A multifunctional composite thin film and its preparation method

Abstract

This invention discloses a multifunctional composite film and its preparation method. The film is composed of an inner porous hydrophobic radiative cooling film and an outer hydrophilic film loaded with phase change microcapsules. The hydrophobic layer is prepared by non-solvent-induced phase separation of a hydrophobic radiative cooling material and a pore-forming agent, providing high solar reflectivity and high-to-medium infrared emissivity, enabling all-weather passive radiative cooling. The hydrophilic layer is composed of a hydrophilic radiative cooling material, forming a wetting gradient with the hydrophobic layer, enabling unidirectional sweat drainage and evaporative cooling. The phase change microcapsules are embedded in the hydrophilic layer, mitigating thermal shock caused by sudden temperature changes through phase change heat absorption and release. This invention features a simple process, low cost, and suitability for large-scale production, showing broad application prospects in the field of personal thermal and humidity management.

Description

Technical Field

[0001] This invention belongs to the field of functional materials technology, specifically relating to a multifunctional composite film and its preparation method. Background Technology

[0002] In recent years, global warming and frequent extreme weather events have posed a serious threat to human health and quality of life. While traditional cooling technologies such as fans and air conditioning systems are widely used, their high energy consumption and greenhouse gas emissions contradict the current strategic goal of "carbon peaking and carbon neutrality." In extreme high-temperature environments, the human body is susceptible to heat exhaustion, heat stress, and other diseases; therefore, developing energy-efficient, effective, and sustainable personal thermal management technologies has become a research hotspot.

[0003] As a "second skin" between the human body and the environment, textiles play a crucial role in personal thermal management. Ideal personal thermal management materials should possess the ability to flexibly regulate the thermal and humidity homeostasis of the human body's microenvironment, rather than merely acting as a physical barrier. Human body heat dissipation is primarily achieved through radiation, conduction, evaporation, and convection, among which radiative cooling technology has attracted considerable attention due to its characteristic of requiring no external energy input. This technology utilizes the high reflectivity of materials in the 0.3–2.5 μm solar wavelength range and the high emissivity in the 8–13 μm infrared wavelength range to directly radiate heat into outer space through atmospheric windows, thereby achieving passive cooling.

[0004] However, with increased physical activity and sweat production, radiative cooling alone is insufficient to meet the needs of heat and humidity regulation. If sweat cannot be expelled promptly, it will clog heat dissipation channels, affecting thermal comfort. By constructing an asymmetric wetting structure, sweat can be unidirectionally expelled from the skin side towards the inner and outer layers, evaporating rapidly on the outer layer and utilizing latent heat of vaporization for further cooling. Simultaneously, drastic temperature changes can easily cause discomfort; by introducing phase change materials into the cooling material, temperature buffering can be achieved, mitigating body temperature fluctuations and improving wearing comfort.

[0005] While existing technologies, such as electrospinning, can produce multilayer composite films that achieve some functions, the processes are complex and costly, making large-scale production difficult. Therefore, developing a composite film with a simple preparation process, controllable cost, and multiple functions including radiative cooling, unidirectional moisture conduction, evaporative cooling, and phase change temperature regulation is of significant practical importance and promising application prospects. Summary of the Invention

[0006] This invention discloses a multifunctional composite thin film and its preparation method. The thin film is composed of an inner porous hydrophobic radiation-cooling membrane and an outer hydrophilic membrane loaded with phase change microcapsules, prepared using a solvent-inducing phase separation method. The inner layer provides high solar reflectivity and mid-infrared emissivity to achieve radiation cooling, the hydrophilic-hydrophobic layer forms a wetting gradient to achieve unidirectional moisture conduction and evaporative cooling, and the phase change microcapsules provide a temperature buffer. This invention features a simple process suitable for large-scale production.

[0007] On the one hand, the present invention provides a multifunctional composite film, which adopts the following technical solution: A multifunctional composite film, comprising a hydrophobic layer and a hydrophilic layer, wherein phase change microcapsules are embedded in the hydrophilic layer; The hydrophobic layer is prepared by non-solvent-induced phase separation of hydrophobic radiation cooling material and pore-forming agent, and the hydrophilic layer is prepared by non-solvent-induced phase separation of hydrophilic radiation cooling material. Phase change microcapsules are formed by emulsion polymerization of core and shell materials.

