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

Abstract

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.

LDPE Surface Modifications Induced by Atmospheric Plasma Torches with Linear and Showerhead Configurations

“…In previous studies, 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–O 2 or He–O 2 post‐discharge. For instance, we have shown that an Ar–O 2 flowing post‐discharge treatment induces a decrease in advancing water contact angles (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) . 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.…”

“…As the RF power applied to the two torches is 90 W, the surface plasma power density estimated on Σ 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, this difference can be considered as negligible as plasma power variations over this range does not induce significant surface modification in terms of aWCA, mass losses, roughness, or chemical surface composition . Those values are reported in Table .…”

“…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, and 75° . In a film of slightly oxidized polyethylene, the photoelectron emitted by an O 1 s atom presents a kinetic energy of ≈721 eV corresponding to an attenuation length ( λ ) of 2.5 nm.…”

Molecular Surface Analysis and Depth-Profiling of Polyethylene Modified by an Atmospheric Ar-D2O Post-Discharge

“…Indeed, most of the highly energetic particles of the plasma, such as electrons and ions, are neutralized before reaching the polymer substrate. So the plasma‐polymer interactions mainly occur by means of excited species and radicals and they remain confined in the outermost atomic layers of the polymer . A typical example of surface modification concerns the polyolefins and, more specifically polyethylene, employed in a multitude of applications from the everyday life, such as food packaging and medicine, to the highly technological ones in the automotive and electronics fields.…”

“…A known route to improve the adhesion of the polyolefins consists in the so‐called oxidative functionalization, usually obtained by the use of O 2 as reactive gas, that induces the grafting of O‐containing functional groups. The literature reports a series of recent works carried out with atmospheric Ar/O 2 or He/O 2 post‐discharges for improving the hydrophilicity of LDPE or HDPE (high density polyethylene) . This result is obtained by the grafting of three polar functionalities –C–O, CO and O–CO– whose densities on the polymer surface decrease with increasing the treatment time.…”