2D Self-assembly of an amido-ended hyperbranched polyester induced by platinum ion coordination effect

Article abstract of DOI:10.1039/c3ra42057d

Compared with amphiphilic hyperbranched polymers (HBPs), the self-assembly of hydrophilic HBPs remains a challenging area. In this article, we report the self-assembly of an amido-ended, hydrophilic hyperbranched polyester (HTDA-2), by taking advantage of the induction effect of the platinum ion. Self-assembled HTDA-2-Pt polymers are composed of ordered 2D tree-like structures with a diameter of about 500 μm and a trunk-width of approx. 1-2 μm, as shown by transmission/reflection polarizing optical microscopy (TRPOM), scanning electron microscopy (SEM) and dynamic light scattering (DLS). Factors such as temperature, time, solvent, concentration and humidity which influence the morphology of the self-assemblies were investigated using XPS, XRD, SEM, and FT-IR techniques. It is proposed that the coordination between the platinum and oxygen atoms in the amido groups together with hydrogen bonding may induce individual aggregation of core chains and peripheral chains, resulting in micro-phase separation and formation of micelles. These micelles then aggregate and self-assemble into perfect tree-like structures on the surface of a glass substrate in a limited region. The dimension (D_f) of about 1.59 for the fractal tree-like morphology suggests perfect fractal behavior of the self-assemblies. Another interesting finding is the appearance of crystal behavior of the tree-like self-assemblies as revealed by XRD analysis.

Full text of DOI:10.1039/c3ra42057d

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2D Self-assembly of an amido-ended hyperbranched
polyester induced by platinum ion coordination effect3
Cite this: RSC Advances, 2013, 3,
17073
Daohong Zhang,*a Junna Li,a Jing Wang,a Sufang Chen,b Jiliang Zhou,a
Tingcheng Li,a Junheng Zhang,a Aiqing Zhanga and Chuanjun Liu*c
Compared with amphiphilic hyperbranched polymers (HBPs), the self-assembly of hydrophilic HBPs
remains a challenging area. In this article, we report the self-assembly of an amido-ended, hydrophilic
hyperbranched polyester (HTDA-2), by taking advantage of the induction effect of the platinum ion. Self-
assembled HTDA-2-Pt polymers are composed of ordered 2D tree-like structures with a diameter of about
500 mm and a trunk-width of approx. 1–2 mm, as shown by transmission/reflection polarizing optical
microscopy (TRPOM), scanning electron microscopy (SEM) and dynamic light scattering (DLS). Factors such
as temperature, time, solvent, concentration and humidity which influence the morphology of the self-
assemblies were investigated using XPS, XRD, SEM, and FT-IR techniques. It is proposed that the
coordination between the platinum and oxygen atoms in the amido groups together with hydrogen
bonding may induce individual aggregation of core chains and peripheral chains, resulting in micro-phase
separation and formation of micelles. These micelles then aggregate and self-assemble into perfect tree-
like structures on the surface of a glass substrate in a limited region. The dimension (Df) of about 1.59 for
the fractal tree-like morphology suggests perfect fractal behavior of the self-assemblies. Another
interesting finding is the appearance of crystal behavior of the tree-like self-assemblies as revealed by XRD
analysis.
Received 26th April 2013,
Accepted 19th July 2013
DOI: 10.1039/c3ra42057d
www.rsc.org/advances
a
restrictive ratio of the hydrophilic and hydrophobic
1 Introduction
segments through interfacial/surface self-assembly or using
selective solvent self-assembly,7,12 resulting in the formation
of honeycomb morphologies13,14 and mesoscopic pattern
morphologies.15 Although the preparation of amphiphilic
HBPs involves complex and tedious synthetic processes, such
as grafting modification and copolymerization by the RAFT
process,7,9 self-assembly of amphiphilic HBPs has been highly
successful, and their applications as drug delivery and reaction
vessels for charge storage9 have been well developed.
