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High-Strength Nanocomposite Hydrogels with Swelling-Resistant and Anti-Dehydration Properties Bo Xu 1 , Yuwei Liu 1 , Lanlan Wang 1 , Xiaodong Ge 1 , Min Fu 2 , Ping Wang 1 and Qiang Wang 1, * 1

2

*

Key Laboratory of Eco-Textile, Ministry of Education, College of Textile and Clothing, Jiangnan University, Wuxi 214122, China; [email protected] (B.X.); [email protected] (Y.L.); [email protected] (L.W.); [email protected] (X.G.); [email protected] (P.W.) College of Chemical and Environmental Engineering, Shandong University of Science and Technology, Qingdao 266590, China; [email protected] Corresponding: [email protected]; Tel.: +86-510-8591-2005

Received: 2 July 2018; Accepted: 13 September 2018; Published: 14 September 2018

 

Abstract: Hydrogels with excellent mechanical properties have potential for use in various fields. However, the swelling of hydrogels under water and the dehydration of hydrogels in air severely limits the practical applications of high-strength hydrogels due to the influence of air and water on the mechanical performance of hydrogels. In this study, we report on a kind of tough and strong nanocomposite hydrogels (NC-G gels) with both swelling-resistant and anti-dehydration properties via in situ free radical copolymerization of acrylic acid (AA) and N-vinyl-2-pyrrolidone (VP) in the water-glycerol bi-solvent solutions containing small amounts of alumina nanoparticles (Al2 O3 NPs) as the inorganic cross-linking agents. The topotactic chelation reactions between Al2 O3 NPs and polymer matrix are thought to contribute to the cross-linking structure, outstanding mechanical performance, and swelling-resistant property of NC-G gels, whereas the strong hydrogen bonds between water and glycerol endow them with anti-dehydration capacity. As a result, the NC-G gels could maintain mechanical properties comparable to other as-prepared high-strength hydrogels when utilized both under water and in air environments. Thus, this novel type of hydrogel would considerably enlarge the application range of hydrogel materials. Keywords: nanocomposite hydrogels; alumina; high-strength; anti-dehydration; swelling resistance

1. Introduction Hydrogels are polymeric materials that consist of three-dimensional (3D) networks and a large amount of water [1]. Because of their well-known hygroscopicity, hydrophilicity, biocompatibility, and similarity to native tissues, hydrogels have been widely used in a wide range of applications, including as super-absorbents, tissue engineering, drug delivery, wound dressing, as well as cell culture [2–6]. Recently, hydrogels with excellent mechanical properties have attracted increasing attention due to their potential for use in artificial tissues, bio-actuators, soft robots, and so forth [7–11]. Thus, massive efforts have been made to improve the mechanical properties of hydrogels [12]. Despite several tough and strong hydrogels being fabricated via different strategies [13–18], the practical applications of these hydrogels are still experiencing challenges. On the one hand, as a kind of “soft and wet” material, hydrogels are usually utilized in under water environments. Unfortunately, most hydrogels would absorb water to swell as a result of the difference in osmotic pressure [19]. After swelling, the strength and toughness of hydrogels are drastically weakened. On the other hand, when used in air environments, water is inevitably lost from the hydrogels by evaporation, leading to the drying-out of hydrogels, which also causes degradation of the softness of hydrogels. Therefore, developing hydrogel materials with excellent mechanical properties as well as swelling-resistance Polymers 2018, 10, 1025; doi:10.3390/polym10091025

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and anti-hydration properties is an attractive but highly challenging task. Recently, several research groups have made some outstanding contributions to this area. For example, Sakai et al. reported a high-strength and non-swelling hydrogel with a homogeneous network structure by random copolymerization of hydrophilic and thermoresponsive monomers with tetra-armed polyethylene glycol (PEG) backbone [19]. When the thermoresponsive segment ratio is 0.4, the resultant hydrogels swelled at 10 ◦ C but could recover its original shape at around 37 ◦ C. In another study, Jin et al. fabricated a type of dual-cross-linked (DC) hydrogels composed of tannin acid (TA) cross-linked poly(vinyl alcohol) and chemically cross-linked polyacrylamide. A strong multiple polymer—TA hydrogen bonds not only endow hydrogels with tough mechanical properties, but also suppress the swelling of hydrogels, resulting in good swelling resistance [20]. Our previous studies also demonstrated that hydrogels with low swelling ratios could be realized using titania (TiO2 ) and alumina (Al2 O3 ) nanoparticles as inorganic cross-linking agents [21,22]. The low swelling ratios of these gels were supposed to contribute to the strong complexation interaction between nanoparticles and carboxyl groups on the polymer chains. Although these above-mentioned hydrogels achieved non-swelling or low swelling via different mechanisms, few reports have concentrated on hydrogels with anti-dehydration properties, not to mention that hydrogels possess both of these unique properties. In this study, we successfully developed a novel high-strength nanocomposite hydrogels (NC-G gels) that simultaneously possess anti-dehydration and swelling-resistant properties, via in situ free-radical copolymerization of acrylic acid (AA) and N-vinyl-2-pyrrolidone (VP) in a water-glycerol bi-solvent containing small amounts of alumina nanoparticles (Al2 O3 NPs) as the inorganic cross-linking agents. The obtained hydrogels exhibited excellent mechanical properties comparable to previously reported nanocomposite hydrogels. More importantly, the introduction of glycerol endows the obtained hydrogels with anti-dehydration ability in air due to the strong hydrogen bonding interactions between water and glycerol, which help to lock the water inside the polymer networks. Besides, slight shrinkage instead of swelling was observed when the NC-G gels were immersed in the water environment, which probably contributed to the increased cross-linking density of hydrogels. Due to unique anti-dehydration and swelling-resistant properties, the NC-G gels maintained outstanding softness and high mechanical properties both under water and in air environments, which could greatly expand the practical applications of hydrogels. 2. Materials and Methods 2.1. Materials Acrylic acid (AA, >99%) and N-vinyl-2-pyrrolidone (VP, 99%) were purchased from Aladdin Reagent Co., Ltd., Shanghai, China. Glycerol (99.5%) was obtained from Innochem Co., Ltd., Beijing, China. 2-hydroxy-40 -(2-hydroxyethoxy)-2-methylpropiophenone (Irgacure 2959, 98%) was purchased from Energy Chemical Co., Ltd., Shanghai, China. A colloid solution of Al2 O3 NPs with particle size of 10–20 nm and solid content around 10% was provided by Wanjing New Materials Co., Ltd., Hangzhou, China. Organic cross-linker N,N 0 -methylenebis-(acrylamide) (MBA, 98%) was obtained from Alfa Aesar Co., Ltd., Shanghai, China. All the reagents were used as received without further treatment. 2.2. Preparation of Hydrogels The hydrogels were prepared by photo-initiated in situ free radical polymerization of AA and VP in the mixed solutions containing water, glycerol, and small amounts of Al2 O3 NPs, following the procedure similar to our previous report [22]. Firstly, homogeneous precursor solutions were obtained by mixing a certain amount of raw Al2 O3 colloid solution, deionized water, glycerol, and 20 mg Irgacure 2959 in a 20 mL glass bottle under a nitrogen atmosphere. Then, the precursor solutions were transferred into different molds after being degassed by a vacuum. Finally, free radical polymerization was conducted under 365 nm ultraviolet (UV) light (Scientz03-II, Xinzhi Co., Ltd., Ningbo, China) with

