Chemical and Biomolecular Engineering - Theses
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Developing methods for assessing concept maps of process safety
Process Safety is a key element of engineering that ensures the rapid human advancements can occur safely. It is important for the people who are already working in the process industries, and for those who will join them in the future. Process Safety education is a mandatory component and should be in the chemical engineering syllabus in every university around the world. A concept map is a graphical representation of information that shows the relationships between concepts. Since its development, it has been used to represent ideas and concepts in a simple and holistic manner. This study investigates whether concept maps may be used to assess learning by individual students and by cohorts of students of the Process Safety domain. To achieve that objective, a method of assessing concept maps through a categorical scoring system is proposed by developing appropriate categorisations for the concepts, links and propositions. The concept categories for the Process Safety domain were developed and validated via a novel process to remove human subjectivity. The process involved defining ten categories into which each of the concepts could be assigned. Several sets of concept maps were analysed independently by three assessors, requiring the assessors to assign every concept into one of the proposed categories. Analysis of the assessors’ responses was aided by presenting their responses in a three-way table. The use of the novel table allowed the assessors’ responses to be compared effectively. This comparison tool enabled identification of problematic categories for further refining. The analysis of the distribution of the concepts, with the help of the proposed Link Quality Index, revealed more information about students’ understanding of the topic. This study analysed different types of connections between concepts to obtain more context and understanding of the concept maps, which represents students’ grasp of the Process Safety knowledge. The students generally appreciated the non-physical preventative measures, such as procedures and maintenance, but they did not recognise the importance of the education, training and values, such as responsibility towards Process Safety. This information is useful in helping to redesign the curriculum. This study also proposes a method that may be applied beyond Process Safety domain; the classification of propositions into one of three proposed attributes, Professional Practice, Values, or Technical Knowledge. The application of this method to engineering students’ concept maps revealed that students show high awareness towards Technical Knowledge. Students also demonstrate the ability to use higher order thinking in explaining the relationship of the behaviour of the Professional Practices and Values. Upon applying this method to nursing students’ concept maps, on the topic of Oxygenation, it is found that nursing students had similar patterns in their awareness of technical attribute. However, nursing students generally were more oriented towards using lower order thinking in explaining relationship between concepts. Overall, this study found that concept maps are a useful method to be applied in Process Safety domain; however, students need to be aware of the importance of having complete and clear propositions which are essential in indicating their understanding.
Nanofabrication of patterned surface structures by controlling surface-initiated and crosslinked ultra-thin polymeric films
This thesis reports the successful development and use of surface-initiated polymer films via ring-opening metathesis polymerization (ROMP) and solid-state continuous assembly of polymers reactions as capable bottom-up processes for nanofabrication of organic and inorganic materials. Chapter 1 presents an overview of existing bottom-up nanofabrication methods. Chapter 1 also highlights the drawbacks of current top-down fabrication methods and the opportunities to expand bottom-up methods to nanofabrication, of which are the focus of this thesis. In Chapter 2, the development of a protocol for solid-state continuous assembly of polymers via ring-opening metathesis polymerization (ssCAPROMP) from silanized substrates is presented. This method expands upon previously reported protocols by utilizing an olefinic silane to form highly crosslinked and tailorable polymer thin films which are covalently attached to surfaces. Polymer films constructed using the new anchoring layer showed similar growth characteristics and film thicknesses when compared with previous studies on CAP film growth, but overcome the high degree of steric hindrance and typical minimal macro-cross linker attachment encountered when previously working with silanized silicon surfaces and CAP processes. Films were also successfully reinitiated multiple times, a key property of the CAP process. Optimization of the protocol was carried out via thorough testing of reaction conditions, offering insight into the reactivity of the SI-ROMP process at play. The protocol was further developed to work on the organic material SU-8, a negative photoresist commonly used in microfluidic devices and bio-microelectromechanical systems (bioMEMS) and could be