Synthesis, characterization, and swelling properties of a novel tapioca-g-Poly(Acrylic acid−2−acrylamido−2−methylpropane sulfonic acid)/ammonium polyphosphate superabsorbent polymer

In this study, a novel superabsorbent polymer tapioca starch-g-poly(acrylic acid-2-acrylamido-2-methylpropane sulfonic acid)/ammonium polyphosphate (TS-g-AA-AMPS/APP) was synthesized based on the graft copolymerization of tapioca starch (TS) with acrylic acid (AA) and 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS) via free radical polymerization in aqueous solution with ammonium polyphosphate (APP) additive added for study. The synthesized superabsorbent polymer material was characterized by Fourier transform infrared spectroscopy (FTIR), field emission scanning electron microscopy (FE-SEM), energy-dispersive x-ray spectroscopy (EDS), and thermogravimetric analysis (TGA). Research investigating the material synthesis conditions to absorbance in distilled water and 0.9 wt% NaCl solution has also been studied. Under optimal synthesis conditions, the absorbance in distilled water and 0.9 wt% NaCl solution were 416 g g−1 and 61 g g−1, respectively, for the sample with 3 wt% APP content. The introduction of APP units has improved the absorption properties of the material such as the water retention capacity reaching 55.25% in 10 h at 60 °C. TS-AA-AMPS/APP exhibits reversible swelling ability, with the swelling level not being reduced compared to the initial after 5 swelling-drying cycles at 60 °C, and water absorption reaches swelling equilibrium after about 270 min. Additionally, TGA thermogravimetric analysis results showed an improvement in the thermal stability of TS-AA-AMPS/APP compared to the pure polymer. These results show that the TS-AA-AMPS/APP polymer with excellent swelling properties, low production cost, and environmentally friendly has the potential for practical applications in agriculture, gardening, and water retention materials.