The ability of most amphiphilic HBPs to self-assemble is
attributable to the individual aggregation of two blocks or the
micro-phase separation resulting from the different solubili-
ties of amphiphilic segments in selected solvents.7 The
interactions between the aggregates play an important role
in controlling the size and stability of self-assemblies. On the
contrary, non-amphiphilic HBPs, including hydrophilic and
hydrophobic HBPs, as a majority of HBPs, are still difficult to
assemble into ordered supramolecular structures because the
relative property variation between ‘‘core’’ chains and end
groups chains is not strong enough to induce micro-phase
separation. Therefore, the self-assembly of hydrophilic HBPs is
still a challenging work up to now.
The self-assembly of hyperbranched polymers (HBPs) has
received a great deal of attention because of its fundamental
importance in fabricating novel ordered supramolecular
structures1 and fascinating applications.2,3 HBPs containing
various functional terminal groups, such as carboxyl, hydroxyl,
vinyl, and amino groups, provide unique properties for the
encapsulation of small molecules or inorganic nanoparticles
to form self-assembled structures.4,5 Self-assembled HBPs
materials have been extensively used in the field of biomater-
ials.6–10 Amphiphilic HBPs which have been reported so
far9,11,12 represent only a minority of the HBPs that can be
readily self-assembled into micelles, fibers, tubes, vesicles,
films and large compound vesicles (LCVs). This is done under
aKey Laboratory of Catalysis and Materials Science of the State Ethnic Affairs
Commission & Ministry of Education, South-central University for Nationalities,
Wuhan, Hubei Province, 430074, China. E-mail: zhangdh27@163.com;
Fax: +86-27-67842752; Tel: +86-27-67842752
bKey Laboratory for Green Chemical Process of Ministry of Education, Wuhan
Institute of Technology, Wuhan, Hubei, 430073, China
cDepartment of Electronics, Graduate School of Information Science and Electrical
Engineering, Kyushu University, 744, Motoota, Nishiku, Fukuoka, 819-0395, Japan.
E-mail: liu@o.ed.kyushu-u-ac.jp; Fax: +81-92-8023629; Tel: +81-92-8023629
A
poly(amidoamine) (PAMAM) dendrimer containing
amino-ended groups has shown potential in being able to
electrostatically self-assemble with
virus.16 Moreover,
Electronic supplementary information (ESI) available: FT-IR, 1H NMR and XPS
3
spectra, DLS curve, TGA curve and optical microscope images. See DOI: 10.1039/
c3ra42057d
a
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PAMAM dendrimer aggregates with fractal features have a
crucial impact on the intrinsic emission properties.17
Therefore, it is significant to develop new assembly processes
for amino-ended HBPs to obtain advanced biohybrid materi-
als.16 Our recent paper18 reported the self-assembly of a non-
amphiphilic carboxyl-ended hyperbranched polyester by plati-
num induction, and ordered 1D microrod self-assemblies with
a length of 10–20 mm and a width of 1 mm were obtained. It
was revealed that the coordination induction of platinum ions
played a key role in driving micro-phase separation between
core chains and peripheral segments of the carboxyl-ended
hyperbranched polyester/platinum complex. The coordination
induction of the platinum ion can be used as a new tool to
realize the self-assembly of non-amiphphilic HBPs.
In this article, we reveal that another non-amphiphilic or
hydrophilic HBP containing amido-ended groups (HTDA-2)
can be self-assembled into ordered micrometer 2D tree-like
supramolecular structures by use of the platinum ions’
coordination induction. In comparison to a method for self-
assembling a polymer complex containing a metal with a
certain chemical structure,19,20 the approach in this work is
rather simple and easily available. The 2D self-assembly of
HTDA-2-Pt shows fractal properties and crystal behavior,
which is generally difficult to obtain from amiphiphilic
HBPs due to their soft-loose amorphous morphology. This
discovery may help new applications of hydrophilic self-
assemblies as multi-functional semiconductors materi-
als19,21–23 due to the existence of metal ions, as well as being
optical, magnetic and bioactive materials24 owing to their
fractal and crystal properties.16
2 Experimental
2.1 Materials
Analytical materials, including trimellitic anhydride (TMA),
xylene, diethylene glycol (DEG), tetrabutyl titanate (TBT), urea,
triethylamine, isopropanol, chloroplatinic acid, pyridine, N,N-
dimethylformamide (DMF), tetrahydrofuran (THF), ether,
ethyl acetate and NaHCO3 were purchased from Sinopharm
Chemical Reagent Corporation and used without further
purification unless noted otherwise.
