Differentiating the Role of Osmotic Pressure and Ionic Interactions on Self-Healing Polymers

Abstract

Self-healing polymers can recover from physical and chemical damage autonomously, which improves the durability and performance of systems that rely on these polymers. To design self-healing polymers that work well in practical applications, it is important to understand the impact that the presence of different ions has on self-healing mechanisms. In this paper, we investigate the role of monovalent (Na⁺) and divalent (Ca²⁺) ions in the self-healing efficiency of a model polymer, namely, poly­(2-acrylamido-2-methyl-1-propanesulfonic acid) (PAMPS), whose network is dominated by hydrogen bonding. By pre-embedding ions into the bulk gel, our method systematically eliminates the confounding ion concentration gradients and osmotic pressure differences that complicated previous studies. Through tensile testing, we find that at high concentrations, divalent ions improve the strength and modulus recovery of polymer samples and slightly reduce the strain recovery relative to samples without any ions in them. Monovalent ions did not result in a statistically significant change in strength recovery but increased strain recovery at high concentrations. Using additional rheological measurements, we find that both monovalent and divalent ions decrease the relaxation time of the PAMPS chains, with monovalent ions doing so to a much larger extent. This suggests that changes in chain mobility might be the key factor that controls any improvements in strain and strength recovery. Overall, our results deconvolute the competing roles of ionic cross-linking and chain mobility and highlight the importance of controlling for osmotic artifacts in ion-containing hydrogels.

Introduction

Synthetic polymers are versatile materials widely used in various applications including everyday life packaging, advanced biomedical applications, semiconductors, and water treatment membranes. Incorporating functionalities such as self-healing into synthetic polymers can improve the longevity and resilience of processes that rely on them for critical functions. Self-healing refers to autonomous repair from physical or chemical damage by synthetic materials. Self-healing polymers are broadly categorized into two types: extrinsic and intrinsic. Extrinsic self-healing polymers have embedded microcapsules or microvascular networks , which contain a healing agent. When damage occurs, these encapsulated agents are exposed and react to form a solid layer that restores the material’s integrity. This approach provides effective one-time repairs and is particularly suitable for applications in the production of self-healing structural and coating materials where immediate restoration is critical. − In contrast, intrinsic self-healing polymers rely on reversible chemical and physical interactions, such as dynamic covalent bonds, , supramolecular interactions, thermal remending, and polymer chain entanglements, , to achieve repair. This mechanism has the advantage of enabling repeated healing and can easily be incorporated into many types of polymers, especially hydrogels, as their bulk mechanical properties are governed directly by these intrinsic noncovalent mechanisms. , However, the molecular level interactions that are responsible for intrinsic self-healing, especially the secondary ones such as hydrogen bonding , and ionic interactions, , can be sensitive to environmental factors such as the presence of ions that participate in these interactions.

Several previous studies have demonstrated the self-healing property of polymers (mostly hydrogels) cross-linked with different types of ions. − Charlet et al. reported metal-coordinated hydrogels with tunable mechanical strength, where ion–ligand interactions enable both shape retention and self-healing in aqueous environments. Li et al. introduced hybrid dual-cross-linked poly­(acrylic acid) (PAAc) hydrogels, combining covalent and ionic cross-links. Their approach, using Fe³⁺ ions and saline soaking, significantly improved toughness, strength, and self-healing ability. Several studies also show that self-healing is possible in the presence of seawater, specifically using seawater as the medium in which the polymer or hydrogel is repaired. However, relatively few papers have directly investigated the impact of ions with different valences on a self-healing polymer’s recovery, especially systematically. ,

To systematically investigate these ion-specific effects, we utilize a model self-healing hydrogel poly­(2-acrylamido-2-methyl-1-propanesulfonic acid) (PAMPS). PAMPS is a model system because it is a physical hydrogel composed of linear, uncross-linked polymer chains. As illustrated in Figure a,b, its self-healing capability is governed entirely by two reversible, noncovalent steps: (1) hydrogen bonding and (2) interchain diffusion across the damaged interface (Figure b). This network is primarily stabilized by strong hydrogen bonds, which can form between amide groups as well as between amide and sulfonate groups (Figure g). This H-bond-dominated system allows us to investigate an unresolved question regarding how added ions affect the healing process. The conventional hypothesis is that monovalent ions (like Na⁺) disrupt the network via ionic screening (Figure e). This screening effect also serves to mitigate the electrostatic repulsion between the anionic sulfonate groups (Figure h). Conversely, divalent ions (like Ca2⁺) enhance strength recovery by forming robust ionic cross-linking (Figure f).

