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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Poly(butyl methacrylate) (PBMA) latex labeled with either phenanthrene or anthracene derivatives in low concentrations (1 or 2 mol %) was prepared by semicontinuous emulsion polymerization.The latex film formation process was studied by analysis of the nonradiative energy transfer from phenanthrene (donor) to anthracene (acceptor). Initially, when the latex film dries, there is little energy transfer, indicating that the particles conserve their individual identity. With increasing time at temperatures above the glass transition temperature of PBMA, the extent of energy transfer increases. This is a clear indication that interdiffusion of polymer chains across particle boundaries occurs. The diffusion coefficient D of the polymer was evaluated by using models based upon Pick's second law. In each sample, the magnitude of D decreases with time. Initial D values increase with increasing temperature, taking values between 1 X 10"14 to 1 X 10"18 cm2/s, yielding an apparent activation energy of 38 kcal/mol. The diffusion data also fit well to the Williams-Landel-Ferry equation. The results demonstrate the great potential of fluorescence techniques in the study of polymer diffusion processes during latex film formation.

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Poly(methyl methacrylate)-polystyrene (PS-PMMA) core-shell latex particles were prepared by twostage emulsion polymerization under monomer-starved conditions. These particles were examined by three different transmission electron microscopy techniques: cryosectioning, beam damage in ice, and freeze-fracture (FFTEM). This is the first reported application of FFTEM to the study of composite latex morphology. From these experiments we learn that the second stage polymer forms a shell surrounding the seed. In addition, we infer that some second stage monomer polymerizes within the latex seed. A very interesting observation is that when MMA is polymerized in the presence of PS seed latex, some polystyrene migrates to the surface of the PMMA shell. These results are explained in terms of a combination of thermodynamic and kinetic factors affecting the polymerization and morphology evolution process.Latex materials in which a core of one polymer is surrounded by a shell of a second polymer1-6 are important in many different industrial applications. Core-shell materials are employed as impact modifiers for plastics, in adhesives, and for a variety of coatings applications. It is widely believed that the properties that the latex contribute to these systems are intimately related to the detailed morphology of the latex particles themselves. One of the current challenges in the field of polymeric microspheres is to learn how to characterize these structures more completely so that one can develop better synthetic methods to prepare particles with controlled morphology.