2.2 Preparation of HTDA-2
The synthesis of an amido-ended hyperbranched polyester
(HTDA-2) is shown in Fig. 1. First, an AB2 monomer was
prepared from the reaction between trimellitic anhydride
(TMA, 0.09 mol) and diethylene glycol (DEG, 0.09 mol) using
100 mL xylene as solvent at 393 K for 3 h in a four-necked
reaction flask equipped with stirrer, cooler, nitrogen inlet and
water-trap (Dean–Stark), followed by addition of 0.01 mol
‘‘core’’ TMA and TBT (0.2 g). The reaction temperature was
raised to 413 K and the reaction continued for 6 h. The solvent
was then removed under a pressure of 3–5 mmHg at 413 K for
30 min. A yellowish transparent solid was obtained, which was
proved to be a carboxyl-ended hyperbranched polyester (HTD-
2) containing 12 molar carboxyl groups with number average
21
¯
molecular weight (Mn) of about 4260 g mol and polymeriza-
tion distribution index (PDI) of about 2.2325,26 from gel
permeation chromatography (GPC), and its degree of branch-
ing (DB) was about 0.71.
HTD-2 (3.5 g), urea (2.5 g), triethylamine (20 g) and
deionized water (20 mL) were added to a flask for reaction at
368 K for about 6 h using a mechanical stirrer. The solution
was put into a separating funnel for delaminating, and the
solvent in the up-layer yellow solution was removed under a
pressure of 3–5 mmHg at 363 K for 30 min to give a powdered
Fig. 1 Synthesis of HTDA-2 and HTDA-2-Pt.
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solid, which was proved to be the amido-ended hyperbranched
polyester (HTDA-2). FT-IR and 1H-NMR spectra of HTDA-2 and
China) equipped with a thermal platform were used to
examine the morphology of the self-assemblies. The X-ray
diffraction (XRD) patterns were obtained using a Bruker-D8
diffractometer with monochromatized Cu-Ka radiation (l =
1.54 Å) at 40 kV and 40 mA. The surface compositions of the
samples were determined using a Vacuum Generator Mutilab
2000 X-ray photoelectron spectrometer (XPS) with the C1s
(284.6 eV) peak of the contamination carbon as the internal
standard. Thermogravimetric analysis (TGA) was performed
with a thermogravimetric analyzer (NETZSCH TG209) at a
heating rate of 10 K min21 and a nitrogen flow of 20 mL
min21. Dynamic light scattering (DLS) measurements of
samples with a concentration of 1.0 mg mL21 were performed
on a ZetaSizer Nano ZS90 (Malvern Instrument, Worcs, UK)
equipped with 4 mW He-Ne Laser at about 25.0 uC and at a
scattering angle of 90u. The apparent z-average hydrodynamic
diameter (Dh) and polydispersity index (PDI) were calculated
by the dispersion technology software provided by Malvern.
HTD-2 are shown in Fig. S1 and Fig. S2, ESI respectively. For
3
comparison, absorption peaks of –NH2 in HTDA-2 are
observed at 3460 cm21and 3260 cm21, and a sharp absorption
peak of –C–N appears at 1370 cm21. These characteristic peaks
confirm the formation of amido-ended groups, supporting the
appearance of a blunt peak about –CONH2 at d 6.4–6.7 ppm in
Fig. S2, ESI.
3
2.3 Preparation of HTDA-2-Pt
The synthesis of an amido-ended hyperbranched polyester/
platinum complex (HTDA-2-Pt) is shown in Fig. 1.
Chloroplatinic acid (0.4 g), isopropanol (12 g) and THF (6 g)
were added to a four-neck flask fitted with a mechanical
stirrer, a condenser, a thermometer and a nitrogen inlet.
Stirring continued for 1 h at room temperature. Then HTDA-2
(2 g) and NaHCO3 (0.7 g) were added into the flask. The
reaction took place under ultra-sonication (700 W) for 10 min.
