Colloid Polym Sci 275:760-768 (1997) 9 SteinkopffVerlag 1997 C. Howley D.G. Marangoni J.C.T. Kwak Association and phase behavior of hydrophobically modified photoresponsive poly(acrylamide)s in the presence of ionic surfactant Received: 23 September 1996 Accepted: 11 March 1997 C. Howley 9 Dr. J.C.T. Kwak ( ~ ) Department of Chemistry Dalhousie University Halifax, Nova Scotia, Canada B3H 4J3 D.G. Marangani Department of Chemistry Saint Francis Xavier University Antigonish, Nova Scotia, Canada B2G 2W5 Abstract The extent of association between the cationic surfactant TTAB and a series of hydrophobically modified polyacrylamides (HPAMs) containing an N-n-alkyl and substituted azobenzene hydrophobic sidegroup has been studied utilizing a cationic surfactant-selective membrane electrode. Binding of TTAB to the polymer hydrophobes is found to increase with increasing hydrophobicity of the hydrophobe. In the presence of electrolyte, aqueous solutions of HPAMs and ionic surfactant exhibit an associative phase separation. The temperature or clearing point (CP) at which the system goes from a one phase to two-phase system are reported. The area of the two-phase region is found to increase with increasing electrolyte concentration, hydrophobicity of the hydrophobe for the high molecular weight HPAMs, and decreasing hydrophobicity for low molecular weight HPAMs. Exposure of HPAMs containing an azobenzene hydro- phobe to UV light results in a decrease in interaction between the hydrophobe and surfactant and a corresponding decrease in the CP due to conversion of azobenzene from the more hydrophobic trans form to the less hydrophobic cis isomer. Key words Hydrophobically modified polymers - azobenzene - photoresponsive polymer polymer/ surfactant complexes - associative phase separation Introduction The unusual physio-chemical properties of both dilute and concentrated aqueous mixtures of polymers and surfac- tants have received considerable attention. Current and future applications in such diverse areas as e.g. coatings, enhanced oil recovery, and pharmaceutical formulations are the basis for the continued interest in these systems. In the case of neutral, water soluble polymers, interactions with ionic surfactants are relatively weak, but can be strongly enhanced by the incorporation of hydrophobic sidegroups on the polymer [1]. In aqueous solutions of such hydrophobically modified polymers (HMPs) ag- gregation of the hydrophobic sidegroups results in an enhanced viscosity of the solution, and this viscosity in- crease is strongly affected by the presence of surfactants. These rheological effects have led to an increasing use of HMPs as viscosity enhancers, with applications for in- stance in tertiary oil recovery and coating formulations [2]. Although the rheological properties of a number of HMP- surfactant systems have been reported, it is difficult to obtain quantitative information on the HMP-surfac- tant interactions. In this paper, we use a surfactant-selec- tive electrode method to obtain binding isotherms for an ionic surfactant with a number of HMPs of systematically varied hydrophobicity. In certain polymer systems, association between the polymer and surfactant can result in gel formation and in some instances phase separation. Polymer surfactant