22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Post-treatment stability of aromatic polymers modified by helium atmosphericpressure plasma
C. Borcia, I. Punga and G. Borcia
IPARC, Faculty of Physics, Alexandru Ioan Cuza University, Iasi, Romania
Abstract: A study is carried out on the relation between plasma effects on polymer
surfaces exposed to helium atmospheric-pressure plasma, polymer structure characteristics
and recovery during ageing, where the loss of polarity and the mechanisms related to it are
assessed on samples tested before and after immersion in water, establishing the factors
restraining the post-treatment ageing process.
Keywords: atmospheric-pressure plasma, polymer, surface modification, ageing
1. Introduction
Any material changes its physico-chemical properties at
the interaction with the surrounding, from atmospheric
air, to water, biological liquids, etc. Therefore, the surface
stability is paramount in defining the material
performance in operation. This is particularly accurate for
polymers, since the interface of the polymer with its
working environment is the key of its performance,
marking its behavior in many applications, as packages,
adhesives, lacquers, metallization, dyeing, composites,
membranes and biomedical field.
Polymers display highly dynamic interface with their
working environment, therefore the surface modification
of polymers is a constant challenge, since it induces a
perturbation of the material, which adds to the complexity
of the interaction at the interface [1]. In this context, the
knowledge on the processes by which a treated polymer
surface tends to reorganize, as to achieve stability, on the
relation between the polymer surface characteristics and
the surface dynamics, and on the factors allowing
controlling the interaction, on a time-space scale, is most
important for all polymers applications.
Plasma holds a particular position among various
techniques for processing polymer materials, as complex
source of energy for surface modification, due to the large
variety of components, as excited and ionized particles,
photons, radicals, all these species being capable to
induce chemical reactions, both in the plasma volume and
at its interface with solid surfaces, where the plasma
effect on polymers is mainly the result of combined
functionalization and crosslinking, still, functionalization
being the dominant reaction path. Yet, plasma treatment
has the disadvantage that the surface modification is not
permanent, since the surface tends to restore to the
untreated state, process known as the recovery
mechanism. This ageing effect is due to both short-range
motion, as reorientation of polar chemical groups into the
bulk, and long-range mechanisms, as diffusion and chains
relaxation, acting with different time constants, on
different surface and subsurface layers [2-6].
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Taking this into account, we provide an investigation
on the surface modification processes of polymers treated
by plasma and an assessment on the dynamics and
stability of modified surfaces.
Here, aromatic-structure polymers are selected, known
to exhibit enhanced mechanic characteristics and
chemical stability, related to intrinsic rigidity and
chemical inertness of the aromatic ring, in order to
demonstrate the plasma capability for efficient
modification by creating complex surface structure.
The limiting level of modification attainable and the
factors controlling it are established, also the factors
restraining the post-treatment ageing process, thus
allowing for polymer operational stability.
This investigation is useful for general applications of
treated polymer surfaces, in particular for those implying
various depositions on polymers, as dyes, lacquers,
lamination, metallization etc., when the polymer-material
interface may develop to a new state before the deposited
layer reaches its steady state.
2. Experimental
The surface treatment is performed using atmosphericpressure helium plasma, generated by dielectric barrier
discharge (DBD), in a flow-through configuration,
conducting to an air-enriched discharge environment [7].
As a consequence, the oxygen, present in non-negligible
amount, is to be taken into account when analyzing the
surface modification mechanisms.
The samples are commercial polymer films of
polysulphone (PSU), poly(ethylene terephthalate) (PET),
poly(ether ether ketone) (PEEK) and polystyrene (PS)
(Goodfellow Ltd., Cambridge), 50 µm thick, all bearing
aromatic structure (Fig. 1), also offering variety of
structure, functionality, degree of oxidation and polarity.
The surface treatment is performed for 10 s and 30 s
exposure time, selected in relation to previous
experiments [8]. The surface of the polymers, before and
after treatment, is analyzed by contact angle, X-ray
photoelectron spectroscopy (XPS) and solvent absorption.
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The dynamics of the surface is assessed monitoring the
ageing of treated samples (S-I) and samples treated and
immersed in water for 30 minutes (S-II), respectively, the
measurements being carried out at different intervals, up
to 30 days, after the treatment, to assess the loss of
polarity and the mechanisms related to it.
O
CH3
O
C
O
O
PSU
S
O
CH3
C
C
O
O
O
O
O
CH2
CH2
C
PET
PEEK
O
CH
CH2
PS
Thus, the contact angle measurement on treated
samples (S-I) shows significantly lower values, proving
the increased hydrophilic character after plasma exposure.
Then, the contact angle is used to calculate the adhesion
work of water Wa and the relative increase of the
adhesion work ∆Wa / Wa on treated surfaces, with respect
to untreated samples, for the two treatment times. As
results obtained for the two treatment times are similar
within error bars, only the 10 s treatment results are
presented in Fig. 2. One observes that ∆Wa / Wa has the
highest value of about 90%, meaning practically double
of the initial adhesion work, for PS, which is the polymer
with the lowest level of adhesion prior treatment. On the
other hand, all other polymers, PSU, PET and PEEK,
which start practically with the same Wa value, exhibit
similar level of modification in terms of adhesion.
