Preparation of organic/inorganic hybrid hollow particles based on gelation of polymer vesicles

Article abstract of DOI:10.1021/ma0497097

In a previous communication, we have reported the study of novel organic/inorganic hollow particles based on a reactive amphiphilic diblock copolymer, poly(ethylene oxide)-block-poly[3-(trimethoxysilyl)propyl methacrylate] (PEO-b-PTMSPMA). The preparation

Full text of DOI:10.1021/ma0497097

5
710
Macromolecules 2004, 37, 5710-5716
Preparation of Organic/Inorganic Hybrid Hollow Particles Based on
Gelation of Polymer Vesicles
J ia n zh on g Du a n d Yon gm in g Ch en *
State Key Laboratory of Polymer Physics and Chemistry,
J oint Laboratory of Polymer Sciences and Materials, Institute of Chemistry,
The Chinese Academy of Sciences, Beijing 100080, People’s Republic of China
Received February 11, 2004; Revised Manuscript Received May 10, 2004
ABSTRACT: In a previous communication, we have reported the study of novel organic/inorganic hollow
particles based on a reactive amphiphilic diblock copolymer, poly(ethylene oxide)-block-poly[3-(trimethoxy-
silyl)propyl methacrylate] (PEO-b-PTMSPMA). The preparation of these novel particles involved the
preformed vesicles of the block copolymers in a methanol/water solvent mixture, followed by a gelation
process to fix the vesicular morphology. In this paper, the detailed conditions for the preparations of the
hybrid vesicles, including water contents in the binary solvent, initial copolymer concentration (Cini) in
methanol, compositions of the diblock copolymers, and gelation catalyst, have been explored in great
detail. The results demonstrated that the robust vesicles could be prepared under a variety of conditions,
such as over a broader range of the water content (from ca. 34 to 98.4 wt %) at the Cini of 5.0 mg/mL for
PEO45-b-PTMSPMA59. Furthermore, exclusive vesicles were formed under the Cini ranging from 0.5 to
2
0 mg/mL of PEO45-b-PTMSPMA59 in methanol while the water content was kept constant at 55.8 wt %,
whereas the average size of vesicles and the size distribution increased correspondingly. Before the
exclusive vesicles appeared, spheres, short rods, and lamellae were observed as the coexisted morphologies
when the water content increased gradually. For the block copolymers with a constant PEO length and
different PTMSPMA lengths, i.e., PEO45-b-PTMSPMA (x ) 29, 42, and 180), only vesicles were produced
x
under the conditions applied with the longer PTMSPMA blocks resulting in a thicker vesicle wall. The
gelation catalyst plays a critical role in the morphological fixation. We have found that triethylamine
(TEA) was the best gelation catalyst under the conditions used in this study, while the acid catalysts
destroyed the vesicles. In addition, because of the inorganic components, the nanocapsule was very stable,
and its morphology remained even after calcination at 450 °C.
1
. In tr od u ction
these vesicles were the aggregates of many polymer
chains and were destroyed easily when the environment
changed. If some repeat units in one of block copolymer
segments bear reactive groups, the resulted vesicles can
be fixed by chemical cross-linking. Such hollow nano-
particles are shape-persistent against environmental
Self-assembly of block copolymers has attracted an
increasing interest for applications as nanomaterials
and for nanofabrications. It is known that amphiphilic
diblock copolymers can form micelles by self-assembling
in selective solvents, such as polar solvents for hydro-
philic blocks.
of the micelles include spheres, wormlike rods or
tubules, and vesicles. The morphologies depended on
various factors, such as the composition of the block
1
1
-3
change. There are a few examples in recent years to fix
The commonly observed morphologies
the polymer vesicles by chemical cross-linking.1
2-15
One
typical example from Liu et al. involved the self-
assembly of the polyisoprene-block-poly(2-cinnamoyl-
ethyl methacrylate) diblock copolymer in hexane/THF.15
By exposure of the vesicles in solution to UV light, the
domains containing cinnamoyl groups were cross-linked
by photodimerization, and the stable vesicles were
obtained. However, those nanocapsules had purely
organic structures originating from their precursor
copolymers.
copolymers, the chain architectures, and the conditions
of the aggregation.2
-16
Among these versatile micelles,
the vesicles that enclose a hollow cavity by a bilayer
membrane are of particular interest due to their poten-
tial applications in encapsulation and release.
addition, it needs to lay emphasis on applying water as
the medium of self-organizing block copolymers since
water is required for biomedical applications and is also
environment friendly. The state of the art of the polymer
vesicles has been summarized in some recent re-
8
,15c
In
It is known that the inorganic structures are more
stable compared to the organic ones. The hollow silica
or polyorganosiloxane nanoparticles were prepared
mainly by template approaches in the literature. Schacht
et al. have prepared the hollow silica spheres in the oil/
water emulsions, which resulted from an interfacial
9
,10
views.
