Self-assembled structures in rod-coil block copolymers with hydrogen-bonded amphiphiles

Article abstract of DOI:10.1039/c0sm01385d

We report the synthesis and the self-assembled morphologies of a series of new rod-coil diblock copolymers, poly[2,7-(9,9-dihexylfluorene)]-b-poly(4- vinylpyridine) (PF-b-P4VP). The rod-coil diblock copolymers were synthesized via Suzuki coupling reaction and living anionic polymerization. Probing by transmission electron microscopy, small-angle X-ray scattering and atomic force microscopy, we found that the microphase separation varies from lamellar to cylindrical and then to spherical, depending on the length of P4VP blocks. The P4VP coil blocks were further hydrogen-bonded with 3-pentadecylphenol (PDP) to form rod-comb block copolymers. Similar to conventional coil-coil block copolymers-based supramolecules, the incorporation of PDP transforms the morphologies from lamellar to cylindrical or from cylindrical to spherical due to the increase of volume fraction of P4VP(PDP) comb blocks. The strategy described here can be used to tune the self-assembled structures of rod-coil block copolymers.

Full text of DOI:10.1039/c0sm01385d

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PAPER
Self-assembled structures in rod-coil block copolymers with hydrogen-bonded
amphiphiles†
a
a
c
c
a
Han-Sheng Sun, Chia-Hao Lee, Chia-Sheng Lai, Hsin-Lung Chen, Shih-Huang Tung*
ab
and Wen-Chang Chen*
Received 28th November 2010, Accepted 9th February 2011
DOI: 10.1039/c0sm01385d
We report the synthesis and the self-assembled morphologies of a series of new rod-coil diblock
copolymers, poly[2,7-(9,9-dihexylfluorene)]-b-poly(4-vinylpyridine) (PF-b-P4VP). The rod-coil diblock
copolymers were synthesized via Suzuki coupling reaction and living anionic polymerization. Probing
by transmission electron microscopy, small-angle X-ray scattering and atomic force microscopy, we
found that the microphase separation varies from lamellar to cylindrical and then to spherical,
depending on the length of P4VP blocks. The P4VP coil blocks were further hydrogen-bonded with
3
-pentadecylphenol (PDP) to form rod-comb block copolymers. Similar to conventional coil-coil block
copolymers-based supramolecules, the incorporation of PDP transforms the morphologies from
lamellar to cylindrical or from cylindrical to spherical due to the increase of volume fraction of
P4VP(PDP) comb blocks. The strategy described here can be used to tune the self-assembled structures
of rod-coil block copolymers.
1
. Introduction
phase behaviors. A great number of efforts, experimental or
theoretical, have been made to unravel the systems of rod-coil
Rod-coil block copolymers have been an important class of
nanomaterials, not only because of their potential for the bio-
logical and optoelectronic applications but also the richness of
block copolymers and a variety of nanostructures have been
1–3
reported in solutions, melts, and thin films.
However, the
results are usually interpreted on a case-by-case basis and
a universal theoretical framework for explaining the phase
1
–3
phase behaviors that fascinate many nanotechnologists. In
rod-coil block copolymers, flexible coil-like chains are covalently
bonded to one or more blocks with rod-like chains due to
p-conjugation, helical secondary structures, or aromatic meso-
genic groups along the backbone. The incorporation of rigid rod-
like chains into the block copolymers results in asymmetric chain
conformations. Their self-assembly behaviors are thus very
different from those of conventional coil-coil block copolymers.
In addition to the classical theories for the microphase separation
of the rod and coil components, an important thermodynamic
factor that needs to be taken into account is the strong tendency
of rods to stack and align due to the strong interaction between
rods and the large excluded volume of long rigid rods, which in
fact usually dominates the microphase separation. Moreover, the
anisotropic alignments of rigid rods may lead to liquid crystalline
9
behavior is still under development.
Conjugated polymers are unique organic materials that
possess electronically and optically active properties due to the
delocalization of p-electrons. The p-interaction also imparts
rigidity thereby giving rise to a rod-like chain. In addition to
chemical architectures, the electrical and optical properties of
conjugated polymers are highly dependent on structures, such as
10
the interchain distance and the chain orientation. Thus,
manipulation of the structure is important for conjugated poly-
mers to function efficiently with desired properties. One prom-
ising way to do so is incorporation of flexible chains into
conjugated polymers to form rod-coil block copolymers in which
the aggregation sizes and stack ordering of conjugated blocks can
11
be tailored by the flexible blocks. Various conjugated moieties
4–8
or crystalline phases,
further increasing the complexity of
of p-conjugated rod-coil block copolymers have been
1–3
reported, among which polyfluorene (PF) is widely investi-
gated for optoelectronic applications due to its good thermal and
chemical stability, fluorescence quantum yields, and charge
a
Institute of Polymer Science and Engineering, National Taiwan
University, Taipei, 10617, Taiwan. E-mail: shtung@ntu.edu.tw; chenwc@
ntu.edu.tw
12–14
b
carrier mobility.
