Polymer electrolytes are indispensable in numerous electrochemical systems. Existing polymer electrolytes rarely meet all technical demands by their applications (e.g., high ionic conductivity and good mechanical strength), and new types of polymer electrolytes continue to be developed. In this dissertation, the molecular structure and dynamics of three emerging types of polymer electrolytes featuring Coulombic liquids, i.e., polymerized ionic liquids (polyILs), nanoscale ionic materials (NIMs), and polymeric ion gels, were investigated using molecular dynamics (MD) simulations to help guide their rational design. First, the molecular structure and dynamics of a prototypical polyILs, i.e., poly(1-butyl-3-vinylimidazolium hexafluorophosphate), supported on neutral and charged quartz substrates were investigated. It was found that the structure of the interfacial polyILs is affected by the surface charge on the substrate and deviates greatly from that in bulk. The mobile anions at the polyIL-substrate interfaces diffuse mainly by intra-chain hopping, similar to that in bulk polyILs. However, the diffusion rate of the interfacial mobile anions is much slower than that in bulk due to the slower decay of their association with neighboring polymerized cations. Second, the structure and dynamics of polymeric canopies in the modeling NIMs where the canopy thickness is much smaller than their host nanoparticle were studied. Without added electrolyte ions, the polymeric canopies are strongly adsorbed on the solid substrate but maintain modest in-plane mobility. When electrolyte ion pairs are added, the added counter-ions exchange with the polymeric canopies adsorbed on the charged substrate. However, the number of the adsorbed electrolyte counter-ions exceeds the number of desorbed polymeric canopies, which leads to an overscreening of the substrate's charge. The desorbed polymers can rapidly exchange with the polymers grafted electrostatically on the substrate. Finally, the molecular structure and dynamics of an ion gel consisting of PBDT polyanions and room-temperature ionic liquids (RTIL) were studied. First, a semi-coarse-grained model was developed to investigate the packing and dynamics of the ions in this ion gel. Ions in the interstitial space between polyanions exhibit distinct ordering, which suggests the formation of a long-range electrostatic network in the ion gel. The dynamics of ions slow down compared to that in bulk due to the association of the counter-ions with the polyanions' sulfonate groups. Next, the RTIL-mediated interactions between charged nanorods were studied. It was discovered that effective rod-rod interaction energy oscillates with rod-rod spacing due to the interference between the space charge near each rod as the two rods approach each other. To separate two rods initially positioned at the principal free energy minimum, a significant energy barrier (~several kBT per nanometer of the nanorod) must be overcome, which helps explain the large mechanical modulus of the PBDT ion gel reported experimentally. / Ph. D. / Polymer electrolytes are an indispensable component in numerous electrochemical devices. However, despite decades of research and development, few existing polymer electrolytes can offer the electrochemical, transport, mechanical, and thermal properties demanded by practical devices and new polymer electrolytes are continuously being developed to address this issue. In this dissertation, the molecular structure and dynamics of three emerging novel polymer electrolytes, i.e., polymerized ionic liquids (polyILs), nanoscale ionic materials (NIMs), and polymeric ion gels, are investigated to understand how their transport and mechanical properties are affected by their molecular design. The study of polyILs focused on the interfacial behavior of a prototypical polyILs supported on neutral and charged quartz substrates. It was shown that the structure and diffusion mechanism of the interfacial polyILs are sensitive to the surface charges of the substrate and can deviate strongly from that in bulk polyILs. The study of NIMs focused on how the transport properties of the dynamically grafted polymers are affected by electrolyte ion pairs. It was discovered that the contaminated ions can affect the conformation the polymeric canopies and the exchange between the “free” and “grafted” polymers. The study of polymeric ion gels focused on the molecular and mesoscopic structure of the ionic liquids in the gel and the mechanisms of ion transport in these gels. It was discovered that the ions exhibit distinct structure at the intermolecular and the interrod scales, suggesting the formation of extensive electrostatic networks in the gel. The dynamics of ions captured in simulations is qualitatively consistent with experimental observations.

