Surface functionalization of multi-walled carbon nanotubes with hydrogen bonding functionality for tailored polyurethane nanocomposites

Hydrogen bonding functionalities were introduced onto the surface of multi-walled carbon nanotubes (MWCNTs) using acid oxidation to promote intermolecular interactions and facilitate dispersion in polyurethanes matrices. Chemical oxidation was achieved by refluxing with concentrated nitric acid, which introduced carboxylic acid groups onto the MWCNT surface. These surface-bound reactive carboxylic acid groups were further functionalized into amide-amine and amide-urea derivatives, providing an opportunity for additional hydrogen bonding on the surface of the MWCNT. Segmented poly(tetramethylene oxide) based polyurethanes were prepared using methylene bis-(4- cyclohexylisocyanate) (HMDI) as the diisocyanate and 1,4 butanediol as the chain extender. Functionalized MWCNTs were dispersed in a 45 wt% hard segment polyurethane matrix with sonication, and composite film properties were measured to investigate the influence of surface functionality on thermal and mechanical properties. Transmission electron microscopy was also used to investigate the effect of functionalization on the nanotube structure and compare the dispersibility of the different MWCNTs in the polyurethane matrix. Introduction of hydrogen bonding surface functionality led to an increased dispersibility of the MWCNTs and an improvement in mechanical properties. This is in sharp contrast to non-functionalized MWCNT analogs. Improvements in mechanical properties were independent of the chemical structure of the interactive functional groups on the MWCNT surface.