1.

Schematics of the self-healing mechanisms, competing ionic interactions, and confounding variables in the PAMPS hydrogel system. (a,b) The two-step intrinsic self-healing mechanism: (a) interchain diffusion across the damaged interface, followed by (b) the restoration of the hydrogen bond network. (c) The schematic of a salt concentration gradient and osmotic pressure difference in pristine PAMPS gel with ionic solutions. (d) The pre-embedded ionic bulk PAMPS gel, (e) monovalent ionic screening (Na⁺), and (f) divalent ionic cross-linking (Ca²⁺). (g) Hydrogen bonding between amide groups as well as between amide and sulfonate groups, and (h) charge screening mitigating repulsion between sulfonate groups.

We first attempted to resolve these competing mechanisms in our previous paper. In that study, we examined how different ions (NaCl and MgSO₄) affect the healing of the PAMPS gels. We found that both monovalent ions (Na⁺) and divalent ions (Mg²⁺) improved the ultimate tensile (UT) strain recovery but decreased ultimate tensile strength recovery, compared to samples healed in the presence of deionized water. This challenged our original hypothesis that monovalent ions disrupt strength recovery by screening charges and weakening noncovalent interactions, whereas divalent ions enhance it by forming robust ionic cross-links. One hypothesis for this unexpected result is that the presence of both monovalent and divalent ions at the damage interface creates an ion concentration gradient and osmotic pressure difference that makes water come to the damaged interface (Figure c). As a result, the healed area is softer than the pristine sample and shows enhanced extensibility but reduced strength (Figure S1a).

In this study, we investigated the influence of monovalent (Na⁺) and divalent (Ca²⁺) ions on the self-healing properties of PAMPS hydrogels that are preloaded with these ions (Figure d). This “pre-embedding” approach removes the salt concentration gradient at the healing interface, eliminating the osmotic pressure difference and enabling the isolation of the intrinsic ion effects on the healing process (Figure e–h). We also utilize a dog-bone specimen with the ASTM D412 standard (Figure S1b,c) and compare healing time to ensure complete recovery (Figure f).

3.

Tensile testing results of PAMPS hydrogels. Photograph of the self-healing process of PAMPS hydrogel; (a) pristine and cut, (b) healing, and (c) healed specimen. (d) Stress–strain curves of PAMPS hydrogels without ionic additives, comparing uncut specimens and specimens healed for 1 and 24 h. (e) Stress–strain curves of uncut specimens with ionic solutions of varying concentrations: DI water, 0.01 M NaCl, 0.1 M NaCl, 0.01 M CaCl₂, and 0.1 M CaCl₂. (f–h) Mechanical properties of uncut specimens, demonstrating the influence of monovalent (Na⁺) and divalent (Ca²⁺) ions on ultimate tensile (UT) (f) strain, (g) stress, and (h) modulus.

We found that monovalent ions improved the ultimate tensile strain, while divalent ions enhanced the ultimate tensile strength and elastic modulus. In addition to tensile tests, we use Fourier transform-infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), hydrogen nuclear magnetic resonance (H NMR), and shear rheological measurements to obtain mechanistic insight into these results. Throughout this study, we define self-healing efficiency by comparing the mechanical properties of a sample after it has been cut and allowed to heal (for 1 or 24 h) to the properties of an identical pristine (uncut) sample. Using this method, we found that after healing, monovalent ions improved the ultimate tensile strain, while divalent ions enhanced the ultimate tensile strength and elastic modulus. We find that enhanced chain mobility might be the key factor that controls any improvement in strength recovery, instead of our previous assumption that divalent ions form cross-links that bridge polymer chains together. Overall, our results provide new insight into the impact of monovalent and divalent ions on self-healing polymers and suggest that differences in ion concentration between the damage area and bulk polymer need to be considered in future work.

