Highly ordered luminescent microporous films prepared from crystalline conjugated rod-coil diblock copolymers of PF-b-PSA and their superhydrophobic characteristics

Article abstract of DOI:10.1039/c1sm05712j

We have successfully prepared three crystalline conjugated rod-coil diblock copolymers of poly[2,7-(9,9-dihexylfluorene)]-block-poly(stearyl acrylate) (PF-b-PSA), PF₇-b-PSA₆₄, PF₇-b-PSA ₉₃, and PF₇-b-PSA₁₆₆, via the combination of a Suzuki coupling reaction and atom transfer radical polymerization. The crystalline comb-like PSA coil segment effectively formed the highly ordered microporous films through a "breath figure" (BF) process. On the other hand, the blue-emission band of the PF block was blue-shifting and became narrower as the PSA block was increased. The effects of block ratio, humidity, and solution concentrations on the porous structures were studied. The longer PSA block length, larger humidity, and higher copolymer concentration formed a more regular PF-b-PSA microporous structure. The images of SEM and scanning laser confocal microscope of the PF₇-b-PSA₁₆₆ microporous film showed that all bubbles were independent and perfectly monodisperse with a pore diameter ca. 1.85 μm. Furthermore, a rod-co-valley-like structure exhibited superhydrophobicity (contact angle up to 163 ± 0.3°), which was obtained from the peeled skin layer of the ordered PF₇-b- PSA₁₆₆ microporous surface. The experimental results revealed that multifunctional ordered microporous films could be successfully prepared from crystalline conjugated rod-coil block copolymers.

Full text of DOI:10.1039/c1sm05712j

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PAPER
Highly ordered luminescent microporous films prepared from crystalline
conjugated rod-coil diblock copolymers of PF-b-PSA and their
superhydrophobic characteristics†
a
Yu-Cheng Chiu, Chi-Ching Kuo, Chih-Jung Lin and Wen-Chang Chen
b
a
ab
*
Received 20th April 2011, Accepted 29th June 2011
DOI: 10.1039/c1sm05712j
We have successfully prepared three crystalline conjugated rod–coil diblock copolymers of poly[2,7-
(9,9-dihexylfluorene)]-block-poly(stearyl acrylate) (PF-b-PSA), PF7-b-PSA64, PF7-b-PSA93, and PF7-b-
PSA166, via the combination of a Suzuki coupling reaction and atom transfer radical polymerization.
The crystalline comb-like PSA coil segment effectively formed the highly ordered microporous films
through a ‘‘breath figure’’ (BF) process. On the other hand, the blue-emission band of the PF block was
blue-shifting and became narrower as the PSA block was increased. The effects of block ratio,
humidity, and solution concentrations on the porous structures were studied. The longer PSA block
length, larger humidity, and higher copolymer concentration formed a more regular PF-b-PSA
microporous structure. The images of SEM and scanning laser confocal microscope of the PF7-b-
PSA166 microporous film showed that all bubbles were independent and perfectly monodisperse with
a pore diameter ca. 1.85 mm. Furthermore, a rod-co-valley-like structure exhibited superhydrophobicity
(contact angle up to 163 ꢀ 0.3ꢁ), which was obtained from the peeled skin layer of the ordered PF7-b-
PSA166 microporous surface. The experimental results revealed that multifunctional ordered
microporous films could be successfully prepared from crystalline conjugated rod–coil block
copolymers.