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the intensity of 60 mW/cm2 for 30 min. The resultant hydrogels were termed as NC-Gx gels, in which x% represents the mass percentage of glycerol in the total water-glycerol solution. The molar ratio of AA to VP was fixed at 3:7 based on our experiment, since the hydrogel prepared with the monomer ratio of 3:7 exhibited the best comprehensive mechanical properties compared to its counterparts obtained with other monomer ratios. Unless otherwise specified, the total monomer content and the Al2 O3 concentration in the water-glycerol solutions were fixed at 3 mol/L and 2%, respectively, although they can be altered within a large range in order to meet the requirements of different applications. For comparison, the conventional hydrogel cross-linked by 30 mg organic cross-linker MBA was also prepared and named OR-G gels. The detailed sample names and the corresponding compositions are listed in Table 1. Table 1. Compositions of nanocomposite hydrogels (NC-G gels) and OR-G gel. Sample

Glycerol (g)

Water (g)

AA (g)

VP (g)

Al2 O3 (g)

MBA (mg)

NC-G0 NC-G20 NC-G40 NC-G60 NC-G80 OR-G60

0 2 4 6 8 6

10 8 6 4 2 4

0.63 0.63 0.63 0.63 0.63 0.63

2.33 2.33 2.33 2.33 2.33 2.33

0.2 0.2 0.2 0.2 0.2 0

0 0 0 0 0 30

Note: 20 mg Irgacure 2959 was used in all the samples as the photo-initiator; AA: acrylic acid; VP: N-vinyl-2-pyrrolidone; MBA: N,N 0 -methylenebis-(acrylamide).

2.3. Evaluation of Anti-Dehydration and Swelling-Resistant Properties The anti-dehydration and swelling-resistant properties of the hydrogels were evaluated in air atmosphere (the temperature and humidity were fixed at 27 ◦ C and 60%, respectively) and pure water (water was changed every 12 h). The sheet-like samples with size of 10 × 10 × 2 mm were placed in the above-mentioned circumstances and weighed after 10 days. The weight variations were used to evaluate their anti-dehydration and swelling-resistant properties. The weight variation was calculated as follows: Weight variation (%) = (W 0 - W 10 )/W 0 × 100% (1) where W 0 and W 10 are the weights of the samples before measurement and after 10 days, respectively. Three measurements were conducted for each hydrogel, and the average values were used for discussion. 2.4. Mechanical Tests of Hydrogels The mechanical properties of hydrogels were tested on a MIT-1 universal test machine (Sanfeng Co., Changzhou, China) with a 50 N load cell. The as-prepared sheet-like samples with size of 10 × 50 × 2 mm were clamped between two clamps with a gauge length of 10 mm. Then the tensile tests were conducted at a speed of 100 mm/min until fracture under ambient conditions (about 25 ◦ C). The fracture stress was calculated by the force at the fracture point divided by the initial cross-sectional area of the hydrogel sample (ca. 20 mm2 ). The elastic modulus was calculated from the slope over 50–100% of strain on the stress-strain curve. The fracture stress, elongation at break, and elastic modulus of hydrogels were used for discussion. As for the samples placed in air and under water environments, the as-prepared samples with the size mentioned above were placed under certain conditions described in Section 2.3. followed by the mechanical tests, and their values for mechanical properties were calculated based on their actual sizes after 10 days. 2.5. Characterization of Hydrogels Fourier transform infrared (FTIR) spectra of Al2 O3 NPs, neat copolymer of AA and VP, as well as NC-G0 gel were obtained using an IRAffinity-1S (Shimadzu Co., Kyoto, Japan) FTIR spectrometer

