No / Glass-ionomer cements (GICs) have been widely used for over forty years, because of their desirable properties in dentistry. The most important advantages of the GICs are associated with their ability to release long-term antimicrobial agents. However, GICs used as restorative materials have still lots of challenges due to their secondary caries and low mechanical properties. Recent studies showed that the fluoride-releasing activity of conventional GICs is inadequate for effectual antibacterial conservation in many cases. Therefore, many efforts have been proposed to modify the antibacterial features of GICs in order to prevent the secondary caries. Particularly, for achieving this goal GICs were incorporated into various biomaterials possessing antibacterial activities. The scope of this review is to assess systematically the extant researches addressing the antibacterial modifications in GICs in order to provide with an authoritative, at the same time in-depth understanding of controlled antibacterial release in this class of biomaterials. It also gives a whole perspective on the future developments of GICs and challenges related to antibacterial GICs.

<p> The advantageous properties of siloxanes find use in a wide range of applications. Unfortunately, the dynamic nature of silicones which is responsible for these properties is often a limitation in the controlled synthesis and modification of siloxane materials. Gaining greater control over these processes would allow for the synthesis of siloxane materials with more explicit structures, giving them a narrower range of properties and expanding application. Furthermore, the ability to synthesize more siloxane architectures with greater control would allow for an increased understanding of the relationship between structural features and physical properties.</p> <p> The synthesis of hydrosilane-rich siloxane elastomers and subsequent controlled modification, particularly with poly(ethylene oxide), is described. The effect of chain length and functionality (mono- or di-) was found to influence the morphology of grafted polymer. It was also possible to take advantage of the intrinsic properties of siloxanes to sequester hydrophilic moieties to the interior of the elastomer. Utilizing the same hydrosilane rich elastomers, a method for the independent modification of the interior and exterior of hydrosilane rich elastomers is presented. The careful selection of grafting moieties and solvents is used to provide or deny transport to the interior of the elastomer. This method is used to synthesize PEO modified elastomers with various subsequent internal modifications.</p> <p> A method for controlled synthesis of silicone-carbohydrate composites is also described. Utilizing bifunctional silane linkers, protected carbohydrates were functionalized with bulky diisopropyl hydrosilane groups before linkage to short and long silicones. Alternatively, the linker could first be joined to a silicone, followed by silylation of unprotected saccharides using the resultant hindered chlorosilane functional silicone. This method gave preferential silylation at primary hydroxyl groups.</p> <p> Finally, a method is presented for the synthesis of explicit branched siloxane architectures. The B(C6F5)3 catalyzed dehydrocarbonative coupling of hydrosilanes with alkoxysilanes was used to construct branched siloxane architectures in a stepwise fashion. High levels of control were available through manipulation of steric parameters: careful selection of starting materials and conditions allowed for the synthesis of explicit alkoxysilane functional branched siloxanes. These could be grafted to hydrosilane functional silicone polymers, or used to assemble explicit branched siloxanes. Further explorations demonstrated that the assembly process was not inhibited by the presence of organohalide- or alkene functional groups, allowing for the synthesis of functional siloxane moieties with explicit structures.</p> / Thesis / Doctor of Philosophy (PhD)

<p>In the last forty years, most of researches in casting fields especially in semi-solid metal state were dedicated to find new ways to enable near net shaped casting of Al alloys to improve the product properties and decreases the product cost. The thixoforming and rheocasting processes are presented as a ways by which the microstructure of the alloys can be changed to non-dendritic microstructure leading to improve the mechanical properties by mitigating the defect associated with the dendritic microstructure. Unfortunately, these processes have proved to be capital cost prohibitive and complicated for commercial production. Further, near net shaped casting of Al wrought alloys along with the superior properties and performance of these alloys have been a challenge for conventional casting routes due to the main disadvantage of hot tearing or hot cracking during solidification, which renders the cast component ineffective. To overcome the disadvantages of thixoforming and rheocasting processes, Controlled diffusion solidification (CDS) process was innovated to enable casting aluminum alloys with a non-dendritic morphology of the primary Al phase in the resultant cast microstructure and thus alleviating the problem of hot tearing and obtaining a cost effective product with improved mechanical properties. The CDS is a simple process involving mixing of two precursor alloys of different thermal masses (temperature and solute) and subsequently cast the resultant mixture of the desired solute composition and temperature as a near net shaped cast product. The process lends itself to easy commercialization with a marginal capital cost required for set up such as the addition of an extra holding furnace. Further, the CDS process would prove itself to be unique in its ability to cast Al based wrought alloys into near net shaped components without additional processes and cost.</p><p>The CDS process has been proven to yield a cast product with a non-dendritic Al phase morphology and this dissertation presents the in-depth details and analysis of the various events occurring during the process to obtain a successful cast part. The process involves various inter-related events such as mixing, re-distribution of thermal field, redistribution of solute field, three types of nucleation events and growth of these different nuclei. Further the dissertation aims to present a study of the critical parameters such as temperatures of the two pre-cursor alloys, initial mass ratio of these alloys and the rate of mixing them on the effectiveness of the CDS process.</p> <p>The results from this study shows that mixing two precursor alloys to form the final desired alloy presents a natural environment for copious nucleation events aided by distribution of these nuclei by forced convection followed by the formation of unique cells in the resultant mixture (micro-scale) with significant thermal and solute gradients. The solidification in the CDS process is unique and different from conventional casting process in that initial growth of the nuclei takes place with the solute diffusing towards and temperature diffusing away from the solid/liquid interface which presents a favorable environment for a stable unperturbed growth of the solid/liquid interface resulting in a non-dendritic morphology of the primary AI phase.</p><p>The proposed events in the CDS process has been verified with a few Al based wrought alloys and organic alloy systems.</p> / Thesis / Doctor of Philosophy (PhD)

