SO2 Initiated polymerization of Acrylamide and studies on hydrogels based on resulting Polyacrylamide

 

Reena Bhadani1*, U. K. Mitra2, Rajeshwari Jayaswal

1Department of Chemistry, Ranchi Women’s College, Ranchi-834001, Jharkhand, India

2Department of Chemistry, Gossner College, Ranchi – 834001, Jharkhand, India

3Department of Chemistry, R.L.S.Y. College, Ranchi, Jharkhand, India

*Corresponding Author E-mail: rbhadani@gmail.com

 

ABSTRACT:

The polymerization of AM was carried out under the different concentration of acrylamide and cross linkers in an aqueous solution ofSO2 gas as initiator at 500c. The resulting polyacrylamide was saponified with NaOH solution. The hydrolysed polymers show high degree of water swelling (>500) than that of unhydrolysed polymers (<50). Swellability of the gel decreases with the increasing concentration of N, N’- methylene bisacrylamide – a cross linker which enhances the degree of cross linking in the polymer chains. The increased cross linking restrict the expansion of polymer network which causes the lowering of the water absorbency. It was observed that polymer hydration increases with the molar mass of polyacrylamide. The swellability of polyacrylamide hydrogels is greatly lowered by the addition of ionic salts in the swelling medium. Both ionic strength and valency of the salts are responsible to decrease the water up taking capacity of the gels which remain unaffected by non dessociating nature of Urea. The hydrogels, so prepared, are stable and quite able to undergo swelling- deswelling cycles suggesting their reversible characters. Results are similar to those obtained for polyacrylamide formed during the course of NO2 initiation in H2O.

 

KEYWORDS: Polymerization, Initiator, Crosslinker, Polyacrylamide Hydrogels and ionic salts.

 

 


INTRODUCTION:

Hydrogels are water swellable, but not soluble, three dimensional cross linked hydrophilic polymer network which can absorb and hold large amount of water that can be much more than thousand times the weight of the polymers. They are inherently soft, hydrophilic, porous and elastic polymer system. In order to keep the spatial structure, the polymer chains are usually physically or chemically cross linked. The ability of hydrogels to uptake water arises from hydrophilic functional groups such as hydroxilic (-OH), carboxylic (-COOH), amidic        (-CONH-), primary amidic(-CONH2) and sulphonic          (-SO3H) attached to the polymeric backbone.

 

Some hydrophilic monomers commonly used are acrylamide, acrylic acid, methyl methacrylate, isopropyl acrylamide and so on. It is also possible to form hydrogels by copolymerizinghydrophilic and hydrophobic polymers. Some cross linkers commonly used to crosslink polymeric chains are N,N’ – methylene bisacrylamide, 1,4 butandioledimethacrylate, ethylene glycol dimethacrylate etc.[1-9]

 

Hydrogels responding to external stimuli are often termed as “Intelligent or smart hydrogels.” The stimuli that influence smart gels are physical and chemical factors. Temperature, light, electric forces, magnetic forces and mechanical forces are types of physical interactions on the gels that precipitate a reaction. Chemical stimuli are usually pH and chemical environments. The responsive hydrogels have become an important area of research and development in the field of medicine, pharmacy, biotechnology and bioengineering. The renewable, biodegradable, nontoxic various natural polysaccrides including starch, cellulose, sodium alginate, chitosan, guargum etc are used for the synthesis of environmentally friendly responsive graft copolymer hydrogels hydrophilic with vinyl monomers. Such hydrogels finds wide range of application. Our some investigations on water swelling behaviour of polyacrylamide hydrogels are discussed in the present paper.[10-16]

 

Experimental:

Acrylamide, N, N - methylene bisacrylamide  (BAM) and salts were analytical grade and used without further purification. Water was purified by distillation. SO2 was prepared by treating sodium sulfide with dilute hydrochloric acid. The concentration of SO2 in H2O was determined volumetrically with standard alkali solution.[17]

 

Polymerization:

The polymerization of acrylamide was carried out in an aqueous solution of SO2 as an initiator in presence of BAM at 500c in a glass vessel. When the reaction mixture became very viscous, the polymerization was terminated. The mixture was poured into cold methanol and the polymer was isolated and dried. The polymer so formed was saponified in a known solution of NaOH for two hrs and then precipitated in excess of methanol. The polymers were thoroughly and repeatedly washed with acetone, and dried polymers were stored to determine the degree of swelling.

