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Article

Preparation and Characterization of Supramolecular Bonding Polymers Based on a Pullulan Substrate Grafted with Acrylic Acid/Acrylamide by Microwave Irradiation

by
Salam Abdulla Dhahir
,
Auda Jabbar Braihi
and
Salih Abbas Habeeb
*
Polymer and Petrochemical Engineering Department, College of Engineering Materials, University of Babylon, Babylon 51002, Iraq
*
Author to whom correspondence should be addressed.
ChemEngineering 2024, 8(4), 77; https://doi.org/10.3390/chemengineering8040077
Submission received: 23 May 2024 / Revised: 8 July 2024 / Accepted: 26 July 2024 / Published: 29 July 2024
(This article belongs to the Special Issue Supramolecular Synthesis in Chemical Engineering)
Browse Figures
Versions Notes

Abstract

:
A microwave technique was used to prepare a superabsorbent polymer (SAP) by grafting two hydrophilic monomers onto a polysaccharide substrate. The monomers used were acrylic acid (AA) or acrylamide (AM) and were grafted onto a pullulan (PUL) substrate to form PUL-g-AA (SAP1) and PUL-g-AM (SAP2), respectively. The monomers (AM/AA) were grafted together onto a PUL substrate to form PUL-g-(AM/AA) (SAP3). Grafting parameters such as grafting efficiency with the percentage, the conversion of monomer into polymer, gel content, water retention, water adsorption capacity, and swelling kinetics were determined. Additionally, the effect of environmental pH (2, 4, 7, 9, and 12) and sodium dodecylbenzene sulfonate (SDBS) surfactant was evaluated, where 1, 2, 3, 4, and 5 mM of SDBS was added to form SAP4 to SAP8. The FTIR results show that AM was grafted onto PUL through an aliphatic C-N bond, while AA grafting occurred through a single C-C bond. The grafting efficiency with AM was higher than with AA, as well as showing a superior gel content. Water absorbance capacity and water retention increased with the grafting of AA and AM together for SAP3. The highest absorbent capacity, water retention, gel content, and grafting parameters values were obtained with a 3 mM SDBS content and a pH of 7. The swelling kinetics showed that the increases in the theoretical and experimental swelling equilibriums were 72% and 82%, respectively, for SAP6 compared to the values of these parameters for SAP3. The water absorption capacity of the hydrogel increases upon increasing the pH to 7 and then gradually decreases. XRD demonstrated the improved crystallinity and crystalline size of the hydrogel after grafting polymerization of AM/AA onto PUL, in addition to enhanced thermal stability. On the contrary, FE-SEM demonstrated that SDBS improves the porosity and pore size of the hydrogel surface with SAP6.

1. Introduction

Super absorbent polymers (SAPs) contain large hydrophilic networks that allow them to absorb and retain large amounts of water or aqueous solutions, and it is difficult to remove the absorbed water with simple pressure [1]. These polymers have 3D networks that can absorb large amounts of water, up to 300 times their initial mass or more [2]. SAPs may be natural or synthetic depending on where they came from [3,4].
Ideal SAPs have a high swelling capacity, a rapid water absorption capacity, and high reusability [5]. The ability of SAPs to absorb water depends on the hydrogen bonds with water molecules, the ionic concentration of the aqueous solution, and the type of bonds used to make the gel [6]. Polymers with a high absorption ability are SAPs with a low cross-linking density and lead to the formation of a softer gel [7]. Polymers with high-density cross-linked networks are characterized by low absorption and lead to the formation of a gel that is not flexible under the influence of simple pressure [8].
The biodegradability of natural SAPs such as cellulose, starch, and chitosan is a clear benefit. However, they are affected by requiring a high amount of water despite a low absorption rate [9,10]. In contrast, despite the benefits of their low cost, long service life, and high water absorption rate, the non-degradable characteristics of synthetic SAPs such as poly acrylic acid (PAA) and polyacrylamide (PAM) can have negative effects on the environment and plant growth [11,12]. There are many methods to improve the efficacy of SAPs, improve their hydrophilicity, and develop specialized network architectures, such as interpenetrating polymer networks (IPNs), semi-interpenetrating polymer networks (semi-IPNs), and copolymer networks [13]. Among the many uses of SAP, they are used to manage the release of nutrients and to improve the efficiency with which soil uses water [5]. Therefore, the establishment of a porous structure is the key to enhancing swelling rate and regulating their properties [8,14]. Currently, there are several technologies for the fabrication of porous hydrogels, including freeze-drying [15], foaming [16], water-soluble porogens [17], and the phase inversion technique [18].
Therefore, most of the above techniques are characterized by producing hydrogels with a large pore size distribution and closed pores [19,20]. Grafting polymerization is one important method for the chemical modification of polymers’ specifications. Grafting methods include the use of microwave radiation, plasma, ozone, and photometry [21]. Polymerization is a method to determine the physical and chemical properties of a polymer by polymerizing its surfaces using grafting methods, with good behavior of the polymers resulting after modifying their surfaces using graft polymerization [22]. Polysaccharides such as starch, cellulose, and chitin are polysaccharides that contain biomolecules including long-chain carbohydrates and several smaller monosaccharides [23].
Pullulan is a natural water-soluble polysaccharide with repeating units of malt triose residues extracted from the yeast-like fungus Aureobasidium [24,25]. Due to its significant role in reducing ionic surface tension, sodium dodecylbenzene sulfonate (SDBS) is widely used in household detergents [26]. In addition, SDBS increases hydrogels’ surface porosity and swelling kinetics according to its concentration in superabsorbent polymers [27]. Microwave irradiation is frequently used in polymerization grafting reactions, especially when many monomers are grafted onto natural polymers, such as grafting acrylonitrile monomers onto guar gum, acrylamide onto chitosan, and methyl methacrylate onto bamboo cellulose [28], in addition to the use of microwave radiation in the development of cross-linking in hydrogels, which does not require a catalyst [29], instead using high-frequency electromagnetic waves [30]. Microwave irradiation has the advantages of a low reaction time, an increased production capacity, and high reaction control in terms of producing few by-products and being environmentally friendly [31]. Microwaves can provide the energy necessary to encourage the initiator to enter the reaction by overcoming the energies of some of its bonds, breaking them and forming free radicals, which represent active sites for starting the reaction [32].
This research aims to prepare super absorbent polymers (SAPs) using grafting polymerization (microwave irradiation) of acrylic acid and acrylamide monomers based on pullulan as the substrate and then compare the effects of SDBS surfactant and environmental pH from the viewpoint of their swelling parameters.