to form a highly ordered hole array structure by the BF
1. Introduction
method,8–10 including poly(p-phenyleneethynylene), poly(3-
alkylthiophene), poly(9,90-dialkylfluorene), and poly(p-phenyl-
enevinylene). However, only limited linear conjugated rod–coil
block copolymers were reported to form ordered microporous
films, which used amorphous polystyrene coil.11–14
Highly ordered microporous films have attracted extensive
scientific interest because of the potential applications in
patterned templates, photonic band gap, cell patterning, filtra-
tion membranes, and surface wettability.1–5 ‘‘Breath Figures’’
(BF) process6,7 represents a facile, simple, economic, and non-
templated method for fabricating highly ordered microporous
polymer films, including conjugated polymers,8–10 rod–coil block
copolymers,11–14 star-branched polymers.15–18 It uses a highly
volatile solvent and an applied airflow under a humid environ-
ment, which leads to the water vapor condensing on the surface
to form an ordered pore structure. The porous structure was
controlled by the polymer architecture, molecular weight, and
the casting condition. Several conjugated polymers were reported
The formation of the ordered microporous films through the
BF process is generally to prevent the coalescence of water
droplets, which could be achieved by kinetic or thermodynamic
control.19–21 In the case of kinetic control, the higher segment
density of star or comb-like polymers than the linear polymers
facilitates the faster precipitation at the organic/water interface
and thus the water droplets have no time to coalesce before the
film solidification, such as star polystyrene or comb-like glyco-
polymer.15,16,18,20,21 In addition, the ordered polymer structure
could help the formation of the highly uniform porous structure.
Thus, the proposed crystalline conjugated rod–coil block
copolymers of poly[2,7-(9,9-dihexylfluorene)]-block-poly(stearyl
acrylate) (PF-b-PSA) could represent ideal polymers for such
application. Polyfluorenes have been widely studied due to their
high luminescence yield and good charge-transporting charac-
teristics, which may have potential applications for sensory or
optoelectronic device applications. On the other hand, poly
(stearyl acrylate) has comb-like, crystalline, physical cross-link-
ing, and hydrophobic characteristics due to its long alkyl side
aDepartment of Chemical Engineering, National Taiwan University,
Taipei, 10617, Taiwan. E-mail: chenwc@ntu.edu.tw
bInstitute of Polymer Science and Engineering, National Taiwan
University, Taipei, 10617, Taiwan
† Electronic supplementary information (ESI) available: 1H NMR
spectra and GPC profiles of PF–Br, PF7-b-PSA64, PF7-b-PSA93, and
PSA. TGA and DSC curves of studied copolymers at the heating rate
of 20 ꢁC minꢂ1 and under 10 ꢁC minꢂ1 a nitrogen atmosphere. The
top-view and cross-section SEM images of PF7-b-PSA166 microporous
film. WAXD intensity profiles of the microporous films, spin-coated
films, and PSA powder. See DOI: 10.1039/c1sm05712j
9350 | Soft Matter, 2011, 7, 9350–9358
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chain group.22–24 The structure of the comb-like PSA containing
block copolymers would be similar to the star-shaped or hyper-
branched coil polymers, which would benefit the formation of
the ordered microporous films. Also, the crystalline PSA block
could facilitate the stabilization of water droplets and result in
a highly ordered microporous structure. Thus, it would be of
interest to explore the synthesis and morphology of PF-b-PSA
conjugated rod–coil block copolymers.
2,7-dibromo-9,9-dihexylfluorene, 4-bromobenzyl alcohol, 2-
bromoisobutyryl bromide, tetrakis(triphenylphosphine)palla-
dium(0), sodium carbonate methyl 2-bromopropionate (98%),
chloroform, anisole, N,N-dimethylacetamide, and methanol
were purchased from Aldrich Chemical Co. and used without
further purification. Poly(2,7-(9,9-dihexylfluorene))-block-poly
(methylmethacrylate) (PF7-b-PMMA810) was prepared accord-
ing to our previous report.28
In this paper, we report the synthesis, morphology, and
properties of three new crystalline conjugated rod–coil block
copolymers of PF7-b-PSA64, PF7-b-PSA93, and PF7-b-PSA164
,
Synthesis of PF-b-PSA diblock copolymers
which were prepared by via the combination of a Suzuki coupling
reaction and atom transfer radical polymerization (ATRP)
(Scheme 1). The polymer structure, thermal properties, and
photophysical properties of the block copolymers were studied.
