Self-assembled lamellar nanostructures of wholly aromatic rod-rod-type block molecules

Article abstract of DOI:10.1021/ol061929c

Wholly aromatic rod-rod type di- and triblock molecules, oligo(ether sulfone)-b-oligo(ether ketone)s (OES-OEK), were synthesized to study a solid-state self-assembled nanostructure. The OES and OEK segments in the block molecules form segregated crystalli

Full text of DOI:10.1021/ol061929c

ORGANIC
LETTERS
2006
Vol. 8, No. 24
5453-5456
Self-Assembled Lamellar Nanostructures
of Wholly Aromatic Rod
Block Molecules
−Rod-Type
Teruaki Hayakawa,*,† Raita Goseki,† Masa-aki Kakimoto,† Masatoshi Tokita,†
Junji Watanabe,† Yonngui Liao,‡ and Shin Horiuchi‡
Department of Organic and Polymeric Materials, Graduate School of Science and
Engineering, Tokyo Institute of Technology, 2-12-2-S8-26 O-okayama,
Meguro-ku, Tokyo 152-8552, Japan, and Nanotechnology Research Institute,
National Institute of AdVanced Industrial Science and Technology, Central 5,
1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan
hayakawa.t.ac@m.titech.ac.jp
Received August 4, 2006
ABSTRACT
Wholly aromatic rod−rod type di- and triblock molecules, oligo(ether sulfone)-b-oligo(ether ketone)s (OES-OEK), were synthesized to study a
solid-state self-assembled nanostructure. The OES and OEK segments in the block molecules form segregated crystalline domains. The
energy-filtering transmission electron microscopy images revealed that the di- and triblock OES-OEK co-oligomers formed lamellar nanostructures
with a periodicity of
∼
9 and 13 nm, respectively.
The study of molecular self-assembly enables us to gain an
understanding of the important factors leading to a variety
of functional materials based on nano- or supramolecular
structures.1 Block molecules such as block co-oligomers or
copolymers are well-known self-assembling materials that
offer intriguing tunable built-in nanoscopic domains and
tailored surface or bulk properties.2 A variety of coil-coil-
type3 and rod-coil-type4 block molecules have been widely
studied and found to exhibit various microphase-separated
nanostructures such as spheres, cylinders, lamellae, unique
supramolecular structures such as mushroom-shaped nano-
structures4d and nanoribbons,4e or hierarchical structures over
multiple length scales.4f,g Coil segments are commonly based
on polystyrene or poly(ethylene glycol), while rod segments
are commonly based on liquid crystalline molecules or
aromatic π-conjugated molecules. Very few attempts, how-
† Tokyo Institute of Technology.
‡ National Institute of Advanced Industrial Science and Technology.
(1) Muthukumar, M.; Ober, C. K.; Thomas, E. L. Science 1997, 277,
1225.
(3) (a) Goodman, I., Ed. DeVelopments in Block Copolymers; Elsevier
Applied Science Publishers: New York, 1982; Vol. 1. (b) Hamley, I. W.
The Physics of Block Copolymers; Oxford Science Publications: Oxford,
1998.
(2) For reviews, see, for example: (a) Bates, F. S.; Fredrickson, G. H.
Annu. ReV. Phys. Chem. 1990, 41, 525. (b) Park, C.; Yoon, J.; Thomas, E.
L. Polymer 2003, 44, 6725
10.1021/ol061929c CCC: $33.50
© 2006 American Chemical Society
Published on Web 11/04/2006

