This study presents the development of a series of fully biobased, biodegradable superabsorbent polymers (SAPs) from citric acid, its sodium salt, and sorbitol. An approach combining polycondensation and subsequent thermal cross-linking in the absence of any externally added cross-linker is utilized for the fabrication of these SAPs, followed by comprehensive characterization to analyze their composition and structure and to assess their water absorption capacity and biodegradability. An optimized synthesis of superabsorbent polymers (SAPs) is achieved at a 1:1 molar ratio of the combined citric acid (CA) and its sodium salt to sorbitol. This specific formulation is found to be critical for producing a precursor polymer with the maximum molecular weight, which, in turn, yields SAPs exhibiting the highest water absorption capacity. Furthermore, when the neutralization degree (ND) of CA in the monomer feed is 70%, the resulting extracted SAPs exhibit the highest water absorption capacity of approximately 18 ± 2 g/g SAPs with an approximate yield after an extraction of 75%. Most importantly, the resulting SAPs exhibit superior biodegradability under mild conditions in activated sludge. The findings underscore the possibility of the production of synthetic, renewable, and biodegradable SAPs from citric acid and the biobased sorbitol monomer. The observed biodegradability, thereby avoiding the formation of persistent microplastics, is extremely important with respect to environmental concerns and the advancement of sustainable material development.

Absorption Animal feed Nucleic acid structure Polymers Precursors

superabsorbent polymers biobased biodegradable citric acid polyols neutralization

Superabsorbent polymers (SAPs) represent a class of materials with exceptional water-absorbing capabilities. (1,2) They have attracted considerable interest recently because of their extraordinary capacity to absorb and hold vast amounts of water, far exceeding their own weight. (3) This unique ability of SAPs makes SAPs invaluable in various applications. (4) Originally developed in the 1960s, SAPs have undergone significant advancements in their synthesis, characterization, and performance over the last decades. (5) These advancements have enabled the commercialization of SAPs for diverse applications, ranging from disposable diapers and sanitary napkins to soil moisture retention in agriculture and wound dressings in healthcare. (6) With the ever-growing demand for materials that can efficiently manage liquids and aqueous solutions, the development of SAPs continues to be an area of active research and innovation. (7,8)

From the point view of commercial product ranges, acrylate-based SAPs stand out for their low price and unmatched water absorption capacity, resulting from their extremely high and second to none density of hydrophilic groups in each repeating unit. (9,10) Even though acrylate-based SAPs are the most successfully commercialized products, they still have two major disadvantages: (1) environmental impact: The environmental footprint of manufacturing conventional superabsorbent polymers (SAPs) is a major concern as it relies on petrochemical-derived monomers and cross-linkers, contributing to issues like greenhouse gas emissions, depletion of finite resources, and high energy consumption; (11−13) (2) nonbiodegradability: traditional acrylate-based SAPs are typically nonbiodegradable, meaning they persist in the environment for extended periods after disposal. This characteristic can contribute to environmental pollution and waste accumulation, particularly in landfills. (9,14,15) Additionally, the disposal of these polymers can lead to soil and water contamination and the generated persistent microplastics can end up in our food chain. (16)

To deal with these two shortcomings, a lot of researchers have made significant progress in exploring the use of natural polymers, such as polysaccharides and proteins, as a matrix to create environmentally sustainable and biodegradable SAPs to reduce the environmental footprint of SAP production and promote resource efficiency. (17−21) Because of their biocompatibility, abundance, and renewability, natural polymers are promising materials for developing sustainable superabsorbent polymers (SAPs). However, incorporation of these natural polymers into SAP formulations, usually by graft copolymerization with acrylate monomers, will also cause some problems. To begin with, natural polymers often exhibit variability in their properties due to factors such as source, extraction method, and processing conditions. (22−32) This variability can lead to inconsistencies in the performance of SAPs synthesized from natural polymers, making it challenging to achieve uniformity and predictability in product quality. (33,34) Second, during the process of producing natural polymer-based SAPs, toxic cross-linkers such as N,N′-methylenebis(acrylamide) (MBA), epichlorohydrin, or glutaraldehyde are required to create a three-dimensional network based on the polymer matrix. Such externally added cross-linkers are harmful to human beings. (17,20,35,36) Most importantly, the majority of the natural polymer-based SAPs also contain acrylate moieties, which can ensure satisfactory water-absorbing properties but which, as mentioned above, are completely nonbiodegradable. This implies that the biodegradability reported for such partially biobased SAPs originates from the natural polymer part, while the SAPs are not fully biodegradable. (5,37−41) These SAPs can persist in the environment for a long time after use and pose risks to ecosystems and wildlife if they enter waterways or soil.