2. The Experimental Section

2.1. Materials and Methods

Pullulan (PUL) is used as a polysaccharide substrate, while acrylamide (AM) and acrylic acid (AA) monomers are used for grafting purposes. Sodium dodecylbenzene sulfonate (SDBS) (MW = 348 g/mol) is a surfactant. The chemicals needed to complete the research were purchased from Merck and used without further purification for the following purposes: potassium persulfate (KPS) was used as the initiator in an inorganic chemical reaction, and N, N-methylene bisacrylamide (MBA) was used to form cross-links. N, N-, N-tetramethylene diamine (TEMED) was used as an accelerator for the polymerization reactions, while sodium hydroxide (NaOH) and solvents such as distaste water (DW), methanol, and ethanol were used for acid neutralization and washing, respectively.

2.2. Preparation and Absorption Measurement of SAPs

2.2.1. Preparation of SAPs

Superabsorbent polymers (SAPs) were made by grafting monomers (AM/AA) onto PUL according to the following steps:
  • Mixing monomers step: The AA and AM monomer solutions were mixed, and MBA solution was added (0.04 g in 5 mL of DW). The AM monomer solution was prepared by dissolving 6 g in 12 mL of DW with continuous stirring. The AA solution was prepared in an ice bath to avoid polymerization by dissolving 6 g of AA monomer with partial neutralization (80% by weight) using 5 M NaOH.
  • Substrate preparation step: The pullulan substrate concentration was prepared by dissolving 1.20 g of PUL in 35 mL of distilled water, at about 3.2 wt.%. The solution was mixed by a mechanical mixer at 60 °C for 15 min, and then aqueous solutions of SDBS surfactants at different concentrations (0, 1, 2, 3, and 4 mM) were continuously mixed until a homogeneous solution was obtained.
  • Generation of free radicals: To facilitate the grafting step, free radicals were generated using a redox initiator system composed of two solutions: 0.118 g of KPS and 0.051 g of TEMED (each in 5 mL of DW). These solutions were added dropwise to the PUL solution at a temperature of 60 °C for 10 min in the presence of strong mechanical mixing.
  • Grafting step: The monomer solution (AM/AA) was added to the PUL solution after it was cooled to 50 °C, with mechanical mixing at a rotational speed of 1050 rpm for 15 min. The final solution of the reaction mixture was supplemented with 100 mL of added DW, and the final solution was treated in a microwave oven at 475 watts for 4 min. The temperature and viscosity of the mixture gradually increased, and the gelation point was reached after 250 s; the product was a yellowish elastic gel. The SAP grafting polymerization reactions are explained in Figure 1.
  • Treatment of the final product: To remove the remaining surfactant, the resulting elastic gel was cut into small pieces, and the resulting gel was washed several times with ethanol/water (10:1, v/v). Then, the unreacted materials (PUL, AA, AM, KPS, TEMED, and MBA) were completely removed after immersion in pure methanol for 24 h to dehydrate and dissolve without reacting. The final product was washed with ethanol and then ground and baked in an oven at 60 °C for 24 h [27,33].