patterned using two methods: masking of the silanization step and subsequent lift-off prior to initiation and film growth, or through selective surface functionalization via the use of polymeric stamps and micro-contact printing procedures. The study presented in Chapter 2 translates the significant advantages of the solid-state CAPROMP protocol when forming polymer films into a method that can be integrated into the fabrication of robust sensors, bioMEMS or microfluidic devices which require tailorable yet stable polymer thin films. In Chapter 3, the ssCAPROMP protocol developed in Chapter 2 was used as a reactive ink system for a micro/nano 3D printing platform to access spatially defined, crosslinked polymer features. The printing platform used was a modified atomic force microscope whereby a hollow cantilever and an aperture at the tip was used to deliver material from a reservoir through a nanofluidics channel to the substrate. Material delivery was controlled by applying pressure to the reservoir. Utilizing the control over polymer properties offered by CAP, a reactive polymer crosslinker was created that encompassed a number of critical parameters in order to be printed successfully; the ink must undergo rapid crosslinking, it must have a low glass-transition temperature to be printed, and the crosslinking must be of a living nature to enable the printing of multiple layers continuously. The reactive ink could be used without solvent and was delivered through the aperture of the tip onto an initiated surface where it was found to crosslink almost immediately, even in ambient conditions and without requiring an inert atmosphere. This rapid crosslinking enabled the delivery of several layers of which each crosslinked, allowing the build-up of various line heights. A comparison of line heights after washing and drying found that when printing with both the 4 micrometer aperture and the 300 nm aperture, repeated depositions in the same location resulted in an almost quantitative addition of material based on the number of depositions. Overlapping lines were also printed and showed that the height at the overlapped location was the sum of the heights of both lines, highlighting the ability to print on top of existing lines. Experiments showed that by waiting a short period of time between overlapping lines, line heights could be increased as the feature could “cure” before the next deposition. Print directionality was shown to affect line widths based on the contact between the deflected tip and the direction it moves. Structures with three distinct layers were also created, showing that this method of delivering reactive ink and crosslinking in-situ could create three-dimensional patterns. By combining this printing platform with the versatility in ink and polymer chemistry offered by CAP, a robust platform for creating three dimensional structures with a layer resolution of down to 2 nm and a minimum line width down to 450 nm was demonstrated. In Chapter 4, the surface-initiated ring-opening metathesis polymerization of norbornene was used to direct the bottom-up construction of TiO2 and ZnO via atomic layer deposition (ALD). Norbornene monomers were used in the vapor phase to avoid excessive polymerization and cross-metathesis side reactions that occur when using solution-based SI-ROMP. This process afforded surface-bound polymer films in an extremely rapid fashion, with 100 nm films achievable in less than one minute of vapor exposure. The polymer films were exposed to TiO2 and ZnO thermal ALD processes and then analyzed via XPS to observe any inorganic material growth. It was found that 100 nm of surface-initiated polynorbornene could resist ALD of at least 1200 cycles, challenging a current paradigm of using small molecules to prevent ALD deposition. Several norbornene-based monomers were synthesized as surface-binding initiators that were selective for copper and copper oxide over silicon oxide. These initiators were used in conjunction with coplanar and topographical copper features on silicon oxide wafers in order to selectively attach initiator to the patterned features. Once coated, the functionalized features were used to grow polymer in an area-selective fashion and these substrates were subjected to ALD to test the ability of the polymers to perform area selective deposition (ASD). ASD of ZnO was achieved using a hydroxamic acid-based initiator, enabling the deposition equivalent of 38 nm of ZnO before any nucleation was observed on the polymer surface. This was also tested on large areas, where a large Sierpinski’s triangle 300 micrometers across was created on a substrate using e-beam lithography to demonstrate that large scale ASD could be performed. Polymers grown on the copper surfaces showed inhibition of ALD for up to 675 cycles of ZnO. These results show that SI-ROMP is not only an excellent tool for the bottom-up construction of organic materials as is shown in Chapters 2, 3 and 4, but that long macromolecules can be used to drive the patterned bottom-up construction of inorganic films relevant to semiconductor and device fabrication to great effect. Finally, in Chapter 5 we propose several pathways where expansion upon the work contained in this thesis may lead to further advances in the bottom-up construction of organic and inorganic materials for nanofabrication.
