In the work presented in this dissertation, extensive molecular dynamics (MD) simulations have been performed to investigate various physical problems related to the solid-liquid transitions over a wide range of supercooling and superheating temperatures in model Lennard-Jones systems. The major focus of this work is to investigate the thermodynamics, kinetics, and underlying mechanisms of these problems. There are five topics in this work: (1) The classical nucleation theory (CNT) was tested for both liquid supercooling and solid superheating via different solid-liquid coexistence models. It is found that the CNT is valid for liquid supercooling but invalid for solid superheating. The arising elastic energy plays a significant role in affecting the liquid nucleation in a superheated solid. A new nucleation theory was proposed for describing the internal liquid nucleation of solid superheating. (2) Based on CNT, a new and accurate method was developed for calculating the crystal-melt interfacial free energy and its anisotropy. Our result is very close to Turnbulls experimental results. (3) The face, temperature, and size dependences of the crystallization rate were investigated in this work. The results show that the crystallization rate decreases substantially with the increasing system size. Different from the conventional models, a new model is developed to describe these dependences. (4) Melting from internal nanovoids was investigated in this work. It is found that the mechanism of void melting is quite different from bulk melting and nanoparticle melting. There are four different stages and three local melting temperatures in void melting. The mechanism of the complex melting sequence is systematically explained. (5) The homogenous melting at the upper limit of superheating was investigated in this work. For the first time, the ring diffusion is found to take place in superheated crystals and causes the spontaneous melting. The prevailing instability theories are unsuitable to describe this type of melting. The mechanism of the diffusion-loop mediated melting is carefully discussed in this work.

The molten pool shape and thermocapillary convection during melting or welding of metals or alloys are self-consistently predicted from scale analysis. Determination of the molten pool shape and transport variables is crucial due to its close relationship with the strength and properties of the fusion zone. In this work, surface tension coefficient is considered to be negative, indicating an outward surface flow, whereas high Prandtl number represents a thinner thickness of the thermal boundary layer than that of momentum boundary layer. Since Marangoni number is usually very high, the domain of scaling is divided into the hot, intermediate and cold corner regions, boundary layers on the solid-liquid interface and ahead of the melting front. The results find that the width and depth of the pool, peak and secondary surface velocity, and maximum temperatures in the hot and cold corner regions can be explicitly and separately determined as functions of working variables or Marangoni, Prandtl, Peclet, Stefan, and beam power numbers. The scaled results agree with numerical data, different combinations among scaled equations, and available experimental data.

Reported is a study of the crystallization behavior of syndiotactic polystyrene (sPS) and its copolymers (with 4-bromostyrene as the comonomer or with atactic polystyrene arms grafted on the comonomer sites) via three sets of experiments. The first involves the study of structural identification of negatively birefringent spherulites by means of polarized light microscopy (PLM) and scanning electron microscopy (SEM). Results indicated that the optically positive and optically negative spherulites have same morphological features. Differences in the optical texture are due entirely to differences in orientation of the (anisotropic) sheaf-like precursors: the rigid nature of crystalline lamellae renders incomplete development of spherical symmetry even at the axialitic size of tens of microns. In the second part, we propose a modified approach for more precise determination of the Tm* value by taking advantage of the dual-mode distribution of crystalline lamellae in analyzing small-angle X-ray scattering (SAXS) profiles. This method should be generally applicable to other semi-crystalline polymers with dual-mode distribution in lamellar thickness. Results from wide-angle X-ray diffraction (XRD) suggest the presence of ?'-to-?" phase transformation at ca. 264 oC; no indications for the previously proposed ?-to-? transformation are identified. We therefore conclude that the ?' form is truly metastable; the ?"-form is the entropically favored high temperature phase (with Tm* = 300 oC) whereas the more ordered ?' phase (with Tm* = 288 oC) is enthalpically favored at lower temperatures. In the third set of experiments, identification of effects of copolymerization has been studied via a combination of PLM, differential scanning calorimetry (DSC), XRD, SAXS, and transmission electron microscopy (TEM). Results show that the equilibrium melting temperatures (determined via either Hoffman¡VWeeks or Gibbs¡VThomson plots) of the copolymers are significantly lower than that of the corresponding sPS homopolymer. The PLM observations indicate that the axialitic growth rates in copolymers are drastically lower than that of the corresponding homopolymer at comparable backbone length and supercooling. Both XRD and TEM results indicate preferred formation of the ?" phase upon melt crystallization in the bulk state; however, the ?" phase (instead of ?' phase that is the more commonly observed for sPS homopolymers in the bulk state) is dominant in thin films.

