Structure-Property Relationships in Novel Electrospun Composites for Advanced Applications

Active polymeric materials that alter shape in response to an external stimulus offer unique avenues for the design and study of dynamic structures. This research focused on developing elastomeric polymer composites with multiple functionalities by exploring the design and properties for various applications including controlled drug delivery and shape memory. The first part of this dissertation describes the fabrication and characterization of a soft, elastomeric polymeric composite with inherent shape memory properties capable of localized, long-term tunable drug release. In Chapter 2, the fibers were loaded with a hydrophilic drug model, Rhodamine B, and embedded within a siloxane-based elastomeric matrix to form a composite, which is critical to regulating water transport from the environment to the fibers to liberate the drug. In vitro drug release studies were conducted in PBS under physiological conditions to evaluate the effect of drug concentration, fiber size, fiber crystallinity, drug loading and the addition of the crosslinked siloxane. The effect of the microstructural properties of the fibrous phase on drug release were explored and tuned through thermal treatment of the composite. The findings from Chapter 2 were then applied to Chapter 3 for the development of a vascular graft with controlled and sustained nitric oxide (NO) releasing capabilities and suitable mechanical properties for the prevention of restenosis. To avoid the unwanted systemic side effects associated with a free radical such as NO, our approach delivered NO locally by supplying it from the vascular graft material. It was found that reducing the tin catalyst used for crosslinking the silicone constituent significantly improved cell viability, however, the NO interacted with the catalyst activity, affecting the silicone crosslinking reaction. The NO-releasing composite was demonstrated to be a strong chemottractant to endothelial cells. The next part of this research focused on the development of a shape memory elastomeric composite featuring thermoplastic fibers imbibed by polyanhydride-based elastomer. It was determined in Chapter 4 that the polyanhydride elastomer is capable of dynamic covalent exchange reactions at elevated temperatures among the network chains that allowed near-complete reconfiguration of the permanent shape in the solid state. Together, these features were combined to create a shape memory elastomer capable of arbitrary programming of both temporary and permanent shapes. The degradation properties of this composite were then studied in Chapter 5 under in vitro conditions, where it was revealed that the degradation rate of the PAH matrix was strongly influenced by the selection of the composition of polymeric fibrous phase. The degradation of this composite was found to occur as a modified surface to bulk degradation, although the PAH by itself erodes heterogeneously. A hydrophilic model drug was incorporated in the fibrous phases and used to study the in vitro controlled release properties of these composites, where drug release correlated with the matrix degradation. The shape memory properties of these polyanhdride-based compositions were also examined. Lastly, Chapter 6 investigated the design, fabrication, and characterization of a polymeric composite composed of oriented semicrystalline polymeric fibers embedded within a crosslinked epoxy matrix. This anisotropy enabled the construction of complex three dimensional geometries featuring latent mechanical programming. Rather than relying on specific molds to manipulate a new shape, this system capitalized on strain conditioning to influence a new structure. Additionally, we found that by compressing the oriented fibers of each ply during composite cure, the composites constructed from such plies exhibited actuation. The shape memory composites studied in this dissertation demonstrated the potential be broadly applicable from drug releasing implants, tailorable degradability, and the self-assembly of complex shapes. Chapter 7 provides some recommendations for future directions

Correlative investigations into advanced silicon and silicon hybrid anode microstructures for high capacity Li-ion batteries

There is a continuing need for global attention to focus on further development of devices to enable efficient energy storage. This must align with a new and stringent renewable energy target of 32 % for the European Union by 2030. Present materials used within Li-ion batteries currently have a limitation on the amount of energy they can store and for a specified duration. In order to advance the capacity of their most advanced cylindrical cells, Tesla, Samsung, LG and Sony at present use a small fraction of silicon in graphite-dominant anodes to overcome issues around volume expansion and to extend operational life. However, to date, no successful commercial product has been reported that contains silicon as the predominant lithium host material. The solutions offered so far in literature involve complex chemical synthesis or intricate processing routes, which are not realistic solutions to produce practical or cost-effective devices. The thesis core is based on innovative approaches to stabilising silicon-based anodes via additives, which can be conveniently synthesised or commercially available and are chemically compatible with the electrode components. This research work reports on the use of metal-organic frameworks, namely UiO-66 and UiO-67, to enhance the electrochemical performance of high-capacity silicon anodes in lithium-ion batteries. This research work also studied other hybrid anode systems, based on silicon-graphene and silicon-tin powders, using conventional formulation approaches to compare with an advanced electrode manufacturing technique. This study demonstrates that certain additives improve the flexural capability and mechanical integrity of electrode materials. These additives extend the durability of silicon anodes to enable extended reversible transfer of Li-ions, and hence enable a longer lifespan of the battery. This study reports the use of high-quality physicochemical characterisation from a variety of experimental techniques to correlate the anode’s microstructure, dynamics and atomic-scale structure with the maintained performance of the battery. Focused ion beam-scanning electron microscopy (FIB-SEM) tomography, in conjunction with impedance spectroscopy and associated physical characterisation, has been employed to capture and quantify key aspects of the evolution of internal morphology and resistance build up within anodes. FIB-SEM tomography has been employed to explore the hierarchical structure of battery electrodes and for diagnosing battery failure mechanisms with high-resolution imaging. This approach will enable us to observe and quantify failures in Li-ion batteries at the electrode level. It is anticipated that this study will influence major improvements in the design of Li-ion battery materials and their processing which in turn positively impact cell performance