 

Investigation of degree of swelling:

The swelling experiment was carried out by taking a known amount of dry polymer sample and immersed in 200 ml of distilled water to get it hydrated. At a regular interval the swollen gel was taken out and wiped using fine filter paper and weighed. The procedure was repeated until there was no change in the weight of the gel sample. Dried gels are quite hard while swollen gels are very soft and spongy. The degree of swelling was calculated by the following expression:-

 

Degree of Swelling =(m-m0)/m0

Where

m0 = wt. of dry gel and

m = wt. of water swollen gel

 

RESULTS AND DISCUSSION:

Swelling behaviour of unhydrolysed and hydrolysed hydrogel:

 

The degree of swelling of pure polyacrylamide in water is very low compared to that of alkali hydrolysed sample as is seen from Fig. 1

 

Figure 1: Degree of swelling as a function of time for unhydrolysed (Series 1) and hydrolysed (Series 2) poly (AM) hydrogels in distilled water. Wt. of dry unhyrolyesd gels and hydrolysed gels =0.20gm each, temperature - 500C

 

In both cases degree of swelling increases with time and then becomes constant. The unhydrolysed polymer is neutral and has a randomly coiled configuration having a low swelling. on the other hand, hydrolysed polymer has ionic character with an extended chain configuration. such polymers display a better swelling behaviour.

 

Effect of NaOH concentration:

The effect of NaOH concentration, used to hydrolyse polyacrylamide, on the degree of swelling is exhibited in

figure – 2

 

Figure 2: Degree of swelling vs alkaline hydrolysis as a function of different Concentration of NaOH Reaction conditions are ;i) [AM]=6.1 mole/I, ii0 [SO2] = 5.5x102 mole/I, iii) [BAM]= 1.5x10-2 mole/I and, iv) polymerization Temperature =500C

 

The maximum water absorbency is observed at NaOH concentration of 1mole/l after that it begins to decrease. The hydrolysed polymer contains both amide and carboxyl groups in the chain which are highly polar. The water molecules in the medium are attracted to the polymer molecules by dipole - dipole attraction and hydrogen bonding through nitrogen - oxygen and oxygen - oxygen bonding, thereby giving a high degree of swelling. further increase in NaOH concentration causes increase in the hydrolysis of the amide groups (-CONH2) into the carboxyl group (-COOH) till the equilibrium point is obtained. [16] When the concentration of NaOH is increased beyond the equilibrium concentration, excess Na+ ion in the solution are attracted towards the carboxyl groups in the polymer chain, thus creating effecting local neutral environments along the polymer molecules. This phenomenon is known as charge screening effect. Consequently polymer hydrogel configuration is forced to attain some coiled configuration causing deswelling. In addition a higher cross linked structure is formed at higher hydroxyl concentration.

 

Effect of molecular weights of polyacrylamide on swelling:

The polyacrylamide gel synthesized at different concentrations while concentrations of other reactants was kept fixed. The gels so obtained was allowed to hydrate and the degree of swelling was determined. the results are exhibited in Fig.-3

 

Figure 3: Degree of swelling in distilled water at 500c as a function of [AM] concentration used in polymerizing mixture to form polyacrylamide hydrogels. Experimental conditions are i) [NaOH] = 1.0 mole/I,

ii) [SO2]= 5.5x102 mole/I and iii) [BAM]=1.5x10-2

 

It shows the decrease in swelling ability with decreasing monomer concentration used during polymer formation. The decreased swelling ability with lower monomer concentration is due to a low molecular weight of the polymer chain. In a free radical polymerization, the molar mass of the resulting polymer increases with monomer concentration. We used SO2 as an initiator to polymerize acrylamide. A SO2 molecule contains one unpaired electron shown by its paramagnetism. Therefore, it causes free radical polymerization So the molecular weight per kinetic chain length of resulting polymer increased with monomer concentration. It seems swelling ability depends on the molecular weight of polymer chains and longer polymeric chains holds more water.

 

 

 

Effect of concentration of crosslinker:

The water absorbancy of polyacrylamide hydrogels formed at different concentrations of N,N'-methylene bisacrylamide (BAM), a crosslinking agent was examined. The pertinent results is presented in Fig. 4.

 

Figure 4: Effect of N’-N- methylene bisacrylamide (BAM) concentration on degree of swelling of polyacrylamide hydrogels. Experimental conditions are i) [AM]=6.0 mole/I ii) [SO2]= 5.5x10-2 mole/I, iii) [BAM]=1.5x10-2 mole/I and iv) Temperature of polymerization =500c

 

The figure shows that the degree of swelling decreases with the increase of BAM concentration. The higher concentration of the cross linking agent results in the formation of highly cross linked rigid structure because individual coils are highly tied to each other. Consequently the gel network cannot be expanded to uptake a large quantity of water. Therefore increasing degree of cross linking lowers the swelling ability of gels.

