Dynamic boric acid ester cross-linked self-repairing liquid silicone rubber based on octamethylcyclotetrasiloxane and preparation method of dynamic boric acid ester cross-linked self-repairing liquid silicone rubber

Abstract

The invention discloses dynamic boric acid ester cross-linked self-repairing liquid silicone rubber based on octamethylcyclotetrasiloxane and a preparation method thereof. The preparation method comprises the following steps: carrying out anionic ring-opening copolymerization on octamethylcyclotetrasiloxane and a cyclosiloxane monomer containing a catechol group to prepare polydimethylsiloxane-based rubber with a side chain containing a catechol functional group; the content of catechol groups is 3-8 mol%; the crosslinking density is 0.05-0.15 mmol / g, the tensile strength of vulcanized rubber is 2.5-4.5 MPa, the mechanical recovery rate is larger than or equal to 92% after repairing is conducted for 2 h at the temperature of 60 DEG C, and efficiency attenuation lt can be repeatedly repaired for 10 times or more; the problem that mechanical strength and self-repairing efficiency are difficult to consider at the same time is solved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of silicone polymer materials, in particular to a dynamic borate cross-linked self-healing liquid silicone rubber based on octamethylcyclotetrasiloxane and a preparation method thereof. BACKGROUND

[0002] Liquid silicone rubber (LSR) is an important class of organosilicon elastomer materials, which is widely used in aerospace, electronics, medical devices, automotive industry and flexible electronic devices due to its excellent high and low temperature resistance, electrical insulation, chemical stability and biocompatibility. Traditional LSR is cured by addition or condensation to form a permanent cross-linked network, and the cured product has a stable three-dimensional network structure and excellent mechanical properties. However, the irreversible nature of the permanent cross-linked network makes it impossible for the silicone rubber material to repair itself once mechanical damage occurs during use, which seriously affects the service life and safety reliability of the material.

[0003] Self-healing materials can automatically or under external stimulation to repair their own damage and restore the structural integrity and functional properties of the material. According to the different repair mechanisms, self-healing materials can be divided into two categories: exogenous and intrinsic. Exogenous self-healing materials release repair agents to fill cracks to achieve repair by pre-embedding microcapsules or microvessels containing repair agents. This method has the problems of limited number of repairs, poor uniformity of microcapsule dispersion, and possible introduction of structural defects. Intrinsic self-healing materials introduce dynamic bonds into the polymer network to achieve multiple repairs through the reversible breaking and recombination of dynamic bonds, and have become a hot research direction in recent years.

[0004] At present, a variety of dynamic bonds have been applied to the construction of intrinsic self-healing polysiloxane materials, including hydrogen bonds, metal coordination bonds, Diels-Alder bonds, disulfide bonds, imine bonds and borate bonds. However, the existing intrinsic self-healing polysiloxane materials generally face the contradiction between self-healing ability and mechanical properties: high-density dynamic bond cross-linking can provide good self-healing performance, but the reversibility of dynamic bonds leads to a decrease in the stability of the cross-linked network, and the tensile strength and elastic modulus of the material are significantly reduced; while low-density dynamic bond cross-linking can maintain good mechanical properties, but it is difficult to achieve efficient self-healing.

[0005] In recent years, researchers have attempted to introduce dynamic borate ester bonds into polysiloxane materials to achieve self-healing functionality. Borate ester bonds are formed through the reversible condensation reaction of boric acid groups and glycol groups, exhibiting dynamic and reversible characteristics. However, existing self-healing polysiloxane materials based on borate ester crosslinking generally suffer from the following technical problems: First, the stability of borate ester bonds at room temperature is insufficient, and the material is prone to creep and stress relaxation during room temperature storage, leading to a gradual decrease in mechanical strength; second, the repair temperature is high and the repair time is long, typically requiring heating above 80°C for more than 4 hours to achieve a good repair effect, limiting its application in rapid repair scenarios; third, the repair efficiency decreases significantly with the number of repairs, typically dropping by more than 15% after 5 repairs, failing to meet the requirements for repeated repairs; fourth, the tensile strength is generally below 2.0 MPa, making it difficult to meet the mechanical performance requirements of structural materials.

[0006] The review article "Intrinsic Self-Healing Polysiloxane Materials: From Single Dynamic Crosslinking Networks to Multiple Dynamic Crosslinking Networks," published by Ye Juan et al. in the *Acta Polymerica Sinica*, Vol. 54, No. 7, 2023, systematically summarizes the research progress of intrinsic self-healing polysiloxane materials. It points out that an effective strategy to resolve the contradiction between self-healing ability and mechanical properties is to shift from single dynamic crosslinking to multiple dynamic crosslinking. This involves designing multiple dynamic bond networks with synergistic effects to achieve synergistic regulation of mechanical properties and self-healing performance. However, how to precisely control the ratio and spatial distribution of multiple dynamic bonds, and how to ensure the stability of dynamic bonds under service conditions and their reversibility under repair conditions, remain key scientific problems that urgently need to be solved in this field.

[0007] The paper "Covalently Cross-Linked Elastomers with Self-Healing and Malleable Abilities Enabled by Boronic Ester Bonds" (ACS Applied Materials & Interfaces, 2018, 10, 24224-24231) reports the introduction of dynamic borate ester bonds into a styrene-butadiene rubber network via a thiol-olefin click reaction, achieving self-healing and reprocessable properties in the cross-linked rubber. This work provides an important reference for the application of dynamic borate ester bonds in rubber materials; however, its system is an oil-based rubber rather than a silicone rubber, and it fails to address the balance between mechanical strength and repair efficiency.

