a novel route to organic–inorganic hybrid nanomaterials

Communication

1828

A Novel Route to Organic–Inorganic HybridNanomaterialsa

Bindushree Radhakrishnan, Andrew N. Constable, William J. Brittain*

The in situ formation of functionalized silica nanoparticles is reported. The reactive stabilizersused in the study, [3-(2-bromoisobutyryl)propyl]triethoxysilane and [3-(2-bromoisobutyryl)-propyl]ethoxydimethylsilane, have an atom transfer radical polymerization (ATRP) initiator atthe noncondensable end. Condensation with tet-raethoxysilane yields silica nanoparticles with asurface-immobilized initiator. The size of thesefunctionalized silica nanoparticles can be con-trolled by varying the time of initiator additionand initiator concentration. The silica particlesizes ranged from 10 to 300 nm. With the initiatorfunctionalized silica nanoparticles, ATRP syn-thesis was performed with styrene, tert-butylacrylate, and methyl acrylate to produceorganic–inorganic nanomaterials.

Introduction

Silica nanoparticles are used for a variety of applications

depending on their porosity and hardness. However, the

main challenge is to control the interparticle aggregation.

Aggregation can be controlled by covalently grafting

polymer chains onto the particle. The chemical modifica-

tion of a silica nanoparticle surface with a polymer not

only improves the stability but can also alter the

mechanical, structural, and thermal properties of particle

B. Radhakrishnan, A. N. Constable, W. J. BrittainDepartment of Polymer Science, University of Akron, Akron, Ohio44224, USAFax: þ1 585-586-0331; E-mail: [email protected]. RadhakrishnanCurrent address: The Dow Chemical Company, 2301 N. BrazosportBlvd., B1608, Freeport, Texas 77541, USA

a: Supporting information for this article is available at the bottomof the articles abstract page, which can be accessed from thejournal’s homepage at http://www.mrc-journal.de, or from theauthor.

Macromol. Rapid Commun. 2008, 29, 1828–1833

� 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

and the polymer.[1] Such hybrid organic–inorganic materi-

als find a number of applications in optics and electro-

nics.[2]

Both ‘‘grafting-to’’ and ‘‘grafting-from’’ methods have

been explored for the synthesis of hybrid nanomaterials

from preformed silica nanoparticles. The grafting-to

technique involves the chemical reaction of a reactive

polymer end group to the surface.[3] However the grafting

density, which controls the final properties of the hybrid

nanomaterial, is low due to steric hindrance. This draw-

back is overcome by using a grafting-from technique, in

which the polymer chain is grown from the surface

through a covalently linked monomer[4] or an initiator.[5]

The advantages and disadvantages of both techniques

have been reviewed.[1]

There are a number of reports on the synthesis of hybrid

silica nanoparticles both by grafting-to and grafting-from

techniques. Procedures for the grafting-from technique

commonly involve covalent attachment of a suitable atom

transfer radical polymerization (ATRP) initiator or rever-

sible addition fragmentation transfer (RAFT) agent to

silica. While not reviewed in detail here, the literature

DOI: 10.1002/marc.200800435

A Novel Route to Organic–Inorganic Hybrid Nanomaterials

contains numerous examples of ATRP[6–18] and RAFT[19–22]

for the modification of nanoparticles.

Synthesis of silica nanoparticles is a heterogeneous

condensation polymerization and the particles are stabi-

lized by ‘‘electrostatic stabilization’’ by the charges formed

in situ on the surface during the reaction. An alternative to

electrostatic stabilization is steric stabilization, in which

specific nonreactive molecules are added during the

condensation polymerization and adsorb to the surface

and thus prevent coagulation. A variant of steric stabilizers

is a reactive stabilizer that will participate in the

condensation reaction in addition to functioning as a

stabilizing group. These stabilizing groups can be a

monomer, initiator, or a polymer. The disadvantages of

coagulation or lower grafting densities in the modification

of preformed nanoparticles can be circumvented by the

synthesis of nanosilica in the presence of reactive

stabilizers.[23]

Herein, we report the in situ formation of functionalized

silica nanoparticles. The reactive stabilizer used in our

present study has an ATRP initiator at the noncondensable

end yielding silica nanoparticles with a surface-immobi-

lized ATRP initiator. These were then used for polymeriza-

tion and block copolymerization of styrene, tert-butyl

acrylate (t-BA) and methyl acrylate (MA). This unique

process results in the formation of particles with a size

range of 10–300 nmdepending on stabilizer concentration,

type of the stabilizer, and the synthesis process.