It is of note that the compositions and the
structures of the block copolymers are very important
for the preparations of polymer vesicles as seen in the
limited successful examples. The highly asymmetric
coil-coil block copolymers with a long core-forming
block were the main precursors to produce the vesicles
1
7
reaction. Pinnavaia et al. introduced the silica into the
interlayer regions of the multilamellar vesicles to form
nearly spherical particles with stable lamellar meso-
1
0b
11,12
18
reported in the literature; a few coil-rod
and coil-
structures. The ultrastable mesostructured silica
dendron16 block copolymers were reported to form
vesicles as well in some recent publications. However,
vesicles were also reported by the same group using the
neutral gemini surfactants as the structure-directing
1
9
agents. An organic/inorganic hybrid vesicle was pre-
pared by the gelation in the vesicles given by a lipid-
bearing alkoxysilyl head, and a hierarchical vesicle
*
Corresponding author: phone +0086-10-62659906; Fax +0086-
1
0-62559373; e-mail ymchen@iccas.ac.cn.
1
0.1021/ma0497097 CCC: $27.50 © 2004 American Chemical Society
Published on Web 06/19/2004

Macromolecules, Vol. 37, No. 15, 2004
Organic/Inorganic Hybrid Hollow Particles 5711
Sch em e 1. Sch em a tic of P r ep a r a tion of Hybr id Vesicles by Self-Assem bly of Rea ctive Block Cop olym er sa
a
The yellow part stands for the PEO, the blue part for the PTMSPMA, and the red part for the polyorganosiloxane from the
gelation process.
assembly was made through a layer-by-layer ap-
proach.20 The colloids have been used as the templates
for forming the inorganic shells. The linear polysiloxane
colloids as the templates formed in situ in an emulsion
polymerization were further coated with a cross-linked
polysilsesquioxane shell by the gelation of trimethoxy-
methylsilane, and the hollow particles were formed
2
1a,b
through extracting the soluble core by a solvent.
Functional molecules such as dyes were encapsulated
in the siloxane network and the cavity of the hollow
2
1c
particles. In a very recent report, the hybrid core-
shell particles were prepared by coating the surfaces of
monodisperse polystyrene beads with uniform silica
2
2
shells. Hollow silica spheres could be obtained by
removing the polymer cores via calcination or by wet
etching with toluene. The surfaces of block copolymer
micelles have also been coated with a silica layer by
reacting with an active silicate, making use of few
trimethoxysilane groups born on the corona-forming
blocks of the micelles.23
F igu r e 1. SEM image of the vesicles from PEO45-b-
PTMSPMA59. Conditions: initial copolymer concentration
in methanol is 10 mg/mL, water content is 91.0 wt %, and
the TEA concentration is 0.04 wt %.
Our research focuses on the preparation of soluble
nanocapsules with an organic polymer/polyorganosilox-
ane hybrid structure through self-assembly of block
copolymers. Recently, we have reported the preliminary
results on preparing such hollow nanoparticles using a
novel reactive diblock copolymer, poly(ethylene oxide)-
block-poly[3-(trimethoxysilyl)propyl methacrylate] (PEO-
b-PTMSPMA), which was synthesized by atom-transfer
radical polymerization (ATRP).2 The approach for
preparing this nanocapsule and its structure are il-
lustrated in Scheme 1. It is known that PEO is a
hydrophilic polymer while the PTMSPMA with many
Ta ble 1. Ch a r a cter istics of P EO-b-P TMSP MA Diblock
Cop olym er s Used for P r ep a r a tion of Hybr id Vesicles
compositiona
Mnb
Mw/Mnb
45/29
45/42
8 200
11 600
13 800
45 000
1.25
1.34
1.28
1.10
4
4
4
5/59
5/180
a
The number of repeat unit of EO/TMSPMA, obtained from 1H
NMR in CDCl3. b Obtained from SEC.
-Si(OCH3)3 groups is hydrophobic and readily form
cross-linked polysiloxane by the gelation process. PEO45-
b-PTMSPMA59 diblock copolymers form polymer vesicles
in a binary solvent mixture of methanol and water with
the PEO blocks as the corona and the PTMSPMA blocks
factors to the free energy of aggregation.9,10 Many
factors, such as preparation condition, common solvent,
and copolymer composition, may influence the curvature
of the hydrophobic-hydrophilic interfaces of the self-
assembly from the block copolymers with a given
structure. In this work, four diblock copolymers were
used: PEO45-b-PTMSPMAx, x ) 29, 42, 59, and 180 (the
1
24
as the wall, as confirmed by H NMR analysis. After
the addition of a gelation catalyst, triethylamine (TEA),
which only plays a role to fix the vesicular morphology,
the shape-persistent hybrid vesicles with a cross-linked
polysilsesquioxane wall were obtained. A typical scan-
ning electron microscope (SEM) of the hybrid vesicles
is shown in Figure 1, which gives a clear three-
dimensional vesicular morphology. The formation of
these unique vesicles by this block copolymer was rather
remarkable. In this paper, we had studied how the
preparation conditions and the copolymer compositions
influenced the formation of these novel hybrid vesicles.