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with an attenuated total reflectance (ATR) accessory. All of the samples were dried in a 95 °C oven with an attenuated total reflectance (ATR) accessory. All of the samples were dried in a 95 ◦ C oven before conducting the FTIR characterization. Scanning electron spectroscopy (SEM) was conducted before conducting the FTIR characterization. Scanning electron spectroscopy (SEM) was conducted to to observe the microstructures of the NC-G gels. The as-prepared hydrogels were first placed in water observe the microstructures of the NC-G gels. The as-prepared hydrogels were first placed in water (water was changed every 12 h) for 10 days to replace the glycerol with water. Then, the samples (water was changed every 12 h) for 10 days to replace the glycerol with water. Then, the samples were were immersed in liquid nitrogen and followed by freeze-dry (FD-1A-50, Boyikang Co., Beijing, immersed in liquid nitrogen and followed by freeze-dry (FD-1A-50, Boyikang Co., Beijing, China) for China) for 48 h at −48 °C. The fractured surfaces were observed using a Quanta 250 (FEI Co., 48 h at −48 ◦ C. The fractured surfaces were observed using a Quanta 250 (FEI Co., Hillsboro, OR, USA) Hillsboro, OR, USA) instrument at an acceleration voltage of 20 kV after coating with gold. The instrument at an acceleration voltage of 20 kV after coating with gold. The thermal gravimetric analysis thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) measurement of NC(TGA) and differential scanning calorimetry (DSC) measurement of NC-G gels were performed on a G gels were performed on a Pyris 1 (Perkin Elmer Co., Waltham, MA, USA) and DSC 8000 (Perkin Pyris 1 (Perkin Elmer Co., Waltham, MA, USA) and DSC 8000 (Perkin Elmer Co., Waltham, MA, USA) Elmer Co., Waltham, MA, USA) instruments, respectively. For both of the characterizations, the instruments, respectively. For both of the characterizations, the heating rate was set at 10 ◦ C/min from heating rate was set at 10 °C/min from 30 to 300 °C under N2 atmosphere. 30 to 300 ◦ C under N2 atmosphere. Resultsand andDiscussion Discussion 3.3.Results 3.1.Formation FormationofofNanocomposite NanocompositeHydrogels Hydrogels(NC-G (NC-GGels) Gels) 3.1. Hydrogelshave havebeen beenextensively extensivelyapplied appliedininvarious variousareas areasdue duetototheir theirspecific specificphysicochemical physicochemical Hydrogels characteristics [23–25]. However, the poor mechanical properties of hydrogels has severelylimited limited characteristics [23–25]. However, the poor mechanical properties of hydrogels has severely their application range. Although some tough and strong hydrogels have been prepared via different their application range. Although some tough and strong hydrogels have been prepared via different strategies,their theiroutstanding outstandingmechanical mechanicalperformance performance can canonly onlybe beexhibited exhibitedinintheir theiras-prepared as-prepared strategies, states. Notably, hydrogels are generally utilized either in air or water environments. Under these states. Notably, hydrogels are generally utilized either in air or water environments. Under these circumstances,hydrogels hydrogelswould wouldlose loseor orabsorb absorbaalarge largeproportion proportionofofwater waterwithin withintheir theirpolymer polymer circumstances, networks,which which inevitably influence their mechanical limiting their practical networks, inevitably influence their mechanical properties,properties, limiting their practical applications. applications. addressinthis in developed this study, awe developed novel alumina cross-linked biTo address thisTo problem, thisproblem, study, we novel aluminaa cross-linked bi-solvent NC-G solvent NC-G gel, which both anti-dehydration and swelling resistant properties. The NCgel, which possesses both possesses anti-dehydration and swelling resistant properties. The NC-G gels were G gels were prepared by a feasible photo-initiated free radical copolymerization of AA and VP prepared by a feasible photo-initiated free radical copolymerization of AA and VP in mixtures of waterin mixtures of containing water and glycerol containing amounts of Al2O3 NPs as illustrated in Scheme 1. It and glycerol small amounts of Alsmall 2 O3 NPs as illustrated in Scheme 1. It was found that all thatNC-G all of the NC-G gels,glycerol regardless of their glycerol content, were uniform and ofwas thefound obtained gels,obtained regardless of their content, were uniform and stable, and could stable, and could not be dissolved in water, indicating the formation of effective cross-linking not be dissolved in water, indicating the formation of effective cross-linking structures. To confirm structures. To confirm this hypothesis, was conducted and results Figure 1. It this hypothesis, SEM was conducted andSEM the results are shown in the Figure 1. Itare canshown be seeninthat all the can begels seen that all the NC-G gels and OR-G60 gel possess well-defined porous microstructures with NC-G and OR-G60 gel possess well-defined porous microstructures with the pore size around the pore size aroundregardless several micrometers, regardless the glycerol content. These confirmed several micrometers, of the glycerol content.ofThese results confirmed that results the introduction the introduction of glycerol theinfluence precursors notpolymerization influence the monomer ofthat glycerol in the precursors wouldinnot the would monomer as well aspolymerization the formation as well as the formation of cross-linked 3D polymer network structures. of cross-linked 3D polymer network structures.

Scheme 1. Preparation and cross-linking mechanism of NC-G gels. (a) Materials for the preparation of Scheme 1. Preparation and cross-linking mechanism of NC-G gels. (a) Materials for the preparation NC-G gels. Digital pictures of (b) hydrogel precursor and (c) NC-G gels. (d) Illustration of the network of NC-G gels. Digital pictures of (b) hydrogel precursor and (c) NC-G gels. (d) Illustration of the structure NC-G gels. (e) Proposed cross-linking mechanism of NC-G gels. network structure NC-G gels. (e) Proposed cross-linking mechanism of NC-G gels.

