Molecular aggregations and supramolecular architectures of amphiphilic PEO17-OPV3 and its hybrid with silica

Article abstract of DOI:10.1039/b503393d

The synthesis, luminescence properties and self-assembly of a specifically-designed amphiphilic PEO₁₇-OPV₃ molecule (and its hybrid with silicates) to form ring-like disks are described. The synthesis was begun with the Wittig-Horner

Full text of DOI:10.1039/b503393d

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Molecular aggregations and supramolecular architectures of amphiphilic
PEO17–OPV3 and its hybrid with silica
Chi-Chun Hsieh and King-Fu Lin*
Received 7th March 2005, Accepted 13th July 2005
First published as an Advance Article on the web 9th August 2005
DOI: 10.1039/b503393d
The synthesis, luminescence properties and self-assembly of a specifically-designed amphiphilic
PEO17–OPV3 molecule (and its hybrid with silicates) to form ring-like disks are described. The
synthesis was begun with the Wittig–Horner reaction to form a co-planar p-conjugated PPV
oligomer segment, followed by linking with PEO17 through a sulfonate group that causes a twist
in the molecule between the hydrophobic and hydrophilic segments. Due to its amphiphilic
nature, its photoluminescence behavior is greatly affected by the solvent type and concentration.
The deposition of PEO17–OPV3 molecules on mica with a proper co-solvent was able to form a
ring-like supramolecular architecture of ca. 30 nm in diameter as observed by atomic force
microscopy (AFM). The width of the enclosed peripheral area on both sides of the ring is on the
same order of magnitude as the length of the PEO17–OPV3 molecule. The self-assembly of a
PEO17–OPV3 hybrid with silica to form a ring-like disk of ca. 150 nm diameter and ca. 0.65 nm
thickness was also observed. The latter is roughly equal to the width of rod segment, implying that
p–p stacking governs the self-assembly process in the co-organization between PEO17–OPV3
molecules and silicates, whereas the twist in molecules orients the ring formation. As a result, a
multi-lamellar phase transformation mechanism is proposed for the formation of such a
supramolecular architecture.
segments. PEO17–OPV3 is also a blue-light emitting molecule,
Introduction
owing to the existence of a conjugated OPV3 segment. Its
Among the series of p-conjugated polymers, polyphenylene–
aggregation in both polar and apolar solvents has a great
vinylene (PPV)-based polymers have been widely studied
influence on its light-emitting properties, as revealed by the
ever since their optoelectronic properties were discovered.1,2
fluorescence spectroscopy. By deposition on mica under
Recently, the ability of their oligomers (OPV) to form
proper preparation conditions, a ring-like supramolecular
supramolecular architectures has attracted great attention
structure of ca. 30 nm in diameter was observed by atomic
because of potential application in optoelectronic nano-
devices.3–8 It is commonly accepted that the optoelectronic
have always been observed in di- or tri- block copolymer
force microscopy (AFM). Although the ring structures
properties of p-conjugated molecules are affected not only by
the primary molecular structure (p-conjugation) but by the
assemblies,34,35 they were rarely reported for the amphiphilic
conjugated oligomer systems. Instead, cylindrical, tubular,
supramolecular organization (p-stacking), similar to that of
the proteins and block copolymers.9–15 For example, Stupp
reported.3–8,36
and fibril supramolecular structures were consistently
et al.16 have found that the self-assembly of the conjugated
On the other hand, since the discovery of the M41 family
molecules into supramolecular ribbon nanostructures leads to
of silicate mesoporous molecular sieves by Mobil
researchers in 1992,37 surfactant-templated silica or titania
