Environmentally responsive "hairy" nanoparticles: Mixed homopolymer brushes on silica nanoparticles synthesized by living radical polymerization techniques

Article abstract of DOI:10.1021/ja0422561

This article reports on the preparation of environmentally responsive "hairy" nanoparticles by growth of mixed poly(tert-butyl acrylate) (PtBA)/polystyrene (PS) brushes from silica particles using living radical polymerization techniques and subsequent hy

Full text of DOI:10.1021/ja0422561

Published on Web 04/09/2005
Environmentally Responsive “Hairy” Nanoparticles: Mixed
Homopolymer Brushes on Silica Nanoparticles Synthesized
by Living Radical Polymerization Techniques
Dejin Li, Xia Sheng, and Bin Zhao*
Contribution from the Department of Chemistry, UniVersity of Tennessee,
KnoxVille, Tennessee 37996
Received December 23, 2004; E-mail: zhao@novell.chem.utk.edu
Abstract: This article reports on the preparation of environmentally responsive “hairy” nanoparticles by
growth of mixed poly(tert-butyl acrylate) (PtBA)/polystyrene (PS) brushes from silica particles using living
radical polymerization techniques and subsequent hydrolysis of PtBA to produce amphiphilic mixed poly-
(acrylic acid) (PAA)/PS brushes. Silica particles were synthesized by the Sto¨ber process and were
functionalized with an asymmetric difunctional initiator-terminated monolayer. Surface-initiated atom transfer
radical polymerization of tBA was carried out in the presence of a free initiator. Kinetics study showed that
the polymerization was well controlled. By cleaving PtBA off the particles, the molecular weights of the
grafted and free polymers were found to be essentially identical. Mixed PtBA/PS brushes were obtained
by the nitroxide-mediated radical polymerization of styrene from PtBA particles. The Mn of the grafted PS
was found to be the same as that of the free PS formed in the solution from the free initiator. Amphiphilic
mixed PAA/PS brush-coated nanoparticles were synthesized from mixed PtBA/PS particles by hydrolysis
of PtBA with iodotrimethylsilane. Tyndall scattering experiments and 1H NMR study showed that the mixed
PAA/PS particles can be dispersed and form a stable suspension in CHCl3, a selective solvent for PS, and
also in CH3OH, a selective solvent for PAA, demonstrating the capability of these hairy nanoparticles to
undergo chain reorganization in response to environmental changes.
gels that were surface-functionalized with an azo monolayer.7,8
Controlled/“living” radical polymerizations are advantageous
Introduction
Hybrid nanoparticles composed of an inorganic or metal
particle core and an organic polymer shell possess intriguing
properties associated with the nanoparticles and the desired
properties of polymers, including processability and compat-
ibility to the environment.1-3 When densely tethered to the
surface of particles by one end via a covalent bond, polymer
chains are forced to stretch away from the grafting sites and
assume an extended conformation due to the excluded volume
interaction,1-5 resulting in “hairy” particles (polymer brushes
on the particles). Generally, there are two approaches to
chemically attach polymer chains to a surface: (1) the “grafting
to” method, where the end-functionalized polymers react with
an appropriate surface;6 (2) the “grafting from” method, where
polymer chains are grown from initiator-terminated self-
assembled monolayers (SAMs).1-4 Using the grafting from
approach, Prucker and Ru¨he described in their seminal papers
the growth of polystyrene (PS) from high-surface-area silica
over conventional free radical polymerizations as they can
provide control on polymer architecture, molecular weight, and
molecular weight distribution.9,10 A variety of living polymer-
ization techniques, most notably, atom transfer radical polym-
erization (ATRP) and nitroxide-mediated radical polymerization
(NMRP), have been used in recent years by a number of research
groups to synthesize polymer brushes from flat substrates,11
(7) Prucker, O.; Ru¨he, J. Macromolecules 1998, 31, 592-601.
(8) Prucker, O.; Ru¨he, J. Macromolecules 1998, 31, 602-613.