Next, the BF method was used to form highly ordered micro-
porous structures. The effects of coil length, humidity, and
concentration on the formation of the ordered porous film were
studied. The surface morphology and interior structure of the
ordered microporous film were characterized by field-emission
scanning electron microscope (FE-SEM), two photon laser
confocal microscope, and transmission-mode wide angle X-ray
scattering (WAXS). Finally, a rod-co-valley-like structure was
obtained from the peeled skin layer of the ordered microporous
surface for exhibiting superhydrophobicity. The experimental
results suggested the new crystalline PF-b-PSA could exhibit the
Synthesis of PF-b-PSA diblock copolymer is shown in Scheme 1.
PF–Br macroinitiator with Mn and PDI of 2610 and 1.61,
respectively, was prepared via a Suzuki coupling reaction
according to our previous report.29,30 PF-b-PSA copolymers with
different coil lengths were synthesized from PF–Br via ATRP.
The preparation of PF7-b-PSA166 is exemplified below: a dry
round-bottom flask with a magnetic stir bar was charged with the
mixture of CuBr (14 mg, 0.1 mmol), PF–Br macroinitiator
(258 mg, 0.1 mmol), SA (16.2 g, 50 mmol), PMDETA (21 mL, 0.1
mmol), and anisole (16 mL). The mixture was degassed by
bubbling nitrogen to reꢁmove oxygen for 30 min, then immersed
into an oil bath at 110 C and stirred for 48 h. The polymeriza-
tion was stopped by bubbling air into the crude solution. The
mixture was passed through an Al2O3 column to remove the
copper catalyst. In the following, the reaction mixture was
poured into methanol, precipitated, filtered and dried under
vacuum at 40 ꢁC to obtain PF7-b-PSA166 (2.82 g) as a light
multi-functionality of photoluminescence and
hydrophobic surface.25–27
a
super-
1
yellow solid. H NMR (CDCl3, 500 MHz) d (ppm): 0.86–0.90
2. Experimental section
(3H, –CH2CH2(CH2)15CH3), 1.21–1.34 (30H, –CH2CH2
Materials
(CH2)15CH3),
1.59–1.65
(2H,
–CH2CH2(CH2)15CH3;
2H, –CH2CH–), 1.89–1.96 (2H, –CH2CH–), 2.25–2.39
(1H, –CH2CH–), 3.99–4.13 (2H, –OCH2CH2(CH2)15CH3), and
7.25–7.84 (10H, fluorene aromatic protons and phenyl end
group). The reaction conditions and molecular weight distribu-
tions of the three PF-b-PSA are listed in Table 1.
Stearyl acrylate (SA, Polysciences, Inc.) was re-crystallized from
an ethanol solution. 1,1,4,7,7-Pentamethyldiethylenetriamine,
PMDETA (Acros, 99%), was used as received. CuBr (99%),
Synthesis of poly(stearyl acrylate) (PSA)
PSA were synthesized via ATRP and used methyl 2-bromopro-
pionate as initiator. CuBr (14 mg, 0.1 mmol), methyl 2-bromo-
propionate (16.7 mg, 0.1 mmol), SA (4.86 g, 15 mmol),
PMDETA (21 mL, 0.1 mmol), and anisole (5 mL) were placed in
a 25 mL dry round-bottom flask containing a magnetic stir bar.
The mixture was degassed by bubbling nitrogen for 30 min, and
placed in a 110 ꢁC oil bath for 24 h. After removing metal catalyst
by Al2O3 column, the mixture was precipitated into methanol
and dried under vacuum at 40 ꢁC to obtain PSA as a
white powder. 1H NMR (CDCl3, 500 MHz) d (ppm): 0.82–
0.90 (3H, –CH2CH2(CH2)15CH3), 1.17–1.36 (30H, –CH2CH2
(CH2)15CH3), 1.53–1.67 (2H, –CH2CH2(CH2)15CH3; 2H, –
CH2CH–),
1.84–1.92
(2H,
–CH2CH–),
2.20–2.36
(1H, –CH2CH–), and 3.92–4.10 (2H, –OCH2CH2(CH2)15CH3).
