Dynamic Semi IPNs with Duple Dynamic Linkers: Self-Healing, Reprocessing, Welding, and Shape Memory Behaviors

Recent advances in self-healing polyurethane based on dynamic covalent bonds combined with other self-healing methods

To meet more application requirements, improving mechanical properties and self-healing efficiency has become the focus of current research on self-healing PU. The competitive relationship between self-healing ability and mechanical properties cannot be avoided by a single self-healing method. To address this problem, a growing number of studies have combined dynamic covalent bonding with other self-healing methods to construct the PU structure. This review summarizes recent studies on PU materials that combine typical dynamic covalent bonds with other self-healing methods. It mainly includes four parts: hydrogen bonding, metal coordination bonding, nanofillers combined with dynamic covalent bonding and multiple dynamic covalent bond bonding. The advantages and disadvantages of different self-healing methods and their significant role in improving self-healing ability and mechanical properties in PU networks are analyzed. At the same time, the possible challenges and research directions of self-healing PU materials in the future are discussed.

Hydroxyl-Terminated Polybutadiene-Based Polyurethane with Self-Healing and Reprocessing Capabilities

Hydroxyl-terminated polybutadiene (HTPB)-based polyurethane (PU) networks play indispensable roles in a variety of applications; however, they cannot be reprocessed, resulting in environmental problems and unsustainable industrial development. In this work, recyclable HTPB-based PU vitrimer (HTPB-PU_V) networks are fabricated by introduction of a cross-linker 2,2'-(1,4-phenylene)-bis[4-mercaptan-1,3,2-dioxaborolane] (BDB) with dynamic boronic ester bonds into the network. Meanwhile, the BDB can stabilize the HTPB unit in the network by elimination of double bonds. The novel HTPB-PU_V networks are constructed by a thiol-ene "click" reaction and an addition reaction between HTPB and cross-linker BDB and isocyanates (HDI). The dynamic HTPB-PU_V networks are characterized by dynamic mechanical analysis (DMA) and Fourier transform infrared (FTIR). The obtained dynamic HTPB-PU_V networks possess superior thermostability. Moreover, due to the presence of dynamic boronic ester bonds, the HTPB-PU_V network topologies can be altered, contributing to the reprocessing, self-healing, and welding abilities of the final polymer. Through a hot press, the pulverized sample can be reprocessed for several cycles, and mechanical properties of the reprocessed samples are similar to those of the pristine one, with the tensile strength being even higher. The self-healed sample exhibits almost complete recovery from scratch after the healing treatment at 130 °C for 3 h. Moreover, a welding efficiency of 120% was achieved.

Recent advances in dynamic covalent bond-based shape memory polymers

Dynamic covalent bond-based shape memory polymers (DCB-SMPs) are one of most important SMPs which have a wide potential application prospect. Different from common strong covalent bonds, DCBs own relatively weak bonding energy, similarly to the supramolecular interactions of noncovalent bonds, and can dynamically combine and dissociate these bonds. DCB-SMP solids, which can be designed to respond for different stimuli, can provide excellent self-healing, good reprocessability, and high mechanical performance, because DCBs can obtain dynamic cross-linking without sacrificing ultrahigh fixing rates. Furthermore, besides DCB-SMP solids, DCB-SMP hydrogels with responsiveness to various stimuli also have been developed recently, which have special biocompatible soft/wet states. Particularly, DCB-SMPs can be combined with emerging 3D-printing techniques to design various original shapes and subsequently complex shape recovery. This review has summarized recent research studies about SMPs based on various DCBs including DCB-SMP solids, DCB-SMP hydrogels, and the introduction of new 3D-printing techniques using them. Last but not least, the advantages/disadvantages of different DCB-SMPs have been analyzed via polymeric structures and the future development trends in this field have been predicted.

Interpenetrated triple network polymers: synergies of three different dynamic bonds

Triply interpenetrated networks were made with a unique dynamic linker in each network. The linkers were hydrogen bonds, boronic esters and Diels–Alder adducts. Triply dynamic materials had superior properties compared to doubly dynamic analogues.

Progress in Utilizing Dynamic Bonds to Fabricate Structurally Adaptive Self-Healing, Shape Memory, and Liquid Crystal Polymers

Stimuli-responsive structurally dynamic polymers are capable of mimicking the biological systems to adapt themselves to the surrounding environmental changes and subsequently exhibiting a wide range of responses ranging from self-healing to complex shape-morphing. Dynamic self-healing polymers (SHPs), shape-memory polymers (SMPs) and liquid crystal elastomers (LCEs), which are three representative examples of stimuli-responsive structurally dynamic polymers, have been attracting broad and growing interest in recent years because of their potential applications in the fields of electronic skin, sensors, soft robots, artificial muscles, and so on. We review recent advances and challenges in the developments towards dynamic SHPs, SMPs and LCEs, focusing on the chemistry strategies and the dynamic reaction mechanisms that enhance the performances of the materials including self-healing, reprocessing and reprogramming. We compare and discuss the different dynamic chemistries and their mechanisms on the enhanced functions of the materials, where three summary tables are presented: a library of dynamic bonds and the resulting characteristics of the materials. Finally, we provide a critical outline of the unresolved issues and future perspectives on the emerging developments. This article is protected by copyright. All rights reserved.

Oxime - Urethane Structure-based Dynamically Crosslinked Polyurethane with Robust Reprocessing Property

Crosslinked polyurethane with excellent mechanical property, solvent resistance and transparency has become one of the most widely used materials. However, the presence of chemical crosslinks makes it difficult to be reprocessed once moulded, which largely restricts its recycling and reusing, resulting in the serious waste problems. Therefore, it is of great significance to prepare a new type of crosslinked polyurethane with reprocessing function. In this work, a novel reprocessable polyurethane (DOPUs) based on reversible dibutanone oxime-carbamate bonds was facilely prepared. The gel fraction of DOPUs is all higher than 95%, endowing it with excellent solvent resistance. Meanwhile, the visible light transmittance of DOPUs can reach up to 97.48%. After four thermal recycles, the tensile strength and elongation at break of recycled DOPUs can still remain at 3.21 MPa and 219.09%, respectively. Importantly, the synthesized DOPUs exhibit excellent elastic shape memory and permanent shape reconstruction properties under thermal stimulation. The dibutanone oxime-carbamate bonds can also be degraded under UV irradiation, making this material easily degradable. Hence, this material has potential applications in coatings, elastomers and some other fields. This article is protected by copyright. All rights reserved.

Effect of the Crosslinking Degree on Self-Healing Poly(1,2,3-Triazolium) Adhesive

Dynamic covalent materials are a class of polymer that could be stress-relaxation, reprocessable, and self-healing due to dynamic crosslinks in network. Dynamic crosslinks play an important role in the typical characteristic of self-healing polymers. It is meaningful to understand the effect of crosslinking degree on the properties of poly(1,2,3-triazolium) (PTAM). In this article, the dynamic covalent network of PTAM adhesive has been used to study the effect of crosslinking degree. A series of PTAM adhesive with different crosslinking degrees have been obtained by changing the amount of crosslinker. Adhesion property can first rise then fall down with the increase of crosslinking degree and the best lap-shear strength is above 20 MPa. Creep resistance and solvent resistance can be enhanced with the increase of crosslinking degree. Self-healing studies have shown that crosslinking degree can enhance the ability of self-healing, but too high crosslinking degree raises the temperature of self-healing and causes side reaction which reduces the self-healing efficiency. These results provide some insights for the influence of the crosslinking degree on the self-healing and the structural design of dynamic covalent materials.