Vacuum-ultraviolet photochemically initiated modification of polystyrene surfaces: Chemical changes

Article abstract of DOI:10.1562/2004-11-30-RA-387R.1

Fourier-Transform infrared (FTIR) spectroscopy and surface energy analysis (contact angle measurements) have been performed as a means of identification and quantification of the functionalization of polystyrene surfaces upon vacuum ultraviolet- (VUV-) photochemically initiated oxidation. Photochemical oxidation was performed in the presence of water vapor and molecular oxygen using a pulsed Xe₂-excimer radiation source (λ_exc: 172 nm). Surface oxidation was studied as a function of two parameters: irradiation time and distance between sample and radiation source. During the first 1-2 min of irradiation, an increase of the concentrations of hydroxyl (OH) and carbonyl (C=O) groups on the surface was observed, both reaching limiting values. As expected, the rate of oxidation diminished exponentially with increasing distance between the radiation source and the surface of the polystyrene film. Changes in the surface energy due to the introduction of these polar (i.e. OH and C=O) groups were also determined. The densities of the functional groups decreased upon washing with acetonitrile, and analysis of the washing solution by means of gas chromatography-mass spectrometry (GC-MS) revealed the presence of a large number of products. The application of pulsed Xe₂-excimer radiation sources as a valuable alternative to conventional means (i.e. laser and plasma) for the photochemical oxidation and surface modification of polystyrene is discussed.

Full text of DOI:10.1562/2004-11-30-RA-387R.1

Vacuum-ultraviolet Photochemically Initiated Modification of Polystyrene
Surfaces: Chemical Changes
Author(s): Juan López-Gejo, Hartmut Gliemann, Thomas Schimmel, and André M. Braun
Source: Photochemistry and Photobiology, 81(4):777-782.
Published By: American Society for Photobiology
DOI: http://dx.doi.org/10.1562/2004-11-30-RA-387R.1
URL: http://www.bioone.org/doi/full/10.1562/2004-11-30-RA-387R.1
BioOne (www.bioone.org) is a nonprofit, online aggregation of core research in the biological, ecological, and
environmental sciences. BioOne provides a sustainable online platform for over 170 journals and books published
by nonprofit societies, associations, museums, institutions, and presses.
Your use of this PDF, the BioOne Web site, and all posted and associated content indicates your acceptance of
BioOne’s Terms of Use, available at www.bioone.org/page/terms_of_use.
Usage of BioOne content is strictly limited to personal, educational, and non-commercial use. Commercial inquiries
or rights and permissions requests should be directed to the individual publisher as copyright holder.
BioOne sees sustainable scholarly publishing as an inherently collaborative enterprise connecting authors, nonprofit publishers, academic institutions, research
libraries, and research funders in the common goal of maximizing access to critical research.

Photochemistry and Photobiology, 2005, 81: 777–782
Symposium-in-Print: VIII ELAFOT, La Plata, Argentina, 2004
Vacuum-ultraviolet Photochemically Initiated Modification of
Polystyrene Surfaces: Chemical Changes{ à
,
1
Juan Lopez-Gejo , Hartmut Gliemann , Thomas Schimmel and Andre M. Braun*
2
2
1
´
´
1
¨
Lehrstuhl fu¨r Umweltmesstechnik, Universitat Karlsruhe, 76128 Karlsruhe, Germany
2Institut fu¨r Nanotechnologie (INT), Forschungszentrum Karlsruhe, 76021 Karlsruhe, Germany
Received 30 November 2004; accepted 30 January 2005
(1–11). Compared with low-pressure Hg arcs, principal advantages
of VUV excimer radiation sources are their working temperatures of
less than 808C, hence avoiding infrared (IR) emission and (sample)
heating effects, and higher monochromatic radiant exitances. Ex-
cimer radiation sources may be preferred to laser and plasma
techniques for the treatment of large surface areas as well as for the
simplicity of the irradiation equipment used, taking into account
that plasma systems require most often vacuum lines and controlled
gas mixtures.
