Advanced polymer membrane development in pervaporation dehydration and lateral flow diagnostics

The work in this thesis concerns the advanced development of polymeric membranes of two types; pervaporation and lateral-flow. The former produced from a solution casting method and the latter from a phase separation. All membranes were produced from casting lacquers. Early research centred on the development of viable membranes. This led to a supported polymer blend pervaporation membrane. Selective layer: plasticized 4:1 mass ratio sodium-alginate: poly(vinyl-alcohol) polymer blend. Using this membrane, pervaporation separation of ethanol/water mixtures was carefully monitored as a function of film thickness and time. Contrary to literature expectations, these films showed increased selectivity and decreased flux as film thickness was reduced. It is argued that morphology and structure of the polymer blend changes with thickness and that these changes define membrane efficiency. Mixed matrix membrane development was done using spherical, discreet, size-monodisperse mesoporous silica particles of 1.8 - 2μm diameter, with pore diameters of ~1.8 nm were incorporated into a poly(vinyl alcohol) [PVA] matrix. Inclusion of silica benefitted pervaporation performance for the dehydration of ethanol, improving flux and selectivity throughout in all but the highest silica content samples. Early lateral-flow membrane research produced a membrane from a basic lacquer composition required for phase inversion; polymer, solvent and non-solvent. Results showed that bringing lacquers to cloud point benefits both the pore structure and skin layers of the membranes. Advancement of this work showed that incorporation of ethanol as a mesosolvent into the lacquer effectively enhances membrane pore structure resulting in an improvement in lateral flow rates of the final membranes. This project details the formation mechanics of pervaporation and lateral-flow membranes and how these can be controlled. The principle methods of control can be applied to the formation of any other flat sheet polymer membranes, opening many avenues of future membrane research and industrial application.

Development of large-scale colloidal crystallisation methods for the production of photonic crystals

Colloidal photonic crystals have potential light manipulation applications including; fabrication of efficient lasers and LEDs, improved optical sensors and interconnects, and improving photovoltaic efficiencies. One road-block of colloidal selfassembly is their inherent defects; however, they can be manufactured cost effectively into large area films compared to micro-fabrication methods. This thesis investigates production of ‘large-area’ colloidal photonic crystals by sonication, under oil co-crystallization and controlled evaporation, with a view to reducing cracking and other defects. A simple monotonic Stöber particle synthesis method was developed producing silica particles in the range of 80 to 600nm in a single step. An analytical method assesses the quality of surface particle ordering in a semiquantitative manner was developed. Using fast Fourier transform (FFT) spot intensities, a grey scale symmetry area method, has been used to quantify the FFT profiles. Adding ultrasonic vibrations during film formation demonstrated large areas could be assembled rapidly, however film ordering suffered as a result. Under oil cocrystallisation results in the particles being bound together during film formation. While having potential to form large areas, it requires further refinement to be established as a production technique. Achieving high quality photonic crystals bonded with low concentrations (<5%) of polymeric adhesives while maintaining refractive index contrast, proved difficult and degraded the film’s uniformity. A controlled evaporation method, using a mixed solvent suspension, represents the most promising method to produce high quality films over large areas, 75mm x 25mm. During this mixed solvent approach, the film is kept in the wet state longer, thus reducing cracks developing during the drying stage. These films are crack-free up to a critical thickness, and show very large domains, which are visible in low magnification SEM images as Moiré fringe patterns. Higher magnification reveals separation between alternate fringe patterns are domain boundaries between individual crystalline growth fronts.

Polymer and metallodielectric based photonic crystals

The bottom-up colloidal synthesis of photonic crystals has attracted interest over top-down approaches due to their relatively simplicity, the potential to produce large areas, and the low-costs with this approach in fabricating complex 3-dimensional structures. This thesis focuses on the bottom-up approach in the fabrication of polymeric colloidal photonic crystals and their subsequent modification. Poly(methyl methacrylate) sub-micron spheres were used to produce opals, inverse opals and 3D metallodielectric photonic crystal (MDPC) structures. The fabrication of MDPCs with Au nanoparticles attached to the PMMA spheres core–shell particles is described. Various alternative procedures for the fabrication of photonic crystals and MDPCs are described and preliminary results on the use of an Au-based MDPC for surface-enhanced Raman scattering (SERS) are presented. These preliminary results suggest a threefold increase of the Raman signal with the MDPC as compared to PMMA photonic crystals. The fabrication of PMMA-gold and PMMA-nickel MDPC structures via an optimised electrodeposition process is described. This process results in the formation of a continuous dielectric-metal interface throughout a 3D inverted photonic crystal structure, which are shown to possess interesting optical properties. The fabrication of a robust 3D silica inverted structure with embedded Au nanoparticles is described by a novel co-crystallisation method which is capable of creating a SiO2/Au NP composite structure in a single step process. Although this work focuses on the creation of photonic crystals, this co-crystallisation approach has potential for the creation of other functional materials. A method for the fabrication of inverted opals containing silicon nanoparticles using aerosol assisted chemical vapour deposition is described. Silicon is a high dielectric material and nanoparticles of silicon can improve the band gap and absorption properties of the resulting structure, and therefore have the potential to be exploited in photovoltaics.