Method for preparing lignin Janus gel based on phase separation

Abstract

The invention discloses a method for preparing lignin Janus gel based on phase separation, which is based on the characteristic that lignin has amphoteric groups, a lignin suspension is subjected to phase separation in a drying process to form an electrostatic gradient, and meanwhile, a flexible network is constructed and the electrostatic gradient is fixed by utilizing a dynamic Schiff alkali reaction of gelatin and protocatechualdehyde, so that the lignin Janus gel is prepared. Therefore, the lignin gel with the Janus characteristic and the self-healing property is obtained. In the drying process, hydrophobic groups of the lignin are driven by hydrophobic acting force to migrate to a gas-liquid interface, hydrophilic groups deposit to a solid-liquid interface under the action of gravity and hydrogen bonds to form an electrostatic gradient, and meanwhile, a dynamic network of gelatin-protocatechualdehyde can fix the gradient, so that the electrostatic gradient is formed. Finally, the Janus structure with the hydrophobic and rough upper surface and the hydrophilic and smooth lower surface is constructed. The lignin Janus gel has the advantages of good flexibility, good elasticity, good stability, good adhesion performance and the like, the preparation process is simple and convenient, and the lignin Janus gel is nontoxic and harmless to the environment and easy to recycle.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of natural high molecular materials, and particularly relates to a method for preparing a lignin Janus gel based on phase separation. BACKGROUND

[0002] Lignin, as a renewable aromatic polymer with abundant reserves in nature, widely exists in plant cell walls, has degradability, biocompatibility and rich functional groups, and has attracted much attention in the field of functional materials in recent years. However, traditional lignin-based materials are mostly isotropic structures, and their single interface characteristics limit the application potential in complex environments, such as the urgent need for materials with asymmetric functions in the fields of oil-water separation, intelligent response devices and the like.

[0003] Janus materials have unique anisotropic structures, such as hydrophilic / hydrophobic, conductive / insulating and the like, and exhibit significant advantages in the fields of interfacial catalysis, sensors, biomimetic materials and the like. At present, the preparation of Janus materials mostly depends on synthetic polymers or inorganic nanoparticles. However, these methods often involve toxic solvents or complex processes, and the raw materials are not renewable, which is contradictory to the concept of green chemistry. In addition, the existing lignin modification technology needs to rely on pretreatment to improve the dispersibility or mechanical strength, and is still limited to isotropic structures. The construction of asymmetric Janus structure usually needs complex process, and the spontaneous gradient arrangement of untreated lignin has not been realized.

[0004] As a controllable microstructure construction means, the phase separation method has been successfully used to prepare polymer Janus gel materials, but its application in biomass polymer systems is still blank. This is because the lignin molecular chain has high rigidity and weak self-assembly ability, and it is difficult to form stable asymmetric structures through the conventional phase separation process. Therefore, the development of a Janus gel material based on lignin needs to solve the problem of controllable phase separation of lignin, and also needs to realize the selective distribution of functional groups in space, so as to break through the performance bottleneck of traditional lignin materials and expand its application value in emerging fields such as environmental remediation and flexible electronics. SUMMARY

[0005] The application aims at the deficiency of the existing lignin gel functionality, and provides a method for preparing a lignin Janus gel based on phase separation.

[0006] To achieve the above-mentioned purpose, the application adopts the following technical scheme: A method for preparing a lignin Janus gel based on phase separation, which is based on the characteristics of lignin having amphiphilic groups, causing the lignin suspension to undergo phase separation to form an electrostatic gradient during the drying process, while using the dynamic Schiff base reaction of gelatin and protocatechuic aldehyde to construct a flexible network and fix the electrostatic gradient, thereby obtaining a lignin gel with Janus characteristics and self-healing properties; it specifically includes the following steps: (1) The lignin powder is added to water and stirred uniformly, and water bath heating is performed, to obtain a uniformly dispersed lignin suspension; (2) The lignin suspension obtained in step (1) is mixed with a gelatin solution, and protocatechuic aldehyde is added, and the reaction is carried out under stirring and water bath heating, and then it is poured into a culture dish for room temperature drying, thereby preparing the lignin Janus gel.

