Chemical Modification of Cellulose Nanocrystals: Creating a Novel Toolbox Utilising the Overlooked Sulphate Surface Groups

Doctoral thesis, 2020

The move towards a bio-based economy has created an increasing demand for renewable, sustainably produced materials. For future generations, it is crucial to develop economically, socially and environmentally sustainable materials and processes. Cellulose, as the main component in plant biomass, has been an integral part of society since the dawn of age and still continues to provide new possibilities. The complex hierarchical structure of lignocellulosic materials makes it possible to liberate nano-sized cellulose particles with extraordinary and versatile properties, such as large surface area, transparency and excellent mechanical properties. However, the hydrophilic nature of nanocellulose can cause issues in certain applications, and in other instances it may be desirable to introduce additional properties to the CNCs. This can be achieved by functionalising the nanocellulose surface through chemical modification.

This thesis presents a novel approach to chemical modification of cellulose nanocrystals, by utilising the sulphate half-ester groups that decorate their surface. Cellulose nanocrystals produced by sulphuric acid hydrolysis were functionalised with dialkylamines through a ring-opening reaction with azetidinium salts, as well as through conjugation with dialkyl alkylchloride and dialkyl cyclocarbonate. The impact on thermal and rheological properties of the functionalised CNCs was evaluated and they were also incorporated as reinforcing elements in bio-based composites.

The functionalisation had a significant impact on the thermal stability, improving it by around 100 °C. The functionalised CNCs also exhibited a significantly higher viscosity compared to unmodified CNCs and were prone to network formation at considerably lower solid contents. The conjugation protocol was improved by a more robust synthesis path for the dialkylamine reagents and by shifting from using organic solvents to water, to facilitate scale-up. Incorporation of CNCs into a polymeric matrix resulted in a near three-fold increase in stiffness, depending on matrix, modification and processing techniques used. The modifications also created a stronger interphase between the CNCs and the matrix.