–COOH) as well as the weakening of peak centered at 3441 cm−1 (characteristic band of –OH) in the spectrum of NC-G0 gel imply that a strong interaction exists between the carboxyl groups and Al2O3 NPs. According to the literature [22,26–28], these interactions probably correspond to the topotactic chelation reactions between the dissociated carboxyl groups (–COO–) on the polymer chains and the aluminum atoms on the surface of Al2O3 NPs (Scheme 1e), which contributed to the cross-linking Polymers 2018, 10, 1025 structures of NC-G gels. This result verifies that the Al2O3 NPs acted as cross-linking agents, since5 of 12 extra organic cross-linkers were involved in the preparation procedure [22]. Polymers 2018, 10, x FOR PEER REVIEW

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Figure 2 displays the ATR-FTIR spectra of Al2O3 NPs, neat copolymer of AA and VP, as well as dried NC-G0 gel. The disappearance of the band at 1680 cm-1 (characteristic band of vinyl groups) in the spectra of neat copolymer and NC-G0 gel indicates that almost all the monomers participated in the polymerization during the UV irradiation. In addition, this result also verifies that the incorporation of Al2O3 NPs would not hinder the copolymerization of AA and VP, which is an important prerequisite for the formation of hydrogels. Furthermore, compared to the spectra of neat copolymer and that of Al2O3 NPs, the disappearance of the peak of 1712 cm−1 (characteristic band of –COOH) as well as the weakening of peak centered at 3441 cm−1 (characteristic band of –OH) in the spectrum of NC-G0 gel imply that a strong interaction exists between the carboxyl groups and Al2O3 NPs. According to the literature [22,26–28], these interactions probably correspond to the topotactic chelation reactions between the dissociated carboxyl groups (–COO–) on the polymer chains and the aluminum atoms on the surface of Al2O3 NPs (Scheme 1e), which contributed to the cross-linking structures of NC-G gels. This result verifies that the Al2O3 NPs acted as cross-linking agents, since extra organic cross-linkers were involved in the preparation procedure [22].

Figure 1. typical The typical scanning electron microscopy(SEM) (SEM)images images of of NC-G NC-G and Figure 1. The scanning electron microscopy and OR-G OR-Ggels. gels.(a)(a)NCNC-G0, G0, (b) NC-G20, (c) NC-G40, (d) NC-G60, (e) NC-G80, and (f) OR-G60. (b) NC-G20, (c) NC-G40, (d) NC-G60, (e) NC-G80, and (f) OR-G60.

Figure 2 displays the ATR-FTIR spectra of Al2 O3 NPs, neat copolymer of AA and VP, as well as dried NC-G0 gel. The disappearance of the band at 1680 cm-1 (characteristic band of vinyl groups) in the spectra of neat copolymer and NC-G0 gel indicates that almost all the monomers participated in the polymerization during the UV irradiation. In addition, this result also verifies that the incorporation of Al2 O3 NPs would not hinder the copolymerization of AA and VP, which is an important prerequisite for the formation of hydrogels. Furthermore, compared to the spectra of neat copolymer and that of Al2 O3 NPs, the disappearance of the peak of 1712 cm−1 (characteristic band of –COOH) as well as the weakening of peak centered at 3441 cm−1 (characteristic band of –OH) in the spectrum of NC-G0 gel imply that a strong interaction exists between the carboxyl groups and Al2 O3 NPs. According to the literature [22,26–28], these interactions probably correspond to the topotactic chelation reactions between the dissociated carboxyl groups (–COO– ) on the polymer chains and the aluminum atoms on the surface of Al2 O3 NPs (Scheme 1e), which contributed to the cross-linking structures of NC-G gels. This resultFigure verifies that the Al NPs acted as cross-linking agents, since extra organic Figure 1. The typical scanning electron microscopy images of NC-G and OR-G gels. (a)cross-linkers NC2 O3 infrared 2. Fourier transform (FTIR) spectra(SEM) of alumina nanoparticles (Al2O 3 NPs), neat G0, (b) NC-G20, (c) NC-G40, (d) NC-G60, (e) NC-G80, and (f) OR-G60. were involved in the copolymer andpreparation NC-G0 gel. procedure [22].

FigureFigure 2. Fourier transform infrared nanoparticles(Al(Al 2. Fourier transform infrared(FTIR) (FTIR)spectra spectra of of alumina alumina nanoparticles 2O32 O NPs), neatneat 3 NPs), copolymer and NC-G0 gel.gel. copolymer and NC-G0

