MULTI-SCALE MECHANICAL CHARACTERIZATION AND MODELING OF HIERARCHICALLY-STRUCTURED MATERIALS: SYNTHETIC NANO-ENHANCED POLYMERS AND NATURAL PALMETTO WOOD

Abstract

The development of hierarchical structure in synthetic materials is a new approach to enhancing multifunctional properties in a manner that is similar to natural materials. One means for achieving hierarchical structure is to add nanoscale ingredients to polymers that can either self-assemble in a "bottom-up" approach or can be added to polymer with microscale reinforcement through engineered-assembly in a "top-down" approach. In order to better understand synthetic hierarchical structures developed through the nano-enhancement of polymers and those occurring in natural materials, multi-scale characterization techniques and models are developed to elucidate the structure-property relationship due to hierarchical structure, and the control of this relationship through processing in synthetic materials. A combinatorial approach is applied for rapid characterization of the processing-structure-property relationships that develop in nano-enhanced thermoplastic polymers associated with the hierarchical structure that forms from a combination of dispersed, agglomerated, and percolated nanoscale ingredients. A new approach to multi-scale mechanical characterization and modeling using microtensile testing and nanomechanical characterization of the mechanical properties for these hierarchically-structured materials is developed, as well as for the formation of hierarchical structures in epoxies and carbon fiber - epoxy composites through nano-enhancement. This characterization is used with Rule-of-Mixtures formulations to develop new multi-scale models that can predict macroscale properties from the hierarchical structure. Furthermore, it is possible to characterize the "degree of dispersion" in the hierarchical structure and the control of this structure through processing conditions such as sonication. The incorporation of melt annealing is also applied to hierarchically-structured thermoplastics in order to control properties through relaxation of fiber orientation in the microstructure. After establishing the multi-scale mechanical characterization approach, it is utilized to characterize and model the effects of nano-enhancement on curing kinetics in hierarchically-structured adhesives in order to understand the evolution of adhesion between the epoxy and nanoscale ingredient. In addition, this approach is exploited to characterize and model mechanical behavior in a naturally-occurring hierarchically-structured material, palmetto wood. This knowledge serves as biological inspiration to guide further development of synthetic hierarchically-structured materials.