a b s t r a c t Polypropylene samples were exposed to argon plasma discharge and the changes of the PP surface properties were studied by different methods. Surface wettability was derived from contact angle measured by standard goniometry and chemical structure of the plasma modified PP was studied using X-ray photoelectron spectroscopy (XPS) and by Rutherford backscattering spectroscopy (RBS), surface morphology and roughness of samples using AFM. Zeta potential of pristine and modified PP was determined with the SurPASS. The presence of incorporated oxygen in the PP surface layer, about 60 nm thick, was observed in RBS spectra. Oxygen concentration is a decreasing function of the depth. With progressing aging time the oxygen concentration on the PP surface decreases. Plasma treatment results in a rapid decrease of the contact angle, which increases again with increasing aging time. In XPS measurement the oxygen containing structures, created by the plasma treatment, were found on the very surface of the modified PP and the zeta potential being changed too. The significant difference in zeta potential between pristine and plasma treated PP clearly indicates that the plasma treatment leads to a more hydrophilic PP surface

A Macková

A Vasina

P Malinský

P Slepička

T Luxbacher

V Švorčík

Z Kolská

CiteSeerX

Nuclear Instruments and Methods in Physics Research B 268 (2010) 2111–2114Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B
journal homepage: www.elsevier .com/locate /n imbArgon plasma irradiation of polypropylene
P. Slepička a,*, A. Vasina a, Z. Kolská b, T. Luxbacher c, P. Malinský d, A. Macková d, V. Švorčík a
a Department of Solid State Engineering, Institute of Chemical Technology, 16628 Prague, Czech Republic
b J.E. Purkyně University, Department of Chemistry, 40096 Ústí Nad Labem, Czech Republic
c Anton Paar GmbH, 8054 Graz, Austria
d Nuclear Physics Institute of Academy of Sciences of the Czech Republic, 25068 Rez Near Prague, Czech Republic
a r t i c l e i n f o a b s t r a c tArticle history:
Available online 25 February 2010
Keywords:
Polypropylene
Plasma treatment
RBS
XPS
Zeta potential0168-583X/$ - see front matter  2010 Elsevier B.V.
doi:10.1016/j.nimb.2010.02.012
* Corresponding author. Tel.: +420 220 445 159.
E-mail address: petr.slepicka@vscht.cz (P. SlepičkaPolypropylene samples were exposed to argon plasma discharge and the changes of the PP surface prop-
erties were studied by different methods. Surface wettability was derived from contact angle measured
by standard goniometry and chemical structure of the plasma modified PP was studied using X-ray pho-
toelectron spectroscopy (XPS) and by Rutherford backscattering spectroscopy (RBS), surface morphology
and roughness of samples using AFM. Zeta potential of pristine and modified PP was determined with the
SurPASS. The presence of incorporated oxygen in the PP surface layer, about 60 nm thick, was observed in
RBS spectra. Oxygen concentration is a decreasing function of the depth. With progressing aging time the
oxygen concentration on the PP surface decreases. Plasma treatment results in a rapid decrease of the
contact angle, which increases again with increasing aging time. In XPS measurement the oxygen con-
taining structures, created by the plasma treatment, were found on the very surface of the modified PP
and the zeta potential being changed too. The significant difference in zeta potential between pristine
and plasma treated PP clearly indicates that the plasma treatment leads to a more hydrophilic PP surface.
 2010 Elsevier B.V. All rights reserved.1. Introduction
Polymer based materials have been applied successfully in sev-
eral fields as adhesives, biomaterials, protective coatings, micro-
electronic components, automotive industry and in other thin-
film technologies [1,2]. Owing to the consumer and technological
qualities, polypropylene (PP) has very wide spectrum of applica-
tion and takes the second place after polyethylene on world release
– 20.5%. Polypropylene is applied for manufacture gas and water
pressure head pipes, structures, sheets, film, furniture, technical
products, the goods of cultural and community purpose. In general,
special surface properties with regard to chemical composition,
hydrophilicity, roughness, crystallinity, conductivity, lubricity and
crosslinking density are required for successful applications. Sur-
face modification techniques which can transform polymers into
highly valuable finished products have became an important part
of the plastic technologies [3].
Plasma treatment of polymers, including corona discharges, is
now widely used industrial technique to modify surfaces of many
different materials (metals, semiconductors, polymers, and ceram-
ics). Polymers are often modified using rare gas (He, Ne, and Ar) or
reactive gas (O2 and F2) plasma. Surface modification of polymers
with low-pressure plasma has gained great scientific and industrialAll rights reserved.
).importance and it is often used to improve adhesion of coatings,
wettability, printability, bio-compatibility and other surface re-
lated properties of polymers [4–7].
In this work, we studied surface properties of pristine and plas-
ma treated PP with the main aim to understand surface chemistry
and wettability of PP after the plasma exposure. Rutherford back