Introduction
Superabsorbent polymer (SAP) is a type of hydrogel with a three-dimensional polymer network system capable of absorbing and retaining large amounts of water or liquid [1].Because SAP contains hydrophilic functional groups such as -OH, -NH 2 , and SO 3 H, it can absorb water.In addition, the ability to absorb and retain water also depends on the level of cross-linking and porosity [1].SAP has received great attention due to their special features and are applied in many different fields such as agriculture [2], medical materials [3], hygienic products [4], wastewater treatment [5], food industry [6], construction materials [7,8], and chemical industry [9].However, environmental concerns have prompted the replacement of petroleum-based SAPs with bio-based SAPs [10], polysaccharides in particular have been used significantly to replace or combine with synthetic chemicals [1].
Starch is one of the most abundant polysaccharides, available in nature, low cost, biocompatible, biodegradable, renewable, and has good chemical stability and high response [11].In recent years, starch-based superabsorbent polymer materials due to their novel and attractive combination of properties have received increasing attention and development due to their many good properties such as degradability.bioavailability, biocompatibility, abundance, and low cost [1].Starch-based superabsorbent polymers have a wide variety of applications such as slow-release fertilizers, controlled pesticide release, water storage, road dust suppression, fire retardant, personal hygiene applications (diapers, sanitary napkinsK,), water treatment due to heavy metals, dye absorption, oil recovery agent [1,12,13].
Nowadays, the majority of SAPs produced use acrylic acid and its salts as starting materials applying several techniques such as solution polymerization or inverse suspension [1].Acrylic acid (AA, K a = 5.6 × 10 −5 ) is a fairly cheap and common monomer [14], and 2-acrylamide-2-methyl propane sulfonic acid (AMPS) are anionic monomers consisting of a hydrophilic sulfonic acid group (K a = 2 × 10 −1 ) and a nonionic amide group [15], in which the non-ionic amide group gives the polymer better salt tolerance has attracted attention due to its stability, non-toxicity, and low cost [16].The combination of both ionic hydrophilic groups (carboxyl groups, sulfonic acid groups, tertiary amine groups, etc) and nonionic hydrophilic groups (hydroxyl groups, ester groups, amide groups, etc) greatly improves the ability of salt tolerance and water absorption rate of the polymer [17].Yeqiao Meng and Lin Ye have synthesized superabsorbent polymers based on starch and AA, AMPS has shown several advantages such as improved water absorption, swelling rate, high water retention, and salt tolerance [18,19], Elżbieta Czarnecka and co-workers showed that the -SO 3 H groups present in the AMPS groups of the potato starch-based superabsorbent polymer PVA/PS-g-P(AA-co-AM-co-AMPS) are responsible for the adsorption capacity higher water absorption than PVA/PS-g-P(AA-co-AM ) [20].A semi-interlayer polymer network (semi-IPN) was synthesized by Ganguly et al [16] through the addition of AA and AMPS to starch, the semi-IPN hydrogel can quickly absorb water into the gel network and has the property of adjusting swelling when the pH responsiveness in ambient conditions.Starch-based superabsorbent polymers were prepared by gamma-ray co-irradiation, the introduction of AM and AMPS monomers into starch-g-P(AA-co-AM-AMPS) polymer, where the ratio AA: AMPS ratio is 3:1, increasing salt tolerance from 46 g g −1 (without AMPS) to 81 g g −1 [21].Parisa Moharrami and Elaheh Motamedi [22] developed a bio-based hydrogel nanocomposite material MCNC/starch-g-(AMPS-co-AA) with magnetically functionalized cellulose nanocrystals (MCNC) additive.This material has excellent adsorption capacity with high selectivity, used as an effective and environmentally friendly nano-adsorbent for cationic dye removal.A starch/kaolin-g-(AA-AM-AMPS) superabsorbent polymer was prepared by Wei Aifen and co-workers [23] by ultrasound-assisted secondary polymerization.Studies on the ability to absorb water containing copper sulfate and iron sulfate show that the material can absorb Cu 2+ and Fe 3+ ions.This result shows the potential application of starch/kaolin-g-(AA-AM-AMPS) superadsorbent polymer in the field of wastewater treatment.
Water-soluble ammonium polyphosphate (APP) is an inorganic polymer (low degree of polymerization, n < 20) with outstanding water solubility, non-toxic, and biodegradability.APP is an important raw material in water-soluble, chelated, and controlled-release fertilizers [24], flame retardants [25,26], emulsifiers, and food additives, and are low cost [27,28].According to our observation, studies on the introduction of APP into superabsorbent polymer networks are rarely reported, Wang and co-workers have synthesized new semiinterlayer polymer network (semi-IPN) superabsorbent polymers with slow-release fertilizer based on corn straw cellulose polymers (CSC-g-AA/APP, CSC-g-AA/PVA-APP) [29], and millet straw-based polymer (MSPg-AA/PVA-APP) [30], prepared by solution polymerization has very good adsorption capacity in distilled water, helping to enhance soil water holding capacity and slow release of nutrients.Zheng et al [26] introduced ammonium polyphosphate (APP) and flame-retardant woven basalt fibers into polyacrylamide (PAAm) hydrogel to improve flame-retardant performance.The results indicate that high APP content can prevent dehydration and thermal decomposition of gels at high temperatures.Junping Zhang et al [31] phosphorylated starch and incorporated attapulgite into starch phosphate-graft-acrylamide/attapulgite superabsorbent composite and showed that it can significantly improve water absorption capacity, swelling rate, and salt resistance properties of materials.These show that APP can be used as an inorganic filler to improve the thermal stability, as well as the swelling properties of the material.Furthermore, the introduction of inorganic fillers such as APP into the polymer also reduces production costs [32].
Based on the above observations, we report the investigation and synthesis of a novel superabsorbent polymer by graft copolymerization of tapioca starch with AA and AMPS monomers in the presence of APP.Characteristics of superabsorbent polymers such as absorption ability in distilled water and 0.9 wt% NaCl solution, water retention capacity, reswelling ability, swelling rate, and thermal stability have also been researched.This work promises to provide a novel superabsorbent polymer that has excellent absorption properties, low production costs, abundant raw materials, availability, and environmental friendliness thus allowing them to become practical in agriculture, horticulture and water retention materials.

Preparation of TS-g-AA-AMPS/APP superabsorbent polymer
In a 250 ml three-necked flask with mechanical stirring, a mixture of tapioca starch in distilled water was stirred and heated at 90 °C, gelatinized under a stream of nitrogen for 30 min, and then cooled to 55 °C.Then, AA (with a neutralization degree of 65 mol% by 40 wt% NaOH solution) and 40 wt% AMPS were added respectively.Next, KPS was added and stirred for 15 min at the same temperature, and then MBA and different weight percentages of APP were added.The reaction mixture was slowly heated to 70 °C and maintained for 2 h.The product was soaked in acetone to remove water and unreacted monomers.Finally, the product is dried in an oven to a constant weight at a temperature of 50 °C.The proposed reaction mechanism of the TS-g-AA-AMPS/ APP superabsorbent polymer is shown in figure S1.

