Achieving Ultrastiff Polyampholyte Nanocomposite Hydrogels via the Synergistic Strategy of Effective Nanoparticle Aggregation and Multi-Bond Networks

Abstract

Polyampholyte (PA) hydrogels have attracted considerable attention due to their unique dynamic network structures and favorable biocompatibility. However, their low modulus severely limits applications in load-bearing aspects. Herein, we report ultrastiff PA nanocomposite hydrogels through the synergistic strategy of effective aggregation of hydrophilic silica (SiO₂) nanoparticles and multi-bond networks. Specifically, a high content of SiO₂ nanoparticles is first incorporated into a dynamic ionic PA network via in situ polymerization. The resulting hydrogel is subsequently dialyzed in a zirconium salt solution with strong coordination capability, achieving the ultrastiff nanocomposite hydrogel. In this strategy, the dynamic PA network infiltrated between the aggregated SiO₂ nanoparticles enables effective particle aggregation, while the dynamic PA network, consisting of ionic and metal-coordination bonds, provides efficient energy dissipation, resulting in a synergistic reinforcement effect. The effects of dialysis time, concentration of zirconium salt, and particle content on the swelling and mechanical behaviors of the hydrogels are systematically investigated. The optimized nanocomposite hydrogel exhibits a Young’s modulus and a tensile strength as high as 87.9 ± 5.9 MPa and 7.9 ± 0.1 MPa, respectively, which are 976 and 8.8 times those of the original neat PA hydrogel. This work provides an effective strategy for designing hydrogels with ultrahigh mechanical performance.

1. Introduction

Hydrogels have shown great potential in diverse fields such as tissue engineering [1,2,3,4,5] and flexible electronics [6,7,8,9,10] due to their high-water-content and soft tissue-like properties. However, conventional hydrogels usually suffer from poor mechanical performance, which limits their practical applications [11,12]. Significant efforts have been made to develop strong and tough hydrogels including double-network hydrogel, nanocomposite hydrogel, and dynamic network hydrogel [13,14,15,16,17,18]. For instance, polyampholyte (PA) hydrogels, synthesized by random copolymerization of positively and negatively charged monomers, feature extensive ionic bonds within the network that can act as dynamic sacrificial bonds to dissipate energy, thereby imparting high toughness and self-recovery characteristics [19,20,21,22]. Despite these merits, their modulus and strength are still relatively low, limiting their applications in load-bearing areas.

To enhance the mechanical properties of PA hydrogels, several strategies have been developed, including micro/nanocomposites [23,24] and multi-bond networks [25,26,27,28]. Metal-coordination bonds, owing to their reversibility and high bond energy, can significantly improve strength and modulus while maintaining toughness. In our recent work, we have reported a strategy for strengthening PA hydrogels by introducing multivalent metal ions (Zr⁴⁺) during dialysis, leveraging metal-coordination bonds to induce multiphase microstructural reorganization. In that hydrogel system, the introduced Zr⁴⁺ ions, which exist in solution as hydrophilic [Zr(OH)₂·4H₂O]₄⁸⁺ clusters, exhibited strong coordination capability and could induce multiphase microstructural reorganization in PA hydrogels, resulting in an unusual “swelling-yet-strengthening” behavior (i.e., swelling in volume but strengthening in mechanical strength) [26,29,30,31,32]. The mechanical reinforcement of the hydrogels was mainly attributed to the synergy of ionic and metal-coordination bonds.

In an extension of our recent work on the strengthening of PA hydrogels [26], here we design and fabricate ultrastiff PA-based nanocomposite hydrogels by synergizing effective aggregation of hydrophilic SiO₂ nanoparticles and multi-bond network structure (Figure 1a). In detail, a high content of SiO₂ nanoparticles is first embedded in a dynamic ionic PA network via in situ solution radical polymerization, achieving a PA/SiO₂ hydrogel. The PA network is composed of a copolymer of P(NaSS-co-DMAEA-Q) from an anionic monomer [sodium p-styrenesulfonate (NaSS)] and a cationic monomer [dimethylaminoethylacrylate quaternized ammonium (DMAEA-Q)] (Figure 1b). The as-prepared PA/SiO₂ hydrogel is then dialyzed in ZrOCl₂ solution and deionized water in sequence, where the equilibration should be reached for each dialysis medium, obtaining the ultrastiff PA/SiO₂-Zr⁴⁺ nanocomposite hydrogel. In our design, the effective aggregation of nanoparticles is achieved by in situ penetrated dynamic PA chains between the aggregated SiO₂ nanoparticles during polymerization. The dynamic PA-based network, composed of a multi-bond (i.e., ionic and metal-coordination bonds) network, provides efficient energy dissipation, enabling synergistic mechanical enhancement. Systematic studies on the effects of dialysis time, ZrOCl₂ concentration, and particle content on the swelling and mechanical properties are carried out. The effect of SiO₂ nanoparticles on the chemical structure of the nanocomposite hydrogels is also discussed. After optimization, the Young’s modulus and tensile strength of our fabricated nanocomposite hydrogel reach 87.9 ± 5.9 MPa and 7.9 ± 0.1 MPa, respectively, which are 976 times and 8.8 times those of the neat PA hydrogel. Additionally, compared with our previous PA-Zr⁴⁺ hydrogels [26], our fabricated PA/SiO₂-Zr⁴⁺ nanocomposite gel also shows significant increases in these mechanical parameters. This study provides a novel strategy for developing ultrastiff and strong nanocomposite hydrogels.