enhanced electronic conductivity due to improved p-orbital
overlap. In this aspect, amphiphilic molecules with a proper
with ordered nanostructures become a promising candidate
hydrophilicity or hydrophobicity are needed to proceed with a
for optoelectronic devices such as solar cells.38 Considering
self-assembly process. Researches in the field of self-assembly
that the amphiphilic PEO17–OPV3 molecule itself is also a
techniques usually focus first on the design and synthesis of
surfactant, it might be used as a template to construct the
self-assembling amphiphilic or liquid-crystalline molecules,
hybrid nanostructures with nanopatterns served for the
followed by the investigation of aggregation behavior in the
nano-sized devices. To this end, we prepared a solution of
liquid as well as the solid state.17–33
pre-gelled (PEO17–OPV3)–silica hybrid materials and
In this work, we have synthesized a specifically designed
allowed them to deposit on mica. A thin ring-like disk of
amphiphilic PEO17–OPV3 molecule with a rigid OPV segment
ca. 150 nm in diameter and ca. 0.65 nm in thickness was
linking with a soft PEO segment through a sulfonate group
observed by AFM. The thickness is roughly equal to the
that twists the molecule between hydrophilic and hydrophobic
width of the hydrophobic rod segment of the PEO17–OPV3
molecule. Since the supramolecular structure of the hybrid is
Department of Materials Science and Engineering, National Taiwan
similar to that of the neat amphiphilic molecule, PEO17–OPV3
University, Taipei, Taiwan, Republic of China.
is considered to play a structure-directing role for the hybrid
E-mail: kflin@ccms.ntu.edu.tw; Fax: +886 2 2363 4562;
Tel: +886 2 2392 8290
materials.
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precipitates were filtered and washed with water several times.
Recrystallization from chloroform gave a dark brown solid
product. Yield: 65%. 1H NMR (500 MHz, CDCl3): d(ppm):
8.15 (d, 2H), 7.59 (d, 2H), 7.43 (d, 2H), 7.38 (d, 2H), 7.21 (m,
4H), 7.11 (d, 4H), 1.31 (s, 9H).
Experimental
Materials
All starting materials were purchased from commercial
suppliers (Acros and Aldrich). Tetrahydrofuran (THF) was
dried and distilled over sodium. All used solvents were
degassed by sparkling with nitrogen for 30 min prior to use.
Synthesis of PEO17–OPV3 (5). A solution of 0.3 g (0.4 mmol)
poly(ethylene glycol) methyl ether (Mw = 750 g mol21
)
and 0.22 g (0.5 mmol) OPV trimer (4) in 150 mL dry
dichloromethane was prepared in a flask with a magnetic
stirring bar under nitrogen atmosphere and added dropwise
with pyridine (1 g in 20 mL dry dichloromethane). The mixture
was stirred for 36 h at room temperature. Afterwards, the
mixture was washed with 50 mL water three times and then
dried over MgSO4. After removing the solvent with a vacuum
evaporator, dark viscous oil was obtained. For purification,
the crude product was subjected to column chromatography
using ethyl acetate as an eluent. A purified light yellow oil
product was obtained with a yield of 25%. 1H NMR (500 MHz,
CDCl3): d(ppm): 7.05–8.05 (m, 16H), 3.61 (m, 68H), 3.33 (s,
3H), 1.33 (s, 9H).
Dibutyl(4-tert-butylphenyl)methylphosphonate (1). In a flask
with a magnetic stirring bar was placed 2 mL (10 mmol) 1-tert-
butyl-4-(chloromethyl)benzene and 8 g (30 mmol) tributyl
phosphate. The reaction mixture was stirred and refluxed at
160 uC under nitrogen for 12 h. After cooling to room
temperature, the mixture was concentrated under reduced
pressure at 200 uC to remove the residual tributyl phosphate.
The resulting yellow oil was used for the next Horner–Wittig
reaction without further purification. Yield: 80%. 1H NMR
(500 MHz, CDCl3): d(ppm): 7.08–7.32 (m, 4H), 3.99 (t, 4H),
3.77 (m, 2H), 1.57 (m, 4H), 1.45 (m, 4H), 1.34 (s, 9H), 0.91
(t, 6H).
