The melting behavior of polymers can provide information on their crystallization mechanism. However, the origin of the time-dependent low endotherm, or annealing peak, and the extent of melting-recrystallization-remelting during heating are still debated. The crystallization and subsequent melting behavior of isotactic polystyrene are explored in the context of Stroblâ "s 3-Phase model using differential scanning calorimetry (DSC), small angle X-ray scattering (SAXS), and wide angle X-ray diffraction. DSC experiments confirm the existence of a crystallization time-dependent low endotherm, and melting-recrystallization-remelting processes during heating. SAXS analysis using the correlation function confirms that the lamellar thickness increases with crystallization temperature and is independent of time. The spread between equilibrium melting and crystallization temperatures determined in this work (Tfâ"" = 533K, Tcâ"" = 544K) is much smaller than reported by Strobl et al. (Tfâ"" = 562K, Tcâ"" = 598K). These differences are partially attributed to overestimation in lamellar thicknesses calculated through the interface distribution function. Analysis of diffraction broadening shows that the apparent crystal size decreases with crystallization time, suggesting the formation of smaller/less perfect crystals during secondary crystallization. These results are consistent with observations that the glass transition temperature increases with crystallization time and supports the idea that secondary crystallization leads to increased amorphous conformational constraints. These results also suggest that the upward shift of the annealing peak during secondary crystallization is associated with increased amorphous constraints rather than increased crystal dimensions. The lack of distinction between Tfâ"" and Tcâ"" and the evolution of crystal size during crystallization stand in direct contrast with Stroblâ "s model. / Master of Science

No description available.

The spherulitic growth rate data for a molecular weight series of isotactic polystyrene are analyzed in context of the Lauritzen-Hoffman kinetic theory of polymer crystallization. The primary objectives of the study are to critically test the Lauritzen-Hoffinan theory under conditions not rigorously investigated before and to gain a better understanding of the molecular weight dependence of crystal growth rate for isotactic polystyrene. The analyses yield values for fundamental kinetic and thermodynamic quantities associated with polymer crystallization. The physical meaning of the resulting parameters is assessed by comparing these results to those obtained from methods independent of crystal growth rate or crystallization theory altogether. This study differs from others reported in the literature in a number of ways, such as, the narrow molecular weight distribution and the molecular weight range of polystyrenes investigated. Also, growth rate measurements were extended to higher temperatures and a more appropriate kinetic equation for crystal growth rate analysis was applied. The majority of published studies that have used the Lauritzen-Hoffman theory applied an approximated form of the kinetic equation which does not fully describe the temperature dependence of polymer crystallization. The results of the study show that a transition from molecular weight dependent to independent crystal growth rate occurs at a molecular weight of about 250,000 g/mole for isotactic polystyrene. Also, comparison of viscoelastic and crystal growth rate data indicate that the Vogel form of the transport term in the Lauritzen Hoffman kinetic growth rate equation correctly describes the temperature dependence of molecular transport for the crystallization process of isotactic polystyrene. Furthermore, the study suggests that the equilibrium melting temperature for the polymer is significantly higher than the value that has been generally accepted for the past 25 years. The study also provided the opportunity to investigate various other factors and theories associated with polymer crystallization. For example, the theoretical relationship between the crystal's lateral surface free energy, σ, and the characteristic ratio, C_∞, was evaluated. Also, the spherulitic morphology as a function of molecular weight and temperature was examined by scanning electron microscopy, SEM. / Ph. D.

