Stimuli-responsive materials for self-healing in corrosion protection

The corrosion of metallic materials is a crucial issue on an economical and ecological scale. Corrosion protection becomes then necessarily needed. The main focus of the thesis is to develop stimuli-responsive nanocontainers for self-healing in corrosion protection. A nanocontainer is efficient if distinct payloads can be selectively released via different stimuli because unwanted and unspecific release can be avoided. For anti-corrosion, the wanted nanocontainer is the one able to release its self-healing agents or corrosion inhibitors upon change of pH- or/and redox-potential due to the variation of these two signals at the corroded sites. Conducting polymers such as polyaniline (PANI) were chosen for building the shell of capsules due to their important characteristics of being both pH- and redox responsive.

Polímeros condutores aplicados a sistemas-modelo de liberação controlada eletroquimicamente de drogas

Este trabalho descreve a síntese, caracterização e aplicação de sistemas poliméricos baseados em polímeros condutores em sistemas de liberação controlada de drogas. Esta tese pode ser dividida em duas partes: na primeira se apresentam os resultados da aplicação de filmes de polianilina e polipirrol na liberação de drogasmodelo como a dopamina protonada e o ácido salicílico. Na liberação de salicilato utilizou-se um filme polianilina eletrosintetizado e dopado com íons cloreto. Já para a liberação de dopamina protonada (um cátion) a liberação foi conduzida a partir de um sistema bicamadas, com um filme de polianilina recoberta com uma camada de Náfion. É mostrada a liberação controlada nos dois casos, porém também se discutem limitaçãoes deste tipo de sistema que levaram ao estudo de uma forma alternativa de controle eletroquímico utilizando polímeros condutores. A segunda parte do trabalho mostra então esta nova metodologia que se baseia em compósitos de poianilina eletropolimerizada no interior de hidrogéis de poliacrilamida. É mostrado que este novo material é eletroativo e mantém as características de intumescimento dos hidrogéis, tanto necessárias ao desenvolvimento destes sistemas de liberação controlada. Mecanismos para o crescimento e distribuição da polianilina na matriz isolante e para a atuação do compósito no controle eletroquímico da liberação são propostos com base nos dados de microscopia de força atômica, Raman e eletrônica de varredura, além de testes de liberação controlada com moléculas de diferentes cargas.

Asymmetric hybrid capacitors based on activated carbon and activated carbon fibre–PANI electrodes

Composites consisting of polyaniline (PANI) coatings inside the microporosity of an activated carbon fibre (ACF) were prepared by electrochemical and chemical methods. Electrochemical characterization of both composites points out that the electrodes with polyaniline show a higher capacitance than the pristine porous carbon electrode. These materials have been used to develop an asymmetric capacitor based on activated carbon (AC) as negative electrode and an ACF–PANI composite as positive electrode in H2SO4 solution as electrolyte. The presence of a thin layer of polyaniline inside the porosity of the activated carbon fibres avoids the oxidation of the carbon material and the oxygen evolution reaction is produced at more positive potentials. This capacitor was tested in a maximum cell voltage of 1.6 V and exhibited high energy densities, calculated for the unpackaged active materials, with values of 20 W h kg−1 and power densities of 2.1 kW kg−1 with excellent cycle lifetime (90% during the first 1000 cycles) and high coulombic efficiency.

Adsorption and first stages of polymerization of aniline on platinum single crystal electrodes

The present communication studies the adsorption of aniline on platinum single crystal electrodes and the electrochemical properties of the first layers of polyaniline(PANI) grown on those platinum surfaces. The adsorption process was studied in aqueous acidic solution (0.1 M HClO4) and the electrochemical properties of thin films of PANI in both aqueous (1 M HClO4) and non-aqueous media (tetrabutyl ammonium hexafluorophosphate (TBAPF6) with additions of methanesulphonic acid in acetonitrile). First of all, it was found that the adsorption of aniline on platinum single crystal surfaces is a surface sensitive process, and even more important that the adsorption features found at low concentrations (5 × 10−5 M) can be directly correlated to the electrochemical properties of thin films of PANI in the very early stages of polymerization. The Pt(1 1 0) surface was found to be more suitable to obtain polymers with more reversible redox transitions when studied in aqueous media (1 M HClO4). This is in good agreement with the higher polymerization rates found on this surface compared to Pt(1 0 0) and Pt(1 1 1). Finally the differences in ionic exchange rate were greatly enhanced when they were studied in organic media. The AC 250 Hz response in the case of the thin films synthesized on Pt(1 1 0) is about twice greater than that obtained in the other basal planes using polymer layers with the same thickness.

