Stable macroscopic nanocylinder arrays in an amphiphilic diblock liquid-crystalline copolymer with successive hydrogen bonds

Article abstract of DOI:10.1039/b705748b

In bulk films of a novel well-defined amphiphilic diblock liquid-crystalline copolymer with aramid moieties, a stable perpendicularly orientated hydrophilic nanocylinder array with hexagonal packing was formed in a large area by supramolecular self-assemb

Full text of DOI:10.1039/b705748b

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Volume 17 | Number 33 | 7 September 2007 | Pages 3465–3564
ISSN 0959-9428
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Tomiki Ikeda et al.
Stable macroscopic nanocylinder
arrays in an amphiphilic diblock
liquid-crystalline copolymer with
successive hydrogen bonds
0959-9428(2007)17:33;1-Y

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www.rsc.org/materials | Journal of Materials Chemistry
Stable macroscopic nanocylinder arrays in an amphiphilic diblock
liquid-crystalline copolymer with successive hydrogen bonds{
Haifeng Yu, Atsushi Shishido, Jingze Li, Kaori Kamata, Tomokazu Iyoda* and Tomiki Ikeda*
Received 16th April 2007, Accepted 5th July 2007
First published as an Advance Article on the web 12th July 2007
DOI: 10.1039/b705748b
units (PEO114Br) was used as a macroinitiator to undergo a typical
atom transfer radical polymerization (ATRP).3,4 Fig. 1 shows the
molecular structures of the block copolymer, in which about
70 repeat units of the aramid block were estimated by the
integration of the phenyl proton NMR peak at 6.72 ppm of the
aramid to that of the oxyethylene protons of the PEO at 3.64 ppm.
The GPC curve gives a number-average molecular weight (Mn) of
42 400 and a narrow polydispersity index of 1.15. The obtained
ADLC is only soluble in polar solvents such as chloroform,
dichloromethane, and N,N-dimethylformamide because of the
existence of strong H-bonds among the aramid groups. Such
strong interactions are responsible for broadening an absorption
peak at 3340 cm21 in the FTIR spectrum (stretch vibration of
N–H in aramid, Fig. 2a). When the copolymer was heated, three
phase-transition peaks appeared at 32.2, 124.7 and 134.3 uC in its
thermogram by differential scanning calorimetry (DSC, Fig. 1),
corresponding to the PEO melting point, the aramid block melting
point, and smectic phase to isotropic phase transition, respectively.
On cooling, a wider LC range was observed, and the PEO melting
point appeared at 232.6 uC because of the super-cooling effect. No
obvious glass-transition temperature (Tg) was obtained in the DSC
curve.
In bulk films of a novel well-defined amphiphilic diblock liquid-
crystalline copolymer with aramid moieties, a stable perpendi-
cularly orientated hydrophilic nanocylinder array with
hexagonal packing was formed in a large area by supramole-
cular self-assembly.
Recently, much attention has been paid to well-defined block
copolymers or polymer alloys because they can self-assemble into
ordered periodic nanostructures,1 which have been widely used as
templates or scaffolds for bottom-up nanotechnology.1,2
Unfortunately, good domain order over a large area cannot be
achieved by molecular self-assembly alone, and external forces or
modified substrates have been explored to obtain long-range
order.1–4 Based on the pioneering work of Iyoda et al.,3a we have
combined liquid-crystalline polymers (LCPs) with microphase
separation of block copolymers to obtain macroscopic regular
nanostructures in the designed materials by supramolecular self-
assembly.3,4 In the process of microphase separation, inherent LC
properties such as self-organising nature, fluidity of long-range
order and cooperative motion are involved,5 which enables the
microphase-separated periodic structures to be controlled.4 For
instance, azobenzene moieties can play roles both as a mesogen
and as a photosensitive chromophore in LC block copolymers,3–6
whose microphase-separated nanostructures have been manipu-
lated by either the rubbing technique or polarized laser beams.4
Obviously, the obtained nanostructures have a poor optical
durability, which limits their wide application. To improve the
stability of the obtained nanostructures, in this study, non-
photosensitive aramid moieties were introduced as mesogens to
form successive hydrogen bonds (H-bonds) in the matrix.
Copolymer films with a thickness of about 140 nm were
prepared by spin-coating from a chloroform solution onto clean
glass substrates. After the solvent was removed at room
Owing to their molecular rigidity, aramid groups have been
utilized as mesogens to build LCPs,7,8 and their strong H-bonds
enable polyaramid fibers such as Kevlar and Nomex (Dupont) to
be among the toughest organic materials.8 Since H-bonds have
been extensively explored to tailor self-organized nanostructures,9
introduction of the aramid groups into LC block copolymers
might bring about more stable nanostructures by supramolecular
self-assembly. Here, we design a novel amphiphilic diblock LC
copolymer (ADLC) consisting of flexible poly(ethylene oxide)
(PEO) as a hydrophilic segment and poly(methacrylate) containing
an aramid moiety in the side chain as a hydrophobic LC segment.
To prepare the ADLC, a bromo-terminated PEO with 114 repeat
Chemical Resources Laboratory, Tokyo Institute of Technology, R1-11,
4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan.
E-mail: tikeda@res.titech.ac.jp; iyoda.t.aa@m.titech.ac.jp
{ Electronic supplementary information (ESI) available: Experimental
details. See DOI: 10.1039/b705748b
Fig. 1 Molecular structure and thermal properties of the block
copolymer. The polarizing optical microscopic pictures (500) were
obtained at 30 uC and 120 uC, respectively.
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J. Mater. Chem., 2007, 17, 3485–3488 | 3485