Experimental Part

Materials

Styrene (S, Aldrich, 99%), MA (Aldrich, 99%), t-BA (Aldrich, 98%),

N,N,N0,N0 ,N00-pentamethyldiethylenetriamine (PMDETA, Aldrich

99%), and ethyl 2-bromoisobutyrate (E2Br-iB, Aldrich 98%) were

passed through a column of activated basic alumina prior to use.

CuBr (Aldrich, 98%) was purified as described in the literature.[24]

Anhydrous anisole (Aldrich, 99.8%), toluene (Aldrich, 99.8%),

anhydrous tetrahydrofuran (THF, Aldrich, 99.8%), platinum(0)-

1,3-divinyl-1,1,3,3-tetramethyldisiloxane complex (Karsted’s cat-

alyst, Aldrich, 3 wt.-% solution in xylene), 2-bromoisobutyryl

bromide (Aldrich, 96%), allyl alcohol (Aldrich, 99þ%), triethoxy-

silane (Aldrich, 95%), dimethylethoxysilane (Alfa Aesar, 94%)

hydrofluoric acid (HF, 48%, A.C.S. reagent), tetraethyoxysilane

(TEOS, Aldrich, 99%), ammonia (Aldrich, 28% aqueous solution),

and Aliquat 336 (Aldrich) were used as received. The synthesis of

the reactive stabilizer [3-(2-bromoisobutyryl)propyl]-triethoxysi-

lane (BPTS) and [3-(2-bromoisobutyryl)propyl]ethoxydimethylsil-

ane (BPMS) has been previously reported.[6]

Initiator Coated Silica Particles

Initiator coated silica nanoparticles were synthesized by the

hydrolytic condensation of TEOS in the presence of BPTS or BMPS



as the reactive stabilizer. TEOS (3.0mL, 0.01mol), 50mL of ethanol,

and a basic catalyst (ammonium hydroxide, 3.33 mL, 0.09 mol)

were added to a round bottom flask equipped with a stir bar. The

reactive stabilizer was added (10% concentration, M/M relative to

TEOS) at varying time intervals of 0, 10 min, 1, 3, and 6 h. To study

the effect of concentration, 0, 1, 2, 5, 10, and 20% M/M ratios of

reactive stabilizer to TEOS were used and the reactive stabilizer

was added 1 h after the onset of the reaction. The reaction was

allowed to proceed at ambient temperature for 24 h. After

completion of the reaction, the particles were isolated by

centrifugation from the reaction mixture. The particles were

washed with THF, ethanol, and water (two washings of each) to

remove the catalyst and unreacted reactive stabilizer. The silica

particles were dried overnight in a vacuum oven at 60 8C.

Typical Procedure for Surface Initiated Atom Transfer

Radical Polymerization (ATRP)

The initiator coated silica particle (1 g) was added to a 100 mL

Schlenk flask along with a magnetic stir bar. The flask was

evacuated and back-filledwith nitrogen three times and left under

a nitrogen atmosphere. CuBr (0.037 g, 0.25mmol), anisole (7.0mL),

and monomer (7.0 mL) were added to a separate 100 mL Schlenk

flask along with a magnetic stirrer bar and the flask was sealed

with a rubber septum. Three freeze–pump–thaw cycles were

performed and back filled with nitrogen. PMDETA (0.10 mL,

0.48 mmol) was added to the mixture via syringe and the solution

was allowed to stir at the polymerization temperature until it

became homogeneous. The solution was then transferred to the

flask containing the silica nanoparticles via a cannula, followed by

the addition of the free initiator (E2Br-iB) (0.022 mL, 0.15 mmol)

via syringe. The solutionwas allowed to stir for the duration of the

polymerization. The polymerization times and temperatures are

as follows: styrene: 10 h, 110 8C; t-BA: 24 h, 75 8C;MA: 9 h, 90 8C. Toremove free polymer, the polymerized solution was sonicated and

centrifuged in THF five times. The silica particles were dried

overnight in a vacuum oven at 70 8C.

Degrafting Polymer Brushes from Silica Particles

In a polyethylene beaker, the silica polymer hybrid nanoparticles

(100.0 mg) were dispersed in 1.0 mL of toluene, Aliquat 336 (phase

transfer catalyst, 10.0 mg) and 1.0 mL of 49% aqueous HF solution

was added to the dispersion of particles. The reaction was allowed

to stir at room temperature overnight. The polymerwas recovered

by precipitation into methanol.

