Active composite membrane with bionic structure as well as preparation method and application of active composite membrane

Abstract

The invention relates to an active composite membrane with a bionic structure as well as a preparation method and application of the active composite membrane, and belongs to the technical field of bio-based polymer composite materials. In order to solve the problem that an existing PVA modification method cannot give consideration to mechanical properties, water resistance and safety, the invention provides a preparation method of an active composite membrane with a bionic structure. The preparation method comprises the following steps: preparing a PDMS negative template with a lotus leaf surface negative topological structure, carrying out alkali swelling pretreatment on poplar powder, preparing a deep eutectic solvent, preparing LCNF, preparing a polyvinyl alcohol solution and preparing the active composite membrane with the bionic structure. Through collaborative design of a bionic lotus leaf surface micro-nano structure and a bio-based active component, the comprehensive performance of the composite film is remarkably improved, and the hydrophobic performance of the composite film is improved; the tensile strength and the antioxidant and antibacterial activity of the composite film are improved, the preparation process is safe and non-toxic, the strict requirements of food contact materials are met, and the composite film has wide application prospects in the fields of food preservation and medicine packaging.

Description

Technical Field

[0001] This invention belongs to the field of bio-based polymer composite materials technology, and particularly relates to an active composite membrane with a biomimetic structure, its preparation method and application. Background Technology

[0002] In recent years, with the increasing awareness of environmental protection and health among consumers, the development of "active packaging" materials derived from renewable resources, biodegradable, and possessing multiple active preservation functions has become an important research direction in the food packaging field. Among many biomass substrates, polyvinyl alcohol (PVA) is a water-soluble polymer material that has been widely studied due to its non-toxicity, biodegradability, excellent biocompatibility, film-forming ability, and high transparency. However, pure PVA films have significant drawbacks in practical applications: firstly, their mechanical properties, especially toughness and tear resistance, are insufficient to meet the physical strength requirements of packaging materials during transportation and storage; secondly, the PVA molecular chain is rich in hydroxyl groups, resulting in strong hydrophilicity and hygroscopicity.

[0003] The core objectives of PVA modification are mainly focused on two aspects: firstly, improving its mechanical properties through reinforcement and toughening methods, and secondly, improving its water resistance by introducing hydrophobic components. Currently, the main modification methods for PVA include: (1) Physical blending modification: for example, blending with polylactic acid to improve water resistance, but there is a significant phase separation problem, which easily leads to increased material brittleness and decreased impact strength; (2) Chemical crosslinking modification: for example, using crosslinking agents such as glutaraldehyde and boric acid to react with the hydroxyl groups of PVA to form a three-dimensional network structure. Although this method can improve the water resistance and mechanical strength of the material, the toxicity and potential health risks of glutaraldehyde and boric acid residues do not meet the safety standards for food contact materials; (3) Nanocomposite modification: introducing nanomaterials such as nanocellulose, montmorillonite, or graphene into PVA composites can improve mechanical properties and barrier properties to a certain extent, but the effect on improving water resistance is limited.

[0004] It is evident that current PVA modification methods generally suffer from limited modification effects or side effects, making it difficult to simultaneously meet the comprehensive requirements of food packaging materials in terms of mechanical properties, water resistance, and safety. Summary of the Invention

[0005] To address the problem that existing PVA modification methods cannot simultaneously achieve mechanical properties, water resistance, and safety, this invention provides an active composite membrane with a biomimetic structure, its preparation method, and its application.

[0006] The technical solution of the present invention:

[0007] A method for preparing an active composite membrane with a biomimetic structure, comprising the following steps:

[0008] Step 1: Prepare PDMS negative template:

[0009] Take fresh lotus leaves, wash and remove surface contaminants, dry them and fix them at the bottom of a container. Mix the PDMS precursor and curing agent thoroughly and pour them onto the surface of the lotus leaves. Cure under vacuum. After curing, remove the lotus leaves to obtain a PDMS negative template with a negative topology on the surface of the lotus leaves.

[0010] Step 2: Pre-treatment of poplar wood powder:

[0011] Poplar wood powder was added to a 2% sodium hydroxide solution and reacted at 90°C for 2 hours. The filter cake was collected by filtration and washed with deionized water until neutral. After drying, pretreated poplar wood powder was obtained.

[0012] Step 3: Prepare the eutectic solvent:

[0013] Mix choline chloride and oxalic acid in equal molar amounts, heat and stir continuously until a homogeneous and transparent liquid is formed, then cool to room temperature for later use.

[0014] Step 4: Preparation of LCNF (lignocellulose nanofibers):

[0015] The poplar powder pretreated in step two was mixed with the eutectic solvent prepared in step three at a mass ratio of 1:50. The reaction was stopped after stirring at 80°C for 3 to 6 hours. The resulting reaction product was purified, and then the precipitate was collected by centrifugation. The precipitate was prepared into a suspension and sheared evenly to obtain an LCNF suspension.

