Journal of Surface Engineered Materials and Advanced Technology, 2012, 2, 264-270
http://dx.doi.org/10.4236/jsemat.2012.24040 Published Online October 2012 (http://www.SciRP.org/journal/jsemat)
E-Beam Graft Polymerization of Hydrophilic
PEG-Methacrylate on the Surface of PMMA
Seong-Cheol Kim1, PilHo Huh2*
1Department of Nano, Medical and Polymer Materials, Yeungnam Univeristy, Gyeongsan, Republic of Korea; 2Solar Energy R&D
Center, Samsung Electronics Co., Ltd., Yongin, Republic of Korea.
Received July 23rd, 2012; revised August 29th, 2012; accepted September 9th, 2012
ABSTRACT
The graft polymerization of hydrophilic monomers on the surface of hydrophobic PMMA was performed using an elec-
tron beam (e-beam). The dose of e-beam irradiation, reaction concentration, temperature, and reaction time were used to
study the effect of variables on the graft density of poly (ethylene glycol)-methacrylate. The results demonstrated that
the weight percentage of graft polymer increased with increasing temperature, time and monomer concentration. How-
ever, the weight of the graft polymer did not increase with the increasing dose of e-beam irradiation. The change of the
contact angle of the water droplet on the PMMA surface was monitored as a function of a reaction time. The results
showed that the contact angle decreased up until a specific time and then leveled off to an approximately constant value
after a certain reaction time of the graft polymerization. Transmission electron microscopy proved that the constant
value of the contact angle was due to the local survival of surface radicals followed by the perpendicular diffusion of
monomers only into the bulk of the surface-modified area on the sheet surface.
Keywords: Electron Beam; Surface Graft; Contact Angle; ATR; TEM
1. Introduction
The modifications of the surface of materials such as
commercial films and biomedical materials have at-
tracted many researchers and industrialists due to the
dramatic changes in the original physical properties [1-4].
New material properties such as the self-cleaning of sur-
faces including fingerprint-free protection films for cell
phones, and antimicrobial and antistatic fabrics etc., can
be addressed by manipulating the chemistry and the mor-
phologies of the material surface. Superhydrophobic sur-
face properties can be obtained by adopting specific mi-
crostructure patterns found in nature [5,6]. In addition,
the adhesion of certain cell-types onto the target surface
can be enhanced or prohibited by changing the affinity of
the material surface to the cells [7,8]. When the surface
of any material is chemically modified, the surface of
materials must have reactive functional groups such as
hydroxyl groups, amino groups, thiols, isocyanates, and
carboxyl groups etc. The graft density, therefore, de-
pends on the number of the functional surface groups.
However, the disadvantage of the chemical modification
of the surface is that it generally takes increased amounts
of time to react with binding molecules and to signifi-
cantly alter the surface properties [9]. In addition, por-
tions of the chemical reaction are sensitive to airborne
oxygen and moisture. Finally, the availability of the
binding materials with a functional group is limited.
The other modification technologies available for
treatment of the material surfaces include ozone treat-
ment [10,11], graft polymerization on the surfaces of
materials using deep ultraviolet (UV) [12,13], plasma
[8,14], γ-ray [15,16], and electron beam [17-19]. Each
method has advantages and disadvantages [20]. Deep UV
has shallow depth of penetration and thus abstract elec-
trons are limited to the surface of the materials. There-
fore, the bulk of the materials are not influenced by deep
UV. However, it is necessary to UV irradiate the material
for extended periods of time to generate enough surface
radicals to initiate graft polymerization. In addition, the
structure of materials must lose electrons easily upon UV
irradiation. The advantage of the plasma induced graft
reaction is that the monomers without polymerizable
double bonds can be utilized. Another merit of plasma
induced grafting is that the modified surface is relatively
uniform even when the complex structure of the materi-
als is used [8]. However, the same reaction conditions
may require vacuum as well as inert conditions. The graft
density on the surface as a result of this process is gener-
*Corresponding author.

