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.

To optimize the adhesion of layers presenting strong barrier properties on low-density polyethylene (LDPE) surfaces, we investigated the influence of argon and argon-oxygen atmospheric pressure post-discharges. This study was performed using x-ray photoelectron spectroscopy, atomic force microscopy, optical emission spectroscopy (OES) and dynamic water contact angle (WCA) measurements. After the plasma treatment, a slight increase in the roughness was emphasized, more particularly for the samples treated in a post-discharge supplied in oxygen. Measurements of the surface roughness and of the oxygen surface concentration suggested the competition of two processes playing a role on the surface hydrophilicity and occurring during the post-discharge treatment: the etching and the activation of the surface. The etching rate was estimated to about 2.7 nm s −1 and 5.8 nm s −1 for Ar and Ar-O2 post-discharges, respectively. The mechanisms underlying this etching were investigated through experiments, in which we discuss the influence of the O2 flow rate and the distance (gap) separating the plasma torch from the LDPE surface located downstream. O atoms and NO molecules (emitting in the UV range) detected by OES seem to be good candidates to explain the etching process. An ageing study is also presented to evidence the stability of the treated surfaces over 60 days. After 60 days of storage, we showed that whatever the O2 flow rate, the treated films registered a loss of their hydrophilic state since their WCA increased towards a common threshold of 80°. This 'hydrophobic recovery' effect was mostly attributed to the reorientation of induced polar chemical groups into the bulk of the material. Indeed, the relative concentrations of the carbonyl and carboxyl groups at the surface decreased with the storage time and seemed to reach a plateau after 30 days. IntroductionPolymers such as polyethylene present excellent physical and chemical bulk properties but exhibit poor surface adhesion features. The deposition of a subsequent layer onto a low-density polyethylene (LDPE) surface can be achieved for specific applications, for instance a layer acting as an oxygen barrier for food packaging. In order to improve the adhesion of a subsequent layer onto this polymer, a pretreatment of its surface is therefore usually performed. One of the most promising techniques is the plasma activation which-for instance-consists of exposing the sample surface to the post-discharge of a radio frequency (RF) plasma torch. These plasma treatments can be carried out with different gases and vapours like O2, N2, NH3, H2O, CO2, air and noble gases [1][2][3][4]. Depending on the experimental conditions, it is considered that a plasma treatment leads to one of the four following effects: cleaning (removing of organic contaminants), etching (removing of polymer material and degradation of polymer), cross-linking (formation of free radicals and branching of macromolecules) and functionalization (formation of new chemical functions) [5][6][7][8][9][10]. ...

International audienceLow 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 spectro-scopy, water contact angles measurements, and atomic force microscopy. A comparison between the two post-discharge treatments is achieved for several torch-to-substrate distances (gaps), treatment times, and oxygen flow rates in terms of etching rate, roughening rate, diffusion of oxygen into the subsur-face and hydrophilicity. By correlating these results with the chemical composition of the post-discharges, we identify and compare the 'species which are responsible for the chemical surface functionalization, the surface roughening, and etching

aThe in-depth oxygen diffusion into a low density polyethylene film is performed in the post-discharge of an atmospheric plasma torch, supplied in argon as carrier gas and with or without oxygen as reactive gas. The chemical and structural properties of the polymer surface and bulk are studied in terms of plasma parameters (treatment time, power, and reactive gas flow rate). A good correlation between XPS and Fourier transform infrared spectroscopy analyses is demonstrated. The penetration depth of oxygen into the bulk of the polymer is investigated by angle resolved-XPS and time-of-flight SIMS. It is shown that, depending on the plasma conditions, oxygen could penetrate up to 20-40 nm into the low density polyethylene during the atmospheric plasma treatment.

The aim of this work is to study the transformation of plasma polymerised tetramethyldisiloxane (ppTMDSO) films deposited by microwave induced RPECVD through exposure to a N2/O2 microwave plasma afterglow in a duplex reactor. The film thickness, structure and composition of the as‐deposited or transformed ppTMDSO films are determined by profilometry, interferometry, Fourier transform infrared and X‐ray photoelectron spectroscopies. The analysis of the post‐treatment effect is carried out through a combination of results obtained from FTIR study and subsequent optical interferometry study during CF4 etching of deposits. A carbon removal and an enhancement of cross‐linking of the SiOSi chains are shown. We also propose a model to determine the transformed layer depth with taking the film contraction into account for the first time for this family of polymers.