2.2.2. Swelling Measurements

The water absorbency of the hydrogels is measured using the free swelling method and is calculated in grams of water per 1 g of the hydrogel. Thus, an accurately weighed quantity of the polymer under investigation (0.1 g) is immersed in 500 mL of distilled water at room temperature for at least 4 h. Then, the swollen sample is filtered through a weighed 100-mesh (150 µm) sieve until water ceases to drop. The weight of the hydrogel containing absorbed water is measured after draining, and the water absorbency is calculated according to the following equation [34,35]:
W A C = W s − W d / W d
where W A C   is the water absorbence capacity, and ( W s , W d ) are the weights of the swollen specimen and the dry specimen before washing, respectively. The swelling kinetics was studied at different concentrations of SDBS (0, 1, 2, 3, and 4 mM) for several consecutive periods (1800, 3600, 5400, 7200, 9000, 10,800, 12,600, and 14,400 s). In addition, the effects of pH on swelling capacity were measured for aqueous solutions of various pH values (2, 4, 7, 9, and 12) at room temperature.

2.2.3. Measurement of Grafting Parameters

Gel Content: To measure the gel content, accurately weighed dried samples of the SAPs are dispersed in distilled water to swell completely. Then, the swollen SAPs are filtered and washed with distilled water frequently. The samples are dewatered in excess ethanol for 48 h and dried at 50 C for 12 h until the SAPs have a constant weight. The gel content is defined as the following equation [36]:
G e l % = W d / W i ∗ 100
where W d and W i   are the weight of the dried hydrogel after extraction and the initial weight of the SAP, respectively.
Grafting Polymerization: The weight percentage of the synthetic polymer branches grafted onto the functional groups of the pullulan to the total synthetic polymer, including grafted branches and undrafted photopolymers, is calculated. Grafting percentage (G (%)), grafting efficiency ( E ( % ) ), and the conversion of monomers into polymers (C (%)) are determined by the following relations [37]:
G % = ( W 1 − W 0 ) ∕ W 0 ∗ 100
E % = W 1 − W 0 W 2   ∗ 100
C % =   W 3   /   W 2 ∗ 100
where W0, W1, W2, and W3 denote the weight of the pullulan, the final weight of the grafted hydrogel after production, the weight of the monomers, and the mass of monomers in the grafted polymer.
Water retention: The pre-weighed swollen gels ( W e q ) equilibrated in distilled water were kept at room temperature for 24 h. Then, the mass of the hydrogels was recorded and marked as W h y . The water retention is calculated according to the following relationship [38]:
W R % = W h y / W e q ∗ 100
Surface area measurements (BET): The specific surface area and micropore volume of the composites and components were determined according to the physical adsorption of nitrogen with the BET method at a temperature of 77 K using a Micromeritics ASAP 2400 automatic analyzer (Norcross, GA, USA). The average size and size distribution of the composites and components were determined by screen analysis performed by using Tyler standard sieves (Quantachrome Instruments, Boynton Beach, FL, USA).

2.3. Characterization

FTIR tests was carried out to identify the chemical structure of the PUL, AA, and AM, where IR spectra recorded in the range of 400–4000 cm−1 were recorded. This technique was also used to identify the model grafting (if existent) of these components.
With the XRD technique (D8 Bruker, Berlin, Germany), data were collected at 0.02-degree intervals with a count of 0.5 s per step in the 2° range of 10–80°. This test was conducted to monitor the effects of the surfactant concentrations on the crystallite size and crystallinity percentage. The DSC-TGA technique (SDT Q600 V20.9 Build 20) was used to study thermal transitions and thermal stability. On the other hand, the morphological properties of the samples’ surfaces were studied using the FE-SEM technique (ZEISS Sigma, Jena, Germany).

