Dynamic polymer brushes on the surface of silica particles

Article abstract of DOI:10.1039/c3ra40227d

Synthesis of cleavable PS brushes on the surface of silica particles was reported in this paper. Atom transfer radical polymerization (ATRP) initiator was anchored onto the surface of silica particles via dynamic imine bond, and PS brushes were prepared b

Full text of DOI:10.1039/c3ra40227d

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Dynamic polymer brushes on the surface of silica
particles3
Cite this: RSC Advances, 2013, 3,
7023
Jie Jin,a Jinchuan Liu,a Xueming Lian,a Pingchuan Sunb and Hanying Zhao*a
Synthesis of cleavable PS brushes on the surface of silica particles was reported in this paper. Atom transfer
radical polymerization (ATRP) initiator was anchored onto the surface of silica particles via dynamic imine
bond, and PS brushes were prepared by surface ATRP of styrene. PS brushes could be cleaved from the
surface of silica particles in the presence of trifluoroacetic acid. Based on the dynamic properties of imine
bonds, pyrene-labeled silica particles were prepared through dynamic exchange reaction between PS
brushes and 1-pyrenecarboxaldehyde. Moreover, silica particles with PS and fluorescein-labeled
poly(methyl methacrylate) mixed polymer brushes were synthesized by dynamic exchange reaction. This
research demonstrates that dynamic covalent chemistry provides an efficient method for the preparation
of ‘‘dynamic’’ polymer brushes on silica particles.
Received 3rd September 2012,
Accepted 1st March 2013
DOI: 10.1039/c3ra40227d
www.rsc.org/advances
known as Janus particles,17 have received much attention
due to their applications in colloidal science.18 Janus particles
Introduction
Over the past few decades, synthesis of polymer brushes on the
surface of inorganic nanoparticles has attracted much interest
due to the improvement of the properties of the materials.
Basically, there are two techniques to prepare polymer brushes
on the surface of nanoparticles: (1) the ‘‘grafting to’’ method,
where the polymer chains are grafted to the surface by reaction
with the functional groups on the nanoparticles, and (2) the
‘‘grafting from’’ method, where the polymer chains grow from
the surface of initiator-modified nanoparticles. Because of the
steric hindrance imposed by the grafted polymer chains in the
‘‘grafting to’’ method, polymer brushes with high grafting
density on the nanoparticles are prepared by the ‘‘grafting
from’’ method.1–3 Various polymerization methods, including
anionic polymerization,4 cationic polymerization,5 ring-open-
ing polymerization,6 reversible addition-fragmentation chain
transfer (RAFT) polymerization7 and atom transfer radical
polymerization (ATRP),8–11 have been used to prepare polymer
brushes on the surface of nanoparticles. Because of the better
control over the molecular structures, ATRP was used widely in
the preparation of polymer brushes.10,12–16
can be prepared by many different strategies, such as
modifying the two hemispheres step-by-step after immobiliz-
ing the particles on the substrate, Pickering emulsion method,
and electrohydrodynamic jetting in combination with a two-
phase side-by-side spinneret tip, etc.19 Another type of silica
particle with different polymer brushes was reported by Zhao
and co-workers.11 They anchored a Y-type initiator onto the
surface of silica, and synthesized PtBA/PS polymer brushes by
ATRP and nitroxide-mediated radical polymerization (NMP).
The molar ratio of PtBA chains to PS chains on silica was kept
at 1 : 1. Randomly distributed mixed polymer brushes were
also prepared on silica particles. Minko and co-workers
prepared mixed polymer brushes by free radical polymeriza-
tion.20
Dynamic covalent chemistry relates to chemical reactions
carried out reversibly under conditions of equilibrium
control.21 The formation and dissociation of the ‘‘dynamic’’
bonds can be controlled by the physical or chemical condi-
tions of the environment. For example, disulfide is responsive
to the redox conditions,22–26 alkoxyamine has an equilibrium
at high temperature,27–29 hydrazone and imine bonds have a
broken and reborn balance in acidic conditions,30–34 and a
combination of two of them can allow better control over the
structures and properties of the materials.35–37 Dynamic
covalent chemistry has been used in the synthesis of
functional polymers and nanostructures with dynamic proper-
ties. Lazny and co-workers reported the immobilization of
aldehydes and ketones onto the polymeric supports via
hydrazone.32 Fulton and co-worker synthesized diblock poly-
mers possessing amino or aldehyde functions within one of
their blocks by RAFT polymerization, and core cross-linked
Recently, preparation of silica particles with multi-func-
tional groups or different macromolecular brushes have
aroused a lot of interest for their potential applications in
many fields. particles with asymmetric morphology, also
aKey Laboratory of Functional Polymer Materials, Ministry of Education, Department
of Chemistry, Nankai University, 300071 Tianjin, China.
