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. 2023 Sep 7;15(37):44186–44193. doi: 10.1021/acsami.3c06999

Diapers to Thickeners and Pressure-Sensitive Adhesives: Recycling of Superabsorbers via UV Degradation

Shuai Li †,‡, Johannes M Scheiger †, Zhenwu Wang †, Birgit Huber §, Maxi Hoffmann ∥, Manfred Wilhelm ∥, Pavel A Levkin †,⊥,*
PMCID: PMC10521733  PMID: 37676916

Abstract

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Superabsorbers based on crosslinked sodium polyacrylate polymers cannot be easily recycled, resulting in 2 million tons of superabsorbers being landfilled or burned every year. A fast and efficient strategy to recycle superabsorbers would significantly alleviate environmental pollution and promote a sustainable use of these polymers. Herein, the rapid recycling of crosslinked sodium polyacrylate hydrogels based on their inherent UV degradation is demonstrated without the need for chemicals besides water. A quantitative conversion of crosslinked sodium polyacrylate into soluble sodium polyacrylate is achieved in minutes, almost 200 times faster than a previous approach based on de-esterification. The obtained soluble sodium polyacrylate can be used, for example, as a thickener for aqueous dyes or can be esterified with n-butanol or 2-ethylhexanol to serve as a pressure-sensitive adhesive. The UV photodegradation and esterification of superabsorbers is fast, scalable, safe, and economical and yields polymers with controllable molecular weight in the range of 100–400 kg/mol. It thus offers distinct advantages over the chemical de-crosslinking strategies presented previously.

Keywords: superabsorbers, UV degradation, recycling, thickeners, pressure-sensitive adhesives, hydrogels, polyacrylates

Introduction

Today, it is difficult to imagine a world without plastic items because they are so ubiquitous in office supplies, transportation, medical technology, and many other industries. However, 76 wt % of plastic products are discarded after a single use, resulting in a huge waste of petroleum resources.1 At the same time, waste plastics are seriously damaging the natural environment.2−4 Recycling and reusing waste plastic can not only reduce the pressure on the environment but also ensure an efficient use of the limited resources of petroleum.

In recent decades, various techniques for recycling plastic products have been developed.5−8 These recycling processes are grouped into four types: primary recycling (reusing), secondary recycling (melting and reforming), tertiary recycling (chemical), and energy recovery.5,9 Primary and secondary recycling cannot be applied to crosslinked polymers since they are insoluble and cannot be melted.10−12 Chemical recycling usually requires more time and resources than primary and secondary recycling, such as energy or chemicals, but it is applicable to a wider range of polymers.13−16 Energy recycling is the final option in the recycling of plastic products in which the polymers are burned, some of which produce harmful gases and residues.17 Polymer networks made from crosslinked sodium polyacrylate (net-NaPA) are used as superabsorbers in disposable diapers and hygiene products. Since they decompose at 300 °C instead of melting, secondary recycling is not possible.18 Most of the discarded diapers and hygiene products containing net-NaPA are landfilled or incinerated, resulting in the waste of resources and the pollution of the natural environment with microplastics.19,20 In recent years, society, politics, and researchers turned their attention to difficult-to-recycle polymers. For example, superabsorbers have been used for moisturizing soil, water desalination, and osmotic engines.21−27

In 2021, McNeil and co-workers demonstrated a strategy for the tertiary (chemical) recycling of net-NaPA-based diapers via the acid- or base-catalyzed hydrolysis of ester bonds.27 The authors demonstrated the conversion of net-NaPA into linear sodium polyacrylate (NaPA) followed by an acid-catalyzed re-esterification, the poly(2-ethylhexylacrylate) could be reused as a pressure-sensitive adhesive. However, very long reaction times (≥16 h) and heating (80 °C) were required to convert the superabsorber into NaPA. Inspired by this work and our previous experience with the UV photodegradation of poly(meth)acrylate hydrogels,28−30 we hypothesized that the time, energy, and chemicals required to hydrolyze net-NaPA could be reduced by using UV photodegradation. Herein, we demonstrate the inherent and rapid photodegradation of net-NaPA hydrogels into NaPA and its conversion into polybutylacrylate (PBA) and poly(2-ethylhexylacrylate) (PEHA) for use as lacquers and pressure-sensitive adhesives (PSAs). Using UV irradiation instead of hydrolysis, it is possible to degrade net-NaPA into NaPA within minutes instead of hours. The formed NaPA can be used directly in waterborne pigment additives or further esterified to synthesize PSAs. Synthesized PBA and poly(2-ethylhexyl acrylate) (PEHA) have been used as removable and general-purpose PSAs. The rapid recycling of net-NaPA using UV light could be an important contribution to the ongoing efforts to reduce environmental pollution and to aid the sustainable use of polymers.