2. Results and Discussion

2.1. Design, Fabrication, and Concept Proof

Figure 1a illustrates the design principle and fabrication process of the PA/SiO₂-Zr⁴⁺ hydrogels. Based on our strategy combining effective aggregation of SiO₂ nanoparticles with multi-bond networks for synergistic reinforcement, a high content of SiO₂ nanoparticles was first incorporated into the PA network, composed of dynamic ionic bonds, via in situ polymerization. The resulting hydrogels were subsequently dialyzed in a zirconium salt solution with strong coordination capability, ultimately achieving ultrastiff PA nanocomposite hydrogels. In this design and fabrication, the abundant cationic groups in the PA network interacted electrostatically with the negatively charged hydrophilic SiO₂ nanoparticles, enabling effective aggregation of a high content of nanoparticles. Meanwhile, the metal-coordination bonds introduced through the secondary dialysis step could work synergistically with the existing ionic bonds in the PA network to optimize the hydrogel matrix, facilitating efficient energy dissipation and thereby achieving synergistic reinforcement.

For clarity, the as-prepared hydrogels after initial water equilibration are denoted as PA hydrogels; the composite hydrogels containing SiO₂ nanoparticles are denoted as PA/SiO₂ hydrogels; after the introduction of zirconium ions and subsequent secondary water equilibration, the gels are denoted as PA-Zr⁴⁺-WEQ or PA/SiO₂-Zr⁴⁺-WEQ hydrogels, respectively.

To verify our design concept, representative samples were selected to evaluate their tensile properties, and the results are shown in Figure 1c–e. Here, $C_{{\mathrm{ZrOCl}}_2}$ is 0.5 mol/L and ω_SiO2 is 20 wt%. The tensile data revealed that PA/SiO₂-Zr⁴⁺-WEQ composite hydrogels prepared using our synergistic strategy exhibited a Young’s modulus and a tensile strength as high as 87.9 ± 5.9 MPa and 7.9 ± 0.1 MPa, respectively, which are 976 times and 8.8 times those of the original neat PA hydrogels, demonstrating a remarkable enhancement in mechanical properties. In addition, compared with the PA-Zr⁴⁺-WEQ hydrogel containing only multiple dynamic bonds, the composite hydrogel prepared in this work still showed significantly improved mechanical performance. These mechanical properties are notably superior to those of many reported high-strength and tough hydrogels [33,34,35,36].

The aggregation of SiO₂ nanoparticles within the hydrogel matrix was clearly evidenced by the SEM images (Figure 1f). In addition, we also compared the chemical structures between the neat and composite hydrogels by FTIR spectra (Figure 1g). In the PA hydrogel network, the characteristic peaks of 1170 cm⁻¹, 1123 cm⁻¹, 1036 cm⁻¹, 676 cm⁻¹, and 583 cm⁻¹ are correlated to –SO₃⁻ groups, and the peaks of 2933 cm⁻¹ and 1487 cm⁻¹ are correlated to -C(CH₃)₃N⁺ (i.e., -C-N⁺) groups. The characteristic peaks of –SiO⁻ groups in SiO₂ nanoparticles are located at 1102 cm⁻¹ and 467 cm⁻¹ (Figure S1). After the introduction of SiO₂ nanoparticles, the characteristic peaks of the –SO₃⁻ and -C(CH₃)₃N⁺ groups became weakened, but the peaks of –SiO⁻ groups from the SiO₂ nanoparticles became stronger, indicating the existence of effective interactions (e.g., electrostatic attraction) between the nanoparticles and the hydrogel network. The effective interaction is important for the proposed “effective aggregation of SiO₂ nanoparticles”. These combined results preliminarily confirmed our synergistic stiffening and strengthening strategy.

