Effects of Molecular Architecture of Xylans on Material Properties
Doctoral thesis, 2005
Xylans are among the most abundant polymers on earth. Nature has created xylans with different molecular structures in different plant species. This molecular architecture is not a coincidence but is tailored to create a particular desirable functionality. If we were able to understand the effect of the molecular architecture on the material properties, we could create tailor-made xylan-based materials and utilise this immense natural resource in material applications now fulfilled by non-renewable resources. This is of the utmost importance as oil supplies are reducing rapidly and the greenhouse effect is becoming more and more pronounced.
In this work, xylans were isolated from various sources and modified chemically and physically. The material properties were evaluated with a focus on softening behaviour and film formation.
Aspen glucuronoxylan extracted with alkali was deacetylated. It aggregated in aqueous solution and crystallized when cast from water. In contrast, DMSO-extracted aspen glucuronoxylan contained acetyl groups, had a higher water solubility and was totally amorphous. The irregularity introduced with the acetyl groups impedes crystallization. The same phenomenon was observed for arabinose substituents when using barley husk arabinoxylan.
Aspen glucuronoxylan was acetylated to different degrees of acetyl substitution. Acetylation decreased the interactions with water and introduced a glass transition temperature (Tg). When the effect of esterification on the Tg of corn fibre arabinoxylan was evaluated, it was found that Tg decreased with an increasing degree of substitution and side chain length.
It was also shown that xylan has a potential in film applications, such as food packaging. Plasticization of aspen glucuronoxylan with xylitol or sorbitol enabled the formation of continuous films with low oxygen permeability. Barley husk arabinoxylan formed films without the addition of an external plasticizer. An attempt was made to decrease the interactions with water by surface fluorination, at which the films became more hydrophobic with an increasing fluorine content at the film surface.
This research contributes to the knowledge of relationships between the xylan structure and its material properties, and it is my hope that this will facilitate the use of xylan in industrial applications in the future.