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Thermoplastic composites that have been reinforced by thermotropic liquid crystalline (TLCP) fibrils in the microscale and by nanoparticles in the nanoscale are defined as multi-scale wholly thermoplastic composites (WTCs). Multi-scale WTCs have been proposed as lightweight replacements with high performance for some traditional glass fiber (GF) and carbon fiber (CF) reinforced composites materials in various applications. TLCPs are known for performing mechanical properties similar to those of the lower end of CF but significantly better than those of GF. To enhance the mechanical properties of TLPC reinforced WTCs, carbon nanotubes (CNTs) are considered being used as a secondary enhancement in WTCs. CNTs have gathered significant interest in the last 30 years because of their high aspect ratio, high mechanical properties, and other high-performance properties. The focus of this work is on investigating the processing conditions of generating in situ injection-molded multi-scale WTCs, then extending the technology to dual-extrusion and fused filament fabrication (FFF) and obtain high-performance multi-scale WTC products. This dissertation initially focused on investigating the processing conditions, in particular mixing histories and processing temperature profiles, of generating in situ injection-molded multi-scale WTCs, which consist of a representative TLCP, scCO2 aided exfoliated CNTs, and the thermoplastic matrix polyamide 6 (PA 6). The supercooling behavior of the TLCP and thermal stability of PA 6 are studied by applying the rheological methods of small amplitude oscillatory shear (SAOS). Multiple mixing histories with CNTs and processing temperature profiles are analyzed based on the criterion of maximizing tensile properties of multi-scale WTCs and minimizing thermal degradation of the matrix. Under the optimum processing conditions, the in situ injection-molded multi-scale WTCs exhibit a 26% and 34% tensile modulus and strength enhancement, compared to the in situ injection-molded WTCs with no CNTs. Scanning electron micrograph (SEM) images were used to understand the enhancement. The second part of this work is to extend the scCO2 aided in situ multi-scale WTCs processing technology to dual-extrusion and FFF. Multi-scale WTC filaments, which consists of TLCP, CNTs, and polyamide copolymer (PAc), are generated by dual-extrusion, and 3D printed into rectangular specimens in FFF. The 1 wt% CNTs reinforced multi-scale WTC filaments generated by the means of dual-extrusion exhibit 225% and 80% improvement in tensile modulus and strength, respectively, compared to the WTC filaments with no CNTs. In FFF, 40 wt% TLCP/1 wt% CNT/PAc 3D printed specimens with filament laid in longitudinal direction exhibited excellent tensile modulus and strength of 38.92 GPa and 127.16 MPa, respectively. The well-dispersed exfoliated CNTs show high alignment with TLCP microfibrils in the multi-scale WTC filaments and their laid-down specimens, which causes the significant tensile modulus enhancement. Bridging elements are discovered between TLCP fibrils and PAc matrix to improve interfacial adhesion, which is attributed to the well-dispersed exfoliated CNTs. Finally, the significant improvements in tensile properties attributed to scCO2 aided exfoliated CNTs in WTCs are verified on the multi-scale WTCs based on polypropylene (PP). Moreover, additional tensile properties improvements for exfoliated CNTs reinforced multi-scale WTCs are obtained with the use of maleic anhydride grafted polypropylene (MAPP). With 1 wt% CNTs and 16 wt% MAPP dual reinforcement, 20 wt% TLCP reinforced WTCs based on polypropylene (PP) exhibit 265%, 274%, and 182% improvement in the tensile modulus of the filaments, laid up specimens in the concentric pattern and laid up specimens in ±45° rectilinear pattern, respectively. The dual reinforcement also improves the tensile strength of 20 wt% TLCP reinforced WTC filaments by up to 73%. The high alignment between TLCP fibrils and CNTs are confirmed in the multi-scale WTCs based on PP. Besides the bridging elements attributed to CNTs found in the second part of this work, SEM images show that CNTs are partially trapped in TLCP fibrils. / Doctor of Philosophy / Considering the need for environmentally friendly materials, novel thermoplastic composites with high mechanical performance, lightweight, and potentially high recyclability properties were generated in this work. Two types of thermoplastic matrices, polyamide (PA or nylon) and polypropylene (PP) were reinforced with carbon nanotubes (CNTs) and rigid chain polymers known as thermotropic liquid crystalline polymers (TLCPs). CNTs are known for their high mechanical properties and high aspect ratio, which are helpful to reinforce thermoplastic composite materials. During injection molding and the dual-extrusion processes, TLCPs deform into almost continuous microfibrils and reinforce the thermoplastic matrices. Instead of using traditional glass fibers or carbon fibers to reinforce thermoplastics, TLCP reinforced thermoplastic composites, which are defined as wholly thermoplastic composites (WTCs), can retain their mechanical properties during the recycling process such as in injection molding and have better performance during the lay-down process in fused filament fabrication. The goal of this work was to generate CNTs reinforced WTCs for use in injection molding and fused filament fabrication with high mechanical performance. In the injection molding process for generating CNTs reinforced WTC end-gated plaques, it was determined that the optimum thermal mixing histories for the CNTs could be identified by the inspections of the tensile property measurements and scanning electron microscopy (SEM). With the obtained optimum thermal mixing histories with CNTs, CNTs reinforced WTC filaments were generated by dual extrusion technology and used in fused filament fabrication. With 1 wt% addition of CNTs, the tensile properties of WTCs were significantly enhanced in both the filament materials and the laid-down parts. Especially, the CNT reinforced WTC filaments based on nylon matrices exhibited competitive tensile moduli to long carbon fiber reinforced nylon composite filaments, which was also competitive to the properties of aluminum alloys. In addition, the laid-down parts of CNTs reinforced WTC based on PP presented further tensile strength improvement due to the improved interfacial adhesion between the laid-down filaments and between layers, which was attributed to the addition of maleic anhydride grafted polypropylene.

