22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
A ToF-SIMS study of the polyethylene chemical modification induced by an
atmospheric Ar-D 2 O post-discharge
V. Cristaudo1, S. Collette2, C. Poleunis1, F. Reniers2 and A. Delcorte1
1
Institut de la Matière Condensée et des Nanosciences (IMCN), Bio & Soft Matter (BSMA), Université catholique de
Louvain (UCL), 1 Croix du Sud, BE-1348 Louvain-la-Neuve, Belgium
2
Chimie Analytique et Chimie des Interfaces (CHANI), Université Libre de Bruxelles (ULB), 2 Boulevard du Triomphe,
BE-1050 Brussels, Belgium
Abstract: In an original approach to trace water reactivity upon surface treatment by
ambient “open air” plasma, the H-D exchange and the oxidation processes induced in
polyethylene by an Ar-D 2 O post-discharge are investigated using time-of-flight secondary
ion mass spectrometry (ToF-SIMS), both at the surface and along the sample depth.
Keywords: atmospheric Ar-D 2 O post-discharges, ToF-SIMS, deuteration, oxidation
1. Introduction
The use of atmospheric plasmas for surface
modification of polymers has expanded tremendously
since the last decade [1-3]. In addition to the many
advantages offered by the more conventional low pressure
plasma techniques, atmospheric plasmas allow us to avoid
the constraints of the vacuum, thereby achieving surface
treatments at lower costs. In order to upgrade these
surface plasma treatments to industrial scale, the chemical
and physical properties of the polymers modified in
atmospheric discharges need to be better understood.
However, this investigation is complicated by the
interaction between the plasma and the environment.
Indeed, oxygen, nitrogen and water are always present in
the atmosphere, intervening inevitably in the plasmainduced processes. Often, the presence of water vapor in
the plasma can be considered as problematic as water is
known to destabilize it [4-6]. However, the water vapor
can be deliberately mixed into the plasma to achieve a
milder treatment or to generate radicals of interest [7].
It is known in the literature that the presence of water
vapor in the plasma induces the grafting of -OH groups
onto polymer surfaces, representing a promising
functionalization route for industrial and medical
applications [8]. However, tracing of the water reactivity
in the polymer modification remains challenging. In
order to investigate the reactions involving hydrogen, and
thereby obtain a comprehensive view of the surface
modification by water plasmas, one must be able not only
to detect hydrogen, out of reach of the classical
characterization techniques, such as X-ray photoelectron
spectroscopy (XPS). However, it is also requested the
separation of the diverse hydrogen contributions, deriving
from the water vapors - plasma or atmospheric water and from the polymer itself.
The present work reports an original approach to
elucidate the reactions involving hydrogen, using a
combination of deuterated water in the plasma and
state-of-the-art SIMS analysis. For this purpose, D 2 O
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vapor was injected in the post-discharge of an
atmospheric argon plasma torch for the treatment of
LDPE (low density polyethylene) films. ToF-SIMS was
chosen for the surface and in-depth analysis because it is
the only surface technique possessing sufficient
sensitivity and selectivity to the hydrogen isotopes as
elements and in molecular fragments [9-11]. In addition,
thanks to the recent advent of large Ar cluster ion beams
(clusters constituted by hundreds to thousands of atoms)
for sample sputtering, the modifications of the polymer
molecular structure can be also followed along the sample
depth [12-14]. Therefore in the following sections, a
protocol for the quantification of the deuteration at the
surface is first established by defining a deuteration ratio
based on the SIMS intensities of the protonated repetitive
units of LDPE differing in their H isotopes content.
Based on this parameter, the torch-surface distance is
optimized. In the second part of the work, the evolutions
of the deuteration and the oxidation in a shallow region of
the sample subsurface is followed using molecular depth
profiling, demonstrating the usefulness of Ar cluster
beams for this purpose.
2. Methods
The LDPE film was treated by an atmospheric RF
argon plasma torch (AtomfloTM 250D) from SurfX
Technologies LLC.
Deuterated water vapors were
injected into the post-discharge region of the Ar plasma
by means of a bubbler. In order to investigate the efficacy
of the deuteration process as a function of the gap
between the LDPE film and the plasma source, the
experiments were performed during 5 minutes of
treatment at three different distances of 5, 7 and 10 mm,
respectively.
Secondary ion mass spectra and molecular depthprofiling experiments were performed using an ION-TOF
ToF-SIMS V (Münster, Germany) instrument equipped
with both Bi-LMIG (liquid metal ion gun) and Ar-GCIB
(gas cluster ion beam) primary ion sources mounted at
1
45° to the surface normal. The secondary ions were
collected by a time of flight analyzer perpendicular to the
sample surface. The mass spectra were obtained using a
30 keV Bi 5 + beam on a 200 x 200 µm2 sample area.