[0016] Step 5: Prepare a polyvinyl alcohol solution:

[0017] Mix polyvinyl alcohol and distilled water at a mass ratio of 1:20, seal the mixture, heat and stir until a transparent and homogeneous liquid is formed, and then allow it to cool naturally.

[0018] Step Six: Preparation of biomimetic active composite membrane:

[0019] The LCNF suspension and quercetin / ethanol solution prepared in step four were added to the polyvinyl alcohol solution obtained in step five at a mass ratio of LCNF, quercetin and polyvinyl alcohol dry matter of 1~7:0.5~2:100, and the mixture was stirred at 50°C to crosslink and obtain a film-forming solution.

[0020] The film-forming solution is poured into a mold containing the PDMS negative template obtained in step one. After the film dries, it is peeled off from the template to obtain the active composite film.

[0021] Furthermore, in step one, the PDMS template preparation uses Sylgard 184, the mass ratio of precursor to curing agent is 10:1, the curing temperature is 60℃, and the curing time is 8 hours.

[0022] Furthermore, in step two, the poplar powder has a particle size of 60 mesh, the mass-to-volume ratio of the poplar powder to the sodium hydroxide solution is 1g:50mL, the drying temperature is 100℃, and the drying time is 4 hours.

[0023] Furthermore, the heating temperature in step three is 80°C.

[0024] Furthermore, in step four, the reaction termination involves cooling the reaction system in an ice-water bath and adding distilled water; the impurity removal process involves placing the reaction product in a dialysis bag and dialyzing it with deionized water for 5 days; the centrifugation speed is 5000 rpm and the centrifugation time is 10 minutes; the mass fraction of the suspension is 0.7%; and the shearing is performed using a high-speed blender at 800W power in a 1-second work-1-second intermittent working mode for 30 minutes.

[0025] Furthermore, the heating and stirring temperature in step five is 95°C, and the cooling temperature is reduced to 50°C.

[0026] Furthermore, in step six, the mass concentration of quercetin in the quercetin / ethanol solution is 1 wt%; and the cross-linking time is 30 minutes.

[0027] Furthermore, step six also includes spraying a layer of natural wax onto the surface of the resulting active composite membrane.

[0028] An active composite membrane with a biomimetic structure prepared by the preparation method provided by the present invention.

[0029] The present invention discloses the application of an active composite film with a biomimetic structure in the fields of food preservation and pharmaceutical packaging.

[0030] The beneficial effects of this invention are:

[0031] This invention significantly improves the overall performance of composite membranes through the synergistic design of biomimetic lotus leaf surface micro-nano structures and bio-based active components. Specifically, the biomimetic micro-nano structures on the composite membrane surface effectively reduce the membrane's surface energy, improving its hydrophobic properties; the abundant hydroxyl groups on the surface of lignocellulose nanofibers form a high-strength three-dimensional network structure with polyvinyl alcohol molecular chains, significantly improving the tensile strength and toughness of the composite membrane; and the introduction of quercetin endows the composite membrane with excellent antioxidant and antibacterial activity, effectively inhibiting oxidative deterioration and microbial contamination during food storage.

[0032] This invention employs a green and environmentally friendly process for preparing LCNF using a eutectic solvent, avoiding the environmental pollution problems associated with traditional solvent methods. Furthermore, all raw materials are bio-based or naturally derived, ensuring the biocompatibility and biodegradability of the composite film. An optimized step involving surface coating with natural wax further enhances the water resistance and barrier properties of the composite film, making it promising for applications in food preservation and pharmaceutical packaging. Compared to existing technologies, the composite film prepared by this invention simultaneously achieves enhanced mechanical properties, improved water resistance, and imparted bioactivity, while the preparation process is safe and non-toxic, meeting the stringent requirements for food contact materials. Attached Figure Description

[0033] Figure 1 Comparison of tensile strength of composite films prepared in Examples 1-4 and Comparative Examples 1-3;

[0034] Figure 2 Comparison diagram of water contact angles of composite membranes prepared in Examples 1-4 and Comparative Examples 1-3;

[0035] Figure 3 A comparison chart of the free radical scavenging rates of the composite membranes prepared in Examples 1-4 and Comparative Examples 1-3;

[0036] Figure 4 This is a comparison of the UV-Vis transmittance of the composite films prepared in Examples 1-4 and Comparative Examples 1-3. Detailed Embodiments

[0037] The technical solution of the present invention will be further described below with reference to embodiments, but it is not limited thereto. Any modifications or equivalent substitutions to the technical solution of the present invention without departing from the spirit and scope of the technical solution of the present invention should be covered within the protection scope of the present invention. In the following embodiments, the process equipment or apparatus not specifically specified are all conventional equipment or apparatus in the art. Unless otherwise specified, the raw materials used in the embodiments of the present invention are all commercially available; unless otherwise specified, the technical means used in the embodiments of the present invention are all conventional means well known to those skilled in the art.

