Supramolecular assemblies of starlike and V-shaped PB-PEO amphiphiles

Article abstract of DOI:10.1002/anie.200460906

A star is born in the form of a polybutadiene-poly (ethylene oxide) amphiphile 1. The molecule exhibits remarkable self-assembly properties: it forms regular and reverse micelles in water and hexane, respectively. Comparison of 1 with a V-shaped precursor

Full text of DOI:10.1002/anie.200460906

Angewandte
Chemie
Supramolecular Chemistry
Supramolecular Assemblies of Starlike and V-
Shaped PB–PEO Amphiphiles**
Jun Xu and Eugene R. Zubarev*
Self-assembly of linear amphiphiles is a well-known phenom-
enon,[1–5] and the number of different structures observed
both in the solid-state and in selective solvents is exceedingly
large.[6–10] It is difficult to envision a diblock amphiphile that
would not undergo self-assembly, and in this context, the
terms “linear amphiphilic” and “self-assembling” are nearly
[11]
synonymous.Branched amphiphiles such as dendrimers
and stars[12] also undergo self-organization in selective sol-
vents.[13–18] However, there are examples in which they do not
aggregate,[19–23] but rather behave as unimolecular
micelles.[24–26] Furthermore, the morphological diversity of
aggregates formed by branched structures is significantly
lower than that of linear counterparts.For example, most self-
assembling star-shaped amphiphiles only form spherical
micelles,[27–31] whereas linear diblocks can organize into at
least 30 different morphologies.[9] This creates the impression
that a branched architecture somewhat precludes morpho-
logical diversity and that star-shaped amphiphiles cannot
compete with linear analogues.In contrast, we document here
that the multiarm PB6–PEO6 (PB = polybutadiene, PEO =
poly(ethylene oxide)) molecule 1 self-assembles into non-
spherical structures in water and generates unique “cotton-
ball” morphology in hexane.Such hierarchical structures have
not been observed in PB–PEO diblocks or any other linear
system.
Starlike amphiphiles[32–38] are generally difficult to synthe-
size owing to the limitations of many synthetic techniques,[39]
[*] J. Xu, Prof. Dr. E. R. Zubarev
Department of Materials Science and Engineering
Iowa State University
Ames, IA 50011 (USA)
Fax: (+1)515-294-5444
E-mail: zubarev@iastate.edu
[**] The authors are grateful to the Donors of the American Chemical
Society Petroleum Research Fund for partial support of this research
(ACS-PRF No. 40727 G-10). PB=polybutadiene, PEO=poly(ethy-
lene oxide).
Supportinginformation for this article is available on the WWW
under http://www.angewandte.org or from the author.
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and most of such structures are based on
styrenic and acrylic monomers.These limita-
tions may also explain one seemingly surpris-
ing fact: linear PB–PEO diblocks have been
known and studied extensively for many
decades,[40] whereas the synthesis of star-
shaped PB–PEO molecules has not been
reported to date.Herein, we describe a
modular approach to produce a 12-arm PB–
PEO star with V-shaped chains directly con-
nected to an aryl ester dendrimer.
Commercially available hydroxyl-termi-
nated polybutadiene (3; Mn = 1000, Mw/Mn =
1.12) and poly(ethylene oxide) (6; Mn = 2200,
Mw/Mn = 1.15) were both coupled with
mono(silyl)-protected biphenyl dicarboxylic
acid, 2, under mild esterification conditions.
The TIPS (triisopropylsilyl) group of the
respective products, 4 and 7, were subse-
quently removed to afford the carboxyl-
terminated linear precursors 5 and 8, respec-
tively (Scheme 1).Biphenyl groups were
specifically introduced to have reliable
NMR references in order to analyze and
confirm unambiguously the purity of all the
intermediate compounds.The linear precur-
sors 5 and 8 were then coupled sequentially to
TIPS-protected 3,5-dihydroxybenzoic acid
(DHBA), 9, under the same esterification
conditions to give 11.Deprotection of the
TIPS-protected carboxylate group produced
the V-shaped PB–PEO amphiphile 12 in high
yield and purity (Mw = 6210, Mw/Mn = 1.13).
