Tuning the Color of Photonic Glass Pigments by Thermal Annealing

Abstract

structure are typically correlated. In general, the domain size, and therefore the reflected wavelength, is largely dependent on the polymer molecular weight, with very long backbones required for reflection at visible wavelengths. [5,6] This simple relationship between polymer "size" and domain spacing has allowed for facile tuning of the reflected color, both via intrinsic (e.g., tuning molecular weight) and extrinsic methods (e.g., swelling particular blocks). [7][8][9][10][11][12][13][14][15] Moreover, transient color change can be achieved by dynamically changing the structure in response to external stimuli. For example, thermochromism can be achieved via heatinduced solvent swelling or dissociation of hydrogen-bonding networks (which both alter the domain spacing), [16][17][18] or domain-selective crystallization (which increases the refractive index). [19] However, to develop the large, well-ordered domain structures required for photonics, thermal or solvent annealing is widely employed to enhance the mobility of the polymer chains and enable structural reorganization. [5,20,21] Recently, it has been shown that bottlebrush block copolymers (BBCPs) can be assembled into microparticles comprising an inverse photonic glass structure. [22,23] In such systems the optical response arises from the coherent scattering between uniformly sized pores with short-range order within a BBCP matrix (rather than being related to the refractive index contrast of the constituent blocks of the BCP). [24,25] As such, a broad spectrum of colored pigments can be achieved using a single BBCP by varying the fabrication conditions alone (e.g., emulsification and drying steps). [22,25] However, at present, the interplay between the processing conditions, in particular thermal treatments, and the resultant structures remains poorly understood, especially between different polymer compositions.In this study, we demonstrate how a "thermal annealing" approach can be applied to photonic BBCP pigments with a porous photonic glass architecture, enabling their coloration to be tuned across the visible spectrum. By comparing biocompatible BBCPs with similar composition but different thermal behavior, we show that applying a heat-treatment to an aqueous dispersion of microparticles results in a swelling of the internal pores, which is driven by both a temperature-induced softening of the BBCP (i.e., polymer mobility) and the absence of Thermal or solvent annealing is commonly employed to enhance phase separation and remove defects in block copolymer (BCP) films, leading to well-resolved nanostructures. Annealing is of particular importance for photonic BCP materials, where large, well-ordered lamellar domains are required to generate strong reflections at visible wavelengths. However, such strategies have not been considered for porous BCP systems, such as inverse photonic glasses, where the structure (and thus the optical response) is no longer defined solely by the chemical compatibility of the blocks, but by the size and arrangement of v...

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