Eco Friendly Polymerisation Route for Acrylic Acid via NO2 gas in aqueous medium for Hydrogel Synthesis
Reena Bhadani*, Kumari Neetu Singh
Department of Chemistry, Ranchi Women’s College, Ranchi, Jharkhand, India.
*Corresponding Author E-mail: rbhadani04@gmail.com
ABSTRACT:
Hydrogels are three-dimensional networks of hydrophilic polymer chains between liquid and solid phases1,2. The most important feature of hydrogels is their capacity to swell and shrink. This capacity is surprisingly dependent on their environment 3,4. Hydrogels are cross-linked networks having segments of hydrophilic groups, a cross-linking agent, and a monomer with two or more double bonds, which furnishes the polymer network structure by connecting the long linear chain formed in polymerisation.
Thus, the cross-linking of the long chains produces a three-dimensional, clear polymer network that is elastic in nature5,6. Such a network increases its volume by absorbing water and solute when immersed in water or an aqueous solution. This process is termed swelling 7, 8. This swelling–deswelling ability makes hydrogels special materials for a variety of applications in different fields9–13. Hydrogel networks formed from polyacrylic acid (PAA) have the ability to absorb more than one hundred times their weight. Hence, hydrogels based on PAA are called superabsorbent polymers (SAPs) 14. SAPs are widely used in personal hygiene products and controlled drug delivery 15,16. Researchers have also focused on SAP applications in ion exchange resins, biosensors, soft actuators/valves, and membranes for haemodialysis 17,18. The magnitude of swelling capacity is dependent on the monomer concentration, initiator concentration, density of cross-linking material, temperature, ion in the aqueous solution, pH of the medium and electric and magnetic intensities. Hence, it is called stimulus-responsive or intelligent/smart hydrogels 19,20,21. The present paper is devoted to the synthesis of polyacrylic acid polymer hydrogels using NO2 gas as an initiator and BAM as cross-linkers at ambient temperature. The synthesised moieties were saponified by NaOH solution, and then the hydrogels were characterised by different stimuli toward which the hydrogel is responsive.
Analytical grade Acrylic acid as a monomer, and analytical reagent grade BAM was used as a cross-linker. Twice-distilled de-ionised water was used throughout the experiment. Various salts of analytical grade were used without further purification. NO2 gas was prepared by heating lead nitrate. Its concentration in H2O was determined volumetrically by treating the solution with excess sodium hydroxide solution and titrating the resulting solution with acidified standard potassium permanganate solution 22,23.
Acrylic acid (AA), having the chemical formula CH2CHCOOH (C3H4O2), is an organic compound used in the production of polymer hydrogels. AA is highly soluble in water. Its solubility in water is 1000 g/L. Hence, in the saturated solution of acrylic acid, there will be almost 1000 g of monomer in one litre of acrylic acid aqueous solution in pure water.
Figure 1
Polymerisation of acrylic acid was carried out by adding a known amount of initiator NO2 gas in aqueous solution in a test tube; contrary to acrylic acid in water, a known amount of cross-linker BAM was also added. The test tube was purged with Nitrogen and then stopped. The test tube was thermally shocked for two hours at 500C in a water bath. After some time, the reaction mixture started to turn viscous and finally transformed into a glassy, sticky mass. The obtained glassy and spongy gels were taken out from the test tube and were washed several times to clean the unwanted and unreacted materials and then dried. The dried polymers were stored to determine the degree of swelling in water.
Polymer gels were introduced into 100 ml of 4% (w/v) sodium hydroxide solution in a 250 ml round bottle flask. The flask was loosely stoppered. The gel was kept heating at 500C for 60 minutes. Now the soft gel was taken out, washed several times and then dried. The dried gels were kept for further investigation.
The Known amount of dried gel sample was taken and immersed in 200 ml of water in a glass container at a particular temperature. The gels start getting swollen. The sample was taken out at regular intervals and dried with fine filter paper, and then weighed. This process continued until no weight change was observed in two or three cycles. The degree of swelling was determined by the formulae given below.
