preparation of thermosensitive pnipam-grafted mesoporous silica particles

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Preparation of ThermosensitivePNIPAm-Grafted Mesoporous Silica Particles

Jun-Hwan Park, Young-Ho Lee, Seong-Geun Oh*

In this study, the composites of thermoresponsive poly(N-isopropylacrylamide) (PNIPAm) andmesoporous silica particles were synthesized. 3-(Trimethoxysilyl)propyl methacrylate (MOP)was used to introduce double bonds on silica particles. First, MOP-modified spherical silicaparticles were prepared by the sequential addition of tetraethyl orthosilicate (TEOS) andMOP into W/O (water-in-oil) emulsion. Second, PNIPAm-grafted silica microparticles weresynthesized by the radical copolymerization of MOP-modified silica particles and N-isopropyl-acrylamide (NIPAm) monomers in the ethanol/water mixture. Polymerization was carried outwith the variation of reaction temperature, concentration of crosslinking agent, and weightratio of MOP-modified silicaparticles to NIPAmmonomers.The formations of MOP-func-tionalized silica microspheresand PNIPAm-grafted silicamicroparticles were charac-terized by FE-SEM, TEM, FT-IR, TGA, and BET. The lowercritical solution temperature(LCST) of the PNIPAm-graftedsilica particles was investi-gated by DSC.

Introduction

Stimuli-sensitive polymers show abrupt changes in their

swelling behavior in response to external stimuli such as

change in temperature, pH, solvent composition, and

electric fields. They have received much attention due to

their potential applications in numerous fields. Among

diverse stimuli, temperature is the most broadly used

stimulus in environment-responsive polymer systems

because it is easy to control. The unique property of tem-

J.-H. Park, Y.-H. Lee, S.-G. OhDepartment of Chemical Engineering and Center for Ultrami-crochemical Process System (CUPS), Hanyang University, 17Haengdang-dong, Seongdong-gu, Seoul 133-791, KoreaFax: (þ82) 2 2294 4568; E-mail: [email protected]

Macromol. Chem. Phys. 2007, 208, 2419–2427

� 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

perature-responsive polymers is that they have a lower

or upper critical solution temperature (LCST or UCST). One

of the most extensively studied temperature-responsive

polymers is poly(N-isopropylacrylamide) (PNIPAm) having

the LCST around 32 8C.[1] It swells below 32 8C and shrinks

above 32 8C in aqueous solutions. By utilizing this prop-

erty, PNIPAm can be used in biological applications such as

DDS because it has its own LCST nearby human body

temperature.[2]

However, organic compounds have poor mechanical

properties. In contrast, inorganic substances have good

chemical stability and mechanical strength. By combin-

ing organic substances with inorganic compounds, orga-

nic components can enhance the chemical stability and

mechanical property.[3] That is to say, the desirable prop-

erties, which cannot be obtained when each component

DOI: 10.1002/macp.200700247 2419

J.-H. Park, Y.-H. Lee, S.-G. Oh

2420 �

exists individually, can be achieved by the hybridization of

organic and inorganic components. Because of this charac-

teristic, the composites of stimuli-responsive polymers

and inorganic substances have been studied widely.[4]

Silica has good thermal and mechanical stability[5] and

is used as an inorganic component of organic–inorganic

composites.[6] Silica can also be utilized for DDS because it

offers chemical stability and biocompatibility.[7] Because

of these characteristics of silica, some researches on the

synthesis of thermoresponsive PNIPAm and silica compo-

sites have been studied lately. Akashi and his research

group synthesized PNIPAm-grafted silica and prepared

well-dispersed platinum colloids on the PNIPAm-grafted

silica. They also investigated catalytic properties in hydro-

genation of their products.[8] Moreover, they fabricated

porous PNIPAm hydrogels using silica particles as a temp-

late and studied rapid and controlled deswelling features

of the prepared PNIPAmhydrogels.[9] Zha et al. synthesized

silica/crosslinked-PNIPAm composite particles with a core/

shell structure and monodisperse PNIPAm microcontain-

ers by etching silica particles, and investigated the effects

of the concentration of crosslinker on the hydrodynamic

diameter of PNIPAm.[10] Thermoresponsive PNIPAm-grafted

silica hybrid materials have the potential for various

applications such as controlled release of target materi-

als,[11] chromatography,[12] membrane science,[13] etc.

