22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Optimization of dielectric barrier discharge for atmospheric pressure deposition
of carboxyl-containing polymers
L. Zajíčková1,2, A. Manakhov1, M. Eliáš1,2, M. Michlíček1,2, A. Obrusník1,2, P. Jelínek1,2 and J. Polčák3
1
Plasma Technologies, CEITEC Central European Institute of Technology, Masaryk University, Brno, Czech Republic
2
Department of Physical Electronics, Faculty of Science, Masaryk University, Brno, Czech Republic
3
CEITEC - Central European Institute of Technology, Brno University of Technology, Brno, Czech Republic
Abstract: Maleic anhydride and acetylene diluted in argon were co-polymerized in
atmospheric pressure dielectric barrier discharge (DBD) at 6.6 kHz driving frequency. Gas
dynamics of argon in the DBD system with various configurations of gas supply was
modelled in 3D for a possible improvement of the coating uniformity. Chemical structure
of deposited films was optimized by variation of maleic anhydride and acetylene flow rates.
Keywords: carboxyls, anhydrides, plasma co-polymerization, maleic anhydride
1. Introduction
Surfaces with carboxyl functional groups find
applications in adhesion promotion [1], grafting of
molecules with specific functionalities (e.g. for reversed
adhesion [2]), improvement of cell colonization and
tissure engineering applications [3], immobilization of
biomolecules [4]. Carboxyl (COOH) functional groups
have been prepared by plasma processing in gas feeds
containing simple molecules like CO 2 /H 2 O [5] or
CO 2 /C 2 H 2 [6]. More efficient approach of carboxyl
incorporation employs plasma polymerization of organic
molecules containing carboxyl or anhydride groups such
as acrylic acid [6,7,8] and maleic anhydride [9,10,11],
respectively. High amount of carboxyl groups can be
incorporated at low energetic conditions of acrylic acid
(AA) low pressure plasma polymerization when the
deposition is dominated by oligomers but the films are
oily, sticky, swellable and soluble in water [6]. At
increased power delivery the fragmentation of the AA
monomer becomes more important and the XPS analyses
revealed C/O ratio of 0.55 and percentage of COOR peak
being 25 %.
Atmospheric pressure plasma polymerization is a
competing technology to low pressure discharges. Beck at
al. performed polymerization of AA mixed with He in
dielectric barrier discharge (DBD) at atmospheric
pressure [12]. At low delivered power the peak attributed
to COOR group took up to 29.7 % of the C1s XPS signal.
The carboxyl films are often intended for
bioapplications [3,4] that require a sufficient stability of
the films in aqueous media. The plasma polymer crosslinking improves the layer stability but it is often achieved
at the expenses of a functional group concentration.
Therefore, plasma co-polymerization of two monomers
offers an additional possibility to tune the film stability
and carboxyl functionalization efficiency. Manakhov et
al. studied a pulsed plasma co-polymerization of MA and
vinyltrimenthoxysilane (VTMOS) in atmospheric
pressure DBD [10]. As reported, up to 25% of carboxyl
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groups per carbon atom can be deposited although the
immersion in the de-ionised water during three hours did
not induce a degradation of the COOH amount monitored
by infrared spectroscopy.
Atmospheric pressure DBD can be operated either in
filamentary or homogeneous mode. Stabilization of
homogeneous DBDs requires suppression of filament
formation. It can be achieved by a specially structure
electrodes [13], higher frequency [14] or gas mixture.
Atmospheric pressure glow discharge (APGD) can be
ignited in He or Ne [15,16]. Another type of
homogeneous DBD can be obtained in nitrogen and it is
called atmospheric pressure Townsend-like discharge
(APTD) [17]. The homogeneous DBD discharge is more
suitable for homogeneous plasma treatment of heat
sensitive polymers or for homogeneous deposition [18]
but in varying deposition mixtures it is difficult to avoid
the filaments completely.
In this work, maleic anhydride (MA) and acetylene
(C 2 H 2 ) precursors were co-polymerized in atmospheric
pressure DBD with the goal to prepare crosslinked stable
plasma polymers containg a high concentration of
carboxyl groups. The films were deposited on electrospun
polycaprolactone (PCL) nanofibers aimed at soft tissue
enginering. Therefore, the crosslinking could not be
achieved by second monomer containing Si as already
reported MA and VTMOS but avoiding metalic elements
like Si. that could be used in soft tissue engineering, i.e.
without metalic element like Si. The second precursor,
acetylene, was chosen for an enhancement of the polymer
crosslinking while retaining the structure of MA as much
as possible. The second role of acetylene was to ignite
homogeneous mode of DBD [19].
2. Experimental Details
Plasma co-polymer layers were deposited in nearly
homogeneous mode of atmospheric pressure DBD from
the mixture of MA and C 2 H 2 diluted in Ar. The MA
vapours were delivered into the discharge by Ar flowing
1
through a bubbler with solid MA pellets. The DBD was
ignited by a sinusoidal high voltage (6.6 kHz) supplied
with a tunable generator providing 12 W power input. The
top electrode was connected to the high voltage whereas
the bottom one was grounded. The whole set-up was
enclosed in a metallic cube with the side dimensions of
500 mm. Before starting the experiment this chamber was
pumped down to a pressure of 100 Pa and then filled up to
the pressure of 96 kPa with the mixture used for
depositions.
