Chung Yup Kim

Synthesis of soluble polypyrrole of the doped state in organic solvents

Polypyrrole soluble in m-cresol and conditionally soluble in chloroform, dichloromethane or 1,1,2,2-tetrachloroethane was synthesized chemically by using dodecylbenzene sulfonic acid as the dopant and ammonium persulfate as the oxidant. The polymer was soluble in m-cresol and became soluble in the weakly poor solvents such as chloroform, dichloromethane and 1,1,2,2-tetrachloroethane when equal amounts of dodecylbenzene sulfonic acid and polymer were added to one of the solvents. Soluble polypyrrole showed the same spectra of FT-Raman scattering and UV-Vis light absorption as those of electrochemically polymerized polypyrrole. The intrinsic viscosity of the soluble polypyrrole in m-cresol was 0.17 dl/g. The solution of the polymer in chloroform was cast to a stiff film with a very smooth surface and the electrical conductivity of the film was 5 S/cm. The redox reactivity of the film was sustained for a long time in cyclovoltammetry.

Soluble polypyrrole as the transparent anode in polymer light-emitting diodes

Abstract Polypyrrole (PPy) films processed from solution have been used as semi-transparent anodes in polymer light-emitting diodes. An external quantum efficiency of 0.5% was achieved with poly (2-methoxy,5-(2′-ethyl-hexyloxy)-1,4-phenylene-vinylene) (MEH—PPV) as the luminescent polymer. The hole-injection potential barrier between PPy and MEH—PPV, as determined by Fowler—Nordheim analysis, is≈ 0.23 eV.

Difference in doping behavior between polypyrrole films and powders

Abstract The differences in protonation and charge transfer behavior between polypyrrole (PPY) powders and films doped with inorganic acids, such as H2SO4 and HClO4, or bulky acids, such as dodecylsulfonic acid (DS), toluene-4-sulfonic acid (TSA), dodecylbenzensulfonic acid (DBSA) and poly(styrenesulfonic acid) (PSSA), were assessed quantitatively by the X-ray photoelectron spectroscopic (XPS) technique. PPY-dodecylsulfate and PPY-toluene-4-sulfonate films with [N + ] [N] ratio over 40% can be readily prepared electrochemically. Treatment of these salt films with a base results in the 25% deprotonated PPY. Reprotonation of the 25% deprotonated PPY films (containing 25% =N- units) by excess bulky acids readily results in complexes with surface [N + ] [N] ratios substantially above 50%. For deprotonated PPY films loaded with excess inorganic acids, and deprotonated PPY powders loaded with excess simple or bulky acids, the protonation level or [N + ] [N] ratio remains in the order of 25 to 30%. Raman and UV-Vis spectroscopies show little difference in structures of PPY formed chemically and electrochemically, but morphologies of cross sections of the simple and bulky anion-doped samples look different from each other. The enhanced charge transfer interaction in PPY-bulky anion films is probably attributable to the counter-ion-induced molecular ‘blending’ between the polymer and the dopant.

Studies of the overoxidation of polypyrrole

Resistometry, UV-Vis and Raman Spectroscopy have been employed as in-situ methods for the determination of the overoxidation potential of polypyrrole (PPy). These methods all show that the onset of overoxidation begins at potentials as low as 0.65V v Ag/AgCl for unsubstituted PPy. They also show a strong dependence of overoxidation potential on the pH of the supporting electrolyte, with an increase in stability at lower pHs.

Stability of reduced polypyrrole

Summary form only given. Electrochemically polymerized polypyrrole(PPy) shows changes in visible light spectrum on reduction revealing changes in electron states of the conjugated double bond. The reduced PPy is unstable to show changes in the visible light spectrum in less than one minute after cutting off applied potential. The absorption peak at 420 nm assigned as /spl pi/ - /spl pi//sup */ transition in the conjugated double bond shrinks fast to show a strong absorption around 800 nm assigned as bipolaron. When the reaction system is purged with nitrogen, the spectral changes slow down significantly to prove that oxygen accererates the structural changes in PPy. It is observed that the spectral changes slow down when hydroquinone is added to the system. The addition of hydroquinone in the system produces little polaron on oxidation of PPy and makes the polymer segments trun directly into bipolaron states. Hydroquinone is ionized in an aqueous system to produce proton which seems to stabilize free radicals in the polaron state. A decrease of pH in the system makes PPy become stable in repeated redox reaction while the polymer shows poor repeatability of cyclovoltammometry in the system with high pH. It is suggested that deprotonation of -NH in the pyrrole ring develops into production of an imine structure to loose the electrochemical reactivity.