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3.2.Anti-Dehydration Anti-Dehydrationand andSwelling SwellingResistant Resistant Properties NC-G Gels 3.2. Properties of of NC-G Gels Theevaporation evaporationofofwater waterininairairand andabsorbing absorbingofofwater waterininwater waterenvironments environmentsare aretwo twomajor major The and long-lasting problems that limit the practical application of hydrogels because of their great and long-lasting problems that limit the practical application of hydrogels because of their great influence mechanical properties of hydrogels. Our previous report demonstrated that influence on on the the mechanical properties of hydrogels. Our previous report demonstrated that hydrogels hydrogels with low swelling ratios can be obtained by using AA and N,N-dimethylacrylamide with low swelling ratios can be obtained by using AA and N,N-dimethylacrylamide (DMAA) as (DMAA) as the co-monomer, and 2O3 NPs as the inorganic cross-linkers due to the strong chelation the co-monomer, and Al2 O3 NPs asAl the inorganic cross-linkers due to the strong chelation reactions reactions between polymer chains and Recently, Lu et al.the reported that the between polymer chains and nanoparticlesnanoparticles [22]. Recently,[22]. Lu et al. reported that incorporation incorporation of glycerol in hydrogels could prevent the loss of water and maintain the properties of glycerol in hydrogels could prevent the loss of water and maintain the properties of the hydrogelof ◦ C because hydrogel in a wide temperature −20 to 60 of the strong hydrogen bonding inthe a wide temperature range from −20range to 60from of °C thebecause strong hydrogen bonding interactions interactions between water and glycerol [29]. Based on these reports, we thought that the newly between water and glycerol [29]. Based on these reports, we thought that the newly prepared NC-G prepared NC-G gels could possess both swelling resistant and anti-dehydration properties. To verify gels could possess both swelling resistant and anti-dehydration properties. To verify this hypothesis, this hypothesis, hydrogels with content differentwere glycerol content placedwater in airfor and10under water for 10 hydrogels with different glycerol placed in airwere and under days to observe days to observe their changes in volume and weight. Figure 3a shows the digital pictures of astheir changes in volume and weight. Figure 3a shows the digital pictures of as-prepared NC-G gels prepared NC-G gels with different glycerol contents. The NC-G0 gel, which contained no glycerol, with different glycerol contents. The NC-G0 gel, which contained no glycerol, is opaque, which is is opaque,from which different from the hydrogels form[22]. AAItand [22]. Itacknowledged has been widely different theishydrogels prepared form AAprepared and DMAA hasDMAA been widely acknowledged that the carbonyl groups on the VP segment could form hydrogen bonds groups with the that the carbonyl groups on the VP segment could form hydrogen bonds with the carboxyl carboxyl groups by hydrogen AA [30,31]. These hydrogen bonds would probably of cause the introduced by AA introduced [30,31]. These bonds would probably cause the formation small formation of small polymer aggregates within hydrogels, which leads to the phase separation, thus polymer aggregates within hydrogels, which leads to the phase separation, thus resulting in the resulting in the opaque appearance NC-G0 [32]. by This be verified by the transparent opaque appearance of NC-G0 gels [32]. of This can begels verified thecan transparent appearance of Al2 O3 appearance of Al 2O3 cross-linked hydrogels using AA and DMAA as the monomer with the same cross-linked hydrogels using AA and DMAA as the monomer with the same monomer ratio, in which monomer in which the hydrogen bonding the polymer chains much lower than that the hydrogenratio, bonding between the polymer chainsbetween is much lower than that in theisNC-G0 gel, probably in the NC-G0 gel, probably due to the steric hindrance of the two methyl groups. However, with the due to the steric hindrance of the two methyl groups. However, with the incorporation of glycerol, incorporation of glycerol, a well-known plasticizer, into the hydrogel system, the a well-known plasticizer, into the hydrogel system, the hydrogels gradually transformed fromhydrogels opaque gradually transformed from opaque to totally transparent. This is because the glycerol able to to totally transparent. This is because the glycerol was able to destroy the hydrogen bondswas between destroychains, the hydrogen bonds between ofpolymer preventing thethe formation of polymer polymer preventing the formation polymerchains, aggregates. Therefore, transparence of the aggregates. Therefore, the transparence of the hydrogels increased with increasing glycerol content. hydrogels increased with increasing glycerol content.

Figure 3. The digital pictures of NC-G gels. (a) As-prepared, (b) immersed in water for 10 days, digital pictures of NC-G gels. (a)ofAs-prepared, (c)Figure placed3.inThe air for 10 days, (d) weight variation NC-G gels. (b) immersed in water for 10 days, (c) placed in air for 10 days, (d) weight variation of NC-G gels.

Distinct from most of the hydrogels that would absorb water to swell under aqueous environments, NC-G did notthat swell, but even slightly shrunk after under immersion in Distinct the from mostgels of not the only hydrogels would absorb water to swell aqueous water for 10 days, revealed Figure fromeven the quantitative measurements showin environments, theasNC-G gelsinnot only3b. didThe not results swell, but slightly shrunk after immersion that all of shrank about 10% (w/w) compared to their weight (Figure 3d). This water forthe 10hydrogels days, as revealed in Figure 3b. The results from the original quantitative measurements show phenomenon be explained the(w/w) cross-linking of Alweight cross-linked that all of thecan hydrogels shrankbased abouton 10% comparedmechanism to their original (Figure 3d). This 2 O3 NPs hydrogels. As can we discussed previously, NC-Gofgels contributed to the phenomenon be explained based onthe thecross-linkage cross-linkingwithin mechanism Al2O 3 NPs cross-linked – groups on polymer chains and Al O NPs. In our experiment, the Al O interactions hydrogels.between As we –COO discussed previously, the cross-linkage2 within NC-G gels contributed to2 the 3 3 – NPs were dispersed in –COO acidic media toon result in a transparent with a pH interactions between groups polymer chains andAl Al22O O33 colloid NPs. Insolution our experiment, thevalue Al2O3 around four.dispersed After polymerization, thetoliquid within the hydrogels wassolution also acidic, pH NPs were in acidic media resultphase in a transparent Al2O3 colloid withwith a pHavalue value lower than the pKa (4.75) of –COOH. Under this condition, only part of the –COOH on around four. After polymerization, the liquid phase within the hydrogels was also acidic, with athe pH – , which could interact with Al O NPs to form cross-linkage. polymer chainsthan deprotonated to –COO value lower the pKa (4.75) of –COOH. Under this condition, only 2 3 part of the –COOH on the polymer chains deprotonated to –COO–, which could interact with Al2O3 NPs to form cross-linkage.