The first DBD set-up, schematically depicted in Figure
1, consisted of bottom copper electrode with the
dimensions of 150 mm × 60 mm and the top hard
aluminium electrode 30 mm × 80 mm. They were both
covered by Al 2 O 3 dielectrics, 0.6 mm in thickness. The
gap between the ceramic plates was 1.7 mm. The
deposition mixture was fed by a slit positioned at the
upper electrode. The upper electrode with the gas supply
was horizontally moved above the bottom electrode with
the substrate.
Although the upper electrode was moving the
deposition uniformity was not satisfactory. Therefore,
different set-ups with the gas slit between two top copper
electrodes were investigated by gas flow simulations and
two of them were tested also experimentally.
PCL nanofibers were prepared by electrospinning with
NanospiderTM from ELMARCO s.r.o. (Czech Republic).
Polycaprolactone pellets (Sigma Aldrich, Mw = 80 000,
80 g) were dissolved in 500 ml of formic and acetic acids
in ratio 17:33 and maintained at 40 °C for 6 hours and
then next 12 hours at ambient temperature. The mixture
was subsequently stirred with 100 ml of 1:1 solution of
acetic and formic acids for next 12 hours. The PCL
solution was poured into bath with wired electrode 50 cm
in length. Nanofibers were collected on paper covering a
grounded electrode and travelling with speed of 15
mm/min. Electrospinning was performed with electrode
rotation speed of 4 RPM and voltage of 55 kV between
electrode and collector.
Characterization of surface chemistry of the deposited
films was performed by X-ray photoelectron spectroscopy
(XPS) using the Omicron X-ray source (DAR400) and
electron spectrometer (EA125) fitted on a custom built
UHV system. The narrow scan measurements were
performed at pass energies of 25 eV and the X-ray gun
power was set to 270 W. The electrons take off angle was
50°. The maximum lateral dimension of the analyzed area
was 1.5 mm. The quantification was carried out using
XPS MultiQuant software [20]. The XPS C1s and N1s
signals were fitted with the Casa XPS software after
subtraction of the Shirley-type background. The XPS data
curve fittings were performed in accordance with the
available literature on binding energies (BE) of different
carbon environments [21]. The fitting employed
Gaussian–Lorentzian (G-L) peaks with the fixed G-L
percentage 30%. The full width at half maximum
(FWHM) was set to 1.85 ± 0.05 eV for all the peaks.
Atomic force microscopy (AFM) was measured by a
2
commercial ambient scanning probe microscope (NTMDT Ntegra Prima) in a semicontact mode using
commercial silicon cantilevers NSG-10 (NT-MDT).
Fig. 1. Schematic drawing of the atmospheric pressure
DBD set-up used for the deposition of carboxyl-containg
plasma polymers from maleic anhydride.
3. Results
The optimized conditions for stable carboxyl-rich films
in the first DBD set-up were 3.0 sccm of acetylene and
0.4 sccm of MA (achieved with Ar flow of 1.8 slm
through the bubbler). XPS determined the composition
(without hydrogen) of few top nanometers of the film
surface as 72 at.% of carbon and 27 at.% of oxygen. The
concentration of oxygen in the deposited layers is
relatively high compared to the atomic composition of the
gas mixture supplied into the plasma. The ratio of MA to
C 2 H 2 in the gas mixture is equal to 1:7.5 that corresponds
to O/C ratio of 0.16. Hence, the incorporation of the
oxygen species into the plasma copolymer layer is more
efficient compared to the hydrocarbon species. Further
investigation of the surface chemistry was based on the
fitting of the XPS C1s peak that revealed details about the
carbon chemical environment. The density of COOR
groups (11 at.%) was comparable with the previously
reported results of MA or acrylic acid plasma
polymerization using atmospheric pressure DBD [10,12].
Fig. 2. ATR-FTIR spectra of PCL nanofibers
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The chemistry of deposited layers on PCL nanofibers
was studied by ATR-FTIR (Figure 2). Due to the high
penetration depth of ~1 µm, PCL signal was also visible
in the spectra. The region from 1900 to 2650 cm-1 is
excluded from the graph due to a diamond tip absorption.
The absorption at 1782 cm-1 has confirmed the presence
of a cyclic anhydride group in the polymer structure.
However, after the immersion in water only the traces of
the peak remained in the spectrum. It suggests that the
anhydride groups were hydrolysed under the water
immersion.
4. Conclusions
PCL nano/microfibers can be coated by atmospheric
pressure DBD by co-polymerization of maleic anhydride
and acetylene. The films at optimized conditions
contained about 11 % of COOR groups. ATR-FTIR
revealed that plasma co-polymer layers retained their
structure after water immersion.
5. Acknoledgements
This work was supported by the COST CZ project
LD14036 financed by the Ministry of Education of the
Czech Republic and the SCOPES project “Manufacturing
of biosensors aided by plasma polymerization” funded by
Swiss National Science Foundation. The research was
also supported by the project “CEITEC-Central European
Institute of Technology” (CZ.1.05/1.1.00/02.0068) from
the European Regional Development Funds.
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