Several polyfluorene-based rod-coil block
Department of Chemical Engineering, National Taiwan University,
Taipei, 10617, Taiwan
copolymers have been reported and most of the studies have
focused on the self-assembled structures in solutions or the
15–20
c
Department of Chemical Engineering, National Tsing Hua University,
Hsin-Chu, 30013, Taiwan
dependence of optoelectronic properties on the other blocks.
†
Electronic supplementary information (ESI) available: the GPC data of
PF -b-P4VP22 and the morphologies of more PF-b-P4VP copolymers and
PF-b-P4VP(PDP) supramolecules. See DOI: 10.1039/c0sm01385d
The concept of supramolecules has been increasingly adopted
in the nanotechnology of polymeric materials since it can prevent
9
4
198 | Soft Matter, 2011, 7, 4198–4206
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the complexity of synthesizing new polymers and can create
Aldrich) under reduced pressure. Tetrahydrofuran (THF, 99.9%,
Acros) was distillated from sodium (30ꢀ35 wt % dispersion in
paraffin wax, Aldrich) and benzophenone (>99%, Aldrich)
mixture.
21–23
functional materials with reversible properties.
The incor-
poration of small molecules which graft onto backbone of
polymers via secondary bonds results in a comb-like structure. In
block copolymer system, the added small molecules associated
with one of the blocks can alter the volume fractions of blocks
and a morphological transition occurs. Furthermore, the small
molecule itself can assemble into lamellae in one of the micro-
domains, forming hierarchical structure-within-structure
Synthesis of poly[2,7-(9,9-dihexylfluorene)]-block-poly(4-
vinylpyridine)
The synthesis route is shown in Scheme 1. We first synthesized
poly[2,7-(9,9-dihexylfluorene)] macroinitiator (PF-Vinyl) (3).
The procedure is described as follows: 2-bromo-9,9-di-n-hexyl-
fluoreneboronic (2) was prepared by monolithiation of 9,9-
dihexyl-2,7-dibromofluorene (1) with 1.1 equimolar amounts of
n-butyllithium followed by boronation with triisopropyl borate
24
morphologies. More interestingly, in coil-coil block copolymer
thin films, the graft effects can be used to manipulate the orien-
tation of microdomains with respect to the surface, which
provides a facile route to control the nanostructures and prop-
25–28
erties of thin films.
Poly(4-vinylpyridine) (P4VP) is
ꢂ
16
commonly used to study the self-assembly of block copolymer-
based supramolecules since its pyridine groups can easily form
hydrogen bonds or ionic bonds with other molecules. Mezzenga
and coworkers have reported the morphology of P4VP-based
rod-coil block copolymers, poly(diethylhexyloxy-p-phenyl-
at ꢁ78 C (yield: 52%). Poly[2,7-(9,9-dihexylfluorene)] macro-
31
initiator was synthesized by modified Suzuki coupling. 1.50 g
(3.29 mmol) 2-bromo-9,9-di-n-hexylfluoreneboronic (2), 57.5 ml
(0.53 mmol) 4-bromostyrene, 81.1 mg (0.070 mmol) Pd(PPh ) ,
3
4
15 ml K CO aqueous solution (2.0 M), 20 ml toluene, and 5
2
3
29
ꢂ
enevinylene)-b-P4VP, and the use of supramolecules of poly(3-
drops Aliquatꢀ 336 were mixed. After stirred vigorously at 85 C
for 24 h, a solution of 1.50 g (12.31 mmol) phenylboric acid,
26 mg (0.022 mmol) Pd(PPh ) , and 15 ml THF was added and
hexylthiophene)-b-poly(4-vinylpyridine) with PCBM for photo-
30
voltaic applications. The microphase separation of rod-comb
supramolecules, however, has not yet been systematically
explored.
3
4
reacted for 12 h to cap the bromo-side. The resulting mixture was
cooled to room temperature, and poured into 700 ml methanol
for the polymer to precipitate. The collected solid was re-dis-
solved in 5 ml of THF, re-precipitated twice in 700 ml of meth-
In this study, we synthesized poly[2,7-(9,9-dihexylfluorene)]-b-
poly(4-vinylpyridine) (PF-b-P4VP) rod-coil diblock copolymers
by combining Suzuki coupling reaction and living anionic
polymerization. The PF blocks possess optoelectronic function-
ality while P4VP blocks are the media used to manipulate the
nanostructures. We then prepared the rod-comb supramolecules
by mixing PF-b-P4VP and 3-pentadecylphenol (PDP), an
amphiphilic molecule which associates with 4VP via hydrogen
bonds. The morphologies of PF-b-P4VP and its supramolecules
in bulk states and thin films were studied by transmission elec-
tron microscopy (TEM), small-angle X-ray scattering (SAXS),
grazing incidence small-angle X-ray scattering (GISAXS) and
atomic force microscopy (AFM). A series of morphologies,
lamellar, cylindrical, and spherical, could be obtained by varying
the lengths of P4VP blocks and the amount of added PDP.