Near-net shaping of dense ceramics via freeze-casting
At present, producing dense complex-shaped ceramic parts require time-consuming and labour intensive processes, that are unsuitable for large-scale mass production. This results in prohibitively expensive part costs, in turn limiting the range of practical applications for complex-shaped dense ceramic materials. With process modifications, the freeze-casting process has potential as a suitable near-net shaping process for this application. In freeze-casting, a well-dispersed suspension of particles, such as a dispersion of ceramic powder in a carrier solvent, is frozen in a mould to produce a solid part. The frozen solvent can then be removed by sublimation in a freeze dryer, to produce a ceramic green body. Sintering this part enables us to produce a dense ceramic part retaining the shape of the mould. Current research in the field of freeze-casting elsewhere is primarily focused on tailoring the freezing process, to create solvent-crystals of various morphologies, for use as pore-forming templates in the production of highly porous components. In this thesis, freeze-casting was performed with highly loaded non-aqueous alumina ceramic suspensions, primarily with submicron alumina in cyclohexane carrier solvent, with the end-goal of producing dense ceramic components. Previous progress in this research area has been hindered by the samples exhibiting severe internal cracking. In this thesis the mechanisms and key factors responsible were identified for crack formation within samples, and applied this new understanding to explore selected avenues for mitigating the formation of cracks through changes to the process and to the suspension composition. In this work, the role of each of the primary freeze-casting process steps (freezing, freeze-drying and pressureless sintering) were systematically explored relating to both microstructure development and crack formation. A range of techniques were used to examine internal sample microstructures, including micro computed-tomography (micro-CT) and SEM imaging, with specific attention being given to the effects of freeze-drying conditions on samples. The key conclusions are as follows. By performing dense freeze-casting with highly loaded suspension, and by maximising the freezing rate (using pre-chilled moulds to minimise the templated pore size), it was demonstrated that dense and complex-shaped objects can be formed via this process. The formation of undesirable internal cracks in this process was identified and proven to occur during the freeze-drying step, with cracks being absent in the frozen state, and present prior to sintering. It was found that the rate of sublimation drying influences both the distribution and configuration of the crack network, with fast drying rates (rapid sublimation under vacuum) resulting in both external radial ‘spoke’ cracking and internal discontinuous cracking, while slow drying rates (self-sublimation of samples in a freezer at ambient pressure) resulted in ‘onion-like’ concentric ring cracks. While cracks occur during drying, it was identified that the development of cracks occurs towards the start of drying, and significantly, that crack development occurs independently to the progress of drying and solvent removal. Addition of an acrylic binder to the suspension was shown to significantly increase the mechanical green body strength, and this reduced but did not eliminate cracks. With regards to crack mitigation, the combination of both using slow drying rates, and adding an acrylic binder to the suspension, was demonstrated to successfully mitigate crack formation. Finally, the use of cyclooctane as an alternative carrier solvent also resulted in reduced cracking, confirming that the solvent selection is integral to the effectiveness of the dense freeze-casting process. In this way, the viability of a dense freeze-casting process for rapid production of complex shaped ceramic components was critically evaluated, and potential limitations of this process were identified and considered.
The structural-functional relationship of polymer-surfactant complexes relevant to personal care product
Polymer-surfactant (PS) mixtures are widely used to control both solution and surface properties. The link between the molecular structure of polymers and surfactants and their associative behaviours is of great interest and it is not very well understood. The examination using several different methods in the colloidal systems is to link the function of PS complexes to their microstructure from different aspects. My thesis aims to investigate how oppositely charged PS complexes can affect the interaction and the adhesive force of drops to surfaces and link these attributes to the target functions of a formulation, including shelf life stability and drop deposition or adhesion of an emulsion formulated chemical products, for example, personal care products. This was achieved by using both novel microscopic methods to quantify adhesion interactions and probe the adsorption and microstructure of PS complexes coated on drops and model surfaces as well as correlating these data to macroscopic methods for bulk solution properties. In this work, cellulose based cationic polymer and anionic surfactants, sodium lauryl (or dodecyl) sulphate surfactants were used based on current key ingredients in personal care product formulation. We have studied when drops will stick to surfaces in the presence of PS complexes by systematically varying the components of PS complexes (e.g. polymer type, surfactant concentration and type, and electrolyte concentration) and correlating the observed drop adhesion to hydrophobic surfaces with the phase diagrams of PS complexes. This observed that polydispersity in anionic surfactant headgroup can drive different drop adhesion, which motivated studies on surfactants in the absence of polymer to see how polydispersity of head group affects the micellization of the surfactant by measuring their critical micelle concentration (cmc) as a function of polydispersity degree and added electrolyte as well as the shape and dimension of the micelle using small angle neutron scattering (SANS). These measurements demonstrated that by controlling the degree of polydispersity in surfactant headgroup, the micelle character and their interaction with polymer can be possibility predicted. The measurements of drop adhesion were then compared to the adsorption of the PS complexes in order to explain how the structure of PS complexes on different surfaces can affect drop adhesion. The adsorption of PS complexes onto model surfaces that have more complexity, relevant to skin, hair or textiles were studied by measuring the adsorbed PS layer thickness using AFM imaging as well as force measurements in combination with measures of the adsorbed amount using QCM-D. By combing the observation of the layer thickness and adsorbed mass of PS complexes upon surfactant and electrolyte dilutions, and the effect from surface character, more insights of the mechanism of the structure change of PS complexes is understood.
