(718e) Rheology of Cellulose Nanofibrils in a Viscoelastic Network

Cellulose nanofibrils (CNF) are large macromolecular polymers that are biodegradable, sustainably and renewably produced, easily functionalized, and possess a high tensile modulus. Because of these characteristics, CNF have been used in applications ranging from drug delivery to low-cost composites to coatings and films. Whereas polymers are typically modeled as Gaussian chains, CNF are stiffer with a persistence length comparable to their contour length. For composite materials, as an example, this semiflexible structure results in drastic changes to the mechanical moduli at very low volume fractions. In soft matter systems, however, property enhancement depends not only on structure but also the dynamics of various components. For this reason, polymers of various molecular weights and morphologies are commonly blended together to achieve specific performance targets, resulting in materials with complex multi-scale relaxations. Depending on the molecular weights and interactions between blended polymers, many physical phenomena such as tube dilation, reptation, and arm retraction may all play important roles in controlling the rheological behavior of the melt. Whereas much of the literature focuses on blends of flexible species, we focus on how the relaxations of dispersed semiflexible fibers are affected by the viscoelasticity of the continuous phase.

We disperse TEMPO-oxidized cellulose nanofibers into simple Newtonian fluids, such as water, and viscoelastic fluids, such as supramolecular ionic gels, at a variety of volume fractions. At low volume fractions, the fibers do not strongly interact with each other but at higher volume fractions, the CNF form a network so that the material begins to support applied stresses. We characterize these composite materials using steady and oscillatory rheology and relate the bulk moduli to relaxations of the nanofibers. The viscoelasticity of the dispersed phase perturbs the CNF relaxations of individual fibers but does not prevent the fibers from forming a physical network. Comparing the dynamic relaxations of semiflexible fibers in complex environments, our work provides insight into designing and producing biodegradable and renewable composite materials. These materials can be tailored to exhibit multi-scale relaxations by tuning the dynamics of the dispersed and continuous phases.