Doctoral Dissertations

Date of Award

12-1996

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Polymer Engineering

Major Professor

Roberto S. Benson

Committee Members

Peter K. Liaw, John Dunlap, Edward S. Clark

Abstract

In this study, series of segmented polyurethane copolymers were synthesized and physically blended with semi-rigid polyimide (UltemlOOO™) for the preparation of nanostructured systems. Information regarding the insight of the morphology and structure property relationships of these systems were provided by applying transmission electron microscopy (TEM), small angle x-rays scattering (SAXS), dynamical mechanical analysis (DMA), Fourier transform infrared (FTIR) spectroscopy techniques and mechanical tensile testing. It was shown that nano-structured systems of polyurethane/polyimide blends can be successfully prepared by the dispersion of semi-rigid polyimide chains in the polyurethane matrix and the hydrogen bonding between them. It was also shown that the morphology and the resultant mechanical properties of these polyurethanes and their blends with PI were strongly dependent on the chemical nature of hard segments and the molecular weight of soft segments.

With TEM, all of the samples examined in this study exhibited two-phase morphology due to the chemical incompatibility between the hard and soft segments. Both polyurethanes (4ED and 4BD) and their blends with polyimide (PI) exhibited good dispersion of PI molecules in the matrix of 4ED and 4BD. The dispersion of PI chains was attributed to the hydrogen bonds between the PI molecules and the hard segments of 4ED and 4BD. The presence of PI molecules results in more dissolved hard segments in the matrix of soft segments hence, causing a lower degree of phase separation, smaller domain's size and a progressive increase in the glass transition temperature of soft segments for 4ED/PI blends from the studies of SAXS and DMA. However, a higher degree of phase separation and a larger size of the hard domain were revealed by SAXS studies for 4BD/PI blends. This was due to the incorporation of PI molecules into the structures of hard domains by showing interference to the hydrogen bonding within the hard domains from the study of IR.

However, samples 7ED and 7BD which posses higher molecular weight of soft segment behaved quite differently. A Pl-rich phase was revealed by TEM study for sample 7ED/PI-5 and all of the 7BD/PI blends. In fact, a Pl-rich phase already existed for all the 7ED/PI blends by showing a third relaxation peak in the study of DMA. The formation of the Pl-rich phase was attributed to the less compatible environment of soft matrix for the dispersion of PI molecules. The early stage of the formation of the Pl-rich phase in the case of 7BD was also attributed to the formation of micro-sized gel particles, which were formed during the synthesis stage.

The two-phase morphology and the random dispersion of hard domains in the matrix of soft segments were verified by showing a surface fractal characteristics with rough interfaces for samples 4ED, 7ED, and 7BD and their blends with PI. However, samples 4BD and 4BD/PI blends behaved mass fractally due to the ill-organized domain structure.

Results from tensile testing revealed that nano-structured polyurethanes were successfully prepared by the systems of 4ED/PI and 4BD/PI blends. Results showed that the measured Young's moduli of both systems were beyond the predicted ones using the additive rule-of-mixtures and the modified Halpin-Tsai equations for particulate-filled, fiber-filled, and nano-structured systems. Both systems also exhibited improvements in the strain at breaks (and also the toughness) as compared with the ones predicted by the model of Nielsen. An unexpected yielding behavior was shown by sample 4BD/PI-5. This is believed due to the incorporation of the dispersed PI molecules into the structure of hard domains which results in different destruction mechanism of hard domains during deformation. On the other hand, the Pl-rich phase acted as a particulate filler in the matrix of 7ED and only showed moderate improvement in Young's moduli and strain at break.

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