6533b82efe1ef96bd12944a7

RESEARCH PRODUCT

Scale dependent diffusion in latex films studied by photoinduced grating relaxation technique

subject

description

The transition from an aqueous dispersion of polyacrylate latices into a homogeneous polymer film on drying has been studied by monitoring the diffusion of hydrophobic and hydrophilic photochromic molecular probes with the help of a holographic grating relaxation (forced Rayleigh scattering) technique. Experiments with the hydrophobic probe in wet films result in an unusual spatial scale-dependence of the apparent diffusion coefficient that degenerates into a normal scale-independent diffusion coefficient as the film dries. Employment of a two-state diffusion model allows extracting the diffusion coefficients and mean displacements of the tracer in the polymer cores of the latex particles and in the hydrophilic interface meshes surrounding them. The cores were found to stay essentially intact, whereas the interfacial regions change dramatically with the residual water concentration. Its influence on diffusion disappears on drying but can be partially restored by remoistening the material. The interface can be efficiently controlled by varying the surfactant concentration. A water-soluble tracer was found to diffuse in four different environments. In addition to the already known polymer cores and the hydrophilic interfacial regions, we found a large contribution of fast diffusion in a water rich region and a very small contribution of ultraslow diffusion in a state supposedly related with aggregated surfactant.