posted on 2021-01-15, 15:06authored byBonan Zhu, Villads E. Johansen, Gen Kamita, Giulia Guidetti, Mélanie
M. Bay, Thomas G. Parton, Bruno Frka-Petesic, Silvia Vignolini
Cellulose nanocrystals (CNCs) can
spontaneously assemble into chiral
nematic films capable of reflecting circularly polarized light in
the visible range. As many other photonic materials obtained by bottom-up
approaches, CNC films often display defects that greatly impact their
visual appearance. Here, we study the optical response of defects
in photonic CNC films, coupling optical microscopy with hyperspectral
imaging, and we compare it to optical simulations of discontinuous
cholesteric structures of increasing complexity. Cross-sectional SEM
observations of the film structure guided the choice of simulation
parameters and showed excellent agreement with experimental optical
patterns. More importantly, it strongly suggests that the last fraction
of CNCs to self-assemble, upon solvent evaporation, does not undergo
the typical nucleation and growth pathway, but a spinodal decomposition,
an alternative self-assembly pathway so far overlooked in cast films
and that can have far-reaching consequences on choices of CNC sources
and assembly conditions.