- Title
- Competition in polymer brushes: understanding specific ion effects in complex environments
- Creator
- Johnson, Edwin Christopher
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2021
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- The conformation of polymers in solution is governed by the balance of inter and intramolecular interactions between the polymer and all components of the solvent alongside entropic considerations. Polymer brushes are surface architectures comprising end tethered chains at an interface with high areal grafting density. The behaviour of polymers in the brush regime vary to those in solution resulting from the reduced translation energy and increased excluded volume interactions between tethered chains. Responsive polymers are those whose conformation can be modified by the application of a stimulus such as temperature and pH. Multi-stimulus responsive polymers can alter their conformation with two or more stimuli. The extent to which multi-stimulus responsive polymers are affected by a given stimulus is dependent on polymer properties such as configuration, hydrophobicity, molecular weight as well as their architecture. Despite extensive development of responsive polymers, a lack of clarity in the language used to describe their behaviour is apparent. Copolymerisation is a common route through which multi-stimulus responsive polymers are synthesised. For copolymers, composition and segment configuration (block versus statistical) influence the overall behaviour. Often there is significant interplay between the responses to each stimulus by these polymers; that is, one stimulus may modulate the response to another. Despite extensive work developing multi-stimulus responsive polymers, a paucity still exists in our understanding of the balance of forces which govern the extent to which a polymer will be responsive to a given stimulus. In addition to the polymer properties, the behaviour of neutral polymers and polyelectrolytes in a solvent is often dictated or modified by the presence of electrolytes. Importantly the electrolyte identity determines the nature and magnitude of its effect on the polymer. Despite the presence of these ‘specific ion effects’ in many natural and industrial systems, a predictive theory which accounts for the wide variety of effects is missing. What is more, an understanding of how specific ion effects manifest in complex multi-component systems is lacking. Investigations into the influence of specific ion effects in complex systems is essential in order to exploit or understand the role of these phenomena in applications. This thesis presents original research in two sections. The first section investigates how composition affects the interplay of temperature and pH on a multi-stimulus responsive copolymer brush. Ellipsometry is used to investigate the overall responsive behaviour of each composition while neutron reflectometry is used to measure the polymer volume fraction normal to the substrate. Advances in modelling methods of neutron reflectometry data and numerical self-consistent field theory of multi-stimulus responsive copolymer brushes is presented. The interplay between the stimuli of the multi-stimulus responsive copolymer brushes changed as the composition was varied. At low and high pH values, thermoresponsive behaviour was lost at higher contents of the pH responsive monomer. Interestingly only a relatively low content of the weak polyelectrolyte was required to dominate the responsive behaviour of the copolymer (~ 30 mol%. Neutron reflectometry revealed the presence of a prominent depletion region in the polymer volume fraction profiles. Theory calculations indicated that these depletion regions were likely the result of segment enrichment of one monomer near the surface which results in localised increase in brush solvation. The second section presents advances in the understanding of the influence of ion identity and concentration on the behaviour of a pH and temperature responsive copolymer brush, and the impact of mixed salt environments on a thermoresponsive polymer brush. In this section ellipsometry, quartz crystal microbalance with dissipation and neutron reflectometry are utilised to understand the changes in brush thickness and structure with changing electrolyte conditions. In the case of the multi-stimulus responsive copolymer, the effect of a given salt on the copolymer brush behaviour was heavily dependent on the pH. For example, thiocyanate ions would destabilise the copolymer brush at low pH while promoting brush swelling at high pH thus indicating how properties of the substrate can influence the ordering of ions in the specific ion series. The effect of binary salt mixtures containing ions with similar characteristics or ions with differing characteristics on a thermoresponsive brush were examined. The impact of two salts with similar characteristics was concentration dependent, with the net effect cooperative or antagonistic. For mixtures of different salt type, the overall behaviour was temperature dependent. This research presents a significant contribution to the understanding of the interplay of stimuli in multi-stimulus responsive polymer brushes and the forces which govern brush structure. Additional polymer brushes are used as exemplar systems to investigate how specific ion effects manifest in complex environments. In each chapter, the balance of competing forces determines the behaviour of a system. Understanding how these competing forces manifest in complex multi-component systems is essential in developing our understanding of the role of specific ion effects in natural systems.
- Subject
- polymer brush; specific ion effects; responsive polymers; neutron reflectometry; complex systems
- Identifier
- http://hdl.handle.net/1959.13/1443789
- Identifier
- uon:42104
- Rights
- Copyright 2021 Edwin Christopher Johnson
- Language
- eng
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