Thermodynamics and rheology of polymer/layered silicate nanocomposites: Modeling and measurement

In polymer processing, such as extrusion and injection molding, the rheological properties of polymeric materials, related to the microstructure of macromolecules, play a very important role. In filled polymer system, the extend of the dispersion, the shape and the orientation and/or arrangement of the dispersed particles add to the complexity of the rheology. In polymer/layered silicate (PLS) nanocomposites, the nanometric size of the dispersed particles brings still an additional complexity due to the importance of the polymer-particle and particle-particle interactions.This thesis focuses on the rheology of the nano suspension of silicate layers in a viscoelastic polymer matrix. Non-equilibrium thermodynamics is employed to model its linear and nonlinear rheological behavior. Two rheological models on the mesoscopic level of description are developed for isothermal suspensions of completely and incompletely exfoliated silicate lamellae in polymer melts. The models take into account the polymer chain-polymer chain, polymer chain-lamella and lamella-lamella interactions. By following thermodynamic (GENERIC) framework, these physical features are then incorporated into governing equations of the rheological models. The expressions for the extra stress tensor arise automatically in the framework. The mesoscopic level of description chosen in this thesis appears to be a good compromise between microscopic details and overall simplicity of the governing equations. While still being able to express in the model important features of the physics involved, numerical solutions of the governing equations, needed to derive model predictions, are relatively easily (with the assistance of the software package MATEMATICA) obtained. A new set of experimental data for PLS nanocomposites is also reported. The experimental data contribute to the large pool of the data existing in literature for polymer nanocomposites by an original choice of the polymer (of particular interest due to its biodegradability) and by their completeness (that is needed for the comparison with model predictions). Predictions of the models, obtained by solving numerically their governing equations, are responses of the suspension to oscillatory, transient (start-up and relaxation) and steady shear flows. The calculated rheological data are compared with our measured experimental data and the experimental data taken from the literature. Predictions of the models are found to be in a good agreement with results of experimental observations.