Call number: LD2668 .T4 1964 R21 / Master of Science

The theory of diffusion in gases has been approached in two different ways in the past. The first is by means of the old free path theory initiated by Maxwell [and] Boltzmann, improved by Tait, Meyer, Jeans and others which though unable to give exact quantitative results owing to the assumption that molecules after collision are distributed in the Maxwellian type in non-uniform gases, provides a simple vivid picture of the complex molecular transport phenomena in gases. The second one due to Chapman and Enskog is to find the deviation from the Maxwellian velocity distribution in a non-uniform gas by solving Maxwell and Boltzmann's Collision equation.

Thesis (MSc)--Stellenbosch University, 2004 / ENGLISH ABSTRACT: In an ongoing study an empirical force field that can correctly model N,N'- dialkylimidazolium halide ionic liquids, the Imidazolium Ionic Liquid Force Field - IILFF, was developed based on experimental data obtained from the Cambridge Structural Database (CSD) and data calculated using Gaussian98. Different conformations of the isolated cations were optimised at the Hartree-Fock level using the 6-31G(d) basis set. Structural, vibrational and partial atomic charge data of the lowest energy conformation of each cation were taken as observables during optimisation of the force field parameters. Initial parameters of the IILFF were taken from existing force fields and were optimised using the above mentioned data. The IILFF was used to minimise isolated cations as well as crystals. These results were successfully tested against isolated cations minimised using Gaussian98 and the experimental crystals. Finally, the melting process of the 1,3-dimethylimidazolium chloride crystal was studied using an NPT ensemble starting from an ordered crystal cell and increasing the simulation temperature beyond the experimental melting temperature. The IILFF was then used to calculate the potential energy of the system. / AFRIKAANSE OPSOMMING: In 'n voortgaande studie om 'n empiriese kragveld te bou wat N,N' -dialkielimidasoliumhalied ioniese vloeistowwe korrek kan modelleer is die Imidasolium Ioniese Vloeistof Kragveld (nVK) ontwikkel. Die kragveld is ontwikkel gebasseer op eksperimentele data verkry uit die Cambridge Strukturele Databasis (CSD) asook uit data vanaf Gaussian98 berekeninge. Verskillende konformasies van die geïsoleerde katione is geoptimiseer deur middel van Hartree Fock 6-31G(d) berekeninge. Strukturele data, asook vibrasies en gedeeltelike atoom ladings van die laagste energie konformasie van elke katioon is gebruik as waarneembare veranderlikes vir die bepaling van die optimale kragveld parameters. Beginwaardes vir die nVK is geneem uit bestaande kragvelde en geoptimiseer met behulp van bogenoemde data. Die IIVK is gebruik om geïsoleerde katione asook kristalle te minimiseer. Die resultate is suksesvol getoets teen geïsoleerde katione wat met behulp van Gaussian98 geminimiseer is en eksperimenteel bepaalde kristalle. Laastens is die smeltproses van die 1,3-dimetielimidasolium chloried kristal bestudeer met behulp van 'n NPT ensemble. Daar is begin by 'n geordende kristal en die simulasie temperatuur is verhoog tot meer as die eksperimentele smeltpunt. Die IIVK is dan gebruik om die potensiële energie van die sisteem te bepaal.