Results and Discussion

In this study, we showed the rapid photodegradation of swollen net-NaPA into soluble NaPA, inspired by the photodegradation of polymethacrylate hydrogels and organogels from our previous studies.28−30 A UV lamp using a Hg bulb was used for the UV irradiation. The lamp was calibrated to 15.0 mW cm–2 (1000 W) using an OAI 306 UV power meter measuring at 265 nm prior to the experiments unless otherwise specified. The net-NaPA was extracted from commercially available diapers. From published patents, it can be assumed that the superabsorbent polymers are net-NaPA crosslinked with ∼0.05 mol % poly(ethylene glycol) diacrylate.31 Solid net-NaPA was transformed into a hydrogel upon infusing it with water (net-NaPA:H2O = 1:99, wt/wt %). After UV irradiation for 5 min (gel thickness ∼ 5 mm), the hydrogel transformed into a clear solution due to the degradation of the crosslinked polymer network into a soluble polymer (Figures 1A and S1).28,29 Compared to the degradation via hydrolysis, the UV degradation process did not require additional reagents, and the degradation time was decreased from 16 h to 5 min (Figure 1B).27 The scission of the polymer chain could occur in the main chain, or it also could be the scission of the crosslinked part (Figure S1).28,29 To monitor the degradation process, the change in the shear storage modulus of the net-NaPA gel after different UV irradiation times was measured by steady states and oscillatory shear rheology. A substantial drop of the shear storage modulus from 455 to 1.7 Pa at a frequency of 6.28 rad s–1 was observed after 2 min of UV irradiation. After 5 min irradiation time, the shear storage modulus further decreased to 0.004 Pa, indicating the gel had transitioned to a dilute polymer solution (Figure 1C). The molecular weight of the degraded polymer chains could be controlled by the UV irradiation time. With increasing UV irradiation time, the molecular weight of the polymer chains gradually decreased as determined by size exclusion chromatography (SEC) (Figure 1D, Table S1). Accordingly, the hydrodynamic diameter of the polymer chains decreased with increasing UV irradiation time, as determined by dynamic light scattering (DLS) (Figure 1E). Thus, UV light can rapidly degrade net-NaPA to soluble NaPA, which subsequently alters the molecular properties of the resulting NaPA.

Figure 1.

Figure 1

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(A) Image of diapers, extracted net-NaPA powder, net-NaPA hydrogel (net-NaPA:H2O = 1:99, wt/wt %), and degraded NaPA solution after 5 min of UV photodegradation (15 mW cm–2). The diameter of the petri dish is 5 cm. (B) Degradation of net-NaPA. Blue box: Molecular structure of net-NaPA; Green box: UV degradation of net-NaPA. Gray box: hydrolysis degradation of net-NaPA from a previous study.27 (C) Shear storage modulus (G′) of net-NaPA gel at 6.29 rad s–1 after different UV degradation times as determined by rheometry. Oscillatory frequency sweeps were conducted at 25 °C with a strain amplitude of 2%. (D) Weight average molecular weight (Mw) of NaPA after different UV degradation times as measured by SEC. (E) Hydrodynamic diameter (HDD) of NaPA after different UV degradation times as determined by DLS.

