Triply structured polyacrylonitrile nanofiber composite membrane, preparation method and application

Abstract

This invention discloses a triple-structured polyacrylonitrile nanofiber composite membrane, its preparation method, and its applications. A double-network bonded structure is formed by electrospinning polyacrylonitrile nanofibers and nano-spider webs, interweaving them with nanobeaded fibers loaded with titanium dioxide and reduced graphene oxide photocatalysts obtained through electrostatic spraying. This interweaves the nanobeaded fibers to form a triple-structured polyacrylonitrile nanofiber composite membrane. This results in a fiber membrane carrier with excellent mechanical properties, and a nanofiber composite membrane loaded with photocatalysts exhibiting excellent photocatalytic performance, and it is reusable. The triple-structured polyacrylonitrile nanofiber composite membrane provided by this invention has a simple preparation process, mild reaction conditions, low energy consumption, and is environmentally friendly. It can be used for filtration and photodegradation treatment of dyeing and printing wastewater, and has wide applications.

Description

Technical Field

[0001] This invention relates to a triple-structured polyacrylonitrile nanofiber composite material, its preparation method, and its application, belonging to the field of photocatalytic nanocomposite materials technology. Background Technology

[0002] In recent years, people have gained a clearer understanding and have paid close attention to the serious harm caused by the discharge of dye wastewater into the environment, which affects human and other organisms' health. Photocatalytic oxidation for dye degradation, with its mild reaction conditions and strong oxidizing power, has attracted widespread attention from researchers.

[0003] Semiconductor TiO2 has advantages such as high stability, low cost, non-toxicity, and a high number of redox sites. Combining it with reduced graphene oxide (RGO), which has high light transmittance, high conductivity, and a large specific surface area, can improve the light absorption performance of the material, reduce the recombination rate of photogenerated electron-hole pairs, and enhance the photocatalytic performance of the material.

[0004] Nanoscale photocatalyst particles suffer from drawbacks such as easy agglomeration and difficulty in recycling during use, while polymer nanofibers have advantages such as large specific surface area, numerous reaction sites, and simple preparation, thus they are widely used as photocatalyst support materials. Existing methods for preparing fiber-based photocatalyst materials include in-situ growth, photocatalytic reduction of graphene oxide, and spinning after mixing the photocatalyst with spinning solution. In-situ growth involves first generating immature photocatalyst seeds on the fiber, and then further maturing them through a solvothermal reaction (see references: Angewandte Chemie-international Edition, 2016, 55(42): 13224-8., Journal of Environmental Chemical Engineering, 2021, 9(5): 106250.). However, the high temperature and pressure during the solvothermal reaction process can cause irreversible damage to the fiber membrane, which is detrimental to its subsequent applications. Dispersing the photocatalyst in the spinning solution before spinning affects spinnability, and most of the photocatalyst particles are coated in the fiber, making it difficult for them to receive sufficient light during the photocatalytic reaction, which is not conducive to the photocatalyst fully exerting its photocatalytic effect.

[0005] The complex environment of dye wastewater due to various dye wastewater treatment devices and the impact of water flow makes the preparation of nanofiber membranes with both mechanical and photocatalytic properties essential. Currently, methods for reinforcing electrospun nanofiber membranes mainly include heat treatment, blending with rigid nanoparticles or polymers, chemical crosslinking, and improving the orientation degree of nanofibers. Heat treatment is a post-treatment performed after the fiber membrane is formed, and the process is cumbersome. Methods such as blending with rigid nanoparticles or polymers, chemical crosslinking, and improving the orientation degree of nanofibers to enhance nanofiber membranes suffer from problems such as nanoparticle aggregation causing defects in the nanofiber membrane, compatibility issues between polymer materials, crosslinking agent waste liquid, and complex processes. Existing technology reports a method for preparing polyacrylonitrile (PAN) nanofiber membranes with a nanofiber / spider web structure via electrospinning. The structure of the fiber membrane is a unique double-network structure formed by the fusion of 2D PAN nanospider webs and 3D fiber scaffold networks. The fiber membrane exhibits excellent mechanical properties, with a tensile strength of up to 8.5 MPa and an elongation at break of 70% (see: Advanced Functional Materials, 2019, 29(39): 1904108). Other researchers have used a one-step co-electrospinning method to prepare three-dimensional polyethylene terephthalate / thermoplastic polyurethane composite nanofibers (PET / TPU-CNF) with a beaded structure. The beaded structure interconnects the PET nanofibers, resulting in a tensile strength of 4.33 MPa for the fiber membrane, compared to only 2.33 MPa for the PET nanofiber membrane (see: Powder Technology, 2021, 387: 136-45).

