nitrogen plasma modification and chemical derivatization of polyethylene surfaces - an in situ study...

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Nitrogen Plasma Modification and ChemicalDerivatization of Polyethylene Surfaces –An In Situ Study Using FTIR-ATR Spectroscopy

Claus-Peter Klages,* Alena Hinze, Zohreh Khosravi

Chemical derivatization reactions of nitrogen plasma-treated surfaces with aromaticaldehydes, such as the prototypic 4-trifluoromethyl-benzaldehyde (TFBA), have for a longtime been considered selective for primary amines. Results of an in situ study using FTIR-ATR
spectroscopy challenge the validity of this assumption:Modification of polyethylene surfaces by afterglowsof dielectric barrier discharges in nitrogen–hydrogenmixtures with subsequent hydrogen/deuterium ex-change or TFBA derivatization suggests that the latterdoes not follow the commonly assumed reactionscheme.
Prof. C.-P. Klages, A. Hinze, Z. KhosraviTechnische Universit€at Braunschweig – Institut f€urOberfl€achentechnik (IOT), Bienroder Weg 54 E 38108,Braunschweig, GermanyE-mail: [email protected]

Plasma Process. Polym. 2013, DOI: 10.1002/ppap.201300033

� 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com

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r reaction abilities during chemical derivati-
1. Introduction
In a recent publication we critically examined a basic

precondition on which chemical derivatization analyses of

polymer surfaces treated in nitrogen-containing plasmas

have usually been based. Since about three decades it

has been common usage to assume for several chemical

reagents that they are able to indicate selectively the

presenceofprimaryaminogroupsonsuchsurfacesbecause

these reagents have been deemed reactive onlywith�NH2

groupsofprimaryamines.Weshowedthat thisassumption

cannot be maintained, however, in situations where the

presence of other moieties such as imino groups cannot

be disregarded. Although imines have long been known

to be formed on polymer surfaces exposed to nitrogen

plasmas, thei

zation have generally been ignored.[1]

While theargumentation inour formerpaperwas largely

based on literature studies, the present article reports

selected results of ongoing experimental investigations in

situ on surface treatment of polyolefins by atmospheric-

pressure dielectric barrier discharges (DBDs) in nitrogen-

containing gases, using Fourier transform infrared spec-

troscopy in the attenuated total reflectionmode (FTIR-ATR).

The aim of these investigations is a comprehensive

knowledge and understanding of plasma-chemical gener-

ation of chemical functional groups on polymer surfaces

and their temporal development in the presence of inert or

reactive atmospheres.

More detailed investigations are still required to give a

virtually complete explanation of IR-spectroscopic results

obtained in situ during plasma exposure of polymer

surfaces. The purpose of this paper is to present examples

of nitrogen-plasma-treated polyethylene surfaces which

are able to react with 4-trifluoromethyl-benzaldehyde

(TFBA) and to bind the 4-trifluoromethyl-phenyl moiety

1DOI: 10.1002/ppap.201300033

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C.-P. Klages, A. Hinze, Z. Khosravi

2

REa

with a certain area density rTFBA, although primary amines

with a corresponding density rNH2¼ rTFBA cannot be

detected by FTIR spectroscopy. It will also be shown

that several of the IR spectroscopic results are not in a

qualitative agreement with a selective reaction of primary

amines with TFBA furnishing the corresponding imines,

see Equation (1):

CF3CH

ONH2 +R CF3C

H

N + H (1)2O

R

Thereby additional, experimental arguments are pre-

sented against the thesis of primary-amine selectivity of

TFBA.

Figure 1. Schematic view of the experimental setup used in thisstudy, not to scale. The figure shows a bubbler flask filled withheavy water, D2O. For chemical derivatization this flask is filledwith TFBA.

2. Experimental

2.1. Materials

Decalin (decahydronaphthalene, mixture of cis and trans isomers,

anhydrous, �99%), isopropanol (ultrapure, 99.9%), xylene (p-xylene,anhydrous, �99%), hexadecane (99%), 3-aminopentane, 1-amino-

hexane, 1-aminooctane, TFBA (98%), and heavy water (D2O, deuteri-

um oxide, 99.9at% D) were obtained from Sigma–Aldrich Chemie

GmbH (Schnelldorf, Germany), 2-aminooctane from Ominilab-

Laborzentrum (Bremen, Germany), 1-amino-2-ethylhexane from

Acros Organics (Nidderau, Germany), and used without further

purification. Sodium carbonate was from Merck (Darmstadt,

Germany). LDPE foils were purchased from Goodfellow GmbH (Bad

Nauheim, Germany). Gases were cleaned by purification systems

purchased fromMesser Griesheim GmbH, (Duisburg, Germany) and

Spectron GmbH (Frankfurt am Main, Germany).

2.2. Film Preparation and Characterization

Ultrathin LDPE films were prepared from purified polymer

obtainedbydissolutionof LDPEfoils inhotxyleneandprecipitation

in isopropanol. Typically 10 g of the polymer were dissolved in

200ml xylene at a temperature of 80 8C and precipitated by slowly

pouring the hot solution into 1 L of warm (40–50 8C), well-stirred

isopropanol. The precipitate was collected on a 125mm diameter

filter paper using a B€uchner funnel on a vacuumflask andwashed

thoroughly with small portions of warm isopropanol, 1 L in total.

The polymer was then dried in an oven at 60 8C overnight. This

procedure was repeated twice. Purity checks of the obtained

powder using ex situ FTIR-ATR (diamond, 508) show that the

purified product is virtually free of fatty acid amides.

Hot solutions of the purified LDPE in decalin were used to spin-

coat ultrathin polymer films with thicknesses between 50 and

200nm on the preheated ZnS ATR crystal. In order to obtain a

thickness of about 100nm, 100mg LDPE were dissolved in 10ml

decalin at 80–90 8C under continuous magnetic stirring. The ZnS

crystal, coveredbyaglasscup,waspreheatedonahotplate (150 8C).For film preparation the crystal was transferred to a spin coater

(modelWS-400E-6NPP, Laurell Technologies,NorthWales, PA,USA)

equipped with a suitable holder featuring a rectangular recess to

accommodate the crystal. A fewdrops of thehot solutionof LDPE in

decalin were transferred to the hot ZnS substrate using a glass


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pipette. Substrate rotation was started and accelerated to a final

spinning rate of 1 100 rpm, held for 35 s.

