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Investigations on the effect of reactive surfactants on latex film microstructure and properties via tracer diffusion

Final Report Summary - LATEX FILM FORMATION (Investigations on the effect of reactive surfactants on latex film microstructure and properties via tracer diffusion.)

The formation of polymer films from aqueous dispersions of latex particles has become the major industrial process for a variety of applications like coatings, paints, adhesives etc. as it minimizes the emission of volatile organic compounds (VOCs) in comparison to film formation from polymer solutions. In this process surfactants play an ambiguous role. They are required to impart colloidal stability which is a prerequisite for processing dispersions. At the same time surfactants introduce performance problems in the final coating as they have a tendency to form surfactant clusters in drying latex films which migrate to the polymer-air and polymer-substrate interfaces, causing water resistance and adhesion problems. A strategy to improve upon this situation is the use of reactive surfactants (or “surfmers”) which become chemically attached to the latex particles during synthesis. Thus, surfactant migration is avoided while the stabilization property is still active. In addition, by a judicious choice of surfmers of suitable structure and reactivity, a homogenous distribution of the surfmer in the polymer film should be achievable, opening up the option of introducing some additional functionality (e.g. imparting increased mechanical stability) into coatings via the reactive surfmer. In the past years several reactive surfactants have been developed and some have been tested with respect to coating production and performance. However, the available studies essentially focus on the macroscopic behaviour of latex films. Studies which monitor the influence of surfmers on film formation and film performance on a molecular level are, however, rare. For this reason this project aimed at correlating the role of surfmers on the macroscopic performance of the coating (e.g. mechanical stability, water resistance) with their influence on the drying behaviour at molecular level as observed by a tracer diffusion technique. From the changes of the tracer mobility conclusions should be drawn on the physical mechanism underlying the macroscopically observed surfmer effects. For this purpose a surfmer Na-APDBS (cf. Scheme I) was specifically synthesized and characterized which is structurally analogous to the surfactant sodium dodecyl sulphate (SDS) which is commonly used in industrial latex formulations.

The performance of this surfmer in latex synthesis via emulsion polymerization was found to be comparable to SDS with respect to particle size, polydispersity and stability of the dispersions. Polystyrene (PS) latexes were prepared with surfmer and the microstructure of the obtained PS-surfmer copolymers was then studied with static light scattering in the nonpolar solvent toluene. PS was used as this polymer is very well studied and results could be easily compared with literature. Toluene was chosen as it can be expected that structure formation in this nonpolar solvent will provide insights in structures present in the final latex films after drying. We found a supramolecular structure formation which originates from a combination of aggregation to inverse micelles with a concentration dependent association of these aggregates to long filamentous microstructures.