3. Results and Discussions

3.1. Prepared SAPs

A group of SAPs based on PUL was prepared according to the composition details mentioned in Table 1, such as (SAP1): PUL-g-AA; (SAP2): PUL-g-AM; (SAP3): PUL-g-AM/AA; and (SAP6): PUL-g-AM/AA-3 mM SDBS, respectively.
Figure 2A shows the FE-SEM images of various SAPs, with weak bonds between the SAP1 and SAP2 gel particles. The supramolecular nature of bonding of the superabsorbent polymers as they were prepared in this work involved a three-dimensional network connecting the gel particles at SAP3 and SAP6 and the formation of pores due to the interlocking structures of the hydrogels shown in the PUL-g-AM/AA structure. In addition, hydrogels are a broad class of hydrophilic materials formed by three-dimensional networks held together by covalent bond cross-links and weak cohesive forces in the form of hydrogen or ionic bonds. These cross-linked macromolecular structures can absorb large quantities of water and biological fluids without dissolution [39]. On the other hand, the advantage of the strength of the association between complementary hydrogen bonding units is that it affords stable supramolecular materials. The properties of these new materials were comparable to linear polymers possessing molecular weights far greater than the self-associating oligomeric units involved in supramolecular network formation. Molecular recognition units have been incorporated within macromolecular structures in several different ways to afford supramolecular networks, for example, at polymer chain ends [40], the termini of the arms of combs/brushes [41], or within the polymer main chain. In addition, the polymer architecture and the number of molecular recognition units per polymer chain (referred to as valency [42]) have also been adjusted to afford stable and processable supramolecular materials to permit multiple sites of association per polymer chain.
The SDBS surfactant altered the compact structure of PUL-g-AM/AA into a rough surface with deep pores on the hydrogel surface, which means that SDBS acts as a pore-forming agent; these pores became clearer after washing the hydrogel with ethanol (Figure 2B). This is due to the self-assembly of the SDBS particles in the reaction medium. Microwaves provide the energy needed to activate the initiator (KPS) to generate free radicals and accelerate the polymerization reaction using the TEMED. The radicals start the grafting reaction in vinyl monomers such as AA and AM in the double bond of C=C and then propagate to the polymeric chain [43]. The results of FE-SEM coincide with the proposed models; as the graft increased (such as in SAP3), the surface became rougher and more porous, which made it capable of the absorption of more water.
The FE-SEM, as shown in Figure 2, determined the morphological properties of the hydrogels, such as pore volume (nm3) and porosity (%), which were measured by ImageJ software (version 1.54). The porosity of the hydrogels can be calculated by comparing the average pore volume with the volume in the FE-SEM image using the following equation [44]:
Porosity   ( % ) = v p v t o t a l ∗ 100
where Vp and   V t o t a l are the pore volume and the total SEM volume, according to Equation (7), and the porosity % increases with an increase in the pore volume of the hydrogel. The percentage of the increment in the pore volume was 263.78% after adding 3 mM of SDBS to the PUL-g-AM/AA; the results on the pore volume (µm3) and porosity % of the SAPs are shown in Table 2. The pore volume of the hydrogel increased with SDBS compared to the pore volume of the hydrogel that did not contain SDBS [45].
Surface area measurements (BET):
The results on the adsorption surface area and pore volume measurements are provided in Table 2. The surface area and the pore volume of SAP6 were significantly different from those of SAP1, SAP2, and SAP3, which had a wider BET surface area. The specific surface area of SAP6 and SAP3 was found to be 3.8 m2/g and 2.19 m2/g, respectively. It can be concluded that SDBS can form spherical micelles when using a certain concentration of the surfactant. In the process of grafting copolymerization and cross-linking, these micelles may be enclosed in the network and act as a template for pore formation [27].

3.2. FTIR Analysis

FTIR spectroscopy was used to specify each component’s chemical composition and determine whether these components chemically reacted. Figure 3 shows the FTIR spectra of PUL, PUL-g-AA, PUL-g-AM, and PUL-g-AM/AA before washing and PUL-g-AM/AA after washing. For the PUL spectrum (Figure 3A), the bands from 738 to 933 cm−1 belong to α-(1, 4) and α-(1, 6)-D-glucosidic bonds. The bands at 1148 cm−1 and 1373 cm−1 are due to C-O-C and C-O-H, respectively, while band 1636 cm−1 is due to the stretching vibration of the O-C-O group. The band at 2924 cm−1 is due to SP3 of the C-H bond, while an O-H stretching vibration occurred from 3217 cm−1 to 3420 cm−1 [46]. For the acrylic acid spectrum (Figure 3B), the band at 3417 cm−1 belongs to the O-H stretching vibration mode. The band at 2924 cm−1 is due to the stretching vibration of CH in the methylene group. A carbonyl group (C=O) appeared at 1634 cm−1, and the bending vibration of the CH2 group occurred at 1440 cm−1. Bending of the O-H group happened at 1381 cm−1, while C-C stretching occurred at 1161 cm−1 and C-OH stretching at 1026 cm−1; the out-of-plane vibration mode for =CH and =CH2 occurred at 871 cm−1 and 810 cm−1, respectively [43].
At 1404 cm−1, bending vibration of the CH2 group occurred, while out-of-plane bending of =CH and –CH2 occurred at 779 cm−1 and 849 cm−1, respectively [47]. For the spectrum in Figure 3D, the absorption bands at 738 and 933 cm−1 of the PUL spectrum disappeared. At the same time, bands at 1148 and 1636 cm−1 appear, assigned to –COC- stretching of acrylate groups and O-C-O stretching of acrylamide groups, respectively, which suggests the grafting reaction of the copolymer onto the backbone [29]. It is clear from Figure 3E that all three components are present in the structure of the polymeric blend after washing it with ethanol/water and removing unreacted materials. The appearance of bands at about 1600–1700 cm−1 indicates AA grafting, where AA contains a carboxyl group (COOH) [43]. This group includes the OH and C=O groups, which usually appear at these positions. AM grafting is evident at the bands at about 1000–1250 cm−1, and PUL substrate is evident at bands at about 3000–3600 cm−1 and 450–750 cm−1, respectively.
The AM was grafted onto the PUL substrate through an aliphatic C-N bond, with its band appearing at 1000–1250 cm−1. In contrast, AA grafting occurred through a single C-C bond, but since this bond is nonpolar (no difference in electronegativity), it usually does not show up as peaks in an IR spectrum. A covalent bond consists of two electrons from each of two carbon atoms. It is called a sigma bond (σ) between one orbital of each carbon atom.
By comparing the FTIR spectra of the non-washed hydrogel (Figure 3D) and the washed hydrogel (Figure 3E), the typical peak appearing at around 3700–4000 cm−1 is not clear due to the presence of non-reactive materials (such as PUL, AA, AM, KPS, TEMED, and MBA). Meanwhile, it is clearer in Figure 3E, such that the absorption peaks at about 3850 cm−1 can be assigned to O–H stretching vibration. On the other hand, the absorption peaks at 2932 cm−1 can be ascribed to C-H stretching, and the bending vibration is very clear.