Ellipsometric measurements on polymers thin film were made

using a spectroscopic ellipsometer (SE 850 DUV, SENTECH Instru-

ments GmbH, Berlin, Germany) at incident angles of 508, 608, and708 in thewavelength range from380 to 900nmusing a dispersion

model for the visible region. Thicknesses at minimum three

differentpoints on each layerwere obtainedbyprocessing thedata

using theSpectraRay/3softwarepackage (SENTECH)withaCauchy

model for the polymer layer.

2.3. In Situ FTIR-ATR Measurements

Across-sectionof theexperimental setup is shownschematically in

Figure 1: FTIR-ATR measurements were done on an FTIR

spectrometer with series measurement capability (Nicolet 5700,

ThermoFisherScientificGmbH,Dreieich,Germany), equippedwith

an MCT detector and a ZnS ATR crystal (length�height�width

¼80�4�10mm3, u¼458, from Korth Kristalle GmbH, Hamburg,

Germany) using unpolarized light and 4 cm�1 spectral resolution.

The afterglowplasma treatmentwas achieved by aDBD reactor

made from glass with an open cross-section of 80� 1mm2 and an

active volume of 70� 50�1mm3, mounted above the horizontal

ATR accessory (Pike Technologies, Madison,WI, USA) in the sample

chamber of the IR spectrometer. The gas flow (N2þ 4% H2) of

typically 16 Lmin�1 STP during plasma treatment could pass the

dischargeand reachthepolymersurface,positionedatadistanceof

10mm from the down-stream edge of the discharge within about

3ms after leaving the plasma zone with an average velocity of

roughly 3ms�1. The discharge was powered by a commercial

generator (model 7010 R, SOFTAL electronics GmbH, Hamburg,

Germany). The peak voltage and frequency were typically 12.5 or

12 kV (ignition voltage 6.5 kV) and 20kHz, respectively.

In order to avoid electromagnetic interference between the

high-voltage circuit powering the plasma and the electronics of

the spectrometer, the DBD reactor was encased in a Faraday cage

DOI: 10.1002/ppap.201300033

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Nitrogen Plasma Modification and Chemical Derivatization

fabricated from a wire mesh. This shield and the ATR accessory

were grounded.

Duringandafter exposureof thepolymer surfaceon theZnSATR

crystals to afterglows of DBDs in mixtures of N2 with H2, infrared

spectra were taken in situ. In order to unravel the complex spectra

and to isolate the characteristic ‘‘scissoring’’ deformation vibration

of primary amino groups d(NH2) from interfering C55C and C55N

stretching vibrational bands, an exchange reaction with vapor of

heavy water (D2O) was applied, carrying the vibration d(NH2)

(about 1 620 cm�1) to d(ND2), located in the relatively empty

wavelength region around 1200 cm�1. The densities of functional

groups reacting with TFBA were quantified utilizing the C�CF3stretching vibration appearing at 1 324� 1 cm�1. Both, the gas-

phase derivatization and the exchange reaction with D2O or H2O

vapor were performed by exposing the polymer surface to gas

flows, typically 1 Lmin�1 STP, of nitrogen bubbled through a flask

containing the appropriate reagent (TFBA, D2O, or H2O) at room

temperature, see Figure 1.

Zb

2.4. Quantitative Evaluation of IR Spectra

In thefollowingweuse thesymbol e for thedecadicmolarabsorption

coefficient as defined in Lambert–Beer’s law, AT log(I0/I)¼ ecl, forthe absorbance AT of a sample with path length l, containingthe absorbing species at molar concentration c, measured in a

transmissionexperimentwith incident and transmitted intensities

I0 and I, respectively.[2] The absorption coefficient a is defined

by the equation I/I0¼ exp(�al), which for low absorbance can be

approximated by I/I0¼ 1�al. For ATR measurements Harrick

by analogy defined an effective thickness de using the equation

R¼ I/I0 1�ade for the reflectance R in a single-reflection

configuration (ade�1). With N-fold reflections the resulting total

reflectance is RN¼RN¼ (1�ade)N� 1�Nade.[3] As it is common

practice, results of ATR spectra are shown in this paper as

absorbance, A¼ log(1/RN), versus wavenumber n in cm�1.

aLetteroptica

Plasma

� 2013

A ¼ log1

RN

� �ffi N a de=lnð10Þ ð2Þ

The integral of A over an absorption band b of a vibration is

given by the equations
Zb
Adn ¼ N de

Zb

adn=lnð10Þ ¼ N c de

Zb

edn ð3Þ

The integral over e on the right-hand side can be expressed using

the integrated (or integral) absorption intensity;[4]weuse the letter

B for this quantitya:

Zb

Adn ¼ N c de B=lnð10Þ;

B 1

cl

Zb

lnI0Idn ¼ lnð10Þ

Zb

ednð4Þ

A is frequently used in the literature; we reserve this letter forl absorbance.

Process. Polym. 2013, DOI: 10.1002/ppap.201300033

WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Early View Publication; these are NO

For many infrared absorption bands integrated intensities B

can be found in the literature. It is important to pay attention to

the units used: While path length is generally given in cm and

concentration inmol L�1, themolarabsorptioncoefficientmayeither

be based on decadic logarithms like in the ‘‘practical units’’ used by

Wexler[5] or on natural logarithms as, for example, in the papers by

Ramsay[4] as well as by Yagudaev et al.,[6] and in this paper.

Harrick’s definition of an effective thickness is useful for

samples which are either thick, compared with the penetration

depth of the radiation dp (bulk samples), or thin with respect

to dp (thin film samples). For thin films of thickness d, the ratio

de/d f is a function of the refractive indices ni of the three

materials involved (i¼1: ATR element, i¼2: thin film, i¼3: third

phase – usually air), the angle of incidence u, and the state of

polarization of the light with respect to the plane of incidence.

Owing to the virtually constant electric field of the evanescent

wave over a thin film sample of thickness d�dp the absorbance

is independent of the detailed distribution of absorbers over

the thickness, as long as the concentration average cav over d is

the same. Therefore in case of a non-uniform distribution over dthe product cd can be replaced by cavd, which is the area density r

of absorbers:

T the

Adn ¼ N c d f B=lnð10Þ ¼ N r f B=lnð10Þ ð5Þ

If B is known, Equation (5) can be used to determine r from

the measured vibrational band area and f as calculated from

Harrick’s formulae.[3]

If literature data forB areunavailable, it canbedetermined from

ATR measurements on solutions (index s) containing known

concentrations cs of suitable reference molecules featuring the

oscillator: After Equation (4), B can be calculated from a plot of

the integrated absorbance as a function of molar concentration:

The slope of this plot equals the product NdesB/ln(10). In the

reference measurements the sample (solution) thickness on the

ATR element must exceed the penetration depth dp by far and desis the effective thickness for a solution as a bulk sample.

