A structural damage self-detecting lipoic acid-based fluorescent elastomer, a preparation method and application thereof

Abstract

The present application relates to structural damage fluorescent visualization detection technical field, disclose a kind of structural damage self-detection type lipoic acid-based fluorescent elastomer and its preparation method and application.The elastomer is prepared by solution mixing, casting and solvent evaporation of lipoic acid, C4-OH configuration IPOH fluorescent probe and lithium salt, wherein lipoic acid forms poly-lipoic acid-based dynamic network, IPOH is dispersed and limited in network, and lithium salt controls network stability and local microenvironment of probe.When local structural damage such as scratch occurs in the material, the damaged area presents significantly enhanced fluorescence signal under ultraviolet light irradiation, realizing instant, in-situ and visual self-detection of structural damage.The preparation process of the present application is simple, without introducing additional sensing unit, without pre-excitation, energy storage, dark adaptation or delayed readout, with rapid damage response, intuitive positioning, convenient operation, room temperature self-healing and solution reconstruction recovery performance, with wide application prospect.

Description

Technical Field

[0001] This invention belongs to the technical field of intelligent polymer materials and fluorescent damage detection materials, specifically relating to a self-detecting structural damage thioctic acid-based fluorescent elastomer, its preparation method, and its application. Background Technology

[0002] Flexible elastomers, transparent protective layers, flexible encapsulation layers, and sandwich structures are susceptible to mechanical forces such as tension, scratches, friction, or interfacial delamination during service, resulting in scratches, pinholes, microcracks, or interfacial damage. These minute damages are often difficult to detect under natural light, especially when located within the material or at interlayer interfaces, making identification and localization even more challenging. Existing nondestructive testing methods largely rely on imaging equipment, electrical or acoustic testing devices, and additional sensing units, resulting in complex testing systems and operational procedures that fail to meet the demands for rapid, in-situ, and intuitive identification of localized micro-damage.

[0003] Existing thioctic acid-based elastomers and ionic elastomers primarily enhance material stability, mechanical properties, and self-healing capabilities through dynamic disulfide bonds, hydrogen bonds, ionic bonds, or coordination interactions. For example, Chinese invention patent application CN116444807A discloses a self-healing supramolecular elastomer constructed from thioctic acid and a tertiary amine crosslinking agent; Chinese invention patent application CN114989332A discloses a polyacrylate ionic elastomer containing lithium bis(trifluoromethanesulfonylimide). These technologies mainly focus on the material's stable forming properties, mechanical properties, self-healing properties, adhesive properties, or ionic conductivity, without addressing the direct identification of mechanical micro-damage using locally enhanced fluorescence signals generated within the material itself.

[0004] Studies have also reported on the use of fluorescence changes to visualize elastomer damage and self-healing processes. Shen Q., Liu H., Peng Y., Zheng J., and Wu J., in their paper "Visualization of the self-healing process by directly observing the evolution of fluorescence intensity,"... Polymer ChemistryIn 2021, 12: 494–500, a method was disclosed to introduce fluorescent molecules with aggregation-induced quenching effects into dynamically hydrogen-bonded crosslinked elastomers, and to utilize fluorescence enhancement in the damaged area to display the damage and healing process. Chen X. et al., in their paper "Extremely Tough, Puncture-Resistant, Transparent, and Photoluminescent Polyurethane Elastomers for Crack Self-Diagnose and Healing Tracking", ACS Applied Materials & Interfaces In 2020, 12(27): 30847–30855, a polyurethane elastomer was disclosed that utilizes the photoluminescence difference between cracked and intact regions for crack diagnosis and healing tracking. The above research provides a technical basis for the fluorescence visualization of elastomer damage, but its elastomer matrix, fluorescent unit introduction method and response mechanism are different from the dynamic disulfide bond network of polythioctic acid, and cannot be directly applied to the damage detection of thioctic acid-based elastomer materials.

[0005] Furthermore, fluorescent self-warning technology has also been applied to intelligent anti-corrosion coatings. For example, Chinese invention patent CN116376402A discloses an intelligent anti-corrosion coating based on pH-responsive fluorescent MOF composite materials; Chinese invention patent application CN111849327A discloses a self-warning and self-healing anti-corrosion coating utilizing hollow microspheres loaded with fluorescent probes and corrosion inhibitors. This type of technology typically relies on microcapsules, MOFs, or porous carriers to load functional components, and generates fluorescent signals through the penetration of corrosive media, local pH changes, metal ion generation, or release of active components. Its detection targets and triggering mechanisms differ from those of localized mechanical damage in non-corrosive flexible elastomers.

[0006] Therefore, for thioctic acid-based dynamic elastomers, problems still need to be solved, such as the stable dispersion and effective confinement of small molecule fluorescent probes in polythioctic acid networks, the control of fluorescence background in intact regions, and the formation of high-contrast fluorescence signals in damaged areas. There is an urgent need to develop a thioctic acid-based fluorescent elastomer that is easy to prepare and requires no additional response carrier or independent sensing unit. This would enable localized mechanical damage such as scratches, pinholes, or microcracks, which are difficult to detect with the naked eye, to be instantly converted from the material itself into significantly enhanced and location-identifiable fluorescence signals, thereby achieving rapid, in-situ, and visualized self-detection of structural damage. Summary of the Invention

[0007] The purpose of this invention is to provide a structural damage self-detection type thioctic acid-based fluorescent elastomer, its preparation method, and its application, in order to solve the problems that existing self-healing elastomers focus on mechanical recovery after damage, and existing fluorescent self-early warning coatings rely on corrosion / pH / release systems, making it difficult for the soft elastomer body to generate instantaneous enhanced fluorescence readout for local mechanical damage.

