A polyacrylamide / potato starch composite flexible biodegradable hydrogel film and its preparation method and application

Abstract

The application belongs to the technical field of biomass degradable polymer functional materials, and discloses a polyacrylamide / potato starch composite flexible biodegradable hydrogel film, a preparation method and application thereof. The polyacrylamide / potato starch composite flexible biodegradable hydrogel film is prepared by heating and polymerizing potato starch, acrylamide monomers, a crosslinking agent, an anti-freezing agent and an initiator. The application utilizes acrylamide and N,N'-methylene bisacrylamide to crosslink and modify starch, constructs a starch-polyacrylamide semi-interpenetrating network structure, and introduces lithium chloride as an anti-freezing agent, so as to improve the mechanical ductility, low-temperature resistance, water resistance and structural stability of the polyacrylamide / potato starch composite film, and simultaneously endows the composite film with pH-responsive swelling and strain sensing functions.

Description

Technical Field

[0001] This invention belongs to the technical field of biomass biodegradable polymer functional materials, specifically relating to a polyacrylamide / potato starch composite flexible biodegradable hydrogel film, its preparation method, and its application. Background Technology

[0002] Polyacrylamide (PAM)-based synthetic polymers have shown broad application prospects in agricultural mulch films, food packaging, and flexible devices due to their good film-forming properties and excellent mechanical properties. However, existing pure PAM films have the following drawbacks in practical applications: the high rigidity of the pure PAM film molecular chain leads to insufficient ductility and load-bearing capacity, significant performance degradation after cyclic stretching, and a sharp decrease in flexibility at low temperatures, making them prone to brittle fracture and unable to meet the requirements of low-temperature scenarios such as winter agricultural mulch films and refrigerated food packaging; at the same time, due to the simple network structure and lack of multi-component synergistic effects, the water resistance and rheological stability of pure PAM films need to be improved, and their functions are limited, lacking intelligent response characteristics such as pH-responsive swelling and flexible strain sensing, making it difficult to apply in emerging fields such as drug sustained-release carriers and wearable devices. Summary of the Invention

[0003] To address the shortcomings of existing technologies, this invention provides a polyacrylamide / potato starch composite flexible biodegradable hydrogel film, its preparation method, and its applications. This application uses high-quality starch prepared from local potatoes grown in Guyuan, Ningxia. The starch is modified through in-situ graft copolymerization with acrylamide and crosslinking with N,N'-methylenebisacrylamide to construct a starch-polyacrylamide semi-interpenetrating network structure. Lithium chloride is introduced as an antifreeze agent, thereby improving the mechanical extensibility, low-temperature resistance, water resistance, and structural stability of the polyacrylamide / potato starch composite flexible biodegradable hydrogel film. Simultaneously, the polyacrylamide / potato starch composite flexible biodegradable hydrogel film is endowed with pH-responsive swelling and strain-sensing functions.

[0004] One objective of this invention is to provide a flexible biodegradable hydrogel film composed of polyacrylamide and potato starch, wherein the film is prepared by heating and polymerizing potato starch with acrylamide monomer, crosslinking agent, lithium chloride and initiator. The mass ratio of potato starch to acrylamide monomer, crosslinking agent to initiator is 1:(2~4):(0.0061~0.0135):(0.02~0.0831); the temperature of the heating polymerization reaction is 60~70℃, and the reaction time is 10~30min.

[0005] Furthermore, the crosslinking agent is N,N'-methylenebisacrylamide.

[0006] Furthermore, the amount of lithium chloride used is 50% to 85% of the mass of potato starch.

[0007] Furthermore, the initiator is ammonium persulfate.

[0008] Furthermore, the potatoes are selected from fresh potatoes grown at high altitudes and with large temperature differences in Guyuan, Ningxia.

[0009] A second objective of this invention is to provide a method for preparing the above-mentioned polyacrylamide / potato starch composite flexible biodegradable hydrogel film, comprising the following steps: After washing, peeling, crushing, and sieving potatoes, they are mixed with water to obtain a suspension. The suspension is filtered, the filtrate is collected and allowed to settle. The precipitate is then dried, crushed, and sieved to obtain potato starch. Potato starch was mixed evenly with acrylamide monomer, crosslinking agent, antifreeze agent and initiator to obtain precursor solution; The precursor solution was heated and polymerized. After the reaction was completed, it was cooled to room temperature to obtain a polyacrylamide / potato starch composite flexible biodegradable hydrogel film.

[0010] Furthermore, the ratio of crushed potato to water is 1g:(10~14)mL.

[0011] Furthermore, the settling time is 2-3 hours.

[0012] Furthermore, the drying temperature is 40~60℃, and the drying time is 24~36h.

[0013] The third objective of this invention is to provide an application of the above-mentioned polyacrylamide / potato starch composite flexible biodegradable hydrogel film in low-temperature food packaging, winter biodegradable agricultural mulch film, flexible wearable strain sensor, and pH-responsive drug sustained-release carrier.

