Abstract
Water-insoluble micropatterned films were prepared from poly(vinyl alcohol) (PVA) (or ethylene-vinyl alcohol copolymer (EVOH)) and poly(methacrylic acid) (poly(MAAc)). The carboxy groups in poly(MAAc) underwent dehydration reactions with the hydroxy groups in the vinyl alcohol units during heating at 135 °C, which resulted in crosslinking with ester bonds and formation of a polymeric network in the micropatterned films. The surface structures of the micropatterned films and the shapes of the peeled microplastics in the supernatant were measured after decomposition in an oxidizing environment, after ultrasonic irradiation, and with both. The results revealed that the micropatterns could be peeled off from the films after they were subjected to appropriate decomposition conditions and maintained their patterned shapes.
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Introduction
The annual production of plastics exceeds 400 million tons. The leakage of plastics into rivers, oceans, soil, and the atmosphere, is one of the most pressing problems worldwide [1,2,3,4]. In particular, the effects of micro- and nanoplastics generated by the fragmentation and decomposition of discarded plastics on the environment and living organisms have not been determined. Wei et al. reported that millions of microplastics are generated from 0.1 g of a poly(ε-caprolactone) film via enzymatic hydrolysis in aqueous solutions [5]. After generation for up to 6 days, the microplastics exhibited irregular shapes and had an average size of ~10 μm. Song et al. exposed expanded poly(styrene) (poly(St)) to sunlight and monitored its decomposition for 2 years [6]. After exposure to sunlight for 1 month, the mass of the expanded poly(St) had decreased by 5%, and 6.7 × 107 particles/cm2 of micro- and nanoplastics were generated. Thus, large amounts of irregularly shaped microplastics are generated during the decomposition and fragmentation of plastic products. Micro- and nanoplastics exhibit several morphologies, including fragments, foams, paints, pellets, foils, spheres, fibers, films, beads, flakes, sheets, granules, and nurdles [7]. Notably, the accumulation and toxicity of microsized polymeric materials in the different organs of living organisms vary substantially depending on their shapes and sizes. After zebrafish were exposed to poly(St) microplastic particles for 1 week, microplastic particles measuring 5 μm in diameter had accumulated in the gills, liver, and intestines of the fish, whereas microplastic particles with diameters of 20 μm had accumulated exclusively in the gills and intestines [8]. Choi et al. evaluated the cytotoxicities of poly(ethylene) microplastics based on their shapes [9]. Compared to poly(ethylene) beads with smooth surfaces, poly(ethylene) fragments with rough surfaces exhibited high cytotoxicity and induced proinflammatory cytokine release and hemolysis. Plastic films with micropatterns have been used in various applications, such as sensors, optical materials, medical devices, and surface modifications [10,11,12]. However, research on the shapes of the microplastics generated during decomposition of these micropatterned films is lacking. In an environment such as the ocean, the decomposition of discarded plastics into smaller pieces usually occurs through a combination of chemical and physical processes involving photolysis, oxidation, hydrolysis, and mechanical disruption [13]. Long-term observation periods are required to assess the mechanisms involved in the decomposition of microplastics generated from bulk plastics.
Therefore, in this study, micropatterned polymeric films that decomposed quickly under experimental conditions were fabricated, and their decomposition processes were evaluated (Fig. 1). It was discovered that the micropatterns could be peeled off from the films while maintaining their patterned shapes, depending on the decomposition conditions. The micropatterned films were composed of poly(methacrylic acid) (poly(MAAc)) and either a hydrophilic polymer, poly(vinyl alcohol) (PVA), or a hydrophobic polymer, the ethylene-vinyl alcohol copolymer (EVOH). The carboxy groups in poly(MAAc) undergo dehydration reactions with the hydroxy groups in the vinyl alcohol units during heating, resulting in the introduction of a crosslinked structure with ester bonds into the polymeric networks of the micropatterned films. Vinyl alcohol-based polymers (i.e., PVA and EVOH) were selected because they are known to decompose in an oxidizing environment [14,15,16]. The surface structures of the micropatterned films and the shapes of the peeled microplastics in the supernatant were measured during decomposition via oxidation, ultrasonic irradiation, and a combination. To the best of our knowledge, this is the first study demonstrating that PVA-based micropatterned films release microplastics upon decomposition while preserving their micropatterns. This study provides information on the release of shape-controlled microplastics as well as the methods for fabricating microplastics with micropatterns.