3.3. XRD Analysis

The X-ray diffraction patterns of SAP1, SAP2, SAP3, and SAP6 with a range of 10–80° and the results on the crystalline properties are shown in Figure 4 and Table 3. The broad peaks of all the samples with 2 θ = 18.775–21.94°. The grafting reaction between the PUL, PAA, and PAM monomers changes the crystal structure. The percentage of crystallinity and the crystal size obtained indicate a final structure between the values of the two monomers in the two perpendicular planes of the Miller indices (h k l) (110), according to d-d-spacing of 3.569 and 4.115 for SAP1 and SAP2. However, the orthogonal planes of the structure change to (220) with a spacing of 2.378 Å when grafting the two monomers AM/AA onto the PUL substrate at 2θ = 49.825, also changing the orthogonal planes of the Miller indices (h k l) to (211) after adding SDBS to the hydrogel reaction. Generally, PUL-g-AM/AA with and without SDBS has a partially crystalline hydrogel structure due to the formation of intramolecular hydrogen bonds between the polymer chains in the internal hydrogen structure [27,48,49].

3.4. Thermal Properties

Figure 5 shows the DSC curve (red) and the TGA curve (black) for the prepared SAPs, while Table 4 summarizes the weight loss steps for samples heated up to 800 °C. The results showed that the thermal stability of SAP1 is better than that of SAP2, as it is almost uniform, while in the SAP2 sample, it is irregular, and the sample loses 57.76% of its weight at 300 °C. On the other hand, grafting AA and AM each onto PUL enhances the resistance to thermal decomposition. However, the addition of SDBS works strongly to enhance the thermal behavior of the SAP6 sample, as there is no loss of weight in the sample at 220 °C compared to all the samples, and the thermal decomposition is uniform according to the weight loss steps compared to the SAP3 sample, which lost almost half of its weight at 370 °C. In terms of the influence of the SDBS surfactant on the thermal stability and thermal transitions of PUL-g-AM/AA, the weight loss up to 800 °C was 86.46% (SAP3), while this loss decreased to 72.59% for a sample with 3 wt. % SDBS (SAP6) at 800 °C. It was also noted that weight loss occurred gradually and less severely than in the SDBS-free sample. This indicates that SDBS enhances the stability of the hydrogel [50]. Generally, the first step for the SAPs represented the elimination of the adsorbed free water, and the second step represented the dehydration of the saccharide rings and depolymerization, with the formation of water, CO2, and CH4 [51].
This is because SDBS, during polymerization, promotes the emulsification of droplets of the AA and AM monomers, resulting in the solubilization of these two monomers within the micelles and increasing the number of nucleated particles [52]. This behavior is expected to increase the degree of crystallinity. The same finding can be concluded from the DSC curve, where the enthalpy of the thermal transition increased from 158.1 J/g in SAP1 to 1823 J/g in SAP6.The previous results coincide with the increase in crystallinity (%) and crystallite size obtained in the XRD test.

3.5. The Effect of SDBS on the Absorption and Grafting Parameters

For the prepared samples, many absorption and grafting parameters were calculated, such as water retention (WR), gel content (Gel), percentage of grafting (G), grafting efficiency (E), and water absorption capacity (WAC), as shown in Table 5.
The effects of the SDBS concentrations on these properties have a considerable dependence on the concentration of the surfactant in the samples (SAP3 to SAP8) compared to the variation in these parameters without SDBS and when the grafting used only the AA or AM monomers on PUL (SAP1 and SAP2) [43], indicating that micelle creation encourages both swelling and grafting processes. It can be noted that the percentage of water retention in the presence of SDBS increases to 60%, compared to 20% for the hydrogel without SDBS. Furthermore, Gel (%), G (%), and E (%) are the highest for SAP6 (PUL-g-AM/AA-3 mM SDBS). This means that there is an optimal concentration of the surfactant. Beyond this point, the pore-forming ability of SDBS becomes invalid [27,53]. These results agreed with the results of a previous study [27].

