Rheological and microcalorimetric studies of a model alkali-soluble associative polymer (HASE) in nonionic surfactant solutions

Rheological experiments were carried out on a 1 wt % hydrophobically modifi ed alkali-soluble emulsion (HASE) solutions at pH similar to 9 in the prese nce of nonionic polyoxyethylene ether type surfactant (C12EO23). The low sh ear viscosity and dynamic moduli increases at c > cmc until they reach a ma ximum at a critical concentration, c(m) of approximately 1 mM (similar to 1 7 times the cme of free surfactant) and then decrease. The dominant mechani sm at cmc < c < c(m) is an increase in the number of intermolecular hydroph obic junctions and a strengthening of the overall associative network struc ture. Above c(m), the disruption of the associative network causes a reduct ion in the number of junctions and strength of the overall network structur e. The influence of C12EO23 on HASE before cmc could not be detected macros copically by the rheological technique. However, isothermal titration calor imetry enables the determination of complex binding of surfactant to the po lymer. Isothermal titration of C12EO23 into 0.1 wt % HASE indicates that th e C12EO23 aggregation in water and 0.1 wt % HASE polymer solutions is entro pically driven. A reduction in the critical aggregation concentration (cac) confirms the existence of polymer-surfactant interactions. The hydrophobic micellar junctions cause a decrease in the Delta H and Delta S of aggregat ion of the nonionic surfactant. (C) 2000 John Wiley & Sons, Inc.