Enhancement of the Electrochemical Performance of SWCNT dispersed in a Silica Sol-gel Matrix by Reactive Insertion of a Conducting Polymer

The electroassisted encapsulation of Single-Walled Carbon Nanotubes was performed into silica matrices (SWCNT@SiO2). This material was used as the host for the potentiostatic growth of polyaniline (PANI) to yield a hybrid nanocomposite electrode, which was then characterized by both electrochemical and imaging techniques. The electrochemical properties of the SWCNT@SiO2-PANI composite material were tested against inorganic (Fe3+/Fe2+) and organic (dopamine) redox probes. It was observed that the electron transfer constants for the electrochemical reactions increased significantly when a dispersion of either SWCNT or PANI was carried out inside of the SiO2 matrix. However, the best results were obtained when polyaniline was grown through the pores of the SWCNT@SiO2 material. The enhanced reversibility of the redox reactions was ascribed to the synergy between the two electrocatalytic components (SWCNTs and PANI) of the composite material.

Synthesis, Characterization and Conducting Properties of Nanocomposites of Intercalated 2-Aminophenol with Aniline in Sodium-Montmorillonite

A simple method was used to synthesize poly(2-aminophenol), poly(2-aminophenol-co-Aniline) and polyaniline nanocomposites with sodium-montmorillonite (Na-M) using in situ intercalative oxidative polymerization. Morphology and thermal properties of the synthesized nanocomposites were examined by transmission electron microscopy (TEM) and thermogravimetric analysis. The thermal analysis shows an improved thermal stability of the nanocomposites in comparison with the pure poly(2-aminophenol). The intercalation of polymers into the clay layers was confirmed by X-ray diffraction studies, TEM images and FTIR spectroscopy. In addition, the room temperature conductivity values of these nanocomposites varied between 8.21 × 10−5 and 6.76 × 10−4 S cm−1. The electrochemical behavior of the polymers extracted from the nanocomposites, has been analyzed by cyclic voltammetry. Good electrochemical response has been observed for polymer films; the observed redox processes indicate that the polymerization into Na-M produces electroactive polymers.

Study of optical properties of electropolymerized melanin films by photopyroelectric spectroscopy

Photopyroelectric (PPE) spectroscopy, in the 350-1,075 nm wavelength range, was used to study the optical properties of electropolymerized melanin films on indium tin oxide (ITO) coated glass. The PPE intensity signal as a function of the wavelength lambda, V (n)(lambda) and its phase F (n)(lambda) were independently measured. Using the PPE signal intensity and the thermal and optical properties of the pyroelectric detector, we were able to calculate the optical absorption coefficient beta of melanin in the solid-state. We believe this to be the first such measurement of its kind on this material. Additionally, we found an optical gap in these melanin films at 1.70 eV.

Clay-based polymer nanocomposites: Research and commercial development

This paper reviews the recent research and development of clay-based polymer nanocomposites. Clay minerals, due to their unique layered structure, rich intercalation chemistry and availability at low cost, are promising nanoparticle reinforcements for polymers to manufacture low-cost, lightweight and high performance nanocomposites. We introduce briefly the structure, properties and surface modification of clay minerals, followed by the processing and characterization techniques of polymer nanocomposites. The enhanced and novel properties of such nanocomposites are then discussed, including mechanical, thermal, barrier, electrical conductivity, biodegradability among others. In addition, their available commercial and potential applications in automotive, packaging, coating and pigment, electrical materials, and in particular biomedical fields are highlighted. Finally, the challenges for the future are discussed in terms of processing, characterization and the mechanisms governing the behaviour of these advanced materials.

Synthesis and characterization of nanocomposites based on PANI and carbon nanostructures prepared by electropolymerization

Nanocomposites based on polyaniline (PANI) and carbon nanostructures (CNSs) (graphene (G) and multiwall carbon nanotubes (MWCNTs)) were prepared by in situ electrochemical polymerization. CNSs were inserted into the PANI matrix by dispersing them into the electrolyte before the electropolymerization. Electrochemical characterization by means of cyclic voltammetry and steady state polarization were performed in order to determine conditions for electro- polymerization. Electro-polymerization of the PANI based nanocomposites was carried out at 0.75 V vs. saturated calomel electrode (SCE) for 40 and 60 minutes. The morphology and structural characteristics of the obtained nanocomposites were studied by scanning electron microscopy (SEM) and Raman spectroscopy, while thermal stability was determined using thermal gravimetric analysis (TGA). According to the morphological and structural study, fibrous and porous structure of PANI based nanocomposites was detected well embedding both G and MWCNTs. Also, strong interaction between quinoidal structure of PANI with carbon nanostructures via π–π stacking was detected by Raman spectroscopy. TGA showed the increased thermal stability of composites reinforced with CNSs, especially those reinforced with graphene.

