Recyclable, Self-Healing, and Flame-Retardant Solid–Solid Phase Change Materials Based on Thermally Reversible Cross-Links for Sustainable Thermal Energy Storage

Abstract

Conventional polymeric phase change materials (PCMs) exhibit good shape stability, large energy storage density, and satisfactory chemical stability, but they cannot be recycled and self-healed due to their permanent cross-linking structure. Additionally, the high flammability of organic PCMs seriously restricts their applications for thermal energy storage (TES). Therefore, it is urgently required to explore PCM composites exhibiting superior recyclability, good self-healing capability, and excellent flame retardancy simultaneously. Herein, tri-maleimide end-capped cyclotriphosphazene flame retardant (TMCTP) was synthesized via the nucleophilic substitution between 1,3,5,2,4,6-triazatriphosphorine-2,2,4,4,6,6-hexachloride and N-(2-hydroxyethyl)­maleimide. Then, novel dynamically cross-linked PCM composites (FPCMs) with superior recyclability, good self-healing capability, and excellent flame retardancy were fabricated by bonding PEG and TMCTP to polymeric skeleton via reversible furan/maleimide Diels–Alder (DA) reaction. TMCTP, which covalently and dynamically binding in the polymeric FPCMs, acted not only as an efficient flame retardant for reducing the flammability of PCM composites but also as dynamic cross-linking skeletons for thermally induced self-healing and recycling. Differential scanning calorimetry (DSC) analysis confirmed the reversible energy storage and release ability of FPCMs. Due to its reversible DA covalent bonds, the introduction of TMCTP endowed the FPCMs with considerably increased self-healing efficiency (up to 93.1%) and recyclability efficiency (94.6%). Moreover, with the introduction of TMCTP into FPCMs, the heat release rate (HRR) and total heat release (THR) significantly decreased, while the char residue and limiting oxygen index (LOI) value increased, confirming that the flame retardancy of FPCMs greatly improved. Hence, the synthesized FPCMs show enormous potential in TES applications.

Multifunctional polyurethane hydrogel based on a phenol–carbamate network and an Fe3+–polyphenol coordination bond toward NIR light triggered actuators and strain sensors

“…Similarly, the weakened crystalline behavior of TAPU polymers could also be observed in Fig. 1f, that is, the intensities of the two sharp and narrow characteristic peaks at around 19.3° and 23.3° belonging to the (120) and (132) crystal planes of PEG 38,39 decreased with the increase of TA content. It is mainly due to the introduction of the cross-linked phenol–carbamate structure into the polyurethane skeleton, which restricts the rearrangement and orientation of the PEG segment and ultimately decreases the crystalline capacity.…”

Dopamine-Decorated Ti3C2Tx MXene/Cellulose Nanofiber Aerogels Supported Form-Stable Phase Change Composites with Superior Solar–Thermal Conversion Efficiency and Extremely High Thermal Storage Density

“…Numerous materials, including inorganic materials (e.g., hydroxides, salts, hydrated salts, and metals) and organic materials (e.g., polyethylene glycols, paraffin waxes, fatty acids, and alcohols), are developed for thermal energy storage (TES). − Among the various PCMs, sugar alcohols such as erythritol are considered the most attractive candidate for solar–thermal applications due to their extremely high energy storage density (∼340 J/g), biomass-derived, chemically stable, nontoxic, and noncorrosive properties. − Introducing inorganic thermal conduction into sugar alcohols is an available strategy to enhance the thermal conductivity of organic PCMs. − Nevertheless, the large-scale applications of sugar alcohols in the solar–thermal utilization field are still severely restricted because of their intrinsic liquid leakage issues during thermal storage. Microencapsulating PCMs into shell materials and chemical grafting/cross-linking PCMs into polymeric frameworks have been demonstrated to be effective strategies to address liquid leakage issues and fabricate form-stable PCMs. − However, the thermal energy storage capacities of the microencapsulated PCMs and grafted/cross-linked PCMs significantly decrease due to the introduction of a massive amount of shell materials and cross-linkers.…”

In Situ Visualization of Reversible Diels–Alder Reactions with Self-Reporting Aggregation-Induced Emission Luminogens

“…Driven by their unique characters and broad applications, interests in dynamic covalent bonds have never waned in centuries. − Obtaining the 1950 Nobel Prize in Chemistry, the Diels–Alder (DA) chemistry has been one of the most classical and powerful dynamic reversible reactions in both academic and applied fields, especially for the total syntheses of natural products. − The reversibility of these [4 + 2] cycloaddition reactions via retro-DA (RDA) at elevated temperatures endows their wide applications, such as novel smart materials for self-healing, , catalysis, information storage, and biosensors . The reversible cleavage and reformation of covalent bonds in the dynamic covalent reactions are the most fascinating features, which allow the exchange of molecular moieties at equilibrium and the response to environmental stimuli. , Current methods for monitoring the dynamic covalent bonds largely rely on nuclear magnetic resonance (NMR) spectroscopy, mass spectrometry (MS), Raman spectroscopy, and infrared spectroscopy (IR), which could be hampered by complex labeling treatment, finite compatibility, and expensive precise instruments in some cases, especially limited for in situ visualization of reactions and the direct observation of heterogeneous and solid-state reactions. , …”