Polymer nanocomposites containing superstructures of self-organized platinum colloids

Article abstract of DOI:10.1021/jp004304j

Colloidal platinum (diameter 1-2 nm) in styrene was prepared in situ by reduction of a platinum(II) compound followed by partial evaporation of styrene. The resulting dispersions were purified from the reaction side-products, and ammonium O,O'-dialkyldithiophosphates of different chain length or octadecanethiol was added. Polymerization was started with a radical initiator. Several parameters were varied and, particularly in the presence of long-chain dialkyldithiophosphates, self-assembled superstructures of metal colloids appeared at certain conditions in the resulting poly(styrene)-platinum nanocomposites. Unusual hollow shell structures of typical diameters of 50-300 nm were found, which phenomenologically resembled bilayer vesicles in aqueous solutions, although the formation mechanism of the bilayers and the superstructures in the nanocomposites is considered to differ. It is supposed that the formation of the superstructures in the nanocomposites is induced by crystallization of alkyl chains adsorbed at the platinum surface below monolayer coverage.

Full text of DOI:10.1021/jp004304j

J. Phys. Chem. B 2001, 105, 7399-7404
7399
Polymer Nanocomposites Containing Superstructures of Self-Organized Platinum Colloids
Michel Gianini, Walter R. Caseri,* and Ulrich W. Suter
Department of Materials, Institute of Polymers, ETH Zentrum, CH-8092 Z u¨ rich, Switzerland
ReceiVed: NoVember 28, 2000; In Final Form: April 30, 2001
Colloidal platinum (diameter 1-2 nm) in styrene was prepared in situ by reduction of a platinum(II) compound
followed by partial evaporation of styrene. The resulting dispersions were purified from the reaction side-
products, and ammonium O,O′-dialkyldithiophosphates of different chain length or octadecanethiol was added.
Polymerization was started with a radical initiator. Several parameters were varied and, particularly in the
presence of long-chain dialkyldithiophosphates, self-assembled superstructures of metal colloids appeared at
certain conditions in the resulting poly(styrene)-platinum nanocomposites. Unusual hollow shell structures of
typical diameters of 50-300 nm were found, which phenomenologically resembled bilayer vesicles in aqueous
solutions, although the formation mechanism of the bilayers and the superstructures in the nanocomposites is
considered to differ. It is supposed that the formation of the superstructures in the nanocomposites is induced
by crystallization of alkyl chains adsorbed at the platinum surface below monolayer coverage.
Introduction
Materials consisting of a polymer with incorporated inorganic
particles of dimensions below 50-100 nm (nanocomposites)
1
have been described more than 150 years ago; however, they
have attracted attention particularly in the past decade. In most
cases, the matrix incloses randomly distributed particles or
randomly composed aggregates of particles. Comparatively little
examples are known so far, in which the particles form regular
or anisotropic structures. For instance, uniform silica particles,
surface-modified with methacrylate groups, can crystallize in
methyl acrylate.2 Upon polymerization, induced by light at
room temperature, the regular lattice of the particles is retained.
The nanocomposites, which contained 35-45% w/w of silica,
are birefringeant and diffract light according to Bragg’s equation.
An increase in the particle concentrations results in a decrease
in the lattice spacings which is connected with a hypsochromic
shift of the absorption maximum due to the diffracted light.
Ultrathin nanocomposite films with a regular distribution of
inorganic colloids have been obtained with the help of block
-5
Figure 1. Chemical structures and acronymes of the complexing agents
used in this work.
nanometer range (5-14 nm)20 and that the particles aggregate
to oriented structures, which causes the dichroic behavior of
copolymers in which the blocks segregate into regular
1
4,17,20,21
the samples.
More recently, similar effects have been
structures.6
-11
In these cases, metal ions or metal complexes
described when dodecanethiol-coated silver or gold colloids
were embedded in poly(ethylene), which is subsequently
show a specific affinity to one of the blocks. The metal entities
were then exposed to other reagents in order to prepare the
22-27
subjected to solid-state drawing at 120 °C.
Under appropri-
2+
desired particles. For example, Pb ions were converted with
ate conditions, this process induces an orientation of the metal
particles into arrays parallel to the drawing direction.