E-Beam Graft Polymerization of Hydrophilic PEG-Methacrylate on the Surface of PMMA 265
ally low. Radiation induced graft polymerization using
60Co has been studied widely, but this technique is still
far from commercialization due to the resulting low dose
of γ-rays during the treatment of materials [15,16].
Surface graft polymerization using an electron beam
has many advantages including the ability to mass pro-
duce products and the ease of equipment use. Therefore,
there is high potential for commercial applications. The
high energy electron beam can easily remove electrons
from any organic material and generate radicals on the
surface of the material and in the bulk material. The
number of radicals on the material can be controlled pre-
cisely by modulating the irradiation dose of electrons.
Even when the hydrophobicity of graft materials is
somewhat different from the target surface, the e-beam
induced graft polymerization proceeds relatively easily
compared to the chemical modification process using
surface functional groups.
In this paper, we discuss the e-beam graft polymeriza-
tion of hydrophilic PEG-methacrylate on the hydropho-
bic surface of PMMA. We prove that the characteristic
reaction pathway of an e-beam induced graft polymeriza-
tion of hydrophilic monomers on the hydrophobic sur-
face using contact angle measurement and transmission
electron microscopy.
2. Experimental
The monomers, poly(ethylene glycol)-methacrylate (PE-
G-MA) with molecular weights of 300, 475, and 950
g/mol were purchased from Aldrich Co. Ltd. The organic
solvents used in the experiments were purchased from
Daejung Co. Ltd. (Korea) The PMMA sheet was pur-
chased from Easylabel Co. (Korea).
The PMMA sheet with a thickness of 1 mm was cut
into 2 × 2 cm2 pieces and vacuum sealed with a food
packaging film to reduce contact with oxygen. Next, the
e-beam was utilized to irradiate the sealed samples. The
dose of electron beam irradiation was 25 to 100 KGy.
After the irradiation process, the PMMA sample was
placed into the reaction vessel and the vacuum was ap-
plied for 90 seconds to remove oxygen in the reaction
vessel. Next, the prepared PEG-MA solution with a spe-
cific concentration from 20% to 50% (v/v) was injected
in the reaction vessel. To minimize potential alterations
in the concentration of reaction mixture due to the evapo-
ration of water, septa was used to reduce the void volume
of the reaction vessel during graft polymerization.
The grafted PMMA sheet was impregnated into the
epoxy resin and cured at 70˚C for one day to prepare the
sample for transmission electron microscopy (TEM).
After curing reaction with the epoxy resin, the sample
was sectioned with a microtome to expose the cross sec-
tion of the sheet. The microtomed samples were then
mounted on the Cu grid and the platinum was vacuum
deposited for 25 seconds to increase the contrast between
the bulk PMMA and the PEG-rich region. The TEM im-
ages were obtained using a EM 919 Omega transmission
electron microscope with an acceleration voltage of 120
KV. X-ray photoelectron spectroscopy (XPS) measure-
ments were conducted with a PHI Quantera SXM (UL-
VAC-PHI. Inc.) with a Al Kα X-ray source at 15kV and
25W. The emission angle of the photoelectrons, θ, was
kept constant at 45˚. A Perkin Elmer Fourier transform
infrared spectrometer equipped with a variable angle
PIKE VeeMAX II accessory was used to collect the at-
tenuated total reflectance (ATR) spectra of the grafted
polymer. The incident angle of the ATR through a ZnSe
crystal was changed from 30˚ to 75˚ with an increment of
15˚. Contact angles of water on the PMMA and the
grafted PMMA sheet were measured by the sessile drop
method using the CA-20 measurement apparatus (Data-
Physics Instruments GmbH, Filderstadt, Germany) under
ambient laboratory conditions (20˚C, 40% RH). Ten lo-
cations were measured on each sample and the resulting
contact angle values obtained were averaged.
3. Results and Discussion
To find the optimum reaction conditions for the graft
polymerization, the temperature of the reaction medium
was changed from 30˚C to 70˚C. As reported in the lit-
erature, the rate of radical polymerization increases with
temperature [21]. The rate of graft polymerization was
inferred by measuring the weight of the PMMA sheet.
