Preparation and Properties of Attapulgite/Brucite Fiber-Based Highly Absorbent Polymer Composite

The ATP-BF-P(HEC-AA-AMPS) composite highly absorbent polymer was copolymerized with acrylic acid (AA) and 2-acrylamido-2-methylpropane sulfonic acid (AMPS) using an aqueous solution method with attapulgite (ATP) and attapulgite (ATP) as a matrix. The prepared ATP-BF-P(HEC-AA-AMPS) was characterized in terms of microstructure and tested for its water absorption capacity, water retention properties, and pH dynamic sensing ability. The results showed that the synthesized ATP-BF-P(HEC-AA-AMPS) had a rough and porous surface and a high water absorption capacity and rate, almost reaching the maximum water absorption around 20 min, and demonstrated excellent water retention performance at low and medium temperatures. ATP-BF-P(HEC-AA-AMPS) has a sensitive dynamic sensing ability in different pH solutions, with a high swelling capacity between pH 6.0 and 10.0. When the pH value exceeded 10.0, the swelling rate decreased rapidly. Additionally, the thermal stability and mechanical strength of the highly absorbent polymers were significantly improved after blending with ATP and BF.


Introduction
The shortage of freshwater resources is a significant concern in China and worldwide.In recent years, China's rapid development and urbanization have significantly increased the demand for freshwater.In addition, a large amount of agricultural irrigation, industrial water use, domestic water use, etc., makes freshwater resources even scarcer.And at the same time, it produces a series of problems, such as water pollution, which has a significant impact on the sustainable development of the economy and society [1,2].Therefore, how to use water scientifically, so that the limited water resources are efficiently and rationally applied, has aroused widespread concern among researchers.Highly absorbent polymer is a novel functional polymer material that comprises numerous hydrophilic groups and a mildly cross-linked network structure, resulting in outstanding water absorption and retention capabilities [3][4][5].Highly absorbent polymers can absorb and retain a significant amount of water, even in challenging conditions such as specific pressure, saline, and alkaline environments [6].Their weight often exceeds hundreds or even thousands of times when compared to conventional water-absorbent materials [7][8][9].Thus, they are also known as super-absorbent or high-water-holding agents, and find wide applications in agriculture, forestry, gardening, construction, sanitary products, food, medicine, and other fields [10][11][12][13].
Highly absorbent polymers mainly include natural and modified highly absorbent polymers and synthetic highly absorbent polymers.Natural and modified polymers include starch, cellulose, and other natural products.Synthetic highly absorbent polymers include polyvinylates, polyvinyl alcohols, polyoxyethylene, etc. [14].The first highly absorbent polymer was reportedly prepared by Fanta et al. [15] in 1971 at the Northern

Preparation of ATP-BF-P(HEC-AA-AMPS) Highly Absorbent Polymer
The appropriate amounts of 0.45 g HEC, 0.20 g ATP, 0.05 g SDS, 0.1 g BF, and 30 mL of deionized water were weighed and placed into a three-necked flask under nitrogen protection.The mixture was stirred 70 • C for 45 min to produce a dispersed emulsion, then cooled down to 40 • C, and 5 mL of 8 wt% APS solution was added dropwise.The mixture was stirred for 10 min, then 70% of neutralized AA, 1.42 g APMS, and 0.042 g MBA mixed solution were added.The temperature was raised to 70 • C and the reaction continued for three hours.After completion, the reaction mixture was soaked in ethanol solution for one hour to remove unreacted monomers.The product was transferred to a drying oven at 60 • C until a stable weight was achieved.The dried material was crushed with a pulverizer to produce ATP-BF-P(HEC-AA-AMPS) high-water-absorption polymer powder particles.Figures 1 and 2 show the molecular structure of HEC and the reaction mechanism of the polymerization reaction.

Preparation of ATP-BF-P(HEC-AA-AMPS) Highly Absorbent Polymer
The appropriate amounts of 0.45 g HEC, 0.20 g ATP, 0.05 g SDS, 0.1 g BF, and 30 mL of deionized water were weighed and placed into a three-necked flask under nitrogen protection.The mixture was stirred 70 °C for 45 min to produce a dispersed emulsion then cooled down to 40 °C, and 5 mL of 8 wt% APS solution was added dropwise.The mixture was stirred for 10 min, then 70% of neutralized AA, 1.42 g APMS, and 0.042 g MBA mixed solution were added.The temperature was raised to 70 °C and the reaction continued for three hours.After completion, the reaction mixture was soaked in ethano solution for one hour to remove unreacted monomers.The product was transferred to a drying oven at 60 °C until a stable weight was achieved.The dried material was crushed with a pulverizer to produce ATP-BF-P(HEC-AA-AMPS) high-water-absorption polymer powder particles.Figures 1 and 2 show the molecular structure of HEC and the reaction mechanism of the polymerization reaction.