[0008] Preferably, the hydrophobic radiation cooling material is one or more of polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene, and polyvinylidene fluoride-trifluoroethylene; the hydrophilic radiation cooling material is one or more of polyacrylonitrile, hydroxyethyl cellulose, carboxymethyl cellulose, cellulose nanocrystals, silk fibroin, poly(N-isopropylacrylamide), polyacrylic acid, polymethacrylic acid, sodium polyethylene sulfonate, cellulose acetate and its modified forms, hydroxypropyl methylcellulose, and methylcellulose.

[0009] Preferably, the hydrophilic layer further includes a flexibility modifier, which is one or more of polyethylene glycol, thermoplastic polyurethane, polydimethylsiloxane, anionic / cationic flexibility agent, amino-modified organosilicon, polyether-modified organosilicon, hydroxyl silicone oil emulsion, and epoxy derivative; the mass fraction of the flexibility modifier accounts for 5%-10% of the organic solvent.

[0010] Preferably, the pore-forming agent is a polyvinyl alcohol / dimethyl sulfoxide solution, wherein the mass fraction of polyvinyl alcohol is 15%-30% of dimethyl sulfoxide; and the mass fraction of the pore-forming agent in the hydrophobic layer casting solution is 12%-28%.

[0011] Preferably, the core material of the phase change microcapsule is one or more of paraffin, alkanes, and polyethylene glycol, and the shell material is methyl methacrylate.

[0012] On the other hand, the present invention also provides a method for preparing a multifunctional composite thin film, which adopts the following technical solution: A method for preparing a multifunctional composite thin film includes the following steps: S1. Dissolve the flexibility modifier in an organic solvent and heat and stir at 40℃-70℃ for 1-4 hours until completely dissolved. The mass fraction of the flexibility modifier is 5%-10% of the organic solvent. Add the hydrophilic radiative cooling material and heat and stir at 40℃-70℃ for 1-4 hours until completely dissolved to obtain mixture 1. The mass fraction of the hydrophilic radiative cooling material is 5%-15% of the organic solvent. Coat mixture 1 onto a substrate and immerse it in a non-solvent. Let it stand at 25℃-40℃ for 10-180 minutes to separate the phases and form a hydrophilic layer. S2. Dissolve the hydrophobic radiative cooling material in an organic solvent and stir at room temperature for 1-4 hours until completely dissolved to obtain mixture 2; the mass fraction of the hydrophobic radiative cooling material is 10%-30% of the organic solvent. S3. Add the pore-forming agent to mixture 2, heat and stir at 50℃-70℃ for 2-5 hours until homogeneous, to obtain mixture 3; the mass fraction of the pore-forming agent in mixture 3 is 12%-28%; S4. Apply mixture 3 onto the hydrophilic layer obtained in step S1, immerse it in a non-solvent, and let it stand at 25℃-40℃ for 3-5 hours to separate the phases and form a double-layer composite film. Dry it for 8-24 hours. S5. The core material and shell material are emulsified in an emulsifier at a high speed of 5000 r / min to form an O / W type emulsion. The emulsion is then polymerized under the action of an initiator to obtain phase change microcapsules. The phase change microcapsules are loaded onto the surface of the hydrophilic layer of the composite film obtained in step S4 and allowed to stand for 3-8 hours to permeate, thus obtaining a multifunctional composite film.

[0013] Preferably, the stirring speed in steps S1 and S3 is 300-1000 rpm; the stirring speed in step S2 is 300-1000 rpm.

[0014] Preferably, the non-solvent in steps S1 and S4 is one or more of water or alcohol.

[0015] Preferably, the initiator in step S5 is ammonium persulfate.

[0016] Preferably, the organic solvent in steps S1 and S2 is one or more of N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide, acetone, and tetrahydrofuran.