[0008] In summary, existing self-healing silicone rubber technologies suffer from technical bottlenecks such as low mechanical strength (tensile strength typically below 2.0 MPa), decreasing repair efficiency and number of repairs (efficiency usually drops by more than 15% after 5 repairs), and demanding repair conditions (temperatures above 80℃ or requiring special stimuli). These limitations make it difficult to meet the practical application requirements for high strength and repeated repair capabilities. Therefore, developing a liquid silicone rubber material that combines high mechanical strength with excellent repetitive self-healing properties has significant theoretical and practical value.

[0009] In summary, existing self-healing silicone rubber technologies suffer from technical bottlenecks such as low mechanical strength (tensile strength typically below 2.0 MPa), decreasing repair efficiency and repair cycles (efficiency usually drops by more than 15% after 5 repairs), and demanding repair conditions (temperatures above 80℃ or requiring special stimuli). These limitations make it difficult to meet the practical application requirements for high strength and repeated repair capabilities. Therefore, developing a liquid silicone rubber material that combines high mechanical strength with excellent repetitive self-healing properties has significant theoretical and practical value. Summary of the Invention

[0010] To address the technical problems of existing self-healing silicone rubbers, such as low mechanical strength, decreasing repair efficiency with repeated repairs, and stringent repair conditions, the present invention aims to provide a dynamic borate ester crosslinked self-healing liquid silicone rubber based on octamethylcyclotetrasiloxane and its preparation method.

[0011] The technical solution of the present invention is as follows: The dynamically borate ester crosslinked self-healing liquid silicone rubber based on octamethylcyclotetrasiloxane is characterized by comprising the following components: A polydimethylsiloxane gum with catechol functional groups on its side chains is prepared by anionic ring-opening copolymerization of octamethylcyclotetrasiloxane and a cyclosiloxane monomer containing catechol groups, wherein the catechol group content is 3-8 mol%, and the number average molecular weight is [not specified]. The value is 50,000-150,000 g / mol; A polysiloxane crosslinking agent containing boric acid groups reacts with the catechol groups in the polydimethylsiloxane adhesive to form a dynamic borate ester crosslinking network, wherein the borate ester crosslinking density is 0.05-0.15 mmol / g.

[0012] Furthermore, the cyclosiloxane monomer containing the catechol group is 3-(3,4-dihydroxyphenyl)propylheptamethylcyclotetrasiloxane, with the following structural formula: , in It represents a heptamethylcyclotetrasiloxane ring, with the catechol group linked to the silicon atom via a propyl chain.

[0013] Furthermore, the boric acid-containing polysiloxane crosslinking agent is a polymethylhydrosiloxane with phenylboronic acid groups in its side chains, and its structural formula is: , Where R is The molar ratio of m:n is 1:2-1:5, and the number average molecular weight is... The value is 5000-20000 g / mol.

[0014] Furthermore, in the dynamic borate ester crosslinking network, the molar ratio of catechol groups to boric acid groups is 1:0.8-1:1.2.

[0015] Furthermore, the molecular weight distribution index (PDI) of the polydimethylsiloxane adhesive is 1.1-1.5.

[0016] Furthermore, the self-healing liquid silicone rubber also includes a catalyst and a reinforcing filler. The catalyst is a platinum catalyst, used in an amount of 5-50 ppm; the reinforcing filler is fumed silica, used in an amount of 10-40 parts by weight.

[0017] The present invention also provides a method for preparing the above-mentioned self-healing liquid silicone rubber, characterized by comprising the following steps: Step S1, Synthesis of cyclosiloxane monomers containing catechol groups: 3-(3,4-dimethoxyphenyl)propyltrimethoxysilane and heptamethylcyclotetrasiloxane are reacted in an acidic catalyst to obtain a siloxane equilibrium reaction, namely 3-(3,4-dimethoxyphenyl)propylheptamethylcyclotetrasiloxane intermediate; then, a methoxy deprotection reaction is carried out in the presence of boron tribromide to obtain cyclosiloxane monomers containing catechol groups.

[0018] Step S2, Synthesis of polydimethylsiloxane adhesive with catechol functional groups in the side chain: Under anhydrous and oxygen-free conditions, using tetramethylammonium hydroxide as an initiator, octamethylcyclotetrasiloxane and the cyclosiloxane monomer containing catechol groups obtained in step S1 are subjected to anionic ring-opening copolymerization reaction. The reaction temperature is 80-120℃ and the reaction time is 6-12 hours. After polymerization, hexamethyldisiloxane is added for end-capping treatment, and the solvent is removed to obtain polydimethylsiloxane adhesive.

[0019] Step S3, Synthesis of polysiloxane crosslinking agent containing boric acid groups: Using polymethylhydrosiloxane as raw material, a hydrosilylation reaction is carried out with 4-allylphenylboronic acid in the presence of a platinum catalyst. The reaction temperature is 60-80℃ and the reaction time is 4-8 hours to obtain a polysiloxane crosslinking agent with phenylboronic acid groups in the side chain.

[0020] Step S4, Construction of dynamic borate ester crosslinking network: The polydimethylsiloxane adhesive obtained in step S2 and the polysiloxane crosslinking agent containing borate groups obtained in step S3 are mixed at a molar ratio of catechol groups to borate groups of 1:0.8-1:1.2. Platinum catalyst and fumed silica are added. After mixing evenly at room temperature, the mixture is cured at 120-150℃ for 1-3 hours to obtain self-healing liquid silicone rubber with dynamic borate ester crosslinking.

