Block-copolymer-free strategy for preparing micelles and hollow spheres: Self-assembly of poly(4-vinylpyridine) and modified polystyrene

Article abstract of DOI:10.1021/ma0201330

Noncovalently connected micelles (NCCM) with poly(4-vinylpyridine) (PVPy) as the shell and hydroxyl-containing polystyrene, i.e., PS(OH), as the core were formed in a selective solvent mixture for PVPy by inter-polymer hydrogen bonding. With increase in the hydroxyl content in PS(OH), the structure of the micelles changed from perfect spheres to rodlike and then networklike aggregates. Shell-cross-linked micelles and then hollow spheres were obtained based on the NCCM, with hydrogen bonding rather than covalent bonds between the core and shell. The shell structure was locked in by the reaction of PVPy with the cross-linker of 1,4-dibromobutane, under mild conditions. Cavitation of the spherical micelles was realized just by changing the medium from the selective solvent to a common one to dissolve PS(OH) chains in the interior of the micelles. The resultant spheres were proved to be hollow by TEM observation and by both static and dynamic light scattering. The thickness of the shell could be controlled by changing the micelle composition. DLS, TEM, SEM, and AFM studies demonstrated that the integrity of the assembled spheres was maintained during the cross-linking and cavitation. The size of the hollow spheres increased apparently with dilution of the solution. Compared to the conventional route to shell-cross-linked micelles and hollow spheres from the micelles made of block copolymers, the current approach has the significant advantages of avoiding synthesis of block copolymers and chemical degradation of the core components.

Full text of DOI:10.1021/ma0201330

5980
Macromolecules 2002, 35, 5980-5989
Block-Copolymer-Free Strategy for Preparing Micelles and Hollow
Spheres: Self-Assembly of Poly(4-vinylpyridine) and Modified
Polystyrene
Min Wa n g, Min g J ia n g,* F a n glin Nin g, Da oyon g Ch en , Sh iyon g Liu , a n d
Hon gw ei Du a n
Department of Macromolecular Science and The Key Laboratory of Molecular Engineering of Polymers
(Ministry of Education, China), Fudan University, Shanghai 200433, P. R. China
Received J anuary 28, 2002; Revised Manuscript Received April 25, 2002
ABSTRACT: Noncovalently connected micelles (NCCM) with poly(4-vinylpyridine) (PVPy) as the shell
and hydroxyl-containing polystyrene, i.e., PS(OH), as the core were formed in a selective solvent mixture
for PVPy by inter-polymer hydrogen bonding. With increase in the hydroxyl content in PS(OH), the
structure of the micelles changed from perfect spheres to rodlike and then networklike aggregates. Shell-
cross-linked micelles and then hollow spheres were obtained based on the NCCM, with hydrogen bonding
rather than covalent bonds between the core and shell. The shell structure was locked in by the reaction
of PVPy with the cross-linker of 1,4-dibromobutane, under mild conditions. Cavitation of the spherical
micelles was realized just by changing the medium from the selective solvent to a common one to dissolve
PS(OH) chains in the interior of the micelles. The resultant spheres were proved to be hollow by TEM
observation and by both static and dynamic light scattering. The thickness of the shell could be controlled
by changing the micelle composition. DLS, TEM, SEM, and AFM studies demonstrated that the integrity
of the assembled spheres was maintained during the cross-linking and cavitation. The size of the hollow
spheres increased apparently with dilution of the solution. Compared to the conventional route to shell-
cross-linked micelles and hollow spheres from the micelles made of block copolymers, the current approach
has the significant advantages of avoiding synthesis of block copolymers and chemical degradation of
the core components.
In tr od u ction
than the well-organized polymeric molecular assem-
blies4-8 such as micelles, vesicles and hollow particles.
In this paper, we report a new approach of molecular
assembly to polymeric micelles and hollow spheres. This
approach is novel in many aspects. First, the micelle
formation is based on homopolymer/copolymer pairs by
means of interpolymer complexation instead of using
block copolymers, so that the specific interactions rather
than chemical bonds exist between the core and shell
of the micelles. Second, for producing hollow spheres,
using the micelle as a precursor, simple dissolution
rather than chemical degradation is used. Third, the
aggregates could keep their integrity during cross-
linking of the micelles and subsequent dissolution of the
core component.
