Modifying the rheological behavior of associative triblock copolymers in aqueous media through surfactant additives Stergios Pispas a , Dimitris Vlassopoulos b,c, * , George Fytas b,c , Benoit Loppinet b , Nikos Hadjichristidis d a National Hellenic Research Foundation, Institute of Theoretical and Physical Chemistry, Athens, Greece b FORTH, Institute of Electronic Structure & Laser, Heraklion, Crete, Greece c University of Crete, Department of Materials Science and Technology, Heraklion, Crete, Greece d University of Athens, Department of Chemistry, Panepistimiopolis Zografou, Athens, Greece Received 1 March 2006; received in revised form 24 May 2006; accepted 24 May 2006 Available online 4 August 2006 Abstract The rheological behavior of aqueous micellar solutions of a triblock copolymer, bearing two small hydrophobic end-blocks of polybutadiene and a large hydrophilic middle block of poly(sodium methacrylate), was studied in the presence of a cationic (dodecyl trimethyl ammonium bromide) or an anionic (sodium dodecyl sulfate) surfactant. Depending on the concentration and the charges of the interacting (with the water soluble middle block) surfactants, the rheological behavior of the triblock copolymer micelles (which resemble compact spheres, based on scattering studies) can be altered dramatically. The surfactant additives can either solidify a liquid-like system (low triblock concentration, dom- inated by loops) or alternatively liquefy a gel (high triblock concentration, dominated by bridges). Apparently, the hydrophobic tails of the surfactants prefer to join the hydrophobic polybutadiene cores of the triblocks, whereas by increasing the surfactant concentration the core func- tionality can be changed. In addition interactions between the oppositely charged hydrophilic surfactant heads and middle blocks can yield complexes producing new hydrophobic domains. These findings suggest possibilities for controlling complex fluid rheology. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Rheology; Poly(butadiene-b-sodium methacrylate-b-butadiene); Micelles in water 1. Introduction Solutions of triblock copolymers in selective solvents for the middle block have been the subject of many studies due to their interesting self-assembling behavior (flower-like and bridged micelles, physical gels, etc.) and dynamics, as well as the resulting potential to control fluid rheology [1e10]. Among them associative triblock copolymers in aqueous media have attracted considerable interest (experimental as well as theoretical) as a consequence of their potential use in rheology control of aqueous fluids, super-absorbents, con- trolled drug-release systems, paints, coatings, adhesives, actu- ators and sensors, etc. [1,3,6e16]. In all cases the chemical nature of the blocks plays the major role in affecting the mi- croscopic and macroscopic properties (hydrophilic to hydrophobic content, ionic or neutral character of the water soluble block, interactions with the solvent media, etc.). Thus, a great deal of effort has been directed in understanding the behavior and structure of such systems as well as in finding ways of controlling their rheological properties. Apart from parameters like polymer concentration, temper- ature, salt valency and concentration and pH, that have been found to affect the properties of associative triblock copolymer aqueous solutions and physical gels, the interactions with low molecular weight surfactant molecules have been investigated * Corresponding author. FORTH, Institute of Electronic Structure & Laser, P.O. Box 1527, 71110 Heraklion, Crete, Greece. Fax: þ30 81 391 305. E-mail address: dvlasso@iesl.forth.gr (D. Vlassopoulos). 0032-3861/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2006.05.078 Polymer 47 (2006) 7302e7311 www.elsevier.com/locate/polymer