[0008] To achieve the above objectives, the present invention adopts the following technical solution: A structural damage self-detection type lipoic acid-based fluorescent elastomer is disclosed. The elastomer is formed by solution mixing, casting, and solvent evaporation of lipoic acid, a hydroxyl-modified urazine-based fluorescent probe IPOH, and a lithium salt. The lithium salt is preferably lithium bis(trifluoromethanesulfonyl)imide. The lipoic acid undergoes ring-opening polymerization and / or dynamic reconstruction to form a polylipoic acid-based dynamic network. IPOH, as a fluorescent response unit, is dispersed and anchored within the dynamic network through weak phenolic hydroxyl-related interactions. The lithium salt, such as lithium bis(trifluoromethanesulfonyl)imide, provides Li... + The weak interaction between the polythiooctanoic acid (Polythioctic acid) dynamic network and the IPOH is modulated; under ultraviolet light irradiation, the locally damaged areas of the elastomer produce enhanced fluorescence signals relative to the undamaged areas. For ease of description, this elastomer is simply referred to as a TIL elastomer.

[0009] In the TIL elastomer of this invention, thioctic acid is ring-opening polymerized to construct a polythioctic acid dynamic network. The carboxyl groups on the polymer side chains provide hydrogen bonding sites, which, together with reversible disulfide bonds within the chain, construct multiple dynamic interaction nodes. After damage caused by external forces, these dynamic bonds can undergo reversible breakage and reconstruction, providing crucial support for the material's excellent self-healing properties. IPOH, as a fluorescent responsive unit, can have its phenolic hydroxyl groups dispersed and confined within the polythioctic acid dynamic network through hydrogen bonding, coordination, or other weak interactions. Lithium bis(trifluoromethanesulfonyl)imide serves as an ion control unit, wherein Li... + It can coordinate or interact with the carboxyl groups of polythiooctanoic acid (Polythioctic acid) and oxygen- and nitrogen-containing sites in IPOH, regulating mechanical properties while preventing the depolymerization of thioctic acid, thereby helping to improve network stability and regulate the local microenvironment of IPOH. When structural damage occurs, the chain segment arrangement and local weak interaction states in the damaged area change, which in turn causes a change in the luminescence state of IPOH, making the fluorescence intensity of the damaged area higher than that of the undamaged area under ultraviolet light irradiation. Experimental results show that this fluorescence enhancement is related to the dynamic network and probe microenvironment changes caused by local mechanical damage.

[0010] Preferably, the lipoic acid is DL-lipoic acid with a purity of not less than 95%, more preferably not less than 99%.

[0011] Preferably, the IPOH is a hydroxyl-modified urazine fluorescent probe. To clearly distinguish the positional isomers of IPOH with different hydroxyl substitution positions, the structural formulas of the C4-OH configuration IPOH and the C3-OH configuration IPOH used in this invention are shown in formula (I) and formula (II), respectively; more preferably, the IPOH is the C4-OH configuration IPOH shown in formula (I), named indolizino[6,5,4,3-def]phenanthridin-10-ol.

[0012]

[0013] The C4-OH configuration IPOH shown in Formula (I) is used in the preferred embodiment of the present invention, and the C3-OH configuration IPOH shown in Formula (II) is used for performance comparison with the C4-OH configuration IPOH.

[0014] Preferably, the molar ratio of lipoic acid to lithium bis(trifluoromethanesulfonylimide) is 3:1 to 20:1, more preferably 5:1 to 10:1, and even more preferably 7:1. If no lipoic acid is added or the amount added is too low, the system is prone to depolymerization or insufficient network stability; if the amount added is too high, local ion aggregation or restricted chain segment movement is likely to occur.

[0015] Preferably, the mass ratio of lipoic acid to IPOH is 100:1 to 500:1, more preferably 200:1 to 500:1, even more preferably 300 to 400:1, and even more preferably 300:1, which can balance optimal mechanical properties and optimal fluorescence performance. IPOH is an ACQ molecule; if the amount is too low, the fluorescence response is insufficient; if the amount is too high, it can easily cause local aggregation of the probe and affect the network structure, leading to a decrease in the material's mechanical properties or fluorescence output stability, and even aggregation causing luminescence quenching.

[0016] Preferably, the Li in the lithium bis(trifluoromethanesulfonylimide) + It can coordinate or form ion-dipole interactions with the carboxyl group of polythiooctanoic acid, the hydroxyl group of IPOH, and / or the nitrogen atom of the IPOH heterocyclic ring; TFSI - It can generate weak auxiliary interactions with carboxyl or hydroxyl groups, thereby helping to regulate the network stability, damage fluorescence response, and self-healing behavior of elastomers.

[0017] Preferably, anhydrous ethanol is used as the solvent for the solution mixing.

[0018] Preferably, the elastomer produces damage fluorescence contrast under ultraviolet light irradiation at a wavelength of 300-400 nm, more preferably the ultraviolet light wavelength is 365 nm; the fluorescence signal is a readout signal generated instantly under on-demand irradiation, without the need for pre-storage, long afterglow dark state readout, or corrosion reaction triggering.

[0019] Preferably, under the same excitation wavelength, light source intensity, and signal acquisition conditions, the enhanced fluorescence signal is characterized by a higher fluorescence intensity or image grayscale value in the locally damaged area than in the undamaged area. This difference can be identified through visual observation, fluorescence spectroscopy testing, or image grayscale analysis. The degree of enhancement is characterized by the ratio of fluorescence intensity or image grayscale value between the damaged and undamaged areas, preferably not less than 2.0. In this embodiment, the fluorescence intensity ratio between the scratch-damaged area and the adjacent undamaged area is approximately 3.5, and the ratio of the average grayscale values ​​of the two areas in the 365 nm ultraviolet image is approximately 4.62.

[0020] This invention also provides a method for preparing the above-mentioned elastomer, comprising the following steps: S1: Add lipoic acid, IPOH and lithium bis(trifluoromethanesulfonylimide) to anhydrous ethanol and stir until the mixture is homogeneous to obtain the casting solution; S2: The casting liquid is poured into a mold, and the solvent is evaporated at room temperature to obtain the structural damage self-detection type thioctic acid fluorescent elastomer.

[0021] Preferably, the amount of anhydrous ethanol used per 1 g of lipoic acid is 1.0 to 10.0 mL, more preferably 2.0 to 5.0 mL, and even more preferably 3.0 mL.

[0022] Preferably, each 1 g of lipoic acid corresponds to an IPOH dosage of 2 to 10 mg, more preferably 2 to 5 mg, and even more preferably 3.33 mg.