Characterization methods
FTIR spectra of analyzed samples were recorded with an onboard Fourier transform infrared (FT-IR) spectrometer (FTIR Affinity 1S, Shimadzu, Japan), using the KBr pressing method.FTIR spectra were collected from 4000 to 400 cm −1 with 20 scans in each case, at a resolution of 4 cm −1 .Morphological analysis and elemental composition of tapioca starch and superabsorbent polymer samples observed by field emission scanning electron microscopy (FE-SEM) combined with energy-dispersive x-ray spectroscopy (EDS) with equipment (JSM-IT800/JEOL, Japan).TGA thermogravimetric analysis of the samples (13-18 mg) was performed on a platinum pan under an argon atmosphere, heating rate of 10 °C min −1 , in the temperature range from 25 − 600 °C on a DTG-60H machine from Shimadzu, Japan.

Water absorption capacity
Accurately weigh 0.1 g of dry sample (particle size 60-80 mesh) and put it in a tea bag (100 mesh nylon screen) soaked in excess distilled water and 0.9% NaCl solution until it reaches the state swelling balance for 5 h [19,33].Samples were removed from excess water on the surface using filter paper and weighed.The adsorption capacity of the sample in distilled water or 0.9% NaCl solution by mass is determined as follows: Where, M (g) and M 0 (g) are the mass of the sample reaching swelling equilibrium and the dry sample, respectively [34].

Swelling rate
The tea bag (with 100 mesh nylon screen) containing an exact amount of 0.1 g of sample (particle size 60-80 mesh) was soaked in excess distilled water.After swelling, the sample was removed from excess water with filter paper and weighed.The absorption capacity of the samples was calculated according to formula (1) over a specified period of time to plot the swelling rate [19,33].

Water retention
Accurately weigh 100 g of sample swollen in distilled water, then the sample is placed in an air oven at a temperature of 60 °C.The water retention rate (WR) of the superabsorbent polymer was measured every 1 h at 60 °C according to equation (2).In which, W t is the mass of the sample after drying for different times and W 0 is the initial weight of the swollen sample [19,34].
Re-swelling capacity 0.2 grams of dry polymer sample was immersed in an excess of distilled water until it reached swelling equilibrium, removing unabsorbed water.The absorbance of superabsorbent polymer is calculated according to formula (1).Then, the swollen sample was dried in a 60 °C oven to constant weight.Repeat the above process 5 times and calculate the water absorption capacity of each 'swelling-drying' cycle [35,36].

FTIR spectroscopy
The structure of TS-g-AA-AMPS/APP was confirmed by FTIR analysis, as shown in figure 1.On the IR spectrum of tapioca starch (curve (a)) [37], The absorption maxima at 3299 cm −1 and 2931 cm −1 are attributed to the stretching vibrations of -OH and C-H, respectively.The peaks at 1149, 1077, and 995 cm −1 are attributed to the stretching vibration of C-O-C.
In the spectrum of TS-g-AA-AMPS (curve (b)), in addition to the typical absorption peak of starch, new peaks at 1701 cm −1 , 1654 cm −1 , and 1558 cm −1 were also found which are considered to be stretching vibrations, symmetric stretching vibrations and asymmetric stretching vibrations of carboxyl groups (COO−) [19], indicates the insertion of AA into the starch chain.The new peaks at 1303 cm −1 , 1187 cm −1 , and 1044 cm −1 are attributed to the asymmetric and symmetric stretching vibrations of the −SO 3 − group [19,20,38], indicating the insertion of AMPS into the starch chain.
In the spectrum of TS-g-AA-AMPS/APP (curve (c)), in addition to the absorption peaks already found in TS-g-AA-AMPS, new peaks at 853 cm −1 và 883 cm −1 appear that are assigned for the asymmetric stretching vibration of P-O [27], which indicates the existence of APP (curve (d)) in TS-g-AA-AMPS/APP.Unfortunately, the peak at about 1254-1209 cm −1 [27,29] assigned to the P=O stretching vibration of APP could not be observed due to the overlap with the characteristic absorption peaks of the −SO 3 − group in the range of 1294-1156 cm −1 [19,20,38].In addition, the peak at 1250 cm −1 assigned to the P=O stretching vibration of APP could not be observed because APP (n < 20) is highly water soluble with a low degree of polymerization and has the ability to absorb and water hydrolysis, which was also observed in the work of Kai Zhang et al [25].