[0007] Further, the temperature of the water bath heating in step (1) is 90°C, and the time is 30 min.

[0008] Further, the concentration of the lignin suspension obtained in step (1) is 1wt%-10wt%.

[0009] Further, the concentration of the gelatin solution in step (2) is 5wt%.

[0010] Further, the amount of the gelatin solution used in step (2) is converted according to the mass ratio of the gelatin contained therein to the lignin as 5:1-5:10, and the mass ratio of the protocatechuic aldehyde added to the gelatin is 1:1.

[0011] Further, the temperature of the reaction in step (2) is 80-95°C, and the time is 15-30 min.

[0012] Further, the time of the room temperature drying in step (2) is 24h-48h.

[0013] The upper surface of the obtained lignin Janus gel is hydrophobic and rough, and the lower surface is hydrophilic and smooth.

[0014] Lignin has hydrophilic groups such as phenolic hydroxyl, alcoholic hydroxyl, carboxyl, and hydrophobic groups such as benzene ring and methoxy, and thus has amphiphilicity. During the drying process, the hydrophobic groups of lignin migrate to the gas-liquid interface under the driving force of hydrophobic interaction, while the hydrophilic groups deposit to the solid-liquid interface under the action of gravity and hydrogen bonding, forming an electrostatic gradient, while gelatin and protocatechuic aldehyde construct a flexible network through a dynamic Schiff base reaction and fix the gradient, thereby constructing a Janus structure with an upper surface that is hydrophobic and rough, and a lower surface that is hydrophilic and smooth.

[0015] The present application constructs a lignin Janus gel with a three-dimensional network structure by dynamic covalent cross-linking of the aldehyde group of protocatechuic aldehyde with the amino group of gelatin, in combination with the synergistic reinforcement effect of the rigid structure of lignin. By introducing the aldehyde group of protocatechuic aldehyde into the gelatin-lignin system, a reversible dynamic cross-linking network is formed through Schiff base reaction with the amino group on the gelatin molecular chain. The breaking and reconstruction mechanism of this dynamic bond endows the lignin Janus gel with excellent rebound performance and self-healing properties. At the same time, by adjusting the composite ratio of lignin and gelatin, the rigid characteristics of the phenylpropane structural unit in lignin and the auxiliary cross-linking effect of the polyphenol hydroxyl group, the mechanical strength of the lignin Janus gel is significantly improved. The introduction of the Janus group of lignin realizes the gradient construction of the gel network, and finally a Janus gel with high mechanical properties and Janus lignin is obtained, which shows important application value in the fields of flexible electronic devices and biomedicine.

[0016] The present application has the following advantages: The present application does not require pretreatment of lignin, and can realize the electrostatic gradient arrangement of the Janus group through spontaneous phase separation and dynamic network locking in a one-step drying process, realize the functional heterogenization of the two sides of the gel, and construct a Janus gel with good flexibility, good elasticity, good stability, good adhesion and other advantages. The present application solves the problems of weak mechanical properties and single function of traditional gel materials, and has the advantages of simple preparation process, green and non-toxic, and easy product recovery. BRIEF DESCRIPTION OF DRAWINGS

[0017] Figure 1 The scanning electron microscope (SEM) image of the cross section of the lignin Janus gel obtained in Example 1.

[0018] Figure 2 The scanning electron microscope (SEM) images and 3D profile images of the upper and lower surfaces of the lignin Janus gel obtained in Example 1.

[0019] Figure 3 The contact angle change of the upper and lower surfaces of the lignin Janus gel obtained in Examples 1-3.