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J. AM. CHEM. SOC. 2005, 127, 6248-6256
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Environmentally Responsive “Hairy” Nanoparticles
A R T I C L E S
carbon nanotubes,12 and nanoparticles including silica and gold
nanoparticles,13 quantum dots,14 and magnetic nanoparticles.15
Using Langmuir-Blodgett technique, Ohno et al. demonstrated
that the distance between the gold nanoparticles can be well
controlled by the chain length of the grafted polymer, providing
an excellent example on the potential applications of hairy
particles in nanotechnology and advanced materials.13j
In this article, we report on the synthesis, characterization,
and properties of environmentally responsive hairy particles,
mixed homopolymer brushes on silica nanoparticles. Mixed
brushes composed of two different homopolymer chains ran-
domly or alternately immobilized on planar solid surfaces have
been intensively studied in recent years because of their
intriguing phase behavior and potential applications in “smart”
materials.16-45 It has been theoretically predicted and experi-
mentally demonstrated by several research groups that mixed
homopolymer brushes undergo reorganization in response to
external stimuli, exhibiting different surface wettability and
surface morphology.16-45 By tuning parameters including graft-
ing density, molecular weight, chemical composition, solvent,
and temperature, a variety of surface structures and properties
could be achieved by mixed brushes. We have initiated an effort
to synthesize well-defined mixed homopolymer brushes by
controlled/living polymerization techniques and to explore their
self-assembly behavior in selective solvents and under equilib-
rium melt conditions.36-40 ATRP and NMRP, which are two
different controlled radical polymerization techniques and are
performed under different conditions,9,10 have been used to grow
two distinct homopolymers from either mixed initiator-
terminated monolayers36,40 or asymmetric difunctional initiator-
terminated SAMs (Y-SAMs)37-39 on silicon wafers. Y-SAMs
were designed to ensure that the two initiators are well-mixed
in the initiator monolayer. The effects of relative molecular
weights and relative grafting densities of the two grafted
polymers on the self-assembly have been studied.38-40 In this
work, we applied our strategy to synthesize amphiphilic mixed
brushes on silica nanoparticles. It should be pointed out that no
prior experimental work has been reported in the literature on
mixed brushes on the surface of nanoparticles. One can envision
that the success of the synthesis of well-defined mixed homo-
polymer brushes on nanoparticles would open an avenue to
investigate their responsive properties, to explore the applications
in nanotechnology and advanced materials, and to study how
the two polymer chains phase separate in a confined geometry.
Silica particles with an average diameter of 180 nm were
prepared using the Sto¨ber process46,47 and functionalized with
a Y-SAM. Mixed poly(tert-butyl acrylate) (PtBA)/PS brushes
were synthesized by sequential surface-initiated ATRP of tBA
and NMRP of styrene (Scheme 1). Removal of the tert-butyl
group of PtBA produced amphiphilic mixed poly(acrylic acid)
(PAA)/PS brushes. Infrared spectroscopy (IR), scanning electron
microscopy (SEM), thermogravimetric analysis (TGA), 1H
NMR, and gel permeation chromatography (GPC) were em-
ployed to characterize the nanoparticles and the polymers.
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Synthesis of Y-Initiator-Functionalized Silica Nanopar-
ticles. Spherical silica nanoparticles were prepared using the
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of tetraethoxysilane in an ammonia/ethanol solution.46,47 This
process is well-known to produce silica nanoparticles with a
relatively uniform size distribution, and the size of the particles
can be conveniently controlled by the initial concentrations of
the reactants. In this study, silica particles with an average
diameter of 180 nm, determined from scanning electron
microscopy (SEM) images (Figure 1a), were used to prepare
environmentally responsive organic-inorganic hybrid nanopar-
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Li et al.
Figure 1. Scanning electron microscopy images of (a) bare silica nanoparticles, (b) Y-initiator particles, (c) poly(tert-butyl acrylate) particles, (d) dehalogenated
poly(tert-butyl acrylate) particles, (e) poly(tert-butyl acrylate)/polystyrene particles, and (f) poly(acrylic acid)/polystyrene particles. Scale bars: 200 nm.
Scheme 1. Synthesis of Amphiphilic Mixed Poly(acrylic acid)/Polystyrene Brushes on Silica Nanoparticles
Scheme 2. Synthesis of an Asymmetric Difunctional
Initiator-Terminated Monochlorosilane, Y-Silane
ticles by growing mixed homopolymer brushes from the surface.
A Y-initiator-terminated monolayer was deposited on the surface
of the particles through the reaction between the surface silanol
groups and the Si-Cl of an initiator-terminated organosilane.
Although denser SAMs can be obtained by use of organot-
richlorosilanes, the reaction process is complex, and cross-linked
polymeric species formed in the solution might precipitate on
the surface.48 To avoid this issue, an asymmetric difunctional
initiator-terminated monochlorosilane, Y-silane (Scheme 2), was
designed. Y-silane was prepared by a four-step procedure as
outlined in Scheme 2. 4-Vinylbenzyl chloride reacted with the
free radicals generated from benzoyl peroxide (BPO) at 80 °C
in the presence of 2,2,6,6-tetramethylpiperidinoxy (TEMPO),
producing 1. The reaction between allylmagnesium chloride and
1 afforded compound 2, which was then esterified with 2-bromo-
2-methylpropionyl bromide to incorporate the moiety of an
ATRP initiator. Y-silane was then obtained by hydrosilylation
of the precursor 3 with dimethylchlorosilane using platinum
divinyl tetramethyldisiloxane complex as catalyst. Detailed
synthetic procedures and characterization data can be found in
the Supporting Information.
