ABC Heterotelechelic Associative Polyelectrolytes. Rheological Behavior in Aqueous Media Constantinos Tsitsilianis,* Ilias Katsampas, and Vasiliki Sfika Department of Chemical Engineering, University of Patras, 26500 Patras, Greece, and Institute of Chemical Engineering and High-Temperature Chemical Processes, ICE/HT-FORTH, P.O.Box 1414, 26500 Patras, Greece Received April 19, 2000 ABSTRACT: A polystyrene-poly(sodium acrylate)-poly(n-butyl methacrylate) (PS-PANa-PnBMA) ABC triblock copolymer was synthesized by sequential anionic polymerization and modification of the poly- (tert-butyl acrylate) middle block by selective hydrolysis and neutralization to its ionic functions. Because of the fact that both outer hydrophobic blocks are very short comparable to the middle one, this polymer can be viewed as a “heterotelechelic” polyelectrolyte. The rheological behavior of the PS-PANa-PnBMA aqueous solutions was explored by steady state and oscillatory shear flow measurements. Dilute solutions (c < 1 wt %) exhibit very high viscosity, showing that these kinds of polymers can be used as effective thickeners. This behavior is attributed to the formation of a transient network constituted from PS and/ or PnBMA physical cross-links interconnected by stretched polyelectrolyte chains. An unusual observation is that the viscosity of the system seems to be invariant over a wide temperature range. Finally, the differences between heterotelechelic and telehelic polyelectrolytes of the type PS-PANa-PS are also discussed. Introduction During the past decades block copolymers have been widely explored due to their ability to self-assemble into a wide variety of ordered structures. The observed morphologies are governed by several factors such as molecular weight interactions between the different chemically blocks, composition, and, finally, the macro- molecular architecture. The simplest cases of the block copolymers are the AB and ABA types, and their phase behavior have been extensively studied both in solution and in the bulk. More recently, a novel type of linear triblock copoly- mer, bearing three blocks of different nature (ABC type), exhibiting novel ordered structures and therefore new potential applications, has attracted the interest of several research groups. 1-22 In most cases the research efforts have been focused on the solid-state properties of linear ABC triblock copolymers, and only very few studies have been devoted to their solution proper- ties. 23-29 Different types of ABC amphiphilic block copolymers have been studied in aqueous solutions where micelli- zation phenomena have been observed. The obtained micellar structures depend strongly on the nature, the position, and the mutual interactions between the different blocks. For instance, when the ABC is consti- tuted from one hydrophobic (A) and two hydrophilic (B, C) blocks, the micelles exhibit a core-shell-shell struc- ture with (A) core and two sequential (B), (C) coronas. On the other hand, when one hydrophilic (C) and two incompatible hydrophobic (A, B) blocks constitute the ABC copolymer, a core-shell structure was observed. In the latter case, the hydrophobic core is constituted from an inner (A) part and an outer (B) core layer, surrounded from the (C) corona. 28 Concerning the morphology of the micelles described above, they exhibit a spherical shape. Very recently, other types of mor- phologies such as rod and vesicular-like can be formed in water-organic solvent mixtures by hydrophobic- hydrophobic-hydrophilic ABC triblock copolymers, pro- vided that the hydrophilic blocks are short. 29 In the present paper preliminary results concerning the aqueous solution properties of an amphiphilic ABC triblock copolymer constituted from a central long polyelectrolyte (PANa) end-capped with two different incompatible short hydrophobic blocks (PS, PnBMA) will be reported. In fact, this polymer can be viewed as a hydrophobic end-capped telechelic polymer, with the novelty being that the hydrophobic ends are of different nature. To distinguish this type of polymers from the classical ones, they can be termed as “heterotelechelic”. In aqueous media the hydrophobic end-blocks are anticipated to aggregate leading to the formation of an infinite transient network beyond a characteristic con- centration denoted C gel . Since the end-blocks PS and PnBMA are incompatible, this fact may have funda- mental consequences on their aqueous properties (i.e., micelles, gels). Very recently, the rheological behavior of a PS- PANa-PS (ABA) triblock copolymer (i.e., telechelic polyelectrolyte) in aqueous media has been reported for the first time. 30 This system exhibits some unique properties compared to those obtained from the classical associative aphiphilic telechelic polymers (e.g., end- capped poly(ethylene oxide)). 31-37 In dilute solutions (concentrations lower than 1 wt %) a transient network is formed, leading to the formation of a physical gel, characterized by low percolation concentration 0.2 wt %, yield stress, complex steady-shear viscosity profile, high plateau modulus, and long relaxation times. These characteristics are ascribed to the highly hydrophobic character of the PS junctions and the stretched confor- mation of the polyelectrolyte bridges of the network. A first attempt to compare the rheological behavior of the ABC with those of the corresponding ABA will also be presented. 9054 Macromolecules 2000, 33, 9054-9059 10.1021/ma0006777 CCC: $19.00 © 2000 American Chemical Society Published on Web 11/10/2000