| Polymer-clay nanocomposites (PCNs) have recently attracted attention in the scientific community, due to the profound enhancement of end-use, material properties obtained with clay loadings substantially less than required for traditional, micron-sized fillers. Resultantly, a fundamental understanding of the physics and chemistry governing these property changes is of paramount interest. Firstly, we devote efforts towards optimizing and characterizing the compounding methods used to exfoliate and disperse clay domains in a polymer matrix, essential to achieving desired property improvements. Melt-blending in a twin-screw extruder is contrasted with single-screw extrusion incorporating in-situ addition of supercritical carbon-dioxide, in preparing 1, 3, and 5 wt % polypropylene-clay nanocomposites in the presence and absence of a maleic anhydride-based compatibilizer. Dispersion is characterized by XRD, TEM, and rheology. Secondly, we probe the terminal rheology of these PCNs, which was found to be time-dependent and highly sensitive to deformation history. Specifically, they exhibited logarithmic increases in storage modulus and complex viscosity with time. Furthermore, shear flow was found to orient clay domains, in a manner that is not erased by annealing time. This rheology is ubiquitous for several clay loadings and dispersion sates, indicating an evolving mesoscale clay network, similar to colloidal glasses. Lastly, we study the crystallization of PCNs under both quiescent and flow-induced (FIC) conditions. Calorimetry and microscopy show that the clay generally hinders the kinetics of crystallization and spherulitic growth. Interestingly, a late-stage nucleation of tiny spherulites occurs in the highly loaded and well-dispersed PCNs, indicating the clay in these systems may contribute to a buildup of local stresses, triggering crystal nucleation. FIC is studied with a custom-built extruder, utilized to subject the samples to a finite shear pulse. Crystallization is monitored in-situ, by measuring the turbidity and birefringence signals or wide angle X-ray diffraction pattern, following deformation. At higher clay loadings, the shear-enhanced kinetics are significantly accelerated, signifying the clay filler may increase local stresses in the melt, thereby enhancing homogeneous nucleation. Furthermore, data reveal that poorly dispersed, agglomerated clay stacks imbue considerably more melt distortion than their well-exfoliated counterparts, evidenced by a higher orientation fraction and reduction in critical stress denoting skin-core morphology. |
