[Technical Paper] Surface Modification of Polyethylene ...
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47
Kasahara et al.: Surface Modification of Polyethylene Terephthalate (PET) (1/8)
1. IntroductionPolyethylene terephthalate (PET) films have been
widely used in flexible substrates for organic light emitting
diode (OLED) displays,[1] tactile sensors,[2] and roll to
roll UV imprint lithography[3] because they have attrac-
tive properties, including a high melting temperature, low
dielectric constant, and good mechanical strength. On the
other hand, the low surface free energy and the chemical
inertness of the PET often lead to poor adhesive bonding
and poor adhesion of printing and coatings in practice.
Surface modification techniques such as ion implanta-
tion,[4] laser ablation,[5, 6] plasma treatments,[7–12] ultra-
violet-ozone (UV/O3) cleaning,[13] and wet-processes[14]
have been utilized to overcome this problem. Most of
these processes can change the wettability and the chemi-
cal functional groups while increasing the surface rough-
ness. It is essential for the surface modification of the poly-
mers to affect the uppermost surface layer only and not
alter the bulk properties.
Recently, irradiation with UV excimer lamps for the pho-
tochemical modification has been attracted attention. Sev-
eral polymers have been modified by using UV excimer
lamps at different wavelengths, such as 126 nm using
Ar2*,[15] 172 nm using Xe2*,[16–18] and 222 nm using
KrCl*[19] in various gas environments. Additionally, vac-
uum ultraviolet treatments using Xe2* excimer lamps were
utilized to improve the bond strength of the flip chip and
three dimensional (3D) interconnections.[20–22] UV
lamps can provide large area exposures and short reaction
times at low temperature and only require simple and inex-
pensive apparatus. However, the surface modification
effects depend on the lamp parameters such as the wave-
length and the intensity as well as on the chamber pres-
sure and atmosphere.
In this study, PET films were modified by using a 172
nm Xe2* excimer lamp. Two kinds of treatment techniques
were applied. The first was vacuum ultraviolet (VUV) light
irradiation, and the other was VUV irradiation in the pres-
ence of oxygen gas (VUV/O3).[23] The contact angles
were measured to evaluate the wettability and to calculate
[Technical Paper]
Surface Modification of Polyethylene Terephthalate (PET) by
172-nm Excimer LampTakashi Kasahara*, Shuichi Shoji*, and Jun Mizuno**
*Major in Nano-Science and Nano-Engineering, Waseda University, 3-4-1 Okubo, Shinjuku, Tokyo 169-8555, Japan
**Institute for Nanoscience and Nanotechnology, Waseda University, 513 Wasedatsurumakicho, Shinjuku, Tokyo 162-0041, Japan
(Received July 30, 2012; accepted October 11, 2012)
Abstract
We studied the effects of 172 nm Xe2* excimer lamp irradiation on polyethylene terephthalate (PET) surfaces. Two kinds
of techniques were applied: vacuum ultraviolet (VUV) light irradiation and VUV irradiation in the presence of oxygen gas
(VUV/O3). The modified PET surfaces were investigated by using contact angle measurements which enabled the sur-
face free energy to be calculated, X-ray photoelectron spectroscopy (XPS), nano-thermal analysis (nano-TA), and atomic
force microscopy (AFM). The surface free energy increased significantly after the treatments. The results of XPS analy-
sis showed that the elemental ratio of oxygen on the surface increased, whereas that of carbon decreased. From the
deconvoluted C1s and O1s spectra, it was revealed that new oxidized functional groups such as alcoholic and carboxyl
groups were generated. The nano-TA results showed that a low melting temperature (Tm) layer had formed on the VUV
and VUV/O3 treated PET surfaces. The results of AFM measurements showed there were no remarkable changes after
the treatments compared with untreated PET. In summary, the VUV and VUV/O3 treatments using a Xe2* excimer lamp
not only change the surface functionalities but also reduce the Tm of the PET surfaces without significantly affecting the
surface morphologies.
Keywords: Surface Modification, Vacuum Ultraviolet, PET, Surface Free Energy, XPS, Nano-TA
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the surface free energy. The surface chemical structures
were investigated in detail by X-ray photoelectron spec-
troscopy (XPS). Nano-thermal analysis (nano-TA) was
used to evaluate the local thermomechanical properties of
the uppermost surface layer. The surface morphologies
were analyzed by atomic force microscopy (AFM).