Schematic representation of the micropatterned films and the generation of micropatterned microplastics
Experimental
Materials
Methacrylic acid (MAAc) was purchased from Tokyo Chemical Industry Co., Ltd (Tokyo, Japan) and purified by passing it through a basic alumina column. PVA (Mw = 89,000–98,000, 99+% hydrolyzed) and EVOH (ethylene content = 32 mol%) were purchased from Sigma Aldrich (Missouri, USA) and were used as received. All other chemicals and solvents were used as received.
Preparation of poly(MAAc)
Poly(MAAc) was synthesized by free-radical polymerization. MAAc (7.75 g, 90.0 mmol) and 2,2′-azobis(isobutyronitrile) (AIBN) (73.9 mg, 4.50 × 10−1 mmol) ([MAAc]0/[AIBN]0 = 200/1) were dissolved in 60 mL of N,N-dimethylformamide. After degassing with nitrogen for 60 min, the solution was polymerized for 20 h at 70 °C. The resulting poly(MAAc) was purified by dialysis, first against ethanol/water (1/1 v/v) and then against water, and finally dried by lyophilization. Poly(MAAc) was obtained as a white powder in 89% yield.
Preparation of thermally crosslinked films with micropatterns
PVA (or EVOH) and poly(MAAc) were combined to give a weight of 150 mg. For example, 135 mg of PVA and 15 mg of poly(MAAc) (10 wt%) were dissolved in 5 mL of water. For the EVOH/poly(MAAc) films, 135 mg of EVOH and 15 mg of poly(MAAc) (10 wt%) were dissolved in 1 mL of dimethyl sulfoxide. Both the PVA/poly(MAAc) and EVOH/poly(MAAc) films were prepared with 20 wt% poly(MAAc) at the same polymer concentration. The mixture was poured into a microporous template (35 mm dish, low-binding MH or MS patterns, Organogenix Co., Ltd, Kanagawa, Japan). After drying, the films peeled from the templates were heated at 135 °C for 48 h. The film thicknesses (poly(MAAc), 10 wt%) were 0.156 ± 0.027 mm (PVA/poly(MAAc)) and 0.115 ± 0.047 mm (EVOH/poly(MAAc)).
Treatment of micropatterned films in an oxidizing environment
Thermally crosslinked PVA/poly(MAAc) and EVOH/poly(MAAc) films were cut and immersed in sodium hypochlorite pentahydrate (NaClO·5H2O) aqueous solutions at a given concentration and immersion time. After degradation, all of the films were washed with water and dried under atmospheric pressure.
Treatment of micropatterned films by ultrasonication
Thermally crosslinked PVA/poly(MAAc) and EVOH/poly(MAAc) films were cut (1 cm2) and immersed in 3 mL of water. All of the samples were ultrasonicated (with an MCS-10 single-frequency ultrasonic cleaner) (200 W, 40 kHz; AS ONE CORPORATION, Osaka, Japan) at 25 °C for 5 or 24 h. After the treatment, the films were dried under atmospheric pressure.
Treatment of the micropatterned films by a combination of oxidation and ultrasonication
Thermally crosslinked PVA/poly(MAAc) and EVOH/poly(MAAc) films were cut (1 cm2) and immersed in a NaClO·5H2O aqueous solution at a given concentration and immersion time. After degradation, all of the films were washed with water and dried under atmospheric pressure. The films were immersed in 3 mL of water and ultrasonicated at 25 °C for 5 h or 24 h. After the treatment, the films were dried under atmospheric pressure. After degradation (ultrasonication for 10 min), the supernatant was added to a plastic tube and centrifuged at 1000 rpm for 5 min to remove large pieces of the degraded films. The supernatant was centrifuged at 3000 rpm for 5 min. The supernatant was removed, and 1 mL of water was added to disperse the precipitated pieces of the film. A total of 20 μL of the dispersion was added to a silicon wafer and dried under atmospheric pressure.
