Search

Polyesters are an attractive class of polymer that can be readily modified with a wide range of different comonomers, during polymerization or with melt blending, to achieve a wide variety of physical properties. This research primarily focuses on polyesters that incorporate rigid aromatic structures that have excellent potential to enhance thermal and mechanical properties. Copolyesters were prepared through melt polycondensation of diesters and diols in the presence of an exchange catalyst. Monomer incorporation was verified with nuclear magnetic resonance (NMR) and molecular weights were obtained by measuring inherent viscosity (ninh). Physical properties were assessed with thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA) and rheology. Mechanical properties were assessed with tensile and impact testing. Copolyesters of poly(ethylene terephthalate) (PET) were synthesized by substituting dimethyl terephthalate (DMT) with dimethyl 4,4'-biphenyldicarboxylate (4,4'BB) resulting in enhanced glass transition (Tg) temperatures relative to PET while affording melting temperatures (Tm) low enough to allow facile melt processing. Further modification with dimethyl isophthalate (DMI) or dimethyl 3,4'-biphenyldicarboxylate (3,4'BB) slowed crystallization sufficiently to allow biaxial orientation, leading to further studies assessing the permeability of oriented films. Novel amorphous polyesters were synthesized with 3,4'BB or 4,4'BB in combination with neopentyl glycol (NPG), 1,4-cyclohexandimethanol (CHDM) and ethylene glycol (EG). Use of multiple diols produced clear, amorphous copolyesters with Tgs as high at 129 C. A series of novel high temperature(Tm) copolyesters were synthesized from dimethyl 2,6-naphthalenedicarboxylate (DMN) and 4,4'BB combined with CHDM. Studies were performed with standard DSC and thin film calorimetry to show the convergence of multiples melting endotherms in an effort to determine their origin. Preliminary work was performed on the modification of poly(1,4-cyclohexylenedimethylene terephthalate) (PCT), poly(1,4-cyclohexylenedimethylene 2,6-naphthalate) (PCN) and poly(1,4-cyclohexylenedimethylene 4,4'-bibenzoate) (PCB) with dimethyl p-terphenyl-4,4''-dicarboxylate.

The design of microarrays rely on studies geared towards sequence-specific recognition between complementary probe and target molecules in bulk solution. However, this proves to be insufficient to understand the duplex formation reaction on solid-phase. In this dissertation, influence of the surface on DNA duplex stability and melting temperature were theoretically and experimentally investigated. The theoretical approach represents electrostatic and entropic repulsions experienced by hybridizing targets. Electrostatic blocking stemming from surface charge was modeled through Electric Double Layer Theory and Surface Partition Model. Entropic blocking due to steric effects was modeled using polymer physics. Investigated experimental parameters were target concentration, spacer length and probe density. All the experiments gave reproducible melting temperatures with values lower on-surface than in-solution. In a representative set, a target concentration increase from 0.5nM to 15nM with 0.82pmoles of probe at 5*10^12 molecules/cm2 density on 15 dT spacer resulted in approximately 8°C decrease in melting temperature, compared to 5°C increase in solution. This decreasing trend was supported by theory with increasing steric and electrostatic effects at increasing target concentrations leading to higher hybridization efficiencies. Additionally, at low target concentrations (0.0165nM), we observed a multiple melting process in low temperature domains of melting curves due to low stability truncated probes; an indirect indication of synthesis quality. It was observed that as spacer length increases from 2 dT to 25 dT with 0.82pmoles of probe at 5*10^12 molecules/cm2 density with target concentrations ranging from 0.36nM to 1nM, melting temperature increases; an observation theoretically explained by possible entropic blocking dominance. Probe density effect was tested at 5*10^12 molecules/cm2 and 5*10^13 molecules/cm2, on 15 dT spacer and target-to-probe concentration ratios of 0.61:1 to 1.7:1. It was observed that high probe density resulted in lower melting temperature. This trend was theoretically supported by increasing electrostatic and crowding effects. Previously observed dependence of melting temperature on target concentration was also confirmed in all experiments. Melting temperature dependence on probe density seems to be stronger than the dependence on spacer length. The results of this work would lead to better experimental design and correct use of microarrays.

Molecular Dynamics simulations have been used to follow the rate of growth and recession of the prismatic surface of a hexagonal ice-water interface. The fluctuating charge, four-site transferable intermolecular potential model, TIP4P-FQ, was used at temperatures between 265 K and 310 K in a series of isobaric isothermal (NPT) Molecular Dynamics simulations. Using appropriate order parameters, an interface response function that captures the speed of the moving interface as a function of temperature was constructed that covers the melting and growth of hexagonal ice. From the interface response function (Tv=0), the melting temperature was found to be within 8 K of 303 K and the maximum crystallization velocity was estimated to be ~1 m/s at 260 K (14% undercooling; in line with the kinetics of other systems such as Si). Changing the Lennard-Jones sigma parameter from 3.159 angstroms to 3.173 angstroms, in line with a previous parameterization by Rick, confirms results from Gibbs-Duhem integration that predicted a reduction in the melting temperature to 276 K. While this value corresponds well with experiments, given the relative simplicity of the model, it comes at the expense of accurately predicting the properties of liquid water. Crystallization from the prism interface involves at least two layers, as determined by density and dipole analysis and is in line with results found from previous TIP6P results.

A molecular level understanding of nucleation occurring in condensed phases is important in a number of industrial applications like polymer foaming, sonolysis, manufacturing of drugs, polymorphic transformations, and metallic alloy production. The recently developed statistical mechanical approaches based on density functional theory and Monte Carlo (MC) simulation can provide valuable insight into these nucleation processes. In this work, we applied density functional theory to study bubble nucleation in a micellar solution. Our results showed that the presence of surfactant molecules lowers the free energy barrier of bubble nucleation. We also found that under moderate negative pressures, the stable micelle may evolve to form the critical bubble and the resulting free energy barrier is lower than that in the absence of this mechanism. A kinetic consideration revealed that the above mechanism is less effective at lower pressures closer to spinodal. Further, the mechanism of bubble nucleation from the stable micelle correlated well with the liquid-liquid miscibility. In another study, we aimed at understanding crystal nucleation in binary Lennard-Jones (LJ) mixtures. An important factor dictating the crystal nucleation behavior is the underlying phase diagram. Thus, we developed thermodynamic integration (TDI) methods that predict accurately the coexistence points on the phase diagram. In particular, we introduced a method to directly calculate Gibbs free energy difference between the solid and the liquid phases by connecting these phases by a reversible path. Further, we showed that this technique extends naturally to a binary mixture, which allows us to predict its melting temperature. We also developed a new technique to calculate sublimation temperature of single component and binary systems by TDI method. We used these methods to predict eutectic and spindle-shaped phase diagram for the LJ binary mixtures. We then studied the free energy surface for crystal nucleation using MC simulation aided by umbrella sampling technique at a few representative points on these phase diagrams. The emphasis in this work was to identify proper order parameters that correctly describe the crystal nucleation. Our work provides a useful starting point to study how the phase diagram affects the crystal nucleation behavior.