Metalated diblock and triblock poly(ethylene oxide)-block-poly(4- vinylpyridine) copolymers: Understanding of micelle and bulk structure

Article abstract of DOI:10.1021/jp053333x

The paper provides new insights into the structure of Pt-containing diblock and triblock copolymers based on poly(ethylene oxide) (PEO) and poly(4-vinylpyridine) (P4VP), using a combination of atomic force microscopy (AFM), X-ray diffraction (XRD), transmission electron microscopy (TEM), and anomalous small-angle X-ray scattering (ASAXS). Parallel studies using methods contributing supplemental structural information allowed us to comprehensively characterize sophisticated polymer systems during metalation and to exclude possible ambiguity of the data interpretation of each of the methods. AFM and TEM make available the determination of sizes of the micelles and of the Pt-containing micelle cores, respectively, while a combination of XRD, TEM, and ASAXS reveals Pt-nanoparticle size distributions and locations along with the structural information about the polymer matrix. In addition, for the first time, ASAXS revealed the organization of Pt-nanoparticle-filled diblock and triblock copolymers in the bulk. The nanoparticle characteristics are mainly determined by the type of block copolymer system in which they are found: larger particles (2.0-3.0 nm) are formed in triblock copolymer micelles, while smaller ones (1.5-2.5 nm) are found in diblock copolymer micelles. This can be explained by facilitated intermicellar exchange in triblock copolymer systems. For both systems, Pt nanoparticles have narrow particle size distributions as a result of a strong interaction between the nanoparticle surface and the P4VP units inside the micelle cores. The pH of the medium mainly influences the particle location rather than the particle size. A structural model of Pt-nanoparticle clustering in the diblock PEO-b-P4VP and triblock P4VP-b-PEO-b-P4VP copolymers in the bulk was constructed ab initio from the ASAXS data. This model reveals that nearly spherical micellar cores of about 10 nm in diameter (filled with Pt nanoparticles) aggregate forming slightly oblate hollow bodies with an outer diameter of about 40 nm.

Full text of DOI:10.1021/jp053333x

18786
J. Phys. Chem. B 2005, 109, 18786-18798
Metalated Diblock and Triblock Poly(ethylene oxide)-block-poly(4-vinylpyridine)
Copolymers: Understanding of Micelle and Bulk Structure
,
†
‡
‡
‡
Lyudmila M. Bronstein,* Stanislav N. Sidorov, Vasilii Zhirov, Denis Zhirov,
‡
‡
‡
†
§
Yuri A. Kabachii, Sergey Y. Kochev, Peter M. Valetsky, Barry Stein, Olga I. Kiseleva,
Sergey N. Polyakov, Eleonora V. Shtykova, Elena V. Nikulina, Dmitri I. Svergun, and
§
|
|
|,⊥
Alexei R. Khokhlov§
Department of Chemistry, Indiana UniVersity, 800 East Kirkwood AVenue, Bloomington, Indiana 47405, A.N.
NesmeyanoV Institute of Organoelement Compounds, 28 VaViloVa Street, 119991 Moscow, Russia, Physics
Department, Moscow State UniVersity, GSP-2, Leninskiye Gory, 119992 Moscow, Russia, Institute of
Crystallography, Russian Academy of Sciences, 14 Leninskii pr. 59, 117333 Moscow, Russia, and European
Molecular Biology Laboratory, Hamburg Outstation, Notkestrasse 85, D-22603 Hamburg, Germany
ReceiVed: June 21, 2005; In Final Form: August 11, 2005
The paper provides new insights into the structure of Pt-containing diblock and triblock copolymers based on
poly(ethylene oxide) (PEO) and poly(4-vinylpyridine) (P4VP), using a combination of atomic force microscopy
(AFM), X-ray diffraction (XRD), transmission electron microscopy (TEM), and anomalous small-angle X-ray
scattering (ASAXS). Parallel studies using methods contributing supplemental structural information allowed
us to comprehensively characterize sophisticated polymer systems during metalation and to exclude possible
ambiguity of the data interpretation of each of the methods. AFM and TEM make available the determination
of sizes of the micelles and of the Pt-containing micelle cores, respectively, while a combination of XRD,
TEM, and ASAXS reveals Pt-nanoparticle size distributions and locations along with the structural information
about the polymer matrix. In addition, for the first time, ASAXS revealed the organization of Pt-nanoparticle-
filled diblock and triblock copolymers in the bulk. The nanoparticle characteristics are mainly determined by
the type of block copolymer system in which they are found: larger particles (2.0-3.0 nm) are formed in
triblock copolymer micelles, while smaller ones (1.5-2.5 nm) are found in diblock copolymer micelles. This
can be explained by facilitated intermicellar exchange in triblock copolymer systems. For both systems, Pt
nanoparticles have narrow particle size distributions as a result of a strong interaction between the nanoparticle
surface and the P4VP units inside the micelle cores. The pH of the medium mainly influences the particle
location rather than the particle size. A structural model of Pt-nanoparticle clustering in the diblock PEO-
b-P4VP and triblock P4VP-b-PEO-b-P4VP copolymers in the bulk was constructed ab initio from the ASAXS
data. This model reveals that nearly spherical micellar cores of about 10 nm in diameter (filled with Pt
nanoparticles) aggregate forming slightly oblate hollow bodies with an outer diameter of about 40 nm.
Introduction
not soluble in a selective solvent, the micelle core can be treated
like a quasi-solid, thus, additionally stabilizing the nanoparticles.
A block containing no functional groups but providing solubility
and micelle stability in the solution should form the micelle
corona.
Amphiphilic block copolymers form micelles in selective
solvents (a good solvent only for one block), yet the size and
shape of micelles depend on the block chemical structure,
molecular weight of each block, and solvent type.1-7 The for-
mation of metal, metal oxide, and metal chalcogenide nano-
particles in block copolymer micelles has received considerable
attention as a result of their unique properties. Block copolymer
micelles filled with nanoparticles displayed exceptional cata-
The synthesis of metal or semiconductor nanoparticles in the
cores of amphiphilic block copolymer micelles in organic
1
9,22-27
solvents was reported by many research groups.
Using
block copolymer micelle cores as nanoreactors allows the
synthesis of mono- and bimetallic nanoparticles, yet bimetallic
particle morphology depends on a metal pair, that is, on the
relative ability of the metal species to be reduced in the particular
lytic,8 magnetic,
-12
13-15
and optical
16,17
properties and were used
18
for nanolithography, for improvement of tribological perfor-
19
mance of mineral oil, and in biological and pharmaceutical
2
0,21
applications.
If the block containing the functional groups
9
conditions. These different morphologies significantly change
forms the micelle core, it can be loaded with a corresponding
metal compound and serve as a nanoreactor for nanoparticle
formation and stabilization. Because the core-forming block is
the catalytic properties of such systems, although the nanopar-
ticle sizes are similar. When block copolymer micelles are cross-
linked, this provides an additional degree of stabilization to
nanoparticles and allows the modification of micelle morphol-
*
To whom correspondence should be addressed: lybronst@indiana.edu.
Indiana University.
A.N. Nesmeyanov Institute of Organoelement Compounds.
Moscow State University.
Russian Academy of Sciences.
European Molecular Biology Laboratory.
†
28-30
ogy.
The possibility of using such nanospheres as nanore-
‡
§
|
⊥
actors for inorganic nanoparticles was demonstrated by the
3
0
formation of iron oxide magnetic particles and catalytic Pd
29
nanoparticles. The spatial distribution of nanoparticles in block
1
0.1021/jp053333x CCC: $30.25 © 2005 American Chemical Society
Published on Web 09/20/2005

