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IntroductionPolyurethane elastomers are microphase-separated multiblock copolymer of the type (AB) n where at the service temperature one block of the material is glassy or semicrystalline, and the other block is soft or rubbery. Polyurethane chemistry offers tremendous versatility, and an almost limitless variety of chemical components and compositions are possible. The resulting wide range of physical properties have led to commercial applications of polyurethanes in such areas as flexible foams, tough elastomers, engineering thermoplastics, and durable coatings. They owe their good elastomeric properties to microphase separation of the blocks driven by segmental incompatibility. Micro-phase separation can be further enhanced by interurethane hydrogen bonds and/or crystallization. Consequently, microphase separation can occur at segment molecular weights lower than 1000. This is in marked contrast to the amorphous styrene-diene block polymers where due to weaker van der Waals interactions, microphase separation requires segment molecular weights approaching 10,000. Since phase mixing is entropically favored, an increase in temperature can cause a Microphase Separation Transition (MST) from a microphase separated to a single phase state. Such a MST has been studied extensively in diblock and triblock systems by scattering, calorimetric and rheological methods. There...