Here, we describe the preparation of self-assembled super-
structures of platinum colloids in poly(styrene) which markedly
differ from those mentioned above. The self-organization of the
platinum colloids was induced by addition of sulfur-containing
compounds (Figure 1), which are assumed to coordinate on the
particle surfaces by their sulfur-containing headgroups. The
influence of the compounds themselves as well as the influence
of other parameters on the formation on the superstructures was
investigated.
-
H2S to produce PbS, or [AuCl4] was reduced to gold. The
particles thus obtained remain in the respective blocks, i.e., the
particles adopt the patterns formed by the polymer blocks.
Lamellar or hexagonal arrangements of particles were achieved
in this way.
Between 1890 and 1940, the preparation and characterization
of uniaxially oriented arrays of nanoparticles in natural polymers
1
2-21
was reported.
Typically, plant or animal fibrils were
impregnated with various metal salts, which were subsequently
reduced in situ to yield metal particles. Gold, silver, platinum,
and several other metals were employed. The colors of the
resulting nanocomposites observed upon transmission of polar-
ized light depend on the angle between the polarization plane
and the orientation axis of the fibrils. It was recognized early
that the size of the generated primary particles is in the
Experimental Procedure
Chemical Substances. All chemical substances were pur-
chased from Fluka or Aldrich and used without further purifica-
1
0.1021/jp004304j CCC: $20.00 © 2001 American Chemical Society
Published on Web 07/11/2001

7400 J. Phys. Chem. B, Vol. 105, No. 31, 2001
Gianini et al.
tion, if not otherwise indicated. The ammonium O,O′-
dialkyldithiophosphates were synthesized according to the
2
8
literature.
Synthesis of Tris(styrene)platinum(0). Tris(styrene)platinum-
(
0) was prepared in situ approximately following a procedure
2
9,30
described in the literature.
A mixture of cis-dichlorobis-
(
styrene)platinum(II) (3.00 g, 6.3 mmol, synthesized according
31
to the literature ) and freshly distilled styrene (120 mL) was
exposed to triphenylsilane (3.29 g, 12.6 mmol) and stirred for
30 min. A mixture of concentrated ammonia (30 mL) and water
(250 mL) was added, and the two-phase system was vigorously
stirred for 90 min. The organic phase was separated in a
separatory funnel, vigorously stirred with water (250 mL),
separated again, and filtered through 15 cm Alox basic (activity
I) which was situated in a pipe of 4 cm diameter. After filtration,
the Alox was rinsed with freshly distilled styrene (150 mL),
and the eluate was combined with the previous fraction, yielding
a total mass of 150 ((10) g.
To determine the concentration of platinum, the solvent of 5
g of tris(styrene)platinum(0) solution was evaporated at room
temperature under reduced pressure (ca. 0.01 mbar). The black
residue was dissolved in warm aqua regia, and the orange
solution was diluted in 0.1 M hydrochloric acid (700 g). The
platinum content in these solutions was analyzed by the
analytical service of the Laboratory of Inorganic Chemistry at
ETH Z u¨ rich. Platinum concentrations ranging from 0.47 to
Figure 2. Schematic representation of the nanocomposite synthesis.
0
.58% w/w were found in various tris(styrene) platinum(0)
solutions.
Preparation of Colloidal Platinum. Purified tris(styrene)-
platinum(0) solutions, as described above, were concentrated
to a platinum content of ca. 8% w/w at room temperature under
reduced pressure (ca. 0.01 mbar). During this process, the
individual platinum atoms agglomerate and the originally
slightly yellow solutions adopt the brown color of colloidal
platinum. The exact concentrations indicated in the text for
nanocomposite preparation were obtained by dilution of con-
centrated solutions with freshly distilled styrene.
Preparation of the Nanocomposites. Azobis(isobutyronitrile)
(AIBN, 10 mg, 61 µmol) and ammonium O,O′-dialkyldithio-
phosphate (CxdtpNH4) or octadecanethiol (C18thiol) in the
quantities indicated in the text were placed in a test tube which
had been inserted in a Schlenk tube under argon atmosphere
(Figure 2). To this mixture, 1 g of colloidal platinum dispersion
in styrene with the platinum concentrations indicated in the text
was added under argon and heated to 60 °C, whereupon the
reaction mixture slowly solidified. After 3 days, the test tube
was removed and broken in order to access the nanocomposite.
