While investigating the film properties by polarized light microscopy, we discovered that for films in a given thickness range, the nanocrystals had self-assembled into a structure known as a parabolic focal conic. Although this texture with its symmetrical array of line defects is well-known for thin layers of smectic and chiral nematic low molecular weight liquid crystals, we think that this is the first time that it has been observed in a solid film. ["Parabolic Focal Conics in Self-Assembled Solid Films of Cellulose Nanocrystals", M. Roman and D.G. Gray, Langmuir, 21(12), 5555-5561 (2005)] It seems remarkable that a simple rod-like species can spontaneously self-assemble into such a regular complex extended structure. We think this may have implications in creating biomimetic structures.
(Right) Polarizing microscope image of parabolic focal conic (PFC) texture observed in a cellulose nanocrystal film. (Left) Calculated orientation of layers in a PFC. For clarity, only every fourth layer is shown. Layer spacing (= half chiral nematic pitch), 1.375 µm.