FESEM and EDX analysis
Surface morphology including porosity and average pore size play an important role in the water absorption ability of superabsorbent polymer materials.The material has a porous microstructure that increases the surface area and capillary effect leading to increased water absorption [34].Therefore, one of the most important properties of superabsorbent polymers is the surface microstructural morphology.FE-SEM micrographs of TS, TS-g-AA-AMPS, and TS-g-AA-AMPS/APP are shown in figure 2. It can be seen that TS has a smooth and dense surface morphology with a sphere or polygon.The surface of TS-g-AA-AMPS appears to be relatively smooth, with many wavy and holes appearances.Compared with TS-g-AA-AMPS, the surface of TS-g-AA-AMPS/APP prepared by adding APP showed a rough surface, and many wavy, cavities and sockets.This rough surface, many folds, sockets, and cavities facilitate the absorption of water molecules [34,39].These results indicate that APP was embedded into the polymer matrix as physical cross-linking points.
The EDX spectrum of TS shows two peaks of C (carbon) and O (oxygen) present in the structure of cassava starch.In the case of TS-g-AA-AMPS, in addition to the peaks of C (carbon) and O (oxygen), additional peaks of Na (sodium) and S (sulfur) appear originating from monomers in the structural component.Finally, as expected TS-g-AA-AMPS/APP in addition to the similar elemental compositions TS-g-AA-AMPS includes peaks of C, O, Na, and S. A new peak corresponding to P (Phosphorus) appears, indicating the presence of APP distributed in the polymer matrix.Thus, FE-SEM and EDX analysis data indicate the surface morphology and elemental composition but also demonstrate the formation of TS-g-AA-AMPS/APP.

Thermogravimetric analysis
Thermal stability is an important factor to consider when developing biodegradable superabsorbent polymers for practical applications.The TG and DTG curves of TS, TS-g-AA-AMPS, and TS-g-AA-AMPS/APP (1-3 wt%) ).The thermogram of TS-g-AA-AMPS/APP with 1 wt%, 2 wt%, and 3 wt% APP content was similar to the thermal decomposition of TS-g-AA-AMPS without APP, in which the weight loss process can be divided into three main stages of weight loss.The first stage occurs at a temperature range of 25 °C-250 °C due to the evaporation of water that may be present in the polymer network [40] and the dehydration of acrylic acid to form acid anhydride [41].A large weight loss occurs in the second stage of 200 °C-400 °C, which is thought to be due to the decomposition of starch in the polymer, the removal of CO 2 molecules from the polymer backbone and the amide side group of AMPS, as well as the disintegration of the amide side group of the cross-linker [42,43].In the third stage from 400 °C-600 °C main chain decomposition, anhydride formation removes water molecules from two neighboring carboxylic groups of the polymer chain, destroying the cross-linked network structure, and possibly part of the material APP [42].The TGA curve shows that the thermal stability of polymers containing APP TS-g-AA-AMPS/APP (1, 2, 3 wt%) with residual carbon content is 39.53%, 39.55%, and 42.34%, respectively higher than the pure sample TS-AA-AMPS with a residual carbon content of 35.63%.It can be seen that the sample containing APP (3 wt%) improves thermal stability significantly compared to the sample with lower APP content.Therefore, the sample TS-AA-AMPS/APP (3 wt%) was chosen to determine the swelling properties of the superabsorbent polymer.

Investigation of synthesis conditions 3.4.1. Effect of TS content
Figure 4(a) shows the effect of TS content on the ability of the superabsorbent polymer to absorb distilled water and salt water.Figure 4(a) shows that the absorbance of distilled water and 0.9 wt% NaCl solution have similar trends.The absorbance initially increased and then gradually decreased as the starch content increased.The best absorbance in distilled water and 0.9 wt% NaCl solution was achieved with a starch content of 15 wt%.At low starch content (10 wt%), not enough starch radicals are produced to effectively connect to the polymer chain, leading to an incomplete reaction.Additionally, the excess of monomers relative to starch leads to an increase in the homopolymer percentage, which in turn leads to an increase in soluble materials [44] which also contributes to the low absorbance of the material.As starch weight increases, both the grafting and molecular weight of the polymer increase, resulting in increased water absorption.However, when the TS content is greater than 15 wt%, the number of active sites formed on the starch chain increases, promoting the formation of short chains, and leading to increased cross-linking density, thus reducing the water absorption rate of the polymer [45].

