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Structural elements influencing phase evolution in reactive polyurethanes
The formation of specific phase-separated morphologies is central to achieving high performance polyurethanes. Polyurethanes are composed of various structural elements possessing a mixture of different functional groups, molecular weights, and sequence lengths. The chemistry, the phase behavior, and the kinetics of phase evolution will influence the type of phase-separated morphology formed. In fact, the phase behavior also depends upon the chemical structure and the molecular weight distribution of the components. Despite the importance of chemical structure, it is still not understood quantitatively. In addition, little is known about how the developing structure organizes into different phase-separated morphologies. The work herein addresses these issues. The molecular weight distributions, end groups, and linkages of polyurethane structural elements were quantitatively determined. The structural elements included polyether and polyester macrodiols, polyurethane prepolymers, and polyurea hard segments. Under homogeneous conditions, the molecular weight distribution formed obeys a Schultz-Flory distribution; although when toluene diisocyanates are used as the diisocyanate the effect of change in reactivity narrows the distribution. Under heterogeneous conditions, the phase separation of water causes a change in the local stoichiometry and narrows the distribution further. In the presence of typical polyurethane side reactions, the distribution is broadened. The formation of allophanate linkages was most prevalent in PPG prepolymers prepared at reaction temperatures of 145°C. Infrared spectroscopy was used to study the crystallization behavior of semicrystalline polyurethanes and the reaction and morphological evolution of polyurethane foams. Hydrogen bonding between urethane groups was shown to influence all aspects of the crystallization behavior, including the initial state, nucleation and growth rates, and the final morphology. Hydrogen bonding proves to be less crucial in the onset of phase separation in polyurethane foams. The most crucial parameter was shown to be hard segment anisotropy. Foams prepared from diisocyanates yielding highly anisotropic hard segments phase separate at lower reaction conversion, with a faster rate, and to a higher degree of phase separation and perfection.
Heintz, Amy M, "Structural elements influencing phase evolution in reactive polyurethanes" (2003). Doctoral Dissertations Available from Proquest. AAI3110501.