[0021] Further, in step S1, the acidic catalyst is p-toluenesulfonic acid, and the amount used is 0.1-0.5% of the total mass of the reactants; the equilibrium reaction temperature of the siloxane is 100-130℃, and the reaction time is 8-16 hours.

[0022] Further, in step S2, the molar ratio of the octamethylcyclotetrasiloxane to the cyclosiloxane monomer containing the catechol group is 92:8-97:3; the amount of tetramethylammonium hydroxide used is 0.05-0.2% of the total mass of the monomer.

[0023] Further, in step S3, the Si-H content of the polymethylhydrosiloxane is 0.5-2.0 mol / 100g; the molar ratio of 4-allylphenylboronic acid to Si-H is 0.3-0.5:1.

[0024] The technical effects and advantages of this invention are as follows: First, this invention designs cyclosiloxane monomers with catechol functional groups and uses an anionic ring-opening copolymerization strategy to precisely introduce catechol groups into the side groups of the polydimethylsiloxane main chain, thereby achieving a uniform distribution of functional groups on the polymer chain and avoiding the problem of uneven distribution of functional groups caused by traditional post-modification methods.

[0025] Secondly, this invention utilizes the highly stable borate ester bond formed between catechol and phenylboronic acid under neutral or weakly alkaline conditions as a dynamic crosslinking point. Compared to the traditional 1,2-diol / boronic acid system, the equilibrium constant of the catechol / phenylboronic acid system is significantly higher. It is about 100 times higher than that of other materials, and has sufficient stability at room temperature to ensure the mechanical properties of the material. Under mild heating conditions of 60-80℃, it can reversibly dissociate and recombine to achieve damage repair.

[0026] Third, by precisely controlling the content of catechol groups (3-8 mol%) and the crosslinking density of borate esters (0.05-0.15 mmol / g), this invention achieves a balance between the reversibility and stability of the dynamic crosslinking network, resulting in a tensile strength of 2.5-4.5 MPa and an elongation at break of 350-550% after vulcanization, which is significantly better than existing self-healing silicone rubber materials.

[0027] Fourth, the self-healing liquid silicone rubber of the present invention has a mechanical property recovery rate of ≥92% after repairing at 60°C for 2 hours, can be repeatedly repaired ≥10 times with repair efficiency attenuation of <5%, breaking through the repair number limitation of the existing single dynamic crosslinking system.

[0028] Fifth, the anionic ring-opening copolymerization and hydrosilylation reaction used in this invention are both mature industrial technologies, with readily available raw materials, controllable processes, and easy large-scale production. Attached Figure Description

[0029] Figure 1 This is a schematic diagram of the preparation method of the self-healing liquid silicone rubber of the present invention; Figure 2 The tensile stress-strain curves for Examples 1-3 and Comparative Examples 1-2 are shown. Figure 3 The curves showing the changes in self-repair efficiency over multiple iterations in Example 1 are shown. Detailed Implementation

[0030] The present invention will be further described in detail below with reference to specific embodiments. These embodiments are only used to explain the technical solutions of the present invention and do not constitute a limitation on the scope of protection of the present invention.

[0031] Number average molecular weight The molecular weight distribution index (PDI) was determined by gel permeation chromatography (GPC) with tetrahydrofuran as the mobile phase and polystyrene as the standard sample for calibration.

[0032] The content of catechol groups was determined by 1H NMR spectroscopy (NMR spectroscopy). The determination was performed by ¹H NMR, using deuterated chloroform as the solvent, and the result was calculated by the ratio of the integral area of ​​catechol aromatic protons to the integral area of ​​silanylmethyl protons.

[0033] The crosslinking density of borate esters was determined by the equilibrium swelling method. The vulcanized rubber sample was swollen in toluene until equilibrium was reached, and the crosslinking density was calculated according to the Flory-Rehner equation.

[0034] Tensile properties were tested according to GB / T 528-2009, with dumbbell-shaped standard specimens and a tensile rate of 500 mm / min.

[0035] Self-healing efficiency test method: After cutting a standard specimen in the middle, the cut surfaces are tightly joined together, and the specimen is repaired under set temperature and time conditions. Then, the tensile strength of the repaired specimen is tested. Self-repair efficiency Calculate using the following formula: , in This represents the tensile strength of the original sample.

[0036] Multiple repair efficiency tests: The same sample is repeatedly cut, repaired, and tested at the same location. The tensile strength after each repair is recorded, and the repair efficiency is calculated.

[0037] Example 1 This embodiment provides a dynamic borate ester crosslinked self-healing liquid silicone rubber based on octamethylcyclotetrasiloxane and its preparation method.

[0038] Step S1, Synthesis of cyclosiloxane monomers containing catechol groups: In a 500 mL four-necked flask equipped with a mechanical stirrer, thermometer, reflux condenser, and nitrogen protection, 26.8 g (0.1 mol) of 3-(3,4-dimethoxyphenyl)propyltrimethoxysilane and 28.1 g (0.1 mol) of heptamethylcyclotetrasiloxane were added, along with 0.11 g of p-toluenesulfonic acid (0.2% of the total reactant mass) as a catalyst. Under nitrogen protection, the reaction system was heated to 110 °C to initiate a siloxane equilibrium reaction, and this temperature was maintained for 10 hours. After the reaction was complete, low-molecular-weight volatiles were removed by vacuum distillation to obtain the 3-(3,4-dimethoxyphenyl)propylheptamethylcyclotetrasiloxane intermediate.