[0017] In summary, the beneficial effects of the present invention are as follows: The beneficial effects of this invention are reflected in its synergistic effect of multiple mechanisms and excellent comprehensive performance. Firstly, this composite film integrates four major functions: radiative cooling, unidirectional moisture conduction, evaporative cooling, and phase change temperature regulation. The inner hydrophobic radiative cooling layer constructs a micro-nano hierarchical porous structure through non-solvent-induced phase separation, achieving a solar reflectivity of up to 98.5% and a mid-infrared emissivity of 95%, enabling all-weather passive cooling without energy consumption. The asymmetric wetting structure and pore gradient design formed by the hydrophilic and hydrophobic layers endow the film with excellent unidirectional moisture conduction properties, with a sweat removal rate of up to 0.5 mL·h. -1 The phase change microcapsules embedded in the hydrophilic layer rapidly evaporate on the surface, utilizing the latent heat of vaporization for secondary cooling; providing up to 175.4 J·g -1 The phase transition enthalpy effectively buffers the thermal shock to the human body caused by drastic changes in external temperature, improving wearing comfort and safety.

[0018] Secondly, this invention offers significant advantages in terms of preparation process and structural design. By employing a solvent-induced phase separation method combined with the synergistic effect of pore-forming agents and flexible modifiers, not only was a well-connected porous structure successfully constructed, but the mechanical properties of the film were also improved through the flexible modifiers, solving the problem of film brittleness under high porosity. Simultaneously, the dispersion uniformity and interfacial bonding of phase change microcapsules in the matrix were enhanced. This preparation process is simple, low-cost, and requires no complex equipment, making it suitable for large-scale industrial production. It provides a practical solution for personal thermal and humidity management that combines efficient cooling and humidity control functions. Attached Figure Description

[0019] Figure 1 In Example 1, 'a' represents the water contact angle of the hydrophilic layer of the high-efficiency radiation-evaporation cooling composite film. Figure 1 In the figure, b represents the water contact angle of the hydrophobic layer of the high-efficiency radiation-evaporation cooling composite film prepared in Example 1; Figure 2 Solar reflectance spectra of the radiation-evaporation cooling composite films prepared in Example 1 and Comparative Example 1; Figure 3 Infrared emission spectra of the radiation-evaporation cooling composite films prepared in Example 1 and Comparative Example 1; Figure 4 Image a shows the unidirectional moisture conduction effect of the high-efficiency radiation-evaporation cooling composite film prepared in Examples 1-9; Figure 4 In Figure b, the high-efficiency radiation-evaporation cooling composite thin film prepared in Examples 1-9 is shown at 400 W / m 2 Temperature change under xenon lamp irradiation; Figure 5This is a comparison of the cooling effects of the radiation-evaporation cooling composite film and cotton cloth prepared in Example 1 and Comparative Example 1 with the environment. Figure 6 The comparison of the smoothness of temperature fluctuation between the radiation-evaporation cooling composite film prepared in Example 1 and Comparative Example 1 and cotton cloth and the environment. Detailed Implementation

[0020] The present invention will be further described in detail below with reference to the embodiments.

[0021] Example Example 1 A high-efficiency radiation-evaporation cooling composite thin film, the specific steps of which are as follows: S1. Place 7.5 wt.% thermoplastic polyurethane in N,N-dimethylformamide, heat in a 50°C water bath and stir magnetically at 800 rpm until completely dissolved; then add 7.5 wt.% polyacrylonitrile, heat at 60°C and stir magnetically at 800 rpm until completely dissolved to obtain mixture 1; use a scraper to uniformly coat mixture 1 onto a glass substrate, place it in water and let it stand, and obtain a hydrophilic layer by a non-solvent-induced phase separation method.

[0022] S2. Add 14% polyvinylidene fluoride-hexafluoropropylene to N,N-dimethylformamide, and stir magnetically at 700 rpm at room temperature until completely dissolved to obtain mixture 2; dissolve 25 wt.% polyethylene glycol in dimethyl sulfoxide, and stir magnetically at 1000 rpm at 80°C until completely dissolved to obtain a pore-forming agent. Add the pore-forming agent to mixture 2, heat and stir at 50°C until uniformly mixed to obtain mixture 3. The mass fraction of the pore-forming agent is 14 wt.%. S3. After quickly coating the mixture 3 onto the hydrophilic layer with a scraper, phase separation is performed. The non-solvent is water, and the phase separation time is 40 minutes. The composite film is then dried at room temperature to obtain a radiation-evaporation cooling film.