SEM
To better understand the microscopic structure of ATP-BF-P(HEC-AA-AMPS) highl water-absorbent polymers, the surface morphology of the dried polymer powders wa examined using a scanning electron microscope.

EDS
The elemental content of the samples was measured using EDS analysis to determin whether ATP and BF were successfully introduced into the polymer.

SEM
To better understand the microscopic structure of ATP-BF-P(HEC-AA-AMPS) highly water-absorbent polymers, the surface morphology of the dried polymer powders was examined using a scanning electron microscope.

EDS
The elemental content of the samples was measured using EDS analysis to determine whether ATP and BF were successfully introduced into the polymer.

FT-IR
The solid sample was combined with KBr at a ratio of 1:100 by mass, then crushed into a fine powder and compressed into transparent sheets.The inferable spectra of the resulting samples were analyzed using a Fourier transform inferable spectrometer across the range of 4000-400 cm −1 .

Thermodynamic Analysis
The polymer was subjected to thermodynamic analysis utilizing a synchronized thermal analyzer test, conducted in a temperature range of 30 to 600 degrees Celsius with a temperature ramp of 10 degrees Celsius per minute, and carried out in a nitrogenprotected atmosphere.

Measurement of Swelling Behavior
During the experiment, a custom filter bag was used to contain 0.1 g of the polymer sample, which was then submerged in a beaker filled with 500 mL of deionized water, tap water, a 0.9% NaCl solution, and a saturated Ca(OH) 2 solution.The specimens were left to equilibrate at temperatures of 20 • C, 30 • C, 40 • C, and 50 • C. Following this, the polymer weight was recorded at intervals of 1, 2, 5, 7, 10, 15, 30, 60, 90, and 120 min.The water absorption ratio was then determined for each temperature by applying a specific equation.
where M 1 and M 2 represent the mass of the dry and absorbent polymer, measured in grams, respectively; and Q stands for the water absorption multiplicity, measured in g/g [20].

Determination of Water Retention at Different Temperatures
In the experiment, 0.1 g of the polymer sample was weighed, placed into deionized water to reach dissolution equilibrium, considered, and placed into an environmental chamber at 20 • C, 30 • C, 40 • C, and 50 • C. Weighing was performed at intervals, and the water retention multiplicity, R, was calculated at different temperatures using the equation: where M 1 represents the mass of the dried polymer, measured in grams; M 2 represents the mass of the polymer after reaching dissolution equilibrium, measured in grams; and M 3 represents the mass of the polymer after water loss, measured in grams [21].

pH Dynamic Perception Test
In this step, 0.1 g of polymer sample was weighed and immersed in 0.9% NaCl solution.The pH value, using hydrochloric acid and NaOH solution, was adjusted.Then, the swelling multiplication rates were measured at different pH values.

Mechanical Test
The unformed polymer was placed into a 2 mL centrifuge tube and taken out after the polymerization process was complete.A cylindrical sample with a 10.4 mm diameter was created, and the compression characteristics of the water-absorbent polymer were analyzed using a mass spectrometer with a compression speed of 2 mm/s [22,23].

SEM
SEM images of HEC, P(HEC-AA-AMPS), and ATP-BF-P(HEC-AA-AMPS) are presented in Figure 3, respectively.Hydroxyethyl cellulose (HEC) exhibited an extensive and substantial distribution with a relatively smooth surface, while the surface of P(HEC-AA-AMPS) featured a porous structure with smooth pore walls and an uneven size distribution.Following the incorporation of ATP and BF, the surface of the highly absorbent polymer underwent substantial roughening, featuring numerous tiny pores and significant porosity, signifying the successful involvement of ATP and BF in the polymerization reaction.The porous structure facilitated the infiltration of water molecules into the interior of the material's three-dimensional network, thereby enhancing its water absorption properties.
SEM images of HEC, P(HEC-AA-AMPS), and ATP-BF-P(HEC-AA-AMPS) are presented in Figure 3, respectively.Hydroxyethyl cellulose (HEC) exhibited an extensive and substantial distribution with a relatively smooth surface, while the surface of P(HEC-AA-AMPS) featured a porous structure with smooth pore walls and an uneven size distribution.Following the incorporation of ATP and BF, the surface of the highly absorbent polymer underwent substantial roughening, featuring numerous tiny pores and significant porosity, signifying the successful involvement of ATP and BF in the polymerization reaction.The porous structure facilitated the infiltration of water molecules into the interior of the material's three-dimensional network, thereby enhancing its water absorption properties.

