Structure of Plasma-Deposited Copolymer Films Prepared from
Acrylic Acid and Styrene
Alaa Fahmy1,2, Renate Mix1, Andreas Schönhals1, Jörg F. Friedrich1
BAM Federal Institute for Materials Research and Testing, Unter den Eichen 87, 12200 Berlin,
Germany
2
Department of Chemistry, Faculty of Science, Al Azhar University Al Mokheim Al Daaem Street, Nasr
City, Cairo, Egypt
Abstract:
Changes in structure of copolymers composed of acrylic acid (AA) and styrene (S) (molar ratio AA/S
= 1/1) were studied with respect to duty cycle (DC) used for the film deposition. It is evidenced by
some papers that films produced at low DC show a high preservation of the structure of monomers in
plasma deposited polymers while high DC doses yield to fragmentation and loss in regular structure.
This fact is especially important for the deposition of monomers carrying functional groups. Regarding
plasma copolymerization as feasible method to finish surfaces with a definite number of functional
groups it is necessary to direct the attention to chemical nature and physical properties of deposited
layer. Therefore, different methods were used for the characterization of thin plasma copolymer films
(FTIR, dielectric spectroscopy, differential scanning calorimetry, XPS). Special attention was focused
on unambiguous identification of COOH groups by derivatization with trifluoroethanol (TFE) and
subsequent XPS measurement. The dielectric measurements showed that the crosslinking decreases at
AA/S copolymer in comparison to AA homopolymer. Furthermore a segmental motion was observed
in presence of styrene in the chain.
FTIR and XPS analysis and found that the
density of functional groups varies non-linear
Keywords: Acrylic acid-styrene copolymers •
with respect to the feed gas composition.
dynamic mobility • thin film properties
Dawson et al. [3] found an almost linear
correlation between the concentration of
1. Introduction
functional groups and the feed gas composition
for acrylic acid - 1,7-octadiene mixed plasma
Copolymerization is a method used to create
copolymerization as analyzed by XPS.
polymers with special properties different from
However, deviation from linearity might be
that of the homopolymers of both monomers.
due to the fragmentation and polyNot-withstanding the properties of copolymers
recombination. The effect of the plasma energy
depend on the comonomer units. Many
on the thin films was studied in literature by
chemical copolymerizations are based on
applying XPS, NEXAFS, and FTIR or
radical processes. One difference to plasmasometimes also by mass spectrometry but
assisted polymerization, however, is retention
combination of volume sensitive methods with
of their regular structure. If monomers A and B
surface analytics by XPS or FTIR was first
are copolymerized, the resulting copolymers
demonstrated by Fahmy et al. [4] In this work
can show random (-AA-B-AB-AAA-B-…),
the properties of copolymers of acrylic acid
alternating (-A-B-A-B-…), or block (-AAAand styrene are investigated by different
BB-AAAA-BBB-…) structure.
surface and volume sensitive techniques.
Plasma-copolymerized organic films are of
Concerning the latter point, in addition to
growing interest and have been studied by a
FTIR, dielectric spectroscopy is employed in a
number of groups in the last decade. The
broad range of frequencies and temperatures
interest in plasma copolymers is driven by
using molecular mobility as a probe for the
relevant technological applications in different
structure.
fields where a tailoring of surface properties is
important. Friedrich et al. [1,2] reported on
2. Experimental
tuning the density of functional groups at
surface
of
plasma
polymers
by
Plasma deposition
copolymerization of functional-groups bearing
monomers with neutral chain-extending
The deposition experiments were carried out in
comonomers. They investigated for instance
a stainless steel reactor Ilmvac, Germany with
plasma copolymerized films of ethylene or
volume of 50 dm³. The reactor was equipped
styrene with allyl amine or allyl alcohol by
with a pulsable radio-frequency (rf. 13.56
1
Standard conditions for depositing copolymer
films with 150 nm thickness composed of AA
and S units were: power 100 W, pressure 10
Pa, variable duty cycles (DC) (0.1, 0.2, 0.3,
0.5, 0.7, and 1.0 at a pulse frequency of 1
kHz). The DC and the effective power Weff are
defined as:
t pulse − on
t pulse − on + t pulse − off
DC =
Weff = W ×
(1)
t pulse − on
Fig. 2a and 2b show the C1s peak of AA/S
copolymer before and after derivatization with
TFE respectively:
C-C/C-H
a
intensity [cps.]
MHz) generator with an automatic matching
unit and a flat rf. electrode (5 cm x 35 cm). For
details see ref. [4].