Effect of AMPS content
The effect of AMPS content on the ability of the superabsorbent polymer to absorb distilled water and 0.9 wt% NaCl solution is shown in figure 4(b).The results show that the polymer's ability to absorb distilled water and salt water increases when increasing the AMPS content from 25% to 40% by weight.After reaching the maximum absorption with 40 wt% AMPS content, the absorption efficiency decreased with a further increase in AMPS content.The sulfonic acid group (-SO 3 H) of AMPS has a higher ionization constant than the carboxyl groups (-COOH) of AA, leading to higher hydrophilicity, in addition, the amide group (-CONH 2 ) has better salt tolerance.better due to the carboxyl group (-COOH) of AA [19].Therefore, increasing the AMPS content increases the polymer's ability to absorb distilled water and salt water.However, when the AMPS content increases beyond 40% by weight, there are too many hydrophilic groups and hydrogen bond interactions between large molecules [34], and the increase in cross-linking structure leads to shrinkage of the polymer network, reducing absorption capacity [39].Additionally, an increase in repulsion between charges leads to the deformation of the polymer chain [15].

Effect of initiator content
The effect of initiator concentration on the adsorption capacity of the polymer is shown in figure 4(c).As the initiator concentration increases, the water absorption initially increases and then decreases.This can be explained as follows, with an initiator concentration lower than 0.8 wt% that produces less free radical particles and therefore less active free radical sites, the reaction can proceed.not completely, leading to a reduced ability to absorb distilled water as well as salt water.With an initiator concentration greater than 0.8 wt%, the number of active free radicals increases, leading to an intensification of the reaction and premature termination of chain growth, the chain length will be shorter and the polymer structure network size will be small, leading to reduced absorbance [18,34].

Effect of MBA content
The effect of MBA cross-linking content on water absorption is shown in figure 4(d).With an MBA content of less than 0.03% by weight, there are few cross-linking points leading to the network of the superabsorbent polymer not being able to be completely formed, the superabsorbent polymer will be semi-soluble and water absorption can hardly be measured accurately [44].The absorption capacity in distilled water and brine gradually decreased as the MBA concentration increased from 0.03 wt% to 0.12 wt%.The reason for this result is that increasing the concentration of cross-linking agents leads to increased cross-linking density, thereby reducing the size of the polymer network, and leading to reduced absorption capacity.
3.4.5.Effect of neutralization level of AA Figure 4(e) shows the effect of the neutralization level of AA on the polymer's ability to absorb distilled water and 0.9 wt% NaCl solution.When the neutralization level increases from 45 to 65%, water absorption increases.The reason is that after AA is neutralized by NaOH, the −COOH group is neutralized to −COO − , leading to increased electrostatic repulsion, increasing the spatial network size of the polymer, leading to increased water absorption capacity [18].However, when the neutralization level of AA is greater than 65%, the negative charges of −COO − are strongly shielded by excess Na + , reducing the electrostatic repulsion [46] and hindering the elasticity of the cross-linked network.On the other hand, when increasing the degree of AA neutralization, many −COOH groups are neutralized.These neutral groups have less reactivity, so the reaction efficiency decreases, leading to a decrease in absorption capacity [18].

Effect of APP content
The effect of APP content on the absorption properties of the polymer is shown in figure 4(f).APP content increased from 0 wt% to 3 wt%.In general, samples containing APP (1%-3%) had higher absorbance in distilled water and 0.9 wt% NaCl solution than the superabsorbent polymer without APP and the absorbance was highest with the amount of APP 1% by weight.Absorption in distilled water and 0.9% NaCl solution was 472 g g −1 and 70 g g −1 , respectively.This may be because APP particles with a low degree of polymerization (n < 20) are hydrophilic [24] and electrical current in the polymer structure network when exposed to water molecules will be ionized into ammonium ions ( NH 4+ ) and polyphosphate ions ([PO 3 ] n-) have strong polarity [24,47,48] leading to increased hydrogen bonding with water molecules leading to increased absorption.As the APP content increases, the absorption capacity gradually decreases, because the addition of APP increases the crosslink density in the structural network, leading to a decrease in water absorption capacity.At an APP content of 3 wt%, the absorption in distilled water is 416 g g −1 , and 0.9 wt% NaCl solution is 61 g /g .
3.5.Swelling properties of superabsorbent polymers 3.5.1.Water retention Figure 5 shows that the water retention efficiency at 60 °C gradually decreases over time.The water retention rates of TS-AA-AMPS and TS-AA-AMPS/APP were 48.18% and 55.25%, respectively, after 10 h at a temperature of 60 °C, indicating that TS-g-AA-AMPS/APP has a lower water evaporation rate and also high water retention capacity than the St-g-AA-AMPS sample under the same conditions.As pointed out in previous literature [18,49], hydrogen bonding interactions with water of the superabsorbent molecule and Van der Wall's force affect the water retention performance of the polymer.APP present in the polymer, when exposed to water, will add a strongly polarized NH 4 + ion [47] that will interact strongly with water molecules.In addition, the included APP increases the cross-linking density leading to hindering water evaporation and thus improving water retention.

