Abstract
Photonic materials with angular-independent structural color are highly desirable because they offer the broad viewing angles required for application as colorants in paints, cosmetics, textiles, or displays. However, they are challenging to fabricate as they require isotropic nanoscale architectures with only short-range correlation. Here, porous microparticles with such a structure are produced in a single, scalable step from an amphiphilic bottlebrush block copolymer. This is achieved by exploiting a novel "controlled micellization" self-assembly mechanism within an emulsified toluene-in-water droplet. By restricting water permeation through the droplet interface, the size of the pores can be precisely addressed, resulting in structurally colored pigments. Furthermore, the reflected color can be tuned to reflect across the full visible spectrum using only a single polymer (Mn = 290kDa) by altering the initial emulsification conditions. Such "photonic pigments" have several key advantages over their crystalline analogues, as they provide isotropic structural coloration that suppresses iridescence and improves color purity without the need for either refractive index matching or the inclusion of a broadband absorber.
Highlights
Photonic materials with angular-independent structural color are highly be uniquely used to obtain non-iridescent structural coloration,[3,4,5,6,7,8,9,10,11] enabling the desirable because they offer the broad viewing angles required for applicabroad viewing angles required for their tion as colorants in paints, cosmetics, textiles, or displays
The discovery of bottlebrush block copolymers (BBCPs), whereby a highly extended backbone is densely grafted with polymer branches, has enabled such architectures to be translated to Synthetic photonic materials typically exploit photonic crystal a larger scale.[18,19]
This has unlocked new functionality, particustructures, where the scattering elements are periodic over long larly in photonics—where BBCPs have been shown to microdistances, with common examples ranging from the top-down phase separate into lamellar structures with domain spacings deposition of alternating dielectrics to produce a 1D multi-layer, large enough to produce structurally colored photonic crystal to the self-assembly of colloidal particles to produce 3D col- films[20,21,22,23,24,25,26,27] and microparticles.[28,29]
Summary
Turns, contributes to a broadened reflection peak. By controlling these features, non-primary colors such as purple, pink, yellow and orange can be successfully achieved (Figure S7, Supporting Information).[4,38]. The greater the amount of water that can be dispersed uniformly via chaotic advection within the toluene phase (independent of droplet size), the greater the reverse micelles will swell, resulting in larger pores upon subsequent microparticle formation This is consistent with the observation that small changes in the homogenization conditions give rise to significant changes in the final color, while increasing the time for diffusion to occur for, by example, slowing the drying time from thirty minutes to three days, results in only a moderate red-shift (Figure S9, Supporting Information). We demonstrate that the controlled swelling of amphiphilic BBCP micelles within an emulsified droplet is a convenient process to directly fabricate angular-independent photonic pigments covering the full visible spectrum Such optical properties are the result of a short-range distribution of pores leading to coherent scattering effects. These scalable photonic pigments offer the broad viewing angles and fade-resistance required for applications ranging from colorants in automotive paints and exterior coatings to use as pixels in reflective displays
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