- Title
- Nanostructure of temperature responsive polymer brushes modulated by salt identity
- Creator
- Humphreys, Ben Adam
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2019
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- Interfacial properties of a surface can be extensively modified through the addition of a polymer brush layer; a dense array of end-tethered polymers grafted to a surface. When a polymer brush is prepared utilising stimulus responsive polymers, the surface properties can be tailored to respond, often reversibly, to external stimuli such as temperature, light, pH, solvent or electric fields. While stimulus responsive polymer brushes, as a whole, are attractive for a wide range of biological and industrial applications, brushes that are temperature responsive have gained considerable interest due to their ability to trigger a brush response without the need of any further additives to the system. However, the thermoresponse of these polymer brushes, like many other physicochemical phenomena can be influenced by the concentration as well as the identity of the salt present. Unlike for untethered polymers, research into specific ions effects for thermoresponsive brushes has be sparse. This knowledge is crucial when considering new applications. Throughout this thesis, advances in the understanding of specific ion effects on the entropically driven thermoresponse of polymer brushes tethered to planar substrates and colloidal silica particles are highlighted. The neutral, thermoresponsive polymer brushes were synthesised via the surface initiated-activators regenerated by electron transfer (SI-ARGET) variant of traditional atom transfer radical polymerisation (ATRP). This synthetic method was optimised for each system and performed in alcohol/water mixtures. Furthermore, this method required significantly reduced concentrations of catalyst compared to traditional ATRP, whilst still having excellent control over the uniformity and thickness of the resulting polymer brush. In situ ellipsometry, atomic force microscopy and neutron reflectometry (NR) were used to thoroughly investigate the specific ion effects on the surface properties and internal structure of two thermoresponsive polymer brushes, poly(N-isopropylacrylamide) (PNIPAM) and poly(di(ethylene glycol) methyl ether methacrylate (PMEO2MA), tethered to planar silicon substrates. It was revealed that strongly hydrated anions reduce the temperature of the conformational swollen-collapsed transition while weakly hydrated anions, such as thiocyanate, not only increase the temperature of the transition, but also influence its overall shape. The temperature response and stability of hybrid PNIPAM brush modified colloidal silica particle dispersions was then investigated using dynamic light scattering and small angle neutron scattering (SANS). Experiments were performed as a function of molecular weight and in the presence of an extended range of salts and concentrations. This research revealed a molecular weight dependent transition temperature which was shifted to higher or lower temperatures depending on the salt identity and concentration. More importantly, unlike planar polymer brushes, the presence of a master curve was evident for all salts indicating a similar mechanism operating in each electrolyte. Overall, this research has provided valuable insight into the physicochemical behaviour of thermoresponsive polymer brushes. The influence of the chosen polymer is emphasised along with the impact of brush thickness and surface curvature. Subtle variations in specific ion effects are reported between all systems. The internal structure of the polymer brush layer was also revealed using firstly, NR for planar substrates and secondly, for the first time, SANS for the colloidal dispersions. Vertical phase separation was evident for PNIPAM brushes throughout the swollen-collapsed transition while PMEO2MA brushes simply displayed a gradual transition from an extended dilute conformation at low temperature through to a block-like dense structure at high temperature.
- Subject
- polymer brush; PNIPAM; Hofmeister; thermoresponsive; PMEO2MA; specific ion effects; thesis by publication
- Identifier
- http://hdl.handle.net/1959.13/1407888
- Identifier
- uon:35790
- Rights
- Copyright 2019 Ben Adam Humphreys
- Language
- eng
- Full Text
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| Thumbnail | File | Description | Size | Format | |||
|---|---|---|---|---|---|---|---|
| View Details Download | ATTACHMENT01 | Thesis | 20 MB | Adobe Acrobat PDF | View Details Download | ||
| View Details Download | ATTACHMENT02 | Abstract | 501 KB | Adobe Acrobat PDF | View Details Download |