Proton exchange membrane fuel cells (PEMFCs) have been extensively studied as clean, sustainable and efficient power sources for electric vehicles, and portable and residential power sources. As one of the key components in PEMFC system, proton exchange membranes (PEMs) act as the electrolyte that transfers protons from the anode to the cathode. The state-of-art commercial PEM materials are typically based on perfluorinated sulfonic acid containing ionomers (PFSAs), represented by DuPont's Nafion®. Despite their good chemical stability and proton conductivity at high relative humidity (RH) and low temperature, several major drawbacks have been observed on PFSAs, such as high cost, high fuel permeability, insufficient thermo-mechanical properties above 80°C, and low proton conductivity at low RH levels. Therefore the challenge lies in developing alternative PEMs which feature associated ionic domains at low hydration levels. Nanophase separated hydrophilic-hydrophobic block copolymer ionomers are believed to be desirable for this purpose Three series of hydrophobic/hydrophillic, partially fluorinated/sulfonated multiblock copolymers were synthesized and characterized in this thesis. The hydrophilic blocks were based upon the nucleophilic step polymerization of 3, 3′-disulfonated, 4, 4′-dichlorodiphenyl sulfone (SDCDPS) with an excess 4, 4′-biphenol (BP) to afford phenoxide endgroups. The partially fluorinated hydrophobic blocks were largely based on 4, 4′-hexafluoroisopropylidenediphenol (6F-BPA) and various difluoro monomers (excess). These copolymers were obtained through moderate temperature (~130-150°C) coupling reactions, which minimize the ether-ether interchanges between hydrophobic and hydrophilic telechelic oligomers via a nucleophilic aromatic substitution mechanism. The copolymers were obtained in high molecular weights and were solvent cast into tough membranes, which had nanophase separated hydrophilic and hydrophobic regions. The performance and structure-property relationships of these materials were studied and compared to random copolymer systems. NMR results supported that the multiblock sequence had been achieved. They displayed superior proton conductivity, due to ionic, proton conducting channels formed through the self-assembly of the sulfonated blocks. The nano-phase separated morphologies of the copolymer membranes were studied and confirmed by transmission electron microscopy (TEM) and small angle X-ray scattering (SAXS). Through control of a variety of parameters, including ion exchange capacity and sequence lengths, performances as high, or even higher than those of the state-of-the-art PEM, Nafion®, were achieved. Another series of semi-crystalline hydrophobic poly(ether ether ketone)-hydrophilic sulfonated poly(arylene ether sulfone) (PEEK-BPSH100) multiblock copolymers was first synthesized and characterized. However due to their semi-crystalline structure, PEEK blocks are insoluble in most organic solvents at relatively low reaction temperatures, which prevents the coupling reaction between PEEK and BPS100. In order to facilitate the synthesis and processing, removable bulky ketimine was introduced to synthesize amorphous pre-oligomers poly(ether ether ketimine) (PEEKt). The synthetic procedure first involves the synthesis of hydrophobic poly(ether ether ketimine)-hydrophilic sulfonated poly(arylene ether sulfone) (PEEKt-BPS100) multiblock pre-copolymers via coupling reactions between phenoxide terminated hydrophilic BPS100 and fluorine terminated hydrophobic PEEKt blocks. The membranes cast from PEEKt-BPS100 were boiled in 0.5M sulfuric acid water solution to hydrolyze the amorphous PEEKt blocks to semi-crystalline PEEK blocks and acidify BPS100 blocks to BPSH100 blocks simultaneously. FT-IR spectra clearly showed the successful hydrolysis and acidification. The proton conductivity, water uptake and other membrane properties of the acidified semi-crystalline PEEK-BPSH100 membranes were then evaluated and compared with those of the state-of-the-art PEM, Nafion®. / Ph. D.

