Elaboration and characterization of biodegradable poly(aspartic-co-glutamic acid) hydrogels as soil additives with different crosslinkers

Abstract

This work introduces a promising biodegradable copolymer synthesized through the fusion of D,L-aspartic acid (ASP) and L-glutamic acid (GA) utilizing melt polymerization. Employing infrared spectroscopy (IR), nuclear magnetic resonance spectroscopy (¹H NMR, ¹³C NMR), and X-ray diffraction (XRD), the copolymer’s structural characterization highlights its distinctive physicochemical attributes. The synthesis, conducted with a facile and controllable melt polymerization method, yielded a remarkable product yield of up to 81%. The optimization of water absorption properties involved a meticulous exploration of various diamine cross-linking agents—hexamethylene diamine (HMD), lysine (LYS), and a synthesized tartaric acid dihydrazide (TD) derivative. Fourier transform infrared spectroscopy (FT-IR), nuclear magnetic resonance spectroscopy (NMR), and scanning electron microscopy (SEM) were employed to confirm the copolymer’s structure, morphology, and cross-linking efficiency. The findings underscore exceptional water retention capabilities, with a peak swelling ratio of 11.874% achieved at a 10% concentration of hexamethylene diamine. Beyond advancing superabsorbent materials, this study contributes to mitigating environmental concerns associated with non-biodegradable alternatives.

Keywords

Poly(aspartic-co-glutamic acid)copolymer biodegradation cross-linking superabsorbent swelling kinetics

Introduction

Superabsorbent polymers (SAPs) serve a crucial role in various industries, notably in agriculture, where effective water retention is paramount for sustainable crop production. However, the predominant use of non-biodegradable SAPs, exemplified by acrylate polymers, raises significant environmental concerns. Despite their straightforward synthesis and remarkable water-absorption properties, these polymers persist indefinitely in the soil, contributing to environmental pollution.^1–4 In contrast, poly (amino acid) and its copolymers present a promising avenue for sustainable development. Biodegradable, biocompatible, and non-toxic, these polymers can act as soil amendments, enhancing plant production while minimizing soil erosion.^5,6

The category of polyamino acids, characterized by peptide bond linkages and reactive side chains containing functional groups such as –COOH and –CONH₂, has produced various biodegradable SAP polymers, including poly (aspartic acid) (PASP),^7,8 poly (glutamic acid) (PGA),⁹ and poly (lysine) (PLYS).¹⁰ The synthesis of PASP can be divided into two different reaction methods: The first is based on the thermal condensation of aspartic acid monomers under vacuum conditions, requiring temperatures between 160 and 260°C for a period of one to 4.5 h with H₃PO₄ (85%) as a catalyst. The second method involves the use of monomers such as maleic acid and urea with a mixture of phosphoric and sulfuric acid.^11–13 To create the superabsorbent polymer, succinimide (PSI) was used as a raw material, and various diamines were employed as cross-linking agents. Tests have shown that this PASP product is capable of absorbing large quantities of water.¹⁴ Meanwhile, PGA, a high molecular weight polymer, can be produced through microbial¹⁵ and chemical processes.¹⁶ Additionally, PGA finds application in the agricultural, pharmaceutical, and chemical industries as a resin with a high water absorption capacity.¹⁷

In this study, our focus is on the development of a new copolymer composed of D,L-aspartic acid (ASP) and L-glutamic acid (GA). Utilizing melt polymerization for its ease of management and high yields of up to 81%, this copolymer aims to amalgamate the distinct properties of PASP and PGA, yielding a material with improved characteristics suitable for a broader range of applications. Our investigation extends beyond synthesis, exploring the effects of catalysts, reaction parameters, and the molar ratio of (ASP) to (GA). The structural characterization of our material involves a comprehensive analysis using techniques such as infrared spectroscopy (IR), Nuclear Magnetic Resonance Spectroscopy (¹H NMR, ¹³C NMR), and X-ray diffraction (XRD).