E-mail: hyzhao@nankai.edu.cn; Tel: +8622 23498703
bKey Laboratory of Functional Polymer Materials, Ministry of Education, Institute of
Polymer Chemistry, Nankai University, 300071 Tianjin, China
Electronic supplementary information (ESI) available. See DOI: 10.1039/
3
c3ra40227d
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star polymers can be formed by using the dynamic imine
bonds as the linker.33 Lehn and co-workers reported the
change of color and fluorescence at the interface of two
different polyhydrazone polymer films, induced by hydrazone
bond exchange and component recombination through the
interface.35
The synthesis of cleavable polymer brushes provides direct
characterization of the polymer brushes on solid substrates
and facile modifications of the surface of the solid sub-
strates,38 so the dynamic polymer brushes will find potential
applications in surface and medical sciences. Herein, we
report the preparation of the cleavable polymer brushes on
silica particles based on the dynamic properties of the imine
bond. An imine bond formed by the condensation of an amine
and an aldehyde group can be used as a dynamic covalent
bond. ATRP initiator was anchored onto the surface of the
silica particles by reaction of 4-formylphenyl 2-bromoisobutyr-
ylate (FPBIB) and amine-modified silica particles. After ATRP
of styrene, PS brushes grafted to the surface of silica particles
via imine bonds were prepared by the ‘‘grafting from’’ method,
and the structure of the polymer brushes was analyzed after
cleavage of the polymer brushes from solid substrates. In this
research, silica particles with PS brushes and pyrenyl groups
on the surface can be prepared through dynamic exchange
reaction between PS coated silica particles and 1-pyrenecar-
boxaldehyde. Scheme 1 shows an outline for the preparation of
cleavable PS brushes on silica nanoparticles and dynamic
exchange reactions.
Scheme 1 Schematic representation for the preparation of cleavable PS brushes
on silica particles, and dynamic exchange reactions between PS brushes and
1-pyrenecarboxaldehyde, and PS and aldehyde-terminated fluorescein-labeled
poly(methyl methacrylate) (f-PMMA).
Experimental
anhydrous DMF (20 mL) in a round-bottom flask, and excess
triethylamine (7.0 mL, 50.33 mmol) was added. The solution
was cooled in an ice bath, and 2-bromoisobutyryl bromide (6
mL, 48.5 mmol) was added slowly to the solution. After stirring
for 48 h, the white precipitate was filtered off, and the solution
was concentrated under reduced pressure. The concentrated
solution was further purified by silica gel column chromato-
graphy with a mixture of petroleum ether/ethyl acetate/
dichloromathane (10 : 1 : 1, v/v/v) as the eluent. The yield of
the product is about 33%. 1H NMR, d (CDCl3, TMS, ppm):
10.02(s, 1H); 7.97, 7.95(s, 2H); 7.34, 7.32(s, 2H); 2.11(s, 6H).
Materials
Tetraethyl orthosilicate (TEOS) was purchased from Tian Jin
Institute of Chemical Agents and was distilled before use.
