Fluorescence Decay Methods for the Characterization of Latex Film Formation

By J.M.G. Martlnho , J.P.S. Farinha

Fluorescence decay methods have been used extensively to characterize the formation of polymer films from latex dispersions, a process widely used in the coatings industry. By measuring the polymer chain diffusion at the nanometer scale, this technique enables the quanti­fication of the amount of mixing between polymer particles that ultimately contrib­utes to the formation of a polymer coating with good mechanical properties. This article provides an introduction to fluo­rescence decay methods, with a strong emphasis on their application to the characterization of latex film formation. Examples are a/so provided, demonstrat­ing how this technique has been used to help in the understanding of this process in different latex coatings.

The different stages involved in the formation of a film from a water disper­sion of polymer nanoparticles, or latex, are schematically shown in Figure 1.14 After water evaporation and drying, the polymer particles form a densely close ­packed arrangement. At temperatures above the “minimum film formation temperature” (MFFT, which is close to the glass transition temperature of the polymer in the presence of water), a transparent film is formed by deformation of the particles, through a combination of capillary, osmotic, and surface forces that overcome the elastic modulus of the particles.1 To form a mechanically strong film, the polymer chains have to further diffuse across the interfaces between ad­jacent particles, in a process driven by the increase in entropy associated with the healing of these interfaces. This last stage is of utmost importance to the final proper­ties of the film. Therefore, to modify coat­ing formulations and design more efficient coatings, it is necessary to understand the characteristics of polymer diffusion for the particular formulation of interest.