A polyimide / indium tin oxide composite transparent conductive flexible film material based on molecular pulley effect enhancement and a preparation method thereof

Abstract

The application discloses a polyimide / indium tin oxide composite transparent conductive flexible film material based on molecular pulley effect enhancement and a preparation method thereof, and belongs to the technical field of polyimide films. In the application, polyrotaxane is introduced into a transparent polyimide base through graft copolymerization reaction, a sliding crosslinking network is constructed, and a polyrotaxane enhanced polyimide material is obtained; a nanoscale indium tin oxide film is deposited on the surface of the polyrotaxane enhanced polyimide material by using a room temperature magnetron sputtering process, and a flexible transparent conductive film of a polyimide substrate is prepared. The transparent conductive film prepared in the application has excellent conductivity and optical transmittance, and shows excellent stability after 180-degree bending cycle fatigue test, and can be used as a flexible transparent electrode and has the advantages of good transparency, high strength, high toughness, anti-relaxation and anti-creep, and is suitable for the field of flexible optoelectronic devices such as flexible organic solar cells.

Description

Technical Field

[0001] This invention belongs to the field of polyimide film technology, specifically relating to a polyimide / indium tin oxide composite transparent conductive flexible film material enhanced by molecular pulley effect and its preparation method. Background Technology

[0002] As display and flexible electronics technologies rapidly evolve towards thinner, foldable, and even wearable forms, the market is placing unprecedented demands on the comprehensive performance of core cover glass. As a key component of flexible display devices, cover glass materials not only need to provide reliable mechanical support for the delicate light-emitting and sensing layers beneath, but also must withstand bending, stretching, and impact during long-term service, playing a crucial role in physical protection. Therefore, an ideal cover glass material must possess extremely high optical transparency, excellent thermal stability, low dielectric constant, and outstanding toughness, strength, and fatigue resistance under dynamic deformation.

[0003] Among numerous candidate materials, polyimide has long been considered a representative of high-performance engineering plastics due to its strong designability of molecular structure, outstanding high-temperature resistance, excellent electrical insulation, good mechanical properties, and high chemical stability. However, traditional aromatic polyimides often exhibit a deep yellow color and poor transparency due to the easy formation of charge transfer complexes between molecular chains, making it difficult to meet the high light transmittance requirements of display cover plates. With the rapid growth in demand for flexible optoelectronic devices and ultra-flexible devices that can achieve three-dimensional curved surface bonding and adapt to complex dynamic environments, there is an urgent need to develop CPI films with both high strength and high toughness as transparent conductive substrates. To suppress the formation of charge transfer complexes and improve transparency, common strategies include introducing strong electron-withdrawing groups such as trifluoromethyl (-CF3), constructing twisted and non-coplanar molecular structures, incorporating large-volume side groups, and using alicyclic monomers. Among these, introducing -CF3 is widely considered one of the preferred methods for preparing high-performance colorless polyimides: it can effectively disrupt electron conjugation, suppress charge transfer complexes, and endow the material with good optical transparency, high thermal and chemical stability, excellent solubility and processability, and low hygroscopicity.

[0004] However, while this strategy significantly improves optical and processing performance, it also brings severe challenges to mechanical properties: the introduction of CF3 often significantly reduces intermolecular forces and packing density, leading to decreased material toughness, insufficient modulus and strength, and elongation at break generally below 4%. This makes fluorinated transparent polyimide, while possessing excellent transparency as a flexible cover material, prone to creases, microcracks, and even breakage during repeated bending, failing to meet the stringent mechanical durability requirements of highly reliable flexible displays. Furthermore, when depositing ITO on a CPI substrate to prepare transparent conductive films, insufficient interfacial adhesion and poor interfacial matching between CPI and ITO can cause conductive film failure, thus limiting its application in flexible optoelectronic devices. Therefore, there is an urgent need to develop CPI films that combine high transparency, excellent mechanical properties, and strong interfacial adhesion with ITO.