Characterization
To confirm the synthesis and determine the chemical structure of the synthesized poly(MAAc), 1H nuclear magnetic resonance spectra of the poly(MAAc) were obtained with a JNM-GSX300 spectrometer operating at 300 MHz (JEOL, Tokyo, Japan).
The molecular weights and polydispersities of the synthesized copolymers were determined with gel permeation chromatography (GPC) at 40 °C (0.2 M phosphate buffer at a pH of 8, 0.7 mL/min) with Shodex SB-802.5 HQ and Shodex SB-804 HQ columns (Showa Denko K. K., Tokyo, Japan) and a RID-20A refractive index detector (Shimadzu Co., Kyoto, Japan). Poly(ethylene oxide) and poly(ethylene glycol) were used as standards to construct the calibration curves.
Scanning probe microscopy (SPM) was performed with an E-sweep system (Hitachi High-Tech Co., Tokyo, Japan) in noncontact mode with a Si cantilever (spring constant: 1.6 N m−1).
The morphologies of the micropatterned films were observed with scanning electron microscopy (SEM) (JSM-IT 100, JEOL, Tokyo, Japan) at an accelerating voltage of 10 kV. The samples were sputter-coated with Au before the SEM measurements. Elemental analyses were performed with energy dispersive X-ray spectroscopy (EDS).
Statistical analyses
All the data are presented as the mean ± standard deviation.
Results and discussion
Preparation of water-insoluble micropatterned films
The poly(MAAc) synthesized by free-radical polymerization (with a molecular weight (Mn) of 35,300 and molecular weight distribution (Mw/Mn) of 1.40) was mixed with PVA or EVOH in solution, and the mixed solutions were poured into a substrate with micropores. After drying, the polymeric films were peeled off from the substrate and thermally crosslinked at 135 °C for 48 h. In our previous studies, water-insoluble films, sponges, microfibers, and nanopillar films consisting of PVA and poly(MAAc) were fabricated with the thermal crosslinking process [17,18,19,20,21]. Thermal crosslinking caused dehydration between the carboxy groups of poly(MAAc) and the hydroxy groups of PVA or EVOH, resulting in the formation of covalent bonds.
Figure 2 shows SEM images of the fabricated PVA/poly(MAAc) and EVOH/poly(MAAc) films with micropatterns after thermal crosslinking (poly(MAAc) content: 10 or 20 wt%). The side sections of the films were measured with broken pieces after immersion in liquid nitrogen for several minutes. The SEM images show that the micropores of the substrates were transferred to the polymeric films as hexagonal micropatterns. The hexagonal micropatterns were also observed by SPM (Fig. S1). PVA/poly(MAAc) and EVOH/poly(MAAc) films with square micropatterns were fabricated with substrates with square micropores (Fig. S2). These thermally crosslinked films did not dissolve and maintained their micropatterned structures after immersion in water.
SEM images of the fabricated A PVA/poly(MAAc)_10wt%, B PVA/poly(MAAc)_20wt%, C EVOH/poly(MAAc)_10wt%, and D EVOH/poly(MAAc)_20wt% films with micropatterns after thermal crosslinking (10 or 20 wt%: poly(MAAc) contents)
To evaluate the thermodynamic properties of the PVA/poly(MAAc) and EVOH/poly(MAAc) films, the flat films were analyzed via differential scanning calorimetry (Table S1). The changes in the glass transition temperatures (Tg), melting points (Tm), and enthalpy (ΔHm) were attributed to increased poly(MAAc) contents in the films, which resulting in decreased crystallinity and hydrogen bonding of the PVA [18]. Similar thermodynamic properties were observed for the flat films of EVOH/poly(MAAc). Similar phenomena resulting in decreased crystallinity and hydrogen bonding of PVA upon mixing with other materials have been reported by several research groups. For example, a dual physically cross-linked hydrogel consisting of PVA crystallites and hyaluronic acid-Fe3+ networks showed decreases in the Tm and ΔHm values with increasing hyaluronic acid contents [22]. Uslu et al. reported disappearance of the Tm of PVA upon the addition of boron compounds as crosslinkers [23]. A conjugated material consisting of PVA and keratin with hydrogen-bonding interactions exhibited lower enthalpy values than PVA [24].