[0038] Example 1

[0039] This embodiment provides a method for preparing an active composite membrane with a biomimetic structure, the steps of which are as follows:

[0040] Step 1: Prepare PDMS negative template:

[0041] Take fresh lotus leaves and wash their surface with ethanol and deionized water in turn to remove contaminants. Then let them air dry naturally and fix the clean lotus leaves at the bottom of the petri dish.

[0042] The PDMS template was prepared using Sylgard 184. The precursor and curing agent were weighed at a mass ratio of 10:1, thoroughly mixed, and then poured onto the surface of the lotus leaf to ensure even coverage. The entire culture dish was placed in a vacuum drying oven and cured at 60°C for 8 hours, during which a vacuum was maintained to remove air bubbles. After curing, the lotus leaf was carefully peeled off from the cured PDMS to obtain a PDMS negative template with a negative topology on the lotus leaf surface.

[0043] Step 2: Alkali swelling pretreatment of poplar wood powder:

[0044] Take 2g of 60-mesh poplar powder, add 100mL of 2% sodium hydroxide solution, and react at 90℃ for 2 hours. After the reaction is completed, filter the mixture and wash the filter cake repeatedly with deionized water until the pH of the filtrate is greater than 6. Place the pretreated poplar powder in a 100℃ oven and dry for 4 hours to obtain pretreated poplar powder.

[0045] Step 3: Prepare the eutectic solvent:

[0046] Choline chloride and oxalic acid were mixed in a molar ratio of 1:1, heated and stirred continuously at 80°C until a homogeneous and transparent liquid was formed, and then cooled to room temperature for later use.

[0047] Step 4: Preparation of LCNF

[0048] The poplar powder pretreated in step two was mixed with the eutectic solvent prepared in step three at a mass ratio of 1:50, and the mixture was magnetically stirred in an oil bath at 80°C for 3 hours to obtain a black slurry. The reaction apparatus was then moved into an ice-water bath to cool and 50 mL of distilled water was added to stop the reaction.

[0049] The reaction product was transferred to a dialysis bag and dialyzed with deionized water for 5 days to remove impurities. The dialysis suspension was centrifuged at 5000 rpm for 10 minutes, the supernatant was discarded, and the precipitate was collected. The precipitate was reconstituted into a suspension with a mass fraction of 0.7% using distilled water. Finally, the suspension was obtained by shearing after using a high-speed blender at 800W power with a working mode of 1 second on and 1 second off for 30 minutes.

[0050] Step 5: Prepare a polyvinyl alcohol solution:

[0051] Weigh solid polyvinyl alcohol granules and distilled water at a mass ratio of 1:20, seal and heat at 95°C with stirring until a transparent and homogeneous liquid is formed, then allow it to cool naturally to 50°C.

[0052] Step Six: Preparation of biomimetic active composite membrane:

[0053] The LCNF suspension prepared in step four and the quercetin / ethanol solution were added to the polyvinyl alcohol solution obtained in step five at a mass ratio of 3:2:100 for lignocellulose nanoparticles, quercetin and polyvinyl alcohol dry matter. The mixture was stirred and crosslinked at 50°C for 30 min to obtain the film-forming solution.

[0054] Take 20g of film-forming solution and pour it into a mold containing the PDMS negative template obtained in step one. After the film dries, peel it off from the template to obtain a preliminary composite film with a thickness of 52μm. Then, spray a layer of natural wax with a thickness of 5μm onto the surface of the obtained preliminary composite film to obtain an active composite film with a lotus leaf biomimetic hydrophobic structure.

[0055] Example 2

[0056] This embodiment provides a method for preparing an active composite membrane with a biomimetic structure, the steps of which are as follows:

[0057] Step 1: Prepare PDMS negative template:

[0058] Take fresh lotus leaves and wash their surface with ethanol and deionized water in turn to remove contaminants. Then let them air dry naturally and fix the clean lotus leaves at the bottom of the petri dish.

[0059] The PDMS template was prepared using Sylgard 184. The precursor and curing agent were weighed at a mass ratio of 10:1, thoroughly mixed, and then poured onto the surface of the lotus leaf to ensure even coverage. The entire culture dish was placed in a vacuum drying oven and cured at 60°C for 8 hours, during which a vacuum was maintained to remove air bubbles. After curing, the lotus leaf was carefully peeled off from the cured PDMS to obtain a PDMS negative template with a negative topology on the lotus leaf surface.

[0060] Step 2: Alkali swelling pretreatment of poplar wood powder:

[0061] Take 2g of 60-mesh poplar powder, add 100mL of 2% sodium hydroxide solution, and react at 90℃ for 2 hours. After the reaction is completed, filter the mixture and wash the filter cake repeatedly with deionized water until the pH of the filtrate is greater than 6. Place the pretreated poplar powder in a 100℃ oven and dry for 4 hours to obtain pretreated poplar powder.