[0014] Compared with the prior art, the present invention has the following beneficial effects: (1) This invention involves in-situ graft copolymerization of acrylamide monomer and crosslinking agent with potato starch under the action of an initiator. The mass ratio of potato starch to acrylamide monomer and crosslinking agent to initiator is 1:(2~4):(0.0061~0.0135):(0.02~0.0831). The in-situ graft copolymerization reaction temperature is 60~70℃, and the reaction time is 10~30min. Within this range of mass ratio, reaction temperature, and reaction time, a starch-polyacrylamide semi-interpenetrating network structure is constructed, forming a large number of hydrogen bonds between molecular chains. This effectively solves the problems of insufficient mechanical strength and poor ductility of pure polyacrylamide films, resulting in a tensile strength of 317.02 for the film obtained by this invention. The film exhibits a high kPa and elongation at break of 1111.78%, with no significant performance degradation after cyclic stretching. Simultaneously, lithium chloride is introduced as an antifreeze agent, forming hydrogen bonds with starch molecules and polyacrylamide segments to synergistically impart excellent low-temperature flexibility. This allows the film to remain soft and stretchable even after 15 days of storage at -16℃, overcoming the low-temperature brittleness problem of pure polyacrylamide films. The semi-interpenetrating network structure enhances intermolecular hydrogen bonding, resulting in a static water contact angle of 93.3° and weak hydrophobicity, thus improving the film's rheological stability (storage modulus reaches 15650 kPa). The interpenetrating network structure (Pa, with the most gradual structural decay under high strain) improves the water resistance and rheological stability of the polyacrylamide / potato starch composite obtained in this invention. In addition, the interpenetrating network structure endows the film with pH-responsive swelling properties (swelling rate of 2238.374% under alkaline conditions and 449.447% under acidic conditions) and stable strain sensing performance (generating identifiable and cyclically stable electrical signals within the strain range of 100% to 500%), thus enabling the polyacrylamide / potato starch composite film to also have intelligent response functions, expanding its application in the fields of drug sustained-release carriers and wearable devices.

[0015] (2) This invention uses local potatoes from Guyuan, Ningxia as raw materials, and the preparation process is simple and the cost is controllable, which lays the foundation for the industrial application of starch-based multifunctional biodegradable flexible films. Attached Figure Description

[0016] Figure 1 The figures show the flexibility test results of the polyacrylamide-modified potato starch composite flexible biodegradable hydrogel film obtained in Example 1 of this invention under different stretching states, where (a) is the unstretched state, (b) is the stretched state, (c) is the curled state, and (d) is the tortuous state.

[0017] Figure 2 The diagram shows the polyacrylamide-modified potato starch composite flexible biodegradable hydrogel film obtained in Example 1 of this invention withstanding a static load of 500g.

[0018] Figure 3The images shown are scanning electron microscope (SEM) images of starch granules in Example 1 and Comparative Example 2 of the present invention, wherein (a) to (c) are SEM images of starch granules in Comparative Example 2, and (d) to (f) are SEM images of starch granules in Example 1.

[0019] Figure 4 For the present invention Figure 4 The tensile stress-strain curves of the films obtained in Example 1 and Comparative Examples 1-2 are shown.

[0020] Figure 5 The images show the antifreeze test results of the films obtained in Example 1 and Comparative Example 3 of the present invention. (a) is the original stretched shape of the film obtained in Comparative Example 3, (b) is the stretched shape of the film obtained in Comparative Example 3 at -16℃, (c) is the original stretched shape of the film obtained in Example 1, and (d) is the stretched shape of the film obtained in Example 1 at -16℃.

[0021] Figure 6 The image shows the XRD pattern of the thin film obtained in Example 1 of this invention.

[0022] Figure 7 The images show the FT-IR spectra of the thin films obtained in Example 1 and Comparative Examples 1-2 of this invention.

[0023] Figure 8 The images show the contact angles of the films obtained in Example 1 and Comparative Examples 1-2 of the present invention, where (a) is Comparative Example 1, (b) is Comparative Example 2, and (c) is Example 1.

[0024] Figure 9 This is a response curve of the relative resistance change ((R-R0) / R0) of the thin film obtained in Example 1 of the present invention as a function of time.

[0025] Figure 10 The cyclic voltammetry curves of the thin films obtained in Example 1 and Comparative Examples 1-2 of the present invention in the potential range of 0.0-1.0 V (vs. RHE) are shown.

[0026] Figure 11 The images show the electrochemical impedance spectroscopy (EIS) spectra of the thin films obtained in Example 1 and Comparative Examples 1-2 of this invention.

[0027] Figure 12 This is a graph showing the change in swelling ratio of the film obtained in Example 1 of the present invention over time.

