Small‐angle scattering analysis of hard‐microdomain structure and microphase mixing in polyurethane elastomers

The microdomain structure of a series of segmented polyurethane block copolymers is characterized by small‐angle x‐ray and neutron scattering analyses. The materials contain hard segments formed from 4,4′‐ diphenylmethane diisocyanate (MDI) and butanediol (BD), and range in hard‐segment content from 20 to 80% by weight. The results provide evidence for a transition from discrete to continuous hard‐microdomain morphology as the hard‐segment content is increased above ca. 50%. The measured concentration dependences of the interdomain spacing, specific interfacial area, diffuse microphase boundary thickness, and scattering invariants are used to examine the validity of present models for hard‐microdomain structure. The observed behavior corresponds well with the general predictions of a lamellar model wherein partially coiled hard‐segment sequence configurations are allowed. The thickness of the hard microdomains extracted from the model corresponds to approximately four hard‐segment repeat units. Scattering invariant calculations are used together with determinations of the soft‐microphase glass transition temperatures to examine possible models for microdomain mixing. These calculations suggest that both the hard and soft microphases are phase mixed.