ꢂ
anol, and then dried under vacuum at 50 C for 24 h. 986.8 mg of
green powder PF-vinyl (3) was obtained (yield: 66%). The
number-average molecular weight (M ) from GPC measure-
n
ꢁ1
ments (THF as the carrier solvent) was 3002 g mol and the
polydispersity index (PDI) was 1.87. The degree of polymeriza-
tion was also determined by the ratio of fluorene units to the
1
vinyl end group calculated from H NMR spectrum and the
value of 8.8 is consistent with the molecular weight determined
by GPC. The degree of polymerization was thus estimated to be
1
9. H NMR (CDCl ): d (ppm) ¼ 0.76–0.85 (m, 3H, -(CH ) CH ),
3
2 5
3
1.11–1.26 (m, 8H, -CH (CH ) CH ), 2.08–2.17 (m, 2H,
2
2 4
3
-CH (CH ) CH ), 5.29 and 5.81 (2d, 2H, CH ]), 6.70–6.85 (m,
2
2
2 4
3
1H, CH]), 7.38–7.84 (m, 15H, protons on fluorene and phenyl
end groups).
We then synthesized poly[2,7-(9,9-dihexylfluorene)]-block-
2
. Experimental section
poly(4-vinylpyridine) (4) by living anionic polymerization,
32
similar to that reported in the literature. 102.4 mg (0.032 mmol)
Materials
PF -vinyl macroinitiator was placed in a 250 ml reaction flask
9
9
,9-Dihexyl-2,7-dibromofluorene (97%, Aldrich), triisopropyl
borate (>98%, Aldrich), tetrakis(triphenylphosphine) palla-
dium(0) (Pd(PPh 99%, Aldrich), potassium carbonate
CO , >99%, Acros), 4-bromostyrene (98%, Aldrich), phe-
and then evacuated to 10ꢀ20 mtorr for 3 h in order to remove the
residual solvent and undesired moisture. 100 ml purified THF
was transferred into the reactor, stirred for 10 min and cooled to
3 4
) ,
ꢂ
(
K
2
3
ꢁ78 C. Excess sec-butyllithium was used as initiator and added
nylboronic acid (97%, Aldrich), n-butyllithium (n-BuLi, 2.5 M in
hexane, Chemetall), sec-butyllithium (sec-BuLi, 1.3 M in cyclo-
hexane/hexane (92/8), Chemetall), Aliquatꢀ 336 (tri-
octylmethylammonium chloride, Aldrich), hydrochloric acid
to the well-stirred solution using a syringe. The color of the
solution was changed from green to black immediately, indi-
cating that the PF macroinitiators were initiated. The reaction
flask was warmed up to room temperature for 1 h for the excess
sec-butyllithium to react with THF to form ethylene and
(
HCl, 37%, Scharlab S.L.), and 3-pentadecylphenol (PDP, 90%,
Aldrich) were used without further purification. Toluene
anhydrous, Acros), hexane (Mallinckrodt Chemical, Inc.),
ꢂ
a lithium salt. The solution was cooled to ꢁ78 C again. The
(
purified 4-vinylpyridine monomers were then injected into the
reaction flask quickly and polymerized for 1 h. Finally, the living
anionic polymerization was terminated by an aliquot of meth-
anol and then precipitated in hexane. The PF-b-P4VP rod-coil
diblock copolymer was collected and dried under vacuum at
methyl alcohol (MeOH, >99%, Aldrich), chloroform (Mal-
linckrodt Chemical, Inc.) were used as received. 4-Vinyl pyridine
(
2
4VP, 95%, Acros) was distillated from CaH powder (93%,
Acros) and dibutylmagnesium (1.0 M solution in heptane,
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Scheme 1 The synthetic route of PF-b-P4VP linear rod-coil diblock copolymers.
ꢂ
8
0
C for 24 h (yield: 95%). The molecular weights of P4VP
chloroform. The PF-b-P4VP solutions were then added dropwise
to the PDP solution, followed by stirring for at least 3 days at
room temperature to form stock solutions. The bulk samples
were prepared following the procedures for PF-b-P4VP bulk
blocks in copolymers were determined based on the copolymer
1
compositions measured by H NMR and the molecular weight of
PF macroinitiators, which are listed in Table 1. Since PF-b-
P4VPs are unable to completely dissolve in any solvents
commonly used for GPC measurements, the determination of
ꢂ
samples but annealed at 105 C for 48 h. Annealing at lower
temperature is to prevent the PDPs from evaporation. The thin
film samples were prepared following the procedures described
above.
