Due to the high daily demand for plastics, plastic production is growing at an alarming rate. Globally, over 100 million tons of plastic waste are dumped into the environment annually, causing environmental pollution. Furthermore, an increasing number of terrestrial and marine animals are harmed by the accumulation of plastic waste in the environment. Therefore, plastic waste not only disrupts the ecological balance but also pollutes human food sources. To effectively alleviate the serious problems caused by plastic waste accumulation, some research has focused on its degradation. Traditional plastics, such as polyethylene (PE) and polyethylene terephthalate (PET), are extremely inert and require hundreds of years to completely degrade in the natural environment. While PE and PET plastics can be degraded through catalytic pyrolysis, by microorganisms isolated from the natural environment, and by synthetic enzymes, degradation typically requires harsh and strictly controlled conditions. Moreover, the number of plastic types that can be completely and effectively degraded through this method is very small. Therefore, the most effective way to solve the problem of plastic waste accumulation lies in replacing traditional plastics with new types of plastics that can be completely degraded into environmentally friendly substances in the natural environment. Polylactic acid (PLA) can be degraded into CO₂ and H₂O by microorganisms under specific, controlled humidity and temperature conditions. Currently, biodegradable PLA is widely used in biomedical devices, food packaging, and disposable tableware. However, the development of novel biodegradable plastics remains crucial to meet the demands of these applications. In particular, it is necessary to develop plastics that can completely degrade under natural environmental conditions.
Polyvinyl alcohol (PVA) is cost-effective to produce, and the material itself possesses desirable biodegradability, non-toxicity, high tensile strength, and excellent flexibility. In the natural environment, PVA can be completely degraded into CO₂ and H₂O through the microbial oxidation of hydroxyl groups to diketones and further hydrolysis. Therefore, PVA-based plastics are environmentally friendly and biodegradable, possessing broad application potential. However, PVA itself is water-soluble and readily absorbs water in aquatic environments, leading to a significant decrease in mechanical strength, limiting its applications and increasing storage costs. Commercially available polyvinyl formal (PVF) and polyvinyl butyral (PVB) are prepared through an acid-catalyzed acetalization reaction of PVA with aldehydes. They exhibit good water resistance, but due to their high acetalization rate (typically above 70%), they are difficult to degrade in soil. Therefore, there is an urgent need to develop biodegradable PVA-based plastics that can maintain sufficient mechanical strength in aquatic environments.
Non-toxic humic acid (HA) is a class of natural substances that plays an important role in maintaining ecological balance and reducing environmental pollution. HA is rich in oxygen-containing functional groups such as phenolic hydroxyl groups, aliphatic hydroxyl groups, and carboxyl groups, and some segments or side chains are hydrophobic. The research team envisioned using hydrogen bonding to construct a composite of PVA and HA, which could potentially prepare a mechanically stable biodegradable PVA-based supramolecular plastic in aquatic environments. The resulting supramolecular plastic has a tensile strength of approximately 85.0 MPa in air, and even after being immersed in water for 7 days, it still retains a tensile strength of approximately 26.2 MPa, comparable to polyethylene in air. This PVA-based plastic can be completely degraded in soil into humic acid nanoparticles and non-toxic small molecules, while also being reprocessable and easily molded into desired shapes.
Vanillin is an edible flavoring widely used in the food, beverage, and pharmaceutical industries. Vanillin-grafted modified polyvinyl alcohol (VPVA) can be prepared through an acid-catalyzed acetal reaction between PVA hydroxyl groups and vanillin. PVA-based supramolecular plastics were prepared using a one-pot method: a DMSO solution of PVA, HA, vanillin, and p-toluenesulfonic acid (PTSA) was heated at 80 °C for 6 hours. Then, a DMSO solution of FeCl₃ was added dropwise under vigorous stirring at 80 °C. After cooling to room temperature, the mixture was poured onto a clean glass plate and coated using a blade coating method. The resulting plastic film was peeled off the glass plate, dialyzed against deionized water to remove PTSA, DMSO, and unreacted vanillin, and dried at 25 °C for 24 hours and then at 50 °C for 4 hours to obtain the PVA-based supramolecular plastic. The sample with a PVA:vanillin:HA:FeCl₃ mass ratio of 1:0.35:0.05:0.01 was designated VPVA-HA-Fe, and the control sample without Fe³⁺ was designated VPVA-HA.
Fourier transform infrared spectroscopy confirmed the successful grafting of vanillin onto the PVA chains. VPVA molecular chains can be cross-linked via hydrogen bonds between the phenolic hydroxyl groups of vanillin and the aliphatic hydroxyl groups of PVA. In VPVA-HA, HA and VPVA chains also interact through hydrogen bonds. In VPVA-HA-Fe, Fe³⁺ can coordinate with the phenolic hydroxyl and carboxyl groups of HA, as well as the phenolic hydroxyl and ether bonds of vanillin. The synergistic effect of hydrogen bonding and coordination is the main driving force for the construction of the VPVA-HA-Fe supramolecular plastic. Infrared spectroscopy also showed that DMSO and PTSA were completely removed from the system after dialysis. In VPVA-HA, HA nanoparticles were uniformly dispersed with an average particle size of approximately 3.2 nm. After the introduction of Fe³⁺, HA nanoparticles aggregated through coordination, forming composite nanoparticles with an average particle size of approximately 41.2 nm, which were uniformly dispersed in the matrix. Compared with pure PVA, vanillin grafting significantly inhibited PVA crystallization. Thermogravimetric analysis (TGA) showed that VPVA-HA-Fe exhibited good thermal stability below 270 °C, with a moisture content of approximately 3.0 wt%.
