Exploring the nonlinear rheological behavior and optical properties of cellulose nanocrystal suspensions

Doctoral thesis, 2023

Cellulose nanocrystals (CNCs), with their versatile properties, offer immense potential in a range of applications, whether used independently or as sustainable reinforcements in polymers. They also find utility as renewable rheology modifiers in industries such as cosmetics, paints, and foods, where precise control over rheological characteristics is crucial for factors like product stability, prevention of splattering, and efficient processing and transportation. To enhance their properties and unlock new applications, surface modification of CNCs is essential. However, studying flow-induced structuring requires the use of accurate and reliable analysis methods, particularly when dealing with fast and large deformations in suspensions, multiphase systems, and composites.
This thesis presents a novel approach for studying the interactions between flow fields and CNCs by investigating nonlinear rheological parameters using a combination of Fourier-Transform rheology (i) and Polarized Light Imaging (PLI) techniques (ii). The utilization of (i) allows for the capture of nonlinear parameters that cannot be obtained through conventional rheological characterization. Concurrently, (ii) provides visual insights into flow-induced CNC structuring and optical properties.
By employing these two distinct techniques, it becomes possible to discern alterations in the microstructure of CNCs, enabling the determination of critical concentrations for phase transitions, percolation, and gelation. To validate the proposed methodology, several different CNC systems were examined, categorized as either (1) self-assembling or (2) non-self-assembling CNC suspensions. These systems varied in terms of surface charge, concentrations, surface modification with azetidinium salts or monovalent counterions, and aspect ratio.
This comprehensive investigation expands our understanding of CNC behavior under flow conditions and offers valuable insights into the rheological properties of CNC suspensions, potentially paving the way for the development of improved materials and applications in various industries.