Materials and Methods

Materials

PAMPS hydrogels were synthesized using 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS, 99%). Sodium chloride (NaCl) and calcium chloride (CaCl₂) were sourced from Millipore Sigma and used as monovalent and divalent ion sources, respectively. Deionized (DI) water, with a conductivity of 18.2 MΩ·cm at 25 °C, was obtained from a Millipore Milli-Q water purification system. Silicon molds used for the hydrogels were fabricated with Dragon Skin 30 from Smooth-On, using a 3D-printed mold design based on ASTM D412 standards for dog-bone-shaped specimens. To ensure ease of demolding, a nonstick dry-film lubricant aerosol from Lacquer was applied to the molds.

Fabrications of PAMPS Hydrogel Samples

The PAMPS hydrogel was prepared by dissolving AMPS powder in deionized (DI) water or a NaCl or CaCl₂ solution at a 1:1 ratio as shown in Figure a. Three distinct types of hydrogels were fabricated for this study, as summarized in Table . The pristine PAMPS hydrogel was synthesized using DI water. The ion-preloaded hydrogels referred to as PNaAMPS (preloaded with monovalent Na⁺ from NaCl) and PCaAMPS (preloaded with divalent Ca²⁺ from CaCl₂) were synthesized. The solution was stirred at 500 rpm for 15 min at room temperature. To remove any dissolved gases, the solution was degassed in a sonicator for 30 min. For the polymerization process, coated molds were placed into a glass container, and 4.5 mL of the AMPS solution was dispensed into each mold (Figure b). The container was sealed securely using tape to prevent the lid from opening. The solution in the molds was polymerized at 77 °C for 24 h (Figure c). After polymerization, the hydrogels were frozen to prepare them for further processing. Each hydrogel, still within its mold, was individually placed in a vacuum-sealing container (Figure d). These containers were then placed into a freezer for 3 h for better demolding. The hydrogels were quickly demolded one by one after freezing.

2.

Fabrication process of PAMPS hydrogel. Step-by-step schematic of the fabrication process for PAMPS hydrogels, including synthesis, ion incorporation, and the self-healing procedure. (a) Solution preparation, (b) specimen molding with the dog-bone silicon mold, (c) thermal polymerization, (d) specimen demolding after drying, (e) 3 h vacuum drying, (f) damaging and treating, (g) healing and storage process, and (h) actual image of PAMPS hydrogel.

1. Summary of PAMPS Hydrogel Sample Formulations.

abbreviation	sample description	ion type	salt used	final ion concentration
pristine	pristine PAMPS gel	none	DI water	0 M
0.01 M PNaAMPS	PAMPS pre-embed with Na+	monovalent (Na+)	NaCl	0.01 M
0.1 M PNaAMPS	PAMPS pre-embed with Na+	monovalent (Na+)	NaCl	0.1 M
0.01 M PCaAMPS	PAMPS pre-embed with Ca2+	divalent (Ca2+)	CaCl2	0.01 M
0.01 M PCaAMPS	PAMPS pre-embed with Ca2+	divalent (Ca2+)	CaCl2	0.1 M

The demolded hydrogels were then dried in a vacuum chamber for 3 h, followed by a stabilization period of 1 h within the same vacuum chamber (Figures e and S2,3). After drying, the hydrogels were cut immediately, and 4.5 μL of DI water was applied between the cut surfaces (Figure f). The pieces were pressed together and stored individually in sealed containers (Figure g,h). The samples were left to heal for 24 h in a fume hood.

Tensile Testing

Tensile tests were performed to evaluate the mechanical properties of the samples using an Instron testing machine with a 2 kN load cell. The specimens were dog-bone-shaped and prepared according to ASTM D412 standard dimensions (Figure S1b,c). The tested area dimensions are 20 mm in length × 6 mm in width × 3 mm in thickness. Tests were conducted at a loading speed of 100 mm/min, until the specimen was fully fractured. Given the 20 mm gauge length, this speed corresponds to a nominal strain rate of 5 min^–1. To prevent further water evaporation during testing, a very thin layer of oil was applied to the tested area immediately before each test (Figure S4). For consistency, each specimen was carefully aligned with the grip edges of the machine. Three specimens were tested for each data point to ensure reliable and repeatable results, as shown in Figure S5.

