The Effect of Silver Nanoparticles/Titanium Dioxide in Poly(acrylic acid-co-acrylamide)-Modified, Deproteinized, Natural Rubber Composites on Dye Removal

This work aims to enhance the dye-removal performance of prepared poly(acrylic acid-co-acrylamide)-modified, deproteinized, natural rubber ((PAA-co-PAM)-DPNR) through incorporation with silver nanoparticles/titanium dioxide. The (PAA-co-PAM)-DPNR was prepared by emulsion-graft copolymerization with a grafting efficiency of 10.20 ± 2.33 to 54.26 ± 1.55%. The composites based on (PAA-co-PAM)-DPNR comprising silver nanoparticles and titanium dioxide ((PAA-co-PAM)-DPNR/Ag-TiO2) were then prepared by latex compounding using the fixed concentration of AgNO3 (0.5 phr) and varying concentrations of TiO2 at 1.0, 2.5, and 5.0 phr. The formation of silver nanoparticles was obtained by heat and applied pressure. The composites had a porous morphology as they allowed water to diffuse in their structure, allowing the high specific area to interact with dye molecules. The incorporation of silver nanoparticles/titanium dioxide improved the compressive modulus from 1.015 ± 0.062 to 2.283 ± 0.043 KPa. The (PAA-co-PAM)-DPNR/Ag-TiO2 composite with 5.0 phr of TiO2 had a maximum adsorption capacity of 206.42 mg/g, which increased by 2.02-fold compared to (PAA-co-PAM)-DPNR. The behavior of dye removal was assessed with the pseudo-second-order kinetic model and Langmuir isotherm adsorption model. These composites can maintain their removal efficiency above 90% for up to five cycles. Thus, these composites could have the potential for dye-removal applications.


Introduction
Functional polymeric materials have received much attention for their potential in various applications.Since they contain functional groups, they can bind with therapeutic agents that carry them to the target in drug-delivery systems [1,2] and adsorb pollutants such as heavy metals and dyes in wastewater treatment [3][4][5].For the removal of contaminants, functional materials have been used as templates to interact with specific molecules to remove them from a mixture.In the case of separation in aqueous conditions, functional polymeric materials should also be stable in an aqueous medium.They should maintain a dimensional structure and not deform in the medium to be reused [6].Additionally, the templates fabricated from bio-based polymeric materials have advantages because they are environmentally friendly and have low toxicity.Among natural polymers, natural rubber (NR) is a biopolymer from Hevea brasiliensis rubber trees, which is interesting for the construction of bio-based devices [7,8].NR has been extensively used in many applications because it has good mechanical properties and high elasticity.It possesses hydrophobic characteristics due to the cis-1,4-polyisoprene in its chemical structure.Therefore, the fabrication of green adsorbent materials based on natural rubber, which is a renewable biopolymer, is more suitable in terms of environmental friendliness and the ability to maintain structural stability in an aqueous environment.It can also be modified with other components to have desired functional groups for specific binding with adsorbate molecules [9,10].
In our previous work, deproteinized natural rubber was modified by grafting it with poly(acrylic acid-co-acrylamide) through emulsion-graft copolymerization in the presence of N ,N -methylenebisacrylamide as a crosslinking agent [11].This method is a water-based system with an environmentally friendly process.The modified natural rubber was then mixed with silica and employed as a coating material for fertilizer.After modification, the natural rubber-based material had additional functional groups, including carboxylic acid, amide, and hydroxyl groups.The presence of these functional groups not only improved water absorption capacity but also interacted with the other molecules.Maijan et al. prepared polymeric network hydrogel based on polyacrylamide and natural rubber [12].The addition of natural rubber improved the toughness of polyacrylamide-based hydrogel and showed a higher compressive modulus compared to pure polyacrylamide hydrogel.The prepared hydrogel served as an adsorbent for dye removal that could adsorb methylene blue with a maximum adsorption capacity of 538.3 mg/g.Its removal efficiency remained at 70% after reuse for 10 cycles.
To prepare a high-performance adsorbent, photocatalyst particles were added to improve the efficiency of the dye removal.Titanium dioxide (TiO 2 ) is an inorganic compound that is used in a wide range of applications.It has been reported that TiO 2 exhibits excellent photocatalytic activity [13,14].TiO 2 can produce reactive oxygen radical species under UV irradiation.The radicals can degrade organic pollutants in wastewater into harmless products.However, there are some drawbacks; for example, the use of TiO 2 powder for treatment may not be recyclable, and it can only absorb UV light (band gap 3.2 eV).Many research works have reported the use of TiO 2 incorporated into metals to improve efficiency in visible regions.Suriyachai et al. prepared modified TiO 2 particles for photocatalytic glucose conversion [15].The TiO 2 was modified by doping with metal or metalloid (B and Ag) and nonmetal (N).The increase in absorbance in the visible region was found with the single doping (Ag) and co-doping (Ag/N and B/N).The use of Ag-doped TiO 2 enhanced the glucose conversion to 63.5%, while the glucose conversion of bare TiO 2 was only 24.9%.The conversion was further improved with co-doping (>90%).Therefore, the modified TiO 2 could improve performance in terms of productivity.Generally, metallic particles can be formed by the reduction of metal ions.However, strong reducing agents are often added in this method that may be harmful.Wongpreecha et al. prepared silver nanoparticles through a one-step green process [16].The silver nanoparticles were generated under high temperature and pressure using AgNO 3 as a metal precursor and chitosan as a reducing agent and stabilizer.The synthesized silver nanoparticles exhibited good long-term stability and antibacterial activity.In addition, it has been reported that polyacrylic acid can act as a host material for silver nanoparticles since the carboxylic groups in its structure can form coordination or ionic bonds with silver ions [17,18].Fahmy et al. prepared a thin film of polyacrylic acid incorporated into silver nanoparticles by plasma polymerization [19].The aluminum substrates covered with polyacrylic acid were saturated with Ag + ions (from an AgNO 3 solution).The reduction process to generate silver nanoparticles was performed by two methods, which were (1) the use of NaBH 4 as a reducing agent and (2) sunlight exposure.The backbone structure of the polymer layer was not changed by exposure to sunlight.Silver nanoparticles with a size of less than 5 nm were formed and homogeneously dispersed in the polymer layer.
In this work, poly(acrylic acid-co-acrylamide)-modified, deproteinized, natural rubber was prepared via emulsion-graft copolymerization and used as a polymer matrix to fabricate an adsorbent material for dye removal.Various factors in polymerization were then studied, including crosslinking agent contents, ratio of comonomer, and monomer contents.Novel composite materials were prepared by incorporating silver nanoparticles and titanium dioxide into the modified natural rubber through a simple process to enhance 2.2.Preparation of Poly(acrylic acid-co-acrylamide)-Modified, Deproteinized, Natural Rubber The procedure for preparing the poly(acrylic acid-co-acrylamide)-modified, deproteinized, natural rubber ((PAA-co-PAM)-DPNR) was carried out via emulsion-graft copolymerization of the comonomers of acrylic acid and acrylamide, according to previous work [11].The DPNR latex was mixed with Terric16A using a mechanical stirrer at 100 rpm in a three-necked round-bottom reactor.The dispersion was purged with nitrogen gas for 45 min.Subsequently, CHP, acrylic acid, acrylamide, MBA, and TEPA were injected into the reactor.Acrylic acid (50 mol%) was neutralized with a 20 wt.% of NaOH solution.CHP and TEPA were fixed at 1 phr.The total solid content was adjusted to 20 wt.% with deionized water.Polymerization parameters were set at 50 • C for 6 h under a nitrogen atmosphere.Finally, the (PAA-co-PAM)-modified DPNR latex was formed.