[0005] The defects of these materials are rooted in their chemical nature. The rigid molecular chains of traditional polyimide films are tightly packed, making it difficult to disperse stress effectively through energy dissipation mechanisms under external forces, resulting in low elongation at break and insufficient toughness. Especially with the introduction of structures such as fluorinated groups to pursue high transparency, their mechanical ductility is sometimes further sacrificed. Therefore, the modification of polyimide materials is no longer just a matter of process optimization, but also requires breakthroughs at the molecular structure design level to balance its inherent rigidity with the flexibility required for flexible applications. Summary of the Invention

[0006] One technical problem solved by this invention is to provide a polyimide / indium tin oxide composite transparent conductive flexible thin film material enhanced by the molecular pulley effect. This composite transparent conductive flexible thin film material maintains high strength and high toughness while also possessing excellent heat resistance and light transmittance. Another technical problem to be solved by this invention is to provide a method for preparing a polyimide / indium tin oxide composite transparent conductive flexible thin film material enhanced by the molecular pulley effect. This method uses magnetron sputtering technology to deposit an ITO conductive film on its surface to obtain a flexible transparent conductive film.

[0007] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows:

[0008] A method for preparing a polyimide / indium tin oxide composite transparent conductive flexible thin film material enhanced by the molecular pulley effect includes the following steps:

[0009] 1) Polyrotaxane was dissolved in an organic solvent, and N,N'-carbonyldiimidazole and ethylenediamine were added to react the mixture. The mixture was then freeze-dried to obtain amino-modified polyrotaxane powder.

[0010] 2) React diamine monomers and dianhydride monomers in an organic solvent to obtain a polyamic acid precursor solution;

[0011] 3) Add the aminated modified polyrotaxane powder obtained in step 1) to the polyamic acid precursor solution prepared in step 2) to react and obtain a polyrotaxane-reinforced polyamic acid precursor solution;

[0012] 4) Add a chemical imidizing agent to the polyrotaxane-reinforced polyamic acid precursor solution obtained in step 3) to carry out an imidization reaction. After coating the reaction solution, perform heat treatment to obtain a polyrotaxane-reinforced polyimide film.

[0013] 5) Deposit a nanoscale indium tin oxide film on the surface of the polyrotaxane-reinforced polyimide film obtained in step 4) to obtain a flexible transparent conductive film on the polyrotaxane-reinforced polyimide substrate.

[0014] Further, in step 1), the mass ratio of polyrotaxane, N,N'-carbonyldiimidazole to ethylenediamine is 1:0.3~0.6:1~2; the freeze-drying temperature is -20℃ and the time is 48~72h.

[0015] Furthermore, in step 2), the dianhydride monomer is a tetracarboxylic acid dianhydride, the structure of which is shown in general formula (1): (1);

[0016] The groups are groups containing benzene rings, which may be the same or different, and are selected from:

[0017] ;

[0018] The diamine monomer is an aromatic diamine, and its structure is shown in general formula (2): (2);

[0019] The groups are groups containing benzene rings, which may be the same or different, and are selected from:

[0020] .

[0021] Furthermore, in step 3), the mass ratio of the amino-modified polyrotaxane powder to the total mass of the diamine monomers and dianhydride monomers in step 2) is 0.001~0.02:1.

[0022] Furthermore, in step 3), the reaction temperature is 25℃ and the reaction time is 4~8h.

[0023] Further, in step 4), the chemical imidizing reagent is acetic anhydride and triethylamine, and the volume ratio of acetic anhydride to triethylamine is 1:1 to 4:1.

[0024] Furthermore, in step 4), the reaction temperature is 25℃ and the reaction time is 0.5~2h; the heat treatment temperature is 100℃ and the treatment time is 8h.

[0025] Further, in step 5), oxygen plasma is first used to clean the surface of the polyrotaxane-reinforced polyimide film, and then a nanoscale indium tin oxide ceramic target is used as the target material to deposit a nanoscale indium tin oxide film by radio frequency magnetron sputtering. Finally, the film is annealed to obtain a flexible transparent conductive film on the polyrotaxane-reinforced polyimide substrate.

[0026] Furthermore, in the nanoscale indium tin oxide ceramic target, the indium oxide content is 80~90 wt%, and the tin oxide content is 10~20 wt%; the power of the radio frequency magnetron sputtering is 50~60W; the thickness of the nanoscale indium tin oxide film is 30~100 nm; the annealing conditions are: heating to 250℃ at a heating rate of 1~3℃ / min, holding at that temperature for 0.5~2h, and then cooling to room temperature at a cooling rate of 1~5℃ / min.

