Photoinduced reversible liquefaction and solidification of polymers enable processing and healing of polymers with light. Some azobenzene-containing polymers (azopolymers) exhibit two types of photoinduced liquefaction properties: photoinduced reversible solid-to-liquid transition and directional photofluidization. For the first type, light switches the glass transition temperature (T_g) values of azopolymers and induces reversible solid-to-liquid transitions. For the second type, polarized light guides solid azopolymers to flow along the polarization direction. Here, we compare the two types of photoliquefaction and discuss their mechanisms. Recent progresses and applications based on photoliquefaction of azopolymers are also highlighted.

Owing to the advantages of non-volatility, outstanding fluidity and easy recyclability, ionic liquid-based electronics, such as thermometer, strain sensors and thermoelectric converters, have been growing as attractive alternatives to traditionally solid electronics. The fluidic character endows the ionic liquid-based circuit with self-healing ability, satisfying the needs of longer lifetime and less waste generation for electronics, while at the same time brings the risk of leakage. Avoiding the leakage without sacrifice of self-healing ability is one of the major challenges for constructing ionic liquid-based electronic devices. In this feature article, we summarize our recent progresses in developing two types of self-healing electrical devices based on ionic liquids with little risk of leakage. One type involves the encapsulation of ionic liquids in self-healing polymers, and the other type uses ionic polymers or free-standing ionic liquids which are successfully formulated as intrinsically conductive, self-healing, and recyclable electronic devices without additional encapsulation. In the end, a comprehensive outlook is prospected for the future development of ionic liquid-based self-healing electronics, which is expected to spur more innovative work in this field.

Self-healing hydrogels have attracted growing attention over the past decade due to their biomimetic structure, biocompatibility, as well as enhanced lifespan and reliability, thereby have been widely used in various biomedical, electrical and environmental engineering applications. This feature article has reviewed our recent progress in self-healing hydrogels derived from mussel-inspired interactions, multiple hydrogen-bonding functional groups such as 2-ureido-4[1H]-pyrimidinone (UPy), dynamic covalent bonds (e.g., Schiff base reactions and boronic ester bonds). The underlying molecular basics of these interactions, hydrogel preparation principles, and corresponding performances and applications are introduced. The underlying reversible intermolecular interaction mechanisms in these hydrogels were investigated using nanomechanical techniques such as surface forces apparatus (SFA) and atomic force microscopy (AFM), providing fundamental insights into the self-healing mechanisms of the hydrogels. The remaining challenging issues and perspectives in this rapidly developing research area are also discussed.

The concept of self-healing that involves a built-in ability to heal in response to damage wherever and whenever it occurs in a material, analogous to the healing process in living organisms, has emerged a couple of decades ago. Driven primarily by the demands for life-like materials and soft smart materials, therefore, the development of self-healing polymeric hydrogels has continually attracted the attention of the scientific community. Here, this review is intended to give an in-depth overview of the state-of-the-art advances in the field of self-healing polymeric hydrogels. Specifically, recently emerging trends in self-healing polymeric hydrogels are summarized, and notably, recommendations to endow these hydrogels with fascinating multi-functionalities including luminescence, conductivity/magnetism and shape memory etc. are presented. To close, the current challenges and future opportunities in this field are also discussed.

Oxime-urethane bond featuring with high reversibility even at room temperature and multiple reactivity is an emerging dynamic covalent bond, and has shown great potential for self-healing polymers, which are one of the most attractive development directions for next generation of polymeric materials. In this review, recent progresses on the oxime-urethane-based self-healing polymers, including their designs and applications in diverse fields such as biomedicine, flexible electronics, soft robots, 3D printing, protective materials, and adhesives, are summarized, and outlooks on the future development of this field are discussed.

The “solid-liquid” behavior of vitrimers have not been systematically investigated. Herein, a series of “solid-liquid” vitrimers bearing varying contents of dynamic boronic ester bonds were synthesized via thiol-ene click reactions. These vitrimers allow for flexibile modulation of their network structures and thus show a range of intriguing properties including high stretchability, flexible transition from elasticity to plasticity, strong strain rate dependence, and solid-liquid performance. The dynamic association rate of boronic ester bonds within these vitrimers could be apparently accelerated via increasing the content of boronic ester, which could be used to shape-program the flat vitrimer films into various complex 3D structures just with external force. Materials with such versatile dynamic behavior may open up a range of new applications.