1
composition was determined by H NMR in CDCl3,
which is a good solvent for both segments) as shown in
Table 1. The solutions of the vesicles based on PEO-b-
PTMSPMA were made by adding water into a block
copolymer solution of methanol. The gelation catalyst,
TEA, was added 5 h after the vesicles formed. The
following factors for the vesicle formation were ex-
plored: (1) the solvent composition in the binary solvent
mixture of water and methanol; (2) the initial copolymer
concentration (Cini) in methanol; (3) the PTMSPMA
length of the block copolymers; (4) the effect of catalyst
and gelation time. Finally, the mechanism of vesicle
formation and the structure of vesicles were discussed
briefly.
2
. Resu lts a n d Discu ssion
Depending upon the interfacial curvature between the
core and the corona of the micelles, spheres, rods, and
vesicles could be formed as a result of the balance of all

5
712 Du and Chen
Macromolecules, Vol. 37, No. 15, 2004
F igu r e 2. Plot of the turbidity of PEO45-b-PTMSPMA59 in
methanol against the water contents. The initial copolymer
concentrations in methanol are 1.0, 5.0, and 20.0 mg/mL.
2
.1. Vesicle F or m a tion u p on P r ep a r a tive Con d i-
tion s. 2.1.1. Effect of Wa ter Con ten t. The composi-
tion of binary solvents has been shown to have a great
influence on the morphology of the crew-cut micelles as
reported in the literature.3 In this section, such an
effect was explored for the vesicle formation of PEO-b-
PTMSPMA.
,10
Turbidity studies were performed to explore the
possible transition of different morphologies during self-
assembling with increasing of water content based on
PEO45-b-PTMSPMA59. We performed this measurement
without adding TEA at three different Cini’s in metha-
nol: 1.0, 5.0, and 20.0 mg/mL. Figure 2 shows the
turbidity against the weight fractions of water at
ambient temperature. All the three curves show a
similar evolution trend, i.e., having a maximal turbidity
value and some inflections as the water content in-
creases. As indicated by the appearance of turbidity in
the solution, the aggregations of the block copolymers
generally occurred at ca. 20 wt % water content. Further
addition of water into methanol led to the turbidity
increasing gradually, implying the transformation of the
micelle structure or the size increase of the micelles.
Then the turbidity declined after reaching the maximum
as the water content reached some level, possibly due
to the decrease of the micelle concentrations or the sizes.
For the curve at 5.0 mg/mL of Cini, there are two
obvious jumps at ca. 16 and 32 wt % water, respectively.
The latter step corresponded to the vesicle formation
as indicated by transmission electron microscopy (TEM)
analysis shown below. At 1.0 mg/mL of Cini, it was a
relatively smooth curve over the entire range of water
contents. The turbidity increased gradually as the water
content increased then declined at ca. 35 wt %. For 20.0
mg/mL of Cini, only one slight jump appeared at 44 wt
F igu r e 3. TEM images of the gelated polymer micelles from
PEO45-b-PTMSPMA59 at different water contents in the solvent
mixture. The initial copolymer concentration in methanol is
5
.0 mg/mL. (A) irregular objects and few vesicles, 31.3 wt %;
(B) vesicles, coexisted with spheres and lamellae with some
protruding rods, 33.6 wt %; (C) vesicles, 38.7 wt %; (D) vesicles,
48.7 wt %. TEM images of vesicles at higher water contents
are not presented.
for PEO -b-PTMSPMA diblock copolymer. The mor-
4
5
59
phological transition points agreed with the turbidity
study. Exclusive vesicles were observed from ca. 40 to
98.4 wt % water content. A few vesicles that coexisted
with spheres, rods, and lamellae were found at 33.6 wt
% water content. It was of note that it was difficult to
find recognizable particles in the TEM window when
the water content was less than 30 wt %.
Figure 4A-E shows the aggregates after gelation at
different water contents when PEO45-b-PTMSPMA42
diblock copolymer was used as the precursor. A more
obvious effect of the water content on the morphological
change was observed compared with that of PEO45-b-
PTMSPMA59. As shown in Figure 4A, spheres coexisted
with some short rods at 31.3 wt % water. The diameter
of the spheres was 19 ( 1 nm, the same as the rod
diameter. When slightly increasing water to 33.6 wt %,
the lamellae appeared and coexisted with some protrud-
ing rods and separated rods, as shown in Figure 4B. At
this point, the vesicles emerged. One may notice the
diameter of rod is the same as the wall thickness of
vesicle (20 ( 2 nm), slightly thicker than the wall
thickness (∼16 nm) of vesicles that formed at higher
water content. Apparently, this figure had captured the
transition intermediate, i.e., from the rods, the lamellae,
to the vesicles. The content of vesicles became more at
38.7 wt % water; however, the lamellae could still be
found (Figure 4C). From 48.7 wt % water content,
exclusive vesicles were obtained, as demonstrated in
Figure 4D,E. Up to 98.4 wt % water, the vesicles were
still obtained exclusively (TEM images not shown).
Compared with PEO45-b-PTMSPMA59, it appeared that
%
water. The maximum appeared at ∼65 wt % water,
which was higher than the other two cases.
