I. Leppänen: Inherent and Tailored Properties of Cellulose - A Versatile Toolbox for Materials Engineering

Abstract: 

Cellulosic materials exhibit significant potential to support the transition towards a circular bioeconomy, providing solutions to major challenges, such as plastic pollution, and resource sufficiency. Cellulose displays attractive properties, including high mechanical strength, biodegradability, and a broad chemical modifying capacity, further intensified in cellulose's nanoscaled forms due to their hierarchical structure and large surface area. This doctoral thesis sourced inspiration from these inherent features and aimed to exploit them in novel material solutions. The first target of this thesis was to investigate the effect of different chemical modifications on these properties focusing on mechanical strength and biodegradability. We showed that both of these properties for the different cellulose derivatives decreased with an increase in the degree of modification. The biodegradability was significantly decreased already at a degree of substitution of one (three being the maximum). These results underlined the importance of controlled modification to retain the inherent properties of cellulose, which we intended to exploit in novel material solutions. This approach was first exemplified using the inherent hygroscopicity and porosity of cellulose nanofibril (CNF) assemblies. We showed that these features, which generate water transport functions, could be utilized to capture nano- and microplastic particles smaller than 1 µm, which is considered the most challenging colloidal plastic fraction. Furthermore, as we introduced novel analytical methods for nanoparticle detection, we were able to bridge the significant methodological gap related to the collection and detection of submicron plastic particles. Next, the high surface reactivity of cellulose nanocrystals (CNCs) was utilized in a controlled manner to tune the attractive interactions between CNCs and silk proteins to demonstrate their suitability for filament formation. Here, we emphasized the detailed characterization of the introduced functionalities using novel analytical techniques and utilized a fresh perspective on determining the degree of substitution on the surface of the crystallites. In addition, using surface-sensitive techniques, we showed that ionic interactions induced sufficient binding between the CNCs and silk, promoting the shear-induced alignment of silk with the CNCs. Finally, CNFs and CNCs were combined in films with varying ratios to tailor the films' properties. With a 50 % CNC content, we could tune the films' optical properties without compromising their mechanical and barrier performance, which are characteristic features induced by cellulose nanofibrils. By exploiting cellulose's inherent and tailored properties and introducing complementary characterization methods for cellulose-based materials, this thesis provides a new angle to redefine the material bioeconomy, paving the way for new avenues for the whole forest sector.

Follow the remote defence: https://aalto.zoom.us/j/68304564882

Opponent: Professor Tsuguyuki Saito, University of Tokyo, Japan 

Supervisor: Professor Monika Österberg,  Aalto University, Department of Bioproducts and Biosystems

Link to electronic thesis: Inherent and Tailored Properties of Cellulose — A Versatile Toolbox for Material Engineering