Additionally, we endeavor to identify optimal conditions for water absorption by poly (aspartic-co-glutamic acid) using various diamine cross-linking agents, including commercially available amino acids hexamethylene diamine (HMD) and lysine (LYS), as well as tartaric acid dihydrazide (TD) synthesized from tartaric acid. The structural integrity, morphology, and efficiency of the cross-linking procedure are meticulously examined through Fourier transform infrared spectroscopy (FT-IR), NMR measurements of T₂ relaxation times (for three different times: 0.5, 3, and 8 s), and scanning electron microscopy (SEM).

Experimental section

Materials

This study utilized D,L-Aspartic acid (ASP ≥99%), l-glutamic acid (GA ≥99%), 1,6-hexanediamine (HMD ≥99%), orthophosphoric acid (H₃PO₄, 85%), l-lysine (LYS ≥98%), L-(+)-Tartaric acid (≥99.5%), Hydrazide (N₂H₄.H₂O, 80%), sodium hydroxide (NaOH ≥97%), N,N-dimethylformamide (DMF ≥99.9%), dimethyl sulfoxide (DMSO ≥99.9%) were all of analytical grade and purchased from Sigma-Aldrich and used as received. Organic solvents such as methanol (≥99.8%), and ethanol (≥99.9%) were supplied from Sigma-Aldrich. Distilled water was used throughout the experiments.

Synthesis and optimization

Synthesis of pre-polymer

For the synthesis of Poly (succinimide-co-glutamic acid) (PSI/GA), we used the approach illustrated in Figure 3(a). In a 100 mL round bottom flask, ASP and GA were placed, and then 3 g (0.03 mol) of phosphoric acid (85% w/w) was introduced as a catalyst. The copolymerization took place under specific temperature conditions in a vacuum. After being allowed to cool to room temperature, the product was washed multiple times with distilled water to remove the catalyst and reactants. Subsequently, the copolymer was filtered and ultimately dried at 80°C for a period of 4 h.

Selection of the molar ratio of ASP to GA

Previous studies have indicated that maintaining a higher molar concentration of D,L-aspartic acid than its grafting monomer, l-glutamic acid, during polymerization is advisable.^19–21 The remaining parameters were held constant, including a reaction temperature of 180°C, a reaction time of 180 min, and a consistent concentration of 85% (w/w) phosphoric acid (3g).

Table 1 reveals that an increase in the glutamic acid content in PSI/GA synthesis leads to a decrease in yield. Nonetheless, even at levels of approximately 10%–15% GA-content, the yields for the pre-polymer remained acceptable. The optimal molar ratio of D,L-aspartic acid to L-glutamic acid was found to be 4.5:0.75, resulting in the highest product yield. However, beyond the ratio of 4.5:0.75 for D,L-aspartic acid to l-glutamic acid, there was a decrease in product yield, possibly due to reactant saturation and the reaction reaching equilibrium concentration. Therefore, the subsequent reactions were conducted with a molar ratio of 4.5:0.75.

Table 1.

Selection of the molar ratio of ASP to GA.

Sample name	ASP (%)	GA (%)	Yield (%)
PSI	100	0	89
PSI/GA1	85	15	81
PSI/GA2	70	30	60
PSI/GA3	55	45	55
PSI/GA4	45	55	50
PSI/GA5	30	70	24.7
PSI/GA6	15	85	17.6

Selection of reaction time

All reactions were carried out under identical conditions, including a fixed concentration of 85% (w/w) phosphoric acid (3g), and constant amounts of D,L-aspartic acid (6g) and L-glutamic acid (1.1 g). Reaction times varied between 90 and 180 min. It is clear that an increase in reaction time led to an increase in product yield, as shown in Table 2. As the reaction time was extended, the product yield continued to increase. Maximum yield was reached at 180 min; consequently, the same reaction time was applied for all subsequent reactions.

Table 2.

Selection of the reaction time.