[0039] The above intermediate was dissolved in 200 mL of anhydrous dichloromethane. 60 mL of a 1.0 M boron tribromide solution in dichloromethane was slowly added dropwise at -78 °C. After the addition was complete, the temperature was allowed to rise naturally to room temperature, and the reaction was continued with stirring for 8 hours. After the reaction was complete, 100 mL of methanol was added to quench excess boron tribromide. The solvent was removed under reduced pressure, and the mixture was extracted with ethyl acetate, dried over anhydrous sodium sulfate, and distilled under reduced pressure to obtain 38.5 g of 3-(3,4-dihydroxyphenyl)propylheptamethylcyclotetrasiloxane (a cyclosiloxane monomer containing a catechol group), with a yield of 78%.

[0040] Product structure confirmation: H NMR (400 MHz, CDCl ) : 6.78-6.65 (m, 3H, Ar-H), 5.52(s, 2H, Ar-OH), 2.52 (t, 2H, Ar-CH ), 1.68 (m, 2H, -CH -), 0.58 (t, 2H, Si-CH ), 0.08-0.12 (m, 21H, Si-CH ).

[0041] Step S2, Synthesis of polydimethylsiloxane gum with catechol functional groups in the side chain: In a 1000 mL three-necked flask that has been anhydrous, 300 mL of anhydrous toluene is added as a solvent. Under nitrogen protection, 88.9 g (0.30 mol) of octamethylcyclotetrasiloxane and 9.8 g (0.020 mol, corresponding to 5 mol% catechol group content) of the cyclosiloxane monomer containing catechol groups obtained in step S1 are added, so that the molar ratio of the two monomers is 95:5.

[0042] 0.20 g of a 25% tetramethylammonium hydroxide aqueous solution (equivalent to 0.1% of the total monomer mass) was added as an anionic ring-opening polymerization initiator. Under nitrogen protection, the reaction system was heated to 100°C to carry out the ring-opening copolymerization reaction, and this temperature was maintained for 8 hours.

[0043] During the reaction, samples were taken to monitor the conversion rate. When the conversion rate reached 95% or higher, 2.0 g of hexamethyldisiloxane was added for end-capping, and the reaction was continued for 1 hour. The reaction system was heated to 150°C and maintained for 2 hours to decompose the tetramethylammonium hydroxide in the initiator. After cooling to room temperature, the solvent and low molecular weight cyclic compounds were removed by vacuum distillation to obtain 75.2 g of polydimethylsiloxane gum with catechol functional groups on the side chains.

[0044] Product characterization: Number-average molecular weight determined by GPC = 85000 g / mol, molecular weight distribution index (PDI) = 1.28; The content of catechol groups was determined to be 4.8 mol by H NMR.

[0045] Step S3, Synthesis of polysiloxane crosslinking agent containing boric acid groups: In a 250 mL three-necked flask equipped with a mechanical stirrer, thermometer, and nitrogen protection device, add 30.0 g of polymethylhydrosiloxane (Si-H content 1.0 mol / 100 g, i.e., containing 0.30 mol of Si-H) and 100 mL of anhydrous toluene. The repeating unit structure of polymethylhydrosiloxane is -[Si(CH )(H)-O-]-, with a number-average molecular weight of approximately 2000 g / mol.

[0046] 16.2 g (0.10 mol, with a molar ratio of 1:3 to Si-H) of 4-allylphenylboronic acid and 30 ppm (as Pt, relative to polymethylhydrosiloxane) of Karstedt platinum catalyst (xylene solution of divinyltetramethyldisiloxane platinum complex containing 2% platinum) were added.

[0047] Under nitrogen protection, the reaction system was heated to 70°C to carry out a hydrosilylation reaction, and the reaction was maintained at this temperature for 6 hours. After the reaction was completed, the solvent was removed by vacuum distillation to obtain 35.8 g of a polysiloxane crosslinking agent with phenylboronic acid groups in the side chain.

[0048] Product characterization: through H NMR and B NMR confirmed that the phenylboronic acid group had been successfully grafted onto the polysiloxane side chain; GPC determined the number-average molecular weight. = 8500 g / mol; boric acid group content (determined by titration) is 1.8 mmol / g.

[0049] Step S4, Construction of the dynamic borate ester crosslinking network: Under vacuum conditions, 50.0 g of the polydimethylsiloxane adhesive obtained in step S2 was mixed with fumed silica (specific surface area 200 m²). Mix 12.5 g (25 parts by weight) in a planetary mixer to obtain a compound.

[0050] Based on the content of catechol groups in the polydimethylsiloxane rubber (4.8 mol%, approximately 0.52 mmol / g base rubber) and the content of boric acid groups in the crosslinking agent (1.8 mmol / g), calculate the amount of crosslinking agent required when the molar ratio of catechol groups to boric acid groups is 1:1. Weigh 14.4 g of the polysiloxane crosslinking agent containing boric acid groups obtained in step S3 (containing 25.9 mmol of boric acid groups) and mix it with the rubber compound.

[0051] Add 20 ppm (as Pt) of Karstedt platinum catalyst and mix thoroughly at room temperature. Pour the mixture into a mold and cure at 130°C for 2 hours. After demolding, obtain a self-healing liquid silicone rubber vulcanizate with dynamic borate crosslinking.

[0052] Performance testing: Tensile properties: tensile strength 3.8 MPa, elongation at break 480%, stress at 100% elongation 0.52 MPa.

[0053] Self-healing performance: After cutting and butt-jointing the vulcanized rubber sample, it was repaired at 60℃ for 2 hours. The tensile strength after repair was 3.55 MPa, and the self-healing efficiency was 93.4%.

[0054] Multiple repair performance: After 10 consecutive cut-repair-test cycles, the tensile strength after the 10th repair was 3.42 MPa, the self-repair efficiency was 90.0%, and the efficiency decreased by 3.6% compared with the first repair.

