Three series of chemically well-defined polyurethanes were synthesized with the same hard segments but different soft-segment chemistries of interest in biomedical applications. The multiblock polyurethanes have soft segments composed of either an aliphatic polycarbonate [poly(1,6-hexyl 1,2-ethyl carbonate)], polytetramethylenoxide, or a mixed macrodiol of polyhexamethylenoxide and hydroxyl-terminated poly(dimethylsiloxane) and the same hard-segment chemistry [4,4′-methylenediphenyl diisocyanate and 1,4-butanediol]. Analysis using small-angle X-ray scattering and other methods demonstrates that demixing of the hard and soft segments varies greatly between the three series of copolymers. For example, the PDMS/PHMO-based copolymers exhibit a three-phase, core−shell morphology, while the other two series exhibit a typical two-phase structure. In addition to quantitative measurements of hard/soft-segment demixing for the two-phase copolymers, FTIR spectroscopy was used to assess inter- and intracomponent hydrogen bonding, and tapping mode AFM was used to characterize the nanoscale morphology.

Microphase separation is investigated for segmented polyurethane copolymers synthesized from 4,4′-methylenediphenyl diisocyanate and 1,4-butanediol as the hard segments and poly(hexamethylene oxide diol) and hydroxyl-terminated polydimethylsiloxane as soft segments. The neat PDMS-based diol exhibits two segmental relaxations corresponding to the principal repeating unit, Si(CH 3 ) 2 O, and to the hydroxyl end-group segments. When incorporated in the polyurethanes, the siloxane units form their own phase, while the end-group segments are mixed with the second macrodiol and some short hard segment sequences. This is demonstrated by the results of dynamic mechanical analysis and wide-angle X-ray diffraction experiments. The microdomain morphology was characterized principally by small-angle X-ray scattering, and the scattering data were analyzed with a pseudotwo-phase model as well as a modified three-phase core-shell model.