The mixture was heated to approximately 315 K and stirred for
2 h under nitrogen protection. The solvent was then removed
under a 3–5 mmHg vacuum at 338 K. The remaining solid was
washed twice with 5 mL of deionized water in a Buchner
funnel, resulting in a yellowish fluffy solid amido-ended
hyperbranched polyester/platinum complex (named HTDA-2-
Pt). The content of elemental platinum in HTDA-2-Pt is about
3 Results and discussion
3.1 Self-assembly of HTDA-2-Pt under different conditions
3.1.1 Platinum ion effect. Fig. 2 compares the morphology of
HTD-2, HTDA-2 and HTDA-2-Pt self-assemblies. HTD-2 can be
self-assembled into uniform star and crew-cut micelles with
diameter about 1.0-1.5 mm as shown in Fig. 2a. This formation
of the micelles can be attributed to the secondary aggregation
of unimolecular micelles stabilized by intermolecular hydro-
gen bonding27. A rudimentary tree-like structure is shown by
self-assembling HTDA-2 in Fig. 2b, indicating that the HTDA-2
should be easier to self-assemble than HTD-2 due to the much
stronger hydrogen bonds in HTDA-2. If HTDA-2-Pt was used to
self-assemble under the same conditions as HTDA-2, a
perfectly ordered tree-like supramolecular structure was
obtained as shown in Fig. 2c. This result demonstrates that
the platinum ion plays an important role in promoting the
self-assembly of HTDA-2.
7.78 wt%, taken from its TGA curve in Fig. S3, ESI and close to
3
its theoretical content of 7.00 wt%.
2.4 Preparation of hybrid self-assemblies
In a typical experiment, HTDA-2-Pt (0.1 g) was dispersed in the
corresponding solvent (7.9 mL) and followed by ultra-sonica-
tion for 20 min. The solution was dropped onto a clean surface
of glass substrate and controlled at given conditions to allow
for self-assembly. The morphology of the self-assemblies was
measured real-time by TRPOM under a designated tempera-
ture and relative humidity.
2.5 Characterization
FT-IR measurements were performed on a Bruker Vector 33
spectrometer using sealed cell (KBr 0.5 mm). 1H NMR
measurements was conducted on an AVANCE III-400
(Bruker) nuclear magnetic resonance spectrometer using
DMSO-d6 as the solvent. A low-voltage scanning electron
microscope (SEM, XL-30FEG, Philip) and a transmission/
reflection polarizing optical microscope (TRPOM, XPV-203E,
3.1.2 Solvent effect. It is well known that a solvent with
similar polarity and solubility parameter to partial chains of
HTDA-2-Pt helps its dissolution or dispersion according to the
theory of similarity and compatibility. Herein, the formation of
micelles resulting from the dissolution of partial chains and
the aggregation of other chains in HTDA-2-Pt is key to the self-
assembly of HTDA-2-Pt. The core segments of HTDA-2-Pt have
Fig. 2 Optical micrographs of self-assemblies at 298 K and 40% relative humidity: (a) HTD-2 (0.10 g)/pyridine (7.90 mL), (b) HTDA-2(0.10 g)/pyridine (7.90 mL), and
(c) HTDA-2-Pt (0.10 g)/pyridine (7.90 mL).
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relative lower solubility than that of the end groups in pyridine
or DMF, one reason is that the core segments are made up of a
large number of ester groups with a low solubility parameter,
and another reason is that the end groups have chemical
structures similar to pyridine or DMF. Consequently, the
solubility difference between the end groups and core
segments of HTDA-2-Pt may result in micro-separation and
the formation of micelles in pyridine or DMF solvent. The self-
assemblies obtained have uniformly ordered supramolecular
a tree-like structure at a relatively low platinum ion content
(3.81 wt%) in Fig. S7a, ESI, compared with only micelle
3
aggregates being obtained at a high platinum content (15.1
wt%) in Fig. S7b, ESI, indicating that an appropriate loading
3
amount of platinum ions on HTDA-2 is necessary for the
formation of ordered self-assemblies. When the loading
amount of platinum ions on HTDA-2 is 7.78wt% (see Section
2.3) in fact, the molar ratio of the amido groups to platinum is
just about 10 : 1 in HTDA-2-Pt in Fig. 2c, close to the
6-coordination number of Pt4+, resulting in coordination
between the platinum atom and most of the amido groups.