Styrene (99%) was purchased from Tian Jin Institute of
Chemical Agents. It was purified by washing with aqueous
solution of NaOH, drying over MgSO4, and distilling under
reduced pressure. Triethylamine (TEA, Tianjin Chemical
Reagent Co., AR) was dried with CaH2 and distilled before
use. CuBr (99.5%) purchased from Guo Yao Chemical
Company, was purified by washing with glacial acetic acid
and dried in a vacuum oven at 100 uC. p-Toluenesulfonic acid
(p-TsOH, 99.5%) was purchased from Tianjin Jinke Chemical
Co.; 4-hydroxybenzaldehyde (99%) was purchased from J&K
Scientific Ltd.; fluorescein methacrylate (FMA, 97%),
Preparation of silica particles
The spherical silica particles used in this research were
40
¨
prepared by using the Stober method. To a 1000 mL flask
were added 500 mL of ethanol, 10 mL of water, and 36 mL of
ammonia solution (25%). After stirring at 40 uC for 2 h, 30 mL
of TEOS was added to the solution. The mixture was stirred at
40 uC overnight, and silica particles were prepared. The silica
particles were collected by centrifugation (10 000 rpm, 15 min)
and washed by ethanol and water for at least five times. The
particles were then dried at 110 uC under vacuum.
N,N,N9,N99,N99-pentamethyldiethylenetriamine
(PMDETA,
99%) and 3-(triethoxysilyl)propylamine were purchased from
Aldrich; 2-bromoisobutyryl bromide (98%), 1-pyrenecarboxal-
dehyde (99%) and trifluoroacetic acid (TFA, 99%) were
purchased from Alfa Aesar. All the above chemicals were used
as received.
Synthesis of 4-formylphenyl 2-bromoisobutyrylate (FPBIB)
Preparation of ATRP initiator-modified silica particles
4-Formylphenyl 2-bromoisobutyrylate was synthesized by
using
a
procedure similar to previous literature.39
In a round bottom flask, bare silica particles (2.15 g) were
dispersed in dry toluene after 30 min of sonication, and 3-
4-Hydroxybenzaldehyde (1.06 g, 8.68 mmol) was dissolved in
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(triethoxysilyl)propylamine (7.5 g) was added to the solution
dropwise. The solution was stirred at 95 uC for 22 h. After the
reaction, the particles were filtered and washed by ethanol
until no blue precipitants were observed upon addition of Cu2+
to the upper solution, which indicates that all the excess 3-
(triethoxysilyl)propylamine was removed.
In order to prepare ATRP initiator-modified silica particles,
the amine-modified silica particles (0.82 g) were dispersed in
10 mL of THF; after 1 h of sonication, p-TsOH (9.8 mg, 0.057
mmol) and FPBIB (0.10 g 0.38 mmol) were added into the
dispersion at room temperature. The reaction was conducted
at 70 uC for 20 h. After the reaction, the particles were collected
by centrifugation.
Dynamic reaction between PS-coated silica particles and
1-pyrenecarboxaldehyde
PS-coated silica particles (20 mg) were dispersed in 1 mL of
THF under sonication, and 1-pyrenecarboxaldehyde (0.0034 g,
1.5 6 1025 mol) was added to the dispersion. After stirring at
room temperature for 30 min, 0.1 mL of TFA was added to the
dispersion. The dynamic exchange reaction was conducted at
50 uC for 24 h. The silica particles were collected by
centrifugation (10 000 rpm, 15 min), washed by THF and
dried under vacuum.
Dynamic reaction between PS-coated silica particles and
aldehyde-terminated f-PMMA
PS-coated silica particles (8.60 mg) were dispersed in 1 mL of
THF under sonication, and then 0.3 mL of TFA was added to
the dispersion. After stirring at room temperature for 30 min,
aldehyde-terminated fluorescein labeled PMMA (25.4 mg) was
added to the dispersion. After stirring at room temperature for
4 h, TEA was added to stop the reaction. The silica particles
were collected by centrifugation (10 000 rpm, 15 min), washed
by THF and dried under vacuum.
ATRP of styrene on the surface of silica particles
ATRP initiator-modified silica particles (30 mg), styrene (4.0
mL, 35 mmol) and toluene (2 mL) were mixed in a Schlenk
flask, the dispersion was degassed by one freeze-pump-thaw
cycle. CuBr (2.8 mg, 0.019 mmol) and PMDETA (4.0 mL, 0.019
mmol) were dissolved in 2 mL of dry DMF, and 0.2 mL of the
solution was added into the dispersion under nitrogen
atmosphere. The solution was degassed by three freeze-
pump-thaw cycles. The surface ATRP of styrene was conducted
at 110 uC for 24 h. The polymerization was stopped by
exposure of the solution to air.
