Chemical Functionalization of Single-walled Carbon Nanotubes for Compatibilization with Unsaturated Polyester Resin

Abstract

This research represents the first fundamental investigation into the tailored surface chemistry of single-walled carbon nanotubes for incorporation with unsaturated polyester resin and provides a foundation for the development of a novel polymer nanocomposite or dispersion based on these two materials. The primary phenomenon limiting the applicability of single-walled carbon nanotubes (SWNT) in polymeric nanocomposites is a lack of nanotube dispersibility due to their intrinsically strong van der Waals attraction. Overcoming this problem for an industrially relevant polymer, such as unsaturated polyester resin, will enable great potential for applications requiring high-strength and lightweight materials. In order to facilitate these advances, the chemical functionalization of SWNT was examined for compatibilization with unsaturated polyester resin. Guided by potential physicochemical interactions between the components, both covalent and non-covalent nanotube functionalization routes were investigated. Utilizing two known methods for individualization, reduction and sonication, nanotube surfaces were exposed for functionalization. It was found that the accessibility of desired surface chemistries was dependant on the individualization method used. For example, ester surface functionalization was not possible by nanotube reduction. Therefore, chemical functionalization schemes deemed appropriate for the method of individualization were pursued and developed in this work. The sidewall esterification of SWNT was achieved through sonication using the Bingel reaction. Due to strong adsorption, the presence of 1,2-dichlorobenzene sonopolymer by-product limited the dispersibility of the Bingel functionalized nanotubes. Based on resonant Raman spectroscopy, it was discovered that sonication of CoMoCAT SWNT in 1,2-dichlorobenzene did not induce doping, but damaged sp2 hybridized carbons. The mechanical instability of so-called “non-SWNT” carbons towards sonication was discovered using careful thermogravimetric measurements. Furthermore, sonication degradation residues were found to skew the interpretation of functionalization effects. These “non-SWNT” carbons were identified to be high purity double-walled carbon nanotubes and multi-walled carbon nanotubes by resonant Raman spectroscopy and high-resolution transmission electron microscopy. A functionalization scheme enabled by nanotube reduction was adapted for the in-situ polymerization functionalization of styrene, and optimized to achieve a high extent of sidewall modification. This methodology was evaluated using previously well-characterized SWNT reactions, such as reductive alkylation. The potential use of this single-pot functionalization method with a variety of unsaturated monomers extends the range of SWNT applications in polymers. Finally, the passivation of single-walled carbon nanotubes with polyvinylpyrrolidone was an effective means for compatibilization. This was a result of this polymer’s molecular solubility in unsaturated polyester resin, as discovered spectroscopically. By focusing on fundamental physicochemical interactions such and hydrogen bonding and doping, the decoupling and understanding of processing effects was possible for this system. Based on the overall results of this study, two schemes were recommended for SWNT dispersion in unsaturated polyester resin: chemical modification based on the in-situ polymerization functionalization methodology and non-covalent stabilization with polyvinylpyrrolidone.