Fig. 2 Absorption spectra of the block copolymer films. (a,b) FTIR and
UV-Vis spectra of the block copolymer films before and after annealing,
respectively. (c) In-situ FTIR spectra of the annealed films.
temperature, the films were annealed at 140 uC for 24 h by
controlling both the heating and cooling rates at 1 uC min21
.
Upon annealing, the UV-Vis maximum absorbance at 271 nm
shown in Fig. 2b decreased greatly, probably attributed to the out-
of-plane alignment of the aramid mesogens. Such perpendicular
orientation of the mesogens also gives rise to the in-plane
orientation of the carboxyl groups inside the aramid mesogens,
which is indicated by an obvious increase in the intensity of the
FTIR absorption peak at 1646 cm21 (stretch vibration of CLO in
aramid, Fig. 2a). Furthermore, a blue shift of 24 nm occurred in
the UV-Vis maximum absorption owing to the formation of
H-aggregates of the mesogens.
Fig. 3 AFM images of the annealed block copolymer films. (a,b) AFM
image of surface and cross section, respectively. Phase images are on the
left, whereas topological ones are on the right. Inset into the phase picture
is the fast Fourier transform image. (c) AFM phase image of the block
copolymer film with a large area of 3 mm 6 3 mm.
segments of the ADLC, which makes its microphase separation
different from other block copolymers with relatively weak
molecular interactions.
Generally, the covalently-connected immiscible block copoly-
mers thermodynamically show microphase-segregated nanostruc-
tures including spheres, cylinders, gyroid and lamellae, driven by
the positive free energy of mixing of the polymeric composites.10 In
the present ADLC films, both the H-bonds and the LC ordering
affect the microphase-separation process, enabling them to form
hierarchical structures by supramolecular self-assembly. As shown
in Fig. 3, unambiguous regular nanostructures were clearly
observed in the atomic force microscopic (AFM) images.
Considering the composition of the block copolymer, although
the repeat units of PEO are much more than those of the aramid
block, the PEO block still constitutes the minority component in
the microphase separation of the ADLC bulk films. The PEO
block is well known for its crystallization and strong inter- or intra-
molecular interaction (H-bonds), while the block of aramid
mesogens has the inherent LC properties of self-organization
and long-range ordering,6 as well as H-bonds.8,9 As a result, strong
molecular interactions exist in both of the polymeric constituent
By supramolecular self-assembly, the PEO blocks segregated
into hydrophilic nanocylinders of 9 nm diameter and a 21 nm
periodicity dispersed in a matrix of aramid mesogens, and the
hexagonal packing of the PEO nanocylinders was indicated by the
fast Fourier transform shown in Fig. 3a. Although no external
driving forces were exerted on the sample films, such regularly
periodic arrangements of the nanocylinders were not limited to the
film surface alone; three-dimensional (3-D) arrays were confirmed
by a cross-sectional image (Fig. 3b). The PEO nanocylinders of
140 nm in length are normal to the glass substrate, just the same as
the film thickness, showing a perpendicularly orientated nanocy-
linder array (PONA) with hexagonal packing in the bulk films.
Such well-ordered arrays of nanoscale phase domains can be
regarded as analogous to ‘‘a polymeric single crystal’’, which may
extend to a macroscopic scale. Owing to the AFM resolution, an
image of PONA in an area of 3 mm 6 3 mm is provided in Fig. 3c.
3486 | J. Mater. Chem., 2007, 17, 3485–3488
This journal is ß The Royal Society of Chemistry 2007

