Influence of Strain-Holding Conditions on Shape Recovery and Secondary-Shape Forming in Polyurethane-Shape Memory Polymer

Abstract

By applying strain at high temperature to polyurethane-shape memory polymer film, followed by holding the strain under various thermomechanical conditions, the behavior of strain recovery by heating after these processes was investigated. In the experiments, the influence of the strain-holding conditions on shape recovery and secondary-shape forming was discussed. The results obtained are summarized as follows. (1) If strain is held at high temperature for short time, irrecoverable strain starts to appear at the holding time th =0.5 h and strain is not recovered at all at th =8 h in the care of holding temperature Th = Tg + 20 K. In the case of Th = Tg + 10 K, irrecoverable strain appears for short holding time if initial strain is large and the rate of secondary-shape forming S is 42% at th =8 h. (2) If initial strain of 50% is held at low temperature for long time, strain becomes not to be recovered and S is 93% at th=12 h in the care of Th = Tg + 10 K. In the care of Th = Tg, the increasing rate of S increases if th becomes longer than 40 h. If Th is lower than Tg - 10 K for th=72 h, strain is recovered perfectly by heating and secondary-shape forming does not appear.