[0006] Currently, the method of establishing connections between fibers to enhance the mechanical properties of fiber membranes is limited to nanofibers of two sizes. Materials with three sizes of nanofibers interconnected to enhance the mechanical properties of fiber membranes have not yet been reported, nor have fiber-based photocatalytic materials that combine mechanical and photocatalytic properties been reported. Summary of the Invention

[0007] This invention addresses the shortcomings of existing technologies by providing a triple-structured polyacrylonitrile nanofiber membrane with excellent photocatalytic and mechanical properties, a simple preparation process, its preparation method, and its applications.

[0008] The technical solution for achieving the objective of this invention provides a method for preparing a triple-structured polyacrylonitrile nanofiber composite membrane, comprising the following steps:

[0009] 1. Graphene oxide is added to deionized water and ultrasonically treated to obtain a graphene oxide dispersion with a mass fraction of 0.1–0.25 g / L. Acetic acid with a mass fraction of 11.8–21% is then added. Tetrabutyl titanate is dissolved in anhydrous ethanol to prepare an anhydrous ethanol solution of tetrabutyl titanate with a concentration of 75–90 g / L. The graphene oxide dispersion containing acetic acid is added dropwise to the anhydrous ethanol solution of tetrabutyl titanate. After stirring and mixing evenly, a solvothermal reaction is carried out at a temperature of 145–155 °C for 8–15 h. The solvent is then removed by filtration, and the product is washed and dried to obtain a granular photocatalyst.

[0010] 2. Disperse the photocatalyst prepared in step 1 in an N,N-dimethylformamide solution at a mass fraction of 1.2-5.5%, then add polyacrylonitrile at a mass fraction of 5-10%, stir until fully dissolved to obtain an electrostatic spraying solution; dissolve polyacrylonitrile in DMF at a mass fraction of 10-18%, then add tetrabutylammonium chloride at a mass fraction of 2-4% to obtain an electrospinning solution;

[0011] 3. The electrospinning solution and the electrostatic spraying solution are placed in the propulsion pumps of the electrospinning and electrostatic spraying processes, respectively, and a triple-structured polyacrylonitrile nanofiber composite membrane is obtained by using the electrospinning and electrostatic spraying processes in opposite directions.

[0012] The preferred embodiment is as follows: in the electrospinning solution, the mass fraction of polyacrylonitrile is 18%, and the mass fraction of tetrabutylammonium chloride is 2%; the polyacrylonitrile is industrial grade with a weight average molecular weight Mw of 50,000 and a ratio of 1:1; in the electrostatic spraying solution, the mass fraction of photocatalyst is 2.3%, and the mass fraction of polyacrylonitrile is 7.5%; the polyacrylonitrile is industrial grade.

[0013] The electrostatic spraying process conditions are: voltage 9–11 kV, and injection pump feed rate 0.7–0.9 mL / h. -1 The distance between the needle and the roller receiving device is 4–6 cm; the electrospinning process conditions are a voltage of 12–15 kV and an injection pump feed rate of 0.1–0.3 mL·h. -1 The distance between the needle tip and the roller receiving device is 18-20cm, and the ambient humidity is 33-37%.

[0014] The technical solution of the present invention also includes a triple-structured polyacrylonitrile nanofiber composite membrane obtained by the above preparation method. The nanofibers obtained by electrospinning process are fused together with the nanospider web to form an adhesive double network structure, which is interwoven with the nanobeaded fiber structure obtained by electrostatic spraying process to form a triple-structured polyacrylonitrile nanofiber composite membrane.

[0015] The present invention discloses a triple-structured polyacrylonitrile nanofiber composite membrane, wherein the diameter of the nanofibers is 750-800 nm, the diameter of the nanospider web fibers is 20-30 nm, and the diameter of the nanobead fibers is 100-150 nm.