The electrochemical regeneration of granular activated carbons in situ of permeable reactive barriers
Permeable reactive barriers have proven to be an effective and cost efficient remediation technique for the clean-up of petroleum hydrocarbon contaminated sites in extreme regions such as the Antarctic. The materials within these barriers, namely granular activated carbon, decontaminate migrating groundwaters via adsorption processes and prevent further spread of pollutants into the environment. However, with long operational periods, the activated carbon becomes saturated and is no longer effective at capturing contaminants. In an effort to prevent this, this thesis investigated the possibility of using in situ electrochemical treatments as a means of regenerating the activated carbon in these barriers such that the continuous replacement of saturated material is not necessary. Aqueous phase studies were first conducted to assess which electrochemical reactions aid in the degradation of solubilized petroleum hydrocarbons. Due to the natural presence of chloride and iron at the contaminated sites in the Antarctic and sub-Antarctic, the active chlorine and electro-Fenton pathways were chosen. Similarly, naphthalene, a high priority pollutant for removal in these regions, was chosen as a model compound to investigate the efficacy of the selected reactions. Upon application of an electric current in a near-saturated naphthalene solution, both reaction pathways achieved full contaminant removal within 3 hours of treatment. Further analysis showed that the naphthalene was electrochemically transformed into species of lesser toxicity with minimal energy usage that is appropriate for use in remote regions. Varying operational conditions were assessed to determine the underlying mechanism for which naphthalene was removed, and a dynamic kinetic model was developed for each reaction that could accurately predict treatment outcomes over a range of reagent concentrations, treatment timeframes, and applied electric currents. Due to the success for which the active chlorine and electro-Fenton pathways degraded naphthalene in the aqueous phase, the reactions were applied to naphthalene loaded granular activated carbon to determine the extent of regeneration that could be achieved. Regardless of the reaction applied, only 30 % regeneration could be achieved under any of the regenerative trials conducted, indicating that only the exterior surface of the porous granular activated carbon was likely being regenerated. As the micropores within the activated carbon were essentially unaffected by electrochemical treatments, macroporous or non-porous materials may be better suited for achieving high regeneration efficiencies. Although complete regeneration of the activated carbon was not reached, the developed technology can still prolong the longevity for which granular activated carbon can perform within permeable reactive barriers; over four cycles of treatment, the exterior surface was continually restored and freed up adsorptive sites for further adsorption processes. Thus, an ideal method for applying electrochemical treatments in situ of existing permeable reactive barriers is recommended.
Protein–Polyphenol Networks: From Fundamentals to Biomedical Applications
Naturally occurring building blocks have attracted scientific interest for the assembly of functional materials due to their intrinsic biocompatibility and biodegradability. Proteins are a particularly crucial class of functional biomacromolecules involved in most fundamental processes of living organisms that can be assembled into nanomaterials for various biomedical applications. Another ubiquitous class of biomacromolecules are polyphenols, which have traditionally been referred to as “vegetable tannins”, have recently been employed in engineering advanced materials, owing to their available physicochemical and biological properties and capability of assembly through diverse interactions. This thesis aims to introduce protein–polyphenol networks (PPNs), namely interconnected networks of proteins and polyphenols that can be deposited on a wide array of substrates. The polyphenol-mediated protein assembly of materials such as films, capsules, or nanoparticles (NPs) are introduced in this thesis because self-assembly approaches allow for the rapid generation of tailorable materials under mild conditions. This thesis also focuses on exploring the fundamentals of the interactions between proteins and polyphenols, which helps in understanding the assembly mechanism of PPNs. The binding affinity between polypeptides and polyphenols is studied by analytical chemistry techniques, focusing on the interactions between side chains of proteins and polyphenols, which is crucial for the controllable design of protein-based materials. Then, a straightforward and versatile strategy through interfacial polyphenol-mediated protein assembly is introduced to create a library of functional PPN materials, including bioactive surface coatings and functional capsules. Moreover, the PPN capsules not only can be used to clarify the governing interaction(s) between different proteins and polyphenols, but also can be employed in various applications (e.g., enzymatic catalysis, fluorescence imaging, and cell targeting). Next, a template-mediated supramolecular assembly method is developed to synthesize PPN NPs capable of endosomal escape and subsequent protein release in the cytosol. The versatility of this strategy in terms of NP size and protein type makes this a promising platform for potential applications in protein therapeutics. Finally, the protein–polyphenol interactions related to actual biological environments are investigated by the studying protein corona formed around different polyphenol-modified gold NPs (AuNPs). Protein corona compositional analysis demonstrates the binding preference of serum proteins with various polyphenols, and cellular uptake behaviors of polyphenol–AuNPs can elucidate the role of polyphenols in bio–nano interactions, which can act as reference works for the future implementation of polyphenols in biomedical applications.
