Dynamic covalent polymer networks have long been recognized. With the initial focus on the unintended impact of dynamic covalent linkages on the viscoelasticity of commercial rubbers, efforts in modern times have transitioned into designing dynamic covalent polymer networks with unique adaptive properties. Whereas self-healing and thermoset reprocessing have been the primary motivations for studying dynamic covalent polymer networks, the recent discovery of the vitrimeric rheological behavior and solid-state plasticity for this type of material have opened up new opportunities in material innovations. This, coupled with the revelation of the dynamic characteristics of commercially relevant polymer building blocks such as esters and urethanes, suggests a promising future for this class of materials.

Thermoset polymers are known for their superior thermomechanical properties, but the chemical crosslinking typically leads to intractability. This is reflected in the great differences between thermoset and thermoplastic shape-memory polymers; the former exhibit a robust shape memory but are not capable of redefining the permanent shape. Contrary to current knowledge, we reveal here that a classical thermoset shape-memory polyurethane is readily capable of permanent reshaping (plasticity) after a topological network rearrangement that is induced by transcarbamoylation. By employing the Jianzhi technique (also known as kirigami), unexpected shape-shifting versatility was observed for this otherwise classical material. As the essential carbamate moiety in polyurethanes is one of the most common polymer building units, we anticipate that our finding will have significant benefits beyond shape shifting.

Shape memory polymer with thermally distinct elasticity and plasticity enables highly complex shape manipulations.

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.

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.T he ability of shape memory polymer (SMP) to fix a temporary shape and recover to its permanent shape has shown unique advantages toward various applications, including aerospace structures and biomedical devices. 1−6 Expanding the material properties beyond the traditional shape memory effect can drastically open up new opportunities. On this front, much of the recent progress is centered on the temporary shape fixing (e.g., multishape memory effect 7 ) and the reversibility of the polymer shape memory effect. 8−10 In contrast, the attention on the permanent shapes has been largely nonexistent beyond the simple distinction between thermoset SMP and thermoplastic SMP, with the latter remoldable for permanent shape resetting. This is in sharp contrast to the requirement of many real world device applications that often demand sophistication in manipulating the permanent geometry.Plasticity in thermoset SMP allows covalent bond exchange in the network, consequently, permanent shape reconfiguration without melting through the network topological rearrangement. 2−5,11−16 Thus, the permanent shape can be reconfigured repetitively without using mold, more importantly, this offers a new way to access extremely sophisticated permanent shapes that cannot be made otherwise. 2 However, the types of permanent shapes accessible via plastic deformation are limited to the solid state deformation of the prior permanent shape. In this work, we reveal that the permanent shape of an SMP network containing Diels−Alder (DA) adducts can be redefined by both liquid molding and solid state plasticity in two different temperature ranges by taking advantage of their dynamic equilibrium nature.The click and reversible nature of DA reactions has historically attracted much attention for designing nonconventional thermosets. 17−22 Typically, the network material consisting of furan/maleimide adducts is heated to a high temperature (above 110°C) to trigger the reverse Diels−Alder reaction (retro-DA) such that it can reach a liquid state t...

Dynamic covalent polymer networks exhibit unusual adaptability while maintaining the robustness of conventional covalent networks. Typically, their network topology is statistically nonchangeable, and their material properties are therefore nonprogrammable. By introducing topological heterogeneity, we demonstrate a concept of topology isomerizable network (TIN) that can be programmed into many topological states. Using a photo-latent catalyst that controls the isomerization reaction, spatiotemporal manipulation of the topology is realized. The overall result is that the network polymer can be programmed into numerous polymers with distinctive and spatially definable (thermo-) mechanical properties. Among many opportunities for practical applications, the unique attributes of TIN can be explored for use as shape-shifting structures, adaptive robotic arms, and fracture-resistant stretchable devices, showing a high degree of design versatility. The TIN concept enriches the design of polymers, with potential expansion into other materials with variations in dynamic covalent chemistries, isomerizable topologies, and programmable macroscopic properties.

The ability to undergo bond exchange in a dynamic covalent polymer network has brought many benefits not offered by classical thermoplastic and thermoset polymers. Despite the bond exchangeability, the overall network topologies for existing dynamic networks typically cannot be altered, limiting their potential expansion into unexplored territories. By harnessing topological defects inherent in any real polymer network, we show herein a general design that allows a dynamic network to undergo rearrangement to distinctive topologies. The use of a light triggered catalyst further allows spatio-temporal regulation of the network topology, leading to an unusual opportunity to program polymer properties. Applying this strategy to functional shape memory networks yields custom designable multi-shape and reversible shape memory characteristics. This molecular principle expands the design versatility for network polymers, with broad implications in many other areas including soft robotics, flexible electronics, and medical devices.

Thermoset polymers are knownf or their superior thermomechanical properties,b ut the chemical crosslinking typically leads to intractability.T his is reflected in the great differences between thermoset and thermoplastic shapememory polymers;t he former exhibit ar obust shape memory but are not capable of redefining the permanent shape.C ontrary to current knowledge,w er eveal here that ac lassical thermoset shape-memory polyurethane is readily capable of permanent reshaping (plasticity) after atopological network rearrangement that is induced by transcarbamoylation. By employing the Jianzhi technique (also knowna s kirigami), unexpected shape-shifting versatility was observed for this otherwise classical material. As the essential carbamate moiety in polyurethanes is one of the most common polymer building units,w ea nticipate that our finding will have significant benefits beyond shape shifting.