Double ordered layers within mierophase-separated lamellar structure of double liquid crystalline side-chain type block copolymer

Article abstract of DOI:10.1246/cl.2008.1174

A new hierarchical nanostructure was created by self-assembly of a double liquid crystalline side-chain type block copolymer. The block copolymer was successfully prepared via a nitro-xide-mediated living free radical polymerization and formed double ordered layers within a phase-separated lamellar structure of the cast film. Copyright

Full text of DOI:10.1246/cl.2008.1174

1174
Chemistry Letters Vol.37, No.11 (2008)
Double Ordered Layers within Microphase-separated Lamellar Structure
of Double Liquid Crystalline Side-chain Type Block Copolymer
Rina Maeda,1 Teruaki Hayakawa,ꢀ1 Masatoshi Tokita,1 Masa-aki Kakimoto,1 and Hiroaki Urushibata2
1Department of Organic and Polymeric Materials, Tokyo Institute of Technology,
2-12-1-S8-26 Ookayama, Meguro-ku, Tokyo 152-8552
2Department of Electrical and Electronic Engineering, Tokyo Institute of Technology,
2-12-1-S3-22 Ookayama, Meguro-ku, Tokyo 152-8552
(Received September 2, 2008; CL-080836; E-mail: hayakawa.t.ac@m.titech.ac.jp)
A new hierarchical nanostructure was created by self-assem-
bly of a double liquid crystalline side-chain type block copoly-
mer. The block copolymer was successfully prepared via a nitro-
xide-mediated living free radical polymerization and formed
double ordered layers within a phase-separated lamellar struc-
ture of the cast film.
Figure 1. Chemical structures of liquid crystalline styrenic
monomers, M1 and M2.
As the size of advanced technology devices such as elec-
tronic, optic, and magnetic devices or biosensors is scaled down,
suitable new organic and polymeric materials with precisely
controlled structures must be developed for these requirements.
Block copolymers (BCPs) are great candidates for these materi-
als because they can spontaneously self-assemble with periodic
nanostructures, of which the size and morphology can be easily
controlled by the molecular weight and composition of copoly-
mers.1 In a variety of BCPs, side-chain type BCPs have advan-
tages for use in these devices because the functionality and phys-
ical properties based on the nanostructures can be tuned by alter-
ing the side-chain moieties of the polymer.2 Furthermore, by in-
troducing liquid crystalline or crystalline molecules as the side-
chain into the blocks, the formation of higher order hierarchical
structures can be created over multiple length scales.3–6 Al-
though numerous side-chain type BCPs with a single side-chain
substituted block have been prepared, little attention has been
paid to synthesize BCPs with different liquid crystalline side-
chains on each polymer block.7 Double liquid crystalline side-
chain substituted BCPs (DLC-BCPs) can form unique hierarch-
ical periodic nanostructures, in which different liquid crystalline
layers are generated within the microphase-separated nanostruc-
tures. Materials incorporating two or more different functional
groups can display not only their own properties independently
but also new properties by combining the different functionali-
ties for a cooperative effect. Herein, we report the synthesis of
novel liquid crystalline diblock copolymers incorporating differ-
ent side-chains in each block via a nitroxide-mediated living free
radical polymerization (NMP) and characterization of their hier-
archical periodic nanostructures by small- and wide-angle X-ray
scattering (SAXS and WAXS).
cal structures of M1 and M2 were characterized by 1H,
13C NMR, IR spectroscopy and elemental analysis, and all of
the data showed the desired monomers were obtained without
a side product.
In order to prepare the BCPs with well-controlled molecular
weight and polydispersity, we applied a nitroxide-mediated liv-
ing free radical polymerization (NMP) to polymerize the mono-
mers obtained using 2,2,5-trimethyl-3-(1-phenylethoxy)-4-
phenyl-3-azahexane (ꢀ-H Unimer) as the initiator.9 The homo-
polymerization of M1 was carried out in the presence of the
ꢀ-H Unimer, acetic anhydride as an accelerator, and o-dichloro-
benzene at 100 ꢁC for 24 h under reduced pressure. The product
was purified by reprecipitation into a large amount of methanol.
Size exclusion chromatography (SEC) indicated a low polydis-
persity index (PDI) of 1.08 and a number-average molecular
weight Mn of 20800 (versus polystyrene standard in tetrahydro-
furan). The polymer obtained P1 was used as the macroinitiator
to prepare the target DLC-BCPs P1-b-P2 as shown in Scheme 1.
The polymerization of M2 was conducted in the presence of P1
under similar conditions to the homopolymerization of M1. IR,
1H and 13C NMR spectroscopy and SEC chromatogram showed
that the desired BCP was obtained with Mn of 48800 and a low
PDI of 1.10 (DLC-BCP1).
To create the desired DLC-BCPs, two kinds of liquid crys-
talline styrenic monomers M1 and M2 shown in Figure 1 were
designed and synthesized.8 The liquid crystalline structures are
composed of typical biphenyl mesogenic groups and alkyl
spacers. The principal difference of M1 and M2 was the order
of mesogens and spacers from the styrene units in the primary
monomer structures, which was expected to induce micro-
phase-separation to produce the desired DLC-BCPs. The chemi-
Scheme 1. Synthesis of DLC-BCPs via a nitroxide-mediated
living free radical polymerization.
Copyright Ó 2008 The Chemical Society of Japan

