Polymer hydrogels have been extensively studied and used in tissue engineering ^{[1, 2]},carriers for drug delivery ^{[3, 4]},wastewater treatment,and disposable diapers ^{[5, 6]} for their good biocompatibility and stimuli-response ability. Generally,hydrogels are moderately cross-linked networks of a hydrophilic polymer which can absorb and retain a large amount of water or other fluids ^[1]. In fact,the swelling capacity strongly depends on the cross-linking density,and moderately cross-linked hydrogels exhibit the best swelling performance ^[2]. Meanwhile,the cross-linking density and swelling capacity also influence the mechanical properties of hydrogels. Although some nanocomposite hydrogels ^{[7, 8, 9, 10]},especially those based on inorganic nanofillers (e.g.,mica,kaolin,silicate,rectorite,vemiulite,attapulgite,and montmorillonite) raise much attention because of their reduced production costs and improved swelling capacity and swelling rate for superabsorbent polymers,their mechanical properties are seldom reported. Recently,silica nanoparticles reinforced nanocomposite hydrogels with good mechanical properties were reported,but their swelling capacities were neglected ^{[11, 12]}.

However,many applications require hydrogels that have exceptional mechanical properties,which are generally limited by low extensibility and toughness,so many strategies to focus on the improvement of the mechanical properties have been investigated recently ^{[13, 14, 15]}. In particular,nanocomposite hydrogels composed of both an organic polymer and an inorganic compound hold the unique properties of organic-inorganic materials and may be used as high-performance materials ^[13]. Recently we proposed a "single network,dual (or hierarchical) cross-linking" strategy to prepare various tough and highly stretchable nanocomposite hydrogels ^{[16, 17, 18, 19, 20]}. For example,nanocomposite physical hydrogels (NCP gels) based on vinylhybrid silica nanoparticles (VSNPs) have been prepared by in situ free radical polymerization ^{[18, 19]}. The networks of NCP gels are first enabled by the intermolecular hydrogen bonds among the VSNP/polymer nanobrushes. Once the hydrogen bonds are formed,the VSNP can act as multiple covalent "analogous cross-linking points",which translate into excellent mechanical properties. Some reports have been published for the improvement of swelling capacity ^{[16, 17, 18, 21]}. However,the mechanical properties (stress and strain) of the hydrogels are relatively low. Therefore,hydrogels with both high toughness and swelling capacity are still a huge challenge.

In this work,we report a facile one pot method for the preparation of tough and stretchable VSNP/poly(acrylic acid) (PAA) nanocomposite physical hydrogels (NCP gels) with superior swelling capacity. The effect of the content of VSNPs on the mechanical properties and swelling behavior are investigated. We find that the NCP gels have excellent and balanced mechanical properties (tensile strength = 370 kPa,elongation at break = 2200%) and excellent swelling capacity (-2300 g/g in deionized water and 220 g/g in saline). Furthermore,when the NCP gels are in a swollen state,they can be stretched by more than 400%. We believe this method may highlight a new way to prepare hydrogels having excellent and balanced mechanical properties coupled with an excellent swelling capacity,which may find their applications in bioengineering,water treatment,chemical separation,as well as hygienic fields.

