Defence of doctoral thesis: Ratchawit Janewithayapun – Thermoplastic arabinoxylan derivatives: A study of their structure and dynamics

The structure and dynamics of polymers at the molecular and nanometer scale controls how the material responds to stimuli such as heat and strain. To advance the development of polysaccharide-based thermoplastics require that we also further our understanding of their molecular scale properties and how these are altered by modification processes. This thesis aims to understand the mechanisms that allow for a flexible thermoplastic polysaccharide material to be produced via oxidation-reduction and etherification modification of arabinoxylan (AX). The effect of these modifications on the structure and dynamics of linear polysaccharides, such as cellulose and alginate, have been studied but with few experimental measurements of changes at the molecular scale. Characterization of modifications on branched polysaccharides such as AX are even more limited.

AX studied in this work, is a polysaccharide consisting of ß-(1→4)-D-xylopyranosyl repeating units partially substituted with α-L-arabinofuranosyl units, and is a major hemicellulose component in wheat bran. To understand the changes in structure, we utilize small and wide-angle X-ray scattering techniques to characterize solid films and water dispersions. To study the dynamics, we combine thermomechanical characterization, NMR and quasielastic neutron scattering techniques to obtain information across different timescales.

A higher degree of arabinose substitution in AX was correlated with a more extended chain conformation when dispersed in water, and the absence of ordered crystalline regions in the solid films. Oxidation-reduction that results in the ring-opening of the AX does not significantly alter the conformation in water, however, increased chain mobility was observed in the ring-opened solid materials, specifically at the modified positions of the carbohydrate ring. Etherification leads to nanoscale phase separation in the system’s structure, and adds sub-glass transitions to the dynamics. Lastly, when combining oxidation-reduction with etherification, we observed synergistic effects from the mobility of the ring-opened AX chain and the additional motions provided by the side chains.