[0062] Step 3: Prepare the eutectic solvent:

[0063] Choline chloride and oxalic acid were mixed in a molar ratio of 1:1, heated and stirred continuously at 80°C until a homogeneous and transparent liquid was formed, and then cooled to room temperature for later use.

[0064] Step 4: Preparation of LCNF

[0065] The poplar powder pretreated in step two was mixed with the eutectic solvent prepared in step three at a mass ratio of 1:50, and the mixture was magnetically stirred in an oil bath at 80°C for 3 hours to obtain a black slurry. The reaction apparatus was then moved into an ice-water bath to cool and 50 mL of distilled water was added to stop the reaction.

[0066] The reaction product was transferred to a dialysis bag and dialyzed with deionized water for 5 days to remove impurities. The dialysis suspension was centrifuged at 5000 rpm for 10 minutes, the supernatant was discarded, and the precipitate was collected. The precipitate was reconstituted into a suspension with a mass fraction of 0.7% using distilled water. Finally, the suspension was obtained by shearing after using a high-speed blender at 800W power with a working mode of 1 second on and 1 second off for 30 minutes.

[0067] Step 5: Prepare a polyvinyl alcohol solution:

[0068] Weigh solid polyvinyl alcohol granules and distilled water at a mass ratio of 1:20, seal and heat at 95°C with stirring until a transparent and homogeneous liquid is formed, then allow it to cool naturally to 50°C.

[0069] Step Six: Preparation of biomimetic active composite membrane:

[0070] The LCNF suspension prepared in step four and the quercetin / ethanol solution were added to the polyvinyl alcohol solution obtained in step five at a mass ratio of 3:1.5:100 for lignocellulose nanoparticles, quercetin and polyvinyl alcohol dry matter. The mixture was stirred and crosslinked at 50°C for 30 min to obtain the film-forming solution.

[0071] Take 20g of film-forming solution and pour it into a mold containing the PDMS negative template obtained in step one. After the film dries, peel it off from the template to obtain a preliminary composite film with a thickness of 52μm. Then, spray a layer of natural wax with a thickness of 5μm onto the surface of the obtained preliminary composite film to obtain an active composite film with a lotus leaf biomimetic hydrophobic structure.

[0072] Example 3

[0073] This embodiment provides a method for preparing an active composite membrane with a biomimetic structure, the steps of which are as follows:

[0074] Step 1: Prepare PDMS negative template:

[0075] Take fresh lotus leaves and wash their surface with ethanol and deionized water in turn to remove contaminants. Then let them air dry naturally and fix the clean lotus leaves at the bottom of the petri dish.

[0076] The PDMS template was prepared using Sylgard 184. The precursor and curing agent were weighed at a mass ratio of 10:1, thoroughly mixed, and then poured onto the surface of the lotus leaf to ensure even coverage. The entire culture dish was placed in a vacuum drying oven and cured at 60°C for 8 hours, during which a vacuum was maintained to remove air bubbles. After curing, the lotus leaf was carefully peeled off from the cured PDMS to obtain a PDMS negative template with a negative topology on the lotus leaf surface.

[0077] Step 2: Alkali swelling pretreatment of poplar wood powder:

[0078] Take 2g of 60-mesh poplar powder, add 100mL of 2% sodium hydroxide solution, and react at 90℃ for 2 hours. After the reaction is completed, filter the mixture and wash the filter cake repeatedly with deionized water until the pH of the filtrate is greater than 6. Place the pretreated poplar powder in a 100℃ oven and dry for 4 hours to obtain pretreated poplar powder.

[0079] Step 3: Prepare the eutectic solvent:

[0080] Choline chloride and oxalic acid were mixed in a molar ratio of 1:1, heated and stirred continuously at 80°C until a homogeneous and transparent liquid was formed, and then cooled to room temperature for later use.

[0081] Step 4: Preparation of LCNF

[0082] The poplar powder pretreated in step two was mixed with the eutectic solvent prepared in step three at a mass ratio of 1:50, and the mixture was magnetically stirred in an oil bath at 80°C for 3 hours to obtain a black slurry. The reaction apparatus was then moved into an ice-water bath to cool and 50 mL of distilled water was added to stop the reaction.

[0083] The reaction product was transferred to a dialysis bag and dialyzed with deionized water for 5 days to remove impurities. The dialysis suspension was centrifuged at 5000 rpm for 10 minutes, the supernatant was discarded, and the precipitate was collected. The precipitate was reconstituted into a suspension with a mass fraction of 0.7% using distilled water. Finally, the suspension was obtained by shearing after using a high-speed blender at 800W power with a working mode of 1 second on and 1 second off for 30 minutes.

[0084] Step 5: Prepare a polyvinyl alcohol solution:

[0085] Weigh solid polyvinyl alcohol granules and distilled water at a mass ratio of 1:20, seal and heat at 95°C with stirring until a transparent and homogeneous liquid is formed, then allow it to cool naturally to 50°C.

