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

Shape Memory Polymers (SMP) are materials that are capable of change their shape when an external stimulus is applied to them [1]. The polymer can be deformed and fixed in a temporary shape by means of a thermomechanical cycle called programming. The programming includes heating up the material to a high temperature (T_high), stretching it into a ε_max or σ_max and cooling it down to a low temperature (T_low) in order to fix the temporary shape.

The programming can be performed in a stress-controlled or strain-controlled mode. The thermomechanical conditions of the programming affect the shape memory properties in a different way at each mode of the programming. One of the parameters which significantly affects shape recovery in a stress-controlled test is the holding time (t_h) at T_high. This paper studies how the holding time under different levels of stress affects the shape memory properties.