Birefringence in spin-coated films containing cellulose nanocrystals

Layer-by-layer (LbL) self-assembled polyelectrolyte multilayer films containing cellulose nanocrystals and poly(allylamine hydrochloride) were prepared by spin-coating. From 6 to 25 bilayers were deposited on a silicon substrate. The films displayed birefringence as a result of the intrinsic shape and optical anisotropy of rod-like cellulose nanocrystals which were oriented by the spin-coating process. The birefringence varied with thickness and with location relative to the spin axis, ranging from completely isotropic at the spin axis to 0.065 at the film edge. Uniformity and thickness of these composite films were measured by ellipsometry, atomic force microscopy and angle- and wavelength-dependent optical reflectometry. In order to circumvent the difficulties of inferring film thickness and birefringence in anisotropic thin films, a novel approach based on angle-dependent optical reflectometry was used to determine directional refractive indices.