[0023] S4. The core material and shell material are shear-emulsified at high speed in an emulsifier to disperse them into an O / W type emulsion. Under the action of an initiator, stable phase change microcapsules are formed. The phase change microcapsules are coated onto the hydrophilic layer of the composite film with a glass rod and left to stand until they are completely embedded in the hydrophilic layer. Finally, a high-efficiency radiation-evaporation cooling composite film with a slow-temperature effect is formed.

[0024] Example 2 A high-efficiency radiation-evaporation cooling composite film, the specific steps of which differ from those of Example 1 are as follows: in step S2, the mass fraction of the pore-forming agent is 12 wt.%, and the remaining steps are the same as those in Example 1.

[0025] Example 3 A high-efficiency radiation-evaporation cooling composite film, the specific steps of which differ from those of Example 1 are as follows: in step S2, the mass fraction of the pore-forming agent is 16 wt.%, and the remaining steps are the same as those in Example 1.

[0026] Example 4 A high-efficiency radiation-evaporation cooling composite film, the specific steps of which differ from those of Example 1 are that the mass fraction of the pore-forming agent in step S2 is 18 wt.%, and the remaining steps are the same as those in Example 1.

[0027] Example 5 A high-efficiency radiation-evaporation cooling composite film, the specific steps of which differ from those of Example 1 are as follows: in step S2, the mass fraction of the pore-forming agent is 20 wt.%, and the remaining steps are the same as those in Example 1.

[0028] Example 6 A high-efficiency radiation-evaporation cooling composite film, the specific steps of which differ from those of Example 1 are as follows: in step S2, the mass fraction of the pore-forming agent is 22 wt.%, and the remaining steps are the same as those in Example 1.

[0029] Example 7 A high-efficiency radiation-evaporation cooling composite film, the specific steps of which differ from those of Example 1 are as follows: in step S2, the mass fraction of the pore-forming agent is 24 wt.%, and the remaining steps are the same as those in Example 1.

[0030] Example 8 A high-efficiency radiation-evaporation cooling composite film, the specific steps of which differ from those of Example 1 are that the mass fraction of the pore-forming agent in step S2 is 26 wt.%, and the remaining steps are the same as those in Example 1.

[0031] Example 9 A high-efficiency radiation-evaporation cooling composite film, the specific steps of which differ from those of Example 1 are as follows: in step S2, the mass fraction of the pore-forming agent is 28 wt.%, and the remaining steps are the same as those in Example 1.

[0032] Comparative Example Comparative Example 1 A radiation-evaporation cooling composite thin film, the specific steps of which are as follows: S1. Place 7.5 wt.% thermoplastic polyurethane in N,N-dimethylformamide, heat and stir in a 50°C water bath until completely dissolved; then add 7.5 wt.% polyacrylonitrile, heat and magnetically stir in a 60°C water bath until completely dissolved to obtain mixture 1, with a stirring speed of 500 rpm; use a scraper to uniformly coat mixture 1 onto a glass substrate, place it in water and let it stand, and obtain a hydrophilic layer by a non-solvent-induced phase separation method.

[0033] S2. 14% polyvinylidene fluoride-hexafluoropropylene was added to N,N-dimethylformamide and magnetically stirred at 700 rpm at room temperature until completely dissolved to obtain mixture 2. 25 wt.% polyethylene glycol was dissolved in dimethyl sulfoxide and magnetically stirred at 1000 rpm at 80°C until completely dissolved to obtain a pore-forming agent. The pore-forming agent was added to mixture 2, and the mixture was heated and stirred at 50°C until uniformly mixed to obtain mixture 3. The pore-forming agent had a mass fraction of 14 wt.%. S3. After quickly coating the mixture 3 onto the hydrophilic layer with a scraper, phase separation is performed. The non-solvent is water, and the phase separation time is 40 minutes. The composite film is then dried to obtain a radiation-evaporation cooling film.

[0034] Test case Test Example 1 The water contact angle of the high-efficiency radiation-evaporation cooling film prepared in Example 1 was tested by capturing continuous dynamic images of water droplets on the film surface using a high-speed camera.

[0035] Figure 1 The test results of the water contact angle of the hydrophilic layer and the hydrophobic layer of the composite film of the present invention are shown, wherein the water contact angle of the hydrophobic layer is 110°, exhibiting typical hydrophobic properties. Figure 1 a), while the water contact angle of the hydrophilic layer is 56°, exhibiting good hydrophilicity ( Figure 1 b); This significant difference in wettability confirms that the bilayer film successfully constructed an asymmetric wetting structure, providing a key driving force for the unidirectional export of sweat from the skin side (hydrophobic layer) to the outer layer (hydrophilic layer) and its rapid evaporation and diffusion.

