R.M. Faria

Poly (o-methoxy aniline): solubility, deprotonation-protonation process in solution and cast films

Abstract Solubility tests were performed at room temperature for poly ( o -methoxyaniline) (POMA) in N,N -dimethylformamide (DMF), 1-methyl-2-pyrrolidone (NMP), m -cresol, acetonitrile, chloroform, dichloromethane and xylene, and compared with those for polyaniline. A reversible solvatochromic transition from green to blue was observed in dilute POMA solutions in DMF, NMP and m -cresol indicating the occurrence of deprotonation-protonation processes. The solvent NMP more readily induced deprotonation processes at higher concentrations than those observed in DMF solutions. On the other hand, m -cresol exhibited an ability to protonate POMA in solution regardless of its undoped or doped state. Ultraviolet-visible spectra confirmed modifications of electronic transitions in agreement with the color changes. Correlations among crystalline properties, electron paramagmetic resonance results and electric conductivity of POMA films processed from DMF, NMP and m -cresol solutions are discussed.

Characterization of Langmuir-Blodgett films of parent polyaniline

Conducting Langmuir-Blodgett (LB) films have been fabricated from parent polyaniline (PAni) which was doped with functionalized acids. In order to optimize experimental conditions for the formation of stable Langmuir monolayers and their subsequent transfer onto solid substrates, PAni was dissolved in ten different combinations of chloroform solutions. Use was made of camphor sulfonic acid, dodecyl benzene sulfonic acid, and toluenesulfonic acid, and of the solvents N-methyl pyrrolidine and m-cresol as processing agents. Because acidic subphases have been employed, as-deposited LB films were already doped, which was confirmed by the appearance of a polaronic band in the UV-Vis absorption spectra. The absorbance peak increases with the number of deposited layers indicating that a suitable multilayer buildup is accomplished. When analysed by atomic force microscopy, PAni LB films show a fibrillar structure with the fibril width ranging from ≈60 to 160 nm.

Conformational dynamics of phenylene rings in poly(p-phenylene vinylene) as revealed by 13C magic-angle-spinning exchange nuclear magnetic resonance experiments

Poly(p-phenylene vinylene) (PPV) has shown a great potential for electro-optical applications due to its electroluminescent and semiconducting properties. Such properties are directly related with the polymer chain conformation and dynamics. Then, it is important to understand in detail the local chain motions. In this work, three 13C solid-state magic-angle-spinning (MAS) exchange NMR techniques were used to study conformational dynamics of phenylene rings in PPV. The standard 2D MAS exchange experiment was used to identify exchange processes between equivalent and nonequivalent sites. Centerband-only detection of exchange (CODEX) experiments were applied to determine the amplitude of the phenylene ring flips and small-angle oscillations. Additionally, a new version of the CODEX technique, which allows for the selective observation of segments executing exchange between non-equivalent sites, is demonstrated and applied to determine the flipping fractions and the activation energies of the phenylene ring ...

Chloroform-soluble poly(o-methoxyaniline) for ultra-thin film fabrication

Abstract One of the main difficulties in spreading Langmuir monolayers from conjugated (conducting) polymers is the lack of solubility of these polymers in common organic solvents. In this letter we report on a chemically synthesized polyaniline derivative, poly( o -methoxyaniline), which is soluble in chloroform and has been used to frabricate ultra-thin films in a Langmuir trough. The synthesized poly( o -methoxyaniline) comprised two parts, one of which has a low molecular weight (LW) (it is in fact an olygomer with a few monomers) whereas the other has a high molecular weight (HW) of approximately 28000 g mol −1 . When separated, the LW fraction yields Y-type LB films by vertical dipping whereas films could only be deposited with the HW fraction if the horizontal lifting method was employed. When examined through the scanning electron microscope, the latter films presented less defects than the LB films obtained with the LW fraction. This is probably because pores which may have been present in the films obtained with the vertical dipping were not present in the more rigid monolayer of the HW fraction.

Line shape of emission spectra of the luminescent polymer poly(p-phenylene vinylene)

The influence of electron-phonon coupling in the emission spectra of self-assembly (SA) films of poly(p-phenylene vinylene) (PPV) is discussed. PPV is an important polymeric system because it exhibits strong luminescence effect. A semi-empirical model was used to analyze the photoluminescence spectra of PPV. This model assumes that defects along the molecule give rise to a distribution of conjugated segments of different lengths. In addition, it was considered that electronic transition was essentially coupled with three effective phonon modes with correspondent energies at 62, -139 and 192 meV, which were observed experimentally.