[0055] The cross-linking density of borate ester was determined to be 0.085 mmol / g by equilibrium swelling method.

[0056] Example 2 The difference between this embodiment and Example 1 lies in adjusting the content of catechol groups and the crosslinking density.

[0057] In step S2, the molar ratio of octamethylcyclotetrasiloxane to cyclosiloxane monomer containing catechol groups is adjusted to 97:3, so that the catechol group content in the polydimethylsiloxane adhesive is 2.9 mol.

[0058] In step S4, the amount of crosslinking agent is adjusted so that the molar ratio of catechol groups to boric acid groups is 1:1.

[0059] Performance testing: Polydimethylsiloxane adhesive: GPC determination = 92000 g / mol, PDI = 1.25; The content of catechol groups was determined to be 2.9 mol by H NMR.

[0060] Tensile properties: tensile strength 2.6 MPa, elongation at break 520%, stress at 100% elongation 0.38 MPa.

[0061] Self-healing performance: After repairing at 60℃ for 2 hours, the tensile strength after repair is 2.45 MPa, and the self-healing efficiency is 94.2%.

[0062] Performance after multiple repairs: The self-repair efficiency was 91.5% after the 10th repair, which is 2.9% lower than the efficiency of the first repair.

[0063] Borate ester crosslinking density: 0.052 mmol / g.

[0064] The results of this embodiment show that reducing the content of catechol groups can achieve higher elongation at break and more stable multiple repair efficiency, but the tensile strength decreases accordingly.

[0065] Example 3 The difference between this embodiment and Example 1 is that the content of catechol groups and the crosslinking density are increased.

[0066] In step S2, the molar ratio of octamethylcyclotetrasiloxane to cyclosiloxane monomer containing catechol groups is adjusted to 92:8, so that the catechol group content in the polydimethylsiloxane adhesive is 7.6 mol.

[0067] In step S4, the amount of crosslinking agent is adjusted so that the molar ratio of catechol groups to boric acid groups is 1:1.

[0068] Performance testing: Polydimethylsiloxane adhesive: GPC determination = 78000 g / mol, PDI = 1.35; The content of catechol groups was determined to be 7.6 mol by H NMR.

[0069] Tensile properties: tensile strength 4.3 MPa, elongation at break 360%, stress at 100% constant elongation 0.78 MPa.

[0070] Self-healing performance: After repairing at 60℃ for 2 hours, the tensile strength after repair is 3.96 MPa, and the self-healing efficiency is 92.1%.

[0071] Performance after multiple repairs: The self-repair efficiency was 87.8% after the 10th repair, which is 4.7% lower than the efficiency of the first repair.

[0072] Borate ester crosslinking density: 0.142 mmol / g.

[0073] The results of this embodiment show that increasing the content of catechol groups can significantly improve tensile strength, but decrease the elongation at break and slightly increase the decay of the efficiency of multiple repairs.

[0074] Example 4 The difference between this embodiment and Example 1 is that the molar ratio of catechol groups to boric acid groups is adjusted.

[0075] In step S4, the amount of crosslinking agent is adjusted so that the molar ratio of catechol groups to boric acid groups is 1:0.8.

[0076] Performance testing: Tensile properties: tensile strength 3.2 MPa, elongation at break 510%.

[0077] Self-healing performance: After 2 hours of repair at 60℃, the self-healing efficiency is 95.1%.

[0078] Performance after multiple repairs: The self-repair efficiency was 92.3% after the 10th repair, which is 3.0% lower than the efficiency of the first repair.

[0079] Borate ester crosslinking density: 0.068 mmol / g.

[0080] The results of this embodiment show that reducing the amount of boric acid groups (excess catechol) is beneficial to improving the repair efficiency, but the tensile strength is slightly reduced.

[0081] Example 5 The difference between this embodiment and Example 1 is that the molar ratio of catechol groups to boric acid groups is adjusted.

[0082] In step S4, the amount of crosslinking agent is adjusted so that the molar ratio of catechol groups to boric acid groups is 1:1.2.

[0083] Performance testing: Tensile properties: tensile strength 4.0 MPa, elongation at break 420%.

[0084] Self-healing performance: After 2 hours of repair at 60℃, the self-healing efficiency is 91.5%.

[0085] Performance after multiple repairs: The self-repair efficiency was 86.2% after the 10th repair, which is 5.8% lower than the efficiency of the first repair.

[0086] Borate ester crosslinking density: 0.102 mmol / g.

[0087] The results of this embodiment show that increasing the amount of boric acid groups (excess boric acid) can improve tensile strength, but the repair efficiency decreases slightly, and the efficiency decays more with repeated repairs.

[0088] Example 6 The difference between this embodiment and Embodiment 1 lies in adjusting the repair temperature and time.

[0089] The self-healing efficiency of the vulcanized rubber sample prepared in Example 1 was tested under different repair conditions.

[0090] Test results: Repairing at 50℃ for 4 hours: self-repair efficiency 85.2%.

[0091] Repairing at 60℃ for 1 hour: self-repair efficiency 82.6%.

[0092] Repairing at 60℃ for 2 hours: self-repair efficiency 93.4%.

[0093] Repairing at 60℃ for 4 hours: self-repair efficiency 96.8%.

[0094] Repairing at 70℃ for 1 hour: self-repair efficiency 91.5%.

[0095] Repairing at 70℃ for 2 hours: self-repair efficiency 97.2%.

[0096] Repairing at 80℃ for 1 hour: self-repair efficiency 95.8%.