Metalated Diblock and Triblock Copolymers
J. Phys. Chem. B, Vol. 109, No. 40, 2005 18787
copolymers was also extensively studied by a number of
provide structural information both about nanoscale inhomo-
geneities (particles or clusters) and about the internal ordering
research groups.3
1-35
The selective separation of nanoparticles
between blocks was achieved as a result of the presence of
functional groups in a selected block.
5
0
in the sample. The size and shape of the nanoparticles can be
assessed from the portion of the scattering pattern close to the
primary beam (central scattering). When studying metal nano-
particles in nanostructured polymer matrixes, ASAXS is of
special importance because it allows one to separate and analyze
scattering contributions from the metal nanoparticles and their
Large numbers of amphiphilic block copolymers form mi-
celles with a functionalized core in the organic medium. When
aqueous solutions are favored (from the environmental point
of view or for particular applications), the choice of block
copolymers is very limited, and nanoparticle formation is
normally more complicated as the pH of the medium should
be taken into consideration. A few examples of such block
copolymers include poly(ethylene oxide)-block-poly(2-vinylpy-
5
1,52
polymer environment,
composite materials.
similar to other metal-containing
An examination of these contributions
5
3-55
permits the calculation of the volume distribution functions of
the metal nanoparticles from the central scattering and the
determination of the internal structure of the polymer using
information contained in the characteristic Bragg peaks at higher
36
ridine) (PEO-b-P2VP), polybutadiene-block-poly(ethyelene
oxide) (PB-b-PEO),37 polystyrene-block-poly(2-vinylpyridine)-
5
6-59
38
scattering angles.
Furthermore, recently developed meth-
block-poly(ethyelene oxide) (PS-b-P2VP-b-PEO), and poly-
methoxyhexa(ethylene glycol) methacrylate]-block-[2-(dieth-
6
0
ods allow one to determine the ab initio low-resolution struc-
[
6
1-64
39
ture of particles.
In the present paper, ab initio modeling
ylamino)ethyl methacrylate] (PHEGMA-b-PDEAEMA). When
together with the comprehensive analysis of the ASAXS profiles
is applied for the first time to reveal the structural model of
Pt-nanoparticle organization in the diblock PEO-b-P4VP and
triblock P4VP-b-PEO-b-P4VP copolymers in the bulk, which
is compatible with the results of TEM, AFM, and XRD. In
addition, here we present the first example when, in block
copolymer micelles, nanoparticle size is independent of the
reducing-agent type (similar to nanoparticle growth in a confined
environment), which is attributed to a strong interaction of the
vinyl pyridine units with the growing nanoparticle surface in
the micelle cores cross-linked with platinic acid.
the core-forming block is pH sensitive (such as P2VP or
PDEAEMA), micellization depends on pH.40 For example, at a
pH below 5, PEO-b-P2VP forms a molecular solution in water,
whereas a decrease of pH of the PEO-b-P2VP micellar solution
after the incorporation of metal compounds or metal nanoparticle
formation results in no micelle decomposition as a result of the
interaction with metal species.36
The formation of spherical assemblies of CdS-containing
block copolymer reverse micelles in aqueous solution was
reported in ref 41. These stable assemblies were formed by the
slow addition of water to mixtures of the reverse micelles formed
by PS-b-PAA [PAA stands for poly(acrylic acid)] and single
PS-b-PAA chains. This resulted in large compound micelles
containing quantum-confined CdS nanoparticles. This method
allows the transferring of the CdS nanoparticles formed in the
micelle cores in organic medium to aqueous medium without
the loss of stability or nanoparticle aggregation.
Experimental Section
Materials. CH2Cl2 (99.6%, Aldrich) was purified by shaking
it with a small amount of H2SO4 twice until the acid remained
nearly colorless. Then CH2Cl2 was washed with water (twice),
then with a 5% solution of NaHCO3, and then again with water
(twice) and then left over anhydrous CaCl2 overnight. CH2Cl2
was then mixed with CaH2 for 1 h, boiled for 2 h, and distilled
over CaH2. Toluene (99.5%, Aldrich) was kept over CaCl2
overnight and then distilled. H2PtCl6‚6H2O (Reakhim, Russia)
and hydrazine hydrate (N2H4‚H2O, 98+%, Aldrich) were used
as received.
For the templating of platinum nanoparticles, in this work
we have chosen poly(ethylene oxide)-block-poly(4-vinylpyri-
dine) (PEO-b-P4VP) prepared by atom transfer radical polym-
erization (ATRP) using a PEO macroinitiator. This method is
cheaper and more robust compared to anionic polymerization,
which is used for the synthesis of PEO-b-P2VP.36 The choice
of P4VP as the core-forming block allows easy coordination
with a wide range of metal ions.42 As was reported, the
interaction between P4VP and metal compounds is less sterically
hindered compared to that of P2VP, thus, P4VP should result
in more stable nanoparticles. In our preceding publication we
reported the formation of gold nanoparticles in the diblock PEO-
b-P4VP copolymer.44 This paper describes micellar character-
istics and Pt-nanoparticle formation in the diblock PEO-b-P4VP
and triblock P4VP-b-PEO-b-P4VP micelles using a combination
of transmission electron microscopy (TEM), atomic force
microscopy (AFM), X-ray diffraction (XRD), and anomalous
small-angle X-ray scattering (ASAXS). Each of the methods
provides valuable structural information, and their simultaneous
use gives comprehensive insights into the organization of
complicated polymer systems. Moreover, each technique has
both its intrinsic capabilities and its inherent limitations. Among
these methods, ASAXS has particular significance.
PEO monomethyl ether (M ) 2000 g/mol, Aldrich) and poly-
n
43
(ethylene glycol) (PEG, M ) 12 000 g/mol, Fluka) were
n
dehydrated by the distillation of the water as the azeotrope with
toluene. For this, PEO and PEG were mixed with toluene (7
mL of toluene/1 g of polymer) and distilled at 125-130 °C.
After the evaporation of the toluene, the polymers were dried
under vacuum at 65 °C for 1 h. The products were stored under
argon.
4-Vinylpyridine (4VP, Aldrich) was stirred with a small
amount of CaH at 40-50 °C for 2 h. Then 4VP was decanted,
2
degassed via three freeze-thaw cycles, and distilled under
vacuum into a Schlenk tube. The Schlenk tube containing the
4VP was filled with argon and was stored in a refrigerator at
-15 °C. 2-Propanol (i-PrOH) was purified in a similar manner
as that used for 4VP and stored under argon at room temper-
ature. 2-Chloropropionyl chloride (97%, Aldrich), triethylamine
(TEA, >99.5%, Fluka), and CuCl (98+%, Aldrich) were used
as received. Tris(2-dimethylaminoethyl)amine (Me6TREN) was
For the development of novel nanomaterials with special
properties, the problem of nondestructive quantitative analysis
of structural characteristics determining the properties of the
materials is very important. Small-angle X-ray scattering
6
5
obtained as described elsewhere.
Synthesis. Modification of PEO and PEG with 2-Chloro-
propionyl Groups. PEO and PEG with 2-chloropropionyl
terminated groups (PEO-CP, CP-PEG-CP) were prepared using
a modified version of the technique described by Jankova et
(SAXS) is a diffraction method widely and successfully
employed to analyze the structure of various disperse systems
45-49
66
at resolutions from about 1 to 100 nm.
The SAXS patterns
al. The modification consisted of the use of only TEA as the