Sample name	Reaction time (min)	Yield (%)
PSI/GA1-90	90	40
PSI/GA1-120	120	59
PSI/GA1-150	150	66
PSI/GA1-180	180	81

Selection of the reaction temperature

A consistent concentration of 85% (w/w) phosphoric acid (3g) was employed in the experiments, with 6g of aspartic acid and 1.1 g of glutamic acid as the reactants. The reaction duration was held at a constant 3 h, while the reaction temperature was systematically varied within the range of 160 to 210°C, with 10°C increments. As the reaction temperature increased, there was a gradual increase in the product yield. However, when the reaction temperature exceeded a certain point, product charring occurred, along with the formation of by-products due to the decomposition of reactants and catalyst. Conversely, if the temperature was too low, the reaction rate was insufficient, leading to a diminished product yield. The data presented in Table 3 leads to the conclusion that the maximum product yield was attained when the reaction temperature was set at 180°C.

Table 3.

Selection of the reaction temperature.

Sample name (°C)	Reaction temperature (°C)	Yield (%)
PSI/GA1-160	160	66,64
PSI/GA1-170	170	72.04
PSI/GA1-180	180	81
PSI/GA1-190	190	75.64
PSI/GA1-200	200	72.04
PSI/GA1-210	210	68.4

Crosslinker synthesis

In addition to the readily available commercial crosslinkers HMD and LYS, another crosslinker TD was synthesized in the laboratory by reaction of diethyl tartrate with hydrazide to produce diamino compound.

L-(+)-Tartaric Acid Dihydrazide crosslinker

A mixture of N₂H₄.H₂O (4.9 g, 152 mmol, 80%) and diethyl L-(+)-tartrate (5.15 g, 25 mmol) was stirred for 8 h to make a white precipitate. The precipitate was separated through filtration and washed with Et₂O to produce L-(+)-tartaric Acid Dihydrazide (4 g, 89%), with a melting point of 185°C.¹⁸ Figure 1 shows the reaction between diethyl L-(+)-tartrate and hydrazine.

Figure 1.

Synthesis of tartaric acid dihydrazide (TD).

The synthesis of the tartaric acid dihydrazide (TD) crosslinker was confirmed using ¹H-NMR and ¹³C-NMR spectroscopy (Figure 4).

¹H NMR spectra (D₂O, 300 MHz): δ (ppm) 4.54 (s, 2H, H-2, H-3), 4,70 (s, 8H, 2NHNH₂. 2OH).

¹³C NMR spectra (D₂O, 300 MHz): δ (ppm) 71.95 (C-2, C-3), 171.85 (C(O)NH).

Crosslinking and hydrolysis to clPASP/GA

PSI/GA’s succinimide ring structure can be easily reacted with an amine group. This allows bifunctional amines to link two of the polymer chains together (Figure 3(b)), forming a hydrogel network (Figure 3(c)). As illustrated in Figure 2, the most commonly used crosslinking agents are hexamethylene diamine (HMD) and lysine (LYS) amino acid, which are available commercially. Additionally, the crosslinking agent based on tartaric acid dihydrazide (TD) was synthesized and used. The choice of crosslinking agent could influence the hydrogel’s swelling and mechanical properties.

Figure 2.

Diamine crosslinkers used for PASP/GA-based hydrogels.

Crosslinking by HMD

PSI-co-glutamic acid (PSI/GA) was weighed and dissolved in DMF or DMSO. Meanwhile, a 10% ethanol solution of DMF was prepared and diluted with the appropriate solvent before being combined with the PSI-co-glutamic acid solution. After around 15-25 min, a firm elastic gel was formed. After a total of 2.5 h, the gel was crushed, washed thoroughly with ethanol and dried in vacuum at 80°C in order to remove ethanol and any solvent residues of DMF or DMSO Table 4.

Table 4.

Various crosslinker variations (HMD) for synthesis of cross-linked PSI/GA₁ (clPSI/GA₁).