2.2. Effect of Dialysis Time in ZrOCl₂ Solution

Our recent study revealed that, owing to the strong coordination capability of highly hydrated Zr⁴⁺ ion clusters and the multiphase structural characteristics of the neat PA hydrogel, the dialysis of the PA hydrogel in ZrOCl₂ solution required several months to reach equilibrium, accompanied by increases in both size and mechanical properties [26]. In contrast, the subsequent re-equilibration in deionized water took only a few days (around 7 days). This process ultimately achieved a multiphase network structure reconstructed through the synergistic effect of ionic and metal-coordination bonds, exhibiting a swelling-yet-strengthening behavior. Therefore, in this section, we systematically investigate the effect of dialysis time on the swelling and mechanical behaviors of the fabricated nanocomposite hydrogels.

Figure 2 illustrates the swelling behavior of neat PA hydrogel and PA/SiO₂-20 nanocomposite hydrogel after dialysis in 0.5 mol/L ZrOCl₂ solution for different time, followed by secondary water equilibration. After the introduction of Zr⁴⁺ ions, the color change in PA-Zr⁴⁺ gels by the introduction of Zr⁴⁺ is mainly attributed to the reorganization of the multiphase microstructure of the PA network (Figure 2a), agreeing well with our recent study [26]. The color change in PA/SiO₂-Zr⁴⁺ gels should be due to the combination effect between the structure color of PA-Zr⁴⁺ network and the surface reflection of aggregated SiO₂ nanoparticles. The swelling plots reveal that the swelling process for both the neat and nanocomposite hydrogels can be divided into three stages (Figure 2b), in good agreement with our recent study on PA-Zr⁴⁺ hydrogels [26]. Stage I (0–3 d) is a rapid swelling stage, during which Zr⁴⁺ diffused into the gel network, disrupting the original ionic bonds and leading to a rapid increase in volume. Stage II (3–40 d) is a slow deswelling stage, where Zr⁴⁺ began to form metal-coordination bonds with the -SO₃⁻ groups in the PA network, making the network gradually contract. Stage III (40–200 d) is a network reorganization and stabilization stage, during which the gel volume approached plateau values and the multiphase microstructure was essentially formed. After the secondary water equilibration of around 7 days, both the gel volumes showed clear increases compared to their initial states. This specific swelling behavior is consistent with recently reported data [26]. Compared with the neat PA hydrogel, the nanocomposite hydrogel exhibited a marked decrease in volume swelling ratio (Q_v) during Stages I and II. However, the final swelling ratios of the two samples were comparable. The inorganic SiO₂ nanoparticles are stable and cannot be compressed in volume in the hydrogel networks. In this case, it is reasonable that although the polymer network deswelled into a smaller volume for the nanocomposite hydrogel due to the effective interaction, the total volume change in the nanocomposite hydrogel could be comparable to the neat one. This result also suggests that the interaction between the effectively aggregated nanoparticles and the hydrogel network suppressed the swelling of the PA hydrogel network. This interaction probably benefits the mechanical enhancement of the nanocomposite hydrogels.

We further investigated the effect of dialysis time (t_dia) on the tensile properties of the hydrogels. Figure 3 presents the tensile stress–strain curves of neat PA hydrogels and PA/SiO₂-20 nanocomposite hydrogels, along with the corresponding changes in tensile properties as a function of t_dia. Detailed tensile data are also presented in

Table 1. Summary of volume swelling ratio (Q_v) and tensile properties of neat PA hydrogel, PA-Zr⁴⁺ hydrogels, and PA/SiO₂-Zr⁴⁺ nanocomposite hydrogels with different t_dia and ω_SiO2. $C_{{\mathrm{ZrOCl}}_2}$ = 0.5 mol/L.