The weight of the polymer graft on the PMMA sheet
surface increased with temperature exponentially as
shown in Figure 1. The dose of electron beam irradiation
was 50 KGy and the reaction time was 24 hours. The
molecular weight of the monomer in this experiment was
300 g/mol. While, as the MW of the monomers increased
from 300 to 950 g/mol, the amount of graft polymer on
the PMMA surface decreased dramatically (see the sup-
plementary data). The decreased amount of the graft
polymer may be explained by the steric effect of the high
Temperature (oC)
30 40 50 60 70
Weight Increase (%)
10
20
30
40
50
60
Temperature (˚C)
Figure 1. The weight increase of the PMMA sheet with dif-
ferent rea c t i o n temperatures.

E-Beam Graft Polymerization of Hydrophilic PEG-Methacrylate on the Surface of PMMA 266
MW PEG side chain hampering the close approach of the
double bond toward the growing polymer radicals [22].
Figure 2 shows the effect of the solution concentration
on the graft polymerization. The temperature of the reac-
tion was 60˚C and the e-beam dose was 50 KGy. The
dark circle represents the total weight of the sheet con-
taining the water and unreacted trapped monomers inside
the bulk sheet. The dry weight of the surface grafted-
PMMA sheet was measured by immersing it into metha-
nol for one day at room temperature followed by drying
the sheet in a drying oven at 70˚C for one day. As the
concentration of the monomer solution increased, the
weight percentage of the grafted polymer increased
gradually. The weight of the wet sheet increased more
rapidly with respect to that of the dry sheet, which is
probably due to the swelling of the hydrophilic region
containing more PEG repeating units.
The influence of dose on the graft percentage of the
sheet was also studied by measuring the total weight of
both the wet and dry samples. There was only a slight
difference between the total dry weight of the PMMA
sheet. It is still under investigation; however, it might be
explained as follows: the concentration of the surface
radicals seems to be constant even when the dose from
the electron beam is different due to the radical scaveng-
ing reaction of oxygen. In addition, although there is a
high concentration of surviving radicals inside the
PMMA sheet, some of these radicals may not survive
until they come in contact with the hydrophilic mono-
mers that diffuse only into the modified hydrophilic area
in the PMMA sheet. However, the wet weight of the
sheet was slightly higher when the irradiated dose from
the e-beam was 50 KGy (see the supplementary data).
Therefore, most of the sample was irradiated with 50
KGy of e-beam irradiation. When the e-beam dose in-
creased above 50 KGy, the fractal char formation was
also observed on the surface of the PMMA sheet. Some
Concentration (wt %)
20 25 30 35 40 45 50
Increased Weight (%)
0
5
10
15
20
25
30
35
Increased wet wt (%)
Increased dry wt (%)
Figure 2. The dry & wet weight increase of the PMMA sh-
eet as a function of the solution concentration of the mono-
mer.
of the hard PMMA sheets generated cracks in the bulk by
rapid heat generation when a high dose of electron beam
was irradiated onto the samples.
The graft reaction of PEG-MA on the PMMA surface
was studied using ATR. Figure 3 shows the C=O
stretching vibration of the PMMA-graft-PEO. After the
polymerization process, the sample was washed with
distilled water and dried for 2 hours at 60˚C before
measuring the attenuated total reflection of the samples.