[0036] Test Example 2 The reflectivity and emissivity of the radiation-evaporation cooling films prepared in Example 1 and Comparative Example 1 were tested. The infrared emissivity and reflectivity were measured using a Fourier transform infrared spectrometer.

[0037] Figure 2 The solar reflectance spectrum shows that the solar reflectance of the film in the 0.3~2.5μm band is slightly improved after the addition of phase change microcapsules, while the overall reflectance remains at a high level of over 98.5%. Figure 3 The infrared emission spectrum shows that the film maintains a high emissivity of over 95% in the 7–14 μm atmospheric window band after the addition of phase change microcapsules. The results indicate that the introduction of phase change microcapsules has virtually no negative impact on the radiative cooling performance of the film, ensuring the effectiveness of its all-weather passive cooling function.

[0038] Test Example 3 The one-way moisture conduction effect of the high-efficiency radiation-evaporation cooling composite films prepared in Examples 1-9 was tested at 400 W / m. 2 Temperature tests were conducted under xenon lamp irradiance.

[0039] Figure 4 The performance variations of the composite films prepared in Examples 1-9 under different pore-forming agent mass fractions are shown. Figure 4 Figure a shows the unidirectional moisture conduction effect. The results indicate that as the pore-forming agent content increases from 12% to 28%, the unidirectional moisture conduction rate of the film first increases and then tends to stabilize, reaching the optimal value (0.5 mL·h) in the range of 20%~24%. -1 This indicates that an appropriate pore-forming agent content helps to construct a well-connected pore structure and improve the efficiency of directional sweat transport. Figure 4 Figure b shows the temperature change under xenon lamp irradiation at 400 W / m². The results show that the temperature of all films rises rapidly in the initial stage of irradiation and then tends to reach equilibrium. Among them, the films with a pore-forming agent content of 20%~24% have the lowest equilibrium temperature, exhibiting the best radiative cooling performance. Combining the two figures, it can be seen that when the pore-forming agent content is 20%~24%, the films possess both excellent unidirectional moisture conduction and radiative cooling performance.

[0040] Test Example 4 The cooling effects of the radiation-evaporation cooling films prepared in Example 1 and Comparative Examples 1-2 were compared. The cooling effect of the films was tested at a weather station throughout the day as time changed.

[0041] Figure 5 This study demonstrates a comparison of the actual cooling effects of a multifunctional composite film (BMF) with added phase change microcapsules, a composite film (BCM) without added phase change microcapsules, and ordinary cotton fabric in an outdoor environment. Test results show that, due to their high solar reflectivity and high infrared emissivity, the surface temperatures of BMF and BCM are consistently significantly lower than the ambient temperature and the cotton fabric temperature, exhibiting excellent passive radiative cooling performance. Furthermore, the BMF, due to the heat storage function of the phase change microcapsules, shows more gradual temperature fluctuations during the midday high-temperature period, further verifying that the film of this invention achieves both efficient cooling and temperature buffering capabilities.

[0042] Figure 6 Three samples—a multifunctional composite film (BMF) with added phase change microcapsules, a composite film (BCM) without added phase change microcapsules, and ordinary cotton cloth—were shown, along with their temperature change smoothness curves compared to ambient temperature fluctuations. The results showed that BMF exhibited the smallest temperature fluctuation amplitude and the smoothest curve due to the heat storage and release function of the phase change microcapsules, demonstrating excellent temperature buffering capacity. In contrast, BCM and cotton cloth showed more drastic temperature fluctuations, proving that the introduction of phase change microcapsules significantly improved the thermal comfort and resistance to thermal shock of the film.

[0043] The above are all preferred embodiments of the present invention and are not intended to limit the scope of protection of the present invention. Therefore, all equivalent changes made in accordance with the structure, shape and principle of the present invention should be covered within the scope of protection of the present invention.

