Introduction1)

Recently, organic/inorganic composite particles prepared

through sol-gel processing have attracted a great deal of

attention because of the novel and beneficial synergy of

properties of these materials [1-8], which have been used

as specific stationary phases for gas or liquid chromatog-

raphy and in industrial applications that have expanded

into biotechnology and pharmacy.

A variety of techniques have been developed to prepare

organic/inorganic composite particles. Two of the

general approaches are grafting from and grafting to the

inorganic particles. The grafting from technique involves

the polymerization of monomer from active compounds

(initiator, comonomer) covalently attached to the inor-

ganic surface. The grafting to technique involves the

production of a polymer independently of the inorganic

particles; this polymer is reacted with the surface of the

inorganic particles through its end-functional group to

form organic/inorganic composite particles. Generally, a

monomer is more easily diffused to react with the

inorganic particles than a polymer is. Therefore, in case

To whom all correspondence should be addressed.

(e-mail: seongoh@hanyang.ac.kr)

of the grafting to technique, although not a lot of

polymers are attached to the solid surface, this method is

simple and versatile [8-11].

Many studies have been performed using the grafting

from and grafting to techniques. Zhu and coworkers and

Perruchot and coworkers reported the preparation of pol-

ymer-grafted silica particles using the grafting from tech-

nique. They substituted the hydroxyl groups of silica with

Br and Cl atoms through reactions with Cl2 and ATRP

initiator, respectively [11,12]. Many examples of the

grafting to technique have been reported. Papra and co-

workers prepared PEG monolayers on silicon substrates

using trimethoxysilane-terminated PEG molecules [13],

and Chujo and coworkers reported the synthesis of tri-

methoxysilyl-terminated poly(N-acetylethylenimine) (PAEI)

and the preparation of PAEI-silica gel [14]. Maitra and

coworkers prepared PEG-nanosized fumed silica through

the use of a silane coupling agent [15].

When using the grafting to technique, many research

groups have prepared polymer-silica composites using

previously prepared silica particles, such as fumed silica

or silicon substrate, and polymers that initially contain

silanol groups. In this study, we prepared PEG-grafted

silica particles with spherical shapse in water-in-oil

(W/O) emulsions.

Preparation and Characterization of PEG-Grafted Silica Particles

Using Emulsion

Yi-Jeong Hwang, Young-Ho Lee, Chul Oh, Young-Doo Jun, and Seong-Geun Oh

Department of Chemical Engineering and Center for Ultramicrochemical Process System (CUPS),

Hanyang University, Seoul 133-791, Korea

Received September 5, 2005; Accepted March 2, 2006

Abstract: In this study, aimed at preparing PEG-grafted silica particles, PEGME-IPTES with a silane coupling

agent was synthesized by the reaction of poly(ethylene glycol) methyl ether (PEGME) with 3-

(triethoxysilyl)propyl isocyanate (IPTES). The molecular weight of PEGME was varied (350, 750, or 2000).

By utilizing a sol-gel method in a W/O emulsion, both spherical silica particles and PEG-grafted silica

particles were obtained. The syntheses of PEGME-IPTES and the PEG-grafted silica particles were confirmed

using FT-IR spectroscopy. As the molecular weight of PEGME used in the synthesis of the PEGME-IPTES

precursor increased, the amount of PEG grafted onto the surface of the silica particles increased. In addition,

the surface areas of the PEG-grafted silica particles were smaller than that of the bare silica particles. The

surface morphologies were characterized by FE-SEM, TGA and BET surface area measurements.

Keywords: PEG, silica, sol-gel method, W/O emulsion

Preparation and Characterization of PEG-Grafted Silica Particles Using Emulsion 381

Figure 1. Reaction scheme for the synthesis of PEGME-IPTES.

