3 Chemical Science
Nonvolatile Photonic Block Polymer Films Swollen with AcidicLiquid
We report on the nanostructures and optical properties of block copolymer photonic films swollen with a nonvolatile
acid/nonvolatile protic solvent. Photonic films reflecting ultraviolet or visible light were prepared by spin-coating polystyrene-b-poly(2-vinylpyridine) (PS-P2VP) block copolymer (Mn = 121000, PS = 0.60) and immersing the spin-coated films
in 1,3-bis(sulfopropoxy)propane (SA)/tetraethylene glycol (TEG) solutions as nonvolatile acidic solutions. Ultra-smallangle X-ray scattering revealed that the domain periodicity of the PS-P2VP/(SA/TEG) film increases with an increasein the concentration of SA. Moreover, the wavelength of reflected light from PS-P2VP/(SA/TEG) films was found to be
tunable by varying the concentration of SA.
Block copolymers that form periodic, ordered nanostructures (i.e., nanophase separated structures) haveattracted a lot of attention, since they are useful for
preparing high functional materials. One application
example of nanophase-separated block copolymers is
photonic crystals. A photonic crystal is a structural array
where materials with different refractive indices are arranged periodically [1, 2]. The simplest photonic crystal
is a one-dimensional photonic crystal, also referred toas an optical multilayer stack. Lamellar nanophaseseparated structures of block copolymers can act assuch one-dimensional photonic crystals [3]. Recently,
we reported the fabrication of nonvolatile photonic crystal films that reflect light in the near-ultraviolet light tonear-infrared range by immersing the lamellar-formingpolystyrene-b-poly(2-vinylpyridine) (PS-P2VP) block
copolymer in a nonvolatile protic ionic liquid, where theP2VP blocks are selectively dissolved in the ionic liquid[4]. In this study, by immersing PS-P2VP thin films into
a nonvolatile protic liquid containing a nonvolatile acid topartially protonate the pyridyl groups of P2VP, we fabricated nonvolatile, anhydrous photonic films with large
interdomain distance D (> 100 nm) (see also Fig. 1).

To prepare photonic films, PS-P2VP (Mn = 121k, fPS= 0.60) thin films were prepared on a glass or polyimide
substrate by spin-coating. Then, PS-P2VP thin films
were immersed in a nonvolatile protic liquid, i.e., a mixture of nonvolatile sulfonic acid of 1,3-bis(sulfopropoxy)propane (SA) and nonvolatile protic solvent of tetraethyleneglycol (TEG)) at 40°C for 12 h, producing a lightreflecting photonic thin film.
Ultra-small angle X-ray scattering (U-SAXS) measurements were performed at BL-15A2, to determine
D quantitatively. The camera length was 3.6 m with an
X-ray wavelength of approximately 0.172 nm. In all theprofiles (Fig. 2), integer order peaks were observed,indicating lamellar structures. Swelling occurred byaddition of SA/TEG, while the lamellar structure wasmaintained. Furthermore, as the acid concentration
was increased up to around 10 mM, the first order peak
position, q1, of the U-SAXS profile shifted to the lower
q side, which means an increase in D from 114 nm toapproximately 210 nm, as deduced from the relational
expression D = 2π/q1. For the sulfonic acid concentration of 10 mM and above, the peak position became
substantially constant.

Figure 2: U-SAXS profiles of PS-P2VP/(SA/TEG). The concentration of SA in TEG was varied from 0 mM to 30 mM. See also reference [5].

Figure 3: (a) Appearance of PS-P2VP/(SA/TEG). (b) Reflectivity
spectra of PS-P2VP/(SA/TEG). The concentration of SA in TEG
was varied from 0 mM to 30 mM. See also reference [5].

Figure 3a shows the appearance of the PS-P2VP/(SA/TEG) photonic films on a black cloth as background. No visible color appeared from the film swollen
by TEG alone (0 mM) probably because of reflectionin the UV. On the other hand, with an increase in acidconcentration, reflections in the visible regime appearedin the order of blue, green, yellowish green, and red.
Reflectivity measurements were also performed anddemonstrated that the peak wavelength of the reflected
light shifts from the ultraviolet region to the red light region with increasing acid concentration (Fig. 3b). The
wavelength values of the reflected light became substantially constant above an SA concentration higher
than approximately 10 mM, which is in agreement withthe results of U-SAXS.
According to Bragg’s condition, incident light with aspecific wavelength λ from the direction perpendicular
to the film is reflected by the layered structure, where
λ = 2(n1d1 + n2d2) for the alternating multilayer stackof components 1 and 2 [3]. Here, di is the thickness ofcomponent i and ni is the refractive index of componenti. Since the refractive index of a typical organic material is approximately 1.5, λ is approximately 3D, whereD = d1 + d2. λ estimated by reflectivity measurements
and D determined by U-SAXS nearly satisfied the relationship λ ~ 3D, which proves that these films swollen
with SA/TEG are quantitatively 1D photonic crystals reflecting UV/vis light.

In summary, by immersing neat PS–P2VP films in
the nonvolatile liquid containing a protic solvent andsulfonic acid molecules that can protonate the pyridylgroups, photonic films that reflect visible light were prepared. The combination of U-SAXS and reflectivity measurements also confirmed that by varying the degree ofswelling by changing the concentration of sulfonic acid,the interdomain distance of internal structure could betuned in the range of 114–210 nm as well as the wavelength of the reflection light in the range of 340–620 nm.
The findings of this study will facilitate the preparation of
nonvolatile, anhydrous block copolymer photonic films[5].

REFERENCES[1] E. Yablonovitch, Phys. Rev. Lett. 58, 2059 (1987).
[2] S. John, Phys. Rev. Lett. 58, 2486 (1987).
[3] A. C. Edrington, A. M. Urbas, P. DeRege, C. X. Chen,T. M. Swager, N. Hadjichristidis, M. Xenidou, L. J. Fetters,
J. D. Joannopoulos, Y. Fink and E. L. Thomas, Adv. Mater.
13, 421 (2001).
[4] A. Noro, Y. Tomita, Y. Shinohara, Y. Sageshima, J. J. Walish,
Y. Matsushita and E. L. Thomas, Macromolecules 47, 4103
(2014).
[5] A. Noro, Y. Tomita, Y. Matsushita and E. L. Thomas,Macromolecules 49, 8971 (2016).

BEAMLINE
BL-15A2

A. Noro, Y. Tomita and Y. Matsushita (Nagoya Univ.)

Figure 1: Schematic illustration of photonic films swollen with a neat protic solvent or acid/protic solvent. See also reference [5].

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