[0023] Preferably, the amount of lithium bis(trifluoromethanesulfonylimide) corresponding to 1 g of lipoic acid is 60 to 500 mg, more preferably 140 to 280 mg, and even more preferably 198.0 mg.

[0024] Preferably, step S1 uses magnetic stirring with a rotation speed of 400-600 r / min, more preferably 500 r / min, and a stirring time of 5-60 min, more preferably 15 min.

[0025] Preferably, the mold in step S2 is a polytetrafluoroethylene mold; the solvent evaporation is carried out at room temperature for 48 hours.

[0026] The present invention also provides an application of the above-mentioned self-detection thioctic acid-based fluorescent elastomer for structural damage visualization detection or self-healing process monitoring, or for its application in the preparation of adhesive functional layers, transparent protective layers or interlayer fixing materials.

[0027] Optionally, the structural damage includes scratches, cuts, pinholes, microcracks, localized interface damage, or a combination thereof.

[0028] Optionally, the elastomer can be made into a thin film, coating, patch, interlayer, or transparent protective layer for use as a protective layer for flexible devices, a protective layer for sensors, a smart packaging film, a visual damage location patch, or a structural health monitoring material; when used as an adhesive functional layer on the surface of a glass substrate or as an interlayer fixing material between glass substrates, the interfacial adhesion properties are used to provide auxiliary fixing; the above-mentioned damage identification uses the enhanced fluorescence signal generated by the damaged area under ultraviolet light irradiation as the main identification method.

[0029] Optionally, the elastomer can be recycled and reprocessed by ethanol dissolution-recasting. Specifically, the elastomer waste is placed in anhydrous ethanol, stirred at room temperature for 1–24 h, recast, and allowed to stand at room temperature for 1–5 days to obtain a recycled, regenerated structure-damage fluorescence-enhanced self-detection elastomer.

[0030] Optionally, when the elastomer is used on the surface of a glass substrate or between glass substrates, it can form an interfacial bond through room temperature contact, standing, or light pressure; the interfacial bond performance is used to illustrate the material's auxiliary fixing ability, and the structural damage fluorescence enhancement self-detection performance is characterized by the damage test results of the independent elastomer film.

[0031] Compared with the prior art, the beneficial effects of the present invention are as follows: (1) This invention does not rely on microcapsules, core-shell fibers, MOF nanofillers, metal corrosion products, pH changes, pre-stored release agents or long afterglow pre-excitation processes, and can directly use the fluorescence enhancement of the damaged area as a visual self-detection signal of the damage to the flexible elastomer.

[0032] (2) The present invention can convert scratches, pinholes, cuts or microcracks that are not easily identifiable under natural light into fluorescence signals that are significantly enhanced under ultraviolet light. Without destroying the sample or performing complex pretreatment, the damage location can be quickly located and read on demand. It has the advantages of simple detection process, rapid response, convenient operation and suitability for in-situ non-destructive testing.

[0033] (3) The elastomer described in this invention can self-heal at room temperature by relying on a dynamic network; during the healing process, the fluorescence signal of the damaged area changes with the reconstruction of the damaged interface, which can be used to provide optical feedback for the healing process.

[0034] (4) The elastomer of the present invention can still identify local structural damage by enhanced fluorescence signal in the damaged area after being stored at -20 ℃ for 24 h (low temperature) or soaked in 0.9% physiological saline for 2 h (wet state); the elastomer can also be recycled and reprocessed by dissolving in ethanol and recasting, and retains the ability to read out damage fluorescence after reprocessing.

[0035] (5) The elastomer described in this invention has interfacial adhesion to the glass substrate, which can provide auxiliary performance support for it as an adhesive film, transparent protective layer or interlayer fixing material. Attached Figure Description

[0036] Figure 1 This is a schematic diagram of the preparation process of TIL elastomer and the damage fluorescence enhancement self-detection process in Example 7 of the present invention.

[0037] Figure 2 These are fluorescence spectra of TIL elastomers with different thioctic acid / IPOH mass ratios obtained in Examples 4 and 6-9 of this invention.

[0038] Figure 3 This is a comparison of the fluorescence spectra of the C4-OH configuration TIL elastomer obtained in Example 7 of the present invention and the C3-OH configuration TIL elastomer obtained in Comparative Example 1.

[0039] Figure 4 This is a quantitative result of fluorescence spectrum and image grayscale of the scratch-damaged area and the adjacent undamaged area of ​​the TIL elastomer in Comparative Example 1 of the present invention.

[0040] Figure 5 These are the fluorescence spectrum and time-resolved fluorescence decay curve of the TIL elastomer in Test Example 1 of this invention.

[0041] Figure 6 These are comparative photos of the damage readout of the TIL elastomer in Test Example 2 of this invention under natural light and 365 nm ultraviolet light.

[0042] Figure 7 The normalized fluorescence spectra of the TIL elastomer scratch-damaged area and the adjacent undamaged area in Test Example 2 of this invention, as well as the comparison results of the average gray values ​​of the two areas under natural light and 365 nm ultraviolet light obtained by ImageJ processing; both fluorescence spectra are normalized with the maximum original fluorescence intensity of the damaged area spectrum as a common benchmark.

[0043] Figure 8 This is a comparison diagram of the fluorescence response of the TIL elastomer in Test Example 3 of this invention to local damage and overall tensile conditions.

[0044] Figure 9 This is a diagram showing the damage identification results of the TIL elastomer under low temperature and humid conditions in Test Example 3 of this invention.

[0045] Figure 10 This is a graph showing the changes in fluorescence intensity at 456 nm and self-healing efficiency during the self-healing process of the TIL elastomer in Test Example 4 of this invention.

[0046] Figure 11This is a graph showing the results of TIL elastomer ethanol dissolution-recasting recovery and the fluorescence reading of the scratches after recovery in Test Example 5 of this invention. Detailed Implementation

[0047] To better clarify and understand the objectives, process solutions, and advantages of this invention, the technical solutions and implementation methods of this invention will be further described clearly, completely, and in detail below through specific embodiments and in conjunction with necessary accompanying drawings. It should be noted that the embodiments described in this invention are implemented under the premise of the technical solutions of this invention, providing detailed implementation methods and specific operating procedures, but are only some embodiments of this invention, not all embodiments; the specific implementation methods described are only for illustrating and explaining this invention and do not limit this invention. Other implementation methods obtained by those skilled in the art based on the embodiments of this invention without creative effort are all within the scope of protection of this invention.

