  Structural colors are produced by wavelength-dependent scattering of light
from nanostructures. While living organisms often exploit phase separation to
directly assemble structurally colored materials from macromolecules, synthetic
structural colors are typically produced in a two-step process involving the
sequential synthesis and assembly of building blocks. Phase separation is
attractive for its simplicity, but applications are limited due to a lack of
robust methods for its control. A central challenge is to arrest phase
separation at the desired length scale. Here, we show that solid-state
polymerization-induced phase separation can produce stable structures at
optical length scales. In this process, a polymeric solid is swollen and
softened with a second monomer. During its polymerization, the two polymers
become immiscible and phase separate. As free monomer is depleted, the host
matrix resolidifies and arrests coarsening. The resulting PS-PMMA composites
have a blue or white appearance. We compare these biomimetic nanostructures to
those in structurally-colored feather barbs, and demonstrate the flexibility of
this approach by producing structural color in filaments and large sheets.
