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Title: Electrical transport measurements of Phytantriol mesophases
Author: Brown, Rosemary
Awarding Body: University of Bath
Current Institution: University of Bath
Date of Award: 2017
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Phytantriol is an amphiphilic lipid capable of self-assembly. It exhibits a range of mesophases including a bicontinuous cubic phase, which is stable in excess water. The cubic phase has potential applications in soft matter, such as cubosome carriers for drug delivery, and as a template for creating hard matter, such as its use as scaffolding templates in electrodeposition. Key to these applications is ion transport through the mesophases, which is the subject of this thesis. Ion transport is determined by physical specifics of the mesophases such as nanostructure geometry, domain boundaries, and Debye length. The study of ion transport therefore, can shed light on the mesophases themselves. This thesis presents experimental studies of the conductance of phytantriol mesophases and their lyotropic and thermotropic phase transitions. In addition, a method to investigate how conductance through phytantriol is affected by strain is presented and demonstrated. The conductance of the various thermotropic and lyotropic mesophases, along with their respective transitions, have been compared by continuous conductance measurements with varying hydration and temperature. Simultaneous imaging with cross-polarized microscopy enabled the conformation of transitions to and from optically anisotropic phases. Both discontinuous and continuous changes were observed across transitions, reflecting their structural reconfiguration. Unexpectedly, it is shown that the conductance of the inverse micellar phase can become comparable to the lamellar phase. Also unexpected was the observation of a birefringent hysteretic phase not reported in previous literature, based on studies restricted to the steady state. While these measurements provide a global comparison between mesophases, there are restrictions to interpretation due to the physical complexity of the system. The remainder of the thesis focuses on the Q224 phase, which is stable in excess water and most relevant to known potential applications. Four-probe electrical measurements on a micropore filled with Q224 phase provided accurate conductivity values and show that transport is reduced by a factor of 30-40 compared to bulk solution. The effect of temperature and high electrostatic bias were also investigated. By increasing the temperature, it is shown that a transition to the HII phase can be induced, leading to a switch in resistivity. With large electrostatic bias, if the Q224 phase is applied as an asymmetrical deposit, it is shown that the deposit can be deformed also leading to a different resistance, demonstrating that the resistance can be electrically switched. An experimental set-up was developed in order to investigate the effects of stress and deformation on the ion transport of the cubic phase. A size variable pore is used in order to apply strain to the mesophase material. Upon changing pore size, the cubic phase exhibits transient behaviour showing positive and negative piezoresistance. The negative piezoresistance is up to 4 times greater than the expected resistance due to changing pore geometry. When the pore is stretched slowly by small increments, the behaviour can be compared against an open pore and the data suggest that the meso-phase conducts more with stretching. With further development ion transport and a size variable pore could be used to measure the electro-rheological response of mesophase materials.
Supervisor: Takashina, Kei ; Marken, Frank ; Edler, Karen Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available