In recent years, we have devoted effort to obtain
regular assemblies, rather than irregular aggregates,
based on inter-polymer complexation.9 Two approaches
have been studied. In the first, the key point is restrict-
ing one kind of the interaction sites in desired positions
along a polymer chain. For example, when carboxyl-
ended polybutadiene (CPB) and poly(4-vinylpyridine)
(PVPy) were mixed in a suitable common solvent,
hydrogen-bonded “graft” copolymer with PVPy backbone
and CPB grafts was produced. When the common
solvent was switched to a selective one, e.g., for PVPy,
micelles with PVPy shell and CPB core were formed.9c
The principle of the second approach is as follows. When
a solution of polymer A is added into a solution of
polymer B which is a precipitant for A, chains of A
aggregate, but their precipitation can be prevented
provided A and B could form interpolymer hydrogen
bonding. Thus, micellelike particles with a compact A
core surrounded by B shell are formed.9d-g Both ap-
proaches lead to noncovalently connected micelles
(NCCM). Very recently, we found that the NCCM
composed of a PVPy coil and polyimide rods, unexpect-
edly, formed large hollow spheres in a common solvent.10
Inter-polymer complexes, i.e., inter-macromolecular
associates of a polymer pair bound together by second-
ary binding forces, in particular, hydrogen bonding,
have received a great deal of attention since the
1970s.1-3 In the past decade, we found that by introduc-
ing hydrogen bonding into the polymers which originally
lack donor or acceptor groups,3 the transition from
separated coils to complex aggregates took place in
solution when the content of the introduced interaction
sites reached a certain level. However, the complexes
formed in solution are often in the form of irregular
aggregates because each proton-donating polymer chain
can interact with many donor-accepting polymer chains
and vice versa. Therefore, although the complexes are
products of spontaneous “molecular assembly” of the
polymers, their application potential was seriously
limited and consequently received much less attention
In this paper, the NCCM study is extended to a
polymer pair with “controllable interactions”, i.e., poly-
{styrene-co-[p-(1,1,1,3,3,3-hexafluoro-2-hydroxylpropyl)-
R-methylstyrene]} PS(OH), (Scheme 1) and PVPy. Spe-
cial attention has been paid to the effects of the
interaction density, which is adjustable by using PS-
(OH) with different hydroxyl contents, on the formation,
structure, and morphologies of the micelles.
* Corresponding author. E-mail: mjiang@fudan.edu.cn.
10.1021/ma0201330 CCC: $22.00 © 2002 American Chemical Society
Published on Web 06/21/2002

Macromolecules, Vol. 35, No. 15, 2002
Preparing Micelles and Hollow Spheres 5981
Ta ble 1. Ch a r a cter iza tion Da ta of P S(OH)
Sch em e 1. Ch em ica l Str u ctu r e of P S(OH)
sample code
OH content/mol %
Mn/104 g/mol
Mw/Mn
PS(OH)-1
PS(OH)-2
PS(OH)-3
PS(OH)-5
PS(OH)-12
PS(OH)-20
PS(OH)-27
PS(OH)-35
PS(OH)-42
PVPy
1.0
1.67
3.2
4.88
11.7
19.4
27.3
34.6
42.0
3.7
2.0
4.7
2.4
1.9
1.8
1.4
0.9
0.5
12a
1.51
1.60
1.31
1.48
1.50
1.68
1.74
1.80
1.92
a
Viscosity-average molecular weight.
The final aim of this paper is the preparation of
polymeric hollow spheres using NCCM as a precursor.
Among a variety of molecular assembled structures,
hollow spheres in a size range from nanometer to
micrometer are particularly interesting and much in
demand due to their potential uses in encapsulation of
guest molecules.11 Besides the classical approach of
microemulsions to produce hollow spheres, which was
described mostly in patent literature and encountered
difficulties in preparing spheres of very small size, in
recent years, several different routes including those
based on template synthesis12-14 and block copoly-
mers15-18 have been produced. The block copolymer
route includes the following steps: designing and
preparing a block copolymer containing a cross-linkable
block and a degradable block; preparing the micelles
from the copolymer in its selective solvent; locking the
shell by cross-linking; and, finally, creating a central
cavity by degrading the blocks contained in the core.
Besides, J enekhe et al. reported that rod-coil block
copolymers could form hollow spheres via direct self-
assembly in selective solvents.8 As our present method
is based on NCCM, it has significant advantages of
avoiding the preparation of block copolymers and chemi-
cal degradation of the core component.
Although in this paper we concentrate on the prepa-
ration of micelles and hollow spheres from homopoly-
mer/copolymer pairs with interpolymer hydrogen bond-
ing in solution, it should be mentioned that organic
materials of ordered nanometer-sized structure in the
solid state self-assembled by small molecules due to
hydrogen bonding have received much attention in the
literature. Very recently Kim and co-workers19 reported
on preparing supermolecular polymeric material with
hexagonal channels which were created by removing the
template core after the formation of a cross-linked
matrix.
F igu r e 1. Relative viscosity of the blend solutions of PVPy
and PS(OH)-X in chloroform as a function of weight fraction
of PS(OH)-X.
were conducted by mixing solutions of PS(OH) and
PVPy of the same concentration in chloroform. The
appearance of the blend solutions depended on the
hydroxyl content in PS(OH). The solution remained
clear when PS(OH)-1, -2, and -3 were used but turned
turbid, and precipitation occurred as the hydroxyl
content reached and exceeded 5 mol %. The measure-
ments were conducted 10 min after mixing the solutions
for the former case while filtration of the solutions was
done prior to the measurements for the latter.
Figure 1 shows the variation of the relative viscosity
of PS(OH)/PVPy blend solutions in chloroform as a
function of weight fraction of PS(OH). For the blends
with hydroxyl contents being 1, 2, and 3, the viscosity
changed smoothly, and is close to the corresponding
values calculated by the additivity rule (for clarity,
curves of PS(OH)-1 and PS(OH)-3 are not shown). As
the total concentration of polymers in the blend solution
is 3.0 × 10-3 g/mL, much lower than its overlap
concentration C*, obeying the additivity law may mean
that the component coils keep independent and no
substantial complex formed.