When the platinum content in the complex was increased,
excess platinum dispersed in the matrix of the complex (in
addition to the formation of the complex with a 6-coordination
number). This made the formation of regular micelles very
difficult, so only disordered micelle aggregates could be
structures when pyridine (Fig. 2c) or DMF (Fig. S4d, ESI ) is
3
used as the solvent, but the self-assemblies are irregular and
the morphology is vague (Fig. S4a and Fig. S4c, ESI ) when
3
using acetone (d 20.50 (J cm
)
23 0.5, b.p. 329.6 K) or ether (d
15.40 (J cm23)P0.5, b.p. 307.7 K) as solvents. This interesting
result may be attributable to the solubility parameter (d 18.60
(J.cmP
19.44 (J cm
)
23 0.5, b.p. 350.3 K) of ethyl acetate being close to that (d
)
23 0.5) of the core segments of HTDA-2-Pt, resulting
obtained (Fig. S7b, ESI ). In contrast, a HTDA-2-Pt solution
3
in separation and aggregation of the core segments and end
groups in HTDA-2-Pt and the formation of clear self-
with lower platinum ion content may contain non-coordinated
HTDA-2, resulting in the tree-like self-assemblies with some
assemblies (Fig. S4b, ESI ). However, the lower boiling points
vague aggregates (Fig. S7a, ESI ).
3
3
of acetone and ether may result in their easy evaporation, and
so HTDA-2-Pt micelles have not enough time for self-assembly
on the surface, thus producing vague morphologies.18
3.1.6 HTDA-2-Pt concentration effect. The morphologies of
the hybrid self-assemblies using various HTDA-2-Pt concentra-
tions are all shown in Fig. S8, ESI. With the increase of the
3
3.1.3 Humidity effect. Another factor, relative humidity,
which affects the self-assembled morphology of HTDA-2-Pt
also is discussed. As we known, the higher the relative
humidity is, the lower the volatilization rate of the solvent is.
Micelle particles spreading on surface of glass substrate do not
have enough time to self-assemble in low relative humidity
because they can not move freely and aggregate without
solvent, so the resulting self-assemblies are composed of small
HTDA-2-Pt concentration in the HTDA-2-Pt/pyridine solution,
the density of self-assemblies on the surface increases notably
in Fig. S8, ESI. Many micelles can not self-assemble into an
3
ordered tree-like structure at low HTDA-2-Pt concentration,
which is attributed to an absence of enough opportunities for
the micelles’ aggregation on the surface (Fig. S8a, ESI ). Using
3
an HTDA-2-Pt/pyridine solution with a high concentration of
HTDA-2-Pt for self-assembly, worm-like self-assemblies (Fig.
branch-like (Fig. S5a, ESI ) and not tree-like. On the contrary,
3
S8d, ESI ) were obtained on the surface because there is not
3
the micelle particles may move easily and but can not root in
some regions in high relative humidity, and a vague tree-like
enough space for rearrangment and aggregation of a large
number of micelles. Therefore, HTDA-2-Pt can self-assemble
into uniformly ordered structures with fractal properties at a
morphology of the self-assemblies is formed (Fig. S5c, ESI ).
3
Therefore, in medium relative humidity between 40% (Fig.
medium concentration in solution (Fig. S8b–c, ESI ).
3
S5b, ESI ) and 70% (Fig. 2c), it is suitable to obtain uniformly
3
3.2 Mechanism of self-assembly
ordered structures.
