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Two sets of polyether-polyurethane block polymers based on poly(tetramethylene oxide) (PTMO), 4,4'-methylenebis(phenyl isocyanate) (MDI), and butanediol (BD) were prepared in different ways to produce materials with equivalent stoichiometries but different hard segment length distributions. One set of materials was prepared by a one-step polymerization with butanediol as the chain extender. The second series was synthesized by a multistep method using butanediol and/or bis(4-hydroxybutyl) 4,4'-methylenebis(phenylcarbamate) (BMB) as the chain extender. The single-step polymers are shown to have fewer hard segments containing a single MDI unit than the corresponding multistep samples. The result of this is that the multistep materials exhibit a greater degree of phase mixing, as the very short hard segments are more likely to be dissolved in the soft phase than are longer hard segments. The evidence for this comes from the behavior of the sample ET-20M, an MDI/PTMO alternating copolymer. The hard phase volume fraction and crystallinity are greater in the single-step materials due to the lower degree of phase mixing in these polymers. The results of infrared spectroscopy, differential scanning calorimetry, dynamic mechanical analysis, stress-strain testing, and small-angle X-ray scattering are all shown to be consistent with the differences in hard segment length distributions and the differences in phase mixing which accompany the distributional differences. diffraction to study the crystal structure of MDI/BD/ PTMO-2000 polyurethanes. These materials had the same chemical constituents as those studied by Seymour et al.but had longer segmental lengths. Abouzahr found no detectable crystalline diffraction for samples with less than 35 wt % MDI. On the basis of small-angle X-ray scattering and stress relaxation studies, Abouzahr et al. also proposed that polyurethanes have an interlocked domain morphology at moderate MDI content (35 and 45 wt %).Bonart5,6 also examined the packing of MDI/BD hard segments using X-ray scattering and suggested that hard segments were laterally associated forming lamellae with a thickness limited by the average hard segment length. Using electron microscopy and X-ray diffraction analysis, Schneider et al.7 proposed that the MDI/BD hard segment domain existed in a micelle-like structure which was made

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