Self-Healing Procedure

Dog-bone-shaped specimens were cut using a stainless-steel razor blade (Figure a). The cut samples were healed with 4.5 μL of DI water as shown in Figure b and left for 24 h in the vacuum container. The force during the healing process was manually applied by pressing the hydrogel pieces together until uniform visual contact across the interface was consistently achieved (Figure c). Control samples that were not cut were also “stored” for 24 h in the vacuum container. Self-healing efficiency was calculated using eq below. All conditions were tested in at least triplicate.

$${\mu }_{\mathrm{S}\mathrm{H}\mathrm{E}}(\%)=\frac{\mathrm{A}\mathrm{v}\mathrm{e}\mathrm{r}\mathrm{a}\mathrm{g}\mathrm{e}\mathrm{o}\mathrm{f}\mathrm{M}{\mathrm{P}}_{\mathrm{h}\mathrm{e}\mathrm{a}\mathrm{l}\mathrm{e}\mathrm{d}}}{\mathrm{A}\mathrm{v}\mathrm{e}\mathrm{r}\mathrm{a}\mathrm{g}\mathrm{e}\mathrm{o}\mathrm{f}\mathrm{M}{\mathrm{P}}_{\mathrm{u}\mathrm{n}\mathrm{c}\mathrm{u}\mathrm{t}}}\times 100$$ 1

where μ_SHE is the mean of self-healing efficiency, MP_uncut is the mechanical property of an uncut sample with the same salt concentration, and MP_healed is the mechanical property of a healed sample. The standard deviation of the self-healing efficiency is calculated by propagating the uncertainty using eq :

$${\sigma }_{\mathrm{S}\mathrm{H}\mathrm{E}}={\mu }_{\mathrm{S}\mathrm{H}\mathrm{E}}\times \sqrt{{\left(\frac{{\sigma }_{\mathrm{u}\mathrm{n}\mathrm{c}\mathrm{u}\mathrm{t}}}{{\mu }_{\mathrm{u}\mathrm{n}\mathrm{c}\mathrm{u}\mathrm{t}}}\right)}^2+{\left(\frac{{\sigma }_{\mathrm{h}\mathrm{e}\mathrm{a}\mathrm{l}}}{{\mu }_{\mathrm{h}\mathrm{e}\mathrm{a}\mathrm{l}}}\right)}^2}$$ 2

where σ_SHE is the standard deviation of self-healing efficiency, while σ_uncut and σ_heal represent the standard deviation of the mechanical properties of the uncut and healed samples, respectively. Similarly, μ_uncut and μ_heal denote the mean values of the mechanical properties of the uncut and healed samples, respectively.

Fourier Transform-Infrared (FTIR) Analysis

Attenuated total reflectance (ATR) Fourier transform-infrared (FTIR) analysis was conducted to examine the chemical functional groups of each type of hydrogel sample (PAMPS, PNaAMPS, and PCaAMPS). An infrared spectrometer (Nicolet FTIR Infrared Microscope, Thermo Fisher Scientific) equipped with a diamond window attenuated total reflectance (ATR) accessory (Smart iTX, Thermo Fisher Scientific), and a deuterated triglycine sulfate (DTGS) detector was used to obtain the IR spectra, set at a resolution of 4 cm^–1 with 64 scans per spectrum. Samples were cut into 1 × 1 cm pieces from the dog-bone-shaped specimens and dried for 48 h in the fume hood. The intensity of the IR spectrum of each sample was normalized by the respective N–H stretching peak (∼3287 cm^–1).

XPS Analysis

The chemical structures of the three samples (PAMPS, PNaAMPS, and PCaAMPS) were analyzed by using X-ray photoelectron spectroscopy (XPS). The samples were prepared as a thin film (50 μm) on a Si wafer using a casting knife. PHI Quantera SXM (ULVAC-PHI. Inc.) equipped with a monochromatic Al Kα radiation source (1,486.7 eV) was used to acquire S 2p, Ca 2p, N 1s, O 1s, Na 1s, and C 1s core-level photoelectron spectra of the samples under ultrahigh vacuum conditions. The binding energy and intensity of each core-level spectrum were calibrated and normalized by the respective C 1s of the C–C bond (284.6 eV). The pass energy and energy step were fixed at 26 and 0.1 eV, respectively. For each sample, analysis was carried out in three different spots, which showed consistent results. The samples were polymerized and fully dried by heating at 77 °C for 24 h before XPS measurements.