Number-averaged molecular weight (Mn) and polydispersity
index (PDI) estimated from GPC were 27000 and 1.38,
respectively.
Scheme 1 Synthesis of crystalline conjugated rod–coil block copoly-
mers, PF7-b-PSAn (n ¼ 64, 93, 166).
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Table 1 Polymerization conditions, molecular weights, and thermal properties of PF7-b-PSA166 copolymers
b
Mn (g molꢂ1
)
a
c
Calculated composition
[M]0/[I]0
Reaction time (h)
PF
PSA
PDIb
Td (ꢁC)
Tc (ꢁC)
Tm (ꢁC)
PF7-Br
—
—
48
48
48
2610
2610
2610
2610
—
1.61
1.63
1.67
1.88
—
—
—
PF7-b-PSA64
PF7-b-PSA93
PF7-b-PSA166
150
300
500
20810
30250
53920
383
376
367
37.2
38.0
38.7
44.4
45.5
46.1
a
b
Feed molar ratio of the monomer [M]0 to initiator [I]0. Mn and PDI determined by GPC using THF eluent. Onset decomposition temperature
c
measured by TGA under N2.
Preparation of microporous films
spectrum of PF7-b-PSA166 in CDCl3. The proton signals of the
long alkyl chain and oxymethylene on PSA are clearly observed
in the regions of 0.86–0.90, 1.21–1.34, 1.59–1.65 and 3.99–
4.13 ppm, implying the successful incorporation of the PSA. The
weak signals between 7.25–7.84 ppm are attributed to the
protons on aromatic fluorene and phenyl end group, as
compared to those of the PF–Br macroinitiator, as shown in
Fig. S1(a). Note that PF7-b-PSA166 has a much longer PSA chain
length than that of PF and thus weak signals of the fluorene
proton resonances are observed. Also, the smaller peak d than
that of peak e in Fig. 1 is probably the split resulting from the
tacticity of the methylene group.31 The 1H NMR of the other two
PF-b-PSA (PF7-b-PSA64 and PF7-b-PSA93) are shown in Fig. S1
(b) and S1(c), which are also consistent with the proposed
structures. The GPC profiles of the block copolymers shift to the
high molecular weight regions with enhancing the feed ratio, as
shown in Fig. S2 of the Supporting Information. The Mn of these
PF-b-PSA copolymers are 23417, 32860, and 56530, respectively,
with corresponding PDI values of 1.63, 1.67, and 1.88, as listed in
Table 1. The estimated SA repeating units of three copolymers
from Mn are 64, 93, and 166, and thus denoted as PF7-b-PSA64,
Block copolymers of PF-b-PSA were dissolved in chloroform
solution with a fixed concentration of 5, 10, 30, or 50 mg mlꢂ1
.
The polymer solution was cast on an untreated glass substrate in
ꢁ
an adequate moist air flow hood at room temperature (30 C).
Characterization
All the composition, molecular weight, and PDI of synthesized
polymers were determined by NMR and GPC. 1H nuclear
magnetic resonance (NMR) data was recorded at room
temperature on a Bruker AV 500 MHz spectrometer using the
residual proton resonance of the deuterated chloroform. Gel
permeation chromatographic analysis was performed on a Lab
Alliance RI2000 instrument (two column, MIXED-C and D
from Polymer Laboratories) connected with one refractive index
detector from Schambeck SFD GmbH. All GPC analyses were
performed on polymer/THF solution at the flow rate of 1 ml
minꢂ1 at 40 ꢁC and calibrated with polystyrene. Thermogravi-
metric analysis (TGA) and differential scanning calorimetry
(DSC) measurements were performed under a nitrogen atmo-
sphere at a heating rate of 20 and 10 ꢁC minꢂ1 using a TA
instrument (TGA-951 and DSC-910S), respectively. UV-visible
absorption and PL spectra of the copolymer solutions were
recorded by a UV-visible spectrophotometer (Jasco V-570) and
Fluorolog-3 spectrofluorometer (Jobin Yvon), respectively.