Sample Code [x, y] (a)	Young’s Modulus, E [MPa]	Tensile Strength, σb [MPa]	Work of Tension, Wb [MJ/m3]	Qv [m3/m3]
PA	0.09 ± 0.01	0.90 ± 0.10	2.87 ± 0.17	0.71 ± 0.01
PA-Zr4+-3	0.04 ± 0.01	0.27 ± 0.15	0.64 ± 0.33	2.46 ± 0.11
PA-Zr4+-7	0.04 ± 0.02	0.64 ± 0.19	1.42 ± 0.22	2.29 ± 0.04
PA-Zr4+-25	0.17 ± 0.02	0.90 ± 0.10	2.28 ± 0.24	1.97 ± 0.09
PA-Zr4+-60	3.33 ± 0.07	1.60 ± 0.25	3.02 ± 0.95	1.69 ± 0.04
PA-Zr4+-90	6.00 ± 0.10	1.73 ± 0.07	3.10 ± 0.28	1.72 ± 0.11
PA-Zr4+-120	10.5 ± 0.1	2.24 ± 0.13	4.43 ± 0.60	1.68 ± 0.03
PA-Zr4+-200	22.6 ± 0.7	2.64 ± 0.03	4.66 ± 0.37	1.56 ± 0.40
PA-Zr4+-WEQ	52.6 ± 3.1	5.09 ± 0.17	4.20 ± 0.28	1.44 ± 0.01
PA/SiO2-20	1.24 ± 0.13	1.63 ± 0.12	2.65 ± 0.27	1.07 ± 0.02
PA/SiO2-20-Zr4+-3	0.14 ± 0.01	0.88 ± 0.13	1.75 ± 0.63	2.05 ± 0.09
PA/SiO2-20-Zr4+-7	0.37 ± 0.01	1.53 ± 0.47	1.49 ± 0.83	1.71 ± 0.10
PA/SiO2-20-Zr4+-25	0.75 ± 0.01	1.81 ± 0.15	3.05 ± 0.63	1.72 ± 0.10
PA/SiO2-20-Zr4+-60	8.48 ± 0.04	2.70 ± 0.28	2.41 ± 0.53	1.75 ± 0.01
PA/SiO2-20-Zr4+-90	11.0 ± 0.1	2.76 ± 0.15	2.60 ± 0.60	1.78 ± 0.12
PA/SiO2-20-Zr4+-120	21.6 ± 1.3	3.71 ± 0.01	3.04 ± 0.58	1.73 ± 0.54
PA/SiO2-20-Zr4+-200	36.3 ± 1.2	3.51 ± 0.15	1.15 ± 0.31	1.48 ± 0.19
PA/SiO2-20-Zr4+-WEQ	87.9 ± 5.9	7.90 ± 0.10	1.60 ± 0.70	1.45 ± 0.09

^(a) x and y in the code of PA/SiO₂-x-Zr⁴⁺-y represent ω_SiO2 (wt%) and t_dia (d), respectively.

. With increasing t_dia in ZrOCl₂ solution, both the neat PA hydrogels and nanocomposite hydrogels exhibited a similar trend in mechanical properties. In the initial stage (0–3 d), the mechanical properties showed a certain degree of decline; with extending t_dia (3–40 d), they were gradually improved; upon further extension of t_dia (40–200 d), the mechanical properties were further enhanced. This trend agrees well with the three stages proposed to explain the swelling behavior of the hydrogels (Figure 2b). In Stage I, the substantial introduction of Zr⁴⁺ ions and their counter ions disrupted the original ionic network, resulting in slight decreases in mechanical properties. In Stage II, the gradual formation of metal-coordination bonds and ionic bonds led to progressive increases in mechanical properties. In Stage III, further reinforcement of metal-coordination bonds and ionic bonds as well as the network rearrangement and optimization contributed to additional enhancements in mechanical properties.

After transferring the above ZrOCl₂-equilibrated hydrogels to deionized water for secondary equilibration, the mechanical properties of the hydrogels reach their maximum values. Compared with the neat PA gel, PA/SiO₂ nanocomposite hydrogel exhibits improved tensile properties, although the enhancement effect is much lower than PA/SiO₂-Zr⁴⁺-WEQ nanocomposite hydrogel. Meanwhile, compared to the PA-Zr⁴⁺-WEQ gel, the significantly higher mechanical properties of PA/SiO₂-Zr⁴⁺-WEQ nanocomposite hydrogel are attributed to the synergistic reinforcement effect of the effective aggregation of SiO₂ nanoparticles and the multi-bond network within the composite hydrogel. The above results demonstrate that the dialysis time in ZrOCl₂ solution could effectively regulate the mechanical properties of the PA/SiO₂-Zr⁴⁺-WEQ nanocomposite hydrogels.

2.3. Effect of Concentration of ZrOCl₂ Solution

Our recent studies have indicated that the concentration of metal ions could affect both the quantity and quality of metal-coordination bonds, influencing the dissociation, reorganization, and optimization of the original ionic PA network, which could ultimately affect the strengthening of the hydrogels [37]. Therefore, we systematically investigated the effect of $C_{{\mathrm{ZrOCl}}_2}$ (0–1 mol/L) on the swelling and mechanical properties of the nanocomposite hydrogels in this section.