NMR Analysis

Nuclear magnetic resonance (NMR) spectra were obtained using a Bruker AVANCE III HD 500 MHz spectrometer equipped with a broadband (15N to 31P) room-temperature probe. Samples included pristine PAMPS, 0.1 M NaCl-treated (PNaAMPS), and 0.1 M CaCl₂-treated (PCaAMPS) hydrogels. All samples were prepared in deuterium oxide (D₂O) containing 3-(trimethylsilyl)­propionic acid-d₄ (TSP) as an internal standard. Initial hydrogels were synthesized by dissolving AMPS monomer in ionic D₂O solutions (0.1 M NaCl or 0.1 M CaCl₂), followed by stirring at 500 rpm for 2 h at 25 °C. The stirred solutions were polymerized in the vacuum heat oven at 77 °C for 24 h. To remove air bubbles, these solutions were degassed for 30 min using a sonicator bath. For the NMR measurement, the hydrogels were diluted in D₂O to maintain a consistent water content and transferred into glass vials. A total of 64 scans were acquired to improve the signal-to-noise ratio, and all spectra were normalized to the TSP peak for quantitative comparison.

SEM Imaging

Scanning electron microscopy (SEM) was conducted using the FEI Helios NanoLab 660 DualBeam system to observe surface morphology and in-act healing behavior. The electron voltage was set to 5 kV, and the current was maintained at 100 pA. Samples were cut into 1 × 1 cm pieces from the dog-bone-shaped specimens and dried in the fume hood for 48 h. Prior to imaging, a thin carbon layer (∼15 nm) was applied to the surface using a carbon coater at 6.5 V and 20 mA for 6 s to enhance image quality without surface charging. The carbon-coated specimens were damaged by using a stainless-steel razor blade to create cuts with a width of 15 μm. First, to observe macroscale healing, the damaged samples were allowed to dry in a fume hood under ambient conditions. The damaged site was imaged by repeatedly drying the sample and relocating the original scratch. Following this drying healing, 5 μL of DI water was applied onto the damaged area, and the samples were left to dry in the fume hood for 24 h. The sample was then imaged again to observe the effect of the water-mediated healing.

Rheological Measurements

Rheological measurement was performed on a rotational rheometer (ARES G2, TA) with a parallel plate geometry of 25 mm in diameter to investigate the impact of ionic interactions on polymer chain mobility. The samples for the rheometer were prepared by dissolving 1 g of PAMPS dried gel in 7 g of DI water to yield a 12.5 wt % percolated gel. Frequency sweeps in the ranges of 1–600 rad/s at the strain rate of 10% were performed to identify changes in the storage and loss moduli of the solution. This amplitude was confirmed to be within the linear viscoelastic region (LVR) via a preliminary strain sweep. This test was used to identify changes in the storage (G′) and loss (G″) moduli and complex viscosity (η). As G′ > G″ for all samples, they were analyzed as percolated physical gels. The characteristic relaxation time (τ) was determined from the crossover frequency (ω_c), where (G′ = G″), using the relationship τ = 1/ω_c (Figure c).

6.

Rheological analysis of 12.5 wt % dissolved PAMPS gels (pristine, PNaAMPS, and PCaAMPS) showing the impact of ions on polymer chain mobility. (a) Complex viscosity (η) as a function of angular frequency (ω). (b) Tan δ (G′′/G′) as a function of angular frequency, indicating the viscoelastic response. (c) Characteristic relaxation time (τ) derived from the crossover frequency (ω_c), quantifying chain mobility.

Results and Discussion

PAMPS Self-Healing and Impact of Ions on Pristine Gel Mechanical Properties

PAMPS hydrogels polymerized without any ions showed Ultimate Tensile (UT) stress and strain recovery. As shown in Figure d and Table S1, after 24 h of healing, the PAMPS specimens without any ions exhibited UT strain values of 1450.1 ± 146.7 and UT stress values of 0.16 ± 0.012 MPa, closely matching the performance of the uncut specimens (p = 0.83 and p > 0.99, Figure S7). The recovery was dependent on time, with samples healed for only 1 h showing significantly reduced UT strain 549.4 ± 13.51 and UT stress 0.07 MPa (p < 0.001). Interestingly, the stiffness of the samples was not dependent on time. Since the stiffness is more reflective of the hydrogen bonds that hold the bulk sample together while the final strength and extensibility indicate the level of interchain entanglement, this indicates that the reversible hydrogen bonds form relatively quickly. The samples that are healed longer have more opportunity for chains to diffuse or reptate and entangle more while the samples that have only had 1 h of contact have interface chains that are not as entangled. SEM images of the gels also showed the disappearance of the gap at the cut interface, providing visual and microscopic evidence of the repair at the interface.