ABSTRACT Karst regions of the world that receive relatively similar amounts of precipitation display a wide variety of landscapes. It has been suggested (Groves and Meiman, 2005) that climates exhibiting larger discrete storm events have more dissolving power and consequently higher rates of conduit growth than climates with more uniform precipitation distributions. To study this concept, a computer program “Cave Growth” was developed that modeled the growth of a cross-section of a cave passage under dynamic flow and chemical conditions. A series of 46 simulation datasets were created to represent different climatic conditions. These simulations had the same total annual discharge, but demonstrated a range of flow distributions quantified by use of a gamma distribution index, along with two special theoretical cases. After simulating a year of conduit growth for each of the various flow distributions in a series of model runs, and repeating these sets of simulations for three different passage cross-section geometries, it was evident that the annual temporal distribution of flow did indeed impact the amount of cave growth. However, an increase in the “storminess” of the climate did not simply equate to more dissolution and thus conduit growth. Rather, the quantity and duration of surface contact between water and the conduit walls combined with dissolution rates to affect the total growth. The amount of wetted perimeter (contact between fluid and passage floor/walls) generated by specific conduit to capacity were shown to be very effective at growing the cave. Above this level, the dissolving power of additional water was essentially wasted. This investigation suggests that the maximum amount of passage flow levels depended upon the shape of the passage. Flow conditions that filled the growth occurs under flow conditions that result in the most wetted perimeter for the longest period of time at the highest dissolution rate.

No description available.

Cette thèse porte sur la construction de nouveaux exemples de surfaces minimales dérivées de la famille de surfaces de Costa-Hoffman-Meeks. Il s'agit d'une famille de surfaces minimales complètes plongées avec trois bouts et genre k > 0. Soit M_k la surface de Costa_Hoffman_Meeks de genre k. Dans le chapitre 1, j'ai démontré que M_k est non dégénérée pour k > 37. J'ai donc étendu les résultats de S. Nayatani qui assuraient que la surface M_k est non dégénérée seulement pour k=1,...,37. Ce résultat permet de montrer dans les chapitres 2 et 3 l'existence de nouveaux exemples de surfaces minimales de genre g arbitraire à l'aide d'une procédure de collage d'autres surfaces déjà connues (parmi lesquelles y figure la surface M_k). Sans ceci, ces résultats ne seraient valables que pour k < 38. En particulier dans le chapitre 2, j'ai démontré l'existence, dans H^2 x R, (H^2 étant le plan hyperbolique) d'une famille de surfaces minimales plongées inspirées de M_k, pour tout k > 0. Ce résultat peut être censé un cas particulier d'un théorème générale de désingularisation de l'intersection de deux surfaces minimales annoncé par N. Kapouleas et jamais publié. Le chapitre 3 est consacré à la construction de trois familles de surfaces minimales simplement périodiques plongées dans R^3 dont le quotient a genre arbitraire. Les résultats présentés dans ce chapitre (obtenus en collaborations avec L. Hauswirth et M. Rodríguez) généralisent plusieurs anciennes constructions / This thesis is devoted to the construction of new examples of minimal surfaces derived from the family of surfaces if Costa-Hoffman-Meeks. Surfaces in this family are complete embedded with 3 ends and genus k > 0. Let M_k denote the surface of Costa-Hoffman-Meeks of genus k. In chapter 1 I showed M_k is non degenerate for k > 37. So I extended the results of S. Nayatani which insured M_k is non degenerate only for k=1,...,37. That allows to prove in chapters 2 and 3 the existence of new examples of minimal surfaces by a gluing procedure involving already known surfaces (among which figures M_k). Without it theses results would hold only for k < 38. In particular in chapter 2 I showed the existence in H^2 x R (where H^2 denotes the hyperbolic plane) of a family of surfaces inspired to M_k, for all k > 0, which are complete and embedded. This result can be considered as a particular case of a general theorem of desingularization of the intersection of two minimal surfaces announced by N. Kapouleas and never published. Chapter 3 is devoted to the construction of 3 families of singly periodic minimal surfaces, embedded in R^3, whose quotient has an arbitrary value of the genus. The results showed in this chapter (obtained in collaboration with L. Hauswirth and M. Rodríguez) generalize many previous constructions

In four years, ISIS managed to conquer vast areas in both Iraq and Syria and have proven themselves capable of conducting sophisticated military operations. Using terrorism, they have also shocked the world numerous times, most recently in Sri Lanka during the spring of 2019, in an attack leaving more than 250 people dead. Even today there seems to be a discrepancy in how this organization should be defined. Therefore, this thesis examines ISIS through the years of 2010-2015. The ambition is to explain how this organization can be understood through the contrasting theories of hybrid warfare and insurgency from Frank G. Hoffman and David Galula respectively. This thesis concludes that while ISIS’s initial progression bore strong resemblance to the historically typical insurgent, like gaining publicity through terror and using guerilla warfare to acquire supplies and grow, they became something more. They evolved into a hybrid organization capable of mixing conventional combined arms warfare with the deadly effectiveness of irregular methods, essentially converging terrorism with their conventional units. Using modern technology, they managed to turn both social media and commercial products alike, into effective tools of war. This spectacular metamorphosis that is ISIS, illuminates the requirement for development of new theories, or at the very least, to expand on those present today.

