Substantially Conductive Polymers Part 07 Applications of PPV • • • • • Due to its stability, processability, and electrical and optical properties, PPV has been considered for a wide variety of applications. In 1989 the first polymer-based light emitting diode (LED) was discovered using PPV as the emissive layer. Polymers are speculated to have advantages over molecular materials in LEDs, such as ease of processing, reduced tendency for crystallization, and greater thermal and mechanical stability. Ever since the first breakthrough in 1989, a large number of PPV derivatives have been synthesized and used for LED applications. Although solid-state lasing has yet to be demonstrated in an organic LED, poly[2-methoxy-5-(2’-ethylhexyloxy)-p-phenylene vinylene] (MEH-PPV) has been proven to be a promising laser dye due to its high fluorescence efficiency in solution. Polyphenylene vinylene is capable of electroluminescence, leading to applications in polymerbased organic light emitting diodes. PPV was used as the emissive layer in the first polymer lightemitting diodes. Devices based on PPV emit yellow-green light, and derivatives of PPV obtained by substitution are often used when light of a different color is required. In presence of even a small amount of oxygen, singlet oxygen is formed during operation, by energy transfer from the excited polymer molecules to oxygen molecules. These oxygen radicals then attack the structure of the polymer, leading to its degradation. Special precautions therefore have to be kept during manufacturing of PPV in order to prevent oxygen contamination. PPV is also used as an electron-donating material in organic solar cells. Although PPV-based devices suffer from poor absorption and photodegradation, PPV and PPV derivatives (especialy MEH-PPV and MDMO-PPV) find frequent application in research cells. Polypyrrole • A conjugated polymer based on heterocyclic aromatic units on the main chain • Synthesized by chemical or electrochemical polymerization from pyrrole • Mechanism: oxidative coupling reaction Proposed mechanism for the electrochemical polymerization Polythiophene • Environmental stable and highly resistant to heat • Synthesized by the electrochemical polymerization of thiophene • Can also be obtained by various types of metal catalyzed coupling reaction The solubility and processibility can be enhanced by attaching substitution groups at the 3 position However, the coupling can be either head-to-head (HH), headto-tail (HT), or tail-to-tail (TT) Synthesis of Regioregular Polythiophene Copolymers with aromatic compounds or vinylene group Conductivity of polythiophenes and polypyrroles doped under different conditions Material Dopant s (S cm-1) Polythiophene Poly(3-methylthiophene) Poly(3-ethylthiophene) Poly(3-buthylthiophene) Poly(3-hexylthiophene) Poly(3-hexylthiophene) SO3CF3PF6PF6I2 PF6I2 10-20 510 270 4 30 11 Polypyrrole Polypyrrole Polypyrrole Polypyrrole FeCl3 I2 Br2 Cl2 3-200 2-8 5 0.5 Applications of PT • • • • • A number of applications have been proposed for conducting PTs, but none has been commercialized. Potential applications include field-effect transistors, electroluminescent devices, solar cells, photochemical resists, nonlinear optic devices, batteries, diodes, and chemical sensors. In general, there are two categories of applications for conducting polymers. Static applications rely upon the intrinsic conductivity of the materials, combined with their ease of processing and material properties common to polymeric materials. Dynamic applications utilize changes in the conductive and optical properties, resulting either from application of electric potentials or from environmental stimuli. As an example of a static application, poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonate) (PEDOT-PSS) product Clevios P (Figure) has been extensively used as an antistatic coating (as packaging materials for electronic components, for example). AGFA coats 200 m × 10 m of photographic film per year with Clevios because of its antistatic properties. The thin layer of Clevios is virtually transparent and colorless, prevents electrostatic discharges during film rewinding, and reduces dust buildup on the negatives after processing. PEDOT can also be used in dynamic applications where a potential is applied to a polymer film. The electrochromic properties of PEDOT are used to manufacture windows and mirrors which can become opaque or reflective upon the application of an electric potential. Widespread adoption of electrochromic windows could save billions of dollars per year in air conditioning costs. Finally, Phillips has commercialized a mobile phone with an electrically switchable PEDOT mirror Polyaniline • A conducting polymer that can be grown by using aqueous and non-aqueous route • Can be obtained by electrochemical synthesis or oxidative coupling of aniline • Doping achieved by adding protonic acid • Several forms: leucoemaraldine, emaraldine, emaraldine salt, pernigraniline Electrical Conductivity Medium s (S cm-1) 5-Sulfosalicyclic acid Benzene sulfonic acid p-Toluene sulfonic acid Sulfamic acid Sulfuric acid 0.2-1.0 2.0 5.0 2.0 1.2 Applications of PANI • • • • • • Polyaniline and the other conducting polymers such as polythiophene, polypyrrole, and PEDOT/PSS have a great deal of potential for applications due to their light weight, conductivity, mechanical flexibility and chemical properties. Polyaniline is especially attractive among them because it is less expensive, has three distinct oxidation states with different colors and has an acid/base doping response. This latter property makes polyaniline an ideal option for acid/base chemical vapor sensors. The different colors, charges and conformations of the multiple oxidation states also make the material highly promising for applications such as actuators, supercapacitors and electrochromics. Attractive fields for current and potential utilization of polyaniline is in antistatics, charge dissipation or electrostatic dispersive (ESD) coatings and blends, electromagnetic interference shielding (EMI), anti-corrosive coatings, hole injection layers, transparent conductors, ITO replacements, actuators, chemical vapor and solution based sensors, electrochromic coatings (for color change windows, mirrors etc.), PEDOT-PSS replacements, toxic metal recovery, catalysis, fuel cells and active electronic components such as for non-volatile memory. However, the major applications are in printed circuit board manufacturing (final finishes) and corrosion protection. Commercially polyaniline has been supplied by several companies PANIPOL, Eeonyx, Fibron Technologies Crosslink and Ormecon. ELECTROPOLYMERIZATION Electropolymerization Schematic of a typical electrochemical experiment used to probe conjugated polymer and oligomer electrochemistry Repeated cycling electropolyermization by cyclic voltammetry. The inset is of the first CV scan showing the oxidation of the monomer.