Two melting transitions were observed in linear segmented polyurethane-urea elastomers underextension using thermal

mechanical and X-ray diffraction techniques

and the results are compared.These data indicate both strain-induced and temperature-induced crystallization in the stretchedclastomers

which may result from two different types of crystallites with different melting tempera-tures. These have been assigned as type 1 appearing around 60℃

and type 2 around 30℃. The type 1 crystallization can be induced by stretching at room temperature to large strain

and is mechanically reversible

but the type 2 crystallization is mainly induced by cooling below its crystallization temperature.These two crystalline structures are interchangeable under suitable conditions.At elongations greater than 300%

the low temperature peak observed on fusion thermograms disappears or combines with the high temperature peak.When the temperature of the samplc is over the melting point of the type 1 crystal

irreversible melting occurs and only the type 2 crystal develops on cooling.The results of stress-strain and stress hysteresis experiments at different temperatures indicate the relative importance of strain-induced and temperature-induced crystallization on the mechanical properties of these materials.