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Synthesis, structure and phase behavior of liquid crystalline polyurethanes
Abstract
This dissertation describes the synthesis, structure and phase behavior of polyurethanes based on the mesogenic biphenol 4,4$\sp\prime$-bis(6-hydroxyhexoxy)biphenyl (BHHBP) and meta substituted tolylene/phenylene diisocyanates. The structure-property relationships were determined as a function of hydrogen-bonding, the position of the methyl group in the tolylene diisocyanate moiety (TDI) and the biphenol moiety. The liquid crystalline phase (mesophase) and crystalline phase were investigated primarily with differential scanning calorimetry (DSC), wide angle X-ray scattering (WAXS) and infrared spectroscopy. From this combination of characterization techniques, a more detailed description emerges about the thermodynamic stability and kinetic accessibility of each phase. Previous investigation of the (2,4-TDI/BHHBP) mesogenic polyurethane, 2,4-LCPU-6, has shown that this polymer is a monotropic liquid crystal. The influence of H-bonding on the structure and phase behavior of 2,4-LCPU-6 was determined by the synthesis of high molecular weight N-Methyl 2,4-LCPU-6, using a novel high temperature polymerization of a biscarbamoyl chloride with the BHHBP mesogenic diol. The comparison of the structure and physical properties of these two polymers revealed that H-bonding does not affect the mesophase morphology although its absence disrupts crystallinity and results in an enantiotropic liquid crystal. In addition, it was found that the effect of H-bonding on the mesophase-isotropic transition is enthalpic in nature. In contrast to the "regular" ($\alpha,\omega$-hexane diol) based polyurethanes (PUs), BHHBP derived polyurethanes (LCPUs) crystallize rapidly from their melts. This is due to the strong nucleating power of their thermodynamically unstable mesophases (monotropic L.C.) Hexafluoroisopropanol fast solvent-evaporation casting or rapid cooling from the melt resulted in thin films or bulk samples with a glassy mesophase morphology. During the subsequent heating scan, the mesophase to crystal transition takes place. Considerable amount of effort was expended to understand the nature of this transition. With the combination of vibrational spectroscopy which provides a measure of the localized structure, along with DSC and WAXS (which examine the long range order) we established the microstructural changes occurring in the different phases. Applying the results of previously mentioned analysis (kinetic control and phase perfection), highly oriented fibers were obtained for the mesogenic polyurethanes. Atomistic molecular simulations coupled with X-ray intensity refinement allowed us to determine the crystalline chain conformation and packing characteristics for the 2,6-LCPU-6 and 1,3-LCPU-6 (2,6-TDI and 1,3-Phenylene Diisocyanate (1,3-PDI) derived LCPUs). On the basis of structural similarity and well resolved WAXS powder patterns we extended the similar analysis to the "regular" polyurethanes as well (2,6-PU-6 and 1,3-PU-6). The good correlation between H-bonding distance and melting temperature for these four polymers suggests that melting is primarily controlled by the dissociation of H-bonds in the ordered domains.
Subject Area
Polymer chemistry|Physical chemistry|Organic chemistry|Chemistry
Recommended Citation
Papadimitrakopoulos, Fotios, "Synthesis, structure and phase behavior of liquid crystalline polyurethanes" (1993). Doctoral Dissertations Available from Proquest. AAI9316701.
https://scholarworks.umass.edu/dissertations/AAI9316701