Preparation of Nickel-Gallium based catalysts for carbon dioxide hydrogenation to methanol
Abstract Catalytically converting CO2 to methanol by hydrogenation offers a method to effectively reduce the excessive CO2 emission in the atmosphere and produces value-added chemicals simultaneously. Thus, the investigations on catalysts in methanol synthesis reaction has gained attraction in the past few years. Commercial catalysts based on Copper, Zinc, and Zirconium are popular, but increasingly, researchers are looking for other options with superior conversion and selectivity. From prior literature, catalysts based on Nickel and Gallium, specifically a Ni5Ga3 bimetallic catalyst exhibits a similar CO2 conversion and higher methanol yield compared with commercial Copper-based catalysts. However, the purities of Ni5Ga3 catalysts were found to be restricted during the reported synthesis process. Thus, a simpler and reproducible method to prepare highly pure Ni5Ga3 is desirable. In this study, we developed a method to synthesize highly pure Ni5Ga3 catalyst from hydrotalcite-like compounds (HTlc) precursors for CO2 hydrogenation to methanol. A series of Ni-Ga HTlc precursor was synthesized in the temperature range between 90 C and 150 C. The results indicated the HTlc phase in the nickel-gallium precipitant became better crystallized and the structure became more stable as the synthesis reaction temperature increased. Bimetallic alloy Ni5Ga3 was obtained by reducing the as-prepared HTlc precursors in a hydrogen atmosphere. X-ray absorption spectroscopy (XAS) investigation confirmed that a stable and complete HTlc precursor structure assisted in the synthesis of a steady and perfectly structured Ni5Ga3 alloy, where the bond distance of Ni-Ga and cell volume increased with temperature. Ni-Ga HTlc precursor prepared at a hydrothermal temperature of 110 C resulted in the formation of bimetallic alloy, Ni5Ga3, which demonstrated characteristics such as smaller crystal size and stable structure under optimized conditions. The enhanced performance was demonstrated by an endurance test with a constant CO2 conversion and 100% methanol selectivity at 200 C, and the turnover frequency reached 0.27 s-1. Metal oxide promoters are well known to enhance catalytic properties, thus, a modification by incorporating promoters, such as Mg, Zn and Zr, was investigated. A new series of Ni-Ga-X HTlc precursors (X represented Mg, Zn and Zr) were prepared by a similar synthesis procedure, followed by a H2 reduction process. The results revealed that the main Ni-Ga phase transformed from Ni5Ga3 to Ni3Ga when promoters were incorporated in the Ni-Ga catalytic system, due to an unstable HTlc structure as additional elements were incorporated in the parent precursor. Mg and Zr were present as metal oxides, while ZnGa2O4 structure was present in Zn-promoted Ni-Ga catalysts. The BET surface area was measured for all prepared Ni-Ga-X catalysts, and the surface area exhibited a sharp increase after the promoter modifications. Among all samples, the Ni-Ga-Zr revealed the highest BET surface area. TEM-mapping measurements, for Ni-Ga-Zr catalyst, showed Ni-Ga assembly as a core, while Zr surrounded the core, which isolated and separated Ni-Ga catalysts. Thus, the average particle sizes of Ni-Ga-Zr catalysts were considerably decreased compared with other samples, resulting in a relatively large surface area. However, the promotion effect was not obvious in other samples, because Mg could not be completely precipitated in the catalysts and ZnGa2O4 was formed instead. Furthermore, ZrO2 also facilitated the reduction of Ni-Ga-Zr HTlc precursor due to an enhanced electron transfer. Additionally, incorporation of promoters generated additional strong basic sites in the catalytic system, as demonstrated by CO2-TPD measurement. The catalytic properties were evaluated, and a maximum methanol yield (3.8%) was obtained over a Zr-modified Ni3Ga catalyst at 300 C, 30 bar, which exhibited a similar reactivity of commercial Cu-based catalysts. The Ni-Ga-Zr catalysts were subsequently mixed with a commercial high-temperature CO2 adsorbent (MG50). The Ni-Ga-Zr (NGZr) and MG50 were well-mixed, as revealed from SEM images, and the Ni3Ga phase did not change when MG50 was introduced in the Ni-Ga-Zr catalytic system. A series of mixed samples, with different ratios of MG50 and NGZr, was prepared. The corresponding CO2 conversion exhibited a mild decrease as the amount of Ni-Ga-Zr decreased due to loss of active sites, however, the methanol space-time yield was greatly improved as MG50 increased, which suggested that the catalytic property was considerably promoted in the presence of MG50. The highest space-time yield was observed in 25%NGZr/MG50 mixture, with 123.5 gmeth/gcat/h at 300 C. The promotion was ascribed to enhanced CO2 adsorption on MG50 adsorbent, resulting in higher CO2 concentration adjacent to NGZr active sites, contributing to a higher reaction rate and CO2 conversion. Despite the great improvement in methanol space-time yield in NGZr/MG50, the overall CO2 conversion was lower than that of Cu-based catalysts under moderate temperatures, such as 200 C - 250 C. Thus, the NGZr catalysts were subsequently modified by optimizing the Zr amount in the NGZr catalytic system. The TEM-mapping revealed that once the ZrO2 concentration increased above 15%, ZrO2 experienced a severe agglomeration between the Ni-Ga particles instead of surrounding them. Consequently, the interactions between Ni3Ga and ZrO2 was not further increased as Zr content increased from 15% to 25%. The batches of NGZr catalysts were tested for catalytic performance, respectively. The Ni3Ga catalysts with 15% Zr content exhibited a higher CO2 conversion under the entire reaction temperature range when compared to Cu-based catalysts, which indicated that the Ni-Ga-Zr (15%) catalyst area promising candidate for future catalytical CO2 conversion to methanol.
