Molecular Dynamics Studies of PEGylated Single-Walled Carbon Nanotubes: The Effect of PEG Size and Grafting Density

Single-walled carbon nanotubes (SWNTs) covalently or noncovalently modified with polyethylene glycol (PEG) of different sizes (M-w = 550, 2000, 5000, and 7000) and grafting densities (5-16 PEGs per SWNT) were simulated using coarse-grained force fields. The covalently grafted PEGs are evenly distributed on SWNTs, while the noncovalently PEGylated SWNTs show the random distribution of PEGylated lipids adsorbed to the SWNT, in which the SWNT sidewall is less completely wrapped by PEGs and thus largely exposed to water, as was observed in experiments. For covalently PEGylated SWNTs, longer PEG chains with higher grafting density yield a larger size, more isotropic shape, and lower diffusivity of the SWNT-PEG complex. In particular, at low grafting density, the thickness of the PEG layer on SWNTs nearly equals the size of the mushroom, where the end-to-end distance of PEGs is smaller than the distance between the grafting points, similar to the conformation of an isolated chain in water. However, at high grafting density, the thickness of the PEG layer increases beyond the mushroom regime, indicating a mushroom-to-brush transition, in agreement with the Alexander-de Gennes theory. These findings indicate that the PEGylation method influences the distribution of PEG chains on SWNTs and that the PEG size and grafting density modulate the conformation of the conjugated PEG chains, which helps explain the experimentally proposed transition of the PEG conformation between mushroom and brush.