The mechanical properties of PAMPS hydrogels polymerized with Na⁺ and Ca²⁺ ions (denoted as PNaAMPS and PCaAMPS) are presented in Figure e–h and Figure S7. The presence of divalent ions (Ca²⁺) had a negligible effect on the ultimate tensile (UT) stress and strain and modulus of the undamaged specimens, with no statistically significant differences observed across varying ion concentrations (p = 0.85–0.99). In contrast, monovalent ions (Na⁺) led to a modest increase in UT strain (p = 0.0006 and <0.0001) and a slight reduction in UT stress, although the decrease in strength was statistically significant only at lower Na⁺ concentrations (Figure S7). Na ⁺ ions also reduced the modulus of PAMPS gels on average, but the results were not statistically significant (p = 0.22 and 0.38). These results are useful in interpreting the subsequent self-healing performance in the presence and absence of ions.

Impact of Divalent Ions on Self-Healing

Although there was a minimal difference in the mechanical properties of the uncut PAMPS and PCaAMPS samples, there was a significant difference in the level of self-healing observed between these two samples when the Ca²⁺ concentration was high. The healed PCaAMPS samples have UT stress even higher than the uncut samples (Figures a,b,c and S6), resulting in a self-healing efficiency or strength recovery of 130% ± 3% at 0.01 M (Figure f) and 200% ± 40% at 0.1 M Ca²⁺ (Figure g.) The p-value for the comparison of the healed sample set vs the uncut sample set in the 0.01 M Ca²⁺ case was 0.22, while the p-value for the comparison of the healed sample set vs the uncut sample set in the 0.1 M Ca²⁺ case was <0.0001. Therefore, we can conclude that at high Ca²⁺ levels, there is a notable ability to achieve over 100% strength recovery. At low Ca²⁺ concentration, the divalent ion does not result in a statistically significant change in the strength of the gels after healing, similar to the gels without any ions.

4.

Comprehensive mechanical properties of uncut and 24 h-healed PAMPS hydrogels. (a) Statistical summary of ultimate tensile strength and strain at break and elastic modulus for all uncut and healed samples. Asterisks (*) indicate a statistically significant difference (p < 0.05). (b) Representative stress–strain curves of uncut (bulk) samples with varying ion types and concentrations. (c) Representative stress–strain curves of 24 h-healed samples, comparing all ionic conditions. (d–g) Direct comparisons of representative uncut (black lines) vs 24 h-healed (colored lines) curves for each specific condition: (d) PNaAMPS (0.01 M), (e) PNaAMPS (0.1 M), (f) PCaAMPS (0.01 M), and (g) PCaAMPS (0.1 M).

The UT strain recovery of the healed PCaAMPS samples was 87% ± 4% (p = 0.57) and 74% ± 7% (p = 0.015), for 0.01 and 0.1 M Ca²⁺, respectively, which was lower than the PAMPS UT strain recovery of 111% ± 16% (p = 0.83). Given the lack of statistical significance for the low Ca²⁺ concentration and the nonion case, we can conclude that the UT strains of the undamaged and healed samples were similar. At high Ca²⁺ concentrations, however, there is a significant reduction in the strain recovery. The modulus recovery in the presence of divalent ions was 89% ± 37% (p > 0.99) and 205% ± 110% (p = 0.0017), for 0.01 and 0.1 M Ca²⁺ conditions, respectively, which were higher than the PAMPS modulus recovery of 54% ± 22% (p = 0.84). These results suggest that only high Ca²⁺ gels result in a statistically significant increase in the modulus after healing.