TEM studies based on two diblock copolymers, PEO45-
b-PTMSPMA59 and PEO45-b-PTMSPMA42, were per-
formed to examine the effect of water contents on the
morphological development. Water was added dropwise
into the polymer solution of 5.0 mg/mL in methanol with
vigorous stirring. TEA (0.4 wt %) was added 5 h later
to fix the shape by the gelation. The morphology of the
hybrid particles was examined by TEM several days
later.
Illustrated in Figure 3 were the TEM images of the
gelated aggregates prepared at different water contents

Macromolecules, Vol. 37, No. 15, 2004
Organic/Inorganic Hybrid Hollow Particles 5713
F igu r e 4. TEM images of the gelated polymer micelles from PEO45-b-PTMSPMA42 at different water contents. The initial copolymer
concentration in methanol is 5.0 mg/mL. (A) spheres, coexisted with few rods, 31.3 wt %; (B) lamellae with protruding rods and
a few vesicles, 33.6 wt %; (C) vesicles and lamellae, 38.7 wt %; (D) vesicles, 48.7 wt %; (E) vesicles, 55.8 wt %; (F) vesicles in (E)
stained with uranyl acetate for 8 min. TEM images of vesicles at higher water contents are not presented.
F igu r e 5. TEM images of the gelated vesicles prepared from PEO45-b-PTMSPMA59 at different initial copolymer concentrations
in methanol (the water content is 55.8 wt %). (A-F) 0.1, 0.5, 1.0, 2.0, 5.0, and 10.0 mg/mL in methanol. TEM image of vesicles
from Cini ) 20 mg/mL are not presented.
longer PTMSPMA was favorable for the vesicle for-
mation. The diameters of vesicles from PEO45-b-
PTMSPMA42 decreased gradually from 52 ( 8 to 38 (
expected because the cross-linked walls were covered
by the PEO layer as well as some free -SiOH groups,
which have been stained by uranyl acetate.
9
9
nm when the water content increased from 43.1 to
8.4 wt % (Figure S1 in the Supporting Information).
2.1.2. Effect of th e Cin i. The effects of Cini on the
vesicle formation were explored by varying the Cini over
a broad range, while the water content was kept
constant at 55.8 wt % and the TEA concentration for
gelation was 0.4 wt %. Illustrated in Figure 5 were the
TEM images of the vesicles prepared from PEO45-b-
PTMSPMA59 at the Cini ) 0.1, 0.5, 1.0, 2.0, 5.0, 10.0,
and 20.0 mg/mL in methanol after adding TEA. Exclu-
sive vesicles were produced at all the Cini’s used. The
size of the vesicles at low Cini was relative smaller, and
the size distribution was narrower. The quality of the
The wall thickness is ca. 12 nm and remains constant
for the vesicles under all conditions.
One vesicle sample (Figure 4E) was further stained
with saturated uranyl acetate ethanol solution, as
illustrated in Figure 4F. The boundary between the
outer dark loop and the wall was very clear, so was the
boundary between the inner void and the wall. The
thickness of the wall was 15.7 ( 1.1 nm, ∼4.0 nm
thicker than that of the unstained ones. This result was

5
714 Du and Chen
Macromolecules, Vol. 37, No. 15, 2004
vesicle walls in the TEM images was better when the
Cini was higher than 0.5 mg/mL. For example, as shown
in Figure 5C, the high-quality vesicles with a narrow
size distribution were obtained at 1.0 mg/mL of Cini, and
their diameters were ∼90 nm. The average vesicle sizes
increased with the Cini, whereas the polydispersities
were broadened correspondingly. The contour became
irregular for the larger vesicles due to the collapse of
the vesicles out of solution during TEM analysis. The
larger the vesicles were, the more likely they collapsed.
However, the wall thickness was uniform (around 16
nm), as a character of the bilayer polymer vesicles
through self-assembly. Figure S2 (in the Supporting
Information) shows the dependence of the mean size and
the wall thickness of the vesicles on the Cini.
wall thickness were all increased slightly if the time
was over 1 day (Figure S3 in the Supporting Informa-
tion). It can be explained by the partial hydrolysis of
-Si(OCH3)3 groups in the aqueous media. The hybrid
vesicles were formed after 1 month even without
catalyst, unlike the soft vesicles newly formed without
the addition of TEA. In an extreme case, a vesicle
solution prepared from PEO45-b-PTMSPMA42 has been
left more than 1 year in the absence of catalyst (Figure
S4 in the Supporting Information). This result indicates
that the vesicles can be fixed by the gelation even
without catalyst because the polycondensation occurs
quickly once the partial hydrolysis happens.
Once TEA was added after the vesicles formed for
several hours, the hybrid vesicles could be observed
easily by TEM. No difference had been found in terms
of the sizes and wall thickness of the vesicles regardless
the time of TEM analysis. It seems that the quality of
the TEM image of the vesicles with shorter gelation time
was not as good as those with longer gelation time. This
might be owing to the lower cross-linking degree of the
vesicle walls in a short time. The concentrations of TEA
showed no obvious effect on the gelated vesicles as
studied by TEM, either. (Figure S5 in the Supporting
Information).
