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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