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Solvent-Responsive and Reversible Structural Coloration in Nanostructured Block Polymer Films

Structural
coloration, the vibrant colors in many biological systems,
results from periodically ordered nanostructures, giving rise to a
photonic band gap in which specific wavelengths are either reflected
or transmitted. In many of the current materials, structural coloration
is a result of the static morphology, and modifications of the materials
are directed toward tuning the reflected wavelength. Recently, excitement
in the ability to create materials exhibiting responsive and reversible
structural coloration for manipulating light at the nanoscale has
been motivated by biological systems that utilize changes in coloration
for camouflage or communication between other animals. Here, we establish
the effect of solvent swelling and deswelling on the photonic band
gap of nanostructured films containing a lamellar-forming diblock
copolymer, poly­(1,2-butadiene)-block-poly­(ethylene
oxide) (1,2PBD-PEO). The large chemical incompatibility between 1,2PBD
and PEO domains (resulting in a high-χ system) allows to strategically
and independently swell either one or both domains while maintaining
the same lamellar morphology. The influence of solvent, leading to
wavelength-specific reflection, entails changes in both the domain
spacing and refractive index. A good solvent for both 1,2PBD and PEO
domains, such as tetrahydrofuran, leads to a prominent increase in
the domain spacing, and as a result, a shifting of the reflected light
to green from an initially colorless and translucent film. For selective
solvents such as water and hexane, only one domain swells (PEO or
1,2PBD domain, respectively), resulting in asymmetric changes in domain
spacing. Additionally, the existence of PEO crystals plays an essential
role in the ability of solvents to swell polymer domains. The structural
and refractive index transformations on swelling with solvents leading
to changes in the reflected wavelength and intensity are found to
be reversible on the evaporation of solvents, enabling cyclic swelling
and deswelling of the nanostructure. The work presented here highlights
the necessary parameters for tuning the photonic band gap properties
in self-assembled polymer materials using a combination of solvent
quality (e.g., degree of polymer domain swelling) and variations in
the effective refractive indices between domains.