[0048] Unless otherwise specified, the experimental methods and conditions used in the embodiments of the present invention are conventional methods and conditions. Unless otherwise specified, the materials, reagents or instruments used in the embodiments are conventional substances or equipment known to those skilled in the art and can be obtained commercially or prepared by conventional methods.

[0049] The following is information about the raw materials and instruments used in specific embodiments of the present invention, but is not limited to the specific information described below. Any substance or instrument with equivalent efficacy can achieve the technical solution and technical effect described in the present invention: Raw materials: DL-lipoic acid, 99% purity, purchased from Hangzhou Bangyi Chemical Co., Ltd.; lithium bis(trifluoromethanesulfonyl)imide purchased from Hangzhou Shuangmu Chemical Co., Ltd.; anhydrous ethanol purchased from Shanghai Maclean Biochemical Technology Co., Ltd. Unless otherwise specified, all other reagents were commercially available analytical grade reagents and used directly without further purification. C4-OH and C3-OH configurations of IPOH were purchased from Shanghai Haohong Biomedical Technology Co., Ltd., where the name of the C4-OH configuration IPOH is indolizino[6,5,4,3-def]phenanthridin-10-ol, and the name of the C3-OH configuration IPOH is indolizino[6,5,4,3-def]phenanthridin-9-ol, and the structural formulas are shown in formula (I) and formula (II) below, respectively: .

[0050] Instrumentation: Fluorescence spectroscopy and fluorescence lifetime were tested using an FLS1000 steady-state / transient fluorescence spectrometer (Edinburgh Instruments Ltd., UK); mechanical properties and lap shear properties were tested using an Instron 5567 universal testing machine (Instron, USA); damage fluorescence photographs were taken under 365 nm ultraviolet light.

[0051] Examples 1 to 5 below were used to investigate the effect of the amount of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) on the molding and mechanical properties of the elastomer. Unless otherwise specified, each example used 1.00 g of DL-lipoic acid, 3.0 mL of anhydrous ethanol, and 2.50 mg of C4-OH IPOH, and was magnetically stirred at 500 r / min for 15 min at room temperature. The mixture was then cast into a polytetrafluoroethylene mold and allowed to evaporate the ethanol at room temperature to form a film. The room temperature was 15-40°C, preferably 25°C.

[0052] Example 1, TIL-20-400 1.00 g of DL-lipoic acid, 2.50 mg of C4-OH configured IPOH fluorescent probe, and approximately 69.6 mg of lithium bis(trifluoromethanesulfonyl)imide were weighed and added to a reaction vessel, wherein the mass ratio of lipoic acid to IPOH was 400:1 and the molar ratio of lipoic acid to lithium bis(trifluoromethanesulfonyl)imide was 20:1. Then, 3.0 mL of anhydrous ethanol was added, and the mixture was magnetically stirred at room temperature to ensure complete dissolution and homogeneity of all components, yielding a uniform casting solution. The resulting casting solution was cast into a polytetrafluoroethylene mold and allowed to stand at room temperature for 48 hours to evaporate the solvent, yielding TIL-20-400 elastomer.

[0053] Example 2, TIL-15-400 1.00 g of DL-lipoic acid, 2.50 mg of C4-OH configured IPOH fluorescent probe, and approximately 92.8 mg of lithium bis(trifluoromethanesulfonyl)imide were weighed and added to a reaction vessel, wherein the mass ratio of lipoic acid to IPOH was 400:1 and the molar ratio of lipoic acid to lithium bis(trifluoromethanesulfonyl)imide was 15:1. Then, 3.0 mL of anhydrous ethanol was added, and the mixture was magnetically stirred at room temperature to ensure complete dissolution and homogeneity of all components, yielding a homogeneous casting solution. The resulting casting solution was cast into a polytetrafluoroethylene mold and allowed to stand at room temperature to evaporate the solvent, yielding TIL-15-400 elastomer.

[0054] Example 3, TIL-10-400 Weigh 1.00 g of DL-lipoic acid, 2.50 mg of C4-OH configuration IPOH fluorescent probe, and approximately 139.1 mg of lithium bis(trifluoromethanesulfonyl)imide, and add them together to a reaction vessel. The mass ratio of lipoic acid to IPOH is 400:1, and the molar ratio of lipoic acid to lithium bis(trifluoromethanesulfonyl)imide is 10:1. Then, add 3.0 mL of anhydrous ethanol and stir magnetically at room temperature to ensure that all components are fully dissolved and mixed evenly, resulting in a homogeneous casting solution. Cast the obtained casting solution into a polytetrafluoroethylene mold and allow it to stand at room temperature to evaporate the solvent, yielding TIL-10-400 elastomer.

[0055] Example 4, TIL-7-400 Weigh 1.00 g of DL-lipoic acid, 2.50 mg of C4-OH configured IPOH fluorescent probe, and approximately 198.0 mg of lithium bis(trifluoromethanesulfonyl)imide, and add them together to a reaction vessel. The mass ratio of lipoic acid to IPOH is 400:1, and the molar ratio of lipoic acid to lithium bis(trifluoromethanesulfonyl)imide is 7:1. Then, add 3.0 mL of anhydrous ethanol and stir magnetically at room temperature to ensure that all components are fully dissolved and mixed evenly, resulting in a homogeneous casting solution. Cast the obtained casting solution into a polytetrafluoroethylene mold and allow it to stand at room temperature to evaporate the solvent, yielding TIL-7-400 elastomer.