[0028] Figure 13 The graph shows the changes in storage modulus (G') and loss modulus (G'') of the thin films obtained in Example 1 and Comparative Examples 1-2 of the present invention as a function of strain. Detailed Implementation

[0029] To enable those skilled in the art to better understand and implement the technical solutions of the present invention, the present invention will be further described below in conjunction with specific embodiments and accompanying drawings.

[0030] Unless otherwise specified, all reagents used in this invention are commercially available, and all methods used are conventional techniques in the art.

[0031] Example 1 A method for preparing a polyacrylamide / potato starch composite flexible biodegradable hydrogel film includes the following steps: (1) Preparation of potato starch: Fresh potatoes from Guyuan, Ningxia were washed with tap water, peeled and cut into pieces. They were first coarsely ground with a vegetable grinder and then further ground for 2 minutes with a Philips HL high-speed laboratory mixer until the texture was fine. The ground potato matrix was mixed evenly with 10 times the volume of distilled water. The resulting suspension was filtered and washed through a 100-mesh stainless steel sieve to remove soluble sugars and impurities. The filtrate was collected and allowed to stand for 3 hours to settle. The supernatant was discarded and the precipitate was collected. It was washed several times with distilled water and then placed in a 45℃ reaction oven to dry for 24 hours. The dried starch was then ground and passed through a 100-mesh sieve to obtain potato starch. It was sealed and stored in a plastic container for later use. (2) Preparation of polyacrylamide-modified potato starch composite flexible biodegradable hydrogel film: 4.0g acrylamide (AM) was used as the flexible polymer backbone monomer. 0.0061g of crosslinking agent N,N'-methylenebisacrylamide (MBA), 1.0g of potato starch obtained in step (1), 0.85g of antifreeze lithium chloride and 0.02g of thermal initiator ammonium persulfate (APS) were added to the monomer. The mixture was stirred until a transparent precursor solution was formed. The precursor solution was poured into a glass petri dish and heated in an oven at 70℃ under normal pressure for 20min. After the crosslinking polymerization reaction of the system was completed, the mixture was cooled to room temperature to obtain the polyacrylamide / potato starch composite flexible biodegradable hydrogel film, which was named SMS film.

[0032] Example 2 A method for preparing a polyacrylamide / potato starch composite flexible biodegradable hydrogel film includes the following steps: (1) The preparation of potato starch was the same as in Example 1; (2) Preparation of polyacrylamide-modified potato starch composite flexible biodegradable hydrogel film: 2.0g acrylamide (AM) was used as the flexible polymer main chain monomer. 0.0135g of crosslinking agent N,N'-methylenebisacrylamide (MBA), 1.0g of potato starch obtained in step (1), 0.5g of antifreeze lithium chloride and 0.0831g of thermal initiator ammonium persulfate (APS) were added to the monomer. The mixture was stirred until a transparent precursor solution was formed. The precursor solution was poured into a glass petri dish and heated in an oven at 60℃ under normal pressure for 30min. After the crosslinking polymerization reaction of the system was completed, the mixture was cooled to room temperature to obtain the polyacrylamide / potato starch composite flexible biodegradable hydrogel film.

[0033] Example 3 A method for preparing a polyacrylamide / potato starch composite flexible biodegradable hydrogel film includes the following steps: (1) The preparation of potato starch was the same as in Example 1; (2) Preparation of polyacrylamide-modified potato starch composite flexible biodegradable hydrogel film: 3.0g acrylamide (AM) was used as the flexible polymer main chain monomer. 0.01g crosslinking agent N,N'-methylenebisacrylamide (MBA), 1.0g potato starch obtained in step (1), 0.7g antifreeze lithium chloride and 0.05g thermal initiator ammonium persulfate (APS) were added to the monomer. The mixture was stirred until a transparent precursor solution was formed. The precursor solution was poured into a glass petri dish and heated in an oven at 65℃ under normal pressure for 10min. After the crosslinking polymerization reaction of the system was completed, the mixture was cooled to room temperature to obtain the polyacrylamide / potato starch composite flexible biodegradable hydrogel film.

[0034] Comparative Example 1 The difference between this comparative example and Example 1 is that no potato starch was added; the resulting polyacrylamide-based film is named BC film.

[0035] Comparative Example 2 The difference between this comparative example and Example 1 is that the self-made potato starch was replaced with an equal mass of "Caitianxiang" potato starch produced by Ningxia Guyuan Longxin Starch Co., Ltd.; the resulting polyacrylamide / potato starch composite flexible biodegradable hydrogel film was named CS.

[0036] Comparative Example 3 The difference between this comparative example and Example 1 is that the antifreeze lithium chloride was not added.

