Surface Analysis and Ultra‐Shallow Molecular Depth‐Profiling of Polyethylene Treated by an Atmospheric Ar‐D₂O Post‐Discharge

Abstract

In an original approach to trace water reactivity upon surface treatment by ambient “open air” plasma, the H–D exchange processes induced in polyethylene by an Ar‐D2O post‐discharge are investigated using time‐of‐flight secondary ion mass spectrometry, both at the surface and along the sample depth. The surface characterization points out the strong influence of the sample‐torch distance. The most important chemical modifications, excluding polymer overheating, occur at 5 mm of distance, with 25% of deuterated monomer units at the surface. For the first time, ultra‐shallow molecular depth‐profiling by Ar noble gas clusters shows the detailed variation of the deuteration of the polymer repeat unit in the topmost surface layer. Our results indicate that the fraction of deuterated monomer units is reduced by a factor >2 over a depth of ∼3 nm.

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

“…First, the H‐D exchange (also named deuteration) generates a new pattern of fragmentation reproducing that of the pristine LDPE, but shifted in the m/z scale because of the double mass of the deuterons compared to that of the protons. However, the mass resolution achieved in these ToF‐SIMS spectra (about 5000 for C 2 H 5 + at m/z 29), is insufficient for an accurate determination of the isotopic ratio of a given fragment ion . This issue is illustrated in Figure with the peak at m/z 30.043, whose possible identifications are C 2 DH 4 + , C 2 D 2 H 2 + , and C 2 D 3 + because of the uncertainty on the H isotope content.…”

“…The two main columns correspond to the PC1 loadings > 0, representative of all the treated samples (scores > 0), and PC1 loadings < 0, characteristic of the reference and the 10 mm − 30 s sample (scores < 0). The identification of the molecular ion fragments indicated by the PCA is based on the tables (Table S1 and S2) provided in supporting information of our previous work . They list the peaks in the range 0 ≤ m/z ≤ 70 of the pristine and plasma‐treated LDPE mass spectra, and their most reasonable attributions, for both polarities (taking into account the uncertainty on the H isotope content).…”

“…The ability of SIMS to probe the chemical structure of plasma‐modified LDPE, both at the surface and along the top tens of nanometers, has been already evidenced in a previous work conducted in our research team . In that investigation, H 2 O was replaced by D 2 O in order to discriminate the H contributions coming from the polymer itself and the atmospheric moisture from the water deliberately mixed to the plasmogen gas.…”

“…The LDPE film used for the plasma treatment was purchased from Goodfellow. The polymer manufacturer claims a film thickness of 50 μm and an additive free material . No washing procedure was performed prior to surface treatment, in order to avoid contamination evidenced from preliminary SIMS analyses (since this technique possesses an excellent surface sensitivity <1 p.p.m.).…”

“…The LDPE film was treated by an atmospheric radiofrequency (RF) plasma torch (AtomfloTM 250D) from SurfX Technologies LLC . The schematic representation of the experimental device, called “showerhead plasma torch” because of its configuration, is shown elsewhere . The torch consists in two parallel circular electrodes perforated by 126 holes, each one with a critical diameter of 0.6 mm.…”