In fact, such dotted patterning of PEO nanocylinders can be
perfectly achieved over several centimeters, which have been
confirmed by acquiring AFM images spaced by 100 mm over the
whole substrate.
was detected by AFM after the ADLC films were exposed to room
light for six months (Fig. S3 and S4{).
To check the H-bonds in the annealed block copolymer films
with regular stable PONA structures, in situ FTIR spectra were
measured at different temperatures, which are given in Fig. 2c.
According to the analysis, four possible stages are summarized in
Fig. 4a. Stage (1) shows that strong H-bonds exist among the out-
of-plane aligned aramid mesogens in the annealed copolymer film
at room temperature, which may play an important role in the
stabilization of the microphase-separated nanostructures. Upon
heating, the H-bonds are partly destroyed, leading to a slight
decrease in both absorption peaks at 3340 cm21 and 1646 cm21
(Fig. 2c) prior to the melting, and the alignment of the mesogens
remains in stage (2). When the film temperature is higher than the
melting point but lower than the LC phase-transition temperature,
all of the H-bonds might be destroyed completely, resulting in an
obvious decrease in both of the peaks shown in Fig. 3c. A new
shoulder peak at 1672 cm21 appeared, which is attributed to the
vibration of carbonyl bond in aramid without H-bonds. Due to
the smectic LC ordering, the alignment of the aramid groups was
maintained in stage (3). Evidently, random alignment of the
aramid mesogens was induced because of the loss of LC properties
in an isotropic phase in stage (4), leading to a further decrease in
the peak at 1646 cm21 and a corresponding increase in its shoulder
peak at 1672 cm21. Both the out-of-plane alignment of aramid
mesogens and the formation of H-bonds are reversible upon
cooling the sample films, as shown in Fig. 4a. It is worth
mentioning here that H-bonds might also be formed between the
EO units and the aramid mesogens, and a plausible microphase-
separated structure by supramolecular self-assembly is depicted in
Fig. 4b. Obviously, the introduction of aramid mesogens with
H-bonds in the ADLC not only enables one to obtain macroscopic
regular arrays of PONA without external forces, but also improves
the stability of the self-assembled nanostructures.
To obtain good microphase-separated structures, the annealing
temperature was chosen as 140 uC, slightly higher than the phase-
transition temperature (Fig. 1),4 which acts by destroying the
H-bonds, lowering the viscosity of the block copolymer films and
enabling the microphase separation to proceed completely in an
isotropic state. Furthermore, a wider range of LC temperature was
obtained on cooling than the heating process, permitting the LC
ordering to influence the microphase separation and achieve
regularly ordered nanostructures. In the cooling process, the PEO
domains are confined to the continuous phase of the aramid
mesogens, and mass transport of the PEO domains is prevented.
Contraction of the confined PEO domains might occur on
cooling,11 leading to a dent of several nanometers in the 3-D AFM
image (Fig. 4b). Compared with our previous work,3,4 the stability
of the microphase-separated structures has been improved by
replacing the azobenzene mesogens with the aramid groups, which
have strong H-bonds. No obvious change in the PONA structures
In summary, a novel PEO-based diblock LC copolymer with
aramid mesogens was prepared by ATRP. The bulk films of the
well-defined block copolymer showed macroscopic PONA struc-
tures of PEO phase domains dispersed in the continuous phase of
aramid mesogens, owing to supramolecular self-assembly. Such
periodic nanostructures showed a high stability under room light
because of the existence of successive H-bonds in the matrix. The
obtained block copolymer may function as a reliable nano-
template for restricted chemical or electrochemical reactions to
prepare desirable arrays of nanowires, tube, rods, dots, etc.
Acknowledgements
This work was supported by grants from the Japan Society for
the Promotion of Science (JSPS).
Notes and references
1 S. O. Kim, H. H. Solak, M. P. Stoykovich, N. J. Ferrier, J. J. Pablo and
P. F. Nealey, Nature, 2003, 424, 411.
2 (a) W. A. Lopes and H. M. Jaeger, Nature, 2001, 414, 735; (b)
C. A. Breen, T. Deng, T. Breiner, E. L. Thomas and T. M. Swager,
J. Am. Chem. Soc., 2003, 125, 9942; (c) T. Thurn-Albrecht, J. Schotter,
G. A. Ka¨stle, N. Emley, T. Shibauchi, L. Krusin-Elbaum, K. Guarini,
C. T. Black, M. T. Tuominen and T. P. Russell, Science, 2000, 290,
2126; (d) J. Y. Cheng, C. A. Ross, H. T. Smith and E. L. Thomas, Adv.
Mater., 2006, 18, 2505.
Fig. 4 Schematic illustration of the microphase-separated structures and
the hydrogen bonds in the block copolymer films. (A) Possible scheme of
hydrogen bonds and orientation of the aramid mesogens upon heating. (B)
Three-dimensional AFM image of the block copolymer film and plausible
structures of the microphase separation by supramolecular self-assembly.
This journal is ß The Royal Society of Chemistry 2007
J. Mater. Chem., 2007, 17, 3485–3488 | 3487

