HYDRATION, DYNAMICS, AND TRANSPORT ACROSS THE PHASE-DIAGRAM OF AQUEOUS POLY(ETHYLENE OXIDE)-B-POLY(PROPYLENE OXIDE)-B-POLY(ETHYLENE OXIDE) (PLURONIC L64) BY SPIN-PROBE ELECTRON-SPIN-RESONANCE AND ELECTRON-SPIN-RESONANCE IMAGING

The aggregation of the triblock copolymer poly(ethylene oxide)-b-poly( propylene oxide)-b-poly(ethylene oxide) (EO(13)PO(30)EO(13); commercia l name Pluronic L64) in aqueous solutions and the hydration of the PEO blocks were deduced from ESR spectra of the nitroxide spin probe perd euterio-2,2',6,6'-tetramethylpiperidone N-oxide in the temperature ran ge 293-330 K. The isotropic N-14 hyperfine splitting of the nitroxide, alpha(N), is a key parameter that reflects the local polarity, and it s variation with temperature and polymer concentration is an indicator of aggregation. The effective hydration of the polymer chains was det ermined by comparison of alpha(N) values of the probe across the phase diagram with ''calibration'' curves of alpha(N) in pure PEG, pure PPO , and mixtures of the homopolymers, measured as a function of the wate r content Z(w), where Z(w) is the number of water molecules per monome r unit. The calibration curves indicated that the spin probe is prefer entially located in the vicinity of the EO segments and reports on the local polarity and hydration of these segments. The effective degree of hydration, Z(w,eff), determined from the calibration curves and the alpha(N) values of the probes in the L64 aqueous solutions, decreases with increase in L64 content and is significantly lower than the aver age value deduced on the basis of the water content. The dramatic drop in Z(w,eff) at 300 K, from Z(w,eff) approximate to 23 in 10% (w/w) L6 4 to Z(w,eff) approximate to 7 in 20% (w/w) L64, was taken as an indic ator of micellization. The dependence of alpha(N) and tau(c), the rota tional correlation time of the probe, on temperature and L64 content a re additional parameters that describe the local polarity and the mice llization process. A superposition of two ESR signals is suggested by the line shapes detected at high polymer contents (greater than or equ al to 90% (w/w) L64); we proposed that in these conditions the driving force for separation of the EO and PO blocks is weak, and the probe i s incapable of selecting a preferred site. The translational diffusion of the spin probe was measured by ESR imaging in the L64 solutions co ntaining 10-100% (w/w) polymer. The diffusion coefficients D of the sp in probe decrease with increase in the polymer content, but the decrea se is more prominent for L64 contents in the range 10-30% (w/w). At a polymer content of 90% (w/w), D is similar to that of the polymer chai ns (determined by field gradient NMR). We suggested that water provide s an important pathway for the translational diffusion of the probe.