Compared with the method to utilize natural polymers as the substrate to synthesize SAPs, the production of biodegradable SAPs via polycondensation of biobased monomers seems more promising to reduce environmental impact, since the resulting polymers can degrade into harmless byproducts under mild conditions and the use of externally added cross-linkers can be avoided by choosing multifunctional monomers. In 2017, a research group (42) pioneered an innovative step-growth melt polymerization approach to synthesize polycondensate-type superabsorbent polymers (SAPs) from citric acid (CA). Although their SAPs’ maximum absorbency, reaching approximately 22 g/g of distilled water, was significantly lower than that of acrylic acid-based SAPs, it represented a significant advancement in the development of synthetic and potentially biodegradable SAPs. The authors developed and investigated a new method for synthesizing biobased superabsorbent polymers (SAPs) by cross-linking citric acid, monosodium citrate, and 1,4-butanediol with hexamethylene diisocyanate (HDI) and then thoroughly examining their water absorption properties. However, notably absent was a study on the biodegradability of these SAPs, which is particularly crucial given the upcoming legislation (43) on microplastics.

Inspired by the study mentioned above, we came up with the idea to enhance the water-absorbing property of CA-based SAPs by replacing 1,4-butanediol with biobased polyols bearing more hydrophilic groups per molecule. Using polyols with functionalities higher than 2 in a polycondensation with CA also results in a self-cross-linking system, which would make the addition of external, toxic cross-linkers superfluous. To fill the knowledge gap regarding the biodegradability of polycondensate-type SAPs, especially those made via polyesterification, a detailed study on the synthesis of the SAPs from CA and polyfunctional polyols and the biodegradability of the resulting SAPs is necessary. In our previous study, we reported on SAPs based on CA and glycerol. (44) In the current study, we report the synthesis of a biobased, biodegradable superabsorbent polymer from CA, its sodium salt, and a biobased polyol with an even higher functionality than the previously used glycerol, viz., sorbitol (ST). The replacement of glycerol by sorbitol results in a very similar water absorption capacity (18 ± 2 g/g) as compared to the glycerol-based systems. This SAP was made via polycondensation and subsequent thermal cross-linking in the absence of an extra cross-linker. The feed ratio of (CA + its Na salt) and sorbitol is optimized according to the molecular weight of the resulting SAP precursor polymer. By varying the neutralization degree (ND) or the molar ratio (sodium citrate/CA) in the feed, the water absorbance capacity can be easily tuned. The composition and structure are characterized by elemental analysis, NMR, and FTIR techniques. In addition, the water-absorbing properties and biodegradability in activated sludge of the resulting SAPs are assessed in detail. The aim of this study is to check the possibility of using polycondensation chemistry to produce biodegradable SAPs with a satisfactory water absorption capacity from biobased multifunctional monomers (Scheme 1).

For this study, several reagents were procured and used as received. Citric acid (CA, ≥99.5%), D-sorbitol (ST, ≥99.0% HPLC pure), monosodium citrate (MSC, ≥99.5%), and ethylene glycol (EG, 99.8%) for GPC baseline stabilization were all sourced from Sigma-Aldrich. Various salts for biodegradability tests, including monobasic potassium phosphate (≥99.0%), dibasic potassium phosphate (≥98.0%), disodium hydrogen phosphate dihydrate (≥99.0%), ammonium chloride (≥99.5%), calcium chloride (≥93.0%), magnesium sulfate heptahydrate (≥98%), iron(III) chloride hexahydrate (97%), and sodium acetate (≥99%), were also purchased from Sigma-Aldrich. Deionized water was obtained using WaterPro PS Polishing Systems from LABCONCO. Activated sludge for water biodegradation tests was collected from the water treatment center in Glimmen, The Netherlands. A gift of E-D-SCHNELLSIEB PAINT STRAINER super fine 125 μm filter paper was received from Covestro (Netherlands) B.V. Additionally, a regenerated cellulose filter paper with a pore size of 0.45 μm was acquired from SARTORIUS Biotech GmbH for determining insoluble content. The commercial superabsorbent polymer (SAP) reference material was a potassium salt of poly(acrylamide-co-acrylic acid) (Sigma-Aldrich, product number 432776).