spectrum (Figure 2b) of Y-initiator-functionalized particles
(initiator particles) shows absorbance bands at 2983 and 2899
cm-1 that were characteristic of the alkyl C-H stretching
vibration modes and were not present in the IR spectrum of
bare particles (Figure 2a).49 From SEM images (Figure 1b), the
average diameter of initiator particles was 181 nm, essentially
the same as that of bare particles (180 nm). Thermogravimetric
Bare silica particles were dried in vacuum (∼30 mTorr) at
110 °C for 6-7 h and were dispersed in dry tetrahydrofuran
(THF) by ultrasonication. A solution of freshly synthesized
Y-silane in dry THF was injected via a syringe to the flask
containing THF-dispersed nanoparticles. The surface immobi-
lization reaction was carried out under N2 atmosphere at 70 °C
for 40 h. The particles were then isolated by centrifugation and
analysis (TGA), which was performed in air at a heating rate
were redispersed in dry THF. This washing process was repeated
of 20 °C/min, indicated that the weight retention at 800 °C was
four times, followed by drying with a stream of air. The IR
(49) Pretsch, E.; Buhlmann, P.; Affolter, C. Structure Determination of Organic
(48) Ulman, A. Chem. ReV. 1996, 96, 1533-1554.
Compounds: Table of Spectral Data; Springer: Berlin, 2000.
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A R T I C L E S
Figure 2. IR spectra of (a) bare particles, (b) Y-initiator particles, (c) poly(tert-butyl acrylate) particles, (d) poly(tert-butyl acrylate)/PS particles, and (e)
poly(acrylic acid)/PS particles.
ATRP initiator by a metal complex is a bimolecular process
and the free radicals in NMRP are generated by thermal
decomposition, a unimolecular process, ATRP of tBA was
conducted first. A unimolecular activation mechanism is
preferred for the synthesis of the second type of polymer chains
from the surface because of the possible steric hindrance
presented by the existing polymer chains. Surface-initiated
ATRP of tBA was carried out at 75 °C using CuBr/CuBr2 and
N,N,N′,N′,N′′-pentamethyldiethylenetriamine (PMDETA) as the
catalytic system.52 We confirmed that the NMRP initiator was
stable under typical ATRP conditions. A “free” ATRP initiator,
ethyl 2-bromoisobutyrate (EBiB), was added into the reaction
mixture to control the polymerization.11a,b The ratios of [M]o:
[free EBiB]o:[CuBr]o:[CuBr2]o:[PMDETA]o were 553:1:1.07:
0.033:1.05. CuBr2 (3.1 mol % relative to CuBr) was added to
ensure an efficient exchange between the dormant and active
species. According to the EPR results from the Matyjaszewski
group, about 3% Cu(II) species was formed spontaneously due
to the persistent radical effect in ATRP of methyl acrylate at
the molar ratio [M]o:[RBr]o:[Cu(I)]o ) 200:1:1 at 90 °C.53
The polymerization was monitored by 1H NMR spectroscopy
using the peaks located at 6.87-6.95 ppm, which are from the
solvent anisole, as internal standard. The monomer conversion
was determined by use of the integral value of the peak located
at 6.28 ppm (1H from tBA) at time t and the value of the same
peak at time t ) 0. The polymerization was stopped when the
conversion reached 38.0% at t ) 1805 min. THF was added to
dilute the mixture, and the nanoparticles were isolated by
centrifugation. PtBA formed in the solution from the free
initiator was collected and analyzed by 1H NMR spectroscopy
and GPC relative to PS standards. The particles were repeatedly
dispersed in THF and chloroform, isolated by centrifugation to
remove physically adsorbed polymers, and characterized by
Figure 3. Thermogravimetric analysis (TGA) of (a) bare particles, (b)
Y-initiator particles, (c) PtBA-particle-1 obtained at the monomer conversion
of 14.1%, (d) PtBA-particle-2 obtained at the monomer conversion of 24.6%,
(e) PtBA-particle-3 obtained after the polymerization was stopped at the
monomer conversion of 38.0%, (f) PAA/PS particles, and (g) PtBA/PS
particles. TGA was performed in air at a heating rate of 20 °C/min.
90.5% for bare particles and 86.9% for initiator particles (Figure
3). The weight loss of bare particles might be due to the
continued condensation reaction and associated water loss.13h
If the mass percent of the residue at 800 °C is used as the
reference, the weight increase of initiator particles relative to
that of bare particles would be 4.6%. Calculations show that
the density of Y-initiator on the particle surface is ∼0.33 nm2/
initiator. This result is consistent with the literature report.13a,b
Synthesis of PtBA Brushes from Y-Initiator Nanoparticles.
Mixed PtBA/PS brushes were grown from Y-initiator particles
by sequential ATRP of tBA at 75 °C and NMRP of styrene at
120 °C. We chose to synthesize mixed PtBA/PS brushes because
PtBA can be easily converted to hydrophilic PAA,50,51 producing
amphiphilic particles. Considering that the activation of an
1
FTIR, H NMR, SEM, and TGA.
1H NMR analysis showed that the methylene peak of the free
initiator EBiB completely shifted from 4.21 to 4.09 ppm (Figure
4) after the polymerization was started, suggesting a quantitative
(50) Davis, K. A.; Matyjaszewski, K. Macromolecules 2000, 33, 4039-4047.