PDIs is difficult. We could only obtain the PDI of PF
9
-b-P4VP22,
ꢀ
1.78, which were measured in DMF. Since the PDIs of the
P4VP homopolymers we synthesized using living anionic poly-
merization were ꢀ1.20, PDI values between 1.20 and 1.87 should
Characterization
9
be reasonably expected for PF -based copolymers. The GPC
data of PF
measurements are included in the ESI.† H NMR (CDCl
d (ppm) ¼ 0.77–0.84 (m, 3H, -(CH CH ), 1.13 (broad, 8H,
CH (CH ) CH ), 2.11 (broad, 2H, -CH (CH ) CH ), 6.27–6.43
9
-b-P4VP22 and a detailed discussion of GPC
The molecular weight distribution of the prepared polymers was
determined by the gel permeation chromatography (GPC) using
a Lab Alliance RI2000 instrument equipped with MIXED-C and
D columns and a refractive index detector. THF or DMF was
1
3
):
2
)
5
3
-
2
2 4
3
2
2 4
3
and 8.22–8.40 (broad, 2H, protons on pyridine groups), 7.37–
.83 (m, 15H, protons on fluorene and phenyl end groups).
ꢁ1
ꢂ
used as the carrier solvent at flow rate 1 ml min at 40 C and
7
1
calibrated with polystyrene standards. The H NMR measure-
ments were conducted on a Bruker Avance DRX 500 MHz FT-
NMR system. Deuterated chloroform with standard tetrame-
thylsilane (TMS) was used. Thermogravimetric analysis (TGA)
was performed on a PerkinElmer Pyris 7 TGA. 3ꢀ5 mg powder
samples were heated under flowing nitrogen (flow rate
Sample preparation
The rod-coil diblock copolymer, PF-b-P4VP, was first dried in
vacuum oven for 1 day and dissolved in chloroform to form 1–
3
ꢁ1
ꢂ
ꢁ1
2
% (w/v) stock solutions. Bulk samples were prepared by placing
20 cm min ) at a heating rate of 20 C min from room
ꢂ
the polymer solutions in 1 ml Teflon beakers, and then the
solvents were evaporated at room temperature for several days.
temperature to 800 C. The glass transition temperatures were
determined using a TA Instrument DSC Q100 at a heating rate of
ꢂ
ꢁ1
ꢂ
ꢂ
Samples were further dried and annealed under vacuum at
ꢂ
5 C min from ꢁ30 C to 250 C. FT-IR spectra were recorded
at room temperature using a Jasco Model FTIR 410 spectro-
photometer. Samples were cast on KBr pellets and scanned 64
1
50 C for 48 h. Thin films were prepared by spin-coating the
solutions onto silicon wafers at spinning speed 3000 rpm. The
solutions were diluted in order to obtain desired film thicknesses.
Thin films were then solvent-annealed, which was conducted by
placing films inside an inverted dish (170 mm D ꢃ 90 mm H)
together with a beaker of 40 ml of chloroform at room temper-
ature for 2 days. Film thicknesses were measured using a Fil-
metrics F20 interferometer or a Surfcorder ET3000 profilometer
ꢁ
1
times with resolution 8 cm .
Samples prepared for transmission electron microscopy
ꢂ
(TEM) were embedded in resin and cured at 60 C overnight and
then were microtomed to sections with thickness ꢀ80 nm. The
4
thin sections were exposed to either RuO or iodine vapor that
selectively stain the PF or P4VP block, respectively, to enhance
the contrast. TEM images were collected on a Joel JEM-1230
transmission electron microscope at an accelerating voltage of
(
Kosaka Lab). For the preparation of PF-b-P4VP(PDP)
complexes, the desired amount of PDP was dissolved in
Table 1 Characteristics of PF-b-P4VP rod-coil block copolymers
b
ꢁ1
b
ꢁ1
c
ꢂ
g
Sample
M
n,PF (g mol
)
M
n,P4VP (g mol
)
f
4VP
T
( C)
Morphology
a
PF
PF
PF
9
9
9
-b-P4VP22
-b-P4VP92
3120
3120
3120
2310
9730
24340
0.42
0.76
0.89
84
145
147
lamella
cylinder
sphere
-b-P4VP231
a
b
subscripts denote degree of polymerization. molecular weight determined by NMR. weight fraction of P4VP.
