Electronic Thesis and Dissertation Repository

Degree

Doctor of Philosophy

Program

Physics

Supervisor

Dr. Jeffrey Hutter

Abstract

Polyvinyl alcohol (PVA) is a hydrophilic, biocompatible polymer which can be made into physically cross-linked hydrogels by freezing and thawing PVA solution. These hydrogels can be made with anisotropic mechanical properties closely matching those of porcine aorta, making them a promising material for producing artificial heart valves and heart valve stents.

Small- and ultra small-angle neutron scattering has been used to study the structure of isotropic and anisotropic PVA hydrogels at length-scales of 2 nm to 10 μm. By supplementing the neutron data with data from atomic force microscopy, a large range of length-scales have been probed, within which structural changes responsible for bulk anisotropy occur. The gel is modelled as interconnected PVA blobs of size 20 to 50 nm arranged in fractal aggregates extending to micrometers or tens of micrometers. Bulk mechanical anisotropy appears to be due to the alignment of blobs and connections between blobs.

To further understand the connection between structure and bulk mechanical properties, the uniaxial extension behaviour of isotropic PVA hydrogels was modelled using the 3-chain and 8-chain models, and anisotropic versions of the models were developed for modelling the behaviour of anisotropic PVA hydrogels. The mechanical models are compatible with the structural model described above. The models show that the most highly extended strands dominate the entropy and that there are more dominant strands aligned in the direction for which the gel is strongest than in the other directions.

Nanostructures can be used to reinforce materials, such as PVA hydrogels, providing a new method to alter the properties of materials. The spider mite genome was recently sequenced, possibly allowing for spider mite silks with bioengineered properties and new polymer-silk nanocomposite materials. Despite this progress, little is known about the properties of natural spider mite silks. The fibres have diameters of tens of nanometres in comparison to typical spider silk fibres with diameters of several microns. 3-point bending tests were performed with an atomic force microscope to determine the mechanical properties of single spider mite fibres and a new model which accounts for bending, stretching, and an initial tensile stress was developed. Adult and larval fibres have Young’s moduli of 24 ± 3 GPa and 15 ± 3 GPa, respectively. Both adult and larval fibres have an estimated ultimate strength of 200–300 MPa and a toughness of order 9 MJ/m3.

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