Chemistry Letters Vol.37, No.11 (2008)
1175
Figure 2. Characterization of DLC-BCP1: (a) SAXS and (b)
WAXS profiles and (c) DSC curves of homopolymers P1, P2,
and block copolymer P1-b-P2 (SB: smectic B, SA: smectic A,
I: isotropic, FS: fluid smectic, Semi-Cr: semicrystal).
Figure 3. Schematic representation of the hierarchical ordered
nanostructure composed of double ordered layers within the mi-
crophase-separated lamellar structure for DLC-BCP1.
Additional DLC-BCPs with different molecular weights
were also successfully prepared (DLC-BCP2 (P1-b-P2), Mn of
33400, PDI of 1.11 and DLC-BCP3 (P2-b-P1), Mn of 56900,
PDI of 1.20).8
are thus attributed to the phase transitions taking place in each
segment. The peaks at 107 and 159 ꢁC correspond to SB–SA
and SA–I transitions respectively, in the P1 segment. The iso-
tropization in the P2 segment appears as an endothermic peak
at 203 ꢁC. The melting of the Semi-Cr in P2 has not been found
possibly because of low crystallinity.
The experimental results presented above indicate the for-
mation of hierarchical ordered nanostructure consisting of semi-
crystalline and smectic layers within a microphase-separated
lamellar structure in the cast film (Figure 3).
We have reported the synthesis of new double liquid crystal-
line side-chain substituted BCPs and their self-assembly to yield
hierarchical nanostructures. The DLC-BCPs formed double
ordered structures within a microphase-separated lamellar nano-
structure. These materials can be further designed and tuned
through the choice of the side-chains to offer very promising
properties for advanced optical, electronic, and biosensors appli-
cations.
The formation of phase-separated hierarchical nanostruc-
tures in DLC-BCP1 was confirmed by small-angle and wide-
angle X-ray scattering (SAXS and WAXS) profiles from a cast
film prepared from a 5 wt % solution of the polymer in chloro-
form on a glass plate and then annealed for 3 h at 185 ꢁC. The
two diffractions observed in the smallest q region in the SAXS
pattern (Figure 2a) are attributed to the lamellar structure com-
posed of stacked P1 and P2 domains with a periodic length of
25.7 nm. The other two diffractions with spacings of 4.8 and
4.0 nm are attributed to the smectic structures within P2 and P1
domains, respectively, because these diffraction peaks can be
observed for the P2 and P1 homopolymers. The WAXS pattern
(Figure 2b) includes the second-order reflections of the smectic
or semi-crystalline structures in each domain and a broad outer
diffraction with a spacing of 0.44 nm characteristic to liquid
crystalline-like spacing of the mesogenic moieties within the
layers.
The DSC thermogram of DLC-BCP1 includes three entho-
thermic peaks, suggesting that the BCP forms thermotropic liq-
uid crystals (Figure 2c). The homopolymers P1 (Mn 16800 and
PDI 1.08) and P2 (Mn 19800 and PDI 1.09) also show endother-
mic peaks at 114 and 165 ꢁC and at 173 and 203 ꢁC, respectively,
in the DSC thermograms. These peak temperatures are compara-
ble to or somewhat higher than the temperatures observed in the
BCP, indicating that each segment phase segregates into discrete
domains. Polarized optical microscopy (POM) and WAXS pat-
terns show that P1 forms smectic B (SB), smectic A (SA) and iso-
tropic phases in order of increasing temperature. While P1 does
not form a crystalline phase, P2 does form a semicrystal (Semi-
Cr) indicated in the WAXS pattern from the sharp and diffuse
diffraction peaks around 2ꢁ ¼ 20ꢁ as well as a sharp reflection
with a spacing of 4.8 nm. With increasing temperature, P2 shows
fluidity and the WAXS pattern includes sharp and diffuse reflec-
tions characteristic to a fluid smectic mesophase (FS) such as SA.
The transition from FS to the isotropic liquid (I) phase was not
fully observed because thermal decomposition begins around
210 ꢁC. The three endothermic peaks observed for DLC-BCP1
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Published on the web (Advance View) October 25, 2008; doi:10.1246/cl.2008.1174