22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Myths of chemical functional group derivatization on polymer surfaces
plasma-treated in N 2 , NH 3 , and N 2 /H 2 mixtures
C.-P. Klages, Z. Khosravi and S. Kotula
Technische Universität Braunschweig, Institute of Surface Technology (IOT), Bienroder Weg 54 E, DE-38108
Braunschweig, Germany
Abstract: A myth is “an idea … that is believed by many people but that is not true” [1].
Since decades it was generally assumed that aromatic aldehydes react selectively with
amines, trifluoroacetic anhydride with amines and OH groups present on polymer surfaces
plasma-treated in virtually O-free N-containing gases. There is evidence that these
assumptions are myths and cannot be used to base quantitative surface analyses thereupon.
Keywords: Derivatization, amine, amino, imine, imino, aldehyde, TFBA, TFAA
1. Introduction
Since more than 40 years it is known that nitrogencontaining functional groups can be introduced on
polymer surfaces by plasma-treatment using low- or
ambient-pressure gas discharges in nitrogen or nitrogencontaining gases and gas mixtures [2, 3]. Plasma-based
nitrogenation of polymers is presently of considerable
interest for various applications of polymers; see for
example several articles in a recently published book [4].
Chemical derivatization, usually combined with XPS,
has in subsequent years commonly been used for
analytical studies of polymer surfaces treated by nitrogenplasmas in order to quantify surface functional groups
present. Because the formation of amino groups by the
plasma-exposure was conjectured already in early papers
[2, 3], electrophilic reagents were generally used for
derivatization, especially trifluoroacetic anhydride
(TFAA) [5, 6] or aromatic aldehydes like the frequently
used 4-(trifluoromethyl)benzaldehyde (TFBA) or
pentafluorobenzaldehyde (PFB) [7], supposed to form
trifluoroacetamides and 4-(trifluoromethyl)benzaldimines,
respectively, with the presumed amino groups. Any other
reaction possibilities of aldehydes except with primary
amines, following Eq. 1, were generally neglected.
H
H
R NH2 +
C
O
C
CF3
R N
CF3 + H2O
(1)
From a chemical point of view there can be no doubt
that organic acid anhydrides as well as aldehydes will be
able to react with amino groups present. TFAA was
generally considered to react with primary and secondary
amines and also with -OH groups present on the surface
as well. Although the presence of other nitrogen-bearing
functionalities such as imines -CH=N- was inferred
already some time ago [8, 9], these functional groups
were generally not considered to be potential reaction
partners of the derivatization reagents commonly used for
quantitative XPS analysis of the plasma-treated surfaces.
O-10-3
Based on results from recent experimental studies we
arrived at the conclusion that the usually made
assumptions concerning the reactions of electrophilic
derivatization reagents with polyethylene (PE) surfaces
after plasma treatment in N 2 , NH 3 , and N 2 /H 2 mixtures
are probably wrong. We used streaming post-discharges
(“afterglows”) of dielectric barrier discharges (DBDs) in
mixtures of N 2 with up to 4% H 2 to treat ultrathin films of
LDPE (low-density PE) spun onto a ZnS ATR crystal and
monitored the plasma modification process as well as
subsequent derivatization and other modification
reactions in situ by FTIR-ATR spectroscopy.
2. TFBA-reactive surfaces without amino groups
The objective of our experimental studies was
originally to measure densities of primary amino groups
on the polymer surfaces as a function of various
experimental parameters, using the supposedly proven
TFBA derivatization method in conjunction with a
quantitative evaluation of FTIR spectral band intensities.
To our surprise, however, we found that typical plasmatreated surfaces showed significant reactivity with TFBA
vapor resulting in irreversible bonding of TFBA, although
there were no indications of the presence of any -NH 2
groups in FTIR spectra taken from underivatized surfaces.
Fig. 1 shows a typical result: During exposure of the
plasma-treated surface to TFBA vapor, a strong vibration
appeared at 1324 cm-1, due to the C-CF 3 stretching
vibrations
in
4-trifluoromethyl-phenyl
moieties
immobilized by a reaction of TFBA with groups on the
surface. Quantitative evaluation of the data, using a
calibration with hydrocarbon solutions of N-alkyl-4trifluoromethyl-benzaldimines as model systems, showed
that about 1.6 TFBA molecules had reacted within
5 hours per 1 nm2 of the surface.