When the hydroxyl contents reached 5 mol %, the
viscosity profile of the blend solutions showed an ap-
parent negative deviation from the additivity. This
indicates the chains collapsed and partially precipitated
due to interpolymer complexation. When the hydroxyl
content increased further, the visicosity decreased
dramatically. Being an extreme, the viscosity of PS(OH)-
35/PVPy blend solution displays a minimum close to
that of pure solvent, indicating that nearly all the
component polymers precipitated as complexes.
In ter p olym er Com p lexa tion betw een P S(OH)
a n d P VP y in Com m on Solven t
The hydrogen-bonding complexation of PS(OH) with
a series of proton-accepting polymers has been inten-
sively studied,3,20 as the hydroxyl group in (CF3)2C-
(OH)- is strong in inter-association while weak in intra-
association.3,21 The counterpart PVPy is a common
proton-accepting polymer, which has been employed in
the studies on miscibility and complexation due to
hydrogen bonding in a variety of polymer blends.3,22 The
characterization data of PVPy and PS(OH)-X with
different hydroxyl contents are listed in Table 1. Here
X represents the mol % of hydroxyl content in PS(OH).
Viscometry is a simple way to monitor hydrogen-
bonding complexation of polymers in solution qualita-
tively.3 Measurements of the relative viscosity (η/η0) of
PS(OH)-X/PVPy blends as a function of the composition
Laser light scattering (LLS) has proved to be more
sensitive than viscometry in investigating association

5982 Wang et al.
Macromolecules, Vol. 35, No. 15, 2002
Ta ble 2. P r ep a r a tion Con d ition s a n d Com p osition s of th e
Micelles (M), Cr oss-Lin k ed Micelles (C) a n d Hollow
Micelles (H)
composition
micelle
code
C/PS(OH) × C/PVPy × PS(OH)/PVPy
PS(OH)-X
103 g/mL
103 g/mL
(w/w)
M1 (M6)
M2 (M7)
M3 (M8)
M4 (M9)
M5 (M10)
M11 (M12)
M13 (M14)
2
4 (8)
4 (8)
4 (8)
4 (8)
0.2
0.2
0.2
0.2
0.2
0.2
0.2
2.2/1 (4.4/1)
2.2/1 (4.4/1)
2.2/1 (4.4/1)
2.2/1 (4.4/1)
2.2/1 (4.4/1)
0.67/1 (5.0/1)
0.67/1 (5.0/1)
12
20
27
42
12
20
4 (8)
1.2 (9)
1.2 (9)
M15
M16
2
12
10.75
8
0.5
2.0
2.4/1
0.44/1
C15
H15
C16
H16
H8
cross-linked micelle from M15
hollow spheres from C15
cross-linked micelle from M16
hollow spheres from C16
hollow spheres from C8
F igu r e 2. Hydrodynamic diameter distributions of PVPy, PS-
(OH), and PS(OH)-3/PVPy (1/2, w/w) in chloroform. The total
concentration of polymers is 1.0 × 10-4 g/mL.
PVPy; instead, a bluish tinge appeared. Obviously, the
aggregates of PS(OH) were stabilized by the solvated
PVPy chains which are attached to the aggregate
surface by hydrogen bonding between the pyridine ring
and hydroxyl in PS(OH). This procedure has been used
to prepare stable PS(OH)/PVPy micelles in CHCl3/CH3-
NO2 (v/v, 1/9).
There are many factors affecting the assembly of PS-
(OH) and PVPy, i.e., hydroxyl content in PS(OH), initial
concentrations of PS(OH) and PVPy, final composition,
temperature, and even the mixing procedure and speed
etc. As we are mainly interested in the effect of the
interaction density between PS(OH) and PVPy, a series
of micelle solutions containing PS(OH)-X with X ranging
from 2 to 42 mol % at a given initial concentration were
prepared by adding PS(OH)/CHCl3 into PVPy/CH3NO2
very slowly (Table 2). The morphologies of the micelles
(M1-M5) were observed by TEM as shown in Figure 3.
In this paper, the micelles, cross-linked micelles and
hollow spheres are denoted as M, C, and H respectively
(Table 2).
Figure 3a displays the morphologies of M1 composed
of PS(OH)-2/PVPy. It is interesting to find that the
individual micelles are spherical, and most of them are
in the diameter range of 35-50 nm. As no core-shell
structure is observed and the circular outline seems
distinct and perfect, we are inclined to think that the
spheres observed correspond to the micelle core, and the
shell of PVPy is not discernible as the chain density is
too low.15c,23
and aggregation process, especially in very dilute
solutions.3,22c Figure 2 shows the hydrodynamic diam-
eter ( Dh ) distributions of PS(OH)-3, PVPy, and their
blend solutions with total concentrations of polymers
of 1.0 × 10-4 g/mL. For PS(OH)-3, the peak is below 10
nm; for PVPy, the peak is located at about 30 nm. It is
interesting to note that the distribution of PS(OH)-3/
PVPy are bimodal. Besides the small peak located at
about 20 nm, which is obviously attributed to the
individual component polymers in solution, a large peak
associated with polymer aggregates in a size range
around 400 nm appeared. This is a direct evidence of
complex formation in PS(OH)-3/PVPy. As dynamic light
scattering (DLS) is very sensitive to large particles, the
relative small peak area of the independent coils does
not mean that the number of the individuals is smaller
than that of the complex.
