Interactions sociales et réseaux sociaux en économique : modélisation et estimation

Cette thèse comporte trois essais sur les interactions sociales en sciences économiques. Ces essais s’intéressent à la fois au côté théeorique qu’empirique des interactions sociales. Le premier essai (chapitre 2) se concentre sur l’étude (théorique et empirique) de la formation de réseaux sociaux au sein de petites économies lorsque les individus ont des préférences homophilique et une contrainte de temps. Le deuxième essai (chapitre 3) se concentre sur l’étude (principalement empirique) de la formation de réseau sociaux au sein de larges économies où les comportement d’individus très distants sont aproximativement indépendants. Le dernier essai (chapitre 4) est une étude empirique des effets de pairs en éducation au sein des écoles secondaires du Québec. La méthode structurelle utilisée permet l’identification et l’estimation de l’effet de pairs endogène et des effets de pairs exogènes, tout en contrôlant pour la présence de chocs communs.

Self-organized sociopolitical interactions as the best way to achieve organized patterns in human social systems: going beyond the top-down control of classical political regimes

La monografía presenta la auto-organización sociopolítica como la mejor manera de lograr patrones organizados en los sistemas sociales humanos, dada su naturaleza compleja y la imposibilidad de las tareas computacionales de los regímenes políticos clásico, debido a que operan con control jerárquico, el cual ha demostrado no ser óptimo en la producción de orden en los sistemas sociales humanos. En la monografía se extrapola la teoría de la auto-organización en los sistemas biológicos a las dinámicas sociopolíticas humanas, buscando maneras óptimas de organizarlas, y se afirma que redes complejas anárquicas son la estructura emergente de la auto-organización sociopolítica.

Convergencia de vías de señalización en un modelo de apoptosis

La sepsis es un evento inflamatorio generalizado del organismo inducido por un daño causado generalmente por un agente infeccioso. El patógeno más frecuentemente asociado con esta entidad es el Staphylococcus aureus, responsable de la inducción de apoptosis en células endoteliales debida a la producción de ceramida. Se ha descrito el efecto protector de la proteína C activada (PCA) en sepsis y su relación con la disminución de la apoptosis de las células endoteliales. En este trabajo se analizó la activación de las quinasas AKT, ASK1, SAPK/JNK y p38 en un modelo de apoptosis endotelial usando las técnicas de Western Blotting y ELISA. Las células endoteliales (EA.hy926), se trataron con C2-ceramida (130μM) en presencia de inhibidores químicos de cada una de estas quinasas y PCA. La supervivencia de las células en presencia de inhibidores químicos y PCA fue evaluada por medio de ensayos de activación de las caspasas 3, 7 y 9, que verificaban la muerte celular por apoptosis. Los resultados evidencian que la ceramida reduce la activación de AKT y aumenta la activación de las quinasas ASK, SAPK/JNK y p38, en tanto que PCA ejerce el efecto contrario. Adicionalmente se encontró que la tiorredoxina incrementa la activación/fosforilación de AKT, mientras que la quinasa p38 induce la defosforilación de AKT.

Low-molecular-weight gelators: Elucidating the principles of gelation based on gelator solubility and a cooperative self-assembly model

This paper highlights the key role played by solubility in influencing gelation and demonstrates that many facets of the gelation process depend on this vital parameter. In particular, we relate thermal stability (T-gel) and minimum gelation concentration (MGC) values of small-molecule gelation in terms of the solubility and cooperative self-assembly of gelator building blocks. By employing a van't Hoff analysis of solubility data, determined from simple NMR measurements, we are able to generate T-calc values that reflect the calculated temperature for complete solubilization of the networked gelator. The concentration dependence of T-calc allows the previously difficult to rationalize "plateau-region" thermal stability values to be elucidated in terms of gelator molecular design. This is demonstrated for a family of four gelators with lysine units attached to each end of an aliphatic diamine, with different peripheral groups (Z or Bee) in different locations on the periphery of the molecule. By tuning the peripheral protecting groups of the gelators, the solubility of the system is modified, which in turn controls the saturation point of the system and hence controls the concentration at which network formation takes place. We report that the critical concentration (C-crit) of gelator incorporated into the solid-phase sample-spanning network within the gel is invariant of gelator structural design. However, because some systems have higher solubilities, they are less effective gelators and require the application of higher total concentrations to achieve gelation, hence shedding light on the role of the MGC parameter in gelation. Furthermore, gelator structural design also modulates the level of cooperative self-assembly through solubility effects, as determined by applying a cooperative binding model to NMR data. Finally, the effect of gelator chemical design on the spatial organization of the networked gelator was probed by small-angle neutron and X-ray scattering (SANS/SAXS) on the native gel, and a tentative self-assembly model was proposed.

