22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Electrophilic character of PE surfaces plasma-treated in N 2 or N 2 -H 2 mixtures
at atmospheric pressure
Z. Khosravi, S. Kotula and C.-P. Klages
Institute of Surface Technology (IOT), Technische Universität Braunschweig, Bienroder Weg 54 E, DE-38108
Braunschweig, Germany
Abstract: Atmospheric-pressure plasma-based surface functionalization of polymers with
nitrogen-bearing groups has gained much interest for various applications. Therefore, the
understanding of the characteristics of plasma-treated surfaces is of considerable
importance. The main objective of this study is to examine the chemical nature of the
freshly plasma-treated surfaces. Therefore investigations of polyethylene surfaces treated
in a flowing DBD post-discharge and in a special setup for combinatorial direct DBD
microplasma treatment, respectively, were done. For chemical-derivatization-FTIR
analysis of the surfaces nucleophilic as well as electrophilic reagents were employed.
Keywords: derivatization, FTIR, amine, amino, imine, imino, aldehyde
1. Introduction
Since the probably earliest paper on plasmanitrogenation of various polymers by low-pressure plasma
treatment in NH 3 or N 2 -H 2 mixtures, entitled
“Attachment of Amino Groups to Polymer Surfaces…”,
[1] the attention of subsequent research work was mainly
focussed on generation, detection, and utilization of
primary and secondary amino groups presumed to
dominate or at least to be present on the surface (the first
paper on plasma-nitrogenation of polyethylene (PE) in a
nitrogen “corona” discharge at atmospheric pressure is
more reluctant with conclusions with regard to the surface
chemistry [2]).
In literature, chemical species formed after plasmaassisted surface functionalization of polymers were
generally studied using XPS [3-5]. Due to the insufficient
ability of this analytical tool to distinguish between
functional groups, it was often combined with preceding
chemical derivatization (CD-XPS) [6].
Therefore,
electrophilic reagents such as trifluoroacetic anhydride
(TFAA) [5, 7] or aromatic aldehydes such as the
frequently used 4-(trifluoromethyl) benzaldehyde (TFBA)
or pentafluorobenzaldehyde (PFB) [8] were applied as
derivatization reagents. For example, Choukourov and
his co-workers determined the surface concentrations of
primary and secondary amino groups separately by
chemical derivatization using TFBA and TFAA
(trifluoroacetic anhydride (TFAA) has been routinely
used to label the hydroxyl groups with a trifluoroacetate
moiety [10]) on plasma polymerized thin films. They
assumed that TFBA reacts only with primary amines and
TFAA with both primary and secondary amines [9].
Although formation of imines -CH=N- on plasma
treated PE and PS surfaces was reported in some papers,
see, e.g., ref. [11], its reaction with typically used
derivatization reagents was not considered.
In this work, FTIR-ATR spectroscopic investigations in
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situ and ex situ of ultra-thin low-density polyethylene
(LDPE) films and LDPE foils plasma-treated remotely in
a flowing DBD post-discharge (“afterglow”) and directly
in DBD-type microplasma, respectively, are discussed.
The films or foils were treated with plasmas in N 2 -H 2
mixtures containing up to 4% H 2 . Gas-phase chemical
derivatization with vapors of electrophilic reagents TFBA
and TFAA was applied to the plasma-treated surfaces and
also nucleophilic derivatizations were performed, using
4-trifluoromethylbenzylamine (TFMBA), 2-mercaptoethanol (ME) and 4-(trifluoromethyl)phenylhydrazine
(TFMPH) vapors.
Results from both directly and afterglow-treated
surfaces indicate that a considerable amount of every
nucleophilic reagent used is able to bond to the treated
surfaces. It is also observed that the reaction of treated
surfaces with the amine (TFMBA) is comparable to that
of the aldehyde TFBA. These results indicate that such
surfaces have similar electrophilic and nucleophilic
character, in contrast to the generally assumed and so far
accepted opinion. In our last publication, chemical
reactivity of PE surfaces after plasma treatment was
explained by the presence of imino groups >C=N- [12].
In fact, imino groups are the least complex chemical
moieties which are able to explain several experimental
results obtained previously, including the IR bands
positions observed after plasma treatment, the reaction of
treated surface with TFBA in virtual absence of primary
amino groups as evidenced by the lack of -ND 2 vibrations
in infrared spectra of deuterium-exchanged surfaces [13],
and the electrophilic reactivity of treated surfaces.
Although the presence of imino groups on plasma treated
polymers was reported already decades ago, its reaction
with aromatic aldehydes like TFBA has not been
considered. Several reactions of imine with derivatization
reagents, which are frequently supposed to be selective
for primary amines, were reported [14].
