Film formation from concentrated reactive silicone emulsions. 2. Surfactant distribution

Drying mechanisms of concentrated, reactive poly(dimethylsiloxane) in, water emulsions, stabilized by a nonionic polyethoxylated fatty alcohol, were studied in part I of this series. Upon drying, these emulsions form cross-linked polymeric films. The present paper focuses on the fate of the surfactant during and after film formation, studied by attenuated total reflectance (ATR) and infrared microscopy. The effects of various parameters were investigated, namely thickness of the cast layer, surfactant concentration, presence of a CaCO3 filler, and aging of the film after drying. The concentration profile of the surfactant in the film shortly after drying presents a depleted layer on top, a flat vertical line in the bulk, and a strongly enriched layer in contact with the substrate. The vertical line corresponds in fact to an average over surfactant aggregates with a seemingly very high size polydispersity. Drying and coalescence mechanisms have a major influence on the surfactant distribution. In the dry film, the distribution is unstable and, upon aging, both film-air and film-substrate interfaces are progressively enriched with surfactant. More details on the structure of the film after drying are given in part 3 of this series.