Macroscopic mapping of the linear in-plane anisotropy of nanocellulosic thin films by Mueller matrix polarimetry

The present work focuses on the characterization of the macroscopic optical anisotropy of cellulose nanocomposite multilayer thin films prepared by grazing incidence spraying. The polarization properties of such films composed of in-plane aligned cellulose nanofibrils embedded in a polyelectrolyte multilayer are investigated by Mueller matrix polarimetry (MMP). Linear birefringence is the predominant polarization property resulting from the unidirectional organization of the cellulose nanofibrils, thus, the differential decomposition of the Mueller matrix measured on the films allows the determination of their actual structural birefringence, <Delta n > = 0.032. Moreover, modeling the sample as an effective medium provides a parametrized expression that describes its linear birefringence as a function of the optical constants of the components, the volume fraction of nanofibrils, the distribution of orientation, and the film thickness. Therefore, MMP provides a fast, simple, and nondestructive method to characterize both the structural characteristics and the optical birefringence/dichroism of the films over large areas, which is of great interest for various optoelectronic applications.