VSNPs with 3 nm diameter were prepared following our previous reports ^{[19, 20]}. NCP gels were prepared as follows: a calculated amount of acrylic acid was added to deionized water and neutralized by 40% NaOH solution under stirring in an ice bath. After that,a pre-set amount of VSNPs suspension was redispersed in the above solution by stirring at ambient temperature. After 2 h,0.25 wt% ammonium persulfate (relative to the monomer) was added to the mixture and stirred for a few minutes. After being degassed by bubbling with N₂ for 1 h,the uniform suspension was transferred to several plastic syringes of 5 mm internal diameter. The polymerization was performed at 45 ℃ for 30 h in a water bath under N₂ atmosphere. The chemically cross-linked PAA gels were prepared using the same procedure with N,N₀-methylenediacrylamide (Bis) (0.1 wt% relative to the monomer) as the cross-linker. The total water content and the value of neutralization degree of the hydrogels were fixed at 67 wt% and 70%,respectively. Unaxial tensile tests were performed by a Zwick 005 tensile machine at ambient temperature on the cylindrical hydrogel specimen,at a constant crosshead speed of 100 mm min^-1 and initial sample length between jaws of 15 mm. The tensile strength (from the initial cross-section of 19.62 mm²) and percentage elongation at break were calculated by the recorded fracture tensile force and length. The slope between 20 and 80% strain in the stress-strain curve was used to calculate the initial elastic modulus. The fracture toughness of NCP gels was defined as the work of the fracture,calculated as the integrated area underneath the stress-strain curve of each sample. Hysteresis loops were examined to calculate energy dissipation and were performed on the cell loading-unloading cycles tests using the same experimental setup as unaxial tensile test with the same tensile speed of 100 mm min^-1.

The swelling capacity was examined as follows: a weighed quantity of the as-prepared NCP gels sample was immersed in a large amount of deionized water or saline solution (0.9 wt% NaCl) at ambient temperature to reach swelling equilibrium. The swollen samples were then separated from unabsorbed water by filtering over a 100-mesh screen. The water absorbency (Q) of the hydrogels was determined by weighing the swollen samples,and the Q of the samples was calculated using the following equation: Q = (m₂ × m₁)/m₁ where m₁ and m₂ are the weights of the calculated dry sample and the swollen sample,respectively.

Silica nanoparticles are widely used as "nano-fillers" to obtain inorganic-organic nanocomposites due to their easily controllable size,uniform structure,and stable function in aqueous or organic solutions ^[11]. Uniform and multivalent VSNPs with a diameter of 3 nm are prepared by a universal sol-gel method ^{[19, 20]}. As reported in our previous work,we have prepared physically crosslinked PAA hydrogels by in situ free radical polymerization with acrylic acid as the monomer,which is grafted from the surface of VSNPs,thus obtaining VSNP/PAA nanobrushes. VSNPs serve as multivalent covalent cross-linking points once the physical crosslinks enabled by the hydrogen bonding interactions between the nanobrushes have been established ^{[19, 20]}. Thus,dually crosslinked,single-network hydrogels with multivalent covalent crosslinking points and hydrogen bonding points are obtained.

The influence of the VSNPs content on the hydrogels’ mechanical properties is investigated first. A typical stress-strain curves of the NCP gels with 67% fixed water content is presented in Fig. 1,which demonstrates that the NCP gels have good mechanical properties with tensile strengths in the range of 110-370 kPa and elongations at break of 1600%-3000%. In contrast,the chemically cross-linked hydrogels exhibit a relatively low tensile strength of 100 kPa and stretchability of 700% only. With increasing VSNPs content,the elongation at break decreases steadily. Conversely,the tensile strength,modulus,and toughness increase with the increase of VSNPs content from 0.05 wt% to 0.7 wt% relative to the monomer. Degraded mechanical properties can be observed for the NCP gels when the VSNPs content is higher than 0.7 wt%. When the VSNPs content is 0.70 wt%,the NCP gels exhibit excellent mechanical properties with the tensile strength and elongation at break of 370 kPa and 2200%,respectively. The excellent mechanical properties should result from the interplay between the VSNPs content and the molecular weight of the grafted PAA chains. As mentioned above,VSNPs act as the "analogous cross-linking points" when the hydrogels are stretched,which can thus absorb and relax the applied stress via the numerous flexible polymer chains anchored on their surface. At the same time,the physical cross-links break-recombine to dissipate energy,thus delaying crack propagation. When the VSNPs content increases,the grafted PAA molecular weight should be reduced ^[19],because there are a greater number of initiation points on the surface of VSNPs for polymer growth at a given monomer concentration. Consequently,the average polymer length is reduced as the VSNPs content increases,thus decreasing the elongation at break of the NCP gels. Meanwhile,more VSNPs represent more multivalent covalent cross-linking points and less applied stress that can be absorbed and redistributed.