1
8788 J. Phys. Chem. B, Vol. 109, No. 40, 2005
Bronstein et al.
HCl absorber. CP-PEG-CP would not crystallize in ethanol;
therefore, it was purified by a triple extraction of the polymer
powder with ethanol (7 mL ethanol/1 g of polymer, 2 h at room
temperature). The completeness of the terminal group modifica-
tion and the purity of the polymers were monitored by H NMR.
No polymer chain destruction was observed.
after the addition of platinic acid, the pH of the reaction solution
was adjusted to 10 before reduction. An excess of water was
removed by ultrafiltration. For PEO45-b-P4VP28-Pt (N2H4), pH
) 10, 5-fold molar excess of N2H4‚H2O (0.446 mL of a 10%
1
-4
aqueous solution, 9.3 × 10 mol) was used instead of NaBH4.
For PEO45-b-P4VP28-Pt (NaBH4 + N2H4), pH ) 10, two
reducing agents were used. First, in an argon atmosphere,
Synthesis of Diblock PEO-b-P4VP. The PEO-b-P4VP block
copolymer was synthesized by the ATRP of 4VP with a PEO-
-
4
NaBH4 (0.0106 g, 2.8 × 10 mol) was added, and then, after
3
0 min, the solution was additionally reduced with N2H4‚H2O
CP macroinitiator using an approach described by Matyjasze-
-
4
wski et al.67 In a typical procedure, a Schlenk tube equipped
(0.314 mL of a 10% aqueous solution, 6.5 × 10
mol). For
PEO45-b-P4VP28-Pt (N2H4 + NaBH4), pH ) 10, 0.134 mL of
with a magnetic stir bar was loaded with 0.107 g (1.08 mmol)
of CuCl, 4.54 g (43.2 mmol) of 4VP, and 0.249 g (1.08 mmol)
of Me6TREN, degassed via three freeze-thaw cycles, and stirred
at 25 °C until there was complete dissolution of CuCl. Then,
using a degassed syringe, this mixture was added to a Schlenk
tube containing a degassed solution of PEO-CP (2.88 g, 44
mmol) in 9.6 mL of i-PrOH in argon counterflow at 40 °C under
vigorous stirring. The homogeneous mixture was placed in a
bath at 50 °C, stirred for 8 h, and kept overnight at room
temperature. The Schlenk tube content was then dissolved in
-4
a 10% N2H4‚H2O aqueous solution (2.8 × 10 mol) was added
-
2
-4
first followed by 2.47 × 10 g (6.5 × 10 mol) of NaBH4.
P4VP29-b-PEO272-b-P4VP29-Pt (NaBH4), pH ) 2. In a typical
experiment for P4VP29-b-PEO272-b-P4VP29, 0.1 g of a triblock
copolymer was dissolved in 100 mL of water and stirred for 2
-4
days. Then H2PtCl6‚6H2O (0.0565 g, 1.1 × 10 mol) was added
to the P4VP29-b-PEO272-b-P4VP29 solution and stirred for 2
days. After ultrafiltration, a degassed reaction solution was
reduced in an argon atmosphere with 5-fold molar excess of
NaBH4 (0.021 g, 5.5 × 10 mol). For P4VP29-b-PEO272-b-
P4VP29-Pt (NaBH4), pH ) 10, the same procedure was used,
except that, after the H2PtCl6‚6H2O addition, the pH of the
reaction solution was adjusted to 10 with 0.1 N NaOH. An
excess of water was removed by ultrafiltration. For P4VP29-b-
PEO272-b-P4VP29-Pt (N2H4), pH ) 10, a 5-fold molar excess
-
4
5
0 mL of tetrahydrofuran (THF), and the 4VP conversion was
evaluated by gas chromatography (95%). The solution was
filtered through Al2O3 powder to remove the Cu complex, and
the THF was distilled off until a residual volume of 20 mL
remained. The polymer was precipitated in diethyl ether and
dried under vacuum at 40 °C for 5 h. The yield was 5.6 g (76%).
The number-average degree of polymerization of the P4VP
blocks was determined from the degree of polymerization of
-4
of N2H4‚H2O (0.264 mL of a 10% aqueous solution, 5.5 × 10
mol) was used instead of NaBH4. For P4VP29-b-PEO272-b-
P4VP -Pt (NaBH + N H ), pH ) 10, two reducing agents
1
the PEO (45) using H NMR (CDCl3). In the resulting PEO-
29
4
2
4
were used. First, in an argon atmosphere, NaBH4 (0.0063 g,
b-P4VP copolymer, the molecular weight of the P4VP block
was 2940; that is, the degree of polymerization was 28.
-4
1
.65 × 10 mol) was added, and then, after 30 min, the solution
was treated with N2H4‚H2O (0.184 mL of a 10% aqueous
solution). For P4VP29-b-PEO272-b-P4VP29-Pt (N2H4 + NaBH4),
Synthesis of P4VP-b-PEO-b-P4VP. In a typical experiment,
a Schlenk tube equipped with a magnetic stir bar was loaded
with 0.45 g (4.55 mmol) of CuCl, 18.20 g (0.173 mol) of 4VP,
and 1.00 g (1.08 mmol) of Me6TREN, degassed via three
freeze-thaw cycles, and stirred at 25 °C until there was
complete dissolution of CuCl. Then, using a degassed syringe,
this mixture was added to a degassed solution of CP-PEG-CP
-
4
pH ) 10, 0.079 mL of a 10% aqueous solution (1.65 × 10
mol) of N2H4‚H2O was added first, followed by 0.0146 g (3.85
-
4
×
10 ) mol of NaBH4.
Characterization. Specimens for TEM were prepared either
by placing a drop of solution onto a carbon-coated copper grid
or by sectioning an epoxy-embedded specimen. In the latter case,
the polymer powder was prepared by crushing the films obtained
after water evaporation from the micellar solutions using a
mortar and pestle. Images were acquired at an accelerating
voltage of 60 kV on a JEOL JEM1010.
(26.00 g, 4.33 mmol) in a mixture of 22 mL of i-PrOH and 22
mL of toluene and placed in the Schlenk tube in argon
counterflow at 40 °C under vigorous stirring. The homogeneous
solution was stirred at 50 °C for 16 h. The Schlenk tube content
was then dissolved in 500 mL of THF, and the 4VP conversion
was evaluated by gas chromatography (86%). The solution was
filtered through Al2O3 powder to remove the Cu complex, and
the THF was distilled off until a residual volume of 200 mL
remained. The polymer was precipitated in diethyl ether and
dried under vacuum at 40 °C for 5 h. The yield was 35.0 g
AFM experiments were performed in tapping mode using the
Nanoscope IIIa (Digital Instruments, U.S.A.) multimode scan-
ning probe microscope. Commercially available standard silicon
125-µm tapping mode cantilevers (Nanoprobe) were used. The
resonant frequency was in the interval of 300-350 kHz, and
the scan rate was maintained at 1.5-2.0 Hz. The samples were
applied onto a freshly cleaved mica surface and left to dry in
air at room temperature. Image processing and presentation were
done with the help of the user-friendly software Femtoscan 001
(Advanced Technologies Center, Russia).
(79%). The number-average degree of polymerization of the
P4VP blocks was determined from the degree of polymerization
1
of the PEG (273) using H NMR (CDCl3). In the resulting
P4VP-b-PEG-b-P4VP copolymer, the molecular weight of the
P4VP blocks was 3050; that is, the degree of polymerization
was 29.
For the estimation of micelle diameters using AFM data, three
different approaches were used, depending on the behavior of
the micelles on the surface. The lateral dimensions of the
corresponding protrusion measured at half of its height give an
overestimated value that is due to the finite probe tip size. The
explanation of this broadening effect, conventional in probe
microscopy, can be found in ref 68.
Metalation of PEO45-b-P4VP28 Micelles. PEO45-b-
P4VP28-Pt (NaBH4), pH ) 2. In a typical experiment for PEO45-
b-P4VP28, 0.1 g of a block copolymer was dissolved in 30 mL
of water and stirred for 2 days. H2PtCl6‚6H2O (0.0962 g, 1.86
-
4
×
10 mol) was added to the PEO45-b-P4VP28 solution and
stirred for 2 days. Then, the solution was ultrafiltrated with an
Amicon ultrafiltration device, degassed to remove oxygen, and
reduced in an argon atmosphere with 5-fold molar excess of
For the images in Figures 1b and 2a, where individual
micelles were separated from each other and the underlying
substrate was clearly seen, the vertical dimensions of the
protrusions were taken as the estimated size.
-
4
NaBH4 (0.0353 g, 9.3 × 10 mol). For PEO45-b-P4VP28-Pt
(
NaBH4), pH ) 10, the same procedure was used, except that,

