Photon Factory Activity Report 2018 #36 (2019)

BL-6A/2017G589

Sub-10 nm Scale Microphase Separation in Star-Block Copolymers Consisting ofMaltotriose and Poly(ε-caprolactone)
Takuya Isono,1,* Nao Kwakami,2 Kodai Watanabe,2 Kohei Yoshida,2 Hiroaki Mamiya,3 andToshifumi Satoh1
1Division of Applied Chemistry, Faculty of Engineering, Hokkaido University
Kita-13, Nishi-8, Kita-ku, Sapporo, 060-8628, Japan
2
Graduate School of Chemical Sciences and Engineering, Hokkaido University
Kita-13, Nishi-8, Kita-ku, Sapporo, 060-8628, Japan
3National Institute for Materials Science1-2-1, Sengen, Tsukuba, 305-0047, Japan
1 Introduction
Microphase separation of block copolymers (BCPs) has
attracted much attention for its application in
nanolithographic applications [1]. Thus, creating BCPs
capable of forming microphase-separated structures withsmaller domain-spacing, especially less than 10 nm, is ofparticular importance. To achieve such smaller domainspacing, the BCP needs to possess both high FloryHuggins interaction () parameter and low degree ofpolymerization (N). Our group has recently been focusing
on the design of the high -low N BCPs by employing
oligosaccharide as one of the block components [2-4]. The
strong hydrophilic character and rigid structure of theoligosaccharide segment enable the oligosaccharide-based
BCPs to microphase separate even at very low N region,
leading to the ordered nanostructures with domain-spacing
around 10 nm. However, the low N of such BCPs (typically,a few thousand) would cause many issues including alower glass transition temperature, lower viscosity, lessfilm formability, and reduced mechanical properties,
which eventually made the BCPs unusable innanofabrication applications.
In this study, we turned our attention to apply star-blockarchitecture to the oligosaccharide-based BCP. Most
interesting feature of (AB)x star-block architecture is thatthey show the same morphology and domain-spacing value
in the microphase-separated structures as the constitutional
AB diblock arm [5]. This means that the domain-spacing
can be fixed despite an increase in the total molecularweight with the increasing arm number. Thus, integratinghigh  BCP molecules into a star-block copolymer could
be a new solution to simultaneously enable the sub-10 nm
scale microphase separation and improve the molecularweight. Herein, we investigated the microphase separation
behaviors of three-, four-, and six-armed star-block
copolymers composed of poly(ε-caprolactone) (PCL) andmaltotriose (MT), i.e., (PCL-b-MT)3, (PCL-b-MT)4, and(PCL-b-MT)6, respectively (Figure 1).
2 ExperimentAccording to our previous synthetic scheme, the starblock copolymers, (PCL-b-MT)x, were synthesized
through three step reactions involving ring-opening

polymerization of ε-caprolactone, end-functionalization,and click reaction [6]. The molecular characteristics of thestar-block copolymers and the linear counterparts (PCL-bMT) are summarized in Table 1.

===Figure 1. Chemical structure of (PCL-b-MT)x star-block
copolymers (x = 3, 4, and 6).
Table 1. Molecular characteristics of PCL-b-MT linear
deblock copolymer and (PCL-b-MT)x star-block
copolymers
Mn,NMRa
Mw/MnbΦMTc(g mol-1)
PCL-b-MT
3500
1.08
0.14
(PCL-b-MT)3
8900
1.100.15(PCL-b-MT)4
11400
1.110.17(PCL-b-MT)6
15600
1.100.17aDetermined by 1H NMR in DMSO-d6. bDetermined by
SEC in DMF containing 0.01 M LiCl using polystyrene
standards. cMT volume fraction (ΦMT) was calculated
based on the known density value for the blocks: 1.15 g
cm-3 for PCL and 1.36 g cm-3 for amylose (instead of MT).
The small-angle and wide-angle X-ray scattering(SAXS/WAXS) measurements were conducted on thepowder samples at BL-6A in PF. The BCP sample was put
into a glass capillary. The SAXS and WAXS profiles wereobtained by 20 °C step during the step-by-step coolingprocess from the molten state (140 °C) to -20 °C with theINSTEC temperature controller (cooling rate, 5 °C min-1;
λ = 1.50 Å; acquisition time, 30 sec), in which each scanPhoton Factory Activity Report 2018 #36 (2019)was started just after holding the desired temperature for 1
min. The GISAXS experiments were conducted on theBCP thin films on a silicon wafer at BL-6A in PF (λ = 1.50
Å; acquisition time, 10 sec).
3 Results and Discussion
To observe the microphase-separated structures in thelinear and star-block copolymers, we performed the SAXS
measurements on the powder samples. Initially, thesamples were heated at 140 °C, then the SAXS profiles
were acquired at this temperature. All the samples showed
the scattering pattern corresponding to the hexagonallyclose-packed cylindrical structure (Figure 2). The domainspacing, d = 2π/q*, was calculated to be 7.9 nm for PCL-bMT, 8.6 nm for (PCL-b-MT)3, 7.5 nm for (PCL-b-MT)4,
and 7.6 nm for (PCL-b-MT)6. These results confirmed thatthe arm number in the star-block copolymers had small
impact on the resulting morphology and domain-spacing.
(a) PCL-b-MT√3q*