[0016] This invention provides an application of a triple-structured polyacrylonitrile nanofiber composite membrane for filtering and photodegrading dyeing and printing wastewater.

[0017] Compared with the prior art, the advantages of the present invention are as follows:

[0018] 1. The present invention uses a triple-structured nanofiber composite membrane formed by electrospinning and electrostatic spraying, which has better mechanical properties than electrospun nanofiber membranes with a dual-network structure. Compared with fiber membrane reinforcement methods such as heat treatment, rigid nanoparticle or polymer blending, chemical crosslinking, and improving the orientation degree of nanofibers, the preparation method provided by the present invention has the advantages of being simple, mild, and requiring less chemical consumption.

[0019] 2. Using the preparation method provided by this invention, the loading of the photocatalyst TiO2@RGO is completed simultaneously with the formation of nanofibers, avoiding damage to the mechanical properties of the fiber membrane caused by high temperature and high pressure. The fiber membrane has both excellent photocatalytic performance and mechanical properties. Attached Figure Description

[0020] Figure 1 , 2 These are, respectively, X-ray diffraction patterns and SEM images of samples obtained in each step of the preparation of triple-structured nanofiber composite membranes provided in the embodiments of the present invention;

[0021] Figure 3 These are SEM images and structural schematic diagrams of the triple-structured nanofiber composite membrane sample provided in this embodiment of the invention.

[0022] Figure 4 These are scanning electron microscope (SEM) images of the triple-structured nanofiber composite membrane sample provided in this embodiment of the invention at different locations when the tensile elongation is 50%.

[0023] Figure 5 Tensile stress-strain curves of a triple-structured polyacrylonitrile nanofiber composite membrane sample and a comparative example provided for the implementation of this invention.

[0024] Figure 6 The curves showing the changes in UV-Vis absorption spectra of different dye solutions under different conditions over time for samples of the triple-structured polyacrylonitrile nanofiber composite membrane prepared for the present invention after pretreatment.

[0025] Figure 7This is a curve showing the photodegradation efficiency of the triple-structured polyacrylonitrile nanofiber composite membrane sample prepared in this embodiment of the invention on methyl orange (MO) aqueous solution after 5 repeated uses.

[0026] Figure 8 This is a diagram illustrating the effect of applying the triple-structured polyacrylonitrile nanofiber composite membrane sample prepared in the embodiments of the present invention to the treatment of silk dyeing wastewater.

[0027] Figure 3 Among them, 1. nanofibers; 2. nanospider webs; 3. nanobeaded fibers; 4. TiO2@RGO-1.5 beads. Implementation

[0028] The technical solution of the present invention will be further described below with reference to the accompanying drawings and examples. Example

[0029] Preparation of photocatalyst: 4.5 g of graphene oxide (GO) was added to 30 mL of deionized water and sonicated for 2 hours to obtain a GO dispersion. 1.3 g of tetrabutyl titanate was dissolved in 15 mL of anhydrous ethanol. 4 mL of acetic acid was added to the GO dispersion, and then the GO dispersion containing acetic acid was added dropwise to the anhydrous ethanol solution of tetrabutyl titanate. After mixing and stirring for 2 hours, the mixture was transferred to a polytetrafluoroethylene liner and placed in a stainless steel reactor for solvothermal reaction (150 °C, 12 h). The reaction product was filtered to remove the solvent, washed and dried to obtain a particulate photocatalyst, denoted as TiO2@RGO-1.5.