First, we synthesized PEGME-IPTES with a silane cou-

pling agent through the reaction between the hydroxyl

group of PEG and the isocyanate group of IPTES. Sec-

ond, silica micro-particles with spherical shapes were

prepared in the emulsion by using the Stöber method

[16]; the PEG-grafted silica particles were finally

prepared by the addition of PEGME-IPTES to the emul-

sion. We also have investigated the relationship between

the surface morphology of the PEG-grafted silica

particles and the molecular weight of PEGME.

Experimental

Materials

Poly(ethylene glycol) methyl ether (PEGME, Mw 350,

750, 2000), 3-(triethoxysilyl)propyl isocyanate (IPTES,

95 %), dibutyltin dilaurate (DBDU, 95 %), tetraethyl

orthosilicate (TEOS, 98 %), hydroxypropyl cellulose

(HPC), n-decyl alcohol, and Tween 20 were purchased

from Aldrich. Sorbitan monooleate (Span80) was pur-

chased from Sigma and ammonia solution (NH4OH, 25

%) was obtained from Wako. Ethyl acetate, n-hexane,

chloroform, and methanol, which were used as solvents

for column chromatography, were supplied from Duksan.

Commercial tetrahydrofuran (THF) was purified under

reflux over sodium (Aldrich) for 2 days. All chemicals,

except THF, were used as received without further

purification. The water used in this work was deionized

and double-distilled using a Milli-Q Plus system (Mil-

lipore, France); it had an electrical resistivity of 18.2 M .Ω

Methods

Synthesis of PEGME-IPTES

PEGME-IPTES was synthesized through formation of a

urethane bond in the reaction between the hydroxyl

group of PEGME and the isocyanate group of IPTES. As

shown in Figure 1, PEGME (10 mmol) samples of differ-

ent average molecular weights and IPTES (20 mmol)

were dissolved in dry THF containing DBDU as a cata-

lyst. The reagent solution was mixed with a magnetic

stirrer and heated under reflux at 80oC for 24 h under a

nitrogen gas atmosphere. After the reaction was com-

plete, the THF was evaporated and the product isolated

through column chromatography on silica gel (EA:

hexane=1:3 and CHCl3: MeOH=9:1).

Fabrication of PEG-Grafted Silica Particles Using W/O

Emulsion

In this study, a W/O emulsion was used to prepare

spherical silica particles. NH4OH (1 g, 20 wt%), used as

a catalyst, was added to water (4 g). The oil phase was

n-decyl alcohol (45 g) containing HPC polymer (0.35 g)

as a stabilizer and Span 80 (2.5 g) as a low-HLB sur-

factant to increase the stability of the W/O emulsion.

After stirring the water and oil phases separately to

dissolve the surfactant and stabilizer sufficiently, the

water phase was added into the oil phase. The emulsion

was emulsified through high-shear homogenization at

11000 rpm for 2 min and then agitated by a low-shear

mixer for 1 h to form a W/O emulsion containing very

fine water droplets.

To prepare silica particles using the emulsion-gel meth-

od, TEOS (5.6 g, 27 mmol) was added into the W/O

emulsion and the mixture was stirred using a magnetic

stirrer at 40oC for 1 h. After preparing the silica par-

ticles, PEGME-IPTES (6 mmol) was added to the rea-

gent mixture. The ethoxy groups of PEGME-IPTES

reacted with hydroxyl groups on the surface of the silica

particles via the covalent O-Si linkage so that PEG-

grafted silica particles were prepared. After reaction for 5

h, the sample was centrifuged using a Union32R appa-

ratus (Hanil Science Industrial, Korea) at 2500 rpm for

20 min to obtain the PEG-grafted silica. The particles

obtained were washed with ethanol three times to re-

move n-decyl alcohol, HPC, surfactants, and unreacted

reactants. The collected PEG-grafted silica particles were

dried in a vacuum oven at 40oC for 1 day.

Characterization

FourierTransform Infrared (FT-IR) Spectra

To confirm the synthesis of PEGME-IPTES, FT-IR

spectra were obtained. All FT-IR spectra were recorded

at room temperature on a Magma-IR 760 (Nicolet) spec-

trometer using 32 scans at an instrument resolution of 4

cm-1.

