Cathodic Aqueous Polymerization of Acrylamide with ZnCl2
Reena Bhadani1*, S. N. Bhadani2
1Department of Chemistry, Ranchi Women’s College, Ranchi.
2P. G. Department of Chemistry, Ranchi University, Ranchi.
*Corresponding Author E-mail: rbhadani04@gmail.com
ABSTRACT:
The electroinitiated polymerization has been described here. The polymerization mixture consisted of monomer dissolved in an aqueous solution of ZnCl2, and was subjected to electrolysis in a divided cell. The rates of polymerization increased linearly with increase in applied current level and monomer concentration. Polymers of high molecular weights (≈105) were obtained from intrinsic viscosity measurements that increased with monomer concentration. The cyclic voltammogram of acrylamide in H2O - ZnCl2 solution suggests that the monomer gets directly reduced at cathode and the polymerization mechanism is anoinic.
KEYWORDS: Polymerization, electrochemical process, acrylamide, zinc chlorides and cyclic voltammetry.
The zinc salt acts as both agents to form complexes with a monomer and supporting electrolyte in an electrochemical polymerization (1-5).The complex so formed with a monomer diffuses towards the anode or the cathode and undergoes electrochemical polymerization. In chemical polymerization activation needs to be given. Therefore a high temperature is required to obtain meaningful polymerization rate. On the other hand, in electrolytic polymerization the desired rate is achieved by just passing electric current at a fixed temperature. The present investigation deals with the polymerization of acrylamide in an aqueous solution of Zn salts. With ZnCl2 the locus of polymerization is found to be in the catholyte whereas polymerization occurs only in the anolyte in the aqueous solution of Zn(NO3)2(3). Therefore the change of the anion of Zinc salts alters the locus of polymerization. In the present article we describe the findings on the cathodic aqueous polymerization of acrylamide with ZnCl2. Polyacrylamide finds numerous industrial and commercial applications (6 - 9).
MATERIALS AND METHODS:
MATERIALS
Acrylamide(AM) and ZnCl2 were analytical grade and used without purification. Water was twice distilled prior to use.
METHODS
POLYMERIZATION
The polymerization was carried out in a simple H-shaped cell which could hold 50 ml solution and accommodate two Pt. electrodes having an area of 1.0 cm x 2.5 cm. The cell was divided into two compartment by a fine fritted glass disc of 1.0 cm diameter. The viscosity and average molecular weight of polymers was determined in water at 300C by Ubbelohde Viscometer. The kinetics of polymerization was followed gravimetrically. In order to determine the amount of polymers for a desired time, the electrolysis was terminated at that time and the whole catholyte was poured into an excess of cold methanol. The polymer was recovered by filtration, properly washed with methanol, dried and weighed to a constant weight.
RESULTS AND DISCUSSION:
The polymerization didn’t take place in the absence of electric current. The polymers isolated from catholyte were soluble in water suggesting the absence of formation of crosslinked polymers under the present experimental conditions. The monomer was polymerized as a function of polymerization time at different current levels at a fixed monomer and salt concentration at 00C. The results are presented in figure - 1. Polymerization of AM was also carried out at a constant current of 40 mA. Results are shown in figure - 2.
Kinetic Scheme
The following assumptions are made to derive kinetic expressions that fit the data presented in figure-1 and figure-2 (10 - 11).
1. Initiation is via direct electron transfer from cathode to the monomer leading to the formation of radical anion.
2. Radical anion may not be capable of adding to monomer.
3. Monomeric radical anion dimerizes and the dimerization is the slowest step.
4. Termination is only by reaction with solvent or other agent (Z), and their reaction products are not capable of initiating polymerization.
5. Consumption of monomer is primarily by polymer growth, i.e., high molecular weight polymers are produced.
6. concentration of solvent or chain terminating agent (Z) is essentially constant.
7. Steady concentration of polymeric anion is achieved.
The reaction steps may then be written as:
Where, Kp's are equal and Kt's are also equal. The rates of initiation, growth and termination at steady state are, respectively,
Thus
Since the initiation takes place by the direct cathodic reduction of the monomer, the initiation rate is proportional to the quantity of the current in Faraday per unit volume available in the electrode process, i.e., I/FV where I = constant current in amperes, V = volume of catholyte, and F = faraday.
Therefore, under steady-state condition,
When the number of chain growth initiated equals the number of chain growth arrested, the rate of initiation is equal to the rate of termination, i.e.,
Ri = Rt
Thus
or
substituting the value of
Rate of consumption of monomer, i.e.,
Since the consumption of monomer in the initiation process is negligible compared to the growth process leading to formation of high molecular weight polymer, then
Therefore the rate of polymerization, Rp, is directly proportional to the electrolysis current, I and concentration of acrylamide [AM]. Indeed, polymerization rates at different current levels at a constant [AM] obtained from the data of figure-1 gives a straight line when they are plotted with time as shown in figure-3. The plot of [AM] versus time (t) from the data presented in conversion curve of figure-2, also gives a straight line passing through origin, as shown in figure-4.
CYCLIC VOLTAMMOGRAM
The cyclic voltammogram of aqueous solution of AM containing ZnCl2 was investigated to gain insight into the initiation process. The three electrode system using Pt. microelectrode as working electrode, Pt. wire as auxiliary electrode and Saturated Calomel Electrode (SCE) as a reference electrode was employed. When the cathodic potential was run from +0.1 to -2.5V vs SCE in the solution of Zncl2, no cathodic wave was observed. When the same cathodic potential sweep was made in the aqueous solution of AM containing Zncl2, a cathodic peak current at about -2.1V vs SCE was recorded as shown in figure-5. The cyclic voltammogram result substantiates that the monomer is reduced at the cathode to generate monomeric radical anions which on dimerization to dianions grows to polymer by addition with monomer.
Molecular weight of polymers
It is seen from the table-1 that the high molecular weight of the polymers are obtained from the measurement of limiting viscosity [ɳ] using the following expression and that increase with increasing monomer concentration.
Table - 1
|
S.N.
|
Monomer concentration (AA) in Mole/l |
[ɳ] in dl/g |
Mol. Wt.×10-4 |
|
1. |
1.408 |
0.85 |
10.88 |
|
2. |
2.817 |
0.94 |
12.65 |
|
3. |
4.225 |
1.31 |
20.81 |
|
4. |
5.63 |
1.40 |
22.99 |
|
5. |
7.04 |
1.60 |
28.09 |
CONCLUSION:
The cathodic polymerization of acrylamide in an aqueous solution of ZnCl2 is described. Kinetics of the polymerization and cyclic voltammetry investigation suggest that the polymerization process is anoinic.
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Received on 14.12.2015 Modified on 22.12.2015
Accepted on 25.01.2016 © AJRC All right reserved
Asian J. Research Chem. 9(1): Jan., 2016; Page 29-32
DOI: 10.5958/0974-4150.2016.00006.7