1
2.1. Gas phase reaction of TFBA with imines on solid
surfaces: PS-NH 2 beads
Aminomethylated polystyrene (PS) beads were exposed
to isobutyraldehyde (IB) for 60 min. Next, after one night
under nitrogen, samples were exposed to TFBA vapor.
IR measurements of the beads after exposure to IB
vapor show a strong vibration at 1668 cm-1 which can be
attributed to the newly formed imine, isobutyraldimine.
The broad peak at 1750-1700 cm-1 corresponds to the
carbonyl group of aldehyde moieties, see Fig. 1.
0.35
0.30
Absorbance
0.25
PS-NH2
(PS-NH2/IB - PS-NH2) x 10
(PS-NH2/IB/TFBA - PS-NH2/IB) x 10
0.20
0.15
0.10
0.05
0.00
-0.05
1750 1700 1650 1600 1550 1500 1450 1400 1350 1300 1250
Wavenumber / cm-1
Fig. 1. FTIR-ATR spectra (diamond, 52°) of aminomethylated polystyrene beads (top, offset 0.1), beads after
exposure to isobutyraldehyde (IB) subtracted from
spectrum of beads (middle, offset 0.05) and IB-reacted
beads after exposure to TFBA vapors minus spectrum of
IB-exposed beads (bottom).
The beads were subsequently exposed to the TFBA
vapor for 60 min and IR difference spectra showed a
positive band at 1720-1700 cm-1, with the peak position at
1710 cm-1, probably due to C=O vibration in physisorbed
TFBA. Weak negative absorption at ~1668 cm-1 can be
attributed to the vanishing of isobutyraldimine groups,
while a weak positive absorption at 1647 cm-1 is due to
the newly formed 4-trifluorobenzaldimine (see Eq. 1).
Bands in the range of 1320 to 1325 cm-1 are related to
ν(C-CF 3 ) vibrations in TFBA and 4-trifluorobenzaldimine, respectively [15].
R N
CH
C5H11
R
CH
C6H4 CF3
O
CH
C 6H 4
O
HC
3. Evidence of N=C groups from NEXAFS
In addition to the experiments described above,
demonstrating that imines may in fact react with TFBA,
near-edge X-ray absorption fine structure (NEXAFS)
measurements have been employed to provide insight into
the chemical nature of LDPE thin films plasma-treated by
DBD afterglows in N 2 or N 2 -H 2 and to confirm that
imines or other moieties with N=C double bonds are
actually present on the plasma-treated surfaces. The
nitrogen and carbon K-edge spectra of treated LDPE thin
films show the presence of significant amounts of
nitrogen in N=C or N≡C bonds and carbon in C=C bonds
[16].
4. Nucleophilic derivatization of PE surfaces plasmatreated in N 2 or N 2 -H 2
Unlike amino groups, imino groups have the ability to
react with hydrazines, thiols, amines, hydroxylamines,
and other nucleophilic compounds due to their
electrophilic properties. XPS and FTIR analyses ex situ
of derivatized LDPE foils showed that ME (overnight
exposure to ME vapors at 50 °C in an evacuated glass
tube, then overnight evacuation at room temperature) as
well as TFMPH (same procedure) are bonded to the
plasma-treated surface. XPS investigations showed that
noticeable amounts of sulphur and fluorine were detected
on plasma-treated surfaces derivatized with ME and
TFMPH, respectively.
FTIR-ATR spectra of MEderivatized plasma-treated PE foils indicate chemical
bonding of the thiol, see Fig. 2.
0.4
0.3
0.2
PE/plasma/ME - PE x 20
2-Mercaptoethanol
ATR spectra:
diamond,
s-pol, 52°
0.1
CF3
(1)
N
with TFBA, while a new imine band at 1647 cm-1 is seen
which is in a good agreement with Eq. 1. We also can see
C=O vibrations at 1711 cm-1 from the aldehyde groups.
All the above results confirm the reaction of TFBA with
imines in both solid and liquid phase.
Absorbance / A
2. Reaction of aromatic aldehydes with aliphatic imine
Two experiments related to imine reaction with
aromatic aldehydes in gas and liquid phase are reported
below.
C5H11
0.0
1400
1300
1200
1100
1000
900
Wavenumber / cm-1
2.2. Exchange reaction between aliphatic imine and
TFBA in the liquid phase
Another example of imine synthesis in situ and its
reaction with TFBA has been checked for
ethylhexylamine and hexanal in diglyme.
IR
measurement of this solution shows the formation of an
imine band at 1670 cm-1 which is vanished after reaction
2
Fig. 2. FTIR-ATR spectra (diamond, 52°, s-polarization)
of 2-mercaptoethanol (dash) and of LDPE foil
plasma-treated and derivatized in the vapor of
2-mercaptoethanol overnight at 50 °C (solid).