Degree of swelling = [(m-m0)/ mo]
Where,
m = wt. of swollen gel
m0= wt. of initial (dried) gel
Conditions:
Figure 2(a): (i) Each sample weight =0.25 g
(ii) Temperature of test run = 50oC
Figure 2(b): (i) (AA) = 5.62 mole/litre
(ii) Concentration of (NO2) = 0.10 mole/litre
(iii) Concentration of BAM = 0.02 mole/litre
(vi) Polymerisation Temperature = 50oC
Figure 2(c): (i) Concentration of NO2=0.1 mole/ litre
(ii) Concentration of BAM = 0.02 mole / litre
(iii) Time of swelling = 4 hours
Figure 2(d): (i) (AA) = 5.62 mole/litre
(ii) (NO2) =0.10 mole/litre
(iii) Reaction medium = pure water
(v) Time of swelling = 4 hours
Figure 2(e): (i) (AA) = 5.62 mole/litre
(ii) (NO2) =0.10 mole/litre
(iii) Reaction medium = pure water
(iv) Time of swelling = 4 hours
Figure 2(a) charts how polyacrylic acid hydrogels swell over time in distilled water: Curve I is the untreated gel, and Curve II is the hydrolysed one. Each dry sample weighed 0.25 g, and the test was run at 50 °C.The degree of swelling of hydrolysed acrylic acid in distilled water is much higher in comparison to that of in unhydrolysed sample, as shown in Figure 2(a). This is mainly due to increased hydrophilicity and electrostatic repulsion after hydrolysis. Firstly, hydrolysis converted neutral or hydrophobic groups into ionic or highly carboxyl groups (-COO-). These ionic groups attract water strongly via ion-dipole interaction, resulting in the enhancement of water uptake. Secondly, the negatively charged –COO- groups repel each other, forcing the polymer network to expand and create more space for water to enter. Thirdly and lastly, mobile counter ion (Na+) inside the hydrogel network creates a Donnan osmotic pressure, drawing more water into the network to balance the ionic concentration between the inside and the outside of the hydrogel network. The unhydrolysed hydrogel is neutral and has a randomly coiled configuration, having a low swelling.
The effect of NaOH concentration during hydrolysis of polymer gel on the degree of swelling is depicted in Figure 2(b) The hydrogel absorbs the most water at about 0.75 mol L⁻¹ NaOH. At this point it reaches its equilibrium swelling: any further increase in NaOH actually reduces water uptake. When NaOH concentration rises above 0.75 mol L⁻¹, extra Na⁺ ions flock to the polymer’s carboxyl groups, disturbing their local balance of charge. This “charge-screening” effect makes the polymer chains curl up, so the gel shrinks. Very high hydroxide levels also promote extra cross-linking, tightening the network even more and cutting its ability to swell.
Polyacrylic acid (PAA) gels were synthesised using varying proportions of acrylic acid (AA), with all other reaction conditions held constant. After polymerisation, each gel was immersed in water and its degree of swelling recorded (Figure 2(c)). The results reveal a pronounced relationship: gels made with lower AA content swell less. In free-radical polymerisation, higher monomer concentration drives the formation of longer polymer chains and thus increases molecular weight. In our system, NO₂ acts as the radical initiator—upon heating, it decomposes to •NO₂ radicals, which attack AA monomers to generate growing radical sites that continually add further AA units. When more AA is present, these active chains extend further, yielding a looser network capable of imbibing larger volumes of water. By contrast, gels derived from lower AA concentrations produce shorter chains and a denser network, limiting water uptake. In essence, the gel’s swelling capacity scales with the polymer chain length: longer chains create a more open network structure and therefore absorb more water. Hence, swelling ability depends on the molecular weight of the polymer chains and longer polymer chains hold more water.
The water uptake capacity of polyacrylic acid hydrogels formed at different concentrations of (BAM), a cross-linker agent was examined As seen in Figure 2(d), the gel swells less and less as we add more cross-linker. An extra cross-linker ties the polymer chains together more tightly, creating a stiffer, denser network. Because the chains are locked in place, the gel can’t stretch enough to absorb much water. In short, the more cross-linking there is, the lower the gel’s ability to swell.
From the Figure-2(e), it was clearly observed that there is an optimal temperature range (20o-600C) during polymerisation where the swelling is maximised. This will depend on the specific cross-linker used and the polymerisation conditions. However, higher temperature decreases swelling because it leads to increased cross-linking in the hydrogel matrix, restricting the movement of polymer chains, resulting in the reduction of the ability of hydrogels to absorb water, thus decreasing the degree of swelling.
Figure 3
Results shown in Figure 3 indicate that the degree of swelling was significantly reduced by adding the ionic salts. The reduction of swelling is strongly dependent on the types and concentration of and ionic salt added to the swelling medium.
(i) When extra cations from an added salt surround the negatively charged sites on the polymer chains, they “screen” those fixed charges. Without this screening, like charges repel cations neutralise much of that repulsion, the chains no longer force themselves apart, the network expands less, and the gel’s water uptake drops.
