17.3 Conducting and electroactive polymers

Conducting and electroactive polymers are game-changers in materials science. These polymers can conduct electricity like metals or respond to electrical stimuli by changing shape. Their unique properties stem from their molecular structure and doping processes.

These polymers have revolutionized various fields. They're used in sensors, actuators, and energy storage devices. From artificial muscles to supercapacitors, their applications are diverse and exciting, pushing the boundaries of what's possible in technology and engineering.

Conducting and Electroactive Polymers

Properties of conducting and electroactive polymers

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• Conducting polymers
  • Possess electrical conductivity similar to metals or semiconductors (copper, silicon)
  • Exhibit high electrical conductivity due to conjugated backbone structure
    • Alternating single and double bonds allow for delocalization of electrons along the polymer chain
  • Examples: polyaniline (PANI), polypyrrole (PPy), polythiophene (PT)
• Electroactive polymers
  • Respond to electrical stimuli by changing shape, size, or other properties
  • Can be divided into two categories:
    • Electronic electroactive polymers (EAPs): respond to electric field by exhibiting electrostrictive or electrostatic effects
    • Ionic electroactive polymers (IEAPs): respond to ion flow by swelling or contracting
  • Examples: polyvinylidene fluoride (PVDF), Nafion, polyethylene oxide (PEO)

Mechanisms of polymer conductivity

• Electrical conductivity in conducting polymers
  • Conjugated backbone structure allows for electron delocalization along the polymer chain
  • Doping process introduces charge carriers (electrons or holes) into the polymer
    • p-type doping: oxidation of polymer, creating positive charge carriers (holes)
    • n-type doping: reduction of polymer, creating negative charge carriers (electrons)
  • Charge carriers move along the polymer chain, enabling electrical conductivity
• Electroactivity in polymers
  • Electronic EAPs: electric field induces dipole alignment or ion migration within the polymer
    • Dipole alignment leads to mechanical strain and deformation (piezoelectric effect)
    • Ion migration causes swelling or contraction of the polymer (electrostrictive effect)
  • Ionic EAPs: ion flow causes swelling or contraction of the polymer
    • Cations and anions move towards oppositely charged electrodes
    • Ion movement results in volume change and mechanical deformation (electromechanical coupling)

Synthesis of electroactive polymers

• Synthesis of conducting polymers
  • Chemical polymerization: oxidative coupling of monomers using oxidizing agents (ammonium persulfate, iron chloride)
  • Electrochemical polymerization: oxidation of monomers on an electrode surface by applying an electric potential
    • Allows for precise control over polymer growth and morphology
• Processing methods for conducting polymers
  • Solution processing: dissolving polymer in a solvent (N-methyl-2-pyrrolidone, chloroform) and casting into films or fibers
  • Melt processing: heating polymer above its melting point and shaping it through extrusion or injection molding
  • In-situ polymerization: polymerizing monomers directly on a substrate or template (textile fibers, nanoparticles)
• Synthesis and processing of electroactive polymers
  • Depends on the specific type of EAP (PVDF, Nafion, PEO)
  • Common methods include solution casting, spin coating, and extrusion
  • Post-processing techniques such as stretching or poling may be required to enhance electroactivity

Applications in sensors and energy

• Sensors
  • Conducting polymers: used in chemical and biological sensors
    • Change in electrical conductivity upon exposure to analytes (gases, biomolecules)
    • Examples: gas sensors (ammonia, hydrogen sulfide), biosensors for glucose or DNA detection
  • Electroactive polymers: used in pressure and strain sensors
    • Mechanical deformation induces electrical signal (piezoelectric effect)
• Actuators
  • Electroactive polymers: used in artificial muscles and soft robotics
    • Electrical stimuli cause mechanical deformation (bending, twisting, linear motion)
    • Advantages: lightweight, flexible, large strain (up to 380%), low power consumption
  • Examples: robotic grippers, haptic feedback devices, microfluidic pumps
• Energy storage
  • Conducting polymers: used in supercapacitors and batteries
    • High surface area and conductivity enable fast charge/discharge rates and high capacitance
    • Examples: polyaniline-based supercapacitors, polypyrrole-based lithium-ion batteries
  • Electroactive polymers: used in energy harvesting devices
    • Convert mechanical energy (vibrations, human motion) into electrical energy
    • Example: PVDF-based piezoelectric nanogenerators for self-powered electronics