3.1.4 Temperature effect. The effect of temperature on the
stable morphologies of self-assembled HTDA-2-Pt is shown in
The influence of the different factors described above on the
resulting self-assembled morphologies of HTDA-2-Pt suggest
that platinum ions play an important role in inducing self-
assembly. The platinum atom in chloroplatinic acid can
usually coordinate with the oxygen and nitrogen atoms from
the ether (–C–O–C–), ester (–COO–) and amino (–NH2) groups
in HTDA-2. The FT-IR spectra of HTDA-2, HTDA-2-Pt and self-
Fig. S6, ESI. Tree-like self-assemblies are observed on the
3
surface of the glass substrate at medium and low temperatures
(Fig. S6a–b, ESI ) under a same self-assembly period of one
3
day, however, at high temperatures, the self-assemblies’
morphologies are tree-like skeletons containing vague patterns
(Fig. S6c, ESI ). The self-assembly temperature influences not
3
assembled HTDA-2-Pt are shown in Fig. S1, ESI. Absorption
3
peaks of ester (–COO–) groups is observed at 1730 cm21, and
the peaks at 1622 cm21 and 1575 cm21 which may be
attributed to the stretching vibration of amide groups (–CON–)
bonds in HTDA-2, HTDA-2-Pt and self-assembled HTDA-2-Pt,28
respectively. In comparison with the difference before and
after coordination of platinum, the peak area at 1575 cm21
becomes bigger after adding the platinum to HTDA-2, and all
only the volatilization rate of the pyridine and but also the
moving velocity of the micelles. By increasing the self-
assembly temperature, many micelles do not have enough
time to re-array and aggregate, preferring a stable morphology
because of the increase in both the volatility rate of the
pyridine and the moving velocity of the micelles, resulting in a
vague morphology in Fig. S6c, ESI.
3
of the absorption peaks about –NH2 (3460 cm21, 3260 cm21
)
3.1.5 Effect of the loading amount of Pt ions on HTDA-2.
The morphologies shown in Fig. S7, ESI result from self-
and –C–N (1370 cm21) remain invariably in HTDA-2, HTDA-2-
Pt and self-assembled HTDA-2-Pt, confirming the appearance
of an interaction between the platinum and oxygen in the
amide groups.
3
assembly of a HTDA-2-Pt (0.10 g)/pyridine (7.90 mL) solution
containing various platinum ion content at 298 K and 40%
relative humidity. The HTDA-2-Pt could be self-assembled into
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groups,30 suggest that platinum ions in self-assembled HTDA-
2-Pt have an interaction with the nitrogen element in pyridine
and not in amido groups.
Hyperbranched polymers or star polymers without metal
ions usually can be self-assembled into honeycomb supramo-
lecular structures, and their mechanism was supposed to be a
solvent evaporation, water condensation, and core-shell-
micelles aggregation uniformly process.13–15,31 The self-assem-
bly mechanism of 1D micro-rod-like self-assemblies was
discussed.18 Hitherto, the perfectly ordered 2D tree-like
supramolecular structures obtained from the amido-ended
hyperbranched polyester containing platinum ions has rarely
been reported, but the solvent-evaporation water-concentra-
tion process involved should be similar to those of our
previous papers.18,32 The effect of self-assembly time on the
resulting morphology of HTDA-2-Pt (Fig. 4) suggests that the
ordered tree-like structures should grow gradually and form
stable tree-like self-assemblies with a diameter of about 500
mm (Fig. 4f) and a trunk width of about 1–2 mm (Fig. 5a) with
an increase in self-assembly time. Moreover, an important
discovery is that the tree-like morphology is extremely similar
to the ‘‘hyperbranched structure’’ of conjugated polymers’ self-
assembly33 and dendrimers growth patterns,34 so self-assem-
bly of HTDA-2-Pt may be explained by diffusion-limited
aggregation (DLA) theory.35
According to the chemical properties of HTDA-2-Pt and the
self-assembly process, we propose a possible self-assembly
mechanism by the following schematic representation (Fig. 6):
(1) The solubility parameter of the core chains of the HTDA-2-
Pt is calculated to be 19.44 (J cm
coordinate bonds and the amido-ended groups (–CONH2). The
solubility parameter of pyridine is about 21.90 (J cm23 0.5
and
)
23 0.5 without considering the
)
close to that of the coordinate bonds and the amido-ended
(–CONH2) groups with high polarity. So the HTDA-2-Pt may be
regarded as an amphiphilic polymer in pyridine. When it is
dispersed in pyridine, the peripheral chains would swell and
dissolve gradually and but the core chains may shrink and
deform. Then the two blocks aggregate to form micelles with
an apparent z-average hydrodynamic diameter (Dh) of about
Fig. 3 XPS spectra of C1s, O1s, N1s and Pt4f electrons in HTDA-2, HTDA-2-Pt
and self-assembled HTDA-2-Pt.
