ASSOCIATION AND THERMAL GELATION IN MIXTURES OF HYDROPHOBICALLY-MODIFIED POLYELECTROLYTES AND NONIONIC SURFACTANTS

We studied the association between hydrophobically modified poly(sodium acrylate) and a series of oligoethylene glycol monododecyl ether surfactants (C-12 E(n) with n = 4, 5, and 8). The polymers contain a small fraction of long alkyl side groups (dodecyl or octadecyl). The alkyl groups of the modified polymers associate with the surfactant aggregates, which induces dramatic changes in the rheology of these systems. The viscosity of the mixture may be 4 orders of magnitude higher than that measured in mixtures of the nonmodified poly(sodium acrylate) and the surfactant. The systems containing C12E4 as surfactant exhibit a very interesting thermal behavior. Their viscosity increases and a physical gel is formed by increasing the temperature. This thermal behavior is interpreted as due to the transformation of the surfactant aggregates from micelles to vesicles with increasing temperature. We argue that at least two different types of gel structures can be recognized in solutions containing hydrophobically modified polymers and surfactant aggregates. In gels where the surfactant forms small micellar aggregates, mixed micelles, including the hydrophobic side chains of the polymer, act as cross-links of the polymer network. On the other hand, when the surfactant forms giant vesicle aggregates, the polymer adsorbed on the bilayer membrane can form bridges between adjacent vesicles. This leads to an effective cross-linking of the surfactant aggregates.