4-(4-tert-butylstyryl)benzaldehyde (2). A solution of 3.44 g
(16 mmol) 4-(diethoxymethyl)benzaldehyde and 4.8 g (14 mmol
) phosphonate (1) in 30 mL THF was prepared in a flask with a
magnetic stirring bar and added dropwise with potassium
t-butoxide solution in THF (1.8 g, 60 mL). The mixture was
stirred for 18 h at room temperature under nitrogen. Then,
50 mL of 37.5% HCl was added and the mixture was stirred for
another 3 h. Afterwards, the reaction mixture was poured into
a 500 mL water–ethanol (1 : 1 by volume) for precipitation.
The precipitates were filtered and washed with water and
ethanol for several times. Recrystallization from dichloro-
methane–ethanol gave a pure yellow solid product. Yield: 70%.
1H NMR (500 MHz, CDCl3): d(ppm): 9.97 (s, 1H), 7.84 (d,
2H), 7.62 (d, 2H), 7.47 (d, 2H), 7.40 (d, 2H), 7.23 (d, 1H), 7.10
(d, 1H), 1.33 (s, 9H).
Preparation of amphiphilic PEO17–OPV3 solution samples for
measurements of fluorescence spectroscopy
Stock solutions (100 mL) of 1.08 6 1022 M in toluene and
water were prepared first by dissolving 1.25 g PEO17–OPV3 to
the respective solvents. For toluene, the stock solution was
further diluted to 9.46 6 1023, 8.6 6 1023, 6.75 6 1023
5.56 6 1023, 4.3 6 1023, 3.38 6 1023, 2.15 6 1023, 1.08 6
1023, 9.46 6 1024, 8.6 6 1024, 6.75 6 1024, 5.56 6 1024
4.3 6 1024, 3.38 6 1024, 2.15 6 1024, 1.08 6 1024, 9.46 6
,
,
1025, 8.6 6 1025, 6.75 6 1025, 5.56 6 1025, 4.3 6 1025
,
3.38 6 1025, 2.15 6 1025, 1.08 6 1025 (M). For water, the
stock solution was diluted to 5.56 6 1023, 4.3 6 1023, 3.38 6
1023, 2.15 6 1023, 1.08 6 1023, 9.46 6 1024, 8.6 6 1024
,
6.75 6 1024, 5.56 6 1024, 4.3 6 1024, 3.38 6 1024, 2.15 6
1024, 1.08 6 1024, 9.46 6 1025, 8.6 6 1025, 6.75 6 1025
,
Dibutyl(4-sulfonyl chloride phenyl)methylphosphonate (3). In
5.56 6 1025, 4.3 6 1025, 3.38 6 1025, 2.15 6 1025, 1.08 6
1025 (M). An excitation wavelength of 340 nm was chosen for
the photoluminescence measurements.
a flask with a magnetic stirring bar was placed 1 g (3.7 mmol)
of 4-(bromomethyl)benzene-1-sulfonyl chloride and
3 g
(12 mmol) of tributyl phosphate. The reaction mixture was
stirred and refluxed at 160 uC under nitrogen for 12 h. After
cooling to room temperature, the mixture was concentrated
under reduced pressure at 200 uC to remove the residual
tributyl phosphate. The resulting brown oil was used for the
next Horner–Wittig reaction without further purification.
Preparation of amphiphilic PEO17–OPV3 samples for AFM
observation
The solution was prepared by dissolving PEO17–OPV3 in
water–THF (1 : 1 by volume) at a concentration of 3.38 6
1025 M. An automatically-flat mica disk was placed into the
prepared solution in a Petri dish and maintained in level
position for 48 h. After the solution was removed with a
dropper, the sample-deposited mica was then conditioned at
ambient environment for 24 h, followed by thermal annealing
at 120 uC in vacuum oven for 12 h.