We used solutions of aldimines in hexadecane prepared in situ

from TFBA and a 10–20% excess of 3-pentyl-amine or n-hexyl-amine in order to determine the area of the C�CF3 stretching band

at 1 324 cm�1 by integration over a range of �25 cm�1. Measure-

mentswereperformedwith s-polarized lightusinga single-bounce

diamond ATR crystal at a resolution of 1 cm�1. The Smart

DuraSamplIR accessory used provides a peak incidence angle of

508 (statement of the manufacturer, SensIR Technologies, now

part of Smith’s Detection, Danbury, CT, USA). Figure 2 shows the

resultingband areas for different concentrations. The solvent used,

hexadecane,hasquitesimilar refractive indices inthevisible region

(nD¼1.4345 at 20 8C)[7] and in the far infrared (n(80 cm�1)¼ 1.428

at 20 8C).[8] To calculate the effective thickness for ATR measure-

ments with a diamond crystal (n1¼ 2.4; N¼1) we adopt n2¼1.43

for the hexadecane solutions to arrive at des¼0.39l/2.4¼1.23mm

at a wavenumber of n¼ 1/l¼1 324 cm�1. From the slope of the

linear fit to the data points in Figure 2 the integrated absorption

intensity of the C�CF3 stretching band in the aldimines can be

calculated as B¼ 370kmmol�1.

Correspondingly integrated absorption intensities were deter-

mined for the d(ND2) vibration in three isomeric deuterated

3www.plasma-polymers.org

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0.00 0.02 0.04 0.06 0.08 0.10 0.12

0.00

0.05

0.10

0.15

0.20

0.25

alkyl = 3-pentyl alkyl = 1-hexyl linear fit

Ban

d ar

ea /

cm-1

Concentration / mol/liter

Figure 2. Areas of IR absorption bands at 1 324 cm�1 due toC�CF3 stretching vibrations in two N-alkyl-4-trifluoromethyl-benzaldimines in hexadecane solutions as a function ofconcentration (ATR, diamond, 508, single reflection, s-polarization).The slope of the linear fit is 2.00L cm�1mol�1.

Table 1. Integrated band intensities B and typical peak positions nmaimines, amides, acrolein CH255CH�CH55O as prototypic unsaturat

Vibration, compound class B/kmmol�1

d(NH2), amines 30

d(ND2), amines 18 (theor.)

25 (exp.)

nasym(NH2), amines 5

nsym(NH2), amines 2

n(NH), sec. amines 1–2

n(C55N), alkyl-CH55N-alkyl 85

39

n(C55N), aryl-CH55N-alkyl 58

n(C�CF3), 4-CF3-benzaldimines 370

Amide Iþ II, prim. amides 460

Amide I, sec. acetamides 230

Amide II, sec. acetamides 210

n(C55O), CH255CH�CH55O 220

n(C55O), satur. aliph. ketones 170

n(C55O), a,b-unsat. ketones, trans 185

n(C55C), a,b-unsat. ketones, trans 25

n(C55O), a,b-unsat. ketones, cis 140

n(C55C), a,b-unsat. ketones, cis 90

nasym(CO2), aliphat. carboxylates 830

a)Data from Wexler[5] were multiplied by ln(10)�2.3 to account for

rounded to 0 or 5 in the last digit; b)Average of data obtained withc)Measured on an imine formed in situ from hexanal and 1-amino-2-

emaxDn ln(10) is given.



� 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim4

REarly View Publication; these are NOT the final pag

primary amines with the molecular formula C8H17ND2: 5ml of a

3.1M solution of the corresponding amine C8H17NH2 (1-amino-

octane, 2-aminooctane, or 1-amino-2-ethylhexane) in hexadecane

was intensely shaken two times with 10ml D2O in a separating

funnel, then the organic phasewas thoroughly dried (Na2CO3) and

measured as above, using an integral over the resulting ND2

scissoring band (FWHM�30 cm�1) from 1160 to 1260 cm�1. With

an effective depth des of 1.35mm at 508 and 1200 cm�1, the

resulting average B value is 25 kmmol�1.

Using the described ATR method instead of transmission

measurements for the determination of B brings about a source

of a systematic error, owing to the not exactly known angular

spread of the IR beam in the ATR crystal. We estimate that the

size of this error is in the order of�10%. For themain conclusion of

the present paper to be developed below this error does not play

a role because only the ratio B[n(C�CF3)]/B[d(ND2)]¼15 is of

relevance for it!

Table 1 is a collection of integrated band intensities B and

typical peak positions nmax of several important vibrational

infrared absorption bands which are expected to play a role for

FTIR spectroscopy of nitrogen-plasma-treated polymer surfaces.

An unsaturated aldehyde and open chain ketones are included

x of characteristic vibrational infrared absorption bands of amines,ed aldehyde, ketones and carboxylate ions.

nmax/cm�1 (solvent) Ref.a)

1620–1 625 (heptane) [6]

1 201 (exp., Kr) [9]

1 195–1 205 (hexadecane) This workb)

3 375 [5]

3 310 [5]

3 300 [5]

1 670 (diglyme) Unpubl.c)

1 671 (CHCl3) [10]d)

1 654 (CHCl3) [10]d)

1 324 (hexadecane) This work

1 690 (CHCl3) [5]

1 670 (KBr) [5]

1 540 (KBr) [5]

1 696 (CHCl3) [11]

1 715 (non-polar solvent) [5]





1 550–1600 [5]

different definition of molar absorption coefficient and generally

1-aminooctane-d2, 2-aminooctane-d2, 1-amino-2-ethylhexane-d2;

ethylhexane in diglyme over molecular sieve; d)For B the value of

DOI: 10.1002/ppap.201300033



as possible products of reactions of imines with water or TFBA;

carboxylate ions may form upon exposure of amine, imine, and

other basic groups on the polymer surface to the vapor of volatile

organic acids.

