Mechanochromic and thermally reprocessable thermosets for autonomic damage reporting and self-healing coatings

Thermosets are permanently cross-linked polymer networks valued for mechanical strength, thermal stability, and chemical resistance, but their irreversibility limits repair, reshaping, and recycling. Covalent adaptable networks (CANs) address this by enabling network rearrangement under stimuli, with thermally triggered reactions—particularly thermoreversible Diels–Alder (DA) chemistry between furan and maleimide—offering catalyst-free, mild, and reversible cross-linking. In parallel, autonomic damage-reporting using mechanochromic molecules like spiropyran (SP), which undergoes force-induced ring opening to merocyanine (MC) with color and fluorescence changes, enhances reliability and safety. Despite extensive work on SP in linear and cross-linked polymers, integration of SP into CANs via DA chemistry has not been explored. This study aims to create DA-based reprocessable thermosets that autonomously visualize damage via SP activation and self-heal upon heating, while also being recyclable, and to elucidate the mechanochemical interplay between SP activation and potential force-induced retro-DA reactions.

- Covalent adaptable networks (CANs) enable reprocessing of thermosets by dynamic covalent reactions; thermal stimuli are broadly applicable, and DA reactions are favored for their reversible kinetics and mild, catalyst-free conditions. - Furan–maleimide DA systems provide low coupling and high decoupling temperatures, and multifunctional maleimides/furans have been widely used as cross-linkers with complementary functional polymers. - Mechanochromic spiropyran (SP) has been incorporated into polymers via ATRP-initiated polyacrylates/polystyrenes, step-growth/ring-opening polyurethanes, polyesters, polycarbonates, and silicone rubbers using SP-based initiators or cross-linkers. - Some DA adducts undergo force-induced retro-DA reactions, suggesting a possible dual-mechanophore system when combining SP with DA networks. - Gap identified: no prior integration of SP mechanophores into DA-based CANs to achieve simultaneous mechanochromic damage reporting and thermally driven self-healing/reprocessing.

Materials: Inhibitor-free lauryl methacrylate (LMA) and furfuryl methacrylate (FMA) were polymerized via ATRP using purified CuBr and PMDETA in toluene. Spiropyran and cross-linkers: Mechanically active spiropyran (SP), control spiropyran (Ctrl), and tris-maleimide cross-linkers were synthesized following literature procedures with modifications (details in Supplementary Information). Polymer synthesis (ATRP): Nine SP-linked random copolymers poly(LMA-co-FMA) (denoted P; L, M, H molecular weight series with 1–3 indicating FMA content) were synthesized by ATRP using bifunctional SP as a mid-chain initiator to position SP near the chain midpoint. A Ctrl-linked copolymer (CM3) was synthesized analogously. Example (H3): LMA 2.8 ml (9.6 mmol), FMA 0.29 ml (1.9 mmol), SP or Ctrl 20 mg (0.03 mmol), CuBr 4.3 mg (0.03 mmol), copper wire, toluene 9.3 ml, PMDETA 6.3 µl (0.03 mmol); deoxygenated (freeze-thaw), polymerized at 80 °C under Ar; workup via THF dilution, alumina to remove copper, precipitation in methanol. Molecular weights and FMA contents tuned via monomer feed and total concentration. Resulting ranges: Mn ≈ 13–19 kg/mol (L), 34–38 kg/mol (M), 67–77 kg/mol (H); FMA content ≈ 2.6–3.0% (1), 7.1–9.1% (2), 19–29% (3). Glass transition temperatures ranged from about −70 to −46 °C depending on composition. Network formation (DA cross-linking): SP- or Ctrl-linked copolymers were cross-linked with tris-maleimide via DA reaction to yield xP or xCM3 networks. Example (xL1, furan:maleimide = 1:1): L1 0.5 g (furan 0.056 mmol) and tris(2-maleimidoethyl)amine 7.3 mg (maleimide 0.056 mmol) dissolved in CHCl3 (10 wt%), stirred 1 h, cast on Teflon, dried under vacuum, cured at 50 °C for 12 h, then compression molded between steel plates at 140 °C for 30 min (4 tons). Specimens cut to 15 × 5 × 0.7 mm. Computational simulations (AISMD): Ab initio steered molecular dynamics examined mechanochemical reactivity of SP and DA adducts (endo/exo) as single mechanophores and as SP–DA compound molecules. Constant external forces of 0.7–3.5 nN were applied for 10 ps at 300 K (NVT, Langevin thermostat, friction 5×10^12 s^−1). Forces were applied to terminal atoms along a defined axis (atoms A1/A2 towards fixed points P1/P2). Ten trajectories per condition. Calculations used TeraChem with B3LYP/6-31G*. Characterization and testing: 1H/13C NMR for composition; GPC (THF, PS standards) for Mn and dispersity; FT-IR for functional groups; optical and fluorescence imaging for mechanochromism; DSC (−80 to 200 °C, 10 °C/min, N2) for thermal transitions; DMA (−50 to 120 °C, 1 Hz, 0.1% strain) for thermomechanical properties; uniaxial tensile tests (strain rate 0.05 s^−1) for mechanical performance; stress relaxation in compression (1% strain, 0.5 N normal force) at 100–140 °C to assess network dynamics and reprocessability. Polymerization kinetics: Living radical polymerization assessed via first-order kinetic plots varying total monomer concentration and comonomer ratio; apparent propagation rate kp,app ≈ (1.3 ± 0.1) × 10^−1 s^−1 with R^2 ≈ 0.99 at [M]total = 0.93 M and [M1]:[M2] = 5:1.

