Determination of the transition temperature of shape memory polyurethanes using constrained recovery test

Shape memory polymers (SMPs) are smart materials whose original (permanent) shape can be restored from a temporarily fixed one by applying an external stimulus such as heat, acidity, light, etc. In particular, thermo-responsive shape memory polyurethanes (SMPUs) show shape memory behavior when heated. Here, it is important to characterize the transition temperature at which the materials memorize their permanent shape from the temporary shape. Differential scanning calorimetry (DSC) or dynamic mechanical thermal analysis (DMTA) are employed to determine the transition temperature of SMPUs depending on their molecular conformation. A new method is proposed that can determine the transition temperature by making use of the thermo-mechanical behavior of SMPUs using a general tensile tester with an environmental chamber. This method focuses on the constrained recovery behavior of the SMPUs upon heating, i.e., they are prevented from contracting or deforming during the test, thereby causing them to develop a certain stress. The rate of the developed stress according to the temperature depends on the micro-structures of the SMPUs, which determine their transition temperature.