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Li et al.
Figure 6. GPC curves of PtBA-1, PtBA-2, and PtBA-3.
Table 1. Number-Average Molecular Weights of PtBA-1, PtBA-2,
and PtBA-3 Obtained from the Conversion, 1H NMR, and GPC
Analysis
Figure 4. 1H NMR spectra of the polymerization mixture at t ) 0 (a) and
30 min (b).
free
conv.
(%)
DPc and Mn
from NMR
Mn
Mw/Mn
GPC
a
b
polymer
Mn
Mn
GPC
PtBA-1
PtBA-2
PtBA-3
14.1
24.6
38.0
8800
15500
24000
10000
17400
26900
64 (8200)
121 (15500)
182 (23300)
10900
17600
24200
1.16
1.13
1.09
a Based on total initiator. b Based on free initiator. c DP ) degree of
polymerization.
particles designated as PtBA-particle-3). The GPC peak shifted
from low molecular weight to high molecular weight with the
increase of monomer conversion. The number-average molecular
weights (Mn) of PtBA-1, PtBA-2, and PtBA-3 were 10 900,
17 600, 24 200, respectively, and the polydispersity decreased
progressively from 1.16 (PtBA-1) to 1.13 (PtBA-2) to 1.09
(PtBA-3). To obtain the absolute molecular weights of the free
polymers, 1H NMR analysis was performed.54 The peaks located
at 4.09 ppm can be clearly seen and are attributed to the
methylene group of the residual initiator moiety. Using these
peaks and the peak at 2.20 ppm, which is from the -CH- of
PtBA, the values of Mn of the free polymers were obtained and
are summarized in Table 1. The molecular weights based on
the conversion were also calculated using both of the ratios of
monomer to free initiator and total initiator (free initiator +
surface-immobilized initiator). The values of Mn obtained from
1H NMR analysis were close to those calculated from the
monomer conversion, especially to those calculated using the
ratio of the monomer to the total initiator. Thus, both kinetics
study and molecular weight analysis suggested that the polym-
erization was a living process, producing polymers with
controlled molecular weights and relatively low molecular
weight distributions.
Figure 5. Relationship between ln[M]o/[M] versus time for the ATRP of
tBA at 75 °C. The ratios of [M]o:[free EBiB]o:[CuBr]o:[CuBr2]o:[PMDETA]o
were 553:1:1.07:0.033:1.05.
initiator efficiency. To confirm that the ATRP of tBA under
the described conditions was “living” throughout the course of
the polymerization, ln([M]o/[M]) versus reaction time was
plotted and is shown in Figure 5. A linear relationship indicated
that the polymerization was a first-order reaction with respect
to the monomer concentration, and the number of the growing
chains was a constant throughout the course of the polymeri-
zation. Thus, the presence of silica nanoparticles has no negative
effect on the polymerization.
Characterization of Free Polymers Formed in the Solution
from the Free Initiator. To further study the ATRP of tBA
under the described conditions, two samples were taken at t )
547 min (conversion ) 14.1%, the particles were designated
as PtBA-particle-1 and the free polymer PtBA-1) and 1053 min
(conversion ) 24.6%, the particles designated as PtBA-particle-2
and the free polymer PtBA-2) during the polymerization. The
particles and free polymers were separated by centrifugation
and purified. Figure 6 shows the GPC curves of PtBA-1, PtBA-
2, and PtBA-3 (the polymer obtained after the polymerization
was stopped at the conversion of 38.0%, and the corresponding
Characterization of PtBA Brush-Coated Nanoparticles.
PtBA brush-coated nanoparticles (PtBA particles) were found
to be easily dispersed in organic solvents, such as THF and
CHCl3. It is interesting to observe that the PtBA particles
exhibited a color of slight purple in the presence of THF after
deposition in the bottom of the tubes by centrifugation. The
spectrum of Figure 2c shows the characteristic vibration
(54) The 1H NMR spectrum of PtBA-1 is included in the Supporting Information.
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Figure 8. The mass percent of the grafted PtBA relative to the mass of
the residual silica at 800 °C corrected from Y-initiator particles versus the
degree of polymerization of free polymers calculated on the basis of the
monomer conversion. (9) Degree of polymerization calculated by use of
the ratio of monomer to total initiator; (b) degree of polymerization obtained
by use of the ratio of monomer to the free initiator. Relative mass increase
) [(1 - Wpp)/Wpp - (1 - Wip)/Wip] × 100, where Wpp and Wip represent
the percent weight retention at 800 °C of PtBA particles and Y-initiator
particles, respectively.
