Interfacial films of poly(ethylene oxide)-poly(butylene oxide) block copolymers characterized by disjoining pressure measurements, in situ ellipsometry, and surface tension measurements

Interactions between two air-water interfaces stabilized by poly(ethylene oxide)-poly(butylene oxide) block copolymers were investigated by employing the microinterferometric thin-film balance technique. A series of three block copolymers, two linear diblock copolymers, B8E41 and E106B16, and one linear triblock copolymer, E21B8E21, were investigated. In particular the range of the steric forces operating across the thin foam films was quantified. As expected, higher concentrations, were required of the triblock copolymer than for the diblocks for the formation of a stable foam film. Despite the low polydispersity of the sample (M-w/M-n = 1.03), effects due to polydispersity on the concentration dependence of the disjoining pressure isotherm were noted. Furthermore, the adsorption at the air-water interface was determined by surface tension measurements. It was noted that long equilibrium times are required to obtain (quasi) equilibrium surface tension data. Time-resolved ellipsometry was used to follow the adsorption rates and determine the amounts adsorbed in situ at hydrophobic solid surfaces. The adsorbed amount at the solid-liquid interface determined by ellipsometry was similar to that at the air-water interface estimated from the surface tension isotherm provided polydispersity effects on the surface tension isotherm were taken into account. Moreover, interactions in foam films formed from mixtures of the diblock copolymer, B8E41, and a cationic surfactant, hexadecyltrimethylammonium bromide, were studied. Both electrostatic double-layer forces and, at shorter range, steric forces were present, demonstrating the formation of a mixed layer at the interface.