Non-isothermal crystallization of the PET/PTT copolyesters was studied at five different cooling rates over 1-20oC/min by means of differential scanning calorimetry (DSC). Both the Ozawa equation and the modified Avrami equation have been used to analyze the crystallization kinetics. The non-isothermal kinetics of most copolymers cannot be described by the Ozawa analysis, except the copolyester with a composition of 66.3% trimethylene- (TT) and 33.7 %ethylene- terephthalates (ET). It may be due to the inaccuracy of the Ozawa assumptions, such as the secondary crystallization is neglected. From the kinetic analysis using the modified Avrami equation, the Avrami exponents, n, were found to be in the range of 2.43-4.67 that are dependent on the composition of the copolyesters. The results indicated that the primary crystallization of the PET/PTT copolymers followed a heterogeneous nucleation and a spherulitic growth mechanism during the non-isothermal crystallization. In the cases of the copolyesters with either TT or ET less than 10%, we found the molten temperature is a key factor to decide whether the Ozawa equation can be succeeded in analyzing the dynamic crystallization. For the non-isothermal crystallization, a single exothermic peak was detected in each DSC curve regardless of the composition and the cooling rate. It indicated that a single-mode distribution of the crystallite sizes was formed during the cooling process. After the non-isothermal crystallization, the melting behavior of the specimens was monitored by temperature modulated DSC (TMDSC) in the conventional mode and the modulated mode. Multiple endothermic peaks were observed in both modes. The wide-angle X-ray diffraction (WAXD) patterns of these copolymers showed that the peak height became sharper and sharper as the crystallization temperature increased, but the position of the diffraction peaks did not change apparently. It indicated that the multiple melting behaviors did not originate from the melting of the crystals with different structures. The melting behavior of these PET/PTT copolyesters can be explained logically by using the melt-recrystallization model. From the reversing and non-reversing signals of TMDSC, the melting-recrystallization-remelting phenomena were further verified. In addition, a small endothermic peak was found at the highest melting temperature in the reversing thermogram for TT-enriched copolyesters. It is reasonably to believe that this endotherm is attributed to the melting of the crystals that are formed in regime I during the heating scan. The cocrystallization of the PET/PTT copolyesters was studied using DSC and WAXD. A clear endothermic peak in the DSC thermogram was detected over the entire range of copolymer composition. A minimum melting temperature was found for the copolyester with 50% ET. The WAXD patterns of these copolymers can be divided into two groups with sharp diffraction peaks, i.e., PET type and PTT type crystals. The transition of crystal structure between PET type and PTT type occurred around the eutectic composition (50 % ET and TT), determined from the variation of the melting temperature with the composition. In addition, the fiber diagram and the WAXD pattern of the copolyester with the eutectic composition showed a different crystalline structure. These results indicated that the cocrystallization behavior of the PET/PTT copolyesters was isodimorphic.

The molten pool shape and thermocapillary convection in melting or welding of metals or alloys having negative surface tension coefficients and Prandtl number greater than unity are determined from a scale analysis. Negative surface tension coefficient indicates that the surface flow is in outward direction, while Prandtl number greater than unity represents that boundary layer thickness of conduction is less than that of momentum. Determination of the molten pool shape is crucial due to its close relationship with the strength, microstructure and properties of the fusion zone. Since Marangoni and Reynolds number are usually greater than ten thousands, transport processes can be determined by scale analysis. In this work, the molten pool is divided into the hot, intermediate and cold corner regions on the flat free surface, boundary layers on the solid-liquid interface and ahead of the melting front for analysis. The results find that the pool shape, surface speed and temperature profiles can be self-consistently evaluated as functions of Marangoni, Prandtl, Peclet, Stefan, and beam power numbers. The predictions agree with numerical computations and experimental data in the literature.

No description available.

Polymer-layered silicate nanocomposites based on a variety of polymer matrices and several organoclays were prepared by melt processing. A detailed characterization of the thermal degradation of several commercial and experimental organoclays often used to form polymer nanocomposites was reported. The surfactant type, loading, and purification level of organoclay significantly affect their thermal stability; however, broadly speaking, the results suggest that these differences in thermal stability do not appear to have much effect on the morphology and properties of the nanocomposites formed from them. It seems that the thermal stability of organoclays is not the key factor in organoclay exfoliation in melt processed polymer nanocomposites, since the exfoliation/dispersion process may have been completed on a time scale before the degradation of surfactant progresses to a detrimental level. Polymer nanocomposites have been made from a variety of polymers; however, few matrices have demonstrated the ability to readily exfoliate the organoclay as well as nylon 6, especially for highly hydrophobic materials like polyolefins. Hence, a significant part of this research work was devoted to explore various routes to improve polyolefinorganoclay interactions, and thus, organoclay exfoliation in these systems. Amine grafted polypropylenes and a conventionally used maleic anhydride grafted polypropylene were used as compatibilizers for polypropylene based nanocomposites to improve the organoclay exfoliation. A series of ethylene vinyl acetate copolymers, the polarity of which can be adjusted by varying their vinyl acetate contents, based nanocomposites were prepared as the model system to address the relationship between the polarity of the polymers and their preferences over various organoclay structures. Attempts were made to explore the effect of degree of neutralization of acid groups in ionomers on the morphology and properties of nanocomposites, and it seems that the ionic units on the polymer chain provide a more favorable interaction between the polymer matrix and the organoclay compared to acid units and, thus, lead to better dispersion of the clay particles. It was determined that surfactants whose structure lead to more shielding of the silicate surface result in improved levels of exfoliation in all the above mentioned unmodified and modified polyolefin based nanocomposites. / text