Transmission Electron Microscopy (TEM). To prepare the
samples for investigations with TEM, the nanocomposites were
cut first with a razor blade to a regular tetragonal pyramid. Thin
slices of approximate dimensions of 100 nm × 0.5 mm × 0.5
mm were cut with a Reichert Ultracut Microtome using a
diamond knife. TEM images were recorded with a Philips EM
Figure 3. Reaction scheme for the preparation of the poly(styrene)-
platinum nanocomposites.
composite foams consisting of poly(styrene) with randomly
2
2
dispersed platinum particles (Figures 2 and 3). First, tris-
styrene)platinum(0) dissolved in styrene was prepared in situ
by quantitative reduction of cis-dichlorobis(styrene)platinum-
II) with triphenylsilane in styrene. After removal of the reaction
(
(
side products, the tris(styrene)platinum(0) was destabilized by
partial removal of styrene at room temperature and reduced
pressure, inducing an aggregation of the platinum atoms to
colloids.
3
01 electron microscope operating at an acceleration voltage
of 80 keV.
To induce superstructures of self-organized colloids, alkyl
compounds with a sulfur headgroup (Figure 1) were added to
the platinum-styrene dispersions. We expect that such com-
pounds are able to attach to platinum colloids via their sulfur
Gel Permeation Chromatography (GPC). For GPC measure-
ments, the nanocomposites were dissolved in tetrahydrofuran,
and the molecular weights were determined by the analytical
service of the Institute of Polymers of ETH Z u¨ rich using poly-
32,33
atoms.
The compounds employed were mainly ammonium-
(styrene) standards.
O,O′-dialkyldithiophosphates, termed CxdtpNH4, where x de-
notes the number of carbon atoms in the alkyl chains. These
substances were prepared according to the literature.28 We
assume that the ammonium groups are coadsorbed upon
coordination of the dithiophosphate groups to the platinum
Results
The production of the nanocomposites is derived from a
method that has been described for the preparation of nano-

Self-Organized Platinum Colloids
J. Phys. Chem. B, Vol. 105, No. 31, 2001 7401
TABLE 1: Overview of the Experiments and the Structures Observed in the Nanocomposites
M/La
1:1
2:1
3:1
4:1
c
no L, type
L
type
L
type
L
type
L
type
1
2
4
%
%
%
Ab
A
A
C
C
C
C
C
C
C
C
C
18dtpNH
18dtpNH
4
4
A
A
D
A
A
C
C
C
C
C
C
C
C
C
18dtpNH
18dtpNH
4
4
Bc
B
C
C
C
C
C
C
C
C
C
18dtpNH
18dtpNH
4
A
B
D
C
B
B
E
E
E
C
C
C
C
C
C
C
C
C
18dtpNH
18dtpNH
4
4
A
A
D
D
A
B
E
4
e
2
6
dtpNH
dtpNH
4
4
2
dtpNH
dtpNH
4
4
D
2
dtpNH
dtpNH
4
2
6
dtpNH
dtpNH
4
4
d
6
C
6
4
12dtpNH
18dtpNH
18thiol
18dtpNH
18hiolt
4
4
12dtpNH
18dtpNH
18thiol
18dtpNH
18thiol
4
4
B
B
E
B
E
12dtpNH
18dtpNH
18thiol
4
12dtpNH
18dtpNH
18thiol
4
B, C
4
4
f
E
8
%
A
4
B
E
4
18dtpNH
18thiol
4
18dtpNH
18thiol
4
E
E
a
M/L denotes the molar ratio between platinum atoms and complexing agent molecules, c the platinum concentration [w/w] in the initial styrene
x 4
solution, C dtpNH the ammonium O,O′-dialkyldithiophosphates with the number of carbon atoms x in the alkyl chains, and C18thiol, octadecanethiol.
b
Type A: no superstructures or exceptional features. Type B: hollow shells and networks. d Type C: large disks or thick-walled shells. Type
c
e
f
D: disklike structures with irregular shape. Type E: mainly connected shells and stringlike objects.
surfaces. For comparison, experiments with octadecanethiol
exceptional features appear in transmission electron microscopy
(TEM) images obtained from thin nanocomposite sections, and
such “featureless” nanocomposites are termed here type A. Type
A structures also dominated the micrographs of some materials
prepared in the presence of C18dtpNH4 at low platinum
concentrations (1 or 2% w/w) and of most CxdtpNH4 compounds
at the lowest M/L ratio (1:1).
