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The objective of this thesis is to investigate well-defined cross-linked particles synthesized via the reversible addition fragmentation chain transfer (RAFT) process that can be used for drug delivery. To achieve this aim, a wide range of cross-linked micelle systems have been synthesized and intensively investigated. Various biocompatible monomers were employed, including poly (ethylene glycol) methyl ether methacrylate, 2-hydroxyl ethyl acrylate, functionalized glucosamine and nucleotides containing monomers. Different cross-linked structures were used, for example, core-cross-linked, nexus-cross-linked and shell-cross-linked micelles. Diverse stimuli-responsive particles were used, such as pH-sensitive, thermo-sensitive and thiol-sensitive cross-linked systems. Evidences of the successful synthesis of all the resulting cross-linked products are given. They displayed better properties, as drug carriers, than non-cross-linked micelles. A thermo-responsive seven-arm star glycopolymer, synthesized via the RAFT process, was also investigated.

The objective of this thesis is to investigate well-defined cross-linked particles synthesized via the reversible addition fragmentation chain transfer (RAFT) process that can be used for drug delivery. To achieve this aim, a wide range of cross-linked micelle systems have been synthesized and intensively investigated. Various biocompatible monomers were employed, including poly (ethylene glycol) methyl ether methacrylate, 2-hydroxyl ethyl acrylate, functionalized glucosamine and nucleotides containing monomers. Different cross-linked structures were used, for example, core-cross-linked, nexus-cross-linked and shell-cross-linked micelles. Diverse stimuli-responsive particles were used, such as pH-sensitive, thermo-sensitive and thiol-sensitive cross-linked systems. Evidences of the successful synthesis of all the resulting cross-linked products are given. They displayed better properties, as drug carriers, than non-cross-linked micelles. A thermo-responsive seven-arm star glycopolymer, synthesized via the RAFT process, was also investigated.

Yes / Well-defined N,N’-(dimethylamino)ethyl acrylate (DMAEA) functionalized polymeric stars have been synthesized via an arm-first approach. Utilizing reversible addition–fragmentation chain transfer polymerization, linear homopolymers (PEGA, PHEA) were chain extended with DMAEA and a divinyl crosslinker to produce a series of crosslinked polymeric stars. These stars were characterized using a range of techniques including NMR, SEC, DLS and TEM analysis. The hydrolytic behavior of the DMAEA when tethered within a micellar core was investigated by1 H NMR spectroscopy and was found to be strongly dependent on temperature. At elevated temperatures either a higher crosslinking density or a longer arm length was found to offer greater protection to the amine resulting in slower hydrolysis, with hydrolysis found to level off at a lower final percentage hydrolysis. In contrast, the composition and nature of the arm was found to have little impact on the hydrolysis, with the same trends relating to the effect of temperature and crosslinking density observed with a linear (HEA) and a brush (PEGA) arm. Additionally, the release of DMAE from the polymeric stars was successfully confirmed through the use of an enzymatic assay, producing a concentration of DMAE in good agreement with the theoretical concentration based on the 1H NMR spectroscopic analysis. / Atomic Weapons Establishment (AWE), EPSRC

Spider-silk is known as one of the stronger natural materials, unfortunately it is impossible to farm spiders due to their territorial and cannibalistic nature. To address this issue, researchers have studied spider-silk to discover how it is produced in nature. From their results, spider-silk is composed of large sized proteins produced in two different cell types. Using this knowledge, researchers created transgenic organisms capable of producing spider-silk proteins in large quantities. Using these proteins, several groups have created fibers, films, hydrogels, and adhesives with robust and versatile properties. Wet-spinning is a technique commonly used to create fibers from spider-silk proteins. These fibers unfortunately do not compare to the mechanical properties of natural silk. To address this researchers have used a method known as electrospinning to create spider-silk fibers with substantially smaller diameters. In doing so, these electrospun fibers have increased surface area and enhanced mechanical properties. Using this method, our group has modified the electrospinner to be able to produce continuous fine diameter yarns composed of hundreds of nanofibers with mechanical properties surpassing that of natural silk. Fibers aside, spider-silk proteins can be used to create a variety of different biocompatible materials. To further enhance these materials, our group has utilized a technique traditionally used for observation. This technique employs a high intensity light source to initiate cross-links within the proteins. With this method, our spider-silk protein materials have increased their mechanical properties by a factor of seven. These materials can further be modified through post-treatments, resulting in tunable materials with diverse and robust mechanical properties.

