TRANSITION FROM ELECTROSTATIC TO STERIC STABILIZATION IN FOAM FILMS FROM ABA TRIBLOCK COPOLYMERS OF POLY(ETHYLENE OXIDE) AND POLY(PROPYLENEOXIDE)

The equilibrium thickness of microscopic foam films obtained from aque ous solutions of two different poly(ethylene oxide)/poly(propylene oxi de)/poly(ethylene oxide) triblock copolymers (Synperonic PE P85 and F1 08) is experimentally determined. The amphiphilic triblock copolymers adsorb at the air/solution interface as a brush layer. The foam film m ay be stabilized by electrostatic and/or steric repulsion. At a given copolymer concentration (slightly lower than the CMC) increasing the e lectrolyte concentration induces a transition from electrostatic to st eric stabilization. The thicker foam films are electrostatically stabi lized. The thinnest foam films are sterically stabilized, but their th ickness is unexpectedly larger than twice the brush layer thickness. W hile the thickness h of these films strongly depends on the degree of polymerization N, its scaling structure is essentially the same: two b rush layers (h(1) similar to N) and an aqueous core (h(2) similar to N -3/5). The microscopic foam film is a suitable tool for exploring the transition from electrostatic to steric stabilization.