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However, when NC-G gels were immersed in neutral deionized water, more –COO– groups can be – groups generatedwhen to react with Alwere 2O3 NPs during in theneutral solventdeionized exchangewater, process, which could increase the However, NC-G gels immersed more –COO can be cross-linking density leading to the shrinkage of the hydrogels. Furthermore, from generated to react withofAlhydrogels, O NPs during the solvent exchange process, which could increase the 2 3 Figure 3b, alldensity the hydrogels became opaque again after immersion in water Furthermore, for 10 days. This cross-linking of hydrogels, leading to the shrinkage of the hydrogels. fromis because replacement of glycerol with water the plasticizing effect glycerol; Figure 3b, the all the hydrogels became opaque again after eliminated immersion in water for 10 days. Thisofis because therefore, the phase separation occurred again and the hydrogel turned opaque.therefore, the phase the replacement of glycerol with water eliminated the plasticizing effect of glycerol; Figure 3c displays pictures of NC-G gels opaque. placed in air after 10 days. It can be seen that the separation occurred againthe and the hydrogel turned NC-G0 gel3cshrank significantly due the evaporation of air water. when glycerol was Figure displays the pictures of to NC-G gels placed in afterHowever, 10 days. It can be seen that incorporated, the volume change became increasingly less obvious. According to the quantitative the NC-G0 gel shrank significantly due to the evaporation of water. However, when glycerol was data shown inthe Figure 3d, change the weight variation gradually decreased about 47.5% 1.2% as the incorporated, volume became increasingly less obvious.from According to the to quantitative glycerol content increased 0% to 60%. Asgradually for the NC-G80 gel,from the sample even absorbed data shown in Figure 3d, thefrom weight variation decreased about 47.5% to 1.2% aswater the from the environment. This result confirms that the incorporation of glycerol could indeed inhibit the glycerol content increased from 0% to 60%. As for the NC-G80 gel, the sample even absorbed water water of This waterresult within hydrogels to the strongofhydrogen bonding interactions from theevaporation environment. confirms that due the incorporation glycerol could indeed inhibit between and water, as within well as hydrogels the lower volatility and a higher boilingbonding point of interactions glycerol [29]. the water glycerol evaporation of water due to the strong hydrogen To proveglycerol this conclusion, TGA andas DSC to characterize the boiling NC-G gels and results are between and water, as well thewere lowerused volatility and a higher point of the glycerol [29]. shown Figure 4. It canTGA be seen 4a that NC-G0 gelthe lost moregels thanand 40% ofresults its original To proveinthis conclusion, andfrom DSCFigure were used to the characterize NC-G the are weight at 100 °C due to the evaporation of water. However, when glycerol was introduced, higher shown in Figure 4. It can be seen from Figure 4a that the NC-G0 gel lost more than 40% of its original temperatures to remove the the NC-G Fromwas the introduced, DSC curves higher (Figure weight at 100 ◦were C dueneeded to the evaporation of liquid water. from However, whengels. glycerol 4b), the endothermic peaks 118.7from °C tothe 156.1 °C gels. as theFrom glycerol content increased temperatures were needed to shifted remove from the liquid NC-G the DSC curves (Figure from 4b), ◦ ◦ 0% to 80%. All these results confirm that the introduction of glycerol in the hydrogel systems could the endothermic peaks shifted from 118.7 C to 156.1 C as the glycerol content increased from 0% to delay evaporation of the from the ofhydrogels, imparting the could hydrogels 80%. Allthe these results confirm that liquid the introduction glycerol in thus the hydrogel systems delayantithe dehydration properties. evaporation of the liquid from the hydrogels, thus imparting the hydrogels anti-dehydration properties.

Figure Figure4.4.(a) (a)Thermal Thermalgravimetric gravimetricanalysis analysis(TGA) (TGA)and and(b) (b)differential differentialscanning scanningcalorimetry calorimetry(DSC) (DSC) curves curvesofofNC-G NC-Ggels. gels.

3.3. Mechanical Properties of NC-G Gels 3.2. Mechanical Properties of NC-G Gels The mechanical properties of hydrogels play a key role in their practical applications. The currently The mechanical properties of hydrogels play a key role in their practical applications. The reported NC-G gels with dehydration and swelling-resistant properties also had outstanding currently reported NC-G gels with dehydration and swelling-resistant properties also had mechanical performance. As displayed in Figure 5, the as-prepared NC-G60 gel could withstand outstanding mechanical performance. As displayed in Figure 5, the as-prepared NC-G60 gel could large-scale deformations, e.g., stretch after knotting and compression without breaking. Furthermore, withstand large-scale deformations, e.g., stretch after knotting and compression without breaking. aFurthermore, hydrogel strip with a diameter of a5.5 mm could upcould to a 1.0 showing a hydrogel strip with diameter of 5.5lift mm lift kg up weight, to a 1.0 kg weight,excellent showing mechanical strength. In contrast, the OR-G60 gel cross-linked by organic cross-linker with the same excellent mechanical strength. In contrast, the OR-G60 gel cross-linked by organic cross-linker with monomer and glycerol content as NC-G60 gel were fragile and brittle, which was unable to withstand the same monomer and glycerol content as NC-G60 gel were fragile and brittle, which was unable to the deformations mentioned above. withstand the deformations mentioned above.

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Illustration ofthe the outstanding mechanicalperformance performance (a)Knotted, Knotted,(b) (b)stretch stretch Figure Figure5.5.Illustration Illustrationof theoutstanding outstandingmechanical performanceof ofNC-G NC-Ggels. gels.(a) (c)before (d)under (e)after aftercompression, compression,and and(f) (f)lift liftup after afterknotting, knotting,(c) beforecompression, compression,(d) undercompression, compression,(e) upaa 1.0 kg weight. 1.0 kg weight.

