Hoosung Lee

Solution properties of poly(amic acid)–NMP containing LiCl and their effects on membrane morphologies

Abstract Physical properties of poly(amic acid) (PAA) casting solutions in N -methyl-2-pyrrolidone (NMP) containing lithium chloride (LiCl) were characterized by viscometry and dynamic light scattering (DLS) and were related to the morphological properties of asymmetric membranes prepared from these solutions. At a fixed polymer concentration, the increase in viscosity of the PAA solutions with increasing LiCl content is mainly determined by the viscosity of the salt–solvent medium, implying that the LiCl–NMP interactions are stronger than those between LiCl and PAA. Because of the strong salt–solvent interactions, complexes between LiCl and NMP are formed. The complexes reduce the solvent power of NMP for PAA inducing polymer aggregation (clustering) and/or transient cross-links in the solutions. Dynamic light scattering results for salt-containing solutions at low PAA concentrations support the existence of these aggregations. Solutions without salt showed a single relaxation, but solutions with LiCl exhibit multiple relaxation modes; two diffusional modes of cooperative and aggregates, and one angle independent transient network mode. The polymer aggregates and transient cross-links form a gel-like structure in the casting solution film and hinder macrovoid formation during phase inversion, resulting in asymmetric membranes with a primarily sponge-like structure.

Solution-cast polypyrrole film : the electrical and thermal properties

Polypyrrole (PPy) solution has been prepared by dissolving PPy powder in m-cresol after doping with sodium dodecylbenzenesulfonate, and free-standing film of PPy has been cast from the solution. The PPy prepared in this way has very shiny smooth surfaces and contains approximately 13% of the solvent. The activation energy of electrical conduction was found to be 0.14 eV. It has been concluded that the solvent, i.e. m-cresol, plays an important role in electrical conduction as well as the enhancement of electrical conductivity during annealing at around 120 °C. The decrease in electrical conductivity as the PPy film is heated above 140 °C is due to the loss of solvent.

Electrochemistry and electrochromism of a poly(cyclopentadithiophene) derivative with a viologen pendant

N-Methyl-N′-(6-4H-cyclopenta[2,1-b:3,4-b′]dithiophene-4-ylhexyl)-4,4′-bipyridinium dihexafluorophosphate (CPDT-V2+-Me) was synthesized. The monomer was electropolymerized on a glassy carbon or an ITO electrode in a potentiodynamic mode to form the corresponding polymer P(CPDT-V2+-Me) on the electrodes. During the electropolymerization, two redox peaks of the viologen (V) moiety increased up to several cycles and then decreased while the redox peak of P(CPDT) moiety still increased. Especially, a new oxidative peak a in the range of ca. −0.4 and 0 V versus Ag/Ag+ appeared and increased up to several cycles. Peak a almost disappeared after the redox peaks of the viologen moiety almost disappeared. As a result of cyclic voltammetric study, it was shown that peak a originated from the oxidation of reduced viologen moiety via P(CPDT)-mediated electron transfer mechanism. We also found that the electroactivity of viologen moiety in P(CPDT-V2+-Me) decreased significantly when the potential was scanned to the second viologen redox (V0/V+). In practical applications, the polymer can be used in the potential range from the first viologen redox to P(CPDT) redox. The polymer turned into highly transparent P(CPDTα+-V2+-Me), blue P(CPDT-V2+-Me), dark violet P(CPDT-V+-Me), and violet P(CPDT-V0-Me) approximately at 0.8, −0.4, −0.8, and −1.7 V versus Ag/Ag+, respectively.

Decrease in electrical conductivity upon oxygen exposure in polyanilines doped with HCl

Abstract The electrical conductivity of polyanilines doped with HCl was decreased upon exposure to oxygen and was increased reversibly upon evacuation. The line intensity and the line width of the EPR signals were decreased and increased, respectively, upon exposure to oxygen. The former represents the reduction in the concentration of polarons due to the spin–spin interaction of oxygen with paramagnetic polarons, generated by HCl doping process, and the latter the reduction in the mobility of polarons due to the partial localization of delocalized polarons. Therefore, it was concluded that the decrease in conductivity arose from the reduction in both the concentration and the mobility of polarons, the charge carriers for electrical conductivity. However, the decrease in the electrical conductivity was very small whereas the decrease in the polaron concentration was substantial. It was also found that the time scale for the diffusion of oxygen molecules was much longer than that for the spin–spin reaction of oxygen with polarons. Therefore, the small decrease in electrical conductivity may be associated mostly with the reduced mobility of polarons by localizing reaction instead of the reduced polaron concentration.

Effect of poly(amic acid) imidization on solution characteristics and membrane morphology

Abstract The imidization effect of a poly(amic acid) dope solution on membrane formation has been investigated. Poly(amic acid) solution in N -methyl-2-pyrrolidione has been thermally imidized at 120°C at different curing times. The degree of imidization was determined by infrared spectroscopy. The solution properties have been studied as a function of concentration and curing time by dynamic light scattering. The quality of a solvent changed from good to poor with increasing imidization of poly(amic acid). The reduced polymer–solvent interaction diminishes the membrane formation time. The morphology of a membrane was able to be controlled by the characteristics of the dope solution.