3. Results and Discussion

3.1. Plasma Afterglow Treatment of LDPE Film

Surfaces

Figure 3 shows – from bottom to top – a selection of FTIR

spectra taken in situ at intervals of about 10 s during 30 s

plasma afterglow treatment of an 80nm LDPE film under

standard conditions, using a spectrum taken before the

plasma treatment as background spectrum. The first

spectrum (bottom) was taken about 1 s after the plasma

was ignited, the top spectrum corresponds to the situation

immediately at the end of the plasma treatment, and the

dashed spectrum was obtained 18 s after the plasma

treatment was finished. The spectra reveal effects of the

plasma treatment on the ZnS crystal, the ‘‘bulk’’ of the PE

film and on the PE surface.

Plasma effects on the ZnS crystal become visible in the

baseline shift in Figure 3. These shifts are not due to

offsetting spectra against each other but they are a result of

the exposure of the ZnS crystal to the plasma and are

similarly also observed in the absenceof aPEfilm.Although

the refractive indexn of ZnS in the IR region is temperature-

dependent, the increase of n with temperature, dn/dT¼ 2.9� 10�5 K�1 (5.5mm)[12] is too small to explain the

increase of baseline absorption during plasma exposure

(only)byan increaseof the insertion lossdue toenhanced IR

reflections. During standard treatment the increase of

3500 3000 2500 2000 1500 1000-0.010

-0.008

-0.006

-0.004

-0.002

0.000

0.002

Abs

orba

nce

Wavenumber / cm-1

From bottom to top: 1 s 13 s 23 s 32 s 18 s

Figure 3. Spectra of 80nm LDPE film on ZnS taken at differenttimes (see legend) during and after 30 s plasma treatment.Plasma was shut off after 30 s; the last spectrum (dashed) wastaken 18 s after the end of plasma exposure.




baseline absorbance at 2 000 cm�1 is between 0.001 and

0.007, depending on the individual crystal used. After the

treatment theabsorbance slowly recoverswitha relaxation

time in the order of hours. At the moment we can only

speculate that the origin of these effects are possible very

weakUV-inducedbroadband IR absorptions due to electron

traps which are populated by excited electrons, similar to

UV-induced IR absorption bands observed – albeit many

orders of magnitude stronger – in deliberately doped ZnS

crystals.[13]

Aside from positive bands due to the formation of new

functional groups on the polymers surface the spectra

shown in Figure 3 also reveal the effects of plasma-induced

etching of the LDPE film: Because a spectrum of the film

before plasma action was used as a background, etching

away PE material results in negative absorption bands,

mainly in the regions of stretching vibrations of aliphatic

C�H bonds (n(CH2), 2 800–3 000 cm�1) and of deformation

vibrations (d(CH2), scissoring, 1 400–1 500 cm�1). An ex-

panded view of the latter region is shown in Figure 4

featuring two curves: A spectrum of the 80nm LDPE film

itself (with theuncoated crystal as background, inbold) and

a difference spectrum representing the etched-away

material as a positive spectrum after multiplication of

the original difference (spectrum before� spectrum after

plasmatreatment)bya factorof15.Obviously thespectrum

of theetchedmaterial isnot exactlyadownsized copyof the

original: The two peaks at 1 461 and 1473 cm�1 are much

better separated and quite similar in intensity, while in

the spectrum of the film the 1463 cm�1 component clearly

dominates. Even more pronounced are differences in the

1 330–1 390 cm�1 range: Interestingly the spectrum of

etchedmaterial showsonlyapeakat 1 378 cm�1 –probably

1500 1475 1450 1425 1400 1375 1350 1325-0.02

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

Abs

orba

nce

Wavenumber / cm-1

80 nm LDPE on ZnS Difference before - after

30 s plasma treatment (x 15, arb. offset)

Figure 4. Spectrum of 80nm LDPE film on ZnS before plasmatreatment and difference of spectra taken before and after 30 splasma treatment in the region of deformation modes d(CH2) at1 463 and 1 473 cm�1 and wagging modes and the symmetricmethyl (‘‘umbrella’’) deformation at 1 378 cm�1.


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the symmetric ‘‘umbrella’’ deformationvibrationofmethyl

groups in side chains of the polymer – while bands with

maxima at 1 363 and 1 359 cm�1 of comparable intensity –

wagging vibrations of disordered polymethylene chain

segments of the polymer – are also visible in the original

film. In principle three different possible reasons have to

be considered in order to explain the observed differences:

(i) A stratification of the original film with differences

in structure between the bulk of the film and the etched-

away surface region of typically about 3–6nm thickness

within 30 s plasma treatment – due to segregation and

crystallization processes during film formation or there-

after; (ii) preferential etching of phases or crystallites

with special orientations, and/or (iii) an effect of UV

radiation from the plasma acting not only on the surface

region but more or less through the whole film. For a

more detailed discussion of these issues, more systematic

investigations with films of different thicknesses and

varying etching depths are required, supported by

other methods which are able to characterize the film

structure.

In the context of the present paper the formation of

new vibrational bands due to new functional groups at

the polymer surface is of more interest: Figure 5 shows

the region of 1 350–1 800 cm�1 in which the stretching

vibrations of double bonds C55C, C55N, and C55O, as well

as deformation vibrations of XH2 moieties (CH2, NH2, OH2)

are typically located. The spectral regionbetween2 700and

3 500 cm�1 of X–H stretching vibrations shows broad

maxima at 3 160 and 3 320 cm�1.

In Figure 5, a group of at least four overlapping

absorption bands is developing between 1 550 and

1800 1700 1600 1500 1400

0.000

0.001

0.002

0.003

0.004

0.005

0.006

0.007

0.008

Abs

orba

nce

Wavenumber / cm-1

From bottom to top:Spectra taken at intervals of about 5 s during30 s plasma afterglow treatment in N2 + 4 % H2

Figure 5. X55Y stretching regions of spectra taken at intervals of5 s during plasma afterglow treatment of an LDPE film surfaceunder standard conditions. The first spectrum at the bottom ofthe series was taken at the start of the plasma exposure.Following spectra are offset by 0.0005 absorbance units each.