3 (a) Y. Tian, K. Watanabe, X. Kong, J. Abe and T. Iyoda,
Macromolecules, 2002, 35, 3739; (b) H. F. Yu, K. Okano, A. Shishido,
T. Ikeda, K. Kamata, M. Komura and T. Iyoda, Adv. Mater., 2007, 17,
2184; (c) H. F. Yu, A. Shishido, T. Ikeda and T. Iyoda, Macromol.
Rapid Commun., 2005, 26, 1594; (d) H. F. Yu, A. Shishido, T. Iyoda and
T. Ikeda, Macromol. Rapid Commun., 2007, 28, 927.
4 (a) H. F. Yu, J. Z. Li, T. Ikeda and T. Iyoda, Adv. Mater., 2006, 18,
2213; (b) Y. Morikawa, S. Nagano, T. Watanabe, K. Kamata, T. Iyoda
and T. Seki, Adv. Mater., 2006, 18, 883; (c) H. F. Yu, T. Iyoda and
T. Ikeda, J. Am. Chem. Soc., 2006, 128, 11010; (d) H. F. Yu, S. Asaoka,
A. Shishido, T. Iyoda and T. Ikeda, Small, 2007, 3, 768.
6 (a) D. Deus, J. Goodby, G. W. Gray, H.-W. Spiess and V. Vill,
Handbook of Liquid Crystals, Wiley-VCH, Weinheim, Germany, 1998,
vol. 3, pp. 66; (b) T. Ikeda, J. Mater. Chem., 2003, 13, 2037.
7 R. Ruhmann, J. Ru¨bner, G. Rodekirch, V. Zschuppe, R. Gnauck,
M. Netopilik and D. Wolff, Acta Polym., 1990, 41, 492.
8 T. Yokozawa, M. Ogawa, A. Sekino, R. Sugi and A. Yokoyama, J. Am.
Chem. Soc., 2002, 124, 15158.
9 T. Kato, N. Mizoshita and K. Kishimoto, Angew. Chem., Int. Ed., 2006,
45, 38.
10 (a) M. Fasolka and A. Mayes, Annu. Rev. Mater. Res., 2001, 31, 323; (b)
F. Bates and G. Fredrickson, Phys. Today, 1999, 52, 32.
11 G. Reiter, G. Castelein, P. Hoerner, G. Riess, A. Blumen and
J. U. Sommer, Phys. Rev. Lett., 1999, 83, 38.
5 M. Walther and H. Finkelmann, Prog. Polym. Sci., 1996, 21,
951.
3488 | J. Mater. Chem., 2007, 17, 3485–3488
This journal is ß The Royal Society of Chemistry 2007