Solvent effects on the characteristics of conductive and luminescent polymers

Abstract The electrical conductivity of ES film which was cast first in EB form followed by doping by HC1 was in the metallic range (20 S/cm) while that cast from m -cresol was in the range of semiconductor (8×10 −4 S/cm). On the other hand, PPy film cast from m -cresol showed higher conductivity (1.3×10 −4 S/cm) than the film cast from NMP solution(~10 −10 S/cm). This is because the polymer chains in the film cast from a good solvent have more expanded structures than the those in the film cast from a poor solvent. The PL-intensity of P3ATs increased as the solvent becomes poorer. In a good solvent, as the P3AT chains have expanded structures with longer π-conjugation lengths, the excitons can migrate to quenching sites relatively easily lowering the luminescence efficiency. Conversely, in a poor solvent, the they have compact structures confining the excitons in relatively shorter segments of the chain. Thus the excitons would have more chances to relax via an emissive route. These results indicate that the solvent-solute interactions have significant impacts on the luminescence of the film cast from the solution. It must be emphasized that, in the manufacturing of the devices utilizing such characteristics, the impacts of the solvents should be considered seriously.

Synthesis of a perylenediimide-viologen dyad (PDI-2V) and its electrochromism in a layer-by-layer self-assembled multilayer film with PEDOT:PSS

A new electrochromic molecule, perylenediimide-viologen dyad (PDI-2V), was synthesized and used in the fabrication of layer-by-layer (LBL) self-assembled multilayer films with poly(3,4-ethylenedioxythiophene)–poly(styrenesulfonate) (PEDOT:PSS). Since PDI-2V is a tetracationic organic molecule with high molecular weight, it showed strong electrostatic adhesion to negatively charged PEDOT:PSS. When the electrochromic behaviour of the resulting LBL film was investigated with a single type electrochromic device, the PDI-2V showed a three-stage color change from red to blue to violet under the reduction potential range. A dual type electrochromic device was also fabricated with P3HT spin-coated ITO as a counter electrode. This device exhibited a sharp color change between deep red and dark blue when the applied potential was switched from +2.0 V to −2.0 V.

Correlation between electrical conductivity, thermal conductivity, and ESR intensity of polyaniline

Abstract The thermal conductivity, electrical conductivity, and ESR spin concentration, [s], of chemically synthesized polyaniline have been measured. The thermal conductivity, electrical conductivity, and [s] showed maxima between pH=−1 and 1. The thermal conductivity exhibited a very good linear relationship with the electrical conductivity and log[s] when pH

Solutions and solution-cast films of conducting polymers

When O/M < 0.2 and D/M < 1.0, PPy obtained was partially soluble in m-cresol or NMP. Its solubility decreases as the increase of O/M. When O/M was greater than 0.2, the polymer obtained was totally insoluble in any solvent. The solubility of PPy is not determined solely by its molecular weight. Conductivity, inherent viscosity, and ESR data indicate that the solubility is determined by the molecular weight, oxidation state, and degree of crosslinking. UV-Vis spectra of PPy solution in m-cresol and in NMP were quite different from those of the corresponding solid. The sharp peak at 475 nm and the broad peak at 440 nm correspond to π→π* transitions of the neutral and doped forms, respectively, while the peak absorption at 933 nm (1.33 eV) arises from the transition between defect states. PPy film cast from NMP showed very low conductivity (∼10 -10 S/cm) while the film cast from m-cresol showed 1.3x10 -4 S/cm.

Mono- and Multi-layer Langmuir-Blodgett Films of Maleimide Polymers Possessing Nonlinear Optical-Active Side Chains

A copolymer P[OSA-MI] was synthesized by copolymerization of its corresponding monomers,N-phenyl maleimide (MI) and 2-octen-1-ylsuccinic anhydride (OSA). The polymer (poly[2-[1-(2,5-dioxo-1-phenylpyrrolidin-3-ylmethyl)heptyl]-succinic acid 4-(2-{ethyl-[4-(4-nitrophen-ylazo) phenyl]amino}ethyl) ester]) P[DR1MA-MI] was obtained from the reaction of P[OSA-MI] with 2-[4-(4-nitrophenylazo)-N-ethylphenylamino] ethanol (DR1). A stable monolayer of P[DR1MA-MI] was formed by spreading the solution of the polymer in chloroform. In Y-type Langmuir-Blodgett (LB) films prepared using this Langmuir-Blodgett method, the second harmonic waves generated from adjacent monolayers canceled each other out. In X- and Z-type LB films, the second harmonic intensity increased upon increasing the number of monolayers, but this increase was somewhat smaller than predicted by the square law. This phenomenon is due to defects or imperfect alignment of the dipoles in the LB film. The generation of second harmonic waves from Y-type LB films having an even number of monolayers supports this argument. The degree of imperfection seemed to increase as the number of layers increased. The second-order nonlinear optical properties of spin-cast films of these polymers were also measured. The largest second harmonic coefficient of the poled P[DR1MA-MI] film coated on a glass plate was 19 pm/V.