Determination of Monomer Conversion, Grafting Efficiency, and Grafting Percentage
The obtained latex was dried in a hot-air oven at 60 • C. The dried samples were weighed and then immersed in ethanol for 24 h to eliminate any unreacted monomers.Subsequently, the samples were dried again in a hot-air oven at 60 • C for 24 h.The monomer conversion was calculated using the following equation [22]: Weight of(PAA-co-PAM) formed Weight of total monomer added × 100. (1) To determine the grafting efficiency and grafting percentage, the extraction of samples with DI water was conducted to separate the ungrafted PAA-co-PAM.The samples were immersed in DI water for 72 h, and the DI water was changed three times a day.The samples were subsequently dried at 60 and after extraction were recorded.The grafting efficiency and grafting percentage were calculated as follows [23]: Grafting efficiency(%) = Weight of(PAA-co-PAM) grafted Weight of total polymer formed × 100 Grafting percentage(%) = Weight of(PAA-co-PAM) grafted Weight of DPNR used × 100. (3)

FTIR Analysis
Fourier transform infrared spectroscopy (FTIR) was employed to characterize the chemical functional groups of the prepared samples.The measurements were performed at a resolution of 4 cm −1 with 64 scans using attenuated total reflection (ATR) mode on an FTIR spectrometer (Tensor 27, Bruker, Billerica, MA, USA).The scanning was recorded in the range of 4000 to 500 cm −1 .

Preparation of (PAA-co-PAM)-DPNR/Ag-TiO 2 Composites
The (PAA-co-PAM)-DPNR composites consisting of silver nanoparticles and titanium dioxide were prepared and utilized as adsorbent materials, as shown in Figure 1.The composites were prepared by mixing the (PAA-co-PAM)-modified DPNR latex with a 1% w/v AgNO 3 solution and a 10% w/v TiO 2 dispersion.The concentration of AgNO 3 was fixed at 0.5 phr, while the TiO 2 contents varied at 1.0, 2.5, and 5.0 phr.The mixture was magnetically stirred at 600 rpm for 30 min under dark conditions.Subsequently, the mixture was transferred to an autoclave (HVA-110, Hirayama Manufacturing Corporation, Saitama, Japan).The experiment was conducted at 120 • C under pressure of 15 psi for 50 min.The obtained latex was poured into a silicone mold and dried in a hot-air oven at 60 • C for 24 h.The samples were purified by extraction with DI water before use.Control samples loading either AgNO 3 or TiO 2 alone were also prepared.and after extraction were recorded.The grafting efficiency and grafting percentage were calculated as follows [23]: Grafting ef iciency (%) = Weight of (PAA −  − PAM) grafted Weight of total polymer formed × 100 Grafting percentage (%) = Weight of (PAA −  − PAM) grafted Weight of DPNR used × 100. (3)

FTIR Analysis
Fourier transform infrared spectroscopy (FTIR) was employed to characterize the chemical functional groups of the prepared samples.The measurements were performed at a resolution of 4 cm −1 with 64 scans using attenuated total reflection (ATR) mode on an FTIR spectrometer (Tensor 27, Bruker, Billerica, MA, USA).The scanning was recorded in the range of 4000 to 500 cm −1 .

Preparation of (PAA-co-PAM)-DPNR/Ag-TiO2 Composites
The (PAA-co-PAM)-DPNR composites consisting of silver nanoparticles and titanium dioxide were prepared and utilized as adsorbent materials, as shown in Figure 1.The composites were prepared by mixing the (PAA-co-PAM)-modified DPNR latex with a 1% w/v AgNO3 solution and a 10% w/v TiO2 dispersion.The concentration of AgNO3 was fixed at 0.5 phr, while the TiO2 contents varied at 1.0, 2.5, and 5.0 phr.The mixture was magnetically stirred at 600 rpm for 30 min under dark conditions.Subsequently, the mixture was transferred to an autoclave (HVA-110, Hirayama Manufacturing Corporation, Saitama, Japan).The experiment was conducted at 120 °C under pressure of 15 psi for 50 min.The obtained latex was poured into a silicone mold and dried in a hot-air oven at 60 °C for 24 h.The samples were purified by extraction with DI water before use.Control samples loading either AgNO3 or TiO2 alone were also prepared.The formation of silver nanoparticles was investigated using a UV-vis spectrophotometer (Cary Series, Agilent Technologies, Santa Clara, CA, USA).The diluted latex loaded with AgNO 3, both before and after autoclaving, was added into the cuvette and subjected to the sample holder.Latex without the addition of AgNO 3 was used as a blank.The measurement was conducted from 300 to 800 nm at a scan rate of 600 nm/min.

X-ray Diffraction
The crystal structure of silver nanoparticles was investigated using X-ray diffraction (XRD, D2 PHASER, Bruker, Billerica, MA, USA) with a Cu Kα radiation source operated at 30 kV and a current of 10 mA.The XRD patterns were recorded over the 2θ range of 30 • to 80 • with a scan rate of 2 • /min and a step size of 0.02 • .

Morphology
The morphology of different types of composites was observed by scanning electron microscopy (SEM).The dried samples were frozen in liquid nitrogen.Then, the frozen samples were broken and fixed on a stub.Subsequently, the samples were coated with gold under a vacuum using a sputter coater (EM ACE600, Leica Microsystems, Wetzlar, Germany).The cross-sectional morphology of the samples was investigated using a JSM-7800F field emission scanning electron microscope (JEOL Ltd., Tokyo, Japan).The element composition and distribution of the composites were determined by energy dispersive spectroscopy coupled with SEM (SEM/EDS).Additionally, the morphology of samples in the swollen state was studied.The samples were soaked in DI water for 24 h.The swollen samples were freeze-dried overnight.After that, the freeze-dried samples were immersed in liquid nitrogen and then broken.They were sputter-coated with gold under vacuum, and their morphology was investigated through SEM.

Swelling Degree
The dried samples were weighed and immersed in DI water.The swollen samples were taken out and blotted onto filter paper to remove excess water on their surface.Then, the swollen samples were weighed.The swelling degree was determined by the following equation: where W d is the weight of the dried samples, and W s is the weight of the swollen samples.

Compressive Properties
The compressive properties of the samples were determined using a texture analyzer (TA.XT plus, Stable Micro Systems Ltd., Surrey, UK).Before testing, the samples were immersed in DI water for 24 h and cut into a cylindrical shape with a diameter of approximately 12 mm and a thickness of 5 mm.The swollen samples were placed on the plate and compressed to 80% strain.The measurement was run at a fixed strain rate of 0.05 mm/s.

Effect of Type of Adsorbent
The dye adsorption of different types of prepared samples was studied using methylene blue (MB) as a model dye.The samples were placed into 50 mL of a 50 mg/L dye solution.Dye solutions were taken out at various time intervals.The remaining dye concentration after adsorption was determined using UV-vis spectrophotometry (Cary Series, Agilent Technologies, Santa Clara, CA, USA) at a wavelength of 664 nm.The concentration of dye was obtained by comparison with the calibration curve of the MB solution (Figure S1, see Supplementary Materials).The dye adsorption ability was determined in terms of removal efficiency and adsorption capacity as follows [24]: Polymers 2024, 16, 92 where C 0 is the initial dye concentration in the solution (mg/L), C e is the equilibrium dye concentration in the solution (mg/L), m is the weight of the dried samples (g), and V is the volume of the solution (L).

