Vertical free-standing films of amphiphilic associating polyelectrolytes

Hydrophobically modified, poly(acrylic acid) sodium salt (HMPAANa) copolyme rs are known to provide a huge stabilization of oil in water macroemulsions . An interstitial HMPAANa film is formed between the oil droplets, thus cre ating: repulsion between them. We present an x-ray reflectivity study of ve rtical free-standing films drawn from aqueous solutions of HMPAANa copolyme rs. The vertical HMPAANa films are model systems for the interstitial films between oil droplets and the description of their behavior provides inform ation about the stabilization process. Their thickness was investigated as a function of various parameters such as the solution concentration, the de gree of grafting, the length of the grafts, and the backbone molecular weig ht. Below a solution concentration: threshold (C-t), the film thickness sca les like the square root of the molecular weight and is independent of the degree of grafting and the length of the grafts. Polyelectrolyte chains ado pt a self-screened coil conformation within the films and the thickness is governed by the radius of gyration of the coils. Above C-t, a transition fr om a bimolecular film to a physical gel is observed and the thickness then increases with concentration. Finally, we propose an explanation for the st abilization of macroemulsions by HMPAANa copolymers. [S1063-651X(99)10308-8 ].