Healable Supracolloidal Nanocomposite Water-Borne Coatings

Water-borne coatings often contain nanofillers to enhance their mechanical or optical properties. The aggregation of these fillers may, however, lead to undesired effects such as brittle and opaque coatings, reducing their performance and lifetime. By controlling the distribution and structural arrangement of the nanofillers in the coatings and inserting reversible chemical bonds, both the elasticity and strength of the coatings may be effectively improved, while healing properties, via the reversible chemistry, extend the coating's lifetime. Aqueous dispersions of polymer-core/silica-corona supracolloidal particles were used to prepare water-borne coatings. Polymer and silica nanoparticles were prefunctionalized with thiol/disulfide groups during the supracolloid assembly. Disulfide bridges were further established between a cross-linker and the supracolloids during drying and coating formation. The supracolloidal nanocomposite coatings were submitted to intentional (physical) damages, i.e., blunt and sharp surface scratches or cut through into two pieces, and subsequently UV irradiated to induce the recovery of the damage(s). The viscoelasticity and healing properties of the coatings were examined by dynamic, static, and surface mechanical analyses. The nanocomposite coatings showed a great extent of interfacial restoration of cut damage and surface scratches. The healing properties are strongly related to the coating's viscoelasticity and interfacial (re)activation of the disulfide bridges. Nanocomposite coatings with silica concentrations below their critical volume fraction show higher in situ healing efficiency, as compared to coatings with higher silica concentration. This work provides insights into the control of nanofillers distribution in water-borne coatings and strategies to increase the coating lifetime via mechanical damage recovery.

Improving Sustainability through Covalent Adaptable Networks in the Recycling of Polyurethane Plastics

The global plastic waste problem has created an urgent need for the development of more sustainable materials and recycling processes. Polyurethane (PU) plastics, which represent 5.5% of globally produced plastics, are particularly challenging to recycle owing to their crosslinked structure. Covalent adaptable networks (CANs) based on dynamic covalent bonds have emerged as a promising solution for recycling PU waste. CANs enable the production of thermoset polymers that can be recycled using methods that are traditionally reserved for thermoplastic polymers. Reprocessing using hot-pressing techniques, in particular, proved to be more suited for the class of polyurethanes, allowing for the efficient recycling of PU materials. This Review paper explores the potential of CANs for improving the sustainability of PU recycling processes by examining different types of PU-CANs, bond types, and fillers that can be used to optimise the recycling efficiency. The paper concludes that further research is needed to develop more cost-effective and industrial-friendly techniques for recycling PU-CANs, as they can significantly contribute to sustainable development by creating recyclable thermoset polymers.

Advances and Challenges of Self-Healing Elastomers: A Mini Review

In the last few decades, self-healing polymeric materials have been widely investigated because they can heal the damages spontaneously and thereby prolong their service lifetime. Many ingenious synthetic procedures have been developed for fabricating self-healing polymers with high performance. This mini review provides an impressive summary of the self-healing polymers with fast self-healing speed, which exhibits an irreplaceable role in many intriguing applications, such as flexible electronics. After a brief introduction to the development of self-healing polymers, we divide the development of self-healing polymers into five stages through the perspective of their research priorities at different periods. Subsequently, we elaborated the underlying healing mechanism of polymers, including the self-healing origins, the influencing factors, and direct evidence of healing at nanoscopic level. Following this, recent advance in realizing the fast self-healing speed of polymers through physical and chemical approaches is extensively overviewed. In particular, the methodology for balancing the mechanical strength and healing ability in fast self-healing elastomers is summarized. We hope that it could afford useful information for research people in promoting the further technical development of new strategies and technologies to prepare the high performance self-healing elastomers for advanced applications.