EDS
P(HEC-AA-AMPS) and ATP-BF-P(HEC-AA-AMPS) were subjected to EDS tests at the same scanning magnification mainly to measure the changes in the content of the elements C, O, Al, Mg, and Si.Table 3 shows the results of the elemental tests, and Figure 4 shows the EDS energy spectra.It was found that the content of Mg and Si elements in the modified highly absorbent polymer increased significantly, indicating that ATP and BF were successfully doped into the highly absorbent polymer.

EDS
P(HEC-AA-AMPS) and ATP-BF-P(HEC-AA-AMPS) were subjected to EDS tests at the same scanning magnification mainly to measure the changes in the content of the elements C, O, Al, Mg, and Si.Table 3 shows the results of the elemental tests, and Figure 4 shows the EDS energy spectra.It was found that the content of Mg and Si elements in the modified highly absorbent polymer increased significantly, indicating that ATP and BF were successfully doped into the highly absorbent polymer.substantial distribution with a relatively smooth surface, while the surface of P(HEC-AA-AMPS) featured a porous structure with smooth pore walls and an uneven size distribution.Following the incorporation of ATP and BF, the surface of the highly absorbent polymer underwent substantial roughening, featuring numerous tiny pores and significant porosity, signifying the successful involvement of ATP and BF in the polymerization reaction.The porous structure facilitated the infiltration of water molecules into the interior of the material's three-dimensional network, thereby enhancing its water absorption properties.

EDS
P(HEC-AA-AMPS) and ATP-BF-P(HEC-AA-AMPS) were subjected to EDS tests at the same scanning magnification mainly to measure the changes in the content of the elements C, O, Al, Mg, and Si.Table 3 shows the results of the elemental tests, and Figure 4 shows the EDS energy spectra.It was found that the content of Mg and Si elements in the modified highly absorbent polymer increased significantly, indicating that ATP and BF were successfully doped into the highly absorbent polymer.

FT-IR
In Figure 5, the infrared spectra of hydroxyethyl cellulose (HEC), P(HEC-AA-AMPS), and ATP-BF-P(HEC-AA-AMPS) are shown.The infrared spectrum of HEC is displayed in curves a, showing the flat vibrational peak of C-OH at 1334 cm −1 .The distinct peak at 2926 cm −1 is assigned to the stretching vibration of aliphatic C-H in HEC [24], which is also apparent in graphs b and c.The anticipated peak at 3449 cm −1 , corresponding to the telescopic vibration of O-H on HEC, exhibits lower intensity in comparison to curves b and c.This phenomenon arose because the characteristic peaks in curves b and c resulted from the combined effects of O-H and N-H stretching vibrations [25,26].Within curves b and c, distinctive features include the telescopic vibration peaks of O=S at 627 cm −1 and 623 cm −1 , the asymmetric telescopic vibration peaks of C-O-C at 1035 cm −1 and 1044 cm −1 , and the asymmetric telescopic vibration peaks of COO-at 1408 cm −1 and 1411 cm −1 .These characteristic peaks signify the successful polymerization of P(AA) and P(AMPS) into the HEC backbone [27].Additionally, the peaks observed at 800 cm −1 in curve c correspond to the bending vibrational peaks of Si-O-Si, while the 2974 cm −1 peaks for C-H stretching in BF confirm the effective incorporation of inorganic materials into the HEC polymer [28].
In Figure 5, the infrared spectra of hydroxyethyl cellulose (HEC), P(HEC-AA-AM and ATP-BF-P(HEC-AA-AMPS) are shown.The infrared spectrum of HEC is displaye curves a, showing the flat vibrational peak of C-OH at 1334 cm −1 .The distinct peak at 2 cm −1 is assigned to the stretching vibration of aliphatic C-H in HEC [24], which is a apparent in graphs b and c.The anticipated peak at 3449 cm −1 , corresponding to the t scopic vibration of O-H on HEC, exhibits lower intensity in comparison to curves b an This phenomenon arose because the characteristic peaks in curves b and c resulted fr the combined effects of O-H and N-H stretching vibrations [25,26].Within curves b an distinctive features include the telescopic vibration peaks of O=S at 627 cm −1 and 623 cm the asymmetric telescopic vibration peaks of C-O-C at 1035 cm −1 and 1044 cm −1 , and asymmetric telescopic vibration peaks of COO-at 1408 cm −1 and 1411 cm −1 .These cha teristic peaks signify the successful polymerization of P(AA) and P(AMPS) into the H backbone [27].Additionally, the peaks observed at 800 cm −1 in curve c correspond to bending vibrational peaks of Si-O-Si, while the 2974 cm −1 peaks for C-H stretching in confirm the effective incorporation of inorganic materials into the HEC polymer [28].

