To reverse the trend, researchers functionalize the nanotubes. This can dramatically increase the availability of individual SWNTs (unroping) and improve dispersion. In addition, specific functional groups tightly attached to the SWNTs through covalent chemistry can also become sites for interfacial covalent bonding of the SWNT surface to the polymer, which helps to transfer the mechanical stress from the polymer matrix to the nanotubes.
Recently, scientists from Rice University, Texas Southern University and the University of Houston, all in the US, led by Valery Khabashesku, have extended this idea by using covalent functionalization of SWNTs to produce nanotube derivatives carrying both fluorine and long-chain alkyl or perfluoroalkyl groups. These functional groups can be considered as shortened prototypes of polyethylene and Teflon-like chains.
The team dispersed its "bi-functionalized" SWNTs and pristine SWNTs in medium density polyethylene (MDPE) at 1 wt% loadings by using standard shear mixing melt processing and hot pressing lab equipment. Tensile tests performed on dogbone-shaped samples cut from moulded sheets showed the highest (52%) strength enhancement for polyethylene filled with the perfluorinated derivative, F-SWNT-C11FxHy , and a notable, but smaller (16%) enhancement for polyethylene processed with alkylated fluoronanotubes (F-SWNT-C11H23).
The strengthening of the polymer matrix in these composites has been achieved through the combined effects of interfacial covalent bonding of bi-functionalized SWNTs to polyethylene, which is initiated in situ by cleavage of the sidewall C-F groups during shear mixing and melt processing of the composite, and non-covalent entangling interaction of alkyl and perfluoroalkyl "hairs" on nanotubes with the polymer chains. The best performance observed for perfluoroalkyl functionalized SWNTs is due to the repulsion of Teflon-like chains from sidewall fluorine groups on nanotubes, which results in additional exfoliation of the ropes, making them smaller than the other types of SWNTs as established by AFM.
MDPE is a high-commodity thermoplastic material, which has good shock, drop and resistance properties and is used in gas pipes and fittings, and packaging applications. The group believes that the technology demonstrated for SWNT-reinforced MDPE composites in this work can be easily scaled up and transferred to industry.
The researchers presented their work in Nanotechnology.