  End-group-functionalized hyperbranched polymers were synthesized to act as a
carbon nanotube (CNT) surfactant in aqueous solutions. Variation of the
percentage of triphenylmethyl (trityl) functionalization and of the molar mass
of the hyperbranched polyglycerol (PG) core resulted in the highest measured
surfactant efficiency for a 5000 g/mol PG with 5.6% of the available hydroxyl
end-groups replaced by trityl functions, as shown by UV-vis measurements.
Semiempirical model calculations suggest an even higher efficiency for PG5000
with 2.5% functionalization and maximal molecule specific efficiency in general
at low degrees of functionalization. Addition of trityl groups increases the
surfactant-nanotube interactions in comparison to unfunctionalized PG because
of pi-pi stacking interactions. However, at higher functionalization degrees
mutual interactions between trityl groups come into play, decreasing the
surfactant efficiency, while lack of water solubility becomes an issue at very
high functionalization degrees. Low molar mass surfactants are less efficient
compared to higher molar mass species most likely because the higher bulkiness
of the latter allows for a better CNT separation and stabilization. The most
efficient surfactant studied allowed dispersing 2.85 mg of CNT in 20 mL with as
little as 1 mg of surfactant. These dispersions, remaining stable for at least
2 months, were mainly composed of individual CNTs as revealed by electron
microscopy.