Thermal Stability Analysis
The TG and DTG curves of ATP, BF, P(HEC-AA-AMPS), and ATP-BF-P(HEC-A AMPS) are shown in Figure 6.From the figure, the thermal stability of BF and ATP w much higher than that of organic materials, with only 17.47% and 14.7549% mass los 600 °C.The thermal decomposition process of P(HEC-AA-AMPS) and ATP-BF-P(H AA-AMPS) was divided into four stages, corresponding to temperatures of 30 to 170 170 to 340 °C, 340 to 417 °C, 417 to 600 °C, 30 to 200 °C, 200 to 374 °C, 374 to 467 °C, 467 to 600 °C. the temperatures corresponding to the maximum decomposition rate w 377 °C and 395 °C, respectively.The decomposition process of composite water-absorb materials unfolded across distinct stages.The first stage primarily involved the decom sition of water and unreacted raw materials within these composites.Significant deco position took place primarily in the second and third stages.The second stage invol the decomposition of small molecules, like oligomers, present in the material, while third stage was characterized by the decomposition of branched chains within the po mer.Subsequently, in the fourth stage, mass reduction took place as a result of the bre down of the polymer's main chains and the disintegration of its three-dimensional work structure.
Comparing the TG plots of P(HEC-AA-AMPS) and ATP-BF-P(HEC-AA-AMPS can be found that the thermal stability of ATP-BF-P(HEC-AA-AMPS) was significan better than that of the second one in the remaining stages, except for the first stage, wh was similar to that of P(HEC-AA-AMPS), and the remaining sample masses of the after 600 °C were, respectively, 5.18% and 22.76%.These findings demonstrate that A

Thermal Stability Analysis
The TG and DTG curves of ATP, BF, P(HEC-AA-AMPS), and ATP-BF-P(HEC-AA-AMPS) are shown in Figure 6.From the figure, the thermal stability of BF and ATP was much higher than that of organic materials, with only 17.47% and 14.7549% mass loss at 600 • C. The thermal decomposition process of P(HEC-AA-AMPS) and ATP-BF-P(HEC-AA-AMPS) was divided into four stages, corresponding to temperatures of 30 to 170 • C, 170 to 340 • C, 340 to 417 • C, 417 to 600 • C, 30 to 200 • C, 200 to 374 • C, 374 to 467 • C, and 467 to 600 • C. the temperatures corresponding to the maximum decomposition rate were 377 • C and 395 • C, respectively.The decomposition process of composite waterabsorbing materials unfolded across distinct stages.The first stage primarily involved the decomposition of water and unreacted raw materials within these composites.Significant decomposition took place primarily in the second and third stages.The second stage involved the decomposition of small molecules, like oligomers, present in the material, while the third stage was characterized by the decomposition of branched chains within the polymer.Subsequently, in the fourth stage, mass reduction took place as a result of the breakdown of the polymer's main chains and the disintegration of its three-dimensional network structure.
Comparing the TG plots of P(HEC-AA-AMPS) and ATP-BF-P(HEC-AA-AMPS), it can be found that the thermal stability of ATP-BF-P(HEC-AA-AMPS) was significantly better than that of the second one in the remaining stages, except for the first stage, which was similar to that of P(HEC-AA-AMPS), and the remaining sample masses of the two after 600 • C were, respectively, 5.18% and 22.76%.These findings demonstrate that ATP and BF play a role in the polymerization process and enhance the thermal stability of the materials.
and BF play a role in the polymerization process and enhance the thermal stability of materials.

Measurement of Swelling Behavior
The swelling multiplier and swelling rate are critical performance indicators of hig absorbency polymers, serving as the technical prerequisites for determining their su bility for widespread use.
Figure 7 displays the outcomes of the swelling experiment for ATP-BF-P(HEC-A AMPS) at temperatures of 20 °C, 30 °C, 40 °C, and 50 °C.The swelling rate of ATP-B P(HEC-AA-AMPS) in deionized water demonstrated a trend of initially increasing and th decreasing as the temperature rose, peaking at a maximum water-absorbing capacity of 4 g/g at 30 °C.Beyond this temperature, the water absorption capacity started to decline.N tably, in both 0.9% NaCl solution and saturated Ca(OH)2 solution, the water absorption pacity of ATP-BF-P (HEC-AA-AMPS) increased from 59 g/g and 65 g/g, respectively, to g/g and 71 g/g as the temperature increased from 30 °C to 40 °C.This could be explained the observation that the water absorption rate of the ATP-BF-P(HEC-AA-AMPS) threemensional structure seemed to be somewhat disrupted, allowing for the entry of ions a resulting in an enhanced water absorption capacity in the presence of an ionic solution.
However, with increasing temperature, the degree of destruction of the three-dim sional network structure intensified, leading to a decrease in the water-absorbing capac of ATP-BF-P(HEC-AA-AMPS) in ionic solution.Specifically, at 50 °C, the water-absorb capacities in deionized water, 0.9% NaCl solution, and saturated Ca(OH)2 solution w 381 g/g, 145 g/g, 57 g/g, and 60 g/g.These values represent decreases of 22 g/g, 10 g/g g/g, and 5 g/g compared to those observed at 30 °C, with minimal performance variati