[0037] Performance testing Figure 1The figures show the flexibility test results of the polyacrylamide / potato starch composite flexible biodegradable hydrogel film obtained in Example 1 under different stretching states. (a) represents the unstretched state, (b) the stretched state, (c) the curled state, and (d) the tortuous state. The polyacrylamide / potato starch composite flexible biodegradable hydrogel film prepared in Example 1 was cut to standard size. Two tweezers were used to hold both ends of the sample, and tensile and bending loads were applied horizontally. The deformation process of the sample was recorded using a digital camera. The specific deformation test results are shown below. Figure 1 As shown in (a) to (d) in the diagram. Figure 1 As shown in (a) to (d), the film obtained in Example 1 can undergo significant reversible deformation under external force, and will not break instantly during stretching, curling, and twisting, exhibiting excellent elastic recovery characteristics. After continuous application of external force, the overall structure of the film remains intact, without defects such as surface peeling or pore expansion, demonstrating outstanding resistance to deformation damage. This invention constructs a semi-interpenetrating network structure by in-situ grafting and crosslinking starch and acrylamide, overcoming the common defects of polyacrylamide films such as low temperature and easy cracking under stress through the synergistic effect of intermolecular hydrogen bonds, and significantly improving the extensibility and structural stability of polyacrylamide / potato starch composite flexible biodegradable hydrogel film. Multiple cyclic tensile tests show that the film formed by polyacrylamide and potato starch composite does not show significant decrease in elongation at break and tensile strength after multiple cyclic tensile tests. Its uniform and dense internal structure and enhanced intermolecular interactions give the film both excellent flexibility and reliable mechanical stability, providing reliable mechanical test evidence for the application of polyacrylamide / potato starch composite flexible biodegradable hydrogel film in the packaging field.

[0038] Figure 2 The graph shows the polyacrylamide / potato starch composite flexible biodegradable hydrogel film obtained in Example 1 withstanding a static load of 500g. Figure 2 It can be seen that when the polyacrylamide / potato starch composite flexible biodegradable hydrogel film obtained in Example 1 is subjected to a static load of 500g, it can still maintain good morphological and structural stability without tearing, damage, or breakage. This proves that the polyacrylamide / potato starch composite flexible biodegradable hydrogel film obtained in this invention has excellent mechanical load-bearing capacity and can maintain structural integrity and stability in actual use scenarios such as food storage and transportation and agricultural mulching when subjected to external forces such as compression and friction.

[0039] Figure 3The images show scanning electron microscope (SEM) images of starch granules in Example 1 and Comparative Example 2, where (a) to (c) are SEM images of starch granules in Comparative Example 2, and (d) to (f) are SEM images of starch granules in Example 1. The morphology of the starch granules in Comparative Example 2 and Example 1 was observed using a scanning electron microscope (SEM). The test voltage was 10.0 kV, the working distance was 7.5 mm, and images were acquired at different magnifications of ×100 and ×500. The results are shown below. Figure 3 As shown. From Figure 3 As can be seen from (a) to (b) in Comparative Example 2, the commercially available starch granules are uneven in size and exhibit significant agglomeration, resulting in poor granule dispersion; while the self-made starch granules in Example 1 are relatively uniformly distributed, with significantly reduced agglomeration, and show a better overall dispersion. Figure 3 Further, as shown in (c) to (f), the commercially available starch granules in Comparative Example 2 have rough surfaces and irregular edges, with some granules exhibiting broken or sunken defects. This may lead to stress concentration, which is detrimental to the mechanical properties of the resulting film. In contrast, the self-made starch granules in Example 1 have more regular morphology, appearing as elliptical or polygonal shapes, with smooth surfaces, clear edges, and good particle integrity, rarely showing breakage or obvious defects. The particle size of the self-made starch granules in Example 1 is mainly distributed between 10 and 20 μm, with a narrow particle size distribution, indicating that the modification treatment effectively optimized the microstructure of the starch. This microstructure is the reason why the self-made starch-based flexible biodegradable hydrogel film possesses excellent mechanical properties. The regular and uniform starch granules can promote the orderly arrangement of polymer chains during film formation, forming a continuous and dense internal structure of the film, reducing stress concentration sites, and ensuring the stability of the network structure under low-temperature conditions. This is the microscopic basis for the comprehensive improvement of the mechanical properties and low-temperature resistance of the SMS film obtained in Example 1, giving it significant application advantages in agricultural mulch films and biodegradable food packaging.

