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Title: The local free volume and other properties of synthetic and biopolymers : the effect of chemical and modification, matrix composition and environmental conditions
Author: Hughes, David
ISNI:       0000 0004 6059 0591
Awarding Body: University of Bristol
Current Institution: University of Bristol
Date of Award: 2016
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The focus of this thesis is the investigation of changes in the local free volume and related properties in complex polymeric systems of immediate interest to industry using positron annihilation lifetime spectroscopy (PALS) in conjunction with a number of complementary techniques. PALS and differential scanning calorimetry (DSC) were used to investigate the temperature dependence of the local free volume hole size distribution parameters and the thermal transitions in a series of poly( dimethylsiloxane) based elastomers where meta-carborane was incorporated into the elastomer network via two methods. The presence of significant phase separation in the so-called hybrid elastomers is confirmed by DSC and PALS, suggesting that this is not a suitable method for the production of elastomers for industrial application. By cross-linking a single component system, amorphous monomodal elastomers can be prepared that are not susceptible to crystallisation, making them ideal candidates for high-performance elastomers. The features observed in the local free volume distribution parameters from PALS agree well with the transitions measured by DSC, and are shown to be sensitive to the presence of crystallinity due to the coexistence of the rigid and mobile amorphous fractions. Incorporating ortho-carborane in to the hard segment of a model polyurethane as a chain extender is seen to reduce the level of hard segment - soft segment phase separation and promote the evolution of a mixed amorphous phase. The carborane is seen to reduce the level of hard segment ordering via disruption of the formation of hard segment hydrogen bonds, ultimately converting a bimodal/highly heterogeneous free volume environment into a more uniform amorphous phase with reduced local free volume properties. This study suggests that the micro structure, and thus the macroscopic physical properties, of these polyurethanes can be "tuned" via the level of carborane inclusion. The morphology, phase behaviour and local free volume of complex microencapsulation formulations consisting of hydrophobically modified food starch (MFS) and sucrose were studied as formulation composition and water content was varied. Significant phase separation was observed between the MFS and the sucrose, the compositions of the phases were observed to change significantly when part of the blend passes into the rubbery state. The local free volume from PALS, the first such study in a real and multi-component encapsulation matrix, exhibits features seen for much simpler model carbohydrate systems, displaying a complex dependence of blend water content and a reduction in the average hole volume as the sucrose content is increased. It is predicted that the encapsulation performance of these blends will be dictated and limited by the presence of phase separation.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available