Chemical and Biomolecular Engineering - Theses
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ItemExploring additive manufacturing and non-solvent induced phase separation for fluoropolymer membrane production( 2022)The alarming rise in scarcity of fresh and safe drinking water around the world has prompted an increase in search for new materials to fabricate more efficient water filtration membranes. Poly(vinylidene difluoride) (PVDF) is a popular membrane material however its use as long lasting membranes is hindered by fouling, compaction, and caustic damage. The overarching aim of this thesis was to integrate engineering manufacturing with membrane production in order to explore innovative methods for fabrication of flat sheet and patterned fluoropolymer membranes. The first part of this thesis reports on the bulk synthesis of dehydrofluorinated PVDF (dPVDF), catalysed by a reaction of PVDF with ethylenediamine (EDA), which results in alkene moieties along the backbone of the polymer chain, as confirmed by attenuated total reflectance – Fourier transform infrared (ATR-FTIR) and Raman spectroscopies. The second study of this thesis investigated the suitability of dPVDF solutions (prepared in N,N-dimethylacetamide (DMAc), with or without poly(vinyl pyrrolidone) (PVP)) for direct ink writing (DIW). Steady-state rheology confirmed the non-Newtonian and shear-thinning character of these solutions, whereas oscillatory rheology demonstrated superior gelling behaviour of dPVDF solutions with PVP. Additionally, the presence and changes in microstructure of the solutions were studied via modified Cole-Cole plots and Gurp-Palmen plots. Moreover, a stepwise oscillatory shear rate cycling was carried out to simulate the behaviour of the printing inks during DIW, that indicated complete recovery of microstructure in dPVDF ink post-deposition. The third study presents a hybrid process, implying DIW + ex situ non-solvent induced phase separation (NIPS), for the fabrication of flat sheet dPVDF microfiltration (MF) membranes. To produce inks for DIW, the dPVDF was dissolved in DMAc along with a pore-forming agent, PVP (at 5-30 wt%, relative to dPVDF concentration). Membranes were produced by DIW of the inks into continuous wet films - followed by NIPS in deionised (DI) water. The fabricated dPVDF membranes were more hydrophobic (water contact angle, WCA = 115 degrees) than the similarly fabricated PVDF membranes (WCA = 99 degrees) yet had greater equilibrium water content (EWC) and porosity, which correlated to the morphology of the fabricated membranes. The dPVDF membranes with 30 wt% PVP not only demonstrated stability in a caustic environment (1 M NaOH for 90 min), but also had a pure water flux of approximately 4300 LMH, within the range of commercially available PVDF membranes (approximately 6300 – 8100 LMH). Among the many physical methods of improving fouling resistance is the fabrication of patterned membranes, where the patterns provide the functionality of an integrated spacer, thus providing turbulence to the incoming feed. Despite fluoropolymers being common polymers for membrane fabrication, their commercial production into membranes remains dominated by simple casting and solvent phase separation. The final study of this thesis demonstrated a rapid and simple approach to produce patterned fluoropolymer membranes, with profiled surfaces, via immersion precipitation printing (ipP). This study utilised DIW + in situ NIPS in isopropyl alcohol (IPA), followed by ex situ NIPS in DI water. The direct phase inversion of the patterns during membrane production induces a porous morphology. Further, pure water permeability studies were performed on unpatterned membranes and membranes with patterns fabricated with a combination of different fluoropolymer materials (PVDF and synthesised dPVDF). This simple ipP approach showed potential as a viable alternative for the production of fluoropolymer membranes where the complete control over pattern height, fidelity, and shape is required. In summary, this thesis has developed a hybrid approach comprising of DIW + NIPS for membrane production, in which PVDF has been dehydrofluorinated to produce to dPVDF using EDA, and then fabricated into flat sheet membranes. These membranes have shown to be used as microfiltration (MF) membrane with improved caustic resistance relative to commercial membranes. Moreover, ipP has been utilised for the first time to fabricate patterned membranes with varying pattern shapes and heights from PVDF and/or dPVDF. This potentially points towards fabrication of membranes from different materials with variable geometries and tuneable porosities based on solvents, non-solvents, and pore-formers. The implications of this research are far-reaching as it points towards the integration of engineering manufacturing and membrane preservation. These discoveries will form the basis of future work expanding into smart membranes, bespoke membrane form factors, and new filtration applications.