Figure 7. 1H NMR spectra of (a) PtBA-particle-3 and (b) mixed PtBA/PS
particles dispersed in CDCl3.
absorption of the carbonyl group of PtBA at 1729 cm-1 and
the absorption peaks of alkyl C-H at 2977 and 2934 cm-1
,
confirming that the PtBA chains were successfully grown from
the surface-immobilized Y-initiator. The presence of the grafted
1
PtBA chains on the particle surface is also supported by H
NMR analysis of PtBA-particle-3 dispersed in CDCl3 (Figure
7a). The peaks were broadened because the motion of the
polymer chains was restricted by tethering one chain end to
the solid surface.
TGA of PtBA-particle-1, PtBA-particle-2, and PtBA-parti-
cle-3 shows that the amount of the grafted PtBA increases with
the increase of polymer chain length (Figure 3c-e). The weight
retention at 800 °C decreased from 79.5% (PtBA-particle-1) to
75.0% (PtBA-particle-2) to 70.0% (PtBA-particle-3). Using the
mass of the residual silica at 800 °C as reference, a quantitative
analysis of TGA data indicates that the amount of the grafted
PtBA increases linearly with the degree of polymerization (DP)
calculated on the basis of the monomer conversion (Figure 8),
implying that the number of growing chains on the silica surface
is a constant. This result suggests that the surface-initiated
polymerization was a living/controlled process. Since the amount
of the surface-immobilized ATRP initiator is only 11.3 mol %
of the total initiator, a living polymerization in the solution
provides a good control on the surface-initiated polymerization.11a,b
SEM image analysis shows that the size of the particles increases
noticeably after the polymerization (Figure 1c is an SEM image
of PtBA-particle-3). While the average size of Y-initiator
particles was 181 nm (Figure 1b), the average diameter of PtBA-
particle-3 from SEM analysis was 198 nm, which is reasonably
close to the calculated diameter, 206 nm, by use of TGA data.55
The Molecular Weight and Grafting Density of PtBA
Brushes on Silica Nanoparticles. To determine the molecular
weight and the molecular weight distribution of the grafted
polymer, PtBA-particle-3 was destroyed with hydrofluoric acid
(HF).13a,b,h The cleaved polymer was collected and characterized
by 1H NMR and GPC. 1H NMR analysis shows that HF has no
Figure 9. GPC curves of the free (s) and cleaved polymers (‚‚‚): (a) the
PtBA cleaved from PtBA-particle-3; (b) free PtBA-3; (c) the polymer
mixture cleaved from mixed PtBA/PS particles; (d) cleaved PS; and (e)
free PS.
the PtBA cleaved from the particles was identical to that of
free PtBA.56 Figure 9a and b shows the GPC curves of the
cleaved and free polymers. The molecular weights of the grafted
polymer were 23 200 (Mn) and 26 600 (Mw), essentially the same
as those of free polymer (Mn ) 24 200 and Mw ) 26 400).
However, the molecular weight distribution of the grafted PtBA
was slightly broader (PDI ) 1.15) than that of free polymer
(PDI ) 1.09). This result is consistent with those reported in
the literature.11b,13i
1
effect on the ester bond of PtBA as the H NMR spectrum of
(55) Calculation of the diameters of PtBA particles, PtBA/PS particles, and PAA/
PS particles from TGA data can be found in the Supporting Information.
(56) The 1H NMR spectra can be found in the Supporting Information.
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A R T I C L E S
Li et al.
From the degree of polymerization of PtBA-3 obtained by
1H NMR analysis, the average diameter of bare particles
measured from the SEM image, and the TGA data, the average
cross-sectional area per chain, A, was estimated along with
assumptions that the particles are spherical and the density of
silica nanoparticles is identical to that of bulk SiO2 (2.07
g/cm3).13f Calculations show that the grafting density was ∼2.5
nm2 per PtBA chain, indicating that the polymer chains were
highly stretched away from the surface and were in the brush
regime (the thickness of the PtBA was 8.5 nm). This value is
comparable to the typical grafting density of poly(methyl
methacrylate) (PMMA) (∼1.8 nm2/chain) on silicon wafers
reported in a previous publication, where an asymmetric
difunctional initiator-terminated trichlorosilane was used to make
initiator-terminated monolayers.37
As reported in a previous publication,37 in a control experi-
ment for the synthesis of mixed PMMA/PS brushes on silicon
wafers, a piece of silicon wafer functionalized with a SAM of
11′-trichlorosilylundecyl 2-bromo-2-methylpropionate (ATRP-
SAM) was exposed to a typical NMRP condition. We observed
a ∼2.0 nm PS film grown on the surface. Presumably, the C-Br
bond in the ATRP initiator was weak and acted as a chain
transfer agent in NMRP. To eliminate the possibility of bromine-
capped PtBA chains and unreacted ATRP initiator in the
monolayer acting as chain transfer agents in NMRP, we used
tri(n-butyl)tin hydride to remove the bromine atoms.37,57,58 PtBA
particles were dispersed in anisole, followed by addition of
CuBr, PMDETA, and free initiator EBiB. After degassing, the
flask containing the mixture was placed in a 75 °C oil bath,
and excess tri(n-butyl)tin hydride was injected over a period of
15 min. The reaction continued for an additional 30 min. The
particles were isolated by centrifugation, washed extensively
with hexane and chloroform, and dried in vacuum at 35 °C
overnight. Figure 1d shows the SEM image of dehalogenated
PtBA particles cast from a dilute THF solution. The average
diameter of the particles is 200 nm, close to that of PtBA-
particle-3 (198 nm) shown in Figure 1c. It is interesting to
observe that the particles formed a hexagonal pattern upon
evaporation of the solvent, similar to the observations reported
in the literature.13a,b,d
NMRP of Styrene from Dehalogenated PtBA Particles.