To produce superabsorbent polymers (SAPs) with diverse compositions, a two-step bulk polycondensation method was employed using citric acid (CA), sorbitol (ST), and monosodium citrate (MSC). Various molar ratios of these reactants were used. The synthesis was performed in a 500 mL three-neck flask equipped with a mechanical stirrer, a Schlenk line, and a condenser. The process began by removing air from the flask, which was purged and then vacuum-sealed three times at room temperature. First, the specified molar ratios of CA, ST, and MSC were added to the flask along with a small amount of deionized water to ensure all reactants were dissolved, and the mixture was homogeneous. The reaction proceeded in two stages: Stage 1: Esterification was performed at 130 °C under a mild nitrogen flow for 3–4 h. This step was considered complete when no more condensation water could be collected. Stage 2: Polyesterification was then conducted in the same flask. The condenser was replaced with a glass plug, and a cold trap containing liquid nitrogen was used to collect the water. This stage ran for 1.5–2 h at 130 °C under vacuum (approximately 1 mbar). The reaction was stopped when the precursor polymer’s high viscosity caused the mechanical stirrer to halt. A notable Weissenberg effect was also observed on the stirring blade, indicating a high molecular weight for the precursor. Upon completion, the product was cooled to room temperature and then removed. After pulverization and drying, the molecular weight of the soluble product was analyzed via gel permeation chromatography (GPC) in water. The material was then stored at room temperature awaiting further thermal cross-linking.

SAP precursor polymers, created using the method described earlier, were pulverized into fine powders (50–100 μm) at room temperature using an IKA Tube Mill control for three minutes. A Keyence VHX-7000 series digital microscope confirmed the particle size. These powders were then spread in an open cuboid aluminum box and subjected to thermal cross-linking in a vacuum oven at 140 °C and 0.1 mbar for one hour. This process transformed the powders into a fluffy material. After the fluffy material was ground again for 3 min in the same mill to achieve a similar fine powder size (50–100 μm), the material was extracted with water to isolate the soluble fraction.Gel content, a critical parameter representing the yield of SAPs after cross-linking, was determined by measuring the insoluble portion of the SAPs. A known mass (M

1) of pulverized SAPs was immersed in excess deionized water and stirred for 1 h to ensure complete swelling. The resulting swollen gel mixture was then filtered using a superfine 125 μm filter paper, which had an initial weight of M

2. The filter paper and swollen SAP particles were dried together at 60 °C under vacuum for 24 h. The final combined mass of the filter paper and dried SAPs was recorded as M

3. The following formula was then used:

(1)In the rest of this paper, the SAPs acquired prior to washing are denoted as “unextracted SAPs”, while those obtained postwashing are labeled as “extracted SAPs”. These fine powders were then stored in a desiccator at room temperature until further characterization.

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Even before thermal cross-linking, the SAP precursor polymers can contain an insoluble fraction. To quantify this insoluble content, a specific procedure was followed. A Buchner vacuum filtration setup, connected to a Kitasato flask and a KNF Laboport vacuum pump, was used for the analysis. A precise 3 g sample of the precursor polymer was added to 20 mL of deionized water and stirred for 30 min. The resulting suspension, which contained the insoluble material, was then filtered through a SARTORIUS regenerated cellulose filter paper (0.45 μm pore size) on a Buchner funnel.The insoluble content was calculated by the equation below:

(2)where m1 is the initial weight of the prepolymer, and m0 is the weight of the insoluble part after drying.

Gel permeation chromatography (GPC) was performed in water on the SAP precursor polymers using an Agilent Technologies 1260 series system. This setup included an Agilent 1260 Multidetector and three PPS Suprema M columns (100, 1000, 3000 Å) connected in series, with the column temperature maintained at 40 °C. Samples were prepared by dissolving the precursor polymer in an aqueous eluent consisting of 0.2 M NaCl and 0.01 M Morpholine, at a pH of 8.4. This same eluent was used for elution at a flow rate of 1 mL/min. The molecular weight and polydispersity index (PDI) were determined using Agilent’s GPC/SEC Software (version 2.2.281.39672). A calibration curve was established using conventional calibration with narrow poly(ethylene glycol) standards ranging from 100 to 1,000,000 Da, and detection was performed via a refractive index (RI) detector.