Instrumentation

Gel permeation chromatography (GPC) analysis was carried out

using a Waters 150-C Plus instrument equipped with three HR-

Styragel columns [100 A, mixed bed (50/500/103/104 A), mixed

bed (103/104/106 A)], and a triple detector systemwith THF as the

eluant at a flow rate of 1.0mL �min�1 at 30 8C. The detector systemconsisted of a differential refractometer (Waters 410), a differ-

ential viscometer (Viscotek 100), and a laser light scattering

www.mrc-journal.de 1829

B. Radhakrishnan, A. N. Constable, W. J. Brittain

Figure 1. Transmission electronmicroscope analysis of silica nano-particles: (a) absence of BPTS, (b) presence of 10% BPTS concen-

1830

detector (Wyatt Technology, DAWN EOS, l¼670 nm).Mn andMw

were determined using universal calibration, which were

calibrated with polystyrene standards (Polymer Laboratories).

The bromine content of the initiator coated silica particle was

determined by elemental analysis (Galbraith Laboratories). FT-IR

was recorded on a Digilab, Excalibur 300 series instrument using

KBr pellets. Thermogravimetric analysis (TGA) was performed in

nitrogen atmosphere on a Hi-Res TGA 2950 thermogravimetric

analyzer (TA instruments) over the temperature range of 23–

800 8C at a heating rate of 10 8C �min�1. Transmission electron

microscopy (TEM) was performed using an FEI Techani 12;

samples were prepared on a carbon coated copper grid.

Results and Discussion

Synthesis of Initiator Coated Silica

The synthesis and characterization of the difunctional,

ATRP initiator has been reported by von Werne and

Patten.[6] The synthesis is a two-step process (Scheme 1)

that first involves the reaction of allyl alcohol with an acyl

bromide. The second step is a hydrosilylation to form

either a monoethoxy or triethoxy silane. The product is a

reactive stabilizer that participates in the silicate con-

densation process with TEOS. Silica nanoparticles were

synthesized by a sol–gel technique as described by Stober

et al.[25] The alkoxysilane reactive stabilizer can condense

onto the growing particle during particle formation

producing ATRP initiator functionalized silica nanoparti-

cles (Scheme 2).
tration added at the onset of the reaction. Total reaction time forboth reactions was 24 h.
Effect of Adding Reactive Stabilizer

Silica nanoparticles were synthesized in the presence and

absence of reactive stabilizer to evaluate the relationship

between stabilizer and particle size. It was predicted that

the particle growth would be prevented by decreased

condensation due to lower TEOS concentrations. As

predicted, the addition of BPTS resulted in a marked

Scheme 1. Synthesis of a reactive stabilizer.

Scheme 2. ‘‘In situ’’ synthesis of initiator immobilized silica particles



difference in the size of particles (Figure 1). The size of the

particles in the absence of BPTS was �350 nm. However,

the size reduced to 10–20 nm with the addition of BPTS at

the beginning of the reaction. This confirmed the crucial

role of BPTS as a reactive stabilizer that prevents

coagulation in the silica nanoparticle formation. This

was attributed to the steric stabilization effect. Normally,

.

DOI: 10.1002/marc.200800435


Figure 2. Transmission electron microscope analysis of silicananoparticles synthesized using BPTS: (a) 15% relative to TEOSas stabilizer, (b) 20% relative to TEOS as stabilizer.

Table 1. Effect of reactive stabilizer concentration on particle size(time of addition¼ 1 h).

Concentration

BPTSa) BPMSb)

Particle

size

Particle

size

Elemental

analysis of

bromine

%c) nm nm wt.-%

0 350 350 0.00

1 200 300 0.69

2 180 280 1.15

10 160 200 5.03

15 150 160 6.58

20 150 150 7.55

a)BPTS; b)BPMS; c)concentration relative to TEOS.

Figure 3. Effect of stabilizer addition time on the size of theparticle: BPTS and BPMS.

the stability of the growing particle is controlled by

charged ions on the surface (electrostatic stabilization).

However, in the presence of BPTS which is uncharged, the

concentration of surface charges decreases and this

prevents coagulation. Both BPTS and BPMS showed similar

trends.

The stabilizer effect on silica particle synthesis can also

be seen by varying the stabilizer concentration (with

respect to TEOS) under similar reaction conditions. As the

relative BPTS concentration increased, we observed a

decrease in particle size. This result is explained by particle

steric stabilization and decreased particle growth (due to

the lack of propagating groups on the surface). Because

BPTS contains three condensable groups, we speculate that

not all of the condensable groups are reacting in a single

particle. This resulted in the ‘‘raspberry-like’’ structure

shown in Figure 2b. This structure disappeared and core–

shell structure resulted upon using BPMS, which has only

one condensable group. Tabulated below (Table 1) is the

study on the effect of concentration of the reactive

stabilizer on the size of particle under similar reaction

conditions. Marini et al.[26] observed similar effects in the

condensation of TEOS with vinyltriethoxysilane.