Measurement of Swelling Behavior
The swelling multiplier and swelling rate are critical performance indicators of highabsorbency polymers, serving as the technical prerequisites for determining their suitability for widespread use.
Figure 7 displays the outcomes of the swelling experiment for ATP-BF-P(HEC-AA-AMPS) at temperatures of 20 • C, 30 • C, 40 • C, and 50 • C. The swelling rate of ATP-BF-P(HEC-AA-AMPS) in deionized water demonstrated a trend of initially increasing and then decreasing as the temperature rose, peaking at a maximum water-absorbing capacity of 403 g/g at 30 • C. Beyond this temperature, the water absorption capacity started to decline.Notably, in both 0.9% NaCl solution and saturated Ca(OH) 2 solution, the water absorption capacity of ATP-BF-P (HEC-AA-AMPS) increased from 59 g/g and 65 g/g, respectively, to 73 g/g and 71 g/g as the temperature increased from 30 • C to 40 • C.This could be explained by the observation that the water absorption rate of the ATP-BF-P(HEC-AA-AMPS) three-dimensional structure seemed to be somewhat disrupted, allowing for the entry of ions and resulting in an enhanced water absorption capacity in the presence of an ionic solution.
Materials 2024, 17, x FOR PEER REVIEW 8 of 14 and BF play a role in the polymerization process and enhance the thermal stability of the materials.

Measurement of Swelling Behavior
The swelling multiplier and swelling rate are critical performance indicators of highabsorbency polymers, serving as the technical prerequisites for determining their suitability for widespread use.
Figure 7 displays the outcomes of the swelling experiment for ATP-BF-P(HEC-AA-AMPS) at temperatures of 20 °C, 30 °C, 40 °C, and 50 °C.The swelling rate of ATP-BF-P(HEC-AA-AMPS) in deionized water demonstrated a trend of initially increasing and then decreasing as the temperature rose, peaking at a maximum water-absorbing capacity of 403 g/g at 30 °C.Beyond this temperature, the water absorption capacity started to decline.Notably, in both 0.9% NaCl solution and saturated Ca(OH)2 solution, the water absorption capacity of ATP-BF-P (HEC-AA-AMPS) increased from 59 g/g and 65 g/g, respectively, to 73 g/g and 71 g/g as the temperature increased from 30 °C to 40 °C.This could be explained by the observation that the water absorption rate of the ATP-BF-P(HEC-AA-AMPS) three-dimensional structure seemed to be somewhat disrupted, allowing for the entry of ions and resulting in an enhanced water absorption capacity in the presence of an ionic solution.
However, with increasing temperature, the degree of destruction of the three-dimensional network structure intensified, leading to a decrease in the water-absorbing capacity of ATP-BF-P(HEC-AA-AMPS) in ionic solution.Specifically, at 50 °C, the water-absorbing capacities in deionized water, 0.9% NaCl solution, and saturated Ca(OH)2 solution were 381 g/g, 145 g/g, 57 g/g, and 60 g/g.These values represent decreases of 22 g/g, 10 g/g, 3 g/g, and 5 g/g compared to those observed at 30 °C, with minimal performance variation.In addition, in the range of 20 to 50 °C, the prepared samples all reached the expansion equilibrium state within 15 to 20 min, and the prepared samples met the performance requirements of rapid dissolution and swelling under the medium-to-low temperature environment.
The solubilization data of the prepared samples at various temperatures underwent nonlinear curve fitting and were plotted as depicted in Figure 8.The corresponding fitted curve equations and R 2 values are presented in Table 4.It is evident that the fitted curve closely approximates the actual data, with the lowest R 2 value of 0.95434 observed in the saturated Ca(OH)2 solution at 50 °C.However, with increasing temperature, the degree of destruction of the three-dimensional network structure intensified, leading to a decrease in the water-absorbing capacity of ATP-BF-P(HEC-AA-AMPS) in ionic solution.Specifically, at 50 • C, the water-absorbing capacities in deionized water, 0.9% NaCl solution, and saturated Ca(OH) 2 solution were 381 g/g, 145 g/g, 57 g/g, and 60 g/g.These values represent decreases of 22 g/g, 10 g/g, 3 g/g, and 5 g/g compared to those observed at 30 • C, with minimal performance variation.
In addition, in the range of 20 to 50 • C, the prepared samples all reached the expansion equilibrium state within 15 to 20 min, and the prepared samples met the performance requirements of rapid dissolution and swelling under the medium-to-low temperature environment.
The solubilization data of the prepared samples at various temperatures underwent nonlinear curve fitting and were plotted as depicted in Figure 8.The corresponding fitted curve equations and R 2 values are presented in Table 4.It is evident that the fitted curve closely approximates the actual data, with the lowest R 2 value of 0.95434 observed in the saturated Ca(OH) 2 solution at 50 • C. In addition, in the range of 20 to 50 °C, the prepared samples all reached the expansion equilibrium state within 15 to 20 min, and the prepared samples met the performance requirements of rapid dissolution and swelling under the medium-to-low temperature environment.
The solubilization data of the prepared samples at various temperatures underwent nonlinear curve fitting and were plotted as depicted in Figure 8.The corresponding fitted curve equations and R 2 values are presented in Table 4.It is evident that the fitted curve closely approximates the actual data, with the lowest R 2 value of 0.95434 observed in the saturated Ca(OH)2 solution at 50 °C.