The morphologies of microporous films were investigated
through a field-emission scanning electron microscope (FE-
SEM, JEOL JSM-6330F). SEM samples were sputtered with
platinum prior to the images characterization and analysis was
operated at an acceleration of 5 kV. Fluorescence optical
microscope images were taken using Two Photon Laser
Confocal Microscope (Leica LCS SP2). The polymer structure
was further investigated by transmission-mode wide angle X-ray
scattering (WAXS) at beam line 17A of National Synchrotron
Radiation Research Center, Taiwan. The contact angles of water
droplets to the microporous films were measured with an optical
contact angle meter (CAM-100, Creating Nano Technologies
Inc., Taiwan) at room temperature.
PF7-b-PSA93, and PF7-b-PSA166
, respectively. Chloroform,
THF, and chlorobenzene are common solvents for both blocks
of the copolymers.
Fig. 2 shows the DSC curve of PF7-b-PSA166 while the thermal
decomposition curve is exhibited in the inset figure. As listed in
Table 1, the Td (at 5% sample decomposition) of PF-b-PSA are
3. Results and discussion
Synthesis and characterization of rod–coil block copolymers
1H-NMR spectra and GPC were used to confirm the successful
synthesis of the copolymer PF-b-PSA. Fig. 1 shows the 1H NMR
Fig. 1 1H NMR spectrum of PF7-b-PSA166 in CDCl3.
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Fig. 2 The DSC thermogram of PF7-b-PSA166 at a heating rate of 20 ꢁC
minꢂ1 under a nitrogen atmosphere. The inset shows thermal decompo-
sition temperature at a heating rate of 20 ꢁC minꢂ1 under a nitrogen
atmosphere.
between 367 and 383 ꢁC (Fig. S3 of the Supporting Information),
which are reduced by increasing the PSA block length. On the
other hand, the melting temperature (Tm) and crystallization
temperature (Tc) are slightly increased with a higher PSA block
length, which are 44.4–46.1 ꢁC and 37.2–38.7 ꢁC, respectively
(Fig. S4 of the Supporting Information). Also, the Tm and Tc are
higher than the PSA homopolymer (Tm ¼ 37.2 ꢁC, Tc ¼ 30.1 ꢁC)
due to the different coil length of PSA and the introduction of the
rigid PF segment.
Fig. 3 UV-vis absorption and photoluminescent spectra of the studied
PF-b-PSA copolymers: (a) in chloroform solution and (b) spin-coated
thin films.
Fig. 3(a) shows the optical absorption and photoluminescence
(PL) spectra of the PF-b-PSA, respectively. The absorption
maximum wavelengths (lmaxabs) of PF-b-PSA in chloroform
solution and thin film are observed at 370 and 376 nm, respec-
tively, attributed to the p–p* transition of the PF block.
Although there is no difference on the lmaxabs, the absorption
band becomes narrower with increasing the PSA block length.
The emission maximum wavelengths (lmaxPL) of the PF7-b-
PSA64, PF7-b-PSA93, and PF7-b-PSA166 thin films are observed
at (423, 445), (419, 442), and (413, 436) nm, respectively. The
narrower absorption band and the blue shifting on the emission
band suggest the reduced PF aggregation with increasing the
PSA block length in PF-b-PSA.
bubble arrays formed on the surface of the drop-cast film, which
was white and opaque in appearance. Then, the regular three-
dimensional (3D) rod-co-valley-like film formed after removing
the superhydrophobic top layer.
