Abstract
This work demonstrates that phase-segregated poly(ester urethane) (PEU) with switching segments of crystallizable poly(1,4-butylene adipate) (PBA) can be programmed to generate two separate stress recovery events upon heating under constant strain conditions. For programming, two elongations are applied at different temperatures, followed by unloading and cooling. During the adjacent heating, two-step stress recovery is triggered. The results indicate that the magnitude of the stress recovery signals corresponds to the recovery of the two deformation stresses in reverse order. As demonstrated by further experiments, twofold stress recovery can be detected as long as the elongation at higher temperature exceeds the strain level of the deformation at lower temperature. Another finding includes that varying the lower deformation temperature enables a control over the stress recovery temperature and thus the implementation of so-called “temperature-memory effects”. Moreover, exerting only one elongation during programming enables a heating-initiated one-step stress recovery close to the deformation temperature. Based on these findings, such polymers may offer new technological opportunities in the fields of active assembly when used as fastening elements and in functional clothing when utilized for compression stockings.
Highlights
Some materials can be programmed to fix a temporary shape and either recover shape or build up mechanical stress in response to heating
Exerting only one elongation during programming enables a heating-initiated one-step stress recovery close to the deformation temperature. Based on these findings, such polymers may offer new technological opportunities in the fields of active assembly when used as fastening elements and in functional clothing when utilized for compression stockings
The calorimetric properties of the poly(ester urethane) (PEU) were characterized by a melting transition between 33 and 54 ◦ C, a crystallization transition spreading from 14 to −20 ◦ C, and a glass transition at about −45 ◦ C (Figure 1); all of these phase transitions could be assigned to poly(1,4-butylene adipate) (PBA), which later served as switching segment in the PEU [22]. 4 of 12
Summary
Some materials can be programmed to fix a temporary shape and either recover shape or build up mechanical stress in response to heating. This behavior is characteristic for shape-memory materials [1], like shape-memory alloys [2,3,4] and shape-memory polymers (SMPs) [5,6,7,8,9]. The beneficial thermomechanical behavior derives from network structure and the separation of immiscible hard and soft segments at room temperature [29,30,31]
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