Structure and properties of shape-memory polyurethane block copolymers

Segmented polyurethanes containing soft segments with lower molecular weight exhibit shape‐memorizing properties. Structure and properties of shape‐memorizing polyurethanes (S‐PUs) were studied. S‐PUs are characterized by a rather high glass transition temperature: Tg of S‐PUs is usually in the range of 10–50°C. A Pplot of 1/Tm against–In XA is approximately linear, indicating that the hard segments are randomly distributed along the molecular chain. S‐PUs with a hard segment of 67–80 mole % form negative spheruiites; they give a faint scattering maximum in a small‐angle X‐ray diffraction pattern. On the other hand, S‐PUs with a hard segment of 50 mole % form fine birefringent elements, giving diffuse scattering in its SAXD pattern. A cyclic test of an S‐PUs above Tg indicates that the residual strain increases and the recovery strain decreases with increasing cycle and maximum strain. It has been suggested by dynamic mechanical investigation that the shape‐memorizing property of the S‐PUs may be ascribed to the molecular motion of the amorphous soft segments. © 1996 John Wiley & Sons, Inc.