Effect of doping on the fabrication of Langmuir and Langmuir-Blodgett films of poly(o-ethoxyaniline)

Abstract The properties of Langmuir films are generally dependent on the pH of the subphase. In a previous work, for instance, we observed that Langmuir films from poly(o-ethoxyaniline) (POEA) suitable for Langmuir-Blodgett deposition were only obtained if spread on an acidic subphase. Because POEA is highly soluble in both undoped and doped states, we have now extended these studies to systematically investigate the effect of doping on the fabrication of Langmuir and LB films. The pressure-area isotherms of the Langmuir films display less hysteresis and higher collapse pressures, when the polymer is doped in solution prior to spreading of the monolayer on an acidic subphase. This procedure leads to stable films that can be readily deposited in the form of LB films.

Electrochemical response of poly(o-ethoxyaniline) films produced by different techniques

Abstract Poly(o-ethoxyanilines) (POEA) were synthesized electrochemically and chemically in aqueous acid media. POEA films were readily obtained during the electrochemical deposition on platinum electrodes, while the chemically synthesized material was processed in the form of films either by drop coating or by Langmuir-Blodgett (LB) deposition. The open circuit potential values measured during the chemical synthesis, as well as the color changes observed, are in close agreement with those of the electro-polymerization indicating basically the same changes in oxidation states of the polymer structures. The electrochemical response of all films was investigated by cyclic voltammetry. Two redox processes were observed after stabilization in the electrolyte solution, which are associated with interconversion reactions involving different oxidation and protonation states. Ultra-thin LB films required the shortest time for stabilization. POEA films displayed electrochromism, with color changes from yellow to violet as a function of the potential.

Synthesis and characterization of poly(o-phenetidine) for the fabrication of Langmuir and Langmuir-Blodgett films

Abstract The chemical synthesis of a new soluble polyaniline derivative, poly( o -phenetidine), was carried out with monomer excess and the polymer obtained was characterized by elemental analysis, FTIR, UV-vis spectroscopy, gel permeation chromatography, X-ray diffraction and cyclic voltammetry. The poly( o -phenetidine) has chemical and structural properties which are consistent with a pure polymer in the emeraldine oxidation state. Poly( o -phenetidine) Langmuir monolayers were spread from chloroform or dichloromethane solutions on an aqueous solution whose pH was varied from 6 to 2. Because kinetic effects on monolayer compression are extremely important for monolayers from preformed polymers, the influence of the various experimental parameters on the monolayer characteristics was investigated. It is shown that high transfer ratios are obtained in the LB deposition process when Langmuir monolayers are fabricated from very dilute dichloromethane solutions, spread on a low pH subphase, and compressed at a barrier speed of 30 mm min −1 . The monolayer is then transferred onto a solid substrate by vertical dipping. UV—visible spectroscopy and cyclic voltammetry of the deposited LB films reveal essentially the same behavior as cast poly( o -phenetidine) films.

Conductivity of carbon black-PE composites as a function of temperature and UV aging

We performed a study of mechanisms related to ac electrical conductivity of virgin and aged low-density polyethylene (LDPE) loaded with different concentrations of carbon black (CB). Experimental ac conductivity measurements were carried out at different temperatures on samples with different CB concentrations. Scanning calorimetry (DSC) and thermal expansion measurements were used as auxiliary techniques in this study. We also investigated the aging effect of UV radiation on the electrical properties of the samples. Based on the distribution of the CB in the polyethylene matrix, the ac conductivity is explained in terms of the hopping mechanism considering the material as a system where the energy potential barriers are distributed randomly in the bulk of the material.

Protonation effects in polyaniline langmuir films investigated by surface potential measurements

Abstract Doping parent polyaniline (PANI) and its derivatives has been proven essential for obtaining stable Langmuir monolayers that are amenable to transfer onto solid substrates by the Langmuir-Blodgett (LB) technique. The doping process is generally carried out on the Langmuir trough, by spreading the polymer on an acidic subphase. The degree of doping affects the monolayer surface potential as the positive contribution from the double-layer increases with increasing protonation of the polymer molecules. One could then expect that the surface potential should increase with the acidic concentration in the subphase, but instead the potential diminishes for increasing concentration in the range 10 −4 − 10 .1 M. Interestingly, the results may be discussed in terms of the simple Gouy-Chapman theory, despite the complexity of the macromolecules involved.