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ATR spectra (diamond, 52°) from a TFMPHderivatized PE foil and from a solution of a model
hydrazine are shown in Fig. 3. The band area ratio of the
presumed ν(N=C) at 1616 cm-1, and ν(C-CF 3 ) at
1323 cm-1 (in the reference solution) or 1324 cm-1 (in the
foil) is, within accuracy limits of the measurements, in
both spectra the same (0.31 in the solution and 0.32 in the
foil spectrum), indicating the binding of the derivatizing
molecules as a hydrazone - not as a hydrazine or
hydrazide. The area density is appreciable - about 5 per
nm2.
investigations of directly plasma treated PE using a
gradient of hydrogen (0 to 1%) in nitrogen followed by
chemical derivatization with the nucleophilic TFMBA.
The derivatization was done for three hours in vacuum.
After reaction, the sample was stored in vacuum
overnight to remove any physisorbed residues of
TFMBA. Surface analysis was done by FTIR-ATR
spectroscopy (diamond 52° and parallel polarisation). If
electrophilic groups on the surface react with TFMBA,
for example imine as in the proposed reaction scheme (2),
a vibration appears around 1325 cm-1 from ν(C-CF 3 ).
N
0.15
Hexaνal TFMP-hydrazoνe
ν (C-CF3) 1323
ν (N=C) 1615
0.20
Absorbaνce / A
N
CF3
+
0.25
ν (C-F) iν CF3
TFMPH-deriv.
LDPE x 100
CF3
+
(2)
H2N
The intensity of this absorption band indicates the
amount that has reacted with the surface. Results are
shown in Fig. 5.
0.10
0.05
H2N
0.00
-0.05
99% N2+1%H2
-0.10
1650
1600 1400
1300
1200
1100
1000
-1
Waveνumber / cm
Fig. 3. FTIR-ATR (diamond, 52°) spectra of hexanal-4(trifluoromethyl)phenylhydrazone
(solution
in
hexadecane, dash), and of plasma-treated and gas-phase
TFMPH-derivatized LDPE surface after subtraction of an
LDPE spectrum (multiplied by 100, solid) (Abs. offset
arbitrarily) (After ref. [12]).
5. Combinatorial studies of directly plasma-treated
surfaces followed by nucleophilic derivatization
Our group is developing methods for combinatorial
studies enabling the plasma-chemical generation of
coatings or spot arrays with controlled gradients of
physico-chemical surface properties by plasma-printing
utilizing the diffusive mixing of two gases in a porous
medium. The setup that was used for the presented
results is shown in Fig. 4.
Fig. 4. Plasma-printing setup [17].
Using this setup, it has been shown that the
electrophilic character of nitrogen treated surfaces does
not only exist on surfaces that were treated using a
flowing DBD post-discharge but is also valid for direct
treatment [17].
Here we present combinatorial
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100% N2
intensity n(C-CF3) / a.u.
0.013
0.014
0.015
0.016
0.017
0.018
0.019
Fig. 5. Top view of a selectively plasma-treated PE foil
with a gradient of H 2 in N 2 , followed by TFMBA
derivatization. Plot shows the intensity of IR absorption
at 1325 cm-1. Arrows indicate inlet of gases N 2 (top
right) and N 2 -H 2 (top left). Numbers are in arbitrary
units.
It can be seen that H 2 present in the N 2 plasma leads to
a higher reactivity of the surface towards the TFMBA,
i.e., the electrophilic reactivity increases. There seems to
be a maximum of the intensity around 0.5% H 2 . Similar
behaviour was seen for N 2 -H 2 plasma treatment of PE
followed by electrophilic derivatization with TFBA [16]
possibly indicating that similar groups react with both
reagents. Remarkably a comparable amount of TFBA
and TFMBA reacted with the treated surface.
To take into account the effects of different parasubstituent on the ν(C-CF 3 ) vibrational band intensities
after derivatization we prepared xylene solutions of an
imine, obtained in situ from TFBA and 2-ethylhexylamine, and TFMBA, respectively. It was seen that the
intensity in the TFBAldimine was larger by a factor of
only 1.13, showing that indeed a comparable amount of
3
TFMBA and TFBA had reacted with the plasma-treated
surface.
To exclude the possibility that the electrophilic
character of the surfaces is due to presence of carbonyl
compounds (e.g., ketones) which may have been formed
when oxygen is present in the plasma or by hydrolysis of
N=C groups after treatment in the open humid air, we
compared surfaces that were treated with mixtures of
N 2 -H 2 with such treated in a mixture of N 2 -O 2 both
followed by derivatization with TFMPH using the
previously described procedure. Spectra were taken
directly after plasma treatment and after derivatization
reaction. Results are shown in Fig. 6.