Elemental analysis of bromine was performed on BPMS

particles, Table 1. As the concentration of BPMS increased,

the bromine concentration increased while the particle

size decreased. The decrease in size is due to the BPMS

reactive stabilizer attaching to the particle and stunting its

growth. As the concentration in BPMS is increased, the

reduction in growth is magnified, confirmed by the

increase in bromine content.

Effect of Addition Time

Another parameter that was crucial in controlling particle

size was the time of reactive stabilizer addition. We

observed that particle size could be controlled by the time

of stabilizer addition. Smaller particles were formed with

early stabilizer addition due to the noncondensing end of

the BPTS which hindered particle growth (Figure 3). Both

stabilizers showed a similar trend.


B. Radhakrishnan, A. N. Constable, W. J. Brittain

Table 2. Characterization of polymer silica nanoparticle hybrids.

Sample

GPC Analysis TGA (Final weight loss)

Mn, g �molS1 PDI %

Silica-PS-b-PMA PS 7000 1.3 24

PMA 6200 1.3

Silica-PS-b-P(t-BuA) PS 10 000 1.2 28

P(t-BuA) 16 000 1.3

Silica-PMA-b-PS PMA 10100 1.1 34

PS 27 000 1.2

Silica-PMA-b- P(t-BuA) PMA 11000 1.1 27

P(t-BuA) 17 600 1.1

Silica-P(t-BuA)-b-PMA P(t-BuA) 13 000 1.3 23

PMA 12000 1.2

Silica-P(t-BuA)-b-PS P(t-BuA) 13 000 1.3 24

PS 10 000 1.2

1832

Characterization of the Silica Particle

The initiator coated and the polymer coated silica particles

were also characterized by FT-IR, TGA (to determineweight

loss) and elemental analysis (Table 1). FT-IR spectra and

TGA traces for Si/SiO2/PMA-b-PS silica nanoparticles can

be found in the Supporting Information. FT-IR showed the

characteristic carbonyl stretching of ester group the

bromoisobutyrate initiating moiety at 1 730 cm�1. The

intensity of a carbonyl peak increased upon the poly-

merization of PMA. After the polymerization of the PS

second block, peaks from the aromatic hydrogens were

present above 3 000 cm�1.

Polymerization

The initiator immobilized silica particles were used for

ATRP of MA, styrene, and t-BuA. The results are tabulated

below (Table 2). Six different block copolymer modified

silica particles were synthesized by altering the choice of

monomers and choice of the initial block composition. The

initiator coated silica particles, using TGA, showed a

weight loss of 7%. Weight measured after diblock

copolymer synthesis was in the range of 24–34%. The

grafted polymer chains were cleaved from the silica

surface with HF. This free polymer was used to determine

the molecular weight and polydispersity index. The

polydispersity of the cleaved chains was 1.3 or less,

consistent with a controlled polymerization. Diffuse

reflectance IRwas used to characterize the diblock brushes.



When MA was grafted to the surface, there was a

characteristic rise in the 1 730 cm�1.

Conclusion

Silica nanoparticles with a surface-immobilized ATRP

initiator were prepared by an ‘‘in situ’’ condensation

method using a reactive stabilizer. The reactive stabilizer is

a bromoisobutyrate functionalized alkoxysilane. Reaction

with tetraethoxysilane produces functionalized silica

nanoparticles in one step. Particle size ranges from 10 to

350 nm. Particle size can be controlled by the amount of

reactive stabilizer used and the time period of addition

during the condensation process. At higher concentrations,

the BPTS reactive stabilizer resulted in the formation of

raspberry-like structures; however, the use of BPMS

produced more uniform particles. Thus, BPMS is the

preferred reactive stabilizer. Potentially, other types of

polymerization initiators could be used in this process. The

silica nanoparticles with surface-immobilized initiator

were used to prepare homo- and diblock copolymers via

ATRP. Cleavage of the polymers from the silica nanopar-

ticles revealed molecular weights between 6 000 and

27 000 g �mol�1.

Acknowledgements: We acknowledge the financial support ofNational Starch and Chemical.

Received: July 10, 2008; Revised: August 26, 2008; Accepted:August 27, 2008; DOI: 10.1002/marc.200800435

DOI: 10.1002/marc.200800435


Keywords: atom transfer radical polymerization (ATRP); coat-ings; nanoparticles; silica; surface initiation

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a novel route to organic–inorganic hybrid nanomaterials

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