Sandrine Therias

Anion-exchanging clay-modified electrodes: synthetic layered double hydroxides intercalated with electroactive organic anions

Abstract The layered double hydroxide (LDH) Zn 2 Cr(OH) 6 Cl · 2H 2 O, denoted [Zn-Cr-Cl], which is an anion-exchanging clay, is rapidly exchanged with electroactive organic sulphonate anions such as m -nitrobenzene sulphonate and anthraquinone mono- and disulphonates. High degrees of reaction are attained, and pillar-like layered structures are formed. Clay-modified electrodes have been produced using these materials and the precursor LDH [Zn-Cr-Cl]. The electrochemical behaviour of the substrates listed above, either intercalated or simply adsorbed on the LDH particles, has been studied using cyclic voltammetry.

Photochemical Behavior of Polylactide/ZnO Nanocomposite Films

This study reports the effect of light on PLA/ZnO nanocomposites films produced by melt-extrusion. The attention focused on the discrimination between the photocatalytic degradation of PLA provoked by ZnO and the UV screening effect of the ZnO nanoparticles. The chemical modifications of PLA induced by UV light irradiation were analyzed using infrared spectroscopy and completed through the analysis of the low-molecular-weight photoproducts using IC and SPME and the characterization of chain scissions with SEC. A comprehensive mechanism for the photooxidation of PLA was then proposed. The results indicated that the photocatalytic activity of ZnO nanoparticles induces the oxidation of PLA. Because ZnO limits the penetration of light inside the samples, this effect mainly concerns the first micrometers at the surface of the exposed samples. Cross-sectional analysis using micro-IR and ATR-IR spectroscopies was performed to highlight the degradation profile in the PLA/ZnO nanocomposites.

Photochemical behavior of PVA as an oxygen-barrier polymer for solar cell encapsulation

Polyvinyl alcohol (PVA) is a water-soluble polymer that is anticipated to be a good candidate for incorporation into multilayer coatings of organic solar cells due to its high transparency and ability to form a barrier to oxygen. Because a long lifetime is a prerequisite for successful applications, it was necessary to study the photochemical behavior of PVA under solar light. PVA films were exposed to UV-visible light irradiation (λ > 300 nm) in accelerated aging conditions representative of natural ageing. Modifications in the chemical structure of aged samples irradiated at ambient air were recorded. Due to the low oxygen permeability of PVA films, it was shown that the photooxidative degradation of PVA films is restricted to the surface (<5 μm) and results in a large amount of chain scissions, with a progressive erosion of the surface of the irradiated material. The oxidation products formed along the macromolecular chains, and low molecular weight species trapped in the matrix or emitted in the gas phase were also identified. An oxidation mechanism was then proposed to account for these modifications. However, irradiation in the absence of oxygen demonstrated the high photostability of PVA films, which permits the use of PVA as a sublayer in inorganic/organic multilayer encapsulation systems.

Electrodes modified with synthetic anionic clays

Anionic clays Zn2Cr(OH)6C1·2H2O, denoted [ZnCrCI], and Zn3A1(OH)8C1·2H2O, denoted [ZnAlCl], are rapidly exchanged with electroactive organic sulfonate anion such as 2,2′-azinobis 3-ethylbenzothiazoline-6-sulfonate (ABTS). These samples are prepared by a simple exchange reaction or by the coprecipitation method. High degrees of exchange are obtained and pillar-like layered structures are formed. Clay-carbon paste modified electrodes have been produced using these materials and the precursor [ZnCrCl]. The electrochemical behaviour of the intercalated substrate has been studied using cyclic voltammetry. The electrogenerated radical of intercalated [ZnAl-ABTS] has been characterized by EPR.

Electrochemical study of ferrocene and nitroxide derivatives intercalated in Zn–Cr and Zn–Al layered double hydroxides

Layered double hydroxides (LDH) containing the electrochemical redox anions ferrocene sulfonates (FcMS and FcDS) and a nitroxide radical (NODC) have been prepared and characterized by PXRD, IR and ESR. The electrochemical behaviour of these hybrid materials as well as the role of the electrolyte anions during the oxidation process have been studied by cyclic voltammetry. An electrochemical transfer at LDH modified electrodes via an intracrystallite mechanism is confirmed.