Claims

1. A multifunctional composite film, characterized in that, The composite film includes a hydrophobic layer and a hydrophilic layer, wherein phase change microcapsules are embedded in the hydrophilic layer. The hydrophobic layer is prepared by non-solvent-induced phase separation of a hydrophobic radiation cooling material and a pore-forming agent, and the hydrophilic layer is prepared by non-solvent-induced phase separation of a hydrophilic radiation cooling material. The phase change microcapsules are formed by emulsion polymerization of the core material and the shell material.

2. The multifunctional composite film according to claim 1, characterized in that, The hydrophobic radiation cooling material is one or more of polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene, and polyvinylidene fluoride-trifluoroethylene; the hydrophilic radiation cooling material is one or more of polyacrylonitrile, hydroxyethyl cellulose, carboxymethyl cellulose, cellulose nanocrystals, silk fibroin, poly(N-isopropylacrylamide), polyacrylic acid, polymethacrylic acid, sodium polyethylene sulfonate, cellulose acetate and its modified forms, hydroxypropyl methylcellulose, and methylcellulose.

3. The multifunctional composite film according to claim 1, characterized in that, The hydrophilic layer further includes a flexibility modifier, which is one or more of polyethylene glycol, thermoplastic polyurethane, polydimethylsiloxane, anionic / cationic flexibility agent, amino-modified organosilicon, polyether-modified organosilicon, hydroxyl silicone oil emulsion, and epoxy derivative; the mass fraction of the flexibility modifier is 5%-10% of the organic solvent.

4. The multifunctional composite film according to claim 1, characterized in that, The pore-forming agent is a polyvinyl alcohol / dimethyl sulfoxide solution, wherein the mass fraction of polyvinyl alcohol is 15%-30% of dimethyl sulfoxide; the mass fraction of the pore-forming agent in the hydrophobic layer casting solution is 12%-28%.

5. The multifunctional composite film according to claim 1, characterized in that, The core material of the phase change microcapsule is one or more of paraffin, alkanes, and polyethylene glycol, and the shell material is methyl methacrylate.

6. A method for preparing a multifunctional composite thin film as described in any one of claims 1-5, characterized in that, Includes the following steps: S1. Dissolve the flexibility modifier in an organic solvent and heat and stir at 40℃-70℃ for 1-4 hours until completely dissolved. The mass fraction of the flexibility modifier is 5%-10% of the organic solvent. Add the hydrophilic radiative cooling material and heat and stir at 40℃-70℃ for 1-4 hours until completely dissolved to obtain mixture 1. The mass fraction of the hydrophilic radiative cooling material is 5%-15% of the organic solvent. Coat mixture 1 onto a substrate and immerse it in a non-solvent. Let it stand at 25℃-40℃ for 10-180 minutes to separate the phases and form a hydrophilic layer. S2. Dissolve the hydrophobic radiative cooling material in an organic solvent and stir at room temperature for 1-4 hours until completely dissolved to obtain mixture 2; the mass fraction of the hydrophobic radiative cooling material is 10%-30% of the organic solvent. S3. Add the pore-forming agent to mixture 2, heat and stir at 50℃-70℃ for 2-5 hours until homogeneous, to obtain mixture 3; the mass fraction of the pore-forming agent in mixture 3 is 12%-28%; S4. Apply mixture 3 onto the hydrophilic layer obtained in step S1, immerse it in a non-solvent, and let it stand at 25℃-40℃ for 3-5 hours to separate the phases and form a double-layer composite film. Dry it for 8-24 hours. S5. The core material and shell material are emulsified in an emulsifier at a high speed of 5000 r / min to form an O / W type emulsion. The emulsion is then polymerized under the action of an initiator to obtain phase change microcapsules. The phase change microcapsules are loaded onto the surface of the hydrophilic layer of the composite film obtained in step S4 and allowed to stand for 3-8 hours to allow them to permeate, thus obtaining the multifunctional composite film.

7. The preparation method according to claim 6, characterized in that, The stirring speed in steps S1 and S3 is 300-1000 rpm; the stirring speed in step S2 is 300-1000 rpm.

8. The preparation method according to claim 6, characterized in that, The non-solvent mentioned in steps S1 and S4 is one or more of water or alcohol.

9. The preparation method according to claim 6, characterized in that, The initiator mentioned in step S5 is ammonium persulfate.

10. The preparation method according to claim 6, characterized in that, The organic solvent in steps S1 and S2 is one or more of N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide, acetone, and tetrahydrofuran.

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