In this paper, 3-(trimethoxysilyl)propyl methacrylate

(MOP)-modified silica microspheres were prepared in W/O

(water-in-oil) emulsion through emulsion–gel process. MOP

was employed for the functionalization of bare silica

particles with double bond. In our previous research, silica

microspheres and surface-functionalized silica particles

were prepared by emulsion–gel method in W/O emul-

sion[14] or O/W/O multiple emulsion.[15] Moreover, it is

possible to control the surface area and pore size of silica

particles by regulating the type and amount of stabilizers

and surfactants used in W/O emulsion. These facts are

advantageous to the encapsulation of various target mate-

rials with the functionalization of the surface. In this

study, the surface functionalization of bare silica particles

with MOP was achieved by the sequential addition of

tetraethyl orthosilicate (TEOS) and MOP into W/O emul-

sion. Additionally, PNIPAm-grafted silica microparticles

were synthesized using MOP-modified silica particles.

MOP-modified silica particles and N-isopropylacrylamide

(NIPAm) monomers were radically copolymerized in the

mixture of water and ethanol using potassium peroxodi-

sulfate (KPS), N,N0-methylenebisacrylamide (MBA), and

N,N,N0,N0-tetramethylethylenediamine (TEMED) as an ini-

tiator, a crosslinking agent, and an accelerator. The shape

and the surface morphology of the synthesized particles

were observed through FE-SEM and TEM. The chemical

bonding of MOP and PNIPAm on silica particles was con-

firmed by FT-IR. The grafted weight of MOP and PNIPAm


2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

were analyzed by TGA. The surface area, total pore volume,

and average pore diameter of the samples were charac-

terized by BET. The LCST of PNIPAm-grafted silica particles

was investigated by DSC.

Experimental Part

Materials

Tetraethyl orthosilicate (98%) as a silica source, hydroxypropyl

cellulose (HPC, average Mw ca. 370 000) as a stabilizer in the

external oil phase of W/O emulsion, MBA as a crosslinker, and

TEMED as an accelerator of polymerization were purchased from

Aldrich Chemical Company. MOP as a silane coupling agent and

sorbitan monooleate (Span 80) as a low HLB surfactant were

obtained from Sigma Chemical Company. NIPAm and sodium

dodecyl sulfate salt (SDS) as high HLB surfactants were purchased

from Acros Organics. Ethanol and methanol were obtained from

Duksan Pure Chemical Company. 1-Octanol was purchased from

Junsei Chemical Company. Ammonium hydroxide (NH4OH, 25%

solution) was obtained from Wako Chemical Company. KPS as

an initiator was purchased from Kanto Chemical Company. All

commercial chemicals were usedwithout further purification. The

water used in this study was deionized and double-distilled by

Milli-Q Plus system (Millipore, France), having 18.2 MV electrical

resistivity.

Preparation of 3-(Trimethoxysilyl)propyl

methacrylate-Modified Mesoporous Silica

Microspheres using Water-in-Oil Emulsion

For the preparation of MOP-modified mesoporous silica micro-

spheres, emulsion–gel method was used in this study. Oil phase

was prepared by dissolving HPC (1.4 wt.-%) and Span 80 (5 wt.-%)

in 1-octanol. In the water phase, SDS (3 wt.-%) was dissolved in

deionized water containing NH4OH (4 wt.-%). After complete

dissolution, W/O emulsion was prepared by the addition of the

water phase to the oil phase under magnetic stirring at 40 8C for

1 h. The weight ratio of the water phase to the oil phase in W/O

emulsion was 1:9. Next, TEOS of Rw¼10 (Rw¼ the molar ratio of

water to TEOS) was added into W/O emulsion for the formation

of bare silica particles through emulsion–gel reaction. After the

reaction of the TEOS molecules with water droplets proceeded for

6 h, MOP as a silane coupling agent was added into the reagent

mixture. The molar ratio of TEOS to MOP was 5:1. After the

reactionwas performed for 11 h, the products were centrifuged by

using a Union32R apparatus (Hanil Science Industrial, Korea) at

3 000 rpm for 15 min to obtain MOP-modified silica particles. The

obtained particles were washed several times with ethanol to

eliminate residues and then all the MOP-modified silica particles

were redispersed in a small amount of ethanol.