Synthesis and characterization of biocompatible, thermoresponsive ABC and ABA triblock copolymer gelators

The synthesis of doubly thermoresponsive PPO-PMPC-PNIPAM triblock copolymer gelators by atom transfer radical polymerization using a PPO-based macroinitiator is described. Provided that the PPO block is sufficiently long, dynamic light scattering and differential scanning calorimetry studies confirm the presence of two separate thermal transitions corresponding to micellization and gelation, as expected. However, these ABC-type triblock copolymers proved to be rather inefficient gelators: free-standing gels at 37 degrees C required a triblock copolymer concentration of around 20 wt%. This gelator performance should be compared with copolymer concentrations of 6-7 wt% required for the PNIPAM-PMPC-PNIPAM triblock copolymers reported previously. Clearly, the separation of micellar self-assembly from gel network formation does not lead to enhanced gelator efficiencies, at least for this particular system. Nevertheless, there are some features of interest in the present study. In particular, close inspection of the viscosity vs temperature plot obtained for a PPO43-PMPC160-PNIPAM(81) triblock copolymer revealed a local minimum in viscosity. This is consistent with intramicelle collapse of the outer PNIPAM blocks prior to the development of the intermicelle hydrophobic interactions that are a prerequisite for macroscopic gelation.

Thermally responsive elastomeric supramolecular polymers featuring flexible aliphatic hydrogen-bonding end-groups

The present paper details the synthesis, characterization, and preliminary physical analyses of a series of polyisobutylene derivatives featuring urethane and urea end-groups that enable supramolecular network formation to occur via hydrogen bonding. These polymers are readily accessible from relatively inexpensive and commercially available starting materials using a simple two-step synthetic approach. In the bulk, these supramolecular networks were found to possess thermoreversible and elastomeric characteristics as determined by temperature-dependent rheological analysis. These thermoreversible and elastomeric properties make these supramolecular materials potentially very useful in applications such as adhesives and healable surface coatings.

Effects of hydrocolloid addition and high pressure processing on the rheological properties and microstructure of a commercial ostrich meat product "Yor" (Thai sausage)

"Yor" is a traditional sausage like product widely consumed in Thailand. Its textures are usually set by steaming, in this experiment ultra-high pressure was used to modify the product. Three types of hydrocolloid; carboxymethylcellulose (CMC), locust bean gum (LBG) and xanthan gum, were added to minced ostrich meat batter at concentration of 0-1% and subjected to high pressure 600 Mpa, 50 degrees C, 40 min. The treated samples were analysed for storage (G) and loss (G '') moduli by dynamic oscillatory testing as well as creep compliance for control stress measurement. Their microstructures using confocal microscopy were also examined. Hydrocolloid addition caused a significant (P < 0.05) decrease in both the G' and G '' moduli. However the loss tangent of all samples remained unchanged. Addition of hydrocolloids led to decreases in the gel network formation but appears to function as surfactant materials during the initial mixing stage as shown by the microstructure. Confocal microscopy suggested that the size of the fat droplets decreased with gum addition. The fat droplets were smallest on the addition of xanthan gum and increased in the order CMC, LBG and no added gum, respectively. Creep parameters of ostrich yors with four levels of xanthan gum addition (0.50%, 0.75%, 1.00% and 1.25%) showed an increase in the instantaneous compliance (J(0)), the retarded compliance (J(1)) and retardation time (lambda(1)) but a decrease in the viscosity (eta(0)) with increasing levels of addition. The results also suggested that the larger deformations used during creep testing might be more helpful in assessing the mechanical properties of the product than the small deformations used in oscillatory rheology. (c) 2007 Elsevier Ltd. All rights reserved.