π−π∗
C=O
C-O
O-C=O
(2)
t pulse − on + t pulse − off
296
W is the power input. A quartz microbalance
was used for controlling the deposition rate.
Liquid comonomers were stored in round
bottom flasks connected to the plasma chamber
by heated stainless steel gas-pipes kept at 75°C
and controlled by liquid flow controllers (Fig.
1).
292
288
284
binding energy [eV]
280
Figure 2a. C1s peak of plasma-polymerized acrylic acidstyrene copolymer before derivatization (DC =0.5). The
solid line represents the experimental data. The dashed lines
represent the individual contributions.
C-H/C-C
b
N2 balloons
Plasma
Sample
M1
M2
RF
electrode
Vapor inlet
Pulse
generator
intensity [cps.]
PC
Thickness
sensor
Matching
unit
C-O
CF3
π−π∗
>C=O
O-C=O
Rotating
cylinder
Liquid flow controller
Gas outlet
pump
Heated stainless steel gas-pipe
Figure 1. Plasma reactor.
296
292
288
284
binding energy [eV]
280
Figure 2b. C1s peak of plasma-polymerized acrylic acidstyrene copolymer after derivatization (DC =0.5). The solid
line represents the experimental data. The dashed lines
represent the individual contributions.
3. Results and discussion
1. Estimation of functional groups
The general method to estimate the number of
functional groups on surfaces is based on
reaction of these groups with a substance
containing at least one hetero element in its
structure and following XPS-measurement of
the derivatized surface. COOH groups are
reacted with trifluoroethanol corresponding to
scheme (1):
(CH3)3C N C N C(CH3) 3
COOH
+
HO CH2 CF3
pyridine
COO CH2 CF 3
Scheme 1. Derivatization reaction of COOH groups with
trifluoroethanol (TFE) in presence of pyridine and N, N´-ditertbutylcarbodiimide.
The deconvolution of the C1s peak of the
origin copolymer was done assuming five
components assigned to the following bonds:
C-C/C-H: 285.0 eV, C–O: 286.3 eV, C=O:
287.5 eV COO: 289.1 eV and shake up caused
by π→π*[5] interaction at (291.5 eV) hinting
to aromatic structures (Fig. 2a).
The C1s spectrum of the derivatized AA/S
copolymers layer was deconvoluted into six
components (Fig. 2b), including an additional
component at 293.0 eV for the CF3 bond.
The concentration of COOH groups can be
calculated to [4]:
C( COOH ) =100
[F ]
.
3[C ] − 2 [ F ]
(3)
3. Dynamic mobility and thermal stability
[C] and [F] indicate the concentration of
carbon and fluorine. On the basis of equation 3
the number of COOH groups/100 C atoms of
AA/S copolymer films was calculated. Fig. 3
shows the variation of COOH groups with the
DC:
20
C(COOH)
15
10
2
1
0
0.0
0.2
0.4
0.6
0.8
1.0
DC
Figure 3. Dependence of COOH per 100 C atoms in: squarePAA and star-AA/S copolymer on applied DC.
For more details see ref. [4].
2. ATR-FTIR analysis
νC=O
absorbance [a.u.]
Dielectric relaxation spectroscopy DRS was
used for investigating the molecular dynamics
of thin plasma polymer films. In particular, the
glass transition and if possible the glass
transition temperature (Tg) should be estimated
as well as oxidative and thermal stability.
It was proven by many works that DRS is a
very powerful tool to investigate the molecular
mobility and the structure of polymers [9].
This can be also adapted to thin and ultra thin
polymeric films or layers because the
sensitivity of DRS increases with decreasing
thickness of the capacitor dielectric (polymer
layer) [10].
To start the discussion of the plasma AA/S
copolymer structures, the dielectric loss versus
temperature (T) at a fixed frequency (f) (1 kHz)
for plasma homopolymers AA and S and a
copolymer AA/S are presented in Fig. 5.
A relaxation process indicated by a peak is
observed at low temperature for PAA, which is
assigned to the β-relaxation. β-relaxation
corresponds to localized fluctuation of the
dipoles. For higher T the dielectric loss
increases with clear indication of a further
relaxation process which is identified as
electrode polarization.
electrode polarization
2
1
log ε''
The IR spectra of plasma-polymerized AA,
AA/S, and PS are shown in Fig. 4. All spectra
exhibit stretching vibrations of polymer
backbone produced by νas/sym C–H in the range
2916 to 2844 cm−1 associated with C=O
stretching near 1700 cm-1.
