Synthesis and properties of Polyaniline

by Awad Albalwi

Abstract

Polyaniline which is regarding kind of Inherently Conducting Polymers (ICPs), was prepared by

either chemical and electric­chemical polymerization in acidic medium. Such solvents were used

and compared the affecting on the conductivity between EB solution in DMF and m­cresol

solvents by using UV­ vis spectrum. UV­vis spectra and cyclic voltametry were used to generate

and analyze the spectrum for the polymer films

Introduction

In recent year, polyaniline (PANI) is considered one of the most widely used for the application in

electric devices. That is because PANI has a great electrical conductivity, high chemical stability and

easy synthesis that it can be able to control oxidation state and degree of protonation [1,2]. In addition,

the product of the polymerization (PANI) has been looking forward to develop the application in

different fields including sensors, charge storage systems and protection against corrosion [1]. PANI

can be used to undergo forming dielectric of emeraldine base (EB) with a significant film forming

solvent (N­methyl­2pyrrolidone (NMP) into conductive form of emeraldine salt (ES) by protonic acid

[2]. PANI­ES is due to be solubility with large number of organic solvent depending upon

functionalities of protonic acid such as camphorsulfonic acid (HCSA).

The main properties of polyaniline such as crystallinity, conductivity, molecular weight and

electrochemical behavior rely on the processes and conditions of preparation [3].

Inherently conducting polymers (ICPs) are conjugation of π­ electrons extending all over the length

of polymer backbone which they are prepared by chemical or electrochemical oxidation with large of

appropriate monomeric materials. The unique properties of ICPs might be included:

1­ Tuning the conductivity can possibly employed by adjusting the amount of dopant incorporated within the polymer,

2­ Doping and undoping are reversible process.

3­ Both of the characteristics for the optical and electromagnetic absorption in the UV, visible and near infrared are involved [4].

There are three of the most studied ICPs are defined below:

The formation of entirely organic conducting polymers depends upon the oxidation state of the

polymer. The process of that formation (oxidation or reduction ) which is called ‘doping”, causes

changing in the electronic structure in order to provide conducting electricity.

In this experiment, the polyaniline was prepared by potentiodynamic polymerization (cyclic

voltammetry). Moreover, the change of polyaniline were also examined and compared to their redox

and acid/base chemistry via using a sample of emeraldine base which was provided.

Experiment:

Preparation of stock polyaniline solution

Polyaniline stock solution was prepared by adding 4g of aniline & 20 ml of HCl into 200 mL water in 1L beaker and then stirred for 10 min.

Potentiodynamic synthesis of polyaniline

The parameters of using cyclic voltammetry for 10 mL of the stock polyaniline solution were :

Initial potential = final potential= ­0.2 v Lower potential limit= ­0.2v Upper potential limit= +0.9v Number of growth cycles = 10 Scan rate = 100 m Vs­1 Current = 1 mA/ cm2

Chemical and spectroscopic properties of emeraldine base:

For more details about the procedure refer back to the lab manual.

Result and discussion

Question 1:

From adding different solvent or changing PH of the solvent, the color of the EB solution was

reported as shown in Table 1 below:

Table1. the difference color to EB with different solvent.

Samples color

EB solution + DMF Dark blue Prepared

(EB+DMF)+ HCSA Light green Prepared

(EB+DMF)+ Hydrazine Blue light (clear) Prepared

(EB+DMF)+Ammonium persulfate Violet Prepared

EB+ m­cresol Blue Provided

(EB+ m­cresol) + HCSA Green Provided

Figure1. The UV­Vis spectrum of EB dissolve in DMF, the solution was dark blue and gives

wavelength in between 327­ 626 nm.

Figure2. The low conductivity was confirmed while EB in DMF which added to HSCA

obtaining green light and three bands 354nm (π­ π*), 410nm (polaron – π*) and 789nm (π­ polaron

band). The pollarons of each tetraneric unit are isolated from each other in order to the twist defect

between aromatic rings. Because of that the conductivity of the polymer has little energy.

Figure3. The spectrum showed that EB in DMF with adding hydrazine were given color exchange

where was from dark blue to light blue (clear) solution. Also, there was only one absorption occurred

by 342nm which due to (π­ π*) transition.

Figure4. The spectrum illustrated EB in DMF with ammonium persulfate was converted from

dark blue to give violet color (form pernigraniline state). Moreover, there were two peaks

occurred by 323nm (π ­ π*) and 592nm (π­ polaron band).

Question2.

Electrochemical polymerization of aniline was used containing a three electrode electrochemical cell as shown below:

Figure .A three­ electrode electrochemical cell [3].

The material and size of the working electrode (WE) which PANI film was grown depended on using ITO coated glass electrode

Auxiliary electrodes (AE) which made from platinum or stainless steel, has large surface area therefore they don’t have limit the passage of current.

Reference electrode (RE) used Ag/AgCl electrode in conjunction with NaCl salt bridge as a reference in aqueous solution.

The procedure of electrochemical polymerization of polyaniline can be concluded by:

1. Radical cation of aniline was formed by oxidation on electrode surface. 2. Coupling of radicals is occurred ultimately between N­ and C­ on the cycle and then

subsequent elimination of two protons. 3. The dimmer will be formed by rearomatization step resulting in propagation of the chain. 4. Oxidation and Doping the polymer were occurred by adding acid (HA).

Question 3

Using m­cresol as a solvent, the polymer backbone has positive charge with negative counterions

that are sitting in the proximity of polymer chains. In case of removing counterions from the

polymer chain, the interaction of positive charges on the polymer backbone will head to extend

the polymer chain from a coillike conformation to an expanded coillike conformation.

Therefore, the existence of m­cresol will change the confirmation because the twists and defects

between aromatic rings are removed. On the other hand, with using DMF as a solvent, the

polymer chains have a coillike confirmation and the polarons of each tetrameric unit are isolated

and polaron ­ π* transition possibly occurred. Moreover, half­ filled ‘polarone band’ that formed by

interaction of separating polaron on the fully protonated for PANI emeradline salt therefore the polaron

band has a little dispersion energy.

Based on that, the interaction of m­cresol between the adjacent isolated polaron has increased and

stronger and that led to more scattered in energy. Hence, the observation peak 626 nm for EB in DMF

was corresponded to π­ polaron transitions disappear for EB in m­cresol and then replaced by a broad

free­ carrier tail associated with the intraband transitions among the half filled polaron band.

Conclusion:

In conclusion, it can been seen that the conductivity of polyaniline has mainly influenced by

using and changing the acidity with different electronic structures in different solvents. This kind

of changing can lead to attribute to the differences in geometric structure (conformation) for the

polymer chain of polyaniline. So, the conductivity of m­cresol was greater than DMF solvent

where they were used in this experiment.

Reference

1.Moraes, S. R.; Huerta­Vilca, D.; Motheo, A. J., Characteristics of polyaniline synthesized in phosphate buffer solution. European Polymer Journal 2004, 40 (9), 2033­2041. 2. Xia, Y., et.al., Camphorsulfonic Acid Fully Doped Polyaniline Emeraldine Salt: Conformations in Different Solvents Studied by an UltravioletNisiblel Near­Infrared Spectroscopic Method, chem. Mater, 1995, 7, 443­445. 3. pornputtkul, Y., Development of chiral conducting polymers for asymmetric electrosynthesis, intelligent polymer research institute, 2005, 475 4. http://www.conductivepolymers.com/general.htm