Bruno Sixou

Is granularity the determining feature for electron transport in conducting polymers

From several studies of transport properties of conducting polymers (CP) as a function of a parameter related to their structure or microstructure we show that in strongly disordered CP, electron transport is dominated by hopping between conducting grains separated by insulating barriers. In CSA-protonated polyaniline, a weakly disordered CP which exhibits a metallic-like behaviour, several results indicate that heterogeneities play a major role in the transport process. Thus, we conclude that heterogeneous disorder appears as a key parameter which controls the conductivity of a large majority of CP.

Influence of electron irradiation on the mobility and on the mechanical properties of DGEBA/TETA epoxy resins

Abstract The thermomechanical properties of DGEBA/TETA resins are analysed as a function of the irradiation dose. Irradiation by electrons results in a decrease of the glass transition temperature and the elastic modulus in the rubbery region. The Arrhenius diagrams obtained for the various doses show that the cooperative mobility associated with the α relaxation becomes faster after the irradiation. More local mobility corresponding to the β and γ relaxations is also modified, but in a lesser extent. These results can be interpreted invoking a destruction of the crosslinks within the resin. The effects of irradiation on the stress–strain curves are also discussed and are shown to be connected with both the physical aging kinetics and the spatial heterogeneity of the crosslink density in the resin.

Conductivity evolution of polypyrrole thin films with aging

Abstract Conductivity has been measured as a function of time t a during aging, and as a function of temperature for different aging times, on differently doped polypyrrole (PPy) in various conditions. We show that for the samples aged below 150 °C, a single expression describes both the temperature and the aging time dependences of the conductivity, In σ(t a ,T) α −(t a / T) 1/2 . This corresponds to a hopping process between conducting grains separated by expanding insulating barriers upon aging. Moreover, the aging kinetics for different temperatures can be deduced from each others with scaling laws. The relative stabilities of PPy with various dopants have been compared. Slight deviations from the above behaviour, observed when aging above 150 °C, can be accounted for by a generalization of the model.

Non-linear iterative phase retrieval based on Frechet derivative

Several methods of phase retrieval for in-line phase tomography have already been investigated based on the linearization of the relation between the phase shift induced by the object and the diffracted intensity. In this work, we present a non-linear iterative approach using the Frechet derivative of the intensity recorded at a few number of propagation distances. A Landweber type iterative method with an analytic calculation of the Frechet derivative adjoint is proposed. The inverse problem is regularized with the smoothing L₂ norm of the phase gradient and evaluated for several different implementations. The evaluation of the method was performed using a simple phase map, both with and without noise. Our approach outperforms the linear methods on simulated noisy data up to high noise levels and thanks to the proposed analytical calculation is suited to the processing of large experimental image data sets.

The routes towards three-dimensional conducting polymers: 2. Transport properties of fully conjugated gels of poly(3-n-octylthiophene)

Abstract The conductivity and the thermoelectric power of fully conjugated networks of poly(3- n -octylthiophene) have been measured as a function of the temperature between 15 and 300 K. Despite the interchain conjugated reticulation the conduction process is still governed by hopping. The data are in good agreement with a model of inter-cluster hopping conduction. The cluster size is discussed as a function of the reticulation ratio. It is pointed out that the presence of the reticulation points gives rise to specific properties, in particular the intrinsic intra-cluster conductivity is enhanced as compared to that of poly(3- n -octylthiophene).

Intermolecular and intramolecular contributions to the relaxation process in sorbitol and maltitol

Molecular mobility in sorbitol and maltitol is studied with spectroscopic techniques and molecular dynamics simulations in order to evaluate the relative contributions of the intermolecular and intramolecular interactions involved in the relaxation processes. The results of the molecular dynamics simulations performed on the polyols in the bulk or in vacuum compares well with the results of the analysis of the relaxation diagrams in the framework of the Perez et al. model. They both imply that the difference in the relative contributions of the intermolecular and intramolecular interactions associated with the different chemical architectures of the two polyols must be taken into account. The intermolecular interactions cannot be neglected and they are stronger in sorbitol than in maltitol in relation with the linear structure of this polyol. The intramolecular barrier, higher in the maltitol molecule with a more complex structure, could be at the origin of the higher junction temperature between the α and β relaxation processes.

Molecular mobility of sorbitol and maltitol: A ^13C NMR and molecular dynamics approach

Abstract:Molecular mobility in two similar organic glass formers, namely sorbitol and maltitol, is studied in order to understand their difference in the cross-over between and relaxations, far above their respective glass transition temperatures. In this goal, the individual mobility of each carbon atom of the 6 carbon chain present both in sorbitol and maltitol is studied by means of ^13C nuclear magnetic resonance and molecular dynamics simulations. Both techniques imply that the mobility of carbons located at the end of the 6 carbon chain is greater than that of any other carbon of this chain and that the difference in carbon mobility is greater within the sorbitol moiety of maltitol than in sorbitol. The relaxation time obtained by magnetic resonance for carbons at the end of the 6 carbon chain is related to the relaxation time and the one of carbons in the middle of the chain is in relation with the value of the relaxation time. This result may suggest that the merging between the and relaxation processes in both sugars would be related to the decrease of the differences in mobility between the atoms of 6 carbon chain.

Molecular dynamics simulation of the first stages of the cavitation process in amorphous polymers

The first stages of the cavitation process in amorphous polymers submitted to an hydrostatic deformation in the glassy state are studied with coarse grain molecular dynamics simulations for various intermolecular interactions strengths and flexible and semi-flexible chains. For strong intermolecular interactions, the cavitation process is highly localized and the holes have a marked spherical symmetry. The cavitation regions are more diffuse for weaker intermolecular interactions or when the chain stiffness is increased. The mean Voronoï polyhedra volume and the disorder inside the polymer increase until the stress peak observed below the glass transition. High mobility regions are present before the stress peak that may act as nucleation sites for the cavitation process. The localization of these high mobility zones is enhanced for strong intermolecular interactions or a low chain rigidity. Moreover, the velocity fluctuations are more marked in the vicinity of the holes. For strong intermolecular interactions, the holes are not randomly distributed throughout the system and the nucleation of cavities upon deformation occurs preferentially near the chain ends of the polymer.

Binary Tomography Reconstructions With Stochastic Level-Set Methods

In this work, we propose a stochastic level-set method to reconstruct binary tomography cross-sections from few projections. A first reconstruction image is obtained with a level-set regularization method. The reconstruction is then refined with a stochastic partial differential equation based on a Stratanovitch formulation. The reconstruction results are compared with the ones obtained with the classical simulated annealing method. The methods are tested on a complex bone μ- CT cross-section for different noise levels and number of projections. The best reconstruction results are obtained with the stochastic level set-method.

Effect of electron irradiation on the mechanical properties of DGEBA/DDM epoxy resins

Abstract The thermomechanical properties of diglycidyl ether of bisphenol A (DGEBA)/4-4 diaminodiphenylmethane (DDM) resins are analysed as a function of the irradiation dose. Irradiation results in a decrease of the glass transition temperature and of the elastic modulus in the rubbery region, ascribed to a destruction of crosslinks. The Arrhenius diagrams show that the α relaxation becomes faster, that the β process is enhanced but that γ relaxation is less modified by the irradiation. The effects of irradiation on the stress–strain curves suggest a spatial heterogeneity of the crosslink density.