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ABSTRACT: Using the quasielastic neutron scattering technique on the 10−12 s time scale, the dynamics of highly conducting free-standing films of doped polyaniline have been studied in detail. The latter are protonated with sulphonated organic acids made of flexible molecules which also play the role of plasticizer of the system. While polymer chains appeared immobile, the study of the dynamics of these counter-ions reveals a glass transition temperature which corresponds to the change in the electronic conductivity of the film. Competing interactions in between the counter-ions and between polymer chains and counter-ions limit the available space for alkyl chains to move around. A description of the dynamics is proposed on the basis of a model where protons undergo a local diffusion inside individual spheres whose radii depend explicitly on their position on the molecule. When the temperature is decreased, only protons attached to the aliphatic tails explore a significant space while the others are progressively frozen. Refinement of the neutron data permit one to access the radii of the sphere and the diffusion constant as functions of temperature.
Philosophical Magazine. 05/2004; 84(13-16):1547-1554.
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ABSTRACT: Conducting polyaniline protonated with sulphonic flexible counter-ions was investigated by quasielastic incoherent neutron scattering. In addition to their role in electrical properties, the flexible counter-ions also increase the elasticity of the samples. As in the case of more rigid counter-ions, polymer chains appear as very stiff objects whose dynamics is completely outside the investigated time scale. Conversely, the counter-ion dynamics was proved to be of major importance in charge transport since a dynamical transition is observed precisely in the temperature range where the electronic properties change from a metallic to a semiconducting regime.
Applied Physics A 11/2002; 74:s402-s404. · 1.63 Impact Factor
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ABSTRACT: Static and dynamic properties of supercooled liquid and glassy ethanol are evaluated from computer molecular dynamics simulations and compared with experimental data whenever possible. In particular, a comparison with recent neutron-scattering data is done and the results indicate that the relatively simple model here employed is able to reproduce most of the observed features, at least on semiquantitative grounds. Special attention is paid to monitor how the relevant dynamical properties change through the glass transition region and to relate the phenomena observed with the microscopic information provided by the simulations.
Phys. Rev. B. 02/2000; 61(10).
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ABSTRACT: The nitroxide (referred to as tano) forms channel inclusion compounds with linear hydrocarbons. At room temperature, both tano and chain molecules are dynamically disordered. When the temperature is decreased, one phase transition is observed. The pressure dependence of the temperature of transition and the evolution of the individual motions have been studied by incoherent quasielastic neutron scattering in the case of a tano - octane system. The transition temperature increases with the pressure with a slope . The transition mechanism is connected to the lock-in of both tano (interconversion) and chain (reorientation) molecules, as a result of strong correlations between host and guest substructures. A residual disorder persists in the low-temperature phase confirming that all the tano molecules are not dynamically equivalent.
Journal of Physics Condensed Matter 12/1998; 9(30):L403. · 2.55 Impact Factor
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ABSTRACT: The X-ray diffraction study of polycrystalline sulpholan has been carried out in the T = 200–300 K temperature range, in order to compare with previous NMR results about the existence of two high-temperature phases I and II and of two low-temperature phases III and IV. The transition I–II could be evidenced by the temperature evolution of the relative intensities of the observed diffraction lines. The crystalline phases III and IV were identified as monoclinic, with , , , β = 97.49(5)°, space group P21/c, Z = 8 at T = 230 K. The transition III–IV was observed only as a discontinuity in the temperature evolution of lattice parameters.
Physica B: Condensed Matter.
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ABSTRACT: Molecular motions in mono-hydrated racemic camphor sulphonic acid (±)-10-C10H16O4S−–H3O+ which is abbreviated as (CSA–H2O) were investigated using incoherent neutron scattering techniques. Analysis of the intensity of the purely elastic scattering over a wide temperature range (4–340 K) carried out with a high-resolution backscattering spectrometer revealed the onset of molecular motions at ca. 100 K which could be observed on the 10−10 s time-scale up to T=180 K. These motions were identified as 120° jumps of the methyl groups. Quasielastic measurements using both the backscattering and the time-of-flight techniques enabled to study this movement from 150 to 340 K. The corresponding characteristic time was found to follow an Arrhenius law with an activation energy . All the methyl groups appear as dynamically equivalent. That result is at contrast with earlier studies on conducting polymers where CSA was introduced as a counter-ion and for which the intermolecular effects were found to strongly influence the dynamics. Inspection of the low frequency part of the vibrational spectrum evidences deformations of the C–C–S angle and rotational oscillations of the hydration molecules.