Fourier Transform Infrared (FTIR) spectra were collected on a Shimadzu IR-Tracer-100, which was fitted with a golden gate diamond attenuated total reflectance (ATR) sample unit. The spectra were recorded from 4000 to 500 cm–1 at a resolution of 4 cm–1, averaging 32 scans per measurement.

Nuclear magnetic resonance (NMR) spectra were obtained at room temperature using a Varian VXR spectrometer (400 MHz). DMSO-d6 was used as the solvent for both the 1H NMR and 13C NMR analyses. The sample-to-deuterated solvent volume ratios were 1:5 for 1H NMR and 1:1 for 13C NMR experiments. All spectra were processed by using MestReNova software (version 12.0.0–20,080), with chemical shifts referenced to the solvent signal (DMSO-d6).

Elemental analysis of SAPs was performed using an automated Euro Vector EA3000 analyzer with acetanilide serving as the calibration standard. The elemental composition of carbon (C), hydrogen (H), and sodium (Na) was determined for all samples, including unextracted SAPs, extracted SAPs, and the soluble fractions. Each sample was analyzed in triplicate, and the average values were used for reporting.

Particle size and specific surface area of the extracted superabsorbent polymers (SAPs) were characterized by using two distinct methods.

A Keyence digital microscope (VHX-7000 series) was used to determine the particle size. A small sample (20–50 mg) was dispersed on a microscope slide, and images were captured by a computer connected to the microscope. To ensure accurate measurements, areas with single-layered particles were selected. The system then automatically measured the size of the remaining particles after excluding outliers based on their area and circularity.

The specific surface area of the extracted SAPs was measured via the Brunauer–Emmett–Teller (BET) method. Nitrogen adsorption–desorption isotherms were recorded at −196.15 °C by using a Micromeritics Tristar 3000. Before analysis, samples were pretreated under a nitrogen atmosphere at 40 °C for 5 h to remove any residual moisture.

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The absorption kinetics of the superabsorbent polymers (SAPs) were evaluated by immersing 2 g of the cross-linked and extracted SAP powders in 100 mL of deionized water (DW). Samples were allowed to absorb water for various time intervals up to 30 min. The mixture was then filtered, and the amount of absorbed water was determined using the calculation method described below.

For the total free water absorption test, 1–2 g of the cross-linked and extracted SAP powders were added to 100 mL of deionized water in a glass jar. The dispersion was briefly stirred and then left undisturbed for 30 min to allow the SAPs to swell. The swollen SAPs were subsequently separated from the liquid by filtering until no more water dripped. The mass of the remaining residue was measured for calculation, while the filtrate was set aside for later elemental analysis along with the extracted and unextracted SAPs.The total free water absorption (WA) was calculated as follows:

(3)where W1 is the initial weight of the SAPs, and W2 its weight after swelling.

The cross-link density of the superabsorbent polymers (SAPs) was evaluated indirectly via rheological measurements using a Physica MCR302 rheometer equipped with an 8 mm diameter parallel plate geometry. The temperature was precisely controlled with a Peltier hood and a bottom plate. A sample of the 100% solid, extracted porous SAP powder was placed on the bottom plate. The temperature was set to allow the vitrified, cross-linked powder to compress without inducing any chemical reactions. The upper plate was then lowered to compress the sample into a solid disc with a height between 400 and 700 μm. Following compression, oscillatory shear measurements were initiated. The upper plate applied a sinusoidal shear deformation γ(t) with a strain amplitude (γ0) of 0.05% at a frequency of 1 Hz. The resulting shear stress amplitude was recorded as τ0, and the phase shift between stress and strain was denoted as δ. From these measured parameters, the storage modulus (G′) and loss modulus (G″) were derived.

These rheological characteristics were measured as a function of temperature, while the sample was cooled at a rate of −1 °C/min.For cross-linked polymers, at temperatures significantly above the glass transition temperature (Tg), the elastic resistance to deformation (G′) is primarily due to the stretching of the polymer network chains. In this rubbery regime, the material’s behavior aligns with the theory of rubber elasticity, where the elastic shear modulus (G′) is directly proportional to the effective cross-link density (νe), the number of moles of elastically active network chains per m3 of the sample:

(4)where the shear modulus (G′) is obtained within the rubbery plateau. T is the temperature in K and R is the gas constant (8.314 JK–1 mol–1).