c
4
200 | Soft Matter, 2011, 7, 4198–4206
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1
00 kV. Atomic force microscopy (AFM) imaging was per-
PF macroinitiator in d-chloroform. The proton resonance peaks
at 0.76–0.85, 1.11–1.26 and 2.08–2.17 ppm are assigned to the
methyl and methylene of the alkyl group on the PF moiety,
respectively. The signals between 7.26–7.84 ppm are attributed to
the fluorene aromatic protons and phenyl end group. The proton
signals on the vinyl moiety of macroinitiator are clearly observed
at 5.29, 5.81 and 6.70–6.85 ppm, indicating the successful
incorporation of the end-capper 4-bromostyrene. After poly-
merization, the proton peaks of vinyl group of the PF macro-
initiator at 5.29 and 5.81 ppm entirely disappear and the proton
signals of the pyridine groups at 6.38–6.86 and 8.36–8.44 ppm are
observed in PF-b-P4VP rod-coil block copolymer, as shown
Fig. 1b. These results confirm the successful synthesis of the PF
macroinitiator and block copolymer PF-b-P4VP.
formed on a MultiMode AFM system with a Nanoscope 3D
controller (Digital Instruments) in tapping mode. The spring
constant of the silicon cantilevers (Nanosensor PPP-SEIHR) was
ꢁ1
1
5 N m and the resonant frequency was 130 kHz.
Small-angle X-ray scattering (SAXS) measurements of pure
PF-b-P4VPs were conducted on a Bruker NANOSTAR SAXS
apparatus, equipped with a 3.0 kW X-ray generator operated at
4
0 kV ꢃ 35 mA and a two-dimensional position-sensitive
proportional counter. The Cu-Ka line with wavelength
ꢀ
l ¼ 1.54 A was used. SAXS data of PF-b-P4VP(PDP) complexes
were collected on beamline BL23A1 in the National Synchrotron
Radiation Research Center (NSRRC), Taiwan. A mono-
ꢀ
33
chromatic beam of l ¼ 0.887 A was used. The scattering
intensity profiles were reported as the plots of the scattering
intensity (I) vs. the scattering vector q, where q ¼ (4p/l) sin(q/2),
q is the scattering angle. Grazing incidence small-angle X-ray
scattering (GISAXS) measurements were also conducted on
The thermal properties of the synthesized polymers were
measured by TGA and DSC. Fig. 2a shows the thermal decom-
position temperatures (T
d
, 5% weight loss temperature) of the
polymers. The block copolymers exhibit two thermal degradation
ꢀ
beamline BL23A1 in NSRRC, with a wavelength of 0.887 A at
ꢂ ꢂ
temperatures around 381 C and 441 C, which are attributed to
34,35
ꢂ
an incident angle of 0.2 . The scattering profiles were collected on
the decomposition of P4VP and PF, respectively.
Fig. 2b
33
a Mar-CCD with a diameter of 165 mm.
illustrates the DSC curves of the PF-b-P4VP block copolymers at
ꢁ1
ꢂ
a heating rate of 5 C min under nitrogen atmosphere. The glass
transition temperatures (T ) of the synthesized PF macroinitiator
g
3
. Results and discussion
ꢁ1
ꢂ
and P4VP homopolymer (M ¼ 20788 g mol ) are 84 C and
n
ꢂ
1
47 C, respectively. The T of PF -b-P4VP , PF -b-P4VP , and
g
Chemical structures and thermal properties of PF-b-P4VP
9
22
9
92
ꢂ
ꢂ
ꢂ
PF
9
-b-P4VP231 are 84 C, 145 C, and 147 C, respectively,
The synthesized rod-coil block copolymers with PF of 9 repeat
units and varying P4VP lengths are listed in Table 1. The
chemical structures of the synthesized polymers were confirmed
increasing with P4VP chain length.
1
1
Microphase separation of PF-b-P4VP
by H NMR spectrum. Fig. 1a shows the H NMR spectrum of
The TEM image, SAXS data of bulk samples and the AFM
images of thin films for PF
9
-b-P4VP22 (fP4VP ¼ 0.42) are shown in
1
Fig. 1 H NMR spectra of (a) PF
9
macroinitiator and (b) PF
9
-b-P4VP92
Fig. 2 (a) TGA and (b) DSC thermograms of PF
PF-b-P4VP diblock copolymers.
9
macroinitiator and
diblock copolymer.
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3
8
Fig. 3. The TEM image (Fig. 3a), which was obtained using
are incommensurable. We thus believe that the flat regions are
formed by lamellae as well but their orientation is parallel to the
surface.
RuO to stain the PF blocks, shows striped clusters distributed in
4
homogeneous matrix. Since the two blocks of PF -b-P4VP22 are
9
approximately symmetric, the alternating stripes should be
lamellar microdomains oriented normal to the section plane. In
the wide plain regions, however, no structures are observed,
indicating that microphase separation only occurs locally. A
plausible explanation for this unusual phase behavior is that the
lengths of PF and P4VP of this diblock copolymer are short,
Fig. 3d shows the AFM images of a very thin film (less than
18 nm), in which long fibrils are meandering through flat surfaces
and two different widths of fibrils are observed. The width of
thinner fibrils is ꢀ20 nm and they should be a single lamella
perpendicular to the surface. The thicker ones are ꢀ35 nm in
width and presumably formed by the stacking of two lamellae.