Re-swelling capacity
Re-swelling ability is one of the most important properties for practical application.The reswelling ability of the superabsorbent polymer at 60 °C of TS-AA-AMPS and TS-AA-AMPS/APP is shown in figure 6.After 5 reswelling cycles, the TS-AA-AMPS/APP polymer showed better water absorption compared to the material without APP (TS-AA-AMPS) (figure 7).The water absorption capacity of TS-AA-AMPS/APP increased from cycle 1 to cycle 2 but then decreased.This can be attributed to the fact that during the first swelling-deswelling cycle the polymer network is completely expanded, then impurity ions are removed leading to increased water absorption [34,50].On the other hand, because the swollen polymer is dried in the oven under hightemperature conditions, the polymer structure is degraded to a certain extent and thus reduces its ability to absorb water [51].However, TS-AA-AMPS/APP still retains its outstanding water absorption ability and is not reduced compared to the initial after 5 re-swelling cycles, while the water holding ability after 5 re-swelling cycles of polymer without APP decreased to 83.68% compared to the initial.This improvement may be due to the presence of APP in the polymer which is responsible for a better polymer composite network.The better networks prevented the loss of absorbed water during repeated swelling of the synthetic polymer [52,53].This sample TS-AA-AMPS/APP has excellent recovery ability, showing potential application as a recyclable superabsorbent polymer material in horticulture and agriculture [53].

Swelling rate
The swelling ratio of TS-g-AA-AMPS and TS-AA-AMPS/APP as a function of swelling time is shown in figure 7.
Figure 7 shows, both TS-g-AA-AMPS and TS samples -g-AA-AMPS /APP both absorb water quickly in the early stages.The swelling rate gradually increases over time, the time to reach equilibrium for TS-AA-AMPS/APP is about 4.5 h (270 min) while the time to reach equilibrium for TS-AA-AMPS is about 5.5 h.

Conclusion
In summary, a novel superabsorbent polymer tapioca starch-g-AA-AMPS/APP was synthesized based on the aqueous graft copolymerization of tapioca starch with AA and AMPS monomers, using MBA as cross-linker, KPS as initiator with APP additive with different content were included for research.The optimal conditions for the polymerization reaction were also shown with tapioca starch content (15 wt%), KPS content (0.8 wt%),  MBA content (0.03 wt%), neutralization level (65 mol%), APMS content (40 wt%), and APP content (3 wt%).Synthetic polymer materials obtained under optimal conditions have an absorbance in distilled water and 0.9 wt% NaCl solution of 416 g g −1 and 61 g g −1 , respectively, corresponding to APP content (3 wt%).The physical characteristics of the polymer were confirmed through FE-SEM, EDS, FT-IR, and TGA methods.The introduction of APP units has improved many important properties of superabsorbent polymer such as increased thermal stability, water retention capacity of superabsorbent polymer is up to 55.25%, after 10 h at 60 °C, excellent reswelling ability after 5 times of absorbing and releasing water at 60 °C, and water absorption reaches swelling equilibrium in about 4.5 h (270 min).Therefore, it can be concluded that this novel superabsorbent polymer material has good distilled water and saltwater absorption properties, excellent water retention and resorption capacity, and improved thermal stability.In addition, with the advantage of raw materials such as tapioca starch and APP having low production costs and environmentally friendly properties, it promises great application potential usefulness in agricultural, horticulture, and water retention materials.

Figure 4 .
Figure 4. Effect of synthesis conditions on the absorption capacity of superabsorbent polymer: Effects of (a) TS content, (b) AMPS content, (c) APS content, (d) MBA content, (e) neutralization level of AA, and (f) APP content.