Effect of ZnO nanofillers treated with triethoxy caprylylsilane on the isothermal and non-isothermal crystallization of poly(lactic acid)

The crystallization of PLA-silane surface-treated ZnO nanocomposites was investigated by DSC and compared to that of neat PLA. Several modes of crystallization were considered: isothermal and non-isothermal cold crystallization and also isothermal and non-isothermal melt crystallization. The kinetics of cold crystallization were studied using different methods, namely the Avrami and Ozawa-Flynn-Wall models, to calculate activation energies and kinetic constants. In contrast to what is typically observed when the foreign particles are added in a polymer matrix, the silane surface-treated ZnO delayed the crystallization of PLA and made it more difficult to start. The nucleation activity of the ZnO nanoparticles, ϕ, was calculated and found to be greater than 1 (ϕ = 1.7). This indicated that ZnO played an anti-nucleating role in the crystallization of PLA nanocomposites. This effect has been linked mainly to the interactions between the silane groups onto the surface of nanoparticles and PLA macromolecules. These interactions which reduce the mobility of polymer chains have been evidenced by rheological experiments.

Mechanically stable and photocatalytically active TiO2/SiO2 hybrid films on flexible organic substrates

Photocatalytic porous coatings (micro-, meso- and macroporous) are obtained by the dispersion of TiO2 nanoparticles in sol–gel hybrid matrices. The sol–gel silica matrix is used as a binder stabilizing the nanoparticle dispersion and as a protective layer for the organic substrates. Organic groups are introduced into the matrix to induce the film flexibility and a part of them is used to create the final microstructure allowing remarkable improvement of the photocatalytic properties. The film structure and UV stability are fully characterized. The photocatalytic activity is evaluated through a test with formic acid. Flexible efficient photoactive composites are obtained showing important capabilities for depollution (water and air) and self-cleaning applications.

Multiscale investigation of the poly(N-vinylcarbazole) photoageing mechanism.

During the past decade, the development of polymeric solar cells has received a great deal of attention from both academic and industrial laboratories. In order to enhance the device performances both in terms of power conversion efficiency stability in use conditions, Polycarbazole copolymers have attracted increasing attention. In this paper, the photodegradation of poly(N-vinylcarbazole) (PVK) was investigated from the molecular scale to the nanomechanical properties. It was shown irradiation provoked chain scissions, homolysis of the C-N bond and formation of new covalent bonds between the macromolecular chains. To fully understand the mechanism of the degradation of PVK provoked by exposure to UV radiation, mechanical analyses were performed. The consequences of the cross-linking reactions on the surface modifications were analyzed. Roughness and stiffness measurements were obtained through surface analysis and nanoindentation by atomic force microscopy (AFM), and depth-profiling experiments were also performed. The surface modifications and the shape of the profiles of the degradation photoproducts were explained in light of the chemical modifications of the PVK structure. Quantitative correlations were successfully obtained between the main relevant criteria of degradation, from the chemical structure to the mechanical properties. It was found that cross-linking reactions were prevalent.

A universal route to improving conjugated macromolecule photostability

This predictive study demonstrates that the introduction of aromatic-oxy-alkyl links surprisingly makes materials more resistant to photo-oxidative degradation by reducing hydrogen abstraction. This revelation makes it possible, for the first time, to design a toolbox of substituents for soluble, photostable conjugated materials.

Luminescent Nanocomposites Made of Finely Dispersed Y3Ga5O12:Tb Powder in a Polymer Matrix: Promising Candidates for Optical Devices

This paper reports the initial results of an original and simple method to elaborate flexible, self-standing, and thick luminescent films suitable for optical devices. PVP/Y(3)Ga(5)O(12):Tb(3+) nanocomposite films have been successfully achieved from a sol-gel derived Y(3)Ga(5)O(12):Tb(3+) powder and an alcoholic solution of poly-N-vinylpyrrolidone (PVP). The structural, morphological, and optical properties of these nanocomposite films have been studied and compared to those of a pristine PVP film and Y(3)Ga(5)O(12):Tb(3+) powder. The nanocomposite films were characterized by infrared and Raman spectroscopies as well as scanning and transmission electron microscopies (SEM and TEM) and demonstrated good dispersion of the phosphor particles within the polymer matrix via an alveolar mesostructure. The optical properties of these nanocomposites were fully characterized, and both their excitation and emission spectra and decay curves were recorded. Furthermore, photostability of the nanocomposite films and of the luminescent raw powder has been studied after exposure to an accelerated artificial photoageing at wavelengths higher than 300 nm. The elaboration process used is both tunable and applicable to a large variety of powders and polymers because it does not require any additive to form homogeneous and easily shapeable phosphor/polymer nanocomposites applicable in a large variety of optical devices such as solid-state-lighting.