The structure of the microporous films were investigated by
SEM, fluorescence microscopy, and WAXS. The samples for the
SEM images (Fig. 5(a)–5(d)) of PF7-Br, PF7-b-PSA64, PF7-b-
PSA93, and PF7-b-PSA166 microporous films were prepared from
the concentration of 10 mg mlꢂ1 in CHCl3 at the relative
humidity of 65%. As shown in Fig. 5(a), the SEM image of PF7-
Br macroinitiator film is irregular and broken, due to its low
molecular weight. However, the microporous structures of PF7-
b-PSA64, PF7-b-PSA93, and PF7-b-PSA166 have the average
diameters of 0.44 ꢀ 0.19 mm (coefficient of variation (CV) ¼ 43%,
the CV is a normalized dispersion of a probability distribution),
0.87 ꢀ 0.32 mm (CV ¼ 36%), and 1.78 ꢀ 0.13 mm (CV ¼ 8%),
respectively. Note that the average pore diameter is estimated
based on a statistical average of over two hundred pores on each
sample. The average pore size is increased with enhancing the
PSA block length because the higher molecular weight leads to
a slower rate of precipitation.32 Uniform microporous film of
PF7-b-PSA166, is obtained over the area of 0.3 cm2, which has the
lower CV value of 8% and without an ‘‘island region’’ flat area. It
suggests the comb-like compact structure of the PSA block with
a long alkyl side chain could facilitate the stabilization of the
water droplets and result in an ordered microporous structure.
The morphologies of microporous films
Fig. 4 shows the mechanism of forming highly ordered micro-
porous films via crystalline conjugated rod–coil block copoly-
mers, PF-b-PSA. PF-b-PSA was dissolved in chloroform
(CHCl3) first and then cast on an untreated glass substrate in an
adequate moist air flow hood at room temperature (30 ꢁC) under
the relative humidity of 65–90%. In the following, the surface
temperature was cooled to a low temperature of ꢂ6 to 0 ꢁC
during the evaporation of CHCl3. When air containing moisture
came into contact with the surface of polymer solution, moisture
condensed to form water droplets on the surface. Note that the
contact angles of PF and PSA are around 95ꢁ and 115ꢁ,
respectively. The polymer phase would be composed of the top
layer (conjugated PF block) close to the water drop and the
bottom layer (crystalline PSA). After a few minutes, regular
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Fig. 4 Proposed mechanism for the formation of the highly ordered microporous film from the crystalline conjugated rod–coil polymers of PF-b-PSA.
The ordering of the porous structure could be further improved
by changing the solution concentration or humidity, as discussed
in the following section.
surface, which are the imprint of coalescence between unstable
droplets during water condensation process. As the concentra-
tion increased further to 30 (Fig. 6(c)) and 50 (Fig. 6(d)) mg mlꢂ1
,
The effect of alkyl side chain on the formation of microporous
films was further investigated using PF7-b-PMMA810 with an
amorphous coil for comparison. In a sharp contrast, the surface
structure in the PF7-b-PMMA810 film (Fig. 5(e)) is composed of
50% pores and 50% flat regions while the pores are irregular in
distribution of size. From the literature,32 the film cast from pure
linear PMMA did not form the microporous structure. It indi-
cates the significance of using crystalline comb-like PSA coil in
conjugated rod–coil block copolymers for fabricating ordered
microporous films.
the bubble arrays with highly ordered porous structures with
pore diameters of 2.33 ꢀ 0.096 mm (CV ¼ 4.13%) and 1.85 ꢀ
0.076 mm (CV ¼ 4.10%), respectively, were formed. As expected,
the pore diameter is smaller at a high concentration but with
a higher pore density in Fig. 6. Compared to the low concen-
tration of 10 mg mlꢂ1, the higher concentration of 50 mg mlꢂ1
increases the polymer density and solution viscosity. Thus, the
precipitation rate would be enhanced,33 stabilize water droplets
and inhibit water from aggregating to form interconnected holes.