(
C18thiol) have also been performed. The adsorption of
22-27,32-36
alkanethiols on noble-metal colloids is well established.
Polymerization of styrene-platinum dispersions was actuated
in the presence and absence of the above-mentioned complexing
agents by addition of a radical initiator, azobis(isobutyronitrile)
(AIBN) at 60 °C. According to preliminary experiments,
nanocomposites were produced predominantly in the presence
of C18dtpNH4 at different platinum concentrations (1-8% w/w)
and different ratios of metal atoms and complexing agent
molecules (M/L ratio, varying from 1:1 to 4:1). Ammonium
O,O′-dialkyldithiophosphates with alkyl chains containing less
than 18 carbon atoms or octadecanethiol were also employed.
A survey of the experiments is presented in Table 1. The
number-average molecular weights (Mn) of the poly(styrene)
matrix, measured by gel permeation chromatography (GPC),
are not significantly affected by the presence of platinum and
complexing agents. Typical Mn between 30 000 and 40 000 with
Mw/Mn around 5-7 (Mw ) weight-average molecular weight)
were found in numerous materials with C18dtpNH4; these values
do not differ significantly from those of pure poly(styrene)
prepared under the same conditions or polymer containing only
platinum colloids but no complexing agent. The samples
prepared with M/L ratios of 1:1 are hard, and the specimens
become more and more brittle with increasing M/L ratio. In the
absence of a complexing agent, the nanocomposites tend to
become brittle as the platinum concentration increases, with the
exception of the 8% platinum sample, which consists of a
gumlike product. As evident from TEM images, the individual
platinum colloids are 1-2 nm in size in all experiments. The
TEM pictures also show that the particles’ dimensions are
established upon formation of the pristine platinum dispersion
and the partial evaporation of the liquid before addition of the
stabilizer, i.e., the particles do not grow anymore upon treatment
with stabilizer or polymerization of styrene. The platinum
colloids thus obtained have the same dimensions as the reported
Pt309 clusters37 and the colloids in the poly(styrene)-platinum
nanocomposite foams, in which the platinum particles were
prepared according to the same chemical reactions.22 The extent
of the individual platinum clusters do not change significantly
in the presence of a complexing agent.
Onionlike structures of several shells as well as single shells
of platinum colloids emerge in TEM micrographs of a number
of nanocomposites fabricated in the presence of ammonium
O,O′-dialkyldithiophosphates (Figure 4a-d). These structures,
defined as type B, seem to be favored at intermediate M/L ratios
(in particular 2:1 but also 3:1) and long alkyl chains (C18dtpNH4
and C12dtpNH4), as evident from Table 1. The type B patterns
appear as “rings” of typical diameters of 50-300 nm and
thickness of the borderline of 5-15 nm. These rings consist of
hundreds or thousands of colloidal platinum particles. The cross
sections of the borderlines comprehend several particles, as
evident from TEM pictures at high magnifications (Figure 4b).
Features composed of connected shells are also visible (Figure
4a), and complex multilayer structures appear with C18dtpNH4
at platinum concentrations of 4 and 8% w/w and M/L of 2:1,
and at 8% w/w and M/L of 1:1 (Figure 4d). Finally, concentric
shells emerge occasionally; the diameters of the outermost rings
attained, for instance, 1-2 µm (Figure 4c).