1 720 cm�1 during plasma exposure. Originally this region

was expected to be dominated by a band due to �NH2

groups: Primary aliphatic amines have scissoring defor-

mation vibration d(NH2) bands of medium-to-strong

intensity in the range of 1 580 to 1 650 cm�1, the most

probable wavenumber range of appearance is from

1620 to 1 625 cm�1.[6,9,14] (Secondary amines sometimes

show weaker bending absorptions between 1 490 and

1 580 cm�1.[14])

However, a peak maximum in this range was not seen;

obviously the majority of absorption bands are due to

unsaturated groups. Most probable candidates are C55N

and C55C double bonds but also the formation of carbonyl

compounds featuring C55O bonds must be taken into

account because at atmospheric pressure even gases of

very high purity may contain enough oxygen to form a

monolayer of oxygenatedgroups inquite a short time: Even

at an oxygen content of only 100ppb (O2 partial pressure

¼ 0.01 Pa), the wall collision rate of O2 is still about

300nm�2 s�1 resulting in about 9 000 collisions per nm2

during the plasma treatment.[16] One ‘‘successful’’ collision

on 1nm2 of the active surface, resulting in formation of a

carbonyl group would already give an easily measurable

contribution to the spectrum, due to the high integrated

intensities of C55O vibrations.

The interpretation of the bands in the 1 550 to

1 700 cm�1 region is complicated by the unsaturation of

carbon–carbon bonds occurring along with the formation

of N-containing unsaturated functional groups C55N

and possibly carbonyl. As already noted in a previous

paper,[1] imine groups may play a bigger role on polymer

surfaces plasma-treated in nitrogen atmospheres than

frequently thought and – aside from reactions leading

to an incorporation of nitrogen within functional

groups, also the formation of C55C double bonds by the

interaction of nitrogen atoms with alkyl radicals is

probably a favorable reaction channel. In case of ethyl-

D5 radicals in the gas phase the deuterium abstraction by

nitrogen atoms, which could be considered as a model

reaction for the interaction of a radical center at the

polyethylene chain with atomic N,[17]

D

e nu

D3C� _CD2 þN ! D2C55CD2 þND ð6Þ

accounts for nearly two-thirds of the radicals reacted

while the formation of an iminyl radical and a methyl

radical

3C� _CD2 þN ! D2C55Nþ CD3 ð7Þ

accounts for most of the rest.[18]

For energetic reasons the formationof conjugateddouble

bonds such as >C55CH�CH55N� will be favored; that is

while in interpreting the IR spectra one has to take into

DOI: 10.1002/ppap.201300033

mbers, use DOI for citation !!

Table 2. Peak positions ni, FWHM values wi, and areas Ai of fourGaussian absorption bands used to fit spectra P and PO,respectively.

i¼ 1 i¼ 2 i¼ 3 i¼ 4

ni/cm�1 1 570 1 601 1643 1 675

wi/cm�1 31 37 51 50

Ai in P/cm�1 0.03054 0.0606 0.10293 0.0264

Ai in PO/cm�1 0.02278 0.06245 0.1204 0.12593


account the probable presence of unsaturated imines and

carbonyl compounds.

Spectra shown in Figure 3–5 were obtained from

nominally pure gas atmospheres containing well below

1ppm oxygen. In order to investigate the effect of oxygen

we deliberately added 5ppm oxygen in one experiment in

order tosee ifanychanges in the IR spectrawouldgiveaclue

towards functional groups involving oxygen. Figure 6

shows final spectra after finishing 30 s of plasma afterglow

exposure under standard conditions and with oxygen

added, and also the difference spectrum.

In the wavenumber region between 1 540 and

1 740 cm�1, a reasonable joint fit of both spectra is obtained

using a 4-component sum S(n) of Gaussian functions

(Equation 8)

-0.

0.

0.

0.

0.

0.

0.

0.

Abs

orba

nce

Fig30andspeat

Plasma

� 2013

SðnÞ ¼X4i¼1

Ai

wi

ffiffiffiffiffiffiffiffiln 4

p

rexp �ln 4

n� ni

wi

� �2 !

ð8Þ

with shared peak positions ni and full widths at half

maximum (FWHM) wi.

The obtained parameters are shown in Table 2.

The major effects of adding a trace of oxygen are (i) a

strong intensification of band 4 at 1 675 cm�1 whichmight

be due to amide groups and (ii) a reduction of the band 1

which cannot be assigned so far.

3.2. Hydrogen-Deuterium Exchange Experiments

In order to separate the vibrational signature of primary

amines from overlapping stretching vibrations of double

bonds in the 1 600–1750 cm�1 range we applied an

1800 1700 1600 1500 1400001

000

001

002

003

004

005

006

Wavenumber / cm-1

PO P Difference PO - P

ure 6. X55Y bond stretching regions of spectra taken afters plasma afterglow treatment under standard conditions (‘‘P’’)with 5 ppm oxygen added (‘‘PO’’), respectively. The differencectrum PO–P (offset þ0.003) is dominated by peaks centered1 672 and 1 400 cm�1. Note the negative peak at 1 377 cm�1.

Process. Polym. 2013, DOI: 10.1002/ppap.201300033

WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim


exchange of the nitrogen-bonded hydrogen against deute-

rium: In general, hydrogen atoms bonded to heteroatoms

such as O, N, S, or P atoms are chemically much more

reactive than carbon-bonded hydrogen atoms. In contact

with liquid or gaseous heavy water (D2O) a rapid

equilibration will take place furnishing a nearly statistical

distribution of H and D over the compound carrying the

heteroatom and water.[19] Spectroscopic observation of

H–D exchange between biomolecules and the surrounding

water has become an important method for studying the

structure of biomolecules.[20]

In the present example, deuteration of primary aliphatic

amines shifts the deformation vibration from 1620 to

1 625 cm�1 to a range of 1 190 to 1 240 cm�1 (d(ND2)).[9,21]

According to ourmeasurements the integrated intensity of

this vibration is about 25 kmmol�1, reasonably close to

theoretical results.[9,15]

In preliminary experiments it was seen that spectral

changesmeasured in situ during contact of plasma-treated

LDPE films with D2O vapor carried from a bubbler flask

in an N2 stream of 1 Lmin�1 STP came to an end after

an equilibration time of about 30min. In addition, it

was noted that the spectra of (i) a bare ZnS crystal or (ii) an

LDPE-coated Zn crystal without plasma treatment also

were dependent on the presence or absence of H2O or

D2O. This observation is due to several kinds of functional

groups on the ZnS surface carrying active hydrogen such

as thiol, sulfite, and sulfate moieties.[22] Therefore, in order

to eliminate from the final results any effects due to

functional groups with active hydrogen present on the

ZnS surface or in the LDPE film, a special referencing

procedure was applied: First, the LDPE film on ZnS was

exposed for 30min to a water atmosphere generated by a

streamofH2Ovapor inN2. Then the crystalwas exposed for

30min to dry N2 to remove any physisorbed H2O. Then a

reference spectrum ‘‘Ref. H’’ was measured. The procedure

was repeated with D2O to obtain a reference spectrum of

ZnS and LDPE with all active hydrogen atoms replaced by

deuterium atoms, ‘‘Ref. D.’’

