Chemical and Biomolecular Engineering - Theses
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Item3D printing meets colloidal processing: Hierarchically porous ceramics for bone scaffolds( 2023-08)Hierarchically porous structures in nature, such as bamboo and bone, have remarkable strength and stiffness-to-density properties. Direct Ink Writing (DIW) or robocasting, an Additive Manufacturing (AM) material extrusion technique, is able to synthetically create near-net-shaped complex geometries. In this research, DIW and colloidal processing are synergized to fabricate multiscale porous ceramic scaffolds with tailorable properties. 100-micron scale porosity is produced by the 3D printed scaffold architecture. By applying particle-stabilized emulsions or capillary suspensions as feedstock for printing, a secondary level of smaller porosity is created within the scaffold filaments via soft templating of the oil phase. A third scale of porosity of sub-micron pores is formed by partial sintering of ceramic particles. Several ceramic systems are investigated in this work, namely clay, alumina, as well as hydroxyapatite and beta-tricalcium phosphate (bio-compatible and bio-resorbable ceramics). In Chapter 3, possible criteria for obtaining suitable ceramic paste feedstocks for DIW are developed by studying the relationships between paste formulation, rheological properties and print parameters. The study demonstrates that storage modulus and apparent yield stress from visco-elastic rheological measurements could be indicators of “printability” of the feedstock. This is further strengthened by the results in Chapter 4 of a different material. The particle size, surfactant concentration, oil fraction, and mixing speed are shown to influence the rheological properties, which can be adjusted to improve printing. In Chapter 4, the primary focus is to understand how to control the pore sizes within the filament microstructure. The study shows that by increasing the oil fraction and particle size, but reducing the surfactant concentration and mixing speed, produces larger micropore sizes within the filaments. The pore morphology is also determined to morph from sphere-like pores typical of Pickering emulsions, to more elongated pores of the granular phase-inverted emulsions (referred to as capillary suspensions in later studies), by increasing the oil and/or surfactant concentrations. Next, a potential application of this customizable multi-scale porous structure is explored – as a scaffold for bone tissue regeneration purposes. By varying the two levels of macro- and microporosities, scaffolds with different properties are produced. A comprehensive study of the influence of the different levels of porosity and two micropore morphologies is undertaken. Additionally, the macropore effects from variations in print nozzle diameter and inter-filament spacing are investigated. In Chapter 5, the viability and proliferation of primary human osteoblasts (bone cells) on the scaffolds are studied in vitro, firstly on slip-cast scaffolds for their micropore morphology, then as a 3D construct for cellular interactions with both micro- and macro-porosities. In Chapter 6, the scaffolds’ physical properties, such as strength and elastic modulus under compression and bending, are investigated, as well as how they change after undergoing degradation (simulating resorption in the body). Finally, in Chapter 7, all the results from the studies are discussed as a whole, concluding that this reported process of DIW of colloidal ceramic feedstocks is a promising strategy for highly porous bone tissue engineering scaffolds. Additionally, it offers a high level of customization of mechanical as well as biological properties.
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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.
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Item3D printing of flexible and efficient polymeric piezoelectric energy conversion materials( 2020)The unique capability of piezoelectric materials to convert between mechanical and electri-cal energy holds tremendous potential in enabling a range of emerging applications. Pol-ymers, as soft and biocompatible materials, are excellent candidates for the use in power-ing wearable and implantable electronics, as well as for the primary sensing mechanism in soft robotic interfaces. However, piezoelectric polymers are sparsely utilised due to their chemical and structural complexity, and the tremendous energetic cost to maximise their energy conversion efficiency. Fluoropolymers have piezoelectric figures of merit rivalling those of the widely used ceramics and are therefore promising to investigate. The common processing techniques for fluoropolymers revolve around solution casting from toxic, hazardous, and/or high boiling point solvents, which require lengthy solvent evaporation times and arduous post-processing by electrical poling, applying high electric fields to align the dipoles. Recent advances in three-dimensional (3D) printing show promise in order to process fluoropolymers into piezoelectric devices, inducing shear forces on the polymer chains during extrusion toward greater alignment and tailored architectures. In this work, pathways to improving the piezoelectric output of a fluoropolymer, poly(vinylidene fluoride-co-trifluoroethylene) (PVDF-TrFE) were thoroughly investigat-ed. The solvent evaporation-assisted (SEA) 3D printing technique was adapted to printing fluoropolymers, investigating the effects of layer-by-layer deposition on the optical, pol-ymorphic and electromechanical properties. In combination with 3D printing, two classes of nanoscale additives were further investigated, single-walled carbon nanotubes (SWCNTs) and transition metal carbides (MXenes), to elucidate their role in the evolution and alignment of the piezoelectric polarisation. The first part of the thesis focused on the development and optimisation of 3D printing capabilities for fluoropolymers. A binary solvent mixture was optimised by Hansen sol-ubility parameters and the rheological properties were thoroughly probed to optimise the polymer concentration. The effects of printing parameters were further investigated in or-der to minimise spreading of the resultant ink post-printing. The polymers, 3D printed up to 19 layers were transparent and exhibited piezoelectricity, with minimal changes in the electroactive phase fraction and without electrical poling. These results confirmed that shear stresses impart partial polarisation on the extruded materials, and provided a strong foundation for the further studies investigated in this thesis. The second study of this thesis critically investigated the effects of the incorporation of SWCNTs, as a nanoscale additive, into the PVDF-TrFE coupled with the developed 3D printing process. The composites were printed as single-layer films, found to be transpar-ent at carbon nanotube loadings up to 0.05 wt%, with low haze. The piezoelectric proper-ties were investigated through two techniques, piezoresponse force microscopy (PFM) and bulk electromechanical characterisation, finding the greatest enhancement in piezoelec-tric properties at a 0.02 wt% loading of the SWCNTs from both techniques. Molecular dynamics (MD) simulations of the carbon nanotube interface with the polymer confirmed a polarisation enhancement effect, providing the first report of polarisation in the absence of electrical poling. Furthermore, the composites were found to be recyclable in pure ace-tone, a green and low boiling point solvent, allowing the printed piezoelectric polymers to be reprinted, with minimal changes in the chemical, physical and electroactive properties. The final part of this thesis utilised two-dimensional (2D) MXene nanosheet additives as a model non-piezoelectric system to deduce and provide the first report on the mechanism of physical polarisation locking in the PVDF-TrFE, building on the knowledge of the first two studies. The composites were printed directly from acetone as a physical gel, allowing for a faster solvent evaporation rate and therefore improved crystallisation kinetics. MD simulations found a suppressed electroactive phase fraction of the polymer directly adja-cent the surface of the additive, confirmed experimentally by Raman microscopy and dif-ferential scanning calorimetry. Furthermore, the MD simulations found the polarisation vector direction was locked perpendicular to the basal plane of the MXene, which was governed by electrostatic interactions. PFM results confirmed the dipole locking phenom-enon, whereby the polarisation magnitude increased logarithmically with an increase in the MXene loading, while demonstrating the transition metal carbide had no discernible out of plane polarisation. Direct piezoelectric effect measurements via macroscale electromechan-ical testing showed that the composite material exhibited a larger piezoelectric coefficient relative electrically poled polymer, implying that physical poling and polarisation locking from a nanomaterial template can impart greater polarisation than standard electrical poling techniques. In summary, this thesis has developed and applied a fundamental understanding of the origins of piezoelectricity in fluoropolymers and how this phenomenon can be controlled at the nanoscale. The implications of this research are far-reaching, enabling commercial viability of piezoelectric materials in a multitude of emerging applications.