Mixed PtBA/PS brushes were obtained after growing PS chains
from the dehalogenated PtBA-particle-3 using NMRP. Surface-
initiated polymerization of styrene was conducted at 120 °C in
the presence of a free initiator, 1-phenyl-1-(2′,2′,6′,6′-tetram-
ethyl-1′-piperidinyloxy)-2-benzoyloxyethane (PTEMPO). To
reduce the viscosity of the polymerization mixture and to
mitigate the possible problems associated with a high viscosity,
a large quantity of anisole (1:1 to monomer, w/w) was added
as solvent and a molar ratio of monomer to the free initiator of
644 was used. The polymerization was monitored by 1H NMR
spectroscopy using the peak located at 3.85 ppm, which is from
the methyl group of anisole, as internal standard. The monomer
conversion was determined by use of the integral value of the
peaks located at 5.82 ppm (1H from the double bond of styrene)
at time t and the value of the same peaks at t ) 0. After the
polymerization reached a desired conversion at which the
Figure 10. Relationship between ln[M]o/[M] versus time for the NMRP
of styrene at 120 °C. [M]o:[Free initiator PTEMPO] ) 644:1.
calculated DP of PS is comparable to that of PtBA, the
polymerization was stopped by cooling the mixture to room
temperature and diluting it with CHCl3. The particles were
isolated by centrifugation and repeatedly washed with CHCl3
to remove the physically absorbed polymers. The free polymer
produced in the solution was collected and analyzed by GPC
against PS standards. The Mn and Mw of the free PS were 23 000
and 25 300, respectively, which were slightly higher than the
molecular weights calculated from the conversion using the
molar ratio of monomer to free initiator (Mn ) 22 100). Figure
10 shows the kinetic plot. Except in the beginning of the
polymerization, a linear relationship between ln[M]o/[M] and
reaction time was observed, suggesting that the polymerization
was controlled. It is unclear why the polymerization in the
beginning was slightly slower than anticipated. We could not
offer any explanation for this but have repeatedly observed this
phenomenon in the NMRP of styrene from nanoparticles. When
deposited in the bottom of the tubes after centrifugation from
THF or CHCl3 suspension, the particles exhibited a color of
slight red, in contrast to slight purple of PtBA particles. The
particles were dried in vacuum followed by characterizations
1
with IR, H NMR, TGA and SEM.
The IR spectrum of the obtained particles shown in Figure
2d indicates that the new absorptions appear at 3027, 3065, and
3083 cm-1, which are characteristic for the aromatic C-H,
demonstrating that the polystyrene chains were successfully
grown from the surface. Figure 7b shows the 1H NMR spectrum
of mixed PtBA/PS particles dispersed in CDCl3. Besides the
characteristic peaks of PtBA at 1.42 and 2.20 ppm, two broad
peaks appear in the range of 6.2-7.2 ppm, which are attributed
to the aromatic protons of polystyrene. The average diameter
of mixed PtBA/PS particles determined from Figure 1e is 219
nm, 19 nm greater than that of the dehalogenated PtBA particles.
The calculated diameter of PtBA/PS particles based on the TGA
data is 224 nm.55 From TGA analysis (Figure 3g), the weight
retention at 800 °C decreased from 70.0% for PtBA-particle-3
to 59.7%. If the molecular weight of the free PS obtained from
GPC analysis is used along with the TGA results (the Mn of
the grafted PS is essentially identical to that of the free PS, see
below), the average cross-sectional area per PS chain is 2.7 nm2,
(57) Coessens, V.; Matyjaszewski, K. Macromol. Rapid Commun. 1999, 20,
66-70.
(58) He, T.; Li, D. J.; Sheng, X.; Zhao, B. Macromolecules 2004, 37, 3128-
3135.
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Environmentally Responsive “Hairy” Nanoparticles
A R T I C L E S
which is similar to that of PtBA (2.5 nm2 per PtBA chain).
