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Herein, we report on how to teach a thermoplastic polyurethane elastomer completely stress-free, bidirectional motion. Briefly spoken, we applied training, consisting of extensive tensile deformation in the specimen's viscoelastic state, initiating deformation-induced crystallization. Once unloaded, thermo-reversible specimen expansion and contraction could be detected. Following the microstructural evolution of such a trained specimen by in situ wide-angle X-ray scattering gave evidence for progressive growth of oriented crystallites during cooling. Upon heating, crystallite melting resulted in structural disorder. Beyond this, modification of the training method was used to introduce a geometrically more complex crystalline order. Here, actuation in the form of a decrease and an increase in sample thickness and specimen twisting and untwisting were witnessed. The novel semi-crystalline polymer actuator highlights an enormous potential for realizing versatile reversible shape changes in elastomers.
Phase-segregated PEU exhibits two-way shape changes under various constant stresses. Most distinct actuation (expansion on cooling and contraction on heating) can be detected between +60 and -20 °C when the maximum stress applied during a thermo-mechanical pretreatment approached the point where deformation-induced crystallization of the PBA soft segment sets in. The driving forces for the strain-related two-way shape changes consist of PBA crystallization and PBA melting-induced entropy elasticity. At low constant stresses (<1.5 MPa), entropy elasticity also contributes to specimen expansion on cooling. The results of multiple- cycling experiments imply that PEU is an appropriate candidate for the fabrication of thermoresponsive actuators working under constant stress conditions.