2
.1.3. Effect of P TMSP MA Block Len gth . In
general, the highly asymmetric composition is a pre-
condition for a coil-coil block copolymer to form the
9
,10
vesicles in selective solvents for the short block.
So
far, we have demonstrated that PEO45-b-PTMSPMA42
and PEO45-b-PTMSPMA59 could form vesicles under a
variety of conditions. In this part, the degree of poly-
merization of TMSPMA other than 42 and 59, i.e., 29
and 180, were examined whereas the PEO blocks
remained the same. To prepare micelles, the Cini in
methanol was 2.0 mg/mL for PEO45-b-PTMSPMA29, and
the Cini was 1.0 mg/mL for PEO45-b-PTMSPMA180. The
water content was 55.8 wt %, and TEA concentration
for gelation was 0.4 wt %.
A small amount of acid such as hydrochloric acid,
trifluoroacetic acid, or acetic acid was added into the
vesicle solution. However, the characteristic blue tint
of the vesicle solution disappeared quickly; instead,
insoluble gels were obtained. Only a trace of acetic acid
TEM analyses have shown that exclusive vesicles
were obtained for the block copolymers with the short
and the long TMSPMA blocks under the conditions
applied. In comparison with the results of PEO45-b-
PTMSPMA42 and PEO45-b-PTMSPMA59, the wall thick-
ness of the vesicles increased from 11.4 to 12.2, 16.9,
and 21.7 nm as the degrees of polymerization of
TMSPMA increased from 29 to 42, 59, and 180. This
observation is consistent with the bilayer structure of
vesicles. Therefore, to adjust the length of PTMSPMA
block is an efficient way to control the wall thickness of
vesicles. The vesicle diameters from four samples were
(
pH is very close to 7 in the final solution) was needed
to obtain an unsoiled hybrid vesicle (see Supporting
Information for details). This result was very different
from that of TEA, which works well to fix the vesicle
morphology. In general, both acidic and alkali catalysts
are effective for the hydrolysis and polycondensation of
-Si(OCH3)3, but the catalytic mechanisms are different.
The acid catalyzes the hydrolysis quickly while rela-
tively slowly for the polycondensation and may switch
the polarity of PTMSPMA blocks from the hydrophobic
to the hydrophilic. The vesicular morphologies would
be destroyed due to the rapid hydrolysis before the
polycondensation process starts; as a result, macroscopic
gelation happened. However, the hydrolysis reaction
catalyzed by the base is slower whereas the polycon-
densation is faster. Once the partial hydrolysis reaction
occurred in the PTMSPMA domain, the polycondensa-
tion would proceed simultaneously. As a result, the
vesicle structure was remained and became more stable
after the gelation process.
4
5 ( 3, 49 ( 13, 127 ( 34, and 62 ( 8 nm.
2
.1.4. Effect of Gela tion Con d ition s. The vesicles
of PEO-b-PTMSPMA formed spontaneously once water
was added. However, the vesicles in this stage were
soft due to the low glass transition temperature of
PTMSPMA, and it was difficult to keep their shapes in
2
4
dryness. Once the gelation catalyst was added, a stable
hollow structure with high quality was obtained. It is
known that the gelation of -Si(OCH3)3 groups may
occur even without the catalyst. Therefore, the reactive
groups in the vesicle walls should be hydrolyzed pro-
gressively in a methanol/water mixture. Hence, there
is a possibility that the polarity of the hydrolyzed blocks
switches from the hydrophobic to the hydrophilic. If this
process happened, the vesicles would have been disap-
peared. It is necessary to examine the time interval after
the vesicle formation but before the addition of the
catalyst. Since the acid is also a catalyst of gelation, we
have also tried to fix the vesicles using some acids as
catalysts. The diblock copolymer used was PEO45-b-
PTMSPMA59, unless specified otherwise. The Cini in
methanol was 5 mg/mL, and the water content was at
2
.2. Sta bility of th e Hybr id Vesicles. The gelated
vesicles in solution are very stable, and no change has
been observed after the preparation for over 1 year. If
the hybrid vesicle solution was diluted with water to
0
.001 mg/mL and then followed by a sonication process
for half an hour, neither the size nor the morphology
change was observed by TEM. Such shape persistence
apparently owes to the gelation process, which has led
to the cross-linked polysilsesquioxane within the vesicle
walls. This is different dramatically from the vesicles
without gelation, as discussed in the previous section.
Furthermore, the polysiloxane structure also possesses
another unusual property, i.e., the shape-persistent at
high temperature. After calcination at 450 °C for 7 h in
air, the morphology showed no change except less
shrinkage in the volume of vesicle, which was related
to the decomposition of the organic PEO chains.
5
5.8 wt %.