To quantitatively quantitatively measure the the mechanical mechanical properties properties of of NC-G NC-G gels, gels, uniaxial uniaxial tensile tensile tests tests were were quantitatively measure To conducted ofof hydrogels areare shown in Figure 6. The results indicate that conductedand andthe thestress stressand andstrain straincurves curves of hydrogels are shown inFigure Figure The results indicate conducted stress and strain curves hydrogels shown in 6.6.The results indicate all the prepared NC-G gels could be stretched more than seven times their original length with a tensile that all the prepared NC-G gels could be stretched more than seven times their original length with that all the prepared NC-G gels could be stretched more than seven times their original length with betweenbetween 0.26 to 0.61 MPa, which iswhich comparable to nanocomposite hydrogels cross-linked by atensile tensilestrength strength between 0.26 to0.61 0.61 MPa, whichisiscomparable comparable tonanocomposite nanocomposite hydrogels crossastrength 0.26 to MPa, to hydrogels crossother [14,33,34]. Here, it is worth noting that thenoting hydrogels only linkedinorganic by other othernanomaterials inorganic nanomaterials nanomaterials [14,33,34]. Here, worth noting thatprecursors the hydrogels hydrogels linked by inorganic [14,33,34]. Here, itit isis worth that the contained Alcontained O3 NPs, so of NC-G gels could be further precursors2% only 2%the Almechanical O33 NPs, NPs, so soperformance the mechanical mechanical performance of NC-G NC-G gelsenhanced could be be precursors only 2% Al 22O the performance of gels could 2contained by increasing the Al in22O the precursor solution. the solution. glycerol could further enhanced by increasing the Al Al O concentration in the theBesides, precursor solution.content Besides, the further enhanced by increasing the 33 concentration in precursor Besides, the 2O 3 concentration affect thecontent mechanical hydrogels.properties As shownof in the figure, the gradually glycerol content couldproperties affect the theof mechanical properties of hydrogels. hydrogels. Astensile shownstrength in the the figure, figure, the glycerol could affect mechanical As shown in the increased as the glycerolincreased content increased fromcontent 0% to increased 40%, afterfrom which strength started to tensilestrength strength gradually increased asthe theglycerol glycerol content increased from 0%the to40%, 40%, afterwhich whichthe the tensile gradually as 0% to after decrease. The reason for thisThe phenomenon be explained may as follows. (1) As as mentioned above, strength started started to decrease. decrease. The reason for for may this phenomenon phenomenon may be explained explained as follows. follows. (1) (1) As As strength to reason this be the incorporation of in theof hydrogel could destroy the hydrogen bonds between mentioned above,the theglycerol incorporation of glycerolsystems inthe thehydrogel hydrogel systems could destroy thehydrogen hydrogen mentioned above, incorporation glycerol in systems could destroy the polymer chains,polymer which increases the flexibility the polymerof chains. (2) It has been(2) reported that bondsbetween between polymer chains,which which increasesof the flexibility of thepolymer polymer chains. (2) hasbeen been bonds chains, increases the flexibility the chains. ItIthas the glycerol-water mixtures exhibit stronger interactions the polymers than pure water, reported thatthe theglycerol-water glycerol-water mixtures exhibit strongerwith interactions withthe the polymers thanwhich pure reported that mixtures exhibit stronger interactions with polymers than pure could more noncovalent interactions in interactions the polymer in networks actingnetworks as sacrificial bonds water,introduce which could could introduce more noncovalent interactions in the the polymer polymer networks acting as water, which introduce more noncovalent acting as to improve the toughness ofthe NC-G gels [29]. (3) Thegels glycerol could hydrogen bonds with the sacrificial bonds toimprove improve the toughness ofNC-G NC-G gels [29].(3) (3) Theform glycerol couldform formhydrogen hydrogen sacrificial bonds to toughness of [29]. The glycerol could carboxyl groups on thegroups polymer whichchain, wouldwhich lead would to the reduced cross-linking density of bondswith with thecarboxyl carboxyl groups onchain, thepolymer polymer chain, which would leadto tothe the reducedcross-linking cross-linking bonds the on the lead reduced hydrogels, since the cross-linkage of NC-G gelsof contributed the chelation between Al2 O3 density of of hydrogels, hydrogels, since the the cross-linkage cross-linkage of NC-G gels gelstocontributed contributed toreactions the chelation chelation reactions density since NC-G to the reactions NPs and Al carboxyl groups. These three factors could influence thecould mechanical properties of NC-G between Al22O O33 NPs NPs and carboxyl carboxyl groups. These three factors could influence the mechanical mechanical between and groups. These three factors influence the gels. Whenof the glycerol content under 40%, the strong interactions of glycerol-water mixtures properties of NC-G gels. Whenwas the glycerol glycerol content was under under 40%, 40%, the thethe strong interactions of the the properties NC-G gels. When the content was strong interactions of and polymer networks dominant; therefore, the mechanical strength increased with increasing glycerol-water mixtureswere and polymer polymer networks networks were dominant; therefore, therefore, the mechanical mechanical strength glycerol-water mixtures and were dominant; the strength glycerol However, whencontent. the glycerol content was than 40%,was the higher hydrogen increasedcontent. withincreasing increasing glycerol content. However, when thehigher glycerol content was higherthan thanbonds 40%, increased with glycerol However, when the glycerol content 40%, between glycerol and carboxyl groups caused the decrease of the cross-linking density of NC-G gels. thehydrogen hydrogenbonds bondsbetween betweenglycerol glyceroland andcarboxyl carboxylgroups groupscaused causedthe thedecrease decreaseof ofthe thecross-linking cross-linking the As a result, the mechanical properties decreased accordingly. density ofNC-G NC-G gels.As Asaaresult, result, themechanical mechanical propertiesdecreased decreasedaccordingly. accordingly. density of gels. the properties

glycerolcontent. content. Figure6.6.Stress-strain Stress-straincurves curvesof ofNC-G NC-Ggels gelswith withdifferent differentglycerol Figure