When the alkyl chain length in CxdtpNH4 complexes de-
creases at a platinum concentration of 4% w/w and M/L of 2:1
and 3:1, objects other than the above-described type B structures
dominate the TEM pictures. In the case of C6dtpNH4, large disks
or shells with thick walls of diameters around 5 µm (defined as
type C, Figure 5a) or, when C2dtpNH4 is used, disklike entities
(or thick-walled shells) with irregular shape and diameters of
100 nm-2 µm are found (defined as type D, Figure 5b).
Obviously, the formation of superstructures of platinum colloids
in the presence of CxdtpNH4 is governed by the alkyl chains.
Type D structures also appear with C2dtpNH4 and C6dtpNH4
at M/L ) 4:1 and with C2dtpNH4 at M/L ) 1:1. Hence, it seems
that a certain length of the alkyl chains is required to obtain
self-organization.
Experiments performed with C dtpNH at platinum concen-
18
4
trations of 8% w/w and M/L ratios of 1:3 and 1:4 result in
nanocomposites that typically contain flattened shells of a length
up to 1 µm, some regular shells and stringlike objects (defined
as type E, Figure 5c). A mixture of similar structures was also
observed with C18thiol at a platinum concentration of 8% w/w
and M/L ) 2:1 (Figure 5d). The other nanocomposites contain-
ing C18thiol at platinum concentrations of 4 and 8% w/w also
As indicated in Table 1 and as described below in more detail,
several types of arrangements of platinum colloids were
observed in the nanocomposites, depending on the type of
complexing agent, the ratio between platinum atoms and
complexing agent molecules, and the platinum concentration.
In absence of a complexing agent, no superstructures or

7402 J. Phys. Chem. B, Vol. 105, No. 31, 2001
Gianini et al.
Figure 4. Typical examples of Type B structures observed in poly-
styrene)-platinum nanocomposites in the presence of C18dtpNH . (a)
(
4
Figure 5. Typical examples of: (a) type C structure, C
) 2:1, platinum concentration 4% w/w, (b) type D structure, C
M/L ) 1:1, platinum concentration 4% w/w, (c) type E structure, C18
dtpNH
6
dtpNH
4
, M/L
and (b) M/L ) 2:1, platinum concentration 1% w/w, (c) M/L ) 3:1,
2
dtpNH
4
,
platinum concentration 2% w/w, (d) M/L ) 2:1, platinum concentration
-
8
% w/w.
4
, M/L ) 3:1, platinum concentration 8% w/w, (d) type E
structure, C18thiol, M/L ) 2:1, platinum concentration 8% w/w.
show type E structures, in which, however, chainlike assemblies
of colloids dominate.
membranes (vesicles) formed by self-organization in solution
(
note that such spheric structures appear as circles in the
Discussion
38-42
projections represented by the TEM pictures).
Such mem-
branes can conjoin to assemblies such as networks,39 as also
observed in type B features. In fact, the molecules forming
bilayer membranes typically consist of a polar headgroup which
From the above-described structures, the unusual type B
patterns attract most attention. At first glance, the related TEM
images highly resemble those of the superstructures of bilayer

Self-Organized Platinum Colloids
J. Phys. Chem. B, Vol. 105, No. 31, 2001 7403
is connected to two alkyl chains with typically 12-18 carbon
atoms, i.e., these molecules are of the same type as the long-
chain dialkyldithiophosphates used in this study. Even more,
complexation of the headgroup by metal atoms can stabilize
bilayer aggregates.38 However, there are striking differences
between bilayer membranes and the structures described here:
typical bilayer membranes are essentially formed in aqueous
solutions as a consequence of a balance between hydrophilic
and hydrophobic interactions, and the thickness of the walls is
molecularly controlled. In the systems described here, the
medium is apolar and the walls of the structures vary somewhat
in thickness, which extends over a few particle diameters.
As mentioned in the Introduction, compounds with sulfur
atoms show a high affinity to platinum. Hence, CxdtpNH4
molecules are expected to coordinate to the colloids’ surfaces,
in equilibrium with CxdtpNH4 molecules dissolved in the matrix.
If the platinum surfaces were saturated with CxdtpNH4 molecules
at all M/L ratios applied here, the interaction of the CxdtpNH4-
modified particles with each other and the matrix was the same
for all M/L ratios and, as a consequence, we would expect the
types of structure to be independent of the M/L ratios, which,
however, does not agree with the observations, where differences
in the structures prepared at M/L of 1:1 and 2:1 are common.