Among the myraid energy storage technologies, polymer electrolytes have been widely employed in diverse applications such as fuel cell membranes, battery separators, mechanical actuators, reverse-osmosis membranes and solar cells. The polymer electrolytes used for these applications usually require a combination of properties, including anisotropic orientation, tunable modulus, high ionic conductivity, light weight, high thermal stability and low cost. These critical properties have motivated researchers to find next-generation polymer electrolytes, for example ion gels. This dissertation aims to develop and characterize a new class of ion gel electrolytes based on ionic liquids and a rigid-rod polyelectrolyte. The rigid-rod polyelectrolyte poly (2,2'-disulfonyl-4,4'-benzidine terephthalamide) (PBDT) is a water-miscible system and forms a liquid crystal phase above a critical concentration. The diverse properties and broad applications of this rigid-rod polyelectrolyte may originate from the double helical conformation of PBDT molecular chains. We primarily develop an ionic liquid-based polymer gel electrolyte that possesses the following exceptional combination of properties: transport anisotropy up to 3.5×, high ionic conductivity (up to 8 mS cm⁻¹), widely tunable modulus (0.03 – 3 GPa) and high thermal stability (up to 300°C). This unique platform that combines ionic liquid and polyelectrolyte is essential to develop more advanced materials for broader applications. After we obtain the ion gels, we then mainly focus on modifying and then applying them in Li-metal batteries. As a next generation of Li batteries, the Li-metal battery offers higher energy capacity compared to the current Li-ion battery, thus satisfying our requirements in developing longer-lasting batteries for portable devices and even electric vehicles. However, Li dendrite growth on the Li metal anode has limited the pratical application of Li-metal batteries. This unexpected Li dendrite growth can be suppressed by developing polymer separators with high modulus (~ Gpa), while maintaining enough ionic conductivity (~ 1 mS/cm). Here, we describe an advanced solid-state electrolyte based on a sulfonated aramid rigid-rod polymer, an ionic liquid (IL), and a lithium salt, showing promise to make a breakthrough. This unique fabrication platform can be a milestone in discovering next-generation electrolyte materials. / Ph. D. / Among the myraid energy storage technologies, polymer-based electrolytes have been widely employed in diverse applications such as fuel cell membranes, battery electrolytes, “artificial muscle” mechanical actuators, reverse-osmosis membranes and solar cells. The materials used for each of these applications usually require a specific combination of properties, which include anisotropic orientation, tunable mechanical stiffness (modulus), high ionic conductivity, light weight, high thermal stability and low cost. These critical properties have motivated researchers to find next-generation polymer-based electrolytes, for example “ion gels” that consist of a polymer combined with ionic liquids or salts. This thesis describes development of an ion gel that possesses the following exceptional combination of properties: high ionic conductivity (up to 8 mS cm^-1), widely tunable modulus (0.03 ‒ 3 GPa), ion transport anisotropy up to 3.5×, and high thermal stability (up to 300°C). Thus, this unprecedented material shows liquid-like ion motions inside a matrix with solid-like stiffness, and in a material that can withstand extreme temperatures and will not burn. After obtaining these ion gels, we are then mainly focusing on modifying them for application in safe and high density Li-metal batteries. As a next generation of Li batteries, the Li-metal battery offers higher energy capacity compared to the current Liion battery, thus satisfying our requirements in developing longer-lasting batteries for portable devices and even electric vehicles. However, Li dendrite growth on the Li metal anode has limited the pratical application of Li-metal batteries. This unexpected Li dendrite growth can be supressed by developing polymer electrolytes with high modulus (~ GPa), while maintaining sufficient ionic conductivity (~ 1 mS/cm) for efficient battery operation. In short, this thesis describes an advanced solid-state electrolyte based on a kevlar-like (sulfonated aramid) rigid-rod polymer, an ionic liquid (IL), and a lithium salt, showing promise to make a breakthrough and enable practical Li-metal batteries. Furthermore, the unique fabrication platform for these ion gels represents a new paradigm for discovering next-generation electrolyte materials for a wide variety of applications.

No / AIM: The aim of the study was to determine, objectively and non-invasively, whether changes in accommodative demand modify differentially the peripheral refraction in emmetropic and myopic human eyes. METHODS: Forty subjects (19 male, 21 female) aged 20-30 years (mean 22.7 (SD 2.8) years), 21 emmetropes (mean spherical equivalent refractive error (MSE) -0.13 (SD 0.29) D) and 19 myopes (MSE -2.95 (SD 1.76) D) participated in the study. Ametropia was corrected with soft contact lenses (etafilcon A, 58% water content). Subjects viewed monocularly a stationary, high contrast (85%) Maltese cross at 0.0, 1.0, 2.0 and 3.0 D of accommodative demand and at 0, 10, 20 and 30 degrees field angle (nasal and temporal) through a +3.0 D Badal optical system. Static recordings of the accommodation response were obtained for each accommodative level, at each field angle, with an objective, open-view, infrared optometer. RESULTS: Peripheral mean spherical equivalent (M) data showed that the emmetropic cohort exhibited relative myopic shifts into the periphery, while the myopic group showed hypermetropic shifts. Increasing accommodative demand did not alter the peripheral refractive profile in either the temporal (p = 0.25) or nasal (p = 0.07) periphery with no differential accommodative effect between refractive groups in either the temporal (p = 0.77) or nasal (p = 0.73) field. Significant shifts in the J(0) astigmatic component were seen in the temporal (p<0.0005) and nasal (p<0.0005) fields with increasing eccentricity. Interaction effects between eccentricity and accommodative demand illustrated that increasing accommodative demand significantly altered the peripheral refractive profile in the temporal J(0) astigmatic component (p<0.0005). The nasal periphery, however, failed to show such an effect (p = 0.65). CONCLUSIONS: Alterations in peripheral refraction augmented by changes in ocular accommodation are relatively unaffected by refractive error for young, healthy human eyes.