Adsorption Kinetic
The experiment was carried out by adding the composites at different initial dye concentration levels (100-3000 mg/L).The amount of adsorbed dye was determined.To study dye adsorption kinetics, the pseudo-first-order and pseudo-second-order kinetic models are applied to fit the experimental data.The pseudo-first-order and pseudo-secondorder kinetic models are presented in Equations ( 7) and (8), respectively, as follows [25]: where Q e and Q t represent the adsorption capacity of the dye at equilibrium (e) and time (t), respectively.The k 1 and k 2 are the rate constants of the pseudo-first-order and pseudo-second-order kinetic models, respectively.

Adsorption Isotherm
The adsorption isotherm was studied using Langmuir and Freundlich isotherm models [26].The amount of adsorbed dye was examined by varying the initial concentrations of the dye solution in the range of 100-3000 mg/L.The Langmuir equation is presented by the following equation: where C e is the concentration of dye remaining in the solution at equilibrium, Q m is the maximum adsorption capacity, and K L is the Langmuir constant.The essential parameter of Langmuir isotherm to classify adsorption behavior (R L ) is shown by the following equation: (10) where the value of R L is expressed as follows; R L > 1 (Unfavorable), R L = 1 (Linear), 0 < R L < 1 (Favorable), and R L = 0 (Irreversible).
For the Freundlich adsorption isotherm, it can be used to describe the heterogeneous surface with multilayer adsorption.The Freundlich equation is shown in Equation (11): where K F is the Freundlich constant, and n is the heterogeneity factor.

Reusability
To study reusability, the adsorption was performed in 50 mL of a 100 mg/L dye solution.The used composites were regenerated by immersion in 0.5 M NaOH for 24 h and water for 24 h, respectively [12].The adsorption test was conducted for 5 cycles using the same adsorbents.The removal efficiency was then calculated.Each sample was tested three times.

Conversion, Grafting Efficiency and Grafting Percentage
The chemical compositions and properties of various types of (PAA-co-PAM)-modified DPNR are shown in Table 1.The conversion of all samples was found to range from 79.19 ± 4.40 to 83.34 ± 3.01%.When the monomer content was kept constant at 10 phr (sample N1-N4), the grafting efficiency and grafting percentage tended to increase with increasing MBA content.The MBA contents were varied from 0 to 1.00 by weight percentage of comonomer, resulting in an increase in grafting efficiency from 12.44 ± 1.36 to 48.48 ± 4.32% and a rise in grafting percentage from 1.24 ± 0.14 to 4.85 ± 0.43%.In this system, the grafting of PAA-co-PAM occurred at the natural rubber particle surface.The linkage of PAA-co-PAM chains during polymerization was obtained due to the addition of MBA, leading to the formation of a crosslinked network structure [27,28].Consequently, the PAA-co-PAM chains can hold into the natural rubber-based structure, resulting in an increase in grafting efficiency and grafting percentage.The effect of the comonomer ratio of acrylic acid and acrylamide on the properties of the modified DPNR was also studied.The sample N4-N6 with different comonomer ratios exhibited good colloidal stability.At a ratio of 30:70 by weight percentage of acrylic acid and acrylamide, the grafting efficiency and grafting percentage were found to be 10.20 ± 2.33% and 1.02 ± 0.23%, respectively.When the amount of acrylic acid increased, the grafting efficiency also increased.The interaction between copolymers through hydrogen bonding of polyacrylic acid and polyacrylamide can lead to the formation of self-assembled structures, improving grafting efficiency [29][30][31].However, some of the carboxylic acid groups of acrylic acid were neutralized with NaOH.Thus, the comonomer ratio of acrylic acid and acrylamide at 70:30 by weight percentage gave the highest grafting efficiency (48.48 ± 4.32%) and grafting percentage (4.85± 0.43%).Nevertheless, under the condition of 75:25 by weight percentage of acrylic acid and acrylamide, phase separation during polymerization was observed.This observation was probably due to the deterioration of the colloidal stability of rubber particles in an aqueous medium caused by high acidity [32].The presence of PAA-co-PAM in the structure is a crucial factor that could affect the efficiency of materials for the dye adsorption process.The effect of monomer content on its properties was also studied by varying the monomer contents to 10 and 20 phr, denoted as N4 and N7, respectively.When the monomer content increased to 20 phr, the grafting efficiency and grafting percentage increased to 54.26 ± 1.55% and 10.63 ± 0.31%, respectively.The higher grafting efficiency indicated a greater amount of PAA-co-PAM formed in natural rubber-based materials, which is useful for applications.

FTIR Analysis
The chemical functional groups in the sample structure were investigated by FTIR.The FTIR spectrum of the prepared (PAA-co-PAM)-modified DPNR (sample N7) after extraction with water was compared to that of the DPNR, as illustrated in Figure 2.For DPNR, peaks at 1664, 1446, 1375, and 841 cm −1 were observed, corresponding to C=C, -CH 2 , -CH 3 , and =CH, respectively [33].These peaks are indicative of polyisoprene, which is the chemical constituent of natural rubber.After modification by graft copolymerization, sample N7 exhibited the characteristic peaks of DPNR and additional peaks of OH and NH stretching (3500-3200 cm −1 ), C=O stretching (1663 cm −1 ), NH bending (1611 cm −1 ), carboxylate (1561 cm −1 ), and C-O stretching (1242 cm −1 ) [34].These additional signals were consistent with the functional groups of polyacrylic acid and polyacrylamide, confirming the presence of PAA-co-PAM after modification.These additional functional groups could play an important role in capturing adsorbate molecules [35].

FTIR Analysis
The chemical functional groups in the sample structure were investigated by FTIR.The FTIR spectrum of the prepared (PAA-co-PAM)-modified DPNR (sample N7) after extraction with water was compared to that of the DPNR, as illustrated in Figure 2.For DPNR, peaks at 1664, 1446, 1375, and 841 cm −1 were observed, corresponding to C=C, -CH2, -CH3, and =CH, respectively [33].These peaks are indicative of polyisoprene, which is the chemical constituent of natural rubber.After modification by graft copolymerization, sample N7 exhibited the characteristic peaks of DPNR and additional peaks of OH and NH stretching (3500-3200 cm −1 ), C=O stretching (1663 cm −1 ), NH bending (1611 cm −1 ), carboxylate (1561 cm −1 ), and C-O stretching (1242 cm −1 ) [34].These additional signals were consistent with the functional groups of polyacrylic acid and polyacrylamide, confirming the presence of PAA-co-PAM after modification.These additional functional groups could play an important role in capturing adsorbate molecules [35].

