A Cold-Expansion Polymer with Delayed Actuation

Many thermosensitive polymers have been exploited as actuators, whose thermally actuated process is essentially equivalent to the scenario where the polymer bulk exchanges thermal energy with the environment and subsequently undergoes changes in shape or properties. This transformation typically occurs within seconds or even milliseconds, approximating an instantaneous response, which often suggests limited control over the duration of the actuation. In response to this issue, we propose a strategy to control the actuation time of semicrystalline polyurethanes via exploiting the delayed crystallization response during their two-way shape memory behavior. Specifically, a series of semicrystalline polyurethane materials with different crystallinities were synthesized by adjusting the cross-linking density and introducing irregular copolymer units, allowing a controlled extension of the time window for cold-induced deformation of the materials. These findings hold the promise of enabling the use of customizable shapes in exceptionally low-temperature environments. In contrast to the modulus reduction based on high-temperature triggering, semicrystalline shape memory actuators, based on low-temperature-oriented crystallization, exhibit increased modulus while achieving shape changes, which is expected to alleviate the issue of excessively low shape recovery stress in conventional shape memory polymers.