Internal Dynamics in Drying Latex Films: A Study Based on Fluorescence Anisotropy and Acoustic Dissipation

The internal dynamics of drying latex films was investigated with an instrument combining measurements of fluorescence depolarization and of acoustic dissipation. A soft latex film, containing a hydrophilic fluorescent probe, was deposited on the surface of a torsional quartz crystal resonator, thereby shifting frequency and bandwidth of the resonance. The depolarization of the fluorescence was recorded in parallel to the acquisition of the resonance parameters. As the film dried out, the fluorescence depolarization and the bandwidth of the resonator both changed in step-wise manner. The former effect is related to the arrest of rotation on the molecular scale, whereas the acoustic technique probes the viscous compliance on a macroscopic scale. Latexes containing various amounts of sodium dodecyl sulphate (SDS) were studied. For latexes containing little or no added surfactant, the transition from a soft to hard coating (as reflected in fluorescence anisotropy and acoustic dissipation) is rather abrupt. The decrease of rotational mobility and acoustic dissipation occurs at the same time. Adding excess surfactant to the latex decreases the initial softness of the wet film and makes the transition from soft to hard more gradual.

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