Therefore, we assume that the surfaces are saturated with
complexing agent molecules at most at the lowest M/L ratio
Figure 6. Sketch of a possible formation of self-assembled super-
(1:1), if at all, and that the surface coverage with CxdtpNH4
structures by crystallization of surface-bound alkyl chains.
tends to decrease with increasing M/L. In fact, it is not clear if
there are sufficient CxdtpNH4 molecules in the system at M/L
)
4:1 to cover the colloids completely because the particles
are so small that most of the atoms could be exposed to the
surface and hence be capable to coordinate with CxdtpNH4. If
the surface coverage decreases with increasing M/L, the packing
density of the attached molecules also decreases.
The smallest cross-sectional area of an alkyl chain in all-
2,43
2
trans conformation is 0.19 nm
and an area of 0.24 nm per
alkyl chain was estimated for a densely packed alkyl layer of
44
dimethyldioctadecylammonium ions adsorbed on mica; hence,
the diameter of an idealized alkyl chain is around 0.5 nm, i.e.,
relatively large compared to the diameter of the platinum
colloids of 1-2 nm. Because of the high curvature of the
particles, the brushes of alkyl chains are not dense and the chains
probably not extended, even at surfaces saturated with CxdtpNH4
molecules, and gauche conformations are likely to occur.
Nonetheless, organic layers on very small colloidal powders
can exhibit crystallization and melting as found in silver colloids
with bound dodecanethiol chains.26 If metal surfaces are covered
only partially with CxdtpNH4 molecules, the alkyl packing is
less dense, and hence, the surrounding alkyl layer is probably
“
liquid”. However, crystallization could be induced in an
accumulation of particles by interpenetration of alkyl chains of
adjacent particles (Figure 6), which may act as the driving force
for the formation of superstructures. This balance would depend
on many parameters, in particular on the surface coverage, i.e.,
the M/L ratio, the concentration of the colloids in the matrix,
and the length of the alkyl chains. It appears, according to the
above discussion, that the platinum colloids in the superstruc-
tures observed in this study are covered with CxdtpNH4 below
saturation. When dispersions of gold colloids, whose surfaces
were saturated with a monolayer of dodecanethiol, were dis-
persed in styrene or methyl methacrylate and converted into
nanocomposites under similar conditions as those applied for
the experiments above, materials with randomly distributed
particles were found,4 in agreement with the assumption that
Figure 7. Sketch of a possible formation of concentric shells by shape
fluctuations.
type B superstructures are favored by a partial coverage of
alkanethiols.
Using C thiol instead of C dtpNH , it has to be considered
1
8
18
4
that the equilibrium constant for the complexation to platinum
and the number of alkyl chains per headgroup differ. As a
5,46

7404 J. Phys. Chem. B, Vol. 105, No. 31, 2001
Gianini et al.
consequence, different superstructures, if occurring at all, are
expected to arise when these two complexing agents are applied
under the same reaction conditions, although in both cases alkyl
chains surround the platinum surfaces. Indeed, the structures
observed in TEM frequently differ when C18thiol and C18dtpNH4
were used under the same conditions. Although mainly open-
chain structures have been found when C18thiol was applied, it
cannot be excluded that type B objects might be generated upon
variation of the reaction conditions, such as platinum concentra-
tion, M/L ratio, reaction temperature, or matrix composition.
The existence of concentric shells, such as shown in Figure 4c,
can be explained analogously to related phenomena in bilayer
(6) Spatz, J. P.; M o¨ ssmer, S.; Hartmann, C.; M o¨ ller, M.; Herzog, T.;
Krieger, M.; Boyen, H.-G.; Ziemann, P.; Kabius, B. Langmuir 2000, 16,
07.
4
(
7) Spatz, J. P.; M o¨ ssmer, S.; M o¨ ller, M.; Herzog, T.; Plettl, A.;
Ziemann, P. J. Luminescence. 1998, 76/77, 168.
8) Cibien, J. F.; Clay, R. T.; Sohn, B. H.; Cohen, R. E. New J. Chem.