[0030] Preparation of Triple-Structure Polyacrylonitrile Nanofiber Composite Membranes: The fiber membranes were prepared using a combination of electrospinning and electrospraying. The particulate photocatalyst TiO2@RGO-1.5 prepared above was dispersed in 8 g of N,N-dimethylformamide (DMF), and 0.6 g of industrial-grade PAN was added. The mixture was stirred until the PAN dissolved, yielding an electrospraying solution. 0.9 g of industrial-grade PAN, 0.9 g of PAN (Mw=50000), and 0.2 g of tetrabutylammonium chloride (TBAC) were dissolved in 8 g of DMF to obtain an electrospinning solution. The electrospinning and electrospraying solutions were then transferred to 10 mL syringes equipped with stainless steel needles and fixed to a syringe pump. During the spinning process, the electrospinning end voltage was 15 kV, the injection pump speed was 0.2 mL·h⁻¹, and the distance between the needle and the roller receiving device was 20 cm; the electrostatic spraying end voltage was 10.5 kV, the injection pump speed was 0.8 mL·h⁻¹, and the distance between the needle and the roller receiving device was 5 cm. The spinning time was 6 hours, and the ambient humidity was controlled at 35%. A polyacrylonitrile (PAN) nanofiber membrane loaded with TiO₂@RGO-1.5 was obtained, which is a triple-structured nanofiber composite material.

[0031] See appendix Figure 1 These are the X-ray powder diffraction patterns of the samples obtained in each step of preparing the triple-structured polyacrylonitrile nanofiber composite membrane in this embodiment; wherein, (a) is the X-ray diffraction pattern of TiO2@RGO-1.5 particles, (b) is the X-ray diffraction pattern of the comparative sample (SS7.5) prepared in the same steps without the addition of photocatalyst particles in the electrostatic spraying solution, and (c) is the X-ray diffraction pattern of the triple-structured PAN nanofiber membrane (SS7.5-2.3TR(A)) loaded with TiO2@RGO-1.5. Figure 1 It can be seen that the characteristic peaks of TiO2@RGO-1.5 appear at 25.3°, 37.9°, 48.0°, 54.4°, 56.6°, 62.8° and 68.9°. In the XRD pattern of the PAN nanofiber membrane (S-0), the peaks with 2θ of about 16.9° and 24° belong to the characteristic peaks of PAN. The XRD pattern of the polyacrylonitrile nanofiber membrane loaded with TiO2@RGO-1.5 shows both the characteristic peaks of TiO2@RGO-1.5 and the characteristic peaks of PAN.

[0032] See appendix Figure 2 These are SEM images of the samples obtained in each step of preparing the triple-structured polyacrylonitrile nanofiber composite membrane in this embodiment; wherein, (a) is the SEM image of the PAN nanofiber membrane obtained by electrospinning, (b) is the SEM image of the PAN nanofiber membrane obtained by electrospinning and electrostatic spraying, and (c) is the SEM image of the PAN nanofiber membrane loaded with TiO2@RGO-1.5 obtained by electrospinning and electrostatic spraying. Figure 2 It can be seen that the PAN nanofiber membrane obtained by electrospinning has a double network structure in which nanofibers and nanospider webs are fused together. The diameter of the PAN fibers is about 800 nm and the diameter of the nanospider web fibers is about 20 nm. The nanobeaded fibers formed by electrostatic spraying to support beaded photocatalysts have a beaded structure. The diameter of the formed PAN fibers is about 100 nm. The two interweave to form a triple structure fiber membrane.

[0033] See appendix Figure 3This is a scanning electron microscope (SEM) image and structural schematic diagram of the triple-structured polyacrylonitrile nanofiber composite membrane sample provided in this embodiment. In the figure (a), it is a scanning electron microscope image of the triple-structured PAN nanofiber membrane sample loaded with TiO2@RGO-1.5 obtained by electrospinning and electrostatic spraying. From the figure (a), the microstructure schematic diagram of the triple-structured fiber membrane shown in the figure (b) can be obtained. The nanofiber 1 obtained by electrospinning is fused with the nanospider web 2 to form a double network bonding structure, and interpenetrates with the nanobead fiber 3 loaded with TiO2@RGO-1.5 bead 4 photocatalyst obtained by electrostatic spraying to form a triple-structured polyacrylonitrile nanofiber composite membrane.