Preparation of pickering emulsion
Silica particles with PS and f-PMMA mixed polymer brushes
(1.0 mg) were dispersed in 1 mL of decalin by sonication, and
then 0.1 mL of distilled water was added into the dispersion
forming a stable Pickering emulsion.
Preparation of aldehyde-terminated f-PMMA
Fluorescein-labeled PMMA was synthesized by ATRP of MMA
and FMA using FPBIB as initiator. PMDETA (35 mL, 0.167
mmol) was dissolved in 2 mL of MMA monomer (18.83 mmol),
and then CuBr (0.023 g, 0.16 mmol) was added under nitrogen
atmosphere. After stirring for 15 min, the solution was added
to a mixture of toluene (2 mL), FPBIB (0.052 g, 0.19 mmol) and
FMA (0.0141 g, 0.0353 mmol) under nitrogen atmosphere. The
solution was degassed by three freeze-pump-thaw cycles, and
ATRP of MMA and FMA was conducted at 70 uC for 6.5 h. The
polymerization was stopped by exposure of the solution to air.
After removal of solvent under reduced pressure, copper ions
were removed with neutral alumina column chromatography
by using CH2Cl2 as eluent. The product was isolated by
pouring the polymer solution into cold diethyl ether. After
filtration, the polymer was dried under vacuum. The average
molecular weight and PDI of the polymer are 15 k and 1.11,
respectively. GPC trace of the polymer is shown in Fig. S1
Results and discussion
A scheme for the synthesis of cleavable PS brushes on silica
particles is shown in Scheme 1. FPBIB was synthesized by a
reaction between 4-hydroxybenzaldehyde and 2-bromoisobu-
tyryl bromide.39 Chemical structure and H NMR spectrum of
1
FPBIB were shown in Fig. S2, ESI. Spherical silica particles
3
with an average diameter of 150 nm were prepared by using
40
¨
the Stober method. Amino groups modified silica particles
were prepared by reaction of the silica particles and 3-
(triethoxysilyl)propylamine. The elemental analysis result
shows that the nitrogen content is 0.45 wt%. ATRP initiator
was anchored onto the silica particles through dynamic imine
bond after a reaction between aldehyde groups of FPBIB and
the primary amino groups on silica particles. The bromine
content measured by ionic chromatogram was 0.504 wt%.
Thermogravimetric analysis (TGA) curves of original,
amine-modified, and ATRP initiator-modified silica particles
are presented in Fig. 1. They are found to have 7.2 and 8.7 wt%
volatile materials on amine-modified and ATRP initiator-
modified silica particles, so after reaction, 1.5 wt% FPBIB is
attached to the silica particles. A TEM image of the original
silica particles is shown in Fig. 2(a). The TEM result shows that
the average size of the original silica particles is about 150 nm.
PS polymer brushes on the silica particles were prepared by
in situ ATRP. The possible free PS was removed by washing
with THF and followed by repeated sonication and centrifuga-
tion cycles. After polymerization, PS was successfully grafted to
(ESI ).
3
Cleavage reaction of PS polymer brushes
The cleavage reaction of PS-coated silica particles was
conducted using a method similar to previous literature.3 PS-
coated silica particles (0.015 g) were dispersed in 1 mL of THF
under sonication, and TFA (0.4 mL) was added to the
dispersion. The dispersion was stirred at 50 uC for 24 h. The
silica particles were collected by centrifugation (10 000 rpm,
15 min), washed by THF and dried under vacuum. After
separation, an equimolar amount of triethylamine was added
to the cleaved polymer solution to neutralize TFA. The cleaved
PS was concentrated by evaporating the solvent, and solid PS
was obtained by adding the polymer solution into 6-fold of
methanol.