The sample was plasma-treated and then underwent

three 30min exposures to water vapor atmospheres,




8

REa

startingwith 30min inD2O, then changing toH2O, then the

second D2O exposure followed. After each 30min ‘‘wet’’

period, the samplewas overflownwith 1 Lmin�1 STP of dry

N2 in order to remove physisorbed water, and a spectrum

(labelled D, H, 2D, respectively) was measured. Figure 7

shows results of such experiments after the standard

plasma treatment (‘‘P’’) and treatment with 5ppm oxygen

added (‘‘PO’’), respectively.

The pairs of difference spectra generated frommeasure-

ments after treatments P (middle) or PO (bottom) are

obtained as (i) spectra after H2O exposure minus spectra

after previous D2O exposure (‘‘H�D’’) and (ii) vice versa

after 2nd D2O exposure (‘‘2D�H’’). The corresponding

references (Ref. H or Ref. D) were subtracted. Repeating

the exchange twice or more increases the sensitivity of

the method. In addition, the mirror symmetry within the

pairs of difference spectra shows that spectra D and H or

spectra 2D and H differ only by reversible exchange of

active hydrogen – any irreversible reaction with water is

not visible on the time scale of the experiment. Aside from

the replacement of N�H and possibly O�H stretching

vibrations at n> 3 000 cm�1 by N�D (O�D) bands between

2 000 and 2 800 cm�1, a few exchange peaks in the region

1 300–1 750 cm�1 are observed. A detailed discussion of

these spectral changes is beyond the scope of this paper. For

the moment it is important to note that there is no

indication of the presence of primary amino groups which

should yield a positive (negative) d(NH2) vibrational band

near 1 620–1625 cm�1 in the H�D (2D–H) spectra and a

3500 3000 2500 2000 1500 1000-0.006

-0.004

-0.002

0.000

0.002

0.004

0.006

Abs

orba

nce

Wavenumber / cm-1

P P:H-D P:2D-H PO PO:H-D PO:2D-H

Figure 7. Top: Spectra taken after 30 s afterglow exposure understandard conditions (top pair, black, ‘‘P’’) and with 5 ppm oxygenadded, respectively (top pair, dark gray, ‘‘PO’’). Middle: pair ofdifference spectra during H–D exchange experiment afterstandard afterglow treatment: Spectrum after H2O exposureminus spectrum after previous D2O exposure (broken curve,‘‘P:H–D’’) and vice versa after 2nd D2O exposure (solid curve,‘‘P:2D–H’’). Bottom: As before, but with 5 ppm oxygen addedduring afterglow exposure. (Arbitrary absorbance offsets.)




negative (positive) d(ND2) band in the 1190–1 240 cm�1

range in the H�D (2D–H) spectra. Figure 8 shows these

spectra in the 1 100–1 300 cm�1 range, expanded in order

to make any absorption bands better visible. Again there

is no indication of d(ND2): Theoretical bands calculated

under the assumption of an �ND2 density of 1 nm�2

appearing (disappearing) in the presence of D2O (H2O)

vapor are superimposed as Gaussian functions in Figure 8.

The comparison with measured results shows that the

density of �ND2 which would be detectable in these

spectra is significantly below 1nm�2. We estimated that it

is around 0.3 nm�2.

3.3. Exposure of Plasma-Treated Surfaces to TFBA

Vapors

Figure 9 shows selected spectra measured during a TFBA

derivatization experiment, which was extended over a

period of nearly 5 h. Remarkable features – aside from

the strong C�CF3 peak at 1 325 cm�1 marked by ‘‘A’’ – are

a strong transient band of so far unknown origin at

1 585 cm�1 attaining maximum intensity between 20

and 40min (‘‘B’’) and the C55O stretching peak of

physisorbed TFBAat 1 716 cm�1 (‘‘C’’), immediately appear-

ing in the first spectrum measured after starting TFBA

exposure (5min). The LDPE film stays saturated with

‘‘dissolved’’ TFBA until the TFBA-loaded N2 stream is

replaced by a pure N2 stream after 289min, leading to

immediate disappearance of the corresponding peak

(290min). The band area of the C�CF3 peak in Figure 10,

1300 1250 1200 1150 1100

0.0000

0.0005

0.0010

0.0015

0.0020

Abs

orba

nce

Wavenumber / cm-1

From top to bottom: P:H-D P:2D-H PO:H-D PO:2D-H

Figure 8. Wavenumber region around ND2 deformationvibrations taken from spectra in Figure 7, expanded. (Arbitraryabsorbance offsets.) On each spectrum, a Gaussian functioncentered at 1 200 cm�1 is superimposed as a positive ornegative theoretical spectral band, calculated under theassumption of an appeared or disappeared amino group �ND2density of 1 nm�2, using experimental values B¼ 25 kmmol�1 andFWHM¼ 30 cm�1, see above.

DOI: 10.1002/ppap.201300033


1900 1800 1700 1600 1500 1400 13000.000

0.005

0.010

0.015

0.020

Abs

orba

nce

Wavenumber / cm-1

From bottom to top: 2 min 5 min 10 min 20 min 40 min 59 min 91 min 140 min 181 min 223 min 250 min 289 min 290 min 320 min

A

BC

Figure 9. Selection of spectra taken during 287min exposure ofafterglow-treated LDPE surface to TFBA in an N2 stream. Start ofthe TFBA exposure at 2min. After 289min the TFBA-loaded N2stream was replaced by pure N2. Spectra for t> 2min are shiftedvertically and horizontally for better presentation. See Section 3.3for explanations.


representative of the total density of adsorbed plus

chemically bonded TFBA, rises rapidly in the first 50min

and keeps growing even after nearly 5 h. (In a subsequent

experiment there was a still weakly increasing signal

even after 22h of exposure to TFBA.) Removal of TFBA

from the N2 stream (289min) leads to an immediate

desorption of physisorbed and a slower vanishing of

some loosely bound TFBA.