Evidence indicates seed oil melting point is likely an adaptive quantitative trait in many flowering plant species. An adaptive hypothesis suggests selection has changed the melting point of seed oils to covary with germination temperatures because of a trade-off between total energy stores and the rate of energy acquisition during germination under competition. The predicted differences in relative fitness under different temperatures have not yet been tested and little is known about the genetic basis of differences in oil composition. I used Arabidopsis thaliana to: (1) assess the fitness consequences of high and low melting point seeds germinating at different temperatures, (2) assess what genes underlie natural variation in seed oil composition, and (3) consider how these genes may be used to create oils with particular characteristics. To assess the effects of seed oil melting point on timing of seedling emergence and fitness, I competed high and low melting point lines of A. thaliana under cold and warm temperatures. Emergence timing between these lines was not significantly different at either temperature, which comported with warm temperature predictions but not cold temperature predictions. Under all conditions, plants competing against high melting point lines had lower fitness relative to those against low melting point lines, which matched expectations for undifferentiated emergence times. To assess the genetic basis of naturally occurring variation in seed oil melting point, the seed oil compositions of 391 accessions of A. thaliana were used in a genome-wide association study. Twelve genes were tightly linked with SNPs significantly associated with seed oil melting point variation. Seven encoded lipid synthesis enzymes or regulatory products. The remaining 5 encoded products with no clear relation to seed oil melting point. Results suggest selection can alter quantitative trait variation in response to local conditions through a small set of genes. 268 seed-expressed, candidate genes were linked to 103 SNPs associated with A. thaliana seed oil fatty acids. Eight genes were involved in lipid metabolism, and thirty-four encoded regulatory products. I discuss how knowledge of these genes can be used to breed and engineer desirable oil compositions for industry and nutrition. / text

The state of the art in artificial micro self assembly concepts are reviewed. The history of assembly is presented with a comparison to macro assembly, which has been widely studied, and micro self assembly. Criteria were developed and tested to show that macro assembly is more complex in ways that micro self assembly is not. Self assembly requirements for successful and complex self assembly, which evolved from the macro and micro comparison, are also established and tested. A method to assemble complex structures in the micro scale is proposed and demonstrated at the meso scale. The basic concepts of self assembly and a novel approach to complex multi layer self assembly is analyzed.

This project studied the effect of different parameters on the conjugate heat transfer in tube-in-block heat exchangers for various engineering applications. These included magnetic coolers (or heaters) associated with a magnetic refrigeration system, high heat flux coolers for electronic equipment, and hydronic snow melting system embedded in concrete slabs. The results of this research will help in designing the cooling/heating systems and select their appropriate geometrical dimensions and material for specific applications. Types of problems studied in this project are: steady state circular microchannels with heat source in the gadolinium substrate, transient heat transfer in circular microchannels with time varying heat source in a gadolinium substrate, transient heat transfer in composite trapezoidal microchannels of silicon and gadolinium with constant and time varying heat source, steady state heat transfer in microchannels using fluids suspended with nanoparticl es, and analysis of steady state and transient heat transfer in a hydronic snow melting system. For each of these problems a numerical simulation model was developed. The mass, momentum, and energy conservation equations were solved in the fluid region and energy conservation in the solid region of the heat exchanger to arrive at the velocity and temperature distributions. Detailed parametric study was carried out for each problem. Parameters were Reynolds number, heat source value, channel diameter or channel height, solid materials and working fluids. Results are presented in terms of solid-fluid interface temperature, heat flow rate, heat transfer coefficient, and Nusselt number along the length of the channel and with the progression of time. The results showed that an increase in Reynolds number decreases the interface temperature but increases the heat flow rate and Nusselt number. When the heat source varied with time, by applying and removing the magnetic field, the interface temperature, heat flow rate, and Nusselt number attained a periodic variation with time. The decrease in the diameter at constant Reynolds number decreases the interface temperature and increases the heat flow rate at the fluid-solid interface.