Hydration and Dynamics in Reverse Micelles of the Triblock Copolymer EO₁₃PO₃₀EO₁₃ in Water/o-Xylene Mixtures:  A Spin Probe Study

Abstract

The reverse micellar phase L2 of the triblock copolymer poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) EO13PO30EO13 (commercial name Pluronic L64), where EO is ethylene oxide and PO is propylene oxide, was studied in the ternary mixture L64/water/o-xylene and in the binary mixture L64/water, using ESR spectroscopy with nitroxide spin probes. The spin probes differed in size, structure, and polarity and belong to two main types: (a) cationic probes 4-(N,N-dimethyl-N-alkyl)ammonio)-2,2‘,6,6‘-tetramethylpiperidine-1-oxyl iodide (CATn) with n, the number of carbon atoms in the alkyl substituent, equal to 1, 4, 8, 11, and 16; (b) amphiphilic probes based on x-doxylstearic acid (xDSA) with x, the carbon atom to which the doxyl group is attached, equal to 5, 7, 10, and 16. X-band ESR spectra reflect the intercalation of the probes in the self-assembled L64 system, but different probes choose different locations in the aggregates and report on the local polarity, hydration, and degree of order. The hydration gradient in the poly(ethylene oxide) (PEO) core of the reverse micelles was estimated on a scale of ≤48 Å from the analysis of a N, the isotropic hyperfine splitting of 14N, in the CATn series by comparison with a “calibration curve” based on CAT4 in aqueous solutions of PEO. The lower homologs in the series (n = 1 and 4) are located near the center of the hydrated core, while the higher homologs (n = 11 and 16) are close to the interface between the hydrated core and the o-xylene-swollen poly(propylene oxide) (PPO) blocks. The xDSA probes have their head group near the interface between the core and the PPO regions. The nitroxide group in 5DSA is at the core/PPO interface, but the nitroxide groups in the other doxyl probes are at different depths in the PPO regions, depending on the value of x. The dynamics of the CATn probes was elucidated by comparing the experimental B and C parameters in the expression for the line width in motionally averaged ESR spectra of nitroxides ΔH(m I) = A + Bm I + Cm I 2, with the corresponding values calculated as a function of the direction of the axis of rotation and N, the degree of anisotropy of the rotational reorientation. The comparison suggests that the dominant motional mechanism for all probes is rotation around the N−O bond (x axis), but the anisotropy of the lower spin probe homologs is significantly higher (N = 10) compared to that of the higher homologs (N = 2−3). These results are consistent with the existence of a hydration gradient in the core of the reverse micelles.

Micropolarity and Order in the Reverse Micelles of L62 and L64 Pluronic Copolymers, As Studied by Molecular Probe Techniques

“…In this case, the range of Z values is reduced to above half of the above one, i.e., from 70.3 (NP) to 77.8 kcal mol -1 (CAT 1). Regarding the similar systems of L64 (converting local hydration values 16 into Z), the range of Z values appears shifted toward higher polarities: in the ternary system with W ) 1, Z ) 69.2 (NP) and 87.4 kcal mol -1 (CAT 1), while in the binary system Z ) 73.1 (NP) and 80.4 kcal mol -1 (CAT 1).…”

“…A calibration of a N values as a function of the water content in PEO was accomplished by using the previously measured calibration curve of CAT 4 in TEG/water mixtures with different W values. 12,16 Parts a and b of Figure 10 show the ESR spectra of CAT n in the L62/o-xylene/water systems with W ) 0.4 and W ) 2, respectively. The polarity changes with W for L62 micellar solutions, as can be seen in Table 6.…”

Modification of the Microstructure in Block Copolymer−Water−“Oil” Systems by Varying the Copolymer Composition and the “Oil” Type: Small-Angle X-ray Scattering and Deuterium-NMR Investigation

“…The paper is organized in two parts. First, we present a detailed characterization of the ternary isothermal E 20 P 70 E 20 −butyl acetate−water and E 20 P 70 E 20 −butanol−water systems with respect to their phase behavior and microstructures, and then we compare the structural polymorphism afforded by E 20 P 70 E 20 (30% E) to that of E 100 P 70 E 100 (70% E). , The insight offered by this study and related studies on copolymer−water−oil systems − ,, will facilitate the tailoring of the microstructure in amphiphile-containing systems to meet the demand of diverse practical applications. − Amphiphilic block copolymers of the poloxamer poly(ethylene oxide)−poly(propylene oxide) type are particularly amenable to such “microstructural engineering” because of the variability in their molecular architecture (e.g., E/P composition, molecular weight, and block sequence). , …”

Shear-Induced Transformation of Polymer-Rich Lamellar Phases to Micron-Sized Vesicles

“…Besides, normal short-chain surfactants (e.g., C i E j or SDS) especially amphiphilic copolymers were in the focus of different studies during the past decades. − Owing to their amphiphilic nature, they adsorb at the interface and change the elastic properties of the amphiphilic film if they are used as cosurfactants. Even binary and ternary systems with only a tri- or diblockcopolymer have been investigated. − All these systems exhibit lyotropic mesophases like hexagonal, cubic, or lamellar, which are built from micelles or stacked bilayers . In this contribution, our main focus lays on the lamellar phase and its corresponding spherical structure, the multilamellar vesicle (MLV) or the so called “onion” state.…”