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. They are also called soft matter. Properties of polymer gels depend on the structure of the polymer network that creates gels and the interaction of the network and the 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. Factors favorable to swelling are strong interaction with H2O, osmotic potential, high free volume, high chain flexibility and low cross linking density etc. Factors inhibiting swelling are Weak interaction with H2O, High cross linking density and Low chain flexibility etc(1-2).The polymer hydrogels are nick named “Smart or stimuli responsive” as they are capable of reversibly swelling or shrinking upto 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(3)

Polymer composites with organic fillers are prepared for a wide range of commercial applications. Recently there is growing interest in the field of polymer hydro gel - nanocomposites with metal and / or their salts. The smart gels do not depend on nanotechnology but their effects are greatly enhanced by mixing with nano and micro particles. The uses of such materials are in their infancy in the bio-medical field but there is great promise for the technology(4-5)

Several methods have been developed for preparing polymer based metal nanoparticles composites. In a classical method polymer and metal nanoparticles are formed separately and then they are mechanically or physically mixed to obtain polymer – metal nanocomposites. However, the resulting composite shows heterogeneity of polymer and metal component. In another method the polymerization of monomer and reduction of metal ions are carried out simultaneously in situ. Photolysis, radiolysis or other chemical techniques are also used to form the composites in situ. We used here electrochemical method to polymerize monomer and to reduce metal ions simultaneously at the cathode in the same solution. The solution of acrylamide in water containing ZnCl₂ was subjected to electrolysis in a simple H-shaped cell having the capacity to accommodate 50 ml solution, cathode and anode.

The electrochemical process is one of the simplest techniques having the following advantages (6-10):-

i) The process is generally simple, convenient and inexpensive that can be used to form wide range of polymer- metal nanoparticles composites.

ii) The process offers easy control over yield, size and shape of the products by merely monitoring applied current - density and electrode potential.

iii) Reaction can be easily terminated at will by simply switching off the electric current.

MATERIALS AND METHODS:

Materials:

Acrylamide and metal salts are taken of analytical grade and used without further purification. DMF was distilled before using it in the experiment.

Methods:

Simultaneous reduction of monomer and metal ions are cathodically carried out in situ in an electrolytic cell as shown in figure 1.

Figure-1: Electrolytic cell for the measurement of current-potential curve

a, metal electrodes; b, saturated calome electrode; c, glass coated

magnetic stirring bar; d, cathode cell; e, cylindrical glass jar.

The cathode and anode are made of same metals. Ni, Zn, Fe, Cu, Ag, and Pt were used as electrodes. The surface of electrodes was polished with emery paper.

In a typical run a solution of known concentration of ZnCl₂ and acrylamide in water was taken in electrolytic celland was subjected to electrolysis at a constant current of 20 mA. During course of electrolysis polymer gels formed at the cathode which was the locus of polymerization. Metal particles are also formed by the cathodic reduction of metal ions. On continuous electrolysis the cathode gets heavily coated with polymer gels and also with deposited zinc metal particles.

The electrolysis was terminated at a desired time and the catholyte was poured into cold methanol to recover the polymer gels. The resulting polymer gels were washed several times with acetone to remove the adhered electrolyte and then dried in an air oven at 60⁰ C till constant weight. The experiment was repeated with the electrode of different metals and their salt solutions in water.

RESULTS AND DISCUSSIONS:

The yield of dried polymer hydrogels–metal nanocomposites are presented in the table 1

Table – 1Cathodic polymerization of acrylamide (1.60mole/L) and supporting electrolyte(metal salts) (0.30mole/L) in water at different electrodes at a fixed current of 20 mA, Time of electrolysis – 2hr. and Temperature – 30⁰C.

Nature of electrodes	Dried polymer gels-nanometal composites yield(%conversion)
Ni	60
Zn	60
Fe	50
Cu	35
Ag	15
Pt	10

The adsorption and / or catalytic activity of electrodes may presumabily account for the observed differences of yields obtained at different metal electrodes.

The mechanism of the synthesis of Polyacrylamide hydrogels-metal nanocomposites can be understoodin the following steps:-

(i)Electrochemical formation of metal nano particles composites

Metal ions are available in the electrolyte from supporting salts and also oxidative dissolution of the sacrificial metal anode. A sketch of process is shown in figure 2.