Sample name	m (PSI/GA₁) (g)	Crosslinker weight (%)	m (HMD)/2.0 g PSI/GA (g)
ClPSI/GA-HMD_10.0	2.0	10.0	0.24
ClPSI/GA-HMD_15.0		15.0	0.32
ClPSI/GA-HMD_20.0		20.0	0.48

Crosslinking by LYS

2.0 g of PSI/GA was dissolved in 10 mL of DMSO, and a LYS solution (prepared in a 10% methanolic solution and diluted with 2.0 mL of DMSO before being added to the PSI/GA solution) was introduced at room temperature and thoroughly mixed. Subsequently, the solution was heated to 80°C and stirred for an additional 24 h until it formed an elastic gel. Following this, the gel was crushed, washed thoroughly with ethanol, and dried under vacuum overnight Table 5.

Table 5.

Various crosslinker variations (LYS) for synthesis of cross-linked PSI/GA₁ (clPSI/GA₁).

Sample name	m (PSI/GA₁) (g)	Crosslinker weight (%)	m (LYS)/2.0 g PSI/GA (g)
clPSI/GA-LYS_10.0	2.0	10.0	0.3
clPSI/GA-LYS_15.0		15.0	0.45
clPSI/GA-LYS_20.0		20.0	0.6

With 15.0 and 20.0% crosslinker, no hydrogels were formed.

Crosslinking by TD

2g of PSI-co-glutamic was dissolved in 10 mL of DMSO, and a solution of TD (which had been prepared in a 10% ethanolic solution) was diluted with 2 mL of DMSO before being introduced into the PSI/GA solution. The crosslinker was then added slowly, and the mixture was thoroughly stirred at room temperature. After a 6 h reaction at 60°C, the stirring bar was removed, and the mixture formed an elastic gel. The total reaction time had been 24 h at 60°C. Subsequently, the gel was crushed with a spatula, washed with ethanol, and dried under vacuum overnight Table 6.

Table 6.

Various crosslinker variations (TD) for synthesis of cross-linked PSI/GA₁ (clPSI/GA₁).

Sample name	m (PSI/GA₁) (g)	Crosslinker weight (%)	m (TD)/2.0 g PSI/GA (g)
clPSI/GA-TD_10.0	2.0	10.0	0.36
clPSI/GA-TD_15.0		15.0	0.54
clPSI/GA-TD_20.0		20.0	0.73

With 10.0 % crosslinker, no hydrogels were formed.

Formation of PASP/GA Hydrogels through the Hydrolysis of clPSI/GA

clPSI/GA was introduced into a 1.0 M NaOH solution while being vigorously stirred (Figure 3(c)). The direct result of this reaction was the formation of a cross-linked poly (co-glutamic aspartic acid) (clPASP/GA), as evidenced by the swelling of the solid and rapid uptake of the NaOH solution. The mixture underwent a two-hour reaction period before the resulting product was thoroughly washed with water and then dried at 50°C Table 7.

Table 7.

Hydrolysis of clPSI/GA₁ to clPASP/GA₁ with NaOH.

Sample name	m (clPSI/GA₁) (g)	n (NaOH)/n (PSI-units_theor.) (mol/mol)	Yield (%)
clPASP/GA-HMD_10.0	2.0	1.0	73
clPASP/GA-HMD_15.0			77
clPASP/GA-HMD_20.0			82
clPASP/GA-LYS_10.0			75
clPASP/GA-TD_15.0			82
clPASP/GA-TD_20.0			87

Characterization methods

Product yield, X-ray diffractometer, NMR analysis, swelling ratio, FTIR spectroscopy, swelling ratio, and SEM observation were employed for comprehensive characterization.

Determination of the copolymer yield

The purity of the copolymer product can be determined by dissolving it in DMF and using a mass balance to calculate the amount of impurity which does not dissolve.