Synthesis of Thermoresponsive

Poly(N-isopropylacrylamide)-Grafted

Silica Microparticles

Thermoresponsive PNIPAm-grafted silica microparticles were

synthesized by radical copolymerization of MOP-modified silica

DOI: 10.1002/macp.200700247

Preparation of Thermosensitive PNIPAm-Grafted Mesoporous Silica Particles

Table 1. The various synthesis recipes of PNIPAm-silicamicroparticles. KPS/NIPAmmolar ratio (mol-%)¼ 1.05, TEMED¼0.05mL, 0.4wt.-% ofMOP-modified silica in total solution.

Entry NIPAm/MOP-silica

(weight ratio)

MBA/NIPAm

molar ratio

Reaction

temperature

Weight loss

at 115–700 -Ca)

mol-% -C wt.-%

1 4 11.01 20 61.79

2 4 11.01 40 70.22

3 4 5.51 20 17.47

4 4 0 20 16.83

5 4 5.51 40 40.08

6 4 0 40 19.07

7 2 11.01 40 37.62

8 1 11.01 40 17.18

a)Weight loss of organic part (wt.-%)¼ (weight loss at 115–700 -C)/[100 – (weight loss at 25–115 -C)]T100.

and NIPAm monomers. The MOP-modified silica dispersion in

ethanol and the aqueous solution of NIPAm and MBA were added

into KPS solution. After the reagent mixture was bubbled with

nitrogen at room temperature for 30 min, 0.05 mL of TEMED in

water was injected into the reagent mixture and then themixture

was magnetically stirred at 300 rpm for 15 h. The ethanol/water

weight ratio was kept at 1:9 (PNIPAm in this mixed solvent has

LCST at 28–30 8C). The polymerization was carried out below or

above the LCST of PNIPAm (20 or 40 8C). After the polymerization

was finished, the obtained PNIPAm-grafted silica particles were

centrifuged at 3 000 rpm for 15 min and washed several times

with methanol to remove the residues. The powders were dried

in a vacuum oven at room temperature for 24 h. The reaction

conditions are listed in Table 1.

Characterizations

The morphologies of bare and MOP-modified silica were investi-

gated with FE-SEM (JEOL JSM-6700F). A specimen of the synthe-

sized particles was coated with platinum by sputtering at 15 mA

for 3 min using a coating machine. The grafted PNIPAm on the

silica particles were observed by TEM (JEOL JEM-2000EXII). For the

TEM investigation, a drop of dispersion was placed on the TEM

grid and was dried in drying oven at 40 8C for 24 h. FT-IR analysis

was used to confirm the synthesis of MOP-functionalized silica

microspheres and PNIPAm-grafted silica microparticles. All FT-IR

spectra were recorded at room temperature on a Magna-IR 760

(Nicolet) spectrometer using 32 scans at an instrument resolution

of 4 cm�1. TGA measurement was performed on a TGA7 (Perkin-

Elmer) to analyze the weight percent of the organic components

grafted on silica in the prepared samples. The particles were

heated from room temperature up to 700 8C at the heating rate

of 10 8C �min�1 using nitrogen as a purge gas at the flow rate of

100 mL �min�1. BET measurement (TriStar 3000, Micromeritics)

was used to measure the surface area, total pore volume,

and average pore diameter of the prepared particles. Prior to

measurement, all the samples were outgassed at 110 8C for 4 h.



The measurement was carried out by the sorption of nitrogen gas.

The LCST of PNIPAm-grafted silica microparticles was character-

ized by DSC2010 (TA instrument). The samples dispersed in water

were heated and cooled between 10 and 50 8C at the rate of

2 8C�min�1 under nitrogen.

Results and Discussion

Mechanism

The principle of our approach to the synthesis of PNIPAm-

grafted silica is illustrated in Scheme 1. MOP was used as

a silane coupling agent to introduce a double bond onto

bare silica particles. The molecular structure of MOP is

shown in Scheme 2. The bare silica particlesweremodified

with MOP by adding TEOS and MOP into W/O emulsion

sequentially. Next, thermoresponsive PNIPAm-grafted silica

microparticles were synthesized by radical copolymeriza-

tion of MOP silica microspheres and NIPAm monomers in

themixture of water and ethanol using KPS as an initiator,

MBA as a crosslinking agent, and TEMED as an accelerator.

The reaction temperature, the concentration of crosslink-

ing agent, and the weight ratio of MOP-modified silica to

NIPAm monomers were changed for the observation of

the effects of these three factors on the extent and the

shape of PNIPAm grafted onto MOP-functionalized silica

microspheres.