Formation of Silver Nanoparticles
The (PAA-co-PAM)-modified DPNR was first prepared and then mixed with AgNO3 solution and titanium dioxide dispersion.The silver ions were coordinated with carboxylic acid groups containing (PAA-co-PAM)-DPNR to form the complex.The reduction of Ag + ions to Ag 0 was conducted under high temperatures and applied pressure.In this case, the natural rubber was modified by grafting with (PAA-co-PAM), resulting in the improved polarity of the natural rubber-based matrix.The addition of titanium dioxide dispersion could be compatible with the modified natural rubber.It can interact with the polymeric matrix through polar-polar interaction and hydrogen bonding between the  The (PAA-co-PAM)-modified DPNR was first prepared and then mixed with AgNO 3 solution and titanium dioxide dispersion.The silver ions were coordinated with carboxylic acid groups containing (PAA-co-PAM)-DPNR to form the complex.The reduction of Ag + ions to Ag 0 was conducted under high temperatures and applied pressure.In this case, the natural rubber was modified by grafting with (PAA-co-PAM), resulting in the improved polarity of the natural rubber-based matrix.The addition of titanium dioxide dispersion could be compatible with the modified natural rubber.It can interact with the polymeric matrix through polar-polar interaction and hydrogen bonding between the hydroxylated titanium dioxide surface and carboxyl groups of PAA [36].Therefore, the silver nanoparticles and titanium dioxide were dispersed in the natural rubber-based matrix.The titanium dioxide contents were varied as 1.0, 2.5, and 5.0 phr to produce the (PAA-co-PAM)-DPNR/Ag-TiO 2 (N7/Ag-Ti) composites.Their properties were investigated and compared to samples prepared from modified natural rubber (N7), modified natural rubber containing silver nanoparticles (N7/Ag), and modified natural rubber filled with titanium dioxide (N7-Ti).The formulations and characteristics of all samples after drying are shown in Table 2. Sample N7 had a light-yellow color, while the color of the N7/Ag composite was an opaque dark brown, attributed to the occurrence of silver nanoparticles within the natural rubber-based composite [37].In the case of the incorporation of titanium Polymers 2024, 16, 92 9 of 21 dioxide, the samples became lighter brown with an increasing amount of titanium dioxide.Meanwhile, N7-Ti without silver nanoparticles exhibited an opaque white color.The formation of silver nanoparticles was examined using a UV-vis spectrophotometer, as displayed in Figure 3.As seen from the results, the N7/Ag composite exhibits a strong absorption peak at around 435 nm, while this peak is not observed in the case of N7 mixed with AgNO 3 solution.This peak corresponds to the surface plasmon resonance absorption of silver nanoparticles [38,39], indicating the formation of silver nanoparticles in natural rubber-based composites.Based on the estimation of the particle size of silver nanoparticles from UV-vis spectroscopy, as reported by Alim-Al-Razy et al. [40], it was found that the particle size of silver nanoparticles was approximately 65 nm.Furthermore, the XRD pattern of the N7/Ag composite shows diffraction peaks at 38.33 • , 42.83 • , 64.49 • , and 77.21 • , corresponding to the reflection plane indices of (111), ( 200), (220), and (311), respectively (Figure 4a).These correspond to the face-centered cubic (fcc) structure of metallic Ag.Similar observations have been reported in other research works [41,42].For the N7/Ag-Ti5 composite (Figure 4b), the peaks at 25.60  004), ( 200), ( 105), ( 211), (204), and (116) of anatase TiO 2 , respectively [43].However, the peaks corresponding to silver nanoparticles were less prominent.gated and compared to samples prepared from modified natural rubber (N7), modifi natural rubber containing silver nanoparticles (N7/Ag), and modified natural rubber fil with titanium dioxide (N7-Ti).The formulations and characteristics of all samples af drying are shown in Table 2. Sample N7 had a light-yellow color, while the color of N7/Ag composite was an opaque dark brown, attributed to the occurrence of silver nan particles within the natural rubber-based composite [37].In the case of the incorporat of titanium dioxide, the samples became lighter brown with an increasing amount of ti nium dioxide.Meanwhile, N7-Ti without silver nanoparticles exhibited an opaque wh color.The formation of silver nanoparticles was examined using a UV-vis spectrophoto eter, as displayed in Figure 3.As seen from the results, the N7/Ag composite exhibit strong absorption peak at around 435 nm, while this peak is not observed in the case N7 mixed with AgNO3 solution.This peak corresponds to the surface plasmon resonan absorption of silver nanoparticles [38,39], indicating the formation of silver nanopartic in natural rubber-based composites.Based on the estimation of the particle size of silv nanoparticles from UV-vis spectroscopy, as reported by Alim-Al-Razy et al. [40], it w found that the particle size of silver nanoparticles was approximately 65 nm.Furthermo the XRD pattern of the N7/Ag composite shows diffraction peaks at 38.33°, 42.83°, 64.4 and 77.21°, corresponding to the reflection plane indices of ( 111), ( 200), (220), and (31 respectively (Figure 4a).These correspond to the face-centered cubic (fcc) structure of m tallic Ag.Similar observations have been reported in other research works [41,42].For N7/Ag-Ti5 composite (Figure 4b), the peaks at 25.60°, 38.01°, 48.21°, 54.02°, 55.36°, 62.9 and 68.95° were observed, which were assigned to the (101), ( 004), ( 200), ( 105), ( 211), (20 and (116) of anatase TiO2, respectively [43].However, the peaks corresponding to sil nanoparticles were less prominent.Characteristics gated and compared to samples prepared from modified natural rubber (N7), modifie natural rubber containing silver nanoparticles (N7/Ag), and modified natural rubber fille with titanium dioxide (N7-Ti).The formulations and characteristics of all samples afte drying are shown in Table 2. Sample N7 had a light-yellow color, while the color of th N7/Ag composite was an opaque dark brown, attributed to the occurrence of silver nano particles within the natural rubber-based composite [37].In the case of the incorporatio of titanium dioxide, the samples became lighter brown with an increasing amount of tita nium dioxide.Meanwhile, N7-Ti without silver nanoparticles exhibited an opaque whit color.The formation of silver nanoparticles was examined using a UV-vis spectrophotom eter, as displayed in Figure 3.As seen from the results, the N7/Ag composite exhibits strong absorption peak at around 435 nm, while this peak is not observed in the case o N7 mixed with AgNO3 solution.This peak corresponds to the surface plasmon resonanc absorption of silver nanoparticles [38,39], indicating the formation of silver nanoparticle in natural rubber-based composites.Based on the estimation of the particle size of silve nanoparticles from UV-vis spectroscopy, as reported by Alim-Al-Razy et al. [40], it wa found that the particle size of silver nanoparticles was approximately 65 nm.Furthermor the XRD pattern of the N7/Ag composite shows diffraction peaks at 38.33°, 42.83°, 64.49 and 77.21°, corresponding to the reflection plane indices of ( 111), ( 200), (220), and (311 respectively (Figure 4a).These correspond to the face-centered cubic (fcc) structure of me tallic Ag.Similar observations have been reported in other research works [41,42].For th N7/Ag-Ti5 composite (Figure 4b), the peaks at 25.60°, 38.01°, 48.21°, 54.02°, 55.36°, 62.93 and 68.95° were observed, which were assigned to the (101), ( 004), ( 200), ( 105), ( 211), (204 and (116) of anatase TiO2, respectively [43].However, the peaks corresponding to silve nanoparticles were less prominent.Characteristics gated and compared to samples prepared from modified natural rubber (N7), modified natural rubber containing silver nanoparticles (N7/Ag), and modified natural rubber filled with titanium dioxide (N7-Ti).The formulations and characteristics of all samples after drying are shown in Table 2. Sample N7 had a light-yellow color, while the color of the N7/Ag composite was an opaque dark brown, attributed to the occurrence of silver nanoparticles within the natural rubber-based composite [37].In the case of the incorporation of titanium dioxide, the samples became lighter brown with an increasing amount of tita nium dioxide.Meanwhile, N7-Ti without silver nanoparticles exhibited an opaque white color.The formation of silver nanoparticles was examined using a UV-vis spectrophotom eter, as displayed in Figure 3.As seen from the results, the N7/Ag composite exhibits a strong absorption peak at around 435 nm, while this peak is not observed in the case o N7 mixed with AgNO3 solution.This peak corresponds to the surface plasmon resonance absorption of silver nanoparticles [38,39], indicating the formation of silver nanoparticles in natural rubber-based composites.Based on the estimation of the particle size of silver nanoparticles from UV-vis spectroscopy, as reported by Alim-Al-Razy et al. [40], it was found that the particle size of silver nanoparticles was approximately 65 nm.Furthermore the XRD pattern of the N7/Ag composite shows diffraction peaks at 38.33°, 42.83°, 64.49° and 77.21°, corresponding to the reflection plane indices of ( 111), ( 200), (220), and (311) respectively (Figure 4a).These correspond to the face-centered cubic (fcc) structure of metallic Ag.Similar observations have been reported in other research works [41,42].For the N7/Ag-Ti5 composite (Figure 4b), the peaks at 25.60°, 38.01°, 48.21°, 54.02°, 55.36°, 62.93° and 68.95° were observed, which were assigned to the (101), ( 004), ( 200), ( 105), ( 211), ( 204) and (116) of anatase TiO2, respectively [43].However, the peaks corresponding to silver nanoparticles were less prominent.Characteristics natural rubber containing silver nanoparticles (N7/Ag), and modified natural rubber filled with titanium dioxide (N7-Ti).The formulations and characteristics of all samples after drying are shown in Table 2. Sample N7 had a light-yellow color, while the color of the N7/Ag composite was an opaque dark brown, attributed to the occurrence of silver nanoparticles within the natural rubber-based composite [37].In the case of the incorporation of titanium dioxide, the samples became lighter brown with an increasing amount of titanium dioxide.Meanwhile, N7-Ti without silver nanoparticles exhibited an opaque white color.The formation of silver nanoparticles was examined using a UV-vis spectrophotometer, as displayed in Figure 3.As seen from the results, the N7/Ag composite exhibits a strong absorption peak at around 435 nm, while this peak is not observed in the case of N7 mixed with AgNO3 solution.This peak corresponds to the surface plasmon resonance absorption of silver nanoparticles [38,39], indicating the formation of silver nanoparticles in natural rubber-based composites.Based on the estimation of the particle size of silver nanoparticles from UV-vis spectroscopy, as reported by Alim-Al-Razy et al. [40], it was found that the particle size of silver nanoparticles was approximately 65 nm.Furthermore, the XRD pattern of the N7/Ag composite shows diffraction peaks at 38.33°, 42.83°, 64.49°, and 77.21°, corresponding to the reflection plane indices of (111), ( 200), (220), and (311), respectively (Figure 4a).These correspond to the face-centered cubic (fcc) structure of metallic Ag.Similar observations have been reported in other research works [41,42].For the N7/Ag-Ti5 composite (Figure 4b), the peaks at 25.60°, 38.01°, 48.21°, 54.02°, 55.36°, 62.93°, and 68.95° were observed, which were assigned to the (101), ( 004), ( 200), ( 105), ( 211), (204), and (116) of anatase TiO2, respectively [43].However, the peaks corresponding to silver nanoparticles were less prominent.Characteristics gated and compared to samples prepared from modified natural rubber (N7), modified natural rubber containing silver nanoparticles (N7/Ag), and modified natural rubber filled with titanium dioxide (N7-Ti).The formulations and characteristics of all samples after drying are shown in Table 2. Sample N7 had a light-yellow color, while the color of the N7/Ag composite was an opaque dark brown, attributed to the occurrence of silver nanoparticles within the natural rubber-based composite [37].In the case of the incorporation of titanium dioxide, the samples became lighter brown with an increasing amount of titanium dioxide.Meanwhile, N7-Ti without silver nanoparticles exhibited an opaque white color.The formation of silver nanoparticles was examined using a UV-vis spectrophotometer, as displayed in Figure 3.As seen from the results, the N7/Ag composite exhibits a strong absorption peak at around 435 nm, while this peak is not observed in the case of N7 mixed with AgNO3 solution.This peak corresponds to the surface plasmon resonance absorption of silver nanoparticles [38,39], indicating the formation of silver nanoparticles in natural rubber-based composites.Based on the estimation of the particle size of silver nanoparticles from UV-vis spectroscopy, as reported by Alim-Al-Razy et al. [40], it was found that the particle size of silver nanoparticles was approximately 65 nm.Furthermore, the XRD pattern of the N7/Ag composite shows diffraction peaks at 38.33°, 42.83°, 64.49°, and 77.21°, corresponding to the reflection plane indices of ( 111), ( 200), (220), and (311), respectively (Figure 4a).These correspond to the face-centered cubic (fcc) structure of metallic Ag.Similar observations have been reported in other research works [41,42].For the N7/Ag-Ti5 composite (Figure 4b), the peaks at 25.60°, 38.01°, 48.21°, 54.02°, 55.36°, 62.93°, and 68.95° were observed, which were assigned to the (101), ( 004), ( 200), ( 105), ( 211), (204), and (116) of anatase TiO2, respectively [43].However, the peaks corresponding to silver nanoparticles were less prominent.Characteristics gated and compared to samples prepared from modified natural rubber (N7), modified natural rubber containing silver nanoparticles (N7/Ag), and modified natural rubber filled with titanium dioxide (N7-Ti).The formulations and characteristics of all samples after drying are shown in Table 2. Sample N7 had a light-yellow color, while the color of the N7/Ag composite was an opaque dark brown, attributed to the occurrence of silver nanoparticles within the natural rubber-based composite [37].In the case of the incorporation of titanium dioxide, the samples became lighter brown with an increasing amount of titanium dioxide.Meanwhile, N7-Ti without silver nanoparticles exhibited an opaque white color.The formation of silver nanoparticles was examined using a UV-vis spectrophotometer, as displayed in Figure 3.As seen from the results, the N7/Ag composite exhibits a strong absorption peak at around 435 nm, while this peak is not observed in the case of N7 mixed with AgNO3 solution.This peak corresponds to the surface plasmon resonance absorption of silver nanoparticles [38,39], indicating the formation of silver nanoparticles in natural rubber-based composites.Based on the estimation of the particle size of silver nanoparticles from UV-vis spectroscopy, as reported by Alim-Al-Razy et al. [40], it was found that the particle size of silver nanoparticles was approximately 65 nm.Furthermore, the XRD pattern of the N7/Ag composite shows diffraction peaks at 38.33°, 42.83°, 64.49°, and 77.21°, corresponding to the reflection plane indices of ( 111), ( 200), (220), and (311), respectively (Figure 4a).These correspond to the face-centered cubic (fcc) structure of metallic Ag.Similar observations have been reported in other research works [41,42].For the N7/Ag-Ti5 composite (Figure 4b), the peaks at 25.60°, 38.01°, 48.21°, 54.02°, 55.36°, 62.93°, and 68.95° were observed, which were assigned to the (101), ( 004), ( 200), ( 105), ( 211), (204), and (116) of anatase TiO2, respectively [43].However, the peaks corresponding to silver nanoparticles were less prominent.

