H-1 NMR relaxation studies of the micellization of a poly(ethylene oxide) poly(propylene oxide) poly(ethylene oxide) triblock copolymer in aqueous solution

H-1 nuclear magnetic resonance (NMR) relaxation studies of the temperature-induced micellization of a poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide), PEO-PPO-PEO, block copolymer in aqueous solution were performed in the range 280-345 K. H-1 NMR spectra of the block copolymer were well resolved, thus allowing us to probe specifically the methyl and methylene relaxation processes in the PPO and PEO blocks, respectively. Interpretation of the relaxation data in terms of the Hall-Helfand correlation function leads to four distinct correlation times for the PPO and PEO blocks. The slower correlation time in the PPO block was identified as the Zimm-Rouse first normal mode of the copolymer and served to determine the hydrodynamic radius, R(H), of the unimers and the micelles. On a more local scale, the behavior of the correlation time for segmental motions in the PPO block indicates an extension of the PPO chains in micelles relative to the unimers. This conformational change is related to the formation of a water insoluble liquid-like core created by the PPO chains in the micelle where the trans isomers are favored. The rotational isomeric states model used to interpret the faster correlation time in the PEO chains yields an activation energy of 14.6 kj/mol for the correlated transitions in the PEO blocks, in agreement with previous theoretical calculations. The slower correlation time in the PEO blocks shows a marked increase upon micellization attributed in part to the polymer-polymer interactions between the different PEO blocks constituting the hydrophilic moiety of the micelles. A power law relating this relaxation time to the micelle hydrodynamic radius is predicted and observed experimentally. The concentration dependence of the critical micellization temperature, inferred from the methyl spin-lattice relaxation time in the PPO block, was found to be adequately described by a closed association model. The standard free energy, enthalpy, and entropy of micellization obtained by NMR are in close agreement with the recent experimental thermodynamic studies of P. Alexandridis et al.