However, this reaction cannot be explained by the
presence of primary amino groups because, as shown in
Figure 2, amino groups are not detectable, even in H/D
exchange experiments, performed using exposure to
vapors of D 2 O or H 2 O, with careful inspection of the
1
1300
2
O-10-3
Area of 1324 cm-1 band,
integrated from 1300 to 1350 cm-1
Atmosphere:
2 - 290 min: TFBA vapor
290 - 320 min: N2
50
100 150 200 250 300
Time / min
Fig. 1. Area of C-CF 3 band during exposure of treated
LDPE to TFBA vapor. Start of exposure at 2 min. After
290 min the TFBA-loaded N 2 stream was replaced by
pure N 2 (after ref. [10]).
to 1100 cm-1 wavenumber region where the δ(ND 2 )
vibration should appear if -NH 2 groups were formed by
the plasma. We estimate that the detection limit is about
0.3 groups per nm2.
0.006
P
P:H-D
P:2D-H
0.004
Absorbance

R-CH 2 -CH=N-R’ + (CH 3 CO) 2 O
R-CH=CH-N(COCH 3 )R’ + CH 3 COOH
0
0.002
0.000
-0.002
-0.004
-0.006
3500
3000 2500 2000 1500
Wavenumber / cm-1
1000
Fig. 2. Top: Spectrum after 30 s afterglow exposure,
standard plasma conditions (P).
Bottom: Pair of
difference spectra during H-D exchange experiment after
standard treatment: Spectrum after H 2 O exposure minus
spectrum after previous D 2 O exposure (broken curve
P:H-D) and vice versa after 2nd D 2 O exposure (solid
curve P:2D-H) (arbitrary absorbance offsets) (after ref.
[10]).
The apparent discrepancy of these results lead us to
search for other nitrogen-bearing functional groups which
might be reactive towards TFBA. The most simple
candidate non-amine moiety which may react with TFBA
is the imino group, derived either from an aldehyde
(aldimine -CH=N-) or from a ketone (ketimine >C=N-).
In a recent paper we showed that imines with the general
formula R1R2C=N-R3 (R1, R2, R3 = alkyl, alkenyl, H)
are able to react with aldehydes as well. Imines have been
known to be present after nitrogen-plasma treatment of
polymers and in plasma polymers since decades and, as a
O-10-3
literature search shows, imines are able to react with
probably all derivatization reagents which are normally
considered as selective for primary amines or amines in
general [11]. With aldehydes, imines may undergo an
exchange of the carbonyl components and an aldol
reaction, resp., and with acid anhydrides formation of the
corresponding enamide is observed [12]:
(2)
The corresponding reactions generally proceed
uncatalyzed and smoothly at room temperature and we
have shown experimentally, that TFBA does not only
react rapidly with imines formed in situ in solutions, but
also, as a vapor, with imino groups bound covalently to
polymer surfaces.
These results show that neither the reaction with TFBA
nor derivatization with TFAA can be used to quantify
concentrations or densities of primary and secondary
amino groups unless there are good reasons to assume
that imino groups are absent. Such reasons have, however, to our knowledge never been presented in
corresponding studies.
3. Nucleophilic derivatization of plasma-treated PE
The quite obvious conclusion that imino groups might
be responsible for the binding of TFBA in our studies is
supported
by
experiments
using
nucleophilic
derivatization reagents 2-mercaptoethanol (ME) and
4-(trifluoromethyl)phenylhydrazine
(TFMPH)
[13].
According to XPS and FTIR-ATR analyses, performed ex
situ on PE foils treated in the post-discharge, both
reagents are able to bind to the plasma-treated surface.
Fig. 3 shows ATR spectra (diamond, 52°) obtained from
TFMPH-derivatized PE foil and from a solution of a
model hydrazine.
0.25
Hexanal TFMP-hydrazone
0.20
0.15
Absorbance
Band area / cm-1
0.14
0.12
0.10
0.08
0.06
0.04
0.02
0.00
0.10
0.05
TFMPH-deriv. PE
x 100
0.00
-0.05
-0.10
1650
1600 1400
1300
1200
1100
1000
-1
Wavenumber / cm
Fig. 3. ATR spectra of hexanal-4-(trifluoromethyl)phenylhydrazone (solution in hexadecane, top), and of
plasma-treated and gas-phase TFMPH-derivatized LDPE
3
surface after subtraction of an LDPE spectrum (multiplied
by 100, bottom) (abs. offset arbitrarily) (after ref. [13]).