Preparation of 3-(Trimethoxysilyl)propylMethacrylate-Modified Mesoporous SilicaMicrospheres using Water-in-Oil Emulsion

An emulsion is a heterogeneous system, consisting of at

least one insoluble dispersed phase in the form of droplet

www.mcp-journal.de 2421


Scheme 1. The approach to the synthesis of PNIPAm-grafted silica microparticles.

2422 �

and continuous phase. Emulsion–gel method involves the

generation of emulsions which are subsequently conver-

ted into viscous gels and then to solid materials. This

process is based on the Stober method.[16] By using this

system, silica particles treated with MOP containing a

double bond were prepared for the synthesis of PNIPAm-

grafted silica microparticles. The MOP-modified silica par-

ticles containing double bond can be copolymerized with

NIPAm monomer. In our previous papers, the functional-

ization of bare spherical silica particles were achieved by

the sequential addition of TEOS and silane coupling agent

such as a (3-mercaptopropyl)trimethoxysilane (MPTMS)[14d]

or polymer such as PEGME-IPTES[14e,14g] into W/O emul-

sion. MOP-modified silica particles were fabricated in W/O

emulsion using our previous researched experimental

technique.

The shape and the surfacemorphology ofMOP-modified

silica particles are similar to those of bare silica particles

as shown in Figure 1 and 2. This is because MOP was

Scheme 2. The molecular structure of MOP as a silane couplingagent.



added into the reagent mixture after

the emulsion–gel reaction of TEOS for

the formation of spherical silica parti-

cles was completed. Compared to the

FT-IR spectrumof bare silica [Figure 3(a)],

the carbonyl peak of MOP (C––O stretch-

ing vibration, nearly 1 720 cm�1) ap-

peared clearly in the FT-IR spectrum of

MOP-modified silica [Figure 3(b)]. This

means that the surfaces of the silica

particles were modified with MOP. In

TGA data [Figure (4)], the weight loss at

25–115 8C is caused by the evaporation

of water adsorbed on the surface of

particles. In order to calculate the wei-

ght loss of the organic part at 115–

700 8C, the weight loss (wt.-%) of water

at 25–115 8C was excluded because

each sample contains a different amo-

unt of water and this can affect the

accurate content of organic part in

the composite particles (Table 1). The

weight loss of bare silica decreased

at 25–115 8C more than that of MOP-

modified silica because the hydroxyl group (–OH) of

bare silica existed before the modification with MOP

[Figure 4(a)]. In addition, the weight loss at 115–700 8C of

bare silica was approximately 10 wt.-%. It is expected that

this weight loss is caused by the strongly adsorbed HPC

and surfactants (SDS and Span 80) used in W/O emulsion.

The weight loss at 115–700 8C of MOP-modified silica was

19.1 wt.-% [Figure 4(a)]. This increased weight loss at

115–700 8C of MOP-modified silica confirms that silica was

modified with MOP. As listed in Table 2, BET surface area

and total pore volume of MOP-modified silica decreased

compared to bare silica particles. This indicates that pores

of bare silica were closed by the diffusion and introduction

of MOP molecules into the inner pores. From the above-

mentioned results, bare silica particles weremodified with

MOP successfully.

Synthesis of ThermoresponsivePoly(N-isopropylacrylamide)-GraftedSilica Microparticles

Effects of the Reaction Temperature

The radical copolymerization of MOP-modified silica and

NIPAm monomers was performed in the ethanol/water

mixture. The addition of ethanol into water enhances the

dispersion stability of hydrophobic MOP-functionalized

silica particles in the reaction medium because ethanol

decreases the polarity of solution, and thus MOP-modified

DOI: 10.1002/macp.200700247


Figure 2. TEM images of the synthesized particles and theirmagnified surface parts: (a) bare silica, (b) MOP-modified silica,(c) PNIPAm-grafted silica prepared at 20 8C (entry 1), and (d)PNIPAm-grafted silica prepared at 40 8C (entry 2).

Figure 1. FE-SEM images of (a) bare silica and (b) MOP-modifiedsilica microspheres.

silica particles could be dispersed well in the reaction

medium by adding ethanol into the aqueous solution.[11b]

In addition, ethanol helps the diffusion of all reagents and

growing PNIPAm radicals into the internal pores and

the external surface of hydrophobic MOP-modified silica

particles because ethanol reduces the surface tension of

water.