All intermediate compounds were isolated
and purified by conventional column chro-
matography.The hexafunctional aryl ester
dendrimer (generation 1) was then prepared
by using standard procedures.[41] The key
coupling reaction between the amphiphile
12 and the dendrimer core 13 was found to
proceed very rapidly and in near-quantitative
yield (by GPC and NMR spectroscopy).The
final product 1 was further purified by dialysis
against deionized water through a membrane
Scheme 1. Stepwise synthesis of amphiphile 1. Reagents: a) DPTS/DIPC, CH2Cl2, room
temperature, 1–3 h; b) TBAF, À788C, 2 h; c) HF, THF, room temperature, 12 h. DPTS=4-
(N,N-dimethylamino)pyridinium-4-p-toluenesulfonate, DIPC=1,3-diisopropylcarbodiimide,
TBAF=tetra-n-butylammonium fluoride.
with a molecular-weight cut-off at 30000 gmolÀ1 to remove
low-molecular-weight components.
dendrimer core (see Supporting Information for the fully
assigned spectra).[41] The area of the triplet at 7.6 ppm is also
increased by a factor of 1.5 (9 protons; 6 H from V-shaped
fragments and 3 H from the core) with respect to the biphenyl
signals.The appearance of the singlet at 72. 8 ppm (3 protons)
also confirms the presence of the fully substituted dendritic
core.All these features suggest that the product, 1, is a defect-
free 12-arm starlike molecule that does not contain ten-,
eight-, six-, or four-arm byproducts.The presence of such
impurities would otherwise have been observed as residual
peaks and by unsymmetrical shapes of the signals.The
observed 1.5-fold increase in the areas of the doublet and
triplet signals is the highest value possible and can only be
attained if all the molecules are fully substituted.The GPC
trace of 1 also suggests complete substitution as the peak
Figure 1 shows the aromatic regions of the 1H NMR
spectra of the V- and star-shaped amphiphiles.In the
spectrum of 12 (Figure 1a), the doublet signal at 7.92 ppm
and the triplet signal at 7.45 ppm correspond to protons at C-2
and C-6, and C-4, respectively, of the DHBA hinge.Upon
coupling with the dendrimer core, these signals are shifted
downfield to 8.09 and 7.6 ppm, respectively, whereas the
signals for the protons from the biphenyl system are almost
unaffected (Figure 1b).Most importantly, the doublet at
8.09 ppm in the NMR spectrum of the star-shaped product
represents 18 protons, and therefore includes not only 12 pro-
tons from the DHBA moieties of six V-shaped substituents,
but also an additional 6 protons from DHBA fragments of the
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topological restriction imposed by the rigid aromatic core on
the packing of the flexible arms in the amphiphile 1.It is
important to emphasize that the arms are not emanating from
one specific point, but from six equidistant peripheral points
of a fairly large disklike core (see Scheme 1).Thus, the
presence of an aromatic core in 1 prevents the dense packing
of the PEO chains located in the corona of the supermicelles.
To minimize the interfacial energy and to increase the
shielding of the hydrophobic PB core from water molecules,
the curvature of the micelle surface has to decrease, and this
would favor a sphere-to-cylinder morphological transition.
It is still unclear if the presence of the biphenyl moieties
facilitates the self-assembly process.However, it is reasonable
to assume that aromatic p–p interactions do not play a
significant role in this process as heteroarm amphiphiles can
form supermicelles even with a poorly defined divinyl
benzene (DVB) core.[13] We believe that the main influence
of the biphenyls on the self-organization of 1 arises from their
rigidity, which prevents the dense packing of the arms.As a
result of this loose distribution of arms on the surface of the
supermicelle, a cylindrical morphology is preferred over the
spherical one.In other words, the biphenyls promote the
formation of cylinders, whereas the self-assembly process
itself is mainly promoted by the heteroarm molecular
architecture.This is consistent with the fact that heteroarm
amphiphiles without biphenyl groups undergo self-assem-
bly,[13] but only into spheres, whereas the cylindrical morphol-
ogy has never been observed for such systems.Furthermore,
even the V-shaped amphiphile exclusively forms spheres,
although it has the same volume ratio of hydrophilic and
hydrophobic blocks.