ABSTRACT
Fourier-Transform infrared (FTIR) spectroscopy and surface
energy analysis (contact angle measurements) have been per-
formed as a means of identification and quantification of the
functionalization of polystyrene surfaces upon vacuum ultra-
violet- (VUV-) photochemically initiated oxidation. Photo-
chemical oxidation was performed in the presence of water
vapor and molecular oxygen using a pulsed Xe2-excimer ra-
diation source (kexc: 172 nm). Surface oxidation was studied as
a function of two parameters: irradiation time and distance
between sample and radiation source. During the first 1–2 min
of irradiation, an increase of the concentrations of hydroxyl
(OH) and carbonyl (C5O) groups on the surface was observed,
both reaching limiting values. As expected, the rate of oxi-
dation diminished exponentially with increasing distance be-
tween the radiation source and the surface of the polystyrene
film. Changes in the surface energy due to the introduction of
these polar (i.e. OH and C5O) groups were also determined.
The densities of the functional groups decreased upon washing
with acetonitrile, and analysis of the washing solution by
means of gas chromatography-mass spectrometry (GC-MS)
revealed the presence of a large number of products. The
application of pulsed Xe2-excimer radiation sources as a valu-
able alternative to conventional means (i.e. laser and plasma)
for the photochemical oxidation and surface modification of
polystyrene is discussed.
The pulsed Xe2-excimer radiation source (kexc: 172 6 12 nm) is of
special interest due to its high luminescence efficiency (; 40%).
Irradiation of the polymer surface by high-energetic photons (7.2 eV)
may lead to different electronically excited states than those reached
by conventional UV-C radiation, e.g. low-pressure Hg arc (kexc: 254
nm), which in turn may exhibit different reactivities.
In one of our projects, we focus on the oxidation of surfaces of
polystyrene (PS) films, aiming for a simple but controlled surface
functionalization. Oxidation is initiated by the radiation of a Xe2*-
radiation source in the presence of molecular oxygen and water
(vapor). We are aware of two independent investigations, where
Xe2*-radiation sources have been used to irradiate PS (10,11).
Tanaka et al. (10) investigated the physicochemical changes of
vinyl polymers (polyethylene, PS, and poly(vinyl alcohol))
induced by Xe2-excimer radiation in the presence of dry air or
nitrogen by determining contact angle measurements and using
Fourier-transform infrared spectroscopy (FTIR) and x-ray photo-
electron spectroscopy (XPS). By FTIR analysis of PS samples
irradiated in dry air, only one new absorption band to be associated
with the carbonyl function was observed. It is interesting to note
that, in earlier publications, the formation of hydroxyl groups had
been observed in addition to newly formed carbonyl functions,
when PS surfaces were irradiated at 254 nm in the presence of
oxygen (12–14). Hozumi et al. (11) studied the VUV-photochem-
ical surface modification of PS using as Xe2*-source, focusing on
the impact of atmospheric pressure on surface energy (contact
angle), thickness, and elemental composition (both determined by
XPS) of the photo-oxidized layer.
INTRODUCTION
Vacuum-ultraviolet (VUV) radiation produced by excimer lamps is
inciting increasing interest as means to oxidize polymer surfaces
{Posted on the website on 2 February 2005
àPresented as a poster at the VIII ELAFOT, La Plata, Argentina, 8–12
November 2004
*To whom correspondence should be addressed: Lehrstuhl fu¨r Umwelt-
messtechnik, Universita¨t Karlsruhe, 76128 Karlsruhe, Germany. Fax: 00
49-721-608-6240; e-mail: andre.braun@ciw.uni-karlsruhe.de
Abbreviations: ATR, attenuated total reflectance; DTGS, deuterated
triglycine sulfate; FTIR, Fourier-transform infrared spectroscopy; GC/
MS, gas chromatography/mass spectrometry; PS, polystyrene; SNV,
standard normal variate; VUV, vacuum ultraviolet; XPS, x-ray
photoelectron spectroscopy.
In addition to these published data, the objective of our project
is to elucidate the mechanisms leading to the observed surface
modifications. For that purpose, products of the VUV-photo-
initiated oxidation of PS films were identified, and a more detailed
photophysical characterization of the irradiated samples was un-
Ó 2005 American Society for Photobiology 0031-8655/05
777

´
778 Juan Lopez-Gejo et al.
Figure 2. Typical (IR) spectra of nonirradiated and irradiated polysty-
rene films. Reference absorption bands used for the normalization proce-
dure are indicated.
Figure 1. Schematic diagram of experimental set-up.
functional groups associated to the respective absorption bands and are
presented after subtracting the values of a nonirradiated sample.