2. Experimental Procedure2.1 Material
Commercial, 50 μm thick PET film (Teijin DuPont Films
Japan Ltd., G2) was used. The chemical structure of the
PET is shown in Fig. 1. The film was cut into 10 × 10 mm2
square pieces for XPS and AFM and 20 × 20 mm2 square
pieces for contact angle measurements and nano-TA. The
surface of the PET was cleaned with isopropyl alcohol in
an ultrasonic bath for 10 min before the experiments.
2.2 Surface treatmentsThe VUV and VUV/O3 treatments of the PET films were
carried out using the Xe2* excimer lamp source (Ushio
Inc., UER20-172). A schematic diagram of the experimen-
tal set-up is shown in Fig. 2. The central wavelength and
the intensity at the lamp window were 172 nm and 10 mW/
cm2, respectively. The distance between the lamp window
and PET surfaces was fixed at 13 mm for both VUV and
VUV/O3 treatments. For the VUV treatment, the chamber
was initially flushed with nitrogen gas and then evacuated
to a base pressure of less than 20 mbar. The PET was
directly exposed to 172 nm VUV light at room tempera-
ture. The chamber evacuation continued during the VUV
irradiation. The duration of the VUV treatment was varied
between 10 s and 60 s. The photon energy of VUV light is
larger than that of conventional UV light (e.g., low pres-
sure mercury lamps), and can break the various chemical
bonds in organic molecules (e.g., C-C, C-H). For the VUV/
O3 treatment, highly pure oxygen gas was introduced into
the chamber to a pressure of 500 mbar after the initial
chamber evacuation to 20 mbar, and the PET was irradi-
ated by the VUV light in an oxygen atmosphere at room
temperature for times that varied between 30 s and 300 s.
The chamber was not evacuated, and the chamber pres-
sure was kept at 500 mbar during the VUV/O3 process.
Because VUV irradiation was used instead of UV light,
high-density ozone and excited oxygen atoms O(1D) were
generated from O2, and these could react with organic
molecules on the polymer surface.
2.3 Contact angle measurements and surface free energy calculation
The surface free energy of the PET was characterized
by the contact angles. The contact angles on the PET sur-
faces were obtained using a contact angler (Kyowa Inter-
face Science Co. Ltd., LCD-400S) and the sessile drop
method. The surface free energy of the PET can be deter-
mined by using Young’s equation, which can be written as
follows[24]:
γs = γsl + γl cosθ, (1)
where θ is the contact angle, γs is the surface free energy
of the solid, γl is the surface tension of the liquid, and γsl is
the interfacial energy between the solid and liquid. Accord-
ing to Owens-Wendt theory,[25] γs, γl, and γsl can be
expressed as follows:
γs = γsp + γs
d, (2)
γl = γlp + γl
d, (3)
γsl = γs + γl - 2(γsp γl
p)0.5 - 2(γsd γl
d)0.5, (4)
where γsp and γl
p are the polar components, and γsd and γl
d
are the dispersive components of the solid and liquid.
Equation (5) can be obtained from Eqs. (1)–(4).
γl (1 + cosθ) = 2(γsp γl
p)0.5 + 2(γsd γl
d)0.5. (5)
When the values of γl, γlp, and γl
d of more than two test liq-
uids are known, γs, γsp, and γs
d can be determined by the
contact angles. In our case, four different liquids, i.e.,
water (H2O), glycerol (C3H8O3), diiodomethane (CH2I2),
and formamide (CH3NO), were used. With each of the
four liquids, five measurements were taken at the different
locations.
2.4 XPS analysisThe surface composition and chemical bonds of the PET
were investigated using XPS (JEOL Ltd., JPS-9100TR).
The X-ray source and the applied power were MgKα
Fig. 1 Chemical structure of the PET.
Fig. 2 Schematic diagram of the VUV and VUV/O3 treat-ment system.
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Kasahara et al.: Surface Modification of Polyethylene Terephthalate (PET) (3/8)
(1253.6 eV) and 100 W (10 kV and 10 mA), respectively.