The thesis entitled "Phase Behaviour in Crystalline Solids: Exploring the Structure Guiding Factors via Polymorphism, Phase Transitions and Charge Density Studies" consists of five chapters divided into two parts. A basic introductory section describes the topics relevant to the work and the methods and techniques utilized. Part A contains two chapters that discuss the structural aspects related to polymorphism, solvatomorphism, conformational preferences and phase transitions exhibited by active pharmaceutical ingredients (APIs). It also discusses the structure-property correlations in API crystal forms and the possible utility of second harmonic generation (SHG) for their bulk characterization. Part B has three chapters that discuss experimental and theoretical charge density analyses of intra-and intermolecular interactions that play structure guiding roles in some of the APIs discussed in Part A. The main focus of the present work is to characterize the interaction patterns devoid of strong classical hydrogen bonds. The case studies include multifurcated C - H …O hydrogen bonds, the “carbon bonding” and chalcogen interactions involving Se and S atoms. In addition to charge density studies, in situcryocrystallography and molecular complexation experiments have been employed to examine structural consequences of chalcogen bonding. Further, Appendices 1 and 2 describe phase transition studies on the inorganic mineral kröhnkite and its high temperature phase transitions leading to novel inorganic structural types. Part A: Polymorphism and phase behaviour in Active Pharmaceutical Ingredients (APIs) Chapter 1 discusses case studies of polymorphism, supramolecular preference sand phase transitions exhibited by active pharmaceutical ingredients (APIs). Section 1.1 deals with the polymorphism of an anti-oxidant drug candidate ebselen and its hydroxyl derivative. The potential of organoselenium compounds to form a Se…O chalcogen bonded supramolecular recognition unit (synthon) has been established in these polymorphs and its generality is substantiated with the help of a Cambridge Structural Database (CSD) analysis. Section 1.2 demonstrates the utility of the ‘chalcogen bonded supramolecularsynthon’ in generating molecular complexes of APIs. A series of salts and co-crystals of the amyotrophic lateral sclerosis drug Riluzole have been synthesized in order to evaluate the structure directing role of S…O chalcogen bonded synthon in their crystal structures. Section 1.3adescribes the generation of polymorphs and solvatomorphs of the antidepressant drug candidate fenobamand associated phase transitions. The tautomeric preference in this molecule has been rationalized from the crystal structure analysis and abinitioenergy calculations. Further, section 1.3b utilizes chemical derivatization as a means to experimentally simulate thetautomeric preference and molecular conformations in several derivatives of fenobam and thiofenobam. Section 1.4 describes the issue of solvatomorphism and the generation of the fifth solvatomorph of gallic acid, its structural complexity and temperature induced phase transitions. The ability of solvent water molecules to drive structural diversity, by forming ‘hydration synthons’,is demonstrated in this case. Chapter 2 presents a novel methodology for the detection of polymorphic impurities in APIs based on second harmonic generation (SHG).The SHG based method has been employed to polymorphic mixtures of fenobam, hydrochlorothiazide, pyrazinamide, tolbutamide, curcumin, febuxostat and nimesulide.The conventional methods such as powder X-ray diffraction (profile fitting analysis), FT-IR, Raman spectroscopy and thermal analysesto detect the presence of polymorphic impuritiesin bulk API samples are employed on the mixtures of these API samples and the impurity detection limits are compared with the proposed SHG methodology. The APIs used in these case studies were screened for their SHG efficiency using quantum chemical calculations of hyperpolarizability and HOMO-LUMO charge redistribution behaviour. Further, a correlation with the crystal symmetry, relative packing arrangement of molecules and the observed SHG efficiency have been discussed in of some of these cases. Part B: Exploring the nature and structural consequences of nonbonding interactions in molecular crystals Chapter 3 discusses the electron density features of quasi-trifurcated CH…Cl/CH…O interaction motifs leading to ‘carbon bonding’ and a trifurcated CH…O hydrogen bond motif. Section 3.1 describes the experimental and theoretical charge density analyses of quasi-trifurcated CH…Cl and CH…O motifsand investigates the existence of “carbon bonding” in solid state. The experimental charge density evidence for “carbon bonding” have been analyzed in cases of fenobam and dimethylamine: 4-hydroxybenzoic acid complex. The existence of this unconventional interaction, which roughly mimics the transition state geometry of SN2 (bimolecular nucleophilic substitution) reaction, is further established by a CSD analysis. Section 3.2 describes the experimental and theoretical charge density analyses of ferulic acid and compares the topological features associated with a trifurcated CH…O hydrogen bond motif, with corresponding strong classical OH…O hydrogen bonds. The study demonstrates the “Gulliver effect” of weak interactions in charge density terms. Charge density based interaction energy calculations via EPMM and EML methods have been utilized in this context to evaluate the relative strength of such interactions. Chapter 4 discusses the charge density features of intermolecular chalcogen bonding interactions involving selenium