1
Figure 1. Aromatic regions of H NMR spectra of a) the V-shaped
amphiphile 12 and b) the star-shaped amphiphile 1.
becomes even sharper than that of the V-shaped precursor 12
(polydispersity index, PDI = 1.10 versus 1.13, respectively).[41]
To investigate the self-assembly properties, we studied
aqueous solutions of V- and star-shaped amphiphiles at room
temperature.Figure 2, a and b, show representative TEM
Because water is a selective solvent for PEO, the central
core of the cylindrical micelles should be composed of PB
chains.However, such an arrangement would first require a
spatial separation of multiple hydrophobic and hydrophilic
arms on the scale of one unimer.It can be envisioned that
prior to aggregation the starlike amphiphile 1 adopts a
conformation where six water-insoluble PB arms are posi-
tioned below the plane of the aromatic disklike core and the
hydrophilic PEO arms segregate on the opposite side as
represented schematically in Figure 3.However, the collapse
of six hydrophobic PB arms that belong to a given molecule of
1 may not be enough to avoid unfavorable interactions with
water.This is again because of the presence of the rigid core
that spatially separates the arms and prevents their dense
packing.As a result, the unimers 1 would be driven to
aggregate to further minimize the unfavorable contacts by
placing the PB chains of all the aggregating molecules into the
central core of a supermicelle.
The amphiphile 1 can also be dissolved in solvents that are
selective for PB arms.TEM images of samples cast from 05. -
wt% solutions in hexane revealed giant spherical objects that
can be seen at very low magnification (Figure 4a).These
structures measure ꢀ 2 mm in diameter and have a narrow
size distribution with a standard deviation of about 90 nm,
which is less than 5% of the mean diameter value.[41] These
microstructures represent discrete objects that do not form a
continuous network.At higher magnification, the image
clearly shows that the microstructures are composed of
Figure 2. TEM images of samples cast from 3-wt.% aqueous solutions
of a) the V-shaped amphiphile 12 and b) the star-shaped amphiphile 1.
An aqueous solution of PTA (phosphotungstic acid) was used as a
negative staining agent.
micrographs of samples cast from 3-wt% solutions of com-
pounds 12 and 1, respectively.In the V-shaped precursor, only
densely packed spherical micelles that measure ꢀ 18 nm in
diameter were observed.This size is in good agreement with
numbers predicted by molecular modeling calculations
(Materials Studio Program) and suggests that the self-
assembly proceeds by a closed association mechanism.[41,42]
In contrast, the majority of the star-shaped amphiphile, 1,
exists in the form of one-dimensional structures that measure
ꢀ 20 nm in diameter and up to 300 nm in length.Such
morphology is typically referred to as short-rod micelles.
Furthermore, some spherical micelles as well as toroidal
structures are present.All these objects are composed of
many starlike unimers 1 and can be regarded as supermicelles
rather than unimolecular micelles.[24–26]
Figure 2 demonstrates a significant influence of molecular
architecture on the self-assembly process.Interestingly, the
volume fraction of the hydrophile PEO is nearly the same (ꢀ
0.65) in the V- and star-shaped compounds, and according to
theoretical predictions, they both should form spherical
micelles.[1,2] The observed difference can be related to a
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formation is highly reproducible as confirmed by
multiple control experiments.