998, 685.
9) M o¨ ller, M.; Spatz, J. P.; Roescher, A.; M o¨ ssmer, S.; Selvan, S. T.;
(
1
(
Klok, H.-A. Macromol. Symp. 1997, 117, 207.
(10) Spatz, J. P.; M o¨ ssmer, S.; M o¨ ller, M. Chem. Eur. J. 1996, 2, 1552.
(
(
(
11) Sohn, B. H.; Cohen, R. E. Acta Polym. 1996, 47, 340.
12) Ambronn, H. K o¨ nigl. S a¨ chs. Ges. Wiss. 1896, 8, 613.
13) Ambronn, H.; Zsigmondy, R. Ber. S a¨ chs. Ges. Wiss. 1899, 51, 13.
(14) Ambronn, H. Z. Wiss. Mikrosk. 1905, 22, 349.
15) Braun, F. Ann. Phys., Vierte Folge (Drude’s Ann.) 1905, 16, 238.
16) Frey, A. Z. Wiss. Mikrosk. 1925, 42, 421.
17) Frey, A. Jahrb. Wiss. Bot. 1927, 67, 597.
(18) Frey, A. Naturwissenschaften 1925, 13, 403.
19) Bion, F. HelV. Phys. Acta 1928, 1, 165.
20) Frey-Wyssling, A. Protoplasma 1937, 27, 372.
(
(
(
42
membranes, since the mechanism is based on shape variations
of a given shell with time (Figure 7).
(
(
Conclusions
(21) Frey-Wyssling, A. Protoplasma 1937, 27, 563.
22) Heffels, W.; Friedrich, J.; Darrib e` re, C.; Teisen, J.; Interewicz, K.;
Bastiaansen, C.; Caseri, W.; Smith, P. Recent Res. DeV. Macromol. Res.
997, 2, 143.
23) Bastiaansen, C.; Caseri, W.; Darrib e` re, C.; Dellsperger, S.; Heffels,
W.; Montali, A.; Sarwa, C.; Smith, P.; Weder, C. Chimia 1998, 52, 591.
24) Dirix, Y.; Darrib e` re, C.; Heffels, W.; Bastiaansen, C.; Caseri, W.;
Smith, P. Appl. Opt. 1999, 38, 6581.
25) Dirix, Y.; Bastiaansen, C.; Caseri, W.; Smith, P. AdV. Mater. 1999,
11, 223.
(
Styrene containing dispersed platinum particles of diameter
1
1-2 nm was polymerized in the presence of dialkyldithiophos-
(
phates, CxdtpNH4, and octadecanethiol, C18thiol. The size of
the individual particles (1-2 nm) and the degree of polymer-
ization in the formed poly(styrene) (Mn 30,000-40,000) is not
significantly affected by the presence of CxdtpNH4 or C18thiol
under the conditions applied here. It is assumed that CxdtpNH4
and C18thiol adsorb at the surfaces of the colloids by complex-
ation with sulfur atoms. As a consequence, a number of the
resulting nanocomposites contain superstructures, depending on
the type of complexing agent, the length of the alkyl chain, the
concentration of platinum, and the ratio between platinum atoms
and complexing molecules in the system. Most remarkably,
unusual hollow shell structures with a typical diameter of 50-
(
(
(
26) Dirix, Y.; Bastiaansen, C.; Caseri, W.; Smith, P. J. Mater. Sci. 1999,
3
4, 3859.
(27) Dirix, Y.; Bastiaansen, C.; Caseri, W.; Smith, P. Mater. Res. Soc.
Symp. Proc. 1999, 559, 147.
(28) Gianini, M.; Caseri, W. R.; Gramlich, V.; Suter, U. W. Inorg. Chim.
Acta 2000, 299, 199.
(
(
29) Caseri, W.; Pregosin, P. S. Organometallics 1988, 7, 1373.
30) Caseri, W.; Pregosin, P. S. J. Organomet. Chem. 1988, 356, 259.
(31) Albinati, A.; Caseri, W. R.; Pregosin, P. S. Organometallics 1987,
, 788.