Water Retention Properties
In the case of highly absorbent resins, prioritizing strong water retention properties over high swelling multiplicity is crucial.This is exemplified in soil moisturizers and concrete internal curing agents, where a slow water release is essential for maintaining prolonged humidity within soil and concrete structures.
In order to assess the water retention capability of the material ATP-BF-P(HEC-AA-AMPS), it was dissolved until reaching equilibrium at room temperature.Afterward, the water was extracted and moved to a stable-temperature environment in chambers set at 20 • C, 30 • C, 40 • C, and 50 • C, with the weight variations recorded for a period ranging from 1 to 7 days.The water-holding capacity was determined based on Equation (2), and the findings are shown in Figure 9.The water retention performance significantly decreased with increasing temperature, possibly due to the increased water evaporation at higher temperatures.Additionally, the decrease in water retention may have been caused by the deterioration of the resin structure due to prolonged exposure to high temperatures, resulting in a lower ability to absorb water.The water retention rates of the samples at 20 • C, 30 • C, 40 • C, and 50 • C on the initial day were 85.91%, 78.65%, 72.18%, and 65.86%.The difference in water retention between 20 • C and 50 • C was 20.05%.Over the course of the testing period, the differences between these temperatures on the 2nd to 7th days increased and then decreased, with values of 22.68%, 23.88%, 27.74%, 28.3%, 24.56%, and 19.03%, respectively.After 7 days of experimentation, the water retention rates under environments of 20 • C, 30 • C, 40 • C, and 50 • C were 23.61%, 9.82%, 4.32%, and 2.36%, respectively, demonstrating the material's favorable water retention performance under medium-to-low temperature conditions.
Additionally, since the test was conducted in a controlled environment, it's worth noting that in practical applications, high-water-absorption resin is frequently encapsulated within other materials.For instance, it may be used as a soil moisturizer mixed into dry soil, or as a concrete internal curing agent incorporated inside the concrete structure.This implies that, in real-world scenarios, the water retention effect is likely to be even more pronounced.

pH Dynamic Sensing
Figure 10 illustrates the variation curves of the dissolution test for the highly absorbent polymer in a 0.9% NaCl solution at various pH levels.In order to minimize or eliminate variations in ionic strength resulting from significant changes in factors like solution volume, the pH of the solution was adjusted using 11.5 mol/L HCl or NaOH. Figure 7 illustrates that ATP-BF-P(HEC-AA-AMPS) achieved a maximum swelling capacity of 71 g/g at a pH of 6.0.The swelling diversity of the samples exhibited a more rapid increase as the pH rose from 3.0 to 6.0.Between pH 6.0 and 10, the swelling diversity exhibited no significant change, but rapidly decreased as the pH increased.The swelling behavior of highly absorbent resins can vary in different pH environments due to factors such as hydrogen bonding, electrostatic repulsion, and coordination along the resin chain.Conversely, electrostatic repulsion and high osmotic pressure contribute to increases in the swelling diversity of the highly absorbent polymers [29][30][31].At pH levels below 3.0, the H + in the swelling medium will exchange with the Na + in the ATP-BF-P(HEC-AA-

pH Dynamic Sensing
Figure 10 illustrates the variation curves of the dissolution test for the highly absorbent polymer in a 0.9% NaCl solution at various pH levels.In order to minimize or eliminate variations in ionic strength resulting from significant changes in factors like solution volume, the pH of the solution was adjusted using 11.5 mol/L HCl or NaOH.

