Photon Factory Activity Report 2018 #36 (2019)

BL-6A/2016G612

Lamellar microdomains formed by ABA triblock copolymers
consisting of a main-chain nematic polyester B-block and polystyrene A-blocks
1

Masatoshi Tokita1,* and Maito Koga1
Department of Chemical Science and Engineering,Tokyo Institute of Technology,
Ookayama, Meguro-ku, Tokyo 152-8552, Japan

1 Introduction
Liquid crystalline block copolymers (LCBCPs)
consisting of liquid crystalline (LC) segments haveattracted considerable attention because the self-assembled
structures of the LC segment are easily oriented using anexternal field. A magnetic field can align amorphousmicrocylinders as well as the LC director of the matrix
along the field direction. The microdomain orientation isinduced by the LC orientation via mesogenic units
anchoring parallel to the microdomain interface. Such an
LC anchoring is readily explained for the side-chain typeof LC polymer segments typically adopted in LCBCPs.
The mesogens arrange their long axis parallel to theinterface because they are connected at the end to thesegment’s backbone that stretches perpendicularly to theinterface via the most extended spacers.
We previously reported on LCBCPs using the mainchain-type LC polymers as an LC segment [1,2]. TheLCBCPs were ABA triblock copolymers comprising anLC polyester as the B block connected at both ends topoly(methacrylate) (PMA) A blocks. Although the LCpolyesters had molecular weight distribution with apolydispersity index as large as 2, the BCP with the B blockof a smectic (Sm) LC BB-5(3-Me) polyester formed awell-defined lamellar microdomain structure to display
scattering maxima up to the seventh order in the smallangle X-ray scattering (SAXS) pattern. The LC lamellar
microdomain consisted of an Sm LC with the LC director
perpendicular to the lamellar interface. Such an LCorientation was reasonable for main-chain LC segments
incorporating mesogens within the backbone and agreed
with the tendency of BCP segments extended
perpendicularly to the microdomain interface. However,LC segments with contour lengths longer than the lamellar
thickness must be folded several times to beaccommodated in the lamellar microdomains. When thestem length of the folded LC segment was comparable tothe persistence length, the LC segment could not increase
the number of foldings. Therefore, the segment wasinclined with respect to the lamellae to increase theinterface area [2].
An ABA BCP composed of the B block of a main-chain
nematic HBA10THQ LC polyester connected to the A
segments of poly(styrene) (PS) formed a lamellar structure
at a PS volume fraction of 0.45 [3]. Although the LCsegment was segregated from PS to form lamellae, thenematic director was parallel to the lamellae, indicatingthat the LC segment was elongated along the lamellarboundary. This LC segment orientation was not expectedfrom the tendency of the BCP segments to elongate awayfrom the microdomain interface but could be a feature of aflexible main-chain LC polymer with longer spacers.
HBA10THQ has a longer spacer of decane methylene
connecting two consecutive mesogens in the backbone
than BB-5(3-Me), which has a spacer backbone thatconsists of five carbon atoms.
In this study [4], we synthesized a series of ABA
triblock T5PS-φ copolymers using a main-chain nematicLC HBA5THQ polyester with a pentamethylene spacer.

HBA5THQ with a number average molecular weight (Mn)of 22 k was connected to the PS segments, with Mn ranging
from 3 k to 12 k. The LCBCPs at the PS volume fraction(φ) ranging from 13% to 38% formed lamellar
microdomains. The LCBCP with φ > 34% formed flat
lamellae, with the nematic director parallel to the lamellae.
As φ decreased, the flat lamellae became zigzag at φ = 29%,
and the zigzag lamellae ran perpendicularly to the nematic
director at φ = 13%. In this report, we focus on the lamellar
microdomain morphology of T5PS-φ.
2 Experiment
The T5PS-φ copolymers were prepared by the atomtransfer radical polymerization of styrene at 110 °C using
anisoleandα,ω-di-2-bromoisobutyryl-terminated
HBA5THQ as a solvent and a macroinitiator, respectively[3]. The degree of polymerization (DP) and the number
average molecular weights (Mn,LC and Mn,am) of the central
LCP and amorphous groups were estimated by 1H-NMR
spectra. The PS volume fraction (φ) was estimated on thebasis of the densities. The polydispersity indices (PDIs) ofthe copolymers were obtained by size exclusionchromatography (SEC) (Table 1).
Table 1. Characterization of Polymers
Mn,LCMn,amSample
φ/%/g mol−1/g mol−1
T5PS-13 12.8 22100
2900
T5PS-29 28.9 22100
7900
T5PS-34 34.4 22100
10200
T5PS-38 38.0 22100
12000