Surface Morphology Measurement

The surface morphologies and the sizes of the silica and

PEG-grafted silica particles were studied using a field

emission scanning electron microscope (FE-SEM, Jeol

Co. model JSM-840A). The samples were coated with

platinum by sputtering at 15 mA for 180 s using a

coating machine.

Thermogravimetric Analysis (TGA)

Thermogravimetric analysis was performed using an

Yi-Jeong Hwang, Young-Ho Lee, Chul Oh, Young-Doo Jun, and Seong-Geun Oh382

Figure 2. FT-IR spectra of reactants and product: (a) IPTES;

(b) PEGME; (c) PEGME-IPTES after column chromatography.

SDT 2960 apparatus (TA instruments). The samples

were heated from room temperature up to 900oC at the

heating rate of 10oC/min using nitrogen as a purge gas at

a flow rate of 100 mL/min.

Specific Surface AreaMeasurement

Brunauer-Emmett-Teller surface area measurement (BET,

ASAP2000, Micromeritics) was used to determine the

surface areas and pore size distributions of the silica and

PEG-grafted silica particles. Prior to measurement, all

samples were outgassed at 110oC and 10

-3mmHg. The

measurements were performed through the sorption of

nitrogen gas.

Results and Discussion

Synthesis of PEGME-IPTES

One conventional procedure for organic modification of

silica particles is to utilize the reaction of the hydroxyl

groups of silica particles with silane coupling agents. The

general formula of the coupling agent is represented by

R-Si(R')3-nXn (n=2 or 3). Ligand X is a hydrolyzable

group (alkoxy, acyloxy, amino, or chloride) and the R

and R' groups are unreactive in the modification [17,18].

In this study, we introduced PEG onto the surface of the

silica particles to control the pore size, pore volume, and

surface area of the silica particles. When these properties

are controlled, our ultimate object, improving the encap-

sulation efficiency of encapsulated materials and their con-

trolled release from the silica particles, can be achieved.

In this study, we synthesized PEGME-IPTES polymers

containing silane groups for grafting PEG onto the

surfaces of silica particles. PEGME-IPTES polymers

were synthesized by urethane bond formation through

the reaction between PEGME and IPTES. The triethoxy

Figure 3. FT-IR spectra of silica particles and silica-PEG

composite particles: (a) silica particles; (b) PEG (350)-silica

composites; (c) PEG (750)-silica composites; (d) PEG (2000)-

silica composites.

groups of PEGME-IPTES react readily with hydroxyl

groups on the surfaces of the silica particles through

hydrolysis and condensation reactions.

We used FT-IR spectroscopy to confirm the synthesis

of the PEGME-IPTES samples. Figure 2 shows the FT-

IR spectra of PEGME, IPTES, and PEGME-IPTES. In

the spectrum for PEGME-IPTES (Figure 2(c)), a new

urethane bond, which arose from the reaction between

the isocyanate group of IPTES and the hydroxyl group of

PEGME, was observed at 1718 cm-1. In addition, no

signal for the isocyanate group of IPTES is observed; it

appears at 2270 cm-1in the FT-IR spectrum for IPTES.

Fabrication of PEG-Grafted Silica Particles

PEG-grafted silica particles having spherical shapes

were prepared using an emulsion-gel method. First of all,

spherical silica particles were prepared in a W/O emul-

sion by the sol-gel method through hydrolysis and conden-

sation reactions of TEOS molecules. After preparing the

W/O emulsion by mixing it with a magnetic stirrer, TEOS

was introduced into the W/O emulsion. When TEOS was

added into the continuous oil phase of the W/O emulsion,

the oil phase of which was n-decyl alcohol containing

HPC polymer as a stabilizer and Span 80 as a low HLB

surfactant, the TEOS molecules mixed with n-decyl

alcohol because TEOS is oil-soluble. The TEOS

molecules that initially dissolved in the external oil phase

gathered together at the W/O interface when the emul-

sion was mixed by a magnetic stirrer. At the W/O

interface, the TEOS molecules underwent hydrolysis

reactions, i.e., the conversion of OCH2CH3 groups in

TEOS to OH groups. Because silicone hydroxide is

watersoluble, the silanol groups penetrated from the W/O

interface into the internal water phase through the

Preparation and Characterization of PEG-Grafted Silica Particles Using Emulsion 383