[0086] Step Six: Preparation of biomimetic active composite membrane:

[0087] The LCNF suspension prepared in step four and the quercetin / ethanol solution were added to the polyvinyl alcohol solution obtained in step five at a mass ratio of 3:1:100 for lignocellulose nanoparticles, quercetin and polyvinyl alcohol dry matter. The mixture was stirred and crosslinked at 50°C for 30 min to obtain the film-forming solution.

[0088] Take 20g of film-forming solution and pour it into a mold containing the PDMS negative template obtained in step one. After the film dries, peel it off from the template to obtain a preliminary composite film with a thickness of 52μm. Then, spray a layer of natural wax with a thickness of 5μm onto the surface of the obtained preliminary composite film to obtain an active composite film with a lotus leaf biomimetic hydrophobic structure.

[0089] Example 4

[0090] This embodiment provides a method for preparing an active composite membrane with a biomimetic structure, the steps of which are as follows:

[0091] Step 1: Prepare PDMS negative template:

[0092] Take fresh lotus leaves and wash their surface with ethanol and deionized water in turn to remove contaminants. Then let them air dry naturally and fix the clean lotus leaves at the bottom of the petri dish.

[0093] The PDMS template was prepared using Sylgard 184. The precursor and curing agent were weighed at a mass ratio of 10:1, thoroughly mixed, and then poured onto the surface of the lotus leaf to ensure even coverage. The entire culture dish was placed in a vacuum drying oven and cured at 60°C for 8 hours, during which a vacuum was maintained to remove air bubbles. After curing, the lotus leaf was carefully peeled off from the cured PDMS to obtain a PDMS negative template with a negative topology on the lotus leaf surface.

[0094] Step 2: Alkali swelling pretreatment of poplar wood powder:

[0095] Take 2g of 60-mesh poplar powder, add 100mL of 2% sodium hydroxide solution, and react at 90℃ for 2 hours. After the reaction is completed, filter the mixture and wash the filter cake repeatedly with deionized water until the pH of the filtrate is greater than 6. Place the pretreated poplar powder in a 100℃ oven and dry for 4 hours to obtain pretreated poplar powder.

[0096] Step 3: Prepare the eutectic solvent:

[0097] Choline chloride and oxalic acid were mixed in a molar ratio of 1:1, heated and stirred continuously at 80°C until a homogeneous and transparent liquid was formed, and then cooled to room temperature for later use.

[0098] Step 4: Preparation of LCNF

[0099] The poplar powder pretreated in step two was mixed with the eutectic solvent prepared in step three at a mass ratio of 1:50, and the mixture was magnetically stirred in an oil bath at 80°C for 3 hours to obtain a black slurry. The reaction apparatus was then moved into an ice-water bath to cool and 50 mL of distilled water was added to stop the reaction.

[0100] The reaction product was transferred to a dialysis bag and dialyzed with deionized water for 5 days to remove impurities. The dialysis suspension was centrifuged at 5000 rpm for 10 minutes, the supernatant was discarded, and the precipitate was collected. The precipitate was reconstituted into a suspension with a mass fraction of 0.7% using distilled water. Finally, the suspension was obtained by shearing after using a high-speed blender at 800W power with a working mode of 1 second on and 1 second off for 30 minutes.

[0101] Step 5: Prepare a polyvinyl alcohol solution:

[0102] Weigh solid polyvinyl alcohol granules and distilled water at a mass ratio of 1:20, seal and heat at 95°C with stirring until a transparent and homogeneous liquid is formed, then allow it to cool naturally to 50°C.

[0103] Step Six: Preparation of biomimetic active composite membrane:

[0104] The LCNF suspension prepared in step four and the quercetin / ethanol solution were added to the polyvinyl alcohol solution obtained in step five at a mass ratio of 3:0.5:100 for lignocellulose nanoparticles, quercetin and polyvinyl alcohol dry matter. The mixture was stirred and crosslinked at 50°C for 30 min to obtain the film-forming solution.

[0105] Take 20g of film-forming solution and pour it into a mold containing the PDMS negative template obtained in step one. After the film dries, peel it off from the template to obtain a preliminary composite film with a thickness of 52μm. Then, spray a layer of natural wax with a thickness of 5μm onto the surface of the obtained preliminary composite film to obtain an active composite film with a lotus leaf biomimetic hydrophobic structure.

[0106] Comparative Example 1

[0107] This comparative example provides a method for preparing a pure PVA film, the steps of which are as follows:

[0108] Solid polyvinyl alcohol granules and distilled water were weighed at a mass ratio of 1:20, sealed, and heated at 95°C with stirring until a transparent and homogeneous liquid was formed. The mixture was then allowed to cool naturally to 50°C. The polyvinyl alcohol solution was poured into a common polytetrafluoroethylene mold. After the film dried, it was peeled off from the mold to obtain a pure PVA film with a thickness of 50 μm.