Scheme 1. Synthesis of OES-b-OEK 1 and Chemical Structure of OEK-b-OES-b-OEK 2
ever, have been made to study wholly aromatic rigid-rod
methods such as living radical polymerization7 and atom
transfer radical polymerization.8 However, it is difficult to
synthesize block architechtures in condensation polymers as
they are typically prepared by a step-growth mechanism.
While Yokozawa et al. have developed a polymerization
method of chain-growth polycondensation to yield condensa-
tion block copolymers based on aromatic polyamides with
a narrow molecular distribution,9 it is still too limited in scope
to access polymers desired for this study. Thus, the well-
defined di- and triblock molecules of OES-b-OEK and OEK-
b-OES-b-OEK were synthesized by stepwise aromatic nu-
cleophilic substitution and deprotection reactions (Scheme
1). For constructing OES and OEK segments, we first
synthesized 4-fluoro-4′-(4′′-methoxyphenyloxy)diphenyl sul-
fone (3) and 4-fluoro-4′-(4′′-methoxyphenyloxy)diphenyl
ketone (4) as building molecules from 4,4′-difluoro diphenyl
sulfone or ketone and 4-methoxyphenol, respectively. For
synthesis of diblock OES-b-OEK (1), 5 was prepared from
3 and tert-butyl phenol with subsequent deprotection of the
methoxy group by boron tribromide to access the next
nucleophile.
block molecules, namely, rod-rod-type block molecules.5
Wang et al.5b developed rod-rod conjugated diblock co-
polymers of alkyl-substituted oligo(phenylenevinylene)-b-
oligothiophene that self-assembled to lamellae. The study
of wholly aromatic rod-rod type block molecules is essential
not only for fundamental understanding but also to generate
novel self-assembling nanomaterials with intriguing proper-
ties from a variety of conventional, but little studied, wholly
aromatic condensation polymers. Herein, we report synthesis
and self-assembly of wholly aromatic oligo(ether sulfone)
(OES) and oligo(ether ketone) (OEK) di- and triblock co-
oligomers (OES-b-OEK and OEK-b-OES-b-OEK). Amor-
phous aromatic poly(ether sulfone)s and crystalline poly(ether
ketone)s are well-known super-engineering plastics. Crystal-
lization of the block OEK oligomers is expected to induce
nanostructure formation in the solid state. Characterization
of crystalline nanostructures can be carried out by using wide
and small-angle X-ray scattering (WAXS and SAXS) and
energy-filtering transmission electron microscope (EFTEM).6
Well-defined block co-oligomers and copolymers have
been prepared by a variety of facile and reliable synthetic
The aromatic nucleophilic substitution reaction of 6 with
3 was carried out in the presence of potassium carbonate in
(4) (a) Lee, M.; Cho, B.-K.; Zin, W.-C. Chem. ReV. 2001, 101, 3869.
(b) Widawski, G.; Rawiso, M.; Franc¸ois, B. Nature 1994, 369, 387. (c)
Jenekhe, S. A.; Chen, X. L. Science 1999, 283, 372. (d) Stupp, S. I.;
LeBonheur, V.; Walker, K.; Li, L. S.; Huggins, K. E.; Keser, M.; Amstutz,
A. Science 1997, 276, 384. (e) Sone, E. D.; Zubarev, E. R.; Stupp, S. I.
Angew. Chem., Int. Ed. 2002, 41, 1706. (f) Hayakawa, T.; Horiuchi, S.
Angew. Chem., Int. Ed. 2003, 42, 2285. (g) Hayakawa, T.; Yokoyama, H.
Langmuir 2005, 21, 10288. (h) Hayakawa, T.; Seino, M.; Goseki, R.; Hirai,
T.; Kikuchi, R.; Kakimoto, M.; Tokita, M.; Yokoyama, H.; Horiuchi, S.
Polym. J. 2006, 38 (6), 567. (i) Schleuss, T. W.; Abbel, R.; Gross, M.;
Schollmeyer, D.; Frey, H.; Maskos, M.; Berger, R.; Kilbinger, A. F. M.
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(6) (a) Horiuchi, S.; Harada, T.; Yase, K.; Ougizawa, T. Macromolecules
1999, 32, 1312. (b) Horiuchi, S.; Fujita, T.; Hayakawa, T.; Nakao, Y.
Langmuir 2003, 19, 2963. (c) Horiuchi, S.; Dohi, H. Langmuir 2006, 22,
4607. (d) Hofer, P.; Warbichler, P. In Transmission Electron Energy Loss
Spectroscopy in Material Science and the EELS ATLAS, 2nd ed.; Ahn, C.,
Ed.; Wiley-VCH: Weinheim, 2004; Chapter 6.
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(9) (a) Yokozawa, T.; Ogawa, M.; Sekino, A.; Sugi, R.; Yokoyama, A.
J. Am. Chem. Soc. 2002, 124, 15158. (b) Sugi, R.; Yokoyama, A.; Furuyama,
T.; Uchiyama, M.; Yokozawa, T. J. Am. Chem. Soc. 2005, 127, 10172.
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Org. Lett., Vol. 8, No. 24, 2006