(ii) When the medium’s ionic strength increases, the osmotic-pressure gap between the gel and the surrounding solution becomes smaller. Ionic strength reflects both the concentration of the mobile ions and their charge (valency), as captured in the equation below.
Ionic strength = ½∑ mᵢ X zᵢ2
𝑖
Where mᵢ = modality and zᵢ = Valency
Hence Ionic Strength of NaCl = m, Ionic Strength of CuSO₄ = 3m Ionic Strength of FeCl3 = 6m
Experimental finding reveals that even a small quantity of divalent trivalent ions tremendously reduces the degree of swelling. The valence of the anion of the electrolyte does not affect the water absorbency 20.
Polyacrylic acid hydrogels can undergo several cycles of swelling and deswelling, as shown in Figure-4 declares reversible behaviour Swelling (s) and Deswelling (DS) cycles of hydrogels of polyacrylic acid hydrogels, the y-coordinate represents the degree of swelling.
Figure 4
Figure 4 showed the degree of water absorption recovery after the process of swelling-deswelling-swelling. It also showed that after each new swelling, the gel swells a bit more than it did the time before. The lighter, low-molecular-weight fragments wash out during cycling, so the gel left behind is made of heavier chains that can absorb more water. The figure also makes it clear that the gel lets go of water (deswells) more slowly than it takes it up, implying that the swelling and deswelling steps follow different underlying processes.
Figure 5
The FT-IR spectrum of the NO₂-initiated polyacrylic acid hydrogel is dominated by the characteristic vibrations of its carboxylate network and aliphatic backbone. A broad, slightly sloping baseline from 3600 to 3000 cm⁻¹ (not labeled) reflects extensive O–H stretching from hydrogen-bonded carboxylic acid and water molecules within the gel matrix. The weak band at 2963 cm⁻¹ is due to C–H asymmetric stretching of the –CH₂– backbone. The intense absorption at 1707 cm⁻¹ corresponds to the C=O stretching vibration of the protonated (–COOH) groups, while the pair of bands at 1538 and 1390 cm⁻¹ can be assigned to the asymmetric and symmetric stretches of deprotonated carboxylate (–COO⁻) species—evidence of partial ionization and crosslink formation. In the fingerprint region, the medium band at 1168 cm⁻¹ arises from C–O stretching of the acrylic acid ester linkages, and smaller peaks at 1033 and 875 cm⁻¹ reflect C–C stretching and out-of-plane CH wagging modes of the polymer backbone. The low-frequency bands around 812 and 426 cm⁻¹ are associated with skeletal bending modes of the network. Together, these features confirm successful free-radical polymerization of acrylic acid by NO₂ initiation, retention of abundant carboxylic functionality, and the formation of a hydrated, hydrogen-bonded gel network.
3.9 Thermal properties of PAA hydrogels:
(a) (b) (c)
Figure 6: Thermal Gravimetric Analysis of PAA hydrogel
The TGA trace of the NO₂‐initiated polyacrylic acid hydrogel exhibits a three‐stage mass loss totaling essentially 100 % (–99.98 %) between 34 °C and 675 °C. In the first region (34–200 °C), the sample loses ∼15.2 % of its mass (–0.653 mg), which corresponds to the release of adsorbed and bound water from the hydrogel network. The second, most pronounced decomposition step (200–350 °C) accounts for ∼50.4 % mass loss (–2.168 mg) and reflects the breakdown of the polyacrylic acid backbone—chain scission and decarboxylation of pendant carboxyl groups. Finally, between 350 and 675 °C, an additional ∼34.3 % (–1.475 mg) is lost as the remaining char and crosslinked fragments decompose. The concomitant DTA curve shows an initial broad endothermic feature peaking at 150.8 °C (∆H ≈ 14.8 J) associated with dehydration, followed by a series of exothermic events: a major exotherm at 301.3 °C (∆H ≈ 3.8 J) marking the main polymer depolymerization, a smaller exotherm at 436.0 °C (∆H ≈ 7.3 J) likely due to further oxidation of residual carbonaceous material, and a final minor peak at 539.8 °C (∆H ≈ 1.7 J) corresponding to decomposition of the most thermally stable fragments. Together, these data confirm a highly hydrated network that undergoes stepwise thermal degradation, from water loss through progressive polymer backbone and char decomposition.