A polymer humidity sensor

Abstract Polypyrrole doped with dodecylsulfate ion on electrochemical polymerization of pyrrole keeps the anion in it on reduction of the polymer in an aqueous solution. The free anion on reduction of the polymer attracts a cation to form a salt in the polymer. An attempt was made to use polypyrrole as a sensing material in measurement of humidity. The gold covered electrode was constructed by employing a photo-resist method and the ionic conductivity of the reduced polypyrrole changes from 2 × 10 kΩ·cm to 3 × 103 kΩ·cm depending on humidity.

An alternating copolymer consisting of light emitting and electron transporting units

An alternating copolymer composed of fluorenedivinylene as the light emitting unit and pyridine as the electron transporting one was synthesized by employing the Wittig reaction. The copolymer which has conjugation throughout the molecular chain is soluble in both polar and nonpolar solvents. The copolymer has a band gap energy of 2.85 eV deduced from an ultraviolet-visible absorption spectrum, and ionization potential and electron affinity of –5.67 and –2.82 eV, respectively, deduced from a cyclic voltammogram. The photoluminescence (PL) emission maximum was observed at 440 or 540 nm depending on the solvent used in making the solution for spin-casting. The copolymer was also capable of transporting electrons and could be used as an electron transporting layer. A light emitting diode (LED) fabricated with a blend of polyvinylcarbazole (PVK) with a fluorene-based light emitting material, and this copolymer as an electron transporting layer, exhibited an electroluminescence (EL) emission maximum at 475 nm with a full width at the half maximum (FWHM) of 50 nm and a quantum efficiency of 0.1%, where indium tin oxide (ITO) and Al were used as the anode and cathode, respectively.

Synthesis and properties of novel triphenylamine polymers containing ethynyl and aromatic moieties

Novel polymers composed of TPA (triphenylamine), ethynyl and aromatic units in the main chain were synthesized. These polymers are structurally rigid due to aromatic and acetylene moieties and show high Tg and thermal stability. The polymers 3a and 4 are cross-linked on annealing at ∼220°C and they can be potentially used as a hole-transporting material for multi-layer devices with spin-coating.

White emission from a ternary polymer blend by incomplete cascade energy transfer

Abstract White light emission was obtained from a light-emitting diode (LED) prepared from a ternary polymer blend (19:1:1 by weight) consisting of poly(9-vinylcarbazole) (PVK), poly(9,9′-dihexlyfluorene-2,7-divinylene- m -phenylenevinylene-stat- p -phenylenevinylene) (CPDHFPV), and poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylenevinylene] (MEH-PPV), where the order of bandgap energy is PVK>CPDHFPV>MEH-PPV. The major component PVK acted not only as the matrix or diluent but also as the excitation energy donor to help the blend generate white light with high efficiency. Good miscibility between PVK and CPDHFPV facilitated the Forster-type excitation energy transfer from PVK to CPDHFPV enhancing the quantum efficiency. However, poor miscibility between CPDHFPV and MEH-PPV resulted in partial energy transfer between the polymers causing the blend to emit two colors simultaneously. Consequently, the incomplete cascade energy transfer in the blend generated a pure white color near CIE coordinate (0.33, 0.33) and the emissive color of this system showed a low sensitivity to the drive voltage.

Surface studies of pristine and surface-modified polypyrrole films

Polypyrrole (PPY) films having high conductivity were synthesized electrochemically in acetonitrile at low temperature and low current density. Pristine, deprotonated, and ozone-pretreated PPY films were subjected to either thermally induced or near-UV-light-induced graft copolymerization with acrylic acid (AAc), or sodium salt of 4-styrenesulfonic acid (NaSS). Surface structures and redox states of the pristine, deprotonated, reprotonated, and surface-modified polypyrrole films were studied by angle-dependent X-ray photoelectron spectroscopy (XPS). The morphology of the PPY surface after modification by graft copolymerization was revealed by atomic force microscopy (AFM). The results showed that the density of surface grafting decreased with ozone pretreatment. Surface grafting of the two polymeric acids also gave rise to a self-protonated surface structure. A substantial proportion of the grafted protonic acid groups at the surface remained free for further surface functionalization. The surface characteristics, in particular the charge-transfer interactions and the changes in the intrinsic redox states of the substrate films, associated with the external protonation and surface self-protonation processes were also discussed.