Fig. 1

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Fig. 1.Mechanical properties of the NCP gels: (a) stress-strain curves; (b) tensile strength and elongation at break; (c) initial elastic modulus and fracture energy; the water content was fixed at 67%.

Physical hydrogels can effectively dissipate the applied energy through reversible cross-linking,and can be typically examined by hysteresis loops with loading and unloading stress-strain curves. When the loading-unloading measurement is conducted on the NCP gels,a pronounced hysteresis loop is observed in the first loading-unloading cycle for the NCP gels,as shown in Fig. 2a. Moreover,more energy dissipation is observed when the VSNPs content increases from 0.1 wt% to 0.7 wt% (Fig. 2b). This indicates that upon loading-unloading,break-recombination of hydrogen-bonding interactions of the NCP gels is an effective way to dissipate energy,and VSNPs can absorb and disperse the applied energy,thus translating into high mechanical strength and toughness. When the loading-unloading measurement is conducted immediately after the first cycle on the same NCP gels,the hysteresis loop becomes much smaller. In fact,under five successive loading-unloading cycles the hysteresis loop becomes less pronounced as shown in Fig. 2c and d. This can be ascribed to the breakage of a significant portion of the short network PAA chains cross-linked by hydrogen bonding within the network,indicating that the sudden energy dissipation and the gel network are rearranged and homogenized through the break-recombination of the hydrogen bonding interactions between the PAA chains in the first loading-unloading cycle. Therefore,the homogenized NCP gels no longer dissipate more energy under a fixed deformation,whether the number of applied cycles increases or not ^[20]. As mentioned above,the NCP gels are physically crosslinked via reversible noncovalent interactions which are facilitated by hydrogen bonding and chain entanglement,thus providing the self-recovery capacity of the NCP gels in nature. Subsequent loading-unloading measurements are conducted to examine the self-recovery properties of the NCP gels at room temperature without any external stimuli. The NCP gels are recovered for 1 h after the first cycle measurement,and then subjected to the second loading-unloading cycle. The hysteresis loop becomes much more pronounced as shown in Fig. 2e. Moreover,it is found that the recovery efficiency of the NCP gels increases with increasing VSNPs content (Fig. 2f). From the above discussion,it can be concluded that the network structure of the NCP gels is constructed by a reversible physical cross-linking that is facilitated by the intra- and inter-polymer chain hydrogen bonding of the VSNP/PAA nanobrushes,and multivalent covalent cross-linking through VSNPs. Thus,the dual cross-linking system is sufficient to dissipate energy and maintain the gels’ network during deformation.

Fig. 2

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Fig. 2.Loading-unloading cycles of the NCP gels: (a) with difference content of VSNPs at the maximum strain of 1000%; (b) dissipated energy and toughness of the NCP gels with different VSNPs contents; (c) cyclic loading-unloading curves of NCP gels (0.4 wt% VSNPs) at 1000% strain for six cycles; (d) hysteresis ratio of the energy dissipation of cyclic measurement of NCP gels with VSNPs content of 0.4 wt%; (e) recovery of the NCP gels under successively loading-unloading cycles with the strain of 1000%; (f) the effect of content of VSNPs on the recovery efficiency of the NCP gels; the water content was fixed at 67%.

The swelling behavior of the NCP gels is investigated and shown in Fig. 3. As illustrated in Fig. 3a and b,the NCP gels have good swelling behavior. The NCP gels can maintain their shapes even after 800 times of swelling in deionized water (Fig. 3a) and 150 times of swelling in saline (0.9 wt% NaCl,Fig. 3b),respectively. As shown in Fig. 1,the tensile strength of conventional covalently cross-linked hydrogels with a swelling ratio of 2 is only about 100 kPa. In contrast,the NCP gels (0.7 wt%) with a swelling ratio of 2 can maintain a strength of 370 kPa. The NCP gel sample with a swelling ratio of 10 can also sustain a tensile strength of 65 kPa. Furthermore,the NCP gel sample with a swelling ratio of 100 can still be stretched up to 5 times of the initial length as shown in Fig. 3c and d. When we measure the mechanical properties of the swollen NCP gels (0.7 wt%) with different swelling ratios,it is found that the stress,strain,and modulus decrease steadily as swelling ratio increases (Fig. 3e). It is obvious that the mechanical properties of the NCP gels are seriously influenced by the crosslinking density per unit cross-sectional area. The excellent mechanical properties of the NCP gels result from the unique network structure of the physically cross-linked hydrogels as discussed in our previous reports ^{[19, 20]}.