It is worth noting that the penetration depth of VUV-radiation (optical
path length) into the polymer films is significantly smaller than that of the
probing IR radiation. Based on the Lambert-Beer Law and assuming the
density of molecules in solid PS to be 6 3 1021 molecules cmꢁ3 and the
absorption cross-section per molecule at 172 nm to be 35 3 10ꢁ18 cm2
moleculeꢁ1 (16), an optical path length of 110 nm may be evaluated. In
contrast, the IR probing is taking place with penetration depths of the order
of a few micrometers.
Potential fragmentation of irradiated polymer material was assessed by
washing the irradiated surfaces with acetonitrile (RotisolvÒ, HPLC grade ꢂ
99.9%, Carl Roth GmbH þ Co, Karlsruhe, Germany) subsequent to the first
spectroscopic analysis. Acetonitrile does not contain hydroxyl or carbonyl
groups and would therefore not interfere in repeated FTIR analyses, if some
solvent would remain in the polymer matrix. For the contact angle mea-
surements, irradiated samples were washed with methanol (LiChrosolvÒ,
high-performance liquid chromatography grade, Sigma Aldrich Chemie
GmbH, Taufkirchen, Germany). PS is insoluble in both solvents.
Sample surfaces were washed by immersing the polymer films for
a period of 5 min into a well-stirred volume of solvent. Subsequently, the
films were allowed to dry under normal atmosphere for several hours before
repeating the analytic procedures.
Acetonitrile solutions were analyzed after the washing process
by GC/MS. A HP Series II GC-MS equipped with a HP-5MS column
(30 m30.25 mm30.25 lm) was used (Agilent Technologies Deutschland,
GmbH), (carrier gas: He; flow rate: 1.2 mL/min; temperature program: 508C
[2 min], increase to 1108C at a rate of 108C/min; increase to 2808C at a rate
of 508C/min).
The static contact angle of PS with a polar solvent (water, droplets of 5 3
10ꢁ7 L) was measured with a goniometer (OCA 20, Data Physics
Instruments GmbH, Filderstadt, Germany). All contact angles were de-
termined by averaging measured values at five different points on each
sample surface. The error was calculated from the standard deviation of
these five measurements.
dertaken together with FTIR and contact angle measurements in
function of irradiation time. The results of these mechanistic in-
vestigations were prerequisite to understanding and optimizing the
nanoscale structuring of PS surfaces, already reported (11).
MATERIALS AND METHODS
Polystyrene films of 0.125-mm thickness were purchased (Goodfellow
GmbH, Bad Nauheim, Germany). The films were placed on a horizontally
moving table, ensuring a balanced irradiation of the total surface (x axis, Fig.
1). A XERADEXÔ pulsed tubular Xe2*-radiation source (Radium Lamp-
enwerk, Wipperfuerth, Germany; electrical power: 20 W; diameter: 4 cm;
length: 12 cm, kexc: 172 6 12 nm, radiant power: 8 W, exitance: 50 mW/cm2)
was used as radiation source. The equipment is placed in a glove box,
ensuring control of the gas phase implied in the functionalization processes.
Irradiation was performed at different distances (y axis, Fig. 1) between the
surfaces of the radiation source and the film and for different times of
irradiation. The distance is measured by a high-precision dial indicator.
Homogeneous irradiation of sample surface was always achieved because
the length of the lamp and maximum displacement of the table (615 cm)
are much larger than the typical sample dimensions (1 3 1.5 cm2).
Potential local heating of the sample could be avoided by directing a
constant air flow parallel to the irradiated surface. The surface temperature
during irradiation was monitored by means of a thermosensor and re-
mained generally between 25 and 308C. However, during irradiation exper-
iments at small distances (ꢀ2 mm), the surface temperature reached 508C.
Functionalization of the PS surface was identified and quantified by FTIR
spectroscopy (Equinox 55, Bruker Optik GmbH, Ettlingen, Germany)
equipped with a deuterated triglycine sulfate (DTGS) detector using
a SPECAC (Woodstock, GA, USA) Golden Gate attenuated total re-
flectance (ATR) module with a diamond crystal of 1 3 1 mm2(??)). For
each sample (i.e. each irradiation experiment with defined gas phase, time,
and distance of irradiation) a total of seven spectra were collected, every
spectrum taken at a different area of the total sample surface. IR Spectra
were smoothed (to reduce noise) and a standard normal variate (SNV)
method was used to correct for baseline differences originating from
scattering effects (15).
