V.H. Tran

XPS analysis of thermal and plasma treated polyparaphenylene-vinylene thin films and their interface formed with aluminum layer

Abstract X-ray photoelectron spectroscopy (XPS) was used to investigate the interface formed between thermally and plasma treated polyparaphenylene-vinylene thin films and an aluminium layer deposited on them. Enhancement in adhesion of the metal on thermally treated polymer films is explained by the structural change of aluminum. In contrast, plasma treatments induce morphological modifications of the polymer surface and favor the formation of compounds between the polymer and the metal which in turn improve the adhesive strength.

Thermal stabilisation of PVC via the polaron mechanism Part III: Stabilisation with organotin compounds

Abstract By the polaron theory, it has been shown that the stabilisation of poly(vinyl chloride) (PVC) with organotin compounds is due to many different reactions: (1) substitution of the activated hydrogen in PVC by the stabiliser molecule or by-products of the stabiliser, this reaction being one of the most important because it takes place throughout the stabilisation; (2) substitution by the stabiliser of labile chlorine atoms, which occurs only in the early stages of the induction period; (3) saturation of the degraded PVC by by-products of tin compounds or HCl; and (4) stabilisation due to the interaction of organo groups with the degraded PVC.

Studies of the polymer-metal interface in metal-PPV-metal devices

Abstract We report the results obtained from electrical measurements and surface analysis by photoelectron spectroscopy (XPS) performed in polyparaphenylene-vinylene (PPV) thin films sandwiched between metallic electrodes. Current-voltage characteristics of metal-polymer-metal devices are dissymetrical with rectifying contacts at top electrodes (Cr or Al) and blocking one at bottom electrodes (Cr). Analysis of the core level spectra of carbon, oxygen and metals in the interfacial layers leads to the conclusion that the rectifying behaviour of polymer-metal contact is in close relation with the formation of metallic oxide-carbon complexes at the interfaces.

Optical spectroscopic investigations of phenylene vinylene oligomers

A series of phenylene vinylene oligomers containing from 1 to 6 phenylene rings was studied by absorption, Raman and photoluminescence spectroscopies. The results are discussed by examining the influence of chain length on the spectra and compared to those of PPV films obtained from the Wessling precursor route.

The electromagnetic properties ofpoly(p-phenylene-vinylene) derivatives

We report the studies on the frequency dependence of the complex permittivity (e * = e′ − i.e″) and of theelectromagnetic interference shielding (EMI) of the conducting polymers such as poly(para-phenylene-vinylene) (PPV) and poly(2,5-dimethoxy,1,4-phenylene-vinylene) (DMeO-PPV) doped with different dopants. Permittivity measurements from 10 kHz to 10 GHz point out that all conducting polymers doped with H2SO4 show a conductivity range of 10−1−10+1 S cm−1, but doped DMeO-PPV has conductivity slightly higher than that of PPV. From these experimental data, the reflectivity coefficients of these materials are calculated in a frequency range 3–5 GHz, and have the values up to −30 dB depending on the frequency, on the thickness of the materials, on the dopant agents and specially on the doping levels of the materials. The potential applications of these materials in EMI shielding are discussed.

Characterization of the poly(para-phenylene vinylene)-chromium interface by attenuated total reflection infrared and X-ray emission spectroscopies

Abstract We have studied the interfaces formed between poly( para -phenylene vinylene) (PPV) thin film and a Cr layer deposited under vacuum by thermal evaporation. Comparison of attenuated total reflection infrared spectra obtained in pristine and Cr-covered PPV films shows that new absorption bands emerge at 687, 1026 and 1392 cm −1 as a consequence of Cr deposition. The Cr(3d) valence distribution from electron-induced X-ray emission measurement on a Cr-covered PPV sample is shifted by 0.9 eV with respect to Cr metal. The new features found in both experiments are interpreted as characteristics of the compound in the polymer-Cr interface. In the case of PPV deposited on Cr no change is observed. Positioning the Fermi level relative to the Cr valence spectral distribution in both polymer-metal and metal-polymer interfaces from X-ray and X-ray photoelectron spectroscopy (XPS) analyses yields a possible explanation for the electrical behavior of the PPV-based diodes.

Poly(vinyl chloride) dehydrochlorination via the polaron mechanism

ESR studies show that the paramagnetic species detected during the dehydrochlorination of PVC are associated with the π-electrons, i.e. the solitons or polarons, but not with the conventional active radicals (resulting from σ-bond scission). The interaction of HCl or Lewis acid compounds or their combination with conjugated unsaturated systems (model compounds with two to eleven conjugated double bonds or dehydrochlorinated PVC) substantially supports the polaron mechanism for the thermal degradation of poly(vinyl chloride) (Tran & Guyot, Polym. Deg. Stab., 32 (1991) 93–103).

Spectroscopic studies of the conversion of poly(p-phenylene-vinylene) precursor

Abstract Efforts are made to identify the irregular structures formed in poly(phenylene-vinylene) (PPV), obtained via the conversion of its sulfonium precursor by different physical and chemical methods. Analysis by X-ray photoelectron spectroscopy and Fourier transform infra-red spectroscopy reveals that the ketone structure is a common defect appearing in PPV whenever the conversion is carried out in the presence of oxygen. In contrast to the PPV obtained by pyrolysis of the precursor, the PPV obtained by treatment of the precursor with H 2 SO 4 contains no chlorine residue but much more sulfur residue. Ion bombardment of the sulfonium precursor leads to a PPV containing no trace of sulfur or chlorine in the structure. Finally, the nature of bonding between the carbon atom and oxygen, chlorine or sulfur atoms in PPV is discussed and irregular structures are proposed.

Thermal conversion of poly(paraphenylene-vinylene) precursor films: XPS and ESR studies

The conversion of the precursor into poly(phenylene–vinylene) (PPV) was studied by means of photoelectron spectroscopy (XPS) and electron spin resonance (ESR) on precursor films synthesized from the polymerization of pxylene-tetrahydrophenium halides. The sulfonium precursor containing chlorine or bromine was thermally converted to PPV and analyzed in situ using the spectrometers. It was found by XPS analysis that both halide precursors were thermally converted into polymer in a range of 190–250°C but traces of bromine were still present at >300 °C in the precursor containing Br. Furthermore, a small amount of oxygen in a ketone structure was also present in fully converted films. The ESR results corroborated the XPS experiments, showing a progressive disappearance of the radical distribution related to sulfur sites in a comparable temperature range. A discussion of the conversion process is given in terms of the modification of the core level spectra from XPS and the g distributions from ESR experiments.

Interactions in metal-polymer-metal interfaces

Abstract Interfacial layers in metal-polymer-metal structures have been investigated. Analysis by X-ray photoelectron spectroscopy (X.p.s.) at room temperature of the changes in various core electron spectra associated with the polymer film and the overlayers shows that there is formation of metal oxide-carbide species in the polymer-metal interface in the system obtained by deposition of metal on poly(phenylene vinylene) (PPV) polymer. In contrast, for the system in which metal is deposited on the PPV precursor, no complex species are observed after the transformation of precursor into PPV by heat treatment. This difference can be explained by the side-reactions of the under-products generated during the pyrolysis of the precursor into the polymer. Finally, the role of the interfacial complex species in the origin and in the mechanism of the rectifying contacts observed in the metal-polymer-metal structures is also discussed.