1
Yield: 90%. H NMR (500 MHz, CDCl3): d(ppm): 7.03–7.40
(m, 4H), 4.00 (m, 4H), 3.03 (m, 2H), 1.61 (m, 4H), 1.59 (m,
4H), 0.89 (t, 6H).
OPV trimer end-capped with tert-butyl and sulfonyl chloride
groups (4). A solution of 0.25 g (1.9 mmol) aldehyde (2) and
0.5 g (2.5 mmol) phosphonate (3) in 30 mL THF was prepared
in a flask with a magnetic stirring bar and added dropwise
with potassium t-butoxide solution in THF (0.34 g, 50 mL).
The mixture was stirred for 18 h at room temperature under
nitrogen. Then, 20 mL of 37.5% HCl was added and the
mixture was stirred for another 3 h. Afterwards, the reaction
mixture was poured into a 500 mL water for precipitation. The
Preparation of silicate source solution
In a vial, 2.7 mmol ethanol, 7.2 mmol water, and 0.12 mmol
HCl were mixed with stirring at room temperature, and then
added to 0.9 mmol tetraethyl orthosilicate (TEOS) in an ice
bath by vigorously stirring to prevent gelation. The solution
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was maintained in an ice bath under vigorous stir for 2 h to
afford a silicate source solution.
Preparation of (PEO17–OPV3)–silicate hybrid samples for
AFM observation
A solution of 0.2 g (0.17 mmol) PEO17–OPV3 in 5 mL THF
was vigorously stirred for 30 min while the silicate source
solution (0.9 mmol TEOS, 2.7 mmol ethanol, 7.2 mmol water,
and 0.12 mmol HCl ) was added dropwise. The mixture was
heated to 60 uC with stirring and maintained for 90 min.
Afterward, the mixture was diluted by 2 mL EtOH and 1 mL
water before allowed to react for another 24 h at room
temperature. The molar composition was 1.0 TEOS : 77 THF :
69 H2O : 0.13 HCl : 0.19 PEO17–OPV3 : 51.9 EtOH. To
prepare a thin film for AFM observation, 1 mL of the
above solution was further diluted by 3 mL THF and 6 mL
water in a Petri dish and an automatically-flat mica disk
was placed into the solution and maintained in flat position
for 48 h. Then, the solution was removed with a dropper.
The sample-deposited mica was conditioned at ambient
environment for 24 h, followed by thermal annealing at
120 uC in vacuum oven for 24 h.
Fig. 2 1H NMR spectrum of the amphiphilic PEO17–OPV3 molecule.
Photoluminescence (PL) spectra were obtained by using a
JASCO FP-777 spectrofluorometer at an excitation wave-
length of 340 nm. The nanostructures on mica were
investigated with AFM by using a Digital Instruments
Dimension-3100 Multimode microscopes in tapping mode.
The molecular model was constructed using a Cerius 2 energy
minimization.
Characterization
1H Nuclear magnetic resonance (NMR) spectra were obtained
in deuterated chloroform using a BRUKER DMX-500 MHz
FT-NMR. Chemical shifts were referred to tetramethylsilane.
Results and discussion
Amphiphilic PEO17–OPV3 molecule
The scheme for the synthesis of the PEO17–OPV3 molecule is
shown in Fig. 1, beginning with the Horner–Wittig reaction to
form
a co-planar p-conjugated trans-phenylene vinylene
oligomer, followed by linking with PEO17 through a sulfonate
group. Notably, the PPV trimer is recognized as a co-planar
trans-linkage structure because no resonance at d = 6.5–
6.8 ppm in the 1H NMR spectrum attributable to the cis-
vinylene protons39,40 was detected (Fig. 2). Besides, it is
generally accepted that the conjugated vinylene bond formed
using the Horner–Wittig reaction between an aromatic
phosphonate ester and an aromatic aldehyde is normally in
the trans-form.36,41 Fig. 3 presents its 3-D primary structure,
constructed by a Cerius 2 energy minimization. As seen in
Fig. 1 Scheme for synthesis of the amphiphilic PEO17–OPV3
Fig. 3 Molecular graphics of the amphiphilic PEO17–OPV3 using a
molecule.
