.6.10 - 3 a stronger dissociation using the linear plasma torch since the emission of O is 4400 a.u. while it is 3500 a.u. for the showerhead configuration. A first explanation may lie on the Penning ionization of O2 which only occurs in presence of He metastables, leading to the production of O2 + ions. The subsequent dissociation of these ions is a major pathway for the production of O radicals [32]. Also, the highest O2 dissociation using the linear plasma torch could result from a highest plasma power density as reported in The surface roughness as a function of the mass loss is plotted in Figure 9 in the case of LDPE films treated for different gaps. Two regimes could be distinguished for a fixed treatment time: the first where the roughness increases as a linear function of the mass loss for gaps lower than or equal to 10 mm, and the second regime for gaps higher or equal to 15 mm, where mass losses remain always lower than 5 µg/cm 2 and the roughness close to 4 nm. Both plasma treatments seem efficient only for a gap lower than 10 mm, independently of the carrier gas nature and the configuration. This figure clearly indicates that the showerhead plasma torch (Ar) is more suitable to induce structural surface modifications since the roughness and mass losses operate on wider ranges than those. In the framework of our experimental conditions, the surface etching is always accompanied by an increase in its roughness. The absence of a plateau, whether for the roughness or the mass loss, sustains the assumption of two self-dependent phenomena.

IV.3. Wettability and surface activation

The influence of LDPE surface activation on surface wettability can be evidenced by plotting the aWCA as a function of the O/C ratios, as represented in Figure 10. Whatever the plasma torch treatment, the aWCA are significantly lower than the native value at 94.0°.

In presence of oxygen, the nature of the carrier gas plays a minor role on the variations of O/C ratios and as a result on the aWCA values. However, this issue becomes no more negligible in absence of oxygen since aWCA is equal to 33° for a pure helium treatment (linear torch) while 41° for the pure argon treatment (showerhead torch). In the latter case, the highest aWCA value is directly linked to the lowest O/C ratio, namely 0.335. The lowest emissions of O radicals detected in the showerhead configuration are correlated with the lowest oxidation of the treated samples. Besides, the presence of O2 metastables only detected using the linear plasma torch may also be involved in the mechanisms enhancing the oxidation process. On the contrary, the following considerations cannot be taken into account: (i) the carrier gas flux has no influence since it is the same in the two plasma torches, namely 0.94 L/min/mm 2 , as reported in

IV.4. Subsurface functionalization

As a reminder, our pristine LDPE samples are 37 % crystalline and 63 % amorphous (manufacturer data). The reactive species of the plasma torches are assumed to preferentially etch the LDPE regions which are amorphous rather than crystalline [37]. Then, the diffusion of oxygen atoms into the subsurface may preferentially occur through these amorphous regions (10 times higher than in the crystalline regions [38]), starting from the first microseconds of the plasma treatment and stabilizing 30 s later since the wettability state of LDPE remains unchanged beyond this plasma exposure time. At first sight, the functional groups depth profiles can be considered as surprisingly constant. But if we compare their RRMS parameters (approx.. 40 nm) to the ARXPS analysis depths which is comprised between 0.1 and 9 nm, then the chemical information given by ARXPS corresponds to the upper regions of the rough micro-patterns (or the solidified droplets), i.e. to the upper regions of the LDPE liquid phase that may be formed during the post-discharge exposure. This liquid phase could present a quite homogeneous chemical composition, thus explaining the weak variations in the C-O, C=O and O-C=O profiles.

Regarding the profile's components with more accuracy, the C-O and C=O groups are detected in the same proportions whatever the treatment. However, O-C=O shows a content twice higher with the linear plasma torch (20 %) rather than with the showerhead plasma torch (10 %). Several assumptions can be drawn: (i) the results of optical emission spectroscopy reported in Table 5 indicate that more O radicals are produced using helium in the linear plasma torch. (ii) The temperature measured on the LDPE surface during the first 60 s was 114 °C ± 2 °C when using the showerhead plasma torch and 140 °C ± 2 °C when using the linear plasma torch. As the thermal conductivity of He is 8 times higher than the one of Ar [39], this result seems consistent. Then, the formation of the O-C=O functions could depend on the surface temperature and therefore present higher surface concentrations 10 Plasma Processes & Polymers, 2015, Vol. 12, Issue 8, 771-785, http://dx.doi.org/10.1002/ppap.201400097 when using the linear plasma torch with helium. (iii) The formation rate of O-C=O groups could depend on the plasma power density, which is highest in the linear configuration.