Metalated Diblock and Triblock Copolymers
J. Phys. Chem. B, Vol. 109, No. 40, 2005 18789
For the samples depicted in Figures 1a and 2b,c, where
micelles aggregated in dense layers and vertical dimensions
could not be measured correctly, the second approach was used.
The mean distance D between the centers of adjacent micelles
was taken as the diameter estimation. The details of this
estimation are presented in Supporting Information.
SCHEME 1: Principle of the ab Initio Method of
Simulated Annealing
The exception was image 2d, which contains micellar
aggregates that were strongly deformed on the surface so that
their vertical dimensions were close to triblock monolayer
thickness. In that case, the diameters of the particles in the
solution were evaluated considering the surface area of the
micellar aggregates to remain unchanged and using simple
geometrical formulas.
The ASAXS study of the pristine and metal-containing
diblock and triblock copolymers (crushed polymer films) was
69
performed at the beamline JUSIFA on the storage ring DORIS
III of the Deutsches Elektronen Synchrotron (DESY, Hamburg)
-
1
in the range of the momentum transfer 0.4 < s < 7.3 nm ,
where s ) 4π(sin θ)/λ, 2θ is the scattering angle, and λ is the
X-ray wavelength. The samples were measured in a 1-mm-thick
aluminum cuvette placed in a vacuum to diminish the parasitic
scattering. The scattering profiles were corrected for the
background scattering and fluorescence using standard proce-
dures.70 The scattering data were collected at different photon
energies, E0 ) 11.482, E1 ) 11.530, E2 ) 11.550, and E3 )
grid of K points corresponding to hexagonal packing small
spheres (beads or dummy atoms) of radius r0 , Dmax. The
structure of the dummy atoms model (DAM) is defined by a
binary configuration vector Xj, j ) 1, ..., K, assigning an index
to each bead. The index Xj ) 0 (or 1) means that the atom
does not belong (or belongs) to the volume occupied by Pt
nanoparticles within the region of their localization (Scheme
1
1.559 keV. The first energy, being far enough from the L3
absorption edge of Pt (energy E ) 11.562 keV corresponding
to wavelength λ ) 0.107 nm),5 was used to study the structure
of the pristine copolymers. The anomalous signals, which
become considerable in the vicinity of the L3, were obtained as
the difference in intensities between the scattering curves
recorded at the first energy E0 and those recorded at the
successive ones, Ik(s) ) I(s,E0) - I(s,Ek), where k ) 1, 2, or 3.
These differences are proportional to the anomalous scattering
from the Pt atoms described by the scattering length density
1,52
1
). The scattering intensity I(s) from the DAM is evaluated as
∞
l
2
2
I(s) ) 2π
|A (s)|
∑ ∑ lm
l)0m)-l
(2)
(3)
2
2
∆
Fk ) (N0 - Nk )e/Vat, where N0 and Nk are the numbers of
electrons contributing to the scattering far from the resonance
i.e., at the photon energy E0) and at the energy Ek, respectively,
(
where the partial amplitudes Alm(s) are
e is the Thompson’s radius of the electron, and Vat is the atomic
volume. Important here is that the anomalous differences
correspond to the same population of Pt particles, just with
different contrasts, and the proportionality of the difference
signals to each other is an additional validation that the signal
comes from the Pt particles and not from the matrix. The particle
size distribution functions DV(R) were further computed using
l
Alm(s) ) i
x
2/πVa j (sr )Y* (ω )
∑j
l
j
lm
j
Here, the sum runs over the dummy atoms with Xj ) 1, rj,
3
ωj as their polar coordinates, Va ) (4πr0 /3)/0.74 is the displaced
71
an indirect transform program GNOM. Assuming the particles
to be spherical, the program solves the integral equation
volume per dummy atom, and jl(x) and Ylm(ω) are the spherical
Bessel function and the spherical harmonics, respectively.
Starting from a random configuration, simulated annealing60 is
employed to find a compact interconnected configuration X
minimizing the discrepancy between the calculated and the
experimental curves. In all ab initio calculations, the diameter
of the spherical search volume Dmax (on average, about 40 nm)
was estimated from the analysis of the p(r) functions as
Rmax
2
2
I(s) ) (∆F)
∫
D (R) m (R) i (sR) dR
(1)
V
0
Rmin
where R is the radius of a sphere, Rmin and Rmax are the minimum
and maximum radii, respectively, and i0(x) ) {[sin(x) - x cos-
x)]/x } and m(R) ) (4π/3)R are the sphere form factor and
3
2
3
71
(
evaluated by GNOM. The dummy atoms radii were 1.5-2.0
volume, respectively. In the computations of DV(R), the value
of Rmin was kept at 0 and that of Rmax was selected for each
individual data set by successive runs with different values of
the parameter. The repeating distances of the periodical motifs
in the crystalline regions dh ) 2π/smax, corresponding to the peak
position smax on the scattering patterns, were calculated using
the program PEAK.71
nm, yielding about K ) 2000 beads in the search volume. To
analyze the stability of the reconstructions, results of about a
dozen independent runs were averaged using the program
7
2
DAMAVER.
The alternative analysis of the Pt particles formed in the
copolymer matrix was performed using the program
73
MIXTURE. This program allows one to model mixtures
The low-resolution shapes of the regions of Pt-nanoparticle
localization were reconstructed ab initio from the initial portions
containing up to 10 different components, that is, different types
of particles, each characterized by its volume fraction, type of
form factor, contrast, type of polydispersity, and potential for
interparticle interactions. The experimental scattering pattern
-
1
of the scattering patterns (s < 1 nm ) using the program
DAMMIN.6
1,62,71
A sphere of diameter Dmax is filled by a regular