Whereas, the depth-profiles were performed in dual ion
beam mode. 10 keV Ar 5000 + ions were employed to
sputter a 600 x 600 µm2 area, and 30 keV Bi 5 + ions for
collecting the spectra from a 200 x 200 µm2 area
concentric to the sputtered surface.
zero. Only the C 2 DH 4 + intensity does not level off due to
the interference of the 13CCH 5 + peak. Finally, the fluence
reported in the x-axis of the profile was converted in
depth. With the used experimental conditions, the total
erosion depth was found to be ∼10 nm. The deuteration
ratio, R D , applied to this depth-profile and reported in
Figure 1.b, shows an evolution of the H-D exchange from
an initial value of ∼30% to ∼4% over a depth of ∼10 nm.
3. Results and Discussion
The first objective of the present work is to study the
deuteration of the LDPE surface. To assess the extent of
the deuteration, it is recommended to focus on molecular
fragment ions. Therefore, the ions selected for this
analysis were the protonated repetitive unit of the
polyethylene, C 2 H 5 + at m/z = 29, and its progressive
deuteration reaction products, C 2 DH 4 + (m/z = 30),
C 2 D 2 H 3 + (m/z = 31), C 2 D 3 H 2 + (m/z = 32), C 2 D 4 H+
(m/z = 33) and C 2 D 5 + (m/z = 34). It is important to note
that all these fragment ions, differing only by their
H isotopic ratio, should possess the same ionization
probability. This permits, to some extent, a quantitative
analysis in SIMS. Once these methodological precautions
and protocols are established, the influence of the sampletorch gap on the H-D exchange at the surface can be
studied. Three different torch-surface distances were
investigated - 5, 7 and 10 mm, respectively - keeping
constant the treatment time (5 min). In order to obtain a
synthetic view of the sample deuteration, it is useful to
introduce an H-D exchange indicator or “deuteration
ratio”, R D , which is defined as follows:
5
5
𝑥=1
𝑥=0
𝑅𝐷 = � 𝐶2 𝐻5−𝑥 𝐷𝑥+�� 𝐶2 𝐻5−𝑥 𝐷𝑥+
The results display that 5 mm gap gives a R D of 25 - 30%
depending on the sample, which decreases strongly with
the sample-torch distance, down to a value of ∼5% for a
gap of 10 mm (not shown).
The second objective of this work is to study the
deuteration along the depth of the LDPE film
functionalized in the optimized conditions of 5 min and
5 mm. The related depth-profile is shown in Fig. 1a,
where the evolution of the ions C 2 D x H 5-x + (0 ≤ x ≤ 5) is
now followed along the depth. There is a very strong
decrease of the deuterated species intensities with the
sample depth. The order of the curves follows the order
of deuteration, C 2 D 5 + presenting the steepest decay.
However, there is also a smaller decrease of the C 2 H 5 +
intensity (30%), which might be the result of diverse
causes such as, for instance, the presence of smaller
chains on the surface, more branching of the molecules
and/or oxidation. Furthermore, the C 2 H 5 + intensity
stabilizes with increasing the sputtering fluence, whereas
the decay of the species C 2 D x H 5-x + (2 ≤ x ≤ 5) goes to
2
Fig. 1. Depth-profile of the LDPE film treated for 5 min
at 5 mm. a) Intensity of the C 2 D x H 5-x + fragments
(0 ≤ x ≤ 5) as a function of the sputtering fluence.
b) Evolution of the deuteration ratio (R D ) as a function of
the Ar cluster fluence.
The positive mass spectra of the plasma-deuterated
LDPE also point out the presence of C x H y D z O n +
fragment ions, containing oxygen and deuterium
simultaneously. The plasma-treated LDPE depth-profile
in Fig. 2 reports the molecular species C 2 H 5 O+, where
only one O atom is linked to the repetitive unit of the
polymer, and two other related fragment ions, which
solely differ for their H isotopic ratio. The intensity
evolution of these O-containing molecular ions
reproduces that of the C 2 D x H 5-x + (2 ≤ x ≤ 5), going to
zero for a 10 nm depth.
In conclusion, the LDPE deuteration induced by an
Ar-D 2 O post-discharge was successfully evidenced and
quantified at the sample surface and along the depth using
ToF-SIMS. Finally, the molecular depth-profiling by
P-III-6-12
large Ar clusters shows that the H-D substitution is an
extreme surface process involving only the top 10 nm, in
the applied plasma treatment conditions.
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Fig. 2. Depth-profile of the LDPE film treated for 5 min
at 5 mm. Intensity of O-containing fragment ions as a
function of the sputtering fluence.
4. Acknowledgements
This work was carried out in the framework of the
network on Physical Chemistry of Plasma-Surface
Interactions - Interuniversity Attraction Poles, phase VII
(PSI-IAP7), and supported by the Belgian Science Policy
Office (BELSPO). Finally, the authors thank the Belgian
‘Fonds National de la Recherche Scientifique’ (FNRS) for
financial support to purchase the ToF-SIMS instrument.
A. Delcorte is a senior research associate of the FNRS.
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