[0097] Repairing at 80℃ for 2 hours: self-repair efficiency 98.5%.

[0098] The above results demonstrate that the self-healing liquid silicone rubber of this invention exhibits excellent self-healing properties within a temperature range of 60-80℃. Higher repair temperatures and longer repair times result in higher repair efficiency. Repair efficiency exceeding 92% can be achieved after 2 hours of repair at 60℃, meeting the needs of most practical applications.

[0099] Example 7 The difference between this embodiment and Embodiment 1 is that the amount of fumed silica used is adjusted.

[0100] In step S4, the amount of fumed silica used is adjusted to 5.0 g (10 parts by weight).

[0101] Performance testing: Tensile properties: tensile strength 2.8 MPa, elongation at break 550%.

[0102] Self-healing performance: After 2 hours of repair at 60℃, the self-healing efficiency is 94.8%.

[0103] The results of this embodiment show that reducing the amount of fumed silica will reduce tensile strength, but has little impact on self-healing efficiency.

[0104] Example 8 The difference between this embodiment and Embodiment 1 is that the amount of fumed silica used is adjusted.

[0105] In step S4, the amount of fumed silica used is adjusted to 20.0 g (40 parts by weight).

[0106] Performance testing: Tensile properties: tensile strength 4.5 MPa, elongation at break 350%.

[0107] Self-healing performance: After 2 hours of repair at 60℃, the self-healing efficiency is 90.2%.

[0108] The results of this embodiment show that increasing the amount of fumed silica can significantly improve tensile strength, but the elongation at break and self-healing efficiency decrease slightly.

[0109] Comparative Example 1 This comparative example uses the traditional 1,2-propanediol / phenylboronic acid crosslinking system instead of the catechol / phenylboronic acid system.

[0110] Polydimethylsiloxane with 1,2-propanediol groups in the side chain (1,2-propanediol group content 5 mol%) was mixed with the polysiloxane crosslinking agent with boric acid groups obtained in step S3 of Example 1 at a molar ratio of 1,2-propanediol groups to boric acid groups of 1:1, and vulcanizate was prepared according to the method in step S4 of Example 1.

[0111] Performance testing: Tensile properties: tensile strength 1.8 MPa, elongation at break 420%.

[0112] Self-healing performance: 88.5% self-healing efficiency after 2 hours of repair at 60℃; 95.2% self-healing efficiency after 2 hours of repair at 80℃.

[0113] Performance after multiple repairs: After the 5th repair, the self-repair efficiency dropped to 72.3%, a decrease of 18.3% compared to the first repair.

[0114] Comparative analysis: The boronic acid ester bonds formed by the 1,2-propanediol / phenylboronic acid crosslinking system used in Comparative Example 1 have lower stability (equilibrium constant). Approximately 10 M The crosslinked network is prone to hydrolysis and transesterification at room temperature, resulting in a tensile strength of only 1.8 MPa, significantly lower than the 3.8 MPa in Example 1. Furthermore, due to insufficient stability of the crosslinked network, the efficiency significantly decreases after multiple repairs; the efficiency drops by 18.3% after the fifth repair, failing to meet the requirements for multiple repairs.

[0115] In contrast, the borate ester bonds formed by the catechol / phenylboronic acid crosslinking system used in Example 1 have a higher equilibrium constant. Approximately 830 M It exhibits excellent stability at room temperature, ensuring high mechanical strength; at the same time, it can still undergo reversible dissociation-recombination reaction at 60-80℃, achieving efficient multiple self-repair.

[0116] Comparative Example 2 This comparative example uses a single vulcanization system instead of a dynamic borate ester crosslinking system.

[0117] A vulcanizate was prepared by mixing polydimethylsiloxane (vinyl content 0.2 mol%) without catechol groups on the side chain and polymethylhydrosiloxane (Si-H content 1.0 mol / 100g) at a vinyl to Si-H molar ratio of 1:1.2, adding 20 ppm of platinum catalyst, and following the method in step S4 of Example 1.

[0118] Performance testing: Tensile properties: tensile strength 4.2 MPa, elongation at break 380%.

[0119] Self-healing performance: 8.5% self-healing efficiency after 2 hours of repair at 60℃; 12.3% self-healing efficiency after 4 hours of repair at 80℃.

[0120] Comparative analysis: Comparative Example 2 uses a traditional addition-curing vulcanization system, and the resulting Si-C covalent cross-linked network is a permanent network that does not have dynamic reversible properties. Therefore, its self-healing efficiency is extremely low (only about 10%). This efficiency mainly comes from the molecular motion and physical entanglement recovery of polymer chain segments at high temperatures, and does not belong to true chemical self-healing.

[0121] In contrast, the dynamic borate ester crosslinking system used in Example 1 can undergo a reversible dissociation-recombination reaction under mild heating conditions, rebuilding the crosslinking network at the damaged site and achieving a high self-repair efficiency of 93.4%.

[0122] Comparative Example 3 This comparative example uses a higher content of catechol groups (outside the scope of this invention).

[0123] In step S2, the molar ratio of octamethylcyclotetrasiloxane to cyclosiloxane monomer containing catechol groups is adjusted to 88:12, so that the catechol group content in the polydimethylsiloxane adhesive is 11.5 mol.

[0124] Performance testing: Polydimethylsiloxane adhesive: GPC determination = 62000 g / mol, PDI = 1.52.

[0125] Tensile properties: tensile strength 4.8 MPa, elongation at break 220%.

[0126] Self-healing performance: After 2 hours of repair at 60℃, the self-healing efficiency is 78.5%.