Note that holes are formed on the flat region, ꢀ6 nm in depth,
9
giving rise to a low cN value. This low cN of PF -b-P4VP22
39
happens to locate around the border of isotropic and lamellar
phases, i.e. in the weak segregation limit, resulting in a transi-
tional morphology. Similar morphology has been reported in
poly-(diethylhexyloxy-p-phenylenevinylene)-b-polystyrene rod-
implying a monolayer deposited on the substrate. In the
monolayer, the more hydrophilic P4VP blocks should adhere to
the silicon substrate while the PF blocks are exposed to the air.
Comparing Fig. 3c and 3d, the fraction of the perpendicular
lamellae in thicker films is higher than that in thinner films. It is
well known that the substrate-block copolymer interactions
greatly affect the morphology of a thin layer close to the interface
and if the substrate prefers to interact with one of the blocks,
36,37
coil systems.
In the clusters, the lamellae are bended and
discontinuous and have a varying spacing. The variation of the
lamellar spacing also implies a weak segregation. The spacing of
the lamellae estimated from the relatively regular regions in TEM
image is ꢀ20 nm. The low fraction of lamellar clusters and the
lack of ordering in the clusters are reflected on the SAXS
measurements where no distinct diffraction peak appears, as
shown in Fig. 3b.
40
microdomains are generally forced to be parallel to the surface.
For a very thin film, the entire film is almost influenced by the
preferential interaction between the substrate and P4VP, thereby
forming mostly parallel lamellae; as film thickness is increased,
the effect is gradually vanished away from the substrate and thus
We now move on to the thin film morphology of PF -b-
9
41
P4VP . In thin films, the interactions between substrate and
2
perpendicular lamellae are increasingly formed.
The bulk and thin film morphologies of PF -b-P4VP that has
2
polymers are involved in determining the microphase separation
behaviors. Fig. 3c shows the AFM images of a thin film, ꢀ79 nm
in thickness, which reveals the coexistence of strips and flat
surfaces. The morphologies are essentially the same whether
9
92
coil fraction increased to 0.76 are shown in Fig. 4. In the TEM
image (Fig. 4a), where P4VP were stained with I , the PF blocks
2
self-assemble into cylindrical microdomains and the cylinders are
hexagonally packed, in contrast to the irregular packing of
ꢂ
films were thermal annealed at 150 C or solvent annealed. The
strips should be the lamellar structures oriented perpendicular
the surface. In the flat regions, holes are observed, shown in the
height image, and the depth of the holes is 20–25 nm. This depth
is consistent with the spacing of lamellae obtained from TEM. In
coil–coil block copolymer systems, such holes usually occur on
the surface of thin films with lamellar microdomains parallel to
the surface if film thicknesses and the natural spacing of lamellae
9
lamellae in PF -b-P4VP22. SAXS data, shown in Fig. 4b, reveal
*
ꢀ
ꢁ1
a distinct first-order peak at q ¼ 0.031 A , which corresponds to
a d-spacing of 20.3 nm, and an obscure second-order peak at
*
q ¼ O3 q . The ratio of 1:O3 also suggests a hexagonal packing of
cylinders. In the AFM images of a thin film, ꢀ135 nm in thick-
ness, the structure on the surface is hardly seen in the height
image, shown in Fig. 4c, indicating that the surface is rather
9 4
Fig. 3 Structure characterization of PF -b-P4VP22: (a) TEM image and (b) SAXS curve for bulk samples. The TEM sample was stained with RuO .
AFM images for (c) a thicker film ꢀ79 nm in thickness and (d) a very thin film (less than 18 nm).
4
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Fig. 5 Structure characterization of PF
9
-b-P4VP231: (a) TEM image and
Fig. 4 Structure characterization of PF
9
-b-P4VP92: (a) TEM image and
b) SAXS curve for bulk samples. The TEM sample was stained with I
c) AFM images for a thin film ꢀ135 nm in thickness.
(
b) SAXS curve for bulk samples. The TEM sample was stained with
(
(
2
.
RuO
4
. (c) AFM images for a thin film ꢀ65 nm in thickness.
appears that the transition of the PF-b-P4VP morphology, from
lamella to cylinder to sphere, is dependent on the volume frac-
tions of blocks, which has been found in some rod-coil
9
smooth, which is again different from the rough surface of PF -b-
P4VP22 thin films (Fig. 3c). The phase image shows no significant
feature either. The barely distinguishable phase image is due to
naturally low phase contrast between PF and P4VP blocks since
the hardness and adhesion of PF and P4VP are comparable.
29,42,43
systems.
This relationship has been well known for coil–coil
block copolymers. The difference is that the packing of micro-
domains in PF-b-P4VP is not as ordered as that of coil–coil ones
due to the strong p–p interaction and the steric interaction of PF
rod-like chains.