PDI
2.16
1.75
1.88
1.86

Photon Factory Activity Report 2018 #36 (2019)

Small-angle X-ray scattering (SAXS) patterns wererecorded using a Bruker AXS Nano-STAR-U instrument.
Synchrotron radiation (SR) SAXS measurements were
performed at the BL-6A beam line in Photon Factory(Tsukuba, Japan) equipped with a Dectris PILATUS 300KW detector with a camera length of approximately 2 m. The
X-ray wavelength was 0.15 nm. The scattering intensity
was corrected by the transmission and subtraction ofbackground scattering and plotted against the scatteringvector q = (4π sin θ)/λ.
3 Results and Discussion
The LC segment formed a nematic LC as did theHBA5THQ homopolyester [5]. All the fibrous copolymers
aligned the nematic director along the fiber axis to display
the similar wide-angle scattering pattern. However, thecorresponding SAXS patterns exhibited differentazimuthal directions of the maxima among the copolymers.
All the copolymers displayed scattering maxima at
scattering vectors (q) with integer ratios, demonstratingthat the T5PS-φ copolymers formed lamellar
microdomains. However, the normal direction with respect
to the nematic director orientation was dependent on φ.
T5PS-38 and T5PS-34 displayed SAXS maxima
concentrated on the equator perpendicular to the fiber axis
(Fig. 1a and b). The SAXS pattern did not change as φ wasdecreased to 34%. However, T5PS-29 exhibited scattering
maxima azimuthally extended above and below theequator (Fig. 1c), suggesting that the lamellae adopted azigzag conformation. In addition, when φ was reduced to13%, the scattering maxima spot appeared at a larger
azimuth from the equator (Fig. 1d). Such a SAXS
maximum in the quadrant could be ascribed to the lamellae
running in a zigzag perpendicular to the fiber with theaddition of transmitted electron microscopy. These results
demonstrate that T5PS-φ changed the lamellar orientation
with respect to the LC director orientation.

This lamellar structure was affected by the nematic orderin the LC block. The SR-SAXS profiles of T5PS-38 were
measured during heating and cooling at a rate of 2 °C min−1
as they crossed the LC–isotropic liquid transition
temperature (Ti = 182 C) of the LC segment (Fig. 2b). The
profiles include the first- and second-order scattering
maxima, indicating that the lamellar morphology wasconserved during the heating/cooling cycle. The first-ordermaximum at q = 0.3 nm−1 shifted toward a smaller q = 2.5
nm−1 as the temperature increased from 180 to 200 °C andthen returned to its original position with a subsequent
temperature decrease. The estimated d0 from the first-orderscattering maximum was plotted against the temperature(Fig. 2c). The lamellar spacing increased the isotropizationof the LC segment reversibly from 24.2 to 28.5 nm. A
similar reversible increase in d0 was observed forhomologous copolymers with different LC spacer
lengths.[3]Fig. 1: SAXS patterns of fibrous T5PS-38 (a), T5PS-34(b), T5PS-29 (c), and T5PS-13 (d) samples. The fiberaxis is vertical.
T5PS-38 formed lamellae parallel to the fiber axis sothat the nematic director was parallel to the lamellarboundary. In the LC lamellae, the LC segments should beelongated along the nematic director. Fig. 2a depicts aplausible conformation of the LC segment accommodated
in an LC block lamella. An LC segment with the contour
length of 102 nm primarily stretches along the nematic
direction but folds to meander through a 13-nm-thick LC
lamella and bridges the PS lamellae. In the amorphous
block, the PS segment could extend along the boundary
because both segments must occupy the same interface
area to form a flat lamellar boundary.

Fig. 2: (a) Schematic of lamellar microdomains inT5PS-38. (b) Sets of SR-SAXS profiles of the nonoriented T5PS-38 copolymer recorded during heating
and cooling at a rate of 2.0 °C min−1. (c) Lamellar
spacing determined by the SAXS profiles againsttemperature.

Photon Factory Activity Report 2018 #36 (2019)

References[1] M. Hayashi et al., Polymer 178, 121555 (2019).
[2] M. Koga et al., Macromolecules 47, 4438 (2014).
[3] K. Sato et al., Macromol. Chem. Phys. 214, 1089(2013).
[4] M. Koga et al., Polym. J. 51, 295 (2019).
[5] M. Tokita et al., Polymer 52, 5830 (2011).
* tokita.m.aa@m.titech.ac.jp