Conclusively, the hydrogels retain an appreciable amount of bound water as well as oligomers in acidic medium (HNO₃), indicating a less dense crosslinked network in Comparison to the stable sample. The wide temperature range of weight loss suggests gradual network breakages rather than sharply defined stages. All the main thermal decompositions need energy absorption. It means that the hydrogel structure has the capacity to resist rapid exothermic chain breakage. Owing to the tolerance of this hydrogel in a low pH environment, lower crosslink density and heat-absorbing scissions, such hydrogels may be limited for use where both high acid resistance and thermal endurance are critical.
(a) (b) (c) (d)
(e) (f) (g)
Figure 7
The FESEM series reveals a highly textured, hierarchical morphology characteristic of a NO₂-initiated polyacrylic acid hydrogel. At the lowest magnifications (250–2 k×), the network appears as overlapping, sheet-like flakes with sharp, angular tear-edges evidence of brittle fracture during sample preparation and strong interchain interactions. As we move to intermediate magnifications (5 10 k×), individual “islands” of polymerised material emerge, each roughly 2–5 µm across, with a densely folded, cauliflower-like surface. These nodular domains likely mark regions of high radical concentration where •NO₂ initiation produced localised crosslinking and rapid chain growth. At the highest magnification (25 k×), the surface becomes even more intricate: a labyrinth of wrinkles, ridges, and sub-micron pores adorns each nodule, creating a large specific surface area and suggesting a sponge-like network of voids. Overall, the images confirm that NO₂ initiation yields a porous, rough polymer matrix with multiscale features from macro-flakes to nano-wrinkles ideal for applications requiring high swelling capacity and rapid fluid transport.
Being odd electron molecules, NO2 is paramagnetic. Hence, it causes free radical polymerisation. The following resonating structures are assigned to these molecules 24.
NO2 interacts with acrylic acid to give monomeric free radicals, which further add monomer molecules to propagate polymerisation
ṄO2+ CH2 = CH-COOH →O2N-CH2-ĊH-COOH
O2N-CH2-ĊH-COOH + n (CH2=CH-COOH) →
---(--CH₂-ĊH-COOH--) n₊₁---
Both monomeric and polymeric units are cross-linked with bisacrylamide to yield crosslinked polyacrylic acid 25.
Here, the initiator is NO2 gas in an aqueous solution of pure water,
which makes the medium acidic also, favouring the formation of neutralised carboxyl group. In this case, the molecular configuration of the same polyacrylic acid hydrogels in solution is similar to a non-ionic polymer, which has a randomly coiled rather than extended chain configuration exhibiting a less degree of swelling in comparison to that of pure water medium with other initiator salts like (NH)2S2O₈ or K2S2O₈ or H2O2.
4. CONCLUSION:
Using an aqueous solution of NO2 gas as an initiator and N, N-methylenebisacrylamide as a crosslinker, the hydrogels of three-dimensional crosslinked polyacrylic acid were successfully synthesised. The swelling ability of these hydrogels was studied under various conditions. The effects of NaOH concentration during saponification, monomer concentration, temperature, crosslinker density, concentration and valency of ionic salts, and acid medium reveal that the degree of swelling of the so-formed hydrogels is approximately more than 150 times their weight, meaning that the dried hydrogels will absorb 150 times their weight. Hydrogels exhibiting this level of swelling can be categorised as superabsorbent polymers (SAP), with applications in biomedical, pharmaceutical, agricultural, and drug-delivery fields. The study of temperature effects further shows that PAA hydrogels often display very high swelling, which can lead to deformation and deswelling, showing poor stability and shape integrity. The Thermal Gravimetric Analysis (TGA) study also reveals the same. FTIR was carried out to show the chemical structure of the hydrogels. Morphology of the hydrogel was also studied by FESEM. Thus, swelling of the hydrogel was reversible.
Figure 12 (https://doi.org/10.3390/polym16141990)
1. Omidian, H., Rocca, J. G., and Park, K. Advances in superporous hydrogels. Journal of Controlled Release. 2005; 102: 3–12.
2. Fernandez, E., Lopez, D., Lopez-Cabranes, E., and Miljungac, C. Viscosimetric and swelling properties of glucose oxidase–loaded polyacrylamide hydrogel and evaluation of their properties as glucose sensors. Polymer. 2005; 46: 2211–2217.
3. Gupta, P., Vermani, K., and Garg, S. (). Hydrogels from controlled release to pH-responsive agar delivery. Drug Delivery Today. 2002; 10(9): 569–579.
4. Evmenenko, A., Alexe, V., Budova, T., Bunova, A., and Frenkel, S. Swelling-induced changes of polyelectrolyte gels. Polymer. 1999; 40: 2975–2979.
