'Static' and steady-state foams from ABA triblock copolymers: influence of the type of foam films

The role of foam films in stability of foams formed from solutions of F108 and P85 (polyethylene oxide-polypropylene oxide-polyethylene oxide(PEO-PPO PEO)) ABA block copolymers is analyzed. The Pi (h) isotherms of disjoining pressure are measured for single foam films, under conditions close to those in the foam to clarify the influence of the type of foam films, common thin; black and bilayer sterically stabilized films on foam stability. The experiments with single foam films are performed with the thin liquid film-pressure balance technique (TLF-PBT) developed on the basis of the microinterferometric method of Sheludko-Exerowa. Foam pressure drop technique (FPDT) is applied to determine foam lifetime at constant pressure. This technique involves applying an increased and regulated pressure in the foam liquid phase (plateau borders) which makes it possible to evaluate the effect of foam film type on foam stability under strictly defined capillary pressure values. A combined pneumatic-mechanical method is used to study foam properties under dynamic conditions. Effect of the polymer type and its concentration, velocities of the mixer rotations and gas flow on the foam volume formed and foam stability are studied. The foam lifetime versus applied pressure (tau (p)/DeltaP) dependences along with Pi (h) isotherms indicate that the surface forces (electrostatic, van der Waals and steric), respectively, the type of foam films, play a decisive role in stability of foams from ABA block copolymers. Under identical conditions the copolymer F108 forms mure stable foams than those obtained from the P85 solutions. A parallelism between the properties of dynamic foams and single microscopic roam films is observed. When thicker single foam films are formed (from F108 solutions) the steady-state foam column is higher. (C) 2001 Elsevier Science B.V. All rights reserved.