Doctor of Philosophy (PhD)
Civil and Environmental Engineering
The first aim of this work is developing a procedure for experimental and analytical characterization of nano-scale microstructures which mediate large scale deformation in amorphous polymers. Glassy polymers are extensively used as high impact resistant, low density, and clear materials in industries. Nevertheless, their response under severe loading conditions is yet to be appropriately unraveled. Due to the lack of long-range order in the microstructures of glassy solids, their plastic deformation is different from that in crystalline solids. Shear Transformation Zones (STZs) are believed to be the main plasticity carriers in amorphous solids and defined as the localized atomic or molecular deformation patches induced by shear. Employing Nanoindentation and Atomic Force Microscopy (AFM), the micromechanical and microgeometrical properties of STZs are obtained for different polymers. Moreover, since the nucleation of STZs strongly depends on the initial microstructural state of the material, the possible relationship between the microstructural state variables, such as free volume as inherent defects in amorphous solids, and plasticity sites is investigated using Positron Annihilation Lifetime Spectroscopy (PALS) for a better understanding of the response of polymers during deformation. The second objective of this dissertation is proposing a model for observation of the physico-mechanical properties of amorphous polymers at small scales, i.e., indentation size effect (ISE) based on the STZ mediated theory which is one of the challenging concepts in physics and deformation of non-crystalline solids. Employing the shear transformation mediated flow theory and considering the statistical nature of formation and distribution of the flow sites, a rate dependent ISE model for glassy polymers is developed. This model is based on the possibility of occurrence of discrete shear transformation sites within the deformed volume under the indenter which is controlled by the indentation depth and geometry.
Malekmotiei, Leila, "Microstructural Characterization of Shear Transformation Zones and Modeling Indentation Size Effect in Amorphous Polymers" (2018). LSU Doctoral Dissertations. 4637.