The XPS spectra of HTDA-2, HTDA-2-Pt and self-assembled
HTDA-2-Pt are shown in Fig. S9, ESI. Peaks of binding energy
3
(BE) at about 285 eV and 532 eV are attributed to C1s and O1s
electrons, respectively, and the peaks at around 75.9 eV and
73.1 eV are attributed to their Pt4f5/2 and Pt4f7/2 electrons.
From the XPS spectra (Fig. 3) of the narrow scan in ranges of
526–540 eV, 278–295 eV, 70–80 eV and 388–407 eV, the BEs at
531.3 eV, 531.6 eV and 533.2 eV in the XPS spectrum of HTDA-
2 are attributed to the O1s electrons in –C–O–C–, –CO– and
–CONH2, respectively. After HTDA-2 reacts with chloroplatinic
acid, some distinct changes are shown in Fig. 3a, including an
increase (from 533.2 eV to 533.5 eV) in BE about O1s electron
in –CONH2, an increase from 288.1 eV to 288.5 eV in BE about
C1s electron, and unchanged BE data about N1s (399.8 eV) and
about O1s electrons in –C–O–C– and –CO– in HTDA-2 and
HTDA-2-Pt (Fig. 3b). This indicates that coordination occurs
between platinum and oxygen atom in amide groups (–CON–)
of HTDA-2.29 Comparing the XPS spectra (Fig. 3b) of self-
assembled HTDA-2-Pt and HTDA-2-Pt, the decrease from 73.1
eV to 72.6 eV in BE about Pt4f7/2 electrons, increase from 398.6
eV to 399.3 eV in BE about N1s electrons of pyridine and
invariability of the BE (399.8 eV) about N1s electrons in amido
1718 nm and a PDI of about 0.341 in Fig. S10, ESI. (2) The
3
HTDA-2-Pt solution is dropped and spread on the substrate
homogeneously (Fig. 6a) to form spherical bubbles because of
surface tension. As the solvent evaporates, the surrounding
temperature of the bubbles decreases and the flowing moist
water vapor forms water droplets (Fig. 6a) on the surface of the
bubbles.31 Because the evaporation rate of pyridine is quicker
than that of water concentration, the bubbles shrink all the
time (Fig. 6b). (3) The evaporation of pyridine also forces the
bubbles to move together and aggregate through hydrogen
bonding and coordination of HTDA-2-Pt, resulting in the
formation of a primitive self-assembly in Fig. 6b and Fig. 4b.
With the further evaporation of solvent, some single bubbles
move at random and aggregate while they meet the primitive
self-assembly in a limited region. The moving process of the
single bubbles is similar to that of particles in the diffusion-
limited aggregation (DLA) model;35 therefore, the aggregation
process of the single bubbles can be explained by DLA theory.
DLA theory35 is an idealization of the process by which matter
irreversibly combines to form dust, soot, dendrites, and other
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Fig. 4 Optical micrographs of a HTDA-2-Pt/pyridine solution after self-assembly for different times at 303 K and 50% relative humidity: (a) 5 min, (b) 13 min, (c) 90
min, (d) 160 min, (e) 300 min, (f) local magnification image of Fig. 4e.
random objects in the case where the rate-limiting step is
diffusion of matter to the aggregate. The model was mainly
used to research aggregation of inorganic nanoparticles,36–38
assembly and fractal of block polymers,39,40 fractal behavior of
dendrimers.17 A seed particle, as a single bubble in the self-
assembly system, is situated at the origin of a local region, and
then a second particle (or bubble) is near the original region.
The second particle (or bubble) moves randomly with
evaporation of the solvent until it meets a site adjacent to
the seed. (4) Then the moving particle becomes a part of the
primitive self-assembly. Another particle is now introduced at
a random distance point, and it moves randomly until it joins
the self-assembly in Fig. 6c, and gradually forms ‘‘hyper-
branched structure’’ self-assembly, which can be demon-
strated by an appearance of some nanometer claw-like
aggregates of micelles on sides of the trunks in Fig. 5a. If a
particle touches the boundaries of the surface during its
random move, it is removed and another introduced. The
exposed ends of the self-assembly tend to grow more rapidly
than other perimeter sites because perimeter sites near the
center are ‘‘shadowed’’. Finally, a tree-like self-assembly with
excellent supramolecular structure forms in Fig. 6d.
