Fluorescent poly(p-phenylene vinylene)/poly(ethylene oxide) nanofibers obtained by electrospinning

Poly(p-phenylene vinylene) (PPV)/poly(ethylene oxide) (PEO) hybrid nanofibers were prepared by electrospinning a composite solution of PPV precursor/PEO in a mixture of ethanol and water, followed by thermal conversion. The precursor/PEO composite solutions were successfully electrospun into nanofibers with diverse helical, helical and linear, and helical bead-on-string morphologies by controlling the amount of aqueous PEO solution in a composite solution. Moreover, adding aqueous PEO solution to a precursor ethanol solution decreased the diameters of the fibers. The experimental data suggest that the viscosity, conductivity, and surface tension of the electrospinning solution are the main factors that influence the morphology of the fibers. Fourier transform infrared (FT-IR) and X-ray diffraction (XRD) investigations indicated that the PPV precursor reacts with PEO during thermal conversion. Ultraviolet–visible (UV-vis) and photoluminescence (PL) spectra of the PPV-PEO nanofibers exhibited appreciable blue shifts with the addition of PEO, which made it possible to fabricate nanofibers with fluorescence ranging from yellow-green to blue. These highly fluorescent PPV/PEO nanofibers with various morphologies are potentially interesting for many applications, such as micro- and nanooptoelectronic devices and systems.     

Structural characterization and luminescent properties of poly(p-phenylene vinylene) and poly(ethylene glycol) blends

The luminescent properties of poly(p-phenylenevinylene) (PPV) blending with poly (ethylene glycol) (PEG) were investigated in terms of their structural formation during sample preparation. The blended systems were prepared from an aqueous solution of water-soluble poly (xylylene tetrahydrothiophenium chloride) (PPV precursor) mixed with PEG, followed by heat treatment to remove the tetrahydrothiophene groups from the PPV precursor. Structural analysis showed that PEG could react with PPV precursor to form C-O-C linkage and carbonyl groups in PPV chains, interrupting their conjugated length as suggested by their Infrared, Raman and UV/vis spectroscopes. Wide angle X-ray scattering (WARS) of blended systems also showed that PPV in blends had less packing. As to luminescent properties, the UV/vis and photoluminescent (PL) spectra show that the energy gap needed to produce the excitons increased along with the increase of PL intensity when PPV was blended with PEG. Similar results were also found for the EL properties of ITO/polyblends/Al devices. The EL light emission from blends was blue-shifted (compared to PPV) with a rather low threshold electric field strength. The EL performance of polyblends was better than that of pure PPV. Among them, the PPV-50PEG showed the highest EL intensity. The improved EL efficiency was attributed to the dilution effect, interrupted conjugated length, and lower packing of PPV chains.     

Synthesis and characterization of poly(p-phenylene vinylene) polymers containing the quinoxaline group

Summary Novel poly (p-phenylenevinylene) polymers containing the quinoxaline group were prepared by the Horner-Wadsworth-Emmons reaction of the various diphosphonic acid diethyl ester with a dialdehyde monomer. The spectral properties of products so obtained were characterized by UV-visible and fluorescence spectroscopy. The UV-visible absorbance of these polymers showed absorption bands at ca. 432∼440 nm, which corresponding to the π-π* transition of the conjugated system. Their maximum fluorescence appeared at ca. 488 ∼ 498 nm. The resulting polymers showed greenish blue emission in solution and an orange emission in solid state. Received: 13 December 2002/Revised version: 27 February 2003/ Accepted: 1 March 2003 Correspondence to Jae Yun Jaung     

Mechanism analysis of the PEG-participated Gilch synthesis for soluble poly(p-phenylene vinylene) derivatives

By introducing poly(ethylene glycol) (PEG) into the Gilch reaction system for the synthesis of soluble poly(p-phenylene vinylene)s (PPVs), significant changes were observed. First, with the involvement of PEG, the polymerization degree increases significantly and the molecular weight distribution becomes wider. The multimodial characteristic of the GPC curves at low conversions is quite evident. Second, at low PEG content, gelation dramatically disappears; with an increase of PEG, gelation reappears. From the above experimental results, the existence of various types of polymerization-active species and the anionic polymerization mechanism are confirmed, which are due to the dissociation of the carbanion–cation pairs by the complexing effect of PEG. At the same time, the enhanced dissociation of the base (t-BuO⁻K⁺) ion pairs by PEG may be responsible for the dramatic disappearance of gelation. Free t-BuO⁻ anions have strengthened the basicity and accelerated the conjugation-formation process which competes with the interchain by-reactions. Gelation is therefore avoided. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 263–268, 2001     