The pores shown in Fig. 6(d) are nearly monodisperse and defect-
free over large scale around 0.5 cm2. In addition, the enlarged
SEM image (Fig. S6 of the Supporting Information) exhibits that
the each pore is surrounded by six pores and the distance of
center-to-center between pores is 4.52 ꢀ 0.15 mm (CV ¼ 3.3%).
The relatively small coefficient of variation suggests the similar
distance of any two neighbor pores.
Next, the variation of processing conditions, such as polymer
solution concentration and humidity, was used to obtain highly
ordered bubble array structures of PF7-b-PSA166 microporous
films. Fig. 6(a)–6(d) shows the SEM images of PF7-b-PSA166
microporous film prepared from different concentrations of 5,
10, 30, and 50 mg mlꢂ1 in CHCl3 at a high relative humidity of
90%. As exhibited in Fig. 6(a), small and irregular pores formed
and dispersed on the surface, since the low polymer concentra-
tion was not enough to encapsulate the condensed water drop.
As the concentration increased to 10 mg mlꢂ1 (Fig. 6(b)), bubble
arrays and some bigger pores were formed on the polymer
Generally, bubble arrays do not form in a surrounding that
has less than 45–50% relative humidity or without moisture flow
during the casting process.6 However, the SEM images of PF7-b-
PSA166 show the average pore size of 1.30 ꢀ 0.22 mm
(CV ¼ 16%), 1.56 ꢀ 0.16 mm (CV ¼ 10%), and 1.85 ꢀ 0.076 mm
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Fig. 5 Top view of SEM images of drop-casting film prepared from the
concentration of 10 mg mlꢂ1 at the relative humidity of 65%: (a) PF–Br,
(b) PF7-b-PSA64, (c) PF7-b-PSA93, (d) PF7-b-PSA166, and (e) PF7-b-
PMMA810
.
Fig. 7 The SEM images of PF7-b-PSA166 drop-casting film prepared
from the concentration of 50 mg mlꢂ1 at the relative humidity of (a) 65%,
(b) 75%, and (c) 90%.
the higher relative humidity condition. In addition, the area of
bubble arrays at 90% relative humidity is higher than those of
others, indicating the possibility of constructing highly ordered
microporous structure. The laser confocal microscope images of
the PF7-b-PSA166 microporous film (Fig. 8) show the highly
ordered microporous structure with a strong and uniform blue
emission contributed by the conjugated PF rod block.
Fig. 6 SEM images of PF7-b-PSA166 bubble arrays formed by evapo-
ration of chloroform as concentration varied from 5 mg mlꢂ1 to 50 mg
mlꢂ1 at the relative humidity of 90%: (a) 5 mg mlꢂ1, (b) 10 mg mlꢂ1, (c) 30
mg mlꢂ1, and (d) 50 mg mlꢂ1
.
In order to investigate the interior morphology of the bubble
arrays under a highly ordered PF7-b-PSA166 microporous
surface, the sample was packed into a flask containing liquid
nitrogen to solidify it instantly for a few minutes and then cut off
the film immediately. Next, the film was quenched to room
temperature under a surrounding vacuum until no residual water
remained. After coating with platinum on the surface, the film
(CV ¼ 4.1%) prepared at humidity (65%, 75%, and 90%) at the
same concentration (50 mg mlꢂ1) in Fig. 7(a)–7(c). The pore size
was obviously increased at a higher humidity due to the
enhanced amount of moisture contained in the air, similar to
the literature.34 Besides, the regularity of pores was enhanced at
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shape’’ water droplet sank into the polymer surface before
further evaporation. The precipitating PF7-b-PSA166 layer is
pulled around the water droplet by the high surface tension of the
water, creating an egg shape cavity, and a smaller pore opening
at the film surface.14 Besides, a monolayer of bubble arrays is
formed, resulting from the higher density of chloroform solvent
than that of water. The microporous film thickness is around
7 mm and the wall thickness existed between pores is around
200 nm. Each pore is independent and separated by a polymeric
wall. From the cross-sectional SEM image, it shows the water
leaves an imprint as bubble arrays in the microporous film. All
bubbles arrays are independent and have equal shapes with
regards to both the surface and interior morphologies. The three-
dimensional (3D) reconstruction of PF7-b-PSA166 microporous
film via scanning laser confocal microscopy of bubble arrays
(Fig. 9(b)) also shows that the monolayer and independent pores
with highly ordered arrays are fabricated.