The UV degradation of net-NaPA is a surface erosion process, i.e., the top layer degrades first and changes from a net-NaPA gel into an aqueous NaPA solution. Since the degradation of the soluble polymer chains continues in an aqueous solution of the polymer, this could lead to an undesired gradient of the UV dose from the top to the bottom of the degraded solution and net-NaPA hydrogel mixture. To compensate for the influence of the depth of light penetration (Lambert–Beer law) and to avoid “overdegradation”, the UV degradation was conducted in a Buchner filter equipped with a cellulose filter (Figure 2A). The forming NaPA solution then flows into a collection flask and is shielded from further UV irradiation. The thickness of the gel before UV irradiation is ∼5 mm unless specified otherwise. The obtained NaPA solution was collected and dried under reduced pressure (50 mbar) at 60 °C to obtain NaPA as a light yellow solid, which was ground into a powder for further use. In our previous studies, we found that the rate of degradation of crosslinked polymethacrylate networks was strongly affected by their degree of swelling.28−30 Here, the degradation of net-NaPA with different degrees of swelling was compared. The degradation–time curves were determined using the weight of the obtained degradation solution at different time points. Further, net-NaPA extracted from diapers (Figure 2B,D) and net-NaPA purchased from Sigma-Aldrich (Darmstadt, Germany) (Figure 2C,E) were compared. Hydrogels with a mass fraction of 0.5 wt % net-NaPA were found to degrade within 5 min while hydrogels with a mass fraction of 2.0 wt % net-NaPA required 18 min to degrade (Figure 2B,C). The degradation time normalized to the mass of the degraded net-NaPA was thus fairly constant for net-NaPA hydrogels containing 0.5, 1.0, and 2.0 wt % of net-NaPA. The results also showed that net-NaPA from Sigma-Aldrich is degraded faster than net-NaPA from diapers. Net-NaPA from Sigma-Aldrich is 85% neutralized (i.e., % sodium salt) and net-NaPA from diapers is 75% neutralized (i.e., % sodium salt), net-NaPA from Sigma-Aldrich has a stronger affinity with water, its polymer chains are more stretchable than net-NaPA from diaper under the same mass fraction in water, leading to faster photodegradation.32−34 Despite the different UV irradiation times, the molecular weights of NaPA obtained from the UV degradation of hydrogels with a net-NaPA content of 0.5, 1.0, and 2.0 wt % were similar (e.g., Mw = 100.7; 143.1; and 186.3 kg/mol, respectively), (Figure 2D,E, Table S2). It appears that this is due to the similar UV irradiation time per weight percent of net-NaPA in the hydrogel. The results also indicated that net-NaPA from diapers and Sigma-Aldrich have the same photodegradation characteristics. The yields of recycled degraded NaPA from diapers and Sigma-Aldrich net-NaPA were 96.3 and 98.4%, respectively. Thus, for the following experiments, 1.0 wt % net-NaPA from Sigma-Aldrich was used due to the uncertainty about potential additives or contaminations with cellulose fibers in net-NaPA obtained from diapers, as well as for the sake of convenience.

Figure 2.

Figure 2

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(A) Schematic diagram of rapid and continuous preparation of net-NaPA hydrogel photodegradation solution in a Buchner funnel. Kinetics of the photodegradation of (B) net-NaPA extracted from diapers (Procter & Gamble) and (C) net-NaPA from Sigma-Aldrich for different swelling ratios, as quantified by the mass percentage of the NaPA product from photodegradation. Molecular weight distributions of NaPA from (D) degraded net-NaPA extracted from diapers (Procter & Gamble) and (E) degrade net-NaPA from Sigma-Aldrich, as determined with size-exclusion chromatography (SEC). The UV degradation times of the 2.0, 1.0, and 0.5 wt % samples were 20, 10, and 5 min, respectively. Weight percentages (wt %) refer to the weight of dry net-NaPA relative to the total weight of the gel.

