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Thesis (Ph.D.)--Virginia Commonwealth University, 2009. / Prepared for: Dept. of Biomedical Engineering. Title from title-page of electronic thesis. Bibliography: leaves 122-138.

Thesis (M. S.)--Chemical Engineering, Georgia Institute of Technology, 2009. / Committee Chair: Eckert, Charles; Committee Co-Chair: Liotta, Charles; Committee Member: Meredith, Carson. Part of the SMARTech Electronic Thesis and Dissertation Collection.

The nature of the solvent plays an important role in the conformation and orientation of polymers in solution. A particularly interesting case is when the solvent itself possesses order, such as when dissolving the polymer in a LC. In this dissertation, the morphology and diffusion behavior of the conjugated, stiff polymer MEH-PPV, (poly[2-methoxy-5((2-ethylhexyl)oxy)-1,4-phenylenevinylene]), in liquid crystal (LC) solvents have been investigated. Using polarization sensitive fluorescence correlation spectroscopy, it was found that in a nematic LC the polymer molecules are extended and highly aligned parallel with the nematic director. The conformation and orientational order of MEH-PPV increase with chain stiffness as a result of an interplay among the conformational entropy, solvation anisotropy, and bending energy of the polymer chains. In the smectic phase, about 10% of the MEH-PPV molecules are aligned perpendicular to the director in between the smectice layers, an effect not previously observed for a polymer solute. When applying an external electric field across the LC cell, the LC director changes orientation from a planar to a homeotropic alignment. The MEH-PPV chains remain aligned parallel with the LC director with applied field in the bulk of the LC device. However, the local structure near the LC-substrate interface is more complex. Single molecule polarization distributions measured as a function of distance from the LC device interface allow us to use MEH-PPV as sensitive local probe to explore complex structures in anisotropic media. Furthermore, diffusion anisotropy of the polymer solute in a LC solvent was studied by a novel two-beam cross-correlation technique. The diffusion anisotropy was observed to be about 2. This value is comparable to the diffusion anisotropy of the solvent and suggests that, despite the high degree of alignment, the solute diffusion is governed by the solvent and not the solute.

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The goal of this research was to observe the changes in thickness and chemical characteristics that occur in Kapton® , a type of polyimide film, when exposed to heated solvents commonly used in cleaning and processing of these materials. Interference fringes present in transmission infrared spectra were used to calculate the thickness changes of the film, and attenuated total reflection infrared spectroscopy (ATR) was used to observe spectral shifts of the polymer film. Pieces of Kapton® film were immersed in heated solvents of n-methyl pyrrolidinone (NMP) and dimethylsulfoxide (DMSO) resulting in a rapid initial increase of film thickness by about 2.5 um during the first 20 minutes. After the initial increase and throughout the remainder of the 2 hour exposure, the film thickness varied little in the NMP solvent at all temperatures, 67, 95, and 125 oC. However, in DMSO, the behavior was more varied and this behavior was only observed in the 95 oC bath. The change in film thickness throughout the course of the experiment was about 1 μm less in the 125 oC as compared to the 95 oC, perhaps due to dissolution of the film by the solvent. When the film is exposed to heated solvents, a significant shift in the vibrational frequency of the asymmetric carbonyl stretch absorption to higher wavenumbers, as compared to the unexposed film, is observed. This indicates that there is some interaction between the Kapton® film and the solvent such that the strength of the carbonyl bond is lessened. Interactions could disrupt the intra- and intermolecular electron sharing and hinder the formation of the charged resonance form. / Introduction and background -- Preliminary experiments and method development -- Swelling studies on Kapton. / Department of Chemistry

This thesis focuses on the study of solvent degradation on polyimide films. Polyimides spun on silicon and freestanding polyimide films were exposed to solvents as a function of time at a constant temperature. The polyimide spun on silicon was immersed in dimethyl sulfoxide (DMSO) at 90oC for 30, 90, 270, and 450 min. Infrared spectra were obtained via multiple-bounce attenuated total reflection (ATR) and reflection-absorption (RA) spectroscopy. Thickness calculations were made using interference fringes obtained from RA spectra for each time interval. A reduction of thickness was observed with a total difference of 1.5 μm suggesting film degradation as a function of solvent exposure. Spectral changes in the symmetric and asymmetric carbonyl stretching modes were observed in both the ATR and RA sampling, which could be attributed to further curing of the polymer. Two experiments were performed on the freestanding polyimide film. The first experiment exposed the film to DMSO and the second to n-methyl pyrollidinone (NMP). In both instances, the freestanding film was immersed in the solvent at 98oC for 30, 90, 270, and 450 min. Infrared spectra were obtained via single-bounce ATR and transmission spectroscopy. No noticeable differences in spectra were observed. Thickness calculations were made using interference fringes obtained from transmission spectra. After immersion in DMSO, the thickness of the film increased initially, but then no significant changes in thickness occurred after 30 min. The initial increase in thickness may be due to solvent being trapped inside the film. After immersion in NMP, the thickness calculations showed no change in film thickness. However, our data as well as past researchers suggested some increase in thickness must occur due to solvent absorption. We propose that film degradation may be occurring in the NMP at approximately the same rate as swelling, thus no net decrease in thickness could be observed. This hypothesis is supported by the fact that a yellow coloration was visually observed to be in the solvent after immersion of the freestanding film. A subtraction of a spectrum of fresh NMP solvent from the spectrum of the yellow–colored solvent showed bands consistent with polyamic acid, a starting material for the freestanding film, which could have been formed from a hydrolysis reaction. / Literature review and introduction -- Evaluation of sampling techniques -- Film degradation studies on freestanding films. / Department of Chemistry