Morphology
The cross-sectional morphology and EDS analysis of the (PAA-co-PAM)-DPNR/Ag TiO2 composites in the solid stage are demonstrated in Figure 5.The dried samples were frozen in liquid nitrogen and then broken.All samples exhibited a dense solid cross-sec tion surface, as observed from SEM images (first column).Upon adding titanium dioxide to the system, the roughness of the composites was enhanced compared to N7 and N7/Ag Their EDS spectra are also displayed in the second column.The characteristic signals o C, O, and Na appeared for the sample N7.An additional peak corresponding to Ag was found in the case of N7/Ag and (PAA-co-PAM)-DPNR/Ag-TiO2 composites.For (PAA-co PAM)-DPNR/Ag-TiO2 composites, additional signals corresponding to Ti were also found.The elemental compositions of the composites from the EDS analysis are reported in Table 3.It was clearly observed that the weight and atomic percentages of titanium in creased relative to the amount of titanium dioxide.The EDS mappings corresponding to Ag and Ti are shown in the third and fourth columns, respectively.The good distribution of Ag in the matrix is observed, and the presence of Ti dispersed in the natural rubber based matrix was also noted.The size of the titanium dioxide particles was about 208.56 ± 39.77 nm.However, with an increase in the titanium dioxide content, there was an ob served increase in the extent of particle agglomeration, with sizes in the range of 0.82-6.43µm.In addition, the cross-sectional morphology of samples after immersion in water was also examined.The samples were soaked in water and subjected to freeze-drying.Their cross-section surfaces were also observed from SEM images, as shown in Figure 6.It was noted that the morphology of all samples changed from a dense solid surface to a porous structure after soaking in water.The formation of the porous structure could be explained