Considering both PtBA and PS chains, the average cross-
sectional area is 1.3 nm2 per polymer chain, close to that of
mixed PMMA/PS brushes on silicon wafers (∼1.0 nm2/chain).37
The Molecular Weight and Molecular Weight Distribution
of the Grafted PS. To determine the molecular weight and
polydispersity of the grafted PS, the silica in mixed PtBA/PS
nanoparticles was etched by HF. The grafted polymers were
1
collected and characterized by H NMR and GPC. By use of
the integral values of the peaks from 6.2 to 7.20 ppm (the
aromatic hydrogen atoms of PS) and the peak located at 2.20
ppm (-CH- from PtBA),56 calculations show that the ratio of
the number of the total repeating units of PS to that of PtBA is
1.28:1. Using the molecular weight of PS determined from GPC
analysis and the Mn of PtBA obtained from 1H NMR analysis,
the molar ratio of PS to PtBA, which is equal to the ratio of the
grafting densities of the two polymers (number of polymer
chains per unit surface area) on the surface of silica nanopar-
ticles, is 1.06:1, which is slightly different from the value
obtained from TGA analysis (0.93:1). The discrepancy might
come from the limitations of TGA and 1H NMR analysis. From
GPC analysis shown in Figure 9c, the Mn and Mw of the polymer
mixture cleaved from PtBA/PS particles were 37 500 and
43 200, respectively. These values are much higher than those
of either free PtBA or PS, but lower than those of the sum of
two free homopolymers (Mn ) 47 200 and Mw ) 51 700),
suggesting that a considerable portion of the mixture is the
diblock copolymer composed of PtBA and PS initiated from
one Y-initiator molecule.
Figure 11. Tyndall light scattering experiment shows the stable suspensions
of PAA particles in CH3OH but not in CHCl3, PS particles in CHCl3 but
not in CH3OH, mixed PAA/PS particles in both CHCl3 and CH3OH.
the SEM image in Figure 1f was 219 nm. From TGA data
(Figure 3f), the weight retention at 800 °C changed from 59.7%
(for mixed PtBA/PS particles) to 66.6%. Note that there is a
difference of 1.6% between the two TGA curves at 200 °C.
While CHCl3 is a selective solvent for PS, methanol is a
selective solvent for PAA. To study the dispersability of mixed
PAA/PS particles, ∼1.0 mg of PAA/PS nanoparticles was
ultrasonicated in 2 mL of CHCl3 and in 2 mL of CH3OH,
respectively. For comparison, PS particles (synthesized from
Y-initiator particles, the Mn and polydispersity of the free PS
were 18 700 and 1.14, respectively) and PAA particles (prepared
from PtBA-particle-2) were also ultrasonicated in these two
solvents. While PS particles were easily dispersed in CHCl3
but not in methanol, PAA particles were dispersed in CH3OH
but not in CHCl3. Mixed PAA/PS particles were found to be
dispersed in both CHCl3 and CH3OH, forming stable suspen-
sions. Figure 11 shows a simple Tyndall scattering experiment.
A strong light scattering was observed in the suspensions of
PS particles in CHCl3, PAA particles in CH3OH, and mixed
PAA/PS particles in both CHCl3 and CH3OH.
To obtain the molecular weight and the polydispersity of the
grafted PS, the polymer mixture cleaved from PtBA/PS particles
was hydrolyzed in a solution of NaOCH3 in THF/CH3OH (13:
1, v/v) as the Y-initiator contains a hydrolyzable ester bond.
The reaction was run for 7 days followed by removal of the
solvents in vacuum. Cyclohexane was used to extract PS from
the residual solid.58 1H NMR indicates that a pure PS was
obtained from the cyclohexane extract.56 GPC analysis (Figure
9d) shows that the Mn and Mw of the grafted PS were 23 800
and 26 300, respectively, essentially the same as those of free
polymer (Mn ) 23 000 and Mw ) 25 300) (Figure 9e). This
has confirmed the assumption in our previous publications that
the molecular weights and polydispersities of the grafted and
free polymers are identical.36-40
Preparation of Amphiphilic Mixed PAA/PS Particles from
Mixed PtBA/PS Particles and Their Responsive Properties
in Selective Solvents. Amphiphilic mixed PAA/PS brush-coated
nanoparticles were prepared from mixed PtBA/PS particles by
removing the tert-butyl group of PtBA with iodotrimethyl-
silane.51 A control experiment using a free PtBA showed that
the reaction was complete within 6 h as the peak located at
1.42 ppm in 1H NMR spectrum, which is attributed to the three
methyl groups of PtBA, disappeared, and the ratio of the integral
values of the peak at 2.27 ppm (-CH-) and the peaks at 1.50-
1.80 (-CH2) is 1:2.56 The reaction for the particles was run at
room temperature for 6 h. The IR spectrum shows that a strong
absorption appears at 1716 cm-1, while the carbonyl peaks of
To further study the chain reorganization of mixed PAA/PS
brushes on the particles in selective solvents, we performed 1H
NMR analysis. Figure 12 shows the 1H NMR spectra of mixed
PAA/PS particles in CDCl3, N,N-dimethylformamide-d7 (DMF-
d7), and CD3OD. DMF-d7 is a good solvent for both PAA and
PS. A drop of DMF-d7 was added into the particles prior to
CDCl3 and CD3OD to facilitate the chain reorganization and to
increase the concentration of the dispersed nanoparticles. For
the particles in DMF-d7, the peaks in the range of 6.0-7.4 ppm,
which are characteristic for PS, and the peak located at 2.45
ppm, which is attributed to the -CH- of PAA, were observed.
