Polyaniline: Synthesis, Doping, Properties & Applications

Introduction

Polyaniline (PANI) is one of the most studied intrinsically conducting polymers (ICPs) due to its environmental stability, ease of synthesis, tunable electrical conductivity, and reversible redox behavior. It belongs to the family of conjugated polymers with alternating single and double bonds along the backbone, enabling charge transport.

The Emeraldine Base (EB) form, the most crucial intermediate state, has a 1:1 ratio of amine (−NH−) to imine (=N−) linkages:

−[(C₆H₄)−NH−(C₆H₄)−NH-(C₆H₄)-N=(C₆H₄)=N−]_n

Synthesis of Polyaniline

1. Chemical Oxidative Polymerization (Most Common)

• Monomer: Aniline (C₆H₅NH₂)
• Oxidant: Ammonium persulfate ((NH₄)₂S₂O₈), H₂O₂, FeCl₃
• Medium: Acidic aqueous solution (HCl, H₂SO₄)
• Temperature: 0–5 °C (to control reaction rate and obtain high molecular weight)

Reaction:

n C₆H₅NH₂ + n (NH₄)₂S₂O₈ → [−(C₆H₄)−NH−]_n + byproducts

2. Electrochemical Polymerization

• Performed on conductive electrodes (Pt, ITO, carbon) in an acidic electrolyte.
• Forms adherent, uniform PANI films directly onto the electrode surface.
• Methods: Cyclic voltammetry, potentiostatic, or galvanostatic polymerization.

Key Structural Feature: Polymerization occurs via head-to-tail coupling forming linear, para-linked chains.

3. Other Methods

• Enzymatic polymerization (using horseradish peroxidase)
• Photochemical polymerization
• Plasma polymerization

Doping of Polyaniline

Polyaniline (PANI) is unique among conducting polymers because it can be made electrically conductive through two independent mechanisms:

1. Protonic Acid Doping (Primary & Reversible)
2. Redox Doping (Oxidation/Reduction)

Unlike other conjugated polymers (e.g., polyacetylene, polypyrrole), PANI does not require electron donation or removal for conductivity in its most useful form. Instead, protonation of the nitrogen atoms transforms it from an insulator to a conductor.

Oxidation States of Polyaniline

PANI exists in three main oxidation states, defined by the ratio of amine (−NH−) to imine (=N−) groups:

Leucoemeraldine (0% Ox) ⇌ Emeraldine (50% Ox) ⇌ Pernigraniline

State	Structure	Color (Base)	Conductivity
Leucoemeraldine	Fully reduced: -(C6H4-NH-)n-	Colorless/Yellow	Insulating
Emeraldine Base (EB)	50% oxidized: -(C6H4-NH-)-2-(C6H4=N-)-2-	Blue/Violet	<10−10 S/cm
Emeraldine Salt (ES)	Protonated EB	Green	1 – 100 S/cm
Pernigraniline	Fully oxidized: -(C6H4=N-)n-	Dark Violet	Insulating

1. Protonic Acid Doping (Primary Mechanism)

The emeraldine base (EB) is protonated by acids to form the conducting emeraldine salt (ES). This process does not involve the addition or removal of electrons (non-redox).

[−(C₆H₄)−NH−(C₆H₄)−N=(C₆H₄)=N−(C₆H₄)−]_n + nH⁺ → [−(C₆H₄)−NH⁺−(C₆H₄)−NH⁺−(C₆H₄)=NH⁺−(C₆H₄)=NH⁺−]_n · nA⁻

Key Features:

• No change in electron count — only proton addition
• Reversible: Dedoping with base (NH₃, NaOH) → back to EB
• Conductivity increases by 10¹⁰ (from <10⁻¹⁰ to ~100 S/cm)
• Protons added to imine nitrogens → forms polarons/bipolarons

Emeraldine Base (EB)
Insulating

+ H⁺

Emeraldine Salt (ES)
Conducting

– H⁺

Reversible Cycle

Common Dopants (Protonic Acids):

Acid	Abbrev.	Solubility Effect
Hydrochloric acid	HCl	Water-soluble
Sulfuric acid	H2SO4	Water-soluble
Camphorsulfonic acid	CSA	Organic solvents (NMP, m-cresol)
Dodecylbenzenesulfonic acid	DBSA	Organic solvents, surfactant
Phosphoric acid esters	—	Flexible films

2. Redox Doping

Involves gain or loss of electrons, changing oxidation state.

