Enhanced Superabsorbency of Cellulose-Based Hydrogels in NaOH Solution: Synthesis, Characterization, and Performance Evaluation

Cellulose is a natural polymer that is abundantly available in nature. This study successfully developed cellulose dissolved in NaOH and formed into hydrogels using the freeze-thaw method. NaOH solution concentration as a cellulose solvent varied from 1% (CN1) to 5% (CN5) by weight. Six cycles of freezing and thawing were performed for 20 hours at -23°C and 4 hours at 37°C. Subsequently, morphological analysis, swelling degree, weight loss, and compression testing were conducted to assess the physical properties of cellulose-based hydrogels. The results demonstrate that as the concentration of NaOH increases, the resulting hydrogel exhibits smaller pore sizes, as evidenced by optical microscope images. Additionally, the swelling degree increased with the increasing NaOH fraction. The swelling tests were performed in both distilled water and PBS solutions. Notably, soaking the hydrogels in PBS solution demonstrated their potential as superabsorbent hydrogels (SAH). Furthermore, increased NaOH fraction was associated with higher weight loss, greater Young’s modulus, and reduced compressive strength.


Introduction
Superabsorbent hydrogel (SAH) is a novel polymer type that exhibits a three-dimensional hydrophilic network structure comprising loosely crosslinked polymeric materials.This unique structure enables them to absorb water and other fluids up to thousands of times their dry weight [1], [2].Hydrogels can swell to volumes much more significant than their original size, weighing 10-1000 times higher [3].Most hydrogels exhibit responsiveness to various environmental stimuli, including temperature changes [4], pH [5], ionic strength [6], solvent composition, light, and electric fields [7].The macromolecular chains forming this network will significantly expand their structure to a larger size upon exposure to water or other solutions [8].
Due to their unique characteristics and significant swelling capacity, hydrogels have found extensive application in the field of biomedical area as anti-bacterial materials, sorbents for the removal of heavy metals [9], biosensors [10], contact lenses [11], tissue engineering [12], cell scaffolds [13], controlled drug delivery medium, and catalysis [3].In other field, such as in agriculture, hydrogel can be used as an effective water storage in agriculture, offering a suitable means for both water retention and controlled release [3].Also, hydrogel can be applied as the soil amendment, hygienic napkins [14], disposable diapers [8], sealing, coal dewatering, artificial snow, food additives [15], the construction industry, responsive materials, wastewater treatment [1], dye adsorption [7], and potential edible electronics applications [16].Das et al. manufactured superabsorbent hydrogels (SAH) based on carboxymethylcellulose sodium salt (CMCNa) and hydroxyethylcellulose (HEC) for agricultural applications, utilizing citric acid (CA) as a crosslinker.With a swelling ratio of 600%, hydrogels have the potential to retain water and water-soluble fertilizers that will be slowly released to plants according to their needs.Therefore, the agricultural application of hydrogel brings about substantial benefits, including significant reductions in water consumption, prevention of plant mortality, and enhancement of soil water retention capacity.[14].
Hydrogels are classified into physical and chemical methods based on their crosslinking processes [17].Chemical crosslinking occurs through covalent bonds between the polymer chains achieved through radiation or crosslinking agents.Chemically crosslinked hydrogels have robust and resistant qualities, but their uses are generally restricted by the toxicity of the crosslinking agents [13].On the other hand, physical crosslinking takes place through non-covalent bonds, specifically hydrogen bonding, without the need for crosslinking agents, thus avoiding any potential toxic properties.[17].One of the techniques for physical crosslinking is the freeze-thaw method, which involves subjecting a polymer-liquid system to repeated cycles of freezing and thawing.During this process, ice crystals are formed within the polymer precursor solution, resulting in the formation of crosslinks between polymer chains [18].The hydrogel mould must be firmly sealed during freezing and thawing to prevent water loss and long-term gel shrinkage [19].Hydrogels formed by the freeze-thaw method exhibit improved mechanical properties, greater stability, and are also non-toxic [20].The characteristics of hydrogels are influenced by several variables, such as time, temperature, and the number of cycles.The precursor solution that will be converted into a hydrogel must also be dissolved homogenously [19].
Based on their source, hydrogels are classified into two groups: synthetic and natural polymers [15].Most commercially available hydrogels are derived from synthetic polymers.Superabsorbent hydrogels (SAHs) have been commercially developed in Japan and the USA since the 1980s for hygiene products using fully synthetic polymers [7].Currently, natural polymer-based hydrogels are being extensively developed due to their advantages such as biodegradability, good material strength, environmental friendliness, and cost-effectiveness [21].On the other hand, synthetic polymer-based hydrogels are more challenging in terms of sourcing, techniques, and their applications [22].Natural hydrogels consist of hydrogels derived from polysaccharides such as cellulose, starch, chitosan, hyaluronic acid, as well as hydrogels derived from proteins like gelatine and collagen [14].
Cellulose is the most abundant natural polymer in nature and is a primary component of plants and natural fibers such as cotton and linen [23].The advantages of cellulose are its renewable nature, biodegradability, low cost, and functional versatility [24].Nevertheless, the challenges associated with dissolving cellulose in a solution limit its potential applications.[21].Some solvents that are commonly used such as cuoxam, cuan, cadoxen, N-Methylmorpholine-N-Oxide (NMMO), Lithium Chloride/N,N-Dimethylacetamide (LiCl/DMAc), and ionic liquids exhibit properties of volatility, toxicity, and high cost, which limit their usefulness [25].The NaOH-water-based system is widely recognized as one of the most popular and advantageous solvents for cellulose due to its affordability, ease of recyclability, simple, and environmentally friendly nature [26], [27].The NaOH-water-based system is widely recognized and used as a solvent in the pulp industry [27].Isogai and Atalia investigated the solubility of various types of cellulose in aqueous NaOH solution.They found the optimal conditions by dissolving cellulose in a NaOH solution with a concentration of 8-9 wt.% and then freezing it at -20°C followed by thawing at room temperature.The resulting thawed product was then dissolved in a 5% NaOH solution.This method resulted in complete dissolution of cellulose in the NaOH solution [28].
In this work, we dissolved cellulose in a NaOH solution and transformed it into a cellulose-based hydrogel using the freeze-thaw method.The morphology, swelling behaviour, and mechanical properties of the hydrogels were then observed.The use of the freeze-thaw method for producing cellulose-based hydrogels is not yet widely adopted.It was found that hydrogels produced with different NaOH concentrations exhibited different pore sizes, swelling degrees, and weight loss.Based on these results, the optimal NaOH concentration to produce freeze-thaw cellulose-based hydrogels can be determined.