RESULTS AND DISCUSSION
Absorption bands appearing in the IR spectra that can be associated to
new functional groups on the PS surface were integrated. The integrated
intensities were corrected for the variation of the sample-crystal contact
usually encountered in ATR measurements. For this purpose, integrated
values of the absorption band of interest were normalized by means of the
integrated value of a reference absorption band of the same spectral region,
assumed not to be altered during the irradiation process (Fig. 2). Absorption
bands located between 2800 and 2950 cmꢁ1 (C-H stretching vibration) and
1451 and 1500 cmꢁ1 (5C-H and ring C5C stretching vibration) were taken
as references for OH and for C5O, respectively (see Fig. 2).
VUV-irradiation of PS films under normal atmosphere lead to the
appearance of two new IR-absorption bands, one located between
1600 and 1800 cmꢁ1, the other located between 3000 and 3500 cmꢁ1
.
We associate the absorption between 1600 and 1800 cmꢁ1 to
the presence of carbonyl groups and the one between 3000 and
3500 cmꢁ1 to the presence of hydroxyl groups. Examples of IR
spectra taken from nonirradiated and irradiated samples are shown
in Fig. 2.
Data presented in this report represent the mean values calculated from a set
of seven normalized values obtained from seven IR spectra taken from one
irradiated sample, the error being estimated from the standard deviation of the
seven values. These mean values are proportional to the concentrations of the
Under similar experimental conditions (type of VUV-radiation
source, normal atmosphere, ranges of distance and irradiation time),
the appearance of an absorption band related to hydroxyl groups

Photochemistry and Photobiology, 2005, 81 779
Figure 3. Evolution of the mean value of integration of CO (a) and OH (b)
absorption bands as a function of irradiation time, before and after washing
with acetonitrile. Distance of irradiation: 4 mm.
Figure 4. Evolution of the mean value of integration of CO (a) and OH (b)
absorption bands as a function of irradiation distance, before and after
washing with acetonitrile. Time of irradiation: 2 min.
was not reported (10), and it might be that, under their analytical
conditions, the authors could not detect this rather broad peak
showing a much lower signal-to-noise ratio than the well-structured
absorption band of the carbonyl function.
related to the hydroxyl group. The mean values of integration in
function of irradiation time determined for samples before and after
washing with acetonitrile permit differentiating three stages of
carbonyl functionalization:
Progressing conversion of the functionalization was determined
from the normalized integration of the new IR-absorption bands,
and mean values were calculated from a set of seven normalized
values obtained from seven IR spectra taken from one irradiated
sample. These mean values are proportional to the concentrations
of the functional groups associated to the respective absorption
bands and are presented after subtracting the values of a non-
irradiated sample.
1. In the early stage of the photo-oxidation (0–30 s, Fig. 3),
the concentration of carbonyl groups increases linearly. The
mean values of integration being the same before and after
washing, these carbonyl groups are most probably attached
to the polymer chain and are therefore not eliminated from
the surface by the washing process.
2. The polymer backbone is oxidatively broken and products
of low molecular weight are produced and deposited on the
surface at irradiation times .30 s (Fig. 3). The increasing
difference between the curves established for unwashed and
washed samples may be due to an increasing mass of de-
bris during longer times of irradiation (,120 s, Fig. 3),
which can be dissolved in an appropriate solvent. Inversely,
the slower increase of the rate of functionalization obtained
for the washed samples may be attributed to an increasing
importance of the layer of oxidation products, which pre-
vents further oxidative degradation of the underlying (na-
tive) polymer.
Figure 3 shows these mean values and, hence, the increase of the
concentrations of carbonyl and hydroxyl functions at the surface of
a PS film as a function of irradiation time, at a fixed distance of
irradiation of 4 mm. Concentrations of both groups increase during
the first 2 min of irradiation, reaching limiting values after this time.
In the early stage of irradiation, the polymer is oxidized and func-
tional groups are introduced to the surface. Oxidation apparently
stops at a given degree of functionalization. This limiting value
might also be obtained by pathways of oxidation (e.g. fragmenta-
tion of the polymer backbone) producing low-molecular products
that evaporate from the polymer surface or that are mineralized by a
succession of oxidation reactions. Rates of formation, evaporation,
and/or mineralization of hydroxyl and carbonyl functionalized
products might be in a kind of equilibrium and present apparent
limited rate of conversion.
3. Finally, at longer times of irradiation (.120 s, Fig. 3), oxi-
dation products are formed that evaporate or migrate from
the surface permitting photochemically generated new prod-
ucts from the surface of the native polymer. The thickness
of the layer of these products of low molecular weight is
thought to remain constant, as the difference between the
curves established for unwashed and washed samples re-
mains constant with increasing irradiation time.