The process of degradation was investigated with 1H NMR and 13C NMR using net-NaPA swollen with D2O instead of H2O (Figure 3A,B). In the 1H NMR spectrum of net-NaPA, the signals of the hydrogen atoms of the methylene and methine groups on the network backbone of net-NaPA were very broad. After 10 min UV of irradiation, the hydrogen signals of the methylene and methine groups on the backbone of NaPA became sharper, and the J-coupled splitting of the methine groups became observable (Figure 3A). In the 13C NMR spectrum of net-NaPA, no signals for the methine, methylene, or carbonyl carbons could be detected. After 10 min UV of irradiation, signals of methine carbon, methylene carbon, and carbonyl carbon appeared in the spectrum of the formed NaPA (Figure 3B). The 1H NMR and 13C NMR results indicated that UV irradiation degraded the net-NaPA into water-soluble faster-rotating polymer fragments resulting in sharper NMR peaks. The NMR also indicated that no significant side products were formed. In the ATR-FTIR spectra, no significant difference between net-NaPA and NaPA could be observed, indicating that 10 min of UV irradiation did not cause photooxidation detectable by ATR-FTIR (Figure 3C). However, the formation of oxidized species absorbing UV light (λmax = 275 nm) is detectable already after 10 min UV–vis spectroscopy (Figure 3D), since UV–vis spectroscopy is more sensitive than IR spectroscopy. The UV absorbance in the spectra of NaPA linearly increases with increasing UV degradation time and increasingly absorbs blue light, causing a faint yellow color. Yellowing is indicative of the formation of oxidized moieties, such as aldehydes and ketones. The mechanism of photodegradation of NaPA is thus likely to involve known photooxidative pathways as side reactions, namely the formation of peroxyl radicals (R-O-O·), hydroperoxyls (R-O-OH), oxyl radicals (R-O·), and peroxides (R-O-O-R).35

Figure 3.

Figure 3

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(A, B) 1H NMR and 13C NMR spectra of net-NaPA and degraded NaPA after exposure to 10 min UV, using D2O as a solvent. (C) ATR-FTIR spectrum of net-NaPA and degraded NaPA after 10 min UV exposure. (D) UV–vis spectra of a net-NaPA gel and degraded NaPA solution after different UV degradation times. The concentration of NaPA in water is 10 mg mL–1 (∼1 wt %).

To demonstrate that UV photodegraded NaPA gels are suitable as a resource for applications as lacquers and as PSAs, photodegraded NaPA was further esterified with n-butanol or 2-ethylhexanol (Figure 4A). Alcohols are commonly used in the chemical industry to convert acrylic acid to the respective acrylates.36 The synthesis procedure of McNeil and co-workers27 was followed; however, here an excess sulfuric acid (1.00 instead of 0.25 equivalent) was added to acidify sodium polyacrylate. 1H NMR and 13C NMR measurements confirmed the successful synthesis of both poly(butyl acrylate) (PBA) and poly(2-ethylhexyl polyacrylate) (PEHA) (Figure 4B,C). The peak integral ratio between the methine hydrogen on the main chain and the methylene hydrogen on the side chain was 1–2 in the 1H NMR spectrum, indicating efficient esterification, close to 100%. In the ATR-FTIR spectrum, the peaks of methyl and methylene (C–H stretching) appeared at 2865 and 2964 cm–1, and the peak of C–O–C appeared at 1162 cm–1, which further confirmed the successful synthesis of PBA and PEHA (Figure 4D). At the same time, the peak of the carbonyl group in the products at 1560 cm–1 completely disappeared and reappeared at 1730 cm–1 indicating that all carboxylates were converted to esters. The weight average molecular weight of NaPA increased from 140 to 220 kg mol–1 (both PBA and PEHA), the number average molecular weight increased from 27 to 134 kg mol–1 (PBA) and 73 kg mol–1 (PEHA) (Figure 4E, Table S3). The isolated yield of PBA and PEHA was 79 and 86%, respectively. The loss was due to the usual incomplete reaction of the esterification and purification process afterward.

Figure 4.

Figure 4

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(A) Reaction scheme of the esterification of degraded NaPA with n-butanol and 2-ethylhexanol. (B) and (C) 1H NMR and 13C NMR spectra of poly(butyl acrylate) (PBA) and poly(2-ethylhexyl acrylate) (PEHA) in CDCl3. (D) ATR-FTIR spectrum of degraded NaPA after 10 min UV exposure and the esterification products PBA, PEHA. (E) Molecular weight of the degraded NaPA after 10 min UV exposure, and the esterification products PBA and PEHA, obtained by size-exclusion chromatography using aqueous and tetrahydrofuran as mobile phases, respectively.

