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

Abstract

The reverse micelles of triblock copolymers poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) EO6PO36EO6 (Pluronic L62) and EO13PO30EO13 (Pluronic L64) in ternary (copolymer/o-xylene/water) and binary (copolymer/water) systems with different water contents were evidenced and investigated by fluorescence, absorption, and spin probe techniques. The spectral parameters of the polarity sensitive probes, 1-anilinonaphthalene-8-sulfonic acid (ANS), dansylhexadecylamine (Dansyl), pyrenesulfonic acid (PSA), 4-nitropyridine N-oxide (NP) and 4-(N,N‘-dimethyl-N-alkyl)ammonium 2,2‘,6,6‘-tetramethylpiperidine-1-oxyl iodide (CAT n), were related to the local hydration and polarity by comparison with a series of poly(oxyethylene)/water (TEG/water) calibration mixtures. The data were concordant and complementary, resulting in polarity profiles of the core for all systems and the relative radial positioning of the probes. The order of decreasing hydrophobic character found was Dansyl > NP > CAT 16 > CAT 11 ∼ ANS > CAT 8 > PSA > CAT 4 > CAT 1. A linear relationship was found between the RA (the intensity ratio of two absorption vibronic bands) of PSA in ethanol/water mixtures and Kosower's Z parameter (Z = −29.0RA + 177.83). Thus, by means of the calibration mixtures, converting different spectral parameters of the mentioned probes, the local polarity values were expressed in terms of Z values. The role of water as a prerequisite for micellization was evidenced and so was the minimum quantity of water required in the ternary systems (W = 0.2 for L64 and W = 0.4 for L62). Regarding the water distribution, a more advanced segregation of water is observed in the ternary systems as compared to the binary ones, pointing to the role played by the solvent in this process. Evidence was found for a considerable solvation of the poly(propylene oxide) block with xylene, leading to a looser packing in the corona and in the poly(ethylene oxide) core. In the case of ternary systems, the ordering of the polymer chains in the micelles, measured with a series of x-doxylstearic acid spin probes, indicates the progress of micellar organization with increasing water content, the order increasing and extending further from the polar core and the corona becoming less penetrable. In the corresponding binary systems, the polymer chain rotation is “frozen” at room temperature (295 K) and even at 320 K. The structural characteristics of L62 and L64 ternary systems are rather similar, a conclusion suggested especially by the information the polar probes supplied. There are differences in the microenvironment of more hydrophobic probes, which sense a higher hydrophobicity in the case of L62, the increase of microviscosity being also more marked in the L62 system.