[0027] Furthermore, the method for preparing the polyrotaxane-reinforced polyimide / indium tin oxide composite transparent conductive flexible thin film material based on the molecular pulley effect enhances the flexible transparent conductive film on the substrate.

[0028] Compared with the prior art, the present invention has the following advantages:

[0029] (1) The fluorinated CPI material prepared by this invention maintains high strength and high toughness while also possessing excellent heat resistance and light transmittance, effectively solving the performance trade-off problem commonly found in traditional CPI materials—high light transmittance is often accompanied by a significant decrease in mechanical strength. Polyimide and polyrotaxane form a dynamic physical cross-linking network. When the material is bent or stretched, the polyrotaxane covalently grafted onto the polyimide backbone can slide along the axial direction through cyclic molecules, rapidly dispersing local stress to a larger area. This improves fracture toughness, impact resistance, and creep resistance without sacrificing the overall stiffness of the material, significantly enhancing its fatigue resistance under repeated folding conditions and effectively strengthening its performance in applications such as flexible displays and transparent packaging.

[0030] (2) This invention achieves a dual strengthening effect by introducing PR into CPI to construct a sliding crosslinking network: on the one hand, the molecular pulley properties of PR are utilized to promote the efficient transfer of stress from the CPI matrix to PR, thereby improving the toughness of the material; on the other hand, the mechanical interlocking structure is used as a dynamic crosslinking point to enhance the density of the polymer network, thereby improving the strength of the material. Through this dual effect, a high-strength and high-toughness CPI material has been successfully prepared, which can improve the durability of foldable screen products.

[0031] (3) In this invention, a flexible transparent conductive film is prepared by using the prepared CPI material as a substrate and depositing an ITO conductive film on its surface using magnetron sputtering technology. This conductive film has both high operating temperature and high light transmittance, and exhibits excellent stability after a 180° bending cycle fatigue test. The prepared transparent conductive film can be used as a flexible transparent electrode and is suitable for flexible optoelectronic devices such as flexible organic solar cells. Attached Figure Description

[0032] Figure 1 Stress-strain curves of the flexible transparent conductive film on the PR-enhanced CPI substrate prepared for this application and the flexible transparent conductive film on the CPI substrate without PR.

[0033] Figure 2 The UV-Vis spectra of the flexible transparent conductive film on the PR-enhanced CPI substrate prepared for this application and the flexible transparent conductive film on the CPI substrate without PR. Detailed Implementation

[0034] The present invention will be further illustrated below with reference to specific embodiments. These embodiments are implemented based on the technical solutions of the present invention, and it should be understood that these embodiments are only used to illustrate the present invention and are not intended to limit the scope of the present invention.

[0035] The polyrotaxane (PR) in the following examples is prepared using the method described in Chinese Invention Patent Publication No. CN119463208A: A polyrotaxane with controllable cyclodextrin coverage and its preparation method and Example 1 in application.

[0036] In the following examples, PR stands for polyrotaxane, CPI stands for polyimide, and ITO stands for nanoscale indium tin oxide.

[0037] Example 1

[0038] A method for preparing a polyimide / indium tin oxide composite transparent conductive flexible thin film material enhanced by the molecular pulley effect includes the following steps:

[0039] (1) Weigh 1 g of PR and place it in a 100 mL three-necked round-bottom flask. Dry it overnight in a vacuum oven at 85 °C. Perform nitrogen purging on the system 10 times, and then add 20 mL of anhydrous dimethyl sulfoxide. At room temperature, add 0.37 g of N,N'-carbonyldiimidazole and 1.5 g of ethylenediamine and stir the reaction for 48 h. Transfer the resulting reaction solution to a dialysis bag (molecular weight 10000), dialyze for 3 days, remove most of the water by rotary evaporation, and finally freeze-dry to obtain a white aminated modified PR powder.

[0040] (2) Add 5g of 1,4-bis(4-amino-2-trifluoromethylphenoxy)benzene and 70mL of anhydrous N,N-dimethylacetamide to a 250mL three-necked round-bottom flask purged with nitrogen, and then add 5.19g of 4,4'-(hexafluoroisopropene)phthalic anhydride. The reaction solution is reacted at room temperature and under nitrogen for 5h. During the reaction, N,N-dimethylacetamide is added to dilute to 15%wt to obtain a polyamic acid precursor solution.