For the sample of PF -b-P4VP231, the coil fraction further
9
increased to 0.89, the PF blocks turn into spherical microdomains,
as shown in the TEM image in Fig. 5a, where PF blocks were
4
stained with RuO . In Fig. 5b, the SAXS data show a diffraction
Characterization and microphase separation of PF-b-P4VP(PDP)
ꢀ
ꢁ1
peak at q ¼ 0.014 A , implying an average spacing of 44.9 nm
between spheres. The higher-order peaks, however, are absent due
to the lack of ordering. In thin films, the surface is quite smooth
and the spherical microdomains could only be vaguely observed in
the height and phase AFM images, shown in Fig. 5c. As the coil
length increases, which relatively reduces the effects of p inter-
action and thus increases the mobility of polymer chains during
annealing, the surface tends to become smoother to reduce the
surface area and lower the surface free energy.
PF-b-P4VP(PDP) supramolecules are prepared by hydrogen-
bonding 3-pentadecylphenol (PDP) to the 4VP. The
Table 2 Characteristics of PF-b-P4VP(PDP) supramolecules
a
Sample
f
comb
Morphology
PF
PF
PF
9
9
9
-b-P4VP22(PDP)
-b-P4VP92(PDP)
-b-P4VP231(PDP)
1
0.74
0.92
0.97
cylinder
sphere
sphere
1
9
In addition to the PF -based rod-coil block copolymers, we
1
a
have also synthesized copolymers with longer rod lengths, whose
morphologies are presented in supplementary information. It
weight fraction of P4VP(PDP).
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ꢁ1
band shifts to a higher frequency ꢀ1008 cm . This is evidence
that the pyridines are hydrogen-bonded with phenyl groups of
PDP and the formation of the supramolecule PF-b-P4VP(PDP)
is confirmed.
Fig. 7 shows the AFM images of supramolecular thin films.
With the incorporation of PDP, the PF rich domains and
P4VP(PDP) rich domains can be clearly distinguished in phase
images. This is because the soft PDPs enhance the contrast of
hardness and adhesion between the PF and the P4VP(PDP)
blocks. As shown in Fig. 7a, the morphology of PF
9
-b-
P4VP (PDP) (f
1
¼ 0.74) are entirely different from that of
2
2
comb
pure PF -b-P4VP . The addition of PDP transforms the
9 22
lamellar structures of PF -b-P4VP (Fig. 3c) into straight but
9 22
discontinuous cylinders formed by PF blocks. Different from
regular hexagonally-packed cylindrical microdomains found in
Fig. 6 (a) Schematic of PF-b-P4VP(PDP) supramolecule. (b) FT-IR
spectra of PS-b-P4VP and PS-b-P4VP(PDP)
27,44
1
.
coil–coil block copolymer-based supramolecules,
cylinders are randomly embedded in P4VP(PDP) matrix. The
SAXS data of a bulk PF -b-P4VP22(PDP) sample are shown in
the PF
characteristics of the samples are listed in Table 2. The chemical
structure of the rod-comb PF-b-P4VP(PDP) supramolecule is
illustrated in Fig. 6a. Fig. 6b shows the FT-IR spectra of a PF-b-
P4VP(PDP)1 thin film where the subscript denotes the molar
ratio of PDP to 4VP. Free pyridine groups of PF-b-P4VP
9
1
ꢀ
ꢁ1
Fig. 8. A hump at q z 0.055 A indicates the distance between
cylinders is about 11.4 nm.
In the thin films cast from PF
9
-b-P4VP92(PDP)
1
(fcomb ¼ 0.92),
the PF blocks form mostly spherical microdomains along with
a few short curly cylindrical ones in P4VP(PDP) matrix, shown in
Fig. 7b, which is transformed from the hexagonally-packed
cylindrical microdomains found in pure PF -b-P4VP (Fig. 4c).
ꢁ1
produce an absorption band at 993 cm . Upon the incorpora-
tion of PDP, as seen in the spectrum of PF-b-P4VP(PDP) , this
1
9
92
The morphology of this supramolecule should be in the transi-
tional state between cylindrical and spherical phases. Further
increasing P4VP length, as demonstrated in PF
9
-b-
P4VP231(PDP) thin films, though the fraction of P4VP(PDP) is
1
as high as 0.97, phase-separations between PF and P4VP(PDP)
are still observed and the PF microdomains remain spherical,
9
shown in Fig. 7c. The first order peaks of SAXS data for PF -b-
P4VP (PDP) and PF -b-P4VP (PDP) bulk samples shown
9
2
1
9
231
1
ꢀ
ꢁ1
in Fig. 8 are at q ¼ 0.042 and 0.018 A , respectively, corre-
sponding to center-to-center distances between spheres of
1
4.9 and 34.9 nm.
