Rheology, Structure, and Stability of Carbon Nanotube-Unsaturated Polyester Resin Dispersions

Abstract

In this research, the first detailed study characterizing the dispersion of carbon nanotubes into an isophthalic unsaturated polyester resin was performed. To eludicate the potential for mechanical property enhancement, dispersions of various carbon nanotubes were studied prior to cure by means of bulk rheological behavior. The greatest enhancement in viscoelastic response was found upon the incorporation of single-walled carbon nanotubes over both acid oxidized single-walled carbon nanotubes and vapor grown carbon fibers. The application of high shear mixing was found to be effective in reducing initial nanotube aggregate size, but unable to exfoliate individual single-walled carbon nanotubes alone. For single-walled carbon nanotube dispersions, an intriguing concentration dependant linear viscoelastic response was observed; this resulted from elastic nanotube network formation, through the percolation threshold. Between nanotube loadings of 0.025% vol. and 0.250% vol., the dependence of concentration on viscoelastic response was removed through colloidal scaling; this revealed insight into the network development through mastercurves. Over the range of concentrations from 0.0030% vol. to 0.010% vol., the dispersions behaved as a viscous liquid. Studying the reduced complex viscosity, non-Brownian behavior was observed in response to applied shear stresses, as predicted by the rotational Peclet number. For dilute dispersions of single-walled carbon nanotubes (0.0050% vol.), low shear rate induced aggregation phenomena was observed. The aggregation driving force was determined to be chemical in nature. Surface analysis indicated aggregation was avoided in the presence of hydroxyl/phenolic functionalities, as these groups could hydrogen bond with the isophthalic polyester. However, no sample dispersions were found to be thermodynamically stable. This observation was correlated with estimations of the enthalpy of mixing, indicating a positive Gibbs free energy upon mixing. The potential use of oxidized carbon nanotubes was investigated but unsuccessful for all as-produced samples. The presence of carboxylic acid functionalities was confirmed after these treatments. Acid oxidized nanotubes treated with ethanol did show some degree of mixing, but could not compare in performance to pristine single-walled carbon nanotube dispersions. Finally, the discovery of a lyophilization based method for creating aligned self-assembled films from aqueous, oxidized nanotube dispersions was reported. The nanotube self-assembly was believed to occur during the freezing step. This method appears to be facile and shows great promise as a significant advancement in the field.