A single director switching mode for monodomain liquid crystal elastomer

We report rotation of a single director in a nematic monodomain, acrylate based side-chain elastomer which was subjected to mechanical fields applied at angles in the range to the director, , present at the time of network formation. Time and spatially resolving wide angle X-ray scattering, together with polarised light microscopy measurements revealed a pronounced, almost discontinuous switching mode at a critical extension as the strain was applied at angles approaching to , whereas a more continuous rotation was seen when the strain was applied at more acute angles. This director reorientation was more or less uniform across the complete sample and was accompanied by a modest decrease in orientation parameter . At strains sufficient to induce switching there was some continuous distribution of director orientations with fluctuations of 10 although there was no evidence for any localised director inhomogenities such as domain formation. The observed deformation behaviour of these acrylate-based nematic monodomains was in accord with the predictions of a theory developed by Bladon et al., in that the complete set of data could be accounted for through a single parameter describing the chain anisotropy. The experimentally deduced chain anisotropy parameter was in broad agreement with that obtained from small-angle neutron scattering procedures, but was somewhat greater than that obtained by spontaneous shape changes at the nematic-isotropic transition.

Side-chain liquid crystalline elastomers: the relationship between the orientational ordering of the polymer backbone and the length of the coupling chain

The influence of cross-linking on the phase behaviour of a series of side-chain liquid crystalline elastomers has been studied. For samples cross-linked in the temperature range corresponding to the nematic phase, the phase transition was shifted compared to that observed when an identical sample was cross-linked in the isotropic phase. This shift represented a stabilisation of the nematic phase in the former case, in line with theoretical expectations. By utilising a novel, slow cross-linking method, which allows the polymer backbone to take up an equilibrium conformation prior to network formation, it proved possible to monitor the shifts in phase transition temperature as a function of the length of the methylene chain coupling the mesogenic units to the polymer backbone. The results obtained are related to the backbone anisotropy and indicate that the level of orientational order of the polymer in the nematic phase backbone increases with a reduction in the length of the coupling chain.

Conditional transgenic expression of fibroblast growth factor 9 in the adult mouse heart reduces heart failure mortality after myocardial infarction

BACKGROUND: Fibroblast growth factor 9 (FGF9) is secreted from bone marrow cells, which have been shown to improve systolic function after myocardial infarction (MI) in a clinical trial. FGF9 promotes cardiac vascularization during embryonic development but is only weakly expressed in the adult heart. METHODS AND RESULTS: We used a tetracycline-responsive binary transgene system based on the α-myosin heavy chain promoter to test whether conditional expression of FGF9 in the adult myocardium supports adaptation after MI. In sham-operated mice, transgenic FGF9 stimulated left ventricular hypertrophy with microvessel expansion and preserved systolic and diastolic function. After coronary artery ligation, transgenic FGF9 enhanced hypertrophy of the noninfarcted left ventricular myocardium with increased microvessel density, reduced interstitial fibrosis, attenuated fetal gene expression, and improved systolic function. Heart failure mortality after MI was markedly reduced by transgenic FGF9, whereas rupture rates were not affected. Adenoviral FGF9 gene transfer after MI similarly promoted left ventricular hypertrophy with improved systolic function and reduced heart failure mortality. Mechanistically, FGF9 stimulated proliferation and network formation of endothelial cells but induced no direct hypertrophic effects in neonatal or adult rat cardiomyocytes in vitro. FGF9-stimulated endothelial cell supernatants, however, induced cardiomyocyte hypertrophy via paracrine release of bone morphogenetic protein 6. In accord with this observation, expression of bone morphogenetic protein 6 and phosphorylation of its downstream targets SMAD1/5 were increased in the myocardium of FGF9 transgenic mice. CONCLUSIONS: Conditional expression of FGF9 promotes myocardial vascularization and hypertrophy with enhanced systolic function and reduced heart failure mortality after MI. These observations suggest a previously unrecognized therapeutic potential for FGF9 after MI.

Supramolecular materials for inkjet printing: self-assembling polymer networks

Electronically complementary, low molecular weight polymers that self-assemble through tuneable π-π stacking interactions to form extended supramolecular polymer networks have been developed for inkjet printing applications and successfully deposited using three different printing techniques. Sequential overprinting of the complementary components results in supramolecular network formation through complexation of π-electron rich pyrenyl or perylenyl chain-ends in one component with π-electron deficient naphthalene diimide residues in a chain-folding polyimide. The complementary π-π stacked polymer blends generate strongly coloured materials as a result of charge-transfer absorptions in the visible spectrum, potentially negating the need for pigments or dyes in the ink formulation. Indeed, the final colour of the deposited material can be tailored by changing varying the end-groups of the π electron rich polymer component. Piezoelectric printing techniques were employed in a proof of concept study to allow characterisation of the materials deposited, and a thermal inkjet printer adapted with imaging software enabled a detailed analysis of the ink-drops as they formed, and of their physical properties. Finally, continuous inkjet printing allowed greater volumes of material to be deposited, on a variety of different substrate surfaces, and demonstrated the utility and versatility of this novel type of ink for industrial applications.