As the reaction proceeds, the relative intensity of each
peak changed with reaction time. C-H stretching peaks
near 2850 cm–1 were used as reference peaks to compare
the relative intensity of the C=O stretching peaks. The
variation of an incident angle of the IR beam provides
information regarding the chemical structures of materi-
als with the depth of graft polymers [23]. As the incident
angle is smaller, the ATR spectra provide information
from the deep bulk PMMA sample. When the incident
angle is larger, the IR beam can only pass through the
surface of the films and sheets. The peak at 1724 cm–1 is
due to the C=O stretch of the pure PMMA, while the
peak at 1716 cm–1 is caused by the C=O stretch of the
PEG-MA. Two C=O stretching peaks were identified
using PEG-MA and pure PMMA. When the angle was
30˚, the peak appears at 1724 cm–1, which suggests that
the bulk material of the sheet is mostly composed of
PMMA after two hours of graft polymerization. As the
incident angle increases with an increment of 15˚, the
intensity of the C=O peak decreases gradually and the
peak position shifts to the lower wavernumber. Finally, a
new peak at 1716 cm–1 appears together with the peak at
1724 cm–1 when the incident angle is 75˚. The decrease
of the C=O peak intensity with the increasing incident
angle could be explained by considering the decreased
ratio of the C=O groups vs. the C-H groups in the
PEG-MA with respect to those in the pure PMMA. In
addition, water molecules adsorbed onto the modified
PMMA sheet and resided near the surface with more
Figure 3. C=O stretching peaks of PMMA sheet as function
of incident angle of ATR. [reaction time: 2 hours; 50 KGy;
60˚C]

E-Beam Graft Polymerization of Hydrophilic PEG-Methacrylate on the Surface of PMMA 267
PEG graft chains (see the supplementary data).
Figure 4 demonstrates the X-ray photoelectron spectra
of O1s. The fitted area of two peaks of pure PMMA posi-
tioned at 532.93 and 531.43 eV were approximately 48%
and 52% respectively and is the ratio between the C=O
groups and –OCH3 groups in the PMMA. The difference
observed between each peak might be due to experimen-
tal error. Another possibility suggests that the oxidation
of the PMMA surface generates more carbonyl groups by
exposure against the ozone generated from airborne
oxygen during e-beam irradiation. Figure 5 is the O1s
spectra of the PMMA-graft-PEG after 24 hours of graft
polymerization. The integrated area of the C-O-C and C
=O bonds was 59.7% and 40.3% respectively. Consider-
ing the MW of PEG-MA e.g., 300 g/mol, the area of the
C-O-C peak should be 5.5 times larger than that of the
C=O peak if the surface were fully covered with
PEG-MA. These results prove that the surface is not fully
Figure 4. X-ray photoelectron spectra of pure PMMA (a)
and PEG-graft-PMMA (b). [Reaction time: 24 hours; temp:
60˚C; dose: 50 KGy]
covered with hydrophilic PEG-MA even though the in-
crease of the weight of the sheet was more than 5%,
which can provide coverage with a thickness of at least a
few micrometers.
The contact angle of the modified surface was meas-
ured to monitor the change of hydrophilicity during the
graft reaction of the PMMA. Figure 5 shows the change
of wet and dry weight as well as the contact angle as a
function of reaction time. The wet weight of the PMMA
sheet increases with reaction time. The wet weight of the
PMMA sheet increases more rapidly than the dry weight
of the grafted sheet. While, the contact angle of the
modified PMMA surface decreases until the reaction
time reaches 4 hours and then levels off to a specific
value of approximately 60˚. Considering that the grafting
of the PEG-MA monomers on the PMMA sheet in-
creases with the reaction time, the results obtained with
the constant contact angle are contradictory (contact an-
gle of the PMMA sheet is shown in the supplementary
data). A prolonged reaction time (e.g., 2 days) increased
the graft weight of the PEG-MA on the PMMA sheet,
however, the contact angle did not decrease. The Mohr’s
salt was added to the reaction mixture as an inhibitor [24].
However, unlike the former experiment, as the concen-
tration of the Mohr’s salt increased, the degree of graft
also increased (see the supplementary data). The contact
angle did not change even when small amounts of
Mohr’s salt were added.
Figure 6 shows images from the transmission electron
microscopy of the cross section of grafted PMMA. The
arrow defines the diffusion direction of the PEG-MA and
water molecules through the PMMA sheet. The wave
pattern was observed through all cross sections of the
modified PMMA sheet. The dark region contains the
grafted PEG and the bright area is mostly unreacted
PMMA. The width of the dark area was 100 - 120 nm.