and sulphur atoms.Section 4.1 describes the experimental and theoretical charge density analyses of ebselen and its hydroxyl derivative. The charge density characterization of the conserved chalcogen bond synthon (discussed in chapter 1, section 1.1) has been carried out and electronic nature and geometric dependence of Se…O interactions have been explored. The mechanism of drug action of ebselen has been correlated with the experimentally observed charge density distribution around the intramolecular SeC and SeN bonds. Section 4.2 explores the homochalcogen interactions such as S…SandSe…Se in phenol analogues. In situ cryocrystallographic studies on thiophenol, selenophenol and their solid solutions are described. Veggard’s law-like behaviour observed in these solid solutions have been rationalized and the S…S and Se…Sehomochalcogen interactions have been evaluated in these liquid systems which are devoid of any other packing forces such as strong hydrogen bonds. Chapter 5 discusses the conformation locking potential of intramolecular S…O chalcogen bonding in sulfadrugs. Section 5.1 discusses conformation locking in the antibioticdrugsulfamethizole. A two pronged approach has been adopted in the study; a) generation of cocrystals and salts of sulfamethizole for the ‘experimental simulation’ of the molecular conformation, b) evaluation of charge density distribution around the intramolecular S…O interaction region in sulfamethizole. Section 5.2 describes the effect of ‘simple hybridized orbital geometry’ in the formation of intramolecular S…O chalcogen bonding. The experimental charge density analysis of the carbonic anhydrase inhibitor drug acetazolamide has been carried out and the two different intramolecular S…O geometries have been compared in terms of the charge density topology. The analysis highlights the advantage of “orbital geometry” consideration over the conventional distance-angle criteria in assessing nonbonded interactions.

Electron beam induced current (EBIC) and atom probe tomography (APT) were used in this study to determine electrical activities and impurity compositions at extended defects in multicrystalline silicon (mc-Si) samples. The results provide, for the first time, information regarding the chemical species present at defects whose electrical activity has previously been measured. A new APT specimen fabrication process was developed with the ability to select a specific defect for APT analysis. Development of the APT specimen fabrication process proceeded by first selecting and optimising the preferential etching for nano-scale defect delineation. Three etchants were evaluated, namely Secco, Sirtl and Dash, from which the Secco etch was selected. Three parameters were optimised to produce etch pits with geometries that meet the requirements imposed by APT specimen fabrication methods. The optimum parameters were 0.05M potassium dichromate concentration, 20°C etch temperature, and 30sec etch time. In the second stage, marking techniques were developed in order for the defects to be located throughout the APT specimen fabrication process. However, it became apparent that the conventional APT specimen fabrication method could not be used to fabricate APT specimens containing selected defects in a mc-Si sample. This led to the development of a novel APT specimen fabrication approach which allowed APT specimens to be fabricated, reproducibly, containing grain boundaries and isolated dislocations. In order to evaluate accurately iron contamination in mc-Si, four atom probe parameters were optimised to maximise detection sensitivity: the evaporation rate, the laser beam energy, the pulse repetition rate and the specimen temperature. The optimisation process can be divided in to two parts. In the first part, a matrix of pre-sharpened single-crystal silicon specimens was subjected to a variety of experimental parameters. The optimised parameters were determined to be 0.3% evaporation rate, 0.5nJ beam energy, 160kHz repetition rate and 55K specimen temperature. The second part was to determine the iron detection efficiency –the percentage of detected Fe ions that can be correctly identified as Fe– and sensitivity using these parameters to analyse a specially prepared iron calibration specimen. The values were determined to be a detection efficiency of about 35% and sensitivity of 54ppm or 2.70x10¹⁸ atom/cm³. The APT specimen fabrication process and the optimised APT analysis parameters were used to analyse four extended defects in mc-Si samples subjected to three different processing conditions, namely gold-contaminated, as-grown and phosphorus diffusion gettering (PDG). The important aspects of the analysis are listed below: • Gold was not detected at the grain boundary and its associated dislocations in the gold-contaminated specimen. The binding enthalpy of gold to such defects is thus less than 0.63eV. • Iron was not detected in any specimen. • Copper was observed at the grain boundary in the as-grown specimen in the form of individual atoms as well as clusters with diameters ranging between 4nm and 9nm. The electrical activity of the grain boundary was about 58%. • Nickel and carbon were detected at the grain boundary in the post-PDG specimen with the former having platelet structures with diameters and thicknesses ranging between 4nm-7nm and 2nm-4nm, respectively. The recombination strength of the defect was about 22%. • Two nickel clusters were found at the isolated dislocation in the post-PDG specimen. The clusters were spherical with an average diameter of 10nm. The distance between the two clusters was 35nm. The recombination strength of the defect was about 4%.