Responding to terrorism through networks at sites of critical infrastructure : a case study of Australian airport security networks

The importance of effective multilateral security networks is widely recognised in Australia and internationally as being essential to facilitate the large-scale sharing of information required to respond to the threat of terrorism. Australian national security agencies are currently constructing networks in order to bring the diverse national and international security agencies together to achieve this. This paper examines this process of security network formation in the area of critical infrastructure protection, with particular emphasis on airport security. We address the key issues and factors shaping network formation and the dynamics involved in network practice. These include the need for the networks to extend membership beyond the strictly defined elements of national security; the integration of public and private ‘nodes’ in counter-terrorism ‘networks’; and the broader ‘responsibilisation’ of the private sector and the challenges with ‘enabling’ them in counter-terrorism networks. We argue that the need to integrate public and private agencies in counter-terrorism networks is necessary but faces considerable organisational, cultural, and legal barriers.

Dielectric properties of conducting polymer

The dielectric properties of conducting polymer composites containing polypyrrole (PPy) crushed films, PPy powder, polyaniline (PAn) base and acid powders as the dispersants and silicone rubber and vinyl ester as matrix materials have been investigated in the frequency range 2-18 GHz. The dielectric parameters such as the real part, epsiprime, and imaginary part, epsiPrime, of the permittivity and loss tangent, tandelta, increase with increasing conductivity and concentration of the dispersant. The geometrical shape of the dispersant governs the ability of conductive network formation which is indicated by a large drop in the resistivity of the composite. Also, dispersant/matrix interactions and physical properties of the matrix influence the agglomeration of the dispersant phase which, in turn, affects the dielectric properties of the composites. Flakes of PPy obtained by crushing highly conductive films and large PAn powder aggregates were unable to form a conducting network. The composites without a network of dispersant exhibit low dielectric parameters. On the other hand, high values of tan delta ranging from 0.7–1.1 were achieved for the PPy powder (15 parts)/silicone rubber composites where a conducting network was observed.

Reactive block copolymer modified thermosets : highly ordered nanostructures and improved properties

A highly ordered poly(dimethyl siloxane)-poly(glycidyl methacrylate) (PDMS-PGMA) reactive diblock copolymer was synthesized and used to modify bisphenol A-type epoxy resin (ER). The PDMS-PGMA block copolymer consisted of epoxy-miscible PGMA blocks and an epoxy-immiscible PDMS block. The PGMA reactive block of the block copolymer formed covalent bonds with cured epoxy and was involved in the network formation, and the PDMS block phase separated to give different ordered and disordered nanostructures at different blend compositions. The solvent cast PDMS-PGMA diblock copolymer showed ordered hexagonal cylindrical morphology. A highly ordered morphology consisting of hexagonal cylinders inside the lamellar morphology was observed in the cured PDMS-PGMA block copolymer. In the cured ER/PDMS-PGMA blends, a variety of morphologies including lamellar, cubic and worm-like and spherical nanostructures were detected depending on the blend composition. Moreover, the addition of this reactive diblock copolymer significantly increases the hydrophobicity and the glass transition temperature. It also improves the tensile strength and tensile ductility of the nanostructured thermosets at low diblock copolymer contents.

Engineering molecular self-assembled fibrillar networks by ultrasound

The architecture of self-organized three-dimensionally interconnected nanocrystal fibrillar networks has been achieved by ultrasound from a solution consisting of separate spherulites. The ultrasound stimulated structural transformation is correlated to the striking ultrasonic effects on turning nongelled solutions or weak gels into strong gels instantly, with enhancement of the storage modulus up to 3 magnitudes and up to 4 times more gelling capability. The basic principle involved in the ultrasound-induced structural transformation is established on the basis of the nucleation-and-growth model of a fiber network formation, and the mechanism of seeding multiplication, aggregation suppressing, and fiber distribution and growth promotion is proposed. This novel technique enables us to produce self-supporting gel functional materials possessing significantly modified macroscopic properties, from materials previously thus far considered to be “useless”, without the use of chemical stimuli. Moreover, it provides a general strategy for the engineering of self-organized fiber network architectures, and we are consequently able to achieve the supramolecular functional materials with controllable macroscopic properties.