Interface formation of Ca with poly(p-phenylene α,α′-diphenyl vinylene) and poly(p-phenylene α-phenyl vinylene)

We have investigated the interface formation of Ca with poly(p-phenylene α,α′-diphenyl vinylene) (PPV-DP) and poly(p-phenylene α-phenyl vinylene) (PPV-P) using X-ray photoemission spectroscopy (XPS). Similarly to our earlier findings in metal/PPV interface formation, the O 1s peak shifted toward a lower binding energy as soon as Ca was deposited on to the polymers. This was accompanied by the formation of Ca? O, suggesting a chemical origin for the O 1s shift. By contrast, the C 1s peak shift toward a lower binding energy was observed relatively later, after about 4 Å of Ca deposition. At the same time, a new C 1s component became noticable at about ?1.5 eV relative to the initial C 1s peak. This component signifies the possibility of polymer disruption by the Ca atoms to form Ca? C species. The C 1s peak shift is attributed to Ca induced surface band bending and barrier formation as in the case of metal/PPV interface formation. The disruption of the polymer may also induce changes in the interface electronic states and contribute to the C 1s peak shift. From the intensity attenuation analysis, we conclude that the initial 15 Å of Ca overlayer is contaminated by the Ca? O and Ca? C species and the overlayer is pure beyond 15 Å of Ca coverage.     

Synthesis,doping, and electrical conductivity of high molecular weight poly(p-phenylene vinylene)

High molecular weight poly(p-phenylene vinylene) (PPV) has been synthesized starting from the monomer p-xylylene-bis(dimethylsulphonium chloride). The latter was polymerized to yield a water-soluble sulphonium salt polyelectrolyte, which was converted to PPV by the thermal elimination of (CH₃)₂S and HCl from films cast from aqueous solution. The elimination reaction was studied by elemental analysis and by thermogravimetric analysis and mass spectrometry. The PPV films had good mechanical properties and could be n- and p-doped to yield material with electrical conductivities approaching those of highly doped polyacetylene. The degree of conversion of the intermediate polyelectrolyte to PPV could be controlled and the conductivities of these doped films could be related to the average conjugation length.     

Electrosynthesis of conducting poly(p-phenylene)

The polymerization of benzene was studied in bulk and in nitrobenzene using BF₃O (C₂H₅)₂ as a supporting electrolyte at platinum electrodes to obtain poly(p-phenylene). The polymers were formed as a black thready mass on the anode and their yields were restricted to the area of the anode in the electrochemical cell. The yields of the polymers formed in the bulk and in nitrobenzene were almost equal, although electrical resistances of the polymers obtained from the bulk were relatively lower. Cyclic voltammetry measurements suggest that the polymerization of benzene took place from the species generated anodically in situ. © 1993 John Wiley & Sons, Inc.     

Short poly(phenylene vinylene) chains grafted poly(organophosphazene)

Brominated poly(bis(4-methylphenoxyphosphazene) was allowed to react with 1,4-bischloromethylbenzene or 1,4-bischloromethyl-2,5-dimethoxybenzene in solution using phase transfer catalyst or potassium t-butoxide. Poly(p-phenylene vinylene) or poly(2,5-methoxy-1,4-phenylene vinylene) grafted organophosphazene copolymers were obtained. The UV-Vis absorption, photoluminescent, and thermal properties of the copolymers were measured. The copolymers are complete soluble in common organic solvents and fluoresce in the blue color range. The copolymers were used to build a series of organic light emitting diode (OLED). Only weak to nominated intensities with emission color from blue to red were obtained. The photoluminescent and electroluminescent (EL) spectra indicated there is a distribution in the PPV conjugated length. The compositions of the copolymers before and after the graft reaction were analyzed using NMR.     