The biochemical oxygen demand (BOD) was measured to assess the biodegradability of the materials, employing a Lovibond BD 600 series apparatus. A specified quantity of the material was introduced into a test medium consisting of water and activated sludge. The sealed test containers were then maintained in the dark at a constant temperature of 25 °C for 28 days, in accordance with the OECD 301 standard. The ongoing biodegradation was tracked by monitoring the consumption of dissolved oxygen. The BOD value in this context indicates the amount of oxygen consumed during the biochemical degradation of the organic substance. Comprehensive details on the experimental setup and sample preparation are available in the Supporting Information.

5. Results and Discussion

5.1. Optimization of Synthesis Conditions for Polymerization of Citric Acid and SorbitolIn this work, SAP precursor polymers (labeled as PSCxMy) were synthesized from a mixture of citric acid (CA), sorbitol (ST), and monosodium citrate (MSC). In this designation, variable x corresponds to the molar percentage of CA relative to the total acid content (CA + MSC) in the reaction mixture, as depicted in Scheme 1. It has been reported previously that sorbitol (ST) remains thermally stable until 250 °C. (45) However, both mineral acids and organic acid with strong acidity can favor dehydration at lower temperatures (please see Scheme 2 below) and accordingly convert sorbitol to isosorbide efficiently in the end. (46,47) It is known that CA exhibits catalytic activity for the synthesis of isosorbide from sorbitol at 135 °C. (48) In our previous study, (44) polycondensations of CA and glycerol were performed at 130 °C, which is high enough to ensure a fast reaction rate and low enough to avoid thermal decomposition of CA. As sorbitol is more sensitive than glycerol in acidic environments, it is necessary to check if there is any side reaction at 130 °C, viz., dehydration of sorbitol to isosorbide. By taking samples of the polycondensate mixture at 130 °C during a period of 3 h and submitting these samples to 

13C NMR analysis, the presence of isosorbide (if any) can be easily verified. The 13C NMR spectra are presented in Figures 1 and 2. Comparing the spectrum of the polycondensation product with the spectrum of isosorbide shows no isosorbide formation during the 3 h polycondensation at 130 °C. It can therefore be concluded that 130 °C is a safe temperature at which dehydration of sorbitol (and thermal degradation of CA) will not occur but polycondensation of CA and ST can proceed.

Scheme 2. Two Acid-Catalyzed Dehydration Pathways (46,47) of Sorbitol to Isosorbide via 1,4-Sorbitan or via 3,6-Sorbitan

Figure 1. 13C NMR spectra of the product of the polycondensation of CA and ST at 130 °C for 3 h (above) and isosorbide (below).

Figure 2. Water absorption capacity of SAP-70s (molar feed ratio MSC/CA = 70/30) with different molar feed ratios for (MSC + CA)/ST (S1–4 representing SAPs produced with 1:1, 1.5:1, 2:1, 3:1 feed ratio, respectively). Error bars are based on three times repeating tests on water absorption capacity.To create a homogeneous reaction mixture, 2 mL of deionized water was initially added to the monomers in the flask. The first reaction phase, esterification, took place at 130 °C for 3–4 h. The completion of this step was estimated by the cessation of the byproduct water collection. The reaction then proceeded under vacuum for an additional 1.5–2 h. A representative 1H NMR spectrum for PSC50M50 is shown in Figure 3. Table 1 summarizes the number- and weight-average molecular weights (Mn and Mw) and the polydispersity index (PDI) for all synthesized SAP precursor polymers. Since the precursors were filtered before analysis, the data in Table 1 pertain exclusively to the soluble fractions. The GPC results indicate the presence of oligomeric species.

Figure 3. NMR spectrum of the PSC50M50 SAP precursor polymer (all peaks were assigned according to the literature reported by Rashid et al (50)).

Table 1. Molecular Weight and Insoluble Content of the As-Synthesized CA/GLY-Based and CA/ST SAP Precursor Polymers before the Thermal Cross-Linking Treatment

aBased on PEG standards; values for soluble fractions only.