Materials and Methods
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Materials
The alpha-cellulose powder was purchased from Sigma-Aldrich and sodium hydroxide (NaOH) powder was purchased from CV. Sopyan Jaya Cemerlang, Indonesia.Distilled water was procured from Merck and PT.Brataco, Indonesia.Phosphate buffered saline (PBS) solution was obtained from the School of Pharmacy, ITB, Indonesia.

Preparation of Cellulose Hydrogels
NaOH powder was dissolved in distilled water at various concentrations, starting from 1 wt.% (CN1), 2 wt.% (CN2), 3 wt.%(CN3), 4 wt.%(CN4), to 5 wt.% (CN5).Then it was stirred using a magnetic stirrer at room temperature for 10 minutes until a clear and homogeneous solution was formed.Next, α-cellulose powder with a concentration of 5 wt.% was added to the NaOH solutions and stirred using a magnetic stirrer at room temperature until homogeneous.The next step was to place this precursor solution into a freezer at a temperature of -23°C for 20 hours (freezing) and then transfer it to an incubator at 37°C for 4 hours (thawing) [29].This freeze-thaw process was repeated for 6 cycles until Cellulose/NaOH hydrogels were obtained.The hydrogel was stored in a refrigerator until further characterization.

Morphological Analysis
The morphological structure of the hydrogel was determined using an optical microscope.The Cellulose/NaOH hydrogel was initially blotted with tissue paper to eliminate excess water.Then, it was cut in the transverse direction [18].The internal morphology of the hydrogel was observed by an optical microscope measured at a magnification of 4x (AmScope MUI1000, United States).The pore size visible on each surface of the hydrogel was calculated from the surface area measured using ImageJ software (version 6.4-bit Java 8).