140 °C

√4q*

log I

log I

q*

-20 °C
0.5

1.0

1.5

8

2.0

10-1

1214

1618-1q (nm )q (nm )(b) (PCL-b-MT)4q*

140 °C√3q* √4q*

scattering peak at q of ca.0.4 nm-1 newly appeared at below
20 °C, which might be attributed to the crystallineamorphous lamellar for the PCL domain. The emergenceof the scattering peaks due to the PCL crystal (q = 14.2,14.6, 15.6 nm-1 corresponding to [110], [111], and [200]planes) were observed in the WAXS profiles obtained at
below 20 °C. Therefore, in the case of the linear diblock
copolymer, the microphase-separated structure formed in
the molten state was destroyed during the cooling process
because of the rapid crystallization of the PCL block.
Based on the SAXS and WAXS experiments, we found thatthe star-block architecture has the benefit of regulating thecrystallization behavior of the PCL block, leading toordered microphase-separated structures in the bulk state.
Next, we investigated the microphase separation
behaviors in the thin film state by grazing incidence SAXS(GISAXS) measurements. The thin film samples wereprepared by spin-coating the polymer solution onto ahydrophilic silicon wafer followed by thermal annealing at
130 °C for 30 min under vacuum. Figure 3 shows theGISAXS images for PCL-b-MT and (PCL-b-MT)4 as therepresentative examples. The PCL-b-MT thin film showeda featureless scattering profile in the GISAXS image,suggesting the poor ordering even in the thin film state. On
the other hand, all the star-block copolymer thin films
showed scattering patterns corresponding to thehorizontally orientated nanostructures in their GISAXS
images. The difference in the thin film morphology canalso be explained by the crystallization of the PCL block:the rapid PCL crystallization in the linear diblock
copolymer interfered with the microphase separation in thethin film state during the cooling process.

log I

log I

(a) PCL-b-MT-20 °C0.5

1.0

1.5-1q (nm )2.081012141618-1q (nm )Figure 2. Changes in SAXS (left) and WAXS profiles(right) of (a) PCL-b-MT and (b) (PCL-b-MT)4 during thestep-by-step cooling from 140 to -20 °C.
Next, SAXS and WAXS profiles were acquired during
the cooling process from 140 to -20 °C. Figure 2 shows theSAXS and WAXS results for PCL-b-MT and (PCL-bMT)4 as the representative examples. No apparent change
was observed both in the SAXS and WAXS profiles for thestar-block copolymers even after cooling to -20 °C. Thissuggests that the ordered microphase-separated structure
formed in the molten state was preserved even after thecooling. In addition, no evidence of the PCL crystallization
was observed in the WAXS profiles of the star-block
copolymers throughout the cooling process. On the otherhand, for the linear diblock copolymer, the scattering peakat around q of ca. 0.8 nm-1 due to the microphase separation
diminished with decreasing temperature, and eventuallydisappeared at around 0 °C. At the same time, a new(b) (PCL-b-MT)4

Figure 3. GISAXS images of (a) PCL-b-MT and (b) (PCLb-MT)4 thin films annealed at 130 °C for 30 min.

Photon Factory Activity Report 2018 #36 (2019)

4 Conclusion
In the molten state, the star-block copolymers, i.e.,(PCL-b-MT)x (x = 3, 4, and 6), and their linear diblock
counterpart, i.e., PCL-b-MT, showed hexagonally closepacked cylindrical structures with the domain-spacing ofaround 8 nm. Importantly, the morphology and domainspacing were essentially the same regardless of the arm
number. The arm length and MT volume fraction
determined the resulting microphase-separated structures.
Thus, we succeeded in obtaining extremely small
nanostructures from the BCPs with the molecular weightof higher than 10000 g mol-1. Furthermore, we found thatthe star-block architecture regulate the PCL crystallization
behavior of (PCL-b-MT)x, which was beneficial tofabricate the ordered nanostructures in both the bulk andthin film states after the thermal annealing process.
Acknowledgement
We would like to thank the SAXS beamline staffs fortheir kind supports in the experiments at the BL-6A.
References[1] S.-J. Jeong, et al., Mater. Today 16, 469 (2013).
[2] T. Isono, et al., J. Photopolym. Sci. Technol. 28, 635(2015).
[3] I. Otsuka, et al., Macromolecules 48, 1509 (2015).
[4] T. Isono, et al., Macromolecules 51, 428 (2018)[5] K. Ishizu and S. Uchida, Prog. Polym. Sci. 24, 1439
(1999).
[6] T. Isono, et al., Macromolecules 46, 8932 (2013).
Research Achievements
1. T. Isono, et al., Polym. Chem. 10, 1119 (2019).
* isono.t@eng.hokudai.ac.jp