Internal Structure of Core-Shell Latex Particles Studied by Fluorescence Nonradiative Energy Transfer Elı ´as Pe ´rez and Jacques Lang* Institut Charles Sadron (CRM-EAHP), CNRS-ULP Strasbourg, 6, rue Boussingault, 67083 Strasbourg Ce ´ dex, France Received October 25, 1995. In Final Form: March 18, 1996 X Latex particles have been synthesized in two-steps emulsion polymerizations under starving conditions, and the internal structure of these particles has been investigated by fluorescence nonradiative energy transfer (NRET), in order to check for the formation of core-shell particles. The polymers were based on methyl methacrylate (MMA) and butyl methacrylate (BMA) as monomers. The energy donor monomer was introduced during the first step of the polymerization, and the energy acceptor monomer during the second step. The T g of PBMA is too low, compared to the polymerization temperature Tp (Tp ) 80 °C), to observe a separation between the donor- and acceptor-labeled PBMA chains inside the particles. On the contrary the T g of PMMA is larger than Tp, and a separation between energy donor- and energy acceptor-labeled PMMA chains is observed with this polymer. The separation has the structure of a diffuse interface between the two labeled PMMA polymers. The study of other particles shows that the apparent fraction of mixing, f ′ A, between donor- and acceptor-labeled polymer chains inside the particle decreases, as expected, as the incompatibility and the Tg of the polymers increase. Addition of a cross-linking agent during the first step of the polymerization leads to a decrease of f ′A, even in the case of the PBMA latex particles. Annealing of latex films and dispersions at temperatures above the Tg of the polymers allowed phase separation or mixing inside latex particles to be observed. Introduction Core-shell latex particles are composite particles made of two different polymers; one theoretically composes the core and the other the shell of the particles. These particles are usually synthesized by an at least two-step emulsion polymerization. However, it is known that in practice core-shell particles are difficult to obtain. This happens for instance when the compatibility or the polymer glass transition temperature, T g , is favorable to a partial mixing of the two polymers or when there is no appreciable difference between the affinities of the two polymers for the water phase. Other parameters like the relative solubility of the monomers in the water phase and the relative interfacial tensions between the three phases, namely the two polymers and water, or swelling of the core by the monomer used in the second stage of the polymerization, can also affect their morphology. Thus, other structures than the expected core-shell are often found. Moreover, it is likely that a neat interface between the core and the shell rarely occurs. In most cases a composition gradient forms between the center and the periphery of the particle. In order to reduce penetration of one polymer phase into the other, cross-linking agents are sometimes used. Core-shell particles have many current and potential applications in the chemical, biological, and pharmaceuti- cal industries. They are for example used to confer to a material two kinds of properties, one being given by the core and the other by the shell of the particle, as for instance mechanical resistance and hydrophobicity. An- other example is that of shells carrying active groups in the field of biology. Therefore a great deal of studies have been done, using a variety of methods, in order to determine the shape and the internal structure of core- shell latex particles. Transmission electron microscopy (TEM) methods have been employed in the early seventies to study the morphology of polystyrene latex particles synthesized by a two-stage seeded emulsion polymerization. 1,2 A small quantity of butadiene was added to the styrene in the second stage of the polymerization. Thin cross sections of the particles embedded in a resin were obtained by ultramicrotomy, exposed to osmium tetroxide to stain the butadiene, and finally examined in an electron microscope. The core-shell structure of the particles was clearly visible. This morphology was confirmed in another experiment made on the same polystyrene latex particles, where tritiated styrene was used as a tagging agent in the seed and autoradiography as a detecting method. 2 Thus, the possibility that incompatibility between the polystyrene of the seed and the poly(styrene-co-butadiene) of the second stage could be at the origin of the morphology observed in the first experiment was discarded by the authors. They concluded that in the second stage of the polymerization the monomer did concentrate at the periphery of the swollen particles rather than swelling the particles uniformly. Transmission electron microscopy, in conjunction with the osmium tetroxide staining method, has been used for the morphological characterization of polystyrene particles embedded in a poly(isobutyl acrylate) phase. 3 Foamlike structures were observed where beads of polystyrene were surrounded by a film or a shell of poly(isobutyl acrylate). The structure of poly(vinyl acetate)-poly(butyl acrylate) latex particles has been investigated by other authors 4,5 using the staining method described in ref 3. It has been shown that the batch-polymerized particles have a relatively large butyl acrylate-rich core surrounded by a vinyl acetate-rich shell, whereas particles synthesized by semicontinuous polymerization under starving conditions have a more homogeneous composition. These structures were in agreement with dynamic mechanical spectros- * To whom correspondence should be addressed. X Abstract published in Advance ACS Abstracts, June 1, 1996. (1) Grancio, M. R.; Williams, D. J. J. Polym. Sci., Polym. Chem. Ed. 1970, 8, 2617. (2) Keusch, P.; Williams, D. J. J. Polym. Sci., Polym. Chem. Ed. 1973, 11, 143. (3) Kanig, G.; Neff, H. Colloid Polym. Sci. 1975, 253, 29. (4) Misra, S. C.; Pichot, C.; El-Aasser, M. S.; Vanderhoff, J. W. J. Polym. Sci., Polym. Lett. Ed. 1979, 17, 567. (5) Misra, S. C.; Pichot, C.; El-Aasser, M. S.; Vanderhoff, J. W. J. Polym. Sci., Polym. Chem. Ed. 1983, 21, 2383. 3180 Langmuir 1996, 12, 3180-3187 S0743-7463(95)00935-8 CCC: $12.00 © 1996 American Chemical Society