For the two treatments, the increasing incorporation of oxygenated functions on the surface could result from several mechanisms whose most significant ones are the following. First, the abstraction of a H atom by an O or OH radical from the surface can lead to the formation of an alkyl radical. As it is unstable in standard conditions (atmospheric pressure, ambient temperature), it could react with an O atom to form an alkoxy radical (R1). Second, the reaction of a C-radical site with an oxygen radical can lead to the production of a peroxy radical (-C-O-O * ), as suggested by the reaction (R2). This product can subsequently react with a polyethylene chain (R) to generate hydroperoxides (C-O-OH) obtained in reaction (R3) [38]. More detailed mechanisms based on the correlation between simulations and ARXPS measurements can be found in a previous study where we exposed LDPE surfaces to a He post-discharge [25].

V. Conclusion

We have compared the treatments of LDPE films induced by two atmospheric flowing post-discharges generated by plasma torches with linear and showerhead configurations and respectively supplied with helium and argon (as carrier gas). For this purpose, the etching rate, the roughening rate, the chemical surface composition and aWCA measurements of the plasma-treated surfaces have been characterized. For applications focused on the wettability properties it appears that the choice of the torch does not matter if oxygen is added as a reactive gas since in both cases, the aWCA are almost the same and the O/C ratio as well. A fine study shows however that aWCA are slightly lower (about 1-2°) in a He-O2 post-discharge (linear configuration) due to a stronger production of O-C=O functions than in the Ar-O2. Several reasons have advanced in particular a higher plasma power density. On the contrary, the choice of the plasma torch matters if no reactive gas is mixed: using the linear plasma torch enables to reach a more hydrophilic state. For applications more focused on textural modifications of LDPE surfaces, the two plasma torches can induce similar modifications but not to the same extent. In that respect, the surface roughening could result from the melting of the subsurface (liquid phase), giving rise to new micro-patterns appearing as solidified droplets once the post-discharge is switched off. In the framework of our experimental conditions, we have also shown that the surface etching is always correlated with an increasing roughening: no surface roughness plateau has been reached during the ejection of the polymer fragments.

  The peak fitting of the C 1s components is computed with the Casa XPS software. The bond energies of the C-C, C-O, C=O and O-C=O components have been fixed to 285.0 eV, 286.5 eV, 288.0 eV and 288.9 eV respectively and the FWHMs of each component have been set between 1.5-1.8 eV.

Figure 2 .

 2 Figure 2. (a) Variation of the aWCA and (b) O/C ratio of LDPE surfaces exposed to the linear (He) and showerhead (Ar) plasma torches without the addition of oxygen, vs. the treatment time between 1 s and 60 s, PRF = 90 W, g = 2mm.

Figure 3 .

 3 Figure 3.a shows typical mass losses of LDPE samples after their exposure to a post-discharge as a function of the treatment time.The mass losses seem to vary linearly with the plasma exposure time, since a linear fit is obtained with a correlation coefficient of 0.985. As the density of LDPE (LDPE) is 0.93 g/cm 3 , mass losses (expressed in µg/cm 2 ) can easily be converted into an etching rate (expressed in nm/s) defined as:

Figure 3 .

 3 Figure 3. (a) Mass losses, (b) Root mean squared roughness (Rrms) and (c) Tridimensional AFM images (5  5 µm²) vs. treatment time between 1 s and 60 s, using linear (He) and showerhead (Ar) plasma torches (g = 2 mm, PRF = 90W).

Figure 4 .

 4 Figure 4. Variation of (a) aWCA and (b) O/C ratio of LDPE surfaces treated by the linear and showerhead plasma torches vs. the gap for PRF = 90 W, t = 30 s, (O2) = 0 mL/min.

Figures 6a and 6b

  Figures6a and 6bshow the variations of the aWCA and the O/C ratios versus the volume fraction of oxygen for He and Ar postdischarge treatments with a gap fixed at 2 mm, an RF power of 90W and a treatment time set to 30 s. Whatever the treatment, the aWCA values decrease until a plateau at about 31° which seems to be reached as soon as O2 is introduced in the post-

Figure 6 .

 6 Figure 6. Variations of the (a) aWCA and (b) O/C ratio vs. volume fraction of oxygen (PRF=90W, t =30 s, g=2 mm)for LDPE surfaces treated by the linear and showerhead plasma torches.

Figure 9 .

 9 Figure 9. Surface roughness versus the mass loss of LDPE films treated by the linear (pure He) and showerhead (pure Ar) plasma torches, as in the Figure 5 (t = 30 s, PRF = 90 W). Numbers in parentheses correspond to the values of the gaps.

Figure 10 .