In contrast, no limiting values of the rate of functionalization
were found at even longer times of irradiation, when similar in-
vestigations were carried out with UV-C irradiation (Hg low pres-
sure arc [kexc: 254 nm]). The concentration, instead, was found to
increase linearly over the entire irradiation time (12,17,18). Such
a difference in behavior suggests that the photochemically initiated
oxidation may involve different mechanisms depending on the
wavelength of excitation.
The observation of limited mean values of integration for carbonyl
and hydroxylgroupsbound tothepolymer(curvesofwashedsamples,
Fig. 3) may be explained by a mechanism where early functionaliza-
tion of PS is followed by oxidative fragmentation of the backbone.
The effect of the distance of irradiation on the rate of formation of
functional groups on the surface of the polymer was studied by
irradiating a series of samples during a fixed time of 2 min at dis-
tances between 2 and 28 mm. The corresponding results are shown
in Fig. 4. As expected, highest rates of functionalization were found
at close distances and may be explained by the fact that, under these
conditions, absorption of the radiant exitance by molecular oxygen
and water vapor in the gas phase is minimal. At the same time, there
Formation of low-molecular oxidation products may be shown
by washing the irradiated surfaces and analyzing the washed surface
as well as the resulting solution. In fact, an important decrease of the
degree of functionalization was found upon washing of the irra-
diated surfaces with acetonitrile. The effect of sample washing is
that of a reduction in concentration of both functional groups
involved, as shown in Fig. 3. The carbonyl function exhibiting
a much better defined (i.e. narrow) IR-absorption band, the respec-
tive functionalization may be followed in more detail than that

´
780 Juan Lopez-Gejo et al.
Figure 5. Gas chromatograms of acetonitrile used to wash native polystyrene powder (black line) and irradiated polystyrene powder (grey line). Identification
by mass spectrometry.
is a high probability of impact of photochemically generated atomic
oxygen and hydroxyl radicals at the polymer surface. This inter-
pretation is supported by the observed exponential dependence of
the experimentally determined rates of functionalization from the
distance of irradiation.
backbone fragmentation are acetophenone and benzaldehyde.
Among the many pathways leading to their production, oxidation
of styrene is shown in Fig. 6b. Phenol derivatives indicate the
presence of hydroxyl radicals and their electrophilic addition to the
aromatic system (Fig. 6c). Atomic oxygen and/or hydroxyl radi-
cals may also abstract hydrogen from the aliphatic backbone of the
polymer (Fig. 6d), and the C-centered radicals will subsequently
react with molecular oxygen, yielding peroxyl radicals, known as
key intermediates of oxidative degradation (20).
At short distances, the difference between mean values of
integration is highest, implying that polymer backbone fragmen-
tation is very efficient. As the distance of irradiation increases, this
difference diminishes until conditions are reached (15 mm) where
no apparent fragmentation is taking place and all functional groups
formed are bound to the polymer backbone. The results suggest
that the importance of the layer of polymer debris depends on the
incident VUV radiation and on the surface temperature.
Yellowing of polystyrene upon UV irradiation may be found in
numerous reports and was explained either by the formation of
conjugated p-bondsequences in the polymer backbone (21–23)or by
the formation of conjugated dicarbonyl functions (22). Among the
conjugated p-bond sequences, benzalacetophenone was proposed as
a possible chromophore of this yellowing effect (24). In fact, our
analyses confirm the production of benzalacetophenone (Fig. 5).
Although phenol derivatives were identified, products of sub-
sequent oxidative degradation, such as quinoid compounds or
products of ring-opening reaction sequences, could not be detected.
In contrast with the proposal of Hozumi et al. (11) that active
oxygen species would mainly react with the aromatic systems, the
The analysis of the washing solution revealed a large variety of
oxidation products removed from the irradiated polymer surface.
Figure 5 shows the chromatograms of acetonitrile solutions origi-
nating from the washing process of native polystyrene (black line)
and of irradiated polystyrene (grey line) films. Formation of sty-
rene is a typical result of the photolysis of polystyrene, the unzip
reaction (19) of an end-standing radical leading to the formation
of the monomer (Fig. 6a). However, the main products of the
Figure 6. Main pathways of reaction
of the VUV photochemically initiated
oxidation of polystyrene.