[0040] Figure 4 The tensile stress-strain curves of the films obtained in Example 1 and Comparative Examples 1-2 are shown. Film samples with dimensions of 40 mm × 10 mm × 2 mm were prepared from the films obtained in Example 1 and Comparative Examples 1-2, respectively. Tensile tests were performed using a tensile testing machine at a tensile rate of 50 mm / min. The test results are shown below. Figure 4 As shown. By Figure 4It can be seen that, under the same test conditions, the SMS film obtained in Example 1 has excellent mechanical properties: a tensile strength of 317.02 kPa and a maximum elongation of 1111.78%. The CS film obtained in Comparative Example 2 has a lower tensile strength of only 169.96 kPa and a smaller maximum elongation of only 717.07%. The BC film obtained in Comparative Example 1, lacking starch backbone support, exhibits significantly different tensile strength and maximum elongation compared to the films obtained in Examples 1 and 2. This is mainly because the starch in both the CS and SMS films is modified by polyacrylamide grafting and crosslinking. The starch particles in Example 1 are uniform in size and well-dispersed, with ordered film-forming molecules. After modification, the crosslinking network is more uniform and dense, resulting in significantly better overall mechanical properties for the SMS film compared to the CS film. While the BC film possesses a polyacrylamide network structure, the lack of starch participation weakens the intermolecular hydrogen bonding and interpenetration synergistic effects, leading to a significant decrease in mechanical properties compared to the modified starch film. This demonstrates that the modification of starch with polyacrylamide plays a crucial role in the final mechanical properties of the film.

[0041] Figure 5 The images show the antifreeze test results of the films obtained in Example 1 and Comparative Example 3. In the images, (a) shows the original stretched shape of the film obtained in Comparative Example 3, (b) shows the stretched shape of the film obtained in Comparative Example 3 at -16°C, (c) shows the original stretched shape of the film obtained in Example 1, and (d) shows the stretched shape of the film obtained in Example 1 at -16°C.

[0042] To address the low-temperature brittleness and cracking defects of traditional films, and considering that agricultural mulch films and refrigerated food packaging both require tolerance to low-temperature environments, a comparative test on long-term low-temperature tolerance at -16℃ was conducted. Figure 5 As can be seen from (a) to (d), the film obtained in Comparative Example 3 underwent brittle fracture directly after being placed in an environment of -16℃ for 1 day, with almost no plastic deformation; while the SMS film obtained in Example 1 remained soft and stretchable after being stored in an environment of -16℃ for 15 days, without any embrittlement or cracking. This is mainly attributed to the addition of lithium chloride, an antifreeze agent, in this invention. The lithium salt weakens the hydrogen bonds between starch molecules and polyacrylamide molecular chains, improves the fluidity of polymer chain segments at low temperatures, optimizes the low-temperature flexibility of the polymer network, and endows the film with excellent low-temperature resistance, which can meet the needs of winter farmland covering film and low-temperature food preservation film, and make up for the problem of polyacrylamide film being easily damaged at low temperatures.

[0043] Figure 6 The image shows the XRD pattern of the thin film obtained in Example 1. Figure 6As can be seen, the XRD pattern of the film sample obtained in Example 1 shows significantly broadened diffuse peaks, with only one broadened diffraction peak at 2θ at 22-23°. This indicates that the crystalline structure of the starch granules was destroyed during film formation, and the molecular chains recombine to form a semi-crystalline structure dominated by the amorphous phase with a small number of short-range ordered small crystalline regions. This structure is caused by starch gelatinization and molecular chain rearrangement. The low crystallinity gives the film good flexibility and optical transparency, while the polyacrylamide-modified starch prevents the molecular chains from getting too close, thus effectively improving defects such as low-temperature embrittlement and easy breakage when exposed to water.

[0044] Figure 7 The FT-IR spectra of the thin films obtained in Example 1 and Comparative Examples 1-2 are shown below. The solid samples of the thin films obtained in Example 1 and Comparative Examples 1-2 were subjected to FT-IR spectra at 800–3400 cm⁻¹. -1 FT-IR scanning tests were performed within the range, and the test results are as follows: Figure 7 As shown. By Figure 7 It can be seen that the BC film obtained in Comparative Example 1 has a weaker infrared absorption peak and a much higher background reflectance than the CS film obtained in Comparative Example 2 and the SMS film obtained in Example 1, due to the absence of added starch. These two types of samples can be used as a reference. The hydroxyl (OH) stretching vibration region is at 3400 cm⁻¹. -1 At this point, the transmittance of the CS film was 100%, the SMS film was 50%, and the BC film was 95%. The excessively high transmittance of the CS film, even higher than that of the BC film, may be due to the lower moisture content of the commercially available starch samples or inconsistent sample thicknesses during testing. In contrast, the transmittance of the SMS film was significantly lower than that of the CS and BC films, indicating that the self-made starch of this invention has a transmittance of 3400 cm⁻¹. -1 The stronger absorption of infrared radiation at this point indicates the presence of more hydroxyl groups or more robust hydrogen bonds. This is because polyacrylamide modification disrupts some of the crystalline regions in the starch granules, releasing a large number of free hydroxyl groups and resulting in tighter intermolecular hydrogen bonds. At 3200 cm⁻¹... -1 At this location, the transmittance of the SMS film (60%) is still lower than that of the CS film (80%) and the BC film (85%), which confirms this point. The CH stretching vibration is located at 3000 cm⁻¹. -1 In the specified regions, the transmittance of the CS film was 70% and 60%, the SMS film was 50% and 55%, and the BC film was 80% and 85%. Compared to the CS film, the SMS film had lower transmittance in these two wavelength bands, indicating greater absorption at the CH bonds. The introduction of more alkyl groups or changes in the starch molecular chain during polyacrylamide modification resulted in more significant symmetrical and asymmetric stretching of the methylene (-CH2-) groups. At 1600 cm⁻¹... -1At the bending vibration of bound water (HOH), the transmittance of the CS film was 50%, the SMS film was 55%, and the BC film was 75%. The transmittance of the SMS film was close to that of the CS film, indicating that both materials contain a certain amount of bound water, but the SMS film had a slightly higher transmittance, possibly due to the higher number of hydroxyl groups leading to increased hygroscopicity. At 1400 cm⁻¹ -1 (CH2 bending) and 1200~1000cm -1 Within the range of COC and CO stretching vibrations, the transmittance of the SMS film is lower than that of the CS film at 1000 cm⁻¹. -1 The transmittance of the SMS film was 50%, while that of the CS film was 60%. This indicates that the absorption of glycosidic bonds and related ether bonds in the self-made starch in Example 1 was increased, suggesting that polyacrylamide has an effect on the starch backbone structure.