DMAc at 180 °C for 4 h in nitrogen atomosphere to yield 7.
The methoxy end group was then removed to access
nucleophile 8. The yield for each aromatic nucleophilic
substitution and deprotection reaction pair was 94%. This
procedure was repeated once more to afford nucleophile 10.
OEK was introduced to OES using identical substitution-
deprotection chemistry, but using 4 rather than 3 to propagate
the chain. After two cycles of adding 4, the target OES-b-
OEK was obtained. The triblock oligomer was prepared with
identical methodology by using sulfone 13 as the starting
molecule. After one cycle of 3 and two cycles of 4, the target
OEK-b-OES-b-OEK was obtained.
transition of the OEK homo-dimer from the crystalline to
isotropic phase was observed at 213 °C by DSC.
Optical textures appeared under POM with crossed polar-
izers during annealing. The textures formed at 160 and 200
°C for OES-b-OEK and OEK-b-OES-b-OEK, respectively,
after cooling at 10 °C min-1 from 250 °C. The texture growth
was very slow, occurring over several hours. The WAXS
pattern of the block molecules annealed under the same
conditions includes several reflections (Figure 2a,b). These
Note that the chloro analogues of 3 and 4 yielded
significant scrambling of the starting material and side
products such as 4,4′-(4′′,4′′′-di-tert-butylphenyloxy)diphenyl
sulfone or ketone. Presumably, this is due to the nucleophi-
licity of the chloride ion to facilitate an ether exchange
reaction. No reaction conditions were observed to eliminate
the ether exchange reaction when the chloride analogues were
used.
OES-b-OEK and OEK-b-OES-b-OEK chemical structures
1
were characterized by FT-IR, H NMR, and MALDI-ToF
MS. The MALDI-ToF MS spectra (Figure 1) are consistent
Figure 2. WAXS and SAXS profiles of (a) OEK-b-OES-b-OEK,
(b) OES-b-OEK, (c) homo-OES (trimer), and (d) homo-OEK
(dimer).
Figure 1. MALDI-ToF MS spectra of (a) OES-b-OEK and (b)
dihydroxy-terminated OEK-b-OES-b-OEK.
reflections are explained by the superposition of the respec-
tive diffraction patterns measured for the OES and OEK
homo-oligomers (Figure 2c,d), suggesting that the OES and
OEK segments in the block molecules segregate and crystal-
lize, which is consistent with the optical textures observed
in POM. The SAXS pattern of OES-b-OEK includes a
scattering maximum with a spacing of 91 Å, which corre-
sponds to the molecular length of 92 Å calculated for the
most stretched configuration. These indicate that the more
stretched molecules of OES-b-OEK must be in head-to-tail
(OES-to-OEK) orientation, as if head-to-head or tail-to-tail
configurations were present then the scattering maximum
would indicate a spacing of approximately 184 Å and this
is not seen. However, no such layered structure was not
indicated for OEK-b-OES-b-OEK by the SAXS method.
Further confirmation for the formation of self-assembled
structures of OES-b-OEK and OEK-b-OES-b-OEK was
provided by energy-filtering transmission electron micros-
copy (EFTEM). The annealed films were embedded in epoxy
resin and microtomed to give specimens of 50 nm thickness.
Micrographs of the energy-filtered images at 200 ( 10 eV
with calculated values for OES-b-OEK and deprotected
OEK-b-OES-b-OEK at 1712.4 and 2236.4 m/z. The FT-IR
and NMR spectra are also consistent with the formation of
the desired block oligomers.
The thermal behavior of OES-b-OEK and OEK-b-OES-
b-OEK was examined by thermogravimetric analysis (TGA),
differential scanning calorimetry (DSC), and polarized optical
microscopy (POM). The TGA 10% weight losses in nitrogen
of OES-b-OEK and OEK-b-OES-b-OEK were observed at
500 and 483 °C, respectively. The DSC traces on the second
heating showed two distinct thermal transitions for both OES-
b-OEK and OEK-b-OES-b-OEK. Baseline shifts observed
at 119 and 126 °C are consistent with the glass transition
(Tg) of the OES block of OES-b-OEK and OEK-b-OES-b-
OEK, respectively. Endothermic peaks were also observed
at 202 and 223 °C. A phase transition to an isotropic phase
was found to occur at these temperatures by means of POM
observations. These transition temperatures showed good
agreement with those of the homo-oligomer of OES and
OEK. The Tg of the OES homo-trimer was 128 °C and the
Org. Lett., Vol. 8, No. 24, 2006
5455