While the peak at 2.45 ppm disappeared in CDCl3 suspension,
indicating that the PAA chains collapsed and PS chains were
mobile, the aromatic hydrogen peaks were not present in the
CD3OD suspension, but the peak at 2.45 ppm remained visible,
PtBA particles and mixed PtBA/PS particles are at 1729 cm-1
.
The absorption at 2977 cm-1 was no longer present in the
spectrum, indicating that the tert-butyl was successfully cleaved.
The average diameter of mixed PAA/PS particles measured from
1
suggesting that the PS chains collapsed (a comparison of H
NMR spectra of mixed PAA/PS particles suspended in CD3-
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J. AM. CHEM. SOC. VOL. 127, NO. 17, 2005 6255

A R T I C L E S
Li et al.
1
by a Tyndall scattering experiment and H NMR study. These
well-defined organic-inorganic hybrid nanoparticles are an ideal
system for the study of the phase behavior of the two polymers
in a confined geometry. We are currently using differential
scanning calorimetry, transmission electron microscopy, and
other characterization techniques to study these hairy nanopar-
ticles. Moreover, these environmentally responsive hairy nano-
particles might find applications in the controlled release of
substances and as building blocks in the fabrication of nano-
structured materials and responsive materials.59
Acknowledgment. B.Z. thanks the University of Tennessee
at Knoxville (start-up funds), American Chemical Society
Petroleum Research Fund (PRF# 40084-G7), and Ralph E. Powe
Junior Faculty Enhancement Award for supporting this research.
Supporting Information Available: Experimental section
including materials, characterization, synthesis of silica nano-
particles, synthesis of Y-silane, immobilization of Y-initiator
on silica nanoparticles, surface-initiated ATRP of tBA, cleavage
of the grafted PtBA from the nanoparticles, dehalogenation of
PtBA particles, NMRP of styrene from dehalogenated PtBA
particles, cleavage of the grafted polymers from PtBA/PS
particles, hydrolysis of the polymer mixture cleaved from PtBA/
PS particles, hydrolysis of PtBA by use of iodotrimethylsilane,
synthesis of PAA/PS particles from PtBA/PS particles, Tyndall
scattering experiments, calculation of the diameters of PtBA
particles, PtBA/PS particles, and PAA/PS particles from TGA
data; 1H NMR spectrum of Y-silane; 1H NMR spectrum of free
Figure 12. 1H NMR spectra of PAA/PS particles dispersed in (a) CDCl3,
(b) DMF-d7, and (c) CD3OD. A drop of DMF-d7 was added into the particles
prior to CDCl3 and CD3OD to increase the concentration of the dispersed
nanoparticles.
OD and a free polymer of PAA in DMF-d7 can be found in the
Supporting Information56). These results were consistent with
the Tyndall scattering experiments shown in Figure 11. Both
studies have demonstrated that the mixed PAA/PS brushes can
undergo reorganization in different selective solvents, imparting
these hairy nanoparticles with a capability to respond to
environmental changes.
1
PtBA-1 taken at polymerization time t ) 547 min; H NMR
Conclusions
spectrum of poly(tert-butyl acrylate) cleaved from PtBA-
1
By sequential ATRP of tBA and NMRP of styrene from
Y-initiator-modified silica nanoparticles, we successfully pre-
pared mixed PtBA/PS brushes on silica particles. Kinetics
studies showed that the polymerizations were controlled. By
cleaving the grafted polymers off the nanoparticles, we found
that the molecular weights and polydispersities of both PtBA
and PS were essentially identical to those of free polymers
formed in the solutions from the free initiators. Amphiphilic
mixed PAA/PS particles were prepared from mixed PtBA/PS
particles by removing the tert-butyl group of PtBA with
iodotrimethylsilane. The obtained particles were found to form
stable suspensions in CHCl3, a selective solvent for PS, and in
methanol, a selective solvent for PAA, which were demonstrated
particle-3; H NMR spectrum of the polymer mixture cleaved
from PtBA/PS particles; 1H NMR spectrum of polystyrene
obtained from the cyclohexane extract; 1H NMR spectra of (a)
free poly(tert-butyl acrylate) in CDCl3 and (b) poly(acrylic acid)
in D2O obtained from hydrolysis of poly(tert-butyl acrylate) with
iodotrimethylsilane; 1H NMR spectra of (a) PAA free polymer
dissolved in DMF-d7 and (b) PAA/PS particles suspended in
CD3OD; complete refs 11b,c. This material is available free of
charge via the Internet at http://pubs.acs.org.
JA0422561
(59) (a) Russell, T. P. Science 2002, 297, 964-967. (b) Nath, N.; Chilkoti, A.
AdV. Mater. 2002, 14, 1243-1247.
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