Although DLS proved the presence of the PS(OH)-3/
PVPy complex, the blend solution remained clear and
showed no precipitate formation. As in PS(OH)-3, on
average, there is only one hydroxyl group per 30 styrene
units, when hydroxyl interacts with pyridyl on PVPy,
the inert PS sequence between two neighboring hy-
droxyls will form loops or dangling chains, leading to a
loose structure. When hydroxyl contents in PS(OH)
increased to 5 mol % or more, the complexes gradually
lost their solvation ability due to forming a segment-
pairing structure.
Micelle F or m a tion a n d its Mor p h ology
The morphologies of M2 containing PS(OH)-12 in
Figure 3b resemble that of M1 in both the contour shape
and size range. As the hydroxyl content increases to 20
mol % (M3), an apparent variation in morphology takes
place; i.e., spheres turn to ellipsoids and even wormlike
or short rods (Figure 3c). It is noticeable that the
diameter of the cross sections of the wormlike structures
or rods is similar to that of the large spheres, i.e., about
40 nm. Furthermore, the morphologies of PS(OH)27/
PVPy micelles (M4) shown in Figure 3d are character-
istic of the coexistence of spheres, ellipsoids, and
micronetworks composed of the short rods. Besides, the
cross section size of the short rods decreases signifi-
cantly to about 20-30 nm only. Finally, as the hydroxyl
content increases to 42 mol % (M5), i.e., on average,
there is one interaction site per 2-3 PS(OH) monomer
units, most of the micelles are in the shape of short rods
As the formation of interpolymer complexation in
solution often leads to an irregular precipitate, in order
to allow a complementary polymer pair to assemble into
micelles, one needs to use a selective solvent so that the
hydrogen bonding between the component polymers is
allowed to occur on the core-shell interface area mostly.
PS(OH) and PVPy differ substantially in their solubility
behavior. Chloroform was a good solvent for both, while
nitromethane was found to be a good solvent for PVPy
but a nonsolvent for PS(OH). Therefore, the mixture of
CHCl3/CH3NO2 (v/v, 1/9) was used as a selective solvent
for preparing the NCCM composed of PS(OH) core and
PVPy shell. When a dilute solution of PS(OH) in CHCl3
was added into nitromethane, precipitation occurred
immediately. However, no precipitation was found as
the solution was added into nitromethane containing

Macromolecules, Vol. 35, No. 15, 2002
Preparing Micelles and Hollow Spheres 5983
F igu r e 3. TEM micrographs of (a) M1, (b) M2, (c)M3, (d) M4, (e) M5, and (f) M8.
and some of them combine together to form relatively
large networks extending to 500 nm. In addition, the
transverse size of the short rods decreases further, to
about 15-25 nm (Figure 3e).
micelle preparation, rapid aggregation of PS(OH) gener-
ally prevented sufficient interaction between the hy-
droxyl and pyridine groups, so that no macroscopic
precipitation of the complex took place. Furthermore,
we have reasons to believe that, for the cases with
higher density of the interaction sites, the aggregation
of PS(OH) was accompanied by incorporating PVPy
chains. So the aggregates probably did not have a well-
defined core-shell structure, but the two components
interpenetrated each other with enrichment of PVPy on
the surface area as schematically shown in Figure 4.
This hypothesis is supported by the successive studies
of cross-linking and cavitation.
In short, Figure 3a-e displays a regular change in
the micellar morphologies with the hydroxyl content in
PS(OH). The trends are clear, i.e., with increasing
hydroxyl content and consequently intensifying interac-
tions between the two components, the diameter or
transverse size of the micelles, i.e., spheres or rods,
gradually decreases, and at the same time, the elemen-
tary micelles show an increasing ability to combine into
clusters or networks. This variation of the aggregate
morphologies with the hydroxyl content in PS(OH) is
illustrated schematically in Figure 4.
Micella r Va r ia tion w ith Hyd r oxyl Con ten t
Mon itor ed by DLS
As reported in the previous section, in a common
solvent, PS(OH) and PVPy turned turbid and complex
precipitation occurred when the hydroxyl content in PS-
(OH) reached 5 mol % or more. In the procedure of
As DLS is able to measure the translational diffusion
coefficient of particles in solution and hence Dh of the
corresponding equivalent hydrodynamic spheres, it has

5984 Wang et al.
Macromolecules, Vol. 35, No. 15, 2002
F igu r e 6. Dh of PVPy/PS(OH) micelles in chloroform/
nitromethane (1/9, v/v) as a function of initial concentration
of PS(OH) (or final composition of the micelles).
present similar trends, i.e., Dh increases significantly
with hydroxyl content in PS(OH) over a broad range but
decreases as the content reaches very high values (27-
42 mol %). The morphologies shown in Figure 3a-e
correspond to the aggregates in the lower curve (M1-
M5) and only that shown in Figure 3f is for the PS(OH)-
20/PVPy aggregates (M8) in the upper curve. The Dh
values of M1 and M2 are much larger than those
observed by TEM because the shell, or at least the outer
part of the shell, is invisible in TEM. For the cases of
PS(OH) with hydroxyl contents higher than 12 mol %,
as the micelles deformed and the short rods aggregated
further, the sizes calculated from the translational
diffusion coefficient actually reflected the behavior of
micelle aggregates or clusters as an entirety. Therefore,
the increase of Dh with hydroxyl content over the range
from 2 to 27 mol % is in accordance with the morphology
change shown in Figure 3a-d rather than the trans-
verse size of the individual short rods. The decrease of
Dh in the very high hydroxyl content range, i.e., from
27 to 42 mol %, may reflect that the peripheral shape
of the aggregate composed of the short rods becomes
more highly branched causing a decrease of the outline
size.
