Malleable thermosets are crosslinked polymers containing dynamic covalent bonds, which can be reversibly cleaved and reformed. They have attracted considerable attention in recent years due to their combined advantages of thermosets and thermoplastics. They have excellent mechanical properties and thermal and chemical stabilities like traditional thermosets yet are reprocessable and recyclable like thermoplastics. Although the chemical composition plays an important role in determining the mechanical and thermal properties of materials, the application of dynamic covalent chemistry is the key to achieving the unique properties of malleable thermosets. The mechanism of reversible bond cleavage and reformation, bond activation energies and kinetics, and the conditions triggering such reversibility define the malleable properties of the materials, how and why they can be reprocessed, and when the materials fail. In this review, we introduce fundamental concepts and principles of malleable thermosets, dynamic covalent chemistry, and the characteristic materials properties, including reprocessability, rehealability, and possible recyclability. We categorize the recent literature examples based on the underlying chemistry to demonstrate how dynamic covalent chemistry is exploited in malleable thermosets and how their malleable properties can be achieved and altered; we also discuss intriguing future opportunities based on such exploitation.Highly crosslinked covalent network polymers, commonly called thermosets, generally provide outstanding mechanical properties, chemical and heat resistance, and dimensional stability. Thermosets have found extensive variety of applications ranging from kettle handles and surface coatings to auto bodies. However, since thermosets are cured through the formation of irreversible chemical bonds, 1,2 they cannot be reprocessed or recycled upon failure. 3 Furthermore, any shape change that occurs due to reversible bond-exchange reactions (e.g., disulfide crosslinks) has been known as ''creep'' and has been considered a drawback of polymeric materials. 4 Therefore, it is by design that thermoset networks are irreversible and essentially unrecyclable.Thermoplastics, which consist of linear polymer chains with no crosslinks between them, represent reprocessable and fully recyclable polymeric materials. The thermoprocessing involves weakening of intermolecular forces between polymer chains, and no chemical bonding takes place. As a result, thermoplastics can be reshaped by heating many times without negatively affecting physical properties of the materials. However, they usually exhibit inferior chemical resistance and hightemperature mechanical properties compared with thermosets.Recently, covalent adaptable network (CAN) polymers have been developed, which combine excellent mechanical properties of thermosets with reprocessability of thermoplastics. [5][6][7] This new class of polymer networks incorporates dynamic

Wearable electronics can be integrated with the human body for monitoring physical activities and health conditions, for human-computer interfaces, and for virtual/augmented reality. We here report a multifunctional wearable electronic system that combines advances in materials, chemistry, and mechanics to enable superior stretchability, self-healability, recyclability, and reconfigurability. This electronic system heterogeneously integrates rigid, soft, and liquid materials through a low-cost fabrication method. The properties reported in this wearable electronic system can find applications in many areas, including health care, robotics, and prosthetics, and can benefit the well-being, economy, and sustainability of our society.

No abstract

A novel conductive composite consisting of polyimine vitrimer matrix and multiwalled carbon nanotube (MWCNT) filler that allows bending, stretching, rehealing, and closed-loop recycling was developed. Such composites can be easily prepared by simply heating a mixed solution of the polyimine precursors and less than 10 wt % of MWCNTs. The resulting composites combine both advantages of the polyimine vitrimer (i.e., dynamic covalent bond exchange) and carbon nanotubes (i.e., electron conducting), and the as-fabricated thin films exhibit malleability, rehealability, recyclability and good electron conductivity. Impressively, the electrical conductivity of the composite could remain almost the same after bending, reshaping, rehealing, and reuse, thus making it an excellent candidate for flexible electronics. Moreover, such composites can also be fully recycled at the end of their service life, which would greatly reduce the electronic waste, manufacturing cost, and environmental impact.

The moisture-induced malleability of polyimine-based covalent adaptable networks (CANs) enables the thermosets to be reprocessed and recycled at ambient temperature using only water, which leads to an energy-neutral green processing of the materials. This paper provides both experimental and theoretical studies to understand the effects of temperature and moisture content on the kinetics of bond exchange reactions (BERs), malleability, in situ softening, and damping effects of polyimine-based CANs. The study shows that the temperature and moisture content are equivalent in affecting the time scale of network stress relaxation. A time–temperature–moisture superposition principle is therefore established. The modeling theory identifies the critical thicknesses when the network relaxation is dominated by the moisture diffusion or the intrinsic BER kinetics. It also probes the transition region of polyimine CANs underwater, in which the glassy solid samples gradually transform into the malleable state with an enhanced damping ratio.