bThis is calculated according to eq 1; pore size of the filter paper is 0.45 μm.Since there are six hydroxyl groups present in a sorbitol molecule, it is of significance to optimize the feed ratio of citric acid + its Na salt (acids in total) and sorbitol, which can affect the molecular weight of the SAP precursor polymer and thus influence the final cross-link density and the water-absorbing properties of the resulting SAPs. In this study, the optimal feed ratio was determined by monitoring the corresponding water-absorbing performance. Setting the neutralization degree in the monomer feed as 70% (“SAP-70”, meaning a molar ratio MSC/CA of 70/30), four different molar feed ratios for (MSC + CA)/ST were used, from 1:1 to 3:1 (Table 1). It can be seen that by increasing the (MSC + CA)/ST feed ratio, the number-averaged molecular weight of the SAP precursor oligomers decreases from 0.6 ×10*10*10 g/mol to 0.5 × 10*10*10g/mol (Table 1). Studies have indicated that the two primary hydroxyl groups of sorbitol are much more reactive than the others when involved in esterification, (49) which means that at relatively low polycondensation temperatures, sorbitol almost can be regarded as a difunctional monomer. After cross-linking and extraction, these SAPs were characterized for their water absorption capacity (Figure 2). The extracted SAPs with a (MSC + CA)/ST 1:1 molar feed ratio in the feed have the highest water absorption capacity (18 ± 2 g of distilled water/g of SAPs). Because of this screening, 1:1 is chosen as the optimal initial (MSC + CA)/ST feed ratio for the whole series of SAPs synthesized in this study.Of the polymers synthesized, the PSC superabsorbent polymer (SAP) precursor (produced without monosodium citrate, or MSC) showed high number-average molecular weight (Mn = 3.5 × 10*10*10 g/mol), weight-average molecular weight (Mw = 12.0 × 10*10*10 g/mol), and polydispersity index (PDI = 18.79). This outcome is likely due to the high average functionality of its monomers. Conversely, the molecular weight of the SAP precursor polymers generally decreased as the MSC content increased. This is attributed to the lower reactivity and functionality of the MSC. The PDI values for most precursor polymers were slightly greater than 2. This suggests a branched structure, which is consistent with the low insoluble content indices presented in Table 1, indicating predominantly non-cross-linked structures.

It is important to note that the insoluble content index values are considered approximations. This is because the filter paper used for these measurements had a larger pore size than that of the membrane filter used for the GPC analysis.

Following the procedures for glycerol-based SAPs established in our previous research, (44) all SAP precursor polymers were thermally cross-linked at 140 °C for 1 h. The resulting material was then ground for 3 min, and the soluble fraction was removed via water extraction. The remaining cross-linked structures, termed SAPs, were then characterized by Fourier-transform infrared (FTIR) spectroscopy. The FTIR spectra for the extracted SAPs are presented in Figure 4. The peaks observed at 1722, 1581, and 1172 cm–1 correspond to the carbonyl C═O of the β-ester, the carboxylate carbonyl group linked to sodium ions, and the C–O of the β-ester carbonyl group, respectively. The significantly higher intensity of the β-ester signals at 1722 cm–1 compared to the α-ester signals at 1581 cm–1 suggests that the β-carboxylic group exhibits greater reactivity than its α counterpart. (42)

Figure 4. FTIR spectra corresponding to extracted and thermally cross-linked SAPs. These polymers were synthesized from a 1:1 molar ratio of the total acid monomers (CA + MSC) to sorbitol, with the specific CA:MSC ratio detailed for each sample.

Neutralization is well known to be crucial for improving the absorbency properties of superabsorbent polymers (SAPs). However, due to its lower functionality and the steric hindrance from its sodium counterions, monosodium citrate (MSC) is less reactive than citric acid (CA). This reduced reactivity means that not all of the introduced MSC monomers are fully integrated into the three-dimensional SAP network during synthesis and subsequent thermal cross-linking. Even after the cross-linking step, some unreacted monomers and portions of the SAP particles may remain soluble due to insufficient cross-linking depending on the initial monomer feed ratio. Typically, unreacted MSC persists in the product, either as a free monomer or as part of water-soluble oligomeric chains and can be removed by washing with water. Figure 5 shows that the sodium content of the extracted SAPs increases with a higher degree of neutralization. A discrepancy exists between the theoretical sodium content, calculated from the initial monomer feed, and the experimentally measured value for the extracted SAPs. This difference supports the finding that MSC is less reactive with sorbitol than CA. Furthermore, Figure 5 illustrates that while the carbon (C), hydrogen (H), and oxygen (O) contents of the various extracted SAPs are nearly identical, the sodium content shows a clear increase with the degree of neutralization, which, as will be shown, leads to enhanced absorbency. The gap between the theoretical and measured sodium contents widens as the molar percentage of MSC in the feed increases, indicating that it becomes more difficult to incorporate all of the less reactive MSC moieties into the SAP network at higher concentrations.