3D printing of flexible and efficient polymeric piezoelectric energy conversion materials
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.
Advanced Star Polymers: From Synthetic Developments to Biomedical Applications
The developments in polymer synthesis and their applications over the last 100 years have had a profound effect on the world as we know it. While polymers abound in our everyday lives, significant advances continue to be made in the field of polymer chemistry. The first reversible deactivation radical polymerization (RDRP) method was discovered in the 1980s, enabling the synthesis of polymers with tailored molecular weights and advanced architectures. Reversible addition-fragmentation chain transfer (RAFT) polymerization is one type of RDRP that has shown exceptional promise for the synthesis of advanced materials, with recent developments in photo-mediated RAFT polymerization leading to unprecedented control of polymer size and architecture. Star polymers are one type of advanced architecture readily synthesized by RDRP and other techniques for a range of applications. Many of the recent advances in RAFT polymerization have yet to be applied in advanced architectures such as stars. Thus, the objective of this thesis is to investigate the synthesis of star polymers and their applications, with focus on stars synthesized by photo-mediated RAFT polymerization and N-carboxyanhydride ring-opening polymerization (NCA ROP). We first investigated the synthesis of star polymers via photoiniferter RAFT polymerization using a core-first approach. Multifunctional cores were synthesized for star synthesis via RAFT polymerization and early results demonstrated the highly “living” nature of the star polymers. This highly living nature allowed us to synthesize ultra-high molecular weight (UHMW) star polymers with molecular weights in excess of 20 MDa, the largest reported for any RDRP techniques to date. Although photoiniferter RAFT polymerization resulted in highly living stars, it does suffer from two significant limitations. The polymerizations are sensitive to oxygen and can only be mediated by a small portion of the electromagnetic spectrum. Photoinduced energy/electron transfer (PET)-RAFT polymerization can overcome these limitations by employing a photoredox catalyst or photosensitizer. We synthesized a novel self-assembled photocatalyst with broadband and near-infrared (NIR) absorbance, facilitating polymerization across the UV, visible light, and NIR regions. Using this photocatalyst we reported the first NIR-mediated RDRP in aqueous conditions and NIR-mediated star polymerization. Finally, the application of star polymers with degradable polypeptide segments was investigated for lysosomal escape of a model drug in vitro. Endo-lysosomal escape of therapeutics to reach their site of action remains a challenge in the field of nanoparticle drug delivery. Star polymers have exceptional potential in drug delivery stemming from their nanometer size and compact, unimolecular structure that allows for functionalization of distinct core, arm, and peripheral regions. Dye-loaded polypeptide star polymers with PEG-brush coronas were biocompatible, demonstrated successful internalization by cells, and allowed the dye to escape the lysosome due to enzymatic degradation of the polypeptide arms while a nondegradable control star accumulated in the lysosome, demonstrating the advantage of polypeptide stars for drug delivery. Together, these results mark exciting advances in the synthesis of star polymers and their applications. The versatility and exquisite control of photo-mediated RAFT polymerizations are demonstrated, while the advances reported in this thesis show great potential for numerous biomedical applications including biomaterials, cell surface modification, and drug delivery.
