Interfacial Tension in Binary Polymer Blends in the Presence of Block Copolymers. 2. Effects of Additive Architecture and Composition H. Retsos † and S. H. Anastasiadis* Institute of Electronic Structure and Laser, Foundation for Research and TechnologysHellas, P.O. Box 1527, 711 10 Heraklion, Crete, Greece, and Department of Physics, University of Crete, 710 03 Heraklion, Crete, Greece S. Pispas ‡ and J. W. Mays § Department of Chemistry, University of Alabama at Birmingham, Birmingham, Alabama 35294 N. Hadjichristidis Department of Chemistry, University of Athens, Zografou University City, 157 01 Zografou, Athens, Greece Received September 29, 2003; Revised Manuscript Received November 2, 2003 ABSTRACT: The effect of the macromolecular architecture and composition of block copolymer additives on the reduction of the interfacial tension between two immiscible homopolymers is investigated. A series of (polyisoprene) 2(polystyrene), I2S, graft copolymers with constant molecular weight and varying composition are utilized as additives in polystyrene/polyisoprene blends. The interfacial tension decreases with the addition of small amounts of copolymer and reaches a plateau at higher copolymer concentration. The interfacial tension at interfacial saturation depends on copolymer composition exhibiting a minimum, which is lower than that using a symmetric diblock with the same molecular weight. Moreover, the interfacial tension at saturation depends on the side of the interface the copolymer is introduced; adding it to the polyisoprene phase is much more efficient than adding it to polystyrene. This is due to the asymmetric architecture of the copolymer and points to the fact that a local equilibrium can only be attained in such systems: the copolymer reaching the interface from one homopolymer phase probably does not diffuse to the other phase. The fact that this behavior is not a kinetic effect is verified using (polystyrene) 2(polyisoprene), S2I, grafts, which show the mirror image behavior. The effectiveness of the interfacial modifiers is, thus, controlled by the unfavorable interactions, which drive the additive toward the interface, and the possibility of micelle formation, which hinders its activity. I. Introduction Mixing two or more components with complementary properties is largely utilized to improve the performance of polymeric materials for many important industrial applications. Suitably chosen block or graft copolymers are widely used in immiscible polymer blends as com- patibilizers for controlling the morphology (phase struc- ture) and the interfacial adhesion between the phases to obtain an optimized product. 1 This is due to their interfacial activity, i.e., to their affinity to selectively segregate to the interface between the phase-separated homopolymers, 2-4 which reduces the interfacial tension between the two macrophases, 5-8 prevents coales- cence 9,10 (thus leading to a finer and more homogeneous dispersion during mixing 9,11 ), and enhances interfacial adhesion. 12 The effectiveness of block or graft copolymers as emulsifiers is determined by their partitioning to the blend interface, 2,3 where each block preferentially ex- tends into its respective homopolymer phase. 4,13-15 The tendency for interfacial migration is predicted to depend on the molecular weights of the copolymer blocks relative to those of the homopolymers, 13,16-19 on the interaction parameter balance between the homopoly- mers and the copolymer blocks, 20 and on the macro- molecular architecture/topology and composition of the copolymers. 11b,21-26 There are a few points, however, which are usually overlooked especially in theoretical treatments. One relates to the fact that mixing the additive with one of the components may lead to the formation of copoly- meric micelles in the homopolymer matrix. 27 These would compete with the interfacial region for copolymer chains, and the amount of copolymer at the interface or in micelles would depend on the relative reduction in the free energy; typically, much of the premade copolymer would reside in micelles for high molecular weight additives. The effect of the existence of micelles on the interfacial partitioning of diblock copolymers at the polymer/polymer interface has received little atten- tion. 3,16-18,28-31 As an alternative, in situ formation of copolymers (usually grafts) is utilized 9,32,33 to overcome the “wasting” of the additive into micelles. The second point relates to the possible trapping 14 of copolymer chains at the interface, which leads to partial equilib- rium situations. Finally, since in a typical preparation of homopolymer/copolymer blends the system may be diffusion-controlled, the optimal conditions for the mo- lecular design of interfacially active copolymers obtained from equilibrium considerations may have to be modi- fied. * To whom correspondence should be addressed. E-mail: spiros@ iesl.forth.gr. † Present address: Institut de Biologie Structurale, 41 rue Jules Horowitz, 38027 Grenoble, France. ‡ Present address: Department of Chemistry, University of Athens, 157 01 Zografou, Athens, Greece. § Present address: Department of Chemistry, University of Tennessee, Knoxville, TN 37996-1600. 524 Macromolecules 2004, 37, 524-537 10.1021/ma035463e CCC: $27.50 © 2004 American Chemical Society Published on Web 12/23/2003