Adhesion of latex films. Part IV. Dominating interfacial effect of the surfactant

Our aim was to study the effect of various surfactants on the adhesion properties of latex films. For this purpose, a model monodisperse, surfactant-free, core-shell latex was synthesized. The core was a random copolymer of styrene and butyl acrylate. The shell consisted of a styrene/butyl acrylate/methacrylic acid terpolymer with 10 wt% of methacrylic acid. Sodium dodecyl sulfate (SDS) and various ethoxylated nonionic surfactants were post-added to the latex at increasing concentrations. The adhesion of the corresponding films was measured by peeling at 180°. The shapes of the curves relating the peel energy to the surfactant concentration were similar, irrespective of the nature of the surfactant, namely a decrease to a constant plateau level. This means that in these systems, the influence of a weak boundary layer at the interface dominates all other possible effects of the surfactant. This was confirmed by surface analysis and peel experiments with surfactant-free films contacted with spin-coated SDS layers of increasing thickness. The reasons why the deleterious interfacial effect of the surfactant overwhelms all others are discussed.