We first studied the swelling behavior of neat PA hydrogels and PA/SiO₂ nanocomposite hydrogels in ZrOCl₂ solutions with different $C_{{\mathrm{ZrOCl}}_2}$, as presented in Figure 4. Compared with the ZrOCl₂-un-dialyzed samples (i.e., $C_{{\mathrm{ZrOCl}}_2}$ = 0 mol/L), both the neat and composite hydrogels exhibited the aforementioned three characteristic swelling stages (Stage I: pronounced swelling; Stage II: slow deswelling; Stage III: further deswelling). Meanwhile, both kinds of hydrogels showed obvious swelling behaviors, which were markedly influenced by $C_{{\mathrm{ZrOCl}}_2}$. These swelling differences were primarily attributed to the fact that $C_{{\mathrm{ZrOCl}}_2}$ could determine the magnitude of the osmotic pressure difference, which accordingly affected the rate of zirconium ion diffusion into the PA network and the progress of reconstruction of the multi-bond-based dynamic network.

Specifically, with increasing $C_{{\mathrm{ZrOCl}}_2}$, the neat PA hydrogel samples exhibited more pronounced swelling in Stage I and showed a lower volume swelling ratio (Q_v) after secondary water equilibration (Figure 4a,b). For the PA/SiO₂ nanocomposite hydrogels, the swelling behavior in Stage I followed a similar trend to that of neat PA hydrogels with increasing $C_{{\mathrm{ZrOCl}}_2}$ (Figure 4c,d). However, the composite hydrogels achieved the lowest Q_v at $C_{{\mathrm{ZrOCl}}_2}$ = 0.5 mol/L. Additionally, PA/SiO₂-Zr⁴⁺-WEQ nanocomposite hydrogels exhibited lower Q_vs than PA/-Zr⁴⁺-WEQ hydrogels. This result indicates that the effective interactions between the SiO₂ nanoparticles and the PA network suppressed the swelling of the composite network. Furthermore, only at an appropriate $C_{{\mathrm{ZrOCl}}_2}$ could the effective aggregation of nanoparticles and the multi-bond network effectively constrain network swelling, which should be beneficial for the mechanical enhancement of the nanocomposite hydrogels.

Next, we studied the effect of $C_{{\mathrm{ZrOCl}}_2}$ on the tensile behavior of PA/SiO₂-Zr⁴⁺ nanocomposite hydrogels for the samples equilibrated in ZrOCl₂ solutions and re-equilibrated in deionized water (Figure 5). The detailed tensile data are also presented in

Table 2. Summary of tensile properties of PA/SiO₂ hydrogel and PA/SiO₂-Zr⁴⁺ nanocomposite hydrogels with different $C_{{\mathrm{ZrOCl}}_2}$.

Sample Code [x, y, z] (a)	E [MPa]	σb [MPa]	Wb [MJ/m3]
PA/SiO2-20	1.24 ± 0.13	1.63 ± 0.12	2.65 ± 0.27
PA/SiO2-20-0.1-Zr4+-200	21.6 ± 2.2	2.09 ± 0.38	1.21 ± 0.41
PA/SiO2-20-0.5-Zr4+-200	36.3 ± 1.2	3.51 ± 0.15	1.15 ± 0.31
PA/SiO2-20-1.0-Zr4+-200	31.9 ± 2.5	3.70 ± 0.02	1.35 ± 0.21
PA/SiO2-20-0.1-Zr4+-WEQ	26.1 ± 1.3	2.54 ± 0.08	1.23 ± 0.33
PA/SiO2-20-0.5-Zr4+-WEQ	87.9 ± 5.9	7.90 ± 0.10	1.60 ± 0.70
PA/SiO2-20-1.0-Zr4+-WEQ	56.2 ± 0.1	5.14 ± 0.10	0.86 ± 0.10

^(a) x, y, and z in the code of PA/SiO₂-x-z-Zr⁴⁺-y represent ω_SiO2 (wt%), t_dia (d), and $C_{{\mathrm{ZrOCl}}_2}$ (mol/L), respectively.