(a)

(b)

(c)

(d)

Figure 4. SEM micrographs of silica particles and silica-PEG composite particles: (a) silica particles; (b) PEG (350)-silica

composites; (c) PEG (750)-silica composites; (d) PEG (2000)-silica composites.

Yi-Jeong Hwang, Young-Ho Lee, Chul Oh, Young-Doo Jun, and Seong-Geun Oh384

Figure 5. TGA thermograms of: (a) silica particles; (b)

PEG(350)-silica composites; (c) PEG (750)-silica composites;

(d) PEG (2000)-silica composites, at a heating rate of 10o

C/

min.

surfactant layer formed by Span 80. In the internal water

droplets, condensation reactions of the silicon hydroxides

occurred immediately; thus, silica particles with spherical

shapes, like the water droplets of the emulsion, were

prepared [19,20].

To control the open pores of the silica particles,

PEGME-IPTES was grafted onto the surface of spherical

silica particles. After preparing spherical silica particles

through the emulsion-gel process, PEGME- IPTES was

added continuously into the W/O emulsion containing

the silica particles. At the pore surface of the silica

particles, the structure can terminate in either Si-O-Si

groups with the oxygen link pointing to the surface, or in

several types of Si-OH groups [21]. Hydrolyzed

triethoxy groups of PEGME-IPTES reacted with the

Si-OH groups on the surface of the silica particles to

form new Si-O-Si groups that connected the silica

particles to the PEG were formed. Thus, PEG-grafted

silica particles with spherical shape were obtained.

The preparation of the PEG-grafted silica particles was

confirmed by FT-IR spectroscopy. Figure 3 shows the IR

the spectra for the silica and PEG-grafted silica particles.

In the IR spectra of the PEG-grafted silica particles, new

two peaks appeared that were absent in the spectrum of

the bare silica particles. One peak at ca. 1711 cm-1

indicated the urethane bonds of PEGME-IPTES units

grafted onto the silica particles. The intensity of this peak

was weak, and it decreased upon increasing the

molecular weight of PEGME-IPTES. The other peak at

ca. 2916 cm-1

presented the alkyl groups (-CH2-) of

PEGME-IPTES. Upon increasing the molecular weight

of the PEG, the intensity of this peak increased. When

the molecular weight of PEGME was 2000 (Figure 3(d)),

the intensity of the urethane bond was very weak, and the

peak the of alkyl chain disappeared. These two peaks of

the urethane and alkyl groups, confirmed that PEG-

grafted silica particles were formed.

Effect of PEG on Properties of Silica Particles

Figure 4 shows SEM micrographs of the bare silica and

PEG-grafted silica particles having different molecular

weights of PEG. All particles obtained in this study were

spherical and had sizes in the range 1 3 µm. The

surface morphology of the PEG-grafted silica particles

resembled that of the bare silica particles. No difference

in the morphology of the silica surface was observed

through these SEM images because the alkyl chain

length of PEGME-IPTES was too short to be measured

using FE-SEM.