However, it still remains unknown how the
formation of a continuous 3D network is pre-
vented and why nearly monodisperse cottonballs
form instead.This is a particularly remarkable
observation given the fact that their structural
constituents (reverse cylindrical micelles) are
highly flexible objects as revealed by TEM.It is
important to note that dissolution of 1 in hexane
proceeds at elevated temperatures (ꢀ 608C),
above the melting point of PEO.The cottonballs
only form after cooling the solutions to room
temperature and subsequent aging for several
hours.In a control experiment, we cast a sample
directly from a hot solution of 1 in hexane and
did not observe any microspheres; this suggests
that the formation of these structures is a
thermoreversible process.It is reasonable to
Figure 3. A schematic molecular-graphics representation of the self-assembly of amphiphile 1 in
water. a) The image shows a unimer with the 6 arms of PEO (top) and the 6 arms of PB
(bottom) spatially separated; b) and c) the images show the cross-section and the side view,
respectively, of a cylindrical supermicelle with a PB core and a PEO corona.
assume that either partial crystallization of PEO or a
reduction in the solubility of the PB arms upon cooling to
room temperature drives the association of reverse cylindrical
micelles into well-defined spheres.
Furthermore, we investigated hexane solutions of the V-
shaped precursor 12 under the same conditions and found
completely different structures.Figure 5 shows a representa-
Figure 4. a) Low-magnification TEM image of a sample cast from a
0.5-wt.% solution of 1 in hexane; b) high-magnification TEM image of
the same sample taken from an area near the edge of a microsphere.
Osmium tetroxide was used as a stainingagent to improve the con-
trast.
numerous one-dimensional structures that are ꢀ 20 nm in
diameter (Figure 4b).Thus, the observed microspheres have
internal structural constituents in the form of nanosized
cylindrical supermicelles.
Figure 5. TEM micrograph of a sample cast from a 0.5-wt.% solution
of the V-shaped PB–PEO amphiphile 12 in hexane . Inset: Two isolated
square platelets that do not have a screw dislocation. Osmium tetrox-
ide was used as a stainingagent to enhance the contrast.
Selective staining of the samples with osmium tetroxide,
which reacts only with the PB chains, generated a character-
istic contrast with two thin, dark lines located on the opposite
sides of the cylinders (Figure 4b).These images confirmed
that the PB chains constitute the corona of the cylindrical
supermicelles formed in hexane.Hence, the large spherical
microstructures (Figure 4a) are composed of reverse cylin-
drical micelles (Figure 4b).Interestingly, linear PB–PEO
diblocks can easily form regular micelles in water (PB core/
PEO corona),[43] whereas their reverse analogues (PEO core/
PB corona) have not been reported to date.Therefore, the
star-shaped amphiphile 1 forms both regular and reverse
cylindrical micelles in water and hexane, respectively.Most
importantly, the self-assembly process in hexane proceeds
through two hierarchical levels of self-organization: First, the
unimers 1 form cylinders, and then these cylinders self-
assemble into finite spherical microstructures.Owing to
obvious similarities, we refer to this novel morphology as
“cottonballs”.These structures are very stable, and images
taken from samples cast from hexane solutions that were aged
for nearly 3 months showed that they remained intact.Their
tive TEM image of square-shaped platelets formed by 12 in
hexane.The structures possess a two-dimensional morphol-
ogy; they measure ꢀ 1 micron in lateral dimensions and only
22 nm in thickness (determined by AFM).The square shape
and the presence of faceted edges are indicative of a single-
crystalline structure.However, in the V-shaped amphiphile
12, there is only one crystallizable block (PEO) because
polybutadiene is composed of both 1,4- and 1,2-addition
monomer units randomly distributed along the chains.The
square platelets form in a solvent that is selective for PB,
therefore PEO arms must be located in the midsection of such
structures.In essence, they can be viewed as sandwichlike
structures with a single-crystalline layer of PEO coated with
amorphous PB layers on both sides.If the sample is rinsed
with water, the square platelets are completely destroyed.
However, after staining with osmium tetroxide vapor for only
5 mins, the structures remain intact and retain their original
shape upon multiple rinsings with water.The contrasting
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ꢀ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
Angew. Chem. Int. Ed. 2004, 43, 5491 –5496