. It can be seen that most of the composite hydrogels exhibited enhanced mechanical properties, which increased continuously as $C_{{\mathrm{ZrOCl}}_2}$ increased. At $C_{{\mathrm{ZrOCl}}_2}$ = 0.1 mol/L, the enhanced mechanical properties of the composite hydrogels primarily resulted from the effective aggregation of SiO₂ nanoparticles, because our previous study revealed that the contribution from the multi-bond network was very limited for this $C_{{\mathrm{ZrOCl}}_2}$ [26]. When dialyzed only in ZrOCl₂ solutions, PA/SiO₂-Zr⁴⁺ composite hydrogels exhibited the highest mechanical properties at $C_{{\mathrm{ZrOCl}}_2}$ = 1.0 mol/L. However, after the secondary water equilibration, the mechanical properties of PA/SiO₂-Zr⁴⁺-WEQ composite hydrogels were optimized at $C_{{\mathrm{ZrOCl}}_2}$ = 0.5 mol/L. This difference is mainly attributed to the following: (i) excess Zr⁴⁺ ions might be not beneficial to the mechanical reinforcement of the PA network constructed via multi-bond interactions; (ii) excess Zr⁴⁺ ions and the counter ions might also reduce the interactions between the SiO₂ nanoparticles and the polymer network due to charge screening effect, thereby compromising the synergistic reinforcement effect. Nevertheless, when $C_{{\mathrm{ZrOCl}}_2}$ was even as low as 0.1 mol/L, the Young’s modulus (E) and tensile strength (σ_b) of PA/SiO₂-Zr⁴⁺-WEQ composite hydrogel reached 26.1 MPa and 2.54 MPa, respectively, which are 290 times and 2.8 times those of original neat PA hydrogel. At $C_{{\mathrm{ZrOCl}}_2}$ = 0.5 mol/L, the E and σ_b of PA/SiO₂-Zr⁴⁺-WEQ composite hydrogel reached 87.9 ± 5.9 MPa and 7.9 ± 0.1 MPa, respectively, which are 976 times and 8.8 times those of original neat one. Additionally, the tensile properties of PA/SiO₂-Zr⁴⁺-WEQ composite hydrogels with different $C_{{\mathrm{ZrOCl}}_2}$ clearly surpassed those of PA-Zr⁴⁺-WEQ hydrogels (Figure S2), confirming again the synergistic reinforcement combining effective aggregation of SiO₂ nanoparticles with multi-bond networks. The above results demonstrate that $C_{{\mathrm{ZrOCl}}_2}$ could effectively regulate the mechanical properties of the nanocomposite hydrogels.

2.4. Effect of Content of SiO₂ Nanoparticles

The content of SiO₂ nanoparticles (ω_SiO2) should influence their distribution within the PA hydrogel matrix, accordingly affecting the efficiency of the synergistic reinforcement from the effective aggregation of nanoparticles and the multi-bond network proposed in our design strategy. In this section, we systematically discussed the effect of ω_SiO2 on the swelling and mechanical properties of the fabricated PA/SiO₂-Zr⁴⁺ nanocomposite hydrogels.

We first investigated the volume swelling ratio (Q_v) of PA/SiO₂-Zr⁴⁺-WEQ nanocomposite hydrogels after secondary water equilibration, as shown in Figure 6. Meanwhile, Q_v versus t_dia plots for the composite hydrogels with different ω_SiO2 and $C_{{\mathrm{ZrOCl}}_2}$ (Figures S3–S5) indicate similar swelling processes with that shown in Figure 2. Clearly, the hydrogel without SiO₂ nanoparticles (i.e., ω_SiO2 = 0 wt%) exhibited the largest Q_v. As ω_SiO2 increased, the Q_v of the composite hydrogels initially became markedly lower than that of the original neat PA hydrogel, followed by a gradual increase. At ω_SiO2 = 20 wt%, the composite hydrogel possessed a comparable Q_v with the neat gel. At relatively low ω_SiO2 (0–10 wt%), the lower Q_vs of the composite hydrogels were primarily attributed to the effective interactions between the SiO₂ nanoparticles and the polymer network. At relatively high ω_SiO2 (10–20 wt%), the higher Q_vs of the composite hydrogels were mainly due to the incompressibility of the extensively aggregated SiO₂ nanoparticles, limiting the volume contraction during the secondary water equilibration process. The effective interactions between the SiO₂ nanoparticles and the polymer network served as the driving force for the “effective aggregation of nanoparticles” proposed in our design strategy.

We further discussed the effect of ω_SiO2 on the tensile properties of PA/SiO₂-Zr⁴⁺-WEQ nanocomposite hydrogels with $C_{{\mathrm{ZrOCl}}_2}$ = 0.5 mol/L (Figure 7). The detailed tensile data are also presented in

Table 3. Summary of tensile properties of PA/SiO₂-Zr⁴⁺-WEQ nanocomposite hydrogels with different ω_SiO2. $C_{{\mathrm{ZrOCl}}_2}$ = 0.5 mol/L.

Sample Code [x] (a)	E [MPa]	σb [MPa]	Wb [MJ/m3]
PA-Zr4+-WEQ	52.6 ± 3.1	5.09 ± 0.17	4.20 ± 0.28
PA/SiO2-3-Zr4+-WEQ	52.3 ± 4.6	5.50 ± 0.26	3.27 ± 0.25
PA/SiO2-5-Zr4+-WEQ	61.2 ± 5.5	5.60 ± 0.31	5.30 ± 0.85
PA/SiO2-10-Zr4+-WEQ	65.4 ± 1.9	5.72 ± 0.20	1.90 ± 0.55
PA/SiO2-15-Zr4+-WEQ	66.7 ± 4.8	5.53 ± 0.21	2.40 ± 0.56
PA/SiO2-20-Zr4+-WEQ	87.9 ± 5.9	7.90 ± 0.14	1.60 ± 0.65

^(a) x in the code of PA/SiO₂-x-Zr⁴⁺-WEQ represents ω_SiO2 (wt%).

