The unique capability of topological rearrangement for dynamic covalent polymer networks has enabled various unusual properties (self-healing, solid-state plasticity, and reprocessability) that are not found in conventional thermosets. Achieving these properties in one network in a synergetic fashion can open up new opportunities for shape memory polymer. To accomplish such a goal, the freedom to tune topological rearrangement kinetics is critical. This is, however, challenging to achieve. In this work, two sets of dynamic bonds (urethane and hindered urea) are incorporated into a hybrid network for synthesizing shape memory poly(urea-urethane). By changing the bond ratio, networks with highly tunable topological rearrangement kinetics are obtained. Combining self-healing, solid-state plasticity, and reprocessability in one such shape memory network leads to unusual versatility in its shape-shifting performance.Keywords: dynamic covalent bond; plasticity; reprocessing; self-healing; shape memory polymer; thermoset;

Flexible and stretchable electronics offer a wide range of unprecedented opportunities beyond conventional rigid electronics. Despite their vast promise, a significant bottleneck lies in the availability of a transfer printing technique to manufacture such devices in a highly controllable and scalable manner. Current technologies usually rely on manual stick-and-place and do not offer feasible mechanisms for precise and quantitative process control, especially when scalability is taken into account. Here, we demonstrate a spatioselective and programmable transfer strategy to print electronic microelements onto a soft substrate. The method takes advantage of automated direct laser writing to trigger localized heating of a micropatterned shape memory polymer adhesive stamp, allowing highly controlled and spatioselective switching of the interfacial adhesion. This, coupled to the proper tuning of the stamp properties, enables printing with perfect yield. The wide range adhesion switchability further allows printing of hybrid electronic elements, which is otherwise challenging given the complex interfacial manipulation involved. Our temperature-controlled transfer printing technique shows its critical importance and obvious advantages in the potential scale-up of device manufacturing. Our strategy opens a route to manufacturing flexible electronics with exceptional versatility and potential scalability.Keywords: bioinspired; direct laser-writing; micropatterns; programmable transfer printing; shape memory polymers;

Thermoset shape memory polymer (SMP) with dynamic covalent bonds in the network is a new class of SMPs for which the permanent shape can be reconfigured via topological rearrangement (plasticity). Catalyzed transcarbamoylation has recently been established as an effective exchange reaction for plasticity in cross-linked polyurethane networks. However, ensuring the plasticity severely constrains the network design which adversely affects the ability to tune other classical shape memory properties for practical applications. Facing this new challenge, we design an amorphous polyurethane system for which the cross-linking density can be adjusted in a wide range. We discovered that the use of an aromatic diisocyanate in the synthesis of the polyurethanes facilitates achieving plasticity without requiring any catalyst. The overall network design leads to tunable recovery stress and shape memory transition temperatures without sacrificing the plasticity. The versatility of our polyurethane SMP is further reflected in its triple-shape memory performance. We anticipate that our tunable polyurethanes will benefit a variety of potential SMP device applications.

AbstractSolid-state plasticity by dynamic covalent bond exchange in a shape-memory polymer network bestows a permanent shape reconfiguration ability. Spatio-selective control of thermally induced plasticity may further extend the capabilities of materials into unexplored domains. However, this is difficult to achieve because of the lack of spatio-control in typical polymer network synthesis. Metal–ligand interactions possess the high strength of covalent bonds while maintaining the dynamic reversibility of supramolecular bonds. Metallosupramolecular shape-memory polymer networks were designed and prepared, which demonstrated solid-state plasticity. The metallo-coordination bonds within these networks permit facile tuning of the plasticity behavior across a wide temperature range, simply by changing the metal ion. By controlling the diffusion of two different metal ions during preparation of a polymer film, a plasticity behavior with a spatial gradient was achieved, providing a unique shape-morphing versatility with potential in shape-memory devices.

Controlling the response for stimuli-responsive shape changing polymers is critically important for their device applications. The snapping transformation of the Venus Flytrap has inspired the design of shape changing devices with a unique controlling mechanism in mechanical instability, yet their practical potential has been quite limited due to the irreversible nature. Herein, we report an approach to achieve an unprecedented reversible snapping. The material system is a hydrogel assembly that can be mechanically programmed to exhibit instability based bi-stable states. Taking advantages of the multi-responsiveness of the hydrogels allows reversible switching between the two stable states in an abrupt non-continuous (snap) fashion, with unique benefits in precise time-delayed deployment, accelerated deployment speed, and enhanced output force. The mechanism behind our design can be readily extended beyond hydrogels to enhance the performances of diverse multifunctional smart devices.

The reversible and click nature of Diels–Alder (DA) reactions has made them ideal candidates to design materials with nonconventional properties. Most commonly, the reversibility of DA is utilized for designing thermosets that can be liquefied for reprocessing and self-healing, yet the dynamic equilibrium nature has been largely neglected. In this work, shape memory polymers (SMP) containing DA moieties in the networks were synthesized. In addition to its remoldability at the liquid state at sufficiently high temperatures (above 110 °C), we show uniquely and surprisingly that such a network can undergo plastic deformation in its solid state at intermediate temperatures (60–100 °C) by taking advantage of its dynamic equilibrium for network topological rearrangement. The liquid state remoldability and solid state plasticity represent two distinct yet complementary mechanisms to manipulate the permanent shape of an SMP, leading to unprecedented versatility that can benefit a variety of applications in the future.

Epoxy polymers represent a recently emerged class of thermoset shape memory polymers with superior thermo-mechanical endurance and excellent processability. However, the strains at break are typically low for epoxy shape memory polymers. This severely limits their potential applications. In this article, we report a two component epoxy-amine shape memory polymer system with tunable Tg (between 40 °C and 80 °C) and excellent shape memory properties in terms of shape fixity, shape recovery ratios, and cycling stability. Importantly, its values of strain at break above Tg and at the Tg peak reach 111% and 212%, respectively. We anticipate that such a high strain epoxy system will significantly broaden the opportunities for shape memory device applications.

Recent morphological characterization of Nafion as correlated to its proton transport properties has led to the conclusion that the structural ordering in the ionic phase of Nafion may be quite similar to liquid crystalline polymers. Liquid crystalline polymers, on the other hand, are well-known for their thermoreversible actuation (or two-way shape memory) behavior. This has motivated us to design and conduct thermomechanical experiments to probe Nafion’s potential liquid crystalline characteristics from the standpoint of thermoreversible actuation. Our experimental results surprisingly revealed the thermoreversible actuation behavior of Nafion, buried alongside the irreversible creep behavior commonly observed for some polymers. As such, the current study provides evidence supporting Nafion’s structural similarity to liquid crystalline polymers.