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Loi D.,European Synchrotron Radiation Facility | Mossa S.,CNRS Structure and Properties of Molecular Architectures Laboratory | Cugliandolo L.F.,University Pierre and Marie Curie
Soft Matter | Year: 2011

We use molecular dynamics simulations to study the dynamics of an ensemble of interacting self-propelled semi-flexible polymers in contact with a thermal bath. Our intention is to model complex systems of biological interest. We find that an effective temperature allows one to rationalize the out-of-equilibrium dynamics of the system. This parameter is measured in several independent ways - from fluctuation-dissipation relations and by using tracer particles - and they all yield equivalent results. The effective temperature takes a higher value than the temperature of the bath when the effect of the motors is not correlated with the structural rearrangements they induce. We show how to use this concept to interpret experimental results and suggest possible innovative research directions. © The Royal Society of Chemistry 2011.


Loi D.,European Synchrotron Radiation Facility | Mossa S.,CNRS Structure and Properties of Molecular Architectures Laboratory | Cugliandolo L.F.,University Pierre and Marie Curie
Soft Matter | Year: 2011

We follow the dynamics of an ensemble of interacting self-propelled semi-flexible polymers in contact with a thermal bath. We characterize the structure and dynamics of the passive system and as a function of the motor activity. We find that the fluctuation-dissipation relation allows for the definition of an effective temperature that is compatible with the results obtained by using a tracer particle as a thermometer. The effective temperature takes a higher value than the temperature of the bath when the effect of the motors is not correlated with the structural rearrangements they induce. Our data are compatible with a dependence upon the square of the motor strength (normalized by the average internal force) and they suggest an intriguing linear dependence on the tracer diffusion constant times the density of the embedding matrix. We show how to use this concept to rationalize the experimental results and suggest possible innovative research directions. © 2011 The Royal Society of Chemistry.


Buhot A.,CNRS Structure and Properties of Molecular Architectures Laboratory
Macromolecules | Year: 2010

The viscosity η of polymer melts experimentally scales with the length L of the chains as a power law with an exponent b ≈ 3.4 larger than the prediction b = 3 from de Gennes' theory of reptation. This long-standing controversy is revisited within the repton and necklace models. Exact results from the rigid chain dynamics allow us to predict the leading order viscosities and renewal times for chains with fluctuating lengths and/or center-of-mass initial position fluctuations. The corrections to scaling are determined by numerical simulations to behave as L-1/2 in all cases. Such large effects may explain (a) the apparent exponent observed experimentally, (b) confirm the corrections due to the contour length fluctuations, and (c) highlight the importance of the relaxation of longitudinal modes. © 2010 American Chemical Society.


Mizuno H.,Joseph Fourier University | Mossa S.,CNRS Structure and Properties of Molecular Architectures Laboratory | Barrat J.-L.,Joseph Fourier University
Physical Review E - Statistical, Nonlinear, and Soft Matter Physics | Year: 2013

Glasses exhibit spatially inhomogeneous elastic properties, which can be investigated by measuring their elastic moduli at a local scale. Various methods to evaluate the local elastic modulus have been proposed in the literature. A first possibility is to measure the local stress-local strain curve and to obtain the local elastic modulus from the slope of the curve or, equivalently, to use a local fluctuation formula. Another possible route is to assume an affine strain and to use the applied global strain instead of the local strain for the calculation of the local modulus. Most recently, a third technique has been introduced, which is easy to be implemented and has the advantage of low computational cost. In this contribution, we compare these three approaches by using the same model glass and reveal the differences among them caused by the nonaffine deformations. © 2013 American Physical Society.


Lyonnard S.,CNRS Structure and Properties of Molecular Architectures Laboratory | Gebel G.,CNRS Structure and Properties of Molecular Architectures Laboratory
European Physical Journal: Special Topics | Year: 2012

A molecular level understanding of structure and transport properties in fuel cell ionomer membranes is essential for designing new electrolytes with improved performance. Scattering techniques are suited tools for this purpose. In particular, neutron scattering, which has been extensively used in hydrogen-containing systems, is well adapted to investigate water-dependent complex polymeric morphologies. We report Small-Angle Neutron Scattering (SANS) studies on different types of fuel cell polymers: perfluorinated, radiation-grafted and sulfonated polyphosphazene membranes. We show that contrast variation methods can be efficiently employed to provide new insights on membrane microstructure and reveal ionic condensation effects. Neutrons have been used also as non-intrusive diagnosis tool to probe water properties and distribution inside membranes. Recently, in-situ neutronography and SANS experiments on operating fuel cells have been reported. In-plane cartography of water distribution at the surface of bipolar plates and water profiles across membrane thickness have been obtained and studied as a function of operating conditions. The last section of the article is devoted to the use of Quasi-Elastic Neutron Scattering to study water dynamics at molecular scale. We show that analysis with an appropriate sophisticated diffusion model allows to extract diffusion coefficients, characteristic times and length-scales of molecular motions. This quantitative information is fruitfully integrated in multi-scale modelling and usefully compared with numerical simulations. QENS also permits to compare alternative polymers and relate dynamical properties to chemical composition and membrane nanostructure. © 2012 EDP Sciences and Springer.

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