Morphology
The cross-sectional morphology and EDS analysis of the (PAA-co-PAM)-DPNR/Ag-TiO 2 composites in the solid stage are demonstrated in Figure 5.The dried samples were frozen in liquid nitrogen and then broken.All samples exhibited a dense solid cross-section surface, as observed from SEM images (first column).Upon adding titanium dioxide to the system, the roughness of the composites was enhanced compared to N7 and N7/Ag.Their EDS spectra are also displayed in the second column.The characteristic signals of C, O, and Na appeared for the sample N7.An additional peak corresponding to Ag was found in the case of N7/Ag and (PAA-co-PAM)-DPNR/Ag-TiO 2 composites.For (PAA-co-PAM)-DPNR/Ag-TiO 2 composites, additional signals corresponding to Ti were also found.The elemental compositions of the composites from the EDS analysis are reported in Table 3.It was clearly observed that the weight and atomic percentages of titanium increased relative to the amount of titanium dioxide.The EDS mappings corresponding to Ag and Ti are shown in the third and fourth columns, respectively.The good distribution of Ag in the matrix is observed, and the presence of Ti dispersed in the natural rubber-based matrix was also noted.The size of the titanium dioxide particles was about 208.56 ± 39.77 nm.However, with an increase in the titanium dioxide content, there was an observed increase in the extent of particle agglomeration, with sizes in the range of 0.82-6.43µm.In addition, the cross-sectional morphology of samples after immersion in water was also examined.The samples were soaked in water and subjected to freeze-drying.Their cross-section surfaces were also observed from SEM images, as shown in Figure 6.It was noted that the morphology of all samples changed from a dense solid surface to a porous structure after soaking in water.The formation of the porous structure could be explained by the penetration of water molecules into the crosslinked sample structure.After freeze-drying, the trapped water sublimated, resulting in the porous morphology [44,45].Samples with many pores in their structure had advantages for applications.In the adsorption process, adsorbate molecules were initially absorbed on the outer surface and then diffused into the inner layer [46].Therefore, the formation of a porous framework could provide a high surface area for binding with other substances in an aqueous solution, offering benefits for the adsorption process.Moreover, roughness at the pore wall was observed when titanium dioxide was introduced.In particular, when the amount of titanium dioxide increased to 5.0 phr, titanium dioxide could appear on the wall surface.
ples with many pores in their structure had advantages for applications.In the adsorption process, adsorbate molecules were initially absorbed on the outer surface and then diffused into the inner layer [46].Therefore, the formation of a porous framework could provide a high surface area for binding with other substances in an aqueous solution, offering benefits for the adsorption process.Moreover, roughness at the pore wall was observed when titanium dioxide was introduced.In particular, when the amount of titanium dioxide increased to 5.0 phr, titanium dioxide could appear on the wall surface.

Swelling Degree
Figure 7 shows the swelling degree of the composites against immersion time.As a result, the swelling degree increased with the immersion time of the samples until reaching the equilibrium point.The swelling degree of N7 was found to be 3180.6± 273.0% after 24 h.In this case, the main matrix is natural rubber modified with hydrophilic PAA-co-PAM, allowing water to diffuse into its structure.For the composites comprising silver nanoparticles and titanium dioxide, there was a slight increase in swelling degree (3374.5 ± 40.6 to 3434.2 ± 88.1%) when the titanium dioxide was 1.0-2.5 phr.This increase is attributed to the enhancement of the polarity of the composites.When the titanium dioxide content changed to 5.0 phr, the swelling degree decreased.This decrease may be due to the agglomeration of particles, as seen from the SEM images.It was observed that the composite filled with titanium dioxide without silver nanoparticles at the same amount of titanium dioxide (5.0 phr) exhibited a decrease in swelling degree (2055.8± 76.8%) compared to N7.This is probably because of the weak interaction with the main matrix, and a large number of titanium dioxide particles were deposited on the wall of the matrix, blocking water absorption.The swelling degree of the composite containing only silver nanoparticles remained high swelling degree (3229.0 ± 98.3%).It can be explained by Hou et al., where a stable PAA/silver nanoparticle gel was obtained due to the coordination between PAA and silver ion as well as a self-assembly process [47].Therefore, the strong

Swelling Degree
Figure 7 shows the swelling degree of the composites against immersion time.As a result, the swelling degree increased with the immersion time of the samples until reaching the equilibrium point.The swelling degree of N7 was found to be 3180.6± 273.0% after 24 h.In this case, the main matrix is natural rubber modified with hydrophilic PAA-co-PAM, allowing water to diffuse into its structure.For the composites comprising silver nanoparticles and titanium dioxide, there was a slight increase in swelling degree (3374.5 ± 40.6 to 3434.2 ± 88.1%) when the titanium dioxide was 1.0-2.5 phr.This increase is attributed to the enhancement of the polarity of the composites.When the titanium dioxide content changed to 5.0 phr, the swelling degree decreased.This decrease may be due to the agglomeration of particles, as seen from the SEM images.It was observed that the composite filled with titanium dioxide without silver nanoparticles at the same amount of titanium dioxide (5.0 phr) exhibited a decrease in swelling degree (2055.8± 76.8%) compared to N7.This is probably because of the weak interaction with the main matrix, and a large number of titanium dioxide particles were deposited on the wall of the matrix, blocking water absorption.The swelling degree of the composite containing only silver nanoparticles remained high swelling degree (3229.0 ± 98.3%).It can be explained by Hou et al., where a stable PAA/silver nanoparticle gel was obtained due to the coordination between PAA and silver ion as well as a self-assembly process [47].Therefore, the strong interaction between PAA and silver plays an important role in preparing materials with good structural stability and high water absorption.interaction between PAA and silver plays an important role in preparing materials with good structural stability and high water absorption.

