E-Beam Graft Polymerization of Hydrophilic PEG-Methacrylate on the Surface of PMMA

doi:10.4236/jsemat.2012.24040

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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.

Email: *[email protected]

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 (%)

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 (%)

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

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

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 (%)

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 (%)

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

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].