Treatment of micropatterned films in an oxidizing environment
Next, the fabricated micropatterned films were immersed in oxidizing aqueous solutions of sodium hypochlorite pentahydrate (NaClO·5H2O), after which their decompositions were measured. PVA has been shown to undergo degradation in oxidizing environments, with light, with microorganisms, and with other factors [14,15,16]. NaClO·5H2O was chosen as the oxidizing agent based on its environmentally friendly properties, as it produces water and sodium chloride byproducts [25, 26]. In our previous study, the decomposition of water-insoluble, crosslinked PVA/poly(MAAc) flat films was controlled by immersion in a NaClO·5H2O aqueous solution. The crosslinked films completely dissolved in the NaClO·5H2O aqueous solution within 25 h. The molecular weight of the PVA decreased with decomposition [17]. The degradation mechanism involved the following reactions: NaClO·5H2O first oxidized some of the hydroxy groups of the 1,3-diol moieties to produce β-hydroxy ketone moieties. The resulting β-hydroxy ketones easily underwent retro-aldol reactions to form aldehydes and methyl ketones. In contrast, poly(MAAc) did not decompose under the same oxidizing conditions, and no decrease in its molecular weight was observed. The decomposition rates of the PVA/poly(MAAc) and PVA/EVOH flat films were measured in NaClO·5H2O aqueous solutions with various concentrations. The NaClO·5H2O was added to give three times the equivalent amount of the vinyl alcohol units of PVA, and the immersed flat films were collected after 5 and 20 h. After washing with a large amount of water and drying, the residual weights were calculated (Fig. S3). After 5 h of oxidation, the residual weights of the flat films were 71.2 ± 4.2% for PVA/poly(MAAc)_10wt% and 78.1 ± 9.7% for PVA/poly(MAAc)_20wt%. After 20 h, the residual weights were 20.9 ± 7.2% for PVA/poly(MAAc)_10wt% and 13.9 ± 6.0% for PVA/poly(MAAc)_20wt%, suggesting that decomposition had progressed. In contrast, the residual weights of the EVOH/poly(MAAc) flat films were greater than those of the PVA/poly(MAAc) flat films (89.7 ± 3.7% for EVOH/poly(MAAc)_10wt% and 92.7 ± 1.8% for EVOH/poly(MAAc)_20wt%), even after 20 h under the same immersion conditions (the NaClO·5H2O was added at 2.4 times the number of vinyl alcohol units in EVOH). The higher residual weights of the EVOH/poly(MAAc) films were attributed to the hydrophobicities of the ethylene units of EVOH. When the amount of NaClO·5H2O was increased to 32 times that of the vinyl alcohol units of EVOH, the residual weights of the EVOH/poly(MAAc) flat films were 66.2 ± 7.7% for EVOH/poly(MAAc)_10wt% and 29.4 ± 3.3% for EVOH/poly(MAAc)_20wt% after immersion for 24 h. These results suggested that there was an appropriate concentration of NaClO·5H2O in evaluating decomposition of the films, depending on the polymeric compositions.
