Exploring Dynamic Equilibrium of Diels–Alder Reaction for Solid State Plasticity in Remoldable Shape Memory Polymer Network

Abstract

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...

One-Pot Synthesis and Characterization of Novel Shape-Memory Poly(ε-Caprolactone) Based Polyurethane-Epoxy Co-networks with Diels–Alder Couplings

“…The latter was obtained by the reaction of isocyanate functionalized prepolymer with FA (see Figure 2). To link the FA-end-functionalized prepolymers to the FA_DGEBA oligomer, BMI was added to the reaction mixture, which led to formation of the DA-adducts (see EP-PU [16][17][18][19][20][21][22][23][24]. It is noteworthy that only selected EP-PU samples were subjected to some investigations to keep this contribution in limits.…”

“…The first synthetic way for the preparation of samples EP-PU 1-15 was the incorporation of a HO-ended DA adduct in the networks composed of isocyanate (HDI) terminated polyurethane prepolymer and DGEBA and followed by crosslinking of the EP-component by polyetheramine (Jeffamine ® EDR 176). The other synthetic route utilized the reaction between furanyl end-functionalized PU polymers and FA_DGEBA oligomer in the presence of BMI as the coupling agent (samples EP-PU [16][17][18][19][20][21][22][23][24]. Thus, the two synthetic routes differed from one another in generation of the DA-adducts and in the composition of the co-networks.…”

“…[24]. FA (2.45 mL, 2.8 × 10 −2 mol, 2 equivalent) and DGEBA (5 g, 1.4 × 10 −2 mol, 1 equivalent) were mixed in dry toluene (25 mL) and reacted at 100 °C for 24 h. The obtained dark red solution was used in further reactions after 1 3 1.61 Figure 1.…”

Reversible TAD Chemistry as a Convenient Tool for the Design of (Re)processable PCL‐Based Shape‐Memory Materials

“…This highly elastic behavior demonstrates that a heating at 120 °C does not significantly displace the position of the equilibrium of the AE reaction toward the adducts dissociation, in contrast to conventional DA reactions. Therefore, the (re)processability of the material would rather be made possible by solid‐state plasticity, allowing covalent bond exchange in the cross‐linked material, thanks to the high dynamics of the equilibrium as demonstrated for such fast equilibrated systems or for DA‐based systems . This accounts for the observed successful compression molding at 120 °C during 1 h of the material in a stable and permanent shape.…”

Facile one-pot synthesis and self-healing properties of tetrazole-based metallopolymers in the presence of iron salts

“…The past few decades have witnessed a rapid development in exerting such features into articial systems where polymers are used for self-healing applications. Several self-healing strategies, such as the utilization of healing agents, 13 reversible covalent 14,15 and noncovalent bonds, 16 have been reported in polymer systems. Different dynamic noncovalent bonding interactions, such as hydrogen-bonding, 17,18 ionic interactions, 19,20 p-p stacking 21 and metal-ligand interaction, 22 have been used to improve the properties of materials with self-healing ability.…”