Structure of iridescent self-assembled cellulose nanocrystal films

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.

Structure of parabolic focal conic

(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.