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We extend the model proposed by Merabia et al. [2008] regarding reinforcement mechanisms of filled elastomers. This model is based on the presence of glassy layers around filler particles which may bridge neighboring particles. The model was solved by Merabia et al. for spherical particles only and for a single dispersion state. However, experiments show that mechanical properties depend crucially on the complex shape of fillers as well as on their spatial distribution. We consider extensively both aspects in this article. We show that the distribution state of the fillers is key for controlling the reinforcement at high temperature. For a given distribution state, we show a strong effect of filler morphology on reinforcement. Distances between fillers are smaller with fractal aggregates, which leads to stronger reinforcement. Our model opens the path for the development of systems with tailored properties by tuning the filler distribution state and morphology. [ABSTRACT FROM AUTHOR]

We present a new multiscale hierarchical numerical approach combining ab initio, molecular dynamics, and metadynamics to model molecular motions in polymers over widely different time scales. We demonstrate that molecular motions corresponding to γ and β secondary (subglassy) relaxations can be described precisely in terms of free enthalpy, activation energy, and time scales and may be identified with relaxations detected experimentally by spectroscopic and other techniques. This method is illustrated in the case of aliphatic and semiaromatic polyamides. The highlight of this innovative approach in polymers is the ability to simulating long time scale motions while retaining an all-atomistic description of the polymer material. A complete description of the free energy landscape, in terms of conformational (intramolecular) and interactional (intermolecular) contributions, is obtained (energy barriers and time scales). This method thus may provide a predictive tool to understand secondary relaxations in amorphous polymers. [ABSTRACT FROM AUTHOR]

The Kraichnan flow provides an example of a random dynamical system accessible to an exact analysis. We study the evolution of the infinitesimal separation between two Lagrangian trajectories of the flow. Its long-time asymptotics is reflected in the large deviation regime of the statistics of stretching exponents. Whereas in the flow that is isotropic at small scales the distribution of such multiplicative large deviations is Gaussian, this does not have to be the case in the presence of an anisotropy. We analyze in detail the flow in a two-dimensional periodic square where the anisotropy generally persists at small scales. The calculation of the large deviation rate function of the stretching exponents reduces in this case to the study of the ground state energy of an integrable periodic Schrödinger operator of the Lamé type. The underlying integrability permits to explicitly exhibit the non-Gaussianity of the multiplicative large deviations and to analyze the time-scales at which the large deviation regime sets in. In particular, we indicate how the divergence of some of those time scales when the two Lyapunov exponents become close allows a discontinuity of the large deviation rate function in the parameters of the flow. The analysis of the two-dimensional anisotropic flow permits to identify the general scenario for the appearance of multiplicative large deviations together with the restrictions on its applicability. [ABSTRACT FROM AUTHOR]

Hydroxylated amorphous silica nanoparticles were modeled using a combination of computational techniques at different levels of length scales from Ångström to hundreds of nanometers. Using quantum chemical ab initio methods, force field Monte Carlo methods, reactive force field simulations, and numerical model calculations, including BET theory calculations it was possible to describe and model the physico-chemical properties of hydroxylated amorphous silica. The results are compared with experimental data and found to be in good agreement with the theory, confirming the reliability of the computational method and the silica model structure. [ABSTRACT FROM AUTHOR]

We extend a model that has been proposed for describing mechanical properties of glassy polymers and that takes into account the heterogeneous nature of the dynamics. We describe their nonlinear mechanical properties, up to a few 10% deformation. We propose that the elastic energy stored in the volume ∼ ξ³ of dynamical heterogeneities effectively reduces the free energy barriers present for internal relaxation. It allows to calculate yield stresses of order a few 10 MPa, which are consistent as compared to experimental data without additional adjustable parameters than the scale ξ. The model is solved in 3D by numerical simulations with spatial resolution the scale of dynamical heterogeneities. We show that the prediction of our model as regards to yield behavior is consistent with the value of 3-5 nm for the scale for dynamical heterogeneities. Yield appears as the result of an acceleration of the dynamics of subunits with intermediate relaxation times which relax under stress before subunits with very long relaxation. Our simulations describe the onset of plastic behavior and the reorganization at the scale of dynamical heterogeneities. We predict the appearance of shear bands on a scale of a few tens of nanometers at yield and beyond. [ABSTRACT FROM AUTHOR]

We study the dielectric relaxation of polycarbonate (PC) at room temperature under imposed strain rate γ̇, above the yield stress, and up to 13% strain. We find that the dielectric response of stretched PC behaves as if it was heated up at a temperature just below its glass transition temperature, Tg ≈ 423 K for PC. Indeed, in the frequency range of our experiment (10-2 and 10³ Hz), the dielectric response of the stretched PC at room temperature superimposes to the dielectric response of PC at a temperature Ta(γ̇) < Tg, which is a function of strain rate. Specifically we observe that at Ta the dominant relaxation time τα(Ta) of PC at rest is related to γ̇ in such a way that τα(Ta) ∼ 1/γ̇ at and beyond the yield point. In our experiment, 10-5 s-1 < γ̇ < 10-3 s-1, the temperature shifts Tg - Ta are of a few kelvin. The mechanical rejuvenation modifies the dielectric response at frequencies smaller than 10 Hz, whereas for higher frequencies the spectrum is only slightly modified. [ABSTRACT FROM AUTHOR]