[0056] Example 5, TIL-3-400 Weigh 1.00 g of DL-lipoic acid, 2.50 mg of C4-OH configured IPOH fluorescent probe, and approximately 464.0 mg of lithium bis(trifluoromethanesulfonyl)imide, and add them together to a reaction vessel. The mass ratio of lipoic acid to IPOH is 400:1, and the molar ratio of lipoic acid to lithium bis(trifluoromethanesulfonyl)imide is 3:1. Then, add 3.0 mL of anhydrous ethanol and stir magnetically at room temperature to ensure that all components are fully dissolved and mixed evenly, resulting in a homogeneous casting solution. Cast the obtained casting solution into a polytetrafluoroethylene mold and allow it to stand at room temperature to evaporate the solvent, yielding TIL-3-400 elastomer.

[0057] The samples obtained in Examples 1 to 5 were subjected to formability and mechanical property tests to obtain elastomers with relatively stable mechanical properties. The results showed that, compared to Example 4, when the amount of lithium bis(trifluoromethanesulfonyl)imide was too low, the reinforcing and stabilizing effect of ion interactions on the network was insufficient; when the amount of lithium bis(trifluoromethanesulfonyl)imide was too high, local ion aggregation or chain segment constraint was enhanced, affecting the elastomer chain segment rearrangement and leading to a decrease in macroscopic mechanical properties. Therefore, the sample with a lipoic acid / lithium bis(trifluoromethanesulfonyl)imide molar ratio of approximately 7:1 performed better.

[0058] Examples 6 to 9 below were used to investigate the effect of IPOH dosage on the formability, steady-state fluorescence properties, and mechanical properties of the elastomer. Unless otherwise specified, each example used 1.00 g DL-lipoic acid, 3.0 mL anhydrous ethanol, and 198.0 mg lithium bis(trifluoromethanesulfonyl)imide, and was magnetically stirred at 500 r / min for 15 min at room temperature. The mixture was then cast into a polytetrafluoroethylene mold, and the ethanol was evaporated at room temperature to form a film.

[0059] Example 6, TIL-7-500 1.00 g of DL-lipoic acid, 2.00 mg of C4-OH configured IPOH fluorescent probe, and 198.0 mg of lithium bis(trifluoromethanesulfonyl)imide were weighed and added to a reaction vessel, wherein the mass ratio of lipoic acid to IPOH was 500:1 and the molar ratio of lipoic acid to lithium bis(trifluoromethanesulfonyl)imide was 7:1. Then, 3.0 mL of anhydrous ethanol was added, and the mixture was magnetically stirred at room temperature to ensure complete dissolution and homogeneous mixing of the three components, yielding a homogeneous casting solution. The resulting casting solution was cast into a polytetrafluoroethylene mold and allowed to stand at room temperature for 48 h to evaporate the solvent, yielding TIL-7-500 elastomer.

[0060] Example 7, TIL-7-300 1.00 g of DL-lipoic acid, 3.33 mg of C4-OH configured IPOH fluorescent probe, and 198.0 mg of lithium bis(trifluoromethanesulfonyl)imide were weighed and added to a reaction vessel, wherein the mass ratio of lipoic acid to IPOH was 300:1 and the molar ratio of lipoic acid to lithium bis(trifluoromethanesulfonyl)imide was 7:1. Then, 3.0 mL of anhydrous ethanol was added, and the mixture was magnetically stirred at room temperature to ensure complete dissolution and homogeneity of the three components, yielding a homogeneous casting solution. The resulting casting solution was cast into a polytetrafluoroethylene mold and allowed to stand at room temperature for 48 h to evaporate the solvent, yielding TIL-7-300 elastomer.

[0061] Example 8, TIL-7-200 1.00 g of DL-lipoic acid, 5.00 mg of C4-OH configured IPOH fluorescent probe, and 198.0 mg of lithium bis(trifluoromethanesulfonyl)imide were weighed and added to a reaction vessel, wherein the mass ratio of lipoic acid to IPOH was 200:1 and the molar ratio of lipoic acid to lithium bis(trifluoromethanesulfonyl)imide was 7:1. Then, 3.0 mL of anhydrous ethanol was added, and the mixture was magnetically stirred at room temperature to ensure complete dissolution and homogeneity of the three components, yielding a homogeneous casting solution. The resulting casting solution was cast into a polytetrafluoroethylene mold and allowed to stand at room temperature for 48 h to evaporate the solvent, yielding TIL-7-200 elastomer.

[0062] Example 9, TIL-7-100 1.00 g of DL-lipoic acid, 10.00 mg of C4-OH configuration IPOH fluorescent probe, and 198.0 mg of lithium bis(trifluoromethanesulfonyl)imide were weighed and added to a reaction vessel, wherein the mass ratio of lipoic acid to IPOH was 100:1 and the molar ratio of lipoic acid to lithium bis(trifluoromethanesulfonyl)imide was 7:1. Then, 3.0 mL of anhydrous ethanol was added, and the mixture was magnetically stirred at room temperature to ensure complete dissolution and homogeneous mixing of the three components, yielding a homogeneous casting solution. The resulting casting solution was cast into a polytetrafluoroethylene mold and allowed to stand at room temperature for 48 h to evaporate the solvent, yielding TIL-7-100 elastomer.

[0063] The samples obtained in Examples 4 and 6-9 were tested for formability, steady-state fluorescence properties, and mechanical properties to obtain elastomers with stable output of both mechanical and fluorescence properties. The results showed that the amount of IPOH affected the mechanical properties and fluorescence output of the elastomers; relevant data are shown in Table 1 and... Figure 2 As shown. Considering the overall sample's forming state, fracture strength, elongation at break, and steady-state fluorescence output, the sample with a lipoic acid to IPOH mass ratio of 300:1 described in Example 7 exhibited better overall performance. Therefore, this ratio was selected as a priority for subsequent testing.

