Influence of Holding Time on Shape Recovery in a Polyurethane Shape-Memory Polymer

Shape-memory polymers have attracted a lot of interest in recent years. A shape-memory polymer can be deformed and fixed into a temporary shape and subsequently made to recover its original shape when a suitable stimulus is applied. This is accomplished by means of a thermomechanical cycle called programming. Programming can be performed in a stress- or strain-controlled mode. The thermomechanical conditions of the programming affect shape-memory properties differently in each programming mode. One of the parameters which significantly affects shape-memory properties in a stress-controlled procedure is stress-holding time (t _H) at high temperature. This paper studies how stress-holding time affects the most significant shape-memory properties under successive thermomechanical cycles. The experiments were conducted using two different programming temperatures in the vicinity of the T _g. The shape-recovery ratio decreased dramatically with cycling even when the holding time was just a few seconds, however, the impact of the stress-holding time depends on the temperature at which it has been applied. Shape-fixity ratio and switching temperature were also studied, but stress-holding time and successive cycles do not seem to affect either of these factors.