Evaluation and screening of adsorbents for the separation of carbon dioxide from natural gas
Replacing coal with natural gas is one of the feasible approaches to mitigate CO2 emission at present. To satisfy the increasing global demand for natural gas, more research and development efforts are being put in converting the highly sour natural gas into applicable energy sources. Adsorption processes offer a promising method to remove the bulk sour component, CO2, from high pressure sour natural gas. Adsorptive properties of six zeolites and one silica gel (CBV-760, CBV-780, HSZ-320HOA, HSZ-350HUA, HSZ-385HUA, HSZ-390HUA, and Sorbead WS) were characterised by various methods. Their adsorption isotherms of CO2 and CH4 in the low-pressure regime (0 – 10 bar) were used to determine the isosteric heat of adsorption, while the high-pressure adsorption isotherms (0 – 50 bar for CO2 and 0 – 100 bar for CH4) were fitted to the Toth and the Langmuir adsorption isotherm models. The model fitting parameters were further used in a rigorous simulation model to evaluate the performance of the selected adsorbents in separating a gaseous mixture with 30% CO2 and 70% CH4. The simulated CH4 recovery were used as a major adsorbent performance metric. Besides, another traditional adsorbent performance indicator, IAST (Ideal Adsorbed Solution Theory) selectivity, was employed for the screening of the adsorbents. Both evaluation metrics gave different results about the performance ranking for the same CO2/CH4 separation application. Although HSZ-320HOA was suggested to be the best adsorbent among selected materials by the IAST selectivity, the process simulation preferred Sorbead WS which achieved the highest CH4 recovery (71%) among selected adsorbents. To push the limitations of the traditional adsorbent performance indicators, a new indicator was proposed. The new indicator, adiabatic column working selectivity, contained all features that other indicators have, and also involved a new factor, the components remaining in the void gas phase. The adsorbent evaluation results of the adiabatic column working selectivity were compared with the results of other indicators and the rigorous simulation model. It was found that only the new indicator could match the results of simulations well, while other indicators deviated more or less from the simulation results. The new indicator was proved effective for screening the adsorbents for the application of CO2 removal from natural gas, but to apply this indicator, two process variables, the temperature and bed composition at the end of the desorption step must be determined beforehand. To overcome this difficulty, two simplified simulation models were developed to facilitate the calculation of all critical process variables. The newly developed simplified models could determine the comparable desorption conditions as well as the rigorous simulations, but within much less computation time. A pressure swing adsorption (PSA) plant was operated in the CO2CRC Otway CO2 Capture Research Facility applying Sorbead WS as the adsorbent to separate CO2 from gas mixtures with various compositions (28 mol% to 78 mol% CO2 balanced with CH4) under different conditions from May 2017 to May 2019. Using the recorded process conditions, the process performance predicted by the simplified models could agree well with the actual process performances. This outcome verified the validation of the simplified models. Overall, we found a novel adsorbent performance indicator, adiabatic column working selectivity to evaluate the performance of adsorbents for the application of CO2 removal from natural gas at high-pressure. At the same time, with the help of a newly developed simplified verified by a real PSA plant, the calculation results of the adiabatic column working selectivity can more precisely predict and screen adsorbents for the targeted application.
Two-Dimensional Nanosheet-based Thin Film Composite Membranes for Post-combustion CO2 Capture
Climate change due to anthropogenic carbon dioxide emissions (e.g., combustion of fossil fuels) represents one of the most profound environmental disasters of this century. According to the Special Report completed by Intergovernmental Panel on Climate Change (IPCC) in October 2018, maintaining global warming at 1.5 Celsius degree requires a reduction in CO2 emissions of 49% by 2030. To meet this urgent target, power plants have to equip with carbon capture and storage (CCS) technology. For postcombustion CO2 capture from flue gas, there are four main challenges: (1) the low pressure (ca. 1 bar) of the flue gas produced by power plants; (2) the lower CO2 concentration (15-16%) of flue gas; (3) a high CO2 removal requirement (50-90%); and (4) a low energy consumption of the applied technology. Membrane-based CO2 separation is an attractive technology that meets many of the requirements for economic postcombustion CO2 capture. Within this field, thin-film composite (TFC) membranes are particular attractive due to their high gas permeance. TFC membranes are usually composed of three layers: (1) a bottom porous support layer to provide mechanical strength; (2) a top thin (<1 micrometer) species-selective layer to provide selective function; and (3) a highly permeable intermediate gutter layer to improve the compatibility between the support layer and selective layer. Currently, a key challenge in the development of high performance TFC membranes has been to simultaneously maximize the transmembrane gas permeance (by minimizing the gas permeation resistance of each layer) while maintaining high gas pair selectivities. Two-dimensional (2D) nanosheets are recognized as promising candidates for preparing highly permeable and selective membranes by virtue of their nanosized thickness along with their regular in-plane or interlayer pore arrays, providing minimum transport resistance and maximum selectivity based on the molecular size or solubility. In this thesis, based on 2D nanosheet materials, novel gutter layers and selective layers were developed for the fabrication of flexible, ultrapermeable, and highly processable TFC membranes for economic postcombustion CO2 capture. This includes: (1) the synthesis of ultrathin metal-organic framework (MOF) nanosheets using a surfactant or solvent modulation method; (2) the development of gutter layers employing pristine 2D MOF nanosheets or blending 2D MOF nanosheet into poly(dimethylsiloxane) (PDMS) materials; (3) deciphering the physical aging behavior of polymers of intrinsic microporosity (PIMs) materials; and (4) using PIM@MOF composite as selective layers. The fabricated TFC membranes in this thesis showed much enhanced CO2 permeance (> 1,000 GPU) along with good CO2/N2 selectivity (> 25), meeting the requirements for economic postcombustion CO2 capture. Thus, the research method included in this thesis provides new strategies for the preparation of high performance TFC membranes, which may also be used for other gas or liquid separation applications.