Oxidative Doping (p-doping):

EB → Pernigraniline + 2n e⁻ + 2n H⁺

Uses oxidants: I₂, FeCl₃, H₂O₂ → low conductivity

Reductive Doping (n-doping):

EB + 2n e⁻ + 2n H⁺ → Leucoemeraldine

Uses reductants: Hydrazine, NaBH₄ → insulating

Note: Only protonated emeraldine salt (ES) is highly conducting. Leucoemeraldine and pernigraniline are insulating even when doped.

3. Secondary Doping (Conformational Change)

Discovered by MacDiarmid & Epstein (1990s). Involves conformational change in polymer chains induced by specific solvents or dopants.

Process:

1. Primary doping: Protonation with CSA → partial conductivity
2. Secondary doping: Exposure to m-cresol → chain expansion → crystalline, highly conducting

EB + CSA → (ES-CSA)_compact —^m-cresol→ (ES-CSA)_expanded ↑↑ Conductivity

Effects:

• Conductivity: 100 → 400–600 S/cm
• Improved crystallinity (XRD peaks sharpen)
• Better solubility and processability
• Enhanced charge transport via interchain hopping

Charge Transport Mechanism

Conductivity arises from polarons and bipolarons formed upon protonation:

−NH− + H⁺ → −NH⁺− (Polaron: radical cation) −NH⁺− + H⁺ → −N⁺H− (Bipolaron: dication)

Charge carriers move via:

• Intrachain hopping: Along conjugated backbone
• Interchain hopping: Between polymer chains (rate-limiting)
• Metallic islands in secondary-doped PANI

Temperature dependence: Variable range hopping (VRH) model fits well.

Dedoping (Reversibility)

ES → EB by treatment with base:

[PANI·H⁺ A⁻] + NH₃ → [PANI] + NH₄⁺ A⁻

Applications of reversibility:

• pH sensors
• Ammonia gas sensors
• Electrochromic devices
• Reusable membranes

Summary Table: Doping Types

Doping Type	Mechanism	Conducting?	Reversible?	Example
Protonic Acid	H⁺ addition to imine N	Yes (ES only)	Yes	HCl, CSA
Redox (p-type)	Oxidation (e⁻ removal)	No	Partial	I2, FeCl3
Redox (n-type)	Reduction (e⁻ addition)	No	Yes	Hydrazine
Secondary	Chain expansion	Enhances	Semi	m-cresol

Properties of Polyaniline

Property	Doped (Emeraldine Salt)	Undoped (Emeraldine Base)
Conductivity	1–100 S/cm	~10-8 S/cm
Color	Green	Blue
Solubility	Poor (in water, organic solvents)	Soluble in NMP, DMF, DMSO
Thermal Stability	Stable up to ~200°C	Stable up to ~300°C
Environmental Stability	Good (better than polypyrrole)	Excellent
Processability	Poor (brittle)	Can be cast into films

Unique Feature: Conductivity depends on both oxidation state and protonation level — tunable.

Key Physical Properties:

• Electrochromism: Color changes with redox state (yellow → green → blue → violet)
• pH Sensitivity: Conductivity depends on pH
• Environmental Stability: Better than polypyrrole/polythiophene

Applications of Polyaniline

Due to its low cost, tunability, and environmental stability, PANI is heavily researched for:

• Antistatic Coatings: Textiles, electronics packaging
• EMI Shielding: Electronic enclosures
• Corrosion Protection: Steel structures (chromate-free primer)
• Sensors: Gas (NH₃, NO₂), pH, humidity sensors
• Electrochromic Devices: Smart windows, displays (green ↔ blue)
• Rechargeable Batteries: Cathode material (Li/PANI cells)
• Supercapacitors: High capacitance electrodes
• Actuators & Artificial Muscles
• Membranes: Ion-exchange, fuel cells

Commercial Name: PANIPOL™, Ormecon™

Note: Polyaniline is not thermoplastic — it decomposes before melting. However, it can be processed via solution casting or melt blending with thermoplastics for composites.