- Title
- Advancing our understanding of the physicochemical behaviour of stimuli-responsive polybasic brushes
- Creator
- Willott, Joshua David
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2016
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- Stimuli-responsive polymeric surface coatings, especially polymer brushes, have attracted great interest for deployment in many industrial and biological applications such as non-stick, anti-fouling and low friction interfaces. Elucidating the nanoscale structure of polymer brush coatings and understanding the physicochemical mechanisms causing their desirable macroscopic behaviour is of great importance. In this thesis I outline advances in our understanding of the physicochemical behaviour of water-soluble, stimuli-responsive weakly basic polyelectrolyte brushes. Weak polybasic brushes consist of ionisable basic monomers and therefore their aqueous solution behaviour is dependent on range of environmental triggers including solution pH and salinity. This thesis also describes the surface-initiated polymerisation of a family weak polybasic brushes using the novel oxygen tolerant activators regenerated by electron transfer (ARGET) variant of traditional atom transfer radical polymerisation (ATRP). This polymerisation technique can be performed in alcohol/water mixtures, at significantly reduced catalyst concentrations, and allows for fine control over resultant brush thickness. The existing polyelectrolyte brush literature is dominated by weak polyacid brushes and so a primary focus of this thesis is to address the paucity of knowledge on polybasic brushes. We investigate the effects of solution pH, salt concentration and salt type on brush solvation and conformation, brush interactions with foreign bodies and internal brush structure and depth profiles. Accordingly, behaviours have been identified that have not previously been predicted theoretically, nor observed experimentally. For example, weak polybasic brush pH-response is not simply the reverse of polyacid brushes as one might initially expect. We find that pH-induced brush swelling and collapse, as studied by in situ ellipsometry and quartz crystal microbalance with dissipation measurements, is directly influenced by monomer hydrophobicity; with brush collapse occurring at faster rates for the more hydrophobic polymers. Moreover, polybasic brushes exhibit substantial specific anion effects, where for polyacid brushes specific cation effects are dominant. Specifically, in the presence of strongly hydrated kosmotropic acetate anions the weakly polybasic brushes remain highly swollen. Yet, for poorly solvated chaotropic thiocyanate anions, significant brush collapse is observed. Polymer volume fraction profiles obtained from neutron reflectometry measurements on the hydrophobic poly(2 diisopropylamino) ethyl methacrylate brush reveal that in the presence of thiocyanate anions, the brush collapses, forming a dense slab of polymer at the substrate surface. While, in the presence of acetate anions the brush is swollen with polymer density gradually decaying out into solution. Numerical self-consistent field simulations display good qualitative agreement with our experimental observations and provide further insight into the influence of polymer hydrophilicity on the pH- and salt-response of weak polybasic brushes as well as to the origin of their specific-ion response. Overall, this research has enhanced the understanding of the physicochemical behaviour of weak polybasic brushes. We discover relationships between solution conditions (namely pH and ionic strength), brush solvation, brush conformation, and polymer hydrophobicity. A detailed experimental knowledge of the response of weak polybasic brushes to pH, salt concentration and specific salt identity has facilitated the development of sophisticated numerical self-consistent field calculations. These simulations are used to describe brush behaviour with the results validated with experimental findings.
- Subject
- polyelectrolyte; polybasic brushes; pH; salt concentration; specific ion; responsive
- Identifier
- http://hdl.handle.net/1959.13/1335929
- Identifier
- uon:27512
- Rights
- Copyright 2016 Joshua David Willott
- Language
- eng
- Full Text
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| Thumbnail | File | Description | Size | Format | |||
|---|---|---|---|---|---|---|---|
| View Details Download | ATTACHMENT01 | Thesis | 16 MB | Adobe Acrobat PDF | View Details Download | ||
| View Details Download | ATTACHMENT02 | Abstract | 140 KB | Adobe Acrobat PDF | View Details Download |