Multiscale micromechanical modeling of the thermal/mechanical properties of polymer/clay nanocomposites

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2006.Includes bibliographical references (leaves 210-217).Polymer/clay nanocomposites have been observed to exhibit enhanced thermal/mechanical properties at low weight fractions (We) of clay. Continuum-based composite modeling reveals that the enhanced properties are strongly dependent on particular features of the second-phase "particles"; in particular, the particle volume fraction (f,), the particle aspect ratio (L/t), and the ratio of particle thermal/mechanical properties to those of the matrix. These important aspects of as-processed nanoclay composites require consistent and accurate definition. A multiscale modeling strategy is employed to account for the hierarchical morphology of the nanocomposite: at a lengthscale of thousands of microns, the structure is one of high aspect ratio particles within a matrix; at the lengthscale of microns, the clay particle structure is either (a) exfoliated clay sheets of nanometer level thickness or (b) stacks of parallel clay sheets separated from one another by interlayer galleries of nanometer level height, and the matrix, if semi-crystalline, consists of fine lamella, oriented with respect to the polymer/nanoclay interfaces. Here, quantitative structural parameters extracted from XRD patterns and TEM micrographs (the number of silicate sheets in a clay stack, N, and the silicate sheet layer spacing, d(ool)) are used to determine geometric features of the as-processed clay "particles", including L/t and the ratio of fp to We.(cont.) These geometric features, together with estimates of silica lamina elastic and thermal expansion properties obtained from molecular dynamics simulations, provide a basis for modeling effective thermal/mechanical properties of the clay particle. In the case of the semi-crystalline matrices (e.g., nylon 6), the transcrystallization behavior induced by the nanoclay is taken into account by modeling a layer of matrix surrounding the particle to be highly textured and therefore mechanically anisotropic. Micromechanical models (numerical as well as analytical) based on the "effective clay particle" were employed to calculate the overall anisotropic elastic constants, anisotropic coefficient of thermal expansion (CTE), and anisotropic yield surface of the amorphous and semi-crystalline polymer-clay nanocomposites and to compute their dependence on the matrix and clay properties as well as internal clay structural parameters. The proposed modeling technique captures the strong modulus enhancements observed in elastomer/clay nanocomposites as compared with the moderate enhancements observed in glassy and semi-crystalline polymer/clay nanocomposites.(cont.) For the case where the matrix is semi-crystalline, the enhancements of composite modulus and strength are found to rely on different functions of the clay: while the modulus enhancement can be explained by the conventional role of "stiff filler", the strength enhancement of the nanocomposite mainly lies in the improvements of the matrix property achieved through the matrix transcrystallization induced by nanoclay the "nucleation sites". When the nanocomposite experiences a morphological transition from intercalated to completely exfoliated, an abrupt jump in the composite initial yield strength, as opposed to the moderate increase in the overall composite modulus, was predicted. The elastic moduli and anisotropic CTE for MXD6-clay and nylon 6-clay nanocomposites predicted by the micromechanical models are in excellent agreement with experimental data. In summary, continuum-based micromechanical models can provide robust predictions of the overall thermal/mechanical properties of polymer/clay nanocomposites, with the employment of a reliable method to account for the intrinsically hierarchical morphology of the nanoclay, and for the special matrix morphology and properties adjacent to the nanoclay.by Nuo Sheng.Ph.D