Block copolymer micelle formation was studied by a combination of fluorescent probe and quasi-elastic light scattering (QELS) techniques. The polymers, polystyrene-poly(ethy1ene oxide) diblock and triblock copolymers, with M. values ranging from 8500 to 29 000, form spherical micelles in water over the entire concentration range over which QELS signals can be detected. Pyrene (Py) in water (6 X IO-' M) partitions between the aqueous and micellar phases, accompanied by three changes in the pyrene spectroscopy. There is a red shift in the excitation spectrum, a change in the vibrational fine structure of Py fluorescence (11113 decreases from 1.9 to 1.2), and an increase in the fluorescence decay time (from 200 to ca. 350 ns) accompanying transfer of Py from an aqueous to a hydrophobic micellar environment. From these data, critical micelle concentrations (range: 1-5 mg/L) and partition coefficients (3 x lo6) can be calculated.Block copolymers of polystyrene (PSI and poly(ethy1ene oxide) (PEO) form spherical micelles in water when the length of soluble PEO is significantly longer than that of the insoluble PS portion of the m o l e~u l e .~~~ This behavior is common to both PS-PEO diblock and PEO-PS-PEO triblock copolymers. In analogy with low molecular weight surfactants, one defines the onset of intermolecular association as the critical micelle concentration (cmc), and the theories of polymer mi~ellization~ predict that in the presence of micelles, the concentration of free, unassociated block copolymers is close in magnitude to that of the cmc.There are relatively few studies devoted to determination of cmc values for block copolymer micelles. Scattering techniques, which are very powerful for determining the size and shape of the micelles, are able to detect the onset of association only if the cmc occurs in a concentration region where these techniques are sensitive. For block copolymers in water, this is often not the case. For the examples considered here, the cmc values lie well below the smallest concentrations detectable by either Rayleigh or quasi-elastic light scattering (QELS).Fluorescence techniques have been used with great success in the study of low molecular weight surfactant micelles.6 They are useful not only for cmc determination but also for measuring the aggregation number of the micelles. By comparison, aqueous block copolymer systems have received scant attention7 Several years ago Ikema et al. reported very interesting results using l-anilinonaphthalene fluorescence to probe micelle formation in a water-soluble block copolymer.'* Because the change in the fluorescence signal they observed occurred in a concentration region too small for a corresponding change to be observed by light scattering, the authors chose not to interpret this signal as an indication of the onset of polymer association. This absence of clear-cut results seems to have discouraged others from applying these methods. It isonlyrecently that there has been an outburst 0024-9297/91/2224-1033$02.50/0 of activity in the stu...

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