Synthesis and electrochemical characterization of soluble poly(p-phenylene vinylene) derivatives containing olefinic bonds at the side chain

A new kind of soluble poly(p-phenylenevinylene) (PPV) derivative containing free olefinic bonds at the side chain is prepared and studied by electrochemical measurement. The electrochemical investigations reveal that the free olefinic bonds in these polymers are electroactive; a new redox reaction occurs prior to the oxidation of PPV backbone in the cyclic voltammetry. The lower oxidation potential of the olefinic bonds hints that it is possible for these olefinic bonds to react with oxygen, which is desirable to remove the harmful oxygen in the light-emitting polymer devices. The merits and possibility of such polymers containing olefinic bonds in the fabrication of the light-emitting devices are discussed. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 2535–2539, 1999     

Advances and Challenges in the Synthesis of Poly(p-phenylene vinylene)-Based Polymers

We provide an overview of the current progress in the synthesis of poly(p-phenylene vinylene) (PPV) polymers. The described synthetic approaches can be divided into four main categories: polymerization of quinodimethane intermediates, metathesis polymerization, nucleophilic polycondensation, and palladium-catalyzed cross-coupling. While such variety is remarkable and very high-molecular-weight PPV can be prepared by some methods, the synthetic chemistry of PPV still limits the structural variety and purity of arylene vinylene polymers. In particular, palladium-catalyzed cross-coupling reactions, the method of choice in the contemporary conjugated polymer chemistry, often shows limited success in PPV series. On the other hand, the possibility of using metathesis polymerization creates new opportunities not available for other classes of conjugated polymers.     

Electronic structure of poly(p-phenylene sulfide)

The electronic structure of poly(p-phenylene sulfide) (PPS) was studied through steady state photocurrent spectra and temperature dependence of dark conductivity for undoped film, by changes in the in situ optical absorption and ESR spectra under photoirradiation, and optical absorption spectra for doped film. The charge carriers in the undoped state are electronic since the photoconductivity is large and the kink point in the temperature dependence of dark conductivity is higher than the glass transition temperature. A sharp peak near the absorption edge in the photocurrent spectrum of undoped PPS is caused by the diffusion of photocarriers in a sheet with high surface recombination under conditions of low applied field. The in situ optical absorption and ESR spectra of SO₃-doped PPS were similar to those of di-p-tolyl sulfide (a monomer unit of PPS) and the formation of sulfur-centered radical cations was suggested rather than bipolaron states. The stable conduction of SO₃-doped PPS after exposure to atmosphere was analyzed by the classical Lorentz model and the existence of nearly free carriers within subchains was suggested. Their conductivity and mobility were estimated to be 2 S/cm and 3.6 cm²/Vs, respectively. Inter-chain conduction appears to be very low.     

聚苯硫醚热氧化处理研究

研究了聚苯硫醚在空气环境下经过热处理的合成化学改性.采用高温凝胶渗透色谱、光电子能谱、热失重、红外光谱和高压毛细管流变技术对聚苯硫醚进行表征.结果表明:低相对分子质量的聚苯硫醚和高相对分子质量的聚苯硫醚经过热处理,相对分子质量都会大幅提高;聚苯硫醚经过热处理过程会同时发生氧化反应、氧化交联反应、热交联反应和链增长反应.     

Synthesis of poly(phenylene acetylene) from poly(phenylene vinylene)

Summary The objective of this work was to prepare films of poly(phenylene acetylene) from films of a precursor polymer, namely poly(a, a-dibromoxylylene). This polymer was obtained by selective bromination of the vinylene groups of poly(phenylene vinylene) films in bromine/CHCl₃. Pyrolytic dehydrobromination of the brominated films under argon atmosphere gave dark yellow films which showed about 65 wt% of desired phenylene acetylene units.     

Conductivity anisotropy in oriented poly(p-Phenylene vinylene)

Summary The study of charge transport mechanisms in highly conjugated conducting polymers has historically been hampered by the complex and invariant morphologies of the best conductors. We have prepared amorphous and uniaxially oriented films of poly(p-phenylene vinylene) (PPV) which exhibit a large conductivity anisotropy proportional to the degree of molecular orientation. The conductivity of the AsF₅ doped PPV, together with wide angle x-ray and IR characterization of these samples is reported.     