[0034] See appendix Figure 4 This is a scanning electron microscope (SEM) image of the triple-structured polyacrylonitrile nanofiber composite membrane sample provided in this embodiment at different positions when stretched to an elongation of 50%. (a) is a photograph of the fiber membrane sample at an elongation of 50%; (b) is an SEM image of the sample at position (Ⅰ) at an elongation of 50%; (c) is an SEM image of the sample at position (Ⅱ) at an elongation of 50%; and (e) is an SEM image of the sample at position (Ⅲ) at an elongation of 50%. When the fiber membrane is subjected to stretching, as the tensile stress increases, the spiderweb composed of the smallest diameter fibers will break first (position Ⅰ), followed by the breakage of the small-sized nanofibers formed by electrostatic spraying (position Ⅱ). The nanofibers formed by electrospinning, however, will first slowly orient themselves and then be gradually stretched (position Ⅲ) until they break. The first two deformations consume a large amount of work from the external force, therefore, this fiber membrane material exhibits better mechanical properties compared to nanofiber membrane materials prepared by conventional electrospinning.

[0035] See appendix Figure 5 This is a tensile stress-strain curve of the triple-structured polyacrylonitrile nanofiber composite membrane sample and the comparative example provided in this embodiment. Among them, the tensile strength of the fiber membrane obtained by electrospinning in the comparative example is 2.8 MPa and the elongation at break is 15%; the tensile strength of the PAN nanofiber membrane obtained by electrospinning and electrospraying in the comparative example is 2.75 MPa and the elongation at break is 50%; the tensile strength of the triple-structured PAN nanofiber membrane loaded with TiO2@RGO-1.5 obtained by electrospinning and electrospraying in this embodiment is 3.75 MPa and the elongation at break is 70%.

[0036] See appendix Figure 6This is a graph showing the changes in UV-Vis absorption spectra of different dye solutions under different conditions after pretreatment, obtained in this embodiment, over time. The pretreatment process was as follows: 30 °C aqueous solution, stirred for 7.5 h; then dried at 60 °C for 12 h). The treated fiber membrane was denoted as SS7.5-2.3TR. The concentration of the dye solution was 10 mg / L, and the total volume was 100 mL. After 30 minutes of dark adsorption, the concentration of the dye solution decreased significantly. After 7.5 h of xenon lamp irradiation, the concentration of 99% methylene blue (MB) aqueous solution and rhodamine B (RhB) aqueous solution decreased significantly. The photodegradation rate of methyl orange (MO) aqueous solution was 78% and that of oil red (OR) solution was 70% under the same irradiation time.

[0037] See appendix Figure 7 This is a photodegradation efficiency curve of the sample obtained in this embodiment on MO aqueous solution (10 mg / L, 100 mL) after 5 repeated uses; wherein, (a) is the concentration-time change curve of SS7.5-2.3TR photodegradation of MO solution after five uses, and (b) is the tensile stress-strain curve of SS7.5-2.3TR and the used SS7.5-2.3TR. Figure 7 It can be seen that in the fifth photocatalytic degradation experiment, the degradation rate of MO solution by SS7.5-2.3TR can still reach 70%. The strength of SS7.5-2.3TR after use is 3.48 MPa and the elongation at break is 42%, which is close to the data before the photocatalytic reaction.

[0038] The triple-structured polyacrylonitrile nanofiber composite material provided in this embodiment was used for filtration and photodegradation treatment of dyeing wastewater after silk dyeing. The specific process conditions for the silk dyeing experiment included: (1) The dyeing mother liquor consisted of X mL MO aqueous solution (5 g / L), Pingpingjia O (0.5 mL), disodium hydrogen phosphate (3.9 mL), citric acid (6.1 mL) and Y mL deionized water; Pingpingjia O was used to improve the dyeing rate of silk, and citric acid and disodium hydrogen phosphate were used to adjust the pH of the dyeing mother liquor. (2) The ratio of fiber to mother liquor was 1:100. (3) By adjusting the values ​​of X and Y, four types of mother liquor were obtained: 0.1 owf%, 0.2 owf%, 0.3 owf%, and 0.4 owf%. (4) After the dyeing experiment temperature reached 40℃, the temperature was increased by 1.5℃ min. −1 The temperature was increased to 100°C at a certain rate, and then held at 100°C for 60 min.

