- Title
- Specific ion effects on stimulus-responsive polymer brushes
- Creator
- Murdoch, Timothy James
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2018
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- Stimulus-responsive polymers are useful components of smart materials as their conformation can be switched reversibly by changing environmental conditions such as pH and temperature. End-grafting these polymers at a high areal density to form a polymer brush allows the switchable modification of surface properties such as adhesion and lubricity. These surface coatings have shown particular promise for biomedical applications such as controlled cell adhesion and drug delivery. In many of these applications the concentration of salt is significant. The concentration and, more importantly, the identity of salt present in solution affects a wide range of phenomena including bubble-bubble coalescence, protein stability and even the surface tension at the air-water interface. These specific ion effects have also been observed in stimulus-responsive polymer brushes, but are poorly understood on a theoretical level. Therefore a comprehensive experimental and theoretical understanding of these effects is required for informed design of smart polymer brushes. This thesis outlines advances in the understanding of the influence of ion identity and concentration on two major classes of responsive polymer brushes: pH responsive weak, polycationic brushes and neutral thermoresponsive polymer brushes. Experimental studies utilise thiocyanate and acetate as representative weakly and strongly hydrated anions respectively. The specific ion responses of the two classes show opposing behaviour, with polycationic brushes exhibiting a lower, and thermoresponsive polymers a higher degree of swelling in increasing concentrations of thiocyanate, for example. In this case, the difference arises from the neutralisation of charge by thiocyanate in the polycationic brushes, while binding of thiocyanate increases the surface charge of thermoresponsive polymers. The polymer hydrophilicity was found to have a significant impact on the magnitude of the specific ion response. Hydrophobic polycationic brushes showed a stronger response due to a greater preference for collapsed structures when neutralised while the specific ion response was greater for hydrophilic thermoresponsive polymers as the polar nature of their surfaces favours ion binding. Neutron reflectometry (NR) was utilised as a powerful technique for determining changes in the structure of the brushes perpendicular to the interface. The diffuse nature of the brush can lead to challenges when analysing reflectivity data from NR. However, development of new analytical techniques allows the volume fraction profile of the studied polymer brushes to be determined with confidence. This has been used to provide direct evidence of the theoretically predicted vertical phase separation of thermoresponsive poly(N-isopropylacrylamide). However, even greater insight is achieved when paired with other techniques. For example, complementary atomic force microscopy measurements show that the ions can affect the mechanical properties of a brush beyond their influence on its overall degree of swelling. Neutron reflectometry was also used to validate an extended numerical self-consistent field (nSCF) theory of polycationic brushes. This theory showed that addition of a single Flory-Huggins parameter related to the strength of hydration of the counterion was sufficient to account for the observed specific ion effects. Overall, this research represents a significant step forward to understanding specific ion effects on stimulus-responsive polymer brushes. These effects could not be directly accounted for by prior studies of equivalent ungrafted polymer in the literature. That is, the geometry of the system has been found to be a significant factor. This work also briefly introduces studies of polymer brushes in mixed salt solutions, as the vast majority of existing studies on all stimulus-responsive materials have been carried out in a single electrolyte. More detailed studies of the impact of mixed salts are required if the real-world performance of stimulus-responsive polymer brushes is to be predicted and understood.
- Subject
- polyelectrolyte; polybasic brushes; poly oligo(ethylene glycol methacrylate); specific ion effect; neutron reflectometry; atomic force microscopy; thesis by publication; pH; salt concentration; specific ion; responsive; thermoresponsive polymer; responsive polymer; polymer brush; lower critical solution temperature
- Identifier
- http://hdl.handle.net/1959.13/1393943
- Identifier
- uon:33636
- Rights
- Copyright 2018 Timothy James Murdoch
- Language
- eng
- Full Text
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| View Details Download | ATTACHMENT01 | Thesis | 11 MB | Adobe Acrobat PDF | View Details Download | ||
| View Details Download | ATTACHMENT02 | Abstract | 242 KB | Adobe Acrobat PDF | View Details Download |