Ad-atoms and their enhancement effects on Pt activity for formic acid oxidation

Formic acid oxidation has been widely studied at Pt as a model reaction to understand fundamental aspects of electrocatalytic reactions in fuel cells. Electrocatalytic oxidation of formic acid takes place through two parallel pathways (direct and indirect). The indirect pathway proceeds via CO as an intermediate, which is known to be responsible for the poisoning of Pt and its consequent decrease in activity. Surface modification of Pt with ad-atoms is known to hinder this poisoning and promote the direct pathway. The incorporation of polymers (polyaniline, polycarbazole, polyindole) as supports also increases activity. Irreversibly adsorbed Sb and Bi on Pt are known to show high electrocatalytic activity for formic acid oxidation. This work presents the dependence of Sb and Bi irreversible adsorption on immersion time, metal solution concentration and pH. The activity of Sb and Bi modified Pt was correlated against immersion time and percent coverage of Pt by ad-atoms. Polyaniline support effects in combination with a Bi modified Pt catalyst showed enhancement in oxidation current compared to Pt-Bi.

Improvement of carbon materials performance by nitrogen functional groups in electrochemical capacitors in organic electrolyte at severe conditions

N-doped activated carbon fibers have been synthesized by using chemically polymerized aniline as source of nitrogen. Commercial activated carbon fibers (A20) were chemically modified with a thin film of polyaniline (PANI) inside the microporosity of the carbon fibers. The modified activated carbon fibers were carbonized at 600 and 800 °C, respectively. In this way, activated carbon fibers modified with surface nitrogen species were prepared in order to analyze their influence in the performance of electrochemical capacitors in organic electrolyte. Symmetric capacitors were made of activated carbon fibers and N-doped activated carbon fibers and tested in a two-electrode cell configuration, using triethylmethylammonium tetrafluoroborate/propylene carbonate (TEMA-BF4/PC) as electrolyte. The effect of nitrogen species in the degradation or stabilization of the capacitor has been analyzed through floating durability tests using a high voltage charging (3.2 V). The results show higher stabilizing effect in carbonized samples (N-ACF) than in non-carbonized samples and pristine activated carbon fibers, which is attributed to the presence of aromatic nitrogen group, especially positively charged N-functional groups.

Change in dielectric properties in the microwave frequency region of polypyrrole-coated textiles during aging

Complex permittivity of conducting polypyrrole (PPy)-coated Nylon-Lycra textiles ismeasured using a free space transmission measurement technique over the frequency range of1–18 GHz. The aging of microwave dielectric properties and reflection, transmission and absorptionfor a period of 18 months is demonstrated. PPy-coated fabrics are shown to be lossy over thefull frequency range. The levels of absorption are shown to be higher than reflection in the testedsamples. This is attributed to the relatively high resistivity of the PPy-coated fabrics. Both the dopantconcentration and polymerisation time affect the total shielding effectiveness and microwave agingbehaviour. Distinguishing either of these two factors as being exclusively the dominant mechanismof shielding effectiveness is shown to be difficult. It is observed that the PPy-coated Nylon-Lycrasamples with a p-toluene sulfonic acid (pTSA) concentration of 0.015 M and polymerisation times of60 min and 180 min have 37% and 26% decrease in total transmission loss, respectively, upon agingfor 72 weeks at room temperature (20 C, 65% Relative humidity (RH)). The concentration of thedopant also influences the microwave aging behaviour of the PPy-coated fabrics. The samples with ahigher dopant concentration of 0.027 mol/L pTSA are shown to have a transmission loss of 32.6% and16.5% for short and long polymerisation times, respectively, when aged for 72 weeks. The microwaveproperties exhibit better stability with high dopant concentration and/or longer polymerization times.High pTSA dopant concentrations and/or longer polymerisation times result in high microwaveinsertion loss and are more effective in reducing the transmission and also increasing the longevity ofthe electrical properties.