Final properties of polymeric materials e.g., polymer nanocomposites, hydrogels, etc., significantly depend on the microstructure, e.g., dispersion quality of the nanofillers, of the final product. Hence, in-depth exploration and characterization of the microstructure of these materials is of great importance from both an industrial and an academic point of view. In this regard, rheometry has been widely used as a powerful tool to investigate the micro- and nano-structural features of complex fluids. Regarding structural characterization, rheometry provides more reliable information compared to imaging techniques. Many research studies have been performed on the structure-property relationships for various polymeric systems using linear viscoelastic data obtained under small-amplitude oscillatory shear (SAOS) flow. Although SAOS tests provide useful information about the relationship between the rheological properties and microstructure, it should be noted that SAOS flow is limited to a narrow strain or stress region, i.e., ultra-small deformation and linear viscoelastic behavior. Therefore, rheological measurements under intermediate- and large-strain amplitudes have the potential to reveal important additional information regarding aspects of materials response not accessible via linear rheology.

Because of the mentioned reasons, large-amplitude oscillatory shear (LAOS) flow has attracted much attention from both academia and industry. Hence, this Ph.D. thesis has focused on LAOS behavior of a wide range of complex polymeric materials, including polymer nanocomposites, hybrid polymer nanocomposites, hydrogels, and interfaces. This study has a dual achievement:​ (i) better understanding of the network structure of the polymer/nanomaterial-based fluids using LAOS techniques, and (ii) in-depth knowledge of nonlinear viscoelastic behavior of complex fluids.

That is, upon improvement the network structure of viscoelastic materials, e.g., improving the dispersion quality of the nanofillers in a polymeric matrix, we observed that:​ (1) the onset of inter- and intra-cycle viscoelastic nonlinearity shits to lower deformations, (2) the extent of intra-cycle elastic nonlinearity decreases in LAOS region while the intra-cycle viscous nonlinearity increases, and (3) the nonlinearity of interfaces changes from strain-softening to weak strain overshoot. In addition to characterization of the network structure of the mentioned systems by LAOS, for the first time we showed the effects of confinement and wall-slip on LAOS data of polymeric materials.