This thesis is devoted to the mathematical analysis of models describing the energy of defects in crystalline solids via variational methods. The first part of this work studies a discrete model describing the energy of a point defect in a one dimensional chain of atoms. We derive an expansion of the ground state energy using Gamma-convergence, following previous work on similar models [BDMG99,BC07,SSZ11]. The main novelty here is an explicit characterisation of the first order limit as the solution of a variational problem in an infinite lattice. Analysing this variational problem, we prove a regularity result for the perturbation caused by the defect, and demonstrate the order of the next term in the expansion. The second main topic is a discrete model describing screw dislocations in body centred cubic crystals. We formulate an anti plane lattice model which describes the energy difference between deformations and, using the framework defined in [AO05], provide a kinematic description of the Burgers vector, which is a key geometric quantity used to describe dislocations. Apart from the anti plane restriction, this model is invariant under all the natural symmetries of the lattice and in particular allows for the creation and annihilation of dislocations. The energy difference formulation enables us to provide a clear definition of what it means to be a stable deformation. The main results of the analysis of this model are then first, a proof that deformations with unit net Burgers vector exist as globally stable states in an infinite body, and second, that deformations containing multiple screw dislocations exist as locally stable states in both infinite bodies and finite convex bodies. To prove the former result, we establish coercivity with respect to the elastic strain, and exploit a concentration compactness principle. In the latter case, we use a form of the inverse function theorem, proving careful estimates on the residual and stability of an ansatz which combines continuum linear elasticity theory with an atomistic core correction.

L’exposition prolongée par inhalation à des poussières de taille respirable contenant de la silice cristalline est reconnue pour causer des maladies respiratoires dont le cancer du poumon et la silicose. De nombreuses études ont relevé la surexposition des travailleurs de la construction à la silice cristalline, puisque ce composé est présent dans de nombreux matériaux utilisés sur les chantiers. L’évaluation de l’exposition à la silice cristalline dans cette industrie constitue un défi en raison de la multitude de conditions de travail et de la nature éphémère des chantiers. Afin de mieux cerner cette problématique, une banque de données d’exposition professionnelle compilée à partir de la littérature a été réalisée par une équipe de l’Université de Montréal et de l’IRSST, et constitue le point de départ de ce travail. Les données présentes dans la banque ont été divisées en fonction de la stratégie d’échantillonnage, résultant en deux analyses complémentaires ayant pour objectif d’estimer les niveaux d’exposition sur le quart de travail en fonction du titre d’emploi, et selon la nature de la tâche exécutée. La méthode de Monte Carlo a été utilisée pour recréer les échantillons provenant de données rapportées sous forme de paramètres de synthèse. Des modèles Tobit comprenant les variables de titre d’emploi, tâche exécutée, durée, année et stratégie d’échantillonnage, type de projet, secteur d’activité, environnement et moyens de maîtrise ont été développés et interprétés par inférence multimodèle. L’analyse basée sur le quart de travail a été réalisée à partir de 1346 données d’exposition couvrant 11 catégories de titre d’emploi. Le modèle contenant toutes les variables a expliqué 22% de la variabilité des mesures et la durée, l’année et la stratégie d’échantillonnage étaient d’importants prédicteurs de l’exposition. Les chantiers de génie civil et les projets de nouvelle construction étaient associés à des expositions plus faibles, alors que l’utilisation de moyens de maîtrise diminuait les concentrations de 18% à l’extérieur et de 24% à l’intérieur. Les moyennes géométriques les plus élevées prédites pour l’année 1999 sur 8 heures étaient retrouvées chez les foreurs (0.214 mg/m3), les travailleurs souterrains (0.191 mg/m3), les couvreurs (0.146 mg/m3) et les cimentiers-applicateurs (0.125 mg/m3). 1566 mesures réparties en 27 catégories de tâches étaient contenues dans la seconde analyse. Le modèle contenant toutes les variables a expliqué 59% des niveaux d’exposition, et l’ensemble des variables contextuelles étaient fortement prédictives. Les moyennes géométriques prédites pour l’année 1998 et selon la durée médiane par tâche dans la banque de données étaient plus élevées lors du bouchardage du béton (1.446 mg/m3), du cassage de pièces de maçonnerie avec autres outils (0.354 mg/m3), du décapage au jet de sable (0.349 mg/m3) et du meulage de joints de brique (0.200 mg/m3). Une diminution importante des concentrations a été observée avec les systèmes d’arrosage (-80%) et d’aspiration des poussières (-64%) intégrés aux outils. L’analyse en fonction des titres d’emploi a montré une surexposition généralisée à la valeur guide de l’ACGIH et à la norme québécoise, indiquant un risque à long terme de maladies professionnelles chez ces travailleurs. Les résultats obtenus pour l’évaluation en fonction de la tâche exécutée montrent que cette stratégie permet une meilleure caractérisation des facteurs associés à l’exposition et ainsi de mieux cibler les priorités d’intervention pour contrôler les niveaux d’exposition à la silice cristalline sur les chantiers de construction durant un quart de travail. / Chronic inhalation of respirable inorganic dusts containing crystalline silica is linked to occupational respiratory diseases such as lung cancer and silicosis. Several studies have documented the overexposure to respirable crystalline silica in construction workers, as this compound is found in several building materials and many operations can release fine suspended particles. Comprehensive quantitative exposure assessment in this industry is challenging due to the variety in tasks performed, materials used and environmental