[0064] Comparative Example 1, TIL-C3 To compare the effect of the hydroxyl substitution position of IPOH on the elastomer properties, the C3-OH configuration of IPOH shown in Formula (II) was used, and the TIL-C3 elastomer was prepared using the same method as the elastomer corresponding to the C4-OH configuration of IPOH shown in Formula (I) in Example 7. Specifically, 1.00 g of DL-lipoic acid, 3.33 mg of C3-OH configuration IPOH fluorescent probe, and 198.0 mg of lithium bis(trifluoromethanesulfonyl)imide were weighed and added to a reaction vessel, wherein the mass ratio of lipoic acid / IPOH was 300:1 and the molar ratio of lipoic acid / lithium bis(trifluoromethanesulfonyl)imide was 7:1; then 3.0 mL of anhydrous ethanol was added, and the mixture was magnetically stirred at room temperature to fully dissolve and mix the three components to obtain a homogeneous casting solution. The obtained casting solution was cast into a polytetrafluoroethylene mold and allowed to stand at room temperature for 48 h to evaporate the solvent, thus obtaining the TIL-C3 elastomer.

[0065] Test results show that, under the premise of consistent formulation and test conditions, the fracture strength and fluorescence intensity of the TIL-C3 elastomer are lower than those of the elastomer prepared using the C4-OH configuration IPOH. Furthermore, the TIL-C3 elastomer exhibits a broader fluorescence spectrum peak shape and poorer distinction between the main emission peak and the shoulder peak, reflecting its weaker luminescence response and insufficient spectral resolution. The elastomer corresponding to the C4-OH configuration IPOH possesses higher fluorescence intensity, clearer spectral characteristics, and superior mechanical properties. Therefore, the C4-OH configuration is determined to be the preferred fluorescent probe of this invention. Relevant data are shown in Table 1 and Figure 3.

[0066] Further comparative testing revealed that the fluorescence intensity ratio between the scratch-damaged area and the adjacent undamaged area of ​​the TIL-C3 elastomer was approximately 1.9; in the 365 nm ultraviolet imaging image, the average grayscale ratio of the two areas was approximately 2.17, as shown in Figure 4.

[0067] Comparative Example 2: Control sample without lithium bis(trifluoromethanesulfonylimide) Except for the absence of lithium bis(trifluoromethanesulfonylimide), the conditions were the same as in Example 7. The results showed that the lipoic acid / IPOH system was difficult to form a stable and complete self-supporting elastomer after ethanol evaporation. The samples exhibited depolymerization, stickiness, or pulverization, and could not be subjected to stable tensile, self-healing, and damage fluorescence recognition tests.

[0068] Comparative Example 3: Control sample without added IPOH Except for the absence of C4-OH configured IPOH, all other experimental conditions were consistent with those in Example 7. Test results showed that the elastomer without IPOH exhibited no fluorescence under 365 nm UV irradiation, and compared to the system containing IPOH, the elastomer without IPOH showed significantly reduced mechanical properties, as shown in Table 1. This is mainly attributed to the multiple reinforcing effects of IPOH molecules in the dynamic network. On the one hand, the hydroxyl groups and nitrogen-containing heterocyclic structures in IPOH can enhance the network through hydrogen bonding and Li... + Coordination involves the construction of additional physical crosslinking nodes, increasing the network crosslinking density; on the other hand, its rigid π-conjugated structure can restrict the movement of polythiooctanoic acid (IPOH) segments and enhance inter-chain interactions. Therefore, the introduction of IPOH not only improves the dynamic network stability but also suppresses the segment plasticization effect caused by high LiTFSI content, enabling the elastomer to obtain superior mechanical properties.

[0069] Comparative Example 4: Control Samples at Different Volatilization Temperatures Except for the solvent evaporation temperature, all other experimental conditions were consistent with those in Example 7. In this example, the elastomer was obtained by standing at 50°C for 48 hours. The test results showed that when the evaporation temperature was increased to 50°C, obvious wrinkles appeared on the surface of the elastomer in contact with air. This phenomenon is due to the fact that the solvent evaporation rate at the interface is significantly higher than that inside the material, which causes the polymer on the sample surface to become enriched and dense, while the continuous evaporation of the solvent inside the elastomer causes volume shrinkage. The mismatch between the internal and external shrinkage rates leads to a large internal stress difference. The continuously accumulated stress eventually leads to surface defects, deterioration of mechanical properties, and even microcracks in the elastomer. Considering multiple factors such as the macroscopic mechanical properties of the elastomer, the solvent evaporation cycle, and the energy loss of cyclic deformation, room temperature was finally selected as the optimal solvent evaporation process temperature.

[0070] Uniaxial tensile tests were performed on the elastomers obtained in each embodiment and comparative example. The test specimens were rectangular, measuring 20.0 mm × 3.0 mm × 1.5 mm, with a gauge length of 10 mm, and a tensile rate of 50 mm·min. -1 The molar ratio of lipoic acid to lithium bis(trifluoromethanesulfonylimide), the mass ratio of lipoic acid to IPOH, and the corresponding test results of fracture strength and elongation at break for each example are summarized in Table 1 below: Table 1 Mechanical properties of elastomers with different lithium salt ratios and fluorescent molecule ratios

[0071] Table 1 shows that the lithium salt ratio, the fluorescent molecule ratio, and the solvent evaporation temperature all affect the mechanical properties of the elastomer. When the molar ratio of lipoic acid / lithium bis(trifluoromethanesulfonyl)imide is 7:1, the mass ratio of lipoic acid / IPOH is 300:1, and the solvent evaporation temperature is room temperature, the sample exhibits high tensile strength and elongation. Without the addition of lithium bis(trifluoromethanesulfonyl)imide, the system cannot form a stable elastomer, making mechanical testing impossible. Example 7 was ultimately determined to be the optimal ratio, and the following tests were subsequently conducted.

[0072] Test Example 1: Mechanical and Optical Properties of TIL Elastomers

[0073] Both intact and notched samples were prepared using the TIL elastomer obtained in Example 7. Both types of samples were cut into rectangular specimens with dimensions of 20.0 mm × 3.0 mm × 1.5 mm and a gauge length of 10 mm. For the notched samples, a single-sided notch was pre-made on one side of the middle of the gauge length using a sharp blade. The notch direction was perpendicular to the tensile direction of the specimen, and the notch penetrated the specimen thickness. The notch depth along the specimen width direction was approximately 1.0 mm, which is approximately one-third of the specimen width. The intact samples were not notched.

