Nonlinear rheological characterization of cellulose nanocrystals based systems

Licentiate thesis, 2021

In this thesis, cellulose nanocrystals based systems were investigated using advanced rheological characterization techniques, with a particular focus on nonlinear material response functions and Rheo-PLI (Polarized Light Imaging). Cellulose nanocrystals (CNCs) show enormous potential for many applications, either as material on its own or as renewable reinforcement in polymers. In addition, nanocellulose systems have significant potential as sustainable rheology modifiers for e.g. cosmetics, paints, foods, etc, where rheological properties are essential from designing shelf life, avoid splattering and processing/transport efficiency. In this context, surface modification of CNC is essential to enhance properties and seek new applications. Flow structuring of suspensions, multiphase systems and composites also typically require fast and large deformations. Therefore, it is highly relevant to implement proper and precise analyzing methods sufficiently sensitive to flow-induced structuring.
In this framework, the PhD project focuses on investigating the relevance of nonlinear material rheological parameters on flow-field CNC interactions. Two water based CNC systems were characterized in this licentiate.
The first was CNCs with a lower ionic strength able to self-assemble from an isotropic phase with increasing CNC concentration into biphasic and liquid crystalline phases. The second one, CNCs with higher ionic strength, did not self-assemble. Instead, CNC suspensions with a higher sulfate content agglomerate into isotropic and isotropic gel phases. As nonlinear rheological characterization methods Fourier-Transform Rheology (FT-Rheology) and stress decomposition analysis were performed, to our knowledge, for the first time on the CNC systems analyzed. To investigate the dependence of concentration on self-assembled phase transitions in CNC suspensions, the rheological analysis was complemented by Rheo-PLI.