Electron spin resonance (ESR) spectra of amphiphilic spin probes in the triblock copolymer EO13PO30EO13 (Pluronic L64): hydration, dynamics and orderin the polymer aggregates

Aqueous solutions of the triblock copolymer poly(ethylene oxide)-b-poly(pro pylene oxide)-b-poly(ethylene oxide) EO13PO30EO13 (Pluronic L64) were inves tigated over a wide concentration range (20-100%, (w/w) polymer) in the mic ellar, liquid crystalline and reverse micellar phases, using electron spin resonance (ESR) spectroscopy of spin probes. A series of amphiphilic nitrox ide spin probes based on n-doxyl-stearic acid (nDSA) with n, the carbon ato m to which the doxyl group is attached, equal to 5 and 10 were used to meas ure the local polarity, dynamics and degree of order in the self-assembled system. The N-14 isotropic hyperfine splitting, alpha(N), was the polarity sensitive parameter. The probe location and the corresponding effective loc al hydration, Z(eff), were deduced by comparing ESR spectra of the probes i n L64 solutions with spectra of the probes in aqueous solutions of poly(eth ylene oxide) (PEO), poly(propylene oxide) (PPO) and a mixture of PEO and PP O containing the monomer molar ratio 26:30, as in L64. The results indicate that the probes reside in and provide evidence for the presence of hydroph obic and non-hydrated regions consisting of PO blocks and that the order in the aggregates decreases from the PO/EO interface toward the PO domains. A dditional support for these conclusions was obtained from ESR spectra of th e probes in the lamellar phase as a function of added cholesterol and by si mulations of the ESR spectra of the probes in the lamellar phase of L64. Th is study of the hydrophobic part of the aggregates, together with our previ ous study based on ESR spectra of cationic probes that reside in the polar and hydrated EO regions, lead to a detailed description of the nature of L6 4 aggregates in aqueous solutions. (C) 2000 Elsevier Science Ltd. All right s reserved.