Elastomers with outstanding strength, toughness and healing efficiency are highly promising for many emerging fields. However, it is still a challenge to integrate all these beneficial features in one elastomer. Herein, an asymmetric alicyclic structure adjacent to aromatic disulfide was tactfully introduced into the backbone of polyurethane (PU) elastomer. Specifically, such elastomer (PU-HPS) was fabricated by polycondensing polytetramethylene ether glycol (PTMEG), isophorone diisocyanate (IPDI) and p-hydroxydiphenyl disulfide (HPS) via one-pot method. The molecular mobility and phase morphology of PU-HPS can be tuned by adjusting the HPS content. Consequently, the dynamic exchange of hydrogen and disulfide bonds in the hard segment domains can also be tailored. The optimized sample manifests outstanding tensile strength (46.4 MPa), high toughness (109.1 MJ/m³), high self-healing efficiency after fracture (90.3%), complete scratch recovery (100%) and good puncture resistance. Therefore, this work provides a facile strategy for developing robust self-healing polymers.

Aggregate engineering of non-covalent networks endows supramolecular polymers with thermo-mechanical versatility, stimuli-responsive phase transitions and intrinsic damage-healing capabilities. However, most non-covalent networks are vulnerable at elevated temperatures, which suppresses the robustness of supramolecular polymers. Herein, ureidocytosine (UCy) motifs, which are capable of forming extensive non-covalent networks and thus robust molecular aggregates via multivalent hydrogen bonds and aromatic stackings, are proposed to enable precise programming of the thermo-mechanical versatility. Molecular simulations reveal that the enthalpic contributions from the UCy aggregates play dominant roles to compensate the entropic loss from the redistributions of polymeric spacers and stabilize the non-covalent networks over wide temperature windows. Such aggregate-level strategy offers prospects for applications which require thermo-mechanical versatility of supramolecular polymers, such as 3D printing, microfabrication and damage-healing coating.

The development of adhesive technology is gaining increasing attention in machinery, electronics, aviation, and other fields. However, traditional adhesives are difficult to be peeled and removed after cross-linking and curing due to the limited solubility in common solvents, which causes pollution and damage to the surface of the adherend. In this work, we synthesized random hyperbranched polyamide-amine hot melt adhesives through Michael addition of 1,8-octanediamine and N,N'-methylene diacrylamide (ODA-RHP HMA). Owing to the presence of various polar groups, ODA-RHP HMA exhibited robust lap shear strength to different substrates, including glass (6.6 MPa), ceramics (10.3 MPa), steel (11.5 MPa), and aluminum (11.8 MPa). Due to the hydrogen bonds in ODA-RHP, the HMA demonstrated intrinsic self-healing ability, which can be used repeatedly when being subjected to cyclic heating and cooling. Since ODA-RHP HMA can be swollen or dissolved in ethanol, the adhesives can be easily removed through erasing. In addition, ODA-RHP exhibited aggregation-induced luminescence due to the tertiary amine structure in the molecular structure of ODA-RHP, which can be used in the field of cultural relics restoration.

Thermal conducting materials may be damaged during long-term use, resulting in the increase of thermal resistance and therefore inefficient heat dissipation. The introduction of self-healing ability may solve this problem, but the realization of fast and room-temperature self-healing in thermal conducting composites is quite challenging. Herein, we choose a flexible poly(dimethylsiloxane) polymer material (PDMS-COOH) as the matrix and graphene nanosheets as the thermal conductive filler to prepare a new kind of thermal conductive polymer composite (PDMS-COOH-CG) that can quickly self-heal at room temperature. The thermal conductivity of PDMS-COOH-CG₁₀ with 10% of graphene content is 0.48 W·m⁻¹·K⁻¹, which is 16 times that of PDMS-COOH (0.03 W·m⁻¹·K⁻¹). At room temperature, self-healing efficiency of PDMS-COOH-CG₁₀ based on tensile strength can be 53.8% for 30 s and 84.6% for 24 h. Dynamic infrared thermal imaging dipicted that after 2 min of self-healing at room temperature, the thermal conduction temperature near the damage was basically restored to the level of the pristine sample.

Recently, increasingly growing efforts have been devoted to incorporating dynamic covalent bonds into covalently crosslinked networks to address the persistent trade-offs between chemical crosslinking and malleability. Herein, a series of dynamic aminal bond crosslinked polybutadiene rubbers (PAPB) are prepared by crosslinking aldehyde group-terminated polybutadiene rubber (APB) with piperazine. By varying the molecular weight of APB, the crosslinking density of PAPB is changed, which offers the platform to regulate the mechanical characteristics and dynamic properties. Specially, with the decrease of APB molecular weight, i.e. with the increase of crosslinking density, the modulus of PAPB gradually increases while the elongation at break conversely decreases, and the activation energy for network rearrangement initially decreases and then increases. The resultant PAPB exhibit vitrimer-like behaviors that can alter the network topologies at elevated temperatures without the loss of structural integrity through dissociative aminal exchange reactions, while the protic source can accelerate aminal dissociation and result in network dissolution even at room temperature. Due to the aminal exchange, PAPB are thermally malleable and can almost restore the original mechanical characteristics after recycling; besides, they are capable of healing at a relatively low crosslinking density.