The preparation of nano-structured polymeric systems of polyurethane/polyimide blends

In this study, series of segmented polyurethane copolymers were synthesized and physically blended with semi-rigid polyimide {dollar}rm (Ultem1000sp{lcub}TM{rcub}){dollar} for the preparation of nano-structured 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), dynamic 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. Polyurethanes with lower molecular weight of soft segments (M.W. = 425 g/mole) and their blends with polyimide (PI) exhibited good dispersion of PI molecules in the polyurethane matrix. The dispersion of PI chains was attributed to the hydrogen bonds between the PI molecules and the hard segments of the polyurethanes. However, samples which possess higher molecular weight of soft segment (M.W. = 725 g/mole) behaved quite differently. A PI-rich phase was revealed by TEM. In fact, a PI-rich phase already existed for these samples and their blends with PI by showing a third relaxation peak in the study of DMA. The formation of the PI-rich phase was attributed to the less compatible environment of soft matrix for the dispersion of PI molecules.; Results from tensile testing revealed that nano-structured polyurethanes were successfully prepared by the systems of polyurethanes with lower molecular weight of soft segments. Results showed that the measured Young's moduli of these 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. These systems also exhibited improvements in the strain at breaks (and also the toughness) as compared with the ones predicted by the model of Nielsen. On the other hand, the PI-rich phase acted as a particulate filler in the matrix of polyurethanes with higher molecular weight of soft segments and only showed moderate improvement in Young's moduli and strain at break.