Electrochemical equipment system for metal nano particles formation.

Figure-2: Electrochemical formation of metal nanoparticle in aqueous solution of metal salt

There was reductive formation of zero- valent metal atoms on the cathode.

M⁺² + 2e^- M⁰

Formation of metal nano particles occur via nucleation and growth due to attractive Vander Waals forces between metal atoms. Some metal electro deposition may also take place.

(ii)Electrochemical formation of poly acrylamide hydro gels

Cathodic reduction of monomer acrylamide is reduced at cathode

AM + e^-AM^˚-

Generating radical anions which dimerise to yield dianions. Radical anions and dianions initiate the polymerization. On continuation of electrolysis the polymer concentration increases that causes chemical cross linking and consequently formation of hydrogels as depicted in figure 3.

Figure-3: Chemical cross-linking appears at later when the concentration of polymer chains at the surface is high.

(iii) Electrochemical formation of poly acrylamide hydrogel- metal nanocomposites

Both metal ions and monomer molecules are reduced in situ at the cathode surface. There is a fair chance that metal particles so generated at the cathode get embedded or entrapped in the hydro gel network as outlined in figure 4 below.

Figure-4: Polyacrylamide hydrogel formation and metal nanoparticles deposition simultaneously at cathode resulting inpolyacrylamide hydrogel-metal nanocomposite

Characterization:

We are not yet able to characterize the product so prepared due to the constraints of experimental facilities. However, the characterization of the prepared products will be carried out using Fourier Transformed Infrared Spectroscopy (FTIR), X–ray Diffraction (XRD) and Scanning Electron Microscope (SEM). The result will give valuable information on the morphological structure, swelling behavior, bonding information and physical properties.

ACKNOWLEDGEMENT:

The authors thank the University Grants Commission, New Delhi, India for the financial support.

REFERENCES:

1. Kunzler J. F. Hydrogels in Encyclopedia of Polymer Science and Technology, John Willey and Sons, New York, 2, 691-722 (2002).

2. Osada Y. and Kanjiwara Kanji, Editors, Gel Handbook, Academic Press, USA (2000)

3. Chatterji S., Kwon H. K. and Park k. Smart Polymeric gels: Redefining thelimits of Biomedical devices. Prog. Polym. Sci., 32, 1083 – 1122 (2007)

4. Sambarkar P.P., Patwekars. I., Dudhgaonkar B.M., Polymer Nanocomposites: An overview. Int. J. Pharma. Pharma. Sci., 4, 60-65 (2012)

5. Rangareddy P., Mohnaraju K., SubbaramiReddy N. A Review on Polymernanocomposites:Monometallic and Bimetallic Nanoparticles for Biomedical,Optical and Engineering Applications., Chem. Sci. Rev. Lett., 1, 228-235 (2013)

6. Khaydarov Rashid A., Khaydarov Renat R., Gapurova Olga, Estrin Yuri, Scheper Thomas, Electrochemical method for the synthesis of silver nanoparticles., 11, 1193-1200 (2009)

7. Hassan Hashemipour, Maryam EhteshamZadeh, Rabee Pourakbari, Payman Rahimi, Investigation on synthesis and size control of copper nanoparticle via electrochemical and chemical reduction method., Int. J. Phy. Sci., 6, 4331-4336 (2011)

8. Kodihalli G. Chandrappa, Thimmappa V. Venkatesha, Kanagalasara Vathsala, Chikadasappa Shivakumara. A hybrid electrochemical-thermal method for the preparation of large ZnOnanoparticles., J. Nanopart. Res., 12, 2667-2678 (2010)

9. Muhammad I.I., Asgharzadehahmadi A., Zaidel D.N.A., Supriyanto E., Characterization and Evaluation of Antibacterial Properties of Polyacrylamide Based Hydrogel Containing Magnesium Oxide Nanoparticles., Int. J. Bio. Biomed. Engin.,7, 108-113 (2013)

10. Reuber J., Reinhardt H., Johannsmann D. Formation of Surface-Attached Responsive Gel Layers via Electrochemically Induced Free- Radical Polymerization., Langmuir,22, 3362-3367 (2006)

Received on 27.04.2014 Modified on 25.05.2014

Asian J. Research Chem. 7(6): June 2014; Page 593-595