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The purpose of this research project was to determine the processing conditions necessary for preparing flexible carbon nanofiber electrodes by electrospinning and to explore various applications for those electrodes. It was found that by varying only the relative humidity while electrospinning a poly(acrylonitrile) precursor, fragile or flexible freestanding carbon nanofiber electrodes were prepared. The relative humidity during electrospinning controlled the fiber diameter, the bulk porosity of the material, and flexibility of the final carbon electrode. Higher porosity mats electrospun in a high relative humidity environment prevented fiber sintering, which if not minimized, resulted in non-flexible carbon electrodes. Both flexible and fragile electrodes were freestanding, binderless, and collectorless. Additionally, they required no further processing before use and were 100 wt.% active material. When cycled galvanostatically as a lithium ion battery anode, the flexible electrode exhibited a specific capacity of 379 mAH g-1 at the 100th cycle and capacity retention was 97.4% relative to the fifth cycle. When applied as an active material support electrode for lithium ion battery cathodes, the carbon support was successfully utilized with both micron and nano structured active material and cycled for 100 cycles with limited capacity loss. The same electrodes were also found to be a viable replacement for Pt electrode based actuators/artificial muscles. However, this application requires much further research to understand better the required processing and effects of the physical properties of the electrode on actuator performance. In addition to this, the flexible electrodes have a wide variety of other potential applications including, electrochemical storage and conversion devices, chemical sensing, and filtration. The focus of this work was electrochemical storage and conversion devices in the form of lithium ion battery anodes and cathodes as well as ionic polymer composite actuators. / PHD

Electrospinning of nanofibrous mats and scaffolds enables generation of scaffolding that is not only highly porous, but also has a structure that essentially mimics the natural basement membrane. As a result, the method has proliferated extensively, and is commonly used for diverse applications such as water filtration or tissue engineering, the latter of which may involve the use of natural or synthetic materials. Common laboratory scale electrospinning setups can be built inexpensively with merely a syringe pump, a high voltage supply, and an aluminum foil target. These systems, however, are limited to flat target surface geometries that span several centimeters. While a scaffold can be cut or folded to conform to a bone or other biological surface, spinning directly onto a surface with significant peaks and troughs results in poor fiber uniformity. Furthermore, if an alteration of fiber properties is preferred, the high voltage setup limits user access and customization of parameters during the spinning period. Finally, control of the electric field is compromised by the proximity of grounded electrical components. As its first aim, this project develops a robotic control system to enable custom coatings of arbitrary surfaces. By augmenting the traditional electrospinning system with a three-dimensional robotic control system, electric field focusing fibers, and additional aerodynamic forces terms ‘electroblowing’, the device can be produced across targets with strong topographic anisotropy. The second aim continues to enhance these attributes with biocompatible soy based scaffolds. Craniofacial implants are often complex in geometry, and conformal bandages are particularly hard to produce in these areas. Soy based scaffolds will be produced for 3D-printed replicas of these situations. Finally, the methods developed across this aim enables the development and use of a handheld electrospinning system that combines a coaxial high velocity air flow with the high voltage spinning element to reduce effects of operator error. The final goal of the thesis is to test whether fiber control successfully reduces effects of fiber anisotropy in vitro and to use the enhanced fiber control mechanisms to produce scaffolds with significant anisotropy, depositing aligned fibers at a target point to eventually enable generation of scaffolds with programmable variable spatial alignment similar to tendon. When completed, the systems described will enable custom production of coatings or scaffolds for functionality as scaffolding on medically relevant surfaces. Specifically, this means first, that scaffolds can be used with confidence to improve fixation even of non-cylindrical implants and enhance local tissue integration, and second, that implants can be customized with areas of ‘guidance’ fibers or local drug depots to either promote regeneration and population by surrounding tissue or mimic natural anisotropic cues necessary for mechanical or biological functionality. / Bioengineering

