INTRODUCTION:

Polymer hydrogels are cross linked hydrophilic polymers which are insoluble but absorb, swell and retain large amount of water. They exhibit both liquid and solid like properties,(1-4). They are also called soft matter. Therefore properties of polymer gels depend on the structure of polymer network that creates gels and the interaction of the network and solvent. Gels formed by chemical reaction are called chemical gels or covalent gels. Such gels are permanent and not broken easily. On the other hand, gels formed by the aggregation caused by hydrogen bonding or ionic bonding and by the physical entanglement of polymer chains are known as physical gels. These gels are temporary and are broken easily,(1, 5).

The polymer hydrogels are nick named “ Smart or stimuli responsive “ as they are capable of reversibly swelling or shrinking up to several hundred times their own mass due to small changes in environment like pH , temperature , pressure, light intensity, electric or magnetic field as well as introduction of specific ions,(6, 7).

In polyacrylamide based hydrogels, several applications have been found such as metal extraction, waste water treatment,(8). Cross-linked polymers which can absorb large amount of water can be used in broad various fields such as Biotechnology, Biomedical, Engineering, Food-industry, Agriculture, Water treatment and separation process,(9, 10, 11, 12).

In this paper we describe the preparation of polyacrylamide which is soluble in water but the cross linked polymers form water swelling gels . The polymerization was carried out in a dilute aqueous solution of HNO₃,(13).

MATERIALS AND METHODS:

Acrylamide was analytical grade and used without further purification. Water was purified by distillation.

Polymerization:

The polymerization was carried out in an aqueous solution of acrylamide (known amount) with a known amount of HNO₃as an initiator at 30⁰ in a glass vessel. The polymerization was terminated after a certain period of reaction time. The polymer was isolated and dried. Further the polymer so formed was saponified with NaOH and then precipitated in excess of methanol. The polymers in the form of precipitate so obtained were thoroughly and repeatedly washed in acetone and dried polymers were stored to determine the degree of swelling in water.

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 water. 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 glossy in appearance and quite hard while swollen gels are very soft. The degree of swelling was calculated by the following expression

Degree of swelling = {(m – m₀)/ m₀} Where m₀= weight of initial polymer gel

m = weight of swollen polymer gel

RESULTS AND DISCUSSION:

The degree of swelling of pure poly acryl amide in water is low in comparison to that of alkali hydrolyzed sample as is seen from table 1 and 2.

Table 1:- Degree of swelling of poly acrylamide gel with time in pure H₂O at 30° C.

Time (in days)	Final wt. of swollen polymer gel (g)	Degree of swelling
1	14.76	13.76
2	20.90	19.90
3	24.80	23.80
4	27.90	26.90
5	29.10	28.10
6	29.11	28.11

Conditions-

1.) Initial weight of dry gel- 1g.

2.) [AA]- 10g

3.) [HNO₃]- 10ml(2.0mol/lit).

4.) Time of polymerization- 5 days.

5.) Temp of polymerization- 30° C

Table 2:- Degree of swelling of hydrolyzed poly acrylamide gel with time in pure H₂O at 30° C.

Time (in days)	Final wt. of swollen polymer gel (g)	Degree of swelling
1	36.03	571.22
2	57.92	918.37
3	62.00	983.13
4	63.18	1001.86
5	64.10	1016.46
6	64.10	1016.46

Conditions-

1.) Initial weight of dry gel- 0.063g.

2.) [AA]- 5g

3.) [HNO₃]- 5ml(2.0mol/lit).

4.)[NaOH]- 3 wt. %

5.) Hydrolysis time- 1hr at 60° C.

In both cases degree of swelling increases with time and then becomes constant. The hydrolyzed polymer contains both amide and carboxyl groups in their chains which are highly polar in character as shown below.

The water molecules in the solvent 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. The unhydrolyzed polymer is neutral and has a randomly coiled configuration having a low swelling, on the other hand, hydrolyzed polymer has ionic behavior with an extended chain configuration. Such polymers display a better swelling character.

Effect of pH on swelling:

The result present in the table 3 below shows the significant role of pH on the swelling behavior

Table 3-Effect Of pH on degree of swelling of hydrolyzed polyacrylamide gel. Swelling Time – 24hrs

pH	Initial wt. of polymer gel (gm)	Final wt. of swollen polymer gel (gm)	Degree of swelling
1 (N/10HCl)	0.51	13.91	26.27
7 (Water)	0.52	261.37	501.63
13 (N/10 NaOH)	0.53	48.10	80.52

The gel swells as pH is increased and then deswells again at high pH. In neutral water the degree of swelling is maximum.

At low pH i.e. in more acidic solution

RCOO^- + H⁺ → RCOOH

favors forming neutralized carboxyl group. In this situation the molecular configuration of poly acryl amide hydrogels in solution resemble that of non-ionic polymer which has randomly coiled rather than extended chain configuration exhibiting a low degree of swelling.

When pH increases to 7, the degree of swelling reaches maximum as attributed to ionization of gel fixed charges which is carboxyl group (-COOH). At this stage the hydrogel has a fully elongated chain configuration.

The opposite behavior occurs in basic solution in the range pH – 7 to pH -13. As the pH increases, ionization of carboxyl group is suppressed by the excess of sodium ions in the solution. Consequently the polymer hydrogel configuration is forced to attain coiled configuration causing deswelling.

Studies of reversibility of swelling gels:

The ability of the polyacrylamide hydrogel to undergo several cycles of swelling-deswelling is shown in the figure 1

It reveals that in all the successive cycles it swells more than that of the previous cycles.

Although there is a decrease in the polymer weight in the successive swelling-deswelling cycles that may be due to leaching out the low molecular weight polymers.

Dyes absorption by polyacrylamide hydro gels:

Removal of methylene blue in water with hydro gel was investigated. The hydro gel has been submerged into the aqueous solution of methylene blue as shown in the photographs, figure 2.

Figure- 2 Absorption of methylene blue by Polyacrylamide hydrogel

Here we see that the gel has turned deep blue while the solution has become light blue from deep blue. This shows that gel has extracted the mass of the dye from the solution. Similar behavior has been shown by hydrogels when yellow, green and red dyes are applied .

When the colored gels so obtained were placed separately into acetone, the dye molecule diffuses out from the gels and gels into acetone. The work demonstrates that polyacrylamide hydrogels may be used to extract a valuable solute from its solution,(8,14).

Mechanism of formation of HNO₃initiated polymerization:

HNO₃ initiates a free radical mechanism in presence of acryl amide, as outlined below :

4HNO₃+4NO₂ + O₂ + 2H₂O

NO₂ is a free radical as it is paramagnetic and hence interacts with acrylamide to give monomeric free radical which further add monomer molecules to propagate polymerization.

NO₂ gas efficiently initiates acrylamide polymerization, (15). Further HNO₃ initiated polymerization is completely inhibited by a free radical inhibitor such as p-Benzoquinone inhibitor suggesting a free radical polymerization.

Cross linked network:

The intermolecular imidization of the polymer molecules causes the formation of cross linked network and gelation.

CONCLUSION:

Poly acrylamide hydro gels were prepared in an aqueous solution of HNO_3.Cross linked polymers so formed imbibe large amount of water. Gels are a pH sensitive and undergo several cycles of swelling and deswelling. Hydrogels absorbs dyes.

ACKNOWLEDGMENT:

The financial support of the University Grant Commission, New Delhi to carry out this work is gratefully acknowledged.

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Received on 01.02.2014 Modified on 14.03.2014

Asian J. Research Chem. 7(3): March 2014; Page 345-348