[0109] Comparative Example 2

[0110] This comparative example provides a method for preparing a composite membrane with only LCNF added, the steps of which are as follows:

[0111] Step 1: Alkali swelling pretreatment of poplar wood powder:

[0112] Take 2g of 60-mesh poplar powder, add 100mL of 2% sodium hydroxide solution, and react at 90℃ for 2 hours. After the reaction is completed, filter the mixture and wash the filter cake repeatedly with deionized water until the pH of the filtrate is greater than 6. Place the pretreated poplar powder in a 100℃ oven and dry for 4 hours to obtain pretreated poplar powder.

[0113] Step 2: Prepare the eutectic solvent:

[0114] Choline chloride and oxalic acid were mixed in a molar ratio of 1:1, heated and stirred continuously at 80°C until a homogeneous and transparent liquid was formed, and then cooled to room temperature for later use.

[0115] Step 3: Preparation of LCNF:

[0116] The poplar powder pretreated in step one was mixed with the eutectic solvent prepared in step two at a mass ratio of 1:50, and the mixture was magnetically stirred in an oil bath at 80°C for 3 hours to obtain a black slurry. The reaction apparatus was then moved into an ice-water bath to cool and 50 mL of distilled water was added to stop the reaction.

[0117] The reaction product was transferred to a dialysis bag and dialyzed with deionized water for 5 days to remove impurities. The dialysis suspension was centrifuged at 5000 rpm for 10 minutes, the supernatant was discarded, and the precipitate was collected. The precipitate was reconstituted into a suspension with a mass fraction of 0.7% using distilled water. Finally, the suspension was obtained by shearing after using a high-speed blender at 800W power with a working mode of 1 second on and 1 second off for 30 minutes.

[0118] Step 4: Prepare a polyvinyl alcohol solution:

[0119] Weigh solid polyvinyl alcohol granules and distilled water at a mass ratio of 1:20, seal and heat at 95°C with stirring until a transparent and homogeneous liquid is formed, then allow it to cool naturally to 50°C.

[0120] Step 5: Preparation of composite membrane:

[0121] The LCNF suspension prepared in step three was added to the polyvinyl alcohol solution obtained in step four at a mass ratio of 5:100 between lignocellulose nanoparticles and polyvinyl alcohol dry matter. The mixture was stirred and crosslinked at 50°C for 30 min to obtain a film-forming solution. 20 g of the film-forming solution was taken and poured into a common polytetrafluoroethylene mold. After the film dried, it was peeled off from the template to obtain a composite film with a thickness of 50 μm.

[0122] Comparative Example 3

[0123] This comparative example provides a method for preparing a composite membrane with only quercetin added, the steps of which are as follows:

[0124] Step 1: Prepare a polyvinyl alcohol solution:

[0125] Weigh solid polyvinyl alcohol granules and distilled water at a mass ratio of 1:20, seal and heat at 95°C with stirring until a transparent and homogeneous liquid is formed, then allow it to cool naturally to 50°C.

[0126] Step 2: Preparation of composite membrane:

[0127] The quercetin / ethanol solution was added to the polyvinyl alcohol solution obtained in step one at a mass ratio of 0.5:100 (quercetin to polyvinyl alcohol dry matter). The mixture was stirred and crosslinked at 50°C for 30 minutes to obtain a film-forming solution. The solution was poured into a common polytetrafluoroethylene mold. After the film dried, it was peeled off from the mold to obtain a composite film with a thickness of 50 μm.

[0128] The composite membranes prepared in Examples 1-4 and Comparative Examples 1-3 were characterized, and the results are as follows:

[0129] I. Mechanical Performance Analysis

[0130] The composite membranes prepared in Examples 1-4 and Comparative Examples 1-3 were cut into strips of 20×50mm. The mechanical properties of the composite membranes were tested using a general mechanical testing machine at a testing rate of 5mm / min.

[0131] The tensile strength test results of the composite membrane are as follows: Figure 1 As shown, the tensile strength of the biomimetic active composite films prepared in Examples 1-4 is significantly higher than that of Comparative Examples 1-3. Among them, the tensile strength of Example 1 reaches the maximum value of 87.57 MPa, which is 32.40% higher than the tensile strength of the pure PVA film of Comparative Example 1 (66.14 MPa), 12.65% higher than the tensile strength of Comparative Example 2 (77.74 MPa with only LCNF added), and 67.09% higher than the tensile strength of Comparative Example 3 (52.41 MPa with only quercetin added).

[0132] This indicates that the synergistic effect of lignocellulose nanoparticles, quercetin, and the biomimetic structure effectively enhances the mechanical properties of the composite membrane. Lignocellulose nanoparticles, as a reinforcing phase, can form a good dispersion in the polyvinyl alcohol matrix and transfer stress through interactions such as hydrogen bonding. The addition of quercetin further improves the structural integrity of the membrane through cross-linking reactions with the polyvinyl alcohol molecular chains. The introduction of the lotus leaf biomimetic structure allows the micro-nano-scale rough structure formed on the membrane surface to disperse external forces to a certain extent, thus collectively improving the tensile strength of the composite membrane.

