The dynamics of the polymer matrix in filled rubbers is modified by the presence of solid particles. We used low-field proton NMR to investigate model filled samples consisting of a dispersion of grafted silica particles into an elastomeric matrix. Exploiting magic-sandwich echo experiments, we were able to determine the fraction of polymer with slower dynamics and to correlate it to the silica specific surface. The presence of immobilized polymer;most probably due to a gradient of glass transition temperature around the solid particles;is detected whether there is a covalent bond between the filler and the matrix or not. Moreover, the fraction of immobilized polymer decreases in similar ways with either an increase of the temperature or the addition of solvent. In the case of covalent bonds between the silica and the polymer, multiple-quantum experiments reveal that the cross-link density of the elastomer matrix is locally increased in the vicinity of the particles. This is an observation that was not made in any conventional filled elastomer system and it can be attributed to the good particle dispersion and the covalent links in our model samples.

The slowing-down of the dynamics of a polymer chain near a surface has been observed for many years now. Here we show that the behavior of model nanocomposites can be quantitatively described with a gradient of glass-transition temperature. We describe with a single parameter-the range of this gradient-the temperature and solvent effect on the spin relaxation dynamics. Moreover, this parameter allows a quantitative description of the nanocomposite calorimetric response from the one of the bulk polymer.

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Adding fillers in elastomers is known to increase the elastic modulus and the wear resistance of elastomers, but also to increase nonlinear dissipation, a phenomenon known as the Payne effect. Indeed, when submitted to deformations of the order of a few per cents or more, the elastic modulus can decrease down to values much smaller than the initial one. On the other hand, when submitted to large amplitude oscillatory shear at a frequency ω, frequency analysis shows that the contribution of higher harmonics 3ω, 5ω, ..., to the response is quite small. This might appear somehow as a paradox since the nonlinear behavior of filled elastomers can be strongly marked. We discuss here in detail a possible physical origin of these various features. We do it by comparing experimental results performed on model elastomers to the prediction of a model proposed recently, based on the presence of glassy bridges linking neighboring particles. We show that the kinetics of rupture and rebirth of these glassy bridges can explain these effects.

SUMMARYThis paper discusses the quality of the procedure employed in identifying soil parameters by inverse analysis. This procedure includes a FEM-simulation for which two constitutive models-a linear elastic perfectly plastic Mohr-Coulomb model and a strain-hardening elasto-plastic model-are successively considered. Two kinds of optimization algorithms have been used: a deterministic simplex method and a stochastic genetic method. The soil data come from the results of two pressuremeter tests, complemented by triaxial and resonant column testing. First, the inverse analysis has been performed separately on each pressuremeter test. The genetic method presents the advantage of providing a collection of satisfactory solutions, among which a geotechnical engineer has to choose the optimal one based on his scientific background and/or additional analyses based on further experimental test results. This advantage is enhanced when all the constitutive parameters sensitive to the considered problem have to be identified without restrictions in the search space. Second, the experimental values of the two pressuremeter tests have been processed simultaneously, so that the inverse analysis becomes a multi-objective optimization problem. The genetic method allows the user to choose the most suitable parameter set according to the Pareto frontier and to guarantee the coherence between the tests. The sets of optimized parameters obtained from inverse analyses are then used to calculate the response of a spread footing, which is part of a predictive benchmark. The numerical results with respect to both the constitutive models and the inverse analysis procedure are discussed.

We have been able to design model filled rubbers with exactly the same chemical structure but different filler arrangements. From these model systems, we show that the particle arrangement in the elastomeric matrix controls the strain softening at small strain amplitude known as the Payne effect, as well as the elastic modulus dependence on the temperature. More precisely, we observed that the Payne effect disappears and the elastic modulus only weakly depends on the temperature when the particles are well separated. On the contrary, samples with the same interfacial physical chemistry but with aggregated particles show large amplitudes of the Payne effect and their elastic modulus decreases significantly with the temperature. We discuss these effects in terms of glassy bridge formation between filler particles. The observed effects provide evidence that glassy bridges play a key role on the mechanical properties of filled rubbers.

The microstructure of polymer nanocomposites made with disordered silica filler (Zeosil(R) 1165MP) of industrial relevance and various coating agents is quantitatively analyzed using a combination of SAXS, TEM, and a recently developed structural model. The polymer matrix is formed by an endfunctionalized styrene-butadiene statistical copolymer capable of covalent grafting on the silica nanoparticles. The effect of the coating agents with different alkyl chain length (C 8 , C 12 , and C 18 ) on filler structure quantified in terms of aggregate formation, for different concentrations (up to 8%wt with respect to silica), and the effect of a commonly added catalyzer, DPG, are studied using the structural model. As a result we show that a strongly synergetic effect of both DPG and coating agent exist. Our findings open the road to a fundamental understanding and rational design of model and industrial nanocomposite formulation with optimized coating agents.

Submitted to large sinusoidal strains, filled elastomers not only show a decrease in their storage modulus — the Payne effect, but also a nonlinear behavior — their response is not sinusoidal anymore and involves strain‐stiffening. We show in this study that the two effects can be separated thanks to large amplitude oscillatory shear experiments. The stress signal of filled elastomers consisting of a dispersion of silica particles into a polymeric matrix was decomposed into an elastic and a viscous part and we could observe simultaneously the Payne effect and a strain‐stiffening phenomenon. We showed that the strain‐stiffening was correlated with the Payne effect but came from various intricated effects. It most probably also has its origins in the finite extensibility of the polymer chains confined between solid particles, where the strain is larger. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2010