Figure 3 shows the heights of the hexagonal micropatterns after the micropatterned films were immersed in NaClO·5H2O aqueous solution with different concentrations and for different immersion times. For both the PVA/poly(MAAc)_10wt% and PVA/poly(MAAc)_20wt% micropatterned films, no difference was observed in the heights of the patterns before decomposition. When the PVA/poly(MAAc)_10wt% films were immersed in a 30 mg/mL NaClO·5H2O aqueous solution for 15 min, the height was 0.96 ± 0.04 μm, which was greater than that before decomposition. This was attributed to swelling of the micropattern caused by decomposition of some of the films. This was consistent with the findings of other reports on hydrogel swelling during the early stages of decomposition [27,28,29,30]. When the samples were immersed in a 60 mg/mL NaClO·5H2O aqueous solution, decomposition of the micropatterned films progressed, and the heights of the micropatterns decreased to 0.64 ± 0.07 μm. For the PVA/poly(MAAc)_20wt% films, the heights of the micropatterns after decomposition were 0.55 ± 0.11 μm for the 30 mg/mL NaClO·5H2O aqueous solution and 0.33 ± 0.07 μm for the 60 mg/mL NaClO·5H2O aqueous solution. These values were lower than those for the PVA/poly(MAAc)_10wt% films (Fig. 3A–C). EVOH is one of the most common gas barrier materials used in multilayer food packaging and is known to have relatively low degradability [31]. The thermal stability of EVOH increases with increasing ethylene content [32]. EVOH is degraded by oxidizing environments and microorganisms, depending on the conditions [33,34,35,36]. The heights of the micropatterns in the EVOH/poly(MAAc)_10wt% and EVOH/poly(MAAc)_20wt% were 0.74 ± 0.04 μm and 0.66 ± 0.10 μm, respectively, and no difference was observed in heights of the two micropatterned films before decomposition (Fig. 3D–F). After immersion in a 120 mg/mL NaClO·5H2O aqueous solution for 30 min, the heights of the micropatterns in the EVOH/poly(MAAc)_10wt% and EVOH/poly(MAAc)_20wt% films were 0.76 ± 0.08 μm and 0.67 ± 0.09 μm, respectively, and no change in the height of either film was observed after decomposition. When immersed in a 120 mg/mL NaClO·5H2O aqueous solution for 2 h, the heights of the micropatterned EVOH/poly(MAAc)_10wt% and EVOH/poly(MAAc)_20wt% films were 0.63 ± 0.10 μm and 0.54 ± 0.07 μm, respectively, which were smaller than their heights before decomposition. The micropatterns of EVOH/poly(MAAc) showed less decomposition in the NaClO·5H2O aqueous solutions than those of PVA/poly(MAAc). This difference was attributed to the hydrophobicity of the ethylene units in EVOH. These results suggested that the micropatterns of the PVA/poly(MAAc) and EVOH/poly(MAAc) films underwent gradual degradation when exposed to the oxidizing environment of the NaClO·5H2O aqueous solution, and their heights decreased in response to decomposition.
Heights of the hexagonal micropatterns after the micropatterned films were immersed in NaClO·5H2O aqueous solutions with different concentrations and for different immersion times (n = 20). A–C PVA/poly(MAAc) micropatterned films. D–F EVOH/poly(MAAc) micropatterned films. *p < 0.05
Treatment of micropatterned films by ultrasonication
Next, ultrasonic irradiation was applied to the micropatterned films treated with the NaClO·5H2O aqueous solution, after which their decomposition was observed. Decomposition of polymers with ultrasonic irradiation has been reported previously [37, 38]. Mohod et al. reported that the viscosity and molecular weight of PVA decreased after ultrasonic treatment [38]. First, the ultrasonic decomposition of PVA and poly(MAAc), which were the components of the micropatterned films, were measured. A general ultrasonic device used for cleaning glass instruments in the laboratory was selected as the ultrasonic generator. Aqueous solutions of PVA and poly(MAAc) were prepared, and their Mn and Mw/Mn values were measured via GPC after the ultrasonic treatment (Table S2). The PVA and poly(MAAc) were dissolved in a 0.2 M phosphate buffer solution at pH 8. The polymeric aqueous solutions were ultrasonicated at 25 °C for 24 h. After the treatment, the samples were injected into the GPC system. The Mn and Mw/Mn values measured before and after the ultrasonic treatment were similar. These results suggested that the main chains of PVA and poly(MAAc) were not cut, which prevented decomposition of the component polymers of the micropatterned films under the ultrasonic conditions used in this study. The micropatterned films were then immersed in water and subjected to ultrasonic irradiation. Figure S4 shows SEM images of the micropatterned films after treatment with ultrasonic irradiation for 24 h in aqueous solution. No shape changes were observed in the hexagonal micropatterns.