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Fig. 3 Size distributions of ATRP initiator-anchored silica particles (curve a), PS-
Fig. 1 TG thermograms of unmodified silica particles (curve a), amine-modified
silica particles (curve b), ATRP initiator-anchored silica particles (curve c), PS-
coated silica particles (curve d), silica particles after cleavage reaction (curve e)
and silica particles with mixed polymer brushes (curve f).
coated silica particles (curve b) and silica particles after cleavage reaction (curve
c).
modified silica particles is shown in Fig. 3 (curve a). The
modified silica particles possess a unimodal size distribution
with a hydrodynamic diameter range from 129 to 345 nm.
However, after surface ATRP, the hydrodynamic diameter of
the particles ranges from 140 to 748 nm with an average value
at about 290 nm (Fig. 3). In comparison with ATRP initiator-
modified silica particles, the average size increased about 77
nm. Therefore, the surface ATRP of styrene was successfully
conducted on the silica particles.
the surface of silica particles as proved by TGA, DLS and TEM.
The TGA curve of silica particles with PS brushes is shown in
Fig. 1 (curve d). After ATRP of styrene, it is found to have
y23.6 wt% weight loss, which means the weight percentage of
PS grafted to the surface of silica particles is about 14.9 wt%.
The grafting density of PS is calculated to be about 1.08 chains
nm22. A TEM image of the silica particles after surface ATRP
was presented in Fig. 2 (b). On the image, PS brush layers on
the silica particles are observed clearly, and the thickness of
the PS layer ranges from 8 to 10 nm. DLS results also prove the
success of the polymerization. DLS curve of ATRP initiator-
In order to investigate the dynamic properties of the
polymer brushes, PS brushes were cleaved from the surface of
silica particles in an acidic environment. The cleavage reaction
was conducted by dispersing the PS-coated particles into a
mixture of THF/trifluoroacetic acid (TFA) (v/v, 1 : 0.4) via
vigorous mixing. After stirring at 50 uC for 24 h, the particles
were removed from the cleaved polymer chains by centrifuga-
tion and repeated washing with THF. The cleaved PS was
characterized by GPC (Fig. 4) and 1H NMR (Fig. S3 in ESI ). The
3
GPC result shows that the number average molecule weight of
PS is about 11 k and the molecular weight distribution is about
1.29. TGA curve of silica particles after cleavage of PS is
Fig. 2 (a) A TEM image of the original silica particles prepared by the Stober
¨
method; the average diameter is about 150 nm. (b) A TEM image of PS coated
silica particles. (c) A TEM image of silica particles after cleavage reaction.
Fig. 4 GPC trace of PS cleaved from the surface of PS-coated silica particles.
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presented in Fig. 1 (curve e). TGA results show that the weight
loss of the silica particles after cleavage reaction is about 16.5
wt%, which is smaller than the silica particles before cleavage
reaction (curve d in Fig. 1). After the reaction, the grafting
density of PS decreased from 1.08 to 0.58 chains nm22. This
result means not all PS chains are cleaved from the silica
surface due to the dynamic property of the imine bond in the
acidic environment.41 A TEM image of the silica particles after
cleavage of PS is shown in Fig. 2 (c). In comparison to the PS-
coated silica particles, the disappearance of the polymer layers
on silica particles indicates the cleavage of PS chains from the
silica surface and the decrease of the grafting density of PS
brushes. In the meanwhile, PS phases cleaved from the silica
particles are observed on the TEM image. The DLS result also
supports the successful cleavage of PS from the silica particles.
As shown in Fig. 3 (curve c), after the cleavage reaction, the size
distribution of silica particles is shifted to a smaller size range.
The change in size is attributed to the cleavage of PS from the
silica surface.
Dynamic covalent chemistry is based on reversible covalent
bonds, and the formation and dissociation of the bonds can be
controlled through environmental change, so dynamic covalent
chemistry not only provides a method to prepare cleavable
polymer brushes on silica particles but also provides a method
to prepare surface-labeled functional particles. The amino
groups produced after dissociation of imine groups can be used
in the formation of new imine bonds, and new functional groups
or polymer chains can be grafted to the surface of the silica
particles. In this research 1-pyrenecarboxaldehyde was added
into the dispersion of PS-coated silica particles in the presence of
TFA, and pyrene-labeled silica particles were prepared. On the
surface of particles both PS chains and pyrenyl groups were
grafted to the silica particles via imine bonds (Scheme 1).