Thefinal bandarea obtained in this experiment (and also

in the 22h run) was about 0.1 cm�1. Using the integrated

absorption intensity of this band determined above,

0 50 100 150 200 250 300

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

Area of 1324 cm-1 band,

integrated from 1300 to 1350 cm-1

series 0 - 180 min manual measurements series 260 - 320 min

Atmosphere:2 - 290 min: TFBA vapor290 - 320 min: N2

Ban

d ar

ea /

cm-1

Time / min

Figure 10. Area of C�CF3 stretching band during 287minexposure of afterglow-treated LDPE surface to TFBA in an N2stream. Start of TFBA exposure at 2min. After 289min the TFBA-loaded N2 stream was replaced by pure N2.




B¼ 370 kmmol�1, a density of 1.6 nm�2 is calculated for

the 4-trifluoromethyl-phenyl moieties immobilized to the

surface, roughly a factor 5 beyond theprimary aminogroup

densities which would be detectable by the deuteration

experiments described above. This is quantitative experi-

mental evidence that TFBA is not or not only reacting with

�NH2 groups.

There is also qualitative evidence demonstrating that

the derivatization reaction is proceeding different from

the simple chemical reaction formulated in Equation (1).

If this equation would describe completely what happens

upon exposure of the plasma-treated LDPE surface to

TFBA, the only spectral changes observed would be a

vanishing of the vibrations due to primary amines

and appearance of vibrations of an aromatic N-alkyl-aldimine Ar�CH55N�R (Ar¼ 4-CF3�C6H4�, R¼ alkyl).

In the 1 500–1800 cm�1 region of difference spectra

taken after and before derivatization one would corre-

spondingly expect (see Table 1) (i) a negative d(NH2) band

at 1 620–1 625 cm�1 with about 8% of the band area of

the n(C�CF3) peak at 1 325 cm�1 and (ii) a positive n(aryl-

CH55N�) band around 1655 cm�1 with about twice

the area of the negative NH2 band or roughly 16% of n(C-

CF3). (In our calibration experiments using TFBA and

aliphatic amines in hexadecane the corresponding C55N

peak ofN-alkyl-4-trifluoromethyl-benzaldimines appeared

at 1 652 cm�1.)

The observations do not meet these expectations: The

top spectrum of Figure 11, difference between the final

spectrum after derivatization and the starting spectrum,

shows that the strong characteristic C�CF3 vibration at

1 325 cm�1 is accompanied by a broad band in the region

of 1 690–1 710 cm�1, probably – by reason of position

and intensity – due to carbonyl groups. A peak due to

4-trifluoromethyl-benzaldimines near 1 655 cm�1 is not

visible. In addition, the intensity in the range of X–H

stretching vibrations with wavenumbers beyond

3 000 cm�1 is increased and two maxima appear at 3 370

and 3 210 cm�1. Although this range is relatively low for

O�H vibrations we tentatively attribute these bands to

chelated O�H groups of b-hydroxycarbonyl compounds as

they can be formed by aldol additions. Also the relatively

low carbonyl wavenumber could indicate aldol addition

or aldol condensation products.[1,14]

On the other hand, 24h exposure of a plasma-treated

surface to water vapor also produces a carbonyl band of

similar integrated intensity peaking in the 1 670–

1 710 cm�1 region.[23] Therefore we tentatively attribute

the carbonyl band observed after TFBA exposure, located in

a wavenumber range between the regions typical for

saturated aldehydes or ketones on the one hand and

saturated amides on the other, to structure elements

containing a carbonyl group conjugated with a double

bond. Moieties of this kind could be furnished by an



3500 3000 1800 1700 1600 1500 1400 13000.000

0.002

0.004

0.006

0.008

0.010

0.012

Abs

orba

nce

Wavenumber / cm-1

From bottom to top: 5-2 289-290 (!) 320-2

Figure 11. Selected difference spectra taken during TFBAexposure. A label ‘‘m–n’’ means that the spectrum at nmin issubtracted from the spectrum at mmin. From bottom to topthese difference spectra demonstrate (i) physisorption and (ii)desorption of TFBA characterized by a narrow carbonyl peak at1 716 cm�1. (iii) The top spectrum shows the overall differencebetween the final derivatization product spectrum and thestarting spectrum before TFBA contact. A presumptivecarbonyl peak is located at 1 690–1 700 cm�1. In the n(X–H)region, two peaks are visible at 3 370 and 3 210 cm�1.

1750 1700 1650 1600 1550 1500 1450 1400 1350 1300 1250-0.0015

-0.0010

-0.0005

0.0000

0.0005

0.0010

0.0015

0.0020

Abs

orba

nce

Wavenumber / cm-1

7-5 20-7 49-20 149-49 289-149

Figure 12. Selected difference spectra taken during TFBAexposure. A label ‘‘m–n’’ means that the spectrum at nmin issubtracted from the spectrum atmmin. These difference spectrademonstrate (i) formation of a strong absorption band atabout 1 585 cm�1 due to a transient species (spectra 7-5 and 20-7) and its decay (149-49 and 289-149) and (ii) continuousformation of a new absorption band with a maximum at1 700–1 705 cm�1 along with the continuous increase of C�CF3absorption at 1 325 cm�1.


10

REa

exchange reaction of unsaturated imines (1-aza-buta-

dienes) on the plasma-treated surface with TFBA:

NR O

CF3CH

NRCF3C

H

O

+ +H

H

R' H

H

R'

(9)

Similartounsaturatedketones (R0 ¼ PEchain), seeTable1,

a,b-unsaturated aldehydes (R0 ¼H) are also characterized

by very strong C55O stretching vibrations between 1 685

and 1 705 cm�1.[11,14]

Due to virtually same positions (about 1 655 cm�1) and

comparable intensities of C55N vibrations in unsaturated

imines and arylimines, contributions of the vanishing

unsaturated imine on the left hand side of Equation (9) to

the IR spectrum are cancelled by those of the newly formed

benzaldimine on the right hand side. Therefore the region

between 1 650 and 1 655 cm�1 is virtually devoid of a

positive or negative absorbance.

The scheme in Equation (9) is not meant as a final

description of the chemical nature of a nitrogen-plasma-

treated PE surfaces and its TFBA derivatization but as a

working hypothesis for future investigations. At least some




aspects of the spectroscopic results obtained during TFBA

exposure are interpretable in terms of Equation (9), but not

by the commonly assumed reaction shown in Equation (1).

In addition, one may speculate about formation of

unsaturated imines using as model processes only the

reactions reported by Stief et al.,[18] see Equation (6) and (7):

If the reaction scheme shown in Equation (7) is formally

applied to an allyl radical instead of ethyl, the resulting

radical pair could recombine to the unsaturated secondary

aldimine shown in Equation (10) or to an isomeric primary

ketimine.