Fig. 7 AFM images for (a) a PF
19 nm, (b) a PF -b-P4VP92(PDP)
c) a PF -b-P4VP231(PDP)
thin film, film thickness ¼ 117 nm.
9
-b-P4VP22(PDP)
1
thin film, thickness ¼
1
9
1
thin film, film thickness ¼ 327 nm and
(
9
1
1
Fig. 8 SAXS data of PF-b-P4VP(PDP) bulk samples.
4
204 | Soft Matter, 2011, 7, 4198–4206
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ꢀ
ꢁ1
second-order one at q ¼ 0.308 A , corresponding to a spacing
z
of ꢀ4.1 nm, which is due to the ordering of the lamellae formed
by the P4VP(PDP) comb blocks oriented parallel to the surface.
The alignment of P4VP(PDP) lamellae is schemed in the inset of
Fig. 9b. Since P4VP blocks are more hydrophilic and preferen-
tially interacts with silicon oxide, the hydrophobic alkyl tails of
PDPs are precluded from P4VP(PDP)/substrate interfaces. Also,
the alkyl tails of PDPs tend to be exposed to air and thus the
P4VP(PDP) lamellae are stacked parallel to the surface. Such
44,45
structure has been found in other supramolecular systems.
Note that the diffraction peak of the lamellae is absent in the
SAXS data of bulk samples shown in Fig. 8. The substrate-P4VP
interaction force should play an important role in regulating the
lamellae. Furthermore, we didn’t observe the parallel lamellae in
9 9 1
the PF -b-P4VP22(PDP)1.0 and PF -b-P4VP92(PDP) thin film,
implying that a larger fraction of rod PF blocks may prohibit
P4VP(PDP) comb blocks from regular stacking even though the
interfacial effect exists.
The morphologies of more PF-b-P4VP(PDP) supramolecules
with different fractions of blocks and PDP/4VP ratios are pre-
sented in supplementary information. The phase behaviors of all
the characterized samples are summarized in Fig. 10. It is clear
that PDPs can associate with P4VP blocks and therefore increase
the volume fraction of P4VP(PDP), which in turn, transforms the
PF microdomains from lamella to cylinder or from cylinder to
sphere. In other word, the morphology of rod-coil block copol-
ymers can be tailored by supramolecular strategies. This effect
has been seen in coil-coil block copolymer-based supramolecules.
However, in rod-coil systems, the packing of microdomains are
governed by the p-stacking of rod blocks. The arrangement of
microdomains is thus less ordered and the orientation control in
25–28
Fig. 9 (a) GISAXS pattern of a ꢀ100 nm PF
9
-b-P4VP231(PDP)
1
thin
ꢁ1
thin films is not as efficient as in coil-coil analogues.
ꢂ
ꢀ
film at an incident angle of 0.2 . (b) The q
z
scan at q
y
¼ 0.015 A
,
showing the P4VP(PDP) lamellae with a spacing of 4.1 nm are oriented
parallel to the surface.
4. Conclusions
Fig. 9a shows the GISAXS pattern of a PF
9
-b-P4VP231(PDP)
1
This study has demonstrated that the supramolecular strategy is
a facile way to manipulate the morphology of rod-coil block
copolymers. The strategy involves the incorporation of small
molecules via hydrogen bonding into the coil blocks to form rod-
comb supramolecules. We have synthesized a series of new flu-
orene-based block copolymers (PF-b-P4VP) with different coil
chain lengths through Suzuki coupling reaction and living
anionic polymerization and we show the feasibility of this
strategy by incorporating PDP into P4VP blocks. The
morphology can indeed be transformed from lamellar to cylinder
or from cylinder to spherical upon the addition of PDP. We also
show that the p–p interaction is a crucial factor that governs the
microphase separation in supramolecular assemblies.
thin film, ꢀ100 nm in thickness, where the diffraction spots only
ꢀ
ꢁ1
appear along q axis. The q scan at q ¼ 0.015 A , plotted in
z
z
y
ꢀ
ꢁ1
Fig. 9b, shows the first-order peak at q ¼ 0.153 A and the
z
Acknowledgements
This work was financially supported by the Taiwan National
Science Council (NSC 99-2923-E-002-003). We acknowledge
NSRRC, Taiwan for facilitating the SAXS and GISAXS
experiments performed as part of this work and Dr U-Ser Jeng
and Dr Chun-Jen Su of NSRRC for the assistance in scattering
experiments. We also thank Mr. Nao Xiao at Hokkaido
University for the assistance in GPC experiments.
Fig. 10 Morphology diagram of PF-b-P4VP(PDP) as a function of the
weight fraction of the comb-like block P4VP(PDP) (fcomb) at room
temperature. M.W. is the total molecular weight of the PF-b-P4VP(PDP)
supramolecules. The morphologies of most compositions are shown in
supplementary information.
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2
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