Thesis (PhD)--Stellenbosch University, 2014. / ENGLISH ABSTRACT: The work presented in this thesis throws light on the supramolecular approach in exploration of bi-porphyrin nanorods system wherein self-assembly plays an important role. Porphyrin based nanorods were synthesized via self-assembly of meso-tetrakis (4-phenylsulfonicacid) porphyrin dihydrochloride and Sn (IV) tetrakis (4-pyridyl) porphyrin. Understanding the sizes and growth mechanism of the porphyrin nanorods by self-assembly and molecular recognition is essential for their successful implementation in nanodevices. Spectroscopic and microscopic studies were carried out to investigate the effect that time, concentration and solvents have on the fabrication of the porphyrin nanorods by ionic self- assembly. This study demonstrated that aggregates of the di- acid form of meso-tetrakis (4-phenylsulfonic acid) porphyrin dihydrochloride and Sn (IV) tetrakis (4-pyridyl) porphyrin resulted in porphyrin nanorods with diameters between 20 nm and 60 nm, and μm in lengths. Enhanced optical properties illustrated the potential for slightly modifying the method of synthesis to influence the physical and optical properties of porphyrin nanorods. The porphyrin nanorods reflectance data demonstrated that these structures are good absorbers of light and therefore could potentially be used to harvest light. The nonlinear optical (NLO) properties of the porphyrin nanorods were investigated for the first time in this study by second and third harmonic generation techniques. Such study was influenced by the fact that porphyrins have great thermal stability and extended -conjugated macro cyclic ring which give them large nonlinear optical effects. The NLO results showed that the porphyrin nanorods may have many potential uses in photonic applications due to larger third order nonlinear susceptibility. Single molecule spectroscopy was also used to investigate the dynamics of intermolecular and intramolecular processes. Porphyrin nanorods were incorporated into polymer matrices to achieve an arrangement where they can be directly used as a device. The assembly of porphyrin nanorods on track-etched membranes was achieved through altering the surface charge of the respective membranes. Porphyrin nanorods-polymer composites were produced using latex technology and electrospinning techniques. The fibres were characterized with respect to morphology and optical properties. / AFRIKAANSE OPSOMMING: Die werk wat in hierdie tesis beskryf word werp lig op die supramolekulêre benadering in die ondersoek van bi-porfirien nano-silinders waarin self-versameling ‘n belangrike rol speel. Porifirien nano-silinders was voorberei via self-versameling van meso-tetrakis(4-feniel sulfoonsuur) porfirien dihidrochloried en Sn (IV) terakis (4-piridiel) porfirien. Dit is belangrik om die meganismes wat verband hou met die groei en grootte van die nano-silinder struktuur te ondersoek. Dit het ‘n invloed op die self-versameling asook die uiteindelike toepassing. Spectroskopiese en mikroskopiese studies was uitgevoer om die effek van tyd, konsentrasie en oplosmiddel op die selfversamelling te bestudeer. Die studie dui daarop dat bondels van die disuur vorm van meso-tetrakis(4-feniel sulfoonsuur) porfirien dihidrochloried en Sn (IV) terakis (4-piridiel) porfirien het gelei tot porfirien nano-silinders met lengtes tussen 20 nm en 60 nm asook in die mikro meter skaal. Verhoogde optiese eienskappe het die potensiaal om effense veranderinge in die metode om die nano-silinders voor te berei om sodoende ‘n groter invloed op die fisiese en optiese einskappe te hê. Die reflektansie data wys dat hierdie strukture goeie absorbsies van lig toon en daarom geskik sal wees om lig te stoor. Die nie-liniêre optisie (NLO) eienskappe van die profirien nano-silinders was vir die eerste keer ondersoek deur middel van tweed en derde hormoniese generasie tegnieke. Hierdie studie was beïnvloed deur die feit dat porfiriene goeie stabiliteit by hoë temperatuur en ‘n verlengde -gekonjugeerde makro-sikliese ring bevat wat dan groot nie-liniêre optiese effekte gee. Die NLO resultate wys dat die profirien nano-silinders groot potensiaal het in die gebruik van fotoniese toepassings as gevolg van derde orde nie-liniêre vatbaarheid. Enkel molekuul spektroskopie was ook gebruik om die dinamika van intermolekulêre en intramolekulêre prosesse te ondersoek. Porfirien nano-silinders was geïnkorporeer in polimeer matrikse om ‘n eweredige verspreiding te verkry en om direk as ‘n toestel te gebruik. Die versameling van porfirien nano-silinders op baan-ingeëtse membrane was bereik deur die verandering in oppervlak lading van die membrane. Porfirien nano-silinder / polimeer samestellings was verkry deur lateks tegnologie en elektrospin tegnieke. Die vesels was gekarakteriseer in terme van morfologie en optiese eienskappe.