N2-H2 after plasma
N2-H2 after plasma + TFMPH
N2-O2 after plasma
N2-O2 after plasma + TFMPH
0.012
0.008
0.004
-0.004
1800
1700
1600
~1325
0.000
-0.002
1616
ν(N=C)
0.002
1500
ν(C-CF3)
1530
δ(N-H)
0.006
1730
ν(C=O)
Absorbaνce / A
0.010
1350 1300
Waveνumber / cm-1
Fig. 6. FTIR-ATR spectra (diamond, 52°, p-polarization)
of PE surfaces treated with mixtures of N 2 -H 2 (1 % H 2 ,
dash-dot) and N 2 -O 2 (1% O 2 , dot) followed by
derivatization with TFMPH (solid and dash, respectively).
Treatment with both N 2 -O 2 and N 2 -H 2 lead to reaction
of the surface with TFMPH which can be seen by
emerging peaks at ~1325 cm-1 and 1616 cm-1, δ(N=C).
However changes in the region of N-H and O-H
vibrations (~1800-1450 cm-1), which are characteristic for
polymer treatment in N 2 and O 2 plasmas are very
different. Treatment with N 2 -O 2 plasma followed by
reaction with TFMPH shows a decreasing peak at
1730 cm-1 due to reacting carbonyl compounds. Also new
vibrations appear in the region of 1550-1500 cm-1 which
is not completely understood to date. In contrast the
treatment with N 2 -H 2 plasma followed by reaction with
TFMPH only shows a decreasing vibration around
1570 cm-1. So far it is not understood which reaction has
been occurred. The spectra, however, show no changes in
the range of 1800-1600 cm-1 indicating that no reaction of
carbonyl compounds had taken place. Interestingly, also
the position of ν(C-CF 3 ) differs between treatments with
N 2 -O 2 and N 2 -H 2 plasma (1330 cm-1 and 1325 cm-1
respectively) indicating a different bonding structure of
the CF 3 -containing group or a different environment.
Electrophilic (TFBA) and nucleophilic (ME, TFMPH,
TFMBA) reagents have been applied to the treated
surfaces. In all cases considerable amounts of reagent
reacts with the treated surface, generally even more of a
nucleophilic reagent like TFMPH is bonded to the surface
than electrophilic TFBA.
Remarkably, the plasma
treatment of PE in N 2 -H 2 seems to result in surfaces with
amino-reactive character comparable to the amount of
amine groups.
7. References
[1] J.R. Hollahan and B.B. Stafford. J. Appl. Polymer
Sci., 13, 807 (1969)
[2] G.J. Courval, D.G. Gray and D.A.I. Goring.
J. Polymer Sci, Polymer Lett. Ed., 14, 231 (1976)
[3] N. Vandencasteele and F. Reniers. J. Electron.
Spectrosc., 178–179, 394 (2010)
[4] L.J. Gerenser. J. Adhesion Sci. Technol., 7, 1019
(1993)
[5] Y. Nakayama, T. Takahagi, F. Soeda, K.J. Hatada,
S. Nagaoka, J. Suzuki and A. Ishitani. J. Polymer
Sci. A: Polym. Chem., 26, 559 (1988)
[6] D. Briggs and C.R. Kendall. Int. J. Adhes. Adhes.,
2, 13 (1982)
[7] A. Holländer, S. Kröpke and F. Pippig. Surf.
Interface Anal., 40, 379 (2008)
[8] D.S. Everhart and C.N. Reilley. Anal. Chem., 53,
665 (1981)
[9] A. Choukourov, J. Kousal, D. Slavínska,
H. Biederman, E.R. Fuoco, S. Tepavcevic,
J. Saucedo and L. Hanley. Vacuum, 75, 195 (2004)
[10] A. Chilkoti and B.D. Ratner. Surf. Interface Anal.,
17. 567 (1991)
[11] R. Förch, N.S. McIntyre and R.N.S. Sodhi. J. Appl.
Polymer Sci., 40, 1903 (1990)
[12] Z. Khosravi and C.-P. Klages. Plasma Chem.
Plasma Process., 34, 661 (2014)
[13] C.-P. Klages, A. Hinze and Z. Khosravi. Plasma
Process. Polymers, 10, 948 (2013)
[14] C.-P. Klages, Z. Khosravi and A. Hinze. Plasma
Process. Polymers, 10, 307 (2013)
[15] Z. Khosravi and C.-P. Klages. in: Poster at PSE
2014. (Garmisch-Partenkirchen, Germany) (2014)
[16] Z. Khosravi, C.-P. Klages, A. Lippitz and W. Unger.
to be published (2015)
[17] S. Kotula, A. Hinze, I. Krause, N. RodriguezCarrillo, M. Thomas and C.-P. Klages. in: Lecture
at PSE 2014. (Garmisch-Partenkirchen, Germany)
(2014)
6. Summary
Polyethylene
surfaces
remotely
or
directly
plasma-treated in nitrogen-containing gases and
subsequently chemically derivatized have been studied
utilizing FTIR in situ or ex situ and XPS ex situ.
4
O-5-5