ASPECT RATIO AND SURFACE CHEMISTRY EFFECTS ON POLYMER-NANOTUBE COMPOSITES

Abstract

While carbon nanotubes have demonstrated efficacy as additives in polymer composites due to their superior properties, economic imitations associated with nanotube synthesis have hindered any large-scale applications. Recent advancements in carbon nanotube synthesis on multilayered catalyst supports have greatly surpassed yields of nanotubes grown on traditionally spherical catalysts, potentially removing the need for extra purification processes and thus reducing total cost of production. While most commercially available carbon nanotubes possess aspect ratios (length to diameter ratios) near 100, nanotubes grown on multilayered supports can reach aspect ratios well over 1,000. Hence, this research aimed to enhance the understanding of carbon nanotube aspect ratio on polymer-nanotube composites with additional emphasis on the effects of nanotube surface chemistry. Planetary ball milling, a simple and industrially relevant process, was employed to modify nanotube aspect ratio and surface chemistry. It was found that the kinetic length reduction behavior of carbon nanotubes subject to ball milling could be predicted by a simple exponential expression regardless of nanotube wall morphology, bulk morphology, or initial average length. Additionally, ball milling carbon nanotubes in the presence of species like ammonium carbonate, melamine, and sulfur was found to result in a variety of covalently and noncovalently bonded oxygen, nitrogen, and sulfur containing groups, respectively. Polycarbonate composites containing pristine, ultrahigh aspect ratio carbon nanotubes were found to exhibit high melt viscosities during compounding, leading to notable polymer degradation. Additionally, nanotube dispersion was found to be less than ideal due to the existing interleaved bundle morphology, resulting in higher-than-expected percolation thresholds. Upon ball milling for nanotube length reduction, the processability of composites was found to improve substantially; however, dispersion worsened due to the formation of small, tightly packed agglomerates. Mechanochemically oxygen-functionalized nanotubes exhibited improved dispersion and lower percolation thresholds than plain ball milled nanotubes of similar lengths due to enhanced interactions with the polymer via hydrogen bonding interactions. Tire tread rubber compounds filled with carbon nanotubes and carbon black/nanotube hybrid filler systems displayed increasing degrees of mechanical reinforcement with increasing filler aspect ratio. Mechanochemically sulfur-doped nanotubes resulted in further improved mechanical reinforcement and reduced rolling resistance, suggesting there may be merit to further study in reducing tire filler content toward improving automobile fuel economy. The implications of this research demonstrate potential for using ball milling as a scalable procedure for controlling carbon nanotube length and surface chemistry for targeted polymer composite properties.