[0020] Figure 4 The stress-strain curve graph of the lignin Janus gel obtained in Example 1 before and after self-healing. DETAILED DESCRIPTION

[0021] A method for preparing a lignin Janus gel based on phase separation, comprising the following steps: (1) wood lignin powder is added into water and stirred uniformly, and heated at 90℃ in water bath for 30min to obtain a uniform lignin suspension with a concentration of 1wt%-10wt%; (2) The gelatin is added into water and stirred to obtain a gelatin solution with a concentration of 5wt%, then the obtained gelatin solution is mixed with the lignin suspension according to a mass ratio of gelatin to lignin of 5:1-5:10, and protocatechuic aldehyde is added according to a mass ratio of 1:1 to the gelatin, and the mixture is heated under stirring at 80-95°C for 15-30min in a water bath, then the mixture is poured into a culture dish and dried at room temperature for 24-48h, thereby obtaining the lignin Janus gel.

[0022] In order to make the content of the present application more convenient to understand, the technical solutions of the present application are further described below in combination with specific embodiments, but the present application is not limited thereto.

[0023] Unless otherwise specified, the various raw materials, reagents, instruments and equipment used in the present application can be purchased from the market or can be prepared by existing methods.

[0024] Example 1 5g of lignin is mixed with 95g of water and heated in a water bath at 90°C for 30min to obtain a uniform lignin suspension. 5g of gelatin is added to 95g of distilled water, and the gelatin is dissolved in the water bath at 80°C under stirring to prepare a gelatin solution. Then the prepared lignin suspension and gelatin solution are mixed, and 5g of protocatechuic aldehyde is added, and the mixture is reacted in a water bath at 90°C for 20min to obtain a uniform solution. The solution is poured into a culture dish and dried at room temperature for 36h to obtain a lignin Janus gel.

[0025] Example 2 1g of lignin is mixed with 99g of water and heated in a water bath at 90°C for 30min to obtain a uniform lignin suspension. 5g of gelatin is added to 95g of distilled water, and the gelatin is dissolved in the water bath at 80°C under stirring to prepare a gelatin solution. Then the prepared lignin suspension and gelatin solution are mixed, and 5g of protocatechuic aldehyde is added, and the mixture is reacted in a water bath at 90°C for 20min to obtain a uniform solution. The solution is poured into a culture dish and dried at room temperature for 48h to obtain a lignin Janus gel.

[0026] Example 3 2.5g of lignin is mixed with 97.5g of water and heated in a water bath at 90°C for 30min to obtain a uniform lignin suspension. 5g of gelatin is added to 95g of distilled water, and the gelatin is dissolved in the water bath at 80°C under stirring to prepare a gelatin solution. Then the prepared lignin suspension and gelatin solution are mixed, and 5g of protocatechuic aldehyde is added, and the mixture is reacted in a water bath at 90°C for 20min to obtain a uniform solution. The solution is poured into a culture dish and dried at room temperature for 48h to obtain a lignin Janus gel.

[0027] Figure 1The scanning electron microscope (SEM) image of the cross-section of the lignin Janus gel obtained in Example 1. The asymmetric gel structure of the dense layer and the porous layer is clearly shown in the figure. The pores of the porous layer are uniformly distributed, and have a three-dimensional network structure, which is formed because the internal solvent slowly diffuses out, leaving a coherent channel, forming a stable porous framework. The dense layer has very low porosity, and the surface is smooth, which is formed by the close packing of lignin and gelatin when the gel is formed.

[0028] Figure 2 The scanning electron microscope (SEM) image and 3D profile of the upper and lower surfaces of the lignin Janus gel obtained in Example 1. It can be observed in the figure that the upper surface of the gel presents a rough and porous structure, and the lower surface is smooth and has a much smaller roughness than the upper surface. The 3D profile further confirms that the upper surface has a significant height fluctuation, and contains recesses and holes, and the lower surface is smooth and has no holes, which is consistent with the results of the SEM cross-section image. This is because microphase separation occurs during the gel curing process. That is, when the solvent volatilizes, the upper surface of the Janus gel is first exposed to the air, and the solvent molecules trapped in the polymer network can freely escape, resulting in a rough and porous structure; the bottom is in close contact with the mold, which prevents the solvent from freely volatilizing from the bottom, reducing the possibility of forming holes due to volatilization, and during the curing process, the heavier components or unreacted substances may slightly settle, thereby forming a more dense and uniform layer of smooth surface on the contact surface.

