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

Abstract

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.