Compressive Properties
The mechanical properties of the prepared composites were investigated through compression testing.As these composites were employed as adsorbents in an aqueous medium, their compressive properties were examined under swollen conditions.The stress-strain curve of the swollen samples is displayed in Figure 8a.It was observed that the stress increased when the compression strain increased.The result revealed that the samples had high endurance to external compressive loading.The compressive strength at 80% strain and compressive modulus are shown in Figure 8b and 8c, respectively.The compressive strength and compressive modulus of N7 were found to be 199.65 ± 5.01 and 1.015 ± 0.062 KPa, respectively.The samples incorporating silver nanoparticles exhibited high compressive strength and modulus.Thus, the coordination of carboxylic groups in the composites and silver ions contributes to the enhancement of mechanical properties through the formation of a strong network inside the structure [48].For the N7/Ag composite, its compressive strength and compressive modulus increased to 317.04 ± 5.05 and 1.724 ± 0.035 KPa, respectively.When titanium dioxide was introduced, their compressive strength values slightly increased.They seemed to increase with the increase in titanium dioxide contents.The compressive strength increased from 323.18 ± 10.41 to 347.91 ± 11.42 KPa, and the compressive modulus increased from 1.902 ± 0.033 to 2.283 ± 0.043 KPa when the titanium dioxide increased from 1.0 to 5.0 phr.It was observed that the compressive strength and compressive modulus of the N7/Ag-Ti5.0composite increased by 1.80 and 2.25 times compared to N7.However, the compressive strength of N7-Ti5 slightly decreased from N7.This may be due to the weak interaction between modified natural rubber and titanium dioxide.Therefore, the combination of silver nanoparticles and titanium dioxide in the composites could improve the mechanical properties of natural rubberbased materials.Moreover, the samples could maintain their integrity after applying external force without fracturing.Considering the sample structure, the main component of these samples is composed of natural rubber, which exhibits high elasticity [49].Natural rubber-based materials can quickly return to their original or similar sizes after deformation.This indicates that the prepared composites exhibited good structural stability and mechanical properties, which would be effective for various applications.

Compressive Properties
The mechanical properties of the prepared composites were investigated through compression testing.As these composites were employed as adsorbents in an aqueous medium, their compressive properties were examined under swollen conditions.The stress-strain curve of the swollen samples is displayed in Figure 8a.It was observed that the stress increased when the compression strain increased.The result revealed that the samples had high endurance to external compressive loading.The compressive strength at 80% strain and compressive modulus are shown in Figure 8b,c, respectively.The compressive strength and compressive modulus of N7 were found to be 199.65 ± 5.01 and 1.015 ± 0.062 KPa, respectively.The samples incorporating silver nanoparticles exhibited high compressive strength and modulus.Thus, the coordination of carboxylic groups in the composites and silver ions contributes to the enhancement of mechanical properties through the formation of a strong network inside the structure [48].For the N7/Ag composite, its compressive strength and compressive modulus increased to 317.04 ± 5.05 and 1.724 ± 0.035 KPa, respectively.When titanium dioxide was introduced, their compressive strength values slightly increased.They seemed to increase with the increase in titanium dioxide contents.The compressive strength increased from 323.18 ± 10.41 to 347.91 ± 11.42 KPa, and the compressive modulus increased from 1.902 ± 0.033 to 2.283 ± 0.043 KPa when the titanium dioxide increased from 1.0 to 5.0 phr.It was observed that the compressive strength and compressive modulus of the N7/Ag-Ti5.0composite increased by 1.80 and 2.25 times compared to N7.However, the compressive strength of N7-Ti5 slightly decreased from N7.This may be due to the weak interaction between modified natural rubber and titanium dioxide.Therefore, the combination of silver nanoparticles and titanium dioxide in the composites could improve the mechanical properties of natural rubber-based materials.Moreover, the samples could maintain their integrity after applying external force without fracturing.Considering the sample structure, the main component of these samples is composed of natural rubber, which exhibits high elasticity [49].Natural rubber-based materials can quickly return to their original or similar sizes after deformation.This indicates that the prepared composites exhibited good structural stability and mechanical properties, which would be effective for various applications.The dye adsorption ability of different types of (PAA-co-PAM)-DPNR was examined, as shown in Figure 9.The study of dye adsorption using methylene blue (MB) as a dye model was conducted.Based on the characteristics of the samples, they exhibited a porous structure after immersion in an aqueous medium so that the dye solution could diffuse into the sample structure.According to the chemical structure of (PAA-co-PAM)-DPNR, it possesses specific functional groups with the ability to adsorb MB molecules [50,51].Thus, these materials could effectively remove MB molecules from the aqueous medium.The effect of MBA content on the dye adsorption ability was studied, as displayed in Figure 9a.In the 0-24 h range, the removal efficiency values increased with immersion time due to the abundance of active sites that could interact with dye molecules in the initial period.After 24 h, the removal efficiency values remained constant.It was observed that the (PAA-co-PAM)-modified DPNR with a higher content of MBA crosslinking agent exhibited a higher removal efficiency.The removal efficiency values increased from 14.13 ± 1.51 to 40.87 ± 0.92%, while the MBA content increased from 0 to 1.00%.This can be explained by the addition of MBA, which improves the ability to hold (PAA-co-PAM) on its structure, increasing dye adsorption ability.In addition, the (PAA-co-PAM)-modified DPNR with the weight ratio of acrylic acid and acrylamide at 70:30 showed higher removal efficiency than those of other conditions, as shown in Figure 9b.Under these conditions, the obtained (PAA-co-PAM) modified DPNR had high grafting efficiency, indicating a significant number of active functional groups available to adsorb dye molecules.Moreover, it was probably because the interaction of carboxylate groups and dye was stronger than that of -NH2 groups [52].Furthermore, the result from Figure 9c shows that the (PAA-co-PAM)-modified DPNR prepared using 20 phr of monomer exhibited higher removal efficiency than that of 10 phr of monomer.Its removal efficiency was found to be 88.49± 3.21% after being immersed for 24 h.These indicate that the efficiency of dye adsorption depends on the amount of PAA-co-PAM present in the structure.The dye adsorption ability of different types of (PAA-co-PAM)-DPNR was examined, as shown in Figure 9.The study of dye adsorption using methylene blue (MB) as a dye model was conducted.Based on the characteristics of the samples, they exhibited a porous structure after immersion in an aqueous medium so that the dye solution could diffuse into the sample structure.According to the chemical structure of (PAA-co-PAM)-DPNR, it possesses specific functional groups with the ability to adsorb MB molecules [50,51].Thus, these materials could effectively remove MB molecules from the aqueous medium.The effect of MBA content on the dye adsorption ability was studied, as displayed in Figure 9a.In the 0-24 h range, the removal efficiency values increased with immersion time due to the abundance of active sites that could interact with dye molecules in the initial period.After 24 h, the removal efficiency values remained constant.It was observed that the (PAA-co-PAM)-modified DPNR with a higher content of MBA crosslinking agent exhibited a higher removal efficiency.The removal efficiency values increased from 14.13 ± 1.51 to 40.87 ± 0.92%, while the MBA content increased from 0 to 1.00%.This can be explained by the addition of MBA, which improves the ability to hold (PAA-co-PAM) on its structure, increasing dye adsorption ability.In addition, the (PAA-co-PAM)-modified DPNR with the weight ratio of acrylic acid and acrylamide at 70:30 showed higher removal efficiency than those of other conditions, as shown in Figure 9b.Under these conditions, the obtained (PAA-co-PAM) modified DPNR had high grafting efficiency, indicating a significant number of active functional groups available to adsorb dye molecules.Moreover, it was probably because the interaction of carboxylate groups and dye was stronger than that of -NH 2 groups [52].Furthermore, the result from Figure 9c shows that the (PAA-co-PAM)-modified DPNR prepared using 20 phr of monomer exhibited higher removal efficiency than that of 10 phr of monomer.Its removal efficiency was found to be 88.49± 3.21% after being immersed for 24 h.These indicate that the efficiency of dye adsorption depends on the amount of PAA-co-PAM present in the structure.