18790 J. Phys. Chem. B, Vol. 109, No. 40, 2005
Bronstein et al.
SCHEME 2: Schematic Representation of Triblock
P4VP29-b-PEO273-b-P4VP29 Copolymer Micelles
2. Incorporation of Platinic Acid and Its Reduction: AFM.
As a Pt precursor, we chose platinic acid, H2PtCl6‚6H2O,
because incorporation of this compound is accompanied by the
protonation of pyridine units, facilitating interaction with PtCl62
ions resulting from electrostatic attraction. By AFM, interaction
of both diblock and triblock PEO-b-P4VP copolymer micelles
with H2PtCl6‚6H2O at a molar ratio of N/Pt ) 3:1 yields nearly
no changes in micelles measuring 25 ( 4 nm for diblock (Figure
-
2
a) and 23 ( 4 nm for triblock copolymers (Figure 2c). At the
same time, incorporation of platinic acid causes even stronger
interaction between triblock copolymer micelles: even at a 50-
fold dilution of the reaction solution, no individual micelles were
observed. Subsequent reduction of micellar solutions with
NaBH4 does not induce any changes in the PEO45-b-P4VP28
micelles (a mean diameter is 24 ( 4 nm; Figure 2b), while
P4VP29-b-PEO273-b-P4VP29 micelles change dramatically. First,
interaction between micelles is suppressed so individual micelles
can be easily observed (Figure 2d). In addition, the micelle size
increases to 50 ( 10 nm. These facts suggest that (i) each
P4VP29-b-PEO273-b-P4VP29 macromolecule is located within a
single micelle (flowerlike morphology) and (ii) the aggregation
number should increase when the particles are formed. Con-
sidering that a pH change and interaction of 4VP units on the
Pt-nanoparticle surface are the same for both diblock and
triblock copolymer systems, the only difference is likely the
suppression of extended PEO chain conformation between
micelles favoring formation of individual flowerlike micelles.
is fitted by a weighted combination of individual curves from
the components, and the parameters of the best fit are
determined.
Results and Discussion
1
. Block Copolymers: Synthesis and Micellization. The
synthesis of the diblock PEO45-b-P4VP28 and triblock P4VP29-
b-PEO273-b-P4VP29 copolymers was carried out using ATRP
of 4VP with modified PEO or PEG as the macroinitiators (see
Experimental Section). First, PEO or PEG were reacted with
halogenated acid chloride to provide the required functionality
for ATRP. An example of diblock copolymer formation is given
below:
3
. Micelle Characteristics and Pt-Nanoparticle Forma-
tion: TEM and XRD. The metal nanoparticle formation along
with the micelle characterization was monitored by TEM.
Incorporation of platinic acid in PEO45-b-P4VP28 micelles results
in a strong interaction between micelles yielding the micelle
coalescence (Figure 3a), and the observable micelle diameter
is about 12 nm. This value differs from that provided by AFM,
suggesting that the TEM image primarily shows a micelle core
Then this compound was employed in 4VP ATRP using CuCl
as a catalyst:
7
6
containing Pt species, while AFM yields the size of the entire
micelle. Indeed, if the P4VP block had a fully extended
conformation, the micelle core should have measured about 11.2
nm, which would agree with the TEM data. It is worth
mentioning that the P4VP block is pH sensitive; that is, it
molecularly dissolves in water at a pH below 4.7 (see above).
At the same time, block copolymer micelles loaded with platinic
acid do not deteriorate at low pH, similar to PEO-b-P2VP
For both block copolymers that are based on P4VP, water is
a selective solvent. However, while the PEO-b-P2VP becomes
molecularly soluble at pH 5.0,40 the PEO45-b-P4VP28 requires
a slightly lower pH (4.5-4.7) for its dissolution. Because in
P4VP29-b-PEO273-b-P4VP29 the core-forming blocks are termi-
3
6
micelles reported by us earlier. Moreover, the micelle sizes
derived from the AFM data stay unchanged, revealing cross-
linking of the micelle cores due to complexation with PtCl62
ions.
nal, in aqueous solutions they should form flowerlike mi-
-
celles,7
4,75
suggesting also a high probability of connections
between the micelles as shown in Scheme 2.
Indeed, this block copolymer forms a stable gel at a
concentration of 25 g/L, but at lower concentrations, a micellar
solution is obtained. Micelles of diblock and triblock copolymers
were characterized with AFM. Figure 1a shows a layer of
diblock copolymer micelles with a mean diameter of 21 ( 2.5
nm. The micelles do not coalesce despite being deposited from
the reaction solution with a concentration of 3 g/L. In contrast,
triblock copolymer micelles tend to coalesce, even when the
covering of the mica surface is very low (Figure 1b). The
estimated mean diameter for these micelles is 22 ( 3 nm.
Apparently, despite a much longer PEO block in the triblock
copolymer, a similar size of core-forming blocks determines
the size of the dried block copolymer micelles. Higher surface
coverage for this sample yields scalelike films instead of a
micelle monolayer (Figure 1c), suggesting that participation of
a triblock copolymer macromolecule in the formation of two
micelles facilitates further micelle aggregation.
To study the influence of reaction conditions on the particle
and micelle sizes, we varied the pH of the reaction medium
after platinic acid incorporation. Naturally, the pH is about 2
when the ratio of N to Pt is 3 to 1. To reach a pH of about 10,
NaOH was added. In Figure 3b, one can see that the PEO45-
b-P4VP28 micelles that were reduced at pH ) 2 are rather diffuse
and ill-defined, tending to form larger aggregates. Evidently,
despite an observable micelle core size of 11-13 nm (similar
to the H2PtCl6-filled micelles), the micelles are loose and tend
to aggregate at low pH. At pH ) 10, the micelles are better
defined, although the sizes discerned from the TEM (where
mainly micelle cores are seen) are nearly the same. These
micelles are also inclined to aggregate but in a more regular
fashion, forming necklacelike (Figure 3c) or wormlike (not
shown) micelles. Similar micelle morphologies were reported
4
4
by us earlier for Au-filled PEO45-b-P4VP28. The Pt nanopar-
ticles formed in these micelles at pH 2 and 10 have mean

Metalated Diblock and Triblock Copolymers
J. Phys. Chem. B, Vol. 109, No. 40, 2005 18791
Figure 1. AFM images of PEO45-b-P4VP28 (a) and P4VP29-b-PEO273-b-P4VP29 with lower (b) and higher (c) degrees of covering.
2 6 2 6
Figure 2. AFM images of PEO45-b-P4VP28-H PtCl (a), PEO45-b-P4VP28-Pt (b), P4VP29-b-PEO273-b-P4VP29-H PtCl (c), and P4VP29-b-PEO273-
b-P4VP29-Pt (d).
diameters of 1.8 and 1.3 nm, respectively (see Table 1), and
their distribution within micelles is different. At pH 10 the
nanoparticles are evenly distributed through the micelles (no
empty micelles are observed), whereas micelles obtained after
Pt-nanoparticle formation at low pH show uneven particle
distribution: some micelles look completely empty. We think
that, at pH ) 10, dense micelle cores allow no exchange
between the micelles during particle growth; thus, micelles are
evenly filled with nanoparticles. Oppositely, at pH ) 2 exchange
is facilitated between swollen micelles, so even fast reduction
with NaBH4 results in an uneven distribution of particles within
micelles.
Pt nanoparticles formed after NaBH4 reduction measure about
2.2 nm (Table 1). They form nanoparticle ensembles (Figure
4b), which should represent the micelle cores. However, the
block copolymer phase is practically invisible as a result of
low electron density. When the particles are small, they provide
an electron contrast to the micelles (similar to staining), while
larger nanoparticles do not assist in contrasting the polymer
phase. Because the sizes of the particle ensembles differ, this
should reflect uneven distribution of nanoparticles in the
micelles, but unlike the diblock copolymer, this uneven distribu-
tion is observed both in acidic and in basic media. Apparently,
restructuring of the micelles during nanoparticle formation
facilitates an exchange between micelles and uneven nanopar-
ticle distribution. The largest nanoparticle ensembles measure
about 21 nm, confirming the AFM data which shows an in-
The TEM image of P4VP29-b-PEO273-b-P4VP29 micelles
filled with platinic acid shows dark circles with diameters of
about 14 nm essentially attached to each other (Figure 4a). The

