Rheological and morphological characterization of hierarchically nanostructured materials

Abstract

Hierarchically nanostructured materials exhibit order on multiple length scales, with at least one of a few nanometers. The expected enhancements for applications using these materials include improved mechanical, thermal and electrical properties; however, control of the morphology which governs material performance and fabrication remains a challenge. The development of novel quantitative characterization techniques is important to connect the underlying morphology to relevant processing parameters and macroscopic behavior. Rheological and morphological analysis can illustrate these governing structure-property relationships for hierarchically nanostructured materials based on "O-D" polyhedral oligomeric silsesquioxane (POSS) particles, "l-D" carbon nanotubes (CNTs), and "2-D" clay nanoparticles. We develop a technique, using small-angle X-ray scattering, which provides quantitative measurements of the morphological characteristics of CNT films, including shape, orientation, CNT diameter, and spacing between CNTs. The method reflects a locally averaged measurement that simultaneously samples from millions of CNTs while maintaining the necessary precision to resolve spatial morphological differences within a film.

(cont.) Using this technique we elucidate spatial variation in pristine films and study changes in the film structure as a result of mechanical manipulations such as uniaxial compression and capillarity-induced densification. We study the rheological properties of blends formed from POSS and clay nanoparticles incorporated into PMMA in shear and extensional flow fields. Relevant morphological parameters, such as volume fraction, aspect ratio of the clay particles, and POSS miscibility are determined using wide angle X-ray scattering and transmission electron microscopy. The interdependence between melt rheology and morphology are understood within a theoretical framework for percolated physical networks, providing for comprehensive guidance regarding the performance and processing of POSS and clay based nanocomposites.