[0074] The intact and notched samples were fixed in the upper and lower clamps of the universal testing machine, respectively, ensuring that the length direction of the sample was aligned with the loading direction and that the pre-made notch was located in the middle of the gauge length. The loading was carried out at 50 mm / min. -1 The tensile rate was used to perform uniaxial tensile tests until the specimen completely broke, and the breaking strength and elongation at break of each specimen were recorded.

[0075] Test results show that the intact sample has a breaking strength of 147.39 kPa and an elongation at break of 1059.74%; the notched sample has a breaking strength of 136.56 kPa and an elongation at break of 605.49%. Compared with the intact sample, the notched sample retains approximately 92.65% of its breaking strength and approximately 57.14% of its elongation at break. These results indicate that although the pre-formed notch reduces the elongation at break of the sample, the sample still maintains high load-bearing capacity and a certain degree of ductility even with the notch, demonstrating that the TIL elastomer has a certain notch tolerance.

[0076] Steady-state fluorescence assays showed that the main emission peak of the sample under 365 nm excitation was located at approximately 456 nm. Further time-resolved fluorescence assays were used to measure the fluorescence lifetime of the sample, with 456 nm as the monitoring wavelength. The obtained fluorescence decay curves, after fitting, showed an average fluorescence lifetime of 2.40 ns, indicating that the luminescence of this material is a nanosecond-level fluorescence process, rather than a long-afterglow luminescence dependent on energy storage and slow release. The relevant results are as follows: Figure 5 As shown.

[0077] Test Example 2: Fluorescence-enhanced self-detection performance of structural damage

[0078] Scratches or pinholes were prepared on the surface of the TIL elastomer obtained in Example 7 using a blade or needle tip. Under natural light, the damaged and undamaged areas were difficult to distinguish; when irradiated with 365 nm ultraviolet light as needed, the damaged areas immediately showed enhanced blue fluorescence, as shown in the results. Figure 6 As shown.

[0079] The original fluorescence spectra of the scratch-damaged region and its adjacent undamaged region of the TIL elastomer obtained in Example 7 were collected separately. The results showed that at the emission peak of 456 nm, the original fluorescence intensities of the adjacent undamaged region and the scratch-damaged region were 783.8 and 2792.5 au, respectively, with the fluorescence intensity of the scratch-damaged region being approximately 3.5 times that of the adjacent undamaged region. To facilitate a direct comparison of the relative fluorescence intensity differences between the two regions, Figure 7Both fluorescence spectra were normalized using the maximum original fluorescence intensity of the damaged area as a common benchmark; the 3.5-fold enhancement ratio was calculated based on the original fluorescence intensity before normalization. Further grayscale analysis was performed on the natural light and 365 nm UV light images using ImageJ. Regions of interest (ROIs) of equal area were selected at corresponding positions in both types of images, including the scratch-damaged area and the adjacent undamaged area, and their average grayscale values ​​were calculated. The results show that under 365 nm UV illumination, the ratio of the average grayscale value of the scratch-damaged area to that of the adjacent undamaged area is approximately 4.62. Related results are as follows: Figure 7 As shown in the figure. The above results indicate that the local structural damage area of ​​the TIL elastomer exhibits significant fluorescence enhancement compared to the adjacent undamaged area, which can be used for the visual identification and localization of structural damage.

[0080] Test Example 3: Local Damage Response and Environmental Adaptability

[0081] The fluorescence responses of samples with pinhole damage and samples stretched to 100% strain were tested separately. The results showed that pinhole damage caused a significant increase in fluorescence, while the change caused by overall stretching was smaller. This indicates that the system is more sensitive to local network disruption and local interface defects, rather than simply reflecting macroscopic overall deformation. The results are as follows: Figure 8 As shown.

[0082] After being stored at -20 °C for 24 h, the sample was immediately removed, and scratches were prepared and observed under 365 nm ultraviolet light. The damaged area still showed significant fluorescence enhancement. After immersion in 0.9% physiological saline for 2 h, the damaged area also maintained clear fluorescence contrast. The results are as follows: Figure 9 As shown.

[0083] Test Example 4: Optical Feedback During Self-Healing

[0084] The TIL elastomer obtained in Example 7 was cut, and the newly formed cut interfaces were brought into contact and allowed to self-heal at room temperature. The cut area was irradiated with 365 nm ultraviolet light, and fluorescence changes at different self-healing times were observed and recorded. The results showed that the fluorescence signal in the cut area changed significantly with the self-healing process. The fluorescence intensity at 456 nm reached a high level after about 1 hour of self-healing, and then gradually decreased. The relevant results are as follows: Figure 10 As shown above, the changes indicate that the damage fluorescence signal can provide optical feedback on the reconstruction process of the local microenvironment and dynamic network at the cutting interface during self-healing.

[0085] Simultaneously, uniaxial tensile tests were performed on samples with different self-healing times. The self-healing efficiency was calculated according to the following formula:

[0086] In the formula, η is the self-healing efficiency; σ t The fracture strength of the sample after self-healing time t; σ 0 represents the fracture strength of the original intact sample, which is the value used in this test. σ 0 is 147.39 kPa.

[0087] Mechanical testing results showed that the fracture strengths of the samples after 1 min, 1 h, and 2 h of self-healing were 99.49 kPa, 137.92 kPa, and 142.63 kPa, respectively, corresponding to self-healing efficiencies of approximately 67.50%, 93.57%, and 96.77%. The self-healing efficiency further increased to 98.15% after 8 h of self-healing, corresponding to a fracture strength of approximately 144.66 kPa based on the original sample's fracture strength.

[0088] The above results indicate that the fluorescence signal mainly reflects the reconstruction process of the local microenvironment and dynamic network at the damaged interface, while the fracture strength and self-healing efficiency reflect the degree of recovery of the macroscopic load-bearing network of the material. These two types of signals characterize the self-healing process of the elastomer at different scales, enabling the TIL elastomer to simultaneously achieve enhanced fluorescence detection of damage and optical feedback of the self-healing process.

[0089] Test Example 5: Recycling and Reprocessing Performance

[0090] The TIL elastomer obtained in Example 7 was sheared and placed in anhydrous ethanol, and stirred at room temperature for 6 hours to form a homogeneous solution. The resulting solution was recast and allowed to stand at room temperature for about 3 days to obtain the regenerated TIL elastomer. The specific recycling process is as follows: Figure 11 As shown.