[0029] Figure 3 The contact angle change of the upper and lower surfaces of the lignin Janus gel obtained in Examples 1-3. Figure 3 It is shown that as the lignin content increases, the difference in contact angle between the upper and lower surfaces of the gel increases, and the upper surface is hydrophilic and the lower surface is hydrophobic. This is because lignin molecules are rich in aromatic rings and non-polar groups, making them inherently hydrophobic, and the hydrophobic aggregation of lignin is more significant at high concentrations, which increases the degree of phase separation, so the higher the lignin content, the greater the proportion of hydrophobic components in the gel, and the contact angle of the upper surface increases, providing a basis for significant asymmetric wettability.

[0030] Figure 4 The stress-strain curve of the lignin Janus gel obtained in Example 1 before and after self-healing. The figure shows that the self-healing efficiency of the lignin Janus gel is 83.33%, which is due to the dynamic imine bond crosslinking between protocatechuic aldehyde and gelatin and the high-density hydrogen bond network formed between the components, which endows the lignin Janus gel with excellent self-repairing ability.

[0031] The above only describes the preferred embodiments of the present application, and any equivalent changes and modifications made within the scope of the patent application of the present application shall be included in the scope of the present application.

Claims

1. A method for preparing a lignin Janus gel based on phase separation, characterized in that: The lignin suspension is phase-separated to form an electrostatic gradient during the drying process, and a flexible network is constructed and the electrostatic gradient is fixed by using the dynamic Schiff base reaction of gelatin and protocatechuic aldehyde, so that a lignin gel with Janus characteristics and self-healing property is obtained.

2. The method for preparing lignin Janus gel based on phase separation according to claim 1, characterized in that: It comprises the following steps: (1) lignin powder is added into water and stirred uniformly, and water bath heating is performed to obtain a uniformly dispersed lignin suspension; (2) the lignin suspension obtained in step (1) is mixed with a gelatin solution, and protocatechuic aldehyde is added, and reaction is performed under stirring and water bath heating, and then it is poured into a culture dish for room temperature drying, so that the lignin Janus gel is prepared.

3. The method for preparing lignin Janus gel based on phase separation according to claim 2, characterized in that: The temperature of the water bath heating in step (1) is 90°C, and the time is 30 min.

4. The method for preparing lignin Janus gel based on phase separation according to claim 2, characterized in that: The concentration of the lignin suspension obtained in step (1) is 1wt%-10wt%.

5. The method for preparing lignin Janus gel based on phase separation according to claim 2, characterized in that: The concentration of the gelatin solution in step (2) is 5wt%.

6. The method for preparing lignin Janus gel based on phase separation according to claim 2, characterized in that: The amount of the gelatin solution used in step (2) is converted according to the mass ratio of the gelatin contained therein to lignin of 5:1-5:10, and the mass ratio of the protocatechuic aldehyde added to the gelatin is 1:

1.

7. The method for preparing lignin Janus gel based on phase separation according to claim 2, characterized in that: The temperature of the reaction in step (2) is 80-95°C, and the time is 15-30 min.

8. The method for preparing lignin Janus gel based on phase separation according to claim 2, characterized in that: The time of the room temperature drying in step (2) is 24h-48h.

9. A lignin Janus gel prepared according to any one of the methods of claims 1-8, characterized by: The upper surface of the lignin Janus gel is hydrophobic and rough, and the lower surface is hydrophilic and smooth.

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