[0133] II. Water Contact Angle Analysis

[0134] The composite films prepared in Examples 1-4 and Comparative Examples 1-3 were cut into 30×30mm pieces and fixed on the sample stage of the contact angle measuring instrument. 8μL of deionized water was dropped onto the sample surface. After the water droplet stabilized for 3s, the water contact angle was recorded.

[0135] Water contact angle of composite membrane, such as Figure 2 As shown, the water contact angles of the biomimetic active composite membranes prepared in Examples 1-4 are significantly greater than those in Comparative Examples 1-3. Among them, Example 1 achieves the highest water contact angle of 137.34°, significantly higher than the 57.34° of the pure PVA membrane in Comparative Example 1, 72.55° in Comparative Example 2, and 76.69° in Comparative Example 3. This is mainly attributed to the synergistic effect of the micro-nano rough morphology constructed on the membrane surface by the lotus leaf biomimetic structure and the natural wax coating. The unique micro-papillary structure on the lotus leaf surface, after being replicated by the PDMS negative template, forms a similar multi-level rough structure of micron-scale protrusions and nano-scale villi on the composite membrane surface. The low surface energy of the natural wax further reduces the hydrophilicity of the membrane surface. The combined effect of these two factors makes it difficult for water droplets to spread on the membrane surface, thus exhibiting a high water contact angle and endowing the composite membrane with excellent hydrophobicity.

[0136] III. Free Radical Scavenging Performance Analysis

[0137] The composite membrane samples prepared in Examples 1-4 and Comparative Examples 1-3 were placed in 3 mL of ethanol solution and allowed to stand for 2 h. Then, they were thoroughly mixed with 1 mL of 150 μmol / L DPPH / ethanol solution to obtain the DPPH analysis solution. Under the same conditions, a DPPH blank solution without the composite membrane sample was prepared. All mixed solutions were allowed to stand for 30 min in a light-protected environment at 25 °C. Finally, the composite membrane was filtered out of the mixed solution, and the absorbance of the mixed solution was measured at 517 nm using a UV-Vis spectrophotometer.

[0138] Composite membrane free radical scavenging rate = (1-A) 样品 / A 空白 ) × 100%, A 样品 The absorbance of the DPPH analysis solution at 517 nm after the addition of the composite membrane; A 空白 The absorbance value at 517 nm is the DPPH blank solution without the composite membrane.

[0139] The free radical scavenging rate calculation results are as follows: Figure 3As shown, the free radical scavenging rates of the biomimetic active composite membranes prepared in Examples 1-4 were significantly higher than those in Comparative Examples 1-3. Among them, Example 1 achieved the highest free radical scavenging rate of 89.72%, far exceeding the 4.1% of the pure PVA membrane in Comparative Example 1, 55.71% in Comparative Example 2, and 79.22% in Comparative Example 3. This is mainly because quercetin, as a natural antioxidant, contains multiple phenolic hydroxyl groups in its molecular structure that can effectively provide hydrogen atoms to react with DPPH free radicals, reducing them to a stable molecular structure, thus exhibiting excellent free radical scavenging ability. Simultaneously, the addition of lignocellulose nanofibers provides a good loading platform for quercetin, stably dispersing it in the composite membrane system through hydrogen bonding and other interactions, preventing quercetin aggregation and loss, and prolonging its antioxidant activity. Furthermore, the introduction of the biomimetic structure did not negatively impact the free radical scavenging performance of the composite membrane; on the contrary, by increasing the specific surface area of ​​the membrane, it increased the contact opportunities between quercetin and free radicals, further improving the scavenging efficiency.

[0140] IV. Analysis of Ultraviolet-Visible Light Absorption Capacity

[0141] The composite films prepared in Examples 1-4 and Comparative Examples 1-3 were cut into 10×40mm pieces, placed in quartz cuvettes, and the light transmittance at wavelengths of 200–800nm ​​was recorded using a UV-Vis spectrophotometer. There was no significant difference in thickness among all samples.

[0142] The results of the UV-Vis transmittance analysis of the composite film are as follows: Figure 4 As shown, the biomimetic active composite films prepared in Examples 1-4 exhibit significantly lower light transmittance in the ultraviolet region (200-400 nm) compared to Comparative Examples 1-3, while maintaining a certain transmittance in the visible light region (400-800 nm). Specifically, Example 1 showed 0% ultraviolet transmittance at 280 nm, significantly lower than the 27.2% of the pure PVA film in Comparative Example 1, 3.4% in Comparative Example 2, and 7.6% in Comparative Example 3. This indicates that the composite film possesses excellent ultraviolet light absorption capabilities while having minimal impact on visible light transmittance. This is mainly due to the flavonoid functional groups contained in the quercetin molecule, which can absorb ultraviolet light energy through a conjugated system, thereby effectively blocking the penetration of ultraviolet light. The addition of lignocellulose nanofibers, through their own nanoscale structure, forms a synergistic effect with quercetin, further enhancing the scattering and absorption of ultraviolet light. The surface micro-nano rough morphology formed by the biomimetic structure increases the reflection and refraction paths of light on the membrane surface, allowing more ultraviolet light to be absorbed or scattered by the membrane material, thus jointly improving the ultraviolet and visible light absorption performance of the composite membrane, especially the blocking effect in the ultraviolet region.