However, differences were observed in the TGA ther-

mograms and BET measurements. The TGA thermograms

of the bare silica and PEG-grafted silica particles are

shown in Figure 5. Both the bare silica and PEG-grafted

silica particles nearly lost their retaining water contents

below 100oC. In the TGA thermograms of the PEG-

grafted silica particles, the other weight loss occurred at

ca. 390oC. This peak indicated that thermal decompo-

sition of the PEG blocks grafted onto the surface of silica

occurred. However, the decomposition of pure PEG

generally occurs at ca. 360oC [22]. This difference

implies that the thermal stability of PEG grafted onto the

surface of silica particles is improved over that of pure

PEG. Moreover, the TGA-DTA thermograms confirmed

that the decline in weight of the PEG-grafted silica

particles increased as the molecular weight of PEGME

increased. Therefore, the PEG-grafted silica particles

prepared from higher-molecular-weight PEGME had

more PEG than did the samples containing with low-

molecular-weight PEG. In general, with increasing the

molecular weight of PEG, steric hindrance increases and

the number of molecules grafted onto the surface

reduces. However, when the molecular weight of PEGME

Preparation and Characterization of PEG-Grafted Silica Particles Using Emulsion 385

Table 1. BET Surface Areas, Desorption Cumulative Pore Volumes, and Desorption Average Pore Diameters of Silica Particles and

PEG-Silica Composite Particles

Name BET surface area (m2/g) Desorption cumulative pore volume (cm

3/g) Desorption average pore diameter (nm)

SilicaPEG (350)-silicaPEG (750)-silicaPEG (2000)-silica

252 ± 0.5095171.7 ± 0.5407142.5 ± 0.4992115.4 ± 0.3207

0.8486450.4664380.4443640.384105

11.213298.03418.359178.69042

Figure 6. BET results: (a) nitrogen adsorption/desorption iso-

therms; (b) desorption pore size distributions; ( ) silica

particles; ( ) PEG (350)-silica composites; ( ) PEG (750)-

silica composites; ( ) PEG (2000)-silica composites.

is considered, the amounts of PEG grafted onto the

surface of the silica particles was barely affected by the

molecular weight of PEGME because the alkyl chain

length of PEGME was too short to cause severe steric

hindrance. Nitrogen adsorption/desorption isotherms and

the corresponding pore size distribution graphs are shown

in Figure 6. The adsorption/desorption isotherms [Figure

6 (a)] indicated that whether PEG was grafted on the

surfaces of the silica particles or not, the isotherm graphs

of all of the particles were of Type IV of the Brunauer,

Emmett, and Teller (BET) classification; this type is

associated with capillary condensation in mesopores

[23]. In the case of the bare silica particles, the surface

area was 252 m2/g and the average pore diameter was 11.2

nm. When PEG was grafted onto the silica particles, the

surface areas and the pore volumes decreased, as shown

in Table 1. Moreover, the surface areas and pore volumes

of the PEG-grafted silica particles decreased upon

decreasing the molecular weight of PEGME. Suzuki and

coworkers found that porous silica particles modified

with poly(acrylic acid) showed the same trend: an

increase in the polymer concentration on the silica par-

ticles leads to a reduction of the surface area and the

average pore volume [24]. In our experiment, the addi-

tion of PEGME-IPTES did not affect the morphology of

the pore because PEGME-IPTES was added after the

formation of the silica particles. Thus, this result suggests

that the PEG units attached to the surfaces of the silica

particles closes up the pores on the surfaces and that

more pores are filled with PEGME chains as the chain

length of PEGME increases.

Conclusions

PEG-grafted silica particles were prepared through sol-

gel processing using a W/O emulsion as the reaction

media. To graft PEG onto the surface of silica particles,

PEGME polymers of various molecular weights were

reacted with IPTES. PEGME-IPTES conjugates with ure-

thane groups were synthesized by reacting the isocya-

nate group of IPTES with the hydroxyl group of

PEGME. The peak of a urethane group was observed at

1718 cm-1in the FT-IR spectrum to confirm the synthesis

of PEGME-IPTES.

The silica and PEG-grafted silica particles obtained

from the W/O emulsion were spherical shape and were

sized in the range 1 3 µm. SEM images of the surfaces

of the PEG-grafted silica particles were similar to that of

bare silica particles because the alkyl chain length of

PEGME-IPTES was very short when compared with the

diameter of the silica particles. However, as the molecu-

lar weight of the PEGME polymer used in the PEGME-

IPTES precursor increased, the content of PEG in the

PEG-grafted silica particles increased. In addition, the

surface area of the PEG-grafted silica particles and their

Yi-Jeong Hwang, Young-Ho Lee, Chul Oh, Young-Doo Jun, and Seong-Geun Oh386

average pore diameter decreased. These results con-

firmed that PEG was attached onto the surfaces of the

silica particles and decreased the volume of the meso-

pores upon increasing the PEG chain length.