Swelling Degree and Weight Loss
The swelling degree was determined after the hydrogel was dried in an oven at 50°C for 5 days until the water inside evaporated.Then, the dry hydrogel was soaked for 48 hours in phosphate buffered saline (PBS) solution and distilled water, and its weight was measured at 0, 3, 6, 9, 12, 24, and 48 hours.Just prior to weighing, the soaked hydrogel was carefully blotted against filter paper until it was no longer dripping water.The ratio of the weight of the hydrogel before and after the swelling process was calculated as the swelling degree using the following equation (1).
Where W2 and W1 are the weight of the hydrated hydrogel at time t and the initial weight before immersion, respectively.After the 48-hour swelling process, the hydrated hydrogel was dried again in an oven at 50°C for 5 days.The weight of the hydrogel after being re-dried was defined as W3.The remaining gel content indicates the amount of cross-linking that occurred in the hydrogel.Weight loss can thus be calculated using the following equation (2). (2)

Fourier transform infrared (FTIR) Spectroscopy
Functional groups and interactions in the cellulose hydrogels and the cellulose powder were identified using a Fourier transform infrared (FTIR) spectrometer (Thermo Scientific Nicolet iS10) with a wavenumber range of 400−4000 cm −1 .The FTIR spectra of the hydrogels were analyzed to assess the changes in functional groups within the hydrogel structure.

Mechanical Properties
The mechanical properties of the cellulose hydrogels, including compressive modulus and compressive strength, were evaluated using the Sinowon Universal Material Testing Machine SM-200.The compressive modulus was determined by calculating the slope of the linear stress-strain curve.

Statistical Analysis
Swelling degree, gel fraction, and compression test measurements were performed in triplicate for each sample, and the results are presented as the mean ± standard deviation.The statistical analysis was conducted using One-Way Analysis of Variance (ANOVA) followed by Tukey's Honest Significant Difference (HSD) test on IBM SPSS 20 software (IBM, USA) to determine significant differences between groups.The confidence interval for this statistical test was set at more than 95% (p < 0.05).Values with different superscript letters indicate significant differences (p < 0.05) [30].

Morphological Analysis
Figure 1 illustrates the morphological characteristics of Cellulose/NaOH hydrogels generated through the freeze-thaw approach employing different concentrations of NaOH, specifically 1 wt.% (CN1), 2 wt.% (CN2), 3 wt.%(CN3), 4 wt.%(CN4), and 5 wt.% (CN5).Hydrogel CN1, exhibits a white appearance and is the most fragile and easily crumbled among the tested hydrogels.Hydrogels CN2 and CN3 have a tougher and more sponge-like texture and are also white in colour.CN4 and CN5 have a smoother texture and are more transparent.The texture of the hydrogel becomes stronger, and the colour becomes clearer as the concentration of NaOH increases.This is caused by increased crosslinking in the hydrogel network at a higher NaOH concentration, and a more dense composition makes the texture vulnerable to damage [31].The greater the density of crosslinking, the smaller the pores, which leads to lower water absorption capacity.This will be illustrated using optical microscopy images and swelling tests [32].The results of the optical microscope images and the pore distribution curve are presented in Figure 2. The percentage of pores at specific sizes relative to the total number of pores is calculated as the frequency percentage.The pore size distribution of each hydrogel was determined by fitting a nonlinear Gaussian function to obtain the average pore size.The porosity of the hydrogel is a crucial parameter as it influences the hydrogel's performance in terms of swelling degree, weight loss, and mechanical properties [29].The CN1 hydrogel exhibits the most diverse and irregular pore sizes, with the following distribution: 0-0.005 mm² (7%), 0.006-0.01mm² (34%), 0.011-0.015mm² (24%), 0.016-0.02mm² (12%), 0.021-0.025mm² (5%), 0.026-0.03mm² (7%), 0.031-0.035mm² (2%), 0.036-0.04mm² (5%), and 0.045-0.05mm² (2%).The average pore size of the CN1 hydrogel is 0.015 ± 0.010  mm².On the other hand, the CN5 hydrogel exhibits a more limited range of pore sizes, with pores only in the following distribution: 0-0.005 mm² (98%) and 0.006-0.01mm² (2%).Consequently, the CN5 hydrogel has an average pore size of 0.0026 ± 0.001 mm².
Additionally, the percentage of porosity for each hydrogel can be calculated by dividing the total pore area by the observed surface area of the hydrogel.The differences in these porosity values are illustrated in Figure 3. Hydrogel CN1 exhibits the largest porosity, reaching 26%, while the CN5 hydrogel only has a porosity of 5%.Notably, both porosity and average pore size decrease with an increasing NaOH fraction.This phenomenon is attributed to the significant number of cellulose chains broken by NaOH molecules, resulting in shorter and more numerous cellulose chains.As a consequence, the crosslinks formed in the hydrogel become denser, leading to the formation of smaller pores [33].The small pore size leads to a lower absorption capacity, which will be confirmed by the results of the swelling test.[26].