Linear sodium polyacrylate is commercially used as a thickener and water-retention agent for aqueous dyes.37,38 Soluble NaPA obtained through UV degradation can be used directly as a dye thickener. The viscosity of methylene blue aqueous solution with and without degraded NaPA was measured by oscillatory and steady-state shear rheology (Figure 5A). After adding NaPA to the aqueous dye solution consisting of methylene blue in water, the zero shear viscosity of the solution increased significantly, by more than a factor of 2000. Digital photographs and optical microscope images showed that the aqueous solution containing only methylene blue will be completely soaked into the paper (Figure 5B). After adding a NaPA thickener, the methylene blue solution hardly penetrated into the paper and the coating adhered to the surface of the paper, forming a film with high gloss. Because a possible reason might be that the dye and moisture were tightly bound to the polymer after the addition of NaPA, the dye will not penetrate into the paper with the moisture. Meanwhile, NaPA contained a large amount of carboxylate to make it stably attached to the paper fiber.

Figure 5.

Figure 5

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(A) Steady-state viscosity of methylene blue aqueous solution (10 mg mL–1) with and without degraded NaPA (0.5 g mL–1) at 25 °C. (B) Drawings on paper using an aqueous solution of methylene blue with and without degraded NaPA. Left pictures: acquired from the digital camera; right pictures: acquired from the optical microscope. (C) and (D) Oscillatory shear storage and loss moduli of PBA and PEHA. Oscillatory frequency sweeps were conducted at 25 °C with a constant strain of 1% and applying an angular frequency from 0.01 to 100 rad s–1. (E) Peel force of paper coated with PBA and PEHA on the surface of different materials compared to commercially available sticky notes (error bars are standard deviations, N ≥ 5 independent test samples).

PBA and PEHA obtained by esterification have the potential as PSAs. Here, the shear storage and loss moduli of PBS and PEHA were determined via rheology (Figure 5C,D). The molecular weight of PBA and PEHA used here was 220 kg mol–1 each, and they were low molecular weight PSAs. The shear storage and loss modulus of PBA reached values between 103 and 105 Pa applying an angular frequency of 0.1–100 rad s–1 at 25 °C, and the transition from the rubber to the flow zone (i.e., the crossover of G′ and G″) was found at 0.5 rad s–1 at 25 °C, indicating that it could be used as removable and general-purpose PSAs.39 Compared to PBA, the shear modulus of PEHA was lower by a factor of 10–102 and G″ was greater than G′ over the investigated frequency range. Dahlquist criterion stipulates that the shear elastic modulus (G′) of PSAs at the bonding frequency must be lower than 0.1 MPa for the layer to be able to form a good contact within the contact time.40G′ of both PBA and PEHA are well below 0.1 MPa at a frequency of 0.01 rad s–1, indicating that they both meet the minimum conditions for use as PSAs. Afterward, PEHA and PBA were coated on paper as PSAs, and their adhesion properties on the surface of different materials such as paper, isotactic polypropylene (i-PP), aluminum (Al), and glass were tested (Figure 5E). Compared to commercially available sticky notes, PEHA as a PSA had similar maximum peel strength. Due to the higher shear moduli of PBA, its maximum peel strength on different materials was much higher than both PEHA and commercially available sticky notes. The composition of PSA and the amount of free carboxy groups in the final esterified polyacrylate/polymethacrylate can change the adhesive properties of the PSA.41 The polar carboxylic acid groups can provide additional polar, hydrogen bonding, and/or charge interactions with surfaces. In our case, the amount of such groups can potentially be fine-tuned by partial esterification. In addition to the fraction of carboxylic acid in the final PSA, another important way of fine-tuning the properties of PSA is to control the amount of crosslinkers in the product of the degradation. Using our method, this could be achieved by varying either the UV light intensity or time of irradiation. These results indicate that the two polyacrylates synthesized from the photodegradation products of crosslinked sodium polyacrylate have the potential as PSAs or adhesives in general.