conditions between work sites, the mobile workforce and the temporary nature of construction sites. An occupational exposure database of silica exposure was compiled from the literature by a research group from the Université de Montréal and the Institut de recherche en santé et en sécurité du travail (IRSST) to address this issue. The exposure data contained in the database were separated on the basis of sampling strategies, which resulted in two separate – but complementary – analyses. The first analysis was restricted to samples collected to compare levels with an occupational exposure limit, in order to estimate work-shift respirable crystalline silica exposure by construction trade. The second analysis used measurements collected under a task-based sampling strategy in order to estimate the exposure levels associated with specific activities. Monte-Carlo simulation was used to recreate individual exposures from measurements reported as summary statistics. Modeling was performed using Tobit models within a multimodel inference framework, with construction trade, task, sampling duration, year and strategy, project type, construction sector, workspace and control methods as potential predictors. The dataset for the analysis by construction trade was comprised of 1346 exposure measurements and included 11 trade categories. The model containing all the variables explained 22% of the exposure variability and the sampling duration, year and strategy were identified as important predictors. Civil engineering and roadwork sites as well as new construction projects were associated with lower exposure levels, while the use of control methods reduced silica concentrations by 18% outdoors and 24% indoors. Predicted geometric means (GM) for year 1999 were the highest for drillers (0.214 mg/m3), underground workers (0.191 mg/m3), roofers (0.146 mg/m3) and cement grinders/finishers (0.125 mg/m3), based on a 8-hour shift. Heavy equipment operators (0.041 mg/m3) and foremen (0.047 mg/m3) had the lowest predicted GMs. 1566 task-based measurements, representing 27 task categories, were included in the activity-specific dataset. The proportion of variance explained by the model containing all the variables was 59%, and all the variables investigated had a strong influence on the exposure levels. Predictions were made based on the year 1998 and the median duration by task in the dataset. The largest predicted GMs were associated with the following operations: scabbling concrete (1.446 mg/m3), chipping with other tools (0.354 mg/m3), abrasive blasting (0.349 mg/m3) and tuck point grinding (0.200 mg/m3). Important reductions in exposure levels were found with the use of tool-integrated water sprays (-80%) and local exhaust ventilation (-64%). Important overexposure to the ACGIH Threshold Limit Value and the Québec exposure limit was found for all the trades investigated, indicating a long-term risk of silica-related occupational diseases. The results of the task-based analysis suggest that this sampling strategy provides a better characterization of the factors affecting exposure and the impacts of engineering dust control methods to control long-term exposure levels.

Tissue Engineering is a scientific discipline that aims to regenerate tissues and organs that are diseased, lost or congenitally absent. It encompasses the use of suitable synthetic equivalents of native extracellular matrix that may or may not be supplemented with cells or relevant growth factors. Such scaffolds are designed to reside at the site of implantation for a variable period of time during which they induce the regeneration of native tissue. During this time, they also provide a template for new cells to attach, infiltrate, differentiate into appropriate phenotype and eventually restore function of the concerned tissue. Among the factors that affect the outcome are the composition of scaffold, methods of fabrication, bulk properties of the scaffold and topography and architecture at the cellular level. Bone is unique in the body in that it is one of the few tissues capable of complete regeneration even in adults, as seen during fracture healing. However, certain conditions (non-union of fractures, congenital and acquired bone deficiencies) exist in which the regenerative capacities of bone are exceeded and appropriate intervention becomes necessary. Current treatment options include autologous bone grafts harvested from iliac crest or de-cellularized allografts or synthetic substitutes made from metals, ceramics and polymers. However these options have serious limitations: while autografts are limited in supply, necessitate second surgery and show inadequate vascularization, allografts can transmit viral infections. Metals, ceramics and polymers are in essence structural replacements without performing any biological function. Other problems associated with these synthetic materials include adverse immune reactions, corrosion, stress-shielding and secondary fractures due to inadequate osseo-integration. Bone tissue engineering is a specialized field of research that provides an alternative strategy to repair bone defects by exploiting the advances in engineering and better understanding of bone biology. Scaffold-based tissue engineering approach is a promising field that involves implantation of a biomaterial that is specifically matched in terms of biological and material properties to the tissue it replaces. This study explores the feasibility of using electrospinning as a potential fabrication strategy for bone tissue engineering applications, more specifically intended for cleft palate repair. This model represents a congenital deformity that affects both hard and soft tissues and presents unique challenges and opportunities. Among the challenges are: the need for the implant allow growth of the most complex areas of the facial skeleton, integrate and grow with the patient through adolescence, the ability of the implant to not interfere with vital functions including breathing and feeding. Further the implant should provide a flexible matrix that can effectively support erupting teeth. In spite of these extreme demands, maxilla is a non load-bearing membranous bone, a favorable consideration