Swelling Degree and Weight Loss
The swelling degree refers to how much water can be absorbed by the hydrogel and is an important parameter for evaluating performance of hydrogels [29], [34].Differences in the swelling behavior of the hydrogel are observed when it is immersed in PBS solution and distilled water, suggesting variations in its capacity to absorb different solutions.Figure 4 (a) shows the swelling degree and weight loss results of the five hydrogels immersed in distilled water.The results obtained show a decrease in the swelling degree with increasing NaOH fraction.Hydrogel CN1 can absorb water with a swelling degree of 595.75 ± 105.60 % at 12 hours, but it undergoes a significant decrease to 191.83 ± 111.26 % after 24 hours.This is due to the degradation and fragmentation of hydrogel CN1 over time during immersion.The brittleness of the hydrogel is caused by weak cross-linking in CN1 hydrogel.There is no cross-linking between the polymer chains.Since the NaOH concentration in CN1 hydrogel is very low, its cross-linking density is smaller than that of the other samples.The mass of CN1 hydrogel remains constant during the last 24 hours because it has reached its maximum swelling equilibrium state [18].Hydrogels CN2, CN3, CN4, and CN5, on the other hand, exhibit a rise in swelling up to 3 or 6 hours before reaching equilibrium.CN5 hydrogel has the smallest swelling degree, which is only 10.75 ± 1.69 % at 3 hours.After 3 hours, the mass of the hydrogel decreased gradually, causing the swelling degree to become smaller.This suggests that CN5 has a lower water absorption capacity compared to other hydrogels.Water absorption is caused by pores in the hydrogel [29].A high swelling degree indicates a high hydrophilic carboxyl group that can absorb water to increase the space inside the hydrogel [9].The CN5 hydrogel exhibits limited water absorption, suggesting a potential low or absence of a pore structure.This has been confirmed by optical microscope images, demonstrating that higher concentrations of NaOH result in smaller pores within the hydrogel.During the swelling process, water enters the hydrogel and relaxes the polymer chains [35].This study's results align with Ciolacu et al.'s findings that the NaOH-water system can break intra and intermolecular hydrogen bonds in cellulose chains.The higher the NaOH concentration, the more cellulose polymer chains are disrupted, resulting in shorter bonds and smaller cross-linking, decreasing pore size.Consequently, the water absorption capacity is reduced [26].The swelling degree in PBS solution exhibits a significantly higher value compared to distilled water, while following the same trend of reduction with increasing NaOH fraction, as depicted in Figure 4 (b).PBS is a buffer solution with ion concentrations and osmolarity comparable to those of bodily fluids [36].The CN1 hydrogel exhibits the highest swelling degree reaching 1098.51 ± 94.18% at the 3rd hour.This significant swelling degree indicates that the hydrogel possesses superabsorbent properties, capable of absorbing a large amount of fluid.As time progresses beyond the peak point, the degree of swelling gradually decreases.This decrease is attributed to the immersion process in the PBS solution, during which some portion of the hydrogel becomes detached from the bulk material, resulting in a reduction of its mass.Hydrogels CN2, CN3, CN4, and CN5 exhibit lower swelling degrees, measuring 738.29 ± 72.16%, 477.03 ± 25.06%, 95.41 ± 28.56%, and 43.70 ± 22.22%, respectively, within the first hour.After the third hour, the swelling degree tends to remain constant or reaches an equilibrium value until the 48th hour.This indicates that hydrogels CN2, CN3, CN4, and CN5 have stronger cross-linking between polymer chains and a higher cross-linking density.These results exhibit a similar trend as observed during immersion in distilled water.The higher swelling value in the PBS solution can be attributed to the ionization of carboxyl groups in the hydrogel when exposed to ions present in the PBS solution.This ionization leads to increased electrostatic repulsion, resulting in larger swelling of the hydrogel.This outcome is consistent with Sreeja et al.'s findings, which compared the CMC hydrogel's ability to swell in distilled water and PBS solution [37].
Weight loss represents the gel fraction of dried hydrogel [35].Gel fraction is the ratio between the cross-linked polymer of the dry hydrogel before and after the swelling process [30].Weight loss is one of the fundamental characteristics of hydrogels to assess their ability to absorb water without dissolving.Cross-linking prevents the hydrogel from dissolving in the solution and only undergoes expansion during the swelling process [38].The higher the weight loss value, the more weight hydrogel is released during swelling.Figure 4 (a) and (b) shows the weight loss values of Cellulose/NaOH hydrogels after being immersed in distilled water and PBS solution.When immersed in PBS, the hydrogel experiences less weight loss than submerged in distilled water.However, an increased NaOH concentration in both cases leads to higher weight loss.The weight loss of the hydrogel indicates its degradability.The weight loss suggests that the structure of the network and the type of degradable linkages play a significant role in determining the degradation behavior [39].
CN5 had the largest weight loss of 58.76 ± 1.25 % in distilled water and 53.36 ± 6.48 % in PBS solution, indicating that many parts of the hydrogel had detached into the liquid.Therefore, CN5 is the weakest hydrogel in maintaining its structure.This phenomenon can be caused by the amount of NaOH which can weaken the crosslinking points, resulting in greater weight loss [38].The hydrogel with the lowest weight loss was CN1 in the PBS solution (5.96 ± 3.83%), followed by CN2 in distilled water (23.28 ± 3.22%).It demonstrated the lowest degradability among the two hydrogels and retained its shape even after being soaked for 48 hours.Meanwhile, the CN1 hydrogel exhibited significant weight loss in distilled water, primarily due to degradation during immersion, particularly after 12 hours.In contrast, hydrogels soaked in the PBS solution demonstrated better preservation of their gel structure without significant degradation or dissolution.It can be observed that increasing the NaOH fraction in the hydrogel leads to a decrease in swelling degree, attributed to the reduction in pore size.Higher weight loss is also observed, indicating a weakening of crosslinking strength between polymer chains [26].