Fig. 8 Laser confocal fluorescence microscopy images of PF7-b-PSA166
bubble arrays formed by the evaporation of chloroform in the micro-
porous film. The inset figure shows the enlarged image of the studied film.
The ordering structure of the PF7-b-PSA166 microporous film
could be further evidenced by XRD. As shown in the synchro-
tron XRD analysis of Fig. 10, a sharp peak is observed at q ¼
1.5 (Aꢂ1) (d-spacing ¼ 0.41 nm), which is attributed to the PSA
block (the inset figure). It indicates that the octadecyl alkyl side
chains of PSA spontaneously arranged in ordered and crystalline
structure without further annealing treatment. On the other
hand, the wide angle X-ray scattering (WAXS) of PF7-b-
PMMA810 porous film shows the amorphous halo (Fig. S7 of the
Supporting Information), but that of PF7-b-PSA166 reveals an
obvious sharp crystal peak.
was characterized by SEM with a different tilted angle. As shown
in Fig. 9(a) (SEM image, tilt angle 45ꢁ), the cross section of
microporous film has a pore depth of 3 mm and an underneath
diameter of 4 mm, which is larger than the open diameter
(1.85 mm) of the pore in the top-surface. It suggests an ‘‘egg
Fabrication of superhydrophobic property by the peeling off
process
The top layer of PF7-b-PSA166 micorporous films was peeled off
by adhesive tape to form a highly ordered rod-co-valley-like
structure, as shown in Fig. 11. Note that the lotus effect exhibits
superhydrophobicity due to the low surface energy and rough
three-dimensional surface structure (hills and valleys).27,35,36
From the top view image of SEM (Fig. 11(a)), the pore diameter
(4 mm) is much bigger than that (1.85 mm) before peeling
Fig. 9 (a) FE-SEM image of cross-section structure of PF7-b-PSA166
order bubble array film tilted at 45ꢁ at the concentration of 50 mg mlꢂ1 at
the relative humidity of 90%. (b) 3D reconstruction image via scanning
laser confocal microscopy of PF7-b-PSA166 microporous film.
Fig. 10 Synchrotron XRD profiles of the PF7-b-PSA166 microporous
film. The inset shows the XRD profile of PSA film.
9356 | Soft Matter, 2011, 7, 9350–9358
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force or hydrogen bonding35,36 between water molecules and C]
O groups of PF-b-PSA. The above results suggest that the highly
ordered rod-co-valley-like36 materials with both super-
hydrophobic and luminescent characteristics could be success-
fully prepared crystalline conjugated rod–coil copolymers.37
4. Conclusions
In summary, we have successfully fabricated novel luminescent
and highly ordered microporous film from crystalline conjugated
rod–coil diblock copolymers of PF-b-PSA. Compared to amor-
phous linear coil block, crystalline comb-like PSA coil blocks
effectively form the highly ordered microporous film through the
BF process due to their long alkyl side chain. The ordering of the
pore structure was enhanced by the PF-b-PSA with a longer PSA
block length, higher polymer solution concentration or humidity.
Furthermore, a rod-co-valley-like superhydrophobic surface
with a contact angle of 163 ꢀ 0.3ꢁ was achieved when the skin
layer of the ordered microporous surface was peeled off. The
above results suggest the highly ordered rod-co-valley-like
materials with both superhydrophobic and luminescent charac-
teristics could be prepared by new crystalline conjugated rod–coil
copolymers.
Acknowledgements
The financial supports of this work from National Science
Council, The Ministry of Economics Affairs, and the Excellence
Research Program of National Taiwan University are highly
appreciated.
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