18792 J. Phys. Chem. B, Vol. 109, No. 40, 2005
Bronstein et al.
Figure 3. TEM images of PEO45-b-P4VP28-H
2
PtCl
TABLE 1: Characteristics of Pt Nanoparticles Obtained from XRD, TEM, and ASAXS Data
block copolymer sample no. and reduction conditions
PEO45-b-P4VP28 1; pH ) 2, NaBH
6 4
(a) and PEO45-b-P4VP28-Pt after NaBH reduction at pH ) 2 (b) and pH ) 10 (c).
1 c
2 d
d
XRD,a nm
d
TEM,b nm
d
ASAXS , nm
d
ASAXS , nm
R
g
4
2.0
amorphous
1.8 ( 0.3
1.4 ( 0.4
1.3 ( 0.3
1.3 ( 0.2
2.6 ( 0.4
2.2 ( 0.3
2.5 ( 0.3
2.2 ( 0.4
2.7 ( 0.4
2.0 ( 0.3
1.6
16.2
2
3
4
5
; pH ) 10, NaBH
; pH ) 10, N + NaBH
; pH ) 10, NaBH
; pH ) 10, N
4
+ N
2
H
4
2
H
4
4
3.1
amorphous
5.4
2.1
1.7
2.5
2.2
3.1
17.3
17.2
17.0
9.8
4
2
H
4
40
20
40
P4VP29-b-PEO273-b-P4VP29
6; pH ) 2, NaBH
4
2.8
3.5
3.0
2.4
7
8
9
1
; pH ) 10, NaBH
; pH ) 10, N
; pH ) 10, NaBH
0; pH ) 10, N
4
+ N
2
H
4
4
19.3
2
H
4
+ NaBH
4
2.5
2.2
17.8
19.3
2
H
4
6.3
40
a
Mean Pt-nanoparticle diameter obtained from XRD data. b Mean Pt-nanoparticle diameter obtained from TEM (100 nanoparticles were calculated).
c
Mean Pt-nanoparticle diameter from ASAXS data; square deviation is 0.4 nm. d Mean diameter of Pt-particle ensembles from ASAXS data.
Figure 4. TEM images of P4VP29-b-PEO273-b-P4VP29-H
in b shows the cross section of the same sample obtained by imbedding the powder (crushed bulk film) in epoxy resin.
2 6 4
PtCl (a) and P4VP29-b-PEO273-b-P4VP29-Pt after NaBH reduction at pH ) 2 (b). Inset
crease of P4VP29-b-PEO273-b-P4VP29 micelles after nanoparticle
formation.
observed,77,78 although as recently reported, amorphization may
be associated with an energy gain. We believe that, in our
case, a strong interaction between the Pt species and the 4VP
units may prevent the formation of the crystalline phase.
7
9
It is noteworthy that, for the majority of the samples (with
nos. 1 and 9 from Table 1 the only exceptions), the nanoparticle
diameters obtained from the XRD data are larger than those
obtained from the TEM images. This could be due to the
presence of a small fraction of larger particles that are not
detected with TEM. Another possible reason for this discrepancy
is an amorphous character of the small Pt nanoparticles (note
that, for sample nos. 2 and 4, no crystalline phase was detected).
A comparison of the TEM and XRD data with those obtained
from ASAXS (see section 4.2) will allow us to further clarify
this issue. In general, amorphous Pt nanoparticles are rarely
For pH 10, further variations of reduction conditions were
explored. Normally, the use of different reducing agents with a
different nucleation rate allows one to change the nanoparticle
characteristics and morphology. In particular, fast-acting reduc-
ing agents yield small particles, while sluggish reducing agents
2
3
give large particles. In this work, we explored the possibility
of combining two reducing agents of different strengths and
compared the results with those for individual reducing agents.
We assumed that if only 30% of the Pt atoms are nucleated

Metalated Diblock and Triblock Copolymers
J. Phys. Chem. B, Vol. 109, No. 40, 2005 18793
with NaBH4 (fast reducing agent), a limited amount of nuclei
should be formed. If the further reduction is directed by a
sluggish reducing agent (N2H4‚H2O), the larger particles should
grow. The opposite addition order of the reducing agents should
lead to a different morphology. One could expect two popula-
tions of particles: the larger ones nucleated by N2H4‚H2O and
the smaller ones nucleated by the further addition of NaBH4.
However, from Table 1, one can see that, for both block co-
polymer systems, the latter case does not result in broader par-
ticle distribution, while the former case does not yield larger
particles. In general, independently of the reducing agent, larger
particles grow in the triblock copolymer micelles than in diblock
micelles; this can be attributed to an easier exchange between
the micelles in the former case. Apparently, only reduction with
hydrazine hydrate (sluggish reducing agent) leads to larger
particles in the diblock copolymer micelles, while in triblock
copolymer micelles, the Pt nanoparticles are even smaller after
hydrazine reduction than in some other cases. In our earlier work
we observed that the nanoparticle size does not depend on the
reducing agent type when nanoparticles grow in a confined
environment5 or when a strong interaction between the poly-
mer matrix and the growing nanoparticles in the strongly cross-
linked environment overpowers kinetically controlled nucleation
and growth.58 Here, we present the first example when a similar
mechanism is observed for the block copolymer micelles, and
it should be attributed to a strong interaction between the vinyl
pyridine units and the growing nanoparticle surface in the
micelle cores cross-linked with platinic acid (when nucleation
is slow, more “cross-links” remain unchanged in the course of
the reduction resembling conditions of ref 58).
7,80
4. ASAXS Investigation of Diblock and Triblock Copoly-
mers Containing Pt Nanoparticles. ASAXS was applied to
study particle size distributions of Pt nanoparticles embedded
in the PEO45-b-P4VP28 and P4VP29-b-PEO273-P4VP29 micelles.
The use of ASAXS allows one to accurately determine the
influence of the reducing agent type, pH, and polymer structure
on the size distributions of the metal nanoparticles formed in
the block copolymer micelles. As the measurements have been
performed on powders acquired by crushing the films obtained
after water evaporation from the micellar solutions, the location
of the nanoparticles was directly analyzed in bulk block
copolymers.
Figure 5. ASAXS profiles from PEO45-b-P4VP28 (a) and P4VP29-b-
PEO273-b-P4VP29 (b) copolymers. In part a: curve 1 is from PEO45
b-P4VP28, pristine; curve 2 is from PEO45-b-P4VP28-H PtCl at pH 10;
curve 3 is from PEO45-b-P4VP28-H PtCl at pH 2; curve 4 is from
-
2
6
2
6
4
.1. Scattering Profiles. The experimental scattering profiles
PEO -b-P4VP -Pt at pH 10, reduced with NaBH ; curve 5 is from
45
28
4
from both systems, measured at the beam energy far from the
Pt absorption edge, are displayed in Figure 5. The observed
central scattering points to the presence of nanoscale structural
heterogeneities in all samples, including pristine block copoly-
mers (Figure 5a,b, curve 1). In the latter case, the central
PEO45-b-P4VP28-Pt at pH 10, reduced with NaBH
4
and N
2
H
4
; curve 6
; curve 7 is
. In part b: curve
is from PEO45-b-P4VP28-Pt at pH 10, reduced with N
from PEO45-b-P4VP28-Pt at pH 2, reduced with NaBH
is from P4VP29-b-PEO273-b-P4VP29, pristine; curve 2 is from P4VP29
2 4
H
4
1
-
b-PEO273-b-P4VP29-Pt at pH 10, reduced with NaBH
P4VP29-b-PEO273-b-P4VP29-Pt at pH 10, reduced with NaBH
4
; curve 3 is from
4 2 4
and N H ;
scattering can be explained by a microphase separation of block
curve 4 is from P4VP -b-PEO -b-P4VP -Pt at pH 10, reduced with
29
273
29
copolymers in the bulk.8
1,82
After incorporation inside the
2 4
N H
; curve 5 is from P4VP29-b-PEO273-b-P4VP29-Pt at pH 2, reduced
with NaBH
4
.
micelle core occurs, platinic acid may form clusters, as discussed
in our preceding papers,5 and these clusters further contribute
to the central maximum.
9,80
incorporation of platinic acid in the diblock copolymer, bringing
the pH of the reaction solution to 2, results in two weak maxima
-
1
Moreover, some of the scattering patterns that occur after
platinic acid incorporation (Figure 5a, curves 2 and 3) display
shoulders or weak maxima pointing to the formation of weakly
ordered quasi-crystalline structures. The positions and ampli-
tudes of these peaks depend on the chemical composition and
pH of the block copolymer systems. When platinic acid is
incorporated in block copolymer micelles, it serves as a cross-
linking agent for the block copolymer cores, which influences
the microphase segregation in the bulk after the evaporation of
water and leads to a partial self-ordering in the samples
compared to that of the pristine block copolymers. The
at 0.2 and 0.5 nm (Figure 5a, curve 2), corresponding to
periodicities of about 30 and 12 nm, respectively. When the
pH is adjusted to 10, after the platinic acid loading, the peaks
for the bulk sample appear at 0.3 and 0.6 (Figure 5a, curve 3),
that is, a periodicity of about 20 and 10 nm. The equidistant
peak positions suggest a lamellar ordering in the system at pH
10. The ordering at pH 2 appears to be more complicated and
cannot be determined from the available data. Comparison of
these results with TEM and AFM data suggests that both for
pH 2 and for pH 10, the larger periodicity is close to the micelle
size while the shorter period matches the micelle core size.