Acknowledgment

This study was supported by the Center for Ultramicro-

chemical Process Systems, sponsored by KOSEF.

References

1. J. Pyun, K. Matyjaszewski, T. Kowalewski, D.

Savin, G. Patterson, G. Kickelbick, and N. Huesing,

J. Am. Chem. Soc., 123, 9445 (2001).

2. K. Dahmouche, C. V. Santilli, and S. H. Pulcinelli,

J. Phys. Chem. B., 103, 4937 (1999).

3. M. Husseman, E. E. Malmstrm, M. Mcnamara, M.

Mate, D. Mecerreyes, D. G. Benoit, J. L. Hedrick, P.

Mansky, E. Huang, T. P. Rusell, and C. J. Haqker,

Macromolecules, 32, 1424 (1999).

4. J. Pyun and K. Matyjaszewski, Chem. Mater., 13,

3436 (2001).

5. T. von Timothy and T. E. Patten, J. Am. Chem. Soc.,

121, 7409 (1999).

6. J. Wen and G. L. Wilkes, Chem. Mater., 8, 1667

(1996).

7. Q. Zhou, S. Wang, X. Fan, and R. Advincula,

Langmuir, 18, 3324 (2002).

8. H. C. Lay, M. J. Spencer, E. J. Evans, and I.

Yarovsky, J. Phys. Chem. B, 107, 9681 (2003).

9. N. Luo, J. B. Hutchison, K. S. Anseth, and C. N.

Bowman,Macromolecules, 35, 2487 (2002).

10. Ph. Espiard and A. Guyot, Polymer, 36, 4391 (1995).

11. X. Y. Zhu, Y. Jun, D. R. Staarup, R. C. Major, S.

Danielson, V. Boiadjiev, W. L. Gladfelter, B. C.

Bunker, and A. Guo, Langmuir, 17, 7798 (2001).

12. C. Perruchot, M. A. Khan, A. Kamitsi, and S. P.

Armes, Langmuir, 17, 4479 (2001).

13. A. Papra, N. Gadegaard, and N. B. Larsen, Langmuir,

17, 1457 (2001).

14. Y. Chujo, E. Ihara, and T. Saequsa, Macromolecules,

22, 2040 (1989).

15. P. Maitra, J. Ding, H. Huang, and S. L. Wunder,

Langmuir, 19, 8994 (2003).

16. W. Stöber and A. Fink, J. Colloid Interface Sci., 26,

62 (1968).

17. T. Sugimoto, Fine Particles: Synthesis, Characterization,

and Mechanisms of Growth, Marcel Dekker, New

York (2000).

18. H. Jo and F. D. Blum, Chem. Mater., 11, 2548

(1999).

19. Ch. Oh, S. Ch. Chung, S. I. Shin, Y. Ch. Kim, S. S.

Im, and S. G. Oh, J. Colloid Interface Sci., 254, 79

(2002).

20. J. Esquena, Th. F. Tadros, K. Kostarelos, and C.

Solans, Langmuir, 13, 6400 (1997).

21. J. D. Wright and N. A. J. M. Sommerdijk, Sol-Gel

Materials: Chemistry and Application, Gordon and

Breach Science Pubilshers, London (2001).

22. F. Zeng-guo and Z. Sanping, Polymer, 44, 5177

(2003).

23. S. S. Gregg, Adsorption, Surface Area and Porosity,

Academic Press, London (1982).

24. K. Suzuki, S. Siddiqui, C. Chappel, J. A. Siddiqui,

and R. M. Ottenbrite, Polym. Adv. Technol., 11, 92

(2000).