Further spectra shown in Figure 11 and in Figure 12

contain a lot of information about the mechanism of the

derivatization reaction, which are still to be worked out in

detail. In brief, the two lower spectra of Figure 11 show the

very rapid uptake and loss of physically dissolved TFBA by

the LDPE film at the start and the end of the derivatization,

respectively. In Figure 12, differences of selected spectra

are shown. These spectra prove that the chemical

DOI: 10.1002/ppap.201300033



immobilization reactionofTFBAproceedsduring thewhole

timeundercontinuousformationofcarbonylproducts. (Even

during amuch longer subsequent derivatization experiment

the gradual increase of the C�CF3 bandwas accompanied by

rising carbonyl absorption near 1700 cm�1).

A very surprising feature in Figure 12 is the growth and

decay of a transient species responsible for the very strong

band – compared with other vibrations – at 1 585 cm�1

which is accompanied by weaker peaks at 1 345 and

1 370 cm�1. The density of this compound goes through a

maximum at about 55min. This transient species is also

characterized by a broad absorption band between 2 600

and 3500 cm�1, having two pronounced peaks at 2 773 and

2 687 cm�1. The nature of this species is so far unknown

and further investigations are required to clarify it.

The weak bands at 2 150 cm�1 to be seen in Figure 7

(top pair of spectra) are due to a functional group

formed during plasma exposure of LDPE which is

reactive towards water vapor and TFBA vapor as well

(corresponding spectra are not shown): Candidates are

some of the groups with cumulated double bonds: azides

�N55N55N (vs–s), carbodiimides �N55C55N� (vs),

ketenimines >C55C55N� (s), or ketenes >C55C55O (m-

s).[14]All of thesevibrations are at least ofmediumintensity

(m), but usually they are strong (s) or very strong (vs).

Therefore the low-intensity of bands seen in Figure 7

indicates that only a small density of this group is present.

4. Conclusion

In the present paper, we have presented experimental

evidence that the reaction of nitrogen-plasma-treated

polymer surfaces with TFBA is not selective for primary

aminogroups. There are twoarguments, a quantitative and

a qualitative one:

(1)

Plasm

� 20

Exposing the plasma-treated surfaces to TFBA

vapor results in a strong absorption band in FTIR-

ATR measurements, due to chemically bonded

4-CF3�C6H4� moieties. Characteristic signatures of

primary amino groups, on the other hand, are missing

in the spectra. Quantitative analysis shows that the

density of TFBAmolecules immobilizedat the surface is

by a factor of about 5 larger than the density of primary

amino groups, which should be detectable by IR

spectroscopy after deuteration by contact with D2O

vapor.

(2)
The IR-spectroscopic changes during the reaction with
TFBA are not in agreement with the so far generally

assumed reaction scheme but they hint towards the

a Process. Polym. 2013, DOI: 10.1002/ppap.201300033

13 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Early View Publication; these are NOT th

dominance of a different reaction scheme possibly

involving unsaturated imines, which remains to be

scrutinized in the future.

Acknowledgements: The authors gratefully acknowledge helpfuldiscussions with Dipl.-Chem. H. Dillmann and Dipl.-Ing. H.Schmolke of IOT as well as Dr. Kristina Lachmann, andDr. Michael Thomas, Fraunhofer IST Braunschweig. This workwas supported by a grant from the Deutsche Forschungsgemein-schaft (DFG-No. Kl 1096/19).

Received: March 18, 2013; Revised: July 7, 2013; Accepted: July 8,2013; DOI: 10.1002/ppap.201300033

Keywords: derivatization; FTIR-ATR in situ; plasma treatment;polyethylene (PE); polymer modification

[1] C.-P. Klages, Z. Khosravi, A. Hinze, Plasma Process. Polym.2013, 10, 307.

[2] V. P. Tolstoy, I. V. Chernyshova, V. A. Skryshevsky, Handbookof Infrared Spectroscopy of Ultrathin Films, John Wiley &Sons, Inc., Hoboken, NJ, USA 2003.

[3] N. J. Harrick, A. I. Carlson, Appl. Opt. 1971, 10, 19.[4] D. A. Ramsay, J. Am. Chem. Soc. 1952, 74, 72.[5] A. S. Wexler, Appl. Spectrosc. Rev. 1967, 1, 29.[6] M. R. Yagudaev, Yu. N. Sheinker, Dokl. AN SSSR 1962, 144,

379.[7] A. Aucejo, M. C. Burguet, R. Munoz, J. L. Marques, J. Chem. Eng.

Data 1995, 40, 141.[8] J. P. Laib, D. M. Mittleman, J. Infrared Milli. Terahz. Waves

2010, 31, 1015.[9] J. R. Durig, C. Zheng, Struct. Chem. 2001, 12, 137.

[10] J. Fabian, M. Legrand, Bull. Soc. Chim. Fr. 1956, 23, 1461.[11] G. M. Barrow, J. Chem. Phys. 1953, 21, 2008.[12] G. Hawkins, R. Hunnemann, Infrared Phys. Technol. 2004,

45, 69.[13] H. Kukimoto, S. Shionoya, T. Koda, R. Hioki, J. Phys. Chem.

Solids 1968, 29, 935.[14] G. Socrates, Infrared and Raman Characteristic Group

Frequencies: Tables and Charts, John Wiley & Sons, Inc.,Hoboken, NJ, USA 2004.

[15] D. Zeroka, J. O. Jensen, A. C. Samuels, J. Mol. Struct. 1999, 465,119.

[16] R. Brdicka, Grundlagen der Physikalischen Chemie, VEBDeutscher Verlag der Wissenschaften, Berlin, Germany1968.

[17] C.-P. Klages, A. Grishin, Plasma Process. Polym. 2008, 5, 359.[18] L. J. Stief, F. L. Nesbitt, W. A. Payne, S. C. Kuo, W. Tao, R. B.

Klemm, J. Chem. Phys. 1995, 102, 5309.[19] W. R. Harp, R. C. Eiffert, Anal. Chem. 1960, 32, 794.[20] F. S. Parker, K. R. Bhaskar, Appl. Spectrosc. Rev. 1970, 3, 91.[21] S. Pinchas, I. Laulicht, Infrared Spectra of Labelled Compounds,

Academic Press, London 1971.[22] W. Hertl, Langmuir 1988, 4, 594.[23] Z. Khosravi, C.-P. Klages, to be published.


e final page numbers, use DOI for citation !! R

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