Thermosets, known for their superior mechanical properties, high thermal stability, and chemical resistance, are widely used in various applications. However, their irreversible cross-linking hinders repair, reshaping, and recycling. To address this limitation, research has focused on developing reprocessable thermosets, also known as covalent adaptable networks (CANs), which utilize dynamic and exchangeable chemical reactions. Thermoreversible Diels-Alder (DA) reactions, with their relatively fast kinetics and mild reaction conditions, have emerged as a promising tool for creating CANs. The DA reaction involves a [4+2] cycloaddition between a diene and a dienophile, forming a stable cyclohexene derivative. The combination of furan as the diene and maleimide as the dienophile is particularly advantageous due to their relatively low coupling and high decoupling temperatures. This research aims to develop self-healing and reprocessable thermosets that incorporate mechanochromic molecules for damage reporting. Spiropyran (SP) is chosen as the mechanochromic molecule because of its force-induced reversible ring-opening reaction to produce a merocyanine (MC) form, which causes a color change. The integration of SP into CANs, particularly those based on DA chemistry, is novel and promises to provide a material that simultaneously self-heals and reports damage through optical changes.

Previous studies have explored the synthesis of SP-linked mechanochromic polymers using various methods. These include homo and block copolymers like polyacrylates and polystyrenes synthesized using SP as a bifunctional initiator via atom transfer radical polymerization (ATRP). Dihydroxy-terminated SPs have also been incorporated into polyurethanes, polyesters, and polycarbonates through step-growth or ring-opening polymerization. SP-linked silicone rubbers have been fabricated using bis-alkene functionalized SP as a cross-linker. However, integrating SP molecules into CANs, especially those prepared with DA chemistry, remained unexplored before this study. The potential interaction between force-sensitive SP and the force-induced retro-DA (rDA) reactions within a single polymeric system has also not been investigated previously. This study addresses this gap by combining SP and DA chemistry for the creation of a novel self-healing and damage-reporting material.

The research involved the synthesis of linear random copolymers containing furfuryl functionalities from bifunctional SP initiators through ATRP. The polymerization kinetics and comonomer compositions were investigated by varying reaction conditions. Nine types of SP-linked copolymers (P) with varying molecular weights and FMA molar contents were synthesized. A control copolymer (CM3) without SP was also prepared. The synthesized copolymers were then mixed with tris-maleimide cross-linkers to create cross-linked polymers (xP and xCM3) via DA reactions. The cross-linking process and the resulting material properties were studied extensively. Ab initio steered molecular dynamics (AISMD) simulations were conducted to investigate the mechanochemical reactivities of SP and DA adducts on an atomistic level. The simulations aimed to understand the bond cleavage reactions under mechanical stress. Various characterization techniques including 1H NMR, 13C NMR, GPC, FT-IR, optical and fluorescence microscopy, DSC, DMA, and uniaxial tensile tests were employed to analyze the polymers' chemical structure, molecular weight, thermal properties, mechanical properties, and self-healing capabilities. The detailed procedures for polymer synthesis, cross-linking, and characterization techniques are thoroughly described in the supplementary material.

The AISMD simulations demonstrated that both the CO bond of SP and the CC bonds of the DA adduct undergo mechanochemical reactions under applied force. The fraction of bond cleavage reactions for both SP and DA adducts increased with the applied force strength. The polymerization kinetics studies showed that the apparent polymerization rate was influenced by the initial total monomer concentration and the comonomer ratio. The synthesized SP-linked copolymers exhibited mechanochromic behavior; upon mechanical stress, the damaged areas changed color and emitted fluorescence due to the SP to MC transition. This color change and fluorescence served as indicators of damage. The thermoreversible nature of the DA cross-links enabled complete self-healing and color recovery upon heat treatment. The material demonstrated excellent self-healing abilities and retained its mechanical, damage-reporting, and self-healing properties even after multiple (up to 15) recycling processes. The results confirmed the successful integration of mechanochromic SP molecules into the thermoreversible DA network. The material’s mechanical properties were also evaluated; data on storage and loss moduli, tensile strength, and stress relaxation were obtained and analyzed.

The study successfully demonstrated the feasibility of creating self-healing and reprocessable thermosets with integrated damage-reporting capabilities. The combination of SP and DA chemistry allows for both autonomic damage visualization and complete material recovery. The AISMD simulations provided valuable insights into the mechanochemical reactions occurring at the molecular level, complementing the experimental observations. The ability to recycle the material multiple times without significant degradation in its properties highlights its potential for sustainable applications. The findings contribute significantly to the field of self-healing and reprocessable materials, potentially leading to more durable and environmentally friendly coatings and other applications. Future research could focus on exploring different mechanochromic molecules or adapting this approach for various applications in diverse fields.

This research successfully demonstrated the creation of a novel mechanochromic and thermally reprocessable thermoset exhibiting autonomic damage reporting and self-healing properties. The material's ability to be recycled multiple times without loss of function highlights its potential for sustainable applications. Future research could explore the use of other mechanochromic molecules and investigate the material's performance in more complex environments.

The study primarily focused on laboratory-scale synthesis and characterization. Further research is needed to evaluate the material's long-term durability and performance under real-world conditions. The scalability and cost-effectiveness of the synthesis process should also be explored for practical applications. The precise mechanism of the combined mechanochemical reactions of SP and the DA adducts could be further investigated through more detailed simulations and experimental studies.