Characterization of Fiber Alignment and Mechanical Properties of Printed Cellulose Nanofibril Films

Abstract

Cellulose nanofibrils (CNFs) are a naturally abundant polymer with exceptional mechanical properties for their low density. Neat CNF materials have been reported with moduli ranging from 4 to 86 GPa, where the variation in moduli results from several preparation parameters, one of which is the fiber alignment. Because of their high aspect ratio (>100), CNFs form an entangled network in the absence of mechanisms for fiber alignment. In this study, the alignment of CNF fibers in films is achieved via control of printing and drying processes used to manufacture neat CNF films from aqueous suspensions containing low volume fractions of CNFs. The alignment of the CNFs is determined both globally and locally within printed CNF thin films and the effect of orientation on mechanical properties is characterized. Polarized light microscopy is used to characterize the orientation of CNFs through the bulk of the material (i.e., over areas >4 mm²) and shows that propagation of drying fronts can significantly impact alignment. The alignment of CNFs at the surface of the materials is imaged and quantified via atomic force microscopy (AFM). Both topographic and phase imaging, as well as different image processing techniques were evaluated for alignment characterization via AFM.