Figure 5. Elemental content (C, H, O, Na) of extracted, thermally cross-linked SAPs (cross-linked at 140 °C and grinded for 180 s) with different NDs.

5.3. Absorption Rate and Capacity CharacterizationThe water absorption rate of the SAPs is mainly influenced by physical factors, including the particle size and the porosity of the cross-linked particles. Usually, SAPs with more porous structures have higher absorption rates than those with a lower porosity because of the higher diffusion rate. The water absorption rate also increases with decreasing particle size of the SAP particles, resulting from an increasing surface area and, accordingly, a higher diffusion rate. In Table 2, it can be found that after the same grinding time, all investigated SAPs have comparable particle size (Dmax in the range of 54–74 μm) and BET specific surface area. The time dependency of the absorption capacity of four SAPs made in this study was investigated in deionized water, and the results are presented in Figure 6. It is observed that the SAPs synthesized from the ST/CA/MSC-based precursor prepolymers all have relatively fast water absorption rates, and all of them achieve swelling equilibrium within a period of 10 min. The different maximum absorptions will be commented further on.

Table 2. Particle Size and BET Specific Surface Area Characterization of Cross-Linked (at 140 °C), Grinded, and Extracted SAP Samples with Different Neutralization Degrees (Grinded afterward)a

aParticle characteristics determined and analyzed using a Keyence digital microscope (VHX-7000 model) and Micromeritics Tristar 3000 analyzer.

bThis is the minimum possible distance between two parallel lines on either side of the particle.

cThe maximum length between any two points that lie on the inner perimeter of the figure.

dThe area of the graphic.

eThis is the diameter of a circle with the same area as the figure.

Figure 6. Plot of water absorption capacity of SAPs (grinded for 180s) with different neutralization degrees versus swelling time.

As anticipated, SAPs synthesized solely from citric acid (CA) and sorbitol (ST) without incorporating monosodium citrate (MSC) demonstrated poor absorption performance, absorbing only about 3.5 g/g in deionized water (DW) (Figure 7). This underscores the necessity of sufficient neutralization for achieving a high absorption capacity. The substitution of some CA with an MSC was a strategic choice to introduce ions into the polymer network. These incorporated hydrophilic −COONa groups fully dissociate in water, causing the SAP’s three-dimensional network to expand via electrostatic repulsion. This not only increases the internal osmotic pressure but also creates more free space for the water molecules. A greater osmotic pressure difference between the interior and exterior of the SAP drives more water into the polymer to reach equilibrium, thereby significantly improving the absorption. As shown in Figure 5, the sodium content of the extracted SAPs increases with the degree of neutralization (ND). This leads to a corresponding enhancement of both electrostatic interactions and internal osmotic pressure within the cross-linked network. Figure 7 demonstrates that the water absorption of the SAPs─both before and after washing─increases with ND up to 70%. Specifically, absorption rose from approximately 3.5 ± 0.5 to 13 ± 2 g/g for unwashed samples and from 3.5 ± 0.5 to 18 ± 2 g/g for washed samples. In contrast, the maximum absorption in a saline solution was less than 10 g per gram of SAP.

Figure 7. Plot of water absorbency, gel content, and storage modulus of CA/ST-based SAPs. Gel content and storage modulus were determined after extraction.The storage modulus (G′) at temperatures well above the glass transition temperature (Tg) is directly proportional to the cross-link density, as noted in Section 2. The gel content results in Figure 7 align with the cross-linking density data: a higher MSC content in the monomer feed leads to both lower gel content and lower cross-link density. This is because MSC has a lower average functionality than CA. As the MSC content increases, the number of non-neutralized carboxylic acid groups decreases, reducing the probability of an esterification reaction between the hydroxyl groups of ST and the carboxylic acid groups of CA. This results in a lower cross-link density, which is reflected in a lower storage modulus of the extracted network.

If the cross-link density becomes too low, then a water-soluble polymer may form, rendering it unsuitable as a SAP. This occurs at ND values exceeding 70–80%. Cross-link density is a key factor influencing both the absorption performance and biodegradability. As shown in Figure 7, the cross-link density (indicated by G′) decreases sharply as the ND increases. This enhances the polarity and swelling capacity of the network, consequently boosting the maximum water absorption.

