Novel Functional Materials from Upgraded Biopolymers : Polysaccharide Aerogels

Aerogel is a highly porous and lightweight solid material that is prepared by replacing the liquid phase of a gel with air in such a way that the three-dimensional polymeric network remains intact in a dry state. Polysaccharides as aerogel-forming materials have attracted attention over the past few years due to their excellent innate properties, such as their non-toxicity, eco-friendliness, and derivation from renewable resources. Therefore, aerogels from polysaccharides have enormous potential for biomedical, pharmaceutical, and food industrial applications. Formation of the gel is the first step towards the aerogel preparation, though not all polysaccharides tend to form a gel in their native form. It is therefore required to make some modifications to their structures. The choice of modification technique is of the greatest importance in developing polysaccharide-based aerogels for food and food-related applications because the modification technique should be safe. In this thesis, galactose oxidase (GaO) was used as a safe and environmentally friendly modification technique to oxidize and cross-link the polysaccharides guar galactomannan (GM) and tamarind seed xyloglucan (XG). Furthermore, the oxidation ability of GaO and the formation of GM and XG hydrogels were studied in the presence of varying quantities (5–25%) of nanofibrillated cellulose (NFC) as a reinforcing agent to obtain composite hydrogels. The liquid phase of the hydrogels was removed using two different drying techniques: lyophilization and supercritical CO2 (sc-CO2) drying, and the relationship of these techniques to the aerogels’ properties, such as volumetric shrinkage, morphology, specific surface area, and mechanical behavior, were also studied. In this thesis, the synchrotron X-ray microtomography (XMT) technique was used as a modern and non-invasive imaging technique to characterize the aerogels for qualitative and quantitative morphological features in relation to their processing conditions. The enzymatic oxidation of GM and XG with and without NFC-reinforcement allowed for the formation of elastic hydrogels. This study showed that the addition of NFC up to 25% did not hinder the enzymatic activity, and in this way, composite hydrogels were obtained. NFCreinforcement significantly increased the elastic modulus (Gʹ) as compared to the corresponding plain GM and XG hydrogels, but this effect depended on the type of polysaccharide (GM or XG). Replacing the liquid phase of the hydrogels with solvent also showed the strengthening effect of NFC in relation to the GM hydrogel matrix. NFC reduces the volumetric shrinkage, which is dependent on the type of solvent. The lyophilization drying technique is more efficient in terms of preserving the hydrogel’s original volume when converting to the aerogel, especially as compared to supercritical CO2 drying, which showed significant volumetric shrinkage during the drying process. However, supercritical CO2dried aerogels displayed mesoporous structures with pores sizes in the range of few nanometers to a few hundred nanometers that resulted in high surface areas up to 330 m/g of such aerogels. In contrast, lyophilized aerogels displayed different morphologies; for example, the size of the pores ranged from 125 to 250 μm, and they exhibited a low surface area (< 10 m/g). The threedimensional structures of the studied polysaccharide-based aerogels were successfully visualized. Quantitative data provided a complete range of pore sizes and pore wall thickness distributions that were dependent on the ice-templating methods and NFC reinforcement. In conclusion, this study revealed that the properties of the studied polysaccharide-based composite aerogels depend on processing parameters, the choice of drying techniques, and the addition of a reinforcing agent. The properties of GMor XG-based aerogels can be tailored to a desired application through the selection of a drying method with carefully controlled processing conditions. XMT proved to be a promising characterization technique for bio-based porous materials for understanding their structural properties in relation to their functional properties.