3.6. The Effect of SDBS on the Absorption Capacity and Swelling Kinetics

Research studies have shown that hydrogel porosity plays an important role in increasing water absorption, and this property has increased the agricultural applications for this type of polymer [54]. Based on the morphological properties, which were confirmed by the FE-SEM images and the results for the adsorption parameters (water absorbing capacity), Figure 6 shows the increase in the amount of water absorbed with an increase in the absorption time to reach the equilibrium limit as a function of the concentration of SDBS. Reaching the equilibrium state means that expansion either due to penetration through osmotic pressure by the solvent or contraction led to a decrease in enthalpy and the formation of the elastic strength of the network [55]. For each kinetic curve, we can distinguish two sections, an initial rapid increase and the stabilized equilibrium absorbency (Seq), with a rate correction factor R2 ≥ 0.99, indicating the precision of the results. The asymptotic value Seq is dependent on the concentrations of SDBS. The BoxLucas1 model equation is used for non-linear regression in Figure 6 [56]:
y = a (   1 − e x p − b x )
where (a) is Seq (g.g−1), y is WAC (g.g−1), x is absorption time (Sec), and b is the slope of the fitting equation.
The hydrogels’ sorption properties have been evaluated in terms of the swelling ratio   S w , defined as follows [57]:
S w = W s W d
Schott’s second-order swelling kinetics model was proposed for studying the absorption kinetics. The relation between the average swelling rate ( t / S w ) and swelling time ( t ) gives straight lines according to the proposed model [58]:
t / S w = = 1 / K i t + ( 1 / S ∞ ) t
where S w   is the swelling ratio at time   ( t ) ; S ∞ is the theoretical equilibrium swelling ratio, which is the reciprocal of slopes of these lines; and K i t   is the initial swelling rate constant, which is equal to the reciprocal of the intercept of the fitted straight lines with the Y axis.
Table 6 shows the effect of the surfactant concentrations on swelling parameters such as S ∞ with S e q at each SDBS concentration and pH value. The results showed that high values for the theoretical equilibrium swelling ratio ( S ∞ ) with stabilized equilibrium absorbency ( S e q ) were obtained with a 3 mM SDBS content and at a pH of 7. As a result, the hydrogel structure was more porous, and the larger the specific surface area, the faster the water diffusion into this structure was. Hydrogel porosity is one of the most important factors affecting the absorption of these polymers [14].
The theoretical values of the equilibrium swelling ratio S ∞ values are consistent with the stabilized equilibrium absorption S e q for the SDBS concentration and pH values. The initial swelling rate constant K i t values change as the SDBS concentration changes, and the highest value was seen at a 3 mM concentration, in addition to the rate correction factor R2 > 0.98 as an effect of the SDBS concentrations, indicating the precision of the results.

3.7. The Impact of pH on Water Absorbency and Swelling Kinetics

After selecting SAP6 as the best SAP to absorb distilled water at a pH of 7, the water absorption capacity of SAP6 was tested at different pH values (2, 4, 9, and 12) at room temperature (Figure 7) and using different absorption time periods (Figure 8). The results showed that the water absorption capacity of the hydrogel increased with an increasing pH up to 7 and then gradually decreased due to the forces of electrostatic repulsion and hydrogen bonding being in equilibrium [50]. The hydrogel’s ability to swell or absorb water is low when the pH levels are low due to the strong hydrogen bonding, and the presence of -COOH and NH2 groups enhances the physical cross-linking, in addition to neutralizing the electrostatic repulsion between the -COO- groups; therefore, the prepared SAP network tends to shrink. Within the pH range of 4 to 8, ionization of some of the carboxylic acid groups occurs, as does electrostatic repulsion between the -COO groups, leading to increased swelling of the hydrogel [59].
When the pH value is high, the hydrogel’s swelling ability is low because it is affected by excess sodium ion charges, which protect the carboxylic anions and prevent the formation of active anion–anion repulsion. The water absorbance capacity increases with increasing pH values until a pH of 7 and then decreases. In addition, the water absorption capacity increases with an increasing immersion time in water until it reaches an equilibrium state for all pH values. Table 6 shows swelling kinetics parameters such as K i t and S ∞ , representing a tendency for the pH value to affect the water absorbed by the hydrogel; these parameters increase with an increasing pH up to 7 and then decrease gradually. The theoretical equilibrium swelling ( S ∞ ) is consistent with the experimental equilibrium swelling ( S e q ) for all the pH values, seeing increments of 133.87% and 130% at a pH of 7 compared with the values of these parameters at a pH of 2. Also, the change in the initial swelling rate constant ( K i t ) is similar to that of the theoretical equilibrium swelling constant and the experimental equilibrium swelling as a function of pH; these results agree with previous studies [60,61]. The rate correction factor is R2 ≥ 0.99 for the effect of the pH values, indicating the precision of the results.

