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The diverse morphing behaviors of living creatures arise from their unlimited pathways. In contrast, the equilibrium-driven morphing pathways of common synthetic shape-shifting materials are very limited. For a shape-memory polymer (SMP), its recovery from the temporary shape(s) to the permanent shape typically requires external stimulation and follows a single fixed route. Herein, a covalently crosslinked SMP is designed with ample ureidopyrimidinone (UPy) supramolecular moieties in the network. The UPy units endow the SMP with strong time-temperature dependency, which is explored as a mechanism for spatio-temporal programming of autonomous shape-shifting pathways. In particular, the use of digitally controlled photothermal heating provides versatility in control via an off-equilibrium mechanism. In addition, cooling/heating across its glass transition introduces a locking/unlocking mechanism for its temporal morphing. The benefits of these unique features are demonstrated by multi-shape-transformation, an "invisible"-color-based clock, a time-temperature indicator, and sequence-programmable 4D printing.
Peng et al. (Sun,) studied this question.