[0041] (3) Add 10 mg of aminated modified PR powder obtained in step (1) to the polyamic acid precursor solution obtained in step (2), wherein the total mass ratio of aminated modified PR to diamine monomer and dianhydride monomer is 0.001:1. Mix and stir at room temperature for 6 h to obtain PR-enhanced polyamic acid solution.

[0042] (4) Add chemical imidizing agent (acetic anhydride: triethylamine = 4:1, volume ratio) to PR-reinforced polyamic acid solution, stir and react for 1 hour, and then degas the solution; use a gap-type coating machine to uniformly coat the above reaction solution onto the surface of a glass plate and then transfer it to a vacuum environment. The coating parameters are: coating speed is 15 cm / min, the substrate is selected from glass substrate, the thickness of the substrate is 200~250 μm, the thickness of the PR-reinforced CPI film is 50~80 μm, and heat treatment is carried out at 100℃ for 8 hours to obtain PR-reinforced CPI film.

[0043] (5) The surface of the PR-enhanced CPI film was cleaned using oxygen plasma; the cleaned film and indium tin oxide (ITO) target were loaded into the vacuum sputtering chamber and the vacuum was evacuated to a background vacuum of ≤3×10. -6 The sputtering process was carried out at room temperature. An ITO film with a thickness of 30–100 nm was deposited on the substrate by sputtering at 50–60 W RF power for 10–40 min. Finally, the sample was transferred to a muffle furnace for annealing: the temperature was increased to 250 °C at a rate of 1–3 °C / min, held for 0.5–2 h, and then cooled to room temperature at a rate of 1–5 °C / min, ultimately obtaining a flexible transparent conductive film on a PR-enhanced CPI substrate.

[0044] Example 2

[0045] A method for preparing a polyimide / indium tin oxide composite transparent conductive flexible thin film material enhanced by the molecular pulley effect includes the following steps:

[0046] The difference from Example 1 is that in step (3), the total mass ratio of the aminated PR to the diamine monomer and the dianhydride monomer is 0.003:1, resulting in a flexible transparent conductive film on a PR-enhanced CPI substrate.

[0047] Example 3

[0048] A method for preparing a polyimide / indium tin oxide composite transparent conductive flexible thin film material enhanced by the molecular pulley effect includes the following steps:

[0049] The difference from Example 1 is that in step (3), the total mass ratio of the aminated PR to the diamine monomer and the dianhydride monomer is 0.005:1, resulting in a flexible transparent conductive film on a PR-enhanced CPI substrate.

[0050] Example 4

[0051] A method for preparing a polyimide / indium tin oxide composite transparent conductive flexible thin film material enhanced by the molecular pulley effect includes the following steps:

[0052] The difference from Example 1 is that in step (3), the total mass ratio of the aminated PR to the diamine monomer and the dianhydride monomer is 0.007:1, resulting in a flexible transparent conductive film on a PR-enhanced CPI substrate.

[0053] Example 5

[0054] A method for preparing a polyimide / indium tin oxide composite transparent conductive flexible thin film material enhanced by the molecular pulley effect includes the following steps:

[0055] The difference from Example 1 is that in step (3), the total mass ratio of the aminated PR to the diamine monomer and the dianhydride monomer is 0.01:1, resulting in a flexible transparent conductive film on a PR-enhanced CPI substrate.

[0056] Example 6

[0057] A method for preparing a polyimide / indium tin oxide composite transparent conductive flexible thin film material enhanced by the molecular pulley effect includes the following steps:

[0058] The difference from Example 1 is that in step (3), the total mass ratio of the aminated PR to the diamine monomer and the dianhydride monomer is 0.02:1, resulting in a flexible transparent conductive film on a PR-enhanced CPI substrate.

[0059] Comparative Example 1

[0060] (1) Under an anhydrous and oxygen-free inert gas protection, 5 g of 1,4-bis(4-amino-2-trifluoromethylphenoxy)benzene and 70 mL of anhydrous N,N-dimethylacetamide were added to a three-necked round-bottom flask, followed by 5.19 g of 4,4'-(hexafluoroisopropene)phthalic anhydride. The reaction was carried out at room temperature for 5 h, during which N,N-dimethylacetamide was added to dilute to 15 wt% to obtain a polyamic acid precursor solution.