Photochemistry and Photobiology, 2005, 81 781
Figure 8. Evolution of the contact angle of polystyrene films as a function
of irradiation distance, before and after washing with methanol. Time of
irradiation: 2 min.
Figure 7. Evolution of the contact angle of polystyrene films as a function
of irradiation time, before and after washing with methanol. Distance of
irradiation: 4 mm.
the polymer. The combination of FTIR analysis and contact angle
measurements ensures a quantitative determination of the rate of
functionalization. Experimental conditions were found that lead to
substantial oxidative fragmentation of the polymer backbone and to
the accumulation of polar polymer debris of low molecular weight
at the surface of the polymer films. Nevertheless, the experimental
parameters could be adjusted to optimize the formation of polar
groups covalently bound to the polymer and, hence, to minimize
backbone fragmentation.
results of this investigation seem to indicate that oxidation reactions
are mainly involving the aliphatic backbone of the polymer.
The contact angle (water) diminished with increasing time of
irradiation from 908 to 258 (Fig. 7, constant distance of irradiation:
4 mm). Carbonyl and hydroxyl functionalization of the polymer
surface increases the polarity of the surface and, consequently, leads
to a decrease of the contact angle with increasing irradiation time.
These results are in excellent agreement with the mean value of
integration of the absorption band surfaces determined by FTIR.
The total change (decrease) in contact angle was practically com-
pleted within the first 2 min of irradiation (Fig. 7), matching the time
of irradiation needed to reach the limiting values of carbonyl and
hydroxyl group functionalization (Fig. 4). Furthermore, no changes
in surface energy could be detected after the limiting values of polar
group concentrations were reached.
The experimental set-up described allows homogeneous irra-
diation of larger samples, and the use of pulsed Xe2*-radiation
sources represent a valuable alternative to laser- or plasma-induced
processes for the oxidation and surface modification of polymers.
Acknowledgement—The authors acknowledge financial support by the
Deutsche Forschungsgemeinschaft (D.F.G.).
After washing the irradiated surfaces with methanol, the surface
energies increased but did not return to their original value of the
nonirradiated samples. Again, this result is in excellent agreement
with the FTIR measurements. Oxidation products of low molecular
weight are removed by the washing process, leading to a decrease
in (surface) polarity and therefore an increase in contact angle. The
fraction of polar groups covalently bound to the polymer backbone
is accounting for the difference in contact angle observed between
nonirradiated and washed, irradiated samples.
REFERENCES
1. Naoto, S., K. Oda, S. Asakura, A. Fuwa, Y. Yokogawa, T. Kameyama and
A. Hozumi (2004) Photoinduced decomposition of alkyl monolayers
using 172 nm vacuum ultraviolet. J. Vac. Sci. Technol. A 22, 1615–1619.
2. Hozumi, A., N. Shirahata, Y. Nakanishi, S. Asakura and A. Fuwa
(2004) Wettability control of polymer surfaces through 126 nm vacuum
ultraviolet light irradiation. J. Vac. Sci. Technol. A 22, 1309–1314.
3. Wei, C., J.-Y. Zhang, Q. Fang, K. Hu and I. W. Boyd (2004) Surface
modification of polyimide with excimer UV radiation at wavelength of
126 nm. Thin Solid Films 453–454, 3–6.
4. Truica-Maraescu, F. and M. R. Wertheimer (2004) Vacuum ultraviolet-
induced photochemical nitriding of polyolefin surfaces. J. Appl. Polym.
Sci. 91, 3886–3898.
5. Ohtsubo, T., T. Azuma, M. Takaura, T. Higashiguchi, S. Kubodera and W.
Sasaki (2003) Removal of oxygen atoms from a SiO2 surface by incoherent
vacuum ultraviolet excimer irradiation. Appl. Phys. A 76, 139–141.
6. Sugimura, H., T. Hanji, O. Takai, T. Masuda and H. Misawa (2001)
Photolithography based on organosilane self-assembled monolayer
resist. Electrochim. Acta 47, 103–107.
7. Hozumi, A., T. Masuda, K. Hayashi, H. Sugimura, O. Takai and
T. Kameyana (2002) Spatially defined surface modification of
poly(methylmethacrylate) using 172 nm vacuum ultraviolet light.
Langmuir 18, 9022–9027.
8. Nakamura, M., S. Tanaka, Y. Naganuma and C. Kato (2002) Surface
modification by excimer lamps. Hyomen Gijutsu 53, 507–510.