Impact of Monovalent Ions on Self-Healing

The PNaAMPS gels had a strain recovery of 83% ± 3% (p = 0.24) and 121% ± 9% (p = 0.013), for 0.01 and 0.1 M Na⁺ conditions, respectively (Figures a,b,d and S6). Therefore, a high concentration of Na⁺ had a statistically significant impact (increase) on the UT strain after healing. PNaAMPS had a UT stress recovery of 185.7% ± 35% (p = 0.23) and 82% ± 12% (p = 0.96), for 0.01 and 0.1 M Na⁺ conditions, respectively. These values show that the UT stresses of the undamaged and healed samples were relatively similar to the PAMPS samples without any ions on them, with no statistically different change. The PNaAPMS modulus recovery was 200% ± 101% (p = 0.99) and 125% ± 27% (p > 0.99), for 0.01 and 0.1 M Na⁺ conditions, respectively. These results show that despite the difference in average values, the moduli of the undamaged and healed PNaAMPS samples were relatively the same. Collectively, the results show that Na⁺ ions do not inhibit self-healing and may increase the recovery of UT strain to >100% at high concentrations.

Mechanism behind Ion Impact on Self-Healing

Collectively, the self-healing results presented in Figure show that high concentrations of divalent ions increase UT stress recovery and decrease UT strain recovery, while a high concentration of monovalent ions enhances UT strain recovery without altering UT stress recovery. One hypothesis for the mechanism behind these results is that divalent ions form ionic bridges that enhance electrostatic binding between PAMPS chains, while Na⁺ ions do this to a lesser extent. An alternative explanation is the Hofmeister effect, where both the charge and size of the ions result in highly specific interactions between PAMPS chains and each type of ion, although the polymer concentration range we are in is unlikely to be dominated by this effect. FTIR analysis of the three different types of gels did not show any difference in the sulfonate stretching region (around 1142 cm^–1) that could shed light on this hypothesis. Since the degree of ionsulfonate group interaction would not differ significantly from mono- to divalent ions (i.e., each sulfonate group interacts with +1 charge of ion), that could be why no noticeable change is observed in the sulfonate region of the FTIR spectra. As shown in Figure a,b, there was a change in the C–H stretching peaks of the FTIR spectra at 2848 cm^–1 and 2915 cm^–1, which were lower or absent in the case of PNaAMPS. Since XPS results revealed consistent binding energies for S 2p, O 1s, and N 1s across all ionic conditions, confirming that the chemical structure of the PAMPS chain remains unchanged when ions are present (Figure c), the absence of C–H peaks can be interpreted as arising from those groups being oriented in a highly random manner in the PNaAMPS samples.

5.

Chemical characterization of ion–hydrogel interactions for pristine PAMPS, PNaAMPS (0.1 M), and PCaAMPS (0.1 M). (a,b) FT-IR spectra highlighting SO₃H and C–H stretching peaks and their intensity variations under different ionic conditions. (c) XPS scans of the oxygen, nitrogen, carbon, and sulfur regions.

To further examine how ionic species influence the polymer environment, ¹H NMR spectroscopy was performed on pristine PAMPS, 0.1 M PNaAMPS, and 0.1 M PCaAMPS hydrogels (Figure S8). Notable changes were observed in the a­(2H) (alkane) and c­(6H) (methyl group adjacent to sulfonate) peaks, with both PCaAMPS and PNaAMPS showing broader and more intense a­(2H) and c­(6H) peaks, indicating the presence of localized variations in electron density that slow proton relaxation dynamics. These alkane and methyl groups are the same regions that showed a difference in the FTIR spectra, further suggesting that this region is impacted by the presence of the ions. Since ¹H NMR does not directly probe the sulfonate group, direct confirmation of sulfonate–ion interactions will require additional alternative investigation.

To obtain more insight into how the two ions affect PAMPS chain mobility, we measured the frequency dependent viscosity (Figure a) and tan δ (Figure b) and calculated the characteristic relaxation time (τ) (Figure c) for each of the samples. As shown in Figure , the most direct measure of chain mobility, the relaxation time (τ), shows that the pristine gel has the longest τ (slowest relaxation), while PNaAMPS has the shortest τ (fastest relaxation). The addition of both divalent and monovalent ions facilitates the interchain mobility, with Na⁺ doing so to a much higher extent. Similarly, the presence of both the Na⁺ and Ca²⁺ ions reduced the storage and loss moduli, and increased the tan delta, with Na⁺ having a more pronounced impact. The rheological results suggest that the improved strength recovery of the gels with Ca²⁺ ions could be due to the combined impact of improved chain mobility at the cut interface and physical cross-linking by the ions. On the other hand, although Na⁺ ions also have higher mobility, the ions do not result in physical cross-linking and only disrupt the hydrogen bonding network, therefore counteracting any improvements in interchain diffusion at the cut interface. In other words, although Na⁺ ions could facilitate faster and therefore further diffusion of polymer chains across the cut interface during tensile testing, these chains also slip past each other more easily and therefore do not result in an overall increased strength recovery. SEM images of the healed samples (Figure ) show that the PNaAMPS healed interface shows a much smoother morphology, which also indicates that these samples are less viscous and flow more in the presence of moisture.