As illustrated in TEM images of Figure 2, the remarkable

differences of the surface morphology of bare silica par-

ticles, MOP-modified silica, and PNIPAm-grafted silica (20

and 40 8C, entries 1 and 2) were observed. The weight ratio

of NIPAmmonomer to MOP-modified silica was fixed at 4,

and the amount of MBA was also kept at 11.01 mol-% to

NIPAmmonomers. Bare silica andMOP-modified silica had

similar surface morphology. After the polymerization of

NIPAm in the presence of MOP-modified silica, PNIPAm

shells were formed on the silica particles (entries 1 and 2),

and Scheme 1 shows that the reaction temperature (above

or below LCST) affects the morphology of PNIPAm shell.


� 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.mcp-journal.de 2423


Figure 3. FT-IR spectra of the PNIPAm-grafted silica: (a) bare silica,(b) MOP-modified silica, (c) entry 1, (d) entry 2, (e) entry 3, (f) entry4, (g) entry 5, (h) entry 6, (i) entry 7, and (j) entry 8.

2424 �

When a polymerization was performed at 20 8C, smooth

shell was formed on the silica [Figure 2(c)], while PNIPAm-

grafted silica composites particles prepared at 40 8C had

the aggregated form of PNIPAm [Figure 2(d)]. This can be

explained by the behavior of PNIPAm oligomers during the

polymerization. When a polymerization was performed

below LCST, PNIPAm propagating radicals were main-

tained in the swollen hydrophilic state and PNIPAm chain

grows on the silica surface regularly. Thus, the smooth

PNIPAm shell is formed at the surface of MOP-modified

silica particles as shown in Figure 2(c). However, when

the polymerization is carried out above LCST, PNIPAm

oligomeric radicals with sulfate end groups act as oligo-

meric surfactants and form micelles because of the aggre-

gation of oligomeric chains. The grafting PNIPAm cover the

surface, or penetrates into the inner pores of silica particles

with tight chain conformation above LCST (40 8C). Thus,PNIPAm is grafted ontoMOP-modified silica particles as an

aggregate form at 40 8C.In FT-IR spectra [Figure 3(a)], the characteristic peaks of

PNIPAm were clearly observed in both PNIPAm-grafted



silica (20 8C, entry 1) and PNIPAm-grafted silica (40 8C,entry 2) compared with FT-IR spectra of bare silica and

MOP-modified silica. The peaks attributed to the N–H

stretching vibration and bending vibration were observed

at 3 500–3 100 cm�1 and 1 550 cm�1. The sp3 C–H and

C––O stretching vibrations also appeared at 2 980 and

1 650 cm�1, respectively. The broad peak at 1 350–

1 000 cm�1 divided from the peak indicating silica was

attributed to the C–N stretching vibration. This observa-

tion of PNIPAm peaks gives an evidence of the grafting of

PNIPAm onto MOP-modified silica microspheres.

TGA thermograms provide the amount of PNIPAm

grafted onto MOP-modified silica particles (Figure 4 and

Table 1). The weight losses at 115–700 8C of bare silica,

MOP-modified silica, PNIPAm-grafted silica (20 8C, entry 1),

and PNIPAm-grafted silica (40 8C, entry 2) were 10.15, 19.1,

61.79, and 70.22 wt.-%, respectively. This result indi-

cates that the composites of MOP-modified silica and

PNIPAmwere successfully formed at both 20 and 40 8C. It isremarkable that PNIPAm-grafted silica composite is

completely degraded around 600 8C while the decomposi-

tion of PNIPAm homopolymer finishes around 450 8C.[17]

This elucidates that the thermal stability of PNIPAm

grafted ontoMOP-modified silicawas improved.Moreover,

the weight loss of PNIPAm-grafted silica microparticles at

25–115 8C is larger than those of bare silica particles and

MOP-modified silica particles. It seems that the grafted

PNIPAm in the inner pores of PNIPAm-grafted silica con-

tains more water molecules than bare silica particles and

MOP-modified silica particles because PNIPAm has the

characteristic of forming a hydrogel, which includes water

molecules in its crosslinked network structure.

