Polymer-surfactant interaction in thermoreversible gels

The influence of low molecular weight surfactants on the sol/gel transition of hydrophobically modified water-soluble associating polymers is studied on the basis of the recent theory of thermoreversible gelation with multiple junctions. It is shown that the existence of a lower and an upper bound in the aggregation number of hydrophobes at the network junctions leads to non-monotonic polymer gelation concentration as a function of the surfactant concentration. There is a certain surfactant concentration where gelation is most promoted, thereby exhibiting a peak in the viscosity and modulus. Relation between this peak concentration and the critical micelle concentration of the added surfactant is detailed. The number of elastically active network chains is calculated in the post-gel regime as a function of the surfactant concentration, and compared with the experimental data on the high frequency storage modulus of HEUR/SDS systems.