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Latex particles containing equal amounts of poly (methyl methacrylate) (PMMA) and polystyrene (PS) were prepared by polymerizing styrene in the presence of PMMA seed particles using two different initiators, potassium persulfate (KPS) and tert-butyl hydroperoxide (t-BHP). Styrene was added either before polymerization (batch) or continuously during the polymerization. Particles prepared using batch addition of styrene showed no signs of a core-shell morphology, and the surface concentration of PS was lower than 50%. Particles with well-defined core-shell structures were obtained when styrene was added at a low feed rate. With t-BHP as initiator, the shell layer was compact and distinct and the surface concentration of PS high, whereas with KPS as initiator the shell layer was thicker and contained domains of PMMA. Composite particles prepared using a low styrene feed rate, but seed particles containing a chain-transfer agent, had PS domains distributed throughout the entire particle volume. Hence, the formation of particles with a core-shell structure was due to the suppression of radical transport to the interior of the seed particles, mainly because of the high internal viscosity of the latter. The structure and composition of the shell layer, however, depends on the chemical nature of the initiator.

The morphology and viscoelastic properties of films prepared from bimodal latex blends containing equal weight fractions of soft and hard latex particles were investigated as a function of the particle sizes and the particle size ratio (soft particle diameter/hard particle diameter). Minimum film formation temperature (MFT) measurements were associated with transmission electron microscopy (TEM) to emphasize the particle size ratio dependence of the film formation properties. A significant increase of the MFT values with the soft/hard particle size ratio was observed. As long as the particle size ratio was low, TEM micrographs showed that the film forming soft particles, undergoing complete coalescence, clearly act as the continuous phase where the non film forming hard particles are found evenly dispersed while keeping their initial spherical shape. At higher values of the particle size ratio, TEM micrographs pointed out that the soft particles are prevented from coming into contact with each other by the surrounding hard particles, therefore dramatically increasing the MFT of the sample and resulting in a non film forming latex blend. The existence of a critical volume fraction of hard particles that is directly related to the soft/hard particle size ratio was then established on the basis of geometrical arguments involving the percolation theory. The higher the particle size ratio, the lower the critical volume fraction of hard particles that leads to a macroscopic phase inversion resulting in a non film forming bimodal latex blend. Subsequently, the mechanical film properties were investigated by solid-state dynamic mechanical analysis. The size of the dispersed hard phase was found to affect the final viscoelastic film properties. The smaller the size of the hard particles, the better the mechanical enhancement of the mechanical film properties. Last, the experimental viscoelastic thermograms were compared with some theoretical predictions based on self-consistent mechanical modeling. The final film properties of the bimodal hard/soft latex blends were then directly connected to the film formation properties.

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