Conclusions

A rapid approach to cleave covalent bonds in the crosslinked sodium polyacrylate found in diapers as a superabsorber is presented. The potential recycling process is based on the inherent UV degradation of crosslinked sodium polyacrylate into soluble sodium polyacrylate. No chemicals are required to quantitatively convert a superabsorber hydrogel into a polymer solution in as little as 5 min. The recycled soluble sodium polyacrylate can be used as a thickener for aqueous dyes or can be esterified in a simple and efficient way. The esterified polyacrylate, PBA and PEHA, synthesized from the recycled NaPA have potential as removable and general-purpose PSAs. Knowing that currently 2 million tons of superabsorbers are wasted every year, this work demonstrates a fast and potentially efficient strategy to recycle these materials. With further research and development, it can be applied to significantly alleviate environmental pollution and promote more sustainable use of polymers.

Experimental Part

Chemicals and Materials

The superabsorbent polymer obtained from commercial diapers (produced by Procter and Gamble (P&G) purchased from Rossmann (Karlsruhe, Germany)) is a sodium poly(acrylate) crosslinked via ∼0.05 mol % poly(ethylene glycol) diacrylate co-monomer, the polymer is 75% neutralized (i.e., % sodium salt) and contains ∼0.5% moisture.31 Another superabsorbent polymer is a sodium poly(acrylate) crosslinked via ∼0.05 mol % poly(ethylene oxide) diacrylate purchased from Sigma-Aldrich, the polymer is 85% neutralized (i.e., % sodium salt). Isopropanol, ethanol, and acetone were obtained from Merck (Darmstadt, Germany). All other chemicals were purchased from Sigma-Aldrich (Darmstadt, Germany) and used without further purification. PTFE plates, i-PP plates, and aluminum plates were purchased from RS Components GmbH (Frankfurt, Germany) and were cut to the desired size for use. NEXTERION B glass slides were obtained from Schott AG (Mainz, Germany).

Methods

UV Lamps

In this study, an UVAcube 2000 from Dr. Hönle AG (Gräfelfing, Germany) using a 1000 W Hg bulb was used for UV irradiation. The lamp was calibrated to 15 mW cm–2 at 260 nm with the OAI 306 UV power meter. The light source at 260 nm (15 mW cm–2) was used for UV irradiation unless otherwise specified.

Aqueous Size-Exclusion Chromatography (SEC)

SEC measurements were performed on a SECcurity GPC System - Polymer Standards Service GmbH, Mainz - Agilent Technologies 1260 Infinity, comprising an autosampler, a Suprema 5 μm particle size guard column (8 × 50 mm, PSS) followed by two Suprema 5 μm columns (8 × 300 mm, subsequently mixed-bed S and mixed-bed M, PSS), and a differential refractive index (RI) detector and a UV detector. The measurements were performed using disodium hydrogen phosphate 0.07 M in water as the eluent at room temperature with a flow rate of 1 mL·min–1. The SEC system was calibrated using linear poly(acrylic acid) sodium salt standards (PSS) ranging from 1250 to 1,484,000 g·mol–1. Samples were dissolved in an aqueous solution of disodium hydrogen phosphate (0.07 M) at a concentration of 2 mg/mL and then filtered with 0.20 μm regenerated cellulose filters.

Size-Exclusion Chromatography

SEC measurements were performed on a TOSOH Eco-SEC HLC-8320 GPC System, comprising an autosampler, an SDV 5 μm particle size guard column (50 × 8 mm, PSS) followed by three SDV 5 μm columns (8 × 300 mm, subsequently 100, 1000, and 105 Å pore size, PSS), a differential refractive index (RI) detector using tetrahydrofuran (THF) as the eluent at 35 °C with a flow rate of 1 mL min–1. The SEC system was calibrated using linear polymethylmethacrylate (PMMA) standards ranging from 800 to 1.82 × 106 g·mol–1. Samples were dissolved in tetrahydrofuran at a concentration of 2 mg/mL and then filtered with 0.20 μm PTFE filters.