Self-Healing Polymers and Composites: Extrinsic Routes

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Polymers have the property to convert the physical stress to covalent bond shuffling, thereby acting as the healing agents. Polymeric coatings, paints, electronic devices, drug delivery, and many other applications find self-healing materials as a smart technique to prolong the life cycle of the end products. The idea behind these artificial materials is to make them behave like the human body. It should sense the failure and repair it before it becomes worse or irreparable. Researchers have explored several polymeric materials which can self-heal through intrinsic or extrinsic mechanisms. This review specifically focuses on extrinsic routes governed by mechanical stress, temperature change in a covalent bond, humidity, variation in pH, optical sensitivity, and electrochemical effects. Each possible mechanism is further supported by the molecules or bonds which can undergo the transformations under given conditions. On a broader scale, bonds that can self-repair by mechanical force, thermal treatment, chemical modifications, UV irradiation, or electromagnetic phenomenon are covered under this review. It brings into the notice the shortcomings or challenges in adopting the technology to the commercial scale. The possible molecules or bonds which can undergo self-healing under certain conditions have been distinctly presented in a well-segregated manner. This review is envisaged to act as a guide for researchers working in this area.

Self-Healing Silver Nanowires and Reduced Graphene Oxide/Polyurethane Composite Film Based on the Diels–Alder Reaction under Infrared Radiation

The hybrid composite of silver nanowires (AgNWs) and reduced graphene oxide (RGO) was synthesized in situ by an improved polyol–thermal method. The AgNWs-RGO with mass contents of 5–37 wt% was added into the thermo-reversible Diels–Alder reaction polyurethane (DA-PU) matrix with the AgNWs as the main conductor and the RGO as the auxiliary conductor to prepare self-healing composite conductive films. Further, the electrical conductivity, thermal conductivity, mechanical properties, infrared thermal response, and self-healing property of the composite film under infrared light irradiation were studied. The experimental results demonstrate that the AgNWs-RGO endows the composite film with good electrical and thermal conductivity and infrared thermal response ability, while the mechanical properties of the composite film decrease as the AgNWs-RGO mass content increases. The self-healing efficiency of the composite film is higher than that of the pure DA-PU under infrared light irradiation due to the good infrared photothermal response ability of the AgNWs-RGO. When the mass content of AgNWs-RGO in the composite film was 25 wt%, the AgNWs-RGO showed good dispersion in composite films, and the resistivity, thermal conductivity, and tensile strength of the composite film were 0.544 Ω·m, 0.3039 W·m⁻¹·K⁻¹, and 9.05 MPa, respectively. The infrared photothermal conversion temperature of the composite film is 158.5 °C (3450 lux for 1 min), and the infrared photothermal self-healing efficiency is 118% (3450 lux for 600 s). The AgNWs-RGO also improves the multiple self-healing ability of the composite film. The use of a high mass content of AgNWs-RGO in the composite film is beneficial in obtaining high multiple self-healing efficiencies. The first and the fifth infrared thermal self-healing efficiencies of the composite film with AgNWs-RGO of 35 wt% are 105% and 86%, respectively, and the resistivity of the composite film changes little and still maintains good conductivity.

Dynamic Covalent Polyurethane Network Materials: Synthesis and Self-Healability

Polyurethane (PU) has not only been widely used in the daily lives, but also extensively explored as an important class of the essential polymers for various applications. In recent years, significant efforts have been made on the development of self-healable PU materials that possess high performance, extended lifetime, great reliability, and recyclability. A promising approach is the incorporation of covalent dynamic bonds into the design of PU covalently crosslinked polymers and thermoplastic elastomers that can dissociate and reform indefinitely in response to external stimuli or autonomously. This review summarizes various strategies to synthesize self-healable, reprocessable, and recyclable PU materials integrated with dynamic (reversible) Diels-Alder cycloadduct, disulfide, diselenide, imine, boronic ester, and hindered urea bond. Furthermore, various approaches utilizing the combination of dynamic covalent chemistries with nanofiller surface chemistries are described for the fabrication of dynamic heterogeneous PU composites.