5. Peppas, N. A. (Ed.). (1987). Hydrogels in medicine and pharmacy (Vol. 2). CRC Press.
6. Amende, M., and Peppas, N. A. Transport of ionizable drugs and proteins in crosslinked polyacrylic acid and poly(acrylic acid-co-2-hydroxyethyl methacrylate) hydrogels. Journal of Applied Polymer Science. 1996; 59(4): 673–685.
7. Byrne, M. E., Park, K., and Peppas, N. A. Molecular imprinting within hydrogels. Advanced Drug Delivery Reviews. 2002; 54: 149–161.
8. Grini, G. Recent developments in polysaccharide-based materials used as adsorbents in wastewater treatment. Progress in Polymer Science. 30: 38–70.
9. Gombotz, R., and Hoffman, A. Immobilization of biomolecules and cells on and within synthetic hydrogels. In N. A. Peppas (Ed.), Hydrogels in Medicine and Pharmacy. 1986; 5. CRC Press.
10. Peppas, N. A., and Mikos, A. G. Preparation methods and structure of hydrogels. In N. Peppas (Ed.), Hydrogels in Medicine and Pharmacy. 1986;1. CRC Press.
11. Güven, Ö., Şen, M., Karadağ, E., and Saraydın, D. A review on the radiation synthesis of copolymeric hydrogels for adsorption and separation purposes. Radiation Physics and Chemistry. 1996; 56: 381–386.
12. Denizli, A., Say, R., Garipcan, B., and Patir, S. Methacryloyl-diglycolamic acid functionalized poly (2-hydroxy methylacrylate) beads for UO₂²⁺ removal. Reactive and Functional Polymers. 2004; 58: 123–130.
13. Zhung, A., Asakura, T., and Uchiyama, G. The adsorption mechanism of uranium (VI) from seawater on a macroporous fiber polymeric adsorbent containing amidoxime chelating functional groups. Reactive and Functional Polymers. 2003; 57: 67–76.
14. Dhodapkar, R., Borde, P., and Nandy, T. Superabsorbent polymer in environmental remediation. Global NEST Journa. 2009; 1(2): 223–234.
15. Mahdavinia, G., Pouryavadi, A., Hosseinzadeh, H., and Zohuriaan-Mehr, M. J. Modified chitosan superabsorbent hydrogels from poly(acrylic acid-o-acrylamide) grafted chitosan with salt- and pH-responsive properties. European Polymer Journal. 2004; 40: 1399–1407.
16. Grodzinsky, A., and Grimshaw, P. Electrolytically and chemically controlled hydrogel for drug delivery. In Pulsed and Self‐Regulated Drug Delivery. 1990: 11–27.
17. Cuscler, F., Stokar, M., and Varberg, J. Gels as size-selective extraction solvents. AIChE Journal. 1984; 30: 578–582.
18. Gehrke, S. A., Andrews, G., and Cussler, E. Chemical aspects of gel extraction. Chemical Engineering Science. 1986; 42: 2153–2160.
19. Bhadani, R., and Mitra, U. K. Synthesis and studies on water swelling behaviour of polyacrylamide hydrogels. In Macromolecular Symposia. 2016; 369(1): 30-34.
20. Bhadani, R., Mitra, U. K., and Jayaswal, R. SO₂ initiated polymerization of acrylamide and studies on hydrogels based on resulting polyacrylamide. Asian Journal of Research in Chemistry. 2019; 12(5): 258–262.
21. Bhadani, R., and Mitra, U. K. Asian Journal of Research in Chemistry. 2014; 7: 345.
22. Vogel, A. I. (1969). Quantitative Inorganic Analysis (3rd ed., p. 297). GBLS and Longman.
23. Bhadani, S. N., and Prasad, Y. K. Makromolecular Chemistry. 1980; 181: 1089.
24. Bhadani, S. N., and Sen Gupta, S. K. (1996). In Polymer Material Encyclopedia (p. 22). CRC Press.
25. Zhang J, Qu D, Wang S, Qi S, Zuo H. Structure, Property Optimization, and Adsorption Properties of N,N′-methylenebisacrylamide Cross-Linked Polyacrylic Acid Hydrogels under Different Curing Conditions. Polymers. 2024; 16(14):1990. https://doi.org/10.3390/polym16141990.
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Received on 12.04.2026 Revised on 10.05.2026 Accepted on 08.06.2026 Published on 04.07.2026 Available online from July 30, 2026 Asian J. Research Chem.2026; 19(4):304-310. DOI: 10.52711/0974-4150.2026.00047 ©A and V Publications All Right Reserved
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