The purpose of this work is to critically examine the Lauritzen-Hoffman (LH) secondary nucleation barrier model of polymer crystallization. One of the major criticisms of the LH theory was that it predicted divergence of the lamellar thickness and crystal growth rate at finite undercoolings - the so-called “δ𝑙 catastrophe." Within this work, it has been shown that the "δ𝑙 catastrophe" can be eliminated by considering all of the implications of the Hoffman-Miller reptation approach. Combination of this approach and the lattice-strain theory of Hoffman and Miller (which predicts curved face crystals) eliminates two of the major criticisms of the LH theory within a single theoretical framework. Through studies performed in this work, the LH theory has been modified in such a way as to extend its utility to higher undercoolings. Physically meaningful nucleation parameters can be obtained with the modified LH theory if the viscoelastic parameters characterizing the transport of chain segments to the growth front are known a priori. Crystal growth and melting behavior were studied in the case of linear and cyclic polydimethylsiloxanes. An equilibrium melting temperature (T_m) of 250K was determined by the Hoffman-Weeks extrapolation method for a linear PDMS fraction with <M_n>=62,700 g/mol. This value is 12°C higher than that previously cited in the literature. From the kinetic studies, a fold crystal/melt interfacial free energy of 10.2 erg/cm² was determined which corresponds to a work of chain folding of 2.5 kcal/mol. Studies performed on the cyclic PDMS fractions confirmed that the configuration entropy decreases with decreasing molecular weight. Additionally, the studies on the cyclic PDMS fractions have shown that the σ-C_∞ relationship of Hoffman and coworkers fails for cyclic systems. The crystal growth rates, T_m, and lamellar thicknesses of polytetrafluoroethylene have been determined in this work. T_m has been shown to be 331±2°C. By atomic force microscopy and theoretical arguments it has been shown that the lamellar thicknesses of polytetrafluoroethylene, over the temperature range studied, is on the order of 1000Å. These thicknesses correspond to quantization of the folds, from which it was shown that meaningful analysis of the growth rate data is impossible. / Ph. D.

Collage is presented as an allegoric art-form sui generis, considering allegory itself as an open-ended form of art. The research provides a suggestion to a different understanding of collage i.e.: as a catalyst for a search for structure and semiotic relationships in an attempt to overcome a constant disordered expansion of an intertextual web and hermeneutic possibilities; and as an open work, providing multi-layered meanings. As an open work collage is typified by the prominent role of its "readers", its ambiguity and the infinite net of references it summons. Collage may be conceived as a bridge between modernism and postmodernism, structuralism and post-structuralism; as a model for constant innovation and suppleness; as a stimulator for meta-artistic questions, acceptance of "soft" universals and reevaluation of the role of the Other within an artwork. Works by Jasper Johns, Michal Na'aman, Dina Hoffman and myself exemplify these ideas. / Visual Arts / M.A. (Visual Arts)

Collage is presented as an allegoric art-form sui generis, considering allegory itself as an open-ended form of art. The research provides a suggestion to a different understanding of collage i.e.: as a catalyst for a search for structure and semiotic relationships in an attempt to overcome a constant disordered expansion of an intertextual web and hermeneutic possibilities; and as an open work, providing multi-layered meanings. As an open work collage is typified by the prominent role of its "readers", its ambiguity and the infinite net of references it summons. Collage may be conceived as a bridge between modernism and postmodernism, structuralism and post-structuralism; as a model for constant innovation and suppleness; as a stimulator for meta-artistic questions, acceptance of "soft" universals and reevaluation of the role of the Other within an artwork. Works by Jasper Johns, Michal Na'aman, Dina Hoffman and myself exemplify these ideas. / Visual Arts / M.A. (Visual Arts)