Nanotube reinforced thermoplastic polymer matrix composites
Shofner, Meisha Lei
Barrera, Enrique V.
Doctor of Philosophy thesis
The inherent high strength, thermal conductivity, and electrical conductivity make nanotubes attractive reinforcements for polymer matrix composites. However, the structure that makes them desirable also causes highly anisotropic properties and limited reactivity with other materials. This thesis isolates these problems in two separate studies aimed at improving mechanical properties with single wall nanotube (SWNT) reinforced thermoplastic polymer composites. The two studies demonstrate the effect of solid freeform fabrication (SFF) and chemical functionalization on anisotropy and limited reactivity, respectively. Both studies showed mechanical property improvements. The alignment study demonstrates a maximum increase of 93% in tensile modulus with single wall nanotubes (SWNTs). The chemical functionalization study shows a larger increase in storage modulus for functionalized SWNTs as compared to purified SVWNTs with respective increases of 9% and 44% in storage modulus. Improved interfacial properties are also observed as a decrease in mechanical damping. Maximum property increases in composites are obtained when nanotubes are aligned, requiring additional processing consideration to the anisotropic structure. Melt spinning and extrusion processing effectively align nanotubes, but the end product of these techniques, composite fibers, requires further processing to be incorporated into finished parts. Extrusion-based SFF is a novel technique for processing nanotube reinforced composites because it allows for the direct fabrication of finished parts containing aligned nanotubes. SFF processing produces parts containing preferentially oriented nanotubes with improved mechanical properties when compared to isotropic composites. Functionalization of the nanotube surface disrupts the rope structure to obtain smaller ropes and promote further interfacial bonding. The chemically inert nature of nanotubes resulting from a structure containing few defects and the formation of larger, ordered ropes of SWNTs limits the amount of interfacial bonding and load transfer that occurs between nanotubes and a polymer matrix. Improved dispersion, interfacial properties, and mechanical properties are achieved through chemical functionalization. Subsequent partial removal of the functional groups created a direct bond between the nanotubes and the polymer matrix. The alignment and functionalization studies in this thesis further the knowledge of the use of nanotubes as reinforcements in polymer composites through understanding the sensitivity of the nanotubes' anisotropic properties and the nanotube/polymer interface.
Engineering; Materials science