Poly(epsilon-caprolactone) polyurethane and its shape-memory property

A series of segmented poly(epsilon-caprolactone) polyurethanes (PCLUs) were prepared from poly(epsilon-caprolactone) (PCL) diol, 2,4-toluene diisocyanate and ethylene glycol. The molecular weight (M(n)) of PCL was 500-10,000, and the soft-to-hard molar ratio was 1:2 to 1:6. Their shape-memory behaviors were investigated as a function of PCL molecular weight, PCLU composition, and thermal/mechanical history. When a deformation temperature 15-20 degrees C below T(m) was chosen, the lowest recovery temperature (LRT) was 15-18 degrees C below T(m), and the recovery ratio was 94-100% for tensile deformation of 300% and for compression of 2.7-fold. The reasons for this deformation-recovery procedure and the mechanism for this shape recovery below T(m) were discussed. The shape recovery was associated with the premelting of the crystals formed during the deformation and fixation, and, thus, it could be accomplished in the solid state. Its driving force was the inner stress stored in the deformed specimen during deformation and crystallization. Therefore, the LRT was a more practical temperature for shape-memory PCLU than T(m). It could be conveniently measured by means of thermal mechanical analysis. By adjusting the molecular weight of the PCL diol and the hard-to-soft ratio, the LRT of PCLU could be adjusted to the range of 37-42 degrees C, and reasonable rigidity could be retained after shape recovery, fulfilling the essential requirements of medical implantations.