H NMR Relaxation Studies of the Micellization of a Poly(ethylene oxide)−Poly(propylene oxide)−Poly(ethylene oxide) Triblock Copolymer in Aqueous Solution

Abstract

1H 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. 1H 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. (Macromolecules 1994, 27, 2414).

Conformational Structure of Triblock Copolymers by FT-Raman and FTIR Spectroscopy

“…This is related to the fact that the block copolymers have more order structure at higher temperatures. The result is in agreement with that observed by NMR (11).…”

Melt, Hydration, and Micellization of the PEO–PPO–PEO Block Copolymer Studied by FTIR Spectroscopy

“…Since the C-O-C stretching band of the pure sample is at 1104 cm −1 , the frequency shift toward a higher wavenumber may be related to the dehydration of the ether backbone. It is known that the hydrophobic PPO blocks apparently reduce contact with water during the micellization process (1)(2)(3)(4)(5)(6)(7)(8). Another explanation of the frequency changes with temperature are associated with conformational changes of the ether backbone of PPO blocks from more polar, gauche conformation of unimers to less polar, trans conformation of micelles during the micellization process (15-18).…”

Synthesis and characterization of a polyrotaxane consisting of β-cyclodextrins and a poly(ethylene glycol)-poly(propylene glycol) triblock copolymer

“…However, no broadening of the signal was observed for 8. Generally, the shift in 1 H NMR signals is recognized not only as the micellization of 1 45) but also as the complexation of CyDs with guest molecules 46,47) . In addition, the broadening peak indicates that many CyDs are threaded onto the alkyl chains 36,48) .…”