Chemical Physics. 277(2):211-224.
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ABSTRACT: The structure of three jeffamine-pyromellitimides based alternate copolymers were studied by small angle X-ray scattering measurements when they are mixed with a lithium salt. The salt addition contributes to increase the elastic domain of compounds with the longest units (B900 and B2000). By contrast, the salt induces the system having the shortest repeat units (B600) to stiffen and to crystallize. La structure de trois copolymères alternés à base de jeffamine et d'unités pyromellitimides mélangés avec un sel de lithium a été étudiée par diffusion des RX aux petits angles. La présence du sel contribue à augmenter le domaine caoutchoutique des composés aux unités les plus longues (B900 et B2000). Par contre, le système le plus court (B600) se rigidifie et cristallise.
http://dx.doi.org/10.1051/jcp:1998324.
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ABSTRACT: The studied materials are used as electrodes in solid-state batteries and electrochromic devices. They are constituted of alternating redox aromatic di-imides and polyethylene oxide (PEO) ion-conducting segments whose length is controlled. Their electroactive properties should be partly correlated with structural features. It is shown that, concerning the semi-crystalline samples, the ratio of lengths between their two components is driving the overall conformation of chains and thus, the way the system is crystallizing towards different condensed phases. This paper is dealing with the modeling and description of these 3-dimensional structures.
Synthetic Metals 102:1412-1413. · 1.83 Impact Factor
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ABSTRACT: We have used a recently developed empirical potential to simulate the molecular dynamics of the complex TANO–heptane inclusion compound. The incoherent neutron scattering laws evaluated from the computed trajectories are compared to their experimental counterparts. With a model corresponding to a large simulation cell with a matrix disorder based on the X-ray description, the agreement is excellent for both the incoherent structure factors and the dynamical structure factors of the whole system and of the included chains separately.The detailed analysis of the computed trajectories has permitted explanation for the existence of two categories of methyl groups suggested by previous neutron scattering experiments. These originate from intramolecular interactions between the methyl hydrogens and the nitroxide part of the same TANO molecule. Potential energy barriers associated to these two categories of methyls differ by approximately 1 kcal/mol, yielding the mean correlations times τ1=17 ps and τ2=49 ps, respectively.The rotational energy barriers for the included molecules have been computed. It appears that the dynamic fluctuations of the channel diameter are sufficient to significantly lower the potential energy barrier and to allow the included chains to explore the full angular range without any correlated change of conformation of the surrounding TANO molecules.
Chemical Physics. 261:75-88.
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ABSTRACT: Using incoherent quasielastic neutron scattering techniques, the molecular motions were investigated in fully hydrogenated and partially deuterated polyaniline protonated by camphor sulfonic acid (CSA) conducting samples. The obtained results show that on the 10-9–10-12s time scale the polymer chains do not exhibit any diffusive motions: the whole observed quasielastic scattering has accordingly to be attributed to motions of CSA ions. From our measurements two molecular movements could be differentiated. A rapid one has been attributed to the three-site rotation of methyl groups present on camphor moieties of CSA and a slower one that has been modeled as a rigid body motion of the whole CSA molecule. Due to the disordered character of the system, the methyl rotors appeared to be dynamically nonequivalent. Their dynamics was then described in terms of a log gaussian distribution of correlation times. This description allowed a good fitting of experimental data and gave an activation energy of 12.5 kJ mol-1. However, two different regimes in temperature could be distinguished. At high temperatures (T>280K) the width of the distribution is nearly zero and thus, the methyl rotors are dynamically equivalent while it turned larger and larger when temperature is decreased below 250 K revealing that the rotors are more and more sensitive to their local environment. In the conducting samples the slowest motion clearly exists in the 280–330 K temperature range and is blocked at temperatures inferior to 250 K. This transition occurs in the temperature range in which the metal-insulator transition also happens.
Phys. Rev. B. 65(18).
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