Hydrophobic Surface Coating of Nanodiamonds by Polyglycerol-Based Polymers with Alkyl Chains for Dispersing in an Organic Solvent

Since nanodiamonds (NDs) have been attracting growing interest in the field of nanomedicine, aqueously dispersed NDs have been numerously prepared by surface functionalization with hydrophilic groups. In contrast, hydrophobic NDs dispersible in organic solvents have been elusive in spite of the various potential applications such as tribology, lubrication and abrasion. Herein, NDs with a diameter of 100 nm (ND₁₀₀) are made to be dispersible in an organic solvent through the following two processes: 1) ring opening polymerization of 1,2-epoxyhexane giving NDs functionalized with poly(1,2-hexylene glycol) (ND-PHG) and 2) ring opening polymerization of glycidol followed by Williamson ether synthesis giving NDs functionalized with octyl and tetradecyl groups (ND-POPG and ND-PTPG, respectively). These functionalized NDs are characterized qualitatively by Fourier transform infrared spectroscopy (FTIR) and quantitatively by thermogravimetric analysis (TGA). All these NDs with hydrophobic polymers are dispersible in chloroform, while no dispersibility is observed in NDs without surface polymer coating. ND-POPG and ND-PTPG are found to show much higher dispersibility (∼5 mg/mL in chloroform) than ND-PHG (∼0.3 mg/mL). The dispersibility or hydrophobicity is correlated with the number of the alkyl chains, determined by TGA, as well as their length.