Superhydrophobicity refers to surfaces with extremely large water droplet contact angles (usually greater than 150°). This phenomenon requires a hydrophobic material with micro or nano-scale roughness. Superhydrophobic surfaces exist in nature (e.g. the lotus leaf) and can be produced synthetically. This project focuses on the development and characterization of superhydrophobic materials with tunable wettability (i.e. smart superhydrophobic materials). In this study, surfaces were prepared by electrospinning thin, aligned polystyrene fibers onto a piezoelectric unimorph substrate. Results showed electric field induced changes in substrate curvature, which produced corresponding changes in surface wettability. From experiments, an average change in water contact angle of 7.2° ± 1.2° with 90% confidence was observed in ~2μm diameter fiber coatings electrospun for 5 minutes with applied electric field. In addition, fiber coatings electrospun with equivalent deposition showed average electric field induced changes in WCA of 2.5° ± 0.92° for lower diameter fibers (~1μm) and 3.5° ± 1.37° for higher diameter fibers (~2μm) with 90% confidence.

In this thesis the production and filtration performance of electrospun nylon-4,6 nanofiber mats was experimentally studied. The average fiber diameter of nylon-4,6 fibers is controlled by altering the polymer concentration in solution. Adding small amount of pyridine to the electrospinning solution controlled fiber beading and other defects. These fibers were then deposited onto a wide variety of conductive and dielectric substrate materials. A corona ion source was used to eliminate surface charging effects observed at the surface of all substrates. The resulting fiber mats, uniform in size and distribution as verified by SEM imaging, are tested for filtration efficiency and pressure drop. A Figure of Merit (FOM) is calculated for each filter produced and compared to high-grade commercial filters.

Electrospinning can be used to selectively process a variety of natural and synthetic polymers into highly porous scaffolds composed of nano-to-micron diameter fibers. This process shows great potential as a gateway to the development of physiologically relevant tissue engineering scaffolds. In this study we examine the structural and functional considerations regarding electrospun scaffolds for dermal template applications using novel quantification techniques. In order to characterize scaffold structure, a technique utilizing the fast Fourier transform was developed to systematically quantify fiber alignment and evaluate how different electrospinning parameters impact the structure and material properties of an electrospun scaffold. Gelatin was suspended at varying concentrations (80, 100, 130 and 150 mg/ml) and electrospun from 2,2,2 trifluoroethanol onto a rotating mandrel (200-7000 RPM). Scaffold anisotropy developed as a function of fiber diameter and mandrel speed and the induction of varying degrees of anisotropy imparted distinctive material properties to the electrospun scaffolds. Fiber alignment was the variable most closely associated with the regulation of peak stress, peak strain and modulus of elasticity. Next, we examined how the chemical and physical composition of the local microenvironment and the unmasking of possible RGD sensitive binding sites through collagen denaturation, independent of scaffold architecture and porosity, impacts cellular processes. We cultured human dermal fibroblasts on electrospun nylon coated with a variety of non-denatured and thermally denatured collagen-based proteins, as well as recovered electrospun collagen and gelatin (in an effort to examine if the electrospinning process degrades the collagen α chain). Differences in adhesion, proliferation and migration were exhibited between collagen-based proteins. Adhesion inhibition assays using a cyclic RGD peptide demonstrated no change in cell adhesion on non-denatured proteins and a significant drop in cell adhesion on thermally denatured proteins. Based on gel analysis and the results of our functional assays we conclude that collagen  chain structure is not directly altered by the electrospinning process. Overall, these results are critical to the understanding of how structure and architecture contribute to the overall properties of a scaffold, as well as how molecular variations can modulate scaffold functionality in a cellular environment.