F igu r e 4. Schematic illustration of the variation of the
aggregate morphologies with the hydroxyl content in PS(OH).
F igu r e 5. Dh of PS(OH)/PVPy micelles as a function of
hydroxyl content in PS(OH) in chloroform/nitromethane (1/9,
v/v).
A comparison between the two curves for two groups
of solutions i.e., M1-M5 and M6-M10, shows clearly
the effect of the initial concentration of PS(OH). A 2-fold
concentration of PS(OH) causes a substantial increase
in Dh . For example, Dh of PS(OH)-20/PVPy from the
low (4 × 10-3 g/mL) and high (8 × 10-3 g/mL) concen-
tration are 102 and 121 nm, respectively. The corre-
sponding difference in the morphologies can be seen by
comparing parts c and f of Figure 3.
been widely used in the studies of polymeric micelle
solutions.9c,9d,24 In the present study, attempts have
been made to explore the effect of the interaction density
between the two component polymers on the character
of the formed micelles. As reported above, spherical
micelles were obtained only when the hydroxyl content
in PS(OH) was low (2-12 mol %), while the high
hydroxyl content in PS(OH) resulted in complex struc-
tures having significant asymmetry in shape and broad
size distributions. Therefore, we could not expect a
quantitative explanation from DLS, although these
results provide interesting information to corroborate
the transmission electron microscopy (TEM) observa-
tions.
Figure 5 shows the variation of Dh of PS(OH)/PVPy
aggregates as a function of hydroxyl content in PS(OH).
The lower and upper curves refer to the micelle groups
of M1-M5 and M6-M10, which were obtained from
different concentrations of PS(OH) and consequently,
different final compositions, i.e., PS(OH)/PVPy of 2.2/1
and 4.4/1 w/w, respectively (Table 2). The two curves
Figure 6 shows the Dh variation over a broad range
of the initial concentration of PS(OH) and accordingly
the final composition. Generally, in both cases of PS-
(OH)-12 and PS(OH)-20, the micellar size significantly
increases with the concentration of PS(OH). The result
is readily understandable, as at higher PS(OH) concen-
trations collapse and aggregation of the PS(OH) chains
becomes more rapid when the medium surrounding the
chains becomes poor. Increasing the PS(OH) concentra-
tion at a given concentration of PVPy results in an
increase in the micelle size. As the Dh distribution
curves for all the micelle solutions prepared at these
concentration ranges of PS(OH) and PVPy show no

Macromolecules, Vol. 35, No. 15, 2002
Preparing Micelles and Hollow Spheres 5985
Ta ble 3. Dep en d en ce of Micelle Size a n d P olyd isp er sity
on P r ep a r a tion P r oced u r es
C/PS(OH) × C/PVPy × Dh
proca PS(OH)-X
103 g/mL
103 g/mL (nm) polydispersity
A
B
A
B
20
20
27
27
3
3
5
5
0.5
0.5
0.5
0.5
66
62
97
73
0.24
0.14
0.29
0.20
a
In procedure A, the micelle is prepared by adding PS(OH)/
CHCl3 into PVPy/CH3NO2 solutions, while in procedure B, it is
prepared by adding PVPy solution into PS(OH) solution.
signals associated with individual PS(OH) and PVPy
chains (curve not shown), we may say that most of the
component polymers have assembled into the micelles.
In other words, for current NCCM, the micelle composi-
tion, i.e., the weight or volume ratio of the core to shell,
is readily adjustable over a certain range by just
changing the feed composition.9c This provides NCCM
a significant advantage over the micelles made of block
copolymers, in which the composition adjustment highly
depends on synthesis, which in most cases is costly and
time-consuming.
All the micelles of M1-M10 were prepared by adding
PS(OH)/CHCl3 solutions into PVPy/CH3NO2 (procedure
A). It is worth noting that the micelles prepared by the
opposite way, i.e., procedure B, adding PVPy/CH3NO2
into PS(OH)/CHCl3 solutions resulted in a smaller and
narrower distribution. Table 3 presents two examples.
This is understandable as in procedure B the degree of
solvation of PS(OH) chains worsens much slowly than
in procedure A; thus, the PS(OH) chains have more
chance to interact with and be stabilized by PVPy
chains. In fact, despite the mixing method of the
component solutions, the micelles or aggregates of PS-
(OH)/PVPy are in essence kinetically trapped morphol-
ogies.
F igu r e 7. Hydrodynamic diameter distributions of M15, C15,
and H15.
Ta ble 4. Size Dep en d en ce of th e Hollow Sp h er es on
Con cen tr a tion
H15
C (×104 g/mL)
2.25
194
0.583
224
0.229
226
0.141
268
Dh (nm)
H16
C (×104 g/mL)
Dh (nm)
9
123
0.363
167
0.257
176
0.189
188
caused by removing the species contained in its interior
was reported previously.16b,17 In the case of block
copolymer micelles, this expansion is of course a result
of releasing the restriction imposed by the insoluble
blocks aggregated in the core on the solvated blocks. The
same phenomenon found for our NCCM reflects the
restriction on the PVPy chains caused by the aggregated
PS(OH). Another reason for this expansion, i.e., increas-
ing the solvation power of the medium for the PVPy
nanocage by adding DMF cannot, of course, be excluded.