Cerius 2 energy minimization (arrow indicates sulfonate linkage).
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Fig. 4 Concentration dependence of surface tension for PEO17–OPV3
aqueous solutions.
Fig. 6 Normalized PL of PEO17–OPV3 in toluene at various
concentrations, c (1.08 6 1025 M , c , 1.08 6 1022 M), individually
as replotted in Fig. 7.
the figure, the size of the OPV rod segment is about 2 nm
in length and 0.5 nm in width, whereas the length of the
extended PEO segment is about 6 nm. Interestingly, the as-
tetrahedral shape of the sulfonate linking group caused a
twist in the molecule between hydrophobic and hydrophilic
segments. Due to the soft nature of the PEO segment, PEO17–
OPV3 is also considered as a rod–coil molecule. Undoubtedly,
both amphiphilic and rod–coil characters play major roles on
the formation of self-assembling supramolecular structure.
474 nm turned into the major peak as a result of forming large
aggregates or supramolecular structures. On the other hand, in
toluene (as shown in Fig. 6), typical 0–0 and 1–0 vibrational
peaks located at 415 and 435 nm were observed in dilute
solution. As concentration was increased, 0–0 vibrational
peak that was often attributed to the molecularly dispersed
emission, gradually disappeared due to the formation of
excimers. Meanwhile, the second peak shifted to 440 nm and
became the major peak in the concentration (c) range: 3.38 6
1024 M , c , 4.3 6 1023 M. Finally, the major peak shifted
to 471 nm as c . 5.56 6 1023 M. At this point, it is reasonable
to make a conjecture that a larger molecular-packing structure
was formed and had a different photophysical emission center
from the excimers. From the plot of relative intensities (as
shown in Fig. 7, I474nm/I445nm for water and I471nm/I440nm
for toluene), two-step slopes for the increase of relative
intensities were found. Interestingly, the intersection for
water was located at ca. 5 6 1024 M near the CAC value
determined by a surface tension meter. It indicates that the PL
Molecular aggregations and luminescence properties in the
solution
A critical aggregation concentration (CAC) value of 3.67 6
1024 M for PEO17–OPV3 in water was determined by surface
tension meter (as shown in Fig. 4). It is believed that above the
CAC, molecular aggregation to larger domains occurs. With a
hydrophilic long segment linking the hydrophobic PPV
conjugated segment, PEO17–OPV3 could dissolve in both
polar and apolar solvents. In water (as shown in Fig. 5),
excimer-like emission at 445 nm was dominant in dilute
solution. With increasing concentration, the shoulder peak of
Fig. 7 Concentration dependences of PL intensity ratios for
PEO17–OPV3 in water estimated by I474nm/I445nm and in toluene by
Fig. 5 Normalized PL spectra of PEO17–OPV3 in water at various
concentrations, c (1.08 6 1025 M , c , 5.56 6 1023 M), individually
as replotted in Fig. 7.
I471nm/I440nm
.
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measurements are able to detect the formation of supramole-
cular structures.
model is schematically shown in Fig. 9 (top). The grey part
represents the rigid OPV core and the black represents the soft
PEO shell. To our knowledge, the formation of these self-
assembled supramolecular structures might be influenced by
the hydrophilicity of the mica surface.
