Block Copolymer Micelle Core T_g Can Be Determined by Corona NMR T₂ Relaxation Measurements

The conformational and dynamic properties of PEG corona chains in PS-PEG, PtBMA-PEG and PLGA-PEG block copolymer micelles were investigated by ¹H NMR spectroscopy. The PEG corona chains of PtBMA-PEG and PLGA-PEG micelles are fully hydrated. The PEG corona chains of PS-PEG micelles, on the other hand, are only partially hydrated; significant portions of the PEG segments are collapsed onto the surface of the PS core domain in order to minimize unfavorable contact between the PS and water. ¹H NMR spin-spin transverse (T₂) relaxation measurements suggest that the mobility of the collapsed PEG corona segments of PS-PEG micelles is about 5 – 8 times slower than that of the fully hydrated PEG segments, and is significantly influenced by the glass transition of the micelle core domain. The micelle core T_g values determined by PEG corona T₂ relaxation measurements for PS-PEG and PtBMA-PEG micelles agreed well with the T_g values obtained by measurements of fluorescence from FCVJ-loaded PS-PEG and PtBMA-PEG micelles. Therefore, this study establishes that corona T₂ measurements can be used to determine the micelle core T_g.