We found that the apparent Dh of the hollow spheres
H15 substantially increases with diluting the solutions
with the mixed solvent of chloroform/nitromethane/DMF
(v/v/v, 1/9/10). Specifically, as the concentration varied
from 2.25 × 10-4 to 0.141 × 10-4 g/mL, the diameter
increased from 194 to 268 nm and accordingly, the
spherical volume expanded by a factor of 2.7 (Table 4).
This is a very interesting phenomenon and has not been
reported and investigated previously. The finding im-
plies that the degree of swelling of the nanocage or
membrane in the shell can be controlled by adjusting
the concentration and it could be very high at extremely
dilute solutions. The micelles having controllable cavi-
ties were demonstrated further by a combination of
static light scattering (SLS) and DLS. The above solu-
tions of H15 in the solvent of chloroform/nitromethane/
DMF (v/v/v, 1/9/10) were measured by SLS giving an
apparent molecular weight of 1.09 × 107 and radius of
gyration Rg of 146 nm. Also, Dh obtained by DLS in
an extremely dilute solution (C ) 1.4 × 10-5 g/mL) was
Sh ell-Cr oss-Lin k in g a n d Ca vita tion of Micelles
The procedure for producing hollow spheres includes
locking the structure in by cross-linking the shell and
then dissolving the core. Two examples, obtaining “thin
shell” and “thick shell”, respectively, are described as
follows.
A thin-shell sphere (M15) was prepared by the
procedure A with a composition of PS(OH)-2/PVPy 2.4/
1. As both initial concentrations of PS(OH)-2 and PVPy
are larger than the corresponding ones in preparing
micelles M1-M10 with similar compositions, larger
micelles with Dh of 152 nm and polydispersity of 0.21
were obtained. (Figure 7).
1,4-dibromobutane was selected as cross-linker.25,26
Then 50 mol % of 1,4-dibromobutane based on the
pyridine units was added into the solution of M15 and
the mixture was kept at 60 °C for 2 days. In a control
experiment, the micelle solution without adding the
cross-linker was treated at the same conditions. After
the reaction, DLS measurement indicated a slightly
smaller diameter of 138 nm for the cross-linked micelle
C15 (Table 2). This means that the shell structure was
successfully locked in, which is called SCK’s.27 Adding
an equivolume of dimethylformamide (DMF) into the
solution, which not only could solubilize both compo-
nents but also could cause dissociation of the interpoly-
mer hydrogen bonding leading to decomplexation,3,22c
led to a significant increase of the Dh up to 194 nm
(Figure 7), while for the control, micelles disappeared
as monitored by DLS. The size expansion of a micelle
268 nm. Therefore, we found that apparent density of
3
the spheres are 1.8 × 10-3 g/mL (F ) Mw/NA(4/3) Rh
,
where NA is Avogadro’s number), which is about 2
orders of magnitude smaller than the ordinary polymer
micelles24 and surfactant-free dispersed polymer par-
ticles.28 This low density is of course in accordance with
the model of hollow spheres. In addition, the Rg / Rh
value of 1.09 is far different from the expected theoreti-
cal values29 for a uniform sphere (0.774) and a polymer
coil (1.50) but fairly close to that for nondraining thin
layer hollow spheres (∼1.0).

5986 Wang et al.
Macromolecules, Vol. 35, No. 15, 2002
F igu r e 8. TEM micrographs of (a) M15 and (b) C15.
The “thick shell” micelles M16 with a composition of
PS(OH)-12/PVPy (0.44/1), i.e., the weight fraction of the
shell being 0.7, was prepared by a similar method but
with a high initial concentration of PVPy. The Dh of
the thick-shell micelle was measured to be 95 nm. After
cross-linking, it changed to 87 nm (C16). Removing the
core by adding equivolume of DMF to the solutions of
C16 resulted in larger hollow micelles with a Dh of 123
nm (H16). It was also observed that the micelle size
increased with dilution as Dh reached as high as 188
nm at an extremely dilute concentration of 1.89 × 10-5g/
mL (Table 4). Meanwhile, SLS gave an apparent mo-
lecular weight of 6.66 × 106 and Rg ) 96 nm.
Therefore, we found that Rg / Rh is 1.02 and the
average density is 3.2 × 10-3 g/mL. Once again, these
data support the model of hollow spheres. In addition,
this density of the “thick shell” is apparently larger than
that of the “thin shell”.
prepared by using higher initial concentrations, pos-
sesses larger diameters. In addition, the microphoto-
graphs clearly show the perfect spherical shape of both
the micelles M15 and C15. In other words, the spherical
micelles maintained their integrity during the cross-
linking reaction, and in particular, no intermicelle
fusion took place.