7.

SEM images of damaged and healed specimens. (a–c) Initial scratches (15 μm depth) on the surface of (a) pristine, (b) 0.1 M PCaAMPS, and (c) 0.1 M PNaAMPS gels at 0 h. (d–f) The same samples after 24 h of drying-induced chain collapse. (d) Pristine and (e) dry-healed 0.1 M PCaAMPS show a rough, collapsed morphology. (f) Dry-healed 0.1 M PNaAMPS shows a smooth, uniform morphology, consistent with its higher chain mobility (insets are EDX mapping for each ion conditions). A small gap remains in all samples. (g–i) The 24 h-dried samples (d–f) immediately after 5 μL of DI water was drop-cast onto the scratch, (g) water-healed pristine, (h) water-healed 0.1 M PCaAMPS, and (i) water-healed 0.1 M PNaAMPS. The scale bars are 10 μm.

Difference between Ions at the Damaged Interface and Ions in the Bulk Gel

Our findings from this study are distinct from our previous paper where we evaluated the impact of monovalent and divalent ions on the self-healing property of PAMPS hydrogels. In the prior study, the solution at the damaged interface contained the ions, while no ions were present in the bulk PAMPS gel. In that case, we found that both mono- and divalent ions reduced the strength recovery compared to the control DI water case. Our current results show that the presence of neither ion reduced the strength recovery, with divalent ions increasing the strength recovery nearly 2-fold. These results suggest that our previous finding was likely due to the osmotic pressure difference that the presence of the ions only at the cut interface introduces, which would draw water toward the cut interface and make the healed area softer. In our previous study, we found that UT strain recovery increased in the presence of both monovalent and divalent ions, which is also consistent with the hypothesis that the healed interfaces were softer due to the impacts of osmotic pressure.

Conclusions

This paper aimed to shed light on the intrinsic impact of monovalent (Na⁺) and divalent ions (Ca²⁺) on a model self-healing polymer, PAMPS. By pre-embedding ions into the bulk gel, our new experimental design successfully eliminated the confounding effect of osmotic pressure and salt gradient that complicated previous studies. Our results show that monovalent and divalent ions impact the self-healing process differentlywith divalent ions increasing strength and modulus recovery, while monovalent ions primarily enhance strain recovery after the healing process. Interestingly, divalent ions impacted the self-healing efficiency without changing the mechanical properties of the pristine gels. Our rheological results suggest that both monovalent and divalent ions increase the mobility of PAMPS polymer chains, with monovalent ions doing so to a much larger extent. Therefore, the observed self-healing results could be a result of divalent ions providing the right level of chain mobility and physical cross-linking to enhance self-healing, while monovalent ions increase it too much, counteracting the potential benefits. Further research is needed to understand the generalizability of these results using a range of monovalent and divalent ions and to determine repeated self-healing performance in the presence of these ionspast studies have shown that PAMPS can experience at least 6 cycles of rehealing. In addition, what role, if any, the Hofmeister effect plays and ion concentration-dependent effects also need to be probed. Our results, along with future studies, will provide insights for developing self-healing polymers that are more functional in a wide range of environments.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsapm.5c03289.

• Photographs of the polymer samples and polymer mold (Figure S1); variations in mechanical property as a function of drying time (Figure S2); relative water content of samples at different stages (Figure S3); tensile test comparison before and after oil coating (Figure S4); triplicate tensile test results for each ionic concentration (Figure S5); tensile test comparison before and after healing at varying ion concentrations (Figure S6); statistical analysis of UT strain, UT strength, and elastic modulus of samples with different ions (Figure S7); ¹H NMR spectra (Figure S8); storage and loss moduli of samples (Figure S9); SEM images of healed samples (Figures S10 and 11); and summary of UT stress, UT strain, and elastic modulus data in table form (Table S1) (PDF)

This work was supported by the National Science Foundation (grant number: 220136).

The authors declare no competing financial interest.