Dynamic Light Scattering

DLS measurements were conducted on a Zetasizer Nano ZS from Malvern (Kassel, Germany). Degraded NaPA solution with a concentration of 1 mg/mL were measured directly after different UV irradiation times.

UV–Vis Spectroscopy

UV–vis spectroscopy was performed with an Infinite M200 Pro (Tecan Trading AG, Switzerland) plate reader.

NMR

NMR spectra were recorded on a Bruker Avance III 400 MHz spectrometer from Bruker (Karlsruhe, Germany). For net-NaPA and soluble NaPA, D2O (99.9 atom% D) from Merck (Darmstadt, Germany) was used as a solvent. For PBA and PEHA, CDCl3 (99.8 atom% D) from Merck (Darmstadt, Germany) was used as a solvent. For the 1H and 13C spectra, 16 and 2048 scans were collected, respectively.

Attenuated Total Reflection Fourier Transform Infrared Spectroscopy (ATR-FTIR)

All IR measurements were performed on a Bruker Alpha II ATR-FTIR with a diamond ATR plate from 400 to 4000 cm–1 (at 4 cm–1) at 25 °C. Samples were dried and then ground into powder before measurement.

Rheometry

Rheological shear measurements were carried out on an ARES G2 rotational rheometer from TA Instruments (Waters GmbH, Eschborn, Germany) equipped with a parallel plate Invar geometry (13 mm diameter) at a gap of approx. 1 mm. Small amplitude oscillatory shear measurements (SAOS) were conducted from 0.1 to 100 rad s–1 with a strain amplitude of 1% using an Advanced Peltier element for temperature control at 25 °C. The sample was covered with a solvent trap to avoid evaporation of the solvent (water).

Mechanical Test

Printer papers were cut into 5 × 10 cm strips. PEHA was directly coated on one end of the paper strip (coating area: 3 × 5 cm), PBA was dissolved in dichloromethane and then coated on one end of the paper strip (3 × 5 cm). 5 cm wide sticky notes (the sticking area was 3 × 5 cm) were purchased and used directly. Then both PEHA and PBA-coated paper strips and sticky notes were attached to clean paper, i-PP, Al, or glass surface and pressed with a 2 kg rubber roller. The peel test was then carried out using AGS-X Series Universal Electromechanical Test Frame (Shimadzu Inc., Japan). The peel test on the paper surface uses the T-type peel test, and the peel test on the i-PP, Al, and glass surface uses the 180-degree peel test. The peel rate was 300 mm/min.

Microscopy

Images were recorded on a Keyence Z7000 from Keyence (Osaka, Japan).

Digital Photography

Digital photos were taken using a Canon EOS 80D digital camera.

Acknowledgments

S.L. would like to thank the China Scholarship Council (CSC) for the Ph.D. scholarship. This project was partly supported by DFG (Heisenbergprofessur Projektnummer: 406232485, LE 2936/9-1). Furthermore, we thank the Helmholtz Program “Materials Systems Engineering” for the support. M.H. gratefully acknowledges the support from the Stiftung der deutschen Wirtschaft (sdw) within the Klaus Murmann fellowship for her Ph.D. We also thank the Soft Matter Synthesis Laboratory (SML) for the help with ATR-FTIR measurement. Thank Maximilian Seifermann and Julius Höpfner for the help with NMR measurement. This work was partly supported by the Impuls- und Vernetzungsfonds der Helmholtz-Gemeinschaft and by the DFG (Germany’s Excellence Strategy 2082/1-390761711, Excellence Cluster “3D Matter Made to Order”). Michael Pollard is acknowledged for editorial corrections as a native English speaker.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsami.3c06999.

  • Schematic diagram of net-NaPA bond scission; molecular weight data of net-NaPA after different UV irradiation time; molecular weight data of net-NaPA degraded under different mass fraction; molecular weight data of PBA and PEHA (PDF)

The authors declare no competing financial interest.

Supplementary Material

am3c06999_si_001.pdf (157.2KB, pdf)

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