Fourier Transform Infrared (FTIR) Spectroscopy
Figure 5 shows the FTIR spectra of cellulose powder and cellulose hydrogel with 5 different concentrations of NaOH as a solvent.In the cellulose powder, several peaks are observed, namely 3325, 3168, 2902, 1567, 1467, 1377, and 1057 cm -1 .The peak at 3325 cm -1 indicates the presence of stretching vibrations from the O-H functional group [40], [41].Meanwhile, the peak at 2902 cm -1 corresponds to the stretching vibrations of the C-H functional group in the glucose unit [40].The bending vibration of the C-H functional group is indicated by the peak at 1377 cm -1 [42].The peak observed at 1057 cm -1 is attributed to the presence of -C-O-C groups, specifically secondary alcohols and ethers, within the cellulose chain structure [42].
In cellulose hydrogels, all samples exhibited similar peaks.The hydroxyl group is indicated by broad peaks observed at 3312, 3318, 3308, and 3302 cm -1 .These functional groups signify O-H stretching vibrations that contribute to strong inter-and intra-molecular hydrogen bonding [43].Meanwhile, the small peaks at 2897, 2901, and 2906 cm-1 indicate stretching vibrations from the C-H group.New peaks appear in the cellulose hydrogels at 1637, 1639, and 1640 cm -1 , indicating vibrations of water molecules trapped between hydrogen bonds [35].The C-O-C group is indicated by peaks at 1053, 1054, and 1063 cm -1 , which represent functional groups of cellulose [42].The results of Fourier transform infrared (FTIR) spectroscopy demonstrate that the formed hydrogel contains