The photoelectron take-off angle was fixed at 90°. The
wide and high resolution scans were measured at pass
energies of 50 eV and 10 eV, respectively. The surface ele-
mental ratios were determined from the peak areas of the
O1s and C1s spectra. The curve fitting was performed
with a Gaussian/Lorentzian ratio of 70/30 using peak-fit-
ting software (JEOL Ltd., SpecSurf) after a Shirley-type
background subtraction.
2.5 Nano-TAThe nano-TA is an AFM-based analysis technique used
to determine the thermomechanical properties of materi-
als.[26–28] In this method, a thermal probe placed in con-
tact with the sample surface is heated. As the temperature
rises, the deflection of the probe increases initially due to
local thermal expansion of the substrate, and then
decreases when the sample temperature reaches the soft-
ening temperature, which is a glass transition temperature
(Tg) for amorphous polymers or a melting temperature
(Tm) for semi-crystalline polymers. In this study, a nano-
TA system (Anasys instruments Co., nano-TA) combined
with AFM (Agilent Technologies Inc., 5500AFM) was used
to evaluate the Tm of the PET surfaces before and after
treatments. The measurements of the local thermal analy-
sis (LTA) were performed using a heating rate of 10°C/s
at five different locations on each sample.
2.6 AFMThe morphology of the PET was investigated by using
AFM equipment (Shimadzu Co., SPM-9600) in dynamic
mode. An area of 2 × 2 μm2 was scanned in the air at room
temperature. A root mean square surface roughness (Rms)
was obtained from the AFM images.
3. Results and Discussion3.1 Contact angles and surface free energy
Table 1 gives the results of the contact angle measure-
ments before and after treatments. The treatment times of
the VUV were 30 s and 60 s, while those of VUV/O3 were
60 s and 300 s. The contact angles of water, glycerol, and
formamide on the PET decreased drastically after both
VUV and VUV/O3 treatments. The calculated surface free
energy of the PET is shown in Fig. 3. The surface free
energy and its polar component of the untreated PET were
38.52 mN/m and 8.11 mN/m, respectively, while the dis-
persive component was 30.41 mN/m. After both VUV and
VUV/O3 treatments, a significant increase in the polar
component was obtained, whereas the dispersive compo-
nent showed no remarkable change. These results indi-
cate that the VUV and VUV/O3 treatments enhanced the
hydrophilicity of the PET by the creation of additional
polar components.
3.2 XPSThe surface elemental ratios of the PET before and after
treatments are listed in Table 2. The treatment times of the
VUV were 30 s and 60 s, while those of VUV/O3 were 60 s
Table 2 XPS elemental analysis of PET surfaces.
SampleTreatment
Chemical composition (%)
time (s) O1s C1s
Untreated 0 27.9 72.1VUV 30 31.6 68.4VUV 60 32.5 67.5VUV/O3 60 32.0 68.0VUV/O3 300 35.7 64.3
Table 1 Changes in contact angles on PET surfaces.
SampleTreatment Contact angle (°)
time (s) Water Glycerol Diiodomethane Formamide
Untreated 0 72.2 58.8 33.6 64.5VUV 30 33.8 31.3 23.9 8.3VUV 60 32.6 33.7 24.3 9.0VUV/O3 60 44.4 40.7 27.1 15.9VUV/O3 300 40.8 37.9 27.0 12.5
Fig. 3 Calculated surface free energies of PET films (total, polar, and dispersive components) before and after VUV and VUV/O3 treatment for different treatment times.
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Transactions of The Japan Institute of Electronics Packaging Vol. 5, No. 1, 2012
and 300 s. After both VUV and VUV/O3 treatments, the
surface elemental ratio of the O1s increased, whereas that
of the C1s decreased. The oxygen concentrations of the
PET treated by VUV for 60 s and VUV/O3 for 300 s
increased from an initial value of 27.9% to 32.5% and 35.7%,
respectively. These results show that the increased surface
free energies were probably attributed to the incorporation
of oxygen functional groups into the PET surface.
In order to analyze the surface functional groups in
more detail, the C1s and O1s spectra were deconvoluted.