<p> This thesis provides examples of new Si-based polymeric materials in three parts. The rapid evolution of hydrogen gas was observed after the combination of an α- or β-hydroxy carboxylic acid with an alkoxyhydrosilane. This unusual reactivity is interpreted to arise from the cooperative nucleophilic attack of the hydroxy group at silicon and the electrophilic attack of the acid proton at the Si-H bond. These systems further react to produce modified sol-gel materials in which the acid units were incorporated into the matrix.</p> <p> The reactions of tetraethoxysilane, (TEOS) and α-hydroxy acids similarly produced modified sol-gels (in solution, DMSO) and solids (precipitated from THF). The reaction kinetics were followed in these systems using 29Si NMR. Extensive characterisation of our sol-gels was carried out by the use of various NMR techniques, FT-IR, elemental analysis and thermal methods of solids characterisation (TGA, DTA, and DSC).</p> <p> Silicones containing vinylsilane units were prepared in order to examine the silicon β-effect in a polymeric system for the first time. The addition of bromine to these systems unexpectedly resulted in no reaction unless vigorous conditions were used. Steric factors as shown by molecular modeling are attributed to this lack of reactivity. Although many silicone systems were studied where more sterically "open" vinylsilanes were present these steric problems could not be overcome.</p> / Thesis / Master of Science (MSc)

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Electrical control over propellant combustion has the opportunity to improve the functionality and performance of various propulsion systems. In solid rocket motors, active burn rate modulation has the potential to enable throttling. In spacecraft propulsion systems, electrolysis of propellants may provide a means to reduce energy requirements and eliminate the need for expensive catalysts. The work presented in this thesis is concerned with fundamental science related to propellant electrolysis and the performance of rocket propulsion systems using electrolytic ignition. Specifically, the present research is concerned with the effect of conductive and energetic additives on the ignition, combustion, and extinction characteristics of lithium perchlorate-based propellants. Particular attention is paid to the relative importance of electrochemistry and ohmic heating during ignition and steady-state combustion as well as the relative influence of pressure and voltage during steady combustion. Research into the development of an electrically initiated propellant and its integration into a rocket motor is presented as well. This work focused primarily on surveying propellants based on ammonium perchlorate, lithium perchlorate, and hydroxylammonium nitrate for use in a small rocket motor. The decomposition processes of propellants based on ionic liquids and gel polymer electrolytes are detailed. Finally, data from motor firings is presented and parameters influencing the motor's performance and consistency are identified for future improvement. / Master of Science / The principal disadvantage of solid rocket motors is the lack of an active throttling and restart capability. Put simply, once a motor is ignited, it will burn until all of the propellant is consumed and there currently isn't a good method to speed up or slow down a solid rocket on command after it has been launched. As a result, the situations in which solid rocket motors can be used are limited. For example, solid rockets are not used in satellite propulsion because satellites need periodic adjustments to attitude and altitude, not a single boost. But solid rockets are relatively cheap, simple, and reliable and so various means of throttling solid propellants are being investigated. The method relevant to this work is throttling by using an electrical stimulus. By applying voltage across a propellant, the propellant can be ignited and the burning rate can be changed. The research here investigates what materials can be added to these propellants to make them ignite and burn faster as well as the development of novel propellants for applications in a small rocket motor.