The TEM results demonstrated that the time to add
TEA after vesicle formation showed no difference within
the initial 5 h. However, the mean diameter and the

Macromolecules, Vol. 37, No. 15, 2004
Organic/Inorganic Hybrid Hollow Particles 5715
PTMSPMA polymer may have a wormlike conformation
due to its bulky side groups. The interfacial curvature
between the core and the corona of the aggregates in
solution would be small enough to form the vesicles in
bilayer structure. Moreover, the PTMSPMA is a me-
dium hydrophobic polymer, and its glass transition
temperature is lower than room temperature.27 During
the self-assembly process, PTMSPMA blocks in the core
are likely to be swollen by the solvent mixture of
methanol and water. Such a factor could also help the
2
8
vesicle formation. On the contrary, it is not easy to
find the conventionally spherical star micelles for this
family of block copolymers, PEO45-b-PTMSPMAx, over
a range of conditions attempted, even in the other
common solvents (unpublished results).
F igu r e 6. 1H NMR spectra in CDCl
: (A) PEO-b-PTMSPMA
3
diblock copolymer; (B) the capped vesicles.
The bilayer vesicles consist of the orderly aggregated
blocks with the solvated PEO blocks stretching toward
both sides of the wall, whereas the gelation occurred
exclusively in the PTMSPMA phases in the presence of
the catalyst. Because of the protection of the PEO
corona, the macroscopic gelation was avoided effec-
However, the dried hybrid vesicles cannot be redis-
persed neither in water nor other polar media, suggest-
ing that the intervesicle coupling has occurred during
the period of dryness. This result is expected since not
all the active -SiOH groups have been consumed
completely during the gelation process in the solution.
The PEO corona prevents such intervesicle interaction
in solution successfully but fails in the absence of
solvents. When the free -SiOH groups were capped by
a termination reagent, e.g., methoxytrimethylsilane,
and then further terminated by a more effective termi-
nation reagent, chlorotrimethylsilane, the soluble solid
2
9
tively. As a result of gelation in the walls, the hybrid
vesicles were not the aggregates composed of many
polymer chains anymore but the hollow microgels in
nanoscale. The outer and the inner surfaces of the
vesicles were covered with PEO chains that chemically
attached to the surfaces. Obviously, there is a solvent
pool enclosed by the nanowall of the hybrid vesicle.
Scheme 1 shows the structure of the nanocapsule.
1
vesicles were obtained. Shown in Figure 6B is the H
NMR spectrum of the vesicles being capped. Compared
in Figure 6A is the 1H NMR spectrum of the block
copolymer precursor, from which we found that all of
protons assigned to -(CH2)3-Si(OCH3)3 groups disap-
peared while the protons assigned to PEO block still
remained unchanged. The peaks assigned to the back-
bone of the PTMSPMA chain were highly broadened
and nearly disappeared, indicating far less mobility of
these polymer chain segments. The monomodal peak at
δ 0.1 ppm was assigned to -OSi(CH3)3, which came
from the capping reagent.
3
. Exp er im en ta l Section
.1. Ma ter ia ls. The diblock copolymers were synthesized
3
by ATRP of TMSPMA monomer using poly(ethylene oxide)
methyl ether 2-bromoisobutyrate (PEO-Br) as macroinitiator.
The detailed kinetic studies and synthetic procedure of the
30
block copolymers were described elsewhere. Triethylamine
(
TEA; >99%, Beijing Chemical Reagent Co.) was dried over
KOH. Anhydrous methanol (>99%, Beijing Chemical Reagent
Co.) and double distillated water were used directly. Chloro-
trimethylsilane (Me
Co.) was used after distillation. Methoxytrimethylsilane
Me SiOCH ) was obtained by the reaction of Me SiCl with
3
SiCl; >98%, Beijing Chemical Reagent
(
3
3
3
2
.3. F or m a tion Mech a n ism of th e Vesicles. From
anhydrous methanol in the presence of equal molar of TEA.
Other reagents used as purchased.
the above studies, we have found the vesicles evolving
from the lamellae as the water content in the binary
solvent increases. In the literature, the morphological
transition of the crew-cut micelles based on the PS-b-
PAA diblock copolymers follows the order of spheres,
sphere and rod mixtures, rods, rod and vesicle mixtures,
and pure vesicles as the water content in a binary
mixture solvent (water/dioxane) increases.4 It is clear
that our vesicles are developed from the enclosure of
the lamellae. Pure hybrid vesicles are formed over a
wide range of the water contents. In general, our results
are consistent with the reported results in the literature.
However, neither pure spheres nor rods were obtained
exclusively under the present conditions.
3
.2. Ch a r a cter iza tion . To prepare the samples for trans-
mission electron microscope (TEM) analysis, a micelle solution
2-10 µL) was dropped onto a carbon-coated copper grid and
(
left until dry. TEM images were obtained using a J EM 100
instrument operated at 100 kV. To obtain a scanning electron
microscope (SEM) image, the micelle solution (5 µL) was placed
onto the surface of a silica wafer and left until dryness. It was
coated with platinum and viewed by a Hitachi S-4300 electron
microscope operated at 15 kV. The images were recorded by a
digital camera. Turbidity measurements were performed on
a UV-1601PC UV-vis spectrophotometer (Shimadzu) using a
,26
1
UVPC Personal Spectroscopy Software (version 3.91). H NMR
spectra were recorded on a Bruker DMX300 spectrometer with
CDCl as solvent at room temperature.
