Synthesis and Properties of Polyaniline

Abstract

Polyaniline is synthesised by pentiodynamic polymerization (cyclic voltammetry) to study the

mechanism of electrochemical polymerization of polyaniline. The redox and acid/base chemistry of

polyaniline was also analysed using emeraldine. . UV-vis spectra and cyclic voltametry were used to

generate and analyze the spectrum for the polymer films.

Introduction

Polyaniline (PANI) is a conducting polymer of the semi-flexible rod polymer family. Although it was

discovered over 150 years ago, only recently has polyaniline captured the attention of the scientific

community due to the discovery of its high electrical conductivity.[1]

Polyaniline

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

solvent (N-methyl-2pyrrolidone (NMP) into conductive form of emeraldine salt (ES) by protonic acid [2].

PANI-ES is due to be solubility with large number of organic solvent depending upon functionalities of

protonic acid such as camphorsulfonic acid (HCSA).

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 [3].

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.

Experimental

The method followed throughout this experiment is as described under experiment 5 in the lab manual but

some expectations were made. For the preparation of stock polyaniline solution 4g of aniline and 20ml of

concentrated HCl were added instead of 8g of aniline and 40ml of HCl.

Results and Discussion

a) Potentiodynamic synthesis of polyaniline

Stock polyaniline solution was used to grow polyaniline onto an ITO coated glass electrode by cyclic

voltammetry. During the process, a series of colour changes occurred as physical properties of polyaniline

changes with its oxidation state.

Solution Colour

EB solution + DMF Dark blue

(EB+DMF)+ HCSA Light green

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

(EB+DMF)+Ammonium persulfate Violet

EB+ m-cresol Blue

(EB+ m-cresol) + HCSA Green

Each colour change corresponds to three oxidation state of polyaniline, i.e, blue represent pernigraniline

base (EB) , green represent emeraldine base (EB), and yellow represent leucoemeraldine base. This

happens because physical properties of polyaniline changes with its oxidation state.

b)Chemical and spectroscopic properties of emeraldine

1) The UV-visible spectrum of EB solution in DMF at 300 – 1100 nm was obtained. The spectrum

gives two peaks at 615 nm and 323.50 nm.

2) The spectrum obtained for camphorphosulfic acid in EB+ DMF solution gave peaks between

802nm, 497nm and 317.50nm. This three peaks can be assigned as the transition from π band

(valence band) to polaron band, polaron band to π* band (conducting band) and π band to π*

band. The transition from π band to π* band will have peak at lowest wavelength (317.50 nm)

due high energy. The transition from polaron band to π* band wil have medium wavelength

(497.00 nm) and the transition from π to polaron band will have highest wavelength (802 nm).

3) The spectra obtained when ammonium persulfate was added to the solution of EB in DMF gave

peaks at 539nm and 412nm.Here the color change occur from blue to violet colour when a small

amount of ammonium persulfate was added. The colour change is due to pernigraniline salt. The

absorption band of the new species formed was at 539.00 nm and only one peak due to absent of

polaron band. Therefore the peak is the transition from π band to π* band. The peak at 412 nm is

due to other unrelated charge transfer. If the mixture was kept for long duration for the reaction to

complete, the wavelength might have shifted to the left.

4) When few drops of hydrazine were added to EB in DMF solution the main peak was observed

around 331nm. During this reaction the colour changes from dark blue to light blue. Change due

to formation of pernigraniline salt. There is only one peak due to absence of polaron band and the

peak is the transition from π band to π* band.

Discussion

1) The colour of emeraldine base solution in DMF changes when different reagents or solvents of

different pH are added as polyaniline can be transformed to different oxidation state by addition

of acids or bases that protonate and deprotonate the base sites within the polymer. This means

that the reactions depend on pH of the solutions. In solutions of pH greater than four, polyaniline

loses it electroactivity entirely since the emeraldine salt which is the only conducting form of

polymer is dedoped to form insulator emeraldine base. Due to undergoing transitions between

different states, its chemical properties can be controlled by application of potentials or an acid or

base. This means, the conductivity, chemical and physical properties of polyaniline changes

between each of the oxidation states.

2) 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 limited 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 re-aromatization step resulting in propagation of the chain.

4- Oxidation and Doping the polymer were occurred by adding acid (HA).

Cyclic voltametry of polyaniline

The cyclic voltammmetry of polyaniline in between -.02 and 0.9 V obtained two oxidations by 0.3and 0.9

V with reversible process two reductions by 0.35 and -.01 V.

At the beginning, a peak -0.1V (leucoemeraldine) with yellow color was oxidized and transformed into

emeraline by 0.35 V that it was caused conducting by change of the color from yellow to blue. The

second oxidation which was occurred at about 0.9V, was converted the oxidation of emeraline to

pernigranline with changing the color into violet. The current density corresponding to the first peak

obtains 6mA and the electro deposition of the polymer on the electrode surface stops.

The reduction process of PANI is sensitive to the proton concentration so, when no enough protons are

supplied by the solution, the PANI will not be completely reduced. The solution of that, the use of a

buffer solution which has a large capacity to provide protons and possibly to supply protons faster,

changes the behavior of reduction process for the PANI [1].

3) Using m-cresol as a solvent, the polymer backbone has positive charge with negative counterions

that are attached 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 coil like conformation.

Therefore, the existence of m-cresol 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 coil like 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 m-cresol between the adjacent isolated polaron has increased

and stronger led to more scattered in energy. Hence, the observation peak 615nm for EB in DMF

was corresponded to π- polaron transitions disappear for EB in m-cresol and then replaced by a

broad free- carrier tail associated with the intraband transitions among the half filled polaron

band.

Conclusion

In the experiment, we have learned that Polyaniline have electronic conduction

mechanism because it is doped by protonation as well as undergoing the p-type doping

and also Polyaniline can undergo two distinct redox processes as well as pH switching

between unprotonated and protonated states. The redox process can be observed using

cyclic voltammetry. And emeraldine salt is the most conductive form. Also that chemical

properties of polyaniline can be controlled by application of a potential or an acid or base

leading to change in colour between each steps. Polyanilines are usually synthesized by

chemical or electrochemical polymerization of aniline. And finally there are three

oxidation state of polyaniline, a fully reduced form (leucoemeraldine), partially reduced

form (emeraldine), and fully oxidized form (pernigraniline). Polyaniline bases may form

polyaniline salts when treated with strong acids. The salts of hydrochloric and various

organic sulfonic acids, such as camphorsulfonic acid (CSA) are most commonly used.

Reference

[1] Wikkipeia – The free encyclopedia

[2] Xia, Y., et.al., Camphorsulfonic Acid Fully Doped Polyaniline Emeraldine Salt: Conformations in Different Solvents Studied by an UltravioletNisiblel Near-Infrared Spectroscopic Method, chem. Mater, 1995, 7, 443-445.

[3] . http://www.conductivepolymers.com/general.htm

[4] Wallace. Gordon, et al, Conductive electroactive polymers, 2nd edn, 2003