[0061] (2) Add chemical imidizing reagent (acetic anhydride: triethylamine = 4:1, volume ratio) to the polyamic acid precursor solution obtained in step (1), stir and react for 1 hour, and then degas the solution; after uniformly coating the above reaction solution onto the surface of a glass plate using a gap-type coating machine, transfer it to a vacuum environment, and heat treat it at 100°C for 8 hours to obtain a CPI film.

[0062] (3) A nanoscale ITO film is sputtered on the surface of the obtained CPI film using magnetron sputtering technology. After annealing in a muffle furnace, a flexible transparent conductive film of the CPI substrate is obtained.

[0063] Example 7

[0064] The performance of the flexible transparent conductive films of the PR-enhanced CPI substrates prepared in Examples 1-6 and the flexible transparent conductive films of the CPI substrates prepared in Comparative Example 1 were tested.

[0065] (1) Mechanical property testing

[0066] Flexible transparent conductive films from PR-enhanced CPI substrates prepared in Examples 1-6 and the flexible transparent conductive film from the CPI substrate prepared in Comparative Example 1 were cut into dumbbell-shaped strips for tensile testing. The thickness of the strips was measured to be 75 μm, and the tensile rate was 5 mm / min. The test results are shown in Table 1 and [Table data missing]. Figure 1 .

[0067] Table 1. Maximum fracture stress and fracture strain of Examples 1-6 and Comparative Example 1

[0068]

[0069] From Table 1 and Figure 1 It can be seen that the flexible transparent conductive film of the PR-enhanced CPI substrate prepared in the embodiments of this application has excellent mechanical properties, and its fracture stress and fracture strain are higher than those of Comparative Example 1. With the increase of PR mass fraction, the fracture stress and fracture strain of the film show a trend of first increasing and then decreasing. Among them, the enhancement effect of Example 4 is the best. Compared with Comparative Example 1, its fracture stress is increased by 99% and its fracture strain is increased by 285%.

[0070] (2) Optical performance testing

[0071] The flexible transparent conductive films of PR-enhanced CPI substrates prepared in Examples 1-6 and the flexible transparent conductive film of CPI substrate prepared in Comparative Example 1 were subjected to optical performance tests. The transmittance of the materials was measured using a UV-Vis spectrometer, with a measurement range of 200-800 nm, a sampling interval of 1 nm, and a test temperature of room temperature. The test results are shown in Table 2 and... Figure 2 .

[0072] Table 2 Optical performance of Examples 1-6 and Comparative Example 1

[0073]

[0074] From Table 2 and Figure 2 As can be seen, the flexible transparent conductive film of the PR-enhanced CPI substrate prepared in this application exhibits excellent optical properties, with a transmittance ≥85%, which is close to the transmittance level of the control sample. This indicates that the introduction of PR has a negligible impact on the optical properties of the CPI material. This application can significantly improve the mechanical properties of the material while ensuring its excellent optical properties, effectively solving the technical problem of simultaneously enhancing the mechanical properties and maintaining the optical properties of traditional CPI materials.

[0075] (3) Bending fatigue test

[0076] Flexible transparent conductive films on PR-enhanced CPI substrates prepared in Example 4 and on CPI substrates prepared in Comparative Example 1 were subjected to 10,000 bending cycle fatigue tests using a flexible electronic testing instrument. The test conditions were: bending radius 1 nm, and bending angles of 30°, 90°, and 180°. The test results are shown in Table 3.

[0077] Table 3. Bending cycle fatigue test results of Example 4 and Comparative Example 1

[0078]

[0079] As shown in Table 3, the flexible transparent conductive film with PR-enhanced CPI substrate prepared in Example 4 of this application exhibits excellent bending cycle stability. After 10,000 cycles of 180° bending, the conductivity of this sample remained intact without any failure. In contrast, the conductivity of Comparative Example 1 failed after less than 100 cycles of 180° bending, indicating that the introduction of PR can significantly improve the bending reliability and cycle life of the flexible transparent conductive film.

