Highly Efficient Hyperbranched CNT Surfactants: Influence of Molar Mass and Functionalization

摘要

End-group-functionalized hyperbranched polymers were synthesized to act as acarbon nanotube (CNT) surfactant in aqueous solutions. Variation of thepercentage of triphenylmethyl (trityl) functionalization and of the molar massof the hyperbranched polyglycerol (PG) core resulted in the highest measuredsurfactant efficiency for a 5000 g/mol PG with 5.6% of the available hydroxylend-groups replaced by trityl functions, as shown by UV-vis measurements.Semiempirical model calculations suggest an even higher efficiency for PG5000with 2.5% functionalization and maximal molecule specific efficiency in generalat low degrees of functionalization. Addition of trityl groups increases thesurfactant-nanotube interactions in comparison to unfunctionalized PG becauseof pi-pi stacking interactions. However, at higher functionalization degreesmutual interactions between trityl groups come into play, decreasing thesurfactant efficiency, while lack of water solubility becomes an issue at veryhigh functionalization degrees. Low molar mass surfactants are less efficientcompared to higher molar mass species most likely because the higher bulkinessof the latter allows for a better CNT separation and stabilization. The mostefficient surfactant studied allowed dispersing 2.85 mg of CNT in 20 mL with aslittle as 1 mg of surfactant. These dispersions, remaining stable for at least2 months, were mainly composed of individual CNTs as revealed by electronmicroscopy.