Tuning the Color of Photonic Glass Pigments by Thermal Annealing.

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 voids within the BCP matrix. In this study, a demonstration of how the concept of "thermal annealing" can be applied to bottlebrush block copolymer (BBCP) microparticles with a photonic glass architecture is presented, enabling their coloration to be tuned from blue to red. By comparing biocompatible BBCPs with similar composition, but different thermal behavior, it is shown that this process is driven by both a temperature-induced softening of the BBCP matrix (i.e., polymer mobility) and the absence of microphase separation (enabling diffusion-induced swelling of the pores). Last, this concept is applied toward the production of a thermochromic patterned hydrogel, exemplifying the potential of such responsive biocompatible photonic-glass pigments toward smart labeling or anticounterfeiting applications.