 

Effect of electrolyte on swelling:

 

Figure 5: Effect of salt solution on the degree of swelling of polyacrylamide Hudrogels. Experimental conditions are

i) Concentration of each salts = 0.25 mole/I,

ii) Temperature = 500c.

 

Results shown in Fig.5 indicate that the swelling capacity of the hydrogel is significantly lowered by the addition of ionic salts. The deswelling is strongly dependent on kind and concentration of an electrolyte added to the swelling medium. This is possibly due to mainly two effects.

 

Firstly it is the screening of the fixed charges in the polymeric chain and consequently lowering the expansion of the gel network. the negatively charged carboxyl groups attached to polyacrylamide chains set up an anion - anion electrostatic repulsion which tends to expand the network which causes more water uptakes in the hydrogels, where as an ionic salts such as NaCl is added, the presence of Na+ ions screen the fixed charges and tremendously reduce the electrostatic repulsion as compared with what it would be in the absence. therefore the expansion of gel network decreased.

 

Secondly it is the decrease of the osmotic pressure difference between the gel phase and the external solution phase when ionic strength of the medium increases. The ionic strength of a solution depends on both the mobile ions concentration and their valency as shown by the equation below:

 

where

mi = molality and

Zi = valency:

 

Therefore, (Ionic strength)NaCl= m, (Ionic strength)Na2SO4 =3m and (Ionic strength)AlCl3 =6m.

 

In fact experimental findings show that small quantity of divalent or trivalent ions drastically decrease the degree of swelling. The valence of the accompanying anion does not affect the water absorbency. However the water absorbency capacity of the gel remains unaffected by the Urea because of its nondessociatingbehaviour and having similar functional group H2N-CO-NH2 as that of acrylamide.

 

Reversibility of swelling gels:

The ability of polyacrylamide hydrogel to undergo several cycles of swelling-deswelling is shown in Fig. 6 suggesting the reversible behaviour.

 

Figure 6: Swelling (S) and Deswelling (DS) cycles of hydrolyzed polyacrylamide Hydrogels. Y-Co-ordinates represent degree of swelling.

It reveals that in all the successive cycles, it swells slightly more than that of previous cycle. This is due to leaching out the low molecular weight polymer gels. High molecular weight polymer gels show higher degree of swelling. hydrogels require more time to deswell compared to the time required for swelling. it seems that the gels follow different mechanisms for the two processes.

 

Mechanism of formation of SO2 initiated polymerization:

SO2 is a colourless gas with suffocating smell, m.p. -75.50C and b.p. 10.020C. It is colourless heavy liquid at 150C under 2.5 atmospheric pressure. The magnitude of the bond angle O-S-O 119.50C indicates that the sulphur atom is in the trigonal valence state. The reaction of SO2 with ethyl magnesium bromide in ethereal solution to give ethyl sulfinic acid indicates the presence of double bonds in SO2. The S-O double bonds originate from Pπ - dπ bonding due to the lateral overlap of p-orbitals of oxygen with d-orbitals of sulphur. The bond angle 119.50 clearly agrees with the structure predicted from sp2-hybridization. Two resonating forms of structure are shown below:-

 

Therefore, liquid SO2 itself copolymerizes with a number of monomers such as alkenes dienes to form polysulfone in presence of free radicals. It further functions as a diluent in the polymerization. However, SO2 gas in low concentration also acts as initiator for the polymerization of some vinyl monomers. Aqueous SO2 or its alkali metals salt also initiates aqueous vinyl polymerization. SO2 is easily prepared by the action of an acid on sulfite or by the action of concentrated acid on copper.

 

Polyacrylamide synthesized by aqueous SO2 initiator shows similar swelling behaviour with comparatively lower degree of swelling in comparison to that synthesized by aqueous NO2 initiator.[18]

 

CONCLUSION:

High degree of swelling of polyacrylamide is due to its hydrophilic nature and the capability of hydrogen bonding of acrylamide molecules with water. The molecular size, coil density and degree of cross linking greatly influence the water absorbency of gels

 

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Received on 12.08.2019                    Modified on 30.08.2019

Accepted on 14.09.2019                   ©AJRC All right reserved

Asian J. Research Chem. 2019; 12(5):258-262.

DOI: 10.5958/0974-4150.2019.00048.8