pH Dynamic Sensing
Figure 10 illustrates the variation curves of the dissolution test for the highly absorbent polymer in a 0.9% NaCl solution at various pH levels.In order to minimize or eliminate variations in ionic strength resulting from significant changes in factors like solution volume, the pH of the solution was adjusted using 11.5 mol/L HCl or NaOH. Figure 7 illustrates that ATP-BF-P(HEC-AA-AMPS) achieved a maximum swelling capacity of 71 g/g at a pH of 6.0.The swelling diversity of the samples exhibited a more rapid increase as the pH rose from 3.0 to 6.0.Between pH 6.0 and 10, the swelling diversity exhibited no significant change, but rapidly decreased as the pH increased.The swelling behavior of highly absorbent resins can vary in different pH environments due to factors such as hydrogen bonding, electrostatic repulsion, and coordination along the resin chain.Conversely, electrostatic repulsion and high osmotic pressure contribute to increases in the swelling diversity of the highly absorbent polymers [29][30][31].At pH levels below 3.0, the H + in the swelling medium will exchange with the Na + in the ATP-BF-P(HEC-AA- Figure 7 illustrates that ATP-BF-P(HEC-AA-AMPS) achieved a maximum swelling capacity of 71 g/g at a pH of 6.0.The swelling diversity of the samples exhibited a more rapid increase as the pH rose from 3.0 to 6.0.Between pH 6.0 and 10, the swelling diversity exhibited no significant change, but rapidly decreased as the pH increased.The swelling behavior of highly absorbent resins can vary in different pH environments due to factors such as hydrogen bonding, electrostatic repulsion, and coordination along the resin chain.Conversely, electrostatic repulsion and high osmotic pressure contribute to increases in the swelling diversity of the highly absorbent polymers [29][30][31].At pH levels below 3.0, the H + in the swelling medium will exchange with the Na + in the ATP-BF-P(HEC-AA-AMPS) polymer.Simultaneously, the presence of numerous carboxyl groups on the highly absorbent polymer leads to hydrogen bonding between these groups, resulting in a lower swelling multiplication rate.As the pH rises, more acid ions join the chain, leading to increased electrostatic repulsion within the highly absorbent resin network.This, combined with heightened osmotic pressure inside and outside the structure, results in the resin network expanding fully and the equilibrium water absorption rising.While the coordination ability of the highly absorbent resin also increases and the equilibrium water absorption decreases, the primary factors driving the enhanced absorption remain the electrostatic repulsion within the macromolecule chain and the heightened osmotic pressure inside and outside the network.Consequently, the equilibrium water absorption continues to increase [32,33].
When the pH exceeds 6.0, the electrostatic repulsion on the polymer chain diminishes, impeding network expansion.Simultaneously, the coordination ability between polymer chains strengthens [34].Consequently, the swelling capacity begins to decrease.Moreover, a further increase in the amount of Na + reduces the oxygen-sodium ratio on the polymer chain.This leads to a decrease in the coordination between sodium and oxygen, causing a shift in the coordination mode from intramolecular to intermolecular.Consequently, the network of the highly water-absorbent polymer expands further.The comprehensive results indicate that the equilibrium water absorption of the highly water-absorbent polymer remains constant as the pH increases from 7.0 to 10.0.Beyond a pH of 10, the water absorption initiates a decline, potentially attributable to low osmotic pressure and diminished electrostatic repulsion.