Tensile testing of the material at 25 °C and 30% relative humidity revealed that VPVA had a yield strength of approximately 90.7 MPa, a tensile strength of approximately 48.2 MPa, and a Young's modulus of approximately 3.6 GPa. After introducing HA, the yield strength and tensile strength of VPVA-HA increased to 102.3 MPa and 55.6 MPa, respectively, with an elongation at break of approximately 59.7%. HA nanoparticles act as nanofillers to enhance the mechanical properties of the material and deform under external forces, thus imparting high ductility. Fe³⁺ coordination further increased the crosslinking density, resulting in yield strength, tensile strength, and Young's modulus of VPVA-HA-Fe reaching 120.1 MPa, 85.0 MPa, and 4.7 GPa, respectively. This material exhibits excellent flexibility, remaining unbroken after 1000 repeated bending cycles; the strength of the undialyzed sample is slightly lower due to the plasticizing effect of residual DMSO. When the vanillin to PVA mass ratio is fixed at 0.35:1, VPVA-HA-Fe containing 5.0 wt% HA and 1.0 wt% FeCl₃ demonstrates the best mechanical properties.
Practical applications require the material to maintain high strength in high humidity or water environments. After 7 days of exposure to 75% and 100% relative humidity, VPVA-HA-Fe exhibits tensile strengths of approximately 50.5 MPa and 44.8 MPa, respectively, significantly higher than conventional polyethylene (15–30 MPa). After immersion in water for 12 hours, the material still retains a tensile strength of approximately 26.2 MPa, with no significant strength decrease after 7 days of immersion. Upon water absorption, some hydrogen bonds and coordination interactions break, and the water acts as a plasticizer, transforming the material from a rigid plastic into an elastomer. In contrast, VPVA-HA softens after 12 hours of immersion in water due to HA dissolution. Fe³⁺ crosslinks VPVA and HA nanoparticles, inhibiting HA loss. Simultaneously, the hydrophobic HA-Fe³⁺ chelate structure enhances the material's hydrophobicity. The water contact angles for VPVA, VPVA-HA, and VPVA-HA-Fe are 71.9°, 77.0°, and 82.8°, respectively. After immersion in water for 3 days, the fracture strengths of VPVA and VPVA-HA are only 14.2 MPa and 12.1 MPa, respectively, while pure PVA is even lower at 6.7 MPa, fully demonstrating the mechanical stability of VPVA-HA-Fe in an aqueous environment.
Thanks to the dynamic reversibility of hydrogen bonding and coordination interactions, VPVA-HA-Fe can achieve efficient welding with water assistance. The material is cut into two sections, the cut surfaces are immersed in water for 1 hour, then overlapped, and hot-pressed at 75 °C and 2 MPa for 5 minutes to achieve a strong weld. The welded sample can still withstand heavy loads, and its mechanical properties are close to those of the original material. At the interface, molecular chains diffuse and reform dynamic cross-linking bonds, achieving reliable welding. Utilizing this welding characteristic, the film can be welded into a plastic bag that can support a 3 kg weight in a dry state and still support 2 kg after being submerged in water for 7 days, meeting daily usage needs.
Soil burial experiments show that the mass of VPVA-HA-Fe continuously decreases with prolonged burial time, reaching complete degradation after approximately 108 days. At 18 days of burial, the material's shape remains largely intact, but microorganisms have already attached to its surface; significant degradation occurs after 32 days, accelerating with further time until complete disappearance. The degradation mechanism is as follows: the material absorbs water and swells, some hydrogen bonds and coordination interactions dissociate, soil microorganisms attach to the surface and secrete dehydrogenases, oxidases, and hydrolases; under the catalysis of Lewis acids such as Fe³⁺ and Ca²⁺, vanillin can be hydrolyzed and removed from the VPVA chains; the aliphatic hydroxyl groups of PVA are oxidized to diketones, followed by carbon-carbon bond breakage, ultimately degrading into HA nanoparticles, vanillin and its derivatives, CO₂ and H₂O, and other environmentally friendly products.
As a dynamic supramolecular crosslinking material, VPVA-HA-Fe can be repeatedly recycled and reused under water and heat. The material can be cut into fragments, fully hydrated with water, and then hot-pressed at 75 °C and 4 MPa to obtain a complete film again. After five cutting-recycling cycles, the material can still recover its original mechanical strength. Furthermore, the fragments can be dissolved in a DMSO/water mixture and reprocessed by coating; the performance remains stable after multiple cycles, and the excellent recyclability significantly reduces raw material consumption.
In summary, the research team prepared a fully biodegradable VPVA-HA-Fe supramolecular plastic with high strength in aquatic environments through vanillin-modified PVA and the complexation of hydrophobic HA with Fe³⁺. High-density hydrogen bonding and coordination, as well as the reinforcing effect of Fe³⁺ chelating HA nanoparticles, give the material excellent mechanical properties; after immersion in water for 7 days, it still maintains a tensile strength comparable to dry polyethylene, meeting the requirements for aquatic applications. Dynamic non-covalent bonds endow the material with excellent recyclability and reprocessability. Furthermore, leveraging the biodegradability of PVA, it can be completely degraded into non-toxic substances in soil. The material's raw materials are environmentally friendly, inexpensive, and abundant. Its preparation process is suitable for large-scale production, and it boasts advantages such as simple preparation, mechanical stability in aquatic environments, and autonomous soil degradation, making it a promising candidate for applications in numerous fields.
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