The model was developed to explain the leveling off
Reaction Time (hour)
0510 15 20 25
Physical Value
0
20
40
60
80
100
Increased wet wt (%)
Increased dry wt (%)
Contact angle
Figure 5. Contact angle and incr eased weight of the PMMA
sheet as a function of reaction time. [e-beam dose: 50 KGy;
temperature: 70˚C]

E-Beam Graft Polymerization of Hydrophilic PEG-Methacrylate on the Surface of PMMA 268
500 n
m
Figure 6. TEM image of the cross-section of the grafted
polymer [the arrow represents the direction of the mono-
mer diffusion].
of the contact angle as shown in Scheme 1. The cylin-
drical column represents the diffusion of hydrophilic
monomers into the PEG-grafted PMMA. The surface
radical survives in limited sections of the PMMA surface
having higher concentrations of radicals after irradiation.
Next, the radicals initiate the graft polymerization and
change the hydrophobic surface to a more hydrophilic
one. These grafted hydrophilic PEG molecules change
the surface to a more hydrophilic state and facilitate the
adsorption and the diffusion of the hydrophilic PEG-MA
inside the hydrophilized PMMA. The diffused monomers
further react with the inside radicals. During the diffu-
sion-induced graft reaction, the modified area of the sur-
face of PMMA does not change, while the depth of the
grafted region becomes deeper as the reaction time in-
creases. Therefore, the contact angle of the surface-
grafted PMMA does not decrease with increasing levels
of grafted PEG chains after a certain reaction time.
4. Conclusion
The graft reaction of PMMA was analyzed with many
variables including reaction time, temperature, electron
dose, monomer concentration, Mohr’s salt concentration,
and the MW of the monomers. The results demonstrated
that the weight percentage of the PEG graft increased
with increasing temperature, time and concentration.
However, the weight percentage of the graft did not in-
crease with the increasing dose of e-beam irradiation. In
addition, when the Mohr’s salt (functioning as an inhibi-
tor for the radical polymerization) was added into the
solution, ironically the percentage of graft polymer in-
creased and the reaction was almost uncontrollable with
high salt concentrations. In addition, when the MW of
the monomer increased, the graft density decreased dra-
matically, most likely due to the decreased diffusion of
the high MW polymers. The graft reaction of the
Scheme 1. The diffusion of hydrophilic monomers into the
PMMA sheet.
PEG-MA on the PMMA was monitored using ATR by
changing the incident angle. The results showed that the
inner layer of the PMMA sheet was mostly PMMA,
while the polymer near the surface had many C=O
groups which originated from the PEG-MA, suggesting
that the graft polymerization of the PEG-MA was suc-
cessful. The contact angle of the grafted polymer leveled
off at a certain value and did not increase with increasing
grafting density. The reason for the leveling off of the
contact angle was explained using the cross section of the
TEM image.
5. Acknowledgments
This research was supported by the Basic Science Re-
search Program through the National Research Founda-
tion of Korea (NRF) funded by the Ministry of Education,
Science and Technology (2012003956).
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E-Beam Graft Polymerization of Hydrophilic PEG-Methacrylate on the Surface of PMMA 270
Supplementary Data
MW of Monomers
PEG 300PEG 475PEG 950
Weight Increase (%)
0
10
20
30
40
50
60
S1. Weight Increase of the PMMA sheet. [ reaction time: 24
hours, Dose: 50 KGy; temperature 70˚C].
MW of Monomers
PEG 300PEG 475PEG 950
Weight Increase (%)
0
10
20
30
40
50
60
S2. Weight increase of the PMMA sheet as a function of
electron dose. [ reaction time: 24 hours, MW of PEG-MA:
300g/mol; temperature 70˚C].
S3. ATR spectra of –OH stretch at 3500 cm–1 as a function
of the incident angle [reaction time: 2 hours].
S4. The change of the contact angle of grafted PMMA sur-
faces [reaction time: 2, 4, 6, 8, 12, 24hours from the left].
Salt Concentration (mg)
02468
Physical Value
0
20
40
60
80
100
Increased dry wt (%)
Contact ang le (degree)
S5. The weight increase and the change of contact angles as
a function of Mohr’s salt concentration. [reaction time: 24
hours, MW of PEG-MA: 300g/mol; temperature 70˚C].