Solid-state self-assembly from a PEO17–OPV3 solution
In this study, AFM was used to investigate the supramolecular
architectures. Fig. 8 shows a ring-like structure with an
average diameter of 30 nm deposited on mica from a solution
of PEO17–OPV3 in water–THF (1 : 1 by volume) at a
concentration of 3.38 6 1025 M. Ring-like assemblies are
not observed by accident but all over the mica substrate. The
width of enclosed periphery area on both side of the ring
ranging from 6 to 12 nm is the same order of magnitude as the
length of PEO17–OPV3 molecule. Thus, we propose that this
enclosed periphery area has a core–shell morphology with the
OPV rigid segments forming the core and the PEO soft
segments forming the shell. The top view of supramolecular
Owing to the strong polarity of the mica surface, hydrophilic
PEO segments have a tendency to lie flat on the surface,
whereas the hydrophobic OPV segments tilts with its co-planar
configuration normal to the surface. As mentioned in the
Introduction section, most of the research groups studying
the supramolecular structures of oligo-PPV observed struc-
tures with high aspect ratios, such as fibril or cylindrical
textures,28–33,36 significantly different from our observations.
Two factors are possibly involved in the formation of ring
structures: (1) the twist of molecules in between hydrophobic
and hydrophilic segments forces them to have only one side
with the same curvature to proceed with p–p stacking, and: (2)
the high interfacial tension between mica and hydrophobic
OPV segments causes the p–p stacking region to curl in order
to reduce the interfacial area. Thus, thermodynamically, the
packing region of hydrophobic OPV segments prefers to form
a ring structure, unless the adhesion strength of hydrophilic
PEO segments to mica is able to overcome this tendency.
Besides, packing under the same curvature would favor the
formation of a ring. After all, since thermal annealing of the
deposited mica at 120 uC afforded a better image of the ring
structures, these self-assembled supramolecular structures are
thermodynamically stable.
Solid-state self-assembly from a (PEO17–OPV3)–silica hybrid
solution
The hybrid solution was prepared by addition of a pre-gelled
silicate source solution (0.9 mmol TEOS, 2.7 mmol ethanol,
7.2 mmol water, and 0.12 mmol HCl) to PEO17–OPV3 in THF.
Fig. 9 Illustration of the supramolecular structure of neat amphi-
philic PEO17–OPV3 molecule (top) and (PEO17–OPV3)–silicate hybrid
(bottom).
Fig. 8 AFM image of amphiphilic PEO17–OPV3 deposited on a mica
substrate. Top: Image size is 2 6 2 mm. Bottom: enlarged image from
the selection area of the top image.
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oligomers that participate in the formation of supramolecular
architectures are allowed to deposit on mica in the second step
of sample preparation. The silicate oligomers that have no
interaction with PEO17–OPV3 molecules remain in solution
and should be removed as much as possible after deposition of
supramolecular architectures.
Because a large amount of water is incorporated with the
solution in the second step of sample preparation, OPV3
segments pack tightly with each other. Some of the silicates are
attracted to the neighborhood of PEO layers and interweave
with the PEO segments to form a mutilamellar mesophase. It is
highly possible that the ring-like thin supramolecular archi-
tectures were preformed in the solution before deposition on
the mica surface, because they stacked with each other instead
of fusing together (see Fig. 10 (bottom)). The width of the
enclosed peripheral area on both sides of the rings is ca. 40 nm,
several times larger than that formed by neat PEO17–OPV3
molecules (see Fig. 8). Since the thickness of ring-like
supramolecular architectures is roughly equal to the width of
co-planar configuration of an OPV3 rod segment, p–p stacking
of OPV3 rod segments should govern the self-assembly
process. The incorporation of silicate oligomers barely
increased the thickness of ring over the width of OPV3 rod
segment. Furthermore, because silicate oligomers can perform
crosslinking reactions, they act as a strong adhesive to bind
several self-assembling PEO17–OPV3 supramolecular layers
into a larger ring. The final supramolecular architecture of the
(PEO17–OPV3)–silica hybrid is shown schematically in Fig. 9
(bottom).
Fig. 10 AFM images of the (PEO17–OPV3)–silicate hybrid deposited
on a mica substrate. Top: image analysis of 1 6 1 mm size. Bottom:
phase image.
