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

Abstract

The aggregation of the triblock copolymer poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) (EO13PO30EO13; commercial name Pluronic L64) in aqueous solutions and the hydration of the PEO blocks were deduced from ESR spectra of the nitroxide spin probe perdeuterio-2,2‘,6,6‘-tetramethylpiperidone N-oxide in the temperature range 293−330 K. The isotropic 14N hyperfine splitting of the nitroxide, aN, is a key parameter that reflects the local polarity, and its variation with temperature and polymer concentration is an indicator of aggregation. The effective hydration of the polymer chains was determined by comparison of aN values of the probe across the phase diagram with “calibration” curves of aN in pure PEO, pure PPO, and mixtures of the homopolymers, measured as a function of the water content Zw, where Zw is the number of water molecules per monomer unit. The calibration curves indicated that the spin probe is preferentially located in the vicinity of the EO segments and reports on the local polarity and hydration of these segments. The effective degree of hydration, Zw,eff, determined from the calibration curves and the aN values of the probes in the L64 aqueous solutions, decreases with increase in L64 content and is significantly lower than the average value deduced on the basis of the water content. The dramatic drop in Zw,eff at 300 K, from Zw,eff ≈ 23 in 10% (w/w) L64 to Zw,eff ≈ 7 in 20% (w/w) L64, was taken as an indicator of micellization. The dependence of aN and τc, the rotational correlation time of the probe, on temperature and L64 content are additional parameters that describe the local polarity and the micellization process. A superposition of two ESR signals is suggested by the line shapes detected at high polymer contents (≥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 is incapable of selecting a preferred site. The translational diffusion of the spin probe was measured by ESR imaging in the L64 solutions containing 10−100% (w/w) polymer. The diffusion coefficients D of the spin probe decrease with increase in the polymer content, but the decrease 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 chains (determined by field gradient NMR). We suggested that water provides an important pathway for the translational diffusion of the probe.