Abstract

Carbon nanotubes (CNTs) are a tubular form of graphene that have diameters as small as 1 nm and lengths that can exceed 1 µm. Their Young’s modulus has been measured to be 1 TPa, their strength up to 150 GPa, and they have been shown to be ballistic conductors of heat and electricity. CNTs’ extraordinary properties make them useful for a wide variety of applications including field emission devices, nanowires, sensors, and composites. In polymer composites carbon nanotubes are being applied primarily for mechanical reinforcement. Most polymers are derived from petroleum and while the known reserves of oil have continued to grow, so has the world’s need. Recently more and more attention has focused on renewable alternatives. Soybean oil is one such alternative that can be used to create polymers comparable to petroleum based ones. The triglyceride molecules that make up soy oil can be easily functionalized to form monomers that are easily polymerized. Acrylated epoxidized soy oil (AESO) is one such monomer.

In this thesis CNTs will be examined for their use as randomly oriented reinforcement in AESO and other plant oil resins. In order to understand the interactions of carbon nanotubes with solvents, monomers, and polymers, two thermodynamic models based on lattice theory are used. The first applies to rigid rods, which some CNTs approximate, while the other is more applicable to larger, more flexible CNTs. A simpler excluded volume approach is also used to determine the volume fractions necessary for isotropic dispersions. All models show that the maximum CNT volume fraction for isotropic dispersion is strongly dependent on the aspect ratio of the CNTs.

Composites made using unmodified CNTs in AESO both with and without styrene all showed signs of aggregation, as examined using scanning electron microscopy, even at low concentrations. Modulus results from dynamic mechanical analysis showed positive or negative effects that were dependent upon the cure conditions. Results were modeled using the Halpin-Tsai model, with occluded polymer contained within the aggregates increasing the effective CNT volume fraction.

CNTs can be functionalized with organic molecules to improve compatibility with AESO and other monomers. Functionalization of the CNTs was accomplished by using an acid based oxidative technique to add carboxylic acid groups to the CNTs’ surfaces and was verified using X-ray photoelectron spectroscopy. The acid groups were then transformed to more reactive acyl chloride groups which can react with hydroxyl groups on larger organic molecules. Composites made containing CNTs functionalized with non-reactive molecules such as decanol and octadecanol contained aggregates of CNTs despite the functionalization. CNTs functionalized with maleinated hydroxylated soy oil could react with the bulk resin and remained dispersed in the resulting composites. The Halpin-Tsai model can only predict the results if the existence of a strong interphase is assumed to exist between the CNTs and the bulk resin.

From this work it was shown that CNTs can be used in an unmodified form for reinforcement in plant oil based thermosets with the appropriate cure profile; however aggregates will still form. Individual, dispersed CNTs can be achieved only by functionalizing the CNTs with polymerizable molecules.