0
α-relaxation
β-relaxation
-1
δCH2
νOH
+
δOH
νC-O
+
δOH
-2
δOH
PAA
AA/S
νCH2
νCHarom
4000
stretching and aromatic δ C–H occurs at
3080/3060/3020 cm−1 [6-8].
3000
ϖadjac.
2000
1000
-1
wavenumber [cm ]
PS
0
Figure 4. FT-IR spectra of the plasma-polymerized
homopolymers PAA and PS and the AA/S copolymer (DC= 0.5).
Regarding the PS segments, styrene absorption
occurs at 700/760 cm−1 aromatic C–C
stretching, at 1494/1601 cm−1 aromatic C=C
-3
100
200
300
T [K]
400
500
Figure 5. Dielectric loss ε'' versus T at a frequency of 1 kHz
for: solid square-PAA, circle- PS and solid star- AA/S
(co)polymer on heating (DC=0.5).
Electrode polarization is produced by
accumulation of charge on electrode surfaces
and the formation of electrical double layers.
The increase of the dielectric loss is related to
conduction phenomena. The latter are related
to the drift motion of charge carriers,
originating from the carboxylic group.
In case of PS and AA/S at the same conditions,
the electrode polarization was observed. Some
additional charges originate from post-plasma
oxidation were accumulated on the electrode
surface which related to electrode polarization
for PS. In the range of DSC-measured glass
transition, a relaxation process, which
corresponds to the dynamic glass transition is
visible (α-relaxation). The α-relaxation is
related to Tg of the system and for that reason
process is called dynamic glass transition. αrelaxation is due to the segmental motion of
the chain. Probably, the distance between the
chains increases due to the presence of the
styrene
repeat
units.
Therefore,
the
crosslinking decreases and segmental motion
was observed.
The dielectric spectra are analyzed by fitting
the model function of Havriliak and Negami
(HN-function) to the data, which serves for
calculating the relaxation rate at maximal
dielectric loss f p and the dielectric relaxation
strength ∆ε. The HN-functions (4) reads
ε*(f) − ε ∞ =
∆ε
( 1 + (if/f 0 ) β )γ
(4)
where f0 is a characteristic frequency related to
the frequency of maximal loss f p (relaxation
rate), β and γ are fractional parameters (0<β≤1
and 0<βγ≤1) characterizing the shape of the
relaxation time spectra. ε ∞ gives ε´ for f→∞.
The relevant details can be found in ref. [11].
Therefore, to analyze the relaxation process
found for AA/S copolymer the HN-function is
again applied to the data. The electrode peak is
taken into consideration by a Debye function
as described in ref. [12]. Therefore the whole
fit function reads as:
ε ' ' (f) = Im{
∆ε
}+ A* f
( 1 + (if/f 0 ) β ) γ
(5)
where A is a fitting parameter. Conduction
effects were treated by adding a conductivity
contribution σ 0 / ε 0 ( 2πf ) to the dielectric
loss. σ 0 is a fitting parameter related to the dc
x
conductivity of the sample and ε 0 is the
dielectric permittivity of vacuum. The
parameter x (0<x≤1) describes for x<1 non-
Ohmic effects in the conductivity. For more
details see [11].
4. Conclusion
AA/S (co)polymers were deposited by pulsedplasma polymerization. Both, the structureproperty relationships and the functionality of
these polymers, were studied with respect to
DC by different techniques and probes.
It was found that the crosslinking decreases at
AA/S copolymer in comparison to AA
homopolymers. Furthermore, α-relaxation in
DRS results was observed which is related to
the segmental fluctuations of the styrene repeat
unit.
5. References
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(2004) Plasma Processes Polym. 1: 28
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Oehr C, Wertheimer MR (2005) Wiley-VCH,
Weinheim, S. :3
[3] Dawson RA, Short RD (1998) Biomaterials
19: 1717
[4] Fahmy A, Mix R, Schönhals A, Friedrich
JF (2011) Plasma Processes. Polym. 8: 147
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[9] Runt JP, Fitzgerald JJ, in Dielectric
Spectroscopy of Polymeric Materials, (1997)
(Editors) ACS books Washington DC
[10] Hartmann L,
Fukao K, Kremer F
”Molecular Dynamics in Thin Polymer Films”
in Broadband Dielectric Spectroscopy, Kremer
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[11] Schönhals A, Kremer F “Analysis of
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(Editors) Springer Verlag Berlin: 59
[12] Labahn L, Mix R, Schönhals A, (2009)
Phys. Rev. 79: 011801