[0045] Figure 8 The image shows the contact angle diagrams of the films obtained in Example 1 and Comparative Examples 1-2, where (a) is Comparative Example 1, (b) is Comparative Example 2, and (c) is Example 1. The average contact angles of the films obtained in Example 1, Comparative Example 1, and Comparative Example 2 were 94.2°, 94.6°, and 93.3° respectively in three tests, all slightly greater than 90°, indicating that the film material has weak hydrophobicity. The consistency of the multiple measurement results is good, indicating that the film surface properties are uniform and there are no obvious local differences in hydrophilicity or hydrophobicity. Figure 8 As shown in (a) to (c), the contact angles of the films obtained in Comparative Examples 1 and 2 are 94.2° and 94.6°, respectively, slightly higher than the 93.3° of Example 1. This indicates that the contact angle of the composite film slightly decreases after the introduction of potato starch blending, meaning that the surface hydrophilicity slightly increases. This phenomenon can be attributed to the presence of a large number of hydroxyl groups (-OH) on the starch molecular chain, which form hydrophilic sites on the film surface, thus slightly enhancing the spreading tendency of water droplets on the composite film surface. The contact angle of Example 1 remains above 90°, indicating that the introduction of starch does not significantly destroy the overall weak hydrophobic characteristics of the film material. This is due to the hydrogen bond anchoring effect of the polyacrylamide network on the starch hydroxyl groups and the regulatory effect of LiCl on the hydrophilic-hydrophobic balance of the system. This weak hydrophobic characteristic helps to improve the water resistance of the film, which is of positive significance for its application in the field of packaging materials. In agricultural applications, improving the hydrophobicity of geomembranes can slow down water infiltration time and reduce evaporation, which is beneficial to crop growth. In the food packaging field, improving hydrophobicity helps reduce condensation and inhibit microbial growth, thereby extending product shelf life. Overall, although the contact angle of the film obtained in Example 1 is slightly lower than that of Comparative Examples 1-2, it still remains in the weakly hydrophobic range, and its wettability control has achieved a certain effect. It still has certain application potential in packaging and agricultural covering materials.

[0046] Figure 9This is a response curve showing the change in relative resistance ((R-R0) / R0) over time for the thin film obtained in Example 1. The strain sensor based on a polyacrylamide-based potato starch flexible biodegradable hydrogel film can clearly generate identifiable electrical signals for large strains (100%, 200%, 300%, 400%, and 500%). Figure 9 It can be seen that the signal amplitude gradually increases with the increase of relative resistance change. Meanwhile, the relative resistance change ((R-R0) / R0) of each group within the 5 stretching cycles is basically the same, indicating that the electrical signal has good repeatability and stability. This demonstrates that the hydrogel film obtained in this invention has the characteristics of fast response, stable cyclic response, and moderate sensitivity, making it suitable for use as a strain sensor for monitoring human motion or soft robot deformation with large strain, thus expanding the application scenarios of polyacrylamide-based potato starch flexible biodegradable hydrogel film materials.