3
It should be mentioned that the present block co-
polymers with various composition could form vesicles
very easily. Actually, the vesicles have been obtained
for the block copolymers with short core-forming block
relative to the long corona-forming block, i.e., PEO45-
b-PTMSPMA29. This is different from the requirement
of highly asymmetric composition (short hydrophilic
block forms the corona) for the vesicles based on PS-b-
3.3. Gen er a l P r oced u r es for th e Vesicle P r ep a r a tion
a n d Gela tion . To prepare vesicles, PEO-b-PTMSPMA (10.0
mg) was dissolved in methanol (2.0 mL), and then water (2.0
mL) was added dropwise by syringe at a rate of 1 drop every
1
0 s under vigorous stirring. After 5 h, TEA (40.0 µL) was
added to induce the hydrolysis and polycondensation within
micelles.
3
.4. Cappin g th e Fr ee SiOH Gr ou ps in Vesicles. Vesicles
∼20 mg in 20 mL of water/methanol) were stirred for 48 h
after the addition of TEA, and then Me SiOCH
(
6
,10b
PAA and PS-b-PEO reported in the literature.
3
3
(∼100 µL) was
Considering the coil-rod block copolymers may self-
added and stirred for 2 days to terminate free -SiOH groups
on the surface of vesicles preliminarily. Then the vesicle
solution was concentrated to ∼5 mL, and then anisole (20 mL)
assemble into the vesicles as well and the asymmetry
requirement appears not necessary,1
1,12
we assume the

5
716 Du and Chen
Macromolecules, Vol. 37, No. 15, 2004
was added to extract vesicles. The organic phase was collected
and dried over anhydrous Na SO . Me
added to further terminate the residual free -SiOH groups.
The solid product was obtained after removing the solvents
Su p p or tin g In for m a tion Ava ila ble: Figures S1-S5 and
the experimental observations when acid was used as the
catalyst. This material is available free of charge via the
Internet at http://pubs.acs.org.
2
4
3
SiCl (∼100 µL) was
and extra Me
solved in CDCl
.5. Tu r bid ity Stu d y. The polymer was first dissolved in
3
SiCl under vacuum. The product can be dis-
3
for NMR analysis.
Refer en ces a n d Notes
3
methanol, and then quantitative water was added gradually
into the sample cell. After stirring for several minutes, which
was long enough for the micelle stabilization, the sample was
scanned from a wavelength of 400-800 nm. The transmittance
(1) (a) Hamley, I. W. Angew. Chem., Int. Ed. 2003, 42, 1692. (b)
F o¨ ster, S.; Konrad, M. J . Mater. Chem. 2003, 13, 2671.
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Discher, B. M.; Won, Y.; Ege, D. S.; Lee, J . C.-M.; Bates, F.
S.; Discher, D. E.; Hammer, D. A. Science 1999, 284, 1143.
(
T) data at 650 nm were collected for the turbidity analysis
(
c) Won, Y. Y.; Davis, H. T.; Bates, F. S. Science 1999, 283,
60.
3) Cameron, N. S.; Corbierre, M. K.; Eisenberg, A. Can. J . Chem.
999, 770, 1311.
because the adsorption is the lowest for both the polymer and
the aggregate solutions. The turbidity data were obtained from
T data, i.e., 100 - T%. Methanol was used as the reference
for all the measurements.
9
(
1
(4) Shen, H.; Eisenberg, A. Macromolecules 2000, 33, 2561.
(5) Yu, K.; Zhang, L.; Eisenberg, A. Langmuir 1996, 12, 5980.
(6) Yu, K.; Eisenberg, A. Macromolecules 1996, 29, 6359.
4
. Con clu sion
A simple approach to prepare organic/inorganic hy-
(7) Iyama, K.; Nose, T. Polymer 1998, 39, 651.
brid nanocapsules has been developed. It is based upon
the gelation of the polymer vesicles preformed by self-
assembly of the reactive block copolymers, PEO-b-
PTMSPMA, in a binary solvent mixture of methanol and
water. Detailed conditions for the vesicle formation were
explored for PEO45-b-PTMSPMA59. It demonstrates that
the hybrid vesicles can be formed exclusively over a wide
range of water contents in the mixed solvent. Spheres,
rods, and lamellae as the coexisting morphologies were
observed at lower water contents. The formation of the
vesicles was believed to be the closure of the lamellae.
Exclusive vesicles were also obtained for the initial
polymer concentrations ranging from 0.1 to 20 mg/mL
when the water content was kept at 55.8 wt %. How-
ever, high initial polymer concentration would lead to
the vesicles in large size with a broader size distribution.
TEA was found to be a good catalyst for the gelation
reaction while the gelation with acid failed to keep the
vesicular structure. The block copolymers with the same
PEO block length while different PTMSPMA lengths;
i.e., 29, 42, 59, and 180 may also produce the vesicles.
The hybrid hollow particles are stable against the envi-
ronmental changes due to the cross-linked polysilses-
quioxane structure in the vesicle walls once the gelation
occurs.