32) Yee, C.; Scotti, M.; Ulman, A.; White, H.; Rafailovich, M.; Sokolov,
6
(
3
00 nm and a wall thickness of 5-15 nm appear in many
J. Langmuir 1999, 15, 4314.
examples with CxdtpNH4. The formation of these objects is
apparently favored by long alkyl chains. With octadecanethiol,
C18thiol, chains of such objects were normally observed, but
single shells were occasionally also found.
(33) Shon, Y.-S.; Gross, S. M.; Dawson, B.; Porter, M.; Murray, R. W.
Langmuir 2000, 16, 6555.
(
34) Templeton, A. C.; Hostetler, M. J.; Kraft, C. T.; Murray, R. W. J.
Am. Chem. Soc. 1998, 120, 1906.
35) Hostetler, M. J.; Wingate, J. E.; Zhong, C.-J.; Harris, J. E.; Vachet,
(
R. W.; Clark, M. R.; Londono, J. D.; Green, S. J.; Stokes, J. J.; Wignall,
G. D.; Glish, G. L.; Porter, M. D.; Evans, N. D.; Murray, R. W. Langmuir
The TEM images of the shells resemble those of bilayer
vesicles in aqueous solution; however, the driving force for the
superstructures is believed to differ. While a balance of
hydrophobic and hydrophilic interactions causes the genesis of
typical bilayer structures in aqueous solution, the superstructures
in the nanocomposites could be established by a crystallization
of alkyl chains on partially covered platinum surfaces in
assemblies of particles.
1
1
7
998, 14, 17.
(36) Johnson, S. R.; Evans, S. D.; Mahon, S. W.; Ulman, A. Langmuir
997, 13, 51.
(
37) Schmid, G.; Morum, B.; Malm, J.-O. Angew. Chem. 1989, 101,
72.
(38) Kunitake, T. In Physical Chemistry of Biological Interfaces;
Baszkin, A., Norde, W., Eds.; Marcel Dekker: New York, 2000; p 283.
39) Needham, D.; Zhelev, D. In Giant Vesicles; Luisi, P. L., Walde,
P., Eds.; John Wiley & Sons: Chichester, 2000; p 103.
40) Seddon, J. M.; Templer, R. H. In Structure and Dynamics of
(
(
Membranes; Lipowsky, R., Sackmann, E., Eds.; Elsevier: Amsterdam, 1995;
Vol. 1, p 97.
Acknowledgment. We thank P. Smith for exceedingly
helpful discussions.
(
41) Sackmann, E. In Structure and Dynamics of Membranes; Lipowsky,
R., Sackmann, E., Eds.; Elsevier: Amsterdam, 1995; Vol. 1, p 213.
42) Seifert, U.; Lipowsky, R. In Structure and Dynamics of Membranes;
(
References and Notes
Lipowsky, R., Sackmann, E., Eds.; Elsevier: Amsterdam, 1995; Vol. 1, p
403.
(1) Shepard, C. U. Am. J. Sci. Arts 1835, 28, 145.
(
2) Pu, Z.; Mark, J. E.; Jethmalani, J. M.; Ford, W. T. Polym. Bull.
(43) Tsao, Y.; Evans, D. F.; Wennerstr o¨ m, H. Science 1993, 262, 547.
(44) Brovelli, D.; Caseri, W. R.; H a¨ hner, G. J. Colloid Interface Sci.
1999, 216, 418.
(45) Gonsalves, K. E.; Carlson, G.; Chen, X.; Gayen, S. K.; Perez, R.;
Jose-Yacaman, M. Nanostruct. Mater. 1996, 7, 293.
(46) Gonsalves, K. E.; Carlson, G.; Kumar, J.; Aranda, F.; Jose-
Yacaman, M. ACS Symp. Ser. 1996, 622, 151.
1
996, 37, 545.
(
(
3) Jethmalani, J. M.; Ford, W. T. Chem. Mater. 1996, 8, 2138.
4) Jethmalani, J. M.; Ford, W. T.; Beaucage, G. Langmuir 1997, 13,
3
338.
5) Sunkara, H. B.; Penn, B. G.; Frazier, D. O.; Ramachandran, N. J.
Mater. Sci. 1998, 33, 887.
(