. It can be seen that, with increasing ω_SiO2 (0–15 wt%), the Young’s modulus and tensile strength of the composite hydrogels exhibited gradual increases. Tensile data versus t_dia plots for the composite hydrogels with different ω_SiO2 ($C_{{\mathrm{ZrOCl}}_2}$ = 0.5 mol/L) further indicate the dynamically adjusted tensile properties with t_dia due to the re-organized network structures (Figure S6), similar to the data shown in Figure 3. At ω_SiO2 = 20 wt%, these tensile properties showed a significant enhancement. This behavior was primarily attributed to the dispersion state of the SiO₂ nanoparticles within the hydrogel matrix. At relatively low ω_SiO2, the nanoparticles were relatively uniformly dispersed, resulting in a moderate improvement in mechanical properties. In contrast, at relatively high ω_SiO2, the effective aggregation of nanoparticles synergistically interacted with the dynamic multi-bond network, leading to a substantial mechanical enhancement of the composite hydrogels. The trend of tensile data against ω_SiO2 for the composite hydrogels with different $C_{{\mathrm{ZrOCl}}_2}$ is slightly different, probably owing to the difference in energy dissipation efficiency of the multi-bond networks (Figures S6 and S7). Notably, the Young’s modulus of the composite hydrogel with ω_SiO2 = 20 wt% increased by approximately three orders of magnitude compared to that of the original neat PA hydrogel. These tensile data further validated our design strategy.

At last, we discussed the effect of ω_SiO2 on the chemical structures of PA/SiO₂-Zr⁴⁺-WEQ nanocomposite hydrogels by FTIR spectra (Figure 8a). It can be seen that, with increasing ω_SiO2, the characteristic peaks of the –SO₃⁻ and -C(CH₃)₃N⁺ groups in the PA network became gradually weakened, whereas the peaks of –SiO⁻ groups from the SiO₂ nanoparticles became gradually stronger. This trend further indicates that the in situ introduced SiO₂ nanoparticles could form effective interactions (e.g., electrostatic attraction) with PA-Zr⁴⁺ hydrogel matrix. We further illustrate the possible electrostatic interactions in both PA-Zr⁴⁺-WEQ hydrogel and PA/SiO₂-Zr⁴⁺-WEQ nanocomposite hydrogel, as presented in Figure 8b. It is clearly seen that more possible interactions were introduced by addition of hydrophilic SiO₂ nanoparticles in the nanocomposite hydrogel system. This result demonstrates that the incorporated SiO₂ nanoparticles could effectively interact with the hydrogel matrix. The effective interactions are critical for achieving the “effective aggregation of nanoparticles” proposed in our design strategy, thereby ensuring the synergistic effect with the multi-bond network in enhancing the nanocomposite hydrogels.

3. Conclusions

In this work, we have successfully developed ultrastiff PA/SiO₂-Zr⁴⁺ nanocomposite hydrogels via the synergistic strategy of effective aggregation of SiO₂ nanoparticles and multi-bond networks. To achieve the nanocomposite hydrogels, a high content of SiO₂ nanoparticles should be first introduced into a dynamic ionic PA network via in situ polymerization, followed by a dialysis in a ZrOCl₂ solution. The dynamic PA network infiltrated between the aggregated SiO₂ nanoparticles ensured the effective particle aggregation, which accordingly cooperated with the multi-bond (i.e., ionic and metal-coordination bonds) networks to enable the synergistic reinforcement. We systematically investigated the effects of dialysis time (t_dia), ZrOCl₂ concentration ($C_{{\mathrm{ZrOCl}}_2}$), and SiO₂ content (ω_SiO2) on the swelling and mechanical properties of the composite hydrogels. The effective aggregation of SiO₂ nanoparticles in the PA network was confirmed by FTIR spectra. When t_dia = 200 d, $C_{{\mathrm{ZrOCl}}_2}$ = 0.5 mol/L, and ω_SiO2 = 20 wt%, the nanocomposite hydrogel possesses the optimized tensile properties—a Young’s modulus of 87.9 ± 5.9 MPa and a tensile strength of 7.9 ± 0.1 MPa, which are 976 and 8.8 times those of the neat PA gel. The underlying strengthening mechanism and scale-up of PA/SiO₂-Zr⁴⁺ nanocomposite hydrogels need to be further explored in future work. This study offers an effective strategy for developing ultrastiff hydrogels.