Fig. 1. Chemical structure of the tested polymers.
3. Results and discussion
Although the surface oxidation is the obvious surface
modification mechanism in most surface processing
applications, the plasma effect in an inert gas environment
encompasses several processes. Thus, in a first step,
plasma promotes the cleaning of the surface, where the
cleaning effect is accompanied by radical formation. The
radicals trigger then secondary reactions, such as
functionalization and intermolecular crosslinking, acting
at different rates. The first process is very fast and it is
limited to the very surface (one layer), whereas the second
process is expected to take place at much lower rate,
involving at least two layers.
The crosslinking plays a particular role on the
functionality and applications of a surface-treated
material, because a higher degree of crosslinking is
beneficial for the stability of the surface properties. Since
the reversal of the surface wettability after surface
treatment, known as hydrophobic recovery, may be due to
reorientation of induced polar chemical groups into the
bulk, as short-range mechanism, and diffusion and chains
relaxation, as long-range mechanisms [6], crosslinking
may ensure surface stability, by creating a barrier layer
that restricts the diffusion to the bulk [9].
An ideal modified surface would be thus the result of a
convenient combination of oxidation/crosslinking, since
the crosslinked structure would support better stability of
the topmost functionalized layer, resulting in enhanced
properties of wettability, adhesion, bondability etc.
Nonetheless, whereas the oxidation can be rendered to
evidence quantitatively, by XPS, the crosslinking, which
is expected to be enhanced in inert gas plasma,
particularly in helium [8,9], can be demonstrated only
indirectly, by the ageing behavior of treated polymers. In
this respect, the immersion of the samples in water, which
accelerates the recovery by reorientation of the surface
polar groups, may offer useful information.
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Fig. 2. Relative variation of the adhesion work for 10 s
treated polymers (S-I), for different ageing times (in
brackets, Wa for untreated samples).
The enhanced surface adhesion is demonstrably due to
the creation of oxidized functionalities, as shown by the
increased O / C values, calculated from the XPS spectra
(Fig. 3), where again the results are very similar for the
two plasma exposure times.
Fig. 3. Oxygen content, expressed as O / C , of untreated
and treated polymers, samples (S-I) and (S-II).
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The same measurement was carried out on aged
samples (S-I) and (S-II), showing the variation of the
adhesion work, due to contact angle increase, whereas the
XPS values suffer no significant modification on aged
samples, compared to samples (S-I). The results obtained
for Wa are presented in Fig. 4, for the 10 s plasmaexposed samples.
Fig. 4. Adhesion work for untreated and 10 s treated and
aged polymers, samples (S-I) and (S-II).
It results that the tested polymers behave differently,
from the point of view of the plasma-modified surface
properties, also of the ageing behavior, depending, to
some extent, on their initial oxygen content. Nonetheless,
the limiting level of surface modification and the
maximum stress supported by the surface due to enhanced
polarity appear as the factors controlling the surface
processing outcomes.
Thus, PS exhibits the highest degree of surface
modification, in terms of both hydrophilic character /
adhesion (Fig. 2) and degree of surface oxidation, which
relates well to the minor level of oxidized functionalities
in its untreated structure (Fig. 3). Yet, the initial oxygen
content, which could imply lower or higher capacity of
the surface to get oxidized further, does not seem the
dominant factor in the process. In comparison, the other
polymers show rather similar degree of modification of
their surface hydrophilic character immediately after
treatment, lower than that of PS, although their oxygen
content is different (Fig. 3), pointing to some limiting
level of modification of the polymer topmost layer, due to
saturation of the surface in terms of incorporation of polar
groups. This also implies a maximum level of surface
oxidation attainable under plasma exposure, which is
confirmed by the XPS measurement (Fig. 3).
The similar results, for both Wa and O / C , for the two
treatment times, are supporting that surface oxidation
takes place at high rate and plasma exposure shorter than
10 s, under present configuration, is sufficient to achieve
the maximum attainable surface modification in terms of
incorporation of oxygen-related moieties, also implying
the saturation of the surface polarity.
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Furthermore, all tested polymers tend to reach the same
level of surface adhesion after ageing, corresponding to
Wa ≈ 113 mJ / m 2 (Fig. 4), demonstrating that the surface
stabilizes as to support a limiting level of stress due to
increased polarity created by the new functional groups.
The topmost layer of the plasma-modified surface is
reaching a new equilibrium, which does not depend on the
bulk properties of the material. Yet, this equilibrium, at
higher surface energy than the untreated polymer, may be
favored by the presence of a crosslinked barrier layer,
limiting the diffusion of polar groups to the bulk.
The restraining of the diffusion process during ageing is
demonstrated by the behavior of the water-immersed
samples (Fig. 4). Thus, samples (S-II) show measurably
lower Wa compared to samples (S-I) and most of the
recovery process appears to be compressed in those 30
minutes of water immersion, at least in terms of
wettability/adhesion, as pointed out by that the polymers
(S-II) undergo only limited subsequent ageing when
exposed to air.
