Chemical Engineering @ChemengFiles.Com

This thesis introduces a new technique—scanning activity gravimetric analysis (SAGA)—for investigating phase transitions in semicrystalline polymers. Isothermal growth and dissolution of polymer crystallites within picogram to milligram samples are manifested by mass changes in response to changes in the activity of sorbed solvent vapor. Single charged particles are levitated and weighed in an electrostatic field, providing access to highly supersaturated states. Phase transitions are inferred from simultaneous equilibrium sorption and light scattering measurements. Analogous to differential scanning calorimetry, scanning solvent activity up and down exposes broad transitions between the semicrystalline solid state and the dissolved state, which are influenced by sample history. We demonstrate dissolution and crystallization of nanogram samples of polyethylene oxide by controlling the activity of sorbed water vapor and observe self-nucleation of crystallites from partially states and fully dissolved states.

Interest in accurate measurements of the time-dependent Poisson's ratio of polymers arises because it is a component commonly needed in stress analysis and it appears in most theories predicting the behavior of filled materials and composites. Because of the paucity of data and the difficulties in determining [...](t) experimentally, it has been customary in the past to treat [...](t) as a constant. This is unsatisfactory theoretically and inadequate for accurate work.

Much is understood about the behavior of perfectly flexible and perfectly rigid polymer chains; however, many polymers, for example DNA, are somewhere in between these two limiting cases. Such polymers are termed semiflexible, and their molecular elasticity can play a significant role in single-chain behavior as well as contribute to collective effects.

The rheology and microstructure of complex fluids are intimately related, and this relationship is explored to gain a deeper understanding of the physics of colloidal dispersions, emulsions and polymer solutions.

The permeability of water swollen segmented polyurethane membranes has been determined for the solutes urea, glucose, sucrose, and raffinose at 26.5[degrees]C. The permeability of segmented polyurethane membranes; based on poly(oxyethylene glycol) grades 600, 1000, 1500, and 1540; was determined for the swollen unstrained films and for the swollen films at several strains. The free volume theory for diffusion through homogeneously swollen polymers was able to predict the observed changes in membrane permeability with strain for all solutes except urea. The free volume theory fails to predict the urea data accurately because the polyurethanes used absorb urea and therefore the urea can diffuse through the polymer as well as through the solvent, an eventuality not provided for in the free volume theory.

Transfer of triplet electronic excitation energy from the purine and/or pyrimidine moities of native DNA and adenine polynucleotides to the acridine dye 9-aminoacridine has been demonstrated at 77 [degrees] K. The occurrence of such transfers indicates that there is pi electron overlap between the purine and/or pyrimidine bases and the dye bound to the polymer.

The acridine dye has then been used as a trap for the polymer triplet excitation energy. The polymer to dye dependence of the base to dye transfer efficiency indicates that triplet energy is delocalized in native DNA and adenine polynucleotides. Kinetic studies provide evidence that the pathlength for triplet energy transfer in native DNA is determined by trapping within the polymer rather than by diffusion.

Delayed fluorescence from the dye bound to DNA has been observed and its origin in the triplet state of the polymer has been confirmed at high polymer to dye ratios. In addition it has been shown that delayed fluorescence can arise from triplet-triplet annihilation between dyes at low polymer to dye ratios.

The proposition is made that simple time-temperature superposition should not be valid for block copolymers exhibiting multiple mechanical transitions; and an explanation of the time temperature behavior, which is more consistent with the behavior of the individual phases, is presented in terms of an equivalent mechanical model. Based on this model, a method for generating time-temperature shifts, which depend on the experimental time as well as temperature, is developed. This method can easily be extended to any mechanical model and should be valid for polymer composites in general.

The storage and loss compliances of three benzene cast polystyrene/l,4-polybutadiene/polystyrene triblock copolymers with different compositions were measured between -85 and 90[degrees]C over a frequency range from 0.1 to 1000 Hz. The measurements suggest the presence of four relaxation processes. Two, the polystyrene and polybutadiene glass transitions, are treated according to the method of time-temperature superposition referred to above. Anomalous behavior appearing between the two glass transitions is attributed primarily to a temperature dependent interlayer between the two phases and can be treated as a compositional change in the composite. Entanglement slippage in the rubbery matrix also contributes to the total relaxation.

Blends of well characterized polystyrene-polybutadiene SB diblock and polybutadiene continuous SBS triblock copolymers provide rubbery network systems with controlled amounts of terminal chains of known molecular weight. Such systems also provide quantitative information on the concentrations of trapped and untrapped chain entanglements which is not available in conventional elastomers. Three different SB diblocks were synthesized using homogeneous anionic polymerization techniques. These diblocks were blended in various-amounts with a single research grade SBS triblock to form three series of samples for mechanical testing.

The mechanical properties of these materials were studied (1) in free oscillation at about 0.2 Hz over a temperature range from -150?C to 100?C, and (2) in dynamic uniaxial compression from 0.1 to 1000 Hz at various temperatures between -87 and 85?C.

The first order approximation of the theory of the so-called simple material describes the viscoelastic behavior of soft polymers in large deformations with twelve time-dependent material functions and three material constants, all functions of the three invariants of the deformation tensor. Restricting consideration to deformations in which time shift invariance is preserved, a series of models was developed which describe the time dependence of the stress through the Boltzmann superposition integral incorporating into it a suitable nonlinear measure of strain. The theory was developed in its most general three-dimensional form. Its predictions for homogeneous deformations were tested in a series of experiments on an uncrosslinked styrene-butadiene copolymer.

For the prediction of the viscoelastic behavior of soft polymers the simplest form of the theory requires only one time function, the relaxation modulus. In addition, it requires a strain parameter which is a characteristic material constant. The dependence of this parameter on temperature and other material and experimental variables was examined on hand of estimates from published data as well as from the experimental results reported here.

In the course of this work a curious anomaly was discovered in the behavior of emulsion polymerized compression molded dicumylperoxide cured SBR. This material showed lack of time shift invariance in the region of very small strains in which elastomers generally follow a linear stress-strain law. Normally, non-preservation of time shift Ultra-high-Q (UHQ) silica microspheres have found research applications in diverse fields ranging from telecommunications to nonlinear optics to biological and chemical sensing. However, despite having quality factors greater than 108, the silica microsphere has not moved to an industrial setting because of several major drawbacks. The most hindering is the manual fabrication technique used that makes tight process control difficult and integration with other optical or electrical components impossible. Despite the strong desire to fabricate an integrated UHQ microresonator on a planar substrate, the highest quality factor achieved for any micro-fabricated planar micro-cavity (at the time of my first publication) was over 4 orders of magnitude lower than for silica microspheres. In this thesis, a process for creating planar micro-cavities with Q factors in excess of 400 million on silicon wafers is demonstrated. The advantage of these planar ultra-high-Q (UHQ) microtoroid resonators is that they successfully overcome the previously mentioned drawbacks of silica microsphere resonators while maintaining nearly identical, if not better, performance characteristics. Additionally, due to the planar nature of these new devices, functionality has been integrated in-situ while maintaining UHQ for the first time, such as active resonant frequency tuning, coupling control, and low-threshold lasing.