The surface characteristics of the synthesized PNIPAm-

grafted silica are shown in Table 2. In all four samples,

mesoporous particles were obtained. The BET area and

total pore volume of bare silica and MOP-modified silica

are 404 and 202m2 � g�1. After polymerization, BET surface

area and total pore volume of PNIPAm-grafted silica

(entries 1 and 2) decreased dramatically. This shows that

PNIPAm was grafted not only to the exterior surface but

also to the interior pores of MOP-modified silica particles.

BET surface area and total pore volume of PNIPAm-grafted

silica prepared at 20 8C (entry 1) were smaller than those of

PNIPAm-grafted silica prepared at 40 8C (entry 2). It seems

that during the polymerization, the hydrophilic PNIPAm

oligomers which swell at 20 8C could block the pores of

MOP-modified silica more effectively than the aggregates

of micelle-like PNIPAm oligomers which are shrunken

at 40 8C.

Effects of the Concentration ofN,N0-Methylenebisacrylamide as a Crosslinking Agent

The effects of MBA concentration on the synthesis of

PNIPAm-grafted silica at 20 and 40 8C (Table 1, entries 1–6)

DOI: 10.1002/macp.200700247


Figure 4. TGA thermograms of the effects of various reaction conditions on the grafting of PNIPAm onto MOP-modified silica: (a) reactiontemperature, (NIPAm)/(MOP-modified silica) weight ratio¼4; (b) MBA concentration at 20 8C; (c) MBA concentration at 40 8C; and (d)weight ratio of MOP-modified silica to NIPAm at 40 8C.

were investigated by FT-IR spectra [Figure 3(c)–3(h)] and

TGA analysis [Figure 4(b) and 4(c)]. The weight ratio of

NIPAm monomer to MOP-modified silica was fixed at 4.

When MBA was not used at both 20 8C (entry 4) and 40 8C

Table 2. BET surface area, total pore volume, and average pore diametat 20 8C, and PNIPAm-grafted silica prepared at 40 8C.

Sample name BET surface

area

Sin

m2 � gS1

Bare silica 404

MOP-silica 202

PNIPAm-silica (20 -C)a) 3.84

PNIPAm-silica (40 -C)b) 6.18

a)Reaction condition¼ entry 1; b)reaction condition¼ entry 2.



(entry 6), the characteristic peaks of PNIPAm were not

observed in FT-IR spectra. These results agree with their

TGA thermograms [Figure 4(b) and 4(c)]. Only a small

amount of PNIPAmwas attached toMOP-modified silica at

er of bare silica, MOP-modified silica, PNIPAm-grafted silica prepared

gle point adsorption total

pore volume of pores

Adsorption average

pore diameter

cm3 � gS1 nm

0.99703 9.86

0.37767 7.49

0.00592 6.17

0.01059 6.86



2426 �

both 20 and 40 8C when MBA was not used. In contrast,

11.01 mol-% MBA made it possible to graft PNIPAm onto

MOP-modified silica particles successfully at both 20 and

40 8C (entries 1 and 2). These results mean that the grafting

of PNIPAm onto MOP-modified silica did not take place

well without MBA and it seems that MBA connects oligo-

meric (or polymeric) chains grafted on the silica surface

and free oligomers (or polymers). That is, MOP-modified

silica particles can ‘‘catch’’ PNIPAm through the polymer-

ization of growing PNIPAm radicals and plentiful amount

of MBA in the internal pores and the external surface of

MOP-modified silica. It is noticed that for 5.51 mol-% MBA,

different results were observed (entries 3 and 5). PNIPAm

peaks did not appear when 5.51 mol-% MBA was added

into the reagent mixture at 20 8C [Figure 3(e)]. As repre-

sented in TGA data [Figure 4(b), entry 3], the weight loss

was similar to the case without MBA. That is, MOP-

modified silica particles and NIPAm monomers were not

copolymerized well at 20 8C when 5.51 mol-% MBA was

used. At this temperature, PNIPAm-grafted silica particles

were obtained when 11.01 mol-% MBAwas added into the

reaction medium because sufficient amount of cross-

linking agent was necessary for MOP-modified silica

particles to catch PNIPAm swollen in the reagent solution.

On the other side, in the case of entry 5, PNIPAm peaks

were shown clearly when 5.51 mol-% MBA was added

although their intensity was weaker than that of entry 2.

The weight loss at 115–700 8C in this reaction condition

was 40.08 wt.-% [Figure 4(c)]. It seems that PNIPAm oligo-

meric radicals with increased hydrophobicity above LCST

are easily adsorbed on the hydrophobic MOP-modified

silica particles and crosslinked by MBA.