While superabsorbent polymers (SAPs) are widely used for their impressive water absorption, there are growing environmental concerns regarding their persistence and potential harm to soil and water, which could lead to them entering the food chain. Therefore, assessing their biodegradability is vital for understanding their environmental fate and ensuring their sustainability. It is well known that certain linear aliphatic polymers like polyhydroxyalkanoates (PHAs), (51,52) polybutylene succinate (PBS), (53) and polylactic acid (PLA) (54) are highly biodegradable. The SAPs in this study are based on aliphatic polyesters with hydrolyzable ester bonds in their main chains. They are primarily cross-linked through esterification between residual carboxylic acid (−COOH) groups from one component (citric acid, CA) and hydroxyl (−OH) groups from another (sorbitol, ST). Due to this structure, we expected the final SAPs to have satisfactory biodegradability. This is because both the polymer backbone and ester-based cross-links are susceptible to hydrolysis. The initial step of biodegradation typically involves the hydrolysis of the polymer into smaller, more manageable fragments that can be broken down by microorganisms found in activated sludge. (55−57)

To assess biodegradability, we subjected two extracted SAP samples (SAP-50 and SAP-70), a commercial acrylate-based SAP, and a readily degradable reference material (sodium acetate) to a continuous Biochemical Oxygen Demand (BOD) test, following the OECD 301F Standard. The specific calculation method is detailed in the Supporting Information. It is important to note that even for sodium acetate, the biodegradability did not reach 100%. This is because a portion of the carbon and oxygen atoms are used by bacteria for biomass production and growth. (58,59) As shown in Figure 8, the extracted SAP samples exhibited a noticeable initiation period before biodegradation began, unlike sodium acetate, which degraded from the outset. This delay is likely because the cross-linked polymers are too large to pass through the microbial cell walls directly. It takes time for the bacteria to secrete enzymes that can hydrolyze the polymer network into smaller fragments suitable for assimilation. (59) After 28 days, the biodegradabilities of the extracted SAP-50 and SAP-70 samples were 24% and 30%, respectively. As anticipated, the commercial SAP, which is lightly cross-linked and has a carbon–carbon bond backbone, showed no biodegradability with a BOD value of zero. It is reasonable to expect that with a longer incubation period a much larger portion of the polyester-based SAPs from this study would biodegrade. This is because all of the linkages between the monomeric residues in the main polymer chains and the cross-links are hydrolyzable ester bonds. The higher biodegradability of SAP-70 compared to SAP-50 after the same incubation time can be explained by its higher content of polar monosodium citrate (MSC), which facilitates faster water absorption, and its lower cross-link density (see Figure 7).

Figure 8. Biodegradability in activated sludge versus incubation time of several SAPs. Sodium acetate (the black line) is used as a readily degradable reference material.

In conclusion, the development of biobased and biodegradable superabsorbent polymers (SAPs) synthesized through the polycondensation of citric acid, sorbitol, and monosodium citrate followed by thermal cross-linking represents a significant advancement in sustainable materials science. Through comprehensive characterization techniques including FTIR, NMR, elemental analysis, and rheology measurements, the structure and composition of these SAPs have been elucidated, providing valuable insights into their molecular properties. With a maximum water absorbency capacity of 18 ± 2 g/g and demonstrated biodegradability, these 100% biobased SAPs hold great promise for diverse applications in agriculture, hygiene products, and environmental remediation while simultaneously addressing environmental concerns associated with conventional petroleum-based polymers. Furthermore, the successful synthesis and characterization of these biobased SAPs underscore the potential of green chemistry principles in fostering the development of innovative and ecofriendly materials for a more sustainable future. The primary application fields where these materials demonstrate significant potential, despite, or even because of, their lower absorption capacity, include controlled release systems for agriculture and pharmaceuticals, where slow and sustained release is desired. They are also ideal for smart packaging in food and electronics, acting as intelligent moisture regulators to extend shelf life or maintain a stable humidity. Furthermore, their controlled swelling makes them suitable for biosensors and diagnostics, allowing for precise and sensitive detection without disrupting the sensor function. Finally, their predictable and limited volume changes are beneficial for controlled swelling actuators and soft robotics, enabling precise and repeatable actuation. In essence, our materials excel in applications demanding controlled swelling and specific, rather than massive, absorption. Continued research efforts in this field are essential to further optimize the properties and performance of biobased SAPs, paving the way for their widespread adoption and contributing to the advancement of sustainable material design and circular economy practices.