4
O-10-3
4. CD-FTIR of directly DBD-treated PE surfaces
Interesting related results have recently been obtained
in experiments using a newly developed method for
combinatorial studies enabling the plasma-chemical
generation of spot arrays with controlled gradients of
physicochemical surface properties by plasma printing on
polymer surfaces [14]. The process is using a gascarrying microporous metal fiber compact of a few mm
thickness playing the role of an electrode and
simultaneously providing exchange of gaseous species
from a gas stream to individual discharges enclosed in
cavities with diameters in the range of typically 100 to
1000 µm. These cavities are defined by via-holes within
a structured dielectric or metallic “masking” layer
attached to the porous electrode. Due to the presence of
the polymeric substrate the process is basically a direct
DBD-type microplasma treatment.
Surface analysis was performed by FTIR-ATR
spectroscopy (diamond, 52°, parallel polarisation) of the
plasma-treated surface and after gas-phase derivatization
using the electrophilic aldehyde TFBA, the nucleophilic
hydrazine TFMPH, and 4-trifluoromethylphenylisothiocyanate TFMPITC, respectively, a reagent which
can undergo addition reactions with secondary and
primary amines (formation of thioureas), but - with
imines - also more complex reactions [13].
Fig. 4 shows the spectra obtained after derivatizing a
plasma-treated PE foil (N 2 + 1% H 2 ) with the three
reagents mentioned.
Although the three different
chemical structures in para position to the CF 3 group
responsible for the vibration band near 1325 cm-1 may
introduce some differences in the integrated intensities of
the bands, it seems clear that much more derivatization
reagent is bound in the reaction with hydrazine, TFMPH,
than in the reaction with the aldehyde, TFBA: Again the
electrophilic reactivity of the surface is more pronounced
than the nucleophilic reactivity, which is usually only
considered on surfaces after plasma treatment in the
N 2 -H 2 mixture.
The characteristic broad feature between 1400 and
1750 cm-1, introduced by the plasma treatment, is hardly
changed by derivatization with TFBA, due to the
relatively small quantity of reaction product formed.
More pronounced changes, however, are observed after
exposure to the vapors of the hydrazine and the
O-10-3
0.0030
after plasma
after TFBA
after TFMPH
after TFMPITC
0.0025
Absorbance
We assign the peaks at 1616 cm-1 in both spectra to the
hydrazone ν(C=N) vibration, which is expected to appear
between 1645 and 1610 cm-1. The strongest bands, due to
the ν(C-CF 3 ) vibrations, appear at 1324 cm-1 and
1323 cm-1 in the foil sample and the hydrazone solution,
respectively. The band area ratio of the presumed
ν(C=N) and ν(C-CF 3 ) vibrations are the same, within
accuracy limits of the measurements, in both spectra (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.
0.0020
0.0015
0.0010
0.0005
0.0000
1800
1700
1600
1400
1300
Wavenumber / cm-1
Fig. 4. ATR spectra of polyethylene foil treated by
DBD-type microplasma and after derivatization with
TFBA, TFMPH, and TFMPITC, respectively (absorbance
values offset arbitrarily).
isothiocyanate and interestingly the largest effect on the
spectra for these reagents is seen as a pronounced
lowering of the absorbance between 1550 and 1600 cm-1.
In case of the hydrazine, the vibrational band of
hydrazone is visible at 1617 cm-1.
Part of the reaction with the isothiocyanate may be due
to secondary amino groups which are difficult to detect by
IR spectroscopy due to small intensities of the
deformation vibration.
The generally observed
absorbance increase between 3200 and 3400 cm-1 upon
the plasma treatment points to the presence of N-H groups
but so far the obtained spectra and spectral changes upon
derivatization cannot be explained simply by reference to
isolated imino groups -CH=N- or secondary amino groups
-CH 2 -NH-CH 2 -.
Very recent experiments show that 4-trifluoromethylbenzylamine (TFMBA) is also able to bind to PE
surfaces treated in direct DBDs in N 2 or N 2 -H 2 mixtures,
with densities comparable to TFMPH [15].
Double bonds with nitrogen are also seen in recent
NEXAFS measurements on afterglow-treated PE [16].
5. Speculations about molecular surface structures
Looking for an explanation for the so far obtained
spectral data one has to take into account that the
chemistry of compounds containing carbon, hydrogen and
nitrogen is much more complex and richer than the
chemistry
of
oxygenated
organic
compounds.
Considering for example the possible isomers of
carbodiimides with the arrangement -N=C=N- of two N
atoms and one C atom, one finds not less than eight
different species, including the carbodiimide itself, with
different arrangements of these three atoms [17].
