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Title: Structural control for the applications of gelators
Author: Nolan, M.
Awarding Body: University of Liverpool
Current Institution: University of Liverpool
Date of Award: 2019
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The self-assembly of a phenylalanine-substituted perylene bisimide (PBI) gelator in aqueous solution triggered using a pH switch is studied in detail. Spectroscopy, rheology and neutron scattering are used as complementary techniques to characterize the anisotropic structures forming in solution as the pH is decreased. We show that the pH-drop results in worm-like micelles and longer fibres being formed. Photocatalytic solutions are prepared by mixing the PBI solutions with a hydrogen evolution catalyst, Pt, and an electron hole scavenger, methanol, to study the photocatalytic potential of the PBI aggregates. We show, through electrochemical and spectroscopic studies, that illumination of PBI solutions results in the formation of the radical anion and dianion reduced species. We provide evidence that the reduced anisotropic PBI aggregates are active photocatalysts for driving the hydrogen evolution reaction. We expand the photocatalysis study to provide further intrinsic comparisons between self-assembled aggregates. A library of amino acid-substituted PBIs has been studied along with a range of solution compositions. We observe a wide range of photocatalytic activity between the different PBIs which can be related to the different packing arrangements. The addition of an anti-solvent also results in aggregation; at the pH values which show low photocatalytic activity, where there is a low concentration of aggregates, the addition of a solvent results in solvent-induced aggregation and a turn on in photocatalytic activity. This provides further evidence that self-assembly should be carefully considered when designing photocatalytic systems. We also show that when the PBI concentration is increased too high the amount of reduced PBI formed in solution reaches a plateau. Above 5 mg/mL PBI, the amount of radical anion formed during illumination remains the same. However, when lowering the PBI concentration, the rate of hydrogen evolution can dramatically increase. We find an optimum of ~150 μmol/g/h H2 for PBI-V at 0.1 mg/mL under white light illumination. Finally, we prepared a selection of gels using pH-switch and solvent-switch gelation methods and investigate their ability to recover their original morphology after shear. The networks of the gels before and after extrusion have been characterised in detail using confocal microscopy and neutron scattering. The solvent-switch gels are comprised of spherical domains which show good recovery after extrusion. On the other hand, the pH-switch gels which are comprised of a homogeneous dense fibrous network become fragmented after extrusion. This work highlights the need to consider the morphology of low molecular weight gels when used for extrusion-based applications such as 3D printing and drug delivery and proposes design considerations when developing new materials.
Supervisor: Adams, Dave ; Cowan, Alex Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral