**1. Introduction**

In the specialized scientific literature, there are reports on a series of composite hydrogels that have various characteristic properties and can have many applications: delivering or controlled releasing of drugs [1–3], adsorbents for water purification [4–6], adsorbents of dyestuffs in various fields [7], and energy-storage devices [8,9]. Most hydrogels containing graphene oxide (GO) in their composition were obtained using chemical crosslinking agents.

Huang et al. developed some superabsorbent hydrogels based on different GO concentrations (GO/poly (acrylic acid-*co*-acrylamide)) by in situ radical solution polymerization, and obtained hydrogels with a swelling capacity of up to 1100% [10,11].

Hydrogel crosslinking using ionizing radiation is a well-established method that is clean and rapid, and that ensures the obtaining of sterile products with a permanent and homogeneous network structure.

By varying the absorbed dose, the degree of crosslinking, on which the swelling degree strongly depends, can be controlled. Since it has many hydrophilic groups on its surface, GO is an excellent material that can be used in mixtures with several polymers in order to obtain new hydrogels with superabsorbent properties [12–14]. Natural polymers such as CMC and XG are more susceptible to degradation when they are irradiated in aqueous solutions due to the indirect effect of water. The effect of e-beam irradiation on XG, which is used as an ingredient in the food industry to determine irradiation stability, has been investigated by Li et al. [15]. The study showed that the viscosity of irradiated aqueous

**Citation:** C˘alina, I.; Demeter, M.; Sc˘aris,oreanu, A.; Micutz, M. Development of Novel Superabsorbent Hybrid Hydrogels by E-Beam Crosslinking. *Gels* **2021**, *7*, 189. https://doi.org/10.3390/ gels7040189

Academic Editors: Yang Liu and Kiat Hwa Chan

Received: 11 October 2021 Accepted: 27 October 2021 Published: 29 October 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

XG-based solutions was reduced as the irradiation dose increased (5–50 kGy), suggesting degradation or depolymerizations of polysaccharide macromolecules.

Comprehensives studies on the γ-radiation of XG in solid and aqueous states were performed by S, en and Hayrabolulu et al. They showed that the effect of γ-radiation in air for XG is chain scission, which is effective when the dose rate is decreased due to the enhanced oxidative degradations during irradiation [16,17]. The same research group developed superabsorbent hydrogels based on XG using γ-radiation crosslinking synthesis at very low absorbed dose, such as 1–2 kGy [18].

Another study investigated how the irradiation dose (0–30 kGy) influenced the viscosity of CMC solutions, and developed methods to control the degradation [19].

Most often, the crosslinking by irradiation of natural polymers is accompanied by the degradation of the polymer chain. Despite this major disadvantage, natural polymers are preferred over the petroleum-derived synthetic polymers for ecological reasons, but also due to the fact that natural polymers have inherent properties of biocompatibility and biodegradability. Thus, materials produced for biomedical purposes, in addition to being easier to process, are much more easily accepted by cells and tissues.

One of the reasons for using graphene oxide (GO) to create hybrid hydrogels with natural polymers such as xanthan gum (XG) and caboxymethylcellulose (CMC) in their composition is primarily related to obtaining a hydrogel with improved mechanical properties compared to the initial polymers. It is well known that natural polymers have low mechanical properties, although they have excellent absorption properties due to their highly hydrophilic nature.

GO induces elasticity of polymer chains, thermal and chemical stability. It is a material suitable for increasing the mechanical strength of natural polymers and due to the ease with which GO can be dispersed in a polymer mixture increasing the miscibility of these compounds [20,21].

A particularly important role that GO can play in the composition of some hydrogels is related to the lyophilization process when the hydrogels lose their water content. GO hydrogels obviously maintain their chemical composition and still have high porosity, a large specific surface area, and a high affinity for various molecules. Recent studies have shown an increased drug-loading capacity and prolonged release when using GO-based hydrogel systems, making them suitable for advanced drug-delivery applications [22]. In our previous study, superabsorbent hydrogels incorporated with GO were obtained by e-beam irradiation in the absence of air, at low radiation doses (0.5–3 kGy), from a high concentration of polymers in aqueous solution (paste-like condition) [23]. The compositions of the hydrogels above were characterized by a lower swelling capacity and a higher crosslinking density.

The present study sought to develop and characterize a new hybrid hydrogel with superabsorbent properties composed exclusively of biodegradable polymers (XG and CMC), incorporated with GO, to obtain a hydrogel with the best structural, rheological, and swelling properties, and that could be used in the field of biomedical engineering [24] and in hygienic products.

### **2. Results and Discussion**

### *2.1. Sol-Gel Analysis*

Figure 1 shows the variation of the gel fraction (GF) depending on the absorbed dose of the XGCMCGO hydrogels with compositions of 50:50, 80:20, and 70:30. For these hydrogel compositions, it was observed that the highest percentage of the GF was obtained at the lowest absorbed dose: 78% for XGCMCGO (50:50) and 73% for XGCMCGO (80:20); while for XGCMCGO (70:30), at the lowest dose, it obtained the lowest percentage of the GF, namely 60%. The XGCMCGO (70:30) hydrogel showed an almost constant value of GF over the entire range of applied radiation doses.

**Figure 1.** Gel fraction as a function of the absorbed dose during e-beam irradiation for the XGCMCGO hydrogels.

For the hydrogel samples having the compositions of 50:50 and 70:30, the percentage of GF decreased with an increasing absorbed dose. The exception to this rule was the hydrogel with equal concentrations of polymers (XGCMCGO (50:50)), for which the determination of the GF was no longer possible at doses above 7.5 kGy, due to the advanced degradation of the samples after irradiation.

For the XGCMCGO (80:20) hydrogel, the percentage of the GF increased with an increasing absorbed dose; the highest percentage of the GF was 78% at an irradiation dose of 15 kGy.

The obtained values were comparable to those of other studies regarding the e-beam crosslinking of similar hydrogel compositions. For example, Sung et al. prepared CMC/GO hydrogels by e-beam irradiation at 30 kGy and obtained a gel fraction of 67.3% [20].

Another study, performed by Said et al., which investigated the formation of CMC/AA hydrogels in aqueous solutions under the effect of e-beam irradiation, showed that the gel fraction increased quickly with a dose of 50 kGy, reaching a value of 70%, then in the dose range of 50–100 kGy, showed a slight increase [25].

The rate of hydrogel formation depends on the *p*0/*q*<sup>0</sup> ratio of the polymer. If the crosslinking process predominates over the degradation process, an insoluble gel is formed as a detriment to the degradation process [26]. When polymers are subjected to ionizing radiation, crosslinking and main chain scission occur simultaneously. The quantitative estimation of the extent of crosslinking and degradation can be made using the Charlesby– Rosiak equation. The *p*0/*q*<sup>0</sup> ratios calculated using the Charlesby–Rosiak equation for the XGCMCGO hydrogels are presented in Table 1.

**Table 1.** Sol-gel parameters calculated according the Charlesby–Rosiak Equation (3).


The lowest value of the *p*0/*q*<sup>0</sup> of 0.64 was obtained for the hydrogel composition of 80:20, which contained the highest XG concentration. The XGCMCGO (50:50) and XGCMCGO (70:30) hydrogels showed *p*0/*q*<sup>0</sup> ratios of 1.04 and 0.66, respectively. It is well known in the radiation-chemistry field that when various polymeric composition subjected to a treatment with ionizing radiation show a *p*0/*q*<sup>0</sup> less than 2, the crosslinking processes predominated [27]. In the present study, these values were slightly higher compared to those obtained for hydrogels prepared with high concentrations of acrylic acid (70% AA) in data presented in our previous study, where the *p*0/*q*<sup>0</sup> ratio was equal to 0.28 [23].

The gelation dose (Dg) is another parameter that shows the minimum dose required to obtain an insoluble gel during irradiation. D<sup>g</sup> values decreased as the concentration of polymers and crosslinking agent (*N*0*N*—methylenebis(acrylamide), NMBA) increased. We observed that the gelation occurred at a very low dose (0.23 kGy) for the XGCMCGO (50:50) hydrogel.

The radiation-chemical yield of crosslinking (G(X)) decreased with an increase in the irradiation dose for all the XGCMCGO hydrogel compositions. The maximum value of this parameter was 132.2 µmol/J for the XGCMCGO (70:30) hydrogel at a dose of 2.5 kGy. For the radiation-chemical yield of scission (G(S)), the maximum value was 252.5 µmol/J for the XGCMCGO (50:50) hydrogel at a dose of 2.5 kGy (Table 2).

**Table 2.** Radiation-chemical yield of crosslinking G(X) and chain scission G(S) calculated for the XGCMCGO hydrogels.


As shown in the results presented in Table 2, G(S) > G(X) for the XGCMCGO (50:50) hydrogel, with approximately double the value. Therefore, it was obvious that in this case, the degradation processes predominated.

On the other hand, we observed that G(X) > G(S) for the XGCMCGO (80:20) and XGCMCGO (70:30) hydrogels. In these cases, the crosslinking predominated.

### *2.2. Swelling Degree*

To characterize the network structure and determine the effective crosslinking density of the XGCMCGO hydrogels, the swelling properties in deionized water (DI) were first investigated.

Figure 2 shows the variation of the swelling degree (SD) at equilibrium, as a function of time and the absorbed dose, for the XGCMCGO hydrogels. All hydrogels showed good swelling ability. The maximum SD obtained at a dose of 15 kGy was ~6000%, which classified these XGCMCGO (70:30) hydrogels as "superabsorbent hydrogels". In the case of hydrogels with low concentrations of CMC, XG, AA, and NMBA, the SD had a value of 2250% for the XGCMCGO (50:50) hydrogel obtained at 4.7 kGy. At irradiation doses greater than 5 kGy, this hydrogel was degraded in deionized water (DI) in less than 2 h. For the XGCMCGO (80:20) hydrogel, the SD showed a maximum value of 2300% at a dose of 7.5 kGy.

**Figure 2.** The swelling degree of the hydrogels in DI water: (**a**) XGCMCGO (50:50); (**b**) XGCMCGO (80:20); (**c**) XGCMCGO (70:30).

Moreover, we can specify that these hydrogels had the ability to reach equilibrium in less than 6 h and were stable in deionized water; of these, the most suitable proved to be the XGCMCGO (70:30) hydrogel. Previous studies of superabsorbent hydrogels based on CMC/AA/montmorillonite obtained by γ-irradiation showed a swelling degree of 16,000% [28]. Compared to our study, the above study used a high concentration of polymers; i.e., 10% CMC and 30% AA. Sultana et al. synthesized copolymer hydrogels from acrylamide/CMC by γ-irradiation, and they obtained an SD value in DI water of 795% [29]. We have shown that by using low concentrations of polymers and GO and appropriate mass ratios between them, with a moderate radiation dose rate, stable hydrogels with different degrees of swelling were obtained.

Superabsorbent hydrogels based on xanthan gum/polyacrylic acid/graphene oxide were prepared as absorbers for removing methylene blue dye from the water. For these hydrogels, the maximum swelling value in the neutral medium was found to be 2100% for a GO concentration of 1% [30]. Superabsorbent resin based on acrylic acid/CMC/GO showed the highest swelling capacity with 0.6% of GO in hydrogel composition, about 750 g g−<sup>1</sup> in distilled water. The above study demonstrated that the incorporation of a moderate amount of GO in a CMC-based hydrogel can improve the water capacity of the material [31]. The CMC/GO composite superabsorbents prepared by e-beam irradiation at 10 kGy showed a swelling capacity of 140 g g−<sup>1</sup> in distilled water [32]. After analyzing the data, we concluded that the XGCMCGO hydrogels presented in this study, in addition to having the lowest concentration of 0.1% GO, had the best swelling properties in a neutral environment.

### *2.3. Diffusion of Water*

The analysis of water diffusion mechanisms in swollen polymeric systems has received considerable attention in recent years due to the important applications of swollen hydrogels in biomedical and pharmaceutical engineering. When a hydrogel is in contact with water, water enters the hydrogel by diffusion and the hydrogel network expands, resulting in the swelling of the hydrogel. Diffusion involves the migration of water into pre-existing or dynamically formed spaces between the macromolecular chains of the hydrogel [33]. To follow this process, Equation (7) was applied to the initial stage of swelling

process, up to 60% of maximum swelling (equilibrium state) [34]. Table 3 shows the values of water diffusion mechanisms characteristic to the XGCMCGO hydrogels.


**Table 3.** Parameters k, *n*, and D for the XGCMCGO hydrogels.

A value of *n* ≤ 0.5 indicated a Fickian diffusion mechanism. In this case, the rate of solvent diffusion was much lower compared to the macromolecular chain relaxation of the polymer. A value of *n* in the range 0.5 < *n* < 1 indicated a non-Fickian transport, in which the diffusion of the solvent into the hydrogel structure was rapid compared to the relaxation rate of the macromolecular chain of the polymer [35].

Figure 3a–c show graphically, in logarithmic scale, the values of the parameter F = f(t). Figure 3d–f show graphically F = f (t 0.5). The values of the coefficients *n*, k, and D were calculated from the slope and intersection of the lines resulting from the swelling kinetics. The calculated values for the diffusion exponents *n* were less than 0.5 for the XGCMCGO (50:50) hydrogel at all irradiation doses, thus indicating that the solvent transport mechanism was of a Fickian type.

The XGCMCGO (80:20) hydrogel also presented a Fickian diffusion mechanism, but only in the cases of irradiation doses of 4.7 and 15 kGy. For the other irradiation doses (2.5 kGy and 7.5 kGy) the diffusion mechanism was non-Fickian. When the diffusion was non-Fickian, the relaxation time of the macromolecular chain and the diffusion had the same order of magnitude. As the solvent diffused into the hydrogel, the rearrangement of the macromolecular chains did not occur immediately.

The mechanism of water diffusion of the XGCMCGO (70:30) hydrogel was of a Fickian type; the exception was the hydrogel crosslinked with 4.7 kGy, in which case a value of *n* greater than 0.5 was obtained, thus exhibiting a non-Fickian diffusion behavior. In another similar study of a hybrid hydrogel obtained from acrylamide and XG, a value of *n* equal to 0.48 was reported [36]. Another study showed the obtaining of a GO-based hydrogel in which the inorganic component was incorporated in different concentrations. For this, values of *n* were obtained in the range of 0.5–1, which indicated a non-Fickian diffusion, which is specific to crosslinked hydrogels [37].

**Figure 3.** Swelling kinetics curves for the hydrogels: (**a**) XGCMCGO (50:50); (**b**) XGCMCGO (80:20); (**c**) XGCMCGO (70:30). Plots of F versus t0.5 for the hydrogels: (**d**) XGCMCGO (50:50); (**e**) XGCMCGO (80:20); (**f**) XGCMCGO (70:30).

The values of the diffusion coefficient (D) were found in the range of 0.02–0.04 cm2/s, depending on the irradiation dose and the composition of the hydrogel. Compared to other studies, these values were much lower, suggesting the formation of high-molecular-weight hydrogels [38], as can be seen in the rheological measurements. For example, following a study on hydrogels obtained from CMC/acrylamide/GO by radical polymerization, the values obtained for the diffusion coefficient were in the range of 0.66–1.26 cm2/s [39].

### *2.4. ATR-FTIR Spectroscopy*

The characteristic FTIR spectra for the natural polymers (XG and CMC) and GO are shown in Figure 4. The FTIR spectrum characteristic of XG showed: 3308 cm−<sup>1</sup> (O–H groups); 2917 cm−<sup>1</sup> (CH<sup>3</sup> and CH<sup>2</sup> functional groups); 1700–1730 cm−<sup>1</sup> (C = O); 1607 cm−<sup>1</sup> (C = O); 1405 cm−<sup>1</sup> (C–O group); and 1025 cm−<sup>1</sup> (C–O group). The FTIR spectra of CMC showed: 3303 cm−<sup>1</sup> (O–H group); 2923 cm−<sup>1</sup> (C–H stretching vibrations); 1586 cm−<sup>1</sup> (COO– group); 1420 cm−<sup>1</sup> (CH2); 1320 cm−<sup>1</sup> (O-H groups); and 1030 cm−<sup>1</sup> (C–O group). Regarding the FTIR spectrum of GO, it showed: 3231 cm−<sup>1</sup> (O–H); 1732 cm−<sup>1</sup> (C = O and C–H groups); 1620 cm−<sup>1</sup> (C = C); 1275 cm−<sup>1</sup> (C–OH); and 1053 cm−<sup>1</sup> (C–O).

Figure 5a,b shows the characteristic FTIR spectra for nonirradiated polymeric blends with various ratios (XG:CMC) without GO (left side) and containing GO (right side). According to the acquired FTIR spectra, the displacement of the characteristic bands located in the range of 3600–600 cm−<sup>1</sup> toward smaller wavenumbers corresponded to the formation of hydrogen bonds due to the miscibility of the polymers. In addition, increases in the band intensities in this area were proportional to the concentrations of natural polymers (XG and CMC) included in the polymeric blend.

**Figure 4.** The FTIR spectra of pure XG, CMC, and pure GO.

**Figure 5.** (**a**) The FTIR spectra of XGCMC without GO dried hydrogel; (**b**) the FTIR spectra of XGCMCGO dried hydrogel.

For the XGCMCGO unirradiated polymeric blend, the hydrophilic functional groups (–COOH, –C = O, –OH, and –C–O–C) of GO played a key role in improving the compatibility between the polymer matrix and GO.

The FTIR-ATR spectra, corresponding to XGCMCGO hydrogels irradiated at doses in the range of 2.5–15 kGy, are shown in Figure 6. It can be seen that the intensity of the absorption bands varied with the irradiation dose.

The intensity of specific absorption bands in the range of 3300–3000) cm−<sup>1</sup> for XGCM-CGO (50:50) increased with the absorbed dose. The position of the bands assigned to the groups: C–H varied from 2929 cm−<sup>1</sup> to 2940 cm−<sup>1</sup> ; C = O varied from 1614 cm−<sup>1</sup> to 1637 cm−<sup>1</sup> ; and COO– varied from 1538 cm−<sup>1</sup> to 1542 cm−<sup>1</sup> , after e-beam irradiation at a dose of 15 kGy.

In the case of the XGCMCGO (80:20) hydrogel, the value of the band assigned to the O-H group decreased with the irradiation dose from 3268 cm−<sup>1</sup> to 3253 cm−<sup>1</sup> . The intensity of the bands assigned to CH groups increased significantly for the irradiated polymeric blends. The decrease of the wavenumber from 1558/1412/1193/1054 cm−<sup>1</sup> , due to the application of a dose of 15 kGy, to 1557/1403/1157/1053 cm−<sup>1</sup> , respectively, may have been caused by the different degrees of crosslinking of the hydrogel. As the absorbed dose increased, the crosslinking density also increased (see Table 4).

**Figure 6.** The FTIR spectra of unirradiated and irradiated dried hydrogels: (**a**) XGCMCGO (50:50); (**b**) XGCMCGO (80:20); (**c**) XGCMCGO (70:30).

**Table 4.** The elastic modulus (G0 ), the molecular weight between two crosslinks of the hydrogel chains (MC), the crosslink density (νe), and network mesh size (ξ) of the XGCMCGO hydrogels.


For the XGCMCGO (70:30) hydrogels, the intensity of the absorption bands in the range of 3400–2800 cm−<sup>1</sup> increased with a dose of 7.5 kGy. At this dose, the position corresponding to the OH group shifted to smaller wavenumbers, from 3333 to 3324 cm−<sup>1</sup> . The band assigned to the COO–group was shifted from 1556 cm−<sup>1</sup> to 1550 cm−<sup>1</sup> when the hydrogel was irradiated with a dose of 15 kGy. A similar trend was observed for the other bands in the range of 1500–800 cm−<sup>1</sup> . The intensity of the characteristic bands for the XGCMCGO (70:30) hydrogel increased when it was irradiated at doses up to 7 kGy due to the fact that the crosslinking density decreased, a direct consequence of increased swelling. These results were supported by the data obtained from swelling experiments (~6000% at a dose of 15 kGy).

### *2.5. Characterization of the Network Structure*

The characterization of the network structure of a hydrogel is a complex procedure due to the many types of networks encountered in the practice; namely: regular, irregular, and weakly or strongly crosslinked networks.

Basic structural parameters for the XGCMCGO hydrogels such as the elastic modulus (G0 ), average molecular weight between two successive crosslinks (MC), crosslink density (νe), and mesh size (ξ) were calculated according to the equations presented in Section 4.7. C<sup>n</sup> was taken as a weighted average of the characteristic ratios of the polymers: XG = 271 [40], CMC = 10 [41] and AA = 6.7 [42]. M<sup>r</sup> is the monomeric unit of XG, CMC, and AA, taken as an average (XG = 933 g/mol, CMC = 234 g/mol, and AA = 72 g/mol) [43].

Using the values of the elastic modulus (G0 ) determined by rheological analysis for each hydrogel composition and based on the theory of rubber elasticity, we determined MC. The network parameters calculated for the XGCMCGO hydrogels are presented in Table 4.

The values of the M<sup>C</sup> parameter for all hydrogel compositions were in the range of 132.7–250.4 kg/mol. For the XGCMCGO (80:20) and (70:30) hydrogel compositions, the corresponding Mc values decreased with an increase in the absorbed dose; while for the XGCMCGO (50:50) hydrogel compositions, the corresponding M<sup>C</sup> values increased. This increase demonstrated once again the degradation of this polymeric mixture under the action of ionizing radiation.

For the XGCMCGO (50:50) and (70:30) hydrogel compositions, the corresponding ξ values increased with an increase in the irradiation dose; while for the XGCMCGO (80:20) hydrogel compositions, the corresponding ξ values decreased with an increase in the irradiation dose. The decrease of the parameter ξ may have been due to the increase in the crosslinking density. Therefore, a large number of active or free radical centers were formed on the polymer chains, and a large number of small chains were formed, which led to crosslinking and the formation of a more compact network structure with a smaller mesh size. Figure 7 shows the appearance of the polymer mixture before and after irradiation with e-beams.

For this reason, ν<sup>e</sup> is one of the most important structural parameters for characterizing a class of hydrogels that have superabsorbent properties. According to the values obtained, it was shown that this parameter increased as the absorbed dose increased in the cases of the XGCMCGO (80:20) and (70:30) hydrogels. The behavior was completely different in the case of XGCMCGO (50:50), where the values for ν<sup>e</sup> decreased with an increase in the absorbed dose. All the values obtained for ν<sup>e</sup> were in the range of 0.41–0.76 mol/m<sup>3</sup> , and showed the obtaining of hydrogels with a moderately crosslinked network structure that allowed the absorption of an increased amount of fluids.

Knowing the hydrogel pore size is important to understanding the mechanism for transport of bioactive substances in the macromolecular network of such materials. For example, controlling the rate of a drug diffusion is essential, because it reflects the space available for a drug molecule to diffuse inside or outside the swollen hydrogel network [44].

**Figure 7.** XGCMCGO hydrogels in different hypostases.

The values of ξ obtained for the XGCMCGO hydrogels were in the range of 123.6–210.6 nm. These values were similar to those obtained by other authors who developed hydrogels with applicability in tissue engineering, with ξ values previous reported in the range of 139–258 nm [45].