18794 J. Phys. Chem. B, Vol. 109, No. 40, 2005
Bronstein et al.
Figure 6. Difference ASAXS data (anomalous signal) from the Pt-
containing PEO45-b-P4VP28 copolymer: curve 1 is from PEO45-b-
Figure 7. Volume distribution functions of the Pt nanoparticles in
PEO45-b-P4VP28 copolymer. The numbering of the distributions and
colors of the symbols correspond to those in Figure 6.
P4VP28-H
pH 2; curve 3 is from PEO45-b-P4VP28-Pt at pH 10, reduced with
NaBH ; curve 4 is from PEO45-b-P4VP28-Pt at pH 10, reduced with
NaBH and N ; curve 5 is from PEO45-b-P4VP28-Pt at pH 10, reduced
with N ; curve 6 is from PEO45-b-P4VP28-Pt at pH 2, reduced with
NaBH . The successive groups of curves are displaced one logarithmic
2 6 2 6
PtCl at pH 10; curve 2 is from PEO45-b-P4VP28-H PtCl at
4
the average radius of about 2.5 nm. After the reduction of the
Pt ions, the magnitude of the distribution functions increases
compared to that of the H2PtCl6 clusters as a result of higher
contrast of the metal particles, and the distributions become
rather narrow with a maximum at about R ) 1 nm (Table 1).
Some larger particles are present in smaller amounts. The
exception is the Pt-containing sample reduced by the sluggish
reducing agent N2H4 at pH 10, which contains a distinct fraction
of large particles with radii of more than 20 nm (Figure 7, curve
4
2 4
H
2 4
H
4
unit down for better visualization. For each sample, the three differences
are between the measured intensity far from the absorption edge and
those near the edge I(s,E
section. Exceptions are PEO45-b-P4VP28-H
0
k
) - I(s,E ) as described in the Characterization
2
PtCl
6
at pH 10 and 2, where
the anomalous difference for the first energy E
1
near the edge was not
reliable.
6
). Our results indicate that the influence of the initial pH on
Apparently, cross-linking of the cores with platinic acid results
in preservation of the micelle core sizes when the micellar
solution is evaporated to obtain bulk material.
the particle formation is negligible for this system. A decrease
in the Pt-nanoparticle size compared to that of the platinic acid
cluster size is consistent with an increase of the Pt-metal density
so platinic acid clusters serve as nuclei for Pt-nanoparticle
formation.
P4VP29-b-PEO273-P4VP29. Figures 8 and 9 present anomalous
signals for the triblock copolymer system and particle size
distribution functions, respectively. The distributions have a
major fraction of small particles with a radius of about 1 nm
After the reduction of the Pt ions and the formation of the
metal nanoparticles, the maxima in the scattering profiles
disappeared, pointing to the further loss of order in the internal
structure of the copolymer matrix (quantitative characterization
of the reorganized structures in the absence of maxima is no
longer possible). Here, an interaction of the core-forming block
with nanoparticles should occur as a result of the adsorption of
pyridine units on the nanoparticle surface. However, this kind
of interaction may involve fewer pyridine units than the inter-
action with platinic acid, so the micelle core containing the
nanoparticles is easier to reshape when the bulk material is
formed.
(Table 1), and nearly all the samples contain some amount of
large particles (note that scattering from the large particles is
significantly more intense than that from small particles, so even
a small fraction of large particles yield a noticeable contribution
to the scattering profiles). Only the sample with the Pt particles
obtained after a NaBH4 reduction at pH 10 (Figure 9, curve 1)
displays a practically monomodal distribution profile without
the larger particles. Interestingly, the reduction by the same
reducing agent at pH 2 leads to the formation of a broad fraction
of large particles displaying a maximum at R ) 10 nm (Figure
4
.2. Particle Size Distributions. PEO45-b-P4VP28. To obtain
the scattering from the Pt-containing particles, differences
between the SAXS intensities at different energies were
calculated. Analyzing the energy dependence allows the separa-
tion of particle and polymer matrix contributions. Figure 6
demonstrates the sets of difference curves between the scattering
profiles at the first energy and those at the three successive
energies.
Most of the differences (three for each sample) are propor-
tional to the expected anomalous signal. The only exception is
the diblock copolymer containing platinic acid at two different
pHs (Figure 6, curve sets 1 and 2), where only two difference
curves were meaningful. The particle size distributions computed
by GNOM for the diblock copolymer systems are presented in
Figure 7. The profiles for Pt-containing clusters (curves 1 and
9
, curve 4). The reduction with hydrazine hydrate and with a
sequential addition of hydrazine hydrate and sodium borohydride
yields yet larger particles with R of about 20 nm (Figure 9,
curves 2 and 3). Because neither TEM nor XRD reveals the
large particles, one may assume that these larger scatterers (with
diameters of ∼20 to 40 nm) are polymer nanostructures filled
with Pt species (see the next sections). A similar phenomenon
5
9
was discussed in our preceding paper.
Comparison of the Diblock and the Triblock Copolymer
Systems. For comparison, the average Pt particle sizes were
computed from the distribution functions obtained for the two
copolymer systems. The distributions calculated from the three
anomalous differences were computed independently for each
2
were multiplied by a factor of 10 for better visualization)
reveal the major relatively broad fraction of the clusters with

