Synthesis of PEG and Quaternary Ammonium Grafted Silicone Copolymers as Nanoemulsifiers

Amphiphilic silicone copolymers are an exciting class of biomedically relevant polymers that can be used for magnetic resonance imaging (MRI)-based cell labeling and oximetry studies. However, the solution characteristics and the ability to form stable nanoemulsions must be first demonstrated. Therefore, a series of amphiphilic siloxanes were successfully synthesized by grafting allylic poly(ethylene glycol) (PEG) with three (PEG(3)) or 11 (PEG(11)) repeat units or allylic triethylammonium bromide (QUAT) substituents onto polyhydromethylsiloxane backbones at three different PEG:ammonium molar ratios by using a one-pot reaction pathway. The PEG length and the PEG:QUAT molar ratio were varied to tune the hydrophilicity and surface tension, and the polymer structures were confirmed by using 1H NMR and FT-IR spectroscopy. The results show that the water contact angle increased upon attaching the PEG and QUAT groups, while the surface tension was most sensitive to the PEG(3) concentration. Also, the critical micelle concentration of the silicone graft copolymers decreased with an increase in the PEG content. Dynamic light scattering (DLS) and cryogenic transmission electron microscopy probed the solution structures and the ability to form nanoemulsions encapsulating polydimethylsiloxane (PDMS) oils. The graft copolymers containing PEG(3) showed consistent sizes by DLS, but the size distribution changed for the PEG(11) samples as the QUAT concentration increased. Finally, the graft copolymers successfully formed stable nanoemulsions containing PDMS with particle sizes that are appropriate for MRI-based cell labeling and oximetry applications.