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1. Recyclable high-performance epoxy based on transesterification reaction

   https://doi.org/10.1039/c7ta06397k  

   Lu, Lu; Pan, Jian; Li, Guoqiang (January 2017, J. Mater. Chem. A)

   Self-healing thermoset epoxy based on dynamic covalent bond chemistry has been developed in the past several years, which warrants the creation of recyclable epoxy. However, the existing systems produce epoxy that has lower strength, stiffness, and glass transition temperature, making them unsuitable for load-bearing structures. In this study, we developed a new recyclable thermoset epoxy through solid form recycling. The epoxy has strength, stiffness, and glass transition temperature similar to those found in conventional thermoset epoxy. The effect of healing temperature, healing time, healing pressure, and powder size on the healing efficiency was experimentally investigated. It was found that the healing efficiency is as high as 88.1%, and the epoxy can be recycled more than one time. 

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2. Dynamic Interfaces in Self-Healable Polymers

   https://doi.org/10.1021/acs.langmuir.3c03696  

   Liu, Jiahui; Urban, Marek W. (April 2024, Langmuir)

   It is well-established that interfaces play critical roles in biological and synthetic processes. Aside from significant practical applications, the most accessible and measurable quantity is interfacial tension, which represents a measure of the energy required to create or rejoin two surfaces. Owing to the fact that interfacial processes are critical in polymeric materials, this review outlines recent advances in dynamic interfacial processes involving physics and chemistry targeting self-healing. Entropic interfacial energies stored during damage participate in the recovery, and self-healing depends upon copolymer composition and monomer sequence, monomer molar ratios, molecular weight, and polymer dispersity. These properties ultimately impact chain flexibility, shape-memory recovery, and interfacial interactions. Self-healing is a localized process with global implications on mechanical and other properties. Selected examples driven by interfacial flow and shape memory effects are discussed in the context of covalent and supramolecular rebonding targeting self-healable materials development. 

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3. Thermoresponsive, Recyclable, Conductive, and Healable Polymer Nanocomposites with Three Distinct Dynamic Bonds

   https://doi.org/10.1021/acsapm.2c00790  

   Dodo, Obed J.; Petit, Leilah; Dunn, Derrick; Myers, Camryn P.; Konkolewicz, Dominik (September 2022, ACS Applied Polymer Materials)

   Integration of multiple types of dynamic linkages into one polymer network is challenging and not well understood especially in the design and fabrication of dynamic polymer nanocomposites (DPNs). In this contribution, we present facile methods for synthesizing flexible, healable, conductive, and recyclable thermoresponsive DPNs using three dynamic chemistries playing distinct roles. Dynamic hydrogen bonds account for material flexibility and recycling character. Thiol-Michael exchange accounts for thermoresponsive properties. Diels–Alder reaction leads to covalent bonding between polymer matrix and nanocomposite. Overall, the presence of multiple types of orthogonal dynamic bonds provided a solution to the trade-off between enhanced mechanical performance and material elongation in DPNs. Efficient reinforcement was achieved using <1 wt % multiwalled carbon nanotubes as nanocomposite. Resulting DPNs showed excellent healability with over 3 MPa increase in stress compared to unreinforced materials. Due to multiple responsive dynamic linkages, >90% stress–relaxation was observed with self-healing achieved within 1 h of healing time. Increased mechanical strength, electrical conductivity, and reprocessability were achieved all while maintaining material flexibility and extensibility, hence highlighting the strong promise of these DPNs in the rapidly growing fields of flexible compliant electrodes and strain sensors. 

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4. Accelerating dynamic exchange and self-healing using mechanical forces in crosslinked polymers

   https://doi.org/10.1039/C9MH01938C  

   De Alwis Watuthanthrige, Nethmi; Ahammed, Ballal; Dolan, Madison T.; Fang, Qinghua; Wu, Jian; Sparks, Jessica L.; Zanjani, Mehdi B.; Konkolewicz, Dominik; Ye, Zhijiang (June 2020, Materials Horizons)

   null (Ed.)

   Dynamically crosslinked polymers and their composites have tremendous potential in the development of the next round of advanced materials for aerospace, sensing, and tribological applications. These materials have self-healing properties, or the ability to recover from scratches and cuts. Applied forces can have a significant impact on the mechanical properties of non-dynamic systems. However, the impacts of forces on the self-healing ability of dynamically bonded systems are still poorly understood. Here, we used a combined computational and experimental approach to study the impact of mechanical forces on the self-healing of a model dynamic covalent crosslinked polymer system. Surprisingly, the mechanical history of the materials has a distinct impact on the observed recovery of the mechanical properties after the material is damaged. Higher compressive forces and sustained forces lead to greater self-healing, indicating that mechanical forces can promote dynamic chemistry. The atomistic details provided in molecular dynamics simulations are used to understand the mechanism with both non-covalent and dynamic covalent linkage responses to the external loading. Finite element analysis is performed to bridge the gap between experiments and simulations and to further explore the underlying mechanisms. The self-healing behavior of the crosslinked polymers is explained using reaction rate theory, with the applied force proposed to lower the energy barrier to bond exchange. Overall, our study provides fundamental understanding of how and why the self-healing of cross-linked polymers is affected by a compressive force and the force application time. 

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5. Strain-Stiffening Mechanoresponse in Dynamic-Covalent Cellulose Hydrogels

   https://doi.org/10.1021/acs.biomac.4c00450  

   Ollier, Rachel C; Webber, Matthew J (July 2024, Biomacromolecules)

   Mechanical stimuli such as strain, force, and pressure are pervasive within and beyond the human body. Mechanoresponsive hydrogels have been engineered to undergo changes in their physicochemical or mechanical properties in response to such stimuli. Relevant responses can include strain-stiffening, self-healing, strain-dependent stress relaxation, and shear rate-dependent viscosity. These features are a direct result of dynamic bonds or non- covalent/physical interactions within such hydrogels. The contributions of various types of bonds and intermolecular interactions to these behaviors are important to more fully understand the resulting materials and engineer their mechanoresponsive features. Here, strain-stiffening in carboxymethylcellulose hydrogels crosslinked with pendant dynamic-covalent boronate esters using tannic acid is studied and modulated as a function of polymer concentration, temperature, and effective crosslink density. Furthermore, these materials are found to exhibit self-healing and strain- memory, as well as strain-dependent stress relaxation and shear rate-dependent changes in gel viscosity. These features are attributed to the dynamic nature of the boronate ester crosslinks, inter-chain hydrogen bonding and bundling, or a combination of these two intermolecular interactions. This work provides insight into the interplay of such interactions in the context of mechanoresponsive behaviors, particularly informing the design of hydrogels with tunable strain- stiffening. The multi-responsive and tunable nature of this hydrogel system therefore presents a promising platform for a variety of applications. 

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