[0080] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing a polyimide / indium tin oxide composite transparent conductive flexible thin film material enhanced by the molecular pulley effect, characterized in that: Includes the following steps: 1) Polyrotaxane was dissolved in an organic solvent, and N,N'-carbonyldiimidazole and ethylenediamine were added to react the mixture. The mixture was then freeze-dried to obtain amino-modified polyrotaxane powder. 2) The diamine monomer and the dianhydride monomer are reacted in an organic solvent to obtain a polyamic acid precursor solution; the dianhydride monomer is a tetracarboxylic acid dianhydride, the structure of which is shown in general formula (1): (1); The groups are groups containing benzene rings, which may be the same or different, and are selected from: ； The diamine monomer is an aromatic diamine, and its structure is shown in general formula (2): (2); The groups are groups containing benzene rings, which may be the same or different, and are selected from: ； 3) Add the aminated modified polyrotaxane powder obtained in step 1) to the polyamic acid precursor solution prepared in step 2) to react and obtain a polyrotaxane-reinforced polyamic acid precursor solution; the mass ratio of the aminated modified polyrotaxane powder to the total mass of diamine monomers and dianhydride monomers in step 2) is 0.001~0.02:1; 4) Add a chemical imidizing agent to the polyrotaxane-reinforced polyamic acid precursor solution obtained in step 3) to carry out an imidization reaction. After coating the reaction solution, perform heat treatment to obtain a polyrotaxane-reinforced polyimide film. 5) Deposit a nanoscale indium tin oxide film on the surface of the polyrotaxane-reinforced polyimide film obtained in step 4) to obtain a flexible transparent conductive film on the polyrotaxane-reinforced polyimide substrate.

2. The method for preparing the polyimide / indium tin oxide composite transparent conductive flexible thin film material based on the molecular pulley effect enhancement according to claim 1, characterized in that: In step 1), the mass ratio of polyrotaxane, N,N'-carbonyldiimidazole and ethylenediamine is 1:0.3~0.6:1~2; the freeze-drying temperature is -20℃ and the time is 48~72h.

3. The method for preparing the polyimide / indium tin oxide composite transparent conductive flexible thin film material based on the molecular pulley effect enhancement according to claim 1, characterized in that: In step 3), the reaction temperature is 25℃ and the reaction time is 4~8h.

4. The method for preparing the polyimide / indium tin oxide composite transparent conductive flexible thin film material based on the molecular pulley effect enhancement according to claim 1, characterized in that: In step 4), the chemical imidizing reagents are acetic anhydride and triethylamine, with a volume ratio of acetic anhydride to triethylamine of 1:1 to 4:

1.

5. The method for preparing the polyimide / indium tin oxide composite transparent conductive flexible thin film material based on the molecular pulley effect enhancement according to claim 1, characterized in that: In step 4), the reaction temperature is 25℃ and the reaction time is 0.5~2h; the heat treatment temperature is 100℃ and the treatment time is 8h.

6. The method for preparing the polyimide / indium tin oxide composite transparent conductive flexible thin film material based on the molecular pulley effect enhancement according to claim 1, characterized in that: In step 5), oxygen plasma is first used to clean the surface of the polyrotaxane-reinforced polyimide film. Then, using a nanoscale indium tin oxide ceramic target as the target material, a nanoscale indium tin oxide film is deposited by radio frequency magnetron sputtering. Finally, after annealing, a flexible transparent conductive film of polyrotaxane-reinforced polyimide substrate is obtained.

7. The method for preparing the polyimide / indium tin oxide composite transparent conductive flexible thin film material based on the molecular pulley effect enhancement according to claim 6, characterized in that: The nanoscale indium tin oxide ceramic target contains 80-90 wt% indium oxide and 10-20 wt% tin oxide; the power of the radio frequency magnetron sputtering is 50-60 W; the thickness of the nanoscale indium tin oxide film is 30-100 nm; the annealing conditions are: heating to 250°C at a heating rate of 1-3°C / min, holding at that temperature for 0.5-2 h, and then cooling to room temperature at a cooling rate of 1-5°C / min.

8. The flexible transparent conductive film on a polyrotaxane-reinforced polyimide substrate prepared by the method for preparing polyimide / indium tin oxide composite transparent conductive flexible thin film material based on the molecular pulley effect enhancement as described in any one of claims 1 to 7.

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