The product was dissolved in 10 mL DMF and left to stand for 24 h at room temperature. The solution was then filtered through a filter paper, which was dried at 40°C for 3 h until a constant weight was achieved. The purity of the product was then determined using equation.¹

P = \frac{(W_{0} \times W_{1})}{W_{0}} \times 100 %

Let W₀ and W₁ represent the weight of the product and the weight of insoluble materials in filter paper respectively, with p being the purity of the product.

The products had to be washed several times with de-ionized water to eliminate the catalyst and non-reacting components, and then filtered. The copolymers obtained were dried at 80°C for 4 h. The product yield can be calculated using the equation:²

Y = \frac{(A \times P)}{B} \times 100 %

Where A is the weight of the experimental, B is the weight of Theoretical, and Y is yield of the product.

X-ray diffractometer

The synthesized PSI/GA polymer was characterized by X-ray Diffraction analysis, which was carried out using an X-ray diffractometer (XRD-D8, Advance, Bruker) with a scanning rate of 0.02° per second. The analysis covered a 2θ range from 5° to 80°.

NMR analysis

The ¹H NMR and ¹³C NMR spectra of the PSI/GA and PSI in DMSO, as well as new crosslinkers from TD in D₂O, were measured using a 300 MHz (Bruker) spectrometer. Additionally, ¹H NMR spectra of PASP/GA hydrogels with varied crosslinking rates in D₂O were measured using a 60 MHz (Spinsolve 60) spectrometer. The water proton NMR relaxation times T₂ was measured on hydrogels with different amounts of crosslinking. The T₂-relaxometry was carried out using the Carr-Purcell-Meiboom-Gill (CPMG) pulse sequence and the experiments were performed at room temperature. The T₂ was determined by fitting the echo amplitude decay according to the following equation:

M_{x y (t)} = M_{x y (0)} (e^{\frac{- t}{T_{2}}}); M_{x y (0, t)} : m a g n e t i z a t i o n i n x y p l a n e a t t i m e t

FTIR-spectroscopic measurement

FTIR spectrometer was used to analyze various samples of PASP/GA hydrogel in order to verify the predicted cross-linking reaction, using Fourier-transformed infrared (FTIR) spectra (ATR-IR, Bruker, TENSOR 27). The powder sample was dried prior to this analysis.

Swelling ratio

Exactly 0.2 to 0.3 g of crushed hydrogel are weighed and inserted into two tea bags, which are then fastened together with bale ties. These bags are then placed in deionized water at 25°C, and the tea bags (minus the cable ties) are re-weighed at different time intervals (0.5, 1, 2, 3, 24 h).

The swelling rate is calculated using the following formula:

S_{t} [%] = \frac{m (s w o l l e n) - m (d r y) - m (tea bags)}{m (d r y)} \times 100

where m (dry) represent the weight of the dry sample, m (swollen) the weight of the tea bag including swollen hydrogel, and m (tea bags) the weight of tea bags.

The Voigt model was utilized to calculate the maximum swelling ratio S_max and t₆₃ (the time elapsed after 63% of Smax had been reached):

S_{t} = S_{\max} \cdot (1 - e^{\frac{- t}{t 63}})

SEM observation

A scanning electron microscope (JSM-IT500HR) was used to analyze the surface morphology of Superabsorbent polymer PASP/GA after it had been swollen, followed by freeze-drying.

Results and discussion

The X-ray diffractometer results suggest that the copolymer is a random copolymer. The NMR spectra confirm the successful synthesis of the copolymer and its composition. FT-IR spectra reveal characteristic bands for the copolymer and the subsequent hydrogel.

Synthesis of PSI/GA₁

The following part presents the formation of hydrogels from PASP/GA with the HMD, LYS or TD crosslinked. As for GA the optimum condition was obtained with the yield of 81% at the reaction temperature about 180°C, for 3 h, with the molar ratio of ASP acid to GA of 4.5:0.75 and catalyst concentration of 0.03 mol, this pre-polymer was used for hydrogel formation (Figure 3).