9. Kogelschatz, U. (2003) Dielectric-barrier discharges: their history,
discharge physics, and industrial applications. Plasma Chem. Plasma
Process. 23, 1–46.
Finally, Fig. 8 depicts the contact angle as a function of distance
of irradiation for experiments using an irradiation time of 2 min.
The smallest value of contact angle was found for samples irra-
diated at shortest distance. Supporting the results of the FTIR
analyses, the contact angle decreases as the surface concentration of
carbonyl and hydroxyl groups is increased. At short distances of
irradiation (,5 mm), where polymer fragmentation is most effi-
cient, a large difference of surface energy values was found between
unwashed and washed samples (Fig. 8). At irradiation distances
ꢂ18 mm, no difference (larger than experimental error) in contact
angle could be observed between washed and unwashed samples,
suggesting that, in this range of irradiation distance, fragmentation
does not take place under the given experimental conditions.
DISCUSSION
10. Tanaka, S., Y. Naganuma, C. Kato and K. Horie (2003) Surface
modification of vinyl polymers by vacuum ultraviolet light irradiation.
J. Photopolym. Sci. Technol. 16, 165–170.
11. Hozumi, A., H. Inagaki and T. Kameyana (2004) The hydrophilization
of polystyrene substrates by 172-nm vacuum ultraviolet light. J. Colloid
Interface Sci. 278, 383–392.
VUV photochemically initiated oxidation using a pulsed Xe2*-
radiation source is a very efficient and fast process for treatment
(functionalization) of PS surfaces. Carbonyl and hydroxyl groups
generated on the surface of PS films decrease the surface energy of

´
782 Juan Lopez-Gejo et al.
12. Geuskens, G., D. Baeyens-Volant, G. Delaunois, Q. Lu-Vinh, W. Piret
and C. David (1978) Photo-oxidation of polymers-I. A quantitative
study of the chemical reactions resulting from irradiation of polystyrene
at 253.7 nm in the presence of oxygen. Eur. Polym. J. 14, 291–297.
13. Nowakowska, M., J. Kowal and B. Waligora (1978) Photo-oxidation
of polystyrene film. 2. Photooxidation of polystyrene film with light
absorbed by the polystyrene-oxygen complex. Polymer 19, 1317–
1319.
14. Ranby, B. and J. Lucki (1980) New aspects of photodegradation and
photo-oxidation of polystyrene. Pure Appl. Chem. 52, 295–303.
15. Barnes, R. J., M. S. Dhanoa and S. J. Lister (1989) Standard normal
variate transformation and de-trending of near-infrared diffuse reflec-
tance spectra. Appl. Spec. 43, 772–777.
18. Otocka, E. P., S. Curran and R. S. Porter (1983) Photooxydation
of polystyrene: irradiation at 254 and 365 nm. J. Appl. Polym. Sci. 28,
3227–3233.
19. McKellar, J. F. and N. S. Allen (1979) Photochemistry of Man-Made
Polymers. Applied Science Publishers, London, UK.
20. Legrini, O., E. Oliveros and A. M. Braun (1993) Photochemical
processes for water treatment. Chem. Revs. 93, 671–698.
21. Grassie, N. and N. A. Weir (1965) The photooxidation of polymers. II.
Photolysis of polystyrene. J. Appl. Polym. Sci. 9, 975–986.
22. Rabek, J. F. and B. Rangy (1974) Studies on the photooxidation
mechanism of polymers. I. Photolysis and photooxidation of poly-
styrene. J. Polym. Sci., Polym. Chem. Edn. 12, 273–294.
23. Fox, R. B. and T. R. Price. (1978) Photoyellowing of polystyrene and
poly(styrene-alt-methyl methacrylate). In Stabilization and Degrada-
tion of Polymers (Edited by D. L. Allara), pp. 96–108, Washington, DC
(Advances in Chemistry Series; 169).
16. Partridge, R. H. (1967) Vacuum-ultraviolet absorption spectrum of
polystyrene. J. Chem. Phys. 47, 4223–4227.
17. Onyiriuka, E. C. (1993) The effects of high-energy radiation on the
surface chemistry of polystyrene: a mechanistic study. J. Appl. Polym.
Sci. 47, 2187–2194.
24. Wall, L. A. and D. W. Brown (1957) c–Irradiation of poly(methyl
methacrylate) and polystyrene. J. Phys. Chem. 61, 129–136.