Treatment of micropatterned films with a combination of oxidation and ultrasonication
Figure 4 shows SEM images of the micropatterned PVA/poly(MAAc) and EVOH/poly(MAAc) films treated with a combination of aqueous NaClO·5H2O and ultrasonic irradiation. In both films, the micropatterns were peeled off, and micropatterned footprints were observed on the surfaces of the residual films. After the decomposition treatment, the supernatant was collected, and the shapes and components of the microplastics were analyzed. Figure 5 shows the microplastics collected from the supernatant liquid after decomposition of micropatterned PVA/poly(MAAc)_10wt% films by aqueous NaClO·5H2O and ultrasonic irradiation. The micropatterned films were treated a 60 mg/mL NaClO·5H2O aqueous solution for 30 min. The treated films were washed with water and dried under atmospheric pressure. The films were ultrasonicated for 10 min in 3 mL of water. The suspension was centrifuged at 1000 rpm for 5 min to remove large pieces of the degraded films. The supernatant was centrifuged at 3000 rpm for 5 min, and the precipitated pieces were dispersed in 1 mL of water. A total of 20 μL of the dispersion was added to a silicon wafer and dried under atmospheric pressure for measurement via SEM and EDS. Clear hexagonal structures derived from the micropatterned films were observed. Elemental analyses revealed that the hexagonal micropatterns contained carbon, suggesting that the hexagonal micropatterns had peeled from the polymeric micropatterned films. Similar peeling behavior was also observed for the EVOH/poly(MAAc) films with square micropatterns (Fig. S5). These results suggested that the combination of an oxidizing environment and ultrasonic treatment caused peeling of the micropatterned plastics from the polymeric films.
SEM images of the micropatterned A PVA/poly(MAAc)_10wt%, B EVOH/poly(MAAc)_10wt%, and C EVOH/poly(MAAc)_20wt% films after treatment with a combination of NaClO·5H2O aqueous solution and ultrasonic irradiation
A, B SEM and C EDS images of the microplastics collected from the supernatant liquid after decomposition of the micropatterned PVA/poly(MAAc)_10wt% films by a combination of aqueous NaClO·5H2O and ultrasonic irradiation. The micropatterned film was treated with the 60 mg/mL aqueous NaClO·5H2O solution for 30 min and then ultrasonicated for 10 min
Conclusions
In conclusion, water-insoluble micropatterned films were prepared from vinyl alcohol-based polymers (PVA or EVOH) and poly(MAAc). The micropatterns of the PVA/poly(MAAc) and EVOH/poly(MAAc) films gradually were degraded in the oxidizing environment of a NaClO·5H2O aqueous solution, and their heights decreased in response to the decomposition process. In contrast, no shape changes were observed in the hexagonal micropatterns of the PVA/poly(MAAc) or EVOH/poly(MAAc) films after the ultrasonication treatment. When the micropatterned PVA/poly(MAAc) and EVOH/poly(MAAc) films were treated with a combination of NaClO·5H2O aqueous solution and ultrasonic irradiation, the micropatterns were peeled from their surfaces. The peeled micropatterns were collected from the supernatant after decomposition. This study provides insights into the effects of degradation on peeling of micropatterned plastics from polymeric films.
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Acknowledgements
This work was partially supported by the Technical Research Aid Projects 2019 (JFE 21st Century Foundation) and Research Grant 2021 (Iketani Science and Technology Foundation). We are grateful to Nippon Light Metal Co., Ltd, for providing NaClO·5H2O. We are grateful to Dr. Take-aki Koizumi and Dr. Naohiro Sugasawa of the Advanced Instrumental Analysis Center at the Shizuoka Institute of Science and Technology for their technical support.
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Uchida, H., Uchiyama, W., Kurita, E. et al. Micropatterned microplastic generation via degradation of polymeric films with micropatterned structures. Polym J 56, 677–684 (2024). https://doi.org/10.1038/s41428-024-00897-7
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DOI: https://doi.org/10.1038/s41428-024-00897-7