This implies accelerated ageing due to reorientation of
polar groups in water, which decreases the surface energy.
Further ageing, under exposure in air for 30 days,
proceeds at much lower rate compared to the recovery
rate corresponding to the initial 30 minutes contact with
water. Such behavior is consistent to the creation of a
crosslinked barrier layer onto treated polymers, which
limits the diffusion.
PS presents again the highest rate of recovery after
water immersion, which may be explained by the
important perturbation of the PS surface by plasma
treatment. PS is the polymer undergoing the highest
degree of modification, both in terms of polarity and
oxidation, incorporating considerable amount of oxygenrelated polar groups in a non-polar structure, thus creating
high level of stress. The degree of perturbation is such
that the reorientation of the polar groups is not completed
in those 30 minutes of water immersion and continues, at
a lower, yet, significant rate, during further ageing in air.
Importantly, all samples are reaching, after 30 days of
ageing, comparable Wa values to samples (S-I), aged only
in air. This behavior demonstrates that the oxidized
groups created onto the surface are stable and are not
removed from the surface by interaction with water. This
is also demonstrated by XPS, where the oxygen content is
the same on samples (S-I) and (S-II) (Fig. 3).
The tests on solvent absorption offer additional data,
indirectly related to increased chemical stability of the
plasma-modified surface layer. Thus, the solvent
absorption time is significantly higher for plasma treated
samples, compared to untreated ones (Table 1), which is
consistent with the creation of a surface barrier layer by
crosslinking. The formation of chemical links between the
molecular chains yields a stable ordered threedimensional structure, with high density and cohesion,
and enhanced chemical resistance.
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Table 1. Solvent absorption time (in seconds).
Polymer
untreated
PSU
195
PEEK
97
PS
83
treatment time
10 s
30 s
10 s
30 s
10 s
30 s
(S-I)
215
234
111
132
139
164
(S-I) 30 days
213
230
108
129
130
158
Moreover, the fact that the absorption time displays
higher values for the 30 s treated samples than for the 10 s
treated ones, could relate to crosslinking being a slow
process, compared to functionalization, and proceeding on
more than one layer. Crosslinking occurs at lower rate
than oxidation, which is confined mainly to the topmost
layer. Then, although the oxygen species in the discharge
have high reactivity, their relative amount, due to air
entrained by the helium gas flow, is low compared to that
of high energy inert gas species (helium metastables [8]).
Since the radicals are continuously created at the surface,
these are not all converted to carbon-oxygen groups, and
participate, at a given rate, to crosslinking. Thus, stable
crosslinked layer forms in time.
Yet, the relation between surface stability and treatment
time is not observable by contact angle measured on aged
samples, because this measurement refers practically only
to the first monolayer at the surface.
Interestingly, the post-treatment recovery of the tested
samples can be described using a simple model, suggested
by the graphical representation of ln ( cosθ ) versus
ageing time, for the data presented in Fig. 2, where θ is
the water contact angle. From Fig. 5 it results that
ln ( cosθ ) has linear variation with time, as
ln ( cosθ ) = − K t , which implies that cos θ obeys a
natural decay law.
Fig. 5. Evolution of ln ( cosθ ) versus ageing time, for 10
s treated and aged polymers, samples (S-I).
Two phases of ageing can be distinctly separated by the
abrupt change of slope of the graph, from K1 to K 2 ,
associated to a first fast recovery sequence, dominated by
short-range polar groups orientation, and a second slow
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sequence, dominated by long-range diffusion/chain
relaxation mechanisms, respectively.
It results that short-range mechanisms occur at higher
rate, during ageing, compared to the long-range
mechanisms, for the polymers treated by helium
atmospheric-pressure plasma.
These results are to be confirmed by further studies,
where samples (S-I) and (S-II) will be monitored under
more varied ageing conditions, which could modify the
surface recovery rates.
Generally, combined short-range and long-range
mechanisms should be taken into account, their respective
rates during surface recovery depending on the particular
combination of plasma and polymer surface.
4. Conclusion
Helium atmospheric-pressure plasma allows efficient
and stable surface modification by creating complex
structure, most probably by combined functionalization/
crosslinking, taking place at different rates.
The limiting level of surface modification, in terms of
wettability and oxidation, and the maximum stress
supported by the surface due to enhanced polarity appear
as the factors controlling the surface processing outcomes.
More, the stability of the surface properties on plasmatreated polymers is depending on the degree of
perturbation of the surface, where the highest degree of
recovery corresponds to the polymers most perturbed by
increase of the polarity.
The dominant mechanism during ageing is the shortrange reorientation of surface polar groups, whereas the
diffusion process is considerably limited.
5. Acknowledgement
This work has been carried out in the CASPIA project,
funded by the Executive Agency for Higher Education
Research Development and Innovation, Romania, PN-IIPT-PCCA-2013 programme, grant 254/2014.
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