Even if one rules out exotic and less stable
configurations and considers just structures obtained by
combining two (or in one case three) of the more classical
“building blocks” -NH- and C=N, one ends up with the
following configurations, all leading to different chemical
reactivities towards aldehydes, hydrazines, and
isothiocyanates (Fig. 5). There are two reasons to focus
5
the speculations on structures with nitrogen atoms
inserted into the polymer chain: Aside from an argument
based on IR spectral evidence (missing of spectral
changes expected for H/D exchange in primary ketimine,
i. e. >C=N-H moieties) one can formulate a quite simple,
reasonable chemical mechanism by which the
combination of a ground state nitrogen atom N4S with
a -CH*- radical center on the polymer chain would form
secondary aldimine configurations -CH=N- within the
chain [10]. Secondary amine groups -CH 2 -NH- could
then hypothetically be formed by additions of two
hydrogen atoms to -CH=N-:
N
N
1,3-diazabutadiene
N
glyoxal diimine
N
N
azine
N
N
N
H
N
N
H
N
NH
hydrazone
H
N
amidrazone
amidine
H
N
N
H
sec. diamine
H
N
H
N
aminal
H
N
N
H
hydrazine
Fig. 5. Chemical configurations obtained by combining
two (in case of the amidrazone three) simple building
blocks C=N and -NH- as conceivable substructures of a
polyethylene chain after plasma modification in mixtures
of N 2 and H 2 .
At present, based on the (derivatization) FTIR data,
none of these structures can be excluded and possibly a
highly nitrogenated polymer surface has a combination of
all these and maybe more and even more complex
structures. In such a case it would be virtually impossible
to analyze the surface composition in terms of densities of
small individual functional groups.
6. Summary
So far frequently made assumptions concerning the
amino selectivity of several derivatization reagents used
for the analysis of polymer surfaces plasma-treated in N 2 ,
NH 3 , and N 2 /H 2 mixtures are wrong because they are
based on the wrong presumption that other potential
N-bearing groups like imines are unreactive with these
reagents. That, however, is not true; imines, for example,
6
are able to react with aldehydes like TFBA and with
anhydrides like TFAA.
There is evidence from FTIR measurements,
experiments with different derivatization reagents, and
recent NEXAFS measurements that C=N groups actually
play an important role in the surface chemistry of
polyethylene treated plasma-chemically in
N 2 /H 2
mixtures - using either flowing post-discharges or direct
DBD treatment.
However, N-H groups are also present, and the large
variety of chemical structures which can be generated by
just combining a few of the simple building blocks C=N
and -NH- give an impression of the formidable task of
revealing the true chemical nature of these surfaces.
7. References
www.merriam[1] Merriam-Webster
Online
webster.com
[2] J.R. Hollahan and B.B. Stafford. J. Appl. Polym.
Sci., 13, 807 (1969)
[3] G.J. Courval, D.G. Gray and D.A.I. Goring.
J. Polym. Sci, Polym. Lett. Ed., 14, 231 (1976)
[4] M. Thomas and K.L. Mittal, (Eds.). Atmospheric
Pressure Plasma Treatment of Polymers Relevance to Adhesion. (Weinheim: Wiley VCH)
(June 2013)
[5] Y. Nakayama, T. Takahagi, F. Soeda, K.J. Hatada,
S. Nagaoka, J. Suzuki and A. Ishitani. J. Polym.
Sci. A: Polym. Chem., 26, 559 (1988)
[6] A. Holländer, S. Kröpke and F. Pippig. Surf.
Interface Anal., 40, 379 (2008)
[7] D.S. Everhart and C.N. Reilley. Anal. Chem., 53,
665 (1981)
[8] R. Förch, N.S. McIntyre and R.N.S. Sodhi. J. Appl.
Polym. Sci., 40, 1903 (1990)
[9] P. Favia, M.V. Stendardo and R. d’Agostino.
Plasmas and Polymers, 1, 91 (1996)
[10] C.-P. Klages, A. Hinze and Z. Khosravi. Plasma
Process. Polym., 10, 948 (2013)
[11] C.-P. Klages, Z. Khosravi and A. Hinze. Plasma
Process. Polym., 10, 307 (2013)
[12] H. Breederveld. Recueil 79, 401 (1960)
[13] Z. Khosravi and C.-P. Klages. Plasma Chem.
Plasma Process., 34, 661 (2014)
[14] S. Kotula, A. Hinze, I. Krause, N. RodriguezCarrillo, M. Thomas and C.-P. Klages. Lecture at
PSE 2014, Garmisch-Partenkirchen 2014.
[15] S. Kotula. to be published
[16] Z. Khosravi, C.-P. Klages, A. Lippitz and W. Unger.
to be published
[17] D. Bégué, G.G. Qiao and C. Wentrup. J. Am.
Chem. Soc., 134, 5339 (2012)
O-10-3