  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{\textdegree}. 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.