4. Conclusions

In this work, several superabsorbent polymers (SAPs) were prepared using PUL as the substrate and implementing grafting polymerization of AA and AM monomers, which was carried out in the presence of a redox pair initiator consisting of KPS/TEMED and MBA using microwave irradiation. The FTIR analysis demonstrated that AM/AA grafted onto PUL’s macromolecular chains has higher cross-linking than that of PUL-g-AA or PUL-g-AM. The FE-SEM images confirmed that SDBS improves the surface porosity of the hydrogel, which enhances the ability of the hydrogel to absorb large amounts of distilled water. In addition, SDBS improved the grafting parameters and swelling kinetics parameters. A 3 MM concentration was the best concentration of SDBS, as it had the highest theoretical equilibrium swelling and experimental equilibrium swelling as a function of SDBS and environmental pH.

Author Contributions

S.A.H.: Conceptualization, Methodology, Software, Data Curation, Writing—Original Draft Preparation; A.J.B.: Visualization, Investigation, and Supervision; S.A.D.: Software, Validation, Writing—Reviewing and Editing. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Acknowledgments

The authors express their sincere thanks and gratitude to everyone who helped complete this work, especially the employees in the department’s laboratories, CAK Company, and Ali Hussain.

Conflicts of Interest

The authors declare no conflict of interest.

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Chemengineering 08 00077 g001
Figure 1. Grafting polymerization reactions of SAP1, SAP2, and SAP3 using potassium persulfate (KPS) as an initiator.
Figure 1. Grafting polymerization reactions of SAP1, SAP2, and SAP3 using potassium persulfate (KPS) as an initiator.
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Figure 2. (A) FE-SEM images (1 µm) for SAP1, SAP2, SAP3, and SAP6, respectively; (B) schematic of the formation of the SDBS micelle template during the reaction and after washing with ethanol.
Figure 2. (A) FE-SEM images (1 µm) for SAP1, SAP2, SAP3, and SAP6, respectively; (B) schematic of the formation of the SDBS micelle template during the reaction and after washing with ethanol.
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Figure 3. FTIR analysis of (A) PUL, (B) PUL-g-AA, (C) PUL-g-AM, and (D) PUL-g-AM/AA before washing and (E) PUL-g-AM/AA after washing. For the acrylamide spectrum (Figure 3C), bands from 3240 cm−1 to 3418 cm−1 belong to the symmetrical and asymmetrical stretching vibrations of the N-H group. In contrast, the band at 2932 cm−1 belongs to the stretching vibration of the methylene (CH2) group. The bending vibration of the N-H group appears at 1582 cm−1, while the symmetrical stretching vibration of the carbonyl group (C=O) appears at 1620 cm−1.
Figure 3. FTIR analysis of (A) PUL, (B) PUL-g-AA, (C) PUL-g-AM, and (D) PUL-g-AM/AA before washing and (E) PUL-g-AM/AA after washing. For the acrylamide spectrum (Figure 3C), bands from 3240 cm−1 to 3418 cm−1 belong to the symmetrical and asymmetrical stretching vibrations of the N-H group. In contrast, the band at 2932 cm−1 belongs to the stretching vibration of the methylene (CH2) group. The bending vibration of the N-H group appears at 1582 cm−1, while the symmetrical stretching vibration of the carbonyl group (C=O) appears at 1620 cm−1.
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Figure 4. XRD patterns of SAP1, SAP2, SAP3, and SAP6, respectively.
Figure 4. XRD patterns of SAP1, SAP2, SAP3, and SAP6, respectively.
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Figure 5. DSC-TGA curves for SAP1, SAP2, SAP3, and SAP6, respectively.
Figure 5. DSC-TGA curves for SAP1, SAP2, SAP3, and SAP6, respectively.
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Figure 6. Time-dependent water absorption capacity curves of SAPs generated by different concentrations of SDBS in distilled water at a pH of 7.
Figure 6. Time-dependent water absorption capacity curves of SAPs generated by different concentrations of SDBS in distilled water at a pH of 7.
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Figure 7. Effect of environmental pH on water absorbency using 3 mM of SDBS (SAP6).
Figure 7. Effect of environmental pH on water absorbency using 3 mM of SDBS (SAP6).
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Figure 8. Relation between water absorbency and swelling time for different pH values using 3 mM SDBS (SAP6).
Figure 8. Relation between water absorbency and swelling time for different pH values using 3 mM SDBS (SAP6).
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Table 1. Composition of samples of the prepared SAPs.