An investigation of casein and whey protein interactions during rennet gelation to effectively incorporate whey proteins into cheddar cheese matrices
Cheddar cheese is formed by enzyme-induced coagulation of milk proteins, known as ‘rennet gelation’. During rennet gelation of milk, only the casein proteins are coagulated while the whey proteins are expulsed from the coagulum. This results in approximately 20% of the protein being lost during cheese production. Although whey proteins are nutritionally valuable and can be converted to commercial products such as powder, they are often disposed of as a waste. Incorporating them back into cheese is an attractive in-situ method of utilizing the whey protein released during cheese making. It also offers the opportunity to produce cheese with elevated protein and nutritional properties. Concentrating native milk proteins using membrane filtration and denaturing whey proteins using heat are conventionally used to increase cheese yields and reduce protein loss. However, the impact of formulating cheese milk with altered whey protein contents and functionality on the rennet gelation stage is not yet fully understood. This knowledge is vital to producing cheese curds with acceptable coagulation times and curd properties. Therefore, this thesis aimed to deepen our understanding of the influence of whey proteins on the rennet gelation process. Interactions between whey proteins (in native and denatured forms), the rennet enzyme, and coagulating para-casein micelle particles were investigated experimentally. Previous studies report that denaturing whey proteins in the presence of caseins impairs the coagulation of para-casein micelle particles during rennet gelation. However, less is known about the impact of whey proteins in their native form, which is relevant to the use of cheese milk with a protein composition altered using membrane filtration. Therefore, the first phase of this study aimed to investigate the effect of native whey proteins on rennet gelation kinetics. Cheese milks with a wide range of whey protein:casein ratios (with standardised casein concentrations) were formulated using native protein concentrate powders produced by membrane filtration. Oscillatory rheometry and casein macropeptide release measurements during rennet gelation demonstrated that native whey protein impaired enzymatic hydrolysis and significantly delayed the subsequent aggregation of para-casein micelle particles. These observations were independent of changes in the ionic balance or the viscosity of the different milk systems. Binding between whey protein and casein micelles or whey protein and rennet was not observed by dynamic light scattering particle size measurements or native poly-acrylamide gel electrophoresis. While there was no evidence of binding between native whey proteins (approximately 5nm in size) and casein micelles (approximately 200 nm in size) that could be responsible for the impaired rennet gelation, it was instead proposed that whey proteins passively occupy the gaps in the ‘kappa-casein hairy layer’ on the casein micelle surface, which arise as a result of enzyme hydrolysis of the kappa-casein. The whey proteins thereby provide a steric hindrance to rennet reaching the casein micelle surface and a barrier to intimate contact between destabilised casein micelles leading to slower gelation. Incorporating denatured whey protein aggregates into cheese gels has been previously proposed as a means to improve the overall cheese yield. However, the potential of modifying whey protein aggregate properties to mitigate the impaired rennet gelation caused by native whey proteins, has not yet been properly studied. Therefore, in the second phase of the study, protein aggregates with a wide range of sizes were produced by heat and power ultrasound. The effects of size and hydrophobicity differences in the whey protein aggregates produced by heat and heat coupled with ultrasound were investigated in relation to the kinetics of rennet gelation and protein retention in model non-fat cheddar cheeses. Rheological measurements showed that sufficiently large, denatured whey protein aggregates could avoid impairment of rennet gelation caused by native whey proteins, irrespective of changes in the soluble calcium concentration or the surface hydrophobicity of the aggregates. Whey protein aggregates formed by the combined heat and ultrasound treatment were more hydrophobic than the larger heat-treated aggregates and were better retained in the cheese. However, inclusion of sufficiently large aggregates in cheeses milks conferred an openness to the cheese microstructure, and showed promise in terms of improving the otherwise rigid non-fat cheese microstructure. In the third phase of the study, the potential of power ultrasound to generate protein-stabilised water-in-oil-in-water double emulsions containing encapsulated whey protein was investigated as a means of incorporating whey proteins into cheese. Ultrasound was successfully applied to form whey protein-enriched water-in-oil-in-water double emulsions using minimal amounts of food-grade emulsifiers. These emulsions had a markedly higher rate of protein encapsulation than previously reported studies. The size distributions and protein encapsulation of the double emulsions could be tailored by manipulating the emulsion formulation and ultrasonic emulsification parameters. Whey protein-rich double emulsions were successfully incorporated into cooked curds formed by rennet gelation, without impairing gelation kinetics, to increase the retention of whey proteins that are otherwise lost during syneresis. In this body of work, fundamental understanding of the role of native and denatured whey proteins during rennet gelation was developed. The specific interactions among whey proteins, casein micelles and the rennet enzyme during the different stages of rennet gelation and cheese making were systematically studied. Further, power ultrasound was successfully used to formulate whey enriched double emulsions that can markedly improve the whey protein retention in cheeses. This understanding can be useful to implementing strategies to better incorporate whey protein into cheese matrices.