A dramatic morphological change for C15 caused by
adding equivolume DMF into its solutions in chloroform/
nitromethane (v/v, 1/9), has been explored by TEM. A
remarkable feature of the micrographs shown in Figure
9, parts a and b, is the obvious contrast between the
periphery and center in most of the spheres. This
feature is characteristic of the projection images of
hollow spheres.10,12a,30,31 The results demonstrate that
the PS(OH)-2 chains have been removed from the
interior of the micelles simply by changing the medium
without any chemical degradation. The micrographs
display micellar aggregates rather than individual
particles. This aggregation probably took place during
solvent evaporation during the preparation of TEM
specimens, as DLS measurements for the C15 and H15
solutions indicate smaller particle sizes in accordance
with the individual spheres. A close examination of the
micrograph provides a relatively rough evaluation of the
ratio of the outer radius R to the inner radius Ri of the
shell being about 1.10. Thus, the volume ratio of the
shell to the core (R/Ri)3 - 1, would be 0.33. This value
is close to the weight ratio of PVPy to PS(OH) in M15,
i.e., 0.42. In addition, we noticed that a minority of
spheres do not show the hollow structures but rather
uniform contrast over the spheres. For these particles,
perhaps the local cross-linking density is too high to
remove the PS(OH) chains. The exact reasons for this
coexistence of hollow spheres and homogeneous ones
remains unclear.
In short, DLS and SLS studies demonstrated that we
have successfully designed and performed a method for
preparing NCCM and based on which, the SCK’s with
immobile but permeable cross-linked peripheral shell
and finally, hollow spheres by simply dissolving the
polymer chains contained in the interior of SCK’s.
Mor p h ology of Hollow Sp h er e
TEM, scanning electron microscopy (SEM), and atomic
force microscopy (AFM) have been used to explore the
morphological variation of the NCCM caused by the
shell-cross-linking and subsequent core-removal. TEM
is capable of examining the internal structure of the
aggregates while AFM and SEM allow for visualization
of three-dimensional shape.
The morphologies of the M15 micelles and its cross-
linked product C15 are shown in Figure 8, parts a and
b, respectively. As the number of the micelles is not
large enough, accurate evaluation of average diameter
and its distribution cannot be carried out. However,
compared to M1 shown in Figure 3a, M15, which were
The three-dimensional morphology of H15 was stud-
ied by AFM on mica substrate. As clearly shown in
Figure 9c, most hollow spheres are individual particles

Macromolecules, Vol. 35, No. 15, 2002
Preparing Micelles and Hollow Spheres 5987
F igu r e 9. Micrographs of hollow spheres: (a and b) TEM of H15, (c) AFM of H15, and (d) SEM of H15 observed using sample
stage with tilting angle 40°. Both were prepared in chloroform/nitromethane/DMF (1/9/20, v/v/v).
without substantial interparticle fusion. More impor-
tantly, differing from the morphologies of hollow spheres
made from block copolymers showing flattened or col-
lapsed spreading structure,16b,17 the present spheres are
standing on the substrate. In other words, the hollow
spheres made of cross-linked poly(4-vinylpyridine) shells
are shape-persistent, thus maintaining their integrity.
This integrity persistence was confirmed by SEM ob-
servations as all the particles keep their spherical shape
even after vacuum-drying. This superior mechanical
property of the hollow spheres is perhaps caused by a
relatively larger chain density of the shell compared to
that of the ordinary micelles of block copolymers in
which one insoluble block connects one soluble chain
only. This valuable property of the shell may well
provide significant advantages in encapsulation applica-
tions of such hollow spheres.
The same procedure for preparing hollow structures
from the micelles with high hydroxyl contents in PS-
(OH) was performed as well. One example is based on
M8 with PS(OH)-20/PVPy 4.4/1. (Figure 3e) After cross-
linking and changing the solvent for removing soluble
PS(OH)-20 takes place, the final structure is shown in
Figure 10. The short-rod shape completely disappeared
and was replaced by irregular aggregates with loose
structure. This irregular structure is of course composed
of cross-linked PVPy. This result supports our opinion
regarding the structure of the aggregate mentioned
above; namely, for the aggregates containing PS(OH)
with high hydroxyl content, there is probably no clear
core-shell structure, and instead, PVPy and PS(OH)
interpenetrate each other. Therefore, cross-linking and
the subsequent dissolution of PS(OH) resulted in ir-
regular loose structures. This change in morphology of
the hollow bodies with increasing hydroxyl content in
PS(OH) is also schematically shown in Figure 4. This
result indicates that the core-shell structure of NCCM
is an important premise for the preparation of the
desired hollow structures.
M16 is a micelle with “thick shell”, which was
demonstrated by LLS studies. However, we failed to
confirm the presence of an inner cavity for H16 by TEM
observations. In fact, the differences in the electron
scattering between passing through the central part and
shell part becomes less as the shell thickness increases.
In the present “thick-shell” case, this difference turns
to be too small to present a discernible contrast. Similar
results were reported for the hollow spheres, in which

5988 Wang et al.
Macromolecules, Vol. 35, No. 15, 2002
tional diffusion coefficient distribution G(D) or a hydrodynamic
diameter distribution f(Dh) via the Stokes-Einstein equation,
Dh ) (kBT/3πη)D-1, where kB, T, and η are the Boltzmann
constant, the absolute temperature, and the solvent viscosity,
respectively.
TEM was performed on a Philips CM 120 electron micro-
scope at an accelerating voltage of 80 kV. The specimens were
prepared by placing 5 µL micelle solutions on copper grids
coated with a thin carbon film and allowing them to dry freely.
