Photoinduced alignment of nanocylinders by supramolecular cooperative motions

Article abstract of DOI:10.1021/ja064148f

A linearly polarized laser beam was used to control nanocylinders self-assembled in an amphiphilic diblock liquid-crystalline copolymer consisting of flexible poly(ethylene oxide) as a hydrophilic block and poly(methacrylate) containing an azobenzene moie

Full text of DOI:10.1021/ja064148f

Published on Web 08/02/2006
Photoinduced Alignment of Nanocylinders by Supramolecular Cooperative
Motions
Haifeng Yu, Tomokazu Iyoda, and Tomiki Ikeda*
Chemical Resources Laboratory, Tokyo Institute of Technology, R1-11, 4259 Nagatsuta, Midori-ku,
Yokohama 226-8503, Japan
Received June 13, 2006; E-mail: tikeda@res.titech.ac.jp.
Molecular and supramolecular orderings have attracted much
attention because of their potential applications in multidisciplinary
fields of nanotechnology.1-12 At a molecular level, chromophore
molecules such as azobenzenes (AZs) are well-known for photo-
induced alignment with their transition moments almost perpen-
dicular to the polarization of the actinic light by repetition of the
trans-cis-trans isomerization cycles,1 and such ordering can be
transferred to other photoinert mesogens by molecular cooperative
motions coinciding with the AZs.2 On the other hand, supramo-
lecular ordered nanostructures self-assembled in block copolymer
films have been widely used as templates or scaffolds to prepare
Figure 1. Chemical structure and microphase-separated scheme of the block
nanomaterials.3,4 In the microphase-separated processes, the supra-
copolymer. Inset of the AFM phase image is the FFT of a small area. Both
azobenzenes and PEO cylinders are perpendicular to the substrate.
molecular function between incompatible phases plays an important
role in the formation of diverse nanostructures such as sphere,
cylinder, double gyriod, and lamellae.3 The orderings of one phase
LC matrix with a periodicity of 15 nm. Although multigrain was
domain might endow the other microphase-separated domain with
some kind of regularity by the supramolecular cooperative motions,
which gives hints to incorporate the photoresponsive AZs into block
copolymers and transfer the molecular ordering to the supra-
molecular level. Therefore, well-ordered nanostructures in the
AZ-containing block copolymers might be obtained by the photo-
control method.
observed in Figure 1, the fast Fourier transform (FFT) image in a
small area indicated a hexagonal packing of the PEO cylinders
perpendicular to the substrates in each grain.
The block copolymer films with a thickness of about 100 nm
were prepared by spin-coating their toluene solution on glass
substrates. After the solvent was removed at room temperature, the
films were irradiated with a linearly polarized beam from an Ar+
laser (488 nm) at an intensity of 100 mW/cm2. The transmittance
(T) of a He-Ne laser at 633 nm with weak intensity as a probe
beam was measured simultaneously during irradiation through two
crossed polarizers with the sample films between them (Figure S6).
Generally, T is defined by T ) sin2(πd∆n/λ), where d is the film
thickness, ∆n is the photoinduced birefringence, and λ is the
wavelength of the probe light. The ∆n of 0.26 was obtained after
irradiation for 1000 s. Such a film exhibited intensive anisotropy
in its polarized UV-vis spectra (Figure S8), indicating that the
homogeneous LC alignment was achieved perpendicular to the
polarization of the actinic light.1 The order parameter (S) of 0.19
at 350 nm is calculated by S ) (A - A|)/(A + 2A|), where A
and A| are the absorbance perpendicular and parallel to the
polarization direction of the actinic laser beam, respectively. Upon
the film being annealed at 140 °C, the S increased to 0.56 owing
to the improvement of LC alignment by lowering viscosity of the
smectic LC phase. No homogeneous alignment in the unirradiated
area was observed in the polarized UV-vis spectra (S ) 0).
The highly ordered AZs showed great influence on the PEO
domains by the supramolecular cooperative motions upon annealing
at an LC temperature, and a perfect regular array of PEO
nanocylinders, corresponding to the photoinduced alignment direc-
tion of the AZs, was indicated as shown in the AFM images (Figure
2a,b). The inset FFT demonstrated a parallel patterning of the PEO
cylinders in the plane of substrate. The periodic nanostructures
might be achieved by the interplay process between the photoin-
duced ordering of AZs and the self-assembly of block copolymers,15
leading to a parallel array of 40 nanocylinders aligned perpendicu-
Generally, the ordered periodic structures in block copolymers
are formed at 5-50 nm scale only by the supramolecular self-
assembly.3 In the past two decades, electric or magnetic field,5
temperature gradient,6 crystallization,7 modified substrate surface,8
shearing,9 solvent evaporation,10 roll casting,11 photoinduced mass
transfer upon holographic irradiation,12 and mixing with homopoly-
mer13 have been explored to control the nanostructures. However,
a perfect parallel pattern of such nanostructures by noncontacted
approaches is still remaining challengeable. Although the introduc-
tion of AZs into well-defined block copolymers has given the
designed materials some interesting properties,14 photocontrol of
the microphase-separated nanostructures by the supramolecular
cooperative motions has never been tried. In this paper, we com-
municate a noncontacted optical method by using a polarized laser
beam to control poly(ethylene oxide) (PEO) nanocylinders in an
amphiphilic diblock liquid-crystalline (LC) copolymer consisting
of flexible PEO as a hydrophilic block and poly(methacrylate)
containing an AZ moiety in the side chain as a hydrophobic LC
segment.
Figure 1 shows the chemical structure of the amphiphilic diblock
LC copolymer used in this paper, which was prepared by a typical
atom transfer radical polymerization.14d When the copolymer was
heated, two phase-transition peaks appeared at 40.1 and 158.4 °C
in its thermogram by differential scanning calorimetry (Figure S4),
corresponding to PEO melting point-smectic phase-isotropic
phase, respectively. A Tg of 79.6 °C was obtained on cooling. Upon
being annealed at 140 °C, the PEO block self-assembled into
nanocylinders with a diameter of about 7 nm dispersed in the AZ
9
11010
J. AM. CHEM. SOC. 2006, 128, 11010-11011
10.1021/ja064148f CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
In summary, we proposed an optical method to control a parallel
patterning of PEO nanocylinders in an amphiphilic diblock LC
copolymer. The PEO nanocylinders are aligned perpendicular to
the polarization direction of the actinic light by the supramolec-
ular cooperative motions between the ordered AZ LCs and the
microphase separation. By the simple and convenient way of
photocontrol, the macroscopic parallel array of nanocylinders can
be easily obtained in an arbitrary area. Furthermore, the noncon-
tacted method might provide a novel opportunity to control the
nanostructures even on curve surfaces.
Acknowledgment. This work was supported by the grants from
the Japan Society for the Promotion of Science (JSPS).
Supporting Information Available: Synthesis and characterization
of monomer and the block copolymer; optical setup for photoinduced
alignment. This material is available free of charge via the Internet at
http://pubs.acs.org.
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