Figure 3.

Part (a) shows the reaction scheme for the preparation of PSI/GA, part (b) displays the chemical structure of the cross-links in PSI/GA, and part (c) details the cross-linking of PSI/GA and hydrolysis for the creation of PASP/GA hydrogels.

X-ray diffractometer

In Figure 4, the X-ray diffraction analysis demonstrates the absence of crystalline peaks. Instead, only two broad peaks are evident, corresponding to the amorphous phase of PSI/GA₁. This finding suggests that both PSI and PSI/GA₁ exhibit similar XRD patterns, with only slight variations in peak positions. As a result, it can be concluded that the introduction of glutamic acid (GA) into PSI did not substantially alter the amorphous nature of the material or its XRD pattern.^19,21

Figure 4.

X-ray spectra of PSI and PSI-co-glutamic acid.

NMR

Figure 5 displays the ¹HNMR spectra of PSI and PSI/GA₁. For PSI, signals at 2.7 and 3.21 ppm, and 5.27 ppm corresponded to the methylene and methine groups, respectively. Furthermore, the lack of any absorption in the region of 7.8-9ppm suggested the presence of branched and/or opened amide protons. PSI/GA’s ¹HNMR spectrum featured signals at 2.7-3.21 and 5.27 ppm due to the methine protons of the succinimide units. Moreover, signals at 7.88 and 8.74 ppm, 4.91 ppm, 1.9-2.14, and 2.28 ppm were assigned to the amide, methine, and methylene protons of the glutamic acid units. Additionally, it is worth noting that GA introduced by 10% from the integration.

Figure 5.

¹HNMR spectra of PSI (a) and PSI/GA₁ (b) in DMSO-d₆.

The ¹³C NMR Figure 6 of the copolymer showed four peaks at δ = 173.95, 172.67, 171.85, and 169.67 ppm associated with C = O, two peaks at δ = 47.93 and 52.20 ppm correlated to -CH-, and three peaks at δ = 32.81, 24.02, and 23.76 ppm connected to -CH2-, indicating the presence of both PSI and polyglutamic acid.^19,20

Figure 6.

¹³C NMR of PSI/GA in DMSO-d₆.

Syntheses of tartaric acid dihydrazide (TD)

In addition to the HMD and LYS crosslinkers, tartaric acid dihydrazide (TD) crosslinker was prepared from tartaric acid using the method outlined above.

In the ¹H-NMR spectrum of Tartaric Acid Dihydrazide (TD), two distinct signals were evident. The singlet at 4.54 ppm corresponds to the methine group (H-2, H-3), while another singlet at 4.70 ppm is attributed to both hydrazide protons and hydroxyl protons (Figure 7(a)).

Figure 7.

¹H-NMR spectra of tartaric acid dihydrazide (TD) (a) and ¹³C-NMR spectra of tartaric acid dihydrazide (TD) (b).

The ¹³C-NMR spectrum of Tartaric Acid Dihydrazide (TD) revealed distinctive peaks. A peak at 71.95 ppm indicates carbon atoms at positions 2 and 3, emphasizing their aliphatic nature. Another peak at 171.85 ppm aligns precisely with the carbonyl carbon of the amide group in the hydrazide moiety, confirming the presence of an amide linkage (Figure 7(b)).

The NMR spectra demonstrate the successful synthesis of the crosslinker, which was then employed in the subsequent reaction step to create PASP/GA-based hydrogels.