All spectra were referred to the C1s neutral carbon peak at
284.6 eV. Figs. 4 (a)–(e) show the C1s spectra of the
untreated, 30 s and 60 s VUV treated, and 60 s and 300 s
VUV/O3 treated PET films. Based on its chemical struc-
ture, the untreated PET consists of three different carbon
environments,[7–11, 18] because it has binding energies at
284.6 eV corresponding to C-C bonding (C1), at 286.2 eV
corresponding to C-O bonding (ethers) (C2), and at 288.6
eV corresponding to O=C-O bonding (esters) (C3). The
broad peak at around 291 eV was a shake-up satellite due
to the p → p* transitions of the phenyl groups. After VUV
and VUV/O3 treatments, increases in C-O (ethers and
alcoholic group) and O=C-O bonds (esters and carboxyl
groups) were observed.[7, 18] However, the C-C bonding
with bond energy of approximately 340 kJ/mol decreased
slightly, which was probably because of the chain scission
induced by the photon energy of the Xe2* excimer lamp
(697.5 kJ/mol) and/or the oxidative decomposition by the
excited oxygen atoms O(1D). These results indicated that
the Xe2* excimer lamp has sufficient energy to break the
C-C bond effectively, while the excited oxygen atoms
O(1D) is expected to create oxygen functionalities of C-O
Fig. 4 C1s XPS spectra of PET surfaces: (a) untreated; VUV treated for (b) 30 s and (c) 60 s; and VUV/O3 treated for (d) 60 s and 300 s.
Fig. 5 O1s XPS spectra of PET surfaces: (a) untreated; VUV treated for (b) 30 s and (c) 60 s; and VUV/O3 treated for (d) 60 s and 300 s.
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Kasahara et al.: Surface Modification of Polyethylene Terephthalate (PET) (5/8)
and O=C-O bonds on the PET surfaces with the oxidative
decomposition and volatilization.[21]
The O1s spectra obtained from the untreated, VUV, and
VUV/O3 treated PET are shown in Figs. 5 (a)-(e), respec-
tively. According to Ref.,[11, 18] the O1s spectrum of the
untreated PET contains two peaks at 531.6 eV and 533.2
eV, which are assigned to O=C (esters) bonding (O1) and
O-C (ethers) bonding (O2), respectively. The O1s spectra
corresponding to VUV and VUV/O3 treated PET showed a
significant increase in O=C bonding of esters and carboxyl
groups and O-C bonding of ethers and carboxyl groups.[10]
Consequently, polar components such as alcoholic and car-
boxyl groups were formed on the PET surfaces by both
VUV and VUV/O3 treatments, indicating that the obtained
XPS spectra are in agreement with the results of the sur-
face free energy calculation shown in Fig. 3.
3.3 Nano-TAThe results of the nano-TA of the VUV and VUV/O3
treated PET films with various treatment times are shown
in Figs. 6 (a) and (b), respectively. In the case of the
untreated PET, the increase in the probe temperature ini-
tially lead to an increase in the deflection because of the
local thermal expansion of the PET surface, and then the
probe penetrated into the material at approximately 240°C,
which is taken to be Tm. The Tm of the VUV treated PET
shifted downwards with increasing treatment times. When
60 s VUV treatment was carried out, the Tm decreased to
approximately 224°C. The formation of the low Tm layer on
the PET surfaces may be due to the change in the chemi-
cal structures induced by the photochemical modification
of the VUV light.[20] These results also indicated that long
treatment times were important for the modification of the
thermomechanical properties of the PET in the case of the
VUV treatments. For the VUV/O3 treatments, a low Tm
layer had also formed on the PET surfaces, and a value of
approximately 227°C was reached for treatment times lon-
ger than 60 s. Moreover, the deflections increased slowly
compared with the untreated PET, and slow penetrations
into the sample were observed. These changes in the ther-
momechanical properties were probably caused by the
chain scission and additional components induced by the
excited oxygen atoms O(1D), which were also observed in
the results of the surface free energy calculation and XPS.
From the nano-TA studies, we can conclude that the photo-
chemical modification using a Xe2* excimer lamp changed
the thermomechanical properties, indicating that the for-
mation of a low Tm layer can be controlled by the treatment
time of VUV and with and without introduction of oxygen
gas into the chamber.