Effect of Type of (PAA-co-PAM)-DPNR/Ag-TiO2 Composites
The comparison in dye adsorption ability of the prepared composites is illustrated in Figure 10.In the initial stage, the removal efficiency increased with the immersion time due to numerous available active sites interacting with dye molecules.After that, the removal efficiency gradually reached a constant value.At 12 h, the N7 had a removal efficiency of 67.96 ± 2.20%.The removal efficiency was enhanced when using the (PAA-co-PAM)-modified natural rubber consisting of silver nanoparticles and titanium dioxide.The removal efficiency of N7/Ag composite was 92.37 ± 1.18% after being immersed in dye solution for 12 h.As seen from the results, the N7/Ag-TiO2 composites exhibited higher removal efficiency than N7 and N7/Ag and seemed to increase with the increase in titanium dioxide content.The removal efficiency values of N7/Ag-TiO2 composites were found to be 93.47 ± 1.87, 94.19 ± 1.71, and 96.28 ± 1.38% when the titanium dioxide content was adjusted to 1.0, 2.5, and 5.0 phr, respectively.However, the modified natural rubber containing only titanium dioxide did not show an increase in adsorption efficiency.Generally, titanium dioxide is used as a photocatalyst for the photocatalytic degradation of various pollutants, capable of degrading chemical contaminants.However, its performance is notably enhanced under UV radiation [53].The synergistic removal efficiency for modified natural rubber containing silver nanoparticles/titanium dioxide was observed.This is probably because the incorporation of silver nanoparticles can enhance the response to visible light [54].The transfer of electrons from Ag to TiO2 is mediated by localized surface plasmon resonance, which then reacts with adsorbed molecules [55].Additionally, the holes in plasmonic silver nanoparticles can capture conduction electrons of TiO2, therefore preventing the charge recombination phenomenon [56].Therefore, the presence of silver nanoparticles and titanium dioxide distributed in the natural rubberbased matrix promotes dye-removal efficiency.The comparison in dye adsorption ability of the prepared composites is illustrated in Figure 10.In the initial stage, the removal efficiency increased with the immersion time due to numerous available active sites interacting with dye molecules.After that, the removal efficiency gradually reached a constant value.At 12 h, the N7 had a removal efficiency of 67.96 ± 2.20%.The removal efficiency was enhanced when using the (PAAco-PAM)-modified natural rubber consisting of silver nanoparticles and titanium dioxide.The removal efficiency of N7/Ag composite was 92.37 ± 1.18% after being immersed in dye solution for 12 h.As seen from the results, the N7/Ag-TiO 2 composites exhibited higher removal efficiency than N7 and N7/Ag and seemed to increase with the increase in titanium dioxide content.The removal efficiency values of N7/Ag-TiO 2 composites were found to be 93.47 ± 1.87, 94.19 ± 1.71, and 96.28 ± 1.38% when the titanium dioxide content was adjusted to 1.0, 2.5, and 5.0 phr, respectively.However, the modified natural rubber containing only titanium dioxide did not show an increase in adsorption efficiency.Generally, titanium dioxide is used as a photocatalyst for the photocatalytic degradation of various pollutants, capable of degrading chemical contaminants.However, its performance is notably enhanced under UV radiation [53].The synergistic removal efficiency for modified natural rubber containing silver nanoparticles/titanium dioxide was observed.This is probably because the incorporation of silver nanoparticles can enhance the response to visible light [54].The transfer of electrons from Ag to TiO 2 is mediated by localized surface plasmon resonance, which then reacts with adsorbed molecules [55].Additionally, the holes in plasmonic silver nanoparticles can capture conduction electrons of TiO 2 , therefore preventing the charge recombination phenomenon [56].Therefore, the presence of silver nanoparticles and titanium dioxide distributed in the natural rubber-based matrix promotes dye-removal efficiency.

Adsorption Kinetic
The dye adsorption kinetic was studied using N7 and N7/Ag-Ti5.0composites.Figure 11 shows the removal efficiency and adsorption capacity of both samples at different MB concentrations.It can be observed that the equilibrium-adsorption capacity (Q e ) of N7 and N7/Ag-Ti5.0increased with increasing dye concentration.The N7/Ag-Ti5.0 composite exhibited a higher rate of dye adsorption compared to N7.The Q e value of N7/Ag-Ti5.0was higher than that of N7 at the same initial concentration of dye.To study the dye adsorption kinetic, the pseudo-first-order and pseudo-second-order models were applied to fit the experimental data.Table 4 displays the calculated kinetic parameters of MB onto N7 and N7/Ag-Ti5.0at different initial dye concentrations.As seen from the results, the pseudo-second-order kinetic model provided better agreement due to higher correlation coefficient (R 2 ) values.The adsorption capacity values of the experiment were also close to those of theoretical adsorption capacity calculated from the pseudo-second-order kinetic model.These indicate that the adsorption process occurred through chemisorption [57].

Adsorption Kinetic
The dye adsorption kinetic was studied using N7 and N7/Ag-Ti5.0composites.Figure 11 shows the removal efficiency and adsorption capacity of both samples at different MB concentrations.It can be observed that the equilibrium-adsorption capacity (Qe) of N7 and N7/Ag-Ti5.0increased with increasing dye concentration.The N7/Ag-Ti5.0 composite exhibited a higher rate of dye adsorption compared to N7.The Qe value of N7/Ag-Ti5.0was higher than that of N7 at the same initial concentration of dye.To study the dye adsorption kinetic, the pseudo-first-order and pseudo-second-order models were applied to fit the experimental data.Table 4 displays the calculated kinetic parameters of MB onto N7 and N7/Ag-Ti5.0at different initial dye concentrations.As seen from the results, the pseudo-second-order kinetic model provided better agreement due to higher correlation coefficient (R 2 ) values.The adsorption capacity values of the experiment were also close to those of theoretical adsorption capacity calculated from the pseudo-second-order kinetic model.These indicate that the adsorption process occurred through chemisorption [57].Table 4. Pseudo-first-order kinetic and pseudo-second-order kinetic parameters of dye onto N7 and N7/Ag-Ti5.0.
Pseudo-First-Order Pseudo-Second-Order

Figure 1 .
Figure 1.Schematic illustration of the preparation of the (PAA-co-PAM)-DPNR/Ag-TiO2 composites for the adsorption of dye molecules.

Figure 1 .
Figure 1.Schematic illustration of the preparation of the (PAA-co-PAM)-DPNR/Ag-TiO 2 composites for the adsorption of dye molecules.

Figure 9 .
Figure 9.Effect of (a) MBA crosslinker content, (b) weight ratio of acrylic acid and acrylamide, and (c) monomer content on dye adsorption ability.

Figure 9 .
Figure 9.Effect of (a) MBA crosslinker content, (b) weight ratio of acrylic acid and acrylamide, and (c) monomer content on dye adsorption ability.

Polymers 2024 , 22 Figure 10 .
Figure 10.Effect of type of adsorbents on the removal efficiency after being immersed in the dye solution.

Figure 10 . 22 Figure 11 .
Figure 10.Effect of type of adsorbents on the removal efficiency after being immersed in the dye solution.Polymers 2024, 16, x FOR PEER REVIEW 17 of 22
* part per hundred rubber; ** percentage by weight of monomer added.

Table 2 .
Composition of ingredients and physical appearances of the (PAA-co-PAM)-DPNR/A TiO2 composites.