 10 Figure 10. Advancing WCA versus the O/C ratio of LDPE films treated by the linear and showerhead plasma torches, from Figure 6 (t = 30 s, PRF = 90 W, gap = 2 mm). The numbers in parenthesis correspond to 10 -3 of the O2 volume fraction.

Table 1 . Parameters and variables of the linear and showerhead plasma torches used for the treatment of LDPE.

 1

		Carrier gas nature -	Helium	Argon
		Output area	mm²	16	35.61
		Flow rate	L.min -1	15	33.37
		Flux	L.min -1 .mm -2	0.94	0.94
		Flow velocity	m.s -1	15	15
		Power	W	90	90
			mm	10	20
		 (gap=2 mm)	mm 2	250	314
		Plasma power density on 	W.mm -2	0.36	0.28
		Treatment time	s	1-60	1-60
	Variables	Number of scans (NS) O2 flow rate	-mL.min -1	1-150 0-40	1-75 0-80
		Gap	mm	2-30	2-30

Table 2 . Limitations of the two plasma sources imposed by the manufacturer.

 2

	Linear	Showerhead
	plasma	plasma
	torch	torch

Table 3 . Sampling depth analysis (nm).

 3

Table 1

 1 , i.e. 0.35 W.mm -2 for the linear plasma torch while 0.28 W.mm -2 for the showerhead plasma torch. Besides, O2 metastables solely produced in presence of He and enhanced by increasing the O2 flow rate may participate to the roughening and etching rates as well. Last but not least, the heating of the sample exposed to the post-discharge (and for longer treatment times, e.g. NS = 150) may induce a surface melting (liquid phase), giving rise to new micropatterns appearing as solidified droplets once the post-discharge is switched off.

			Linear plasma		Showerhead
			torch (He)		plasma torch (Ar)
		0	O2 2.6.10 -3	Trend	0	O2 2.4.10 -3	Trend
	O radicals	1250 4400 +266% 1000 3500 +250%
	O2 metastable species	200	850	+325%	0	0	-
	Roughening rate (nm/s)	0.94	1.86	+98% 1.24	1.88	+52%
	Etching rate (nm/s)	9.0	29.5	+222% 13.9	37.3	+168%

Table 5 . Optical emission of oxygenated species, roughening.

 5

Table 2 (

 2 ii) The oxygen impurities from the He and Ar bottles present the same concentrations according to the Air Liquide company. Moreover, they seem negligible (< 3 ppm) if compared to the molecular oxygen concentration from the ambient air (approximately 21 % mol.).

		42	(0)
		40	Plasma torch Linear (He)
		38	Showerhead (Ar)
	aWCA (°)	34 36	(0.6) (1.3)	(2.6)
		32	(0)
		30	(0.6) (1.3) (2.6)
		0.335 0.340 0.345 0.350 0.355 0.360 0.365 28
			O/C ratio

VI. Acknowledgements

This work was part of Mons-Brussels collaboration supported by the European Commission/Région Wallonne FEDER program (Portefeuille 'Revêtements Fonctionnels'). Research in Mons is also supported by the Science Policy Office of the Belgian Federal Governement (PAI 7/05), the OPTI2MAT Excellence Program of Région Wallonne, and FNRS-FRFC. The authors also thanks the I.A.P/7 (Interuniversitary Attraction Pole) program 'PSI-Physical Chemistry of Plasma Surface Interactions', financially supported by the Belgian Federal Office for Science Policy (BELSPO). The authors gratefully thank Mr. Philippe De Keyzer for his technical support.

VII. References

Second, the plasma treatment (Ar or He) induces the production of O-C=O groups which do not exist in the native subsurface. This functional group is lower than 10% on the whole analysis depth after a treatment with the showerhead plasma torch while approaching 20% with the linear plasma torch. The C=O functional groups are more present after a He treatment (> 10 %) than an Ar treatment (< 10 % but always higher than the value of the native LDPE which is 2-3 % over the whole analysis depth) LDPE surface.

Figure 8. Depth profiles of the C-C, C-O, C=O and O-C=O chemical bonds for (a) native LDPE sample and (b) LDPE samples treated by the linear and showerhead plasma torches (PRF=90W, t=30 s, g=2 mm).

III.5. Synthetic table

The most relevant results are summarized in Table 4 and discussed in the next section to compare which plasma treatment is the most suitable depending on the desired surface properties. The emissivity values of O and O2 metastable species (in the postdischarge, as measured by OES) have been taken from our previous works [32], [33].

Parameters

Plasma torch configuration Fig.

Linear Showerhead He He-O2 Ar Ar-O2

Plasma

Oxygen volume fraction O2 0 2.6.10 -3 0 2.6.10