scattering (RBS), X-ray photoelectron spectroscopy (XPS) were
used for the characterization of the chemical structure of the mod-
ified PP and AFM for study of surface morphology and roughness of
samples. The zeta potential and contact angle determination were
used as methods for characterization of polymer surface.2. Experimental
Polypropylene (PP, supplied by Visteon–Autopal, CR) in the
form of 1 mm thick foils was used for the present experiments.
The samples were modified in diode plasma discharge on Balzers
SCD 050 device for 0–240 s using Ar+ plasma (gas purity was
99.997%). The reaction chamber was evacuated to the pressure of
2 Pa and the working pressure during the plasma discharge was
10 Pa. The discharge powers were 3.1 and 8.3 W and the treatment
was accomplished at room temperature.
Contact angle characterizing the surface wettability, was mea-
sured at six positions with distilled water at room temperature
using surface energy evaluation system. The aging studies at labo-
ratory conditions were performed on samples by measuring the
0 10 20 30 40 50 60 70 80
20
30
40
50
60
70
C
on
ta
ct
 a
ng
le
 (°
)
Aging time (hour)
 8.3
3.1
PP/plasma
Fig. 2. The dependence of contact angle of the plasma modified PP on the time
elapsed from the plasma treatment (aging time). Discharge powers were 8.3 and
3.1 W.
0 20 40 60 80 100
0
2
4
6
8
10
12
C
on
ce
nt
ra
tio
n 
(a
t %
)
 50
100
150
300
PP/plasma
2112 P. Slepička et al. / Nuclear Instruments and Methods in Physics Research B 268 (2010) 2111–2114dependence of contact angle on the time after the plasma modifi-
cation [2,8].
Structural and compositional changes induced by the plasma
treatment were examined about 7 days after the exposure. Mean-
time the plasma modified samples were stored on the air at room
temperature. Oxygen concentration in the surface layer of the plas-
ma modified PP was determined from X-ray photoelectron spectra
(ARXPS), measured on Omicron Nanotechnology ESCAProbeP spec-
trometer. The XPS spectra were taken at different detection angles
of photoelectrons which escape from the surface [9]. The spectra
evaluation was carried out by CasaXPS program. Before the mea-
surement, the samples were stored under standard laboratory
conditions.
The concentration depth profile of oxygen in the modified PP
surface layer was studied using RBS technique. The RBS analysis
was performed in a vacuum target chamber with 2.72 MeV He+
ions. Elemental depth profiles in the inspected polymeric samples
were determined with the typical depth resolution less than 10 nm
and accessible depth of a few lm. The RBS spectra were evaluated
by the GISA3.99 code [10]. The typical RBS detection limit was
0.1 at.% for oxygen.
Zeta (f) potential of pristine and plasma treated PP was deter-
mined by SurPASS Instrument (Anton Paar). Samples were studied
inside the adjustable gap cell in contact with the electrolyte
(0.001 mol dm3 KCl). Also the pH dependence of f-potential for
both of PP was determined (by titration with 0.1 mol dm3 HCl)
in the range from 2 to 6.
We examined the surface morphology and roughness of pristine
and plasma treated samples by AFM using VEECO CP II device
working in tapping mode. Si probe RTESPA-CP with the spring con-
stant 20–80 N/m was used. By repeated measurements of the same
region (1  1 lm) we certified that the surface morphology did not
change after three consecutive scans. The mean roughness value
(Ra) represents the arithmetic average of the deviations from the
centre plane of the sample [14].Depth (nm)
Fig. 3. Oxygen concentration depth profiles for the PP modified by the plasma
discharge at 8.3 W power and exposure times (50, 100, 150 and 300 s). The profiles
were obtained from RBS measurement.
24
26
28
tio
n 
(a
t %
) treated PP
 aged PP3. Results and discussion
The dependence of contact angle of PP on the exposure time of
the plasma treatment for discharge powers (8.3 and 3.1 W) is
shown in Fig. 1. Rapid decrease of the contact angle was observed
immediately 20 s after the modification for both plasma discharge
powers. Contact angle declines to ca. 40 for PP modified at 3.1 W
and to ca. 20 at 8.3 W in the first 50 s of modification. For the long-
er exposure times the contact angle achieves a saturated value in0 50 100 150 200 250
0
20
40
60
80
100
C
on
ta
ct
 a
ng
le
 (°
)
Exposure time (s)
8.3
 3.1
PP/plasma
Fig. 1. The dependence of contact angle of PP on the time of the plasma treatment
for discharge powers 8.3 and 3.1 W.
0 20 40 60 80 100
16
18
20
22
O
xy
ge
n 
co
nc
at
ra
Angle from the normal (0)
Fig. 4. Dependence of the oxygen concentration (at.%) on the ARXPS detection
angles of photoelectrons which escape from the surface for plasma treated (1 h) and
aged (170 h) PP.the range of 20–30 regardless of the plasma discharge power.
The slight increase of the contact angle for higher exposure times
may be related with the ablation and oxidation of the PP surface
[11].
2 3 4 5 6
-60
-40
-20
0
20
Z
et
a 
po
te
nt
ia
l (
m
V
)
pH 
 pristine PP
 treated PP
Fig. 5. The pH dependence of f-potential of pristine and plasma treated PP (8.3 W,
240 s) determined by SurPASS device.
P. Slepička et al. / Nuclear Instruments and Methods in Physics Research B 268 (2010) 2111–2114 2113The change of the contact angle as a function of the time from
the plasma treatment (aging time) is shown in Fig. 2. With increas-