4. Materials and Methods

4.1. Materials

Sodium p-styrene sulfonate (NaSS, 90 wt%; anionic monomer) was purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. (Shanghai, China) Methyl chloride quaternized N,N-dimethylamino ethylacrylate (DMAEA-Q, 80 wt%; cationic monomer) was obtained from J&K Chemical Ltd. (Beijing, China). [2-(methacryloyloxy)ethyl]trimethylammonium chloride (MPTC, 50 wt%; cationic monomer), N,N’-methylene-bis-acrylamide (MBAA, chemical crosslinker) were supplied by Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). α-ketoglutaric acid (α-keto; UV photo-initiator), ZrOCl₂ and SiO₂ nanoparticles (20 nm) were acquired from Shanghai Macklin Biochemical Technology Co., Ltd. (Shanghai, China).

4.2. Preparation of PA-SiO₂ Hydrogels

PA/SiO₂ hydrogels were prepared via free radical polymerization. The ionic monomers NaSS and DMAEA-Q (total monomer concentration C_m = 2.5 mol/L), crosslinker MBAA (0.10 mol%), and initiator KA (0.10 mol%) were dissolved in deionized water. The molar ratio of anionic NaSS and cationic DMAEA-Q was 0.51:0.49. Various mass fractions (0–20 wt%) of SiO₂ nanoparticles were added, and the mixture was stirred in a 65 °C water bath for 15 min. The precursor solution was injected into a glass mold and irradiated with a UV lamp (365 nm, 4 W/cm²) for 10 h to obtain the as-prepared PA/SiO₂ (ASP-PA/SiO₂) gels. The gels were then dialyzed in deionized water for 5 days, with the water changed every 12 h, to obtain the water-equilibrated PA/SiO₂ gels.

4.3. Preparation of PA/SiO₂-Zr⁴⁺ Hydrogels

The PA/SiO₂ gels were immersed in ZrOCl₂ solutions at different concentrations (0.1 mol/L, 0.5 mol/L, 1.0 mol/L) for various durations (3–200 d). After 200 d, the gels were re-immersed in deionized water for 5 d to obtain the secondarily water-equilibrated PA/SiO₂-Zr⁴⁺-WEQ hydrogels.

4.4. Swelling Ratio of Hydrogels

The as-prepared (ASP) cylindrical hydrogel samples were immersed in deionized (DI) water to attain a secondary re-equilibrium. The original sample volume V₀ was determined based on the initial geometry (diameter d₀ = 10 mm × thickness t₀ ≈ 1 mm), while the volume Vs at different immersion time were calculated from the measured dimensions (d × t). The volume swelling ratio (Q_v) of the sample is defined as [26]

$$Q\mathrm{v}={\displaystyle \frac{V}{V_0}}={\displaystyle \frac{d^2·t}{{d_0}^2·t_0}}$$

(1)

4.5. Tensile Tests

Uniaxial tensile tests of hydrogel samples were performed on a universal testing machine (MTS, E43.104) equipped with a 250 N load cell. Prior to testing, the samples were cut into a dumbbell shape with dimensions of length [l] = 12 mm × width [w] = 2.5 mm × thickness [t] ≈ 1 mm. Tests were conducted at room temperature with a crosshead speed of 100 mm·min⁻¹. The detailed sample geometry and method for the tests are illustrated in Figure S9. To prevent dehydration of the hydrogels, a humidifier was used during testing. The Young’s modulus (E) was calculated from the initial slope of the stress–strain curve within the 10% strain range. The work of tension during testing, denoted as W_b, was determined by integrating the area under the stress–strain curve.

$$W_{\mathrm{b}}={\int }_0^{{\epsilon }_{\mathrm{b}}}\sigma d\epsilon$$

(2)

where σ and ε represent the stress and strain of the sample, respectively, and ε_b denotes the strain at fracture. Each sample was tested at least three times, and the average value was calculated and the standard deviation was obtained as error bar.

4.6. Fourier Transform Infrared Spectroscopy (FTIR)

FTIR spectral analysis of the hydrogel samples was performed on a Tensor 2 FTIR spectrometer (NeXus, Houston, TX, USA). Sample preparation involved the standard KBr pellet method, with spectral data acquired across the 400–4000 cm⁻¹ range at 4 cm⁻¹ resolution. All measurements were conducted under ambient temperature conditions.

Other experimental details are presented in the Supplementary Materials.