[0091] Scratch-enhanced fluorescence was observed in the original sample, the first-recovery sample, and the second-recovery sample under 365 nm UV light, with scratch / intact fluorescence enhancement factors of approximately 3.5, 2.1, and 1.6, respectively. The relevant spectra are shown below. Figure 11 As shown, for ease of observation, Figure 11 The three sets of fluorescence spectra were shifted along the vertical axis. The fracture strengths of the first and second recovered samples were 138.22 kPa and 124.92 kPa, respectively, indicating that the elastomer retains its damage-enhanced fluorescence self-detection and mechanical properties after ethanol dissolution and recasting, and has the potential for repeated processing and reuse.

[0092] Test Example 6: Interface Adhesion Performance Test

[0093] The casting solution obtained in Example 7 was uniformly coated onto the overlapping area of ​​a glass slide that had been cleaned and dried with anhydrous ethanol. The glass slides used were 70 mm × 25 mm × 1.5 mm in size, and the overlapping area of ​​the two glass slides was 25 mm × 10 mm, corresponding to an overlapping area of ​​250 mm².2 Then, another piece of glass is placed over the coating area, so that the two pieces of glass form a single overlap structure along the length direction, and light pressure is applied to make the casting liquid spread evenly at the overlap interface, while removing obvious air bubbles.

[0094] The assembled glass overlap specimen was left to stand at room temperature for 24 hours to allow the ethanol to gradually evaporate and form a TIL elastomer adhesion layer in situ between the two glass pieces. The specimen was then fixed in the upper and lower clamps of a universal testing machine, with the loading direction aligned with the length of the glass pieces, and loaded at a rate of 5 mm / min. -1 The lap shear test was performed at the tensile rate until the adhesive interface was damaged or the two pieces of glass were completely separated, and the maximum load was recorded.

[0095] The lap shear strength is calculated using the following formula:

[0096] In the formula, τ is the lap shear strength, in MPa; F max The maximum load recorded during the test, in N; A The effective overlap area of ​​the glass sheets, in mm. 2 In this test A 250 mm 2 .

[0097] The test results show that the lap shear strength of the glass / glass overlap sample is approximately 0.99 MPa, indicating that the TIL elastomer has a certain interfacial adhesion ability to the glass substrate, which can provide experimental basis for its use as an adhesive functional layer, transparent protective layer, or interlayer fixing material. This test example is only used to characterize the material's interfacial fixing ability to the glass substrate; the fluorescence-enhanced self-detection performance of structural damage was verified by the independent elastomer film damage experiments in Test Example 2 and Test Example 3.

[0098] Unless otherwise specified, the raw materials and equipment used in this invention are all commonly used in the field; unless otherwise specified, the methods used in this invention are all conventional methods in the field.

[0099] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Any simple modifications, alterations, and equivalent transformations made to the above embodiments based on the technical essence of the present invention shall still fall within the protection scope of the present invention.

Claims

1. A structural damage self-detection type lipoic acid-based fluorescent elastomer, characterized in that, The elastomer is formed by mixing lipoic acid, a C4-OH hydroxyl-modified urazine fluorescent probe IPOH, and lithium salt in a solution, followed by casting and solvent evaporation. Under ultraviolet light irradiation, the locally damaged areas of the elastomer produce enhanced fluorescence signals relative to the undamaged areas.

2. The elastomer according to claim 1, characterized in that, The lipoic acid is DL-lipoic acid with a purity of not less than 95%; the lithium salt is lithium bis(trifluoromethanesulfonyl)imide; the chemical structural formula of the C4-OH hydroxyl-modified urazine fluorescent probe IPOH is shown below: 。 3. The elastomer according to claim 1, characterized in that, The molar ratio of lipoic acid to lithium salt is 3:1 to 20:1, and the mass ratio of lipoic acid to IPOH is 100:1 to 500:

1.

4. The elastomer according to claim 3, characterized in that, The molar ratio of lipoic acid to lithium salt is 7:1, and the mass ratio of lipoic acid to IPOH is 300~400:

1.

5. The elastomer according to claim 1, characterized in that, The wavelength of the ultraviolet light is 365 nm, and the local structural damage includes scratches, cuts, pinholes, microcracks, local surface damage, or a combination thereof.

6. The elastomer according to claim 1, characterized in that, Under the same excitation wavelength, light source intensity, and signal acquisition conditions, the enhanced fluorescence signal is characterized by higher fluorescence intensity or image grayscale value in the locally damaged area than in the undamaged area. The difference can be identified by visual observation, fluorescence spectroscopy testing, or image grayscale analysis.

7. The elastomer according to claim 6, characterized in that, Wavelength is 300-400nm; And / or, the degree of enhancement is characterized by the ratio of fluorescence intensity of the damaged area to that of the undamaged area or the ratio of image grayscale values, wherein the ratio is not less than 1.

5.

8. A method for preparing a self-detecting structural damage lipoic acid-based fluorescent elastomer as described in any one of claims 1 to 7, characterized in that, Includes the following steps: S1: Add thioctic acid, IPOH and lithium salt to anhydrous ethanol and stir until well mixed to obtain casting solution; S2: The casting liquid is poured into a mold, and the solvent is evaporated at room temperature to obtain the structural damage self-detection type thioctic acid fluorescent elastomer.

9. The preparation method according to claim 8, characterized in that, In step S1, the amount of anhydrous ethanol used is 1.0–10.0 mL / g based on the mass of lipoic acid, the amount of IPOH used is 2–10 mg / g based on the mass of lipoic acid, and the amount of lithium salt used is 60–500 mg / g based on the mass of lipoic acid; the stirring speed in step S1 is 400–600 r / min, and the stirring time is 5–60 min; the solvent evaporation time in step S2 is 48 h.

10. The application of the structural damage self-detection thioctic acid-based fluorescent elastomer according to any one of claims 1 to 7 in the field of structural damage visualization detection or self-healing process monitoring, or in the field of preparing adhesive functional layers, transparent protective layers or interlayer fixing materials.

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