Polyurethanes having shape memory effects

Segmented polyurethanes (PUs) were prepared from polycaprolactone diols (PCLs), 4,4′-diphenylmethane diisocyanate, and 1,4-butanediol, and tested for shape memory effects. Effects of soft segment molecular weight (M_n = 2000, 4000 and 8000), soft segment content (50–90%), and maximum strain (_m = 100, 200, and 600%) on the cyclic tensile properties as well as the dynamic mechanical, and mechanical properties below (25°C) and above (65°C) the shape recovery temperatures were studied. With increasing soft segment contents: i) glassy state modulus increased and rubbery state modulus decreased; ii) hardness increased at room temperature, and decreased at 65°C; iii) recovery strain decreased with PCL 2000, and increased with PCL 8000 based PUs. On the other hand, the increase in soft segment length resulted in: i) increased rubbery state modulus as well as glass state modulus; ii) increased hardness at room and high temperatures; iii) increased recovery strain at high soft segment content. Tensile yielding became clear with increasing soft segment length and content. Strain upon cooling and unloading (_u) and residual strain (_p) increased, and recovery strain (_r) decreased with cycling. Among these, residual strain was most sensitive to the cycling. Most of the cycling effects were confined during the first one or two cycles. These results were interpreted in terms of soft segment-hard segment phase separation and soft segment crystallization.