Characterization of PSI/GA, PASP/GA and the Chemical Structure of Hydrogels Using FT-IR Spectroscopy

In Figure 8(a), FTIR spectra of PSI, PSI/GA₁ and PASP/GA are displayed. The spectra of PSI^13,22 and PSI/GA present similar peaks, with the latter showing an additional absorption peak at 1352 cm⁻¹ which is attributed to the -CONH group, indicating the presence of glutamic acid in the copolymer. Both spectra display characteristic bands at 1388 cm⁻¹ (stretching vibration of -CONH groups), 1715 cm⁻¹ (C = O) and 2952 cm⁻¹ (stretching vibration of -CH2 groups).^19–21 For PASP/GA, N–H groups (stretch corresponding to secondary amides) can be seen between 3250 cm⁻¹ and 3300 cm⁻¹, a peak at 1588 cm⁻¹ corresponds to the -NH vibration from “amino acid”, and at 1392 cm-1 the -C = O vibrations of COO group are observed. The FT-IR spectra of clPASP/GA-HMD hydrogels created with varying quantities of 1,6-hexane diamine (crosslinker) are displayed in Figure 8(b). All samples contained a cross-linked CONH stretching band at 1518 cm⁻¹. It was observed that when excessive amounts of cross-linking agent were used, the absorption peak at 1518 cm⁻¹ was enhanced. Figure 8(c) of the FTIR results shows that the peak at 1518 cm⁻¹ confirms the presence of –NH in the clPASP/GA-LYS 10.0, which is the end group of the cross-linkers. Finally, Figure 8(d) shows evidence that the Tartaric Acid Dihydrazide (TD) was successfully incorporated in the hydrogel structure due to the appearance of new bands at 1656 cm^{− 1} and 1518 cm^{− 1}, which are stretching vibrations of the -CONH groups.

Figure 8.

FT-IR spectra of PASP/GA cross-linked with various cross-linking agents.

Influence of the crosslinking agent concentration on the swelling rate of PASP/GA hydrogels

The influence of the type and quantity of crosslinker can be seen clearly from the swelling ratio and kinetic parameters of the measurements. Figure 9 displays the kinetic data for the water absorption of various PASP-based hydrogels. The S_max and t₆₃ parameters were calculated using the Voigt model, as shown Figure 10.

Figure 9.

Swelling kinetics of clPASP/GA-HMD, clPASP/GA-LYS and clPASP/GA-TD hydrogels.

Figure 10.

Dependency of experimentally determined maximum swelling degree on crosslinker content of (a) clPASP/GA-HMD, (b) clPASP/GA-LYS and (c) clPASP/GA-TD.

Swelling degrees between 1000% and 12000% were achieved when clPASP/GA was crosslinked with HMD, LYS, and TD respectively.

The addition of crosslinkers to hydrogels with HMD and TD demonstrated a decrease in the maximum water absorption (S_max) as the amount of crosslinker increased. This was accompanied by a decrease in t₆₃, which indicated a slower uptake of the swelling medium. A decrease in water uptake with higher crosslinker concentrations is to be expected. In comparison to HMD, the incorporation of crosslinkers using TD was worse since no hydrogels could be formed at 10%. Unfortunately, there is insufficient data to provide information regarding 15% and 20% of the crosslinker (LYS). However, the swelling ratio of 9134% was achieved in the LYS hydrogel with 10%.

NMR measurements of T₂ relaxation times for 0.5, 3, and 8 s

Figure 11 illustrates NMR measurements of T₂ relaxation times performed on poly (aspartic-co-glutamic acid) (PASP/GA) hydrogels for durations of 0.5, 3 and 8 s, revealing a clear relationship between the degree of cross-linking and water mobility in the hydrogel Table 8. When the degree of cross-linking was increased from 10% to 20% in clPASP/GA-HMD samples, notable changes in T₂ relaxation times occurred, indicating modifications in the mobility and behavior of water molecules within the hydrogel. The decrease in T₂ relaxation times at the 20% degree of cross-linking implies a reduction in water mobility compared to lower degrees of cross-linking. Similarly, in clPASP/GA-TD samples, variations in T₂ relaxation times were detected with increasing cross-linking from 15% to 20%. Specifically, T₂ relaxation times decreased at higher cross-linking levels, signifying a decrease in water mobility in the hydrogel as cross-linking intensity increased.^23–26

Figure 11.