3.4 AFMFigure 7 shows the AFM images and Rms roughness val-
ues of the untreated, VUV, and VUV/O3 treated PET sam-
ples with various treatment times. The surface of the
untreated PET was generally smooth, and its Rms was 1.894
nm (Fig. 7 (a)). It can be clearly seen that after both VUV
for 30 s and 60 s and VUV/O3 for 60 s and 300 s, the mor-
phologies of the PET has no remarkable change although
sphere-like aggregates were formed on the surfaces. The
Rms values of VUV treated PET for 30 s and 60 s were 2.193
nm and 1.884 nm, while those of VUV/O3 treated PET for
60 s and 300 s were 1.943 nm and 1.714 nm. These results
were probably due to the effect of the photon energy of the
VUV light and/or the excited oxygen atoms O(1D) on
chain scission. The changes in roughness were not signifi-
cant in comparison with other treatments such as plasma
methods, which indicating that polymer surface was
etched by physical erosion by ion bombardments during
plasma treatments,[8] while the excited oxygen atoms
O(1D) and 172 nm photon energy modified PET surface at
room temperature without ion bombardment. These
results showed that the VUV and VUV/O3 treatments
using the Xe2* excimer lamp can modify the functional
groups and thermomechanical properties of the PET sur-
faces without significantly changing the surface rough-
ness.Fig. 6 Nano-TA measurements of (a) VUV and (b) VUV/O3 treated PET with various treatment times.
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Transactions of The Japan Institute of Electronics Packaging Vol. 5, No. 1, 2012
4. ConclusionVUV and VUV/O3 treatments of PET surfaces have
been carried out with a 172 nm Xe2* excimer lamp. The
wettability of the PET was dramatically improved due to a
significant increase in the surface free energy. The results
of the XPS analysis of the C1s and O1s spectra showed the
formation of newly oxidized components on the PET sur-
faces, which agreed with the calculated PET surface free
energy. The modified PET surfaces showed the formation
of a low Tm layer on the PET surfaces, as observed in the
nano-TA results. After the surface treatments, the mor-
phologies of the PET showed no remarkable changes. In
conclusion, low Tm layers and oxygen functionalities of
C-O and O=C-O can be formed on PET surfaces without
significantly affecting the surface profiles by VUV and
VUV/O3 treatments using a 172 nm Xe2* excimer lamp.
AcknowledgementsThis work was partly supported by Japan Ministry of
Education, Culture, Sports Science & Technology Grant-
in-Aid for Scientific Basic Research (S) No. 23226010 and
by the Japan Society for the Promotion of Science (JSPS)
through the “Funding Program for World-Leading Innova-
tive R&D on Science and Technology (FIRST Program),”
initiated by the Council for Science and Technology Policy
(CSTP). The authors thank the Nanotechnology Support
Project of Waseda University for their technical advice.
The authors also thank Toyo Co. for the use of nano-TA
equipment and technical advice.
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Takashi Kasahara was born in Saitama Prefecture, Japan, in 1987. He received his BS and MS degree in the field of microsys-tems from Waseda University in 2010 and 2012, respectively. He is presently Ph.D. stu-dent at Waseda University. His current inter-ests are polymer microdevice technologies
such as OLED, flexible sensor, and surface modification.
Shuichi Shoji received his BS, MS and Ph.D. degree in electronic engineering from Tohoku University in 1979, 1981 and 1984, respectively. He had been with Tohoku Uni-versity as a research associate and associate professor from 1984 to 1992. In 1994 he moved to Waseda University as an associate
professor and he is currently a professor of Department of Elec-tronic and Photonic Systems, and Major in Nano-Science and Nano-Engineering, Waseda University. His current interests are micro-/nano-devices and systems for chemical/bio applications.
Jun Mizuno received his Ph.D. degree in applied physics from Tohoku University in 2000. He is currently an associate professor at Waseda University and works at the nano-technology research center where is a research institute of nano-science and engi-neering. His current interests are MEMS-
NEMS technology, bonding technology at a low temperature using plasma activation or excimer laser irradiation, printed elec-tronics, and composite technology for UV or heat nanoimprint lithography combined with electrodeposition.