Angular‐Independent Photonic Pigments via the Controlled Micellization of Amphiphilic Bottlebrush Block Copolymers

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. In this article, porous microparticles with such a structure are produced in a single, scalable step from an amphiphilic, low molecular weight bottlebrush block copolymer (290 kDa). 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 that can be tuned to reflect across the visible spectrum. 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.

Self-assembly of colloidal particles into amorphous photonic crystals

“…6c and d). 123,[136][137][138] Using amphiphilic bottlebrush block copolymers (BBCPs) as surfactants, the controlled swelling of the reverse BBCPs micelles was induced by the confinement of toluene-water microdroplets to make the self-assembled droplets comparable in size to the visible light wavelength. With the evaporation of toluene, shortrange ordered porous structures with self-assembled water droplets as templates were formed.…”

Recent Advances in Block Copolymer Self‐Assembly for the Fabrication of Photonic Films and Pigments

“…So far, this review has primarily focused upon photonic structures formed from self-assembled lamellar architectures. However, the recent discovery of photonic glasses formed from the micelle-driven assembly, [142] as discussed in Section 3.2, shows that structures templated from other phases can also offer interesting optical properties. [10] Focusing upon lamellae formed from LBCPs, the observed color in reflection can be tuned by the domain spacing.…”

“…[168] Recently, such a correlated photonic structure was obtained using BBCPs in the confined environment of a microdroplet to produce photonic pigments. [142] A low MW BBCP with a DP of 72, poly(I-PS)-b-poly(N-PEO), was used to prepare an inverse photonic glass. In this case, the small molecular weight of the BBCP cannot produce structural coloration in the visible range when forming a lamellar structure.…”

“…Adapted under the terms of the CC-BY license. [142] Copyright 2020, The Authors, published by Wiley-VCH. and a photonic glass are considered in Figure 18.…”

“…Block copolymer-based photonic systems have already been demonstrated to be compatible with additive manufacturing methodologies, such as ink-jet and 3D printing, [76,101,113,126] or for direct application as photonic pigments. [111,142,162] In general, the applications envisioned for BCP-based photonic materials center around two strategies: i) large-area coloration and ii) smart stimuli-responsive devices. As a colorant, BCPs could not only be employed as a continuous film but also in the form of a pigment within a formulation (e.g., in paints and cosmetics), with the latter envisaged in either platelet or spherical geometries.…”