Figure 5. DSC thermograms of PNIPAm-grafted silica containingmore PNIPAm relative to the variation with the reaction tempera-ture and the MBA concentration: (a) entry 1 (20 8C, 11.01 mol-%MBA), (b) entry 2 (40 8C, 11.01 mol-% MBA), and (c) entry 5 (40 8C,5.51 mol-% MBA).

Effects of the Weight Ratio of 3-(Trimethoxysilyl)propylMethacrylate-Modified Silica Microspheres toN-Isopropylacrylamide Monomers

The effect of the amount of NIPAm monomer on the

synthesis of PNIPAm-grafted silica was studied using

11.01 mol-% MBA (entries 2, 7, and 8). In FT-IR spectra

[Figure 3(d), 3(i), and 3(j)], when the weight ratio of NIPAm

monomer to MOP-modified silica was decreased from 4

to 2, the intensity of the characteristic peaks of PNIPAm

became weaker. In the case of entry 8, the characteristic

peaks of PNIPAm are not observed. TGA data shows same

tendency, that is, the amount of the weight loss at 115–

700 8C increases with the weight ratio of NIPAm to MOP-

modified silica as shown in Figure 4(d). The weight losses

at 115–700 8C of entries 2 and 7 are 70.22 and 37.62 wt.-%,

respectively. It is expected that, when the amount of

NIPAm monomer is lowered, the possibility of polymer-

ization of NIPAm monomer with MOP-modified silica

particles diminishes, and PNIPAm that is not grafted onto

MOP silica is formed. This indicates that it is necessary



to add NIPAm monomer more than MOP-modified silica

particles for the synthesis of PNIPAm-grafted silica micro-

particles in this study.

DOI: 10.1002/macp.200700247


Investigations on the Lower Critical SolutionTemperature of Poly(N-isopropylacrylamide)-GraftedSilica Microparticles

DSC measurement was used to investigate the thermo-

responsive properties of the synthesized PNIPAm-grafted

silica particles with variable factors of the reaction tem-

peratures and the MBA concentrations at fixed weight

ratio of NIPAm to MOP-modified silica (4:1) as shown in

Figure 5. When the concentration of MBA was fixed at

11.01 mol-%, PNIPAm-grafted silica prepared at 40 8C does

not have the transition temperature (LCST), while DSC

curve of PNIPAm-grafted silica prepared at 20 8C has a

maximum endothermic point at 36 8C. However, in the

case of entry 5, the LCST of PNIPAm-grafted silica was

found clearly at 35 8C [Figure 5(c)], while PNIPAm has the

LCST at 32 8C. It is suggested that the chemical bonding

between PNIPAm and silica suppresses the thermody-

namic behavior of PNIPAm[9b] Thus, it is supposed that

MBA content in PNIPAm-grafted silica prepared above

LCST (entry 2) is larger than that in PNIPAm-grafted silica

prepared below LCST (entry 1).

Conclusion

In summary, PNIPAm-grafted silica microparticles were

synthesized by radical copolymerization of MOP silica

particles and NIPAmmonomers. The modification of silica

withMOPwas achieved by the sequential addition of TEOS

and MOP into W/O emulsion. In the synthesis of PNIPAm-

grafted silica, the effects of reaction conditions such as

reaction temperature, concentration of crosslinking agent

and weight ratio of NIPAm monomer to MOP-modified

silica on the grafting of PNIPAm onto MOP-modified silica

were studied. From the DSC curves, it was confirmed that

the reaction temperature and the concentration of MBA

affected the thermal behavior of PNIPAm-grafted silica

composites. The pore size of silica particles can be controlled

by the addition of a specific amount of one type of stabil-

izer and surfactant into W/O emulsion in emulsion–gel

method. This fact is useful for the encapsulation and the

controlled release of various target materials such as drug,

vitamin, protein, etc.

Acknowledgements: This research was funded by Center forUltramicrochemical Process Systems sponsored by KOSEF.

Received: May 4, 2007; Revised: July 19, 2007; Accepted: July 25,2007; DOI: 10.1002/macp.200700247

Keywords: emulsion–gel method; PNIPAm; radical polymeriza-tion; silicas; thermoresponsive



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preparation of thermosensitive pnipam-grafted mesoporous silica particles

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