In the last two decades, a rapidly growing polymer processing technology, electrospinning, has attracted great interests as it provides a viable and simple method to create ultra-fine continuous fibres. Despite the potential utilization of electrospun nanofibres in many fields, their success is limited so far due to their poor mechanical properties compared to corresponding textile fibres made from the same polymers, which is mainly ascribed to the low degree of orientation and chain extension of the macromolecules along the fibre axis in such fibres. In this thesis, first an in-depth review of the mechanical properties of electrospun fibres and recent developed methodologies to generate high strength and high modulus nanofibres will be presented. In the experimental work, electrospinning of rigid polymer PPTA was attempted and mechanical properties of obtained fibres were evaluated (Chapter 3). It was shown that the electrospinning process cannot be easily operated in a controllable and continuous manner although some high performance fibres were obtained. Chapter 4 dealt with the electrospinning of reactive mesogens (liquid crystal monomers) by employing polymers (PMMA and PA6) as matrix. The mechanical properties of the resulting composite nanofibres (PA6/RM257) showed dependence on the reactive mesogen (RM257) content and the phase separation between PA6 and RM257. In Chapter 5, a high performance polymer BPDA/PDA/ODA was synthesized and electrospun; the nanofibres were characterized using FTIR and WAXD and their mechanical tests were carried out based on unidirectional mats and multifilament bundles. A Weibull modulus based model was introduced to estimate the tensile strength of single nanofibres in such bundles. Subsequently, composites based on BPO nanofibres in a rubbery thermoplastic matrix were fabricated and evaluated in Chapter 7 using composite mechanics theories for off-axis properties and „Rule of Mixture‟ which were used to back-calculate the Young‟s modulus of single BPO nanofibres. From this it could be concluded that the developed co-polyimide BPO nanofibres exhibit among the highest mechanical properties of electrospun nanofibres reported in literature so far. It can be concluded that the electrospun BPO co-polyimide nanofibres and p-aramid fibres possess among the highest mechanical properties reported for electrospun fibres so far.

"The effects of molecular weight (Mw) and concentration (c) on the structure of electrospun PVA have been studied. Experiments have been conducted for Mw values ranging from 9000 g/mol to 124,000 g/mol. The concentration was varied from 5 to 35 wt %. Data were acquired for several solvents including water, Dimethyl Sulfoxide, Ethylene Glycol and N-Methyl Pyrrolidone. The transient phenomena occurring during jet breakdown were examined by high speed digital photography. The structure in the electrospun polymer was analyzed by scanning electron microscopy. The fiber diameter distribution for various conditions was characterized by optical image analysis. The effects of additives such as NaCl and Poly Ethylene Glycol on the structure have been studied. The results indicate that a minimum Mw and c corresponding to [h]c ~ 5 or Capillary number, Ca ~ 0.5 is necessary for forming a fibrous structure. As Mw or c increase, the fiber diameter becomes larger and a broader distribution of fibers may be obtained. The average diameter of the fiber, D, follows a Power law relationship: D (nm) = 18.6([h]c)1.11. Round fibers may be obtained at low Mw and c, while flat fibers are observed at high Mw and c. The transition from round to flat fibers appears to begin at [h]c ~ 12. At any [h]c, there is a minimum Capillary and Ohnesorge numbers at which fibers are stabilized and a maximum at which viscous effects become dominant. The addition of NaCl lowers the average fiber diameter in PVA samples with a high molecular weight. Electrospinning can be used to produce nanofibers of PVA with various architectures. "

The objectives of the present work are to produce porous polymeric scaffolds with Poly (ƒÕ-Caprolactone), PCL, by electrospinning. The structure in the electrospun polymer has been characterized by scanning electron microscopy. The effects of process variables such as voltage, solution concentration and deposition distance on the structure have been studied. The physical phenomena associated with the electrospinning process have been highlighted through high speed digital photography. The feasibility of using additives to the solution to control the structure of the porous construct has been examined. The data indicate that a range of structural morphologies can be produced in the electrospun polymer. Solid and hollow sub-micron beads can be produced by electrospraying of dilute solutions. Beyond a critical solution concentration of about 4 wt% PCL, elongational flow stabilizes the fibrous structure and a web of interconnected sub-micron fibers may be obtained. The average fiber diameter increases with concentration. A combination of elongated beads and fibers, known as the bead-on-string morphology is also observed under many conditions. The fibrous structure is stabilized at high voltages. The fiber diameter in the electrospun polymer typically exhibits a bimodal distribution. The addition of DMF (N,N-dimethylformamide) to the solution increases the deposition rate significantly and leads to extensive splaying, thereby reducing the fiber diameter to about 150 nm. DSC data indicate that electrospinning may lower the degree of crystallinity in the polymer. The wide of range of structural characteristics that may be obtained in the electrospun polymer make it suitable for many biomedical applications including medical textiles, drug delivery, membrane separation, tissue engineering and organ regeneration.