Fig. 3

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Fig. 3.Swelling behavior of the NCP gels: (a) photographs of the highly swelled NCP gels in deionized water (about 800); and (b) saline (about 150),respectively; (c) photograph of the swollen hydrogel (Q = 100) and (d) it can be stretched for 5 times as initial length; (e) mechanical properties of the NCP gel with a different swelling ratio (VSNP = 0.7 wt%); (f) swelling capacity of the NCP gel with various VSNPs content,(g) swelling kinetics of the NCP hydrogel with content of 0.7 wt% VSNPs in deionized water.

As shown in Fig. 3f,when the NCP gels are immersed in a large amount of water or saline (0.9 wt% NaCl),the swelling capacity increases initially and then declines with increasing VSNP content.

When the VSNPs content is 0.7 wt%,the swelling capacity can reach a maximum of 2300 g/g and 220 g/g in deionized water and saline,respectively,far more than that of conventional chemically cross-linked PAA gels. This can be explained by the fact that increasing VSNPs content means increased multiple covalent "analogous cross-linking points",thus improving the cross-linking density. Generally,the swelling capacity of hydrogels is influenced by the cross-linking density. It is accepted that a moderate crosslinked network can provide the hydrogels with a relatively high swelling capacity because there is interplay between the effects of network size and cross-linking density ^[22]. The influence of VSNPs content on the swelling capacity is in agreement with the mechanical properties of the NCP gels. Furthermore,it is found that the NCP gels can dissolve completely within a few days,implying the NCP gel is recyclable. Typically,as shown in Fig. 3g,the swelling kinetics ratio was found to initially increase smoothly,becoming more rapid until arriving at a peak,after which it dropped. Ultimately the NCP gel disrupted to a sol completely. This phenomenon can also prove the dynamic rearrangement of the NCP gels’ network. During swelling,a high concentration of the ions within the hydrogels relative to the bulk water results in a high osmotic pressure,accelerating the speed at which water diffuses into the hydrogel network. With the decreasing density of the NCP gels’ network,the movement of polymer chains becomes more rapid and leads to greater rearrangement of the NCP gels’ network. In a dilute polymer solution,the physical interactions between the polymer chains are very weak,so they cannot hold the NCP gels’ network together for a long time. Ultimately,the bulk physical hydrogels return to nano-polymer brushes of the gelators ^[19]. The swelling behavior of the NCP gels thus indicates that the hydrogels are physically cross-linked in nature,which is beneficial for recycling and reuse.

We provide a facile one-pot method to prepare tough and highly stretchable physical hydrogels using VSNPs as multivalent covalent cross-linking points. The network structure of the NCP gels is constructed through reversible physical cross-linking,intraand inter-polymer chain hydrogen bonding of the polymer chains and multivalent covalent cross-linking through VSNPs,and it is sufficient to dissipate energy and maintain the gels’ network during deformation. When the VSNPs content is 0.7 wt%,the hydrogels exhibit excellent mechanical properties with a fracture tensile strength of -370 kPa and elongation at break of -2200%. Furthermore,the swelling capacity is also studied,and a high swelling ratio of -2300 g/g in deionized water or -230 g/g in saline is exhibited. This unique combination of properties is likely to enable new applications in biotechnology for these superabsorbent hydrogels.

This research was financially supported by the National Natural Science Foundation of China (No. 21474058),State Key Laboratory for Modification of Chemical Fibers and Polymer Materials,Donghua University (No. LK1404) and Tsinghua University Scientific Research Project (No. 2014Z22069).