[0143] The results show that the synergistic effect of wood nanoparticles, quercetin, and surface biomimetic structures forms a strongly cross-linked network structure, which significantly improves the mechanical and hydrophobic properties of polyvinyl alcohol films, and synergistically enhances the antioxidant and UV absorption capacity of polyvinyl alcohol, making it a potential candidate for food preservation packaging.

Claims

1. A method for preparing an active composite membrane with a biomimetic structure, characterized in that, The steps are as follows: Step 1: Prepare PDMS negative template: Take fresh lotus leaves, wash and remove surface contaminants, dry them and fix them at the bottom of a container. Mix the PDMS precursor and curing agent thoroughly and pour them onto the surface of the lotus leaves. Cure under vacuum. After curing, remove the lotus leaves to obtain a PDMS negative template with a negative topology on the surface of the lotus leaves. Step 2: Pre-treatment of poplar wood powder: Poplar wood powder was added to a 2% sodium hydroxide solution and reacted at 90°C for 2 hours. The filter cake was collected by filtration and washed with deionized water until neutral. After drying, pretreated poplar wood powder was obtained. Step 3: Prepare the eutectic solvent: Mix choline chloride and oxalic acid in equal molar amounts, heat and stir continuously until a homogeneous and transparent liquid is formed, then cool to room temperature for later use. Step 4: Preparation of LCNF The poplar powder pretreated in step two was mixed with the eutectic solvent prepared in step three at a mass ratio of 1:

50. The reaction was stopped after stirring at 80°C for 3 to 6 hours. The resulting reaction product was purified, and then the precipitate was collected by centrifugation. The precipitate was prepared into a suspension and sheared evenly to obtain an LCNF suspension. Step 5: Prepare a polyvinyl alcohol solution: Mix polyvinyl alcohol and distilled water at a mass ratio of 1:20, seal the mixture, heat and stir until a transparent and homogeneous liquid is formed, and then allow it to cool naturally. Step Six: Preparation of biomimetic active composite membrane: The LCNF suspension and quercetin / ethanol solution prepared in step four were added to the polyvinyl alcohol solution obtained in step five at a mass ratio of LCNF, quercetin and polyvinyl alcohol dry matter of 1~7:0.5~2:100, and the mixture was stirred at 50°C to crosslink and obtain a film-forming solution. The film-forming solution is poured into a mold containing the PDMS negative template obtained in step one. After the film dries, it is peeled off from the template to obtain the active composite film.

2. The method for preparing an active composite membrane with a biomimetic structure according to claim 1, characterized in that, The PDMS template preparation in step one uses Sylgard 184, with a precursor to curing agent mass ratio of 10:1, a curing temperature of 60°C, and a curing time of 8 hours.

3. The method for preparing an active composite membrane with a biomimetic structure according to claim 1 or 2, characterized in that, The poplar wood powder in step two has a particle size of 60 mesh, the mass-to-volume ratio of the poplar wood powder to the sodium hydroxide solution is 1g:50mL, the drying temperature is 100℃, and the drying time is 4 hours.

4. The method for preparing an active composite membrane with a biomimetic structure according to claim 3, characterized in that, The heating temperature described in step three is 80℃.

5. The method for preparing an active composite membrane with a biomimetic structure according to claim 4, characterized in that, Step four involves stopping the reaction by cooling the reaction system in an ice-water bath and adding distilled water; the impurity removal process involves placing the reaction product in a dialysis bag and dialyzing it with deionized water for 5 days; the centrifugation speed is 5000 rpm and the centrifugation time is 10 minutes; the mass fraction of the suspension is 0.7%; and the shearing is performed by using a high-speed blender at 800W power in a 1-second work-1-second intermittent working mode for 30 minutes.

6. The method for preparing an active composite membrane with a biomimetic structure according to claim 5, characterized in that, The heating and stirring temperature in step five is 95°C, and the cooling temperature is reduced to 50°C.

7. The method for preparing an active composite membrane with a biomimetic structure according to claim 6, characterized in that, In step six, the quercetin / ethanol solution has a quercetin concentration of 1 wt%; the cross-linking time is 30 minutes.

8. The method for preparing an active composite membrane with a biomimetic structure according to claim 7, characterized in that, Step six also includes spraying a layer of natural wax onto the surface of the resulting active composite membrane.

9. An active composite membrane with a biomimetic structure prepared by the preparation method according to any one of claims 1-8.

10. The application of an active composite film with a biomimetic structure as described in claim 9 in the fields of food preservation and pharmaceutical packaging.

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