The UV-vis absorption (spectrum a) and fluorescence
emission spectrum (spectrum b) of the pyrene-labeled silica
particles are shown in Fig. 5. In the UV-vis spectrum, the
absorptions of PS and pyrenyl groups are observed. The
Fig. 6 1H NMR spectrum of PS and f-PMMA mixed polymer brushes cleaved
from silica particles. The spectrum was recorded in CDCl3.
amount of pyrenyl groups on silica particles was determined
quantitatively by comparing the absorption at 343 nm for the
pyrene-labeled silica particles to a standard absorption curve
made from known amounts of 1-pyrenecarboxaldehyde in THF
(Fig. S4, ESI ). Based on the UV-vis results, the content of
3
pyrenyl groups on silica particles was calculated to be about
0.05 mmol g21. The emission spectrum of the silica particles
shows the typical monomer emission peaks at 377, 383, 388,
397 and 418 nm. The UV-vis and fluorescence results
demonstrated that the fluorescence-labeled silica particles
were successfully prepared by the dynamic exchange reaction
between polymer-modified silica particles and 1-pyrenecarbox-
aldehyde.
Silica particles with mixed polymer brushes were also
prepared by dynamic exchange reaction between PS-coated
silica particle and aldehyde-terminated f-PMMA. f-PMMA was
synthesized by ATRP of MMA and FMA by using FPBIB as
initiator.42 The mixed polymer brushes were cleaved by TFA in
THF solution, and purified by precipitating in methanol. Fig. 6
shows the 1H NMR spectrum of the mixed polymer brushes
dissolved in deuterated chloroform. On the spectrum, the
signals representing the phenyl protons on PS chains and
methyl protons on f-PMMA were observed, which demonstrated
the successful synthesis of mixed polymer brushes on silica
particles. TGA results indicated that in comparison to PS-coated
silica particles, the weight loss of the silica particles with mixed
polymer brushes increased by about 5.6 wt% (curve f in Fig. 1).
Fluorescein is a fluorophore commonly used as a fluorescent
tracer for many applications. After grafting of fluorescein-
labeled PMMA onto the surface of silica particles, the silica
particles will be able to be tracked by using a fluorescence
microscope. Because the average size of the silica paerticles is
about 150 nm, it is impossible to observe the isolated particles
on a fluorescence microscope, however, it is possible to observe
aggregation or self-assembly strutures of the particles. In a
mixture of water and oil, solid particles are able to self-assemble
at the liquid–liquid interface and a Pickering emulsion is
prepared. In this research, silica particles with PS and f-PMMA
mixed polymer brushes were dispersed in 1 mL of decalin by
sonication, and then 0.1 mL of distilled water was added into
the dispersion forming a stable Pickering emulsion. In the
Pickering emulsion, decalin forms the continuous phase, and
water forms the discontinuous phase. The water droplets are
Fig. 5 The UV-vis absorption (a) and fluorescence emission spectrum (b) of the
silica particles after dynamic reaction with 1-pyrenecarboxaldehyde in THF.
Excitation wavelength was set at 343 nm.
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Fig. 7 A fluorescence microscope image of a Pickering emulsion prepared by
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fluorescence microscope. In the image, water droplets stabi-
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Conclusions
In summary, we have demonstrated a practical route for the
synthesis of dynamic polymer brushes, which permits post
polymerization analysis. Based on the the dynamic properties
of imine bonds, pyrene-labeled silica particles and silica
particles with mixed polymer brushes were prepared by
dynamic exchange reaction. This research offers a straightfor-
ward method to characterize polymer brushes on the surface
of solid substrates and prepare multi-functional silica particles
with diverse properties.
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Acknowledgements
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This project was supported by the National Natural Science
Foundation of China (NSFC) under Contracts 20974047 and
21074058, and National Basic Research Program of China (973
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