Mechanical Performance Test
Highly absorbent polymers undergo extrusion and friction during usage, leading to destructive damage that impacts their internal structure.Hence, they must possess a certain level of mechanical strength.To ensure experimental rigor, the prepared samples underwent washing with ethanol and deionized water before being directly subjected to a stress-strain test.
Figure 11a displays field test images of ATP-BF-P(HEC-AA-AMPS) at 0%, 30%, and 50% compressive deformation levels, illustrating a gradual thinning of the samples which was observed visually.Subsequently, Figure 11b presents stress-strain curves of P(HEC-AA-AMPS) and ATP-BF-P(HEC-AA-AMPS) at a 50% deformation level.The graphs indicate that the maximum positive stress of the water-absorbing polymers increased from 5.74 N to 8.07 N with the addition of ATP and BF, resulting in a notable 40.59% improvement in mechanical properties.Additionally, at the same level of positive stress, the deformation of the modified polymer was smaller.For instance, the deformation before and after modification was 1.631 mm and 1.701 mm at 4 N, corresponding to compressions of 46.71% and 42.19%, respectively.This demonstrates that the incorporation of ATP and BF enhances the strength and stability of the polymer structure, rendering it more suitable for application in challenging environments.
Materials 2024, 17, x FOR PEER REVIEW 1 AMPS) polymer.Simultaneously, the presence of numerous carboxyl groups o highly absorbent polymer leads to hydrogen bonding between these groups, result a lower swelling multiplication rate.As the pH rises, more acid ions join the chain, le to increased electrostatic repulsion within the highly absorbent resin network.This bined with heightened osmotic pressure inside and outside the structure, results resin network expanding fully and the equilibrium water absorption rising.While t ordination ability of the highly absorbent resin also increases and the equilibrium absorption decreases, the primary factors driving the enhanced absorption rema electrostatic repulsion within the macromolecule chain and the heightened osmotic sure inside and outside the network.Consequently, the equilibrium water absorptio tinues to increase [32,33].When the pH exceeds 6.0, the electrostatic repulsion on the polymer chain d ishes, impeding network expansion.Simultaneously, the coordination ability be polymer chains strengthens [34].Consequently, the swelling capacity begins to dec Moreover, a further increase in the amount of Na + reduces the oxygen-sodium ra the polymer chain.This leads to a decrease in the coordination between sodium and gen, causing a shift in the coordination mode from intramolecular to intermolecular sequently, the network of the highly water-absorbent polymer expands further.The prehensive results indicate that the equilibrium water absorption of the highly wat sorbent polymer remains constant as the pH increases from 7.0 to 10.0.Beyond a 10, the water absorption initiates a decline, potentially attributable to low osmotic sure and diminished electrostatic repulsion.

Mechanical Performance Test
Highly absorbent polymers undergo extrusion and friction during usage, lead destructive damage that impacts their internal structure.Hence, they must possess tain level of mechanical strength.To ensure experimental rigor, the prepared sampl derwent washing with ethanol and deionized water before being directly subjecte stress-strain test.
Figure 11a displays field test images of ATP-BF-P(HEC-AA-AMPS) at 0%, 30% 50% compressive deformation levels, illustrating a gradual thinning of the samples was observed visually.Subsequently, Figure 11b presents stress-strain curves of P( AA-AMPS) and ATP-BF-P(HEC-AA-AMPS) at a 50% deformation level.The graph cate that the maximum positive stress of the water-absorbing polymers increased 5.74 N to 8.07 N with the addition of ATP and BF, resulting in a notable 40.59% imp ment in mechanical properties.Additionally, at the same level of positive stress, t formation of the modified polymer was smaller.For instance, the deformation befor after modification was 1.631 mm and 1.701 mm at 4 N, corresponding to compressi 46.71% and 42.19%, respectively.This demonstrates that the incorporation of ATP a enhances the strength and stability of the polymer structure, rendering it more su for application in challenging environments.

Conclusions
(1) The successful polymerization of ATP-BF-P(HEC-AA-AMPS), a high-absorbency polymer, using an aqueous solution was demonstrated through SEM and FTIR.The polymer exhibited increased roughness and a higher number of micropores after the introduction of ATP and BF, significantly enhancing its liquid-absorbing capacity.(2) ATP-BF-P(HEC-AA-AMPS) exhibited superior thermodynamic stability in the temperature range of 30-600 • C compared to P(HEC-AA-AMPS), with a 17.58% reduction in mass loss at 600 • C, signifying a noteworthy improvement in thermal stability.(3) ATP-BF-P(HEC-AA-AMPS) exhibited better adaptability to the pH range, with minimal changes in dissolution multiplicity and maximum water absorption multiplicity between pH 6.0~10.0,indicating a broad range of applicability.Conversely, beyond a pH value of 10.0, the ability to absorb liquids decreased rapidly.(4) The mechanical properties of ATP-BF-P(HEC-AA-AMPS) improved by 40.59% at a 50% deformation level with the addition of ATP and BF.The overall strength of the polymer was significantly enhanced, rendering it more suitable for use in complex scenarios.

Figure 1 .
Figure 1.Schematic diagram of the molecular structure of HEC.

Figure 1 . 1 Figure 2 .
Figure 1.Schematic diagram of the molecular structure of HEC.

Figure 10 .
Figure 10.Dissolution multiplicity of samples as a function of pH value.

Figure 10 .
Figure 10.Dissolution multiplicity of samples as a function of pH value.

Figure 10 .
Figure 10.Dissolution multiplicity of samples as a function of pH value.

Table 3 .
EDS elemental content test results.

Table 3 .
EDS elemental content test results.

Table 3 .
EDS elemental content test results.