Metalated Diblock and Triblock Copolymers
J. Phys. Chem. B, Vol. 109, No. 40, 2005 18795
reducing agent type. Second, reduction with sluggish hydrazine
hydrate always leads to a bimodal profile of the volume
distribution function independently of polymer systems and pH
conditions, whereas the fast reducing agent (NaBH4) yields
different results for different systems. Third, the distribution
functions for the P4VP29-b-PEO273-P4VP29 system are broader
than those for diblock copolymers. This observation can be
explained by the facilitated intermicellar exchange for the
triblock copolymers as compared with that of the diblock
copolymers. Last, all the samples contain a certain amount of
large particles and, thus, yield relatively high values of the radii
of gyration. The reason is that the experimental radius of
gyration, Rg, of a polydisperse system is biased toward the large
particles being a z average
∞
∞
2
2
6
6
〈
R 〉 )
∫
R (R) D (R)R dR/
∫
D (R)R dR )
g
z
g
V
V
0
0
∞
2
6
6
∫
R
g
(R) D (R)R dR/〈R 〉
V
0
2
8
6
Figure 8. Difference ASAXS data (anomalous signal) from the Pt-
containing P4VP29-b-PEO273-b-P4VP29 copolymer: curve 1 is from
(for polydisperse spheres 〈Rg 〉z ) 3〈R 〉/5〈R 〉). As one can see
from Table 1, the experimental Rg values exceed 16 nm,
reflecting this bias. The only exception is the sample P4VP29-
b-PEO273-P4VP29 after a NaBH4 reduction at pH 2, which
displays no maximum at 10 or 20 nm, leading to the mean Rg
value of 9.8 nm. Generally, for PEO45-b-P4VP28, the amount
of large particles seems to be negligible with the exception of
the Pt-containing sample reduced by the sluggish reducing agent
N2H4, but almost all triblock copolymer samples contain a larger
number of particles with sizes of about 40 nm. These large
particles are attributed to polymer regions filled with Pt
nanoparticles, the scattering of which occurs as the scattering
4
VP29-b-PEO273-b-P4VP29-Pt at pH 10, reduced with NaBH
is from P4VP29-b-PEO273-b-P4VP29-Pt at pH 10, reduced with NaBH
and N ; curve 3 is from P4VP29-b-PEO273-b-P4VP29-Pt at pH 10,
reduced with N ; curve 4 is from P4VP29-b-PEO273-b-P4VP29-Pt at
pH 2, reduced with NaBH . The successive curves are displaced one
4
; curve 2
4
2
H
4
2 4
H
4
logarithmic unit down for better visualization. For each sample,
differences were calculated using scattering profiles recorded at different
energies (see Figure 6).
5
8,59
of individual Pt-containing structures.
Interestingly, for all the samples, the particle diameters
obtained from XRD are systematically larger than those from
ASAXS. Because both methods, unlike TEM, provide an
average over large numbers of particles, the observed difference
indicates that the smaller Pt particles in the ensembles are
amorphous. The amorphous particles are detected by ASAXS
but not by XRD, thus, leading to smaller effective sizes provided
by the former method.
4
.3. Shape Reconstruction of the Regions Containing Pt
Nanoparticles. An attempt was further made to reconstruct the
low-resolution Pt-particle shapes from the ASAXS data using
60-62
the ab initio simulated annealing procedure.
The nanoscale
structure is represented by a set of densely packed beads of
two kinds, matrix and particle, as in Scheme 1. The shape, in
terms of the beads belonging to the particle, is obtained by fitting
the experimental data without a priori information, except for
the maximum size of the object (Rmax) as calculated by the
program GNOM. In principle, this approach is valid for
systems of identical particles but, under certain conditions, can
also be used for polydisperse systems. In this particular case,
the use of the ab initio method can be justified by the TEM
data, showing the rather homogeneous populations of Pt-con-
taining micelles in all the samples.
The simulated annealing method was applied to the difference
ASAXS data, and independent reconstructions were averaged
as described in Experimental Section (see Scheme 3). The radius
of the beads representing the shape of the Pt-rich aggregates
was selected to be 1 nm, that is, the average radius of the small
Pt particles. For both copolymer systems and in all conditions
of nanoparticle formation, slightly oblate hollow shapes were
obtained (a typical shape is displayed in Scheme 3c). These
hollow bodies have the following average diameters: diblock
Figure 9. Volume distribution functions of the Pt nanoparticles in the
P4VP29-b-PEO273-b-P4VP29 copolymer. The numbering of the distribu-
tions and colors of the symbols correspond to those in Figure 8.
7
1
sample, and the value of Rmax in GNOM calculations71 was
varied as described in Experimental Section. The average data
presented in Table 1 indicate that Pt nanoparticles prepared in
PEO45-b-P4VP28 micelles are systematically smaller than those
1
in P4VP29-b-PEO273-P4VP29 micelles: dASAXS ) (1.5-2.5) (
1
0
.4 nm for diblock copolymers and dASAXS ) (2.0-3.0) ( 0.4
nm for triblock copolymers. This result is in good agreement
with the TEM data.
Overall, formation of Pt nanoparticles in PEO45-b-P4VP28 and
P4VP29-b-PEO273-P4VP29 copolymers reveals both common
features and clear differences. First, all the samples contain
major fractions of small particles with the maximum at about
R ) 1 nm, independent of the polymer matrix, the pH, and the

18796 J. Phys. Chem. B, Vol. 109, No. 40, 2005
Bronstein et al.
SCHEME 3: Computation of Shape Models of PEO45-b-P4VP28 and P4VP29-b-PEO273-b-P4VP29 Copolymers for the
Screen Shot of the nth Run of the Computations (a), the Accumulation of the Intermediate Models (b), and the Averaged
Shapes (c)
copolymers, Douter ≈ 40 ( 5 nm, Dinner ≈ 25 ( 5 nm; triblock
copolymers, Douter ≈ 45 ( 5 nm, Dinner ≈ 25 ( 5 nm.
scatterers in the size distribution functions in Figures 7 and 9
(maxima at R ) 20 nm). As the shapes deduced from the
anomalous signal reflect the distribution of the pure Pt ag-
gregates in the system and are not sensitive to the scattering
from the polymer phase, the following model of the overall
organization of the Pt-containing micelles in the bulk can be
suggested. The micelle cores, with sizes of about 10 nm,
aggregate to form slightly oblate hollow bodies with an outer
Given the low resolution of the reconstruction, the thickness
of the spherical layer is about h ) (Douter - Dinner)/2 ) 7-12
nm for both systems, which is in agreement with the sizes of
the micelle cores as determined by TEM. Moreover, the sizes
of the spheroids approximating the shapes of the regions of the
nanoparticle localization coincide with those of the larger

Metalated Diblock and Triblock Copolymers
J. Phys. Chem. B, Vol. 109, No. 40, 2005 18797
SCHEME 4: Model of the Micelle Organization in the
PEO45-b-P4VP28 and P4VP29-b-PEO273-b-P4VP29
Copolymers in the Bulk
single micelle (flowerlike morphology). From TEM, the observ-
able micelle core size for diblock copolymer micelles after Pt-
nanoparticle formation is in the range 11-13 nm.
For most samples, the diameters of the Pt nanoparticles
obtained from XRD data are larger than those obtained from
TEM images. This could be due to the presence of a small
fraction of larger particles that are not detected with TEM
and/or due to the amorphous character of small Pt nanoparticles.
The comparison with the ASAXS data on particle sizes confirms
the latter suggestion. Yet, the particle size is practically
independent of the type of reducing agent, resembling a “cage”
8
0
effect that was observed by us earlier. This can be attributed
to a strong interaction between the vinyl pyridine units and the
growing nanoparticle surface in the micelle cores cross-linked
with platinic acid.
Shape simulation of the regions containing nanoparticles
showed that spherical micelle cores, with sizes of about 10 nm,
aggregate forming slightly oblate hollow bodies with an outer
diameter of about 40 nm. Apparently, the Pt-deficient interior
of these spheroids is filled by the PEO phase.
A contribution from each applied method of structural
characterization allowed us to reconstruct a general picture of
the metalation of diblock PEO45-b-P4VP28 and triblock P4VP29-
b-PEO273-b-P4VP29 copolymers and to offer the entire model
of the copolymer configurations containing Pt nanoparticles.
diameter of about 40 nm (Scheme 4); the center of the spheroid
is filled with a PEO block, while the Pt particles are located at
its periphery. This result is particularly valuable because no such
detailed structural information can be obtained from TEM. As
can be seen from the inset to Figure 4b, showing a cross section
of the triblock copolymer sample with Pt nanoparticles embed-
ded in the epoxy resin, nanoparticle distribution in the bulk looks
very similar to that of the micelles dried on the carbon-coated
grid, but no details can be discerned. Thus, ASAXS provides
important structural information which cannot be obtained from
other techniques.
The above model of the Pt-rich clusters consists of small
particles forming large aggregates, that is, two types of objects
with distinctly different dimensions. The scattering from the
entire system should be a sum of the scattering contributions
from the two types of particles, and this model can be further
verified using an alternative fitting by the program MIXTURE
Acknowledgment. We sincerely thank NATO Science for
Peace Programme (SfP-974173) and Russian Foundation for
Basic Research (Grant RFBR-04-03-32928) for financial support
of this research. We would like to acknowledge the HASYLAB
(DESY, Hamburg) for providing the beam time at JUSIFA
beamline and Dr. G. Goerigk for assistance in the anomalous
scattering measurements and data processing.
Supporting Information Available: Estimation of D from
the AFM data. This material is available free of charge via the
Internet at http://pubs.acs.org.
(see Experimental Section). The two particle types were small
solid spheres and large spheres with two variable densities in a
concentric core and shell, and both types of particles could be
polydispersed (Gaussian distribution). The best fits to the
experimental data (not shown) were provided by a mixture of
fairly monodisperse (10%) small particles with R ≈ 1 nm and
large particles with R of about 20 nm, an outer-shell thickness
of about 8 nm, and a ratio of shell to core density of about 2:1.
These results further corroborate the ab initio model in Scheme
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