T₂ relaxation times NMR measurements of PASP/GA cross-linked with various cross-linking agents for three different t: (a) 0.5 s, (b) 3 s and (c) 8 s.

Table 8.

NMR measurements of T₂ relaxation times (for three different times: 0.5, 3, and 8 s).

Sample name	T₂ (0.5S)	T₂(3S)	T₂(8S)
clPASP/GA-HMD 10,0	0.23	1.16	1.94
clPASP/GA-HMD 15,0	0.24	1.16	1.98
clPASP/GA-HMD 20,0	0.22	0.97	1.35
clPASP/GA-LYS 10,0	0.23	1.18	1.98
clPASP/GA-TD 15,0	0.21	1.07	1.82
clPASP/GA-TD 20,0	0.22	0.98	1.40

Morphological analysis

Figure 12 illustrates the morphology of hydrogels based on poly (aspartic-co-glutamic acid) with varying quantities of crosslinkers. The hydrogel exhibits a microporous structure, and as the proportion of hexamethylene diamine to poly (aspartic-co-glutamic acid) within the hydrogel rises, its pore size diminishes. This occurs because an excessive concentration of crosslinking agent results in an increased crosslinking density, consequently diminishing the hydrogel’s capacity to absorb water.^27,28

Figure 12.

Surface SEM images of (a) PASP/GA-HMD 10%, (b) PASP/GA-HMD 15% and (c) PASP/GA-HMD 20% hydrogels.

Conclusions

In conclusion, this study presents a biodegradable superabsorbent copolymer, merging D,L-aspartic acid (ASP) and L-glutamic acid (GA) through melt polymerization. Structural analyses confirm the copolymer’s unique physicochemical properties and efficient synthesis with yields reaching 81%. Strategic blending of ASP and GA capitalizes on their inherent advantages, resulting in a copolymer exhibiting remarkable water retention capabilities. The exploration of diamine cross-linking agents identifies optimal conditions, achieving a maximum swelling ratio of 11.874% with 10% hexamethylene diamine. Beyond contributing to copolymer synthesis and water absorption knowledge, this research stands out for its innovative combination of ASP and GA, offering a promising solution in the realm of sustainable superabsorbent materials. The copolymer’s potential applications underscore its significance, marking a noteworthy advancement in the pursuit of environmentally friendly alternatives to non-biodegradable polymers.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iDs

Youssef Hafidi

Hicham El Hatka

Markus Biel

Najim Ittobane

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Elaboration and characterization of biodegradable poly(aspartic-co-glutamic acid) hydrogels as soil additives with different crosslinkers

Abstract

Keywords

Introduction

Experimental section

Materials

Synthesis and optimization

Synthesis of pre-polymer

Selection of the molar ratio of ASP to GA

Selection of reaction time

Selection of the reaction temperature

Crosslinker synthesis

L-(+)-Tartaric Acid Dihydrazide crosslinker

Crosslinking and hydrolysis to clPASP/GA

Crosslinking by HMD

Crosslinking by LYS

Crosslinking by TD

Formation of PASP/GA Hydrogels through the Hydrolysis of clPSI/GA

Characterization methods

Determination of the copolymer yield

X-ray diffractometer

NMR analysis

FTIR-spectroscopic measurement

Swelling ratio

SEM observation

Results and discussion

Synthesis of PSI/GA1

X-ray diffractometer

NMR

Syntheses of tartaric acid dihydrazide (TD)

Characterization of PSI/GA, PASP/GA and the Chemical Structure of Hydrogels Using FT-IR Spectroscopy

Influence of the crosslinking agent concentration on the swelling rate of PASP/GA hydrogels

NMR measurements of T2 relaxation times for 0.5, 3, and 8 s

Morphological analysis

Conclusions

Footnotes

Declaration of conflicting interests

Funding

ORCID iDs

References

Synthesis of PSI/GA₁

NMR measurements of T₂ relaxation times for 0.5, 3, and 8 s