Electrical conductivity of polyblends of poly(1,4-phenylene vinylene) and poly(2,5-dimethoxy-1,4-phenylene vinylene)

Summary A series of polyblends of poly(1,4-phenylene vinylene), PPV, and poly(2,5-dimethoxy-1,4-phenylene vinylene), PDMPV, were prepared in film form from precursor polyblends of the respective sulfonium salt polymers, which were separately prepared from the respectivebis(sulfonium salt) monomers. The blend films were doped with I₂ at room temperature to obtain a wide range of electrical conductivities (10^–2 to 10²Scm^–1) depending on the blend composition. The higher the content of PDMPV in the blends the higher was the conductivity.     

Electronic processes in poly(p-phenylene) and related compounds I. Paramagnetic and electrical properties of doped poly(p-phenylene)

Poly(p-phenylene) (PPP) was prepared by polymerization of benzene and was exactly purified from oligomers and the catalyst. IR spectra of the resulting material indicated only para substitution and a DP about 20. PPP is a polycrystalline substance with a small degree of crystallinity and is thermally resistant up to 798 K. The annealing of pristine polymer produced a material with higher crystallinity and crystallite size. In our investigations we examined the electrical properties of doped (AlCl₃) and undoped PPP at different temperatures. The mechanism of conductivity is explained assuming the existence of the Schottky barriers between polycrystalline grains and intergrain regions. The WAXS and ESR data are compared with the electrical results.     

Polymer-supported copolymerization of poly(phenylene vinylene)

Summary A series of phenyl ring di-substituted ,-dichloro-p-xylene monomers were copolymerized in situ onto a cross-linked chloromethylated polystyrene resin. A phase-transfer catalyst was employed to facilitate this multiphase heterogeneous polymerization. The electrical as well as the thermogravimetric properties of the conducting grafted resins are reported.     

Mass spectral analysis of poly (p-phenylene sulphide)

Low molecular weight and high molecular weight samples of poly(p-phenylene sulphide) were extracted exhaustively with methylene chloride. The extracts were examined by h.p.l.c. and by solid probe mass spectroscopy. Four families of compounds have been identified in these extracts: (1) cyclic (p-phenylene sulphide) oligomers; (2) chlorine terminated linear (p-phenylene sulphide); (3) single chlorine terminated linear (p-phenylene sulphide); and (4) linear (p-phenylene sulphides). The fully extracted polymer did not exhibit any mass spectrum. In addition to the compounds specified above, dibenzothiophene was found in all samples while phenyl biphenyl ether was found only in the extract of the high molecular weight material.     

Fabrication of poly(p-phenylene) by powder-forming techniques

Application of powder-metallurgical forming techniques to poly(p-phenylene), PB, resulted in solid objects with tensile strengths as high as 35 MPa (5000 psi). Powders were characterized by BET surface area, x-ray crystallinity, scanning electron microscopy (SEM), and impurity levels in an effort to relate intrinsic powder properties with strengths attainable on fabrication. High surface area (>35m²/g) and low Cl impurity levels were generally associated with highest strengths. Forming variables were studied in some detail. Tough objects were obtained in a narrow sintering range of about 580°–615°C. Perchloropoly(p-phenylene) was also successfully fabricated.     

Electrochemical synthesis of poly(p-phenylene) and poly(p-phenylene)/TiO2 nanowires in an ionic liquid

This study shows for the first time that poly(p-phenylene) (PPP) nanowires can be easily obtained by electrochemical synthesis at room temperature. The method involves the template assisted electropolymerization of benzene in the air and water stable ionic liquid 1-hexyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate ([HMIm]FAP). Track-etched polycarbonate membranes (PC) with an average pore diameter of 90 nm were used as templates. Dense and highly flexible bundles of PPP nanowires with a high aspect ratio (>160) were easily obtained by this method. In addition, we present here our first results to obtain PPP/TiO₂ nanowires by the combination of a sol–gel technique with electropolymerization. HR-SEM, TEM and EDX were used for the structural characterization of the nanowires.