ing aging time the contact angle increases (surface polarity and
wettability decreases). After the exposure a rather rapid increase
is observed in the first ca. 30 h for both discharge powers, the in-
crease being slower for the discharge power of 3.1 W. Regardless
of the power of the plasma discharge the full surface relaxation
and contact angle restoration are achieved after about 70 h of aging.
Enhanced surface hydrophilicity of the plasma treated polymers is
due to the presence of oxygen containing groups [8,11,12] and the
subsequent relaxation is connected with their reorientation andFig. 6. AFM images of the pristine (PP) and 240 s plasma modified PP by 3.possibly also with rearrangement of degraded macromolecules on
the polymer surface [9,13].
Since more pronounced changes of the surface polarity are ob-
served on the samples treated at the power of 8.3 W, these samples
were used in the following experiments. The presence of oxygen in
the surface layer of the plasma treated PP was examined using the
RBS method. The oxygen concentration depth profile in the surface
layer is shown in Fig. 3. Presence of oxygen is observed up to the
depth of about 60 nm with the oxygen concentration decreasing
from the surface to the bulk. From Fig. 3 it is further apparent that
with increasing time of plasma treatment the total oxygen concen-
tration increases on the sample surface. Obviously, the oxygen
from ambient residual atmosphere in the plasma reaction chamber
interacts with the activated PP surface and, in this way, different
oxidized structures are created on the PP surface (carbonyl, car-
boxyl and ester groups has been proven on the plasma treated
polyolefine [11,12]).
The oxygen concentration on the surface of the plasma modified
PP sample was determined using XPS method too. ARXPS measure-
ment gives information from the very ‘‘top” of the surface. The oxy-
gen concentration on the plasma modified PP was measured after 1
and 170 h of aging at different detection angles of photoelectrons
which escape from the surface [9]. The results are shown in
Fig. 4. The oxygen concentration decreases with increasing aging
time due to rearrangement of the degraded macromolecules con-
taining oxygen groups produced by the plasma treatment. Present
results are in agreement with those reported earlier [9]. It can be
also concluded that the highest concentration of oxygen is on the
‘‘top” of the surface (maximum angle with respect to the surface
normal).1 (PP/3.1) and 8.3 W (PP/8.3). Ra is the mean surface roughness in nm.
2114 P. Slepička et al. / Nuclear Instruments and Methods in Physics Research B 268 (2010) 2111–2114The zeta potential of polymer samples was applied as a comple-
mentary method to characterize the polymer surfaces. The pH
dependence of f-potential for pristine and plasma treated PP is
shown in Fig. 5. The significant difference in the f-potential in all
pH range between pristine and plasma treated PP clearly indicates
the effect of PP surface modification leading to a more hydrophilic
surface. It is known that after plasma modification of polyolefins
oxygen polar groups (e.g. C@O, ACOOH, ACOO and –OH groups
are produced [8]). The isoelectric point (IEP = pH where f = 0 mV) is
found at pH ca. 2.5 and 4 for pristine and modified PP, respectively
indicates an inert polymer surface without any dissociable groups.
The AFM images of the surface morphology of the pristine and
plasma modified PP (3.1 and 8.3 W, both 240 s) are shown in
Fig. 6. After the plasma treatment the surface roughness does not
show any significant change in contrast to the surface morphology.
Granular structure becomes apparent on the polymer surface, the
structure being more pronounced on the PP modified with higher
irradiation power.
4. Conclusion
It was found that the plasma treatment results in pronounced
changes in the surface properties of PP. Higher discharge power
leads to more significant decrease of the contact angle in compar-
ison with pristine polymer, thus to the dramatic increase of the PP
surface wettability. With the increasing time from the plasma
treatment the contact angle increases and after about 70 h a full
relaxation takes place and the contact angle reaches a saturated va-
lue. RBS and XPS measurements show the presence of the oxygen
containing groups on the PP surface modified by the plasma dis-
charge. According to the RBS measurement the oxygen depth pro-
file extends up to the depth of about 60 nm, the oxygen
concentration being a decreasing function of the layer depth. Thef-potential determination proved that plasma treatment of PP
leads a more hydrophilicity of surface. Granular structure becomes
apparent on the polymer surface, the structure being more pro-
nounced on the PP modified with higher irradiation power.Acknowledgements
This work was supported by the GA of the CR under the Projects
Nos. 106/09/0125 and 106/09/P046, Ministry of Education of the
CR under Programs No. 6046137302 and LC 06041, and AS of the
CR under the Projects KAN400480701 and KAN200100801.References
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Argon plasma irradiation of polypropylene

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Argon plasma irradiation of polypropylene

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