[0127] Performance after multiple repairs: After the 5th repair, the self-repair efficiency dropped to 58.2%.

[0128] Comparative analysis: The catechol group content in Comparative Example 3 was too high (11.5 mol%), resulting in an excessively high crosslinking density (0.21 mmol / g). Although the tensile strength increased to 4.8 MPa, the elongation at break dropped significantly to 220%, making the material brittle. Simultaneously, the excessively high crosslinking density caused steric hindrance to the rearrangement of the dynamic crosslinking network, reducing the self-healing efficiency to 78.5%, and the efficiency deteriorated significantly with repeated repairs.

[0129] Therefore, the present invention optimizes and controls the catechol group content within the range of 3-8 mol%, achieving the best balance between mechanical strength and self-healing performance.

[0130] Comparative Example 4 This comparative example uses a lower content of catechol groups (below the scope of this invention).

[0131] In step S2, the molar ratio of octamethylcyclotetrasiloxane to cyclosiloxane monomer containing catechol groups is adjusted to 99:1, so that the catechol group content in the polydimethylsiloxane adhesive is 0.9 mol.

[0132] Performance testing: Tensile properties: tensile strength 1.5 MPa, elongation at break 620%.

[0133] Self-healing performance: After 2 hours of repair at 60℃, the self-healing efficiency is 72.3%.

[0134] Borate ester crosslinking density: 0.018 mmol / g.

[0135] Comparative analysis: The catechol group content in Comparative Example 4 was too low (0.9 mol%), resulting in insufficient crosslinking density (0.018 mmol / g) and a tensile strength of only 1.5 MPa, which failed to meet the mechanical performance requirements of the structural material. Furthermore, the excessively low dynamic crosslinking point density made it difficult for the damaged area to form an effective crosslinking network for reconstruction, reducing the self-healing efficiency to 72.3%.

[0136] This invention introduces catechol functional groups into the side groups of the polydimethylsiloxane backbone, which react with a polysiloxane crosslinking agent containing phenylboronic acid groups to form a dynamic borate ester crosslinking network. This achieves a synergistic improvement in the high strength and high repair efficiency of self-healing liquid silicone rubber. The comprehensive mechanism analysis is as follows: First, the thermally reversible nature of the dynamic borate ester bond between catechol and phenylboronic acid is the core mechanism for achieving efficient self-repair. The borate ester bond formed between catechol and phenylboronic acid under neutral conditions exhibits temperature-responsive reversible dissociation-recombination characteristics. At room temperature, the equilibrium constant of the borate ester bond... Approximately 830 M The stable cross-linked network ensures the material's mechanical properties. When heated to 60-80℃, the borate ester bonds undergo reversible dissociation, generating free catechol and phenylboronic acid groups. These free groups re-pair at the damaged interface to form new borate ester cross-linking points, thereby rebuilding the cross-linked network and achieving damage repair. Since the dissociation-recombination process does not involve the breaking and formation of covalent bonds but is achieved through coordination exchange, it does not produce byproducts or lead to a decrease in molecular weight, ensuring that the material can undergo multiple repairs.

[0137] Second, the high flexibility of the polysiloxane backbone promotes molecular chain movement and dynamic bond rearrangement at the damaged interface. The low Si-O bond energy and large bond angle of the polydimethylsiloxane backbone endow the molecular chain with extremely high flexibility (glass transition temperature). Approximately -125℃). At the repair temperature (60-80℃), it is much higher than... The temperature allows the polymer chains sufficient molecular mobility, promoting interdiffusion and penetration of molecular chains on both sides of the damaged interface. This provides more opportunities for catechol groups and phenylboronic acid groups to contact and pair, improving repair efficiency. Simultaneously, the compliant main chain structure facilitates topological rearrangement of the dynamic cross-linking network, resulting in a more uniform distribution of newly formed cross-linking points and preventing stress concentration.

[0138] Third, the anionic ring-opening copolymerization strategy ensures the uniform distribution of catechol functional groups on the polymer chain. Using the anionic ring-opening copolymerization mechanism, octamethylcyclotetrasiloxane and cyclosiloxane monomers containing catechol groups jointly participate in the chain growth reaction, with the functional monomers randomly distributed statistically on the polymer chain. Compared to traditional post-modification methods (such as side-linking), anionic ring-opening copolymerization avoids local aggregation or chain-end enrichment of functional groups, ensuring a uniform spatial distribution of crosslinking points, and giving the entire material consistent mechanical properties and self-healing capabilities.

[0139] Fourth, precise control of crosslinking density achieves synergistic optimization of mechanical strength and self-healing ability. This invention precisely controls the crosslinking density of borate ester within the range of 0.05-0.15 mmol / g by controlling the content of catechol groups (3-8 mol%) and the catechol / boric acid molar ratio (1:0.8-1:1.2). This crosslinking density range can form a sufficiently stable three-dimensional crosslinked network to provide high mechanical strength (2.5-4.5 MPa) while ensuring sufficient network dynamism to achieve efficient self-healing (≥92%). When the crosslinking density is too high, although the mechanical strength increases, molecular chain movement is restricted, dynamic bond rearrangement is difficult, and the repair efficiency decreases; when the crosslinking density is too low, the network stability is insufficient, the mechanical strength decreases, and it is difficult for the damaged area to form effective crosslinking for recovery.