(8) Photos, P. J .; Bacakova, L.; Discher, B.; Bates, F. S.; Discher,
D. E. J . Controlled Release 2003, 90, 323.
(9) Antonietti, M.; F o¨ rster, S. Adv. Mater. 2003, 15, 1323 and
references therein.
10) (a) Discher, D. E.; Eisenberg, A. Science 2002, 297, 967. (b)
Soo, P. L.; Eisenberg, A. J . Polym. Sci., Part B: Polym. Phys.
(
2
004, 42, 923 and references therein.
(
11) (a) Kukula, H.; Schlaad, H.; Antonietti, M.; F o¨ rster, S. J . Am.
Chem. Soc. 2002, 124, 1658. (b) J enekhe, S. A.; Chen, X. L.
Science 1998, 279, 1903. (c) J enekhe, S. A.; Chen, X. L.
Science 1999, 283, 372.
12) Vriezema, D. M.; Hoogboom, J .; Velonia, K.; Takazawa, K.;
Christianen, P. C. M.; Maan, J . C.; Rowan, A. E.; Nolte, R. J .
M. Angew. Chem., Int. Ed. 2003, 42, 772.
13) Nardin, C.; Hirt, T.; Leukel, J .; Meier, W. Langmuir 2000,
16, 1035.
(14) Discher, B. M.; Bermudez, H.; Hammer, D. A.; Discher, D.
(
(
E.; Won, Y. Y.; Bates, F. S. J . Phys. Chem. B 2002, 106, 2848.
(
15) (a) Ding, J .; Liu, G.; Yang, M. Polymer 1997, 38, 5497. (b)
Ding, J .; Liu, G. Macromolecules 1997, 30, 655. (c) Ding, J .;
Liu, G. J . Phys. Chem. B 1998, 102, 6107.
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M. H. P.; Meijer, E. W. Science 1995, 268, 1592.
(17) Schacht, S.; Huo, O.; Voigt-Matin, I. G.; Stucky, G. D.; Schuth,
F. Science 1996, 273, 768.
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(
(
(
1
302.
20) (a) Katagiri, K.; Ariga, K.; Kikuchi, J .-I. Chem. Lett. 1999,
8, 661. (b) Katagiri, K.; Hamasaki, R.; Ariga, K.; Kikuchi,
J .-I. J . Am. Chem. Soc. 2002, 124, 7892.
(
The vesicles reported in this paper possess the fol-
lowing novelties: (1) the vesicles are produced by self-
assembling of an amphiphilic block copolymer bearing
reactive trimethoxysilane groups along one block in a
solution; (2) the simple gelation in the wall of the
preformed vesicles can stabilize the vesicle permanently;
2
(21) (a) Emmerich, O.; Hugenberg, N.; Schmidt, M.; Sheiko, S.;
Baumann, F.; Deubzer, B.; Weis, J .; Ebenhoch, J . Adv. Mater.
1
999, 11, 1299. (b) J ungmann, N.; Schmidt, M.; Maskos, M.;
Ebenhoch, J .; Weis, J . Macromolecules 2002, 35, 6851. (c)
J ungmann, N.; Schmidt, M.; Ebenhoch, J .; Weis, J .; Maskos,
M. Angew. Chem., Int. Ed. 2003, 42, 1713.
(3) the product is soluble, which will benefit further
characterizations and applications; (4) water is the
medium for vesicle formation, and the products com-
posed of PEO and polysilsesquioxane can be made
biocompatible. These shape-persistent nanocapsules will
also become a scaffold to stimulate further research on
the preparation and application of nanomaterials. The
robust hollow nanoparticles may find applications not
only in encapsulation and release just like other types
of polymer vesicles but also in encapsulation uses for
high-temperature application.
(22) Lu, Y.; Mclellan, L.; Xia, Y. Langmuir 2004, 20, 3464.
(23) Koh, K.; Ohno, K.; Tsujii, Y.; Fukuda, T. Angew. Chem., Int.
Ed. 2003, 42, 4194.
(
24) Du, J . Z.; Chen, Y. M.; Zhang, Y. H.; Han, C. C.; Fischer, K.;
Schmidt, M. J . Am. Chem. Soc. 2003, 125, 14710.
(
(
(
25) Luo, L. B.; Eisenberg, A. Langmuir 2001, 17, 6804.
26) Shen, H.; Eisenberg, A. J . Phys. Chem. B 1999, 103, 9473.
27) This polymer was a sticky material and could not precipitate
in methanol.
(
28) Yu, K.; Zhang, L.; Eisenberg, A. Macromolecules 1998, 31,
1144.
(29) B u¨ t u¨ n, V.; Wang, X. S.; de Paz B a´ n˜ ez, M. V.; Robinson, K.
L.; Billingham, N. C.; Armes, S. P. Macromolecules 2000, 33,
Ack n ow led gm en t . Financial support by the
BAIREN Project, the Directional Innovation Project
KJ CX2-SW-H07), and Molecular Center of the CAS is
1
.
(
30) Du, J . Z.; Chen, Y. M. Macromolecules, in press.
(
greatly acknowledged.
MA0497097