Gas transport properties of PIM-1 (the first ladder polymer with intrinsic microporosity) and TC-PIM-1 (thermally cross-linked PIM-1) at 35°C and different pressures were thoroughly studied. The purpose of this study was to evaluate and compare the performance of the TC-PIM-1 membranes with PIM-1 for natural gas separation. The TC-PIM-1 polymer was prepared by post-modification of PIM-1 at 300°C for a period of two days. Sorption isotherms of seven gases, including N2, O2, CH4, CO2, C2H6, C3H8 and n-C4H10, were determined for PIM-1 and TC-PIM-1 using the dual-volume barometric sorption technique at 35°C at different pressures. The sorption isotherms followed the dual-mode sorption model, which is typical for glassy polymers. Moreover, permeability (P) of eight gases, including He, H2, N2, O2, CH4, CO2, C3H8 and n-C4H10, were determined for PIM-1 and TC-PIM-1 at 35°C and 2.0 atm. Furthermore, average diffusion coefficients (D ̅) were calculated from the permeability and solubility data for all tested gases for both polymers. The sorption (S), permeability (P) and average diffusion coefficients (D ̅) for the TC-PIM-1 membrane exhibited lower values than the PIM-1 membrane. However, the TC-PIM-1 membrane showed exceptional gas separation performance. The TC-PIM-1 membrane had a helium (He) permeability of 1218 barrer with He/CH4 and He/N2 ideal selectivities of 27.1 and 23.9 respectively, and carbon dioxide (CO2) permeability of 1088 barrer with CO2/CH4 and CO2/N2 ideal selectivities of 24.2 and 21.3 respectively. Additionally, the TC-PIM-1 membrane showed a hydrogen (H2) permeability of 2452 barrer with an ideal H2/CH4 selectivity of 54.5.

Polystyrene samples prepared in emulsion at 55 C were carefully fractionated. The resulting fractions were in turn fractio­nated, combined in groups having similar intrinsic viscosities, and re-­fractionated. As the intrinsic viscosity of these fractions increased, the value of Huggins' k' in methyl ethyl ketone was found to increase a slightly ( from 0.39 to 0.41). Polystyrene samples prepared in exactly the same way except for the addition of small amounts of divinylbensene, a cross-linking agent, were similarly fractionated. The value of k' in methyl ethyl ketone for these fractions increased considerably (from 0.41 to 0.68) as the intrinsic viscosity increased, even though the amount of divinylbensene added was very small (from 0.003to 0.05%). Relations were then established for these cross-linked fractions between k' and the proportion of divinylbensene present, and between k' and the intrinsic viscosity of the fractions. It was concluded that polystyrene prepared in emulsion at 55 C. is essentially linear in structure, and that the value of Huggins' k* seems to provide a convenient and sensitive test for the presence of any appreciable branching that might occur in polystyrene. / Thesis / Master of Science (MS)

Collagenous tissues such as ligaments and tendons are viscoelastic materials. They exhibit a slow continuous increase in strain over time, or creep, when subjected to a constant stress and a slow continuous decrease in stress over time, or stress relaxation, when subjected to a constant strain. Moreover, the loading and unloading stress-strain curves are different when the tissues are subjected to cyclic loading, showing hysteresis and softening phenomena. The micro-structural origin of the viscoelasticity of these tissues is still unknown and the subject of debate among experts in biomechanics. Therefore, formulating viscoelastic models by accounting for the mechanical contributions of the structural components of these tissues can help in understanding the genesis of viscoelasticity. A nonlinear viscoelastic modeling framework has been developed to describe the elastic and viscoelastic properties of ligaments and tendons by considering their main structural components, the collagen fibers and proteoglycan-rich matrix. The mathematical models derived within this framework can illustrate the tensile behavior, stress relaxation and creep by as suming that the collagen fibers are elastic and the surrounding proteoglycan-rich matrix is viscoelastic. The collagen fibers are represented by linear elastic springs that are engaged to support load at different values of the tissue's strain according to a Weibull distribution function. The mechanical contribution of the matrix is introduced via a Maxwell-type viscoelastic element arranged in parallel with the collagen fibers. According to the proposed mathematical framework, both the collagen fibers and the proteoglycan-rich matrix are responsible for resisting tensile loads. However, the collagen fibers play a significant role in creep while the proteoglycan-rich matrix has a dominant role in stress relaxation. The model parameters that define the stress relaxation and strain stiffening phenomena are estimated by using published experimental on rabbit medial collateral ligaments and are then used to predict creep. The above modeling framework has been also extended to capture the in uence of preconditioning on the mechanical properties of ligaments and tendons. The stress softening and decrease in hysteresis that are observed during successive loading cycles in preconditioning are assumed to be determined by a decrease in the elastic properties of the collagen fibers and proteoglycan-rich matrix. Preliminary data collected on stress relaxation and preconditioning on rat medial collateral ligaments by collaborators are used to evaluate the model parameters and analyze its predictions. The elastic and viscoelastic properties of single collagen fibers are studied by formulating a nonlinear viscoelastic framework by accounting for their main components: microfibrils, cross-links and proteoglycan-rich matrix. The model illustrates tensile behavior and stress relaxation of a single collagen fiber by assuming that the microfibrils and the cross-links are elastic and the surrounding proteoglycan-rich matrix is viscoelastic. The mechanical contribution of the microfibrils is included via a linear elastic spring while the cross-links are represented by linear elastic springs that progressively fail at different values of the tissue's strain according to an exponential distribution function. The matrix is defined by linear dashpots arranged in parallel with each single spring that represents an individual cross-link. The viscous properties of the matrix associated with the unbroken and broken cross-links are assumed to have different values. In the model formulation, the microfibrils and the cross-links are assumed to determine the elastic response of the fibers while the proteoglycan-rich matrix determines the stress relaxation. Microfibrils, cross-links and the proteoglycan-rich matrix are responsible for resisting the loading force during tensile behavior. Experimental data collected by performing incremental stress relaxation tests by other investigators on reconstituted rat tail tendons are used to estimate the parameters in the model and evaluate its performance. / Ph. D.