[0047] Figure 10 This is a cyclic voltammogram of the thin films obtained in Example 1 and Comparative Examples 1-2 in the potential range of 0.0-1.0 V (vs. RHE). Figure 10 It can be seen that the cyclic voltammetry curves of the films obtained in Example 1 and Comparative Examples 1-2 did not show obvious redox peaks, and the overall curves exhibited a rectangular characteristic, indicating that the film samples mainly exhibited double-layer capacitance behavior. Among them, the pure polyacrylamide-based film (BC film) obtained in Comparative Example 1 had the highest current density and the largest area enclosed by the CV curve, indicating that it had the largest electrochemical active area or capacitance contribution. In contrast, after the introduction of potato starch, the current density of the commercially available starch composite film (CS film) obtained in Comparative Example 2 and the self-made starch composite film (SMS film) obtained in Example 1 both decreased significantly, and the current response of the SMS film was lower than that of the CS film. This indicates that the addition of starch has a certain inhibitory effect on the electrochemical activity of the film system, and the electrochemical performance of the self-made starch composite film is weaker than that of the commercially available starch composite film. In addition, the cyclic voltammetry curves of the three film samples all showed a weak cathode current near 0V, which may be related to the background hydrogen evolution reaction; while in the high potential region (about 1.0V), the anodic current density reached its peak, with the anodic current density of the BC film being about 0.025 mA·cm. -2 The anodic current density of the CS thin film is approximately 0.020 mA·cm⁻¹. -2 The anodic current density of the SMS thin film is approximately

[0048] 0.012 mA·cm -2 In summary, the introduction of starch reduced the electrochemical response of polyacrylamide-based films, and the effects of starch from different sources on electrochemical performance varied.

[0049] Figure 11The electrochemical impedance spectroscopy (EIS) spectra of the films obtained in Examples 1 and Comparative Examples 1-2 are shown. The EIS spectra of the films obtained in Examples 1 and Comparative Examples 1-2 were measured under an open-circuit voltage of 2V. Figure 11 It can be seen that the films obtained in Example 1 and Comparative Examples 1-2 all exhibit an approximately semi-circular arc in the high-frequency region, while they appear as approximately vertical straight lines in the low-frequency region, generally conforming to the typical characteristics of an electric double-layer capacitor. The high-frequency intercept reflects the solution resistance. The solution resistances of the BC film, CS film, and SMS film are quite similar, all around 4-5 ohms, indicating that the electrolyte system and electrode contact conditions are consistent. The semi-circular arc diameter corresponds to the charge transfer resistance (Ro). ct The BC film has the smallest arc diameter, R ct The lowest value indicates that the charge transfer kinetics of the pure polyacrylamide-based film are the fastest; the arc diameter of the CS film is slightly larger than that of the BC film, R ct Slightly higher; the SMS film has the largest arc diameter, R ct The highest slope indicates that the self-made starch composite film has the greatest charge transfer resistance and the lowest interfacial charge transport efficiency. The slope of the linear curve in the low-frequency region reflects ion diffusion behavior. The slopes of the linear curves for the BC and CS films are similar, both exhibiting a relatively steep, near-vertical characteristic, indicating good capacitance characteristics and a fast ion diffusion rate. The slope of the linear curve for the SMS film is significantly smaller, and it does not quickly enter the vertical segment after the high-frequency semicircle, but rather shows a gradual transition before becoming vertical. This suggests that the ion diffusion path of the self-made starch composite film is longer or the diffusion resistance is greater, possibly related to the densification of the network structure. In summary, the introduction of starch increases the charge transfer resistance and reduces the ion diffusion efficiency. The self-made starch composite film (SMS) shows the most significant deterioration in impedance characteristics, while the commercially available starch composite film (CS film) falls between the pure polyacrylamide-based film (BC film) and SMS. This result is consistent with the conclusion that the SMS film has the lowest current response in the cyclic voltammetry test, indicating that the source and dispersion state of starch have a significant impact on the electrochemical performance of the composite film.

[0050] Figure 12 The graph shows the change in swelling ratio of the film obtained in Example 1 over time. The film obtained in Example 1 was immersed in deionized water, hydrochloric acid aqueous solution (pH=3), and sodium hydroxide aqueous solution (pH=9) for 6 days, respectively, and the change in its swelling ratio over time was recorded. Specific results are shown below. Figure 12 As shown. By Figure 12As can be seen, the hydrogel film obtained in Example 1 exhibits significant swelling differences under different pH conditions: the swelling rate is highest in an alkaline environment (pH=9, maximum 2238.374%), followed by an acidic environment (pH=3, maximum 449.4472%), and lowest in neutral water (maximum 346.6679%). This phenomenon is attributed to the protonation / deprotonation balance of carboxyl groups (-COOH) in the gel network: under alkaline conditions, -COO- generates electrostatic repulsion, promoting molecular chain extension and water absorption; under acidic conditions, -COOH is protonated, hydrogen bonding is enhanced, and the network relatively shrinks; under neutral conditions, the electrostatic effect is weakest, and the swelling rate is lowest. Furthermore, the swelling rate slightly decreases after reaching its peak in deionized water and hydrochloric acid aqueous solution (pH=3), possibly due to network relaxation or dissolution of low-crosslinked segments caused by prolonged immersion. Overall, the hydrogel film obtained in this invention exhibits significant pH-responsive swelling behavior and good stability under neutral and acidic conditions, making it suitable for use in pH-sensitive drug delivery or smart hydrogel devices.