[0140] Fifth, the reinforcing effect of fumed silica and the synergistic effect of the dynamic crosslinking network. The silanol groups on the surface of fumed silica can form hydrogen bonds with the polysiloxane backbone, providing additional physical crosslinking points and further improving the tensile strength of the material. Simultaneously, the silica nanoparticles form a filler network in the polymer matrix. When the material is subjected to tensile stress, the filler network can effectively disperse the stress, preventing rapid crack propagation caused by stress concentration. During the self-healing process, the physical crosslinking points and the filler network remain relatively stable, providing skeletal support for the reconstruction of the dynamic borate ester crosslinking network, which is beneficial for precise alignment of the damaged interface and the recovery of mechanical properties after repair.

[0141] In summary, this invention successfully developed a dynamic borate crosslinked self-healing liquid silicone rubber with high tensile strength (2.5-4.5 MPa), high elongation at break (350-550%), high self-healing efficiency (≥92%), and excellent stability after multiple repairs (efficiency decay <5% after 10 repairs) by leveraging the thermally reversible properties of the catechol / phenylboronic acid dynamic borate ester bond, the high flexibility of the polysiloxane backbone, precise control of anionic ring-opening copolymerization, and synergistic optimization of crosslinking density. This breakthrough overcomes the technical bottleneck of existing self-healing silicone rubber materials where mechanical strength and repair performance are difficult to balance.

[0142] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. For those skilled in the art, the present invention can be modified and varied in various ways. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of the claims of the present invention.

Claims

1. A self-healing liquid silicone rubber based on octamethylcyclotetrasiloxane and dynamically borate ester crosslinking, characterized in that, The product comprises the following components: a polydimethylsiloxane gum with catechol functional groups on its side chains, which is prepared by anionic ring-opening copolymerization of octamethylcyclotetrasiloxane and a cyclosiloxane monomer containing catechol groups, wherein the catechol group content is 3-8 mol%, and the number average molecular weight is 3%. The crosslinking density is 50,000-150,000 g / mol; a polysiloxane crosslinking agent containing boric acid groups reacts with the catechol groups in the polydimethylsiloxane adhesive to form a dynamic borate ester crosslinking network, and the borate ester crosslinking density is 0.05-0.15 mmol / g.

2. The self-healing liquid silicone rubber according to claim 1, characterized in that, The cyclosiloxane monomer containing the catechol group is 3-(3,4-dihydroxyphenyl)propylheptamethylcyclotetrasiloxane.

3. The self-healing liquid silicone rubber according to claim 1, characterized in that, The boric acid-containing polysiloxane crosslinking agent is a polymethylhydrosiloxane with phenylboronic acid groups in its side chain, and its structural formula is: ， Where R is The molar ratio of m:n is 1:2-1:5, and the number average molecular weight is... The value is 5000-20000 g / mol.

4. The self-healing liquid silicone rubber according to claim 1, characterized in that, In the dynamic borate ester crosslinking network, the molar ratio of catechol groups to boric acid groups is 1:0.8-1:1.

2.

5. The self-healing liquid silicone rubber according to claim 1, characterized in that, The molecular weight distribution index (PDI) of the polydimethylsiloxane adhesive is 1.1-1.

5.

6. The self-healing liquid silicone rubber according to claim 1, characterized in that, It also includes a catalyst and reinforcing filler, wherein the catalyst is a platinum catalyst, and the dosage is 5-50 ppm; the reinforcing filler is fumed silica, and the dosage is 10-40 parts by weight.

7. The method for preparing the self-healing liquid silicone rubber according to any one of claims 1-6, characterized in that, Includes the following steps: Step S1: 3-(3,4-dimethoxyphenyl)propyltrimethoxysilane and heptamethylcyclotetrasiloxane undergo a siloxane equilibrium reaction in the presence of an acidic catalyst, followed by a methoxy deprotection reaction in the presence of boron tribromide to obtain a cyclosiloxane monomer containing a catechol group. Step S2: Under anhydrous and oxygen-free conditions, using tetramethylammonium hydroxide as an initiator, octamethylcyclotetrasiloxane and the cyclosiloxane monomer containing a catechol group obtained in Step S1 undergo an anionic ring-opening copolymerization reaction. After polymerization, [further details omitted]. In step S2, polydimethylsiloxane is used for end-capping treatment to obtain polydimethylsiloxane adhesive; in step S3, polymethylhydrosiloxane is used as raw material and undergoes hydrosilylation reaction with 4-allylphenylboronic acid in the presence of a platinum catalyst to obtain a polysiloxane crosslinking agent containing boric acid groups; in step S4, the polydimethylsiloxane adhesive obtained in step S2 is mixed with the polysiloxane crosslinking agent containing boric acid groups obtained in step S3, a platinum catalyst and fumed silica are added, and the mixture is cured to obtain a self-healing liquid silicone rubber with dynamic borate crosslinking.

8. The preparation method according to claim 7, characterized in that, In step S1, the acidic catalyst is p-toluenesulfonic acid, and the amount used is 0.1-0.5% of the total mass of the reactants; the equilibrium reaction temperature of the siloxane is 100-130℃, and the reaction time is 8-16 hours.

9. The preparation method according to claim 7, characterized in that, In step S2, the molar ratio of the octamethylcyclotetrasiloxane to the cyclosiloxane monomer containing the catechol group is 92:8-97:3; the anionic ring-opening copolymerization reaction temperature is 80-120℃, and the reaction time is 6-12 hours; the amount of tetramethylammonium hydroxide used is 0.05-0.2% of the total mass of the monomer.

10. The preparation method according to claim 7, characterized in that, In step S3, the Si-H content of the polymethylhydrosiloxane is 0.5-2.0 mol / 100g; the molar ratio of 4-allylphenylboronic acid to Si-H is 0.3-0.5:1; the hydrosilylation reaction temperature is 60-80℃, and the reaction time is 4-8 hours.

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