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Streamer characteristics, breakdown strengths and gassing behaviour of insulating liquids under electric stresses are taken into account for a reliable design and safe operation of the transformer. Ester liquids which are biodegradable and have high fire point have been widely used in distribution transformers and some power transformers in recent years. It is also interesting to introduce ester liquids into High Voltage Direct Current (HVDC) converter transformers due to the fast development of HVDC transmission lines. Therefore, this thesis aims to investigate the pre-breakdown, breakdown characteristics and gassing behaviour of a synthetic ester liquid under DC and various impulse voltages where a mineral oil is tested as the benchmark. A comprehensive study of streamer characteristics and breakdown strength of the mineral oil and the synthetic ester liquid under both positive and negative DC voltages was carried out in the point-plane electric fields. Characteristics of streamer length, propagation velocity and shape were analysed based on shadowgraph images obtained at a gap distance of 10 mm, using a multi-channel ultra-high speed camera. Streamer inception voltages with the tip radii of 5 µm, 10 µm, 20 µm and 50 µm and breakdown voltages at various gaps of 2 mm, 5 mm, 10 mm, 20 mm and 30 mm were also investigated. The results indicate that there is no obvious streamer propagation (less than about 10% of the gap distance) under negative polarity even when the applied voltage approaches breakdown voltage. At the same applied voltage level, the streamer in the synthetic ester liquid propagates faster and further than that in the mineral oil. As a result, the breakdown voltages of the synthetic ester liquid are lower than those of the mineral oil at all the gap distances investigated under both polarities. Experimental and modelling studies of pre-breakdown and breakdown phenomena in the mineral oil and the synthetic ester liquid under impulse waveforms with different tail-time were carried out in the point-plane electric fields. A compact solid-state switch based impulse generator was used to provide different impulse waveforms from short tail-time to 'step-like' tail-time: 0.8/8 µs, 0.8/14 µs, 0.8/30 µs and 0.8/3200 µs. A point-plane electrode configuration with a small gap distance of 10 mm and a tip radius of 10 µm was used. The results indicate that the shorter tail-time impulse waveform results in a shorter stopping length and higher breakdown voltage; however it does not affect the instantaneous breakdown voltage and time to breakdown. A mathematical model is therefore described to predict the breakdown voltage under different impulse waveforms. In addition, with the similar stopping length, higher energy injected from the short tail-time impulse caused the streamers to have more branches than those under the long tail-time impulse. The characteristics of fault gas generation in the mineral oil and the synthetic ester liquid under various levels of electrical faults were studied. A test platform with functions of automatic spark fault control and data acquisition was developed. The effects of spark numbers (from 20 to 500), gap distance (5 mm and 10 mm) and voltage levels (Vb-99.9% and 1.5Vb-99.9%) on fault gas generation in liquids were studied. The key gases in the mineral oil are H2 and C2H2, while the key gases in the synthetic ester liquid are H2, C2H2 and CO. The amount of fault gas generation increases linearly with the number of sparks. However, the number of sparks does not have an obvious effect on fault gas pattern and gas generation per unit fault energy in µL/J. Spark at a larger gap distance or under a higher applied breakdown voltage generates more fault gases due to higher injected fault energy.

Gas-expanded liquids (GXLs) are a new and benign class of pressure-tunable liquid solvents which show tremendous promise as the next sustainable processing medium. In order to realize the potential of GXLs fully, it is necessary to elucidate their cybotactic region and gain an understanding of where properties are different in the bulk and micro-scales and how local structure and order affect both reactions and separations. This work explores the cybotactic region of GXLs and probes the existence and implications of those differences. This study is started by exploring the cybotactic region of ambient liquid mixtures. Thermodynamic models based on intermolecular forces are used to predict the solubility of multi-functional solids in a variety of solvent mixtures. While this part does not lend any insight into GXLs directly, it acts as a stepping stone in both understanding the intermolecular forces that govern the cybotactic region and by opening the gateway to studying solid solubility in GXLs. The rest of the study focuses on the differences between bulk and local properties of GXLs. Different probes of polarity in the cybotactic region are compared and the solute dependence of the local structure is explored. Bulk transport properties are measured with different probes in an effort to see if molecular interactions play a role in governing diffusion processes in GXLs.

Thesis (M.S.)--University of Central Florida, 2008. / Adviser: Michael Sigman. Includes bibliographical references (p. 118-122).