Memory effects in schematic models of glasses subjected to oscillatory deformation

Davide Fiocco, Giuseppe Foffi
Iop Publishing Ltd, 2015
Article

Résumé

We consider two schematic models of glasses subjected to oscillatory shear deformation, motivated by the observations, in computer simulations of a model glass, of a nonequilibrium transition from a localized to a diffusive regime as the shear amplitude is increased, and of persistent memory effects in the localized regime. The first of these schematic models is the NK model, a spin model with disordered multi-spin interactions previously studied as a model for sheared amorphous solids. The second model, a transition matrix model, is an abstract formulation of the manner in which occupancy of local energy minima evolves under oscillatory deformation cycles. In both of these models, we find a behavior similar to that of an atomic model glass studied earlier. We discuss possible further extensions of the approaches outlined.

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https://infoscience.epfl.ch/record/208086?ln=fr

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Highly filled polymer nanocomposite films derived from novel nanostructured latexes

Riccardo Ruggerone

The overall aim of this thesis has been to assess the potential of latex-based technologies for the preparation of polymer/clay nanocomposites. The key feature of latex-based technologies is that they offer the possibility of improved control of the final nanocomposite morphology at significantly higher clay loadings than can be obtained with more conventional processing techniques, such as melt blending or in situ polymerization. The idea is to exploit swelling of the clay in either the aqueous or the monomer phase of a water-based latex, depending on the clay surface functionalization, to produce hybrid polymer/clay latex particles with controlled diameters of the order of 100 nm, which may then be consolidated to produce solid nanocomposite films. The materials considered in this work were based on styrenic matrices, considered to be a model system, and acrylics, which are of more interest for commercial coating applications. Two different polymerization techniques were investigated, namely conventional emulsion polymerization and miniemulsion polymerization. The thermal and mechanical properties of films produced from the resulting latexes were then studied in detail. Conventional emulsion polymerization was found to be particularly suitable for the preparation of particles with a well-defined "armoured" morphology, in which the clay formed a more or less complete shell around a matrix core, providing the focus for the remainder of the project. Clay contents of up to about 50 wt % were obtained for both the styrenic and the acrylic latexes using this approach, with excellent degrees of dispersion, the average clay aggregate thickness not exceeding 10 nm. The armoured morphology of the latex particles resulted in a cellular arrangement of the clay in the consolidated films, which became better defined as the clay content increased. The reinforcing effect of the clay on mechanical properties varied according to the physical state of the matrix. Increases in Young's modulus by a factor of 3 to 4 were observed in styrenic films with the cellular morphology in the glassy state, and the degree of exfoliation of the clay was found to be a critical parameter under these conditions, samples containing 5 to 7 wt % of clay showing increased moduli with respect to those obtained at somewhat higher clay contents, for which aggregation was more apparent. In the rubbery state, on the other hand, the Young's modulus increased by more than 2 orders of magnitude for clay contents above 20 wt % and was strongly correlated with the overall filler content. Thermal analysis showed that a significant proportion of the matrix remained immobilized in the rubbery state, i.e. did not contribute to the glass transition. This was argued to be due to strong physical confinement of regions of the matrix intercalated in the clay aggregates. While the increases in Young's modulus in the glassy state could be accounted for in terms of classical micromechanical models, such as those of Halpin-Tsai and Mori-Tanaka, the same models failed to predict the behaviour in the rubbery state. Models based on foam mechanics were therefore developed incorporating an immobilized matrix fraction in the cell walls, whose elastic properties were treated as fitting parameters. Although somewhat different values of the Young's modulus for this immobilized matrix fraction were required to fit the data, depending on the details of the model, they were consistently found to be two to three orders of magnitude greater than that of the neat matrix in the rubbery state (but not to exceed the Young's modulus of the matrix in the glassy state), providing direct evidence for the importance of this interphase for the overall nanocomposite properties. 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