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
Loading...
Files
Date
2011
DOI
Open Access Location
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
Massey University
Rights
The Author
Abstract
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.
Description
Keywords
Biopolymers, Polysaccharides, Structure