Table 1. Composition of samples of the prepared SAPs.
SamplesComposition of SAPs
SAP1PUL-g-AA
SAP2PUL-g-AM
SAP3PUL-g-AM/AA
SAP4PUL-g-AM/AA-1 mM SDBS
SAP5PUL-g-AM/AA-2 mM SDBS
SAP6PUL-g-AM/AA-3 mM SDBS
SAP7PUL-g-AM/AA-4 mM SDBS
SAP8PUL-g-AM/AA-5 mM SDBS
Table 2. Results of pore volume (µm3) and porosity % of SAPs produced by ImageJ software for SEM images and pore volume (µm3/g) and surface area (m2/g).
Table 2. Results of pore volume (µm3) and porosity % of SAPs produced by ImageJ software for SEM images and pore volume (µm3/g) and surface area (m2/g).
SamplesPore Volume
(µm3)		Porosity
%		Pore Volume (µm3/g)Surface Area (m2/g)
SAP10.0225 ± 0.01135.3186.392.25
SAP20.0329 ± 0.01039.2237.181.83
SAP30.0626 ± 0.010211.8228.872.19
SAP62.276 ± 0.081815.90323.473.80
Table 3. Crystallinity percent and average crystalline size of some prepared SAPs.
Table 3. Crystallinity percent and average crystalline size of some prepared SAPs.
SAPsPos. [°2Th.]d-Spacing [Å]FWHM [°2Th.](h k l)Crystallinity
(%)		Crystallite
Size (nm)
SAP121.593.5690.1659311015.0248.46
SAP221.944.1150.1588411011.3540.86
SAP318.78
49.83		4.725
2.378		0.35080
0.16320		110
220		22.6151.60
SAP619.73
50.43		3.335
1.809		0.17640
0.16211		110
211		23.8754.30
(h k l): Miller indices.
Table 4. Weight loss results of SAP samples according to many degradation steps.
Table 4. Weight loss results of SAP samples according to many degradation steps.
SAPsTw (%)Step 1Step 2Step 3Step 4Step 5
ABABABABAB
SAP181.781501.882208.3837014.748035.0080021.82
SAP290.921254.2330053.5347022.280010.96--
SAP386.461501.5637042.6080042.3----
SAP672.59220none2605.0337023.547018.2080025.86
A: temperature (°C); B: weight loss (%); Tw: total weight lost (%).
Table 5. Absorbance and grafting parameters for the prepared samples at room temperature and a pH of 7.
Table 5. Absorbance and grafting parameters for the prepared samples at room temperature and a pH of 7.
Sample C (%)G (%)E (%)Gel (%)WAC (g/g)WR (%)
SAP172.07325.065.0051.7032.063.70
SAP273.19333.366.6761.8058.068.00
SAP383.57743.374.3373.0074.173.44
SAP487.03760.076.0074.39123.574.93
SAP588.32783.378.3378.72143.676.00
SAP689.95838.383.8381.40200.579.50
SAP789.13820.082.0074.81137.873.09
SAP888.86800.080.0078.88128.771.54
C: conversion of monomer into polymers; G: grafting percentage; E: grafting efficiency; Gel: gel content; WAC: water absorption capacity; WR: water retention.
Table 6. Schott’s pseudo-second-order swelling kinetics model results using different SDBS concentrations on SAPs with a pH of 7 and different pH values with 3 mM SDBS only.
Table 6. Schott’s pseudo-second-order swelling kinetics model results using different SDBS concentrations on SAPs with a pH of 7 and different pH values with 3 mM SDBS only.
Parameters for Different SAPs and a pH of 7 Parameters for Different pHs and 3 mM SDBS
SBDS
(mM)		 R 2 K i t S ∞ S e q pH R 2 K i t S ∞ S e q
00.9350.01695.2474.320.9870.06792.5987.6
10.9870.050139.86101.340.9860.079197.24168.7
20.9930.075150.38137.870.9940.183217.39201.5
30.9970.148212.77202.590.9850.068166.39142.0
40.9970.104152.44143.5120.9820.086120.63109.9
50.9980.121134.41116.9
R 2 : correction factor; K i t : initial swelling rate constant;   S ∞ : theoretical equilibrium swelling;   S e q : stabilized equilibrium absorbency.
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Dhahir, S.A.; Braihi, A.J.; Habeeb, S.A. Preparation and Characterization of Supramolecular Bonding Polymers Based on a Pullulan Substrate Grafted with Acrylic Acid/Acrylamide by Microwave Irradiation. ChemEngineering 2024, 8, 77. https://doi.org/10.3390/chemengineering8040077

AMA Style

Dhahir SA, Braihi AJ, Habeeb SA. Preparation and Characterization of Supramolecular Bonding Polymers Based on a Pullulan Substrate Grafted with Acrylic Acid/Acrylamide by Microwave Irradiation. ChemEngineering. 2024; 8(4):77. https://doi.org/10.3390/chemengineering8040077

Chicago/Turabian Style

Dhahir, Salam Abdulla, Auda Jabbar Braihi, and Salih Abbas Habeeb. 2024. "Preparation and Characterization of Supramolecular Bonding Polymers Based on a Pullulan Substrate Grafted with Acrylic Acid/Acrylamide by Microwave Irradiation" ChemEngineering 8, no. 4: 77. https://doi.org/10.3390/chemengineering8040077

APA Style

Dhahir, S. A., Braihi, A. J., & Habeeb, S. A. (2024). Preparation and Characterization of Supramolecular Bonding Polymers Based on a Pullulan Substrate Grafted with Acrylic Acid/Acrylamide by Microwave Irradiation. ChemEngineering, 8(4), 77. https://doi.org/10.3390/chemengineering8040077

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