Design and Synthesis of Reactive Aramid Nanostructures for Advanced Nanocomposites with Tailored Morphology and Properties.

Abstract

The higher strength-to-weight ratios of nanocomposites have made them novel materials to replace metals in applications requiring high material performance. Nanofiller-reinforced polymer nanocomposites are formed primarily through dispersing strong and stiff nanoparticles in various polymeric matrices, and much of the recent work on nanocomposite mechanics only focuses on improving the reinforcement phase. The design of high-performance nanocomposites, however, also requires polymeric matrices with superior mechanical properties and versatile techniques to tailor these matrices. Suitable polymeric nanoscale building blocks for advanced composite matrices can be prepared from conventionally strong materials. In this dissertation, a high performance polymer poly-paraphenylene terephtalamide, better known as Kevlar or aramid polymer, is employed as a matrix material in the form of nanoscale building blocks for nanocomposites. The synthesis of reactive aramid nanoscale structures has addressed the limitations in the field of synthetic nanocomposites, which has traditionally relied on a set of polymeric building blocks with low reactivity and of limited variability. Reactive nanoscale aramid structures are created to bond with various moieties to form aramid networks with tailored nanostructures, morphologies, and mechanical properties. Stable dispersions of nanoscale Kevlar fibers were first obtained through deprotonating macroscale, commercial Kevlar yarns. The high-aspect-ratio aramid nanofibers were then functionalized to improve their reactivity for bonding by surface treating with phosphoric acid (PA). The PA hydrolysis treatment resulted in two aramid building block nanostructures: nanofibers or nanosheets, depending on the treatment extent. Thirdly, the reactive aramid nanostructures were used as polymeric building blocks that can form strong interactions with citrate-stabilized gold nanoparticles (AuNPs). The resulting Kevlar/gold nanocomposites have an optimized combination of stiffness, strength and strain to failure owing to the reinforcement effect of the metal particles (i.e. AuNPs) through their bonding with the aramid matrix. A structure-property relationship was obtained to systematically tailor and optimize the mechanical properties of these advanced aramid composites. These results are the first demonstration of the possibility for aramid nanoscale fibers to form versatile nanosized building blocks that can then be conjugated to fabricate composite structures involving materials previously thought to be impossible to use, and thus creating a new generation of nanostructured aramd materials.