Mechanical Properties
The mechanical properties of the hydrogel were examined to assess its structural integrity.Mechanical stability is essential to materials, particularly those intended for biomedical applications [37].The NaOH concentration influences the mechanical properties of the cellulose based hydrogels.Figure 6 displays the compression stress-strain curves of the hydrogels with different NaOH fractions, with each measurement performed three times.The curves exhibit a characteristic 'J' shape during the compression process, with an initial linear region at low strain followed by an increasing slope after  2. The compressive strength increases with higher NaOH fractions, although the difference is insignificant.The higher compressive strength indicates the presence of stronger cross-linking or connections between the polymer networks and due to higher crosslinking density [37], [44].Young's modulus was analyzed from the slope of the stress-strain curve in Figure 6.CN4 and CN5 hydrogels exhibit the highest Young's modulus, at 1.99 ± 0.15 MPa and 1.95 ± 0.10 MPa, respectively, as shown in Table 2. On the other hand, CN1 hydrogel has the lowest Young's modulus at 1.53 ± 0.05 MPa.This indicates that the higher the NaOH concentration in the cellulose hydrogel, the stiffer its structure becomes.In contrast, hydrogels with lower NaOH concentrations exhibit more excellent elasticity.This is demonstrated by the ability of CN1, CN2, and CN3 hydrogels to return to their original shape after being subjected to pressure.The presence of NaOH makes the cellulose hydrogel stiffer, creating a more compact and less deformable structure.Takara et al. obtained similar results in producing chitosan films treated with NaOH.As the NaOH concentration increases, the resulting films become more brittle with a higher Young's modulus.[45].
Fracture in hydrogels occurs after reaching a particular strain, as shown in Figure 6.The strain at maximum stress represents where the fracture occurs in the hydrogel [37].Hydrogel CN1 experiences maximum stress at 127.99% strain, while CN5 experiences failure at 106.23%.This indicates that NaOH affects the fracture behavior at maximum stress.A lower NaOH fraction results in a higher strain value before a fracture occurs.This is consistent with the elasticity indicated by the Young's modulus values.

Conclusions
Hydrogels based on cellulose were successfully synthesized using the freeze-thaw method with various concentrations of NaOH.Optical microscopy results revealed a reduction in pore size as the NaOH concentration increased, which correlated with a decrease in the swelling degree due to reduced water absorption by smaller pores.The hydrogels exhibited more significant swelling when soaked in PBS solution than distilled water, indicating their potential as superabsorbent hydrogels (SAHs).Weight loss tests showed an increasing trend with higher NaOH concentrations, attributed to the lower water absorption capacity, and reduced structural stability of hydrogels with smaller pores.Hydrogels with lower NaOH fractions exhibited higher Young's modulus values and demonstrated shape recovery after compression, while their compressive strength decreased.In conclusion, excessive NaOH concentrations can lead to reduced pore formation, decreased swelling capacity, increased weight loss, and lower Young's modulus values, despite the ability of NaOH to dissolve cellulose.

10th
Asian Physics Symposium (APS 2023) Journal of Physics: Conference Series 2734 (2024) 012036 IOP Publishing doi:10.1088/1742-6596/2734/1/0120369 functional groups of cellulose.The increase in NaOH concentration in the hydrogel leads to a shift of O-H and C-H stretching towards lower wavenumbers.

Figure 5 .
Figure 5. FTIR spectra of cellulose powder and cellulose hydrogels with different concentrations of NaOH.

Table 1 .
FTIR peak assignments in cellulose powder and hydrogels.

Table 2 .
Mechanical properties of cellulose based hydrogel.