from materials engineering perspective. The present study is organized into three independent sections. The first section investigates developing strategies intended to improve the material properties of electrospun bone scaffold. Bone is composed of a high volume fraction (50%) of inorganic hydroxyapatite nanocrystals that is closely associated with collagen. The dispersal of brittle mineral is critical in not only strengthening the bone in compression but also contributes to the osteoconductivity of the matrix. Since loading of mineral in a bone scaffold is a serious limitation, we attempted to achieve improved loading of bone mineral by dual mineralization approach. We first incorporated nanocrystalline hydroxyapatite (nHA) directly into the scaffold by adding it to the electrospinning polymer solution. The second step involves inducing biomimetic mineralization of electrospun scaffolds by incubating them in simulated body fluid (SBF) for 2 weeks. The hypothesis was that the nanocrystalline hydroxyapatite seeded during electrospinning would act as sites for nucleation and further crystal growth when incubated in solution supersaturated with respect to calcium and phosphate ions. We tested this approach in two synthetic, biocompatible polymers-polydioxanone and poly (lactide: glycolide) and four formulations of SBF with differential loading of nHA (0-50% by wt. of polymer). A modified Alizarin Red S (ARS) staining that specifically binds to calcium was developed that allowed us to quantify the mineral content of 3D scaffold with great accuracy. Results indicated a unique combination of factors: PDO scaffolds containing 50% nHA incubated in 1x revised-SBF incubated under static conditions gave maximum mineralization over a period of two weeks. We then sought to exploit these findings to engineer a stiffer scaffold by stacking multiple layers together and cold welding them under high pressure. Electrospun scaffolds (1, 2 or 4 layered stacks) were either compressed before or after mineralizing treatment with SBF. After two weeks, scaffolds were analyzed for total mineral content and stiffness by uniaxial tensile testing. Results indicated while compression of multiple layers significantly increases the stiffness of scaffolds, it also had lower levels of mineralization partly due to increased density of fibers and loss of surface area due to fiber welding. However this can be offset to a reasonable degree by increasing the number of stacks and hence this strategy can be successfully adopted to improve the mechanical properties of electrospun scaffolds. The second section introduces a novel infrared imaging technique to quantify and characterize the biological activity of biomaterials, based on cell adhesion. Cells attach to the surface by the formation of focal contacts where multiple proteins including vinculin and talin assemble to signal critical processes like cell survival, migration, proliferation and differentiation. After allowing MG-63 osteoblasts to adhere to 2D biomaterial surface coated with extracellular matrix proteins (collagen, gelatin, fibronectin) cells were fixed and probed with antibodies for vinculin and talin. Secondary antibodies, tagged with infrared-sensitive fluorescent dyes, were used to quantify the molecules of interest. In addition, the kinetics of focal contact formation in these different substrates was followed. Successful quantification of focal contacts were made and further research revealed phosphorylation of vinculin at pY-822 as one potential mechanism for recruitment of vinculin to focal contacts. Hence it could represent a subset of vinculin and might serve as a specific molecular marker for focal contacts. As an extension, we evaluated the possibility of using such an assay to quantify 3D electrospun tissue engineering scaffolds. We fabricated scaffolds of graded biological activity by electrospinning blends of polydioxanone and collagen in different ratios. Vinculin and talin expressed by MG-63 cultured on these scaffolds for 24 hours were quantified in a similar manner. Results indicate that while talin does not show a significant difference in expression among different scaffolds, vinculin showed a positive correlation with increasing biological activity of scaffolds. In conclusion, we have identified vinculin as a reliable marker of focal contacts in 3D scaffolds while phosphovinculin (pY-822) was more specific to focal contacts in coated 2D substrates. In both instances, infrared imaging proved to be reliable in study of focal contacts. The third section aims to make the bone scaffolds osteoinductive- a property of a material to induce new bone formation even when implanted in subcutaneous and intramuscular heterotopic sites. Bone morphogenetic proteins (BMP) are potent cytokines that can induce migration, proliferation and differentiation of stem cells along osteoblastic lineage. The therapeutic efficacy of BMPs in the treatment of severe bone defects has been identified and is currently FDA approved for specific orthopedic applications. BMPs are clinically administered in a buffer form that not only makes the treatment expensive but less effective. Suitable delivery systems for BMP delivery have been an intense area of investigation. We rationalized electrospinning as a strategy to incorporate BMP within the scaffold and that would enable controlled release when implanted. One of the drawbacks of using electrospinning to deliver bioactive molecules is the potential denaturing effect and eventual loss of activity of BMPs. The final section of this dissertation tries to develop sensitive and relevant assays that could answer intriguing questions about solvent-protein interaction. We chose to use the BMP-2/7 heterodimer as the osteoinductive molecule of choice because of its superior potency compared to homodimer counterparts. We characterized the detection and quantification of BMP-2/7 using a slot blot technique. Further, we used a novel cell line (C2C12 BRA) to test the retention of activity of BMP-2/7 that has been exposed to organic solvents. Results indicate significant loss of activity when BMPs are exposed to organic solvents but complete recovery was possible by diluting the solvent with an aqueous buffer.