A well self-assembled ring-like thin architecture of the
(PEO17–OPV3)–silica hybrid with a diameter of ca. 150 nm
and a thickness of ca. 0.65 nm will have many potential
applications to nano-sized optoelectronic devices, such as
nano-photodiodes, nano-switches, nano-computation devices,
organic light emitting diodes, photovoltaic devices, etc., if each
ring can be manipulated individually. To realize these future
applications, further researches in cooperation with physicists
and electronic engineers are ongoing.
After reaction and dilution at proper conditions (see the
Experimental section for a detailed description), a ring-like
thin disk form of supramolecular architecture was deposited
on mica as presented in Fig. 10. Although the supramolecular
structure of the (PEO17–OPV3)–silica hybrid is approximately
similar to that prepared from the neat PEO17–OPV3 solution,
the diameter (ca. 150 nm) is much larger and more uniform.
The thickness of ca. 0.65 nm measured from the image
analysis of AFM (see Fig. 10 (top)) was also uniform. Notably,
the thickness is roughly equal to the width of co-planar
configuration of an OPV3 rod segment (see Fig. 3), indicating
that the p–p stacking of amphiphilic PEO17–OPV3 molecules
acts as a template for the formation of the supramolecular
structure of (PEO17–OPV3)–silica hybrid.
Conclusions
An amphiphilic rod–coil PEO17–OPV3 molecule was specifi-
cally designed with
a sulfonate group linking between
hydrophobic and hydrophilic segments that twists the mole-
cule in situ. Its photoluminescent properties in solution were
dependent on the solvent type and concentration in association
with molecular aggregation. As it was deposited on mica
surface, the p–p stacking of apolar co-planar OPV segments
was forced to tilt normal to the mica surface, driving the self-
assembly process to form a ring-like architecture with a
diameter of ca. 30 nm. As it was blended with silicates in
solution, a uniform ring-like thin disk of ca. 150 nm in
diameter and ca. 0.65 nm in thickness was deposited on mica.
Because the thickness is roughly equal to the width of
co-planar OPV configurations, the silicates were believed to
act as an adhesive that binds several self-assembled PEO17–
OPV3 lamellae to form a larger ring-like supramolecular
architectures. Its potential applications in nano-sized opto-
electronic devices are under exploration.
The exact mechanism of ring-like thin supramolecular
architecture formation is not clear. However, we believe that
at least three criteria listed below should be followed for this
formation. First, PEO17–OPV3 molecules dissolving in solu-
tion in the first step of sample preparation should not have
a tight packing for either the hydrophobic or hydrophilic
segments. Otherwise, further organization to a larger supra-
molecular architecture becomes difficult. Thus, THF, a good
solvent for both segments, was used. Second, the condensation
reactions of TEOS through hydrolysis by HCl to form silicate
oligomers should not go over 25% of total reacting groups.
Otherwise, a three dimensional network forms and condenses
as a small gelled particle.42 A large scale gelation occurs as the
extent of reactions reaches 33%.42 Third, only the silicate
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16 B. W. Messmore, J. F. Hulvat, E. D. Sone and S. I. Stupp, J. Am.
Chem. Soc., 2004, 126, 14452.
Acknowledgements
17 A. K. Brannan and F. S. Bates, Macromolecules, 2004, 37, 8816.
18 R. B. Prince, J. G. Saven, P. G. Wolynes and J. S. Moore, J. Am.
Chem. Soc., 1999, 121, 3114.
19 M. P. Nieh, S. K. Kumar, R. H. Fernando, R. H. Colby and
J. Katsaras, Langmuir, 2004, 20, 9061.
The authors acknowledge the financial support of the National
Science Council in Taiwan, Republic of China, through Grant
NSC 92-2216-E-002-011.
20 M. S. Gin, T. Yokozawa, R. B. Prince and J. S. Moore, J. Am.
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