SEM and AFM were conducted with a Philips XL30 and
Nanoscope III-microscopes, respectively. The sample prepara-
tion for SEM was similar to that for TEM. Samples for AFM
imaging were prepared by placing a 5 µL drop of micelle
solution on freshly cleaved mica and allowing it to dry in a
desiccator.
Ma ter ia ls. Poly(4-vinylpyridine) was synthesized by free
radical polymerization of 4-vinylpyridine (Aldrich) which was
distilled from CaH2 three times. The viscosity-average molec-
ular weight of the polymer (Mη) of the polymer was 1.2 × 105.
The hydroxy-containing monomer p-(hexafluoro-2-hydroxyiso-
propyl)-R-methylstyrene (HFMS) was synthesized from 4-chloro-
R-methylstyrene via a Grignard reaction with hexafluoroace-
tone. Both IR and NMR confirmed the existence of HFMS: IR
3600, 3520, 1629, 1611-1513, 1150-1126 cm-1; NMR δ 2.0
(CH3, 3H), δ 4.0-5.0 (-CH, 1H), δ 4.9-5.2 (H in double bond,
2H), δ 7.0-8.0 (aromatic ring, 4H). Random copolymers of
styrene and HFMS (PS(OH)) were synthesized and purified
as described previously.33 By variation of the feed composition,
a series of PS(OH) samples with the HFMS content ranging
from 1.0 to 42 mol % were produced. The HFMS content in
PS(OH) was determined by fluorine analysis. The molecular
weight of PS(OH) samples were measured by SEC. All the
results are listed in Table 1. The samples were denoted by
PS(OH)-X where X represents the HFMS content (in mol %).
F or m a tion of Micelles. To prepare the micelle solution,
PVPy and PS(OH) were dissolved in nitromethane and chlo-
roform separately. To prepare M1-M5, 1 mL PS(OH)-X/
chloroform solutions (4 × 10-3 g/mL) were added dropwise very
slowly (ca. 0.2 mL/min) into 9 mL PVPy/nitromethane solu-
tions (0.2 × 10-3 g/mL). The solutions turned from completely
colorless and transparent to slightly bluish, indicating the
formation of micelles. The mixture was stirred for ca. 24 h at
room temperature and then kept for 5 days before measure-
ments. DLS and TEM were used to characterize the micelle
solutions. M6-M16 were prepared via the same procedure but
with different polymer concentrations as listed in Table 2.
Sh ell-Cr oss-Lin k in g of Micelles. To the micelle solution
of PS(OH)/PVPy was added a 50 mol % of cross-linker 1,4-
dibromobutane based on the pyridine units. The reaction
mixture was stirred for 48 h at 60 °C. The average hydrody-
namic diameter Dh was determined by DLS, and the mor-
phology of the micelles was detected by TEM.
F igu r e 10. Micrograph of H8.
the cavity was created by ozonolysis of polystyrene
blocks.15c
Exp er im en ta l Section
Mea su r em en ts. 1H NMR spectra were acquired on a
Bruker AMX-400 NMR spectrometer with tetramethylsilane
as the internal standard. FTIR spectra were obtained on a
Nicolet Magna 550 spectrophotometer as KBr pellets.
Size exclusion chromatography (SEC) measurements were
conducted using a Waters model 510 HPLC Pump and an
ERMA ERC-7512 refractive index detector, and a set of
columns from Polymer Standard Service. Calibration was
carried out with polystyrene standards. THF as eluent at a
flow rate of 1.0 mL min-1 was used and the fluorine content
of the copolymers (PS(OH)) was measured by anionic chroma-
tography using a Diones 2110I.
Measurements of the relative viscosity (η/η0) of the blend
solutions in chloroform were made as a function of composition
using an Ubbelodhe viscometer at 30.0 ( 0.1 °C. The original
concentrations of the component polymers were 3.0 × 10-3 mg/
mL.
A commercial laser light scattering (LLS) spectrometer
(Malvern Autosizer 4700) equipped with a multi-τ digital time
correlation (Malvern PCS7132) and a solid-state laser (ILT
5500QSL, output power ) 100 mW at λ0 ) 514.5 nm) as light
source was used. The measurements were preformed at 30.0
( 0.1 °C, where the relative error of Dh is less than 2%. The
solutions were respectively clarified using a 0.45-µm Millipore
filter before the measurements and then poured into the cell.
In DLS, the line-width distribution G(Γ) can be calculated from
the Laplace inversion of the intensity-intensity-time cor-
relation function G(2)(q,t).32 The inversion was carried out by
the CONTIN program supplied with the Malvern PCS7132
digital time correlator. G(Γ) can be converted into a transla-
Ca vit a t ion of Sh ell-Cr oss-Lin k ed Micelles. To one
volume of the shell-cross-linked micelle solution was added
one volume of DMF. The mixture was stirred for 12 h at room
temperature and kept for 5 days before measurements. The
micelle solutions were characterized by LLS, TEM, SEM, and
AFM.
Ack n ow led gm en t. This work was supported by the
National Natural Science Foundation of China (NNSFC
No.29992590, 50173006) and the National Basic Re-
search Project-Macromolecular Condensed State Pro-
gram.
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