Doctor of Philosophy / Department of Grain Science and Industry / Yong Cheng Shi / This study focused on in-vitro digestibility and pasting properties of cross-linked (CL) starch with different amylose contents and different cross-linking levels, as well as physicochemical properties of extruded wheat flours with different amylose contents and functionality of amylopectin and cross-linking in improving the textural and physical properties of oat flour extrudates. Starch was CL by phosphorylation using a mixture of sodium trimetaphosphate and sodium tripolyphosphate at the ratio of 99:1 under alkaline condition. The digestibility of highly CL maize starches with different amylose contents was determined by Englyst, Available Carbohydrate Dietary Fiber and Association of Official Analytical Chemists (AOAC) Method 991.43 methods. CL waxy and normal maize starch granules swelled much more at higher temperatures, resulting in significantly lower total dietary fiber content than high-amylose maize starch. The alkali treatment study on low levels of cross-linking in starch affected the ratios of total bound phosphate esters which changed the degree of starch swelling, crytallinity and pasting properties of the CL starch. The study on the mechanism of the digestive enzymes and the extent of digestion during the incubation of CL wheat starch in AOAC Method 2009.01 showed progressive digestion after 16 h of incubation, which may not reflect in-vivo response in human. Extrusion of normal and waxy wheat flours resulted in the breakdown of starch and an increase in the insoluble protein, which affected the textural and structural properties of extrudate. High energy input played a major role in radial expansion of normal hard wheat extrudate, whereas higher amylopectin in soft waxy wheat flour was a dominant factor in determining the radial expansion when compared to normal soft wheat. Inclusion of soft waxy wheat in the oat flour formulations at 18% moisture content improved the textural and structural properties of extrudates. Low cross-linking level of CL waxy maize starch in oat flour formulation increased the void fraction and reduced the breaking strength of extrudates, whereas higher cross-linking levels of CL starches improved the resistant starch level on oat flour formulation but had very poor structural and textural properties.The study offers a good insight on the properties and digestibility of CL starch, as well as using low levels of CL starch to improve textural properties of nutritional extruded products. In addition, study on the extrusion of wheat flours with different amylose and protein contents provides knowledge on the influence of chemical compositions and energy input on the physico-chemical properties of extrudates.

Condition monitoring (CM) of electrical equipment is an important field in electrical engineering and a considerable amount of research is dedicated to this field. Power cables are one of the most important parts of any electrical network and the variety of techniques available for CM of electrical cables is therefore no surprise. Electrical cables are exposed to operational and environmental stressors which will cause degradation of the insulation material. The degradation will continue to the point where the cable fails. Blackouts caused by failing cables will have an effect on the safety, efficiency and production of an electrical network. It is therefore important to constantly monitor the condition of electrical cables, in order to prevent the premature failure of cables. The research presented in this dissertation sets out to investigate CM techniques for power cables and to design and implement a basic cable CM technique based on the principles of partial discharge (PD) measurements. A comprehensive literature study introduces the fundamental concepts regarding the CM of power cables. The basic construction of electrical cables, as well as the variety of different types is researched in order to lay a foundation for the research that follow. CM techniques for electrical equipment are investigated, with the emphasis on techniques used on cables. Conducted research led to the decision to focus on CM by means of PD measurements. PD as a phenomenon is investigated to be able to better understand the origins and effects of discharge activity. From there the focus shifts to the available techniques for monitoring the condition of electrical cables by means of PD measurements. The research conducted in the literature study chapter forms the basis from which the rest of the study is conducted. Simulation models were used to study PD characteristics. The models are derived from engineering and mathematical principles and are based on the well-known three-capacitor model of PD. The simulations were performed in order to study the effects of discharge activity. The designed simulation models allows for a variety of PD characteristics to be studied. The simulations were performed in the MATLAB® Simulink® environment. The research conducted in the dissertation was used to design an elementary CM technique which can be used to detect the presence of PD within electrical cables. The designed CM technique was used for the practical measurement of PD data. MATLAB® programs were designed in order to analyse the PD data in both the time- and frequency-domain. The analysis of the measured data revealed PD characteristics of the test specimen used for the measurements. The designed CM is used for the detection of PD activity within electrical cables and in combination with other techniques, may be used for complete CM of electrical cables. The experimental setup which was used to take practical PD measurements adds another dimension to the work presented in this dissertation. / MIng (Electrical and Electronic Engineering), North-West University, Potchefstroom Campus, 2015