[0039] See appendix Figure 8 This is a curve showing the effect of applying the triple-structured polyacrylonitrile nanofiber composite material sample prepared in this embodiment, after being folded into a bottomed cylindrical shape, to silk dyeing wastewater. Figure 8Figures a through d show the UV-Vis absorption spectra of silk dyeing wastewater before and after filtration at mother liquor concentrations of 0.1 owf%, 0.2 owf%, 0.3 owf%, and 0.4 owf%, respectively. Figure (e) shows the C / C0 ratio of MO in the four filtrates as a function of light exposure time. The filtration experiment not only removed broken silk from the wastewater, but also simultaneously caused the adsorption of dye by SS7.5-2.3TR. Therefore, the filtered solution not only had a lower baseline, but also a reduced absorbance at its UV-Vis absorption spectrum characteristic peak. As the dye concentration increased during the dyeing experiment, the concentration of dye remaining in the wastewater after reaching dyeing equilibrium increased, and the time required for complete photodegradation gradually lengthened. As shown in Figure (e), when the dye concentration was 0.1 owf%, SS7.5-2.3TR could degrade 95% of MO in just 4.5 hours; while when the dye concentration was 0.4 owf%, the degradation rate of MO was 85% after 9 hours of light exposure.

[0040] The triple-structured polyacrylonitrile nanofiber composite membrane prepared in this embodiment has a significant photodegradation effect on solutions of common organic dyes such as methylene blue (MB), rhodamine B (RhB), methyl orange (MO), and oil red (OR), and can be applied to the filtration and photodegradation of dyeing and printing wastewater.

Claims

1. A method for preparing a triple-structured polyacrylonitrile nanofiber composite membrane, characterized in that... Includes the following steps: (1) Add graphene oxide to deionized water and treat it with ultrasound to obtain a graphene oxide dispersion with a mass fraction of 0.1-0.25 g / L. Then add acetic acid with a mass fraction of 11.8-21%. Dissolve tetrabutyl titanate in anhydrous ethanol to prepare an anhydrous ethanol solution of tetrabutyl titanate with a concentration of 75-90 g / L. Add the graphene oxide dispersion containing acetic acid dropwise to the anhydrous ethanol solution of tetrabutyl titanate. After stirring and mixing evenly, carry out a solvothermal reaction at a temperature of 145-155℃ for 8-15 h. Then remove the solvent by filtration, wash and dry to obtain a granular photocatalyst. (2) Disperse the photocatalyst prepared in step (1) in N,N-dimethylformamide solution at a mass fraction of 2.3%, and then add polyacrylonitrile at a mass fraction of 7.5%, wherein the polyacrylonitrile is industrial grade, stir until fully dissolved to obtain electrostatic spraying solution; dissolve polyacrylonitrile in DMF at a mass fraction of 18%, and add tetrabutylammonium chloride at a mass fraction of 2% to obtain electrospinning solution, wherein the polyacrylonitrile is industrial grade with a mass average molecular weight Mw=50000 in a ratio of 1:1; (3) The electrospinning solution and the electrospraying solution were placed in the propulsion pumps of the electrospinning and electrospraying processes, respectively, and a triple-structured polyacrylonitrile nanofiber composite membrane was obtained by using the electrospinning and electrospraying processes in opposite directions; wherein, the process conditions for electrospraying were a voltage of 9-11 kV and a pump propulsion speed of 0.7-0.9 mL·h. -1 The distance between the needle and the roller receiving device is 4–6 cm; the electrospinning process conditions are a voltage of 12–15 kV and an injection pump feed rate of 0.1–0.3 mL·h. -1 The distance between the needle and the roller receiving device is 18-20cm, and the ambient humidity is 33-37%. The nanofibers (1) obtained by electrospinning process and the nanospider web (2) are fused together to form a double network bonding structure, which is interwoven with the nanobead fiber (3) structure obtained by electrostatic spraying process to form a triple structure polyacrylonitrile nanofiber composite film.

2. The triple-structured polyacrylonitrile nanofiber composite membrane obtained by the preparation method of the triple-structured polyacrylonitrile nanofiber composite membrane according to claim 1, characterized in that: The diameter of nanofibers is 750–800 nm, the diameter of nanospider web fibers is 20–30 nm, and the diameter of nanobead fibers is 100–150 nm.

3. The application of the triple-structured polyacrylonitrile nanofiber composite membrane as described in claim 2, for filtering and photodegrading dyeing and printing wastewater.

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