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Due to the anti-dehydration and swell-resistant properties as well as the excellent mechanical properties of their as-prepared states, we thought that the NC-G gels could also possess considerable mechanical performance both in air and water environments. Therefore, mechanical tests were conducted on hydrogels after being placed either in air for 10 days or immersed in water for 10 days. As shown in Figure 7, the NC-G0 gels exhibited excellent mechanical toughness after immersion in water for 10 days, and could withstand twisting without breaking. However, after being placed in air for 10 days, the hydrogel became stiff and brittle because of the loss of water within the hydrogels. Thus, the hydrogel was easily fractured upon deformation. In contrast, the NC-G60 gel maintained its outstanding mechanical performance both in air and water, which simultaneously overcame the two long-lasting problems inhibiting the practical applications of hydrogel materials. Based on these findings, we also measured the mechanical properties of hydrogels under different conditions, and the results are summarized in Figure 8. All of the NC-G gels demonstrated similar mechanical properties after immersion in water for 10 days. This is because the glycerol within hydrogels was replaced with water, which causes the different NC-G gels to have similar compositions after immersion in water. Furthermore, as discussed above, the volumes of the NC-G gels shrank 10% after immersion in water due to the increased cross-linking density. Therefore, the tensile strength and elastic moduli of the hydrogels significantly increased, while the elongations at break slightly decreased. Notably, the tensile strength of the hydrogels immersed in water were as high as 1 MPa, making these gels prospective for application in load-bearing systems. As for the hydrogels placed in air for 10 days, their mechanical properties revealed a significant relationship with their glycerol content. For the NC-G0 gel, most of the water evaporated during the 10 days. Thus, no mechanical data could be obtained due to its stiff and fragile nature. When 20% of the water was replaced by glycerol, the NC-G20 gel was still soft after being placed in air for 10 days. It could be stretched more than 200% (234 ± 10.2%) its original length before fracture because of the reduced water loss from the hydrogels, showing tensile strength and elastic modulus around 0.91 ± 0.05 MPa and 520.2 ± 25.6 kPa, respectively. Further Polymers 2018, 10, x FOR PEER REVIEW 10 of 12 increasing the glycerol content led to the increasing in the softness and extensibility of the resultant hydrogels, whereas the elastic modulus gradually decreased. In addition, the tensile strength of addition, the tensile strength of NC-G40 gel, NC-G60 gel, and NC-G80 gel after being placed in air NC-G40 gel, NC-G60 gel, and NC-G80 gel after being placed in air for 10 days were measured to be for 10 days were measured to be 0.64 ± 0.04 MPa, 0.53 ± 0.03 MPa, and 0.29 ± 0.01 MPa, respectively. 0.64 ± 0.04 MPa, 0.53 ± 0.03 MPa, and 0.29 ± 0.01 MPa, respectively. These values are almost the same These values are almost the same as those recorded in their as-prepared states and comparable to as those recorded in their as-prepared states and comparable to other high-strength hydrogels reported other high-strength hydrogels reported in the literature [33–35]. In general, the data displayed in in the literature [33–35]. In general, the data displayed in Figure 8 strongly verified that the NC-G gels Figure 8 strongly verified that the NC-G gels could be utilized in air and water environments because could be utilized in air and water environments because of their excellent mechanical performance of their excellent mechanical performance under these conditions. under these conditions.

Figure 7. Illustration of NC-G gels at different conditions. Figure 7. Illustration of NC-G gels at different conditions.

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Figure 8. Comparison of mechanical properties of NC-G gels placed in air and under water for 10 days. Figure 8. Comparison of mechanical properties of NC-G gels placed in air and under water for 10 (a) Elongation at break; (b) Tensile strength; (c) Elastic modulus. days. (a) Elongation at break; (b) Tensile strength; (c) Elastic modulus.

4. Conclusions 4. Conclusions In summary, we prepared a novel type of high-strength nanocomposite hydrogel with In summary, and we prepared a novel type of high-strength nanocomposite hydrogel with anti-dehydration swelling-resistant properties. The hydrogels were prepared by in situantifree dehydration and swelling-resistant properties. The hydrogels prepared by inAlsitu radical copolymerization of AA and VP in mixtures of water were and glycerol using NPsradical as the 2 O3free copolymerization of AAagents. and VP in topotactic mixtures of water and glycerol using Al Al22O33 NPs as inorganic cross-linking The chelation reactions between andthe theinorganic polymer cross-linking agents. The topotactic chelation reactions between Al 2 O 3 NPs and the polymer matrix matrix not only contributed to the outstanding mechanical properties of hydrogels, but also endowed not only contributed to the outstanding mechanical of hydrogels, butinteractions also endowed them them with swelling resistance. Simultaneously, theproperties strong hydrogen bonding between with resistance. Simultaneously, strong hydrogen bonding interactions between water waterswelling and glycerol were responsible for the the anti-dehydration capacity of the resultant hydrogels. Based and glycerol were responsible the anti-dehydration capacityhigh of the resultant hydrogels. Based on these unique properties, thefor prepared hydrogels maintained mechanical performance bothon in these unique the which prepared hydrogels mechanical performance both in air and waterproperties, environments, would largelymaintained enhance thehigh practical applications of hydrogels. air and water environments, which would largely enhance the practical applications of hydrogels. Author Contributions: B.X., P.W. and Q.W. conceived and designed the experiments. Y.L., L.W. and X.G. performed the experiments. andand M.F.Q.W. analyzed data. B.X. Y.L. wrote paper. All authors discussed the Author Contributions: B.X.,B.X. P.W. conceived andand designed the the experiments. Y.L., L.W. and X.G. results and improved the final text of the paper. performed the experiments. B.X. and M.F. analyzed data. B.X. and Y.L. wrote the paper. All authors discussed Acknowledgments: This the research wasoffinancially the results and improved final text the paper. supported by the National Natural Science Foundation of China (Grant No. 31771039); the Program for Changjiang Scholars and Innovative Research Teams in Universities (Grant No. IRT_15R26) and Fundamental Research Funds for the Universities (Grant No. Foundation JUSRP115A03, Acknowledgments: This research was financially supported byCentral the National Natural Science of JUSRP51717A). China (Grant No. 31771039); the Program for Changjiang Scholars and Innovative Research Teams in Universities No. IRT_15R26) and Fundamental Conflicts of (Grant Interest: The authors declare no conflict of Research interest. Funds for the Central Universities (Grant No. JUSRP115A03, JUSRP51717A).

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