3.3 Crystal and fractal properties of self-assemblies
The self-assemblies from amphiphilic HBPs reported pre-
viously are amorphous,5,9,41,42 except for the addition of
crystalline polyethylene glycol (PEG)43 and polyhedral oligo-
meric silsesquioxane (POSS)44 segments. Recently, we
obtained a crystal structure by self-assembling a carboxyl-
ended hyperbranched polyester/platinum complex18 and a
HTD-2-Cu complex.32 The self-assembled micrometer tree-like
structure may have crystal properties from the above ordered
self-assemblies’ morphology, especially the smooth micro-
Fig. 6 Schematic mechanism description for the hybrid self-assembly of HTDA-
Fig. 5 SEM micrographs of self-assembled HTDA-2-Pt in Fig. 4f.
17078 | RSC Adv., 2013, 3, 17073–17080
2-Pt.
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As seen from Fig. 4, the tree-like self-assemblies with self-
similarity should have fractal properties. The dimension of the
fractal micrographs is between 1D and 2D in many clusters
and aggregates from DLA, commonly between 1.46 and 1.71,
and a higher fractal dimension has a higher order structure.48
According to the DLA model and the following equation:49
log NðrÞ
Df ~ lim
r?0
logðrÞ
where r is the length unit, N(r) is the size of the geometric object
measured with the unit r. A relationship between logN(r) and log(r)
in the morphology of self-assembled HTDA-2-Pt (Fig. 4e) is shown
in Fig. 8, and its Df is about 1.59 that demonstrates perfect and
uniform supramolecular structure of the self-assemblies.
Fig. 7 XRD spectra of HTD-2, HTDA-2, HTDA-2-Pt and self-assembled HTDA-2-
Pt.
4 Conclusions
A new method was used to prepare micrometer scale 2D
ordered tree-like supramolecular structures by hybrid self-
assembly of an amido-ended hyperbranched polyester/plati-
num complex on the surface of a glass substrate. The
morphology of self-assemblies is influenced significantly by
temperature, time, solvent, concentration and relative humid-
ity. A hybrid self-assembly mechanism was also analyzed by
XPS, FT-IR, TRPOM, DLS and SEM, indicating that the
formation of ordered tree-like self-assemblies with hyper-
branched structure could be attributed to the synergy of the
platinum coordination induction and micro-phase separation
between core segments and end group of HBPs resulting from
the solubility parameters of the solution, supporting DLA
theory. The new hybrid self-assembly method and mechanism,
as well as crystal behavior and fractal property of self-
assemblies may play important role in developing the self-
assembly and applications of hyperbranched polymers in the
field of biomaterials.
structure in Fig. 5b. Therfore we investigated the XRD spectra
(Fig. 7) of HTDA-2, HTDA-2-Pt and self-assembled HTDA-2-Pt.
In comparison with the XRD spectrum of amorphous HTD-2
which has a wide and blunt peak at 2h degrees between 18u
and 30u, some strong crystalline peaks (marked with a
rectangle) at 2h degrees between 9u and 30u are observed for
HTDA-2 (Fig. 7), which may be attributed to the crystal
structures of amido groups45 and phenyl46 and ethylene
oxide47 chains. The resultant HTDA-2-Pt complex from
HTDA-2 and chloroplatinic acid shows some additional
crystalline peaks larger than 30u. As shown in the XPS
spectrum (Fig. S9, ESI ), some chorine atoms remain in the
3
HTDA-2-Pt complex because the Cl–Pt bonds can not be
completely converted into a platinum coordination bond; this
results in the additional crystalline peaks (marked with black
circle). The peaks at 2h degrees between 9u and 30u for self-
assembled HTDA-2-Pt are stronger than those in HTDA-2-Pt,
indicating a higher degree of crystal order.
Acknowledgements
The authors gratefully acknowledge the financial support of
the National Natural Science Foundation of China (No.
50903094) and Hubei Provincial Department of Education
Science Research Program (Q20131505).
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