- SP successfully incorporated near chain midpoints of poly(LMA-co-FMA) via ATRP, yielding well-defined copolymers with tunable molecular weights (≈13–77 kg/mol), FMA contents (≈2.6–29 mol%), and Tg values around −70 to −46 °C. - DA cross-linking with tris-maleimide produced thermoreversible networks (xP and xCM3) that could be molded and reprocessed by heating. - AISMD simulations showed two primary force-activated events: SP C–O bond scission (ring opening to MC) and DA adduct C–C bond cleavage (retro-DA). Under 0.7–3.5 nN constant forces at 300 K, the probability of SP ring opening and DA bond cleavage increased with force. Post-cleavage bond metrics: lCO ≈ 4 Å for SP ring opening; lCC increased from ≈4 to 12 Å as DA adducts separated. Endo/exo stereochemistry was investigated, revealing different susceptibilities to force (trend shown in fraction vs force plots). - Experimentally, mechanically damaged areas of xP thermosets exhibited visible color change and fluorescence due to SP→MC conversion, confirming mechanochromic damage reporting within the DA network. - Thermal treatment reversed the MC back to colorless SP and simultaneously enabled DA retro/cycloaddition to reorganize the network, restoring optical and mechanical integrity (self-healing). - The thermoreversible covalent networks allowed recycling/reprocessing up to 15 times without measurable degradation in mechanical performance, mechanochromic response, or self-healing capability. - Polymerization kinetics exhibited living/controlled behavior with kp,app ≈ (1.3 ± 0.1) × 10^−1 s^−1 and R^2 ≈ 0.99 at representative conditions, enabling predictable tuning of composition and molecular weight.

The study addresses the challenge of creating thermosets that autonomically report damage and can be repaired and recycled. By embedding SP mechanophores within DA-based CAN networks, mechanical stress triggers SP ring opening, providing immediate visual and fluorescent indication of damage. Heating reverses both the SP mechanochemical transformation and the DA network connectivity, enabling self-healing and reprocessing. Simulations corroborate that both SP activation (C–O cleavage) and DA retro-reactions (C–C cleavage) are accessible under force, offering insight into how dual mechanophores respond to load. The synergy between mechanochromism and thermoreversible cross-links yields coatings that can warn of incipient failure and be fully restored, extending service life and reliability while improving sustainability through multiple recycling cycles (up to 15 cycles). The controlled polymerization strategy ensures precise placement of SP and tunable network properties, and the thermomechanical testing confirms robust performance and repeatability.

This work demonstrates a unified platform of mechanochromic, self-healing, and reprocessable thermosets by integrating spiropyran mechanophores into Diels–Alder covalent adaptable networks. The materials autonomically visualize damage via color/fluorescence changes, heal upon heating via reversible DA reactions, and retain performance after up to 15 recycling cycles. Ab initio simulations elucidate the mechanochemical pathways of SP ring opening and DA retro-DA under force, informing design rules for dual-mechanophore systems. Future research directions include optimizing SP and DA stereochemistry and placement to tailor sensitivity and selectivity, expanding to other dynamic covalent chemistries and mechanophores, quantifying long-term cycling stability under environmental exposures, and translating to practical coating formulations and large-area manufacturing.