[0051] Figure 13 This is a graph showing the storage modulus (G') and loss modulus (G'') of the thin films obtained in Examples 1 and Comparative Examples 1-2 as a function of strain. Figure 13 It can be seen that throughout the entire test strain range, the G' of all three is greater than G'', indicating that the films obtained in Example 1 and Comparative Examples 1-2 are dominated by elastic response. In terms of modulus values, the G' of the BC film is the highest in the linear viscoelastic region (24870 Pa), followed by the SMS film (approximately 15650 Pa), and the CS film is the lowest (approximately 12240 Pa); the G' of the BC film is approximately 1.6 times that of the SMS film and 2.0 times that of the CS film, respectively. As the strain increases, all three enter the nonlinear region, and G' begins to decrease. Among them, the G' of the empty BC film decreases most rapidly; when the strain exceeds approximately 10%, its G' has dropped to less than 50% of its initial value, indicating that although the pure PAM (polyacrylamide) crosslinked network has high initial rigidity, its structure is most thoroughly destroyed under shear. The SMS film obtained in Example 1 exhibited the most gradual decrease in G', maintaining a good modulus level even under high strain conditions. Its rate of decrease was significantly lower than that of the BC film, indicating that the synergistic effect of hydrogen bonds and interpenetrating network structure formed by starch and PAM in the SMS film effectively delayed the destruction of the film's network structure. The CS film had the lowest initial modulus, and its rate of decrease was between that of the BC and SMS films. The above data comparison shows that although the BC film had the highest initial modulus, it had the weakest resistance to shear failure; the SMS film had a moderate initial modulus, but exhibited the best structural stability under high strain; the CS film had the weakest overall performance due to the poor compatibility between starch and the polyacrylamide matrix.

[0052] It should be noted that when numerical ranges are involved in this invention, it should be understood that the two endpoints of each numerical range and any value between the two endpoints can be selected. Since the steps and methods used are the same as in the embodiments, preferred embodiments are described in this invention to avoid redundancy. Although preferred embodiments of this invention have been described, those skilled in the art, once they understand the inventive concept of this invention, can make other changes and modifications to these embodiments, and all such changes and modifications fall within the scope of this invention.

[0053] Obviously, those skilled in the art can make various modifications and variations to this invention without departing from its spirit and scope. If such modifications and variations fall within the scope of equivalents of this invention, then this invention is also intended to include such modifications and variations.

Claims

1. A flexible biodegradable hydrogel film composed of polyacrylamide / potato starch, characterized in that, The film is prepared by heating and polymerizing potato starch with acrylamide monomer, crosslinking agent, lithium chloride and initiator. The mass ratio of potato starch to acrylamide monomer, and crosslinking agent to initiator is as follows: 1:(2~4):(0.0061~0.0135):(0.02~0.0831); The heating polymerization reaction is carried out at a temperature of 60~70℃ and a reaction time of 10~30min.

2. The polyacrylamide / potato starch composite flexible biodegradable hydrogel film according to claim 1, characterized in that, The crosslinking agent is N,N'-methylenebisacrylamide.

3. The polyacrylamide / potato starch composite flexible biodegradable hydrogel film according to claim 1, characterized in that, The amount of lithium chloride used is 50-85% of the mass of potato starch.

4. The polyacrylamide-modified potato starch composite flexible biodegradable hydrogel film according to claim 1, characterized in that, The initiator is ammonium persulfate.

5. A method for preparing a polyacrylamide / potato starch composite flexible biodegradable hydrogel film as described in any one of claims 1 to 4, characterized in that, Includes the following steps: After washing, peeling, crushing, and sieving potatoes, they are mixed with water to obtain a suspension. The suspension is filtered, the filtrate is collected and allowed to settle. The precipitate is then dried, crushed, and sieved to obtain potato starch. Potato starch was mixed evenly with acrylamide monomer, crosslinking agent, antifreeze agent and initiator to obtain precursor solution; The precursor solution was heated and polymerized. After the reaction was completed, it was cooled to room temperature to obtain a polyacrylamide / potato starch composite flexible biodegradable hydrogel film.

6. The polyacrylamide / potato starch composite flexible biodegradable hydrogel film according to claim 1, characterized in that, The ratio of crushed potato to water is 1g:(10~14)mL.

7. The method for preparing the polyacrylamide / potato starch composite flexible biodegradable hydrogel film according to claim 5, characterized in that, The settling time is 2-3 hours.

8. The method for preparing the polyacrylamide / potato starch composite flexible biodegradable hydrogel film according to claim 5, characterized in that, The drying temperature is 40~60℃, and the drying time is 24~36h.

9. The application of a polyacrylamide / potato starch composite flexible biodegradable hydrogel film as described in any one of claims 1 to 4 in low-temperature food packaging, winter biodegradable agricultural mulch film, flexible wearable strain sensor, and pH-responsive drug sustained-release carrier.

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