Photon Factory Activity Report 2017 #35 (2018)

BL-6A/2018G027

Regular Ordering of Spherical Microdomains in a Dewetted Monolayer Film byThermal Annealing of a Superthin Film of a Triblock Copolymer

Rasha Ahmed Hanafy BAYOMI1,2, Takashi AOKI1, Sono SASAKI1, and Shinichi SAKURAI1,*
Department of Biobased Materials Science, Graduate School of Science and Technology, Kyoto
Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan2
Department of Apparel Design and Technology, Faculty of Applied Arts, Helwan University, Orman,Giza 12111, Egypt
1

1 Introduction
In the block copolymer system, it is well known thatthe dewetting takes place when the film tends to satisfy
the commensurability between the film thickness and thealternating period of the microdomains. Dewetting
phenomena of the block copolymers have been widely
studied for the cylindrical and lamellar systems, but few
studies have focused on the spherical system. In thispresentation, we investigated the dewetting phenomena ofa monolayer film of the block copolymer forming
spherical microdomains. Especially, we focused on thepacking manner of the spherical microdomains.
Fundamentally, it has been reported that the sphericalmicrodomains order on a hexagonal (hcp) lattice for thethickness of the layer up to three or four times larger than
the alternating period of the microdomains [1, 2].
Interestingly, we have found that the ordering manner inthe dewetted monolayer film varies from hcp to deformed
hexagonal. Furthermore, the number of grains in thedewetted film has been examined as a function of the sizeof the dewetted film. We investigated the ordering
regularity of spherical microdomains of block copolymers
in confined spaces (in a dewetted specimen on a Si wafer).
2 Experiment
We used a polymethylmethacrylate-block-poly(nbutylacrylate)-block-polymethylmethacrylate (PMMAPnBA-PMMA; MAM) triblock copolymer, in which
PMMA spherical microdomains are formed in a PnBA
matrix. The volume fraction of PMMA in this specimen
was 0.20. An ultrathin film was prepared by spin coating.
Before annealing, the film exhibits a homogeneous statewith a full coverage on the Si wafer (the film thickness ~15 nm). In order to measure the film thickness before
annealing, we engraved a line in the film using astationery cutter, then we conducted the contact-mode
atomic force microscopy (AFM).
TheSAXS(small-angle
X-rayscattering)measurements were conducted at the BL–6A beamline ofPF, using X-ray with λ= 0.150nm. The sample-to-detector
distance was 2.184m, which was calibrated using achicken tendon collagen having a d-spacing of 65.3nm.
The beam size was measured at the detector position as
0.245mm (an FWHM value) in the vertical direction and
0.498mm (an FWHM value) in the horizontal direction. APILATUS3-1M (DECTRIS Ltd., Baden, Switzerland)
was used as a two-dimensional detector.

3 Results and Discussion
The 1 d-SAXS profile for the MAM specimen, which
was annealed for 5 h at 180 °C under a nitrogen
atmosphere. Many lattice peaks are observed. The
relative peak positions can be assigned as 1: √2: √4: √5:√6: √7, exhibiting the bcc lattice. It is striking to observethe √7 lattice peak for the direct confirmation of the bcc
symmetry because the √7 lattice peak should not appear
for the simple cubic system. Furthermore, the lack of the√3 reflection peak is remarkable, which is ascribed to thefact that its peak position completely matches with theposition of the destructive interference of the particle
scattering. From this relationship, we can evaluate theradius of the spherical microdomains by modeling theparticle scattering function for the sphere, because it onlydepends on its radius (R). The particle scattering function
with R = 8.9 ± 1.25 nm can meet the extinction conditionof the √3 reflection peak.
After five hours thermal annealing at 180°C, thedewetting took place, forming islands with a height ~22nm. AFM revealed many spherical microdomains
regularly ordered in the island. The Fourier transform(FT) pattern clearly displays the six spots. In order todiscuss the ordering manner of the sphericalmicrodomains in the dewetted islands, the FT patterns ofmany islands were examined in details. The packing
manner of the spheres is examined in terms of the island
shape (degree of circularity of the island). The islands
having a higher circularity (mostly complete round shape)
exhibit a well-ordered arrangement of the sphericalmicrodomains mainly ordering on a symmetric hexagonallattice. According to Black et al., over time the defects in
the islands diffuse towards the island’s perimeter andvanish, resulting in a well-ordered structure, which attests
to the validity of our results. On the contrary, the islands
having less circularity exhibiting a deformed hexagonallattice, forming more than one grain (polygrain state)
inside of one island (not showing the result in thisabstract). The polygrain state could be a result of theislands coalescence.
* shin@kit.ac.jp