with the energy windows of 20 eV were used for the pre-
and postedge images, respectively.6c Those maps allows us
to assign the brighter lamellae to the OES blocks. The images
for both OES-b-OEK and OEK-b-OES-b-OEK films clearly
show a layered nanosacle structure. However, the orientation
along the long axis of OEK-b-OES-b-OEK is lower than that
of OES-b-OEK. This is agreement with the absence of the
scattering maximum in the SAXS of OEK-b-OES-b-OEK.
The layer spacings in the nanostructures of OES-b-OEK and
OEK-b-OES-b-OEK films were approximately 94 and 131
Å, respectively. The layer spacing of OES-b-OEK is in good
agreement with that of the SAXS characterization. The
EFTEM images confirmed the formation of lamellar nano-
structures in OES-b-OEK and OEK-b-OES-b-OEK films.
In summary, we have studied syntheses and self-assembled
structures for new block molecules of wholly aromatic rod-
rod-type block molecules, OES-b-OEK and OEK-b-OES-
b-OEK. The well-defined block molecules were successfully
synthesized in high yields. The OES-b-OEK and OEK-b-
OES-b-OEK formed self-assembled lamellar nanostructures
with a periodicity of approximately 9 and 13 nm, respec-
tively. These architectures are very promising to applications
such as nano- or microelectronics, membranes, and templat-
ing through manipulation of the block length and composi-
tion.
Figure 3. EFTEM images of (a) OES-b-OEK and (b) OEK-b-
OES-b-OEK. Schematic illustrations of the lamellar nanostructures
are oriented with respect to the EFTEM image.
Acknowledgment. We gratefully acknowledge the finan-
cial support for these studies by the Tokuyama Science
Foundation. We thank Dr. Stephen J. Grunzinger of the
Tokyo Institute of Technology for helpful discussions.
energy losses are shown in Figure 3. These nanostructures
were unobservable in the corresponding conventional TEM
images with conventional staining techniques. Sulfur distri-
bution images could be created by the “two-window jump
ratio” method6d using the sulfur L2,3-ionization edge at 160
eV, in which two energy-loss images at 130 and 200 eV
Supporting Information Available: Synthetic details and
characterization data for all new compounds. This material
is available free of charge via the Internet at http://pubs.acs.org.
OL061929C
5456
Org. Lett., Vol. 8, No. 24, 2006