Rheological Characterization of Cellulose Nanomaterials for Quality Control and Processing
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Cellulose nanomaterials (CNM), have emerged as promising sustainable materials that are produced from renewable sources, such as wood and plant materials. The uniquely high strength and low density of CNM make them promising candidates to replace petroleum-based materials in a wide range of applications such as rheological modifiers, composites and hydrogels. The commercialization of CNM is hindered by its limited production capacity; a significant challenge for scale-up is the lack of robust, rapid and reliable quality control method to ensure product reproducibility and quality. In this thesis, we explore the use of rheology as a method to characterize cellulose nanomaterials. Detailed preparation and test protocols are developed to obtain reliable and reproducible rheological results for aqueous suspensions of cellulose nanocrystals (CNC) and TEMPO oxidized cellulose nanofibril (TEMPO-CNF). We develop a rheological model that accurately describes the flow curves across the full range of the shear rates for all tested concentrations for both CNC and TEMPO-CNF. This model can estimate the concentration of an uncharacterized CNM sample, which greatly reduces the testing time by performing a viscosity test compared to the conventional gravimetric oven-drying test. The model was also used to define a rheological ‘flow index’ parameter for TEMPO-CNF in the viscoelastic regime to describe and compare the effects of morphology and surface charge more effectively. The flow index of all homogenized samples collapse onto a single curve when plotted against cumulative homogenization energy, exhibiting a power law scaling, which can be used for quality control and/or benchmarking of cellulose nanomaterials, and which can guide the selection of optimum processing conditions. We also explore extending the preparation and test protocols for viscosity measurements of CNC suspensions with a Brookfield viscometer, which is commonly used for the industrial quality control. In addition to the quality control, the dewater/drying process is another key barrier to the commercialization of CNM materials. CNM materials are typically produced in aqueous suspension with high water content, which requires subsequent water removal step to reduce transportation cost or as a pre-processing step for applications such as rheological modifiers and composites. In this thesis, reverse dialysis is developed as an alternative dewatering method for CNM that avoids common dewatering issues like irreversible aggregation and sample heterogeneity. The redispersibility in a dewatering-redilution cycle is quantified by viscosity, and the redispersibility of the TEMPO-CNF dewatered via reverse dialysis is much better than via rotary evaporation. The method can also be applied to dewatering of CNC, chitin nanofiber and composites of polyvinyl alcohol (PVA) and TEMPO-CNF. Concentrated and well-dispersed PVA/TEMPO-CNF composite gels can be obtained at component concentrations that are difficult to achieve by other methods. Reverse dialysis thus increases the processing range for these sustainable nanomaterials while preserving their beneficial morphological properties.