MICROPOLARITY AND ORDER IN THE REVERSE MICELLES OF L62 AND L64 PLURONIC COPOLYMERS, AS STUDIED BY MOLECULAR PROBE TECHNIQUES

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/w ater) and binary (copolymer/water) systems with different water conten ts were evidenced and investigated by fluorescence, absorption, and sp in probe techniques. The spectral parameters of the polarity sensitive probes, 1-anilinonaphthalene-8-sulfonic acid (ANS), dansylhexadecylam ine (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 polarit y by comparison with a series of poly(oxyethylene)/water (TEG/water) c alibration mixtures. The data were concordant and complementary, resul ting 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 similar to ANS > CA T 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 e thanol/water mixtures and Kosower's Z parameter (Z = -29.0R(A) + 177.8 3). Thus, by means of the calibration mixtures, converting different s pectral parameters of the mentioned probes, the local polarity values were expressed in terms of Z values. The role of water as a prerequisi te 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 o f water is observed in the ternary systems as compared to the binary o nes, pointing to the role played by the solvent in this process. Evide nce 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 ord ering of the polymer chains in the micelles, measured with a series of x-doxylstearic acid spin probes, indicates the progress of micellar o rganization with increasing water content, the order increasing and ex tending further from the polar core and the corona becoming less penet rable. In the corresponding binary systems, the polymer chain rotation is ''frozen'' at room temperature (295 K) and even at 320 K. The stru ctural characteristics of L62 and L64 ternary systems are rather simil ar, a conclusion suggested especially by the information the polar pro bes supplied. There are differences in the microenvironment of more hy drophobic probes, which sense a higher hydrophobicity in the case of L 62, the increase of microviscosity being also more marked in the L62 s ystem.