Hierarchical structure function models of biopolymer networks : thesis submitted to the Institute of Fundamental Sciences, Massey University, New Zealand, in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Physics, Palmerston North, October 2011

Thumbnail Image
Open Access Location
Journal Title
Journal ISSN
Volume Title
Massey University
The Author
This project aimed to bridge the structure-function divide in polysaccharide networks so that the rheological properties of multi-chain assemblies might be predicted from the ne structures of the constituent polymers and their mode of assembly. The polysaccharide pectin is an important constituent of the plant cell wall and when cured into a gel the mechanical properties of its networks have recently come into the focus of research via extensive microrheological studies, in which interesting connections between the gel's mechanical response, gelation conditions and the pectin ne structure were discovered. This tunability makes it therefore a promising model system for further experiments and computer-aided investigations, and accordingly it is the focus of this thesis. Firstly, a small angle X-ray scattering study of di erent microrheologically wellcharacterized ionotropic pectin gels was undertaken to gain insights into the structures of the assembled elementary network strands. The SAXS results paired with molecular modelling con rm that gels which are semi exible from a microrheological point-of-view contain large bundles of aggregated dimers compared to the more exible networks, where predominantly single chain sections and dimers are found to contribute. These later gels can be formed among other ways using a biomimetic methodology exploiting plant enzymes. Secondly, after learning that networks could be experimentally manifest where single chains form the majority of links between nodes, in contrast to the better known hierarchical structures of polysaccharide gels, a computational approach was pursued to investigate the behaviour of biopolymer networks comprised of single polysaccharide chains using the experimentally measured force extension relation for pectin. This exhibits interesting force-induced conformational transitions that have been investigated in their own right. A 2-dimensional model was initially chosen for practical purposes. The study supports the hypothesis that conformational transitions could have biological signi cance as stress-switches in signalling processes, but that they are unlikely to a ect the bulk rheological properties of tissue. Finally, the model was further expanded into 3-dimensions to test quantitatively its predictions of the shear moduli of such systems. To